Neurochemical characterization of insulin receptor-expressing primary sensory neurons in wild-type and vanilloid type 1 transient receptor potential receptor knockout mice

The insulin receptor (IR) is expressed by a subpopulation of primary sensory neurons (PSN), including a proportion of cells expressing the nociceptive transducer vanilloid type 1 transient receptor potential receptor (TRPV1). Recent data suggest functional links between the IR and other receptors, including TRPV1, which could be involved in the development of PSN malfunctions in pathological insulin secretion. Here we used combined immunohistochemical labelling on sections from L4-5 dorsal root ganglia of wild-type (WT) and TRPV1 knockout (KO) mice to examine the neurochemical properties of IR-expressing PSN and the possible effect of deletion of TRPV1 on those characteristics. We found that antibodies raised against the high-molecular-weight neurofilament (NF-200) and the neurofilament protein peripherin distinguished between small and large neurons. We also found that the IR was expressed predominantly by the small peripherin-immunopositive cells both in the WT and in the KO animals. IR expression, however, did not show any preference between the major subpopulations of the small cells, the calcitonin gene-related peptide (CGRP)-expressing and Bandeiraea simplicifolia isolectin B4 (IB4)-binding neurons, either in the WT or in the KO mice. Nevertheless, a significant proportion of the IR-expressing cells also expressed TRPV1. Comparison of the staining pattern of these markers showed no difference between WT and KO animals. These findings indicate that the majority of the IR-expressing PSN are small neurons, which are considered as nociceptive cells. Furthermore, these data show that deletion of the TRPV1 gene does not induce any additional changes in neurochemical phenotype of nociceptive PSN.     

Effect of reflux-induced inflammation on transient receptor potential vanilloid one (TRPV1) expression in primary sensory neurons innervating the oesophagus of rats

A possible mechanism of oesophageal hypersensitivity is the acid-induced activation of transient receptor potential vanilloid receptor 1 (TRPV1) in the primary sensory neurons. We investigated TRPV1 expression and its colocalization with substance P (SP) and isolectin B4 (IB4)-positive cells in the thoracic dorsal root ganglia (DRGs) and nodose ganglia (NGs) of rats with reflux-induced oesophagitis (RO). RO was developed by fundus ligation and partial obstruction of the pylorus of Sprague-Dawley rats. Four groups of rats were used; fundus ligated acute (RO 48 h), chronic 7 days (RO 7D), RO 7D + omeprazole (7D + Omz, 40 mg kg(-1), i.p.) and sham-operated controls. Immunohistochemical analysis of TRPV1, SP and IB4 expression were carried out in spinal cord (SC), DRGs and NGs. RO rats exhibited significant inflammation and increase in TRPV1-ir and SP-ir expressions in the SC, DRGs and NGs. The maximum colocalization of TRPV1 and SP was observed in RO 7D rats, but Omz prevented inflammation and over expression of TRPV1 and SP. TRPV1-ir significantly increased in IB4-positive cells in DRGs and SC, but not in the NGs. Results document that acid-induced oesophagitis increases TRPV1 expression in both SP- and IB4-positive sensory neurons. The over expression of TRPV1 may contribute to oesophageal hypersensitivity observed in gastro-oesophageal reflux disease (GORD).     

Activation of transient receptor potential vanilloid 2‐expressing primary afferents stimulates synaptic transmission in the deep dorsal horn of the rat spinal cord and elicits mechanical hyperalgesia

Probenecid, an agonist of transient receptor vanilloid (TRPV) type 2, was used to evaluate the effects of TRPV2 activation on excitatory and inhibitory synaptic transmission in the dorsal horn (DH) of the rat spinal cord and on nociceptive reflexes induced by thermal heat and mechanical stimuli. The effects of probenecid were compared with those of capsaicin, a TRPV1 agonist. Calcium imaging experiments on rat dorsal root ganglion (DRG) and DH cultures indicated that functional TRPV2 and TRPV1 were expressed by essentially non‐overlapping subpopulations of DRG neurons, but were absent from DH neurons and DH and DRG glial cells. Pretreatment of DRG cultures with small interfering RNAs against TRPV2 suppressed the responses to probenecid. Patch‐clamp recordings from spinal cord slices showed that probenecid and capsaicin increased the frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents in a subset of laminae III–V neurons. In contrast to capsaicin, probenecid failed to stimulate synaptic transmission in lamina II. Intrathecal or intraplantar injections of probenecid induced mechanical hyperalgesia/allodynia without affecting nociceptive heat responses. Capsaicin induced both mechanical hyperalgesia/allodynia and heat hyperalgesia. Activation of TRPV1 or TRPV2 in distinct sets of primary afferents increased the sEPSC frequencies in a largely common population of DH neurons in laminae III–V, and might underlie the development of mechanical hypersensitivity following probenecid or capsaicin treatment. However, only TRPV1‐expressing afferents facilitated excitatory and/or inhibitory transmission in a subpopulation of lamina II neurons, and this phenomenon might be correlated with the induction of thermal heat hyperalgesia.     

面部炎症痛诱发大鼠三叉神经节神经元中辣椒素受体表达的改变

目的探讨辣椒素受体（transient receptor potential vanilloid receptor 1,TRPV1）参与和面部炎症痛相关的热痛觉过敏与冷痛觉感受的可能机制。方法于大鼠面部皮下注射松节油造成面部炎症痛模型,分别应用热测痛和冷测痛装置测量热缩头潜伏期（head withdrawal thermal latency,HWTL）和冷缩头潜伏期（head withdrawal cold lentency,HWCL）的变化,每天测量一次,连续21天。应用免疫组织化学染色,细胞大小频率分析和平均光密度值分析来研究面部炎症痛后第3、5、7、14、21天支配大鼠面部表皮区三叉神经节（trigeminal ganglion,TG）初级感觉神经元、触须部皮肤末梢神经纤维和投射至三叉神经感觉尾侧亚核（trigeminal sensory nuclei caudalis,Vc）中枢突TRPV1表达的改变。结果注射松节油后第1至14天,热退缩反应潜伏期与冷退缩反应潜伏期均明显下降,分别于注射后第5天和第3天达到最低,第21天恢复到正常水平;注射松节油后第1至14天,TRPV1表达的细胞数量增加,并于第7天达到最大,第21天恢复到正常水平。正常大鼠TRPV1主要表达于TG的中小神经元,触须部皮肤以及三叉神经尾侧亚核含丰富的TRPV1阳性末梢;面部炎症痛后2周内,TG的中小神经元,触须部皮肤末梢以及Vc的Ⅰ和Ⅱ外层均可见明显的TRPV1表达增加。结论面部炎症痛可以引起大鼠对伤害性热刺激和冷刺激的痛觉过敏,并导致三叉神经节中TRPV1阳性神经元和外周与中枢阳性神经纤维末梢数目增加,表明TRPV1在三叉神经节的中小神经元和末梢轴突表型的改变可能对松节油引起面部炎症痛时热痛觉过敏和冷痛觉感受的形成与维持起重要作用。     

NGF and GDNF differentially regulate TRPV1 expression that contributes to development of inflammatory thermal hyperalgesia

The transient receptor potential ion channel, TRPV1 plays an essential role in the development of inflammatory thermal hyperalgesia. We investigated the dependence of inflammatory TRPV1 induction on neurotrophic factor. Rat dorsal root ganglia (DRG) neurons were classified according to immunostaining for trk-A and IB4 and the effects of antibodies against NGF or GDNF on TRPV1 expression within the groups were then analysed by immunohistochemical means. The data were compared with the time course of trophic factor expression and the effects of their antibodies on thermal hyperalgesia against radiant heat after inflammation. Although the levels of both NGF and GDNF were increased by inflammation, NGF rapidly and transiently increased whereas GDNF increased gradually over a period of approximately one week. TRPV1 expression was increased within both trk-A positive and IB4 positive neurons after inflammation. Increased TRPV1 expression within trk-A positive neurons was prevented by anti-NGF but not by anti-GDNF, whereas TRPV1 induction within the IB4 positive group was blocked by anti-GDNF but not by anti-NGF. Both antibodies prevented the short latency of withdrawing an inflamed paw from radiant heat. These results suggest that inflammation differentially increases both NGF and GDNF, which facilitate TRPV1 expression within distinctive neurons to induce thermal hyperalgesia.     

面部炎症痛诱发大鼠三叉神经节神经元中辣椒素受体表达的改变(英文)

目的探讨辣椒素受体(transient receptor potential vanilloid receptor 1,TRPV1)参与和面部炎症痛相关的热痛觉过敏与冷痛觉感受的可能机制。方法于大鼠面部皮下注射松节油造成面部炎症痛模型,分别应用热测痛和冷测痛装置测量热缩头潜伏期(head withdrawal thermal latency, HWTL)和冷缩头潜伏期(head withdrawal coldlentency, HWCL)的变化,每天测量一次,连续21天。应用免疫组织化学染色,细胞大小频率分析和平均光密度值分析来研究面部炎症痛后第3、5、7、14、21天支配大鼠面部表皮区三叉神经节(trigeminal ganglion,TG)初级感觉神经元、触须部皮肤末梢神经纤维和投射至三叉神经感觉尾侧亚核(trigeminal sensory nuclei caudalis,Vc)中枢突TRPV1表达的改变。结果注射松节油后第1至14天,热退缩反应潜伏期与冷退缩反应潜伏期均明显下降,分别于注射后第5 天和第3 天达到最低,第21 天恢复到正常水平;注射松节油后第1至14天 ,TRPV1表达的细胞数量增加,并于第7天达到最大,第21 天恢复到正常水平。正常大鼠TRPV1主要表达于TG的中小神经元,触须部皮肤以及三叉神经尾侧亚核含丰富的TRPV1阳性末梢;面部炎症痛后2周内,TG的中小神经元,触须部皮肤末梢以及Vc的Ⅰ和Ⅱ外层均可见明显的TRPV1表达增加。结论面部炎症痛可以引起大鼠对伤害性热刺激和冷刺激的痛觉过敏,并导致三叉神经节中TRPV1阳性神经元和外周与中枢阳性神经纤维末梢数目增加,表明TRPV1在三叉神经节的中小神经元和末梢轴突表型的改变可能对松节油引起面部炎症痛时热痛觉过敏和冷痛觉感受的形成与维持起重要作用。     

Research progress in transient receptor potential vanilloid 1 of sensory nervous system

The transient receptor potential vanilloid subfamily member 1 (TRPV1) is a protein mainly expressed in sensory neurons and fibers, such as in trigeminal ganglion and dorsal root ganglion, and has been indicated to be involved in several physiological and pathological processes. Studies on thermal activation have revealed that phosphorylation is involved in TRPV1 activation and 2 putative phosphorylation sites, Ser residues 502 (Ser-502) and Ser residues 800 (Ser-800), have been recently confirmed to possess the capability of resensitizing TRPV1. In addition to acidification, alkalization has also been proved to be a highly effective stimulator for TRPV1. TRPV1 could be regulated by various physical and chemical modulators, as well as the chronic pain. TRPV1 plays a crucial role in the transmission of pain signals, especially under inflammation and the neoplasm conditions, and it can also modulate nociceptive afferents by reinforcing morphine tolerance. The present review mainly focused on the structural and functional complexities of TRPV1, together with its activation and modulation by a wide variety of physical and chemical stimuli. Its pharmacological manipulation (sensitization/desensitization) and therapeutical targets were also discussed.     

Increased gene and protein expressions of the transient receptor potential vanilloid receptor 4 following sustained pure mechanical pressure on rat dorsal root ganglion neurons

Dorsal root ganglion(DRG) neurons from newborn Wistar rats cultured in vitro were pressurized with 20,40,80 or 120 mm Hg compressive loadings(1 mm Hg = 0.133 kPa) for 12,24,48 or 72 hours,respectively.The 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide test showed that pressures less than 80 mm Hg had no obvious impact on the activity of DRG neurons.The protein expression levels of transient receptor potential vanilloid receptor 4(TRPV4),transient receptor potential vanilloid receptor 1,transie...     

ORL-1 and Mu opioid receptor antisera label different fibers in areas involved in pain processing

Mu opioid receptors (MOR) mediate the analgesic effects of opioid drugs such as morphine. The opioid receptor-like (ORL-1) receptor is structurally related to opioid receptors and the ORL-1 receptor agonist, orphanin FQ/nociceptin, induces analgesia at the spinal level, but appears to recruit different circuitry than that used by mu opioids. When administered intracerebroventricularly, orphanin FQ/nociceptin produces hyperalgesia and/or reverses opioid analgesia. The functionally distinct actions elicited by MOR and ORL-1 receptors, which activate similar intracellular signaling systems and show similar regional distributions, could be explained by their differential cellular localization. By using double label immunohistochemistry and confocal microscopy, the present study investigates the distribution of MOR and ORL-1 receptors in regions of the rat nervous system that are involved with nociceptive processing. In general co-localization of MOR and ORL-1 receptor immunoreactivity was not observed in either perikarya or neuropil in the dorsal root ganglia, nor in the Lissauer's tract and superficial laminae of the spinal cord. Likewise, there was no evidence for co-localization of these receptors within the periaqueductal gray, the nucleus raphe magnus, the gigantocellular reticular nucleus, and the nucleus of the solitary tract. These observations indicate that MOR and ORL-1 receptors are expressed predominantly on different fiber systems in these regions. This differential distribution is consistent with the distinct pharmacology of ORL-1 and MOR receptor agonists and suggests that the antisera to MOR and ORL-1 receptors may provide useful markers for further investigations of analgesic and counteranalgesic pathways modulating pain perception. J. Comp. Neurol. 399:373–383, 1998. © 1998 Wiley-Liss, Inc.     

Inhibitory effects of CB1 and CB2 receptor agonists on responses of DRG neurons and dorsal horn neurons in neuropathic rats

Cannabinoid 2 (CB2) receptor mediated antinociception and increased levels of spinal CB2 receptor mRNA are reported in neuropathic Sprague-Dawley rats. The aim of this study was to provide functional evidence for a role of peripheral, vs. spinal, CB2 and cannabinoid 1 (CB1) receptors in neuropathic rats. Effects of the CB2 receptor agonist, JWH-133, and the CB1 receptor agonist, arachidonyl-2-chloroethylamide (ACEA), on primary afferent fibres were determined by calcium imaging studies of adult dorsal root ganglion (DRG) neurons taken from neuropathic and sham-operated rats. Capsaicin (100 nm) increased [Ca2+]i in DRG neurons from sham and neuropathic rats. JWH-133 (3 microm) or ACEA (1 microm) significantly (P<0.001) attenuated capsaicin-evoked calcium responses in DRG neurons in neuropathic and sham-operated rats. The CB2 receptor antagonist, SR144528, (1 microm) significantly inhibited the effects of JWH-133. Effects of ACEA were significantly inhibited by the CB1 receptor antagonist SR141716A (1 microm). In vivo experiments evaluated the effects of spinal administration of JWH-133 (8-486 ng/50 microL) and ACEA (0.005-500 ng/50 microL) on mechanically evoked responses of neuropathic and sham-operated rats. Spinal JWH-133 attenuated mechanically evoked responses of spinal neurons in neuropathic, but not sham-operated rats. These inhibitory effects were blocked by SR144528 (0.001 microg/50 microL). Spinal ACEA inhibited mechanically evoked responses of neuropathic and sham-operated rats, these effects were blocked by SR141716A (0.01 microg/50 microL). Our data provide evidence for a functional role of CB2, as well as CB1 receptors on DRG neurons in sham and neuropathic rats. At the level of the spinal cord, CB2 receptors have inhibitory effects in neuropathic, but not sham-operated rats suggesting that spinal CB2 may be an important analgesic target.     

Glycolytic metabolite methylglyoxal inhibits cold and menthol activation of the transient receptor potential melastatin type 8 channel

Methylglyoxal (MG) is a reactive dicarbonyl compound involved in protein modifications linked to diabetes mellitus. The plasma level of MG is elevated in diabetic patients, particularly those with painful diabetic neuropathy. Diabetic neuropathy is often associated with spontaneous pain and altered thermal perception. This study assesses effects of MG on TRPM8, an ion channel involved in innocuous cold sensing and cold allodynia and also in cold‐mediated analgesia. Acute treatment with MG inhibited the activation of recombinant rat and human transient receptor potential melastatin type 8 (TRPM8) by cold and chemical agonists. A similar effect was observed when native TRPM8 was investigated in cultured rat dorsal root ganglion (DRG) neurons. DRG neurons treated with MG for 16–24 hr displayed a significant reduction in the fraction of cold‐ and menthol‐sensitive neurons, most likely expressing TRPM8. The fraction of allyl isothiocyanate‐sensitive neurons was also reduced, and the coexpression among different neuronal populations was affected. The same prolonged exposure to MG significantly reduced the expression of TRPM8 at the mRNA level. Overall, our data provide evidence for decreased activity and expression level of TRPM8 in the presence of MG, which may be linked to some of the alterations in pain and temperature sensing reported by diabetic patients. © 2015 Wiley Periodicals, Inc.     

Phenylarsine oxide as a redox modulator of transient receptor potential vanilloid type 1 channel function

Transient receptor potential vanilloid type 1 (TRPV1) channels are capable of detecting and integrating noxious stimuli and play an important role in nociceptor activation and sensitization. It has been demonstrated that oxidizing agents are capable of positively modulating (sensitizing) the TRPV1 channel. The present study investigates the ability of the thiol‐oxidizing agent phenylarsine oxide (PAO) to modulate TRPV1 currents under voltage‐clamp conditions. We assessed the ability of PAO to modulate both proton‐ and capsaicin‐activated currents mediated by recombinant human TRPV1 channels as well as native rat and human TRPV1 channels in dorsal root ganglion (DRG) neurons. Experiments with other oxidizing and reducing agents having various membrane‐permeating properties supported the intracellular oxidizing mechanism of PAO modulation. The PAO modulation of proton‐activated currents was consistent across the cell types studied, with an increase in current across the proton concentrations studied. PAO modulation of the capsaicin‐activated current in hTRPV1/Chinese hamster ovary cells consisted of potentiation of the current elicited with low capsaicin concentrations and inhibition of the current at higher concentrations. This same effect was seen with these recombinant cells in calcium imaging experiments and with native TRPV1 channels in rat DRG neurons. Contrary to this, currents in human DRG neurons were potentiated at all capsaicin concentrations tested after PAO treatment. These results could indicate important differences in the reduction–oxidation modulation of human TRPV1 channels in a native cellular environment. © 2014 Wiley Periodicals, Inc.     

The putative role of vanilloid receptor-like protein-1 in mediating high threshold noxious heat-sensitivity in rat cultured primary sensory neurons

High threshold noxious heat-activated currents and vanilloid receptor-like protein-1 expression were studied in rat cultured primary sensory neurons to find out the molecule(s) responsible for high threshold noxious heat-sensitivity. The average temperature threshold and amplitude of high threshold noxious heat-activated currents were 51.6 +/- 0.13 degrees C and -2.0 +/- 0.1nA (at a holding potential of -60 mV), respectively. The current-voltage relationship of high threshold noxious heat-activated currents was linear at positive membrane potentials, while it showed a weak inward rectification at negative membrane potentials. The average reversal potential measured in control intracellular and extracellular solutions was 4.5 +/- 0.9 mV (n = 6). Ionic substitutions revealed that the high threshold noxious heat-activated current is a nonselective cationic current with calculated ionic permeabilities of Cs+ : Na+ : Ca2+ (1 : 1.3 : 4.5). Consecutive stimuli reduced the heat threshold from 52.2 +/- 1 to 48.4 +/- 1.4 degrees C and then to 44 +/- 0.7 degrees C (n = 3). High threshold noxious heat-activated currents could dose-dependently and reversibly be reduced by ruthenium red (100 nm-10 micro m) but not by capsazepine (10 micro m). The average longest diameter of high threshold noxious heat-sensitive neurons was 31.48 +/- 0.5 micro m (A = approximately 778 micro m2; n = 77). Twenty-three percent of the total neuronal population expressed vanilloid receptor-like protein-1. The average area of the vanilloid receptor-like protein-1-immunopositive cells was 1,696 +/- 65.3 micro m2 (d = approximately 46 micro m). Vanilloid receptor-like protein-1-expressing neurons did not express the vanilloid receptor 1. Comparison of our data with results obtained in vanilloid receptor-like protein-1-expressing non-neuronal cells and previous immunohistochemical findings suggests that high threshold noxious heat-activated currents are produced by vanilloid receptor-like protein-1 and that high threshold heat-sensitive dorsal root ganglion neurons are the perikarya of type I noxious heat-sensitive fibers.     

Distribution of a specific calcium-binding protein of the S100 protein family,S100A6 (calcyclin), in subpopulations of neurons and glial cells of the adult rat nervous system

S100A6 (calcyclin) is a member of the large S100 Ca²⁺-binding protein family, considered to activate several processes along the calcium signal transduction pathway including the regulation of cell growth, proliferation, secretion, and exocytosis. In the present study, the distribution of S100A6 in the rat nervous system was examined by immunohistochemistry with a goat antiserum against recombinant human S100A6, which recognizes the rat S100A6 homologue. The main S100A6-immunoreactive elements were 1) neuronal somata and dendrites in some specific regions of the limbic system (e.g., the basolateral amygdaloid nucleus, ventral tip of the CA1-subicular border region, entorhinal cortex, and parasubiculum), most of which were identified as a subpopulation of pyramidal cells; 2) olfactory receptor cells and olfactory nerve fibers and terminals in the olfactory bulb; 3) some tracts of the hindbrain and spinal cord (e.g., the spinal trigeminal tract, solitary tract, dorsal root fibers, and the tract of Lissauer) and their terminals (e.g., the principal sensory trigeminal nucleus, spinal trigeminal nucleus, nucleus of the solitary tract, marginal zone, substantia gelatinosa, and proper sensory nucleus of the dorsal horn), as well as some sensory neurons of their origins in the dorsal root and trigeminal ganglia; 4) a subpopulation of astrocytes in the white matter (e.g., the corpus callosum, cingulum, external capsule, internal capsule, and fimbria of the hippocampus) and around the ventricles; 5) some ependymal cells, especially around the central canal; and 6) Schwann cells. These results will improve our understanding of the diverse function of Ca²⁺-binding proteins in the CNS. J. Comp. Neurol. 404:235–257, 1999. © 1999 Wiley-Liss, Inc.     

Local inflammation increases vanilloid receptor 1 expression within distinct subgroups of DRG neurons

Vanilloid receptor 1 (VR1) is essential to the development of inflammatory hyperalgesia. We investigated whether inflammation can increase in VR1 positive neuronal profiles in rat DRG neurons using histochemical methods. We also used size frequency analysis and double staining with several neuronal markers to investigate whether or not inflammation alters VR1 expression. Inflammation induced a 1.5-fold increase in percentage of VR1-like immunoreactivity (LI) positive profiles per total neuronal profiles, suggesting that the number of heat and pH sensitive neurons increase during inflammation. Area frequency histograms showed that VR1 expression increased in small and medium-sized neurons after inflammation. Double labeling of VR1 with NF200 showed that VR1 positive neurons with NF200 positive profiles significantly increased, indicating that the medium-sized VR1 positive neurons were neurons with myelinated A-fibers. Local inflammation thus increases in VR1 protein level within distinct subgroups of DRG neurons that may participate in the development and maintenance of inflammatory hyperalgesia.     

Characterization of rat transient receptor potential vanilloid 1 receptors lacking the N-glycosylation site N604

Whole-cell patch-clamp recordings were performed on HEK293 cells transiently transfected with the rat (r) wild-type transient receptor potential vanilloid 1 (TRPV1) (rTRPV1) receptor or with a mutant that lacks the potential N-glycosylation site at position N604 (rTRPV1-N604T). Replacement of Asn by Thr (N604T) depressed the maximum of the concentration-response curve for capsaicin and decreased the EC50 value of this agonist. Further, such a manipulation modified the sensitivity to the TRPV1 receptor-antagonist capsazepine and altered the dependence of the capsaicin effect on extracellular pH. Hence, glycosylation may affect the basic functional characteristics of the rTRPV1 receptor channel in accordance with the knowledge that N-glycosylation may regulate ligand binding or gating properties of ionotropic neurotransmitter receptors.     

Expression and localization of insulin receptor in rat dorsal root ganglion and spinal cord

The expression and localization of the insulin receptor (IR) was examined in rat dorsal root ganglia (DRG) and spinal cord using Western blotting, in situ hybridization and immunocytochemistry. Western blotting showed that the molecular weight of the IR beta subunit was higher in PNS than that found in CNS. Both IR mRNA and protein expressions were highest in small-sized sensory DRG neurons and myelinated sensory root fibers expressed higher levels of IR protein than myelinated anterior root fibers. In the spinal cord, IR immunoreactive neurons were present in lateral lamina V and in lamina X, suggesting the presence of IR in nociceptive pathways. Electronmicroscopy of DRGs revealed a polarized localization of the IR in abaxonal Schwann cell membranes, outer mesaxons in close vicinity to tight junctions of both myelinating and non-myelinating Schwann cells and to plasma membranes of sensory neurons. From these findings, we speculate that insulin may play a role in sensory fibers involved in nociceptive function often perturbed in diabetic neuropathy. The high expression of IR localizing to tight junctions of dorsal root mesaxons of DRGs may suggest a regulatory role on barrier functions compensating for the lack of a blood-nerve barrier in dorsal root ganglia. This is consistent with the colocalization of IR with tight junctions of the paranodal barrier and endoneurial endothelial cells in peripheral nerve.