Distribution of sensory neurones of the pudendal nerve in the dorsal root ganglia and their projection to the spinal cord

Summary The morphology and distribution of the sensory neurones of the pudendal nerve within the spinal ganglia of rats were investigated by use of horseradish peroxidase (HRP). The labelling was visualized in diaminobenzidine (DAB) or tetramethyl-benzidine (TMB)-stained sections. Injection of HRP directly into the pudendal nerve labelled perikarya predominantly in the sixth lumbar DRG (L6). Following injection of HRP into the scrotal skin, however, additional cells were labelled in L5 and SI. Labelling was invariably unilateral. Approximately equal numbers of small (<30 m) and large neurones (>40 m) were labelled following subcutaneous injections although injections into the nerve marked twice as many small cells as large cells. This suggests that, in the rat, most of the small-diameter fibres within the pudendal nerve ascend through L6. Although a cluster of neurones was observed in one experiment, the remaining 25 experiments did not reveal any somatotopic arrangement since the labelled perikarya were distributed evenly throughout the ganglion. Similar numbers of retrogradely labelled neurones (somatopetal transport of the tracer) were observed in both DAB- and TMB-stained sections, although TMB allowed the demonstration of anterograde (somatofugal) HRP transport by terminal labelling in the superficial laminae of the lumbar spinal cord, extending into laminae II–IV.Partially supported by grants from the DFG to HWK (Ko 758/1)     

K252a Modulates the Expression of Nerve Growth Factor-Dependent Capsaicin Sensitivity and Substance P Levels in Cultured Adult Rat Dorsal Root Ganglion Neurones

Abstract: K252a, an inhibitor of trk phosphorylation and nerve growth factor signal transduction in PC12 cells, blocked nerve growth factor-induced responses in cultured adult rat dorsal root ganglion sensory neurones. The nerve growth factor-dependent appearance of capsaicin sensitivity and accumulation of the neuropeptide substance P were inhibited when dorsal root ganglion neurones were grown in the presence of low concentrations (100 n M ) of K252a. At higher concentrations (3 µ M ), however, K252a stimulated the development of capsaicin sensitivity and the accumulation of substance P even in the absence of nerve growth factor. By using a wide dose range, therefore, we showed that K252a could either inhibit or mimic nerve growth factor's actions on sensory neurones. These results may explain the apparent paradox in the literature that some groups show a blocking effect of K252a on nerve growth factor-dependent survival of dorsal root ganglion sensory neurones, whereas others report that K252a can substitute for nerve growth factor or other trophic factors and promote neuronal survival.     

Effect of capsaicin on voltage-gated currents of trigeminal neurones in cell culture and slice preparations

Effects of capsaicin on voltage-gated currents were examined in vitro by whole-cell patch-clamp recordings from small neurones of rat trigeminal ganglia either in slice preparations or in different cell cultures. Cells were classified as sensitive to capsaicin if they responded with inward current and/or conductance change to the agent in nanomolar concentration. Capsaicin (150 to 330 nM) in sensitive cells reduced the mixed inward current evoked by depolarizing step or ramp commands in all preparations. In cultured cells, the inward current was depressed to 32.78 +/- 26.42% (n = 27) of the control. Both the tetrodotoxin-sensitive and -resistant inward currents were affected. The data support the concept that capsaicin besides acting on VR-1 receptors inhibits also some voltage gated channels. In 34 cultured cells, capsaicin increased the slope conductance to 170.5 +/- 68%. Percentage of capsaicin sensitive cells observed in nerve growth factor-treated cultured cell populations was higher (77.8%) than in the two other preparations (14.3 or 38.8%). It is concluded that 1) depression of the voltage-gated currents may play an important role in the functional desensitization of the sensory receptors and in the analgesic effect induced by the agent and 2) cell body of sensory neurones under native condition seems less sensitive to capsaicin then that of cells cultured in the presence of nerve growth factor.     

Neurotoxic effect of capsaicin in mammals

Capsaicin is now widely used to explore and/or prove the role of peptide-containing primary afferent neurones in different somato- and viscerosensory functions. The present paper deals with the morphological effects of capsaicin administered according to currently used experimental paradigms. As it has been repeatedly confirmed in the recent literature, administration of capsaicin to newborn mammals results in a highly selective degeneration of a particular population of small sized, B-type primary afferent neurones located in spinal and cranial sensory ganglia. Chemosensitive i.e. capsaicin sensitive primary sensory neurones (CPSNs) correspond to primary sensory ganglion cells which contain neuropeptides. The permanent functional impairments and the decrease in the peptide contents of the sensory neurones observed after neonatal capsaicin treatment may be accounted for an irreversible loss of CPSNs. Direct application of capsaicin to peripheral nerves results in an apparently irreversible functional impairment of unmyelinated afferent fibres implicated in nociceptive, viscerosensory and neurogenic inflammatory mechanisms. Morphological observations indicate that perineural treatment with capsaicin initiates a selective but delayed degeneration process of unmyelinated afferent nerve fibres presumably due to an inhibition of intraneuronal transport mechanisms. In contrast with perineural capsaicin treatment affecting the chemistry and function of the whole sensory neurone, injection of capsaicin into the subarachnoid space results in an irreversible abolition of the "afferent" but not the "efferent" function of CPSNs. Accordingly, noxious thermal or chemical stimuli applied to the peripheral innervation areas of the trigeminal nucleus caudalis or the affected segments of the spinal cord fail to induce nociceptive reflexes because of the degeneration of the central terminals of CPSNs. However, in these same skin areas, application of chemical irritants invariably evoked the neurogenic inflammatory response, indicating that CPSNs deprived of their central terminals maintain their capacity to synthesize and release the peptide(s) responsible for the initiation of that response. In contrast with previous findings, our recent studies furnished evidence for a selective neurodegenerative action of systemically injected capsaicin in adult mammals, as well. Therefore, some of the irreversible functional impairments produced by capsaicin in adult animals may result from the degeneration of a particular subpopulation of CPSNs.(ABSTRACT TRUNCATED AT 400 WORDS)     

Association of overactive bladder and stress urinary incontinence in rats with pudendal nerve ligation injury

Approximately one-third of patients with stress urinary incontinence (SUI) also suffer from urgency incontinence, which is one of the major symptoms of overactive bladder (OAB) syndrome. Pudendal nerve injury has been recognized as a possible cause for both SUI and OAB. Therefore, we investigated the effects of pudendal nerve ligation (PNL) on bladder function and urinary continence in female Sprague-Dawley rats. Conscious cystometry with or without capsaicin pretreatment (125 mg/kg sc), leak point pressures (LPPs), contractile responses of bladder muscle strips to carbachol or phenylephrine, and levels of nerve growth factor (NGF) protein and mRNA in the bladder were compared in sham and PNL rats 4 wk after the injury. Urinary frequency detected by a reduction in intercontraction intervals and voided volume was observed in PNL rats compared with sham rats, but it was not seen in PNL rats with capsaicin pretreatment that desensitizes C-fiber-afferent pathways. LPPs in PNL rats were significantly decreased compared with sham rats. The contractile responses of detrusor muscle strips to phenylephrine, but not to carbachol, were significantly increased in PNL rats. The levels of NGF protein and mRNA in the bladder of PNL rats were significantly increased compared with sham rats. These results suggest that pudendal nerve neuropathy induced by PNL may be one of the potential risk factors for OAB, as well as SUI. Somato-visceral cross sensitization between somatic (pudendal) and visceral (bladder) sensory pathways that increases NGF expression and alpha(1)-adrenoceptor-mediated contractility in the bladder may be involved in this pathophysiological mechanism.     

The morphological consequences of neonatal treatment with capsaicin on primary afferent neurones in adult rats

A comparison was made using our work and that reported in the literature of the losses of myelinated and unmyelinated fibres in a variety of nerves and also of losses of nerve cells in dorsal root ganglia, after treatment of neonatal rats with capsaicin. In L3 and L4 dorsal roots 85-93% of unmyelinated fibres and 9-33% of myelinated fibres were lost after 50-100 mg/kg capsaicin neonatally. In rats treated with 85 mg/kg capsaicin, percentage losses of unmyelinated (89%) and myelinated (36%) fibres of L4 dorsal roots were remarkably similar to the calculated losses of small dark (92%) and large light (34%) neurones respectively in these ganglia. Studies with monoclonal antibody RT97 which labels the large light neurones only, confirmed that some RT97 negative cells (i.e. small dark neurones) remain after capsaicin treatment. At present no evidence exists to suggest that the cell death of small dark neurones or C fibres after neonatal capsaicin treatment is completely selective for subgroups of these neurones, either in relation to sensory modality, or in relation to immunocytochemical cell markers and peptide content. However much more data is required to establish whether this cell death is really nonselective as regards immunocytochemical markers.     

Serotonin-immunoreactive neurones in the brain of Locusta migratoria innervating the corpus cardiacum

Summary The serotoninergic innervation of the corpus cardiacum (CC) of Locusta migratoria was investigated using two antisera against serotonin. A dense network of immunoreactive nerve fibres was present in the storage lobe of the CC. Immunopositive fibres only sporadically crossed the border between the storage lobe and the glandular lobe of the CC. Immunopositive fibres entered the storage lobe of the CC via the nervus corporis cardiaci I (NCCI); NCCII was immunonegative. Unilateral retrograde fillings of the NCCI with the fluorescent tracer Lucifer yellow, followed by antiserotonin immunocytochemistry, revealed about 20 double-labelled neurones in the anterior part of the pars intercerebralis. The double-labelled neurones were scattered between fluorescent non-immunoreactive neurones. Additionally, 5–7 neurones labelled only with Lucifer yellow were found at the ventrolateral side of the tritocerebrum. No immunopositive neurones were observed in the hypocerebral ganglion. Immunopositive fibres from neurones in the frontal ganglion ran via the recurrent nerve and the neuropile of the hypocerebral ganglion into the paired oesophageal nerve. At most, a few immunopositive nerve fibres occurred in the cardiostomatogastric nerves II, which connect the storage lobe of the CC with the paired oesophageal nerve at the caudal end of the hypocerebral ganglion.     

Response of respiratory-related hypoglossal nerve activity to capsaicin-induced pulmonary C-fiber activation in rats

This study was designed to examine respiratory-related hypoglossal nerve activity in response to activation of pulmonary C-fibers by capsaicin. Rats were anesthetized with urethane (1.2 g/kg, i.p.). Tracheostomy was performed. Catheters were introduced into the femoral vein and artery. Another catheter was placed near the entrance of the right atrium via the right jugular vein. Rats were paralyzed with gallamine triethiodide (5 mg/kg, i.v.), and ventilated artificially. Activities of the phrenic nerve (PNA) and the hypoglossal nerve (HNA) were recorded simultaneously. Varied doses of capsaicin (0.625, 1.25, and 5 µg/kg) were delivered into the right atrium to activate pulmonary C-fibers. Before bilateral vagotomy, apnea, decreases in PNA and HNA were observed in response to pulmonary C-fiber activation by the low and moderate doses of capsaicin. The high dose of capsaicin evoked an increase in PNA, an immediate tonic discharge of the hypoglossal nerve, and a decrease in phasic HNA. The onset time of HNA preceding PNA was abolished and replaced by a time lagged pattern as pulmonary C-fibers were activated. Raising CO₂ concentration did not attenuate the inhibitory effect of pulmonary C-fiber activation upon PNA and HNA. After bilateral sectioning of the vagi, administration of the moderate dose of capsaicin to activate non-vagal C-fibers produced increases in PNA and HNA. These results suggest that pulmonary vagal C-fiber activation may narrow the diameter at the oropharyngeal level by a decrease in phasic HNA, which may be disadvantageous for the maintenance of a patent upper airway.     

Differential effects after repeated treatment with haloperidol, clozapine, thioridazine and tefludazine on SNC and VTA dopamine neurones in rats

The effects of repeated treatment (21 days) with different antipsychotic compounds (haloperidol, clozapine, thioridazine and tefludazine) on dopamine (DA) neurones in substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) were studied in rats using single unit recording techniques. A dose-dependent decrease in the number of spontaneously active DA neurones in SNC and in VTA was observed with haloperidol. Clozapine showed no significant effect on the activity in SNC while a dose-dependent decrease in the number of active DA neurones in VTA was observed. Thioridazine showed no or weak effect in SNC while repeated treatment induced a marked inhibitory effect on the DA neurones in VTA. Tefludazine, a potential antipsychotic compound, induced a dose-dependent decrease in both SNC and VTA DA activity. However, the effect on the DA neurones in VTA was more pronounced at all doses. Since the classical neuroleptic haloperidol is equally effective in both regions, while the atypical neuroleptics clozapine and thioridazine have selective or predominant effect in the VTA area it has previously been thought that the inhibition of spontaneously active DA neurones in VTA should indicate an antipsychotic effect of a compound while the inhibition of DA neurones in SNC should account for the development of neurological side effects. The data suggests that the potential antipsychotic compound tefludazine should not induce neurological side effects at lower doses but still has an antipsychotic activity while repeated treatment with higher doses of tefludazine might cause extrapyramidal side effects.     

Effects of stimulation and damage of afferent nerves on the glucose and free fatty acid levels in the rat blood under various glycemic conditions

Capsaicin stimulation of afferent neurones increased hyperglycaemic responses to glucose and decreased hypoglycaemia in control rats. Elevation of the free fatty acids level occurred following both the stimulation alone and that combined with insulin. Neonatal capsaicin pre-treatment decreased the hypoglycaemic response effect of insulin and the FFA level. Treatment of adult rats with capsaicin did not affect hypoglycaemia following insulin administration but did decrease the FFA level. In capsaicin pre-treated rats, the capsaicin stimulation exerted no effect upon the hypoglycaemia following insulin administration but abolished the insulin effect on the FFA level. The findings suggest that the capsaicin-sensitive nerves play a major role in mediating the glucose and the FFA metabolic responses.     

Spinal cord sensory systems after neonatal capsaicin

Animals with a severe reduction in the number of afferent C-fibres as a consequence of neonatal administration of capsaicin, exhibit a number of neurological and behavioral deficits including increased nociceptive thresholds, altered somato-visceral and viscero-visceral reflexes, depressed cardiovascular and respiratory reflexes and changes in the organisation of spinal cord sensory systems. The reduction in the number of C-fibres produced by neonatal capsaicin does not cause a decrease of similar magnitude in the number of dorsal horn cells driven by the surviving C-fibres. Twenty-two per cent of dorsal horn neurones in capsaicin treated animals respond to electrical stimulation of the surviving afferent C-fibres: a reduction of only 50% from control values. Inhibitory controls on afferent C-fibre evoked responses of dorsal horn neurones are weaker in capsaicin treated rate than in control animals. The cutaneous receptive fields of some dorsal horn neurones can increase in size following stimulation of afferent C-fibres. Tonic descending inhibition on C-fibre evoked responses of dorsal horn neurones is reduced in capsaicin treated rats: fewer neurones show tonic descending inhibition in these animals and those that do are subjected to less powerful inhibitions than similar neurones from control animals. However, some central inhibitory mechanism are unchanged after neonatal capsaicin treatment, specially those that do not involve afferent C-fibres. We suggest that the nervous system develops central inhibition in response to and directed towards the excitations mediated by its afferent drives. Therefore reduced central inhibition in response to a decreased number of afferent C-fibres can compensate for the lost capacity in the signalling of peripheral noxious events.     

Role of pudendal afferents in voiding efficiency in the rat

The reciprocal activities of the bladder and external urethral sphincter (EUS) are coordinated by descending projections from the pontine micturition center but are subjected to modulation by peripheral afferent inputs. Transection of the somatic pudendal nerve innervating the striated EUS decreases voiding efficiency and increases residual urine in the rat. The reduction in voiding efficiency was attributed to the lack of phasic bursting activity of the EUS following denervation. However, transection of the pudendal nerve also eliminates somatic sensory feedback that may play a role in voiding. We hypothesized that feedback from pudendal afferents is required for efficient voiding and that the loss of pudendal sensory activity contributes to the observed reduction in voiding efficiency following pudendal nerve transection. Quantitative cystometry in urethane anesthetized female rats following selective transection of pudendal nerve branches, following chemical modulation of urethral afferent activity, and following neuromuscular blockade revealed that pudendal nerve afferents contributed to efficient voiding. Sensory feedback augmented bladder contraction amplitude and duration, thereby increasing the driving force for urine expulsion. Second, sensory feedback was necessary to pattern appropriately the EUS activity into alternating bursts and quiescence during the bladder contraction. These findings demonstrate that the loss of pudendal sensory activity contributes to the reduction in voiding efficiency observed following pudendal nerve transection, and illustrate the importance of urethral sensory feedback in regulating bladder function.     

阴部神经痛的解剖学基础及治疗进展

阴部神经痛是发生于会阴部阴部神经支配区域的烧灼样,电击样,针刺样疼痛,是典型的神经病理性疼痛,其目前的患病率尚未准确统计、诊断和治疗均比较困难,患者长期遭受精神和身体上的双重折磨、生活质量低下。阴部神经常与阴部血管伴行共同走行于阴部神经管内,可分为三个主要的分支,即直肠下神经,会阴神经和阴茎/阴蒂背神经。不同分支病变将会导致不同部位的疼痛。其治疗方法目前主要包括保守治疗、阴部神经阻滞、阴部神经外科减压以及神经调控。本文就阴部神经解剖结构,阴部神经痛病因和临床表现,诊断及其治疗进展做一简要综述。     

Oral irritation by mustard oil: self-desensitization and cross-desensitization with capsaicin

We investigated the temporal pattern of oral irritation elicited by sequential application of mustard oil (allyl-isothiocyanate), and whether it exhibits self-desensitization and cross-desensitization with capsaicin. Mustard oil (0.125%, 40 micro l) was sequentially applied to one side of the tongue at 1 min intervals, and subjects rated the intensity of the irritant sensation elicited by each stimulus. Ratings successively declined across trials, indicating desensitization. In contrast, sequential application of capsaicin (10 ppm) elicited irritation that increased in intensity across trials (sensitization). To test for self-desensitization by mustard oil, a 10 min hiatus was imposed following the series of unilateral mustard oil stimuli, after which mustard oil was applied to both sides of the tongue. In a two-alternative forced-choice paradigm, subjects chose which side had stronger irritation and also independently rated the irritant intensity on each side. A significant majority of subjects chose the side not previously receiving mustard oil as more intense, and assigned significantly higher intensity ratings to that side, indicating self-desensitization. In two additional sessions, the same paradigm was used to show mustard oil cross-desensitization of irritation elicited by capsaicin, and capsaicin cross-desensitization of irritation from mustard oil. In a final session, sequential application of mustard oil at faster (20 s) intervals initially evoked a sensitizing pattern followed by desensitization. The temporal patterns of oral irritation exhibited by mustard oil, and its reciprocal cross-desensitization with capsaicin, are similar to those of menthol and nicotine.     

Increase in tissue concentrations of histamine and 5-hydroxytryptamine following capsaicin treatment of newborn rats

Treatment of newborn rats with capsaicin is known to result in a permanent deficit of unmyelinated afferent neurones. The present study was concerned with the effect of neonatal capsaicin (50 mg kg^?1 s.c.) on tissue concentrations of histamine and 5-hydroxytryptamine (5-HT) in the adult rat. The amines were determined at the age of 5 to 6 months using high performance liquid chromatography. Histamine and 5-HT concentrations were significantly increased in the dorsal skin of the hind paw and the dorsal spinal cord. Histamine concentrations were also increased in lungs and ventral spinal cord while 5-HT concentrations were unaltered in these tissues. Both histamine and 5-HT concentrations were unchanged in the ventral skin of the hind paw, gastrointestinal tract and brain. It is proposed that the changes in the amine concentrations reflect a secondary response of histamine and 5-HT containing mast cells and neurones to the irreversible degeneration of unmyelinated afferent neurones caused by neonatal capsaicin treatment.     

Permanent alterations of spinal cord reflexes following nerve lesion in newborn rats

Sciatic nerve lesion in newborn rats is known to cause degeneration of a large number of axotomized motoneurones and spinal ganglion cells. Some of the surviving motoneurones exhibit abnormal firing properties and the projection pattern of central terminals of sensory neurones is altered. We report here on long-term changes in spinal cord reflexes in adult rats following neonatal nerve crush. In acutely spinalized and anaesthetized adult rats 4-6 months old in which the sciatic nerve had been crushed on one side at birth, the tibial nerve, common peroneal nerve or sural nerve were stimulated on the reinnervated and control side and reflex responses were recorded from the L5 ventral spinal roots. Ventral root responses (VRRs) to tibial and peroneal nerve stimulation on the side of the nerve lesion were significantly smaller in amplitude representing only about 15% of the mean amplitude of VRRs on the control side. The calculated central delay of the first, presumably monosynaptic component of the VRR potential was 1.6 ms on the control side while the earliest VRR wave on the side of the nerve lesion appeared after a mean central latency of 4.0 ms that seems too long to be of monosynaptic origin. These results suggest that neonatal sciatic nerve injury markedly alters the physiological properties and synaptic connectivity in spinal cord neurones and causes a marked depression of spinal cord responses to peripheral nerve stimulation.     

The Reversible Increase in Tight Junction Permeability Induced by Capsaicin Is Mediated via Cofilin-Actin Cytoskeletal Dynamics and Decreased Level of Occludin

Previous results demonstrated that capsaicin induces the reversible tight junctions (TJ) opening via cofilin activation. The present study investigated the mechanisms underlying the reversible TJ opening and compared the effect to the irreversible opening induced by actin inhibitors. Capsaicin treatment induced the F-actin alteration unique to capsaicin compared to actin-interacting agents such as latrunculin A, which opens TJ irreversibly. Along with TJ opening, capsaicin decreased the level of F-actin at bicellular junctions but increased it at tricellular junctions accompanied with its concentration on the apical side of the lateral membrane. No change in TJ protein localization was observed upon exposure to capsaicin, but the amount of occludin was decreased significantly. In addition, cosedimentation analyses suggested a decrease in the interactions forming TJ, thereby weakening TJ tightness. Introduction of cofilin, LIMK and occludin into the cell monolayers confirmed their contribution to the transepithelial electrical resistance decrease. Finally, exposure of monolayers to capsaicin augmented the paracellular passage of both charged and uncharged compounds, as well as of insulin, indicating that capsaicin can be employed to modulate epithelial permeability. Our results demonstrate that capsaicin induces TJ opening through a unique mechanism, and suggest that it is a new type of paracellular permeability enhancer.     

Sympathetic activation by chemical stimulation of white adipose tissues in rats

Injection of leptin into white adipose tissue (WAT) increases sympathetic outflow. The present study was designed to determine the effects of capsaicin and other chemicals in WAT on the sympathetic outflow and blood pressure and the roles of WAT afferents and hypothalamic paraventricular nucleus (PVN) in the adipose afferent reflex (AAR). The AAR was induced by injection of capsaicin, bradykinin, adenosine, adenosine triphosphate (ATP), or leptin into inguinal WAT (iWAT) or retroperitoneal WAT (rWAT) in anesthetized rats. The iWAT injection of capsaicin increased the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) but not the heart rate. Bradykinin, adenosine, or leptin but not ATP in the iWAT caused similar effects to capsaicin on the RSNA and MAP. Intravenous, intramuscular, or intradermal injection of capsaicin had no significant effects on the RSNA and MAP. The effects of capsaicin in rWAT were similar to that in iWAT on the RSNA and MAP. Furthermore, injection of capsaicin into the iWAT increased the WAT afferent nerve activities, WAT efferent nerve activity, and brown adipose tissue efferent nerve activity. The iWAT denervation or chemical lesion of the PVN neurons with kainic acid abolished the AAR induced by the iWAT injection of capsaicin. These results indicate that the stimulation of iWAT afferents with capsaicin, bradykinin, adenosine, or leptin reflexly increases the RSNA and blood pressure. The iWAT afferents and the PVN are involved in the AAR induced by capsaicin in the iWAT.     

Enkephalin-like immunofluorescence in nerves of the rat iris following systemic capsaicin injection

A network of nerve fibers with an enkephalin-like immunoreactivity was demonstrated in rat iris whole mounts. Systemic administration of capsaicin in doses which caused partial (5 mg/kg) or complete (50 mg/kg) disappearance of substance P-containing fibers in the iris did not cause degeneration of enkephalin-positive nerve fibers. The enkephalin-immunoreactive network seemed intact also after a capsaicin dose of 250 mg/kg. In fact, the fluorescence intensity of the nerve fibers showing enkephalin-immunoreactivity was often increased three days after a capsaicin injection in a dose of 50 mg/kg. The mechanism behind this effect of capsaicin remains to be elucidated, but could be due either to a direct effect on the enkephalin-positive nerves or involve the disappearance of substance P nerves and/or a simultaneous inflammatory response. However, an increased fluorescence intensity of the enkephalin-immunoreactive fibers was sometimes seen also without capsaicin treatment.     

Suramin affects capsaicin responses and capsaicin-noxious heat interactions in rat dorsal root ganglia neurones

The effect of suramin, an inhibitor of G protein regulated signalling, was studied on the membrane currents induced by noxious heat and by capsaicin in cultured dorsal root ganglia neurones isolated from neonatal rats. Whole-cell responses induced by a heat ramp (24-52 degrees C) were little affected by suramin. The noxious heat-activated currents were synergistically facilitated in the presence of 0.3 microM capsaicin 13.2-fold and 6.3-fold at 40 degrees C and 50 degrees C, respectively. In 65% of neurones, the capsaicin-induced facilitation was inhibited by 10 microM suramin to 35 +/- 6% and 53 +/- 6% of control at 40 degrees C and 50 degrees C (S.E.M., n = 15). Suramin 30 microM caused a significant increase in the membrane current produced by a nearly maximal dose (1 microM) of capsaicin over the whole recorded temperature range (2.4-fold at 25 degrees C and 1.2-fold at 48 degrees C). The results demonstrate that suramin differentially affects the interaction between capsaicin and noxious heat in DRG neurones and thus suggest that distinct transduction pathways may participate in vanilloid receptor activation mechanisms.