Sodium currents in vagotomized primary afferent neurones of the rat

Vagotomy blunts cardiorespiratory responses to vagal afferent stimulation via pre- and postsynaptic effects in the nucleus tractus solitarii

Viscerosensory information travels to the brain via vagal afferents, where it is first integrated within the brainstem nucleus tractus solitarii (nTS), a critical contributor to cardiorespiratory function and site of neuroplasticity. We have shown that decreasing input to the nTS via unilateral vagus nerve transection (vagotomy) induces morphological changes in nTS glia and reduces sighs during hypoxia. The mechanisms behind post-vagotomy changes are not well understood. We hypothesized that chronic vagotomy alters cardiorespiratory responses to vagal afferent stimulation via blunted nTS neuronal activity. Male Sprague-Dawley rats (6 weeks old) underwent right cervical vagotomy caudal to the nodose ganglion, or sham surgery. After 1 week, rats were anaesthetized, ventilated and instrumented to measure mean arterial pressure (MAP), heart rate (HR), and splanchnic sympathetic and phrenic nerve activity (SSNA and PhrNA, respectively). Vagal afferent stimulation (2-50 Hz) decreased cardiorespiratory parameters and increased neuronal Ca2+ measured by in vivo photometry and in vitro slice imaging of nTS GCaMP8m. Vagotomy attenuated both these reflex and neuronal Ca2+ responses compared to shams. Vagotomy also reduced presynaptic Ca2+ responses to stimulation (Cal-520 imaging) in the nTS slice. The decrease in HR, SSNA and PhrNA due to nTS nanoinjection of exogenous glutamate also was tempered following vagotomy. This effect was not restored by blocking excitatory amino acid transporters. However, the blunted responses were mimicked by NMDA, not AMPA, nanoinjection and were associated with reduced NR1 subunits in the nTS. Altogether, these results demonstrate that vagotomy induces multiple changes within the nTS tripartite synapse that influence cardiorespiratory reflex responses to afferent stimulation. KEY POINTS: Multiple mechanisms within the nucleus tractus solitarii (nTS) contribute to functional changes following vagal nerve transection. Vagotomy results in reduced cardiorespiratory reflex responses to vagal afferent stimulation and nTS glutamate nanoinjection. Blunted responses occur via reduced presynaptic Ca2+ activation and attenuated NMDA receptor expression and function, leading to a reduction in nTS neuronal activation. These results provide insight into the control of autonomic and respiratory function, as well as the plasticity that can occur in response to nerve damage and cardiorespiratory disease.

Spinal cord injury-mediated changes in electrophysiological properties of rat gastric nodose ganglion neurons

In preclinical rodent models, spinal cord injury (SCI) manifests as gastric vagal afferent dysfunction both acutely and chronically. However, the mechanism that underlies this dysfunction remains unknown. In the current study, we examined the effect of SCI on gastric nodose ganglia (NG) neuron excitability and on voltage-gated Na⁺ (Na_V) channels expression and function in rats after an acute (i.e. 3-days) and chronic (i.e. 3-weeks) period. Rats randomly received either T3-SCI or sham control surgery 3-days or 3-weeks prior to experimentation as well as injections of 3% DiI solution into the stomach to identify gastric NG neurons. Single cell qRT-PCR was performed on acutely dissociated DiI-labeled NG neurons to measure Na_V1.7, Na_V1.8 and Na_V1.9 expression levels. The results indicate that all 3 channel subtypes decreased. Current- and voltage-clamp whole-cell patch-clamp recordings were performed on acutely dissociated DiI-labeled NG neurons to measure active and passive properties of C- and A-fibers as well as the biophysical characteristics of Na_V1.8 channels in gastric NG neurons. Acute and chronic SCI did not demonstrate deleterious effects on either passive properties of dissociated gastric NG neurons or biophysical properties of Na_V1.8. These findings suggest that although Na_V gene expression levels change following SCI, Na_V1.8 function is not altered. The disruption throughout the entirety of the vagal afferent neuron has yet to be investigated.

Unilateral vagotomy alters astrocyte and microglial morphology in the nucleus tractus solitarii of the rat

The nucleus tractus solitarii (nTS) is the initial site of integration of sensory information from the cardiorespiratory system and contributes to reflex responses to hypoxia. Afferent fibers of the bilateral vagus nerves carry input from the heart, lungs, and other organs to the nTS where it is processed and modulated. Vagal afferents and nTS neurons are integrally associated with astrocytes and microglia that contribute to neuronal activity and influence cardiorespiratory control. We hypothesized that vagotomy would alter glial morphology and cardiorespiratory responses to hypoxia. Unilateral vagotomy (or sham surgery) was performed in rats. Prior to and seven days after surgery, baseline and hypoxic cardiorespiratory responses were monitored in conscious and anesthetized animals. The brainstem was sectioned and caudal, mid-area postrema (mid-AP), and rostral sections of the nTS were prepared for immunohistochemistry. Vagotomy increased immunoreactivity (-IR) of astrocytic glial fibrillary acidic protein (GFAP), specifically at mid-AP in the nTS. Similar results were found in the dorsal motor nucleus of the vagus (DMX). Vagotomy did not alter nTS astrocyte number, yet increased astrocyte branching and altered morphology. In addition, vagotomy both increased nTS microglia number and produced morphologic changes indicative of activation. Cardiorespiratory baseline parameters and hypoxic responses remained largely unchanged, but vagotomized animals displayed fewer augmented breaths (sighs) in response to hypoxia. Altogether, vagotomy alters nTS glial morphology, indicative of functional changes in astrocytes and microglia that may affect cardiorespiratory function in health and disease.

TRPM3 expression and control of glutamate release from primary vagal afferent neurons

Vagal afferent fibers contact neurons in the nucleus of the solitary tract (NTS) and release glutamate via three distinct release pathways: synchronous, asynchronous, and spontaneous. The presence of TRPV1 in vagal afferents is predictive of activity-dependent asynchronous glutamate release along with temperature-sensitive spontaneous vesicle fusion. However, pharmacological blockade or genetic deletion of TRPV1 does not eliminate the asynchronous profile and only attenuates the temperature-dependent spontaneous release at high temperatures (>40°C), indicating additional temperature-sensitive calcium conductance(s) contributing to these release pathways. The transient receptor potential cation channel melastatin subtype 3 (TRPM3) is a calcium-selective channel that functions as a thermosensor (30-37°C) in somatic primary afferent neurons. We predict that TRPM3 is expressed in vagal afferent neurons and contributes to asynchronous and spontaneous glutamate release pathways. We investigated these hypotheses via measurements on cultured nodose neurons and in brainstem slice preparations containing vagal afferent to NTS synaptic contacts. We found histological and genetic evidence that TRPM3 is highly expressed in vagal afferent neurons. The TRPM3-selective agonist, pregnenolone sulfate, rapidly and reversibly activated the majority (∼70%) of nodose neurons; most of which also contained TRPV1. We confirmed the role of TRPM3 with pharmacological blockade and genetic deletion. In the brain, TRPM3 signaling strongly controlled both basal and temperature-driven spontaneous glutamate release. Surprisingly, genetic deletion of TRPM3 did not alter synchronous or asynchronous glutamate release. These results provide convergent evidence that vagal afferents express functional TRPM3 that serves as an additional temperature-sensitive calcium conductance involved in controlling spontaneous glutamate release onto neurons in the NTS.NEW & NOTEWORTHY Vagal afferent signaling coordinates autonomic reflex function and informs associated behaviors. Thermosensitive transient receptor potential (TRP) channels detect temperature and nociceptive stimuli in somatosensory afferent neurons, however their role in vagal signaling remains less well understood. We report that the TRPM3 ion channel provides a major thermosensitive point of control over vagal signaling and synaptic transmission. We conclude that TRPM3 translates physiological changes in temperature to neurophysiological outputs and can serve as a cellular integrator in vagal afferent signaling.

Direct Anandamide Activation of TRPV1 Produces Divergent Calcium and Current Responses

In the brainstem nucleus of the solitary tract (NTS), primary vagal afferent neurons express the transient receptor potential vanilloid subfamily member 1 (TRPV1) at their central terminals where it contributes to quantal forms of glutamate release. The endogenous membrane lipid anandamide (AEA) is a putative TRPV1 agonist in the brain, yet the extent to which AEA activation of TRPV1 has a neurophysiological consequence is not well established. We investigated the ability of AEA to activate TRPV1 in vagal afferent neurons in comparison to capsaicin (CAP). Using ratiometric calcium imaging and whole-cell patch clamp recordings we confirmed that AEA excitatory activity requires TRPV1, binds competitively at the CAP binding site, and has low relative affinity. While AEA-induced increases in peak cytosolic calcium were similar to CAP, AEA-induced membrane currents were significantly smaller. Removal of bath calcium increased the AEA current with no change in peak CAP currents revealing a calcium sensitive difference in specific ligand activation of TRPV1. Both CAP- and AEA-activated TRPV1 currents maintained identical reversal potentials, arguing against a major difference in ion selectivity to resolve the AEA differences in signaling. In contrast with CAP, AEA did not alter spontaneous glutamate release at NTS synapses. We conclude: (1) AEA activation of TRPV1 is markedly different from CAP and produces different magnitudes of calcium influx from whole-cell current; and (2) exogenous AEA does not alter spontaneous glutamate release onto NTS neurons. As such, AEA may convey modulatory changes to calcium-dependent processes, but does not directly facilitate glutamate release.

Isolation of TRPV1 independent mechanisms of spontaneous and asynchronous glutamate release at primary afferent to NTS synapses

Cranial visceral afferents contained within the solitary tract (ST) contact second-order neurons in the nucleus of the solitary tract (NTS) and release the excitatory amino acid glutamate via three distinct exocytosis pathways; synchronous, asynchronous, and spontaneous release. The presence of TRPV1 in the central terminals of a majority of ST afferents conveys activity-dependent asynchronous glutamate release and provides a temperature sensitive calcium conductance which largely determines the rate of spontaneous vesicle fusion. TRPV1 is present in unmyelinated C-fiber afferents and these facilitated forms of glutamate release may underlie the relative strength of C-fibers in activating autonomic reflex pathways. However, pharmacological blockade of TRPV1 signaling eliminates only ~50% of the asynchronous profile and attenuates the temperature sensitivity of spontaneous release indicating additional thermosensitive calcium influx pathways may exist which mediate these forms of vesicle release. In the present study we isolate the contribution of TRPV1 independent forms of glutamate release at ST-NTS synapses. We found ST afferent innervation at NTS neurons and synchronous vesicle release from TRPV1 KO mice was not different to control animals; however, only half of TRPV1 KO ST afferents completely lacked asynchronous glutamate release. Further, temperature driven spontaneous rates of vesicle release were not different from 33 to 37°C between control and TRPV1 KO afferents. These findings suggest additional temperature dependent mechanisms controlling asynchronous and thermosensitive spontaneous release at physiological temperatures, possibly mediated by additional thermosensitive TRP channels in primary afferent terminals.

The Nav1.9 channel regulates colonic motility in mice

The colonic migrating motor complex (CMMC) is a major pattern of motility that is entirely generated and organized by the enteric nervous system. We have previously demonstrated that the Na_v1.9 channel underlies a tetrodotoxin-resistant sodium current which modulates the excitability of enteric neurons. The aim of this study was to observe the effect of loss of the Na_v1.9 channel in enteric neurons on mouse colonic motility in vitro. The mechanical activity of the circular muscle was simultaneously recorded from three sites, namely, proximal, mid- and distal, along the whole colon of male, age-matched wild-type and Na_v1.9 null mice. Spontaneous CMMCs were observed in all preparations. The mean frequency of CMMCs was significantly higher in the Na_v1.9 null mice (one every 2.87 ± 0.1 min compared to one every 3.96 ± 0.23 min in the wild type). The mean duration of CMMCs was shorter and the mean area-under-contraction was larger in the Na_v1.9 null mice compared to the wild type. In addition, CMMCs propagated preferentially in an aboral direction in the Na_v1.9 null mice. Our study demonstrates that CMMCs do occur in mice lacking the Na_v1.9 channel, but their characteristics are significantly different from controls. Up to now, the Na_v1.9 channel was mainly associated with nociceptive neurons and involved in their hyperexcitability after inflammation. Our result shows for the first time a role for the Na_v1.9 channel in a complex colonic motor pattern.

Influence of vagotomy on monosynaptic transmission at second-order nucleus tractus solitarius synapses

Manipulations of vagal activity are used to treat medical pathologies, but the underlying CNS changes caused by these treatments are not well understood. Furthermore, heart and lung transplant as well as treatments for many gastrointestinal disorders result in section of the vagus nerve (vagotomy). Following unilateral vagotomy under isoflurane anesthesia of Sprague-Dawley rats, electrophysiological properties were recorded with whole cell patch techniques in horizontal brain stem slices. Vagotomy significantly reduced the median amplitude of evoked excitatory postsynaptic currents (evEPSCs; -121; n = 43) in the nucleus tractus solitarius (NTS) when compared with controls (-157 pA; n = 66; P < 0.05) but had no significant effect on the passive properties or on the average amplitude or frequency of miniature EPSCs. The degree of synaptic failure exhibited during a 50-Hz train of stimuli was used to define two separate classes of synapses: "low failure" and "high failure" (HF); failure rates <5 and > or =5%, respectively. HF synapses had significantly smaller median evEPSCs (-88 vs. -184 pA; P < 0.05). After vagotomy, the percentage of HF synapses nearly doubled to 56% (n = 24/43) when compared with controls (30%; n = 20/66). Additionally, the overall percentage of failures after the second to fifth stimuli significantly increased by at least twofold. These results suggest that vagotomy causes a decrease in synaptic efficacy by both increasing the overall percentage of synaptic failures and shifting the population of NTS synapses toward more HF transmission. In addition, the alterations due to vagotomy are likely to be presynaptic in nature.

Effect of nerve graft porosity on the refractory period of regenerating nerve fibers

OBJECT

In the present study the authors consider the influence of the porosity of synthetic nerve grafts on peripheral nerve regeneration.

METHODS

Microporous (1-13 microm) and nonporous nerve grafts made of a copolymer of trimethylene carbonate and epsilon-caprolactone were tested in an animal model. Twelve weeks after surgery, nerve and muscle morphological and electrophysiological results of regenerated nerves that had grown through the synthetic nerve grafts were compared with autografted and untreated (control) sciatic nerves. Based on the observed changes in the number and diameter of the nerve fibers, the predicted values of the electrophysiological parameters were calculated.

RESULTS

The values of the morphometric parameters of the peroneal nerves and the gastrocnemius and anterior tibial muscles were similar if not equal in the rats receiving synthetic nerve grafts. The refractory periods, however, were shorter in porous compared with nonporous grafted nerves, and thus were closer to control values.

CONCLUSIONS

A shorter refractory period enables the axon to follow the firing frequency of the neuron more effectively and allows a more adequate target organ stimulation. Therefore, porous are preferred over nonporous nerve grafts.

Voltage-gated sodium channels in nociceptive versus non-nociceptive nodose vagal sensory neurons innervating guinea pig lungs

Lung vagal sensory fibres are broadly categorized as C fibres (nociceptors) and A fibres (non-nociceptive; rapidly and slowly adapting low-threshold stretch receptors). These afferent fibre types differ in degree of myelination, conduction velocity, neuropeptide content, sensitivity to chemical and mechanical stimuli, as well as evoked reflex responses. Recent studies in nociceptive fibres of the somatosensory system indicated that the tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels (VGSC) are preferentially expressed in the nociceptive fibres of the somatosensory system (dorsal root ganglia). Whereas TTX-R sodium currents have been documented in lung vagal sensory nerves fibres, a rigorous comparison of their expression in nociceptive versus non-nociceptive vagal sensory neurons has not been carried out. Using multiple approaches including patch clamp electrophysiology, immunohistochemistry, and single-cell gene expression analysis in the guinea pig, we obtained data supporting the hypothesis that the TTX-R sodium currents are similarly distributed between nodose ganglion A-fibres and C-fibres innervating the lung. Moreover, mRNA and immunoreactivity for the TTX-R VGSC molecules Na(V)1.8 and Na(V)1.9 were present in nearly all neurons. We conclude that contrary to findings in the somatosensory neurons, TTX-R VGSCs are not preferentially expressed in the nociceptive C-fibre population innervating the lungs.

Mechano- and chemosensitivity of rat nodose neurones--selective excitatory effects of prostacyclin

Nodose ganglion sensory neurones exert a significant reflex autonomic influence. We contrasted their mechanosensitivity, excitability and chemosensitivity in response to the stable prostacyclin (PGI2) analogue carbacyclin (cPGI) in culture. Under current clamp conditions we measured changes in membrane potential (DeltamV) and action potential (AP) responses to mechanically induced depolarizations and depolarizing current injections before and after superfusion of cPGI (1 microM and 10 microM). Chemosensitivity was indicated by augmentation of AP firing frequency and increased maximum gain of AP frequency (max. dAP/dDeltamV), during superfusion with cPGI. Results indicate that two groups of neurones, A and B, are mechanosensitive (MS) and one group, C, is mechanoinsensitive (MI). Group A shows modest depolarization without AP generation during mechanical stimulation, and no increase in max. dAP/dDeltamV, despite a marked increase in electrical depolarization with cPGI. Group B shows pronounced mechanical depolarization accompanied by enhanced AP discharge with cPGI, and an increase in max. dAP/dDeltamV. Group C remains MI after cPGI but is more excitable and markedly chemosensitive (CS) with a pronounced enhancement of max. dAP/dDeltamV with cPGI. The effect of cPGI on ionic conductances indicates that it does not sensitize the mechanically gated depolarizing degenerin/epithelial Na+ channels (DEG/ENaC), but it inhibits two voltage-gated K+ currents, Maxi-K and M-current, causing enhanced AP firing frequency and depolarization, respectively. We conclude that MS nodose neurones may be unimodal MS or bimodal MS/CS, and that MI neurones are unimodal CS, and much more CS to cPGI than MS/CS neurones. We suggest that the known excitatory effect of PGI2 on baroreceptor and vagal afferent fibres is mediated by inhibition of voltage-gated K+ channels (Maxi-K and M-current) and not by an effect on mechanically gated DEG/ENaC channels.

Electrophysiological and pharmacological validation of vagal afferent fiber type of neurons enzymatically isolated from rat nodose ganglia

An unavoidable consequence of enzymatic dispersion of sensory neurons from intact ganglia is loss of the axon and thus the ability to classify afferent fiber type based upon conduction velocity (CV). An intact rat nodose ganglion preparation was used to randomly sample neurons (n=76) using the patch clamp technique. Reliable electrophysiological and chemophysiological correlates of afferent fiber type were established for use with isolated neuron preparations. Myelinated afferents (approximately 25%) formed two groups exhibiting strikingly different functional profiles. One group (n=10) exhibited CVs in excess of 10 m/s and narrow (<1 ms) action potentials (APs) while the other (n=9) had CVs as low as 4m/s and broad (>2 ms) APs that closely approximated those identified as unmyelinated afferents (n=57) with CVs less than 1m/s. A cluster analysis of select measures from the AP waveforms strongly correlated with CV, producing three statistically unique populations (p<0.05). These groupings aligned with our earlier hypothesis (Jin et al., 2004) that a differential sensitivity to the selective purinergic and vanilloid receptor agonists can be used as reliable pharmacological indicators of vagal afferent fiber type. These metrics were further validated using an even larger population of isolated (n=240) nodose neurons. Collectively, these indicators of afferent fiber type can be used to provide valuable insight concerning the relavence of isolated cellular observations to integrated afferent function of visceral organ systems.

Effect of 8-bromo-cAMP on the tetrodotoxin-resistant sodium (Nav 1.8) current in small-diameter nodose ganglion neurons

We examined whether 8-bromo-cAMP (8-Br-cAMP)-induced modification of tetrodotoxin-resistant (TTX-R) sodium current in neonatal rat nodose ganglion neurons is mediated by the activation of protein kinase A (PKA) and/or protein kinase C (PKC). In 8-Br-cAMP applications ranging from 0.001 to 1.0mM, 8-Br-cAMP at 0.1mM showed a maximal increase in the peak TTX-R Na(+) (Nav1.8) current and produced a hyperpolarizing shift in the conductance-voltage (G-V) curve. The PKC inhibitor bisindolylmaleimide Ro-31-8425 (Ro-31-8425, 0.5microM) decreased the peak Nav 1.8 current. The Ro-31-8425-induced modulation of the G(V)(1/2) baseline (a percent change in G at baseline V1/2) was not affected by additional 8-Br-cAMP application (0.1mM). The maximal increase in Nav 1.8 currents was seen at 0.1microM after the application of a PKC activator, phorbol 12-myristate 13-acetate (PMA) and forskolin. The PMA-induced increase in Nav 1.8 currents was not significantly affected by additional 0.1mM 8-Br-cAMP application. Intracellular application of a PKA inhibitor, protein kinase inhibitor (PKI, 0.01mM), inhibited the baseline Nav 1.8 current, significantly attenuated the 8-Br-cAMP-and PMA-induced increase in the peak Nav 1.8 current, and caused a significant increase in the slope factor of the inactivation curve. The PKI application at a higher concentration (0.5mM) greatly inhibited the PMA (0.1microM)-induced increase in the peak Nav 1.8 current amplitude and further enhanced the Ro-31-8425-induced decrease in the current. These results suggest that the 8-Br-cAMP-induced increase in Nav 1.8 currents may be mediated by activation of both PKA and PKC.

The effect of PKC activity on the TTX-R sodium currents from rat nodose ganglion neurons

To determine how protein kinase C (PKC) activity influences properties of the tetrodotoxin-resistant sodium current (TTX-R I(Na)) in neonatal rat nodose ganglion (NG) neurons, we assessed the effects of phorbol,-12-myristate,13-acetate (PMA), one of the PKC activators, and staurosporine, one of the PKC inhibitors, on the current. PMA (30 and 100 nM) induced an increase in the peak current amplitude of normalized current-voltage curves, a leftward shift in the potential for half activation (V(1/2)) of normalized conductance-voltage curves and a leftward shift of V(1/2) potential for steady-state inactivation curves. The effects of staurosporine (0.1 and 1 muM) on the peak current amplitude and the V(1/2) potential for activation were opposite compared with those seen after PMA application. Staurosporine (1 muM) antagonized PMA (100 nM)-induced modification of TTX-R I(Na). These results suggest that the basal TTX-R I(Na) obtained from neonatal NG neurons is controlled by the level of PKC activity.

Absence of an association between axotomy-induced changes in sodium currents and excitability in DRG neurons from the adult rat

It is generally believed that nerve injury results in neuronal hyperexcitability that reflects in part a change in Na+ currents. However, there are conflicting data on the nature of Na+ current changes and the association between alterations in Na+ currents and increases in excitability. One potential source of conflicting data is that injured and spared neurons may respond differently to nerve injury; these subpopulations of neurons have not been distinguished in previous studies with the axotomy model of nerve injury (complete transection of the sciatic nerve). The present study was performed to determine the relationship between changes in Na+ channels and changes in neuronal excitability in identified injured dorsal root ganglion neurons post-axotomy. Small (< 45 pF) neurons labeled with a DiI injection into the sciatic nerve were studied 10 days and 4 weeks post-axotomy. Ten days post-axotomy, tetrodotoxin-resistant (TTX-R) Na+ current (INa) was decreased and TTX-sensitive (TTX-S) INa was increased, however, excitability was unchanged. Four weeks post-axotomy, neurons had become hyperexcitable while TTX-R INa remained reduced and TTX-S INa had returned to control levels. Thus, axotomy-induced changes in Na+ currents were not correlated with an axotomy-induced change in excitability. Additional analysis of axotomized neurons suggested that concomitant changes in other ionic currents occurred. These results suggest that neuronal excitability following axotomy is dependent on the sum of changes in ionic currents, and the overall effect on excitability may not always correspond to that predicted by a change in a single class of voltage-gated ion channel.

Nerve injury alters profile of receptor-mediated Ca2+ channel modulation in vagal afferent neurons of rat nodose ganglia

Although nerve injury is known to up- and down-regulate some metabotropic receptors in vagal afferent neurons of the nodose ganglia (NG), the functional significance has not been elucidated. In the present study, thus, we examined whether nerve injury affected receptor-mediated Ca2+ channel modulation in the NG neurons. In this regard, unilateral vagotomy was performed using male Sprague-Dawley rats. One week after vagotomy, Ca2+ currents were recorded using the whole-cell variant of patch-clamp technique in enzymatically dissociated NG neurons. In sham controls, norepinephrine (NE)-induced Ca2+ current inhibition was negligible. Following vagotomy, however, the NE responses were dramatically increased. This phenomenon was in accordance with up-regulation of alpha2A/B-adrenergic receptor mRNAs as quantified using real-time RT-PCR analysis. In addition, neuropeptide Y (NPY) and prostaglandin E2 responses were moderately augmented in vagotomized NG neurons. The altered NPY response appears to be caused by up-regulation of Y2 receptors negatively coupled to Ca2+ channels. In contrast, nerve injury significantly suppressed opioid (tested with DAMGO)-induced Ca2+ current inhibition with down-regulation of micro-receptors. Taken together, these results demonstrated for the first time that the profile of neurotransmitter-induced Ca2+ channel modulation is significantly altered in the NG neurons under pathophysiological state of nerve injury.

Voltage-gated sodium channels and hyperalgesia

Physiological and pharmacological evidence both have demonstrated a critical role for voltage-gated sodium channels (VGSCs) in many types of chronic pain syndromes because these channels play a fundamental role in the excitability of neurons in the central and peripheral nervous systems. Alterations in function of these channels appear to be intimately linked to hyperexcitability of neurons. Many types of pain appear to reflect neuronal hyperexcitability, and importantly, use-dependent sodium channel blockers are effective in the treatment of many types of chronic pain. This review focuses on the role of VGSCs in the hyperexcitability of sensory primary afferent neurons and their contribution to the inflammatory or neuropathic pain states. The discrete localization of the tetrodotoxin (TTX)-resistant channels, in particular NaV1.8, in the peripheral nerves may provide a novel opportunity for the development of a drug targeted at these channels to achieve efficacious pain relief with an acceptable safety profile.

Role of nerve growth factor in modulation of gastric afferent neurons in the rat

Recent studies demonstrated that experimental ulcers are associated with changes in the properties of voltage-sensitive sodium currents in sensory neurons. We hypothesized that nerve growth factor (NGF) contributes to these changes. Gastric ulcers were induced by acetic acid injection into the wall of the rat stomach. NGF expression was determined by ELISA and immunohistochemically. Sensory neurons were labeled by injection of a retrograde tracer into the gastric wall. Sodium currents were recorded in gastric sensory neurons from nodose and dorsal root ganglia cultured for 24 h in the presence of NGF or a neutralizing NGF antibody, respectively. Gastric ulcer formation caused a rise in NGF concentration within the gastric wall and an increase in NGF immunoreactivity. Exposure to NGF caused a significant increase in the TTX-resistant sodium current, whereas the TTX-sensitive sodium current remained unchanged. This was associated with an acceleration of the recovery from inactivation in spinal sensory neurons. Production and release of NGF in the gastric wall may contribute to sensitization of primary afferent neurons during gastric inflammation.

Calcium and calcium-activated currents in vagotomized rat primary vagal afferent neurons

Adult inferior vagal ganglion neurons (nodose ganglion neurons, NGNs) were acutely isolated 4-6 days after section of their peripheral axons (vagotomy) and examined with the whole-cell patch-clamp technique. A subset (approximately 25 %) of vagotomized NGNs displayed depolarizing after-potentials (DAPs), not present in control NGNs. DAPs were inhibited by niflumic acid (125 microM) or cadmium (100 microM), and had a reversal potential near E(Cl), indicating that they were due to Ca(2+)-activated chloride current (I(Cl(Ca))). N-type, L-type, T-/R- and other types of voltage-dependent Ca(2+) channels provided about 43, 2, 16 and 40 % of the trigger Ca(2+) for DAP generation, respectively. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was estimated using fura-2 fluorescence. Resting [Ca(2+)](i) and peak [Ca(2+)](i) elevation induced by activating Ca(2+)-induced Ca(2+) release (CICR) stores with 10 mM caffeine were not significantly different among control NGNs, vagotomized NGNs with DAPs and vagotomized NGNs without DAPs, averaging 54 +/- 7.9 (n = 19; P = 0.49) and 2022 +/- 1059 nM (n = 19; P = 0.44), respectively. Blocking CICR with 10 microM ryanodine reduced DAP amplitude by approximately 37 %. Ca(2+) influx induced by action potential waveforms was increased by over 250 % in vagotomized NGNs with DAPs (19.0 +/- 2.1 pC) compared to control NGNs (5.0 +/- 0.8 pC) or vagotomized NGNs without DAPs (7.0 +/- 0.8 pC). L-type, N-type, T-/R-type and other types of Ca(2+) influx were increased proportionately in vagotomized NGNs with DAPs. In conclusion, a subset of vagotomized NGNs have increased Ca(2+) currents and express I(Cl(Ca)). These NGNs respond electrically to increases in [Ca(2+)](i) during regeneration.