Spinal Projection Neurons Research Articles

Astrocyte D1/D5 Dopamine Receptors Govern Non-Hebbian Long-Term Potentiation at Sensory Synapses onto Lamina I Spinoparabrachial Neurons.

Recent work demonstrated that activation of spinal D1 and D5 dopamine receptors (D1/D5Rs) facilitates non-Hebbian long-term potentiation (LTP) at primary afferent synapses onto spinal projection neurons. However, the cellular localization of the D1/D5Rs driving non-Hebbian LTP in spinal nociceptive circuits remains unknown, and it is also unclear whether D1/D5R signaling must occur concurrently with sensory input in order to promote non-Hebbian LTP at these synapses. Here we investigate these issues using cell-type-selective knockdown of D1Rs or D5Rs from lamina I spinoparabrachial neurons, dorsal root ganglion (DRG) neurons, or astrocytes in adult mice of either sex using Cre recombinase-based genetic strategies. The LTP evoked by low-frequency stimulation of primary afferents in the presence of the selective D1/D5R agonist SKF82958 persisted following the knockdown of D1R or D5R in spinoparabrachial neurons, suggesting that postsynaptic D1/D5R signaling was dispensable for non-Hebbian plasticity at sensory synapses onto these key output neurons of the superficial dorsal horn (SDH). Similarly, the knockdown of D1Rs or D5Rs in DRG neurons failed to influence SKF82958-enabled LTP in lamina I projection neurons. In contrast, SKF82958-induced LTP was suppressed by the knockdown of D1R or D5R in spinal astrocytes. Furthermore, the data indicate that the activation of D1R/D5Rs in spinal astrocytes can either retroactively or proactively drive non-Hebbian LTP in spinoparabrachial neurons. Collectively, these results suggest that dopaminergic signaling in astrocytes can strongly promote activity-dependent LTP in the SDH, which is predicted to significantly enhance the amplification of ascending nociceptive transmission from the spinal cord to the brain.

Molecular Organization of Autonomic, Respiratory, and Spinally-Projecting Neurons in the Mouse Ventrolateral Medulla.

The ventrolateral medulla (VLM) is a crucial region in the brain for visceral and somatic control, serving as a significant source of synaptic input to the spinal cord. Experimental studies have shown that gene expression in individual VLM neurons is predictive of their function. However, the molecular and cellular organization of the VLM has remained uncertain. This study aimed to create a comprehensive dataset of VLM cells using single-cell RNA sequencing in male and female mice. The dataset was enriched with targeted sequencing of spinally-projecting and adrenergic/noradrenergic VLM neurons. Based on differentially expressed genes, the resulting dataset of 114,805 VLM cells identifies 23 subtypes of neurons, excluding those in the inferior olive, and five subtypes of astrocytes. Spinally-projecting neurons were found to be abundant in seven subtypes of neurons, which were validated through in situ hybridization. These subtypes included adrenergic/noradrenergic neurons, serotonergic neurons, and neurons expressing gene markers associated with premotor neurons in the ventromedial medulla. Further analysis of adrenergic/noradrenergic neurons and serotonergic neurons identified nine and six subtypes, respectively, within each class of monoaminergic neurons. Marker genes that identify the neural network responsible for breathing were concentrated in two subtypes of neurons, delineated from each other by markers for excitatory and inhibitory neurons. These datasets are available for public download and for analysis with a user-friendly interface. Collectively, this study provides a fine-scale molecular identification of cells in the VLM, forming the foundation for a better understanding of the VLM's role in vital functions and motor control.

Spinal projecting neurons in rostral ventromedial medulla co-regulate motor and sympathetic tone

Many behaviors require the coordinated actions of somatic and autonomic functions. However, the underlying mechanisms remain elusive. By opto-stimulating different populations of descending spinal projecting neurons (SPNs) in anesthetized mice, we show that stimulation of excitatory SPNs in the rostral ventromedial medulla (rVMM) resulted in a simultaneous increase in somatomotor and sympathetic activities. Conversely, opto-stimulation of rVMM inhibitory SPNs decreased both activities. Anatomically, these SPNs innervate both sympathetic preganglionic neurons and motor-related regions in the spinal cord. Fiber-photometry recording indicated that the activities of rVMM SPNs correlate with different levels of muscle and sympathetic tone during distinct arousal states. Inhibiting rVMM excitatory SPNs reduced basal muscle and sympathetic tone, impairing locomotion initiation and high-speed performance. In contrast, silencing the inhibitory population abolished muscle atonia and sympathetic hypoactivity during rapid eye movement (REM) sleep. Together, these results identify rVMM SPNs as descending spinal projecting pathways controlling the tone of both the somatomotor and sympathetic systems.

Functional dissection of parabrachial substrates in processing nociceptive information.

Painful stimuli elicit first-line reflexive defensive reactions and, in many cases, also evoke second-line recuperative behaviors, the latter of which reflects the sensing of tissue damage and the alleviation of suffering. The lateral parabrachial nucleus (lPBN), composed of external- (elPBN), dorsal- (dlPBN), and central/superior-subnuclei (jointly referred to as slPBN), receives sensory inputs from spinal projection neurons and plays important roles in processing affective information from external threats and body integrity disruption. However, the organizational rules of lPBN neurons that provoke diverse behaviors in response to different painful stimuli from cutaneous and deep tissues remain unclear. In this study, we used region-specific neuronal depletion or silencing approaches combined with a battery of behavioral assays to show that slPBN neurons expressing substance P receptor ( NK1R) (lPBN NK1R) are crucial for driving pain-associated self-care behaviors evoked by sustained noxious thermal and mechanical stimuli applied to skin or bone/muscle, while elPBN neurons are dispensable for driving such reactions. Notably, lPBN NK1R neurons are specifically required for forming sustained somatic pain-induced negative teaching signals and aversive memory but are not necessary for fear-learning or escape behaviors elicited by external threats. Lastly, both lPBN NK1R and elPBN neurons contribute to chemical irritant-induced nocifensive reactions. Our results reveal the functional organization of parabrachial substrates that drive distinct behavioral outcomes in response to sustained pain versus external danger under physiological conditions.

A transcriptomic taxonomy of mouse brain-wide spinal projecting neurons

The brain controls nearly all bodily functions via spinal projecting neurons (SPNs) that carry command signals from the brain to the spinal cord. However, a comprehensive molecular characterization of brain-wide SPNs is still lacking. Here we transcriptionally profiled a total of 65,002 SPNs, identified 76 region-specific SPN types, and mapped these types into a companion atlas of the whole mouse brain1. This taxonomy reveals a three-component organization of SPNs: (1) molecularly homogeneous excitatory SPNs from the cortex, red nucleus and cerebellum with somatotopic spinal terminations suitable for point-to-point communication; (2) heterogeneous populations in the reticular formation with broad spinal termination patterns, suitable for relaying commands related to the activities of the entire spinal cord; and (3) modulatory neurons expressing slow-acting neurotransmitters and/or neuropeptides in the hypothalamus, midbrain and reticular formation for ‘gain setting’ of brain–spinal signals. In addition, this atlas revealed a LIM homeobox transcription factor code that parcellates the reticulospinal neurons into five molecularly distinct and spatially segregated populations. Finally, we found transcriptional signatures of a subset of SPNs with large soma size and correlated these with fast-firing electrophysiological properties. Together, this study establishes a comprehensive taxonomy of brain-wide SPNs and provides insight into the functional organization of SPNs in mediating brain control of bodily functions.

Thermosensory thalamus: parallel processing across model organisms.

The thalamus acts as an interface between the periphery and the cortex, with nearly every sensory modality processing information in the thalamocortical circuit. Despite well-established thalamic nuclei for visual, auditory, and tactile modalities, the key thalamic nuclei responsible for innocuous thermosensation remains under debate. Thermosensory information is first transduced by thermoreceptors located in the skin and then processed in the spinal cord. Temperature information is then transmitted to the brain through multiple spinal projection pathways including the spinothalamic tract and the spinoparabrachial tract. While there are fundamental studies of thermal transduction via thermosensitive channels in primary sensory afferents, thermal representation in the spinal projection neurons, and encoding of temperature in the primary cortical targets, comparatively little is known about the intermediate stage of processing in the thalamus. Multiple thalamic nuclei have been implicated in thermal encoding, each with a corresponding cortical target, but without a consensus on the role of each pathway. Here, we review a combination of anatomy, physiology, and behavioral studies across multiple animal models to characterize the thalamic representation of temperature in two proposed thermosensory information streams.

Genetic evidence of the function of Phox2a-expressing anterolateral system neurons in the transmission of chronic pain.

The development of the chronic neuropathic pain state often originates at the level of peripheral sensory neurons, whose abnormal function elicits central sensitization and maladaptive plasticity in the nociceptive circuits of the spinal dorsal horn. These changes eventually reach supraspinal areas bringing about cognitive and affective co-morbidities of chronic pain such as anxiety and depression. This transmission presumably relies on the function of spinal projection neurons at the origin of the anterolateral system (AS). However, the identity of these neurons and the extent of their functional contribution remain unknown. Here, we asked these questions in the context of the mouse AS neurons that require the transcription factor Phox2a for their normal target connectivity and function in transmitting acute nociceptive information to the brain. To this end, we examined the effects of a spinal cord-specific loss of Phox2a (Phox2acKO) on the development of central sensitization evoked by the spared nerve injury (SNI) model of chronic pain. We found that SNI-treated Phox2acKO mice developed normal reflexive spinal responses such as mechanical allodynia evidenced by a decreased withdrawal threshold to von Frey filament stimulation and dynamic brush. On the other hand, Phox2acKO attenuated the development of cold but not mechanical hyperalgesia, in behavioral paradigms that require the relay of nociceptive information to the brain. Furthermore, Phox2acKO attenuated anxio-depressive-like behaviors evoked by SNI, measured by performance in the open field test and tail suspension test. Thus, Phox2a AS neurons play a critical role in the generation and maintenance of chronic neuropathic pain.

Cell type-specific calcium imaging of central sensitization in mouse dorsal horn

Allodynia is a state in which pain is elicited by innocuous stimuli. Capsaicin applied to the skin results in an allodynia that extends to a broad region beyond the application site. This sensitization is thought to be mediated by spinal networks; however, we do not have a clear picture of which spinal neurons mediate this phenomenon. To address this gap, we used two-photon calcium imaging of excitatory interneurons and spinal projection neurons in the mouse spinal dorsal horn. To distinguish among neuronal subtypes, we developed CICADA, a cell profiling approach to identify cell types during calcium imaging. We then identified capsaicin-responsive and capsaicin-sensitized neuronal populations. Capsaicin-sensitized neurons showed emergent responses to innocuous input and increased receptive field sizes consistent with psychophysical reports. Finally, we identified spinal output neurons that showed enhanced responses from innocuous input. These experiments provide a population-level view of central sensitization and a framework with which to model somatosensory integration in the dorsal horn.

Sex-specific characteristics of cells expressing the cannabinoid 1 receptor in the dorsal horn of the lumbar spinal cord.

It is becoming increasingly clear that robust sex differences exist in the processing of acute and chronic pain in both rodents and humans. However, the underlying mechanism has not been well characterized. The dorsal horn of the lumbar spinal cord is the fundamental building block of ascending and descending pain pathways. It has been shown that numerous neurotransmitter and neuromodulator systems in the spinal cord, including the endocannabinoid system and its main receptor, the cannabinoid 1 receptor (CB1 R), play vital roles in processing nociceptive information. Our previous findings have shown that CB1 R mRNA is widely expressed in the brain in sex-dependent patterns. However, the sex-, lamina-, and cell-type-specific characteristics of CB1 R expression in the spinal cord have not been fully described. In this study, the CB1 R-iCre-EGFP mouse strain was generated to label and identify CB1 R-positive (CB1 RGFP ) cells. We reported no sex difference in CB1 R expression in the lumbar dorsal horn of the spinal cord, but a dynamic distribution within superficial laminae II and III in female mice between estrus and nonestrus phases. Furthermore, the cell-type-specific CB1 R expression pattern in the dorsal horn was similar in both sexes. Over 50% of CB1 RGFP cells were GABAergic neurons, and approximately 25% were glycinergic and 20-30% were glutamatergic neurons. The CB1 R-expressing cells also represented a subset of spinal projection neurons. Overall, our work indicates a highly consistent distribution pattern of CB1 RGFP cells in the dorsal horn of lumbar spinal cord in males and females.

Population Imaging of Central Sensitization

Capsaicin applied locally to the skin causes central sensitization that results in allodynia, a state in which pain is elicited by innocuous stimuli. Here, we used two-photon calcium imaging of neurons in the dorsal spinal cord to visualize central sensitization across excitatory interneurons and spinal projection neurons. To distinguish among excitatory neuron subtypes, we developed CICADA, a cell profiling approach that leverages the expression of distinct Gq-coupled receptors. We then identified capsaicin-responsive and capsaicin-sensitized neuronal populations. Capsaicin-sensitized neurons showed emergent responses to low threshold input and increased receptive field sizes consistent with the psychophysical phenomenon that allodynia is observed across an extended secondary zone. Finally, we identified spinal projection neurons that showed a shift in tuning toward low threshold input. These experiments provide a population-level view of central sensitization and a framework with which to model somatosensory integration in the dorsal horn. Grant support from NINDS.

Distinct Populations of Spinal Projection Neurons Mediate Divergent Functions for Pain and Sensorimotor Integration

Pain is believed to be relayed from the spinal cord to brain through two major ascending pathways. One arises largely from the superficial dorsal horn, primarily targets the parabrachial nucleus (PBN), and is thought to mediate the aversive component of pain. The other arises predominantly from the deep dorsal horn, primarily targets the rostral aspect of the ventral posterior lateral (VPL) thalamus, and is thought to mediate the discriminative component of pain. However, the precise roles of these two populations have been a subject of ongoing controversy. Here, we investigate the anatomy and function of these output pathways at a new level of detail. Our study provides evidence that spinal-PBN neurons and spinal rostral VPL (rVPL) neurons are two distinct populations with different molecular characteristics by in situ hybridization and distinct patterns of collateralization through separate axon tracts. Chemogenetic activation of spinal-PBN neurons elicited spontaneous site-oriented nociceptive behavior (flinching/guarding and licking), and their collateral terminals were somatotopically organized in periductal gray (PAG) and caudal VPL. Moreover, optogenetic activation of different terminals of spinal-PBN neurons elicited distinct nociceptive behaviors. Meanwhile, chemogentic activation of spinal-rVPL neurons did not elicit spontaneous nociceptive behavior. Conversely, chemogenetically inhibiting spinal-rVPL neurons or VPL excitatory neurons did not affect pain behaviors but gave rise to a defect in fine motor coordination. Together, these findings challenge the current dogma and suggest that superficial spinal output neurons have a prominent role in conveying pain to the brain, whereas deep layer spinal output neurons are more likely to be involved in sensorimotor integration.

A multisynaptic pathway from the ventral midbrain toward spinal motoneurons in monkeys.

Motivation boosts motor performance. Activity of the ventral midbrain (VM), consisting of the ventral tegmental area (VTA), the substantia nigra pars compacta (SNc) and the retrorubral field (RRF), plays an important role in processing motivation. However, little is known about the neural substrate bridging the VM and the spinal motor output. We hypothesized that the VM might exert a modulatory influence over the descending motor pathways. By retrograde transneuronal labelling with rabies virus, we demonstrated the existence of multisynaptic projections from the VM to the cervical enlargement in monkeys. The distribution pattern of spinal projection neurons in the VM exhibited a caudorostral gradient, in that the RRF and the caudal part of the SNc contained more retrogradely labelled neurons than the VTA and the rostral part of the SNc. Electrical stimulation of the VM induced muscle responses in the contralateral forelimb with a delay of a few milliseconds following the responses of the ipsilateral primary motor cortex (M1). The magnitude and number of evoked muscle responses were associated with the stimulus intensity and number of pulses. The muscle responses were diminished during M1 inactivation. Thus, the present study has identified a multisynaptic VM–spinal pathway that is mediated, at least in part, by the M1 and might play a pivotal role in modulatory control of the spinal motor output.Key points Motivation to obtain reward is thought to boost motor performance, and activity in the ventral midbrain is important to the motivational process.Little is known about a neural substrate bridging the ventral midbrain and the spinal motor output.Retrograde trans‐synaptic experiments revealed that the ventral midbrain projects multisynaptically to the spinal cord in macaque monkeys.Ventral midbrain activation by electrical stimulation generated cortical activity in the motor cortex and forelimb muscle activity.A multisynaptic ventral midbrain–spinal pathway most probably plays a pivotal role in modulatory control of the spinal motor output.

Inhibition of central axon regeneration: perspective from chondroitin sulfate proteoglycans in lamprey spinal cord injury.

Inhibition of central axon regeneration: perspective from chondroitin sulfate proteoglycans in lamprey spinal cord injury.

D1/D5 Dopamine Receptors and mGluR5 Jointly Enable Non-Hebbian Long-Term Potentiation at Sensory Synapses onto Lamina I Spinoparabrachial Neurons

Highly correlated firing of primary afferent inputs and lamina I projection neurons evokes synaptic long-term potentiation (LTP), a mechanism by which ascending nociceptive transmission can be amplified at the level of the spinal dorsal horn. However, the degree to which neuromodulatory signaling shapes the temporal window governing spike-timing-dependent plasticity (STDP) at sensory synapses onto projection neurons remains unclear. The present study demonstrates that activation of spinal D1/D5 dopamine receptors (D1/D5Rs) creates a highly permissive environment for the production of LTP in male and female adult mouse spinoparabrachial neurons by promoting non-Hebbian plasticity. Bath application of the mixed D1/D5R agonist SKF82958 unmasked LTP at STDP pairing intervals that normally fail to alter synaptic efficacy. Furthermore, during D1/D5R signaling, action potential discharge in projection neurons became dispensable for LTP generation, and primary afferent stimulation alone was sufficient to induce strengthening of sensory synapses. This non-Hebbian LTP was blocked by the D1/D5R antagonist SCH 39166 or genetic deletion of D5R, and required activation of mGluR5 and intracellular Ca2+ release but was independent of NMDAR activation. D1/D5R-enabled non-Hebbian plasticity was observed across multiple neuronal subpopulations in the superficial dorsal horn but was more prevalent in spinoparabrachial neurons than interneurons. Interestingly, the ability of neonatal tissue damage to promote non-Hebbian LTP in adult projection neurons was not observed in D5R knock-out mice. Collectively, these findings suggest that joint spinal D1/D5R and mGluR5 activation can allow unfettered potentiation of sensory synapses onto the output neurons responsible for conveying pain and itch information to the brain.SIGNIFICANCE STATEMENT Synaptic LTP in spinal projection neurons has been implicated in the generation of chronic pain. Under normal conditions, plasticity at sensory synapses onto adult mouse spinoparabrachial neurons follows strict Hebbian learning rules, requiring coincident presynaptic and postsynaptic firing. Here, we demonstrate that the activation of spinal D1/D5Rs promotes a switch from Hebbian to non-Hebbian LTP so that primary afferent stimulation alone is sufficient to evoke LTP in the absence of action potential discharge in projection neurons, which required joint activation of mGluR5 and intracellular Ca2+ release but not NMDARs. These results suggest that D1/D5Rs cooperate with mGluR5 receptors in the spinal dorsal horn to powerfully influence the amplification of ascending nociceptive transmission to the brain.

Looking at the bigger PICture: understanding and counteracting the decline of persistent inward currents in older adults.

Looking at the bigger PICture: understanding and counteracting the decline of persistent inward currents in older adults.

Netrin1 and reelin signaling are required for the migration of anterolateral system neurons in the embryonic spinal cord.

Projection neurons of the spinal cord dorsal horn which transmit pain, itch, and temperature information to the brain comprise the anterolateral system (AS). A recent molecular and genetic study showed that many developing AS neurons express the transcription factor Phox2a and provided insights into the mechanisms of their ontogeny and wiring of nociceptive neuronal circuits. Here, we show that the loss of the axonal guidance and neuronal migration signal netrin1 results in impaired migration of mouse Phox2a+ AS neurons into the spinal lamina I. Furthermore, we show that in the absence of Dab1, an intracellular transducer of the neuronal migration signal reelin, the migration of spinal lamina V and lateral spinal nucleus Phox2a+ AS neurons is impaired, in line with deficits in nociception seen in mice with a loss of reelin signaling. Together, these results provide evidence that netrin1 and reelin control the development of spinal nociceptive projection neurons, suggesting a mechanistic explanation for studies that link sequence variations in human genes encoding these neurodevelopmental signals and abnormal pain sensation.

Pain and itch processing by subpopulations of molecularly diverse spinal and trigeminal projection neurons

A remarkable molecular and functional heterogeneity of the primary sensory neurons and dorsal horn interneurons transmits pain- and or itch-relevant information, but the molecular signature of the projection neurons that convey the messages to the brain is unclear. Here, using retro-TRAP (translating ribosome affinity purification) and RNA sequencing, we reveal extensive molecular diversity of spino- and trigeminoparabrachial projection neurons. Among the many genes identified, we highlight distinct subsets of Cck+ -, Nptx2+ -, Nmb+ -, and Crh+ -expressing projection neurons. By combining in situ hybridization of retrogradely labeled neurons with Fos-based assays, we also demonstrate significant functional heterogeneity, including both convergence and segregation of pain- and itch-provoking inputs into molecularly diverse subsets of NK1R- and non-NK1R-expressing projection neurons.

Corticotropin-Releasing Hormone from the Pontine Micturition Center Plays an Inhibitory Role in Micturition.

Lower urinary tract or voiding disorders are prevalent across all ages and affect >40% of adults over 40 years old, leading to decreased quality of life and high health care costs. The pontine micturition center (PMC; i.e., Barrington's nucleus) contains a large population of neurons that localize the stress-related neuropeptide, corticotropin-releasing hormone (CRH) and project to neurons in the spinal cord to regulate micturition. How the PMC and CRH-expressing neurons in the PMC control volitional micturition is of critical importance for human voiding disorders. To investigate the specific role of CRH in the PMC, neurons in the PMC-expressing CRH were optogenetically activated during in vivo cystometry in unanesthetized mice of either sex. Optogenetic activation of CRH-PMC neurons led to increased intermicturition interval and voided volume, similar to the altered voiding phenotype produced by social stress. Female mice showed a significantly more pronounced phenotype change compared with male mice. These effects were eliminated by CRH-receptor 1 antagonist pretreatment. Optogenetic inhibition of CRH-PMC neurons led to an altered voiding phenotype characterized by more frequent voids and smaller voided volumes. Last, in a cyclophosphamide cystitis model of bladder overactivity, optogenetic activation of CRH-PMC neurons returned the voiding pattern to normal. Collectively, our findings demonstrate that CRH from PMC spinal-projecting neurons has an inhibitory function on micturition and is a potential therapeutic target for human disease states, such as voiding postponement, urinary retention, and underactive or overactive bladder.SIGNIFICANCE STATEMENT The pontine micturition center (PMC), which is a major regulator of volitional micturition, is neurochemically heterogeneous, and excitatory neurotransmission derived from PMC neurons is thought to mediate the micturition reflex. In the present study, using optogenetic manipulation of CRH-containing neurons in double-transgenic mice, we demonstrate that CRH, which is prominent in PMC-spinal projections, has an inhibitory function on volitional micturition. Moreover, engaging this inhibitory function of CRH can ameliorate bladder hyperexcitability induced by cyclophosphamide in a model of cystitis. The data underscore CRH as a novel target for the treatment of voiding dysfunctions, which are highly prevalent disease processes in children and adults.

Robo2 Receptor Gates the Anatomical Divergence of Neurons Derived From a Common Precursor Origin.

Sensory information relayed to the brain is dependent on complex, yet precise spatial organization of neurons. This anatomical complexity is generated during development from a surprisingly small number of neural stem cell domains. This raises the question of how neurons derived from a common precursor domain respond uniquely to their environment to elaborate correct spatial organization and connectivity. We addressed this question by exploiting genetically labeled mouse embryonic dorsal interneuron 1 (dI1) neurons that are derived from a common precursor domain and give rise to spinal projection neurons with distinct organization of cell bodies with axons projecting either commissurally (dI1c) or ipsilaterally (dI1i). In this study, we examined how the guidance receptor, Robo2, which is a canonical Robo receptor, influenced dI1 guidance during embryonic development. Robo2 was enriched in embryonic dI1i neurons, and loss of Robo2 resulted in misguidance of dI1i axons, whereas dI1c axons remained unperturbed within the mantle zone and ventral commissure. Further, Robo2 profoundly influenced dI1 cell body migration, a feature that was partly dependent on Slit2 signaling. These data suggest that dI1 neurons are dependent on Robo2 for their organization. This work integrated with the field support of a model whereby canonical Robo2 vs. non-canonical Robo3 receptor expression facilitates projection neurons derived from a common precursor domain to read out the tissue environment uniquely giving rise to correct anatomical organization.

The neurokinin-1 receptor is expressed with gastrin-releasing peptide receptor in spinal interneurons and modulates itch

The neurokinin-1 receptor (NK1R, encoded by Tacr1) is expressed in spinal dorsal horn neurons and has been suggested to mediate itch. However, previous studies relied heavily on neurotoxic ablation of NK1R spinal neurons, which limited further dissection of their function in spinal itch circuitry. Thus, we leveraged a newly developed Tacr1-CreER mouse line to characterize the role of NK1R spinal neurons in itch. We show that pharmacological activation of spinal NK1R and chemogenetic activation of Tacr1-CreER spinal neurons increases itch behavior, whereas pharmacological inhibition of spinal NK1R suppresses itch behavior. We use fluorescence in situ hybridization to characterize the endogenous expression of Tacr1 throughout the superficial and deeper mouse dorsal horn, as well as the lateral spinal nucleus of mouse. Notably, TACR1 expression was evolutionarily conserved in the human spinal cord dorsal. Retrograde labeling studies from the parabrachial nucleus show that less than 20% of superficial Tacr1-CreER dorsal horn neurons are spinal projection neurons, and thus the majority of Tacr1-CreER are local interneurons. We then use a combination of in situ hybridization and ex vivo two-photon Ca2+ imaging of the spinal cord to establish that NK1R and the gastrin-releasing peptide receptor (GRPR) are coexpressed within a subpopulation of excitatory superficial dorsal horn neurons. These findings are the first to describe a role for NK1R interneurons in itch and extend our understanding of the complexities of spinal itch circuitry. NINDS T32NS086749 (TDS), NINDS F32NS110155 (TDS), NINDS T32 NS073548 (CAW), and NIH NINDS R01NS096705 (SER).