Experimental and modeling study of Na+ current heterogeneity in rat nodose neurons and its impact on neuronal discharge

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.

Persistent tetrodotoxin-resistant Na+ currents are activated by prostaglandin E2 via cyclic AMP-dependent pathway in C-type nodose neurons of adult rats

It has been documented that nodose neurons express TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na(+) channels. However, wheteher nodose neurons functionally express persistent TTX-R Na(+) currents has not been reported. The present study first demonstrated persistent TTX-R Na(+) channel activities in 7/19 C-type nodose neurons in the presence of PGE(2) using whole-cell patch. Voltage-dependent property showed that persistent TTX-R Na(+) currents were activated at near -60mV and channels were maintained open. The average peak was approximately 300-500pA. The mid-point of activation exhibited a greater shift to a more hyperpolarized potential in the neurons co-expressing TTX-R and persistent TTX-R Na(+) currents than those expressing TTX-R only. This effect of PGE(2) was also mimicked by Forskolin. The fact that persistent TTX-R Na(+) currents were only activated by PGE(2) suggested that the modulatory effects of PGE(2) on persistent TTX-R Na(+) currents are crucial in PGE(2)-mediated neuronal excitability, and may have a great impact on specifically physiological significance.

Differential Slow Inactivation and Use-Dependent Inhibition of Nav1.8 Channels Contribute to Distinct Firing Properties in IB4+ and IB4− DRG Neurons

Nociceptive dorsal root ganglion (DRG) neurons can be classified into nonpeptidergic IB4+ and peptidergic IB4− subtypes, which terminate in different layers in dorsal horn and transmit pain along different ascending pathways, and display different firing properties. Voltage-gated, tetrodotoxin-resistant (TTX-R) Nav1.8 channels are expressed in both IB4+ and IB4− cells and produce most of the current underlying the depolarizing phase of action potential (AP). Slow inactivation of TTX-R channels has been shown to regulate repetitive DRG neuron firing behavior. We show in this study that use-dependent reduction of Nav1.8 current in IB4+ neurons is significantly stronger than that in IB4− neurons, although voltage dependency of activation and steady-state inactivation are not different. The time constant for entry of Nav1.8 into slow inactivation in IB4+ neurons is significantly faster and more Nav1.8 enter the slow inactivation state than in IB4− neurons. In addition, recovery from slow inactivation of Nav1.8 in IB4+ neurons is slower than that in IB4− neurons. Using current-clamp recording, we demonstrate a significantly higher current threshold for generation of APs and a longer latency to onset of firing in IB4+, compared with those of IB4− neurons. In response to a ramp stimulus, IB4+ neurons produce fewer APs and display stronger adaptation, with a faster decline of AP peak than IB4− neurons. Our data suggest that differential use-dependent reduction of Nav1.8 current in these two DRG subpopulations, which results from their different rate of entry into and recovery from the slow inactivation state, contributes to functional differences between these two neuronal populations.

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.

Multiple types of Na+ currents mediate action potential electrogenesis in small neurons of mouse dorsal root ganglia

Small (<25 microm in diameter) neurons of the dorsal root ganglion (DRG) express multiple voltage-gated Na(+) channel subtypes, two of which being resistant to tetrodotoxin (TTX). Each subtype mediates Na(+) current with distinct kinetic property. However, it is not known how each type of Na(+) channel contributes to the generation of action potentials in small DRG neurons. Therefore, we investigated the correlation between Na(+) currents in voltage-clamp recordings and corresponding action potentials in current-clamp recordings, using wild-type (WT) and Na(V)1.8 knock-out (KO) mice, to clarify the action potential electrogenesis in small DRG neurons. We classified Na(+) currents in small DRG neurons into three categories on the basis of TTX sensitivity and kinetic properties, i.e., TTX-sensitive (TTX-S)/fast Na(+) current, TTX-resistant (TTX-R)/slow Na(+) current, and TTX-R/persistent Na(+) current. Our concurrent voltage- and current-clamp recordings from the same neuron revealed that the action potentials in WT small DRG neurons were mainly dependent on TTX-R/slow Na(+) current mediated by Na(V)1.8. It was surprising that a large portion of TTX-S/fast Na(+) current was switched off in WT small DRG neurons due to a hyperpolarizing shift of the steady-state inactivation (h (infinity)), whereas in KO small DRG neurons which are devoid of TTX-R/slow Na(+) current, the action potentials were generated by TTX-S/fast Na(+) current possibly through a compensatory shift of h (infinity) in the positive direction. We also confirmed that TTX-R/persistent Na(+) current mediated by Na(V)1.9 actually regulates subthreshold excitability in small DRG neurons. In addition, we demon strated that TTX-R/persistent Na(+) current can carry an action potential when the amplitude of this current was abnormally increased. Thus, our results indicate that the action potentials in small DRG neurons are generated and regulated with a combination of multiple mechanisms that may give rise to unique functional properties of small DRG neurons.

KCNQ/M-currents contribute to the resting membrane potential in rat visceral sensory neurons

The M-current is a slowly activating, non-inactivating potassium current that has been shown to be present in numerous cell types. In this study, KCNQ2, Q3 and Q5, the molecular correlates of M-current in neurons, were identified in the visceral sensory neurons of the nodose ganglia from rats through immunocytochemical studies. All neurons showed expression of each of the three proteins. In voltage clamp studies, the cognition-enhancing drug linopirdine (1-50 microM) and its analogue, XE991 (10 microM), quickly and irreversibly blocked a small, slowly activating current that had kinetic properties similar to KCNQ/M-currents. This current activated between -60 and -55 mV, had a voltage-dependent activation time constant of 208 +/- 12 ms at -20 mV, a deactivation time constant of 165 +/- 24 ms at -50 mV and V1/2 of -24 +/- 2 mV, values which are consistent with previous reports for endogenous M-currents. In current clamp studies, these drugs also led to a depolarization of the resting membrane potential at values as negative as -60 mV. Flupirtine (10-20 microM), an M-current activator, caused a 3-14 mV leftward shift in the current-voltage relationship and also led to a hyperpolarization of resting membrane potential. These data indicate that the M-current is present in nodose neurons, is activated at resting membrane potential and that it is physiologically important in regulating excitability by maintaining cells at negative voltages.

Inflammation increases the excitability of masseter muscle afferents

Temporomandibular disorder is a major health problem associated with chronic orofacial pain in the masticatory muscles and/or temporomandibular joint. Evidence suggests that changes in primary afferents innervating the muscles of mastication may contribute to temporomandibular disorder. However, there has been little systematic study of the mechanisms controlling the excitability of these muscle afferents, nor their response to inflammation. In the present study, we tested the hypotheses that inflammation increases the excitability of sensory neurons innervating the masseter muscle of the rat and that the ionic mechanisms underlying these changes are unique to these neurons. We examined inflammation-induced changes in the excitability of trigeminal ganglia muscle neurons following intramuscular injections of complete Freund's adjuvant. Three days after complete Freund's adjuvant injection acutely dissociated, retrogradely labeled trigeminal ganglia neurons were studied using whole cell patch clamp techniques. Complete Freund's adjuvant-induced inflammation was associated with an increase in neuronal excitability marked by a significant decrease in rheobase and increase in the slope of the stimulus response function assessed with depolarizing current injection. The increase in excitability was associated with significant decreases in the rate of action potential fall and the duration of the action potential afterhyperpolarization. These changes in excitability and action potential waveform were associated with significant shifts in the voltage-dependence of activation and steady-state availability of voltage-gated K(+) current as well as significant decreases in the density of voltage-gated K(+) current subject to steady-state inactivation. These data suggest that K(+) channel subtypes may provide novel targets for the treatment of pain arising from inflamed muscle. These results also support the hypothesis that the underlying mechanisms of pain arising from specific regions of the body are unique suggesting that it may be possible, if not necessary to treat pain originating from different parts of the body with specific therapeutic interventions.

Chronic IL-1beta signaling potentiates voltage-dependent sodium currents in trigeminal nociceptive neurons

The proinflammatory cytokine interleukin-1beta (IL-1beta) mediates inflammation and hyperalgesia, although the underlying mechanisms remain elusive. To better understand such molecular and cellular mechanisms, we investigated how IL-1beta modulates the total voltage-dependent sodium currents (INa) and its tetrodotoxin-resistant (TTX-R) component in capsaicin-sensitive trigeminal nociceptive neurons, both after a brief (5-min) and after a chronic exposure (24-h) of 20 ng/ml IL-1beta. A brief exposure led to a 28% specific (receptor-mediated) reduction of INa in these neurons, which were found to contain type I IL-1 receptors (IL-1RI+) on both their soma and nerve endings. In marked contrast, after a 24-h exposure, the total sodium current was specifically increased by 67%, without significantly affecting the TTX-R component. This potentiation of INa was suppressed in the presence of selective inhibitors of protein kinase C and G-protein-coupled signaling pathways, thereby suggesting that INa can be modulated through multiple pathways. In summary, the potentiation of INa through chronic IL-1beta signaling in nociceptive sensory neurons may be a critical component of inflammatory-associated hyperalgesia.

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.

CAP-1A is a novel linker that binds clathrin and the voltage-gated sodium channel Na(v)1.8

The voltage-gated sodium channel Na(v)1.8 produces a tetrodotoxin-resistant current and plays a key role in nociception. Annexin II/p11 binds to Na(v)1.8 and facilitates insertion of the channel within the cell membrane. However, the mechanisms responsible for removal of specific channels from the cell membrane have not been studied. We have identified a novel protein, clathrin-associated protein-1A (CAP-1A), which contains distinct domains that bind Na(v)1.8 and clathrin. CAP-1A is abundantly expressed in DRG neurons and colocalizes with Na(v)1.8 and can form a multiprotein complex with Na(v)1.8 and clathrin. Coexpression of CAP-1A and Na(v)1.8 in DRG neurons reduces Na(v)1.8 current density by approximately 50% without affecting the endogenous or recombinant tetrodotoxin-sensitive currents. This effect of CAP-1A is blocked by bafilomycin A1 treatment of transfected DRG neurons. CAP-1A thus is the first example of an adapter protein that links clathrin and a sodium channel and may regulate Na(v)1.8 channel density at the cell surface.

Prostaglandin E2-induced modification of tetrodotoxin-resistant Na+ currents involves activation of both EP2 and EP4 receptors in neonatal rat nodose ganglion neurones

1 The aim of the present study was to investigate which EP receptor subtypes (EP1-EP4) act predominantly on the modification of the tetrodotoxin-resistant Na+ current (I(NaR)) in acutely isolated neonatal rat nodose ganglion (NG) neurones. 2 Of the four EP receptor agonists ranging from 0.01 to 10 muM, the EP2 receptor agonist (ONO-AE1-259, 0.1-10 microM) and the EP4 receptor agonist (ONO-AE1-329, 1 microM) significantly increased peak I(NaR). The responses were associated with a hyperpolarizing shift in the activation curve. 3 Neither the EP1 receptor agonist ONO-DI-004 nor the EP3 receptor agonist ONO-AE-248 significantly modified the properties of I(NaR). 4 In PGE2 applications ranging from 0.01 to 10 microM, 1 microM PGE2 produced a maximal increase in the peak I(NaR) amplitude. The PGE2 (1 microM)-induced increase in the GV(1/2) baseline (% change in G at baseline V(1/2)) was significantly attenuated by either intracellular application of the PKA inhibitor PKI or extracellular application of the protein kinase C inhibitor staurosporine (1 microM). However, the slope factor k was not significantly altered by PGE2 applications at 0.01-10 microM. In addition, the hyperpolarizing shift of V(1/2) by PGE2 was not significantly altered by either PKI or staurosporine. 5 In other series of experiments, reverse transcription-polymerase chain reaction (RT-PCR) of mRNA from nodose ganglia indicated that all four EP receptors were present. 6 The NG contained many neuronal cell bodies (diameter <30 microm) with intense or moderate EP2, EP3, and EP4 receptor-immunoreactivities. 7 These results suggest that the PGE2-induced modification of I(NaR) is mainly mediated by activation of both EP2 and EP4 receptors.

Cerebellar dysfunction in multiple sclerosis: evidence for an acquired channelopathy

Cerebellar dysfunction in multiple sclerosis (MS) is a significant contributor to disability, is relatively refractory to symptomatic therapy, and often progresses despite treatment with disease-modifying agents. Thus, there is a need for better understanding of its pathophysiology. This chapter reviews a growing body of evidence which suggests that mis-tuning of Purkinje cells, due to expression of an abnormal repertoire of sodium channels, contributes to cerebellar deficits in MS. Within the normal nervous system, sodium channel Na(v)1.8 is expressed in a highly specific manner within spinal sensory and trigeminal neurons, and is not present within Purkinje cells, Na(v)1.8 mRNA and protein are, however, expressed within Purkinje cells both in models of MS (experimenal autoimmume encephalomyelitis; EAE), and in postmortem tissue from humans with MS. Expression of Na(v)1.8 within Purkinje cells in vitro alters electrogenesis in these cells in several ways: first, by increasing duration and amplitude of action potentials; second, by decreasing the proportion of action potentials that are conglomerate and the number of spikes per conglomerate action potential; and third, by supporting sustained, pacemaker-like impulse trains in response to depolarization, which are not seen in the absence of Na(v)1.8. Similar changes are observed in recordings from Purkinje cells in vivo from mice with EAE. Taken together, these results suggest that expression of Na(v)1.8 within Purkinje cells distorts their pattern of firing in MS.

Differential distribution and function of hyperpolarization-activated channels in sensory neurons and mechanosensitive fibers

Sensory neurons express hyperpolarization-activated currents (I(H)) that differ in magnitude and kinetics within the populations. We investigated the structural basis for these differences and explored the functional role of the I(H) channels in sensory neurons isolated from rat nodose ganglia. Immunohistochemical studies demonstrated a differential distribution of hyperpolarization-activated cyclic nucleotide-gated (HCN) protein (HCN1, HCN2, HCN4) in sensory neurons and peripheral terminals. HCN2 and HCN4 immunoreactivity was present in all nodose neurons. In contrast, only 20% of the total population expressed HCN1 immunoreactivity. HCN1 did not colocalize with IB4 (a marker for C-type neurons), and only 15% of HCN1-positive neurons colocalized with immunoreactivity for the vanilloid receptor VR1, another protein associated primarily with C-type neurons. Therefore, most HCN1-containing neurons were A-type neurons. In further support, HCN1 was present in the mechanosensitive terminals of myelinated but not unmyelinated sensory fibers, whereas HCN2 and HCN4 were present in receptor terminals of both myelinated and unmyelinated fibers. In voltage-clamp studies, cell permeant cAMP analogs shifted the activation curve for I(H) to depolarized potentials in C-type neurons but not A-type neurons. In current-clamp recording, CsCl, which inhibits only I(H) in nodose neurons, hyperpolarized the resting membrane potential from -63 +/- 1 to -73 +/- 2 mV and nearly doubled the input resistance from 1.3 to 2.2 GOmega. In addition, action potentials were initiated at lower depolarizing current injections in the presence of CsCl. At the sensory receptor terminal, CsCl decreased the threshold pressure for initiation of mechanoreceptor discharge. Therefore, elimination of the I(H) increases excitability of both the soma and the peripheral sensory terminals.

Voltage-gated ion channels in nociceptors: modulation by cGMP

In tissue or nerve injury, proinflammatory mediators are released that can modulate a variety of ion channels found in nociceptors. The changes in channel activity, which primarily occurs through changes in intracellular pathways, may lead to the pathological states of hyperalgesia and allodynia. To understand further the regulatory mechanisms underlying the changes in channel activity, we used whole cell patch-clamp recordings from capsaicin-sensitive nociceptive neurons in rat trigeminal ganglion neurons to examine how the cGMP-dependent pathways may regulate ion channel function. Addition of the 8-(4-chlorophenylthio)-3',5' (CPT)-cGMP, a membrane permeant modulator of ion channels, decreased the number of evoked action potentials by 36% and inhibited the tetrodotoxin-resistant (TTX-R) sodium currents and IA potassium currents by 37 and 32%, respectively. Delayed rectifier potassium (IK) currents were unaffected, suggesting that the effects of CPT-cGMP are unlikely to arise from a nonspecific effect on channel activity as a consequence of the adsorption of amphipathic CPT-cGMP molecules to the membrane's bilayer component. This conclusion was reinforced by the lack of changes in gramicidin A channel function in the presence of CTP-cGMP. In summary, the activation of the cGMP-dependent pathways reduces nociceptor excitability, in part, by decreasing the activity of voltage-gated TTX-R sodium channels. This pathway may be a target for efforts to produce selective analgesics.

Differential action potentials and firing patterns in injured and uninjured small dorsal root ganglion neurons after nerve injury

The profile of tetrodotoxin sensitive (TTX-S) and resistant (TTX-R) Na(+) channels and their contribution to action potentials and firing patterns were studied in isolated small dorsal root ganglion (DRG) neurons after L5/L6 spinal nerve ligation (SNL). Total TTX-R Na(+) currents and Na(v) 1.8 mRNA were reduced in injured L5 DRG neurons 14 days after SNL. In contrast, TTX-R Na(+)currents and Na(v) 1.8 mRNA were upregulated in uninjured L4 DRG neurons after SNL. Voltage-dependent inactivation of TTX-R Na(+) channels in these neurons was shifted to hyperpolarized potentials by 4 mV. Two types of neurons were identified in injured L5 DRG neurons after SNL. Type I neurons (57%) had significantly lower threshold but exhibited normal resting membrane potential (RMP) and action potential amplitude. Type II neurons (43%) had significantly smaller action potential amplitude but retained similar RMP and threshold to those from sham rats. None of the injured neurons could generate repetitive firing. In the presence of TTX, only 26% of injured neurons could generate action potentials that had smaller amplitude, higher threshold, and higher rheobase compared with sham rats. In contrast, action potentials and firing patterns in uninjured L4 DRG neurons after SNL, in the presence or absence of TTX, were not affected. These results suggest that TTX-R Na(+) channels play important roles in regulating action potentials and firing patterns in small DRG neurons and that downregulation in injured neurons and upregulation in uninjured neurons confer differential roles in shaping electrogenesis, and perhaps pain transmission, in these neurons.

Differentiation of autonomic reflex control begins with cellular mechanisms at the first synapse within the nucleus tractus solitarius

Visceral afferents send information via cranial nerves to the nucleus tractus solitarius (NTS). The NTS is the initial step of information processing that culminates in homeostatic reflex responses. Recent evidence suggests that strong afferent synaptic responses in the NTS are most often modulated by depression and this forms a basic principle of central integration of these autonomic pathways. The visceral afferent synapse is uncommonly powerful at the NTS with large unitary response amplitudes and depression rather than facilitation at moderate to high frequencies of activation. Substantial signal depression occurs through multiple mechanisms at this very first brainstem synapse onto second order NTS neurons. This review highlights new approaches to the study of these basic processes featuring patch clamp recordings in NTS brain slices and optical techniques with fluorescent tracers. The vanilloid receptor agonist, capsaicin, distinguishes two classes of second order neurons (capsaicin sensitive or capsaicin resistant) that appear to reflect unmyelinated and myelinated afferent pathways. The differences in cellular properties of these two classes of NTS neurons indicate clear functional differentiation at both the pre- and postsynaptic portions of these first synapses. By virtue of their position at the earliest stage of these pathways, such mechanistic differences probably impart important differentiation in the performance over the entire reflex pathways.

Veratridine modifies the TTX-resistant Na+ channels in rat vagal afferent neurons

A number of neurotoxins from venoms of invertebrates and plants are ligands for voltage-gated Na+ channels and are useful tools for studying Na+ channel function and structure. Using whole-cell recordings from vagal afferent nodose neurons, we studied neurotoxins that target Na+ channels. We asked whether Ts3 (an alpha-scorpion toxin) and/or veratridine (a lipid-soluble toxin), could modify the TTX-resistant Na+ current generated by vagal afferent nodose neurons. Nodose TTX-resistant current was not affected by Ts3, whereas Ts3 slowed inactivation of the current generated by TTX-sensitive current component. We found that veratridine inhibited the TTX-resistant Na+ currents on rat nodose neurons. Interestingly, veratridine-modified Na+ channels developed a persistent current that accounted for the large tail current observed. We propose that veratridine modifies TTX-resistant Na+ channels through a mechanism distinct from its actions on other voltage-gated Na+ channels.

Patterned electrical activity modulates sodium channel expression in sensory neurons

Peripheral nerve injury induces changes in the level of gene expression for sodium channels Nav1.3, Nav1.8, and Nav1.9 within dorsal root ganglion (DRG) neurons, which may contribute to the development of hyperexcitability, ectopic neuronal discharge, and neuropathic pain. The mechanism of this change in sodium channel expression is unclear. Decreased availability of neurotrophic factors following axotomy contributes to these changes in gene transcription, but the question of whether changes in intrinsic neuronal activity levels alone can trigger changes in the expression of these sodium channels has not been addressed. We examined the effect of electrical stimulation on the expression of Nav1.3, Nav1.8, and Nav1.9 by using cultured embryonic mouse sensory neurons under conditions in which nerve growth factor (NGF) was not limiting. Expression of Nav1.3 was not significantly changed following stimulation. In contrast, we observed activity-dependent down-regulation of Nav1.8 and Nav1.9 mRNA and protein levels after stimulation, as demonstrated by quantitative polymerase chain reaction and immunocytochemistry. These results show that a change in neuronal activity can alter the expression of sodium channel genes in a subtype-specific manner, via a mechanism independent of NGF withdrawal.

Role of tetrodotoxin-resistant Na+ current slow inactivation in adaptation of action potential firing in small-diameter dorsal root ganglion neurons

When acutely dissociated small-diameter dorsal root ganglion (DRG) neurons were stimulated with repeated current injections or prolonged application of capsaicin, their action potential firing quickly adapted. Because TTX-resistant (TTX-R) sodium current in these presumptive nociceptors generates a large fraction of depolarizing current during the action potential, we examined the possible role of inactivation of TTX-R sodium channels in producing adaptation. Under voltage clamp, TTX-R current elicited by short depolarizations showed strong use dependence at frequencies as low as 1 Hz, although recovery from fast inactivation was complete in approximately 10-30 msec. This use-dependent reduction was the result of the entry of TTX-R sodium channels into slow inactivated states. Slow inactivation was more effectively produced by steady depolarization than by cycling channels through open states. Slow inactivation was steeply voltage dependent, with a Boltzmann slope factor of 5 mV, a midpoint near -45 mV (5 sec conditioning pulses), and completeness of approximately 93% positive to -20 mV. The time constant for entry (approximately 200 msec) was independent of voltage from -20 mV to +60 mV, whereas recovery kinetics were moderately voltage dependent (time constant, approximately 1.5 sec at -60 mV and approximately 0.5 sec at -100 mV). Using a prerecorded current-clamp response to capsaicin as a voltage-clamp command waveform, we found that adaptation of firing occurred with a time course similar to that of development of slow inactivation. Thus, slow inactivation of the TTX-R sodium current limits the duration of small DRG cell firing in response to maintained stimuli and may contribute to cross desensitization between chemical and electrical stimuli.

Lidocaine and octanol have different modes of action at tetrodotoxin-resistant Na(+) channels of peripheral nerves

Local anesthetics and alcohols block impulse conduction in peripheral nerves by inhibiting Na(+) currents. In small peripheral nerve fibers, tetrodotoxin-resistant (TTX-r) Na(+) channels play an important role in impulse generation. We investigated the effects of lidocaine and the alcohol octanol on TTX-r Na(+) channels. Currents were recorded with the whole-cell patch-clamp method from enzymatically isolated rat dorsal root ganglion cells (data evaluation: nonlinear least-squares fitting). Lidocaine and octanol blocked the TTX-r Na(+) current in a reversible and concentration-dependent manner (50% inhibitory concentration values: 177 +/- 25 and 455 +/- 25 microM, respectively). Lidocaine additionally produced a strong use-dependent block. Both drugs showed a strong dynamic block (i.e., block developed during the time course of current activation and inactivation). Double-pulse protocols showed a slow dissociation of lidocaine from the channel during repolarization (time constant: 1763 +/- 63 ms; 300 microM). The dissociation of octanol was too quick to be distinguished from normal current repriming kinetics of 2.2 ms. Lidocaine and octanol acted noncompetitively in the Na(+) channel. Lidocaine and octanol have different blocking properties on the TTX-r Na(+) current and bind to different channel sites.

IMPLICATIONS

Lidocaine and octanol have different inhibitory effects on the function of tetrodotoxin-resistant Na(+) channels in rat dorsal root ganglion cells, as well as noncompetitive modes of action, as investigated by the whole-cell patch-clamp method, and therefore are likely to have different binding sites on the channel.

Upregulation of sodium channel Nav1.3 and functional involvement in neuronal hyperexcitability associated with central neuropathic pain after spinal cord injury

Spinal cord injury (SCI) can result in hyperexcitability of dorsal horn neurons and central neuropathic pain. We hypothesized that these phenomena are consequences, in part, of dysregulated expression of voltage-gated sodium channels. Because the rapidly repriming TTX-sensitive sodium channel Nav1.3 has been implicated in peripheral neuropathic pain, we investigated its role in central neuropathic pain after SCI. In this study, adult male Sprague Dawley rats underwent T9 spinal contusion injury. Four weeks after injury when extracellular recordings demonstrated hyperexcitability of L3-L5 dorsal horn multireceptive nociceptive neurons, and when pain-related behaviors were evident, quantitative RT-PCR, in situ hybridization, and immunocytochemistry revealed an upregulation of Nav1.3 in dorsal horn nociceptive neurons. Intrathecal administration of antisense oligodeoxynucleotides (ODNs) targeting Nav1.3 resulted in decreased expression of Nav1.3 mRNA and protein, reduced hyperexcitability of multireceptive dorsal horn neurons, and attenuated mechanical allodynia and thermal hyperalgesia after SCI. Expression of Nav1.3 protein and hyperexcitability in dorsal horn neurons as well as pain-related behaviors returned after cessation of antisense delivery. Responses to normally noxious stimuli and motor function were unchanged in SCI animals administered Nav1.3 antisense, and administration of mismatch ODNs had no effect. These results demonstrate for the first time that Nav1.3 is upregulated in second-order dorsal horn sensory neurons after nervous system injury, showing that SCI can trigger changes in sodium channel expression, and suggest a functional link between Nav1.3 expression and neuronal hyperexcitability associated with central neuropathic pain.

Temporal course of upregulation of Na(v)1.8 in Purkinje neurons parallels the progression of clinical deficit in experimental allergic encephalomyelitis

Multiple sclerosis (MS) is recognized to involve demyelination and axonal atrophy but accumulating evidence suggests that dysregulated sodium channel expression may also contribute to its pathophysiology. Recent studies have demonstrated that the expression of Na(v)1.8 voltage-gated sodium channels, which are normally undetectable within the CNS, is upregulated in cerebellar Purkinje cells in experimental allergic encephalomyelitis (EAE) and MS, and suggest that the aberrant expression of these channels contributes to clinical dysfunction by distorting the firing pattern of these neurons. In this study we examined the temporal pattern of upregulation for Na(v)1.8 mRNA and protein in chronic relapsing EAE by in situ hybridization and immunocytochemistry, respectively. Our results demonstrate a positive correlation between disease duration and degree of upregulation of Na(v)1.8 mRNA and protein in Purkinje neurons in chronic-relapsing EAE. The progressive deterioration in clinical baseline scores (i.e. in clinical scores during remissions) is paralleled by a continued increase in Na(v)1.8 mRNA and protein expression, but temporary worsening during relapses is not associated with transient changes in Na(v)1.8 expression. These results provide evidence that the expression of sodium channel Na(v)1.8 contributes to the development of clinical deficits in an in vivo model of neuroinflammatory disease.

Arylacetamide kappa-opioid receptor agonists produce a tonic- and use-dependent block of tetrodotoxin-sensitive and -resistant sodium currents in colon sensory neurons

We have previously reported that U50,488 [(trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide] enantiomers contribute to visceral antinociception by a nonopioid receptor-mediated blockade of sodium currents in colon sensory neurons. The present experiments were undertaken to examine the effect of arylacetamide kappa-opioid receptor agonists (kappa-ORAs) U50,488 and EMD 61,753 [(N-methyl-N-[1S)-1-phenyl)-2-(13S))-3-hydroxypyrrolidine-1-yl)-ethyl]-2,2-diphenylacetamide HCl] on tetrodotoxin-sensitive (TTX-S) and -resistant (TTX-R) sodium currents, and the mechanism of their sodium channel-blocking actions. Whole cell patch-clamp experiments were performed on colon sensory neurons from the S1 dorsal root ganglion identified by content of retrograde tracer 1.1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine metanesulfonate. The concentration-response curves of U50,488 and EMD 61,753, for tonic inhibition of total, TTX-S, and TTX-R sodium currents were similar (EC50 values for U50,488 and EMD 61,753 were 8.4 +/- 1.69 and 1.2 +/- 1.78 microM, respectively). In contrast, the peptide kappa-ORA dynorphin was without effect in these experiments. U50,488 (10 microM) shifted the voltage dependence of steady-state inactivation curves for total, TTX-S, and TTX-R currents to more negative potentials. Inhibition was present at holding potentials of -100 to -20 mV. After the tonic block elicited by 10 microM U50,488, repetitive stimulation with 5-ms depolarizing pulses at a frequency of 3 Hz further enhanced the inhibition of total, TTX-R, and TTX-S currents by 43.8 +/- 4.9, 46.2 +/- 4.9, and 40 +/- 3.2%, respectively. These results demonstrate that arylacetamide kappa-ORAs nonselectively inhibit voltage-evoked sodium currents in a manner similar to local anesthetics, by enhancing closed-state inactivation and induction of use-dependent block.

Reversal of experimental neuropathic pain by T-type calcium channel blockers

Experimental nerve injury results in exaggerated responses to tactile and thermal stimuli that resemble some aspects of human neuropathic pain. Neuronal hyperexcitability and neurotransmitter release have been suggested to promote such increased responses to sensory stimuli. Enhanced activity of Ca(2+) current is associated with increased neuronal activity and blockade of N- and P-types, but not L-type, calcium channels have been found to block experimental neuropathic pain. While T-type currents are believed to promote neuronal excitability and transmitter release, it is unclear whether these channels may also contribute to the neuropathic state. Rats were prepared with L(5)/L(6) spinal nerve ligation, and tactile and thermal hypersensitivities were established. Mibefradil or ethosuximide was administered either intraperitoneally, intrathecally (i.th.), or locally into the plantar aspect of the injured hindpaw. Systemic mibefradil or ethosuximide produced a dose-dependent blockade of both tactile and thermal hypersensitivities in nerve-injured rats; responses of sham-operated rats were unchanged. Local injection of mibefradil also blocked both end points. Ethosuximide, however, was inactive after local administration, perhaps reflecting its low potency when compared with mibefradil. Neither mibefradil nor ethosuximide given i.th. produced any blockade of neuropathic behaviors. The results presented here suggest that T-type calcium channels may play a role in the expression of the neuropathic state. The data support the view that selective T-type calcium channel blockers may have significant potential in the treatment of neuropathic pain states.

The TTX-resistant sodium channel Nav1.8 (SNS/PN3): expression and correlation with membrane properties in rat nociceptive primary afferent neurons

We have examined the distribution of the sensory neuron-specific Na+ channel Nav1.8 (SNS/PN3) in nociceptive and non-nociceptive dorsal root ganglion (DRG) neurons and whether its distribution is related to neuronal membrane properties. Nav1.8-like immunoreactivity (Nav1.8-LI) was examined with an affinity purified polyclonal antiserum (SNS11) in rat DRG neurons that were classified according to sensory receptive properties and by conduction velocity (CV) as C-, Adelta- or Aalpha/beta. A significantly higher proportion of nociceptive than low threshold mechanoreceptive (LTM) neurons showed Nav1.8-LI, and nociceptive neurons had significantly more intense immunoreactivity in their somata than LTM neurons. Results showed that 89, 93 and 60% of C-, Adelta- and Aalpha/beta-fibre nociceptive units respectively and 88% of C-unresponsive units were positive. C-unresponsive units had electrical membrane properties similar to C-nociceptors and were considered to be nociceptive-type neurons. Weak positive Nav1.8-LI was also present in some LTM units including a C LTM, all Adelta LTM units (D hair), about 10% of cutaneous LTM Aalpha/beta-units, but no muscle spindle afferent units. Nav1.8-LI intensity was negatively correlated with soma size (all neurons) and with dorsal root CVs in A- but not C-fibre neurons. Nav1.8-LI intensity was positively correlated with action potential (AP) duration (both rise and fall time) in A-fibre neurons and with AP rise time only in positive C-fibre neurons. It was also positively correlated with AP overshoot in positive neurons. Thus high levels of Nav1.8 protein may contribute to the longer AP durations (especially in A-fibre neurons) and larger AP overshoots that are typical of nociceptors.

The roles potassium currents play in regulating the electrical activity of ventral cochlear nucleus neurons

Using kinetic data from three different K+ currents in acutely isolated neurons, a single electrical compartment representing the soma of a ventral cochlear nucleus (VCN) neuron was created. The K+ currents include a fast transient current (IA), a slow-inactivating low-threshold current (ILT), and a noninactivating high-threshold current (IHT). The model also includes a fast-inactivating Na+ current, a hyperpolarization-activated cation current (Ih), and 1-50 auditory nerve synapses. With this model, the role IA, ILT, and IHT play in shaping the discharge patterns of VCN cells is explored. Simulation results indicate that IHT mainly functions to repolarize the membrane during an action potential, and IA functions to modulate the rate of repetitive firing. ILT is found to be responsible for the phasic discharge pattern observed in Type II cells (bushy cells). However, by adjusting the strength of ILT, both phasic and regular discharge patterns are observed, demonstrating that a critical level of ILT is necessary to produce the Type II response. Simulated Type II cells have a significantly faster membrane time constant in comparison to Type I cells (stellate cells) and are therefore better suited to preserve temporal information in their auditory nerve inputs by acting as precise coincidence detectors and having a short refractory period. Finally, we demonstrate that modulation of Ih, which changes the resting membrane potential, is a more effective means of modulating the activation level of ILT than simply modulating ILT itself. This result may explain why ILT and Ih are often coexpressed throughout the nervous system.

Phoneutria nigriventer spider venom activates 5-HT4 receptors in rat-isolated vagus nerve

1. The venom of Phoneutria nigriventer spider (PNV) causes intense pain and inflammation following an attack. We have investigated the involvement of capsaicin-sensitive nerve fibres by utilizing an in vitro nerve preparation. Extracellular DC potential recordings were made from the rat-isolated vagus nerve, a preparation that is rich in capsaicin-sensitive, that is, nociceptive, C-fibres. 2. PNV (1-10 microg ml(-1)), capsaicin (0.03-0.3 microM) or 5-hydroxytriptamine (5-HT) (0.3-3 microM) induced dose-dependent depolarizations of vagus nerve fibres. Depolarizing responses to capsaicin were blocked by ruthenium red (RR, 10 microM), but responses to PNV were not. Depolarizing responses to PNV or veratridine (50 microM) were inhibited by tetrodotoxin (TTX, 10 microM), but those to capsaicin were not. This suggests that capsaicin and PNV depolarize the nerve fibres by distinct mechanisms. 3. Depolarization in response to 5-HT (3 microM) was reduced by the 5-HT(3) receptor antagonists Y25130 (0.5 micro M) and tropisetron (10 nM) or, to a lesser extent, by the 5-HT(4) receptor antagonist RS39604 (1 or 10 microM). Depolarizing responses to PNV were not affected significantly by Y25130 or tropisetron, but were blocked by RS39604. 4. These data show that 5-HT(4) receptors play a significant role in the activation of nociceptive sensory nerve fibres by PNV and suggest that this is of importance in the development of the pain and inflammation associated with bites from the P. nigriventer spider.

Annexin II/p11 is up-regulated in Purkinje cells in EAE and MS

The sensory neuron specific sodium channel Na(v)1.8 is normally detectable at only very low levels within cerebellar Purkinje cells. Annexin II light chain (p11) binds to the amino terminus of Na(v)1.8 and facilitates its functional expression within the cell membrane. We previously demonstrated that expression of Na(v)1.8 is up-regulated in cerebellar Purkinje cells in experimental allergic encephalomyelitis (EAE) and multiple sclerosis (MS). In this study we demonstrate that expression of p11 is significantly up-regulated in Purkinje cells in EAE (71 +/- 9.0% vs 21.3 +/- 4.9% in controls) and in MS(65.5 +/- 1.6% vs 21.8 +/- 6.2% in controls). We also demonstrate a high degree of co-expression of p11 and Na(v)1.8 (84.8 +/- 8.9%). Together with earlier results which show that experimental expression of Na(v)1.8 within Purkinje cells perturbs the temporal pattern of impulse generation in these cells, our results extend the evidence for an acquired channelopathy which interferes with cerebellar function in MS.

Tetrodotoxin-resistant sodium channels Na(v)1.8/SNS and Na(v)1.9/NaN in afferent neurons innervating urinary bladder in control and spinal cord injured rats

Tetrodotoxin-resistant (TTX-R) sodium channels Na(v)1.8/SNS and Na(v)1.9/NaN are preferentially expressed in small diameter dorsal root ganglia (DRG) neurons. The urinary bladder is innervated by small afferent neurons from L6/S1 DRG, of which approximately 75% exhibit high-threshold action potentials that are mediated by TTX-R sodium channels. Following transection of the spinal cord at T8, the bladder becomes areflexic and then gradually hyper-reflexic, and there is an attenuation of the TTX-R sodium currents in bladder afferent neurons. In the present study, we demonstrate that Na(v)1.8 is expressed in both bladder and non-bladder afferent neurons, while Na(v)1.9 is expressed in non-bladder afferent neurons but is rarely observed in bladder afferent neurons. In spinal cord transected rats 28-32 days following transection, there is a decreased expression of Na(v)1.8 sodium channels in bladder afferents, but no change in the expression of Na(v)1.8 in non-bladder afferent neurons. Both bladder and non-bladder afferent neurons exhibit limited increases in Na(v)1.9 expression following spinal cord transection. These results demonstrate that the expression of TTX-R channels in bladder afferent neurons changes after spinal cord transection, and these changes may contribute to the increased excitability of these neurons following spinal cord injury.

Sensory and electrophysiological properties of guinea-pig sensory neurones expressing Nav 1.7 (PN1) Na+ channel alpha subunit protein

The TTX-sensitive Na(v)1.7 (PN1) Na(+) channel alpha subunit protein is expressed mainly in small dorsal root ganglion (DRG) neurones. This study examines immunocytochemically whether it is expressed exclusively or preferentially in nociceptive primary afferent DRG neurones, and determines the electrophysiological properties of neurones that express it. Intracellular somatic action potentials (APs) evoked by dorsal root stimulation were recorded in L6/S1 DRG neurones at 30 +/- 2 degrees C in vivo in deeply anaesthetised young guinea-pigs. Each neurone was classified, from its dorsal root conduction velocity (CV) as a C-, Adelta- or Aalpha/beta-fibre unit and from its response to mechanical and thermal stimuli, as a nociceptive, low threshold mechanoreceptive (LTM) or unresponsive unit. Fluorescent dye was injected into the soma and Na(v)1.7-like immunoreactivity (Na(v)1.7-LI) was examined on sections of dye-injected neurones. All C-, 90 % of Adelta- and 40 % of Aalpha/beta-fibre units, including both nociceptive and LTM units, showed Na(v)1.7-LI. Positive units included 1/1 C-LTM, 6/6 C-nociceptive, 4/4 C-unresponsive (possible silent nociceptive) units, 5/6 Adelta-LTM (D hair), 13/14 Adelta-nociceptive, 2/9 Aalpha/beta-nociceptive, 10/18 Aalpha/beta-LTM cutaneous and 0/9 Aalpha/beta-muscle spindle afferent units. Overall, a higher proportion of nociceptive than of LTM neurones was positive, and the median relative staining intensity was greater in nociceptive than LTM units. Na(v)1.7-LI intensity was clearly positively correlated with AP duration and (less strongly) negatively correlated with CV and soma size. Since nociceptive units tend overall to have longer duration APs, slower CVs and smaller somata, these correlations may be related to the generally greater expression of Na(v)1.7 in nociceptive units.

Expression of Nav1.8 sodium channels perturbs the firing patterns of cerebellar Purkinje cells

The sensory neuron specific sodium channel Na(v)1.8/SNS exhibits depolarized voltage-dependence of inactivation, slow inactivation and rapid repriming, which differentiate it from other voltage-gated sodium channels. Na(v)1.8 is normally selectively expressed at high levels in sensory ganglion neurons, but not within the CNS. However, expression of Na(v)1.8 mRNA and protein are upregulated within cerebellar Purkinje cells in animal models of multiple sclerosis (MS), and in human MS. To examine the effect of expression of Na(v)1.8 on the activity pattern of Purkinje cells, we biolistically introduced Na(v)1.8 cDNA into these cells in vitro. We report here that Na(v)1.8 can be functionally expressed at physiological levels (similar to the levels in DRG neurons where Na(v)1.8 is normally expressed) within Purkinje cells, and that its expression alters the activity of these neurons in three ways: first, by increasing the amplitude and duration of action potentials; second, by decreasing the proportion of action potentials that are conglomerate and the number of spikes per conglomerate action potential; and third, by contributing to the production of sustained, pacemaker-like impulse trains in response to depolarization. These results provide support for the hypothesis that the expression of Na(v)1.8 channels within Purkinje cells, which occurs in MS, may perturb their function.

Roles of tetrodotoxin (TTX)-sensitive Na+ current, TTX-resistant Na+ current, and Ca2+ current in the action potentials of nociceptive sensory neurons

Nociceptive sensory neurons are unusual in expressing voltage-gated inward currents carried by sodium channels resistant to block by tetrodotoxin (TTX) as well as currents carried by conventional TTX-sensitive sodium channels and voltage-dependent calcium channels. To examine how currents carried by each of these helps to shape the action potential in small-diameter dorsal root ganglion cell bodies, we voltage clamped cells by using the action potential recorded from each cell as the command voltage. Using intracellular solutions of physiological ionic composition, we isolated individual components of current flowing during the action potential with the use of channel blockers (TTX for TTX-sensitive sodium currents and a mixture of calcium channel blockers for calcium currents) and ionic substitution (TTX-resistant current measured by the replacement of extracellular sodium by N-methyl-D-glucamine in the presence of TTX, with correction for altered driving force). TTX-resistant sodium channels activated quickly enough to carry the largest inward charge during the upstroke of the nociceptor action potential (approximately 58%), with TTX-sensitive sodium channels also contributing significantly ( approximately 40%), especially near threshold, and high voltage-activated calcium currents much less (approximately 2%). Action potentials had a prominent shoulder during the falling phase, characteristic of nociceptive neurons. TTX-resistant sodium channels did not inactivate completely during the action potential and carried the majority (58%) of inward current flowing during the shoulder, with high voltage-activated calcium current also contributing significantly (39%). Unlike calcium current, TTX-resistant sodium current is not accompanied by opposing calcium-activated potassium current and may provide an effective mechanism by which the duration of action potentials (and consequently calcium entry) can be regulated.

Characterization and developmental changes of Na+ currents of petrosal neurons with projections to the carotid body

Carotid body chemoreceptors transduce a decrease in arterial oxygen tension into an increase in spiking activity on the sinus nerve, and this response increases with postnatal age over the first week or two of life. Previous work from our laboratory has suggested a major role of axonal Na(+) channels in the initiation of afferent spiking activity. Using RT-PCR of the petrosal ganglia we identified Na(+) channel TTX-S isoforms Na(v)1.1, Na(v)1.6, and Na(v)1.7 and the TTX-resistant (TTX-R) isoforms Na(v)1.8 and Na(v)1.9 at high levels. Electrophysiologic recordings (at 3 ages: 3 days, 9 days, 18-20 days) of neurons that project to the carotid body exhibited predominantly fast-inactivating sodium currents, with a bimodal recovery from inactivation at -80 mV (fast component approximately 8 ms; slow component approximately 90 ms). Developmental age had little effect with no change in peak current density (approximately 1.4 nA/pF) and was associated with a slight, but significant increase in the speed of recovery from inactivation at -140 and -120 mV but not at other potentials. Assuming that the same Na(+) channel complement is present at the nerve terminal as at the soma, the association of a sensory modality (chemoreception) with a relatively uniform Na(+) channel profile suggests that the rapid kinetics of TTX-S channels may be essential for some aspects of chemoreceptor function beyond mediating simple axonal conduction.

Potassium channels Kv1.1, Kv1.2 and Kv1.6 influence excitability of rat visceral sensory neurons

Voltage-gated potassium channels, Kv1.1, Kv1.2 and Kv1.6, were identified as PCR products from mRNA prepared from nodose ganglia. Immunocytochemical studies demonstrated expression of the proteins in all neurons from ganglia of neonatal animals (postnatal days 0-3) and in 85-90 % of the neurons from older animals (postnatal days 21-60). In voltage clamp studies, alpha-dendrotoxin (alpha-DTX), a toxin with high specificity for these members of the Kv1 family, was used to examine their contribution to K(+) currents of the sensory neurons. alpha-DTX blocked current in both A- and C-type neurons. The current had characteristics of a delayed rectifier with activation positive to -50 mV and little inactivation during 250 ms pulses. In current-clamp experiments alpha-DTX, used to eliminate the current, had no effect on resting membrane potential and only small effects on the amplitude and duration of the action potential of A- and C-type neurons. However, there were prominent effects on excitability. alpha-DTX lowered the threshold for initiation of discharge in response to depolarizing current steps, reduced spike after-hyperpolarization and increased the frequency/pattern of discharge of A- and C-type neurons at membrane potentials above threshold. Model simulations were consistent with these experimental results and demonstrated how the other major K(+) currents function in response to the loss of the alpha-DTX-sensitive current to effect these changes in action potential wave shape and discharge.

Patch clamp electrophysiology in nodose ganglia of adult rat

The patch clamp technique is widely utilized for studying the electrophysiological properties of enzymatically isolated sensory neurons. Unfortunately, dissociation of the ganglion severs the afferent fibers. As a result, isolated neurons can only be broadly categorized according to somatic action potential waveforms, ion channel subtypes, chemical sensitivities and cell diameter. Such restricted classifications contrast with the continuum of conduction velocities (CVs), discharge patterns, sensory modalities and functional properties of visceral and spinal afferents. Previous reports of patch clamp recordings using intact ganglion have been limited to young animal preparations. This raises concerns regarding postnatal development and impedes the use of chronic models of disease or injury, which often necessitate the use of a more mature animal preparation. Here, we present a methodology for preparing nodose ganglion from adult rat (250-400 g) for study using the patch clamp technique. Successful whole cell recordings were obtained from approximately 50% of the cells selected for study, the majority of which had intact afferent fibers. Measures of somatic discharge and afferent fiber CV at both room and physiological temperatures were consistent with investigations using sharp microelectrodes. Voltage clamp recordings of whole cell Na(+), Ca(2+) and K(+) ion channel currents were comparable to those obtained using isolated neuron preparations. The ability to classify voltage- and ligand-gated ion channel type with afferent fiber CV in an adult preparation adds a valuable new dimension to cellular investigations of the diverse functional and chemical properties of the peripheral afferent nervous system.

Oscillatory mechanism in primary sensory neurones

Ectopic spike activity, generated at low levels in intact sensory dorsal root ganglia and intensified following axotomy, is an important cause of neuropathic pain. The spikes are triggered by subthreshold membrane potential oscillations. The depolarizing phase of oscillation sinusoids is due to a phasic voltage-sensitive Na(+) conductance (gNa(+)). Here we examine the repolarizing phase for which K(+) conductance (gK(+)) is implicated. In vivo, gK(+) blockers have excitatory effects inconsistent with the elimination of oscillations. Indeed, using excised dorsal root ganglia in vitro, we found that gK(+) block does not eliminate oscillations; on the contrary, it has a variety of facilitatory effects. However, oscillations were eliminated by shifting the K(+) reversal potential so as to neutralize voltage-insensitive K(+) leak channels. Based on these data, we propose a novel oscillatory model: oscillation sinusoids are due to reciprocation between a phasically activating voltage-dependent, tetrodotoxin-sensitive Na(+) conductance and passive, voltage-independent K(+) leak. In drug-free media, voltage-sensitive K(+) channels act to suppress oscillations and increase their frequency. Numerical simulations support this model and account for the effects of gK(+) block. Oscillations in dorsal root ganglia neurones appear to be based on the simplest possible configuration of ionic conductances compatible with sustained high frequency oscillatory behaviour. The oscillatory mechanism might be exploited in the search for novel analgesic drugs.

Experimental ulcers alter voltage-sensitive sodium currents in rat gastric sensory neurons

BACKGROUND & AIMS

Voltage-dependent Na+ currents are important determinants of excitability. We hypothesized that gastric inflammation alters Na+ current properties in primary sensory neurons.

METHODS

The stomach was surgically exposed in rats to inject the retrograde tracer 1.1'-dioctadecyl-3,3,3,'3-tetramethylindocarbocyanine methanesulfonate and saline (control) or 20% acetic acid (ulcer group) into the gastric wall. Nodose or thoracic dorsal root ganglia (DRG) were harvested after 7 days to culture neurons and record Na+ currents using patch clamp techniques.

RESULTS

There were no lesions in the control and 3 +/- 1 ulcers in the ulcer group. Na+ currents recovered significantly more rapidly from inactivation in nodose and DRG neurons obtained from animals in the ulcer group compared with controls. This was partially a result of an increase in the relative contribution of the tetrodotoxin-resistant to the peak sodium current. In addition, the recovery kinetics of the tetrodotoxin-sensitive current were faster. In DRG neurons, gastric inflammation shifted the voltage-dependence of activation of the tetrodotoxin-resistant current to more hyperpolarized potentials.

CONCLUSIONS

Gastric injury alters the properties of Na+ currents in gastric sensory neurons. This may enhance excitability, thereby contributing to the development of dyspeptic symptoms.

B2 receptor-mediated enhanced bradykinin sensitivity of rat cutaneous C-fiber nociceptors during persistent inflammation

Bradykinin (BK), which has potent algesic and sensitizing effect on nociceptors, is of current interest in understanding the mechanisms of chronic pain. BK response is mediated by B2 receptor in normal conditions; however, findings that B1 receptor blockade alleviated hyperalgesia in inflammation have been highlighting the role of B1 receptor in pathological conditions. It has not yet been clear whether nociceptor activities are modified by B1 receptor agonists or antagonists during inflammation. In addition, previous studies reported the change in BK sensitivity of nociceptors during short-lasting inflammation, and data in persistent inflammation are lacking. Therefore we investigated whether an experimentally induced persistent inflammatory state modulates the BK sensitivity of nociceptors and which receptor subtype plays a more important role in this condition. Complete Freund's adjuvant was injected into the rat-tail and after 2-3 wk, persistent inflammation developed, which was prominent in the ankle joint. Using an in vitro skin-saphenous nerve preparation, single-fiber recordings were made from mechano-heat sensitive C-fiber nociceptors innervating rat hairy hindpaw skin, and their responses were compared with those obtained from C-fibers tested similarly in normal animals. BK at 10(-8) M excited none of the 10 C-fibers in normal animals while it excited 5 of 11 (45%) C-fibers of inflamed animals, and at 10(-6) M BK excited all of the 11 inflamed C-fibers (or 94% of 36 tested C-fibers) but only 4 of 10 (or 45% of 58 tested C-fibers) in normal animals. Thus the concentration-response curves based on the incidence of BK induced excitation, and the total number of impulses evoked in response to BK were significantly shifted to the left. Moreover, an increased percentage of the inflamed C-fibers responded to 10(-6) M BK with bursting or high-frequency discharges. Thirty-percent of inflamed C-fibers had spontaneous activity, and these fibers showed comparatively less tachyphylaxis to consecutive second and third 10(-6) M BK stimulation. A B2 receptor antagonist (D-Arg-[Hyp3, Thi5,8,D-phe7]-BK) completely eliminated BK responses in inflamed rats, while B1 receptor antagonists (B 9958 and Des-Arg9-[Leu8]-BK) had no effect. Selective B1 receptor agonist (Des-Arg10-Kallidin) excited 46% (n = 13) of inflamed C-fibers at 10(-5) M concentration, which is 1,000 times higher than that of BK needed to excite the same percentage of inflamed C-fibers. We conclude that in chronically inflamed tissue, sensitivity of C-fiber nociceptors to BK, which is B2 receptor mediated, is strongly increased and that B1 receptor may not be important to a persistent inflammatory state, at least at the primary afferent level.

Gating properties of Na(v)1.7 and Na(v)1.8 peripheral nerve sodium channels

Several distinct components of voltage-gated sodium current have been recorded from native dorsal root ganglion (DRG) neurons that display differences in gating and pharmacology. This study compares the electrophysiological properties of two peripheral nerve sodium channels that are expressed selectively in DRG neurons (Na(v)1.7 and Na(v)1.8). Recombinant Na(v)1.7 and Na(v)1.8 sodium channels were coexpressed with the auxiliary beta(1) subunit in Xenopus oocytes. In this system coexpression of the beta(1) subunit with Na(v)1.7 and Na(v)1.8 channels results in more rapid inactivation, a shift in midpoints of steady-state activation and inactivation to more hyperpolarizing potentials, and an acceleration of recovery from inactivation. The coinjection of beta(1) subunit also significantly increases the expression of Na(v)1.8 by sixfold but has no effect on the expression of Na(v)1.7. In addition, a great percentage of Na(v)1.8+beta(1) channels is observed to enter rapidly into the slow inactivated states, in contrast to Nav1.7+beta(1) channels. Consequently, the rapid entry into slow inactivation is believed to cause a frequency-dependent reduction of Na(v)1.8+beta(1) channel amplitudes, seen during repetitive pulsing between 1 and 2 Hz. However, at higher frequencies (>20 Hz) Na(v)1.8+beta(1) channels reach a steady state to approximately 42% of total current. The presence of this steady-state sodium channel activity, coupled with the high activation threshold (V(0.5) = -3.3 mV) of Na(v)1.8+beta(1), could enable the nociceptive fibers to fire spontaneously after nerve injury.

Sodium currents in vagotomized primary afferent neurones of the rat

1. Nodose ganglion neurones (NGNs) become less excitable following section of the vagus nerve. To determine the role of sodium currents (I(Na)) in these changes, standard patch-clamp recording techniques were used to measure I(Na) in rat NGNs maintained in vivo for 5-6 days following vagotomy, and then in vitro for 2-9 h. 2. Total I(Na) and I(Na) density in vagotomized NGNs were similar to control values. However, steady-state I(Na) inactivation in vagotomized NGNs was shifted -9 mV relative to control values (V(1/2), -74 +/- 2 vs. -65 +/- 2 mV, P < 0.01) and I(Na) activation was shifted by -7 mV (V(1/2), -21 +/- 2 vs. -14 +/- 2 mV, P < 0.006). I(Na) recovery from inactivation was also slower in vagotomized NGNs (fast time constant, 2.8 +/- 0.4 vs. 1.6 +/- 0.3 ms, P < 0.02). 3. The fraction of I(Na) resistant to 1 microM tetrodotoxin (TTX-R) was halved in vagotomized NGNs (21 +/- 8 vs. 56 +/- 8 % of total I(Na), P < 0.05). This change from TTX-R I(Na) to TTX-sensitive (TTX-S) I(Na) may explain altered I(Na) activation, inactivation and repriming in vagotomized NGNs. 4. The contribution of alterations in I(Na) to NGN firing patterns was assessed by measuring I(Na) evoked by a series of action potential (AP) waveforms. In general, control NGNs produced large, repetitive TTX-R I(Na) while vagotomized NGNs produced smaller TTX-S I(Na) that rapidly inactivated during AP discharge. We conclude that TTX-R I(Na) is important for sustained AP discharge in NGNs, and that its diminution underlies the decreased AP discharge of vagotomized NGNs.

Persistent TTX-resistant Na+ current affects resting potential and response to depolarization in simulated spinal sensory neurons

Small dorsal root ganglion (DRG) neurons, which include nociceptors, express multiple voltage-gated sodium currents. In addition to a classical fast inactivating tetrodotoxin-sensitive (TTX-S) sodium current, many of these cells express a TTX-resistant (TTX-R) sodium current that activates near -70 mV and is persistent at negative potentials. To investigate the possible contributions of this TTX-R persistent (TTX-RP) current to neuronal excitability, we carried out computer simulations using the Neuron program with TTX-S and -RP currents, fit by the Hodgkin-Huxley model, that closely matched the currents recorded from small DRG neurons. In contrast to fast TTX-S current, which was well fit using a m(3)h model, the persistent TTX-R current was not well fit by an m(3)h model and was better fit using an mh model. The persistent TTX-R current had a strong influence on resting potential, shifting it from -70 to -49.1 mV. Inclusion of an ultra-slow inactivation gate in the persistent current model reduced the potential shift only slightly, to -56.6 mV. The persistent TTX-R current also enhanced the response to depolarizing inputs that were subthreshold for spike electrogenesis. In addition, the presence of persistent TTX-R current predisposed the cell to anode break excitation. These results suggest that, while the persistent TTX-R current is not a major contributor to the rapid depolarizing phase of the action potential, it contributes to setting the electrogenic properties of small DRG neurons by modulating their resting potentials and response to subthreshold stimuli.

Inhibition of mechanical activation of guinea-pig airway afferent neurons by amiloride analogues

1. The aim of this study was to investigate a role for Epithelial Sodium Channels (ENaCs) in the mechanical activation of low-threshold vagal afferent nerve terminals in the guinea-pig trachea/bronchus. 2. Using extracellular single-unit recording techniques, we found that the ENaC blocker amiloride, and its analogues dimethylamiloride and benzamil caused a reduction in the mechanical activation of guinea-pig airway afferent fibres. 3. Amiloride and it analogues also reduced the sensitivity of afferent fibres to electrical stimulation such that greater stimulation voltages were required to induce action potentials from their peripheral terminals within the trachea/bronchus. 4. The relative potencies of these compounds for inhibiting electrical excitability of afferent nerves were similar to that observed for inhibition of mechanical stimulation (dimethylamiloride approximately benzamil > amiloride). This rank order of potency is incompatible with the known rank order of potency for blockade of ENaCs (benzamil > amiloride >> dimethylamiloride). 5. As voltage-gated sodium channels play an important role in determining the electrical excitability of neurons, we used whole-cell patch recordings of nodose neuron cell bodies to investigate the possibility that amiloride analogues caused blockade of these channels. At the concentration required to inhibit mechanical activation of vagal nodose afferent fibres (100 microM), benzamil caused significant inhibition of voltage-gated sodium currents in neuronal cell bodies acutely isolated from guinea-pig nodose ganglia. 6. Combined, our findings suggest that amiloride and its analogues did not selectively block mechanotransduction in airway afferent neurons, but rather they reduced neuronal excitability, possibly by inhibiting voltage-gated sodium currents.

Slow inactivation of sodium currents in the rat nodose neurons

Nodose neurons express sodium currents that can be differentiated based on their sensitivity to tetrodotoxin. Several studies have demonstrated significant differences in voltage-dependence and kinetics of activation and inactivation between tetrodotoxin-sensitive and tetrodotoxin-resistant currents. However, little is known about the slow inactivation. Using whole cell patch-clamp technique fast and slow inactivation of sodium currents were studied in cultured rat nodose neurons. Tetrodotoxin-resistant currents recovered much more rapidly after a 15-ms depolarization than tetrodotoxin-sensitive currents. However, repeated 5-ms depolarizations at 10 Hz induced a cumulative inhibition that was more prolonged in tetrodotoxin-resistant compared to tetrodotoxin-sensitive currents. Consistent with these findings, slow inactivation proceeded more rapidly and was more complete for the tetrodotoxin-resistant than for tetrodotoxin-sensitive currents. While the voltage-dependence of fast inactivation differed significantly between the pharmacologically distinct currents, the voltage-dependence of slow inactivation was similar for both sodium currents. We conclude that slow inactivation of sodium currents can be triggered by trains of brief depolarizations. The resulting prolonged decrease in membrane excitability may contribute to the different patterns of action potential generation observed in primary afferent neurons.

Differential effects of capsaicin on rat visceral sensory neurons

Nodose neurons play an important role in the regulation of visceral function. Recent studies demonstrated that about 80% of these neurons contain messenger RNA for the capsaicin receptor, a heat-sensitive ion channel. Nodose neurons express voltage-sensitive sodium currents that can be differentiated based on their sensitivity to tetrodotoxin. Considering the potential role of tetrodotoxin-resistant sodium currents in somatosensory neurons, sodium channel expression and sodium currents were studied in nodose neurons. The results were correlated with the response to capsaicin. Nodose neurons contain messenger RNA for the tetrodotoxin-resistant sodium channel PN3. Consistent with these findings, about half of the neurons predominantly expressed tetrodotoxin-resistant sodium currents. In 54% (47/87) of the cells, capsaicin triggered an increase in intracellular calcium. Similarly, in 42% (18/43) of the cells, capsaicin elicited an inward current. There was no relationship between cell size (r=0.07) or sodium current properties (r=0.14) and the response to capsaicin. Micromolar concentrations of capsaicin inhibited voltage-dependent sodium, calcium and potassium currents. This effect was use dependent and did not involve the capsaicin receptor. In conclusion, capsaicin changed the excitability of visceral sensory neurons by blocking voltage-dependent ion channels, an effect that may contribute to the analgesic properties of capsaicin.

Changes in expression of two tetrodotoxin-resistant sodium channels and their currents in dorsal root ganglion neurons after sciatic nerve injury but not rhizotomy

Two TTX-resistant sodium channels, SNS and NaN, are preferentially expressed in c-type dorsal root ganglion (DRG) neurons and have been shown recently to have distinct electrophysiological signatures, SNS producing a slowly inactivating and NaN producing a persistent sodium current with a relatively hyperpolarized voltage-dependence. An attenuation of SNS and NaN transcripts has been demonstrated in small DRG neurons after transection of the sciatic nerve. However, it is not known whether changes in the currents associated with SNS and NaN or in the expression of SNS and NaN channel protein occur after axotomy of the peripheral projections of DRG neurons or whether similar changes occur after transection of the central (dorsal root) projections of DRG neurons. Peripheral and central projections of L4/5 DRG neurons in adult rats were axotomized by transection of the sciatic nerve and the L4 and L5 dorsal roots, respectively. DRG neurons were examined using immunocytochemical and patch-clamp methods 9-12 d after sciatic nerve or dorsal root lesion. Levels of SNS and NaN protein in the two types of injuries were paralleled by their respective TTX-resistant currents. There was a significant decrease in SNS and NaN signal intensity in small DRG neurons after peripheral, but not central, axotomy compared with control neurons. Likewise, there was a significant reduction in slowly inactivating and persistent TTX-resistant currents in these neurons after peripheral, but not central, axotomy compared with control neurons. These results indicate that peripheral, but not central, axotomy results in a reduction in expression of functional SNS and NaN channels in c-type DRG neurons and suggest a basis for the altered electrical properties that are observed after peripheral nerve injury.

Glial-derived neurotrophic factor upregulates expression of functional SNS and NaN sodium channels and their currents in axotomized dorsal root ganglion neurons

Dorsal root ganglion (DRG) neurons produce multiple sodium currents, including several different TTX-sensitive (TTX-S) currents and TTX-resistant (TTX-R) currents, which are produced by distinct sodium channels. We previously demonstrated that, after sciatic nerve transection, the levels of SNS and NaN sodium channel alpha-subunit transcripts and protein in small (18-30 micrometer diameter) DRG neurons are reduced, as are the amplitudes and densities of the slowly inactivating and persistent TTX-R currents produced by these two channels. In this study, we asked whether glial-derived neurotrophic factor (GDNF), which has been shown to prevent some axotomy-induced changes such as the loss of somatostatin expression in DRG neurons, can ameliorate the axotomy-induced downregulation of SNS and NaN TTX-R sodium channels. We show here that exposure to GDNF can significantly increase both slowly inactivating and persistent TTX-R sodium currents, which are paralleled by increases in SNS and NaN mRNA and protein levels, in axotomized DRG neurons in vitro. We also show that intrathecally administered GDNF increases the amplitudes of the slowly inactivating and persistent TTX-R currents, and SNS and NaN protein levels, in peripherally axotomized DRG neurons in vivo. Finally, we demonstrate that GDNF upregulates the persistent TTX-R current in SNS-null mice, thus demonstrating that the upregulated persistent sodium current is not produced by SNS. Because TTX-R sodium channels have been shown to be important in nociception, the effects of GDNF on axotomized DRG neurons may have important implications for the regulation of nociceptive signaling by these cells.

Vagotomy decreases excitability in primary vagal afferent somata

Standard patch-clamp and intracellular recording techniques were used to monitor membrane excitability changes in adult inferior vagal ganglion neurons (nodose ganglion neurons, NGNs) 5 days following section of the vagus nerve (vagotomy). NGNs were maintained in vivo for 5 days following vagotomy, and then in vitro for 2-9 h prior to recording. Vagotomy increased action potential (AP) threshold by over 200% (264 +/- 19 pA, mean +/- SE, n = 66) compared with control values (81 +/- 20 pA, n = 68; P < 0.001). The number of APs evoked by a 3 times threshold 750-ms depolarizing current decreased by >70% (from 8.3 to 2.3 APs, P < 0.001) and the number of APs evoked by a standardized series of (0.1-0.9 nA, 750 ms) depolarizing current steps decreased by over 80% (from 16.9 APs to 2.6 APs, P < 0.001) in vagotomized NGNs. Similar decreases in excitability were observed in vagotomized NGNs in intact ganglia in vitro studied with "sharp" microelectrode techniques. Baseline electrophysiological properties and changes following vagotomy were similar in right and left NGNs. A "sham" vagotomy procedure had no effect on NGN properties at 5 days, indicating that changes were due to severing the vagus nerve itself, not surrounding tissue damage. NGNs isolated after being maintained 17 h in vivo following vagotomy revealed no differences in excitability, suggesting that vagotomy-induced changes occur some time from 1-5 days after injury. Decreased excitability was still observed in NGNs isolated after 20-21 days in vivo following vagotomy. These data indicate that, in contrast to many primary sensory neurons that are thought to become hyperexcitable following section of their axons, NGNs undergo a marked decrease in electrical excitability following vagotomy.

Sodium channels and their genes: dynamic expression in the normal nervous system, dysregulation in disease states(1)

Although classical neurophysiological doctrine rested on the concept of the sodium channel, it is now clear that there are nearly a dozen sodium channel genes, each encoding a molecularly distinct channel. Different repertoires of channels endow different types of neurons with distinct transduction and encoding properties. Sodium channel expression is highly dynamic, exhibiting plasticity at both the transcriptional and post-transcriptional levels. In some types of neurons within the normal nervous system, e.g. hypothalamic magnocellular neurosecretory neurons, changes in sodium channel gene expression occur in association with the transition from a quiescent to a bursting state; these changes are accompanied by the insertion of a different set of sodium channel subtypes in the cell membrane, a form of molecular plasticity which results in altered electrogenic properties. Dysregulation of sodium channel genes has been observed in a number of disease states. For example, transection of the peripheral axons of spinal sensory neurons triggers down-regulation of some sodium channel genes, and up-regulation of other sodium channel genes; the resultant changes in sodium channel expression contribute to hyperexcitability that can lead to chronic pain. There is also evidence, in experimental models of demyelination and in post-mortem tissue from patients with multiple sclerosis, for dysregulation of sodium channel gene expression in the cell bodies of some neurons whose axons have been demyelinated, suggesting that an acquired channelopathy may contribute to the pathophysiology of demyelinating diseases such as multiple sclerosis. The dynamic nature of sodium channel gene expression makes it a complex topic for investigation, but it also introduces therapeutic opportunities, since subtype-specific sodium channel modulating drugs may soon be available.