Quantitative immunohistochemical co-localization of TRPV1 and CGRP in varicose axons of the murine oesophagus, stomach and colorectum

Organization and morphology of calcitonin gene-related peptide-immunoreactive axons in the whole mouse stomach

Nociceptive afferent axons innervate the stomach and send signals to the brain and spinal cord. Peripheral nociceptive afferents can be detected with a variety of markers (e.g., substance P [SP] and calcitonin gene-related peptide [CGRP]). We recently examined the topographical organization and morphology of SP-immunoreactive (SP-IR) axons in the whole mouse stomach muscular layer. However, the distribution and morphological structure of CGRP-IR axons remain unclear. We used immunohistochemistry labeling and applied a combination of imaging techniques, including confocal and Zeiss Imager M2 microscopy, Neurolucida 360 tracing, and integration of axon tracing data into a 3D stomach scaffold to characterize CGRP-IR axons and terminals in the whole mouse stomach muscular layers. We found that: (1) CGRP-IR axons formed extensive terminal networks in both ventral and dorsal stomachs. (2) CGRP-IR axons densely innervated the blood vessels. (3) CGRP-IR axons ran in parallel with the longitudinal and circular muscles. Some axons ran at angles through the muscular layers. (4) They also formed varicose terminal contacts with individual myenteric ganglion neurons. (5) CGRP-IR occurred in DiI-labeled gastric-projecting neurons in the dorsal root and vagal nodose ganglia, indicating CGRP-IR axons were visceral afferent axons. (6) CGRP-IR axons did not colocalize with tyrosine hydroxylase or vesicular acetylcholine transporter axons in the stomach, indicating CGRP-IR axons were not visceral efferent axons. (7) CGRP-IR axons were traced and integrated into a 3D stomach scaffold. For the first time, we provided a topographical distribution map of CGRP-IR axon innervation of the whole stomach muscular layers at the cellular/axonal/varicosity scale.

Regional characterisation of TRPV1 and TRPA1 signalling in the mouse colon mucosa

Capsaicin and allyl isothiocyanate (AITC) activate transient receptor potential (TRP) vanilloid-1 (TRPV1) and TRP ankyrin-1 (TRPA1), respectively. TRPV1 and TRPA1 expression have been identified in the gastrointestinal (GI) tract. GI mucosal functions remain largely undefined for TRPV1 and TRPA1 with side-dependence and regional differences in signalling unclear. Here we investigated TRPV1- and TRPA1-induced vectorial ion transport as changes in short-circuit current (ΔIsc), in defined segments of mouse colon mucosa (ascending, transverse and descending) under voltage-clamp conditions in Ussing chambers. Drugs were applied basolaterally (bl) or apically (ap). Capsaicin responses were biphasic, with primary secretory and secondary anti-secretory phases, observed with bl application only, which predominated in descending colon. AITC responses were monophasic and secretory, with ΔIsc dependent on colonic region (ascending vs. descending) and sidedness (bl vs. ap). Aprepitant (neurokinin-1 (NK1) antagonist, bl) and tetrodotoxin (Na+ channel blocker, bl) significantly inhibited capsaicin primary responses in descending colon, while GW627368 (EP4 receptor antagonist, bl) and piroxicam (cyclooxygenase inhibitor, bl) inhibited AITC responses in ascending and descending colonic mucosae. Antagonism of the calcitonin gene-related peptide (CGRP) receptor had no effect on mucosal TRPV1 signalling, while tetrodotoxin and antagonists of the 5-hydroxytryptamine-3 and 4 receptors, CGRP receptor, and EP1/2/3 receptors had no effect on mucosal TRPA1 signalling. Our data demonstrates the regional-specificity and side-dependence of colonic TRPV1 and TRPA1 signalling, with involvement of submucosal neurons and mediation by epithelial NK1 receptor activation for TRPV1, and endogenous prostaglandins and EP4 receptor activation for TRPA1 mucosal responses.

Mapping the Organization and Morphology of Calcitonin Gene-Related Peptide (CGRP)-IR Axons in the Whole Mouse Stomach

Nociceptive afferent axons innervate the stomach and send signals to the brain and spinal cord. Peripheral nociceptive afferents can be detected with a variety of markers [e.g., substance P (SP) and calcitonin gene-related peptide (CGRP)]. We recently examined the topographical organization and morphology of SP-immunoreactive (SP-IR) axons in the whole mouse stomach muscular layer. However, the distribution and morphological structure of CGRP-IR axons remain unclear. We used immunohistochemistry labeling and applied a combination of imaging techniques, including confocal and Zeiss Imager M2 microscopy, Neurolucida 360 tracing, and integration of axon tracing data into a 3D stomach scaffold to characterize CGRP-IR axons and terminals in the whole mouse stomach muscular layers. We found that: 1) CGRP-IR axons formed extensive terminal networks in both ventral and dorsal stomachs. 2) CGRP-IR axons densely innervated the blood vessels. 3) CGRP-IR axons ran in parallel with the longitudinal and circular muscles. Some axons ran at angles through the muscular layers. 4) They also formed varicose terminal contacts with individual myenteric ganglion neurons. 5) CGRP-IR occurred in DiI-labeled gastric-projecting neurons in the dorsal root and vagal nodose ganglia, indicating CGRP-IR axons were visceral afferent axons. 6) CGRP-IR axons did not colocalize with tyrosine hydroxylase (TH) or vesicular acetylcholine transporter (VAChT) axons in the stomach, indicating CGRP-IR axons were not visceral efferent axons. 7) CGRP-IR axons were traced and integrated into a 3D stomach scaffold. For the first time, we provided a topographical distribution map of CGRP-IR axon innervation of the whole stomach muscular layers at the cellular/axonal/varicosity scale.

Safety and efficacy of intravesical instillation of resiniferatoxin in healthy cats: A preliminary study

Objectives

To evaluate the safety of intravesical application of resiniferatoxin (RTX) in healthy cats and its effects on calcitonin gene-related peptide (CGRP) and substance P (SP) produced by C-fibers.

Methods

Seven adult female cats received either 25 mL of saline (control; n = 1), or intravesical RTX at 5, 25, or 50 μg in 25 mL of saline to a final concentration of 0.2 μg/mL (318 nM), 1 μg/mL (1,591 nM), and 2 μg/mL (3,181 nM) (n = 2 per group). The treatment was instilled into the urinary bladder for 20 min. Plasma concentrations of RTX were measured at 0, 0.5, 1, and 4 h. Physical exam, complete blood count, and serum biochemical analysis were performed on day 0, 7, and 14. After 14 days, the sacral dorsal root ganglia (DRG) and the urinary bladder were harvested for histological and immunofluorescence analysis.

Results

Intravesical RTX was well tolerated and plasma concentrations were below the quantifiable limits except for one cat receiving 1 μg/mL. Mild to moderate histopathological changes, including epithelial changes, edema, and blood vessel proliferation, were observed at lower doses (0.2 and 1 μg/mL), and were more severe at the higher dose (2 μg/mL). C-fiber ablation was observed in the urinary bladder tissue at all doses, as shown by an apparent reduction of both CGRP and SP immunoreactive axons.

Conclusion

A dose of 25 μg (1 μg/mL) of RTX instilled in the urinary bladder of healthy cats appeared to decrease the density of SP and CGRP nerve axons innervating bladder and induced moderate changes in the bladder tissue.

How should we define a nociceptor in the gut-brain axis?

In the past few years, there has been extraordinary interest in how the gut communicates with the brain. This is because substantial and gathering data has emerged to suggest that sensory nerve pathways between the gut and brain may contribute much more widely in heath and disease, than was originally presumed. In the skin, the different types of sensory nerve endings have been thoroughly characterized, including the morphology of different nerve endings and the sensory modalities they encode. This knowledge is lacking for most types of visceral afferents, particularly spinal afferents that innervate abdominal organs, like the gut. In fact, only recently have the nerve endings of spinal afferents in any visceral organ been identified. What is clear is that spinal afferents play the major role in pain perception from the gut to the brain. Perhaps surprisingly, the majority of spinal afferent nerve endings in the gut express the ion channel TRPV1, which is often considered to be a marker of "nociceptive" neurons. And, a majority of gut-projecting spinal afferent neurons expressing TRPV1 are activated at low thresholds, in the "normal" physiological range, well below the normal threshold for detection of painful sensations. This introduces a major conundrum regarding visceral nociception. How should we define a "nociceptor" in the gut? We discuss the notion that nociception from the gut wall maybe a process encrypted into multiple different morphological types of spinal afferent nerve ending, rather than a single class of sensory ending, like free-endings, suggested to underlie nociception in skin.

Quantification of CGRP-immunoreactive myenteric neurons in mouse colon

Quantitative data of biological systems provide valuable baseline information for understanding pathology, experimental perturbations, and computational modeling. In mouse colon, calcitonin gene-related peptide (CGRP) is expressed by myenteric neurons with multiaxonal (Dogiel type II) morphology, characteristic of intrinsic primary afferent neurons (IPANs). Analogous neurons in other species and gut regions represent 5-35% of myenteric neurons. We aimed to quantify proportions of CGRP-immunopositive (CGRP+) myenteric neurons. Colchicine-treated wholemount preparations of proximal, mid, and distal colon were labeled for HuC/D, CGRP, nitric oxide synthase (NOS), and peripherin (Per). The pan-neuronal markers (Hu+/Per+) co-labeled 94% of neurons. Hu+/Per- neurons comprised ∼6%, but Hu-/Per+ cells were rare. Thus, quantification was based on Hu+ myenteric neurons (8576 total; 1225 ± 239 per animal, n = 7). CGRP+ cell bodies were significantly larger than the average of all Hu+ neurons (329 ± 13 vs. 261 ± 12 μm² , p < .0001). CGRP+ neurons comprised 19% ± 3% of myenteric neurons without significant regional variation. NOS+ neurons comprised 42% ± 2% of myenteric neurons overall, representing a lower proportion in proximal colon, compared to mid and distal colon (38% ± 2%, 44% ± 2%, and 44% ± 3%, respectively). Peripherin immunolabeling revealed cell body and axonal morphology in some myenteric neurons. Whether all CGRP+ neurons were multiaxonal could not be addressed using peripherin immunolabeling. However, of 118 putatively multiaxonal neurons first identified based on peripherin immunoreactivity, all were CGRP+ (n = 4). In conclusion, CGRP+ myenteric neurons in mouse colon were comprehensively quantified, occurring within a range expected of a putative IPAN marker. All Per+ multiaxonal neurons, characteristic of Dogiel type II/IPAN morphology, were CGRP+.

Efficacy of Endoscopic Radiofrequency Ablation for Treatment of Reflux Hypersensitivity: A Study Based on Rome IV Criteria

Objective. Effective therapies for reflux hypersensitivity are lacking. Endoscopic radiofrequency ablation may reduce the sensitivity of the distal esophagus through direct interference with nociceptors or vagal afferent fibers and thus may be useful in reflux hypersensitivity. The aim of this study is to assess the effectiveness and possible mechanisms of endoscopic radiofrequency ablation in reflux hypersensitivity patients. Methods. Patients with reflux hypersensitivity who fulfilled the Rome IV criteria and who wished to receive further treatment were recruited. Endoscopic radiofrequency ablation was delivered to the gastroesophageal junction. Data were collected by questionnaire using a 6-point Likert scale. The primary outcome measure was effect on symptoms including heartburn, regurgitation, and chest pain. The secondary outcomes were degree of satisfaction, medication use, acid exposure time (AET), low esophageal sphincter (LES) pressure, and total reflux episodes. We also assessed positive cell density of transient receptor potential vanilloid type 1 receptor (TRPV1) and calcitonin gene-related peptide (CGRP), both of which are biomarkers of afferent fibers, in biopsies obtained from esophageal mucosa 0.5 cm-1 cm above the Z line. These scales will be administered at baseline, 3-month follow-up, 6-month follow-up, and 12-month follow-up. Results. A total of 22 reflux hypersensitivity patients were enrolled (14 males, median age 50.0 years). A significant improvement in symptom scores (heartburn, regurgitation, and chest pain) was noted at 3 months, 6 months, and 12 months ( $P<0.001$ ). Satisfaction with life increased to 72.7% (16/22), 72.7% (16/22), and 68.2% (15/22) at 3, 6, and 12 mo, respectively, compared with baseline ( $P<0.001$ ). Nineteen patients reduced their medication use after treatment. Of these, 22.7% (5/22), 31.8% (7/22), and 40.9% (9/22) subjects stopped medication use at 3 mo, 6 mo, and 12 mo, respectively. No statistical differences were noted in AET, LES pressure, or total reflux episodes from preoperation to 12 mo postoperation. After treatment, the positive cell density of both TRPV1 and CGRP decreased significantly; however, only TRPV1 had a positive correlation with heartburn ( $r=0.51$ , $P=0.03$ ) and chest pain ( $r=0.77$ , $P<0.01$ ). Conclusion. Endoscopic radiofrequency ablation was an effective and safe therapeutic option in reflux hypersensitivity patients. Further studies with large sample size are required to validate the role of radiofrequency in reflux hypersensitivity.

Mechanisms underlying the prokinetic effects of endogenous glucagon-like peptide-1 in the rat proximal colon

Glucagon-like peptide-1 (GLP-1), a well-known insulin secretagogue, is released from enteroendocrine L cells both luminally and basolaterally to exert different effects. Basolaterally released GLP-1 increases epithelial ion transport by activating CGRP-containing enteric afferent neurons. Although bath-applied GLP-1 reduced the contractility of colonic segments, GLP-1-induced stimulation of afferent neurons could also accelerate peristaltic contractions. Here, the roles of endogenous GLP-1 in regulating colonic peristalsis were investigated using isolated colonic segments. Isolated segments of rat proximal colon were placed in an organ bath, serosally perfused with oxygenated physiological salt solution, and luminally perfused with degassed 0.9% saline. Colonic wall motion was recorded using a video camera and converted into spatiotemporal maps. Intraluminal administration of GLP-1 (100 nM) stimulating the secretion of GLP-1 from L cells increased the frequency of oro-aboral propagating peristaltic contractions. The acceleratory effect of GLP-1 was blocked by luminally applied exendin-3 (9-39) (100 nM), a GLP-1 receptor antagonist. GLP-1-induced acceleration of peristaltic contractions was also prevented by bath-applied BIBN4069 (1 μM), a CGRP receptor antagonist. In colonic segments that had been exposed to bath-applied capsaicin (100 nM) that desensitizes extrinsic afferents, GLP-1 was still capable of exerting its prokinetic effect. Stimulation of endogenous GLP-1 secretion with a luminally applied cocktail of short-chain fatty acids (1 mM) increased the frequency of peristaltic waves in an exendin-3 (9-39)-sensitive manner. Thus, GLP-1 activates CGRP-expressing intrinsic afferents to accelerate peristalsis in the proximal colon. Short-chain fatty acids appear to stimulate endogenous GLP-1 secretion from L cells resulting in the acceleration of colonic peristalsis.NEW & NOTEWORTHY Glucagon-like peptide-1 (GLP-1) activates CGRP-containing intrinsic afferent neurons resulting in the acceleration of colonic peristalsis. Short-chain fatty acids stimulate the secretion of endogenous GLP-1 from L cells that accelerates colonic peristalsis. Thus, besides the well-known humoral insulinotropic action, GLP-1 exerts a local action via the activation of the enteric nervous system to accelerate colonic motility. Such a prokinetic action of GLP-1 could underlie the mechanisms causing diarrhea in patients with type-2 diabetes treated with GLP-1 analogs.

Dysfunctional TRPM8 signalling in the vascular response to environmental cold in ageing

Ageing is associated with increased vulnerability to environmental cold exposure. Previously, we identified the role of the cold-sensitive transient receptor potential (TRP) A1, M8 receptors as vascular cold sensors in mouse skin. We hypothesised that this dynamic cold-sensor system may become dysfunctional in ageing. We show that behavioural and vascular responses to skin local environmental cooling are impaired with even moderate ageing, with reduced TRPM8 gene/protein expression especially. Pharmacological blockade of the residual TRPA1/TRPM8 component substantially diminished the response in aged, compared with young mice. This implies the reliance of the already reduced cold-induced vascular response in ageing mice on remaining TRP receptor activity. Moreover, sympathetic-induced vasoconstriction was reduced with downregulation of the α_2c adrenoceptor expression in ageing. The cold-induced vascular response is important for sensing cold and retaining body heat and health. These findings reveal that cold sensors, essential for this neurovascular pathway, decline as ageing onsets.

Cellular distribution of cannabinoid-related receptors TRPV1, PPAR-gamma, GPR55 and GPR3 in the equine cervical dorsal root ganglia

BACKGROUND

The activation of cannabinoid and cannabinoid-related receptors by endogenous, plant-derived or synthetic cannabinoids may exert beneficial effects on pain perception. Of the cannabinoids contained in Cannabis sativa, cannabidiol (CBD) does not produce psychotropic effects and seems to represent a molecule having great therapeutic potential. Cannabidiol acts on a great number of cannabinoid and cannabinoid-related G-protein-coupled receptors and ionotropic receptors which have, to date, been understudied in veterinary medicine particularly in equine medicine.

OBJECTIVES

To localise the cellular distribution of four putative cannabinoid-related receptors in the equine cervical dorsal root ganglia (DRG).

STUDY DESIGN

A qualitative and quantitative immunohistochemical study.

METHODS

The cervical (C6-C8) DRG of six slaughtered horses were obtained from a local slaughterhouse. The tissues were fixed and processed for immunohistochemistry, and the resulting cryosections were used to investigate immunoreactivity for the following putative CBD receptors: Transient receptor potential vanilloid type 1 (TRPV1), nuclear peroxisome proliferator-activated receptor gamma (PPARγ), and G protein-coupled receptors 55 (GPR55) and 3 (GPR3).

RESULTS

Large percentages of neuronal cell bodies showed immunoreactivity for TRPV1 (80 ± 20%), PPARγ (100%), GPR55 (64 ± 15%) and GPR3 (63 ± 11%). The satellite glial cells (SGCs) were immunoreactive for TRPV1, PPARγ and GPR55. In addition, GPR55 immunoreactivity was expressed by DRG interneuronal macrophages. In addition, microglia cells were observed surrounding the neuron-SGC complex.

MAIN LIMITATIONS

The limited number of horses included in the study.

CONCLUSIONS

Cannabinoid-related receptors were distributed in the sensory neurons (TRPV1, PPARγ, GPR55 and GPR3), SGCs (TRPV1, PPARγ and GPR55), macrophages (GPR55) and other interneuronal cells (PPARγ and GPR55) of the equine DRG. Given the key role of DRG cellular elements and cannabinoid receptors in the pathophysiology of pain, the present findings provided an anatomical basis for additional studies aimed at exploring the therapeutic uses of non-psychotropic cannabinoid agonists for the management of pain in horses.

Effect of Experimental Gestational Diabetes Mellitus on Mechanical Sensitivity, Capsaicin-Induced Pain Behaviors and Hind Paw Glabrous Skin Innervation of Male and Female Mouse Offspring

Purpose

Gestational diabetes mellitus (GDM) induces cardiovascular and metabolic disturbances in offspring. However, the effects of GDM in pain processing in offspring and whether male and female offspring are equally affected is not well known. Thus, we determined: i) whether GDM in mice affects offspring hindpaw mechanical sensitivity, capsaicin-induced spontaneous pain-like behaviors, and epidermal nerve fiber density (ENFD); and ii) whether there is sexual dimorphism in these parameters in offspring from GDM dams.

Methods

GDM was induced in pregnant ICR mice via i.p. streptozotocin (STZ). Then, glucose levels from dams and offspring were determined. Male and female offspring 2–3 months of age were evaluated for: a) baseline mechanical sensitivity of the hind paw by using von Frey filaments; b) number of flinches and time spent guarding induced by intraplantar capsaicin (0.1%); and c) density of PGP-9.5 and CGRP axons in the epidermis from the hind paw glabrous skin.

Results

Prepartum levels of glucose in STZ-treated dams were significantly increased compared to vehicle-treated dams; however, GDM or vehicle offspring displayed normal and similar blood glucose levels. Male and female GDM offspring showed significantly greater mechanical sensitivity and capsaicin-induced pain behaviors compared to vehicle offspring. Male GDM offspring displayed a slightly more intense nociceptive phenotype in the capsaicin test. PGP-9.5 and CGRP ENFD in hind paw glabrous skin were greater in male and female GDM offspring versus their controls. Sexual dimorphism was generally not observed in GDM offspring in most of the studied parameters.

Conclusion

These results suggest GDM induced greater pain-like behaviors in adult offspring regardless of sex along with an increased ENFD of PGP-9.5 and CGRP in the hind paw glabrous skin. We show that GDM peripheral neuropathy differs from diabetic peripheral neuropathy acquired in adulthood and set the foundation to further study this in human babies exposed to GDM.

The enteric nervous system in gastrointestinal disease etiology

A highly conserved but convoluted network of neurons and glial cells, the enteric nervous system (ENS), is positioned along the wall of the gut to coordinate digestive processes and gastrointestinal homeostasis. Because ENS components are in charge of the autonomous regulation of gut function, it is inevitable that their dysfunction is central to the pathophysiology and symptom generation of gastrointestinal disease. While for neurodevelopmental disorders such as Hirschsprung, ENS pathogenesis appears to be clear-cut, the role for impaired ENS activity in the etiology of other gastrointestinal disorders is less established and is often deemed secondary to other insults like intestinal inflammation. However, mounting experimental evidence in recent years indicates that gastrointestinal homeostasis hinges on multifaceted connections between the ENS, and other cellular networks such as the intestinal epithelium, the immune system, and the intestinal microbiome. Derangement of these interactions could underlie gastrointestinal disease onset and elicit variable degrees of abnormal gut function, pinpointing, perhaps unexpectedly, the ENS as a diligent participant in idiopathic but also in inflammatory and cancerous diseases of the gut. In this review, we discuss the latest evidence on the role of the ENS in the pathogenesis of enteric neuropathies, disorders of gut-brain interaction, inflammatory bowel diseases, and colorectal cancer.

Involvement of TRPV1 and TRPA1 in the modulation of pacemaker potentials in the mouse ileum

BACKGROUND

The roles of transient receptor potential cation channel, subfamily V, member 1 (TRPV1) and subfamily A, member 1 (TRPA1) in mechanisms of gastrointestinal motility are complex. This study aimed to clarify the effects of several TRPV1 and TRPA1 ligands on the electrical potentials generated by pacemaker cells in the mouse-isolated ileum.

METHOD

The pacemaker potentials of ileal segments of mice were recorded extracellularly using a 60-channel microelectrode array. The dominant frequencies, average waveform periods and propagation velocities were quantified. The effects of TRPV1 and TRPA1 agonist and antagonist were compared with the baseline recordings.

RESULTS

The electrophysiological recordings showed that capsaicin (30 μM to 3 mM), resiniferatoxin (300 μM), capsazepine (100-300 μM), allyl isothiocyanate (300 μM), isovelleral (300 μM), icilin (300 μM), A-967,079 (10 μM), AP18 (20 μM) and HC-030,031 (50 μM) significantly reduced the pacemaker frequency and increased the waveform period relative to the baseline. Conversely, ruthenium red (300 μM) significantly increased the pacemaker frequency and reduced the waveform period. Capsaicin (3 mM) and AP18 (20 μM) also significantly reduced the propagation velocity. However, all tested antagonists failed to inhibit the effects of agonists. AMG9810 (300 μM), but not A-967,079 (300 μM), significantly inhibited the increases in pacemaker frequency caused by increased temperatures.

CONCLUSION

Our findings suggest that TRPV1 and TRPA1 play a minor role in regulating pacemaker potentials and that at non-specific actions at other TRP and ion channels most likely contributed to the overall effects on the electrophysiological recordings that we observed.

Cannabinoid and Cannabinoid-Related Receptors in the Myenteric Plexus of the Porcine Ileum

An important piece of evidence has shown that molecules acting on cannabinoid receptors influence gastrointestinal motility and induce beneficial effects on gastrointestinal inflammation and visceral pain. The aim of this investigation was to immunohistochemically localize the distribution of canonical cannabinoid receptor type 1 (CB1R) and type 2 (CB2R) and the cannabinoid-related receptors transient potential vanilloid receptor 1 (TRPV1), transient potential ankyrin receptor 1 (TRPA1), and serotonin receptor 5-HT1a (5-HT1aR) in the myenteric plexus (MP) of pig ileum. CB1R, TRPV1, TRPA1, and 5-HT1aR were expressed, with different intensities in the cytoplasm of MP neurons. For each receptor, the proportions of the immunoreactive neurons were evaluated using the anti-HuC/HuD antibody. These receptors were also localized on nerve fibers (CB1R, TRPA1), smooth muscle cells of tunica muscularis (CB1R, 5-HT1aR), and endothelial cells of blood vessels (TRPV1, TRPA1, 5-HT1aR). The nerve varicosities were also found to be immunoreactive for both TRPV1 and 5-HT1aR. No immunoreactivity was documented for CB2R. Cannabinoid and cannabinoid-related receptors herein investigated showed a wide distribution in the enteric neurons and nerve fibers of the pig MP. These results could provide an anatomical basis for additional research, supporting the therapeutic use of cannabinoid receptor agonists in relieving motility disorders in porcine enteropathies.

A Novel Mode of Sympathetic Reflex Activation Mediated by the Enteric Nervous System

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex (CMC). Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2 Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with bursts of viscerofugal firing at 2 Hz that aligned with 2-Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after a 4- to 5-d organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2-Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

Vagus Nerves Provide a Robust Afferent Innervation of the Mucosa Throughout the Body of the Esophagus in the Mouse

The vagal afferent nerves regulate swallowing and esophageal motor reflexes. However, there are still gaps in the understanding of vagal afferent innervation of the esophageal mucosa. Anatomical studies found that the vagal afferent mucosal innervation is dense in the upper esophageal sphincter area but rare in more distal segments of the esophagus. In contrast, electrophysiological studies concluded that the vagal afferent nerve fibers also densely innervate mucosa in more distal esophagus. We hypothesized that the transfection of vagal afferent neurons with adeno-associated virus vector encoding green fluorescent protein (AAV-GFP) allows to visualize vagal afferent nerve fibers in the esophageal mucosa in the mouse. AAV-GFP was injected into the vagal jugular/nodose ganglia in vivo to sparsely label vagal afferent nerve fibers. The esophageal tissue was harvested 4-6 weeks later, the GFP signal was amplified by immunostaining, and confocal optical sections of the entire esophagi were obtained. We found numerous GFP-labeled fibers in the mucosa throughout the whole body of the esophagus. The GFP-labeled mucosal fibers were located just beneath the epithelium, branched repeatedly, had mostly longitudinal orientation, and terminated abruptly without forming terminal structures. The GFP-labeled mucosal fibers were concentrated in random areas of various sizes in which many fibers could be traced to a single parental axon. We conclude that the vagus nerves provide a robust afferent innervation of the mucosa throughout the whole body of the esophagus in the mouse. Vagal mucosal fibers may contribute to the sensing of intraluminal content and regulation of swallowing and other reflexes.

Intrinsic sensory neurons provide direct input to motor neurons and interneurons in mouse distal colon via varicose baskets

Connections from intrinsic primary afferent neurons (IPANs), to ascending motor and interneurons have been described in guinea pig colon. These mono- and polysynaptic circuits may underlie polarized motor reflexes evoked by local gut stimulation. There is a need to translate findings in guinea pig to mouse, a species increasingly used in enteric neuroscience. Here, mouse distal colon was immunolabeled for CGRP, a marker of putative IPANs. This revealed a combination of large, intensely immunofluorescent axons in myenteric plexus and circular muscle, and thinner varicose axons with less immunofluorescence. The latter formed dense, basket-like varicosity clusters (CGRP+ baskets) that enveloped myenteric nerve cell bodies. Immunolabeling after 4-5 days in organ culture caused loss of large CGRP+ axons, but not varicose CGRP+ fibers and CGRP+ baskets. Baskets were characterized further by triple labeling with CGRP, nitric oxide synthase (NOS) and calretinin (CALR) antibodies. Approximately half (48%) of nerve cell bodies inside CGRP+ baskets lacked both NOS and CALR, while two overlapping populations containing NOS and/or CALR comprised the remainder. Quantitative analysis revealed CGRP+ varicosities were most abundant in baskets, followed by CALR+ varicosities, with a high degree of colocalization between the two markers. Few NOS+ varicosities occurred in baskets. Significantly higher proportions of CALR+ and CGRP+ varicosities colocalized in baskets than in circular muscle. In conclusion, CGRP+ baskets in mouse colon are formed by intrinsic enteric neurons with a neurochemical profile consistent with IPANs and have direct connections to both excitatory and inhibitory neurons.

TRPV1 activation alters the function of Aδ and C fiber sensory neurons that innervate bone

The Transient receptor potential cation channel subfamily V member 1 (TRPV1) is a non-selective cation channel that is activated by capsaicin, low pH and noxious heat. It has been suggested to have a pro-algesic role in a range of conditions that present with bone pain, but the mechanisms by which this occurs are not yet clear. In this study we aimed to determine if TRPV1 is expressed in Aδ and/or C fiber bone afferent neurons, and to explore its role in the activation and/or sensitization of bone afferent neurons to mechanical stimulation. A combination of retrograde tracing and immunohistochemistry was used to determine expression of TRPV1 in the soma of bone afferent neurons that innervate the rat tibial marrow cavity. A novel, in vivo, electrophysiological bone-nerve preparation, recently developed in our laboratory, was used to make recordings of the activity and sensitivity of bone afferent neurons in response to application of the TRPV1 agonist capsaicin to the marrow cavity. We found that a substantial proportion of bone afferent neurons express TRPV1. These include both small-diameter myelinated (neurofilament rich) and unmyelinated (neurofilament poor) neurons that are likely to be Aδ and C fiber neurons, respectively. Electrophysiological recordings revealed that application of capsaicin to the marrow cavity increased ongoing activity of C fiber, and to a lesser extent Aδ fiber, bone afferent neurons. Capsaicin also sensitized both Aδ and C fiber bone afferent neurons to mechanical stimulation. This evidence supports a role for TRPV1 in the pathogenesis of pain associated with bone pathology or disease.

QX-314 inhibits acid-induced activation of esophageal nociceptive C fiber neurons

INTRODUCTION

Acid reflux in the esophagus can induce painful sensations such as heartburn and non-cardiac chest pain. These nociceptive symptoms are initiated by activation of TRPV1-positive afferent C fibers in the esophagus. The present study aimed to explore a novel C fiber inhibition approach. We hypothesized that activation of TRPV1 by acid enabled QX-314, a membrane impermeable sodium channel blocker, to inhibit acid-induced activation of esophageal nociceptive C fiber neurons.

METHOD

We determined the inhibitory effect of QX-314 in the presence of acid in guinea pig esophageal nociceptive vagal jugular C fiber neurons by both patch clamp recording in neuron soma and by extra-cellular recording at nerve terminals.

KEY RESULTS

Our data demonstrated QX-314 alone did not inhibit sodium currents. However, when applied along with capsaicin to activate TRPV1, QX-314 was able to block sodium currents in esophageal-specific jugular C fiber neurons. We then showed that in the presence of acid, QX-314 significantly blocked acid-evoked activation of jugular C fiber neurons. This effect was attenuated by TRPV1 antagonist AMG9810, suggesting acid-mediated inhibitory effect of QX-314 was TRPV1-dependent. Finally, we provided evidence at nerve endings that acid-evoked action potential discharges in esophageal jugular C fibers were inhibited by QX-314 when applied in the presence of acid.

CONCLUSION AND INFERENCES

Our data demonstrated that activation of TRPV1 by acid enabled membrane impermeable sodium channel blocker QX-314 to inhibit acid-induced activation in esophageal nociceptive C fibers. This supports a localized application of QX-314 in the esophagus to block esophageal nociception in acid reflux disorders.

Spinal Afferent Innervation of the Colon and Rectum

Despite their seemingly elementary roles, the colon and rectum undertake a variety of key processes to ensure our overall wellbeing. Such processes are coordinated by the transmission of sensory signals from the periphery to the central nervous system, allowing communication from the gut to the brain via the "gut-brain axis". These signals are transmitted from the peripheral terminals of extrinsic sensory nerve fibers, located within the wall of the colon or rectum, and via their axons within the spinal splanchnic and pelvic nerves to the spinal cord. Recent studies utilizing electrophysiological, anatomical and gene expression techniques indicate a surprisingly diverse set of distinct afferent subclasses, which innervate all layers of the colon and rectum. Combined these afferent sub-types allow the detection of luminal contents, low- and high-intensity stretch or contraction, in addition to the detection of inflammatory, immune, and microbial mediators. To add further complexity, the proportions of these afferents vary within splanchnic and pelvic pathways, whilst the density of the splanchnic and pelvic innervation also varies along the colon and rectum. In this review we traverse this complicated landscape to elucidate afferent function, structure, and nomenclature to provide insights into how the extrinsic sensory afferent innervation of the colon and rectum gives rise to physiological defecatory reflexes and sensations of discomfort, bloating, urgency, and pain.

Descending monoaminergic pathways projecting to the spinal defecation center enhance colorectal motility in rats

The central regulating mechanisms of defecation, especially roles of the spinal defecation center, are still unclear. We have shown that monoamines including norepinephrine, dopamine, and serotonin injected into the spinal defecation center cause propulsive contractions of the colorectum. These monoamines are the main neurotransmitters of descending pain inhibitory pathways. Therefore, we hypothesized that noxious stimuli in the colorectum would activate the descending monoaminergic pathways projecting to the spinal defecation center and that subsequently released endogenous monoamine neurotransmitters would enhance colorectal motility. Colorectal motility was measured in rats anesthetized with α-chloralose and ketamine. As a noxious stimulus, capsaicin was administered into the colorectal lumen. To interrupt neuronal transmission in the spinal defecation center, antagonists of norepinephrine, dopamine, and/or serotonin receptors were injected intrathecally at the L6-S1 spinal level, where the spinal defecation center is located. Intraluminal administration of capsaicin, acting on the transient receptor potential vanilloid 1 channel, caused transient propulsive contractions. The effect of capsaicin was abolished by surgical severing of the pelvic nerves or thoracic spinal transection at the T4 level. Capsaicin-induced contractions were blocked by preinjection of D2-like dopamine receptor and 5-hydroxytryptamine subtype 2 and 3 receptor antagonists into the spinal defecation center. We demonstrated that intraluminally administered capsaicin causes propulsive colorectal motility through reflex pathways involving the spinal and supraspinal defecation centers. Our results provide evidence that descending monoaminergic neurons are activated by noxious stimulation to the colorectum, leading to facilitation of colorectal motility. NEW & NOTEWORTHY The present study demonstrates that noxious stimuli in the colorectum activates the descending monoaminergic pathways projecting to the spinal defecation center and that subsequently released endogenous monoamine neurotransmitters, serotonin and dopamine, enhance colorectal motility. Our findings provide a possible explanation of the concurrent appearance of abdominal pain and bowel disorder in irritable bowel syndrome patients. Thus the present study may provide new insights into understanding of mechanisms of colorectal dysfunction involving the central nervous system.

Alterations in Gastric Mucosal Expression of Calcitonin Gene-Related Peptides, Vanilloid Receptors, and Heme Oxygenase-1 Mediate Gastroprotective Action of Carbon Monoxide against Ethanol-Induced Gastric Mucosal Lesions

Carbon monoxide (CO) has been reported to contribute to the maintenance of gastric mucosal integrity, gastroprotection, and ulcer healing. However, involvement of transient receptor potential vanilloid receptor type 1 (TRPV1) located on afferent sensory fibers endings and sensory neuropeptide calcitonin gene-related peptide (CGRP) in CO-mediated gastroprotection against ethanol-induced gastric damage has not been explored. Male Wistar rats with and without denervation of afferent sensory neurons induced by capsaicin (total dose 125 mg/kg within 3 days) were pretreated with vehicle, CO donor tricarbonyldichlororuthenium (II) dimer (CORM-2, 5 mg/kg i.g.), administered alone or with CGRP-α (10 μg/kg i.p.) or TRPV1 antagonist capsazepine (5 mg/kg i.g.), followed 30 min later by intragastric (i.g.) administration of 75% ethanol. The area of gastric damage and gastric blood flow (GBF) were assessed planimetrically and by laser flowmetry, respectively. Microscopic evaluation of ethanol-induced gastric lesions was performed after haematoxylin/eosin (H&E) or alcian blue/periodic acid-Schiff/alcian blue (AB/PAS) staining. Gastric mucosal mRNA fold change for heme oxygenase (HMOX)-1, HMOX-2, CGRP-α, CGRP-β, inducible nitric oxide synthase (iNOS), endothelial (e)NOS, neuronal (n)NOS, cyclooxygenase (COX)-1, COX-2, and protein expression for HMOX-1 and TRPV1 was determined by real-time PCR or Western blot, respectively. Pretreatment with CORM-2 combined or not with CGRP reduced ethanol-induced gastric lesions and elevated GBF. Capsaicin-denervation or co-treatment with capsazepine or CGRP and CORM-2 in capsaicin-denervated animals failed to affect these beneficial effects of CO donor. In rats with intact sensory nerves, CORM-2 increased gastric mRNA level for HMOX-1 and CGRP-α. In capsaicin-denervated rats, CORM-2 increased eNOS mRNA fold change and TRPV1 protein expression while capsaicin denervation itself decreased HMOX-1 protein expression and eNOS mRNA level. We conclude that CO prevents gastric mucosa from ethanol-induced lesions due to activation of TRPV1/CGRP-α system and accompanying increase in gastric microcirculation but independently on afferent sensory nerve activity despite the stimulation of TRPV1 protein and CGRP-α mRNA expression.

The extracellular matrix glycoprotein tenascin-X regulates peripheral sensory and motor neurones

KEY POINTS

Tenascin-X (TNX) is an extracellular matrix glycoprotein with anti-adhesive properties in skin and joints. Here we report the novel finding that TNX is expressed in human and mouse gut tissue where it is exclusive to specific subpopulations of neurones. Our studies with TNX-deficient mice show impaired defecation and neural control of distal colonic motility that can be rescued with a 5-HT₄ receptor agonist. However, colonic secretion is unchanged. They are also susceptible to internal rectal intussusception. Colonic afferent sensitivity is increased in TNX-deficient mice. Correspondingly, there is increased density of and sensitivity of putative nociceptive fibres in TNX-deficient mucosa. A group of TNX-deficient patients report symptoms highly consistent with those in the mouse model. These findings suggest TNX plays entirely different roles in gut to non-visceral tissues - firstly a role in enteric motor neurones and secondly a role influencing nociceptive sensory neurones Studying further the mechanisms by which TNX influences neuronal function will lead to new targets for future treatment.

ABSTRACT

The extracellular matrix (ECM) is not only an integral structural molecule, but is also critical for a wide range of cellular functions. The glycoprotein tenascin-X (TNX) predominates in the ECM of tissues like skin and regulates tissue structure through anti-adhesive interactions with collagen. Monogenic TNX deficiency causes painful joint hypermobility and skin hyperelasticity, symptoms characteristic of hypermobility Ehlers Danlos syndrome (hEDS). hEDS patients also report consistently increased visceral pain and gastrointestinal (GI) dysfunction. We investigated whether there is a direct link between TNX deficiency and GI pain or motor dysfunction. We set out first to learn where TNX is expressed in human and mouse, then determine how GI function, specifically in the colon, is disordered in TNX-deficient mice and humans of either sex. In human and mouse tissue, TNX was predominantly associated with cholinergic colonic enteric neurones, which are involved in motor control. TNX was absent from extrinsic nociceptive peptidergic neurones. TNX-deficient mice had internal rectal prolapse and a loss of distal colonic contractility which could be rescued by prokinetic drug treatment. TNX-deficient patients reported increased sensory and motor GI symptoms including abdominal pain and constipation compared to controls. Despite absence of TNX from nociceptive colonic neurones, neuronal sprouting and hyper-responsiveness to colonic distension was observed in the TNX-deficient mice. We conclude that ECM molecules are not merely support structures but an integral part of the microenvironment particularly for specific populations of colonic motor neurones where TNX exerts functional influences.

Optogenetic Activation of Colon Epithelium of the Mouse Produces High-Frequency Bursting in Extrinsic Colon Afferents and Engages Visceromotor Responses

Epithelial cells of the colon provide a vital interface between the internal environment (lumen of the colon) and colon parenchyma. To examine epithelial-neuronal signaling at this interface, we analyzed mice in which channelrhodopsin (ChR2) was targeted to either TRPV1-positive afferents or to villin-expressing colon epithelial cells. Expression of a ChR2-EYFP fusion protein was directed to either primary sensory neurons or to colon epithelial cells by crossing Ai32 mice with TRPV1-Cre or villin-Cre mice, respectively. An ex vivo preparation of the colon was used for single-fiber analysis of colon sensory afferents of the pelvic nerve. Afferents were characterized using previously described criteria as mucosal, muscular, muscular-mucosal, or serosal and then tested for blue light-induced activation. Light activation of colon epithelial cells produced robust firing of action potentials, similar to that elicited by physiologic stimulation (e.g., circumferential stretch), in 50.5% of colon afferents of mice homozygous for ChR2 expression. Light-induced activity could be reduced or abolished in most fibers using a cocktail of purinergic receptor blockers suggesting ATP release by the epithelium contributed to generation of sensory neuron action potentials. Using electromyographic recording of visceromotor responses we found that light stimulation of the colon epithelium evoked behavioral responses in Vil-ChR2 mice that was similar to that seen with balloon distension of the colon. These ex vivo and in vivo data indicate that light stimulation of colon epithelial cells alone, without added mechanical or chemical stimuli, can directly activate colon afferents and elicit behavioral responses.SIGNIFICANCE STATEMENT Abdominal pain that accompanies inflammatory diseases of the bowel is particularly vexing because it can occur without obvious changes in the structure or inflammatory condition of the colon. Pain reflects abnormal sensory neuron activity that may be controlled in part by release of substances from lining epithelial cells. In support of this mechanism we determined that blue-light stimulation of channelrhodopsin-expressing colon epithelial cells could evoke action potential firing in sensory neurons and produce changes in measures of behavioral sensitivity. Thus, activity of colon epithelial cells alone, without added mechanical or chemical stimuli, is sufficient to activate pain-sensing neurons.

CGRPα within the Trpv1-Cre population contributes to visceral nociception

The role of calcitonin gene-related peptide (CGRP) in visceral and somatic nociception is incompletely understood. CGRPα is highly expressed in sensory neurons of dorsal root ganglia and particularly in neurons that also express the transient receptor potential cation channel subfamily V member 1 (Trpv1). Therefore, we investigated changes in visceral and somatic nociception following deletion of CGRPα from the Trpv1-Cre population using the Cre/lox system. In control mice, acetic acid injection (0.6%, ip) caused significant immobility (time stationary), an established indicator of visceral pain. In CGRPα-mCherry^lx/lx;Trpv1-Cre mice, the duration of immobility was significantly less than controls, and the distance CGRPα-mCherry^lx/lx;Trpv1-Cre mice traveled over 20 min following acetic acid was significantly greater than controls. However, following acetic acid injection, there was no difference between genotypes in the writhing reflex, number of abdominal licks, or forepaw wipes of the cheek. CGRPα-mCherry^lx/lx;Trpv1-Cre mice developed more pronounced inflammation-induced heat hypersensitivity above baseline values compared with controls. However, analyses of noxious acute heat or cold transmission revealed no difference between genotypes. Also, odor avoidance test, odor preference test, and buried food test for olfaction revealed no differences between genotypes. Our findings suggest that CGRPα-mediated transmission within the Trpv1-Cre population plays a significant role in visceral nociceptive pathways underlying voluntary movement. Monitoring changes in movement over time is a sensitive parameter to identify differences in visceral nociception, compared with writhing reflexes, abdominal licks, or forepaw wipes of the cheek that were unaffected by deletion of CGRPα- from Trpv1-Cre population and likely utilize different mechanisms. NEW & NOTEWORTHY The neuropeptide calcitonin gene-related peptide (CGRP) is highly colocalized with transient receptor potential cation channel subfamily V member 1 (TRPV1)-expressing primary afferent neurons, but the functional role of CGRPα specifically in these neurons is unknown in pain processing from visceral and somatic afferents. We used cre-lox recombination to conditionally delete CGRPα from TRPV1-expressing neurons in mice. We show that CGRPα from within TRPV1-cre population plays an important role in visceral nociception but less so in somatic nociception.

Distinct Expression of Phenotypic Markers in Placodes- and Neural Crest-Derived Afferent Neurons Innervating the Rat Stomach

BACKGROUND

Visceral pain is initiated by activation of primary afferent neurons among which the capsaicin-sensitive (TRPV1-positive) neurons play an important role. The stomach is a common source of visceral pain. Similar to other organs, the stomach receives dual spinal and vagal afferent innervation. Developmentally, spinal dorsal root ganglia (DRG) and vagal jugular neurons originate from embryonic neural crest and vagal nodose neurons originate from placodes. In thoracic organs the neural crest- and placodes-derived TRPV1-positive neurons have distinct phenotypes differing in activation profile, neurotrophic regulation and reflex responses. It is unknown to whether such distinction exists in the stomach.

AIMS

We hypothesized that gastric neural crest- and placodes-derived TRPV1-positive neurons express phenotypic markers indicative of placodes and neural crest phenotypes.

METHODS

Gastric DRG and vagal neurons were retrogradely traced by DiI injected into the rat stomach wall. Single-cell RT-PCR was performed on traced gastric neurons.

RESULTS

Retrograde tracing demonstrated that vagal gastric neurons locate exclusively into the nodose portion of the rat jugular/petrosal/nodose complex. Gastric DRG TRPV1-positive neurons preferentially expressed markers PPT-A, TrkA and GFRα₃ typical for neural crest-derived TRPV1-positive visceral neurons. In contrast, gastric nodose TRPV1-positive neurons preferentially expressed markers P2X₂ and TrkB typical for placodes-derived TRPV1-positive visceral neurons. Differential expression of neural crest and placodes markers was less pronounced in TRPV1-negative DRG and nodose populations.

CONCLUSIONS

There are phenotypic distinctions between the neural crest-derived DRG and placodes-derived vagal nodose TRPV1-positive neurons innervating the rat stomach that are similar to those described in thoracic organs.

Imaging activation of peptidergic spinal afferent varicosities within visceral organs using novel CGRPα-mCherry reporter mice

In vertebrates, visceral pain from internal organs is detected by spinal afferents, whose cell bodies lie in dorsal root ganglia (DRG). Until now, all recordings from spinal afferents have been restricted to recording transmission of action potentials along axons, or from cell bodies lying outside their target organ, which is not where sensory transduction occurs. Our aim was to record directly from a major class of spinal afferent within visceral organs, where transduction of sensory stimuli into action potentials occurs. Using novel calcitonin gene-related peptide (CGRP)α reporter mice, DRG neurons expressed mCherry, including nerve axons within viscera. In colon, a minority of total CGRP immunoreactivity was attributed CGRPα. In isolated unstretched colon, calcium imaging from CGRPα-expressing varicose axons did not detect resolvable calcium transients. However, noxious levels of maintained circumferential stretch to the colon induced repetitive calcium transients simultaneously in multiple neighboring varicosities along single mCherry-expressing axons. Discrete varicosities could generate unitary calcium transients independently of neighboring varicosities. However, axons expressing mCherry only generated coordinated calcium transients when accompanied by simultaneous activation of multiple varicosities along that axon. Simultaneous imaging from different classes of myenteric neurons at the same time as mCherry-expressing axons revealed coordinated calcium transients in multiple myenteric neurons, independent of activity in mCherry-expressing axons. CGRPα-expressing axon terminals preferentially responded to heat, capsaicin, and low pH. We show that direct recordings can be made from the major class of peptidergic spinal afferent that contributes to visceral nociception. This approach can provide powerful insights into transduction of stimuli in viscera.

Capsaicin, Nociception and Pain

Capsaicin, the pungent ingredient of the hot chili pepper, is known to act on the transient receptor potential cation channel vanilloid subfamily member 1 (TRPV1). TRPV1 is involved in somatic and visceral peripheral inflammation, in the modulation of nociceptive inputs to spinal cord and brain stem centers, as well as the integration of diverse painful stimuli. In this review, we first describe the chemical and pharmacological properties of capsaicin and its derivatives in relation to their analgesic properties. We then consider the biochemical and functional characteristics of TRPV1, focusing on its distribution and biological effects within the somatosensory and viscerosensory nociceptive systems. Finally, we discuss the use of capsaicin as an agonist of TRPV1 to model acute inflammation in slices and other ex vivo preparations.

Identification of different functional types of spinal afferent neurons innervating the mouse large intestine using a novel CGRPα transgenic reporter mouse

Spinal afferent neurons detect noxious and physiological stimuli in visceral organs. Five functional classes of afferent terminals have been extensively characterized in the colorectum, primarily from axonal recordings. Little is known about the corresponding somata of these classes of afferents, including their morphology, neurochemistry, and electrophysiology. To address this, we made intracellular recordings from somata in L6/S1 dorsal root ganglia and applied intraluminal colonic distensions. A transgenic calcitonin gene-related peptide-α (CGRPα)-mCherry reporter mouse, which enabled rapid identification of soma neurochemistry and morphology following electrophysiological recordings, was developed. Three distinct classes of low-threshold distension-sensitive colorectal afferent neurons were characterized; an additional group was distension-insensitive. Two of three low-threshold classes expressed CGRPα. One class expressing CGRPα discharged phasically, with inflections on the rising phase of their action potentials, at low frequencies, to both physiological (<30 mmHg) and noxious (>30 mmHg) distensions. The second class expressed CGRPα and discharged tonically, with smooth, briefer action potentials and significantly greater distension sensitivity than phasically firing neurons. A third class that lacked CGRPα generated the highest-frequency firing to distension and had smaller somata. Thus, CGRPα expression in colorectal afferents was associated with lower distension sensitivity and firing rates and larger somata, while colorectal afferents that generated the highest firing frequencies to distension had the smallest somata and lacked CGRPα. These data fill significant gaps in our understanding of the different classes of colorectal afferent somata that give rise to distinct functional classes of colorectal afferents. In healthy mice, the majority of sensory neurons that respond to colorectal distension are low-threshold, wide-dynamic-range afferents, encoding both physiological and noxious ranges.