ABSTRACT Itch, in particular chronic forms, has been widely recognized as an important clinical problem, but much less is known about the mechanisms of itch in comparison with other sensory

modalities such as pain. Recently, considerable progress has been made in dissecting the circuit mechanisms of itch at both the spinal and supraspinal levels. Major components of the spinal

neural circuit underlying both chemical and mechanical itch have now been identified, along with the circuits relaying ascending transmission and the descending modulation of itch. In this

review, we summarize the progress in elucidating the neural circuit mechanism of itch at spinal and supraspinal levels. SIMILAR CONTENT BEING VIEWED BY OTHERS A NEUROPEPTIDE CODE FOR ITCH

components1. Itch serves as an important protective mechanism that allows an animal to detect harmful substances invading the skin and remove them by scratching. The resultant scratching

behavior, which is driven by strong emotional and motivational components, can sometimes induce a pleasant feeling, leading to an itch-scratch cycle. This itch-scratch cycle can result in

serious skin damage for patients with chronic itch2. Itch, like many other somatosensations, typically originates from the skin. The itch signals are relayed by peripheral sensory fibers to

the spinal cord, where the information is processed by local interneurons before reaching the spinal projection neurons. The spinal projection neurons then send the itch signals to the brain

via the ascending pathways, and further processing of itch sensation occurs in multiple brain areas and circuits1,3,4,5. Based on peripheral inputs, itch can be classified into mechanical

and chemical itch. Chemical itch can be further classified into histamine-dependent and histamine-independent subclasses according to the response to antihistamine agents6. Progress has been

made in deciphering the molecular and cellular mechanisms of itch in the peripheral nervous system. Several key receptors, including members of the Mas-related G-protein-coupled receptor

(Mrgpr) and serotonin receptor families, were found to be important for detecting chemical itch signals7,8,9,10,11,12. It was shown that MrgprA3 marks a group of itch-selective neurons in

the dorsal root ganglia (DRG)10. In addition, transient receptor potential (TRP) channels have been shown to be recruited by histamine-dependent and histamine-independent itch pathways in

the periphery for itch signal transduction13,14. Consistently, mutation of the TRP channels causes pathological itch15. More recently, it has been shown that sodium channels expressed in

primary sensory neurons also play important roles in itch signal transmission16,17. These developments on the peripheral mechanisms of itch have been discussed in several excellent

reviews4,6,18,19. Using genetic, pharmacogenetic and optogenetic approaches, recent studies have also begun to dissect the itch circuitry within the central nervous system. In this review,

we will focus on these central circuit mechanisms that contribute to the sensation of itch. Note that although it is not possible to determine if an experimental animal feels the sensation

of itch, for the purposes of this review, we interchangeably use the terms itch and scratch for animal studies. SPINAL CIRCUITS OF CHEMICAL ITCH Multiple subtypes of excitatory neurons in

the spinal cord are involved in itch processing. Genetic deletion of the transcription factor Tlx3 or testicular orphan nuclear receptor TR4 causes the loss of excitatory neurons in laminae

I and II of the dorsal spinal cord, leading to significantly impaired pruritogen-induced scratching behavior20,21. Recent studies have identified the molecular markers for excitatory neurons

involved in the processing of chemical itch. Spinal neurons expressing gastrin-releasing peptide receptor (GRPR) are predominantly excitatory neurons, representing a key component of the

spinal itch circuit (Fig. 1a)22,23. This is evidenced by data showing that ablation of spinal GRPR+ neurons in mice almost completely abolishes the scratching behavior induced by both

histamine-dependent and histamine-independent pruritogens23, while optogenetic or pharmacogenetic activation of GRPR+ neurons directly evokes itch-like scratching or biting behaviors24.

Consistently, the number of spinal GRPR+ neurons are substantially decreased in mice lacking TR4, which show deficits in scratch responses21. In contrast, experimental ablation of spinal

GRPR+ neurons does not affect acute pain behaviors, indicating that spinal GRPR+ neurons are selectively involved in processing itch signals. In addition, ablation of spinal GRPR+ neurons

does not affect the number of neurons expressing neurokinin-1 (NK1R), which marks a significant percentage of projection neurons in the rodent spinal cord23. Consistently, studies have shown

that there is limited overlap between spinal GRPR+ neurons and spinal projection neurons labeled by retrograde tracing from the ventral posterolateral or ventral posteromedial nucleus

(VPL/VPM) of the thalamus or lateral parabrachial nucleus (LPBN)25. Moreover, it has been shown that spinal GRPR+ neurons form excitatory synapses with spinal projection neurons, which

convey the itch signal to the brain for further processing26. Thus, GRPR+ neurons represent a subpopulation of excitatory interneurons in the dorsal spinal cord that selectively transmit

itch information. Mice lacking GRPR show impaired responses to different types of pruritogens22, suggesting that GRPR is necessary for itch at the spinal level. Using slice electrophysiology

and optogenetics, Pagani et al. further explored the mechanism underlying GRP/GRPR neurotransmission27. They found that single action potentials in presynaptic GRP+ neurons were not

sufficient to evoke action potentials in postsynaptic excitatory GRPR+ neurons, although these two neuronal types were monosynaptically connected via glutamatergic synapses at a very high

connection rate. Only when the GRP+ neurons were activated in burst firing mode could suprathreshold activation of postsynaptic GRPR+ neurons be achieved, and this depolarized or active

state of GRPR+ neurons persisted for minutes even after the presynaptic optogenetic stimulus was terminated. Furthermore, the progressive depolarization of GRPR+ neurons depends on GRPR but

not glutamate receptor signaling, as blocking GRPR almost completely abolished the suprathreshold activation of GRPR+ neurons induced by burst-like stimulation of GRP+ neurons, while

antagonizing the glutamate receptors did not. Despite the essential role of GRP/GRPR in itch transmission, there has been long-standing controversy about the origin of GRP28. Early studies

using immunohistochemistry showed that GRP was expressed in a subset of DRG neurons22,29, and the expression level of GRP was upregulated in DRGs in chronic itch models30,31. Dorsal root

rhizotomy almost completely abolished GRP immuno-signals in the ipsilateral spinal cord30, confirming that the spinal GRP+ signals mainly originated from DRGs. However, some studies failed

to detect GRP immunostaining signals in the DRG; instead, they found abundant GRP expression in the spinal cord. In addition, neither dorsal rhizotomy nor ablation of spinal cord TRPV1+

terminals altered GRP immuno-signals in the spinal cord28. Measuring GRP mRNA level by qPCR and ISH also confirmed that GRP was localized in the superficial dorsal spinal cord but not in the

DRG28,32. Other groups further reported that GRP mRNA was barely detectable in DRGs of both juvenile and adult mice; instead, the majority of GRP is synthesized locally in the dorsal spinal

cord33. Moreover, directly labeling the GRP+ neurons by using the GRP-EGFP or GRP-Cre transgenic mouse line confirmed the existence of GRP+ neurons in the dorsal spinal cord but not in the

DRG27,32,34,35,36,37. Transcriptional profiling of DRG neurons using qRT-PCR or RNA-seq has revealed in an unbiased manner the molecular diversity of neurons underlying different

somatosensation, yet GRP was not detected in these primary sensory neurons in several cases38,39, further supporting the idea of GRP expression in the spinal cord rather than in the DRG.

However, a recent study in which a GRP-Cre mouse line was generated using a knock-in strategy showed Cre+ signals in a subset of DRG neurons as well as in the spinal cord40. Conditional

deletion of GRP in primary sensory neurons significantly attenuated itch behaviors, supporting the idea that GRP expression in primary sensory neurons is essential for itch signaling. This

study also showed that there are GRP+ neurons in the dorsal spinal cord, the ablation of which does not affect itch or pain responses. Optogenetic activation of GRP+ fibers in the skin was

found to evoke itch-like scratching behavior, further verifying the existence of GRP+ neurons in the DRG, as well as their functional role in itch transmission40. Taken together, these data

strongly suggest that GRP exists in spinal excitatory interneurons, as well as peripheral sensory neurons, both of which act upon downstream spinal GRPR+ neurons for itch transmission. The

differential observation of GRP expression in the DRG in different studies may have resulted from the different efficiency and specificity of GRP-Cre mouse lines, since different strategies

were used to generate these mice. Nevertheless, the question of whether GRP is expressed in the DRG has yet to be fully resolved. The functional role of spinal GRP+ neurons has been

extensively studied. Ablation of spinal GRP+ neurons labeled in a GRP-Cre transgenic mouse line reduced pruritogen-induced scratching behavior, while pharmacogenetic activation of these

neurons directly elicited spontaneous scratching behavior and enhanced the behavioral responses to various pruritogens36. By contrast, these manipulations caused no significant effect on the

behavioral responses to acute noxious stimuli, emphasizing the selective role of spinal GRP+ neurons in itch. Consistent with the properties of the GRP/GRPR signaling pathway, in vivo

optogenetic stimulation of spinal GRP+ neurons in burst mode, but not low-frequency stimulation with single pulses, elicited itch-like aversive behavior in freely moving mice27. Consistent

with these findings, Sun et al. also showed that ablation of spinal GRP+ neurons decreased itch responses. However, they found that manipulation of spinal GRP+ neurons also affected pain

responses37. Despite these conflicting results regarding the role of spinal GRP+ neurons in pain, which could largely result from the different experimental approaches for manipulating this

neuronal population in these two studies, they both suggest that spinal GRP+ neurons play a critical role in spinal itch transmission (Fig. 1a). However, using a different mouse line, Barry

et al. found that the ablation of spinal GRP+ neurons did not affect itch or pain responses40. This is likely due to differential labeling efficiency of the spinal cord neurons resulting

from different mouse lines used in these studies. Thus, additional studies are needed to further address the functional role of spinal GRP+ neurons in itch. In addition to GRP+ and GRPR+

neurons, another major component of the spinal circuit underlying chemical itch has also been identified32. Ablation of neuropeptide natriuretic polypeptide b (Nppb) receptor

(Npra)-expressing neurons in the dorsal spinal cord also selectively eliminates the itch responses induced by pruritogens, suggesting an important role of Npra+ neurons in itch processing.

This manipulation suppresses Nppb-evoked scratching behavior but does not affect scratching behavior evoked by intrathecal injection of GRP. Moreover, Npra is coexpressed with GRP in a

subset of spinal interneurons32. These data suggest that Npra+ neurons are located upstream of GRPR+ neurons along the spinal itch pathway (Fig. 1a). Another potential contributor to itch is

somatostatin (SST), which is expressed in both the primary sensory neurons and the spinal cord. Mice lacking SST in both the peripheral and spinal cord exhibit behavioral deficits in

response to pruritic stimuli. In addition, activation of the SST receptor Sst2a in the spinal cord enhances itch responses evoked by Nppb or GRP41. These data indicate that SST may also play

an important role in modulating the spinal itch circuit. GATING THE SPINAL CIRCUITS OF CHEMICAL ITCH BY LOCAL INHIBITORY NEURONS Spinal inhibitory interneurons play a prominent role in

regulating the spinal itch circuitry42,43,44. Dysregulation of spinal inhibitory interneurons could lead to hyperactivity of the itch circuit. This is seen from studies of mutant mice

lacking the atonal-related transcription factor Bhlhb5, which is transiently expressed in the dorsal spinal cord45. These mice exhibit excessive scratching behavior and elevated itch

responses to different types of pruritogens. Mice lacking Bhlhb5 were found to exhibit a dramatic loss of spinal interneurons expressing galanin and neuronal nitric oxide synthase (nNOS),

but the distribution of two other inhibitory neuronal populations, expressing neuropeptide Y (NPY) or parvalbumin (PV), remained intact46, suggesting that spinal galanin+ and/or nNOS+

interneurons play a key role in gating chemical itch. Indeed, a more recent study examined the functional role of spinal galanin+ and nNOS+ neurons in itch signal processing and found that

galanin+ neurons form predominant inhibitory synapses with spinal GRPR+ neurons and play an important role in gating chemical itch. By contrast, the functional role of spinal nNOS+ neurons

in itch signal processing remains elusive, given the heterogeneous composition of this molecularly defined neuronal population, as well as their complicated synaptic connection with GRPR+

neurons47. Furthermore, genetic deletion of vesicular glutamate transporter type 2 (_Vglut2_) in Nav1.8-expressing nociceptors resulted in pain deficits and enhancement of itch responses48,

supporting the notion that pain can suppress itch at the spinal level49. Consistently, electrophysiological results in the monkeys have shown that scratching suppresses the neural activity

of spinal neurons induced by pruritogens50, indicating an interaction between pain and itch at the spinal level. This finding is further supported by a recent study indicating that local

inhibitory neurons might be involved in the interaction between itch sensation and other somatosensory stimuli in the spinal cord of the rodents51. The Bhlhb5+ interneurons likely receive

direct synaptic input from primary sensory neurons responding to nociceptive and cooling stimuli46. Thus, Bhlhb5+ interneurons are well positioned to mediate the suppressing effect of pain

on itch. Spinal Bhlhb5+ neurons also express opioid peptide dynorphin (DYN)46. It was shown that kappa opioid receptor (KOR) agonists selectively inhibit itch-induced scratching behavior but

not pain-related behaviors, while blocking KOR signaling by KOR antagonists increases pruritogen-induced itch responses. These results suggest that spinal Bhlhb5+ interneurons inhibit itch

through kappa opioid signaling and that DYN acts as an important neuromodulator of itch transmission (Fig. 1a). Furthermore, intrathecal injection of a kappa opioid receptor agonist was

found to attenuate GRP-induced scratching behavior, suggesting that DYN might modulate itch signal processing by acting on GRPR+ neurons directly, or downstream of GRPR+ neurons in the

spinal cord46. Consistently, the application of a KOR antagonist evoked robust scratching behavior, and scratching was greatly reduced after ablating spinal GRPR+ neurons but not Npra+

neurons. These results indicate that DYN acts downstream of Npra+ neurons, probably at the level of spinal GRPR+ neurons41. Moreover, pharmacogenetic activation of spinal DYN+ neurons

significantly impairs pruritogen-induced scratching behavior, although such manipulation also affects mechanical nociception41. Thus, spinal DYN+ neurons also play a critical role in gating

itch processing. Moreover, spinal DYN+ neurons extensively colocalize with galanin rather than nNOS52, and chemogenetic activation of nNOS+ neurons has no effect on pruritogen-induced

behaviors, while modulating nociceptive responses. These data suggest that it is the DYN/galanin+ subpopulation of neurons, rather than the nNOS+ subpopulation of Bhlhb5+ neurons, that is

likely to be more important for itch suppression, which is also consistent with recent findings that spinal galanin+ neurons gate chemical itch transmission47. However, another group

reported that ablation of DYN+ neurons altered mechanical sensation but did not change pruritogen-induced scratching behavior53. This discrepancy is likely due to the difference in the

strategies used to manipulate DYN+ neurons. Spinal inhibitory neurons release gamma-aminobutyric acid (GABA) and/or glycine to tightly control the activity of postsynaptic neurons.

Pharmacological activation of spinal α2 and α3 GABAA receptors suppresses both acute and chronic itch, and chemogenetic activation of cervical spinal GABAergic neurons almost completely

abolishes pruritogen-induced scratching behavior, indicating a powerful inhibitory control of spinal itch circuits by GABAergic neurons47,54. Similarly, pharmacogenetic activation of spinal

glycinergic neurons was found to reduce neuropathic pain as well as pruritogen-evoked behaviors, indicating that spinal glycinergic neurons could also modulate the spinal itch circuitry55.

Different molecularly identified subpopulations of spinal Bhlhb5+ neurons use GABA and/or glycine as fast neurotransmitters; thus, both GABAergic and glycinergic interneurons provide

substantial inhibitory control of itch signal transmission in the spinal cord. SPINAL CIRCUITS OF MECHANICAL ITCH In addition to the chemical itch evoked by various pruritogens, itch

sensation can also be evoked by light tactile stimulation, known as mechanical itch. In the periphery, mechanical stimuli are transduced and transmitted primarily by Merkel cells and Aβ

primary sensory fibers, and the mechanosensitive Piezo channels expressed in Merkel cells functioning as the key mediator of mechanosensation56,57. Mechanical itch is independent of spinal

GRPR+ neurons58, indicating that chemical itch and mechanical itch are processed by independent neural circuits at the spinal level. Recent studies have identified several key components of

the neural circuit for mechanical itch. It has been shown that spinal excitatory interneurons expressing urocortin 3 (Ucn3) represent a key node of the neural circuit for mechanical itch59.

Ablation or pharmacogenetic inactivation of Ucn3+ neurons significantly attenuates the scratching behavior evoked by light touch stimuli but not by pruritogens. Consistent with the critical

role in mechanical itch transmission based on behavioral experiments, Ucn3+ neurons receive synaptic input from peripheral myelinated Toll-like receptor 5 (TLR5+) low-threshold

mechanoreceptors. Another subpopulation of spinal excitatory neurons, which express the neuropeptide Y1 receptor (NPY1R), was also shown to be critical for mechanical itch60. These neurons

receive extensive input from cutaneous low-threshold mechanoreceptors. Ablation or pharmacogenetic silencing of spinal NPY1R+ neurons reduces mechanical itch-dependent scratching behavior

but not scratching behavior evoked by pruritogens60. Consistently, activation of spinal NPY1R+ neurons increases light touch-induced as well as spontaneous scratching behaviors, which are

both GRPR+ neuron-independent. Thus, NPY1R+ neurons are largely specialized for transmitting mechanical itch information. Interestingly, there is only partial overlap between NPY1R+ neurons

and Ucn3+ neurons in the dorsal spinal cord59. However, another study found that intrathecal administration of a NPY1R agonist attenuated the scratching behavior evoked by both mechanical

and histamine-dependent chemical itch61, indicating a partially overlapping pathway for the transmission of these two different itch submodalities. These results emphasize the complexity of

the spinal circuits underlying mechanical itch and chemical itch. The spinal mechanical itch circuit is also gated by local inhibitory neurons58. Ablation of spinal neurons labeled by

neuropeptide Y (NPY) was found to induce spontaneous scratching behavior, which could not be blocked by the depletion of spinal GRPR+ neurons. Moreover, pharmacogenetic inactivation of

spinal NPY+ neurons also enhanced light touch-induced scratching behavior58. These results support the idea that NPY+ inhibitory neurons in the spinal cord play a key role in gating the

neural circuit responsible for mechanical itch. Consistently across studies, it has been shown that NPY+ neurons form functional inhibitory synaptic connections with NPY1R+ neurons and Ucn3+

neurons in the dorsal spinal cord59,60, confirming that NPY+ neurons gate the mechanical itch circuit at the spinal level (Fig. 1b). TRANSMISSION OF ITCH SIGNALS FROM THE SPINAL CORD TO THE

BRAIN Spinal projection neurons, which target multiple brain regions, serve as a key relay for sending various somatosensory information to the brain1,62,63,64,65. Among different pathways,

both spinothalamic and spinoparabrachial pathways are involved in the transmission of itch signals (Fig. 1). The transmission of itch signals from the spinal cord to the thalamus has been

shown by electrophysiological studies in different animal species showing that spinothalamic and trigeminothalamic tract neurons are activated by peripheral pruritic stimuli66,67,68, which

is consistent with the data showing that the number of c-Fos-positive spinal projection neurons increases after application of pruritogens to the skin26. Interestingly, different types of

pruritogens activate distinct subsets of spinothalamic tract (STT) neurons in primates67, suggesting that histamine-dependent and histamine-independent itch are processed by distinct

pathways. The functional role of spinal projection neurons has also been explored with behavioral experiments in rodents. The majority of projection neurons in the superficial dorsal horn

express the neurokinin 1 receptor (NK1R)62,69,70, which is the receptor for substance P. Ablation of spinal NK1R+ neurons by intrathecal application of substance P-saporin significantly

reduces scratching behavior evoked by pruritogens as well as chronic itch71,72. Interestingly, ablation of spinal NK1R+ neurons does not affect mechanical itch responses60, supporting the

segregation between chemical and mechanical itch pathways. Elimination of spinal NK1R+ neurons also attenuates behavioral responses to noxious stimuli72,73, suggesting that nociceptive

signals are also conveyed by these projection neurons. Electrophysiological recording revealed that primate STT neurons respond to peripheral itch, pain, mechanical, and thermal stimuli67,

indicating the polymodal properties of spinal projection neurons. Information from different somatosensory modalities is thus processed and integrated by spinal projection neurons1,43.

Electrophysiological recording experiments in the rats also demonstrated that the majority of trigeminoparabrachial tract (VcPbT) neurons could be activated by various pruritogens74, similar

to the response properties of trigeminothalamic tract (VcTT) neurons. Unlike VcTT neurons, the VcPbT neurons showed a delayed peak as well as a prolonged response pattern after application

of 5-hydroxytryptamine (5-HT)74, which better matched the time course of behavioral responses to pruritogens in awake animals. It was recently discovered that the spinoparabrachial pathway

is activated by peripheral pruritic stimuli and that optogenetic inhibition of this pathway attenuated pruritogen-induced scratching behavior26, suggesting that the spinoparabrachial pathway

plays a critical role in processing itch information. The parabrachial nucleus (PBN)-projecting spinal projection neurons receive direct synaptic inputs from spinal GRPR+ neurons26,

suggesting that itch signals are transmitted from GRPR+ neurons to the PBN via a disynaptic circuit. Thus, PBN serves as a first central relay for itch transmission in the brain. As

mentioned earlier, ascending transmission of distinct somatosensory modalities, i.e., pain and itch, relies on spinal projection neurons. The interaction between pain and itch is widely

distributed along sensory pathways, and there has been limited evidence directly examining the functional roles of spinocerebral projections in pain and itch simultaneously. It is thus

possible that pain and itch recruit a similar population of spinal projection neurons. Whether these projection neurons can be further genetically and functionally divided into subclasses

remains to be explored. BRAIN AREAS INVOLVED IN ITCH PROCESSING The representation of itch signal processing in the brain in earlier studies was mostly investigated by macroscopic imaging

approaches, including positron emission tomography (PET) scans and functional MRI (fMRI). These functional imaging studies in humans have identified many brain regions that are activated by

pruritic stimuli, as well as by itch-associated scratching or emotional changes75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92. Despite some differences in the brain regions revealed

by different studies, most studies have found activation of the thalamus, primary and secondary somatosensory cortex (S1 and S2), prefrontal cortex (PFC), anterior cingulate cortex (ACC),

insular cortex, premotor and motor cortex, and parietal cortex. These distinct brain areas are thought to be involved in different aspects of itch signal processing1,2,3,92. The thalamus is

typically recognized as the relay station for somatosensation from the spinal cord to the primary sensory cortices, and the somatosensory cortex is proposed to encode the spatial, temporal,

and intensity aspects of itch sensation1,2,3,92. Motor areas are thought to be involved in the planning and execution of itch-induced scratching behavior. Higher-order cortices, including

the PFC and ACC, are likely involved in processing the emotional or motivational components of itch sensation. In rodents, by anterograde tracing initiated from itch-selective spinal GRP+

neurons36, several brain areas, including the thalamus, PBN, amygdala, S1, periaqueductal gray (PAG), and rostral ventromedial medulla (RVM), have been suggested to be itch-related brain

areas. However, the functional role of most of these brain areas in itch remains to be examined. Histamine-dependent and histamine-independent itch activate slightly different brain regions,

although a series of core brain areas are activated by both kinds of itch, as revealed by human functional imaging studies87. Histamine-independent itch induced by cowhage more extensively

recruits the insular cortex, claustrum, basal ganglia, thalamic nuclei and pulvinar. Histamine-dependent and histamine-independent itch are thus processed by largely overlapping yet distinct

neural networks. The brain activity patterns responding to itch and pain are very similar88. No itch-selective brain region has been identified in most studies, suggesting that the

differences between itch and pain signal processing possibly exist at the cellular level. Molecular and cellular research strongly supports the existence of labeled lines, at least at the

level of peripheral sensory fibers and spinal cord interneurons, by which different somatosensory modalities are processed by specific receptors, cells or neuronal circuits4,5.

Electrophysiological studies, however, have directly challenged this theory by showing multimodal response properties of neurons at different stages along the sensory pathways to different

types of stimuli66,93. Instead, these data point toward a population coding mechanism that underlies the generation of the specific quality of somatosensation49,94,95. It is challenging to

reconcile the data obtained in behavioral and electrophysiological experiments, since the experimental conditions, animal species, investigative strategies and approaches were all discrepant

in different studies, yet it is hard to resist the idea that parallel pathways do exist to process specific somatosensory information, especially at the level of DRG and spinal

interneurons. Given the complex interactions between itch and pain, or between other somatosensations at multiple neural levels, plus the fact that similar brain structures and neural

networks could be recruited by both itch and pain, population coding might be a more economical and efficient way to differentiate diverse sensory modalities in the central nervous system,

especially at the level of spinal projection neurons and higher brain centers, although we cannot completely exclude the possibility that dedicated labeled lines are retained at the

supraspinal level. During the past few years, tremendous progress in the development of optogenetics, electrophysiology, in vivo imaging and other techniques has led to an unprecedented

opportunity to functionally dissect the role of different brain areas in itch signal processing. It has been shown that PBN neurons are activated by pruritic stimuli and that pharmacogenetic

inactivation of PBN neurons significantly impairs itch-induced scratching behavior26. Furthermore, selective knockout of the glutamate transporter _Vglut2_ in the PBN significantly

decreases the scratching behavior evoked by various types of pruritogens, as well as the scratching behavior in an allergic itch model, which suggests that PBN glutamatergic neurons play an

important role in itch processing and further verifies the essential role of PBN in processing the itch signal (Fig. 2). PBN sends dense projections to the amygdala, and it was shown that

acute inactivation of the central amygdala (CeA) by infusion of a GABAA agonist reduced pruritogen-induced scratching behavior96, suggesting that CeA is also involved in itch processing.

Dopaminergic (DA) neurons in the ventral tegmental area (VTA) play a key role in encoding motivational drive97. It was shown that VTA DA neurons are important for driving itch-induced

scratching behavior98. The activity of the DA neuronal population was immediately elevated after the onset of itch-induced scratching behavior, as recorded with fiber photometry in mice.

This activation pattern of VTA DA neurons was also verified by single-unit electrophysiological recording of opto-tagged DA neurons, in which a significant proportion of VTA DA neurons

increased their firing rate even slightly before the scratching onset, suggesting that VTA DA might encode the driving force for scratching behavior. Indeed, optogenetic inactivation of VTA

DA neurons impaired pruritogen-induced scratching behavior. This is consistent with previous pharmacological studies, in which blocking dopamine D1 receptors suppressed the scratching

behavior induced by compound 48/8099. Moreover, activation of dopamine D2, but not D1 receptors, directly induced scratching behavior in monkeys100, further indicating that DA signaling

could promote scratching behavior. Yuan et al. also investigated the activity of DA projections from VTA to the nucleus accumbens (NAc) during itch-induced scratching behavior98 and found

that DA fibers in the lateral shell of the NAc (NAc LaSh) showed more enhanced activity after scratching onset than in the medial shell of the NAc, suggesting that the VTA-NAc LaSh

projection might be more prominently involved in itch-induced scratching behavior. BRAIN MECHANISMS OF CONTAGIOUS ITCH Watching others scratching themselves or even talking about itchiness

can induce a desire to scratch in humans, despite no chemical or mechanical pruritic input1,91. This phenomenon is referred to as contagious itch. Previous fMRI studies have found that

visually induced itch activates many brain regions that are typically involved in the processing of physical itch91, including the primary somatosensory cortex, insular cortex, and

prefrontal cortex. A recent study demonstrated that contagious itch behavior is also observed in mice101. Mice exhibiting contagious itch behavior show higher level of c-Fos expression, as

well as lower level of GRP expression in the suprachiasmatic nucleus (SCN) of the hypothalamus, indicating that GRP+ neurons in the SCN may be involved in visually contagious itch101. The

receptor of GRP is also highly expressed in the SCN, and conditional knockout of _Grpr_ or ablation of GRPR+ neurons in the SCN reduces imitative scratching behavior. These data suggest a

potential role of GRP–GRPR signaling in the SCN for processing visually contagious itch in mice (Fig. 2). However, other groups102,103 failed to reproduce the imitative scratching behavior

in normal mice observing demonstrator mice receiving histamine application, nor did they detect significant temporally contiguous scratching in observer mice. In addition, the total

scratching bouts during the entire observation period was also not different between observers and control mice103. Similarly, another study also failed to replicate contagious scratching in

mice using an itching video-based paradigm, whereas itch sensation and responses were successfully elicited in humans102. Thus, whether mice can serve as a good animal model for studying

contagious itch sensation is still debatable. Nevertheless, the central circuit mechanisms of contagious itch are much less explored, and it would be interesting and important to investigate

the neural basis of contagious itch, especially in more sophisticated animal models such as primates. EMOTIONAL COMPONENTS OF ITCH SENSATION Itch, like many other somatosensations, is a

complex experience consisting of sensory, emotional and motivational components. Indeed, animal studies have shown that acute pruritic stimuli can induce conditional place aversion (CPA), as

well as anxiety-like behaviors, as revealed by the open field test (OFT) and elevated plus maze (EPM) assay, suggesting a negative emotional component of itch sensation104,105,106. As

indicated by previous human functional imaging studies, multiple brain regions could be involved in processing the affective components of itch sensation1,2. Recent animal studies have

further explored the possible mechanism underlying the negative emotional component of itch sensation. A subpopulation of CeA neurons was activated by various types of pruritogens, and a

subset of these itch-responsive neurons also responded to noxious stimuli104. Moreover, the neuronal activity of amygdala-projecting brain areas such as the PBN and mPFC was also increased

after pruritogen injection104. Importantly, optogenetic activation of histamine-responsive amygdala neurons reduced the duration in the open arm of the EPM and center time in the OFT,

suggesting that the amygdala plays a critical role in encoding the negative emotional component of itch sensation. In addition, VTA GABAergic neurons also play a critical role in encoding

the aversive component of itch sensation. The activity of VTA GABAergic neurons is immediately increased after itch-induced scratching behavior105. Silencing VTA GABAergic neurons has been

found to impair itch-associated CPA105, supporting the prominent role of VTA GABAergic neurons in encoding the negative emotional aspect of itch sensation. Additionally, a recent study

demonstrated that PAG is critical for modulating the affective component of itch. Pharmacogenetic activation of PAG GABAergic neurons impairs itch-associated CPA, suggesting that PAG

GABAergic neurons modulate the affective component of itch107. At the spinal level, the negative emotional signal associated with itch is conveyed by GRPR+ neurons106. Although itch is an

unpleasant sensation, scratching an itch can produce a hedonic experience50, which could facilitate the scratching behavior. VTA is a widely recognized reward center in the brain and is

capable of encoding both value and salience97. Human functional imaging studies have indicated that VTA is involved in the pleasure associated with scratching an itch84,90, and this was

further tested in a recent animal study105. Recording the population activity of DA neurons in the mouse VTA in response to pruritogen-induced scratching behavior with fiber photometry

showed that acute pruritic stimuli activated VTA DA neurons with a several-second delay upon scratching onset. Moreover, the activation of DA neurons vanished during scratching attempts when

the scratching behavior after pruritogen application was prevented with a custom-made collar, suggesting that VTA DA neurons might signal the pleasure associated with scratching an itch105.

Furthermore, inactivating VTA DA neurons attenuated scratch-associated conditional place preference (CPP), further supporting the crucial role of the VTA in regulating the emotional

components of the itch-scratch cycle (Fig. 2). The response of DA neurons exhibited high heterogeneity108, which could contribute to their functional heterogeneity98,105. Further molecular

or circuit-based dissection of VTA DA neurons in itch sensation is required. NEURAL CIRCUITS UNDERLYING MODULATION OF ITCH SIGNAL PROCESSING The processing of itch is gated by top-down

modulation, and multiple brain areas can differentially modulate itch signal processing. Human functional imaging studies have shown that the activity of PAG is altered during itch

processing2,87,89. Recordings from the PAG using fiber photometry in mice have also shown that the activity of PAG increases during itch processing107,109. Given its critical role in the

descending modulation of pain, it has been proposed that PAG could also play an important role in modulating itch1,49,92. This is supported by an animal study showing that electrical

stimulation of the PAG suppresses histamine-evoked responses in spinal dorsal horn neurons93. Recent studies examined the functional role of multiple neuronal subtypes of PAG in itch and

found that glutamatergic and GABAergic neurons in the PAG are differentially involved in itch as well as pain regulation107,109,110. Moreover, a subpopulation of PAG glutamatergic neurons

expressing tachykinin 1 (Tac1) plays an important role in facilitating the itch-scratch cycle, since the ablation or pharmacogenetic inhibition of these neurons reduces itch-induced

scratching behavior. Pharmacogenetic or optogenetic activation of PAG Tac1+ neurons has been found to directly induce a robust itch-like scratching behavior. The scratching behavior induced

by activation of Tac1+ neurons is reduced by ablation of spinal GRPR+ neurons, suggesting that PAG Tac1+ neurons modulate the spinal itch processing through a descending pathway109. PAG

Tac1+ neurons form glutamatergic synapses with spinal cord-projecting RVM neurons, supporting the idea that the PAGTac1-RVM circuit plays an important role in the descending regulation of

itch sensation (Fig. 2). The neuromodulatory system plays important roles in the descending modulation of spinal itch signal processing5. Among all neuromodulators, 5-HT has been most widely

implicated in the descending control of spinal sensory processing. Serotoninergic neurons in the RVM send projections to the trigeminal nucleus caudalis and the dorsal horn of the spinal

cord. Depletion of 5-HT+ fibers in the spinal cord decreases pruritogen-induced scratching behavior111, suggesting that descending serotoninergic projection facilitates itch processing in

the spinal cord. This effect is largely mediated by 5-HT1A receptors, which are coexpressed with GRPR in some spinal cord neurons. Coactivation of 5-HT1A and GRPR increases the excitability

of spinal GRPR+ neurons and greatly potentiates GRP-GRPR signaling. Interestingly, the facilitation of the spinal itch circuit by PAG Tac1+ neurons is not mediated by descending

serotoninergic projections109, indicating the existence of parallel pathways originating from RVM for itch modulation. Indeed, RVM can also form GABA/glycine-mediated fast inhibitory

synapses with spinal GRPR+ neurons, providing another potential descending pathway for itch modulation47. However, this finding cannot explain the facilitation of itch by the PAGTac1-RVM

circuit, as those projections are excitatory109. One possibility is that RVM modulates the spinal itch circuit through multiple pathways, and RVM neurons could facilitate itch signal

processing by a disynaptic disinhibition mechanism via local spinal inhibitory neurons. The essential role of the RVM in the descending modulation of pain has also been extensively

studied112,113. However, the exact identity of RVM neurons responsible for pain or itch modulation is still unknown, and it remains to be elucidated whether the descending modulation of itch

or pain is achieved by distinct subpopulations of RVM neurons or whether it is a shared neuron ensemble exerting a more generalized modulation in the processing of various types of

somatosensory information. In addition, noradrenaline is also involved in the regulation of spinal itch signal processing114,115,116. However, the circuit mechanism underlying the modulation

of the spinal itch circuitry by locus coeruleus-derived noradrenaline remains to be further examined. Higher-order brain areas can also dynamically modulate itch processing. It has been

shown that the projections from ACC to the dorsomedial striatum (DMS) selectively modulate histamine-dependent itch processing117. DMS-projecting ACC neurons are activated by histamine; and

disruption of the ACC–DMS pathway by targeted regional lesions attenuates histamine-induced but not 5-HT-induced scratching behavior, and does not affect the responses to acute noxious

stimuli. In line with these results, synaptic transmission in the ACC was shown to be potentiated by chronic itch118. Furthermore, optogenetic inhibition of the ACC–DMS connections

suppressed the scratching behavior induced by histamine-dependent but not histamine-independent pruritic stimuli and did not change capsaicin-evoked nocifensive behaviors. Moreover,

optogenetic activation of this pathway induced spontaneous scratching, which was greatly abolished after antagonizing histamine H1 or histamine H4 receptors117. These data indicate that the

ACC–DMS circuit exhibits a selective modulatory role in histaminergic itch (Fig. 2), further demonstrating the existence of segregated cerebral circuits mediating histamine-dependent and

histamine-independent itch sensation. FUTURE DIRECTIONS During the past decade, tremendous progress has been made in deciphering the circuit mechanisms of itch sensation in both the spinal

cord and the brain5. Several dedicated neural circuits have been demonstrated to process different forms of itch as well as modulate different aspects of itch sensation. Despite all these

exciting advances in the itch field, several key questions remain to be addressed. There is still debate about the coding mechanism of itch1,42,43,94,95. How is itch discriminated from other

somatosensations? Where is the perceptual distinction between different somatosensory modalities achieved? To address these questions, it will be important to record the activity of neurons

with cellular resolution along the sensory pathways during itch processing in awake animals, which can be achieved by in vivo extracellular recording or calcium imaging with endoscopy or

miniaturized two-photon microscopy119,120. Multiple key components of the neural circuit for itch signal processing have been identified at the spinal level. However, understanding of the

spinal circuit for itch processing and modulation is not complete. We know more about the inhibitory neuronal control of transmission for chemical itch, but less is known about the roles of

different inhibitory neuronal populations in the regulation of mechanical itch5,59,60. Whether those circuits actively involved in chemical itch modulation can also provide substantial

modulation of mechanical itch or whether these two different forms of itch sensation are dynamically regulated by distinct inhibitory neuronal networks remains to be elucidated. How do

different somatosensory modalities interact at the spinal level? The identity of the spinal projection neurons that send different types of itch information to the brain is still unknown.

What is the functional difference between the spinothalamic and spinoparabrachial projections in conveying itch as well as other somatosensory information? Recent sequencing studies have

provided more insight into the classification of neurons in the spinal cord121,122, and such knowledge together with new tools for dissecting local circuits will guide further dissection of

spinal circuits. Furthermore, the PBN is demonstrated to be a first relay station for itch transmission in the brain26, yet the genetic identity of itch-responsive PBN neurons needs to be

determined. Since PBN has been shown to participate in many other physiological processes, such as pain sensation123, it will be important to further investigate the integration or

segregation of different sensory information in the PBN. Finally, our knowledge of the modulatory network of itch sensation, especially the emotional and motivational components of itch, is

still limited and requires further systematic investigation. It is also critical to gain a deep understanding of the circuit mechanism underlying the positive emotional component, which is

one of the major driving forces for the vicious itch-scratch cycle. Developing new paradigms will be the key to address these questions. Additionally, the circuit mechanisms underlying

chronic itch are largely unknown. Dissection of the mechanism of chronic itch will guide the development of new therapeutic approaches for itch. REFERENCES * Ikoma, A., Steinhoff, M.,

Stander, S., Yosipovitch, G. & Schmelz, M. The neurobiology of itch. _Nat. Rev. Neurosci._ 7, 535–547 (2006). Article  CAS  PubMed  Google Scholar  * Dhand, A. & Aminoff, M. J. The

neurology of itch. _Brain_ 137, 313–322 (2014). Article  PubMed  Google Scholar  * Paus, R., Schmelz, M., Biro, T. & Steinhoff, M. Frontiers in pruritus research: scratching the brain

for more effective itch therapy. _J. Clin. Invest._ 116, 1174–1186 (2006). Article  CAS  PubMed  PubMed Central  Google Scholar  * Bautista, D. M., Wilson, S. R. & Hoon, M. A. Why we

scratch an itch: the molecules, cells and circuits of itch. _Nat. Neurosci._ 17, 175–182 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Dong, X. & Dong, X. Peripheral

and central mechanisms of itch. _Neuron_ 98, 482–494 (2018). Article  CAS  PubMed  PubMed Central  Google Scholar  * Han, L. & Dong, X. Itch mechanisms and circuits. _Annu. Rev.

Biophys._ 43, 331–355 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Wilson, S. R. et al. The epithelial cell-derived atopic dermatitis cytokine TSLP activates neurons to

induce itch. _Cell_ 155, 285–295 (2013). Article  CAS  PubMed  PubMed Central  Google Scholar  * Morita, T. et al. HTR7 mediates serotonergic acute and chronic itch. _Neuron_ 87, 124–138

(2015). Article  CAS  PubMed  PubMed Central  Google Scholar  * Liu, Q. et al. Sensory neuron-specific GPCR Mrgprs are itch receptors mediating chloroquine-induced pruritus. _Cell_ 139,

1353–1365 (2009). Article  PubMed  PubMed Central  Google Scholar  * Han, L. et al. A subpopulation of nociceptors specifically linked to itch. _Nat. Neurosci._ 16, 174–182 (2013). Article 

CAS  PubMed  Google Scholar  * Stantcheva, K. K. et al. A subpopulation of itch-sensing neurons marked by Ret and somatostatin expression. _EMBO Rep._ 17, 585–600 (2016). Article  CAS 

PubMed  PubMed Central  Google Scholar  * Solinski, H. J. et al. Nppb neurons are sensors of mast cell-induced itch. _Cell Rep._ 26, 3561–3573 e4 (2019). Article  CAS  PubMed  PubMed Central

  Google Scholar  * Shim, W. S. et al. TRPV1 mediates histamine-induced itching via the activation of phospholipase A2 and 12-lipoxygenase. _J. Neurosci._ 27, 2331–2337 (2007). Article  CAS

  PubMed  PubMed Central  Google Scholar  * Wilson, S. R. et al. TRPA1 is required for histamine-independent, Mas-related G protein-coupled receptor-mediated itch. _Nat. Neurosci._ 14,

595–602 (2011). Article  CAS  PubMed  PubMed Central  Google Scholar  * Lin, Z. M. et al. Exome sequencing reveals mutations in TRPV3 as a cause of olmsted syndrome. _Am. J. Hum. Genet._ 90,

558–564 (2012). Article  CAS  PubMed  PubMed Central  Google Scholar  * Tseng, P.-Y., Zheng, Q., Li, Z. & Dong, X. MrgprX1 mediates neuronal excitability and itch through

tetrodotoxin-resistant sodium channels. _Itch_ 4, e28 (2019). Article  PubMed  Google Scholar  * Salvatierra, J. et al. A disease mutation reveals a role for NaV1.9 in acute itch. _J. Clin.

Invest._ 128, 5434–5447 (2018). Article  PubMed  PubMed Central  Google Scholar  * LaMotte, R. H., Dong, X. & Ringkamp, M. Sensory neurons and circuits mediating itch. _Nat. Rev.

Neurosci._ 15, 19–31 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Akiyama, T. & Carstens, E. Neural processing of itch. _Neuroscience_ 250, 697–714 (2013). Article 

CAS  PubMed  Google Scholar  * Xu, Y. et al. Ontogeny of excitatory spinal neurons processing distinct somatic sensory modalities. _J. Neurosci._ 33, 14738–14748 (2013). Article  CAS  PubMed

  PubMed Central  Google Scholar  * Wang, X. et al. Excitatory superficial dorsal horn interneurons are functionally heterogeneous and required for the full behavioral expression of pain and

itch. _Neuron_ 78, 312–324 (2013). Article  CAS  PubMed  PubMed Central  Google Scholar  * Sun, Y. G. & Chen, Z. F. A gastrin-releasing peptide receptor mediates the itch sensation in

the spinal cord. _Nature_ 448, 700–703 (2007). Article  ADS  CAS  PubMed  Google Scholar  * Sun, Y. G. et al. Cellular basis of itch sensation. _Science_ 325, 1531–1534 (2009). Article  ADS

  CAS  PubMed  Google Scholar  * Bardoni, R. et al. Counter-stimuli inhibit GRPR neurons via GABAergic signaling in the spinal cord. _bioRxiv_, https://doi.org/10.1101/489831 (2018). *

Aresh, B. et al. Spinal cord interneurons expressing the gastrin-releasing peptide receptor convey itch through VGLUT2-mediated signaling. _Pain_ 158, 945–961 (2017). Article  CAS  PubMed 

PubMed Central  Google Scholar  * Mu, D. et al. A central neural circuit for itch sensation. _Science_ 357, 695–699 (2017). Article  ADS  CAS  PubMed  Google Scholar  * Pagani, M. et al. How

gastrin-releasing peptide opens the spinal gate for itch. _Neuron_ 103, 102–117 (2019). Article  CAS  PubMed  PubMed Central  Google Scholar  * Solorzano, C. et al. Primary afferent and

spinal cord expression of gastrin-releasing peptide: message, protein, and antibody concerns. _J. Neurosci._ 35, 648–657 (2015). Article  PubMed  PubMed Central  CAS  Google Scholar  *

Barry, D. M. et al. Critical evaluation of the expression of gastrin-releasing peptide in dorsal root ganglia and spinal cord. _Mol. Pain_ 12, 1744806916643724 (2016). * Zhao, Z. Q. et al.

Chronic itch development in sensory neurons requires BRAF signaling pathways. _J. Clin. Invest._ 123, 4769–4780 (2013). Article  CAS  PubMed  PubMed Central  Google Scholar  * Nattkemper, L.

A. et al. Overexpression of the gastrin-releasing peptide in cutaneous nerve fibers and its receptor in the spinal cord in primates with chronic itch. _J. Invest. Dermatol._ 133, 2489–2492

(2013). Article  CAS  PubMed  PubMed Central  Google Scholar  * Mishra, S. K. & Hoon, M. A. The cells and circuitry for itch responses in mice. _Science_ 340, 968–971 (2013). Article 

ADS  CAS  PubMed  PubMed Central  Google Scholar  * Fleming, M. S. et al. The majority of dorsal spinal cord gastrin releasing peptide is synthesized locally whereas neuromedin B is highly

expressed in pain- and itch-sensing somatosensory neurons. _Mol. Pain_ 8, 52 (2012). Article  CAS  PubMed  PubMed Central  Google Scholar  * Gutierrez-Mecinas, M., Furuta, T., Watanabe, M.

& Todd, A. J. A quantitative study of neurochemically defined excitatory interneuron populations in laminae I-III of the mouse spinal cord. _Mol. Pain_ 12, 1744806916629065 (2016). *

Gutierrez-Mecinas, M., Watanabe, M. & Todd, A. J. Expression of gastrin-releasing peptide by excitatory interneurons in the mouse superficial dorsal horn. _Mol. Pain_ 10, 79 (2014).

Article  PubMed  PubMed Central  CAS  Google Scholar  * Albisetti, G. W. et al. Dorsal horn gastrin-releasing peptide expressing neurons transmit spinal itch but not pain signals. _J.

Neurosci._ 39, 2238–2250 (2019). Article  PubMed  PubMed Central  Google Scholar  * Sun, S. et al. Leaky gate model: intensity-dependent coding of pain and itch in the spinal cord. _Neuron_

93, 840–853 (2017). Article  CAS  PubMed  PubMed Central  Google Scholar  * Chiu, I. M. et al. Transcriptional profiling at whole population and single cell levels reveals somatosensory

neuron molecular diversity. _Elife_ 3, e04660 (2014). * Usoskin, D. et al. Unbiased classification of sensory neuron types by large-scale single-cell RNA sequencing. _Nat. Neurosci._ 18,

145–153 (2015). Article  CAS  PubMed  Google Scholar  * Barry, D. M. et al. Exploration of sensory and spinal neurons expressing GRP in itch and pain. _bioRxiv_, 

https://doi.org/10.1101/472886 (2018). * Huang, J. et al. Circuit dissection of the role of somatostatin in itch and pain. _Nat. Neurosci._ 21, 707–716 (2018). Article  CAS  PubMed  PubMed

Central  Google Scholar  * Koch, S. C., Acton, D. & Goulding, M. Spinal circuits for touch, pain, and itch. _Annu. Rev. Physiol._ 80, 189–217 (2018). Article  CAS  PubMed  Google Scholar

  * Braz, J., Solorzano, C., Wang, X. & Basbaum, A. I. Transmitting pain and itch messages: a contemporary view of the spinal cord circuits that generate gate control. _Neuron_ 82,

522–536 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Polgar, E. et al. Functional differences between neurochemically defined populations of inhibitory interneurons in the

rat spinal dorsal horn. _Pain_ 154, 2606–2615 (2013). Article  CAS  PubMed  PubMed Central  Google Scholar  * Ross, S. E. et al. Loss of inhibitory interneurons in the dorsal spinal cord

and elevated itch in Bhlhb5 mutant mice. _Neuron_ 65, 886–898 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Kardon, A. P. et al. Dynorphin acts as a neuromodulator to

inhibit itch in the dorsal horn of the spinal cord. _Neuron_ 82, 573–586 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Liu, M. Z. et al. Synaptic control of spinal GRPR(+)

neurons by local and long-range inhibitory inputs. _Proc Natl. Acad. Sci. USA_ 116, 27011–27017 (2019). * Liu, Y. et al. VGLUT2-dependent glutamate release from nociceptors is required to

sense pain and suppress itch. _Neuron_ 68, 543–556 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Davidson, S. & Giesler, G. J. The multiple pathways for itch and their

interactions with pain. _Trends Neurosci._ 33, 550–558 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Davidson, S., Zhang, X., Khasabov, S. G., Simone, D. A. & Giesler,

G. J. Jr. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. _Nat. Neurosci._ 12, 544–546 (2009). Article  CAS  PubMed  PubMed Central  Google

Scholar  * Bardoni, R. et al. Pain inhibits GRPR neurons via GABAergic signaling in the spinal cord. _Sci. Rep._ 9, 15804 (2019). Article  ADS  PubMed  PubMed Central  CAS  Google Scholar  *

Sardella, T. C. P. et al. Dynorphin is expressed primarily by GABAergic neurons that contain galanin in the rat dorsal horn. _Mol. Pain_ 7, 1744–8069 (2011). * Duan, B. et al.

Identification of spinal circuits transmitting and gating mechanical pain. _Cell_ 159, 1417–1432 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Ralvenius, W. T. et al. Itch

suppression in mice and dogs by modulation of spinal alpha2 and alpha3GABAA receptors. _Nat. Commun._ 9, 3230 (2018). Article  ADS  PubMed  PubMed Central  CAS  Google Scholar  * Foster, E.

et al. Targeted ablation, silencing, and activation establish glycinergic dorsal horn neurons as key components of a spinal gate for pain and itch. _Neuron_ 85, 1289–1304 (2015). Article 

CAS  PubMed  PubMed Central  Google Scholar  * Woo, S. H. et al. Piezo2 is required for Merkel-cell mechanotransduction. _Nature_ 509, 622–626 (2014). Article  ADS  CAS  PubMed  PubMed

Central  Google Scholar  * Feng, J. et al. Piezo2 channel-Merkel cell signaling modulates the conversion of touch to itch. _Science_ 360, 530–533 (2018). Article  ADS  CAS  PubMed  PubMed

Central  Google Scholar  * Bourane, S. et al. Gate control of mechanical itch by a subpopulation of spinal cord interneurons. _Science_ 350, 550–554 (2015). Article  ADS  CAS  PubMed  PubMed

Central  Google Scholar  * Pan, H. et al. Identification of a spinal circuit for mechanical and persistent spontaneous itch. _Neuron_ 103, 1135–1149 (2019). * Acton, D. et al. Spinal

neuropeptide Y1 receptor-expressing neurons form an essential excitatory pathway for mechanical itch. _Cell Rep._ 28, 625–639 (2019). Article  MathSciNet  CAS  PubMed  PubMed Central  Google

Scholar  * Gao, T. et al. The neuropeptide Y system regulates both mechanical and histaminergic itch. _J. Invest. Dermatol._ 138, 2405–2411 (2018). Article  CAS  PubMed  Google Scholar  *

Todd, A. J., McGill, M. M. & Shehab, S. A. Neurokinin 1 receptor expression by neurons in laminae I, III and IV of the rat spinal dorsal horn that project to the brainstem. _Eur. J.

Neurosci._ 12, 689–700 (2000). Article  CAS  PubMed  Google Scholar  * Polgar, E., Wright, L. L. & Todd, A. J. A quantitative study of brainstem projections from lamina I neurons in the

cervical and lumbar enlargement of the rat. _Brain Res._ 1308, 58–67 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Todd, A. J. Neuronal circuitry for pain processing in the

dorsal horn. _Nat. Rev. Neurosci._ 11, 823–836 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Bernard, J. F., Dallel, R., Raboisson, P., Villanueva, L. & Le Bars, D.

Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat. _J. Comp. Neurol._ 353, 480–505

(1995). Article  CAS  PubMed  Google Scholar  * Davidson, S. et al. Pruriceptive spinothalamic tract neurons: physiological properties and projection targets in the primate. _J.

Neurophysiol._ 108, 1711–1723 (2012). Article  PubMed  PubMed Central  Google Scholar  * Davidson, S. et al. The itch-producing agents histamine and cowhage activate separate populations of

primate spinothalamic tract neurons. _J. Neurosci._ 27, 10007–10014 (2007). Article  CAS  PubMed  PubMed Central  Google Scholar  * Moser, H. R. & Giesler, G. J. Jr. Characterization of

pruriceptive trigeminothalamic tract neurons in rats. _J. Neurophysiol._ 111, 1574–1589 (2014). Article  PubMed  PubMed Central  Google Scholar  * Spike, R. C., Puskar, Z., Andrew, D. &

Todd, A. J. A quantitative and morphological study of projection neurons in lamina I of the rat lumbar spinal cord. _Eur. J. Neurosci._ 18, 2433–2448 (2003). Article  CAS  PubMed  Google

Scholar  * Al-Khater, K. M. & Todd, A. J. Collateral projections of neurons in laminae I, III, and IV of rat spinal cord to thalamus, periaqueductal gray matter, and lateral parabrachial

area. _J. Comp. Neurol._ 515, 629–646 (2009). Article  PubMed  PubMed Central  Google Scholar  * Carstens, E. E., Carstens, M. I., Simons, C. T. & Jinks, S. L. Dorsal horn neurons

expressing NK-1 receptors mediate scratching in rats. _Neuroreport_ 21, 303–308 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Akiyama, T. et al. A central role for spinal

dorsal horn neurons that express neurokinin-1 receptors in chronic itch. _Pain_ 156, 1240–1246 (2015). Article  CAS  PubMed  PubMed Central  Google Scholar  * Mantyh, P. W. et al. Inhibition

of hyperalgesia by ablation of lamina I spinal neurons expressing the substance P receptor. _Science_ 278, 275–279 (1997). Article  ADS  CAS  PubMed  Google Scholar  * Jansen, N. A. &

Giesler, G. J. Jr. Response characteristics of pruriceptive and nociceptive trigeminoparabrachial tract neurons in the rat. _J. Neurophysiol._ 113, 58–70 (2015). Article  PubMed  Google

Scholar  * Ishiuji, Y. et al. Distinct patterns of brain activity evoked by histamine-induced itch reveal an association with itch intensity and disease severity in atopic dermatitis. _Br.

J. Dermatol._ 161, 1072–1080 (2009). Article  CAS  PubMed  PubMed Central  Google Scholar  * Schneider, G. et al. Significant differences in central imaging of histamine-induced itch between

atopic dermatitis and healthy subjects. _Eur. J. Pain_ 12, 834–841 (2008). Article  CAS  PubMed  Google Scholar  * Darsow, U. et al. Processing of histamine-induced itch in the human

cerebral cortex: a correlation analysis with dermal reactions. _J. Invest. Dermatol._ 115, 1029–1033 (2000). Article  CAS  PubMed  Google Scholar  * Valet, M. et al. Cerebral processing of

histamine-induced itch using short-term alternating temperature modulation—an FMRI study. _J. Invest. Dermatol._ 128, 426–433 (2008). Article  CAS  PubMed  Google Scholar  * Yosipovitch, G.

et al. The brain processing of scratching. _J. Invest. Dermatol._ 128, 1806–1811 (2008). Article  CAS  PubMed  Google Scholar  * Herde, L., Forster, C., Strupf, M. & Handwerker, H. O.

Itch induced by a novel method leads to limbic deactivations a functional MRI study. _J. Neurophysiol._ 98, 2347–2356 (2007). Article  PubMed  Google Scholar  * Hsieh, J. C. et al. Urge to

scratch represented in the human cerebral cortex during itch. _J. Neurophysiol._ 72, 3004–3008 (1994). Article  CAS  PubMed  Google Scholar  * Leknes, S. G. et al. Itch and motivation to

scratch: an investigation of the central and peripheral correlates of allergen- and histamine-induced itch in humans. _J. Neurophysiol._ 97, 415–422 (2007). Article  PubMed  Google Scholar 

* Mochizuki, H. et al. Time course of activity in itch-related brain regions: a combined MEG-fMRI study. _J. Neurophysiol._ 102, 2657–2666 (2009). Article  PubMed  Google Scholar  *

Mochizuki, H. et al. The cerebral representation of scratching-induced pleasantness. _J. Neurophysiol._ 111, 488–498 (2014). Article  PubMed  Google Scholar  * Vierow, V. et al. Cerebral

representation of the relief of itch by scratching. _J. Neurophysiol._ 102, 3216–3224 (2009). Article  PubMed  Google Scholar  * Mochizuki, H. et al. Neural correlates of perceptual

difference between itching and pain: a human fMRI study. _Neuroimage_ 36, 706–717 (2007). Article  PubMed  Google Scholar  * Papoiu, A. D., Coghill, R. C., Kraft, R. A., Wang, H. &

Yosipovitch, G. A tale of two itches. Common features and notable differences in brain activation evoked by cowhage and histamine induced itch. _Neuroimage_ 59, 3611–3623 (2012). Article 

PubMed  Google Scholar  * Drzezga, A. et al. Central activation by histamine-induced itch: analogies to pain processing: a correlational analysis of O-15 H2O positron emission tomography

studies. _Pain_ 92, 295–305 (2001). Article  CAS  PubMed  Google Scholar  * Mochizuki, H. et al. Imaging of central itch modulation in the human brain using positron emission tomography.

_Pain_ 105, 339–346 (2003). Article  PubMed  Google Scholar  * Papoiu, A. D. et al. Brain’s reward circuits mediate itch relief. a functional MRI study of active scratching. _PLoS ONE_ 8,

e82389 (2013). Article  ADS  PubMed  PubMed Central  CAS  Google Scholar  * Holle, H., Warne, K., Seth, A. K., Critchley, H. D. & Ward, J. Neural basis of contagious itch and why some

people are more prone to it. _Proc. Natl Acad. Sci. USA_ 109, 19816–19821 (2012). Article  ADS  CAS  PubMed  PubMed Central  Google Scholar  * Mochizuki, H. & Kakigi, R. Central

mechanisms of itch. _Clin. Neurophysiol._ 126, 1650–1660 (2015). Article  PubMed  Google Scholar  * Carstens, E. Responses of rat spinal dorsal horn neurons to intracutaneous microinjection

of histamine, capsaicin, and other irritants. _J. Neurophysiol._ 77, 2499–2514 (1997). Article  CAS  PubMed  Google Scholar  * Ma, Q. Labeled lines meet and talk: population coding of

somatic sensations. _J. Clin. Invest._ 120, 3773–3778 (2010). Article  CAS  PubMed  PubMed Central  Google Scholar  * Prescott, S. A., Ma, Q. & De Koninck, Y. Normal and abnormal coding

of somatosensory stimuli causing pain. _Nat. Neurosci._ 17, 183–191 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Chen, L., Wang, W., Tan, T., Han, H. & Dong, Z.

GABA(A) receptors in the central nucleus of the amygdala are involved in pain- and itch-related responses. _J. Pain_ 17, 181–189 (2016). Article  CAS  PubMed  Google Scholar  *

Bromberg-Martin, E. S., Matsumoto, M. & Hikosaka, O. Dopamine in motivational control: rewarding, aversive, and alerting. _Neuron_ 68, 815–834 (2010). Article  CAS  PubMed  PubMed

Central  Google Scholar  * Yuan, L., Liang, T. Y., Deng, J. & Sun, Y. G. Dynamics and functional role of dopaminergic neurons in the ventral tegmental area during itch processing. _J.

Neurosci._ 38, 9856–9869 (2018). Article  CAS  PubMed  PubMed Central  Google Scholar  * Akimoto, Y. & Furuse, M. SCH23390, a dopamine D1 receptor antagonist, suppressed scratching

behavior induced by compound 48/80 in mice. _Eur. J. Pharm._ 670, 162–167 (2011). Article  CAS  Google Scholar  * Pellon, R., Flores, P., Alling, K., Witkin, J. M. & Katz, J. L.

Pharmacological analysis of the scratching produced by dopamine D2 agonists in squirrel monkeys. _J. Pharm. Exp. Ther._ 273, 138–145 (1995). CAS  Google Scholar  * Yu, Y. Q., Barry, D. M.,

Hao, Y., Liu, X. T. & Chen, Z. F. Molecular and neural basis of contagious itch behavior in mice. _Science_ 355, 1072–1076 (2017). Article  ADS  CAS  PubMed  PubMed Central  Google

Scholar  * Lu, J. S. et al. Contagious itch can be induced in humans but not in rodents. _Mol. Brain_ 12, 38 (2019). Article  PubMed  PubMed Central  Google Scholar  * Liljencrantz, J.,

Pitcher, M. H., Low, L. A., Bauer, L. & Bushnell, M. C. Comment on “Molecular and neural basis of contagious itch behavior in mice”. _Science_ 357, eaan4749 (2017). * Sanders, K. M.,

Sakai, K., Henry, T. D., Hashimoto, T. & Akiyama, T. A subpopulation of amygdala neurons mediates the affective component of itch. _J. Neurosci._ 39, 3345–3356 (2019). Article  CAS 

PubMed  PubMed Central  Google Scholar  * Su, X. Y. et al. Central processing of itch in the midbrain reward center. _Neuron_ 102, 858–872 (2019). Article  CAS  PubMed  Google Scholar  * Mu,

D. & Sun, Y. G. Itch induces conditioned place aversion in mice. _Neurosci. Lett._ 658, 91–96 (2017). Article  CAS  PubMed  Google Scholar  * Samineni, V. K., Grajales-Reyes, J. G.,

Sundaram, S. S., Yoo, J. J. & Gereau, R. W. T. Cell type-specific modulation of sensory and affective components of itch in the periaqueductal gray. _Nat. Commun._ 10, 4356 (2019).

Article  ADS  PubMed  PubMed Central  CAS  Google Scholar  * Morales, M. & Margolis, E. B. Ventral tegmental area: cellular heterogeneity, connectivity and behaviour. _Nat. Rev.

Neurosci._ 18, 73–85 (2017). Article  CAS  PubMed  Google Scholar  * Gao, Z. R. et al. Tac1-expressing neurons in the periaqueductal gray facilitate the itch-scratching cycle via descending

regulation. _Neuron_ 101, 45–59 e9 (2019). Article  CAS  PubMed  Google Scholar  * Samineni, V. K. et al. Divergent modulation of nociception by glutamatergic and GABAergic neuronal

subpopulations in the periaqueductal gray. _eNeuro_ 4, ENEURO.0129-16.2017 (2017). * Zhao, Z. Q. et al. Descending control of itch transmission by the serotonergic system via

5-HT1A-facilitated GRP-GRPR signaling. _Neuron_ 84, 821–834 (2014). Article  CAS  PubMed  PubMed Central  Google Scholar  * Francois, A. et al. A brainstem-spinal cord inhibitory circuit for

mechanical pain modulation by GABA and enkephalins. _Neuron_ 93, 822–839 (2017). Article  CAS  PubMed  PubMed Central  Google Scholar  * Fields, H. L., Bry, J., Hentall, I. & Zorman, G.

The activity of neurons in the rostral medulla of the rat during withdrawal from noxious heat. _J. Neurosci._ 3, 2545–2552 (1983). Article  CAS  PubMed  PubMed Central  Google Scholar  *

Gotoh, Y., Omori, Y., Andoh, T. & Kuraishi, Y. Tonic inhibition of allergic itch signaling by the descending noradrenergic system in mice. _J. Pharmacol. Sci._ 115, 417–420 (2011).

Article  CAS  PubMed  Google Scholar  * Gotoh, Y., Andoh, T. & Kuraishi, Y. Noradrenergic regulation of itch transmission in the spinal cord mediated by alpha-adrenoceptors.

_Neuropharmacology_ 61, 825–831 (2011). Article  CAS  PubMed  Google Scholar  * Kuraishi, Y. Noradrenergic modulation of itch transmission in the spinal cord. _Handb. Exp. Pharm._ 226,

207–217 (2015). Article  CAS  Google Scholar  * Lu, Y. C. et al. ACC to dorsal medial striatum inputs modulate histaminergic itch sensation. _J. Neurosci._ 38, 3823–3839 (2018). Article  CAS

  PubMed  PubMed Central  Google Scholar  * Zhang, T. T. et al. Potentiation of synaptic transmission in Rat anterior cingulate cortex by chronic itch. _Mol. Brain_ 9, 73 (2016). Article 

CAS  PubMed  PubMed Central  Google Scholar  * Luo, L., Callaway, E. M. & Svoboda, K. Genetic dissection of neural circuits: a decade of progress. _Neuron_ 98, 256–281 (2018). Article 

CAS  PubMed  PubMed Central  Google Scholar  * Zong, W. et al. Fast high-resolution miniature two-photon microscopy for brain imaging in freely behaving mice. _Nat. Methods_ 14, 713–719

(2017). Article  CAS  PubMed  Google Scholar  * Haring, M. et al. Neuronal atlas of the dorsal horn defines its architecture and links sensory input to transcriptional cell types. _Nat.

Neurosci._ 21, 869–880 (2018). Article  PubMed  CAS  Google Scholar  * Sathyamurthy, A. et al. Massively parallel single nucleus transcriptional profiling defines spinal cord neurons and

their activity during behavior. _Cell Rep._ 22, 2216–2225 (2018). Article  CAS  PubMed  PubMed Central  Google Scholar  * Han, S., Soleiman, M. T., Soden, M. E., Zweifel, L. S. &

Palmiter, R. D. Elucidating an affective pain circuit that creates a threat memory. _Cell_ 162, 363–374 (2015). Article  CAS  PubMed  PubMed Central  Google Scholar  Download references

ACKNOWLEDGEMENTS We thank Andrew J. Todd, Earl Carstens, and Yong-Jing Gao for comments on the manuscript. This work is supported by the National Natural Science Foundation of China (No.

31825013, 61890952), the Shanghai Municipal Science and Technology Major Project (Grant No. 2018SHZDZX05), and the Strategic Priority Research Program of the Chinese Academy of Sciences

(Grant No. XDB32010200). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Institute of Neuroscience, State Key Laboratory of Neuroscience, Center for Excellence in Brain Science &

Intelligence Technology, Chinese Academy of Sciences, 320 Yue-Yang Road, 200031, Shanghai, China Xiao-Jun Chen & Yan-Gang Sun * University of Chinese Academy of Sciences, 19A Yu-quan

Road, 100049, Beijing, China Xiao-Jun Chen * Shanghai Center for Brain Science and Brain-Inspired Intelligence Technology, 201210, Shanghai, China Yan-Gang Sun Authors * Xiao-Jun Chen View

author publications You can also search for this author inPubMed Google Scholar * Yan-Gang Sun View author publications You can also search for this author inPubMed Google Scholar

CORRESPONDING AUTHOR Correspondence to Yan-Gang Sun. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER’S NOTE Springer

Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. RIGHTS AND PERMISSIONS OPEN ACCESS This article is licensed under a Creative

Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the

original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in

the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended

use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit

http://creativecommons.org/licenses/by/4.0/. Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Chen, XJ., Sun, YG. Central circuit mechanisms of itch. _Nat Commun_ 11, 3052

(2020). https://doi.org/10.1038/s41467-020-16859-5 Download citation * Received: 29 September 2019 * Accepted: 30 May 2020 * Published: 16 June 2020 * DOI:

https://doi.org/10.1038/s41467-020-16859-5 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not

currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative