Calcium-dependent after-potentials in visceral afferent neurones of the rabbit

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Localized IP3-Evoked Ca2+Release Activates a K+Current in Primary Vagal Sensory Neurons

Electrophysiological and microfluorimetric techniques were used to determine whether intracellular photorelease of caged IP3, and the consequent release of Ca2+, could trigger a Ca2+-activated K+current ( IIP3). Photorelease of caged IP3evoked an IIP3that averaged 2.36 ± 0.35 (SE) pA/pF in 24 of 28 rabbit primary vagal sensory neurons (nodose ganglion neurons, NGNs) voltage-clamped at –50 mV. IIP3was abolished by intracellular BAPTA (2 mM), a Ca2+chelator. Changing the K+equilibrium potential by increasing extracellular K+ion concentration caused a predicted Nernstian shift in the reversal potential of IIP3. These results indicated that IIP3was a Ca2+-dependent K+current. IIP3was unaffected by three common antagonists of Ca2+-activated K+currents: bath-applied iberiotoxin (50 nM) or apamin (100 nM), and intracellular 8-Br-cAMP (100 μM) included in the patch pipette. We have previously demonstrated that both IP3-evoked Ca2+release and Ca2+-induced Ca2+release (CICR) are co-expressed in NGNs and that CICR can trigger a Ca2+-activated K+current. In the present study, using caffeine, a CICR agonist, to selectively attenuate intracellular Ca2+stores, we showed that IP3-evoked Ca2+release occurs independently of CICR, but interestingly, that a component of IIP3requires CICR. These data suggest that IP3-evoked Ca2+release activates a K+current that is pharmacologically distinct from other Ca2+-activated K+currents in NGNs. We describe several models that explain our results based on Ca2+signaling microdomains in NGNs.

Imaging stochastic spatial variability of active channel clusters during excitation of single neurons

Topographical maps of membrane voltages were obtained during action potentials by imaging, at 1 microm resolution, live dissociated neurons stained with the voltage sensitive dye RH237. We demonstrate with a theoretical approach that the spatial patterns in the images result from the distribution of net positive charges condensed in the inner sites of the membrane where clusters of open ionic channels are located. We observed that, in our biological images, this spatial distribution of open channels varies randomly from trial to trial while the action potentials recorded by the microelectrode display similar amplitudes and time-courses. The random differences in size and intensity of the spatial patterns in the images are best evidenced when the time of observation coincides with the duration of single action potentials. This spatial variability is explained by the fact that only part of the channel population generates an action potential and that different channels open in turn in different trials due to their stochastic operation. Such spatial flicker modifies the direction of lateral current along the neuronal membrane and may have important consequences on the intrinsic processing capabilities of the neuron.

Calcium regulation of a slow post-spike hyperpolarization in vagal afferent neurons

Activation of distinct classes of potassium channels can dramatically affect the frequency and the pattern of neuronal firing. In a subpopulation of vagal afferent neurons (nodose ganglion neurons), the pattern of impulse activity is effectively modulated by a Ca2+-dependent K+ current. This current produces a post-spike hyperpolarization (AHPslow) that plays a critical role in the regulation of membrane excitability and is responsible for spike-frequency accommodation in these neurons. Inhibition of the AHPslow by a number of endogenous autacoids (e.g., histamine, serotonin, prostanoids, and bradykinin) results in an increase in the firing frequency of vagal afferent neurons from <0.1 to >10 Hz. After a single action potential, the AHPslow in nodose neurons displays a slow rise time to peak (0.3-0.5 s) and a long duration (3-15 s). The slow kinetics of the AHPslow are due, in part, to Ca2+ discharge from an intracellular Ca2+-induced Ca2+ release (CICR) pool. Action potential-evoked Ca2+ influx via either L or N type Ca2+ channels triggers CICR. Surprisingly, although L type channels generate 60% of action potential-induced CICR, only Ca2+ influx through N type Ca2+ channels can trigger the CICR-dependent AHPslow. These observations suggest that a close physical proximity exists between endoplasmic reticulum ryanodine receptors and plasma membrane N type Ca2+ channels and AHPslow potassium channels. Such an anatomical relation might be particularly beneficial for modulation of spike-frequency adaptation in vagal afferent neurons.

The relationship between axonal spike shape and functional modality in cutaneous C-fibres in the pig and rat

Axonal spike shape was examined in identified cutaneous C-fibres dissected from the saphenous nerves of anaesthetized pigs and rats, and was found to vary with functional class. In the pig, the action potential duration for heat nociceptor units (duration at half peak amplitude, 1.25 +/- 0.16 ms, mean +/- S.E.M., n=32) was significantly longer than the duration for polymodal nociceptive units (0.88 +/- 0.11 ms, n=32). Both classes of nociceptive C-fibre had action potentials of longer duration than the low-threshold mechanoreceptor units (0.49 +/- 0.04 ms, n=24) and the inexcitable C-fibres (0.56 +/- 0.06 ms, n=19). Undershoot durations were also longer in nociceptive than non-nociceptive C-fibres. In contrast, spike amplitudes were similar in all classes of C-afferent. In the rat, as in the pig, the polymodal nociceptor units had action potentials of longer duration (0.75 +/- 0.05 ms, n=73) than the mechanoreceptor units (0.60 +/- 0.01 ms, n=23). C-fibres identified as spontaneously active sympathetic efferent units had wider action potentials (main initial peak: 1.01 +/- 0.12 ms, n=22; undershoot: 4.1 +/- 1.23 ms, n=20) than the afferent C-fibres (main peak: 0.69 +/- 0.03 ms, n=130; undershoot: 1.4 +/- 0.09 ms, n=111). All rat C-fibre types had action potentials with main initial peaks of a similar height. However, cold thermoreceptor units had spikes with significantly smaller undershoots compared to nociceptive or inexcitable C-fibres. It is concluded that there are clear differences in axonal spike shape between the different functional classes of C-fibre and, in particular, that nociceptive C-afferents tend to have axonal action potentials of longer duration than non-nociceptive afferents. The ion channels responsible for the longer duration action potentials may provide a target for the development of highly selective analgesic drugs.

Properties and functions of calcium-activated K+ channels in small neurones of rat dorsal root ganglion studied in a thin slice preparation

1. Properties, kinetics and functions of large conductance calcium-activated K+ channels (BKCa) were investigated by the patch-clamp technique in small neurones (Adelta- and C-type) of a dorsal root ganglion (DRG) thin slice preparation without enzymatic treatment. 2. Unitary conductance of BKCa channels measured in symmetrical high K+ solutions (155 mM) was 200 pS for inward currents, and chord conductance in control solution was 72 pS. Potentials of half-maximum activation (V ) of the channels were linearly shifted by 43 mV per log10 [Ca2+]i unit (pCa) in the range of -28 mV (pCa 4) to +100 mV (pCa 7). Open probabilities increased e-times per 15-32 mV depolarization of potential. 3. In mean open probability, fast changes with time were mainly observed at pCa > 6 and at potentials > +20 mV, without obvious changes in the experimental conditions. 4. BKCa channels were half-maximally blocked by 0.4 mM TEA, measured by apparent amplitude reductions. They were completely blocked by 100 nM charybdotoxin and 50 nM iberiotoxin by reduction of open probability. 5. Two subtypes of small DRG neurones could be distinguished by the presence (type I) or absence (type II) of BKCa channels. In addition, less than 10 % of small neurones showed fast (approximately 135 V s-1) and short ( approximately 0.8 ms) action potentials (AP). 6. The main functions of BKCa channels were found to be shortening of AP duration, increasing of the speed of repolarization and contribution to the fast after-hyperpolarization. As a consequence, BKCa channels may reduce the amount of calcium entering a neurone during an AP. 7. BKCa channel currents suppressed a subsequent AP and prolonged the refractory period, which might lead to a reduced repetitive activity. We suggest that the BKCa current is a possible mechanism of the reported conduction failure during repetitive stimulation in DRG neurones.

Cyclosporine A modulation of Ca++ activated K+ channels in cardiac sensory afferent neurons

Whole-cell and single channel recordings were used to characterize the effects of the immunosuppressant cyclosporine A (CsA) on cardiac sensory neurons (CSN) of the nodose ganglia. Application of 10 nM CsA resulted in a 29.1% decrease in CSN input resistance and an average -8+/-3 mV hyperpolarization of membrane potential. Application of 10 nM CsA had no effect on evoked Ca++ currents but increased evoked K+ currents by 158.9+/-24%. Application of 10 nM CsA significantly increased the open probability of KCa channels by 183+/-9%. These results suggest that application of CsA results in the activation of KCa channels in cardiac sensory neurons and this effect may contribute to the cellular mechanisms underlying CsA modulation of vagal afferent neurons.

Ca2+-induced Ca2+ release mediates a slow post-spike hyperpolarization in rabbit vagal afferent neurons

The relation between Ca2+-induced Ca2+ release (CICR) elicited by action potentials (APs) and a Ca2+-dependent slow post-spike hyperpolarization (AHPslow) in acutely dissociated adult rabbit nodose neurons was studied using microfluorimetric calcium measurements in conjunction with standard intracellular current- and voltage-clamp recording techniques. The magnitude of the AP-induced transient increase in [Ca2+]i (DeltaCat) was used to monitor CICR. There was a close correlation between the magnitude of the DeltaCat and the AHPslow current over the range of 1-16 APs (r = 0.985). Functional CICR blockers, ryanodine (10 muM), thapsigargin (100 nM), 2,5-di(t-butyl)hydroquinone (10 muM) or cyclopiazonic acid (10 muM), selectively reduced the peak amplitude of the AHPslow >/=91%. In five neurons, simultaneous recordings of the DeltaCat and the AHPslow revealed that both responses were blocked in parallel. These findings indicate that CICR is necessary for the generation of the AHPslow in rabbit nodose neurons. The DeltaCat rises and decays significantly faster than the AHPslow. This temporal disparity suggests that activation of the AHPslow by Ca2+ may require additional signal transduction steps.

Histamine H1 receptor activation blocks two classes of potassium current, IK(rest) and IAHP, to excite ferret vagal afferents

1. Intracellular recordings were made in intact and acutely dissociated vagal afferent neurones (nodose ganglion cells) of the ferret to investigate the membrane effects of histamine. 2. In current-clamp or voltage-clamp recordings, histamine (10 microM) depolarized the membrane potential (10 +/- 0.8 mV; mean +/- S.E.M.; n = 27) or produced an inward current of 1.6 +/- 0.35 nA (n = 27) in approximately 80% of the neurones. 3. Histamine (10 microM) also blocked the post-spike slow after-hyperpolarization (AHP slow) present in 80% of these neurones (95 +/- 3.2%; n = 5). All neurones possessing AHPslow in ferret nodose were C fibre neurones; all AHPslow neurones had conduction velocities < or = 1 m s-1 (n = 7). 4. Both the histamine-induced inward current and the block of AHPslow were concentration dependent and each had an estimated EC50 value of 2 microM. These histamine-induced effects were mimicked by the histamine H1 receptor agonist 2-(2-aminoethyl) thiazole dihydrochloride (10 microM) and blocked by the H1 antagonists pyrilamine (100 nM) or diphenhydramine (100 nM). Schild plot analysis of the effect of pyrilamine on the histamine-induced inward current revealed a pA2 value of 9.7, consistent with that expected for an H1 receptor. Neither impromidine (10 microM) nor R(-)-alpha-methylhistamine (10 microM), selective H2 or H3 agonists, respectively, significantly affected the membrane potential, input resistance or AHPslow. 5. The reversal potential (Vrev) for the histamine-induced inward current was -84 +/- 2.1 mV (n = 4). The Vrev for the histamine response shifted in a Nernstian manner with changes in the extracellular potassium concentration. Alterations in the extracellular chloride concentration had no significant effect on the Vrev of the histamine response (n = 3). The Vrev for the AHPslow was -85 +/- 1.7 mV (n = 4). 6. These results indicate that histamine increases the excitability of ferret vagal afferent somata by interfering with two classes of potassium current: the resting or 'leak' potassium current (IK(rest)) and the potassium current underlying a post-spike slow after-hyperpolarization (IAHP). Both these effects can occur in the same neurone and involve activation of the same histamine receptor subtype, the histamine H1 receptor.

Tetraethylammonium-sensitive calcium-sensitive potassium current in a subclass of the bullfrog dorsal root ganglion cells

Bullfrog dorsal root ganglion (DRG) cells were classified into three types, As, Ar and C, according to their electrophysiological properties. Actions of tetraethylammonium (TEA; 100 microM-100 mM) on As-type cells were examined using current- and voltage-clamp methods; TEA caused a membrane depolarization or an inward current, associated with a decrease in membrane conductance. These TEA-induced responses reversed in polarity at -85 to -90 mV, and the change in reversal potential followed the Nemst equation as extracellular K+ concentration was changed. The TEA-induced responses were reversibly inhibited by Ca(+2)-free/high-Mg+2 solutions and inorganic Ca blockers. It is concluded that bullfrog DRG As-type cells might be also endowed with Ca-sensitive K channels which may be open at rest and blocked by TEA.

Activity-dependent slowing of conduction velocity provides a method for identifying different functional classes of C-fibre in the rat saphenous nerve

Repetitive firing of nerve fibres results in the slowing of their conduction velocity. The extent of conduction velocity slowing throughout a standard electrical stimulus (20 s, 20 Hz, 2 x electrical threshold) was examined in identified C-fibres dissected from the saphenous nerve of anaesthetized rats. The aim of this study was to establish whether the different functional classes of C-fibre could be identified on the basis of their activity-dependent slowing of conduction velocity. Following 20 s of stimulation at 20 Hz, nociceptive C-fibres showed a significantly greater slowing of conduction velocity (mean +/- S.E.; polymodal and heat nociceptors = 29.2% +/- 0.7, n = 53; mechanical nociceptors = 27.7% +/- 1.7, n =13) than cold thermoreceptive fibres (10.8% +/- 0.6, n = 10), mechanoreceptors (14.4% +/- 0.8, n = 17) and spontaneously active sympathetic efferent units (14.9% +/- 0.8, n = 24). The degree of conduction velocity slowing shown by a unit was not correlated with its resting conduction velocity. There was little overlap of the degree of conduction velocity slowing between the nociceptive and non-nociceptive fibres. Also, there was little overlap of conduction velocity slowing between the mechanoreceptors and the cold units, particularly after just 6 s of stimulation at 20 Hz. Units for which no receptive field to mechanical or thermal stimuli could be found showed a bimodal distribution of conduction velocity slowing. In the saphenous nerve, such inexcitable units will be of three main types--sympathetic efferent units, "sleeping" or "silent" nociceptors and non-cutaneous afferent fibres. Those inexcitable units slowing in conduction velocity by greater than 20% showed a similar distribution to the polymodal nociceptors and those inexcitable units slowing by less than 20% showed a similar distribution to the spontaneously active sympathetic units. Twenty-three of the 61 units without mechanical or thermal receptive fields were investigated using electrical skin stimulation and topical application of 5 or 10% mustard oil. Afferent fields could not be found for any of the nine units that slowed in conduction velocity by less than 20%. Afferent fields were detected for 11 of the remaining 14 insensitive units, which all showed a greater than 20% slowing from resting conduction velocity. Therefore, one can distinguish nociceptive and non-nociceptive afferent fibres simply by looking at the axonal property of activity-dependent slowing of conduction velocity. Moreover, it is possible to use this axonal property to separate the two classes of non-nociceptive afferent C-fibre (i.e. mechanoreceptors and cold thermoreceptors). In addition, one can also use this parameter to differentiate between the afferent and non-afferent populations of inexcitable C-fibres. The ability to identify a particular fibre type on the basis of an axonal property provides a useful tool for the functional classification of fibres in experiments where axons are separated from their terminals.

Role of a Ca(2+)-dependent slow afterhyperpolarization in prostaglandin E2-induced sensitization of cultured rat sensory neurons

To determine if inhibition of a Ca(2+)-dependent slow afterhyperpolarization (AHPslow) contributes to prostaglandin E2 (PGE2)-induced sensitization of DRG neurons, we have used patch-clamp electrophysiological techniques on cultured dorsal root ganglion (DRG) neurons from the adult rat. In support of a role for AHPslow in sensitization of DRG neurons, we demonstrate that: (1) AHPslow expression is restricted to a subpopulation of putative nociceptors; (2) burst duration is controlled by AHPslow in these neurons; and (3) in some neurons, PGE2 decreases AHPslow and produces a concomitant increase in the number of action potentials generated in response to depolarizing current injection. However, our results also demonstrate that AHPslow modulation is not sufficient to explain PGE2-induced sensitization in the majority of DRG neurons because: (1) the size of the population of DRG neurons expressing AHPslow is less than half the size of the population of DRG neurons sensitized by PGE2; (2) PGE2 produces a decrease in action potential threshold as well as an increase in the number of action potentials in response to current injection, while inhibition of AHPslow has little effect on threshold; and (3) the sensitizing effects of PGE2 are dissociated from its effects on AHPslow in more than half of neurons tested. We conclude that PGE2-induced sensitization must involve the modulation of ionic currents in addition to that underlying AHPslow.

Excitable properties and underlying Na+ and K+ currents in neurons from the guinea-pig jugular ganglion

Neurons in the superior vagal (jugular) ganglion relay afferent information from thoracic visceral organs and may be important in inflammatory processes due to the peripheral release of bioactive neuropeptides such as substance P. We characterized the excitable properties and underlying voltage-gated Na+ (INa) and K+ (IKv) currents in acutely dissociated guinea pig jugular ganglion neurons with microelectrode and whole-cell patch-clamp recording techniques. Current clamp recordings revealed a resting potential of approx. -55 mV and input resistance of approx. 100 M ohms. Brief depolarizing steps evoked an overshooting action potential (approx. 2 ms duration), fast (< 20 ms duration) afterhyperpolarization (AHPF) sequence in all neurons, followed by a slow (> 1 s) Cd(2+)-sensitive afterhyperpolarization (AHPS) in 45% of the neurons. The AHPS was implicated in limiting repetitive action potential firing during maintained depolarizing steps. The action potential in 15/17 neurons, and a major component of the whole cell INa in 13/13 neurons were insensitive to TTX (1-10 microM), indicating that jugular neurons express predominantly a TTX-resistant type of INa. Cd2+ (200 microM) did not affect action potential repolarization, while tetraethylammonium (TEA; 10 mM) in the presence of Cd2+ markedly prolonged action potential repolarization, and blocked the AHPF in 11/11 neurons. This suggested that the action potential repolarization and the AHPF are mediated by IKv, with little contribution by Ca(2+)-dependent IK (IK(Ca)). Whole cell IKv activated rapidly (tau < 1.5 ms), and inactivated variably over a time period of seconds. IKv activation and inactivation voltage dependencies and TEA sensitivity were compatible with its availability during the action potential and AHPF. Only 1/26 neurons exhibited current with the rapid inactivation kinetics and voltage-dependencies characteristic of classic IA-type current. These results highlight differences in the properties of jugular neurons (e.g., deficiency of rapid IA, and lack of a TTX-sensitive subpopulation), relative to those known for other visceral and somatic afferents, and thus provide a basis for further functional studies.

Fluorescence imaging of local membrane electric fields during the excitation of single neurons in culture

The spatial distribution of depolarized patches of membrane during the excitation of single neurons in culture has been recorded with a high spatial resolution (1 micron2/pixel) imaging system based on a liquid-nitrogen-cooled astronomical camera mounted on an inverted microscope. Images were captured from rat nodose neurons stained with the voltage-sensitive dye RH237. Conventional intracellular microelectrode recordings were made in synchrony with the images. During an action potential the fluorescence changes occurred in localized, unevenly distributed membrane areas, which formed clusters of depolarized sites of different sizes and intensities. When fast conductances were blocked by the addition of tetrodotoxin, a reduction in the number and the intensities of the depolarized sites was observed. The blockade by tetrodotoxin of voltage-clamped neurons also reduced the number of depolarized sites, although the same depolarizing voltage step was applied. Similarly, when a voltage-clamped neuron was depolarized by a constant-amplitude voltage step, the number of depolarized sites varied according to the degree of activation of the voltage-sensitive channels, which was modified by changing the holding potential. These results suggest that the spatial patterns of depolarization observed during excitation are related to the operations of ionic channels in the membrane.

A-current modifies the spike of C-type neurones in the rabbit nodose ganglion

1. In the rabbit nodose ganglion, C-type fibre neurones (C neurones) can be divided into two subtypes according to their after-hyperpolarizing potential (AHP) i.e. those with a fast AHP only and those with a fast AHP and a subsequent slow AHP produced by a slow calcium-dependent potassium current. In addition we have shown that some C neurones can be divided into two groups according to the effect of membrane hyperpolarization on their spikes i.e. type 1 in which duration and amplitude do not change and type 2 in which duration and amplitude decrease significantly. 2. In the present report we studied the effect of A-current (IA) on spike duration, amplitude and slow AHP using intracellular recording techniques. 3. To detect the presence of IA, we first applied a series of increasing rectangular hyperpolarizing pulses to remove IA inactivation and then a short depolarizing pulse to trigger a spike. In type 1 C neurones the lag time of the spike in relation to hyperpolarization remains constant whereas in type 2 C neurones the spike only appears after IA inactivation and lag time in relation to hyperpolarization is lengthened. Thus, type 2 C neurones have an IA while type 1 C neurones do not. The fact that addition of cadmium did not change the lag time in type 2 C neurones shows that the IA is not calcium dependent. 4. Nodose neurones can be orthodromically activated by stimulation of the vagal peripheral process. In this way, after a hyperpolarizing pulse, IA can be fully activated by the orthodromic spike itself. Under these conditions it is possible to analyse the effects of IA on the spike. This was done by increasing either the hyperpolarizing potential, pulse duration, or the delay of the spike after the end of the pulse. We observed that maximum IA inactivation removal was always associated with the lowest duration and amplitude of the spike. 5. When IA inhibitors, 4-aminopyridine (4-AP) or catechol, were applied to type 2 C neurones, the delay of the spike after the hyperpolarization-depolarization test was no longer observed. In addition 4-AP abolished the shortening of the duration of the spike induced by steady hyperpolarization. 6. In type 2 C neurones with slow AHP, the IA-related decrease in spike duration was associated with a disappearance of the slow AHP. This indicates that IA decreases the calcium influx during the spike.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanical stimulation increases intracellular calcium concentration in nodose sensory neurons

The cellular mechanisms involved in activation of mechanosensitive visceral sensory nerves are poorly understood. The major goal of this study was to determine the effect of mechanical stimulation on intracellular calcium concentration ([Ca2+]i) using nodose sensory neurons grown in culture. Primary cultures of nodose sensory neurons were prepared by enzymatic dispersion from nodose ganglia of 4-8 week old Sprague-Dawley rats. Whole cell [Ca2+]i was measured by a microscopic digital image analysis system in fura-2 loaded single neurons. Brief mechanical stimulation of individual nodose sensory neurons was achieved by deformation of the cell surface with a glass micropipette. In 31 of 50 neurons (62%), mechanical stimulation increased [Ca2+]i from 125 +/- 8 to 763 +/- 89 nM measured approximately 10 s after stimulation. [Ca2+]i then declined gradually, returning to near basal levels over a period of minutes. [Ca2+]i failed to increase after mechanical stimulation in the remaining 19 neurons. The mechanically-induced rise in [Ca2+]i was essentially abolished after the neurons were incubated for 5-10 min in zero Ca2+ buffer (n = 7) or after addition of gadolinium (10 microM), a blocker of stretch-activated ion channels (n = 5). The effect of gadolinium was reversed after removal of gadolinium. The results indicate that: (1) mechanical stretch increases [Ca2+]i in a subpopulation of nodose sensory neurons in culture, and (2) the stretch-induced increase in [Ca2+]i is dependent on influx of Ca2+ from extracellular fluid and is reversibly blocked by gadolinium. The findings suggest that opening of stretch-activated ion channels in response to mechanical deformation leads to an increase in Ca2+ concentration in visceral sensory neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of the excitatory actions of bradykinin by inhibition of PGI2 formation in nodose neurones of the guinea-pig

1. Intracellular recordings were made from neurones in intact guinea-pig nodose ganglia in vitro and from acutely isolated adult guinea-pig and rabbit nodose neurons to study a bradykinin (BK)-mediated block of a slow spike after-hyperpolarization (AHPslow) that is prominent in 30-40% of these neurones. 2. BK (100 nM) reversibly blocked the AHPslow, resulting in an ablation of the spike accommodative properties of these neurones. The B1 BK receptor agonist [des-Arg9]-BK did not mimic or prevent the actions of BK. In contrast, the B2 BK receptor antagonist D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]-BK (HOE 140) prevented BK-induced block of the AHPslow and the effect of BK on spike frequency adaptation. 3. The BK block of the AHPslow in acutely dissociated neurones was prevented by indomethacin, indicating that this BK effect was dependent upon a cyclo-oxygenase metabolite intrinsic to these neurones. 4. One to twenty femtomoles of the prostanoids PGE2, PGD2, 9 alpha, 11 beta-PGF2 (a metabolite of PGD2), PGF2 alpha, TxB2 and PGI2 were released spontaneously from a nodose ganglion in 15 min. BK (100 nM) selectively increased PGI2 release 2.8-fold without affecting the release of the other prostanoids. Treatment with 10 microM tranylcypromine (TCP), a putative PGI2 synthase inhibitor, completely prevented the BK-induced release of PGI2. 5. In the presence of 10 microM TCP, BK no longer produced significant effects on the AHPslow. In contrast, 10 microM TCP did not prevent PGI2 from blocking the AHPslow. 6. These results suggest that vagal afferents that exhibit AHPslow also possess the B2 type of BK receptor. Activation of these BK receptors results in the production of PGI2, which in turn controls spike frequency adaptation by affecting the amplitude of the AHPslow.

Nitric oxide regulates spike frequency accommodation in nodose neurons of the rabbit

A Ca(2+)-dependent slow spike after hyperpolarization (AHPslow) is present in about 35% of the neurons in the nodose ganglion. Although the AHPslow profoundly affects spike frequency accommodation of these neurons, the mechanisms that control the generation and the duration of the AHPslow are unclarified. N omega-Nitro-L-arginine methyl ester (L-NAME; 10 microM), a specific inhibitor of nitric oxide synthase (NOS), reduced the AHPslow by more than 92%. The L-NAME block of the AHPslow was antagonized by application of 50 microM S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide donor. The fast, Ca(2+)-dependent, spike after hyperpolarization preceding the AHPslow and the elevation of intracellular Ca2+ accompanying the AHPslow were unaffected by L-NAME treatment. These findings indicate that products of NOS activity might directly or indirectly activate the AHPslow K+ channels at a step beyond Ca2+ influx or intracellular Ca2+ mobilization.

Calcium-activated potassium channels in rat visceral sensory afferents

The purpose of the present study was to describe, at the single-channel level, the activity of a calcium-sensitive potassium channel in rat visceral-sensory neurons which has been suggested to be involved in sensory neuron excitability. Single-channel recordings in the inside-out configuration identified a 220 pS conductance calcium-activated potassium channel (KCa). From a -20 mV holding potential, increasing [Ca2+]i from 0.01 microM to 1.0 microM increased the open probability of this channel 92% (from 0.12 to 0.23). However, from a +20 mV holding potential, increasing [Ca2+]i from 0.01 to 1.0 microM increased the open probability by 326% (from 0.15 to 0.64). In addition, this large conductance KCa channel was blocked by TEA (1.0 microM) and charybdotoxin (40 microM) when applied to the external surface. These results are the first to characterize a large conductance KCa channel in the sensory afferent neurons of the rat nodose ganglia and should further expand the understanding of the ionic currents involved in the regulation of sensory afferent neuronal activity.

An intermediate conductance calcium-activated potassium channel in rat visceral sensory afferent neurons

Whole-cell and single channel recordings were used to characterize an intermediate conductance calcium-activated potassium (KCa) channel in sensory neurons of the nodose ganglion. From a -80 mV holding potential, the total outward current in these neurons was increased when extracellular calcium was raised from 0.02 to 5 mM. This calcium-evoked outward current was not blocked by either charybdotoxin (50 nM) or apamine (40 nM). In the inside-out patch configuration, the current-voltage relationship for this channel was linear between -60 and +60 mV in symmetrical 145 mM potassium aspartate (KAsp) and possessed a conductance of approximately 60 picosiemens (pS). Increasing [Ca2+]i from 0.01 microM to 1.0 microM markedly increased the cumulative open probability of this channel and the effect of increasing [Ca2+]i on these channels was not voltage dependent. In the outside-out patch configuration, neither tetraethylammonioum (TEA), (1 mM), apamine (40 nM) or charybdotoxin (ChTx) (50 nM) had any effect on the activity of this channel. These results provide new evidence for the existence of pharmacologically distinct intermediate conductance KCa channel in sensory afferent neurons.

Electrophysiological properties and chemosensitivity of acutely isolated nodose ganglion neurons of the rabbit

Neurons in the nodose ganglion were enzymatically dissociated from adult rabbits and electrophysiologically examined within 2 h. About 90% of the neurons survived the dissociation procedure. Their resting membrane potential (-59 +/- 0.6 mV), time constant (5.7 +/- 0.5 ms), input resistance (80 +/- 5.3 M omega), action potential properties (amplitude, 79 +/- 1.2 mV; overshoot 21 +/- 1.0 mV; duration 5.2 +/- 0.1 ms; threshold, -38 +/- 0.9 mV; and fast spike afterhyperpolarization, 10 +/- 0.5 mV, 64 +/- 3.8 ms) were not different from values obtained from nodose neurons in intact ganglion in vitro and in vivo. An unusually long duration spike afterhyperpolarization observed in about 35% of nodose neurons in intact ganglion was demonstrated in about 28% of the acutely isolated neurons. The percentage of isolated neurons responding to bath applied acetylcholine, bradykinin, serotonin and histamine was similar to those reported for neurons in the intact ganglia. We conclude that the electrophysiological and chemoreceptive properties of acutely isolated adult rabbit nodose neurons are similar to those observed for neurons in intact ganglia. Thus the availability of large numbers of acutely isolated adult nodose neurons should allow multidisciplinary studies of vagal afferent C-fiber neurons not readily obtainable in the intact ganglion.

Electrophysiological properties and chemosensitivity of guinea pig nodose ganglion neurons in vitro

Conventional intracellular recording techniques were employed to obtain information on the electrophysiological and pharmacological characteristics of C-type neurons in the guinea pig nodose ganglia. Approximately 90% of the cell bodies gave rise to axons with conduction velocities consistent with C-fibers (0.9-1.1 m/s). The average resting membrane potential and input impedence was about -60 mV and 45 M sigma, respectively. Orthodromic electrical stimulation of the vagus nerve 20-30 mm caudal to the ganglion produced overshooting action potentials in the nodose neurons. The falling phase of the action potential was followed by a transient (50-300 ms) fast hyperpolarization (AHPfast). In 20% of C-type neurons the AHPfast was followed by a slowly developing, long-lasting afterhyperpolarization (AHPslow) that limited the ability of the neuron to fire action potentials at high frequency. The AHPslow magnitude was dependent on the number of spikes, had a reversal potential of -87 mV, and was abolished by 100 microM cadmium chloride, suggesting that it is produced by a calcium-dependent potassium current. In about 30% of the nodose neurons, hyperpolarizing current steps from resting potential produced a time- and voltage-dependent anomalous rectification in the electrotonic potential. External cesium (1 mM), but not barium (100 microM) reversibly blocked this effect. Single-electrode voltage-clamp measurements revealed a slowly developing inward current in these neurons that grows in magnitude with step hyperpolarizations from resting potential, and has an estimated reversal potential of about -44 mV. These properties suggest that this current is analogous to IH observed in many peripheral and central neurons. Autacoids including serotonin, histamine, several prostanoids, peptidoleukotriene, and bradykinin, were examined for their ability to affect the excitability of the nodose neurons. Serotonin was the only autacoid capable of depolarizing the membrane potential to action potential firing threshold. The serotonin-induced membrane depolarization was associated with a significant increase in input conductance. Histamine depolarized the membrane potential of the C-type neurons in 28/30 neurons. Bradykinin, prostacyclin, and leukotriene C4 were found to cause membrane depolarizations in a subset (73%, 31%, and 50%, respectively) of nodose neurons. The AHPslow was virtually abolished by bradykinin, prostacyclin, and in a subset of neurons, leukotriene C4. Inhibition of the AHPslow was accompanied by a change in the accommodative properties of the neurons, reflected by the increased frequency at which the neuron could successfully elicit repetitive action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of subpopulations of rat dorsal root ganglion neurons in vitro

Intracellular recordings were made from rat dorsal root ganglion neurons in vitro. Action potentials with an inflection on the falling phase occurred in all cells conducting up to 5.2 m/s and in a proportion of faster conducting cells which decreased with increasing conduction velocity, until no cells conducting faster than 31 m/s had an inflection. Overall, all C-cells (less than 1.3 m/s), 61% of A delta-cells (1.3-12 m/s) and 23% of A alpha/beta-cells (greater than 12 m/s) had inflections. A-cells with inflections were found to be electrophysiologically distinct from those without as they differed in the mean and distribution of every action potential and afterhyperpolarization parameter measured. C-cells differed from all A-cell groups, but the means and distributions of most parameters were closer to those of A-cells with inflections than of A-cells without. In addition, all A- and C-cell action potentials with inflections were tetrodotoxin resistant, while all those without were sensitive. The only parameters whose means differed between A alpha/beta- and A delta-neurons were ones which correlated with conduction velocity (action potential duration, overshoot and maximum rate of rise and fall). The response pattern to prolonged current injection did not correlate with conduction velocity, but slightly more A-cells with inflections were single firing. A-cells with long afterhyperpolarizations always fired singly, while those with shorter durations fired singly or multiply. Somatic following frequency was most strongly limited by long afterhyperpolarization duration; it was also slightly lower in A delta- than in A alpha/beta-cells, and lower in A-cells with inflections than in those without. Fibre following frequencies were highest in the fastest conduction neurons.

Electrophysiological and morphological properties of type C vagal neurons in the nodose ganglion of the cat

Some passive and active electrical properties of type C neurons were studied intracellularly, in situ, in the nodose ganglia of adult cats. From the neuronal responses to hyperpolarizing and depolarizing rectangular current pulses it was possible to determine the input resistance (34.4 M omega) and specific membrane resistance (2373 omega.cm2). Significant changes in magnitude and duration of the action potential evoked by vagal stimulation result from changes in the resting potential caused by the passage of steady polarizing currents across the cell membrane. The action potentials evoked by infranodose vagal stimulation had a long duration, a long latency and comprised several components. The fast main spike was followed by a long post-hyperpolarization. The double shock technique showed that the fast main potential was composed of an initial segment spike ('A spike') and a somatic spike ('S spike'), and made it possible to determine the somatic refractory periods. After electrical identification of the cells, horseradish peroxidase was injected ionophoretically into the soma, and it was shown that the central processes were about four times smaller in diameter than the peripheral processes.

Calcium-dependent slow outward current in visceral primary afferent neurones of the rabbit

Slow outward currents were recorded from voltage-clamped neurones in nodose ganglia excised from rabbits. In the majority of Type C neurones, a short depolarizing command pulse evoked a slow outward tail current (ISAH) with a decay time constant ranging from 0.5 to 2 s. The ISAH was due to an increase in membrane conductance to K+ because its reversal potential was approximately equal to the Nernst potential for K+. The ISAH was reversibly blocked by removal of external Ca2+ or by Ca2+ antagonists. A Ca2+ ionophore, A23187, produced an outward current which was similar to the ISAH. The ISAH was resistant to tetraethylammonium and depressed by Ba2+, whereas it was not affected by Cs+ and 4-aminopyridine. The ISAH was initially augmented and subsequently depressed by apamin (1-10 nM) and (+)-tubocurarine (100-600 microM). It is concluded that the ISAH in visceral primary neurones may be due to a long-lasting increase in K+ conductance caused by an increase in the concentration of intracellular Ca2+, resulting from Ca2+ entry during the depolarizing command pulse.

Somatostatin presynaptically inhibits transmitter release in feline parasympathetic ganglia

Intracellular recordings were made from neurons in cat parasympathetic ciliary ganglia in vitro. Somatostatin (30 nM-3 microM) reduced the amplitude of excitatory postsynaptic potentials (EPSPs), whereas the peptide did not affect acetylcholine (ACh)-induced depolarizations. Thus somatostatin depressed the EPSPs without changing the postsynaptic sensitivity to ACh. The inhibitory action of somatostatin on the EPSPs was passed off even in the presence of the peptide at concentrations higher than 100 nM. When paired stimuli at an interval of 50 ms were applied to preganglionic nerves, the second EPSP was facilitated, being larger in amplitude than the first one; this facilitation was reversibly inhibited in the presence of the peptide. Somatostatin reversibly reduced the frequency of spontaneous EPSPs without appreciably changing their mean amplitude. All of these results indicate that somatostatin may presynaptically reduce the amount of ACh released. The mechanism underlying this action was discussed.

Long-duration spike afterhyperpolarizations in neurons from the guinea pig superior cervical ganglion

Intracellular recordings from guinea pig superior cervical ganglia maintained in vitro have revealed a protracted spike afterhyperpolarization in approximately 18% of the principal neurons that is substantially longer (greater than 1 s duration) than those previously reported. This long afterhyperpolarization is distinct from shorter duration afterpotentials in these cells because it can be selectively and reversibly blocked by cooling and by bath applied histamine. Cellular excitability is increased when the long-duration afterhyperpolarization is abolished and thus it deserves consideration as a site for modulation of synaptic transmission through the superior cervical ganglion.

Inhibition of calcium-dependent spike after-hyperpolarization increases excitability of rabbit visceral sensory neurones

1. Conventional intracellular recordings were made from rabbit nodose neurones in vitro. Prostaglandins D2 and E2, but not F2 alpha, produced a selective, concentration-dependent (1-100 nM) inhibition of a slow, Ca2+-dependent spike after-hyperpolarization (a.h.p.). Block of the slow a.h.p. was accompanied by an increased membrane resistance and a small (less than 10 mV) depolarization of the membrane potential. Inhibition of the slow a.h.p. produced no change in the voltage-current relationship other than the increased membrane resistance. 2. In C neurones with slow a.h.p.s, trains of brief depolarizing current pulses (2 ms duration, 0.1-10 Hz) could not elicit repetitive action potentials without failure at rates above 0.1 Hz. By contrast, C neurones without slow a.h.p.s could respond at stimulus frequencies up to 10 Hz. The frequency-dependent spike firing ability of slow a.h.p. neurones was eliminated by inhibition of the slow a.h.p. 3. Action potentials were also evoked by intrasomatic injection of paired, depolarizing current ramps (1 nA/10 ms, 0.1-5 s inter-ramp interval). For neurones without a slow a.h.p., the current threshold and number of evoked spikes were the same for both ramps, and the ramps were nearly superimposable. In neurones with a slow a.h.p., the current threshold for the first spike in the second ramp was greatly increased (300-500%) and the number of evoked spikes was reduced. Following inhibition of the slow a.h.p., the current threshold and number of evoked spikes was the same for both ramps. 4. Forskolin, a direct activator of the catalytic subunit of adenylate cyclase, also produced a concentration-dependent inhibition of the slow a.h.p., with 50% block at 30 nM. Prostaglandin D2 and forskolin produced identical enhancement of excitability in C neurones and neither substance produced any effect on C neurones that could not be attributed to inhibition of the Ca2+-dependent K+ conductance associated with the slow a.h.p. We propose that, in some visceral sensory neurones, the level of excitability is regulated by cyclic AMP-mediated control of the slow a.h.p.

The Ca2+-sensitive K+-currents underlying the slow afterhyperpolarization of bullfrog sympathetic neurones

Ca2+-sensitive K+ currents involved in the slow afterhyperpolarization (a.h.p.) of an action potential of bullfrog sympathetic neurones were studied with a single-electrode voltage clamp method. The outward tail current (IAH) generated after the end of a depolarizing command pulse (from the holding potential of -60 mV to 0 mV, 5-20 ms in duration), mimicking an action potential, was separated into at least two exponential components (IAHf and IAHs). They were identified as K+ currents, since their reversal potentials were close to the K+ equilibrium potential and they were sensitive to external K+. The time constant of IAHf (tf; 44 ms at -60 mV) was decreased by membrane hyperpolarization from -40 to -80 mV, while that of IAHs (ts; 213 ms) remained constant. Removal of external Ca2+ or addition of Cd2+ significantly decreased the IAHs amplitude (As) and tf without a change in ts and the IAHf amplitude (Af). On the other hand, increasing Ca2+ influx by applying repetitive command pulses enhanced both Af and As with negligible effects on tf and ts, and produced a much slower component. Intracellular injection of EGTA reduced Af with no effect on tf, and increased As with a decreased ts. Both muscarine and (+/-)-tubocurarine, which reduced IAHs, hardly affected IAHf. These results indicate that a.h.p. is induced by the activation of two distinct Ca2+-dependent K+ channels, which differ in voltage sensitivity, Ca2+-dependence and pharmacology.

The mechanism of action of capsaicin on sensory C-type neurons and their axons in vitro

The selective excitant and neurotoxic action of capsaicin on vagal sensory neurons in the rat has been investigated in vitro using three techniques: extracellular recording of compound spike potentials from the whole nerve; intracellular recording from ganglion cells using single-electrode current and voltage clamp; and electron microscopy of the nerve and nodose ganglion. Capsaicin (0.1-10 microM) depolarized vagal sensory C fibres and cell bodies, and produced an increased conductance. The conductance increase appeared to be due to an increased permeability to sodium and calcium, plus a secondary increase in potassium (and perhaps chloride) conductance consequent upon calcium entry. The early entry of calcium seems to be a significant priming event in the neurotoxic process, since dramatic ultrastructural changes take place within a few minutes of capsaicin application, which are minimized by removing extracellular calcium ions. The observations indicate that in sensory C neurons capsaicin opens a conductance of limited specificity and that a resultant large calcium entry is closely involved in the rapid development of cell injury.

Ouabain augments calcium-dependent potassium conductance in visceral primary afferent neurones of the rabbit

1. The effects of ouabain (1 nM-100 microM) on the membrane properties of rabbit visceral primary afferent neurones (nodose ganglion cells) were studied with intracellular recordings and voltage-clamp techniques in vitro. 2. Ouabain (greater than or equal to 1 microM) often produced a membrane hyperpolarization associated with a fall of membrane resistance in type C neurones. The ouabain-induced hyperpolarization reversed in polarity at about -90 mV. These suggest that the ouabain-induced hyperpolarization is due to an increase in potassium conductance. 3. Both the peak amplitude and the duration of the after-hyperpolarization following an action potential were reversibly increased with increasing concentration of ouabain. In tetraethylammonium (TEA, 10-20 mM) and tetrodotoxin (TTX, 1-10 microM), the duration of both the calcium-dependent action potential and the after-hyperpolarization following the action potential was prolonged by ouabain (greater than or equal to 10 nM). 4. A depolarizing command pulse evoked a slow outward current in TEA (10-20 mM) and TTX (1-10 microM). This was increased in amplitude and prolonged in duration by ouabain (100 nM-1 microM). Such augmentation of the slow outward current by ouabain was usually associated with an increase in a slow inward current during the period of the depolarizing command pulse. 5. An outward current produced by the calcium ionophore A23187 was reversibly augmented by ouabain (greater than or equal to 10 nM). 6. An outward current caused by exchanging a potassium-free superfusion solution for one containing 4.7 or 10 mM-potassium was completely abolished by ouabain (greater than or equal to 10 nM). 7. The hyperpolarization elicited by intracellular injection of calcium was reversibly prolonged by either ouabain (1 microM) or caffeine (10 nM). 8. These results suggest that ouabain augments the after-hyperpolarization both by an increase in calcium influx across the cellular membrane and by an increase in intracellular calcium concentration.

Tetrodotoxin-resistant sodium current of rat nodose neurones: monovalent cation selectivity and divalent cation block

1. Monovalent cation selectivity and divalent cation sensitivity of the tetrodotoxin (TTX)-resistant Na+ current in dissociated adult rat nodose ganglion neurones were investigated using the whole-cell patch-clamp technique. 2. The TTX-resistant Na+ current was isolated using ion substitution and pharmacological agents. Under these conditions, the current reversal potential shifted 52 mV per tenfold change in external [Na+]. 3. Inorganic and organic monovalent cation permeability ratios (Px/PNa) were determined from changes in reversal potential and the Goldman-Hodgkin-Katz equation. The Px/PNa values determined by the former method were HONH3+, 1.38; Li+, 1.00; H2NNH3+, 0.66; NH4+, 0.28; CH3NH3+, less than 0.13; K+, less than 0.13; Rb+, less than 0.12; Cs+, less than 0.10; (CH3)4N+, less than 0.10. The values determined by either method agreed within 10%. 4. The effects of Cd2+, Co2+, Mn2+ and Ni2+ on the TTX-resistant Na+ current were analysed from peak-conductance values. These ions shifted the activation of the current to more positive potentials and decreased the maximal conductance. At 3 mM concentrations, Cd2+, Ni2+, Co2+ and Mn2+ decreased the maximal conductance 64.6, 50.7, 25.0 and 20.3%, respectively. 5. The results indicate that: (a) the monovalent cation selectivity of the TTX-resistant Na+ current is similar to that of the TTX-sensitive Na+ current in other tissues; and (b) the TTX-resistant Na+ current is less sensitive to divalent cations than the Ca2+ current in these neurones. These observations suggest that the structure determining the monovalent cation permeability of the TTX-resistant Na+ current is similar to that of the TTX-sensitive Na+ current in other tissues, and that the channels carrying the TTX-resistant Na+ current are distinct from those responsible for the Ca2+ current.

Intracellular studies of the electrophysiological properties of cultured intracardiac neurones of the guinea-pig

1. The electrophysiological properties of intracardiac neurones cultured from ganglia within the atria and interatrial septum of the new-born guinea-pig heart were studied using intracellular micro-electrodes. 2. Three types of neurones with resting membrane potentials in the range -45 to -76 mV were detected. The first type, AHs cells, had high (15-28 mV) firing thresholds, pronounced slow post-spike after-hyperpolarizations and fired only once to prolonged intrasomal current injection. The second type, AHm cells, were similar to AHs cells, except that they could fire short bursts of spikes (100-400 ms) at the onset of current injection. The third type, M cells, had low firing thresholds (10-15 mV), no slow after-hyperpolarizations and produced non-adapting trains of action potentials in response to depolarizing current injection. 3. The generation of action potentials in M cells was prevented by tetrodotoxin (TTX; 0.3 microM), whereas in AHs and AHm cells action potentials displayed a channel blockade using solutions containing the divalent cations cadmium, cobalt or manganese (0.02-1 mM). 4. The post-spike after-hyperpolarization in AHs and AHm cells was abolished by the removal of extracellular calcium, shortened in solutions containing the calcium entry blockers CdCl2, MnSO4 and CoCl2 (0.02-1 mM) and prolonged by the addition of calcium (5.0 mM), tetraethylammonium (1-3 mM), 4-aminopyridine (1-3 mM), cyanide (10 microM) or caffeine (100 microM) to the perfusate. 5. The reversal potential of the post-spike after-hyperpolarization was -89.1 mV. This value changed by 62.9 mV for a 10-fold increase in extracellular potassium concentration. 6. The peak conductance change during the post-spike after-hyperpolarization (gK,Ca), was largely independent of membrane potential between -50 and -110 mV. The peak increase in gK,Ca and the duration of the after-hyperpolarization increased with the number of spikes preceding it. 7. It is concluded that calcium entry during the action potential is responsible for the activation of an outward potassium current in the two types of AH cells; the roles played by intracellular calcium extrusion as well as sequestration mechanisms in the generation of the response are discussed.

Low concentrations of GABA reduce accommodation in primary afferent neurons by an action at GABAB receptors

The pattern of accommodation of spike activity during sustained membrane depolarization was investigated in primary afferent neurons recorded intracellularly in vitro in the rat. We show that gamma-aminobutyric acid (GABA) and baclofen reduce accommodation in some fast conducting dorsal root ganglion neurons. This effect was restricted to those A delta cells with axons displaying a rather fast conduction velocity (15-25 m/s). GABA-induced blockade of accommodation was not observed in large A beta neurons. Pharmacological studies with baclofen, as opposed to isoguvacine, indicate that this effect is due to GABAB receptors activation. The effect is also shown to be resistant to bicuculline antagonism. In slow conducting afferents, GABAB receptor activation is known to shorten the CA2+ component of action potentials. By contrast, no such component was observed in the A delta cells studied. Furthermore, Ca2+-activated K+ conductances are not implicated in the reduction of accommodation caused by GABAB receptor activation. In conjunction with the actual knowledge about the distribution of GABA receptors on primary afferents, our result indicates that GABAA and GABAB receptors coexist on all categories of A delta and C primary afferents in the rat.

Prostaglandins block a Ca2+-dependent slow spike afterhyperpolarization independent of effects on Ca2+ influx in visceral afferent neurons

The blockade of a slow Ca2+-activated K+-dependent afterhyperpolarization (AHPs) in rabbit visceral sensory neurons by the prostaglandins, PGE1 and PGD2, was investigated to determine whether the blockade was indirectly due to a reduction in Ca2+ influx. The prostaglandins (PGs) could block the AHPs in the absence of any change in Ca2+-dependent spikes elicited in the presence of tetrodotoxin and tetraethylammonium bromide. A PG-induced decrease in Ca2+-dependent spike width observed in some neurons was temporally dissociated from the PG-induced block of the AHPs. In addition, a slow afterhyperpolarization produced by the application of the Ca2+ ionophore, A23187, was blocked by the PGs. It is concluded that a reduction in Ca2+ influx is not responsible for the PG-induced blockade of the AHPs.

On the potassium conductance increased by opioids in rat locus coeruleus neurones

Intracellular recordings were made from locus coeruleus neurones in slices cut from rat pons and superfused in vitro. Membrane currents were recorded with a single-electrode switch-clamp amplifier. Opioids, enkephalin analogues or morphine, caused a concentration-dependent potassium current, which had a maximum value of about 300 pA at -60 mV. The opioid-sensitive potassium conductance was independent of membrane potential between -60 and -130 mV, but became less as the membrane potential was changed from -60 to -30 mV. The opioid outward current was reduced by quinine (100 microM-1 mM) and barium (30 microM-2 mM), but not by 4-aminopyridine (100 microM-1 mM) or tetraethylammonium (10 mM). A potassium current with similar properties flowed for several seconds after a burst of action potentials; this appeared to result from calcium entering the neurone during the action potentials. The alpha 2-adrenoceptor agonists noradrenaline and clonidine caused a concentration-dependent potassium conductance increase which had the same maximum value as that caused by opioids in the same neurones. Experiments in which an opioid and an alpha 2-adrenoceptor agonist were superfused together indicated that the same potassium conductance is increased by both agonists.