Primary Sensory Neurons Research Articles

TRPM8 inhibits substance P release from primary sensory neurons via PKA/GSK-3beta to protect colonic epithelium in colitis

Transient receptor potential melastatin 8 (TRPM8) is a cold sensory receptor in primary sensory neurons that regulates various neuronal functions. Substance P (SP) is a pro-inflammatory neuropeptide secreted by the neurons, and it aggravates colitis. However, the regulatory role of TRPM8 in SP release is still unclear. Our study aimed to investigate TRPM8’s role in SP release from primary sensory neurons during colitis and clarify the effect of SP on colonic epithelium. We analyzed inflammatory bowel disease patients’ data from the Gene Expression Omnibus dataset. Dextran sulfate sodium (DSS, 2.5%)-induced colitis in mice, mouse dorsal root ganglion (DRG) neurons, ND7/23 cell line, and mouse or human colonic organoids were used for this experiment. Our study found that TRPM8, TAC1 and WNT3A expression were significantly correlated with the severity of ulcerative colitis in patients and DSS-induced colitis in mice. The TRPM8 agonist (menthol) and the SP receptor antagonist (Aprepitant) can attenuate colitis in mice, but the effects were not additive. Menthol promoted calcium ion influx in mouse DRG neurons and inhibited the combination and phosphorylation of PKAca from the cAMP signaling pathway and GSK-3β from the Wnt/β-catenin signaling pathway, thereby inhibiting the effect of Wnt3a-driven β-catenin on promoting SP release in ND7/23 cells. Long-term stimulation with SP inhibited proliferation and enhanced apoptosis in both mouse and human colonic organoids. Conclusively, TRPM8 inhibits SP release from primary sensory neurons by inhibiting the interaction between PKAca and GSK-3β, thereby inhibiting the role of SP in promoting colonic epithelial apoptosis and relieving colitis.

Intrinsic Burst-Blinking Nanographenes for Super-Resolution Bioimaging.

Single-molecule localization microscopy (SMLM) is a powerful technique to achieve super-resolution imaging beyond the diffraction limit. Although various types of blinking fluorophores are currently considered for SMLM, intrinsic blinking fluorophores remain rare at the single-molecule level. Here, we report the synthesis of nanographene-based intrinsic burst-blinking fluorophores for highly versatile SMLM. We image amyloid fibrils in air and in various pH solutions without any additive and lysosome dynamics in live mammalian cells under physiological conditions. In addition, the single-molecule labeling of nascent proteins in primary sensory neurons was achieved with azide-functionalized nanographenes via click chemistry. SMLM imaging reveals higher local translation at axonal branching with unprecedented detail, while the size of translation foci remained similar throughout the entire network. These various results demonstrate the potential of nanographene-based fluorophores to drastically expand the applicability of super-resolution imaging.

Gpr149 is involved in energy homeostasis in the male mouse.

GPR149 is an orphan receptor about which little is known. Accordingly, in the present study, we mapped the tissue expression of Gpr149 in mice using three complementary approaches: quantitative PCR, in situ hybridization, and a newly generated Gpr149-Cre reporter mouse model. The strongest expressions of Gpr149 were observed in neurons of the islands of Calleja, the ventromedial hypothalamus, and the rostral interpeduncular nucleus. Moderate-to-low expression was also observed in the basal forebrain, striatum, hypothalamus, brainstem, and spinal cord. Some Gpr149 expression was also detected in the primary afferent neurons, enteric neurons, and pituitary endocrine cells. This expression pattern is consistent with the involvement of GPR149 signaling in the regulation of energy balance. To explore the physiological function of GPR149 in vivo, we used CRISPR-Cas9 to generate a global knockout allele with mice lacking Gpr149 exon 1. Preliminary metabolic findings indicated that Gpr149-/- mice partially resist weight gain when fed with a high-fat diet and have greater sensitivity to insulin than control mice. In summary, our data may serve as a resource for future in vivo studies on GPR149 in the context of diet-induced obesity.

Somatic ion channels and action potentials in olfactory receptor cells and vomeronasal receptor cells.

Olfactory receptor cells are primary sensory neurons that catch odor molecules in the olfactory system, and vomeronasal receptor cells catch pheromones in the vomeronasal system. When odor or pheromone molecules bind to receptor proteins expressed on the membrane of the olfactory cilia or vomeronasal microvilli, receptor potentials are generated in their receptor cells. This initial excitation is transmitted to the soma via dendrites, and action potentials are generated in the soma and/or axon and transmitted to the central nervous system. Thus, olfactory and vomeronasal receptor cells play an important role in converting chemical signals into electrical signals. In this review, the electrophysiological characteristics of ion channels in the somatic membrane of olfactory receptor cells and vomeronasal receptor cells in various species are described and the differences between the action potential dynamics of olfactory receptor cells and vomeronasal receptor cells are compared.

Comparative Gene Signature of Nociceptors Innervating Mouse Molar Teeth, Cranial Meninges, and Cornea.

The trigeminal ganglion (TG) collects afferent sensory information from various tissues. Recent large-scale RNA sequencing of neurons of the TG and dorsal root ganglion has revealed a variety of functionally distinct neuronal subpopulations, but organ-specific information is lacking. To link transcriptomic and tissue-specific information, we labeled small-diameter neurons of 3 specific subpopulations of the TG by local application of lipophilic carbocyanine dyes to their innervation site in the dental pulp, cornea, and meninges (dura mater). We then collected mRNA-sequencing data from fluorescent neurons. Differentially expressed genes (DEGs) were analyzed and subjected to downstream gene set enrichment analysis (GSEA), and ion channel profiling was performed. A total of 10,903 genes were mapped to the mouse genome (>500 reads). DEG analysis revealed 18 and 81 genes with differential expression (log 2 fold change > 2, Padj < .05) in primary afferent neurons innervating the dental pulp (dental primary afferent neurons [DPAN]) compared to those innervating the meninges (meningeal primary afferent neurons [MPAN]) and the cornea (corneal primary afferent neurons [CPAN]). We found 250 and 292 genes differentially expressed in MPAN as compared to DPAN and to CPAN, and 21 and 12 in CPAN as compared to DPAN and MPAN. Scn2b had the highest log 2 fold change when comparing DPAN versus MPAN and Mmp12 was the most prominent DEG when comparing DPAN versus CPAN and, CPAN versus MPAN. GSEA revealed genes of the immune and mitochondrial oxidative phosphorylation system for the DPAN versus MPAN comparison, cilium- and ribosome-related genes for the CPAN versus DPAN comparison, and respirasome, immune cell- and ribosome-related gene sets for the CPAN versus MPAN comparison. DEG analysis for ion channels revealed no significant differences between the neurons set except for the sodium voltage-gated channel beta subunit 2, Scn2b . However, in each tissue a few ion channels turned up with robust number of reads. In DPAN, these were Cacna1b , Trpv2 , Cnga4 , Hcn1 , and Hcn3 , in CPAN Trpa1 , Trpv1 , Cacna1a , and Kcnk13 and in MPAN Trpv2 and Scn11a . Our study uncovers previously unknown differences in gene expression between sensory neuron subpopulations from the dental pulp, cornea, and dura mater and provides the basis for functional studies, including the investigation of ion channel function and their suitability as targets for tissue-specific analgesia.

In vivo calcium imaging shows that satellite glial cells have increased activity in painful states.

Satellite glial cells are important for proper neuronal function of primary sensory neurons for which they provide homeostatic support. Most research on satellite glial cell function has been performed with in vitro studies, but recent advances in calcium imaging and transgenic mouse models have enabled this first in vivo study of single-cell satellite glial cell function in mouse models of inflammation and neuropathic pain. We found that in naïve conditions, satellite glial cells do not respond in a time-locked fashion to neuronal firing. In painful inflammatory and neuropathic states, we detected time-locked signals in a subset of satellite glial cells, but only with suprathreshold stimulation of the sciatic nerve. Surprisingly, therefore, we conclude that most calcium signals in satellite glial cells seem to develop at arbitrary intervals not directly linked to neuronal activity patterns. More in line with expectations, our experiments also revealed that the number of active satellite glial cells was increased under conditions of inflammation or nerve injury. This could reflect the increased requirement for homeostatic support across dorsal root ganglion neuron populations, which are more active during such painful states.

Squalamine reverses age-associated changes of firing patterns of myenteric sensory neurons and vagal fibres

Vagus nerve signaling is a key component of the gut-brain axis and regulates diverse physiological processes that decline with age. Gut to brain vagus firing patterns are regulated by myenteric intrinsic primary afferent neuron (IPAN) to vagus neurotransmission. It remains unclear how IPANs or the afferent vagus age functionally. Here we identified a distinct ageing code in gut to brain neurotransmission defined by consistent differences in firing rates, burst durations, interburst and intraburst firing intervals of IPANs and the vagus, when comparing young and aged neurons. The aminosterol squalamine changed aged neurons firing patterns to a young phenotype. In contrast to young neurons, sertraline failed to increase firing rates in the aged vagus whereas squalamine was effective. These results may have implications for improved treatments involving pharmacological and electrical stimulation of the vagus for age-related mood and other disorders. For example, oral squalamine might be substituted for or added to sertraline for the aged.

Morphological, molecular, and functional characterization of mouse glutamatergic myenteric neurons.

The enteric nervous system (ENS) functions largely independently of the central nervous system (CNS). Glutamate, the dominant neurotransmitter in the CNS and sensory afferents, is not a primary neurotransmitter in the ENS. Only a fraction (∼2%) of myenteric neurons in the mouse distal colon and rectum (colorectum) are positive for vesicular glutamate transporter type 2 (VGLUT2), the structure and function of which remain undetermined. Here, we systematically characterized VGLUT2-positive enteric neurons (VGLUT2-ENs) through sparse labeling with adeno-associated virus, single-cell mRNA sequencing (scRNA-seq), and GCaMP6f calcium imaging. Our results reveal that the majority of VGLUT2-ENs (29 of 31, 93.5%) exhibited Dogiel type I morphology with a single aborally projecting axon; most axons (26 of 29, 89.7%) are between 4 and 10 mm long, each traversing 19 to 34 myenteric ganglia. These anatomical features exclude the VGLUT2-ENs from being intrinsic primary afferent or motor neurons. The scRNA-seq conducted on 52 VGLUT2-ENs suggests different expression profiles from conventional descending interneurons. Ex vivo GCaMP6f recordings from flattened colorectum indicate that almost all VGLUT2-EN (181 of 215, 84.2%) are indirectly activated by colorectal stretch via nicotinic cholinergic neural transmission. In conclusion, VGLUT2-ENs are a functionally unique group of enteric neurons with single aborally projecting long axons that traverse multiple myenteric ganglia and are activated indirectly by colorectal mechanical stretch. This knowledge will provide a solid foundation for subsequent studies on the potential interactions of VGLUT2-EN with extrinsic colorectal afferents via glutamatergic neurotransmission.NEW & NOTEWORTHY We reveal that VGLUT2-positive enteric neurons (EN), although constituting a small fraction of total EN, are homogeneously expressed in the myenteric ganglia, with a slight concentration at the intermediate region between the colon and rectum. Through anatomic, molecular, and functional analyses, we demonstrated that VGLUT2-ENs are activated indirectly by noxious circumferential colorectal stretch via nicotinic cholinergic transmission, suggesting their participation in mechanical visceral nociception.

Changes of Trigeminal Ganglion Neurons Innervating the Temporomandibular Joint in Chronic Pain Rat Model.

Background: Phenotype alterations of nociceptive neurons have been shown to be a key step in the pathogenesis of many pain-related diseases. However, it is unclear if the characteristic changes of temporomandibular joint (TMJ) primary afferent neurons are related to the pathogenesis of temporomandibular joint osteoarthritis (TMJOA) chronic pain. This study aimed to determine the morphological and neurochemical changes in trigeminal ganglion (TG) neurons innervating the TMJ in TMJOA chronic pain rats. Materials and Methods: Monosodium iodoacetate (MIA)-induced TMJOA chronic pain rat model was established (n = 6), and saline was injected in rats of the control group (n = 6). TMJ primary afferent neurons were labeled with retrograde tracing (Dil). The spatial distribution and the expression of calcitonin gene-related peptide (CGRP), isolectin B4 (IB4), and neurofilament 200 (NF200) of TMJ primary afferent neurons in TG were investigated using immunofluorescence. Intracellular calcium signaling was recorded by calcium imaging (n = 20). Results: TMJ primary afferent neurons were located only in the V3 region of the TG from both saline- and MIA-injected rats. The number of TG neurons innervating the TMJ was increased in MIA-injected rats. Elevated number and intracellular calcium concentration of small- and medium-sized instead of large-sized Dil+ TG neurons were found in MIA-injected rats. The upregulated expression of CGRP and IB4, but not NF200, in TG neurons innervating the rat TMJs was accompanied by TMJOA chronic pain. Conclusion: This study suggests that sensitization of small- to medium-sized Dil+ TG neurons and CGRP- and IB4-positive Dil+ TG neurons might contribute to the development of TMJOA chronic pain in rats. This will provide valuable information for more efficient control of TMJOA chronic pain.

20-HETE mediated TRPV1 activation drives allokinesis via MrgprA3+ neurons in chronic dermatitis.

Rationale: Noxious stimuli are often perceived as itchy in patients with chronic dermatitis (CD); however, itch and pain mechanisms of CD are not known. Methods: TRPV1 involvement in CD was analyzed using a SADBE induced CD-like mouse model, and several loss- and gain-of-function mouse models. Trigeminal TRPV1 channel and MrgprA3+ neuron functions were analyzed by calcium imaging and whole-cell patch-clamp recordings. Lesional CD-like skin from mice were analyzed by unbiased metabolomic analysis. 20-HETE availability in human and mouse skin were determined by LC/MS and ELISA. And finally, HET0016, a selective 20-HETE synthase inhibitor, was used to evaluate if blocking skin TRPV1 activation alleviates CD-associated chronic itch or pain. Results: While normally a pain inducing chemical, capsaicin induced both itch and pain in mice with CD condition. DREADD silencing of MrgprA3+ primary sensory neurons in these mice selectively decreased capsaicin induced scratching, but not pain-related wiping behavior. In the mice with CD condition, MrgprA3+ neurons showed elevated ERK phosphorylation. Further experiments showed that MrgprA3+ neurons from MrgprA3;Braf mice, which have constitutively active BRAF in MrgprA3+ neurons, were significantly more excitable and responded more strongly to capsaicin. Importantly, capsaicin induced both itch and pain in MrgprA3;Braf mice in an MrgprA3+ neuron dependent manner. Finally, the arachidonic acid metabolite 20-HETE, which can activate TRPV1, was significantly elevated in the lesional skin of mice and patients with CD. Treatment with the selective 20-HETE synthase inhibitor HET0016 alleviated itch in mice with CD condition. Conclusion: Our results demonstrate that 20-HETE activates TRPV1 channels on sensitized MrgprA3+ neurons, and induces allokinesis in lesional CD skin. Blockade of 20-HETE synthesis or silencing of TRPV1-MrgprA3+ neuron signaling offers promising therapeutic strategies for alleviating CD-associated chronic itch.

Temperature-Dependent Activation of Thermosensitive Transient Receptor Potential Channels.

Temperature detection is essential for the survival and perpetuation of any species. Thermoreceptors in the skin sense the body temperature and also the temperatures of the ambient air and the objects. In 1997, Dr. David Julius and his colleagues found that a receptor expressed in small-diameter primary sensory neurons was activated by capsaicin (the pungent chemical in hot pepper). This receptor was also activated by temperature above 42°C. That was the first time that a thermal receptor in primary sensory neurons has been identified. This receptor is named transient receptor potential vanilloid 1 (TRPV1). Now, 11 thermosensitive TRP channels are known. In this chapter, we summarize the reports and analyze thermosensitive TRP channels in a variety of ways to clarify the activation mechanisms by which temperature changes are sensed.

Histamine H4 receptor and TRPV1 mediate itch induced by cadaverine, a metabolite of the microbiome.

Cadaverine is an endogenous metabolite produced by the gut microbiome with various activity in physiological and pathological conditions. However, whether cadaverine regulates pain or itch remains unclear. In this study, we first found that cadaverine may bind to histamine 4 receptor (H4R) with higher docking energy score using molecular docking simulations, suggesting cadaverine may act as an endogenous ligand for H4R. We subsequently found intradermal injection of cadaverine into the nape or cheek of mice induces a dose-dependent scratching response in mice, which was suppressed by a selective H4R antagonist JNJ-7777120, transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine and PLC inhibitor U73122, but not H1R antagonist or TRPA1 antagonist or TRPV4 antagonist. Consistently, cadaverine-induced itch was abolished in Trpv1-/- but not Trpa1-/- mice. Pharmacological analysis indicated that mast cells and opioid receptors were also involved in cadaverine-induced itch in mice. scRNA-Seq data analysis showed that H4R and TRPV1 are mainly co-expressed on NP2, NP3 and PEP1 DRG neurons. Calcium imaging analysis showed that cadaverine perfusion enhanced calcium influx in the dissociated dorsal root ganglion (DRG) neurons, which was suppressed by JNJ-7777120 and capsazepine, as well as in the DRG neurons from Trpv1-/- mice. Patch-clamp recordings found that cadaverine perfusion significantly increased the excitability of small diameter DRG neurons, and JNJ-7777120 abolished this effect, indicating involvement of H4R. Together, these results provide evidences that cadaverine is a novel endogenous pruritogens, which activates H4R/TRPV1 signaling pathways in the primary sensory neurons.

Corneo‐conjunctival sensory neuron: Types, transduction channels and codification of nerve impulses

The cornea and the conjunctiva are innervated by the peripheral projections of a small number of primary sensory neurons whose somata are located at the ophthalmic region of the trigeminal ganglion. Most trigeminal neurons innervating the ocular surface are small and medium size, and contain neuropeptides such as SP and CGRP. Functionally, ocular surface neurons have been classified into five main neuronal types, based on their response to multiple modalities of stimuli (mechanical forces, heat, cold, H+, chemical substances, etc.). Polymodal nociceptors (activated by several modalities of stimulus, mechanical energy, heat, H+, and chemical irritants), mechanonociceptors (activated by mechanical energy), cold thermoreceptors (activated by temperature reductions and hyperosmolarity) are found in the cornea, the sclera and the conjunctiva. Besides, low‐threshold mechanoreceptors (activated by mechanical forces of low intensity) are found in the corneoscleral limbus and the lid border, and pruriceptors (activated by pruritogen substances) have been described in the tarsal conjunctiva, close to the lid margin.The ability to respond to the different modalities of stimulus lies in the types of transducing channels that each type of neuron expresses at its peripheral nerve endings. Sensory transduction involves the conversion of the physical or chemical stimuli acting on the nerve terminal into electrical energy, that is, into a local change of membrane potential that, if it is large enough, will generate action potentials that will travel along the axon to be processed by the CNS. So that, nerve terminals with TRPV1 channels are directly gated by heat, H+, and several chemical irritants such as capsaicin; nerve terminals with Piezo2 channels will be activated by mechanical forces; TRPM8 channels provide nerve terminals sensitivity to cold and osmolarity increases; et cetera.Ocular sensory neurons also express other ion channels that contribute to coding, that is to help the sensory neuron to establish a correspondence between the characteristics of the stimulus (intensity, temporal course, etc.) and specific parameters of the firing it induces. Voltage‐gated Ca2+, Na+ and K+ channels as well as cyclic nucleotide‐gated channels, among others, contribute to define the firing frequency and pattern to encode the traits of the stimulus. Thus, the ion channels present in the nerve terminals of each class of ocular sensory neuron provide the neurons the ability to detect and encode one or several modalities of stimulus, which constitutes the origin of the different sensations evoked on the ocular surface.(Supported by PID2020‐115934RB‐I00 from DOI: MCIN/AEI/10.13039/50110001103, and CIPROM/2021/48 from the Generalitat Valenciana, Spain)

The role of nociceptive neurons in allergic rhinitis.

Allergic rhinitis (AR) is a chronic, non-infectious condition affecting the nasal mucosa, primarily mediated mainly by IgE. Recent studies reveal that AR is intricately associated not only with type 2 immunity but also with neuroimmunity. Nociceptive neurons, a subset of primary sensory neurons, are pivotal in detecting external nociceptive stimuli and modulating immune responses. This review examines nociceptive neuron receptors and elucidates how neuropeptides released by these neurons impact the immune system. Additionally, we summarize the role of immune cells and inflammatory mediators on nociceptive neurons. A comprehensive understanding of the dynamic interplay between nociceptive neurons and the immune system augments our understanding of the neuroimmune mechanisms underlying AR, thereby opening novel avenues for AR treatment modalities.

Timed sulfonylurea modulation improves locomotor and sensory dysfunction following spinal cord injury.

Traumatic spinal cord injury (SCI) results in immediate tissue necrosis and delayed secondary expansion of neurological damage, often resulting in lifelong paralysis, neurosensory dysfunction, and chronic pain. Progressive hemorrhagic necrosis (PHN) and excessive excitation are the main sources of secondary neural injury. Recent approaches to attenuate PHN by glibenclamide can improve locomotor function after SCI. However, use of glibenclamide can exacerbate development of SCI-induced chronic pain by inhibiting KATP channels to increase neuronal excitation and glial activation. In this study, we explored a treatment strategy involving administration of glibenclamide, which suppresses PHN, and diazoxide, which protects against neuronal excitation and inflammation, at different time intervals following spinal cord contusion. Our goal was to determine whether this combined approach enhances both sensory and motor function. Contusive SCI was induced at spinal segment T10 in adult rats. We found that KATP channels opener, diazoxide, decreased the hyperexcitability of primary sensory neurons after SCI by electrophysiology. Timed application of glibenclamide and diazoxide following contusion significantly improved locomotor function and mitigated development of SCI-induced chronic pain, as shown by behavioral evidence. Finally, we found that timed application of glibenclamide and diazoxide attenuates the inflammatory activity in the spinal cord and increases the survival of spinal matters following SCI. These preclinical studies introduce a promising potential treatment strategy to address SCI-induced dysfunction.

Corneal sensory nerve function: An overview

Somatosensory innervation of the cornea is provided by the peripheral axons of a small number of primary sensory neurons located in the ophthalmic region of the trigeminal ganglion. Corneal sensory nerve fibres are heterogeneous and may express different neurotransmitters and ion channels involved in transduction and coding of the stimuli acting on the tissue. Based on their activation by one or several types of physical and chemical stimuli, corneal nerve fibres are classified as polymodal nociceptors, mechanonociceptors and cold thermoreceptors. Impulse activity originated at corneal sensory nerves reaches second‐order neurons located at the lower brainstem, from where sensory information travels to the contralateral posterior thalamus and, then, to the brain cortex.After processing, the activation of the different classes of corneal neurons results in different sensations that depend on the type of primary sensory neuron initially activated by the stimulus, although pain is the main sensation elicited by corneal stimulation. Sensations originated at the cornea are explored by esthesiometry, allowing to quantify the threshold, intensity and quality of the evoked sensations. Sensitivity of the ocular surface is reduced with age and altered by several pathological processes that affect sensory innervation, either ocular (such as allergic keratoconjunctivitis, infectious keratitis, eye surgery, hereditary corneal dystrophies, dry eye, etc.) or systemic diseases, such as diabetes and peripheral neuropathies.A prominent feature of ocular nociceptors is their sensitization by the inflammatory mediators released after noxious stimulation or tissue injury, while cold thermoreceptor activity is inhibited by inflammatory substances. Both the spontaneous and the stimulus‐evoked activity increases in sensitized nociceptors, which causes sustained pain and hyperalgesia. After corneal sensory nerves damage (produced as a consequence of an ocular or systemic disease, or by accidental or surgical injury), part of sensory nerves degenerates leading to reduced sensitivity to natural stimulation in the affected area. At the same time, axons start to regenerate and to recover the ability to transduce and encode the stimulus. During this regeneration process, cold thermoreceptor and nociceptor nerves may become hyperexcitable due to changes in their expression/activity of ion channels, which give rise to dryness sensation and ocular neuropathic pain.(Supported by PID2020‐115934RB‐I00 from DOI: MCIN/AEI/10.13039/50110001103, and CIPROM/2021/48 from the Generalitat Valenciana, Spain)

Mechanisms of ocular surface pain

Ocular pain is evoked upon activation of a small number of nociceptive neurons located at the trigeminal ganglion whose peripheral projections profusely innervate the cornea and conjunctiva. In healthy conditions, nerve impulse activity originated at peripheral nerve endings of these primary sensory neurons in response to noxious or potentially noxious stimuli acting on the ocular surface travels centrally and reaches second‐order neurons located at the trigeminal brain stem complex. In turn, these brainstem neurons send their axons to the contralateral posterior thalamic nuclei, where the third neurons along the ocular nociceptive pathway are found. Thalamic neurons send their axons to the primary somatosensory area of the brain cortex and, after brain cortex processing if this sensory inflow, nociceptive pain sensation is experienced.The activity of ocular nociceptor neurons is modified under inflammation. Inflammatory mediators released at the ocular surface increase the excitability of the nerve terminals. As a result, the impulse response of primary nociceptor neurons to noxious stimuli is increased, which constitutes the basis of hyperalgesia and leads to pain and other sensations such as gritty eyes, foreign body sensation, etc. experienced under ocular inflammation.After a lesion (including surgery) or disease that affects any nerve or neuron of the ocular somatosensory pathway, the activity and connectivity of the neurons involved in ocular pain is altered, sometimes permanently. The excitability of damaged neurons is long‐lasting increased, leading to ocular dysesthesia and development of neuropathic pain referred to the eye.(Supported by PID2020‐115934RB‐I00 from DOI: MCIN/AEI/10.13039/50110001103, and CIPROM/2021/48 from the Generalitat Valenciana, Spain)

Conscious sensations and protective reflexes driven by eye surface nerves in health and disease

The trigeminal primary sensory neurons that innervate the ocular surface, especially the cornea, have been classified in three functional types: polymodal nociceptors, mechano‐nociceptors and cold thermoreceptors. These sensory nerves are responsible for the conscious sensations evoked in response to ocular surface stimulation as well as for the regulation of tearing and blinking, and they are also involved in the trophic maintenance of the ocular tissues.Depending on the transducing channels they express in their membrane, sensory nerves from the ocular surface respond to different types of stimulus and, after this information is processed by the CNS, different conscious sensations are evoked: corneal mechano‐nociceptors' activation led to pricking pain; polymodal nociceptors evoke burning irritation, and cold thermoreceptors' activation may evoke freshness/cold/dryness/pain depending on the subtype of cold thermoreceptor neuron activated by the stimulus. While reflex blinking and tearing are driven by the activation of polymodal nociceptors as the afferent arm of the reflex, basal blinking and tearing rate are under the control of cold thermoreceptors' activity.During inflammation and after injury (such as UV‐induced keratitis, allergic keratoconjunctivitis, corneal surgery, tear‐deficient dry eye) the activity of corneal sensory nerves is modified in both eyes, even when inflammation or damage affects only one eye. The abnormal corneal sensory nerve activity constitutes the basis for the discomfort, dryness and pain sensations experienced under these conditions. Also, protective mechanisms depending on ocular nerve activity such as blinking and tearing become altered.(Supported by PID2020‐115934RB‐I00 from DOI: MCIN/AEI/10.13039/50110001103, and CIPROM/2021/48 from the Generalitat Valenciana, Spain)

Peripheral Branch Injury Induces Oxytocin Receptor Expression at the Central Axon Terminals of Primary Sensory Neurons.

Considerable evidence suggests that oxytocin, as a regulatory nonapeptide, participates in modulatory mechanisms of nociception. Nonetheless, the role of this hypothalamic hormone and its receptor in the sensory pathway has yet to be fully explored. The present study performed immunohistochemistry, enzyme-linked immunosorbent assay, and RT-qPCR analysis to assess changes in the expression of the neuronal oxytocin receptor in female rats following tight ligation of the sciatic nerve after 1, 3, and 7 days of survival. Oxytocin receptor immunoreactivity was present in both dorsal root ganglia and lumbar spinal cord segments, but not accumulated at the site of the ligation of the peripheral nerve branch. We found a time-dependent change in the expression of oxytocin receptor mRNA in L5 dorsal root ganglion neurons, as well as an increase in the level of the receptor protein in the lumbar segment of the spinal cord. A peak in the expression was observed on day 3, which downturned slightly by day 7 after the nerve ligation. These results show that OTR expression is up-regulated in response to peripheral nerve lesions. We assume that the importance of OTR is to modify spinal presynaptic inputs of the sensory neurons upon injury-induced activation, thus to be targets of the descending oxytocinergic neurons from supraspinal levels. The findings of this study support the concept that oxytocin plays a role in somatosensory transmission.

Optogenetic Activation of Peripheral Somatosensory Neurons in Transgenic Mice as a Neuropathic Pain Model for Assessing the Therapeutic Efficacy of Analgesics.

Optogenetics is a novel biotechnology widely used to precisely manipulate a specific peripheral sensory neuron or neural circuit. However, the use of optogenetics to assess the therapeutic efficacy of analgesics is elusive. In this study, we generated a transgenic mouse stain in which all primary somatosensory neurons can be optogenetically activated to mimic neuronal hyperactivation in the neuropathic pain state for the assessment of analgesic effects of drugs. A transgenic mouse was generated using the advillin-Cre line mated with the Ai32 strain, in which channelrhodopsin-2 fused to enhanced yellow fluorescence protein (ChR2-EYFP) was conditionally expressed in all types of primary somatosensory neurons (advillincre/ChR2+/+). Immunofluorescence and transdermal photostimulation on the hindpaws were used to verify the transgenic mice. Optical stimulation to evoke pain-like paw withdrawal latency was used to assess the analgesic effects of a series of drugs. Injury- and pain-related molecular biomarkers were investigated with immunohistofluorescence. We found that the expression of ChR2-EYFP was observed in many primary afferents of paw skin and sciatic nerves and in primary sensory neurons and laminae I and II of the spinal dorsal horns in advillincre/ChR2+/+ mice. Transdermal blue light stimulation of the transgenic mouse hindpaw evoked nocifensive paw withdrawal behavior. Treatment with gabapentin, some channel blockers, and local anesthetics, but not opioids or COX-1/2 inhibitors, prolonged the paw withdrawal latency in the transgenic mice. The analgesic effect of gabapentin was also verified by the decreased expression of injury- and pain-related molecular biomarkers. These optogenetic mice provide a promising model for assessing the therapeutic efficacy of analgesics in neuropathic pain.