Post-tetanic Hyperpolarization Research Articles

Suppression of spikes during posttetanic hyperpolarization in auditory neurons: the role of temperature, Ih currents, and the Na+-K+-ATPase pump

In vivo recordings from postsynaptic neurons in the medial nucleus of the trapezoid body (MNTB), an auditory brain stem nucleus, show that acoustic stimulation produces a burst of spikes followed by a period of hyperpolarization and suppressed spiking activity. The underlying mechanism for this hyperpolarization and reduced spiking is unknown. Furthermore, the mechanisms that control excitability and resting membrane potential are not fully determined for these MNTB neurons. In this study we investigated the excitability of principal neurons from the MNTB after high-frequency afferent fiber stimulation, using whole cell recordings from postnatal day 15-17 rat brain stem slices. We found that Na(+)-K(+)-ATPase activity mediates a progressive hyperpolarization during a prolonged tetanic train and a posttetanic hyperpolarization (PTH) at the end of the train, when postsynaptic action potentials failed to fire. Raising the temperature to more physiological levels (from 22 to 35°C) depolarized the resting membrane potential of both presynaptic and postsynaptic cells and decreased the latency of action potential firing during PTH. Higher temperatures also reduced the presynaptic calyx action potential failure rates by 50% during presynaptic PTH, thus increasing the safety-factor for presynaptic spiking. The effect of temperature on hyperpolarization-activated cation current (I(h)) is reflected in the resting potential at both pre- and postsynaptic neurons. We thus propose that temperature-sensitive Na(+)-K(+)-ATPase activity and I(h) contribute to set the resting membrane potential and produce a brief period of suppressed spiking (or action potential failures) after a prolonged high-frequency afferent tetanus.

Accumulation of K+ Ions in the Perinodal Space as a Factor Regulating the Functional State of Myelinated Nerve Fibers

Experimental studies on intact nodes of Ranvier in isolated nerve fibers led to the conclusion that the accumulation of K+ ions released into the perinodal space during excitation performs a series of important functions: it decreases K+ release from the axoplasm, induces prolonged post-spike depolarization and its associated exaltation phase, underlies repeated and rhythmic activity, activates the electrogenic membrane Na+,K+-pump, and determines the generation of the prolonged component of post-tetanic hyperpolarization, which is an additional factor responsible for reabsorption of potassium ions into the cytoplasm of nerve fibers, etc.

Presynaptic Ca2+ buffers control the strength of a fast post-tetanic hyperpolarization mediated by the α3 Na+/K+-ATPase

The excitability of CNS presynaptic terminals after a tetanic burst of action potentials is important for synaptic plasticity. The mechanisms that regulate excitability, however, are not well understood. Using direct recordings from the rat calyx of Held terminal, we found that a fast Na(+)/K(+)-ATPase (NKA)-mediated post-tetanic hyperpolarization (PTH) controls the probability and precision of subsequent firing. Notably, increasing the concentration of internal Ca(2+) buffers or decreasing Ca(2+) influx led to larger PTH amplitudes, indicating that an increase in [Ca(2+)](i) regulates PTH via inhibition of NKAs. The characterization for the first time of a presynaptic NKA pump current, combined with immunofluorescence staining, identified the alpha3-NKA isoform on calyx terminals. Accordingly, the increased ability of the calyx to faithfully fire during a high-frequency train as it matures is paralleled by a larger expression of alpha3-NKA during development. We propose that this newly discovered Ca(2+) dependence of PTH is important in the post-burst excitability of nerve terminals.

Roles of Ca2+, hyperpolarization and cyclic nucleotide-activated channel activation, and actin in temporal synaptic tagging.

At crayfish neuromuscular junctions, cAMP increases transmitter released by action potentials by activating two effectors, hyperpolarization and cyclic nucleotide-activated channels (HCNCs) and a separate target that has been tentatively identified as exchange protein activated by cAMP (Epac). Intense electrical activity in the motor neuron induces a long-term facilitation (LTF) of transmitter release in which hyperpolarization from an electrogenic Na+-K+ exchanger activates HCNCs. The coupling of HCNCs to transmission involves actin. After LTF induction, cAMP further increases transmission in an HCNC-independent manner, activating the second target. This relaxation of the requirement for HCNC activation to enhance release is called temporal synaptic tagging. Tagging lasts at least 1 d but develops only in the 10 min period after electrical activity. The HCNCs are activated by the post-tetanic hyperpolarization occurring during this time. Both synaptic tagging and LTF induction depend on presynaptic Ca2+ accumulation during activity; both are blocked by EGTA-AM, and LTF is also prevented by stimulation in a low-[Ca2+] medium. Actin depolymerizers prevent induction of LTF and tagging, with little effect on HCNCs, whose sensitivity to cAMP and HCNC blockers is unaffected by tagging. Enhancement of actin polymerization can rescue tagging from HCNC block, suggesting that actin acts at a step after HCNC activation. These and other recent results suggest a model in which HCNC activation, followed by a process involving actin polymerization, acts cooperatively with [Ca2+] to induce tagging, after which only Epac activation is required for cAMP to further enhance transmission.

Axon–glia interactions modulate axonal excitability in mammalian unmyelinated nerves

A period of electrical activity in unmyelinated nerve fibers is followed by a post-tetanic hyperpolarization (PTH), generated by the hyperactivity of the electrogenic Na +–K + pump. In order to protect the membrane potential against these strong hyperpolarizations, different types of axonal inward currents are activated during the PTH. We investigated in the rabbit vagus nerve one of these currents, which was activated by carbamylcholine (CCh). We observed that the effect of CCh on the PTH amplitude could be blocked or reversed with scopolamine. Moreover, the PTH amplitude increased when scopolamine alone was added to the perfusate, indicating that an endogenous muscarinic agonist was liberated in the preparation during the period of electrical activity. This CCh-activated current was TEA but not Ba 2+ or Cs + sensitive. It has been shown previously that muscarinic acetylcholine receptors (mAChRs) in the rabbit vagus nerve are located on the axonal but not glial membrane and that Schwann cells express several types of purinergic receptors, which activation evoke Ca 2+ transients in Schwann cells. We hypothesise that during electrical activity axons release a transmitter, presumably ATP. This transmitter evoke in the neighbouring Schwann cells a Ca 2+-dependent liberation of a endogenous muscarinic agonist, which in turn activates a TEA-sensitive inward current in axons. We suggest that the major purpose of this mechanism is the control of the membrane potential during and after a period of intense electrical activity when the Na +–K + pump generates a robust PTH.

Differential contribution of Na +-K + pump and K + conductance to the post-tetanic hyperpolarization of the subtypes of tetrodotoxin-resistant C-fibers in the isolated bullfrog sciatic nerve

The post-tetanic hyperpolarizations (PTHPs) of the fast and slow tetrodotoxin-resistant (TTX-R) C-fibers were recorded from isolated bullfrog sciatic nerves using a modified vaseline gap method. Under control solutions, the monophasic fast and slow TTX-R C-fibers compound action potentials (C-CAPs) increased in amplitude up to 133.9±17.9% (mean±SD; n=9) and 167.0±41.5% ( n=7), respectively, shortly after tetanic stimulation. The recovery time constants of the fast and slow ones were 66.5 and 101.2 s, respectively. Significant differences between the rates of amplitude augmentation of the fast and slow TTX-R C-CAPs were found for the most pairs of measured points ( P<0.05 or P<0.01). After application of ouabain solutions, the augmentation of the fast TTX-R C-CAP was inhibited to an insignificant level, while that of the slow TTX-R C-CAP remained significant with the recovery time constant being 90.9 s. After additional application of TEA, the post-tetanic augmentation of the slow TTX-R C-CAP was blocked. These results suggest that Na +-K + pump contributes to the PTHPs of the fast and slow TTX-R C-fibers and that K + conductance contributes only to the PTHP of the slow TTX-R C-fibers, indicating differential modulation of the firing rate of each TTX-R C-fibers group.

Axonal and glial currents activated during the post-tetanic hyperpolarization in non-myelinated nerve.

Changes in membrane potential and potassium concentration in the extracellular space ([K+]e) of rabbit vagus nerve were measured simultaneously during electrical activity and during the period of recovery using a modified sucrose-gap method and potassium-sensitive microelectrodes. After stimulation for 15 s at 15 Hz the main activity-induced increase in [K+]e reached 16.9 mM. This increase in [K+]e was paralleled by a depolarization of the preparation. The period of activity was followed by a post-tetanic hyperpolarization (PTH) lasting tens of seconds, generated by the axonal electrogenic Na+-K+ pump and to a lesser extent by the pump of the surrounding Schwann cells. The amplitude of the PTH dramatically increased in experiments in which inward currents were blocked by removal of Cl– or after application of Cs+ or Ba2+, indicating that under normal conditions the current generated by the Na+-K+ pump is strongly short-circuited. A pharmacological and kinetic study showed that these currents are: (1) the hyperpolarization-activated current I h, and (2) the inwardly rectifying I KIR current. The results show that the latter originates from Schwann cells. Our data indicate that in non-myelinated nerves there is a subtle association of inward ionic channels which (1) helps the cell to maintain an optimal membrane potential after a period of activity, and (2) contributes to the removal of excess K+ from the extracellular space.

Evidence for the involvement of Na +-K + pump and K + conductance in the post-tetanic hyperpolarization of the tetrodotoxin-resistant C-fibers in the isolated bullfrog sciatic nerve

The post-tetanic hyperpolarizations (PTHPs) were recorded from the isolated bullfrog sciatic nerves using a modified vaseline gap method. Repetitive stimulation of tetrodotoxin-resistant (TTX-R) C-fibers produced frequency- and activity-dependent PTHPs. The ouabain solutions (0.1 mM), the solutions (0.1 mM ouabain+2 mM tetraethylammonium (TEA)) and 5 mM TEA solutions reduced the PTHP amplitude to 56.2±10.7% (mean±SD, n=9), to 37.0±12.3% ( n=8), to 76.0±10.9% ( n=5) of the control value, respectively. Each pair of the three solutions revealed a significant difference in their effects on the PTHP ( P<0.01). Furthermore, the Li–Ringer solutions and the solutions (Li–Ringer+2 mM TEA) reduced the PTHP amplitude as well, thus resembling ouabain effects. These results strongly suggest that the repetitive stimulation of TTX-R C-fibers activates the membrane Na +-K + pump resulting in a major part of the PTHP and induces TEA-sensitive K + conductance composing a minor part of the PTHP, which might modulate repetitive firing of TTX-R C-fibers.

A specific blocker reveals the presence and function of the hyperpolarization-activated cation current I h in peripheral mammalian nerve fibres

Electrotonic responses recorded extra- or intracellularly from peripheral nerve preparations show a “sag” to hyperpolarizing current pulses. [10, 11, 20]The biophysical nature of this “inward rectification” is still under discussion since the phenomenon has not been noted at voltage-clamped single nerve fibres, and since Cs +, which reduces inward rectification, is not a specific ion channel blocker. In this study, we found that low micromolar concentrations of ZD 7288, a specific blocker of the hyperpolarization-activated cationic current (I h) in the soma of central mammalian neurones, [8]result in a complete block of inward rectification in the electrotonic responses of isolated rat spinal dorsal roots. In addition, ZD 7288 enhanced the activity-dependent slowing of conduction seen in compound C fibre action potentials of isolated rat vagus nerves and augmented the post-tetanic hyperpolarization following trains of action potentials in unmyelinated and myelinated axons. The data suggest that ZD 7288 is a potent blocker and a useful research tool for the study of hyperpolarization-activated inward rectification (I h) of peripheral nerve preparations.

Altered expression of delayed excitation in medial NTS neurons of spontaneously hypertensive rats

The spontaneously hypertensive (SH) rat has an exaggerated sympathetic discharge which may result from an enhanced neuronal excitability in the central nervous system. To test this hypothesis, we examined the electrophysiological properties of neurons in the medial region of the nucleus of the solitary tract (mNTS), a central nucleus involved in the processing of baroreceptor afferent information, in SH rats and normotensive Sprague–Dawley (SD) rats. An in vitro brainstem slice preparation was used to record intracellularly from 18 neurons in 4–5-month-old SH rats and 16 neurons in 4–5-month-old SD rats. Between the two groups there were no significant differences in resting membrane potential, input resistance, and spontaneous firing frequency, or in action potential amplitude, duration, and after-hyperpolarization (AHP). There were also no significant differences in spike frequency adaptation and post-tetanic hyperpolarization (PTH). Delayed excitation (DE), a manifestation of A-current, occurred in 88% of SH and SD mNTS neurons, but the duration of DE was significantly (P<0.05) shorter in SH mNTS neurons. We propose that attenuated expression of A-current may contribute to increased sympathetic drive in SH rats.

Electrogenic pump and a Ca(2+)- dependent K+ conductance contribute to a posttetanic hyperpolarization in lamprey sensory neurons.

1. Tetanic stimulation of lamprey sensory dorsal cells resulted in a posttetanic hyperpolarization (PTH). The amplitude and duration of the PTH were dependent on the stimulus duration and frequency. The PTH was not reversed at membrane potentials negative to -100 mV, whereas the afterhyperpolarization following single action potentials reversed at approximately -85 mV. There was also a biphasic effect on the input resistance during the PTH, with an early reduction that recovered to control before the PTH had decayed. 2. The amplitude and duration of the PTH were increased in Ringer solution containing tetraethylammonium and 4-aminopyridine, both of which broadened single action potentials, but were reduced after intracellular injection of Cs+. Ca(2+)-free Ringer solution, Cd2+, and Co2+ also reduced the PTH, suggesting the involvement of a Ca(2+)-dependent K+ conductance. However, the PTH was not reduced in Ba2+ Ringer solution, or by the Ca(2+)-dependent K+ channel antagonists apamin and charybdotoxin. 3. The cardiac glycoside ouabain reduced the amplitude and duration of the PTH, as did substitution of Na+ with choline or Li+. K(+)-free Ringer solution also reduced the PTH, whereas high-K+ Ringer solution had more variable effects. The amplitude and duration of the PTH were also dependent on temperature. These results support the involvement of an ouabain-sensitive Na-K pump in the PTH. 4. The PTH was reduced by the tachykinins substance P and physalaemin, and by 5-hydroxytryptamine, which blocks apamin-sensitive Ca(2+)-dependent K+ channels in the lamprey. However, gamma-aminobutyric acid, which has been reported to reduce a Ca(2+)-dependent K+ conductance in the dorsal cells, did not reduce the PTH. 5. These results suggest that a Ca(2+)-dependent K+ conductance and an Na-K electrogenic pump underlie the PTH. The PTH reduces the excitability of the dorsal cells, suggesting that it may act as a mechanism to gate sensory information entering the spinal cord.

Postsynaptic induction of long-term synaptic facilitation in snail central neurones.

Long-term facilitation in molluscs is believed to be induced due to purely presynaptic activations. We recorded excitatory postsynaptic potentials (EPSPs) simultaneously from two identified neurones of snail parietal ganglia. We report a non-decrementing facilitation induced by intracellular tetanization with concomitant presynaptic activation. The mean EPSP amplitude measured in 10 neurones 30-50 min after tetanization was 17% greater than in the non-tetanized control neurones. Only short-lasting (5-10 min) postsynaptic changes were found (post-tetanic hyperpolarization and resistance decrease). The facilitation was especially prominent (34%, 10 min post-tetanus) but decreased within 15 min in preparations with rapid wash-out of the external media. The data suggest that induction of long-term enhancement in molluscs depends on postsynaptic events and, like in mammals, may involve increased postsynaptic Ca2+ and subsequent release of retrograde messengers.

Posttetanic hyperpolarization produced by electrogenic Na(+)-K+ pump in lizard axons impaled near their motor terminals.

1. The hyperpolarization that follows tetanic stimulation was recorded intra-axonally from the internodal region of intramuscular myelinated motor axons. 2. The peak amplitude of the posttetanic hyperpolarization (PTH) that followed stimulation at 20-100 Hz for < or = 35 s increased with increasing train duration, reaching a maximum of 22 mV. PTH decayed over a time course that increased from tens to hundreds of seconds with increasing train duration. For a given frequency of stimulation the time integral of PTH was proportional to the number of stimuli in the train, averaging 3-4 mV.s per action potential. 3. Ouabain (0.1-1 mM) and cyanide (1 mM) depolarized the resting potential and abolished PTH. Tetanic stimulation in ouabain was followed by a slowly decaying depolarization (probably due to extra-axonal K+ accumulation) whose magnitude and duration increased as the duration of the train increased. 4. Axonal input resistance showed no consistent change during PTH in normal solution but increased during PTH in the presence of 3 mM Cs+ (which blocks axonal inward rectifier currents). 5. PTH was abolished when bath Na+ was replaced by Li+ or choline. PTH persisted after removal of bath Ca2+ and addition of 2 mM Mn2+. 6. Removal of bath K+ abolished the PTH recorded after brief stimulus trains and greatly reduced the duration of PTH recorded after longer stimulus trains. 7. A brief application of 10 mM K+, which normally depolarizes axons, produced a ouabain-sensitive hyperpolarization in axons bathed in K(+)-free solution. 8. These observations suggest that in these myelinated axons PTH is produced mainly by activation of an electrogenic Na(+)-K(+)-ATPase, rather than by changes in K+ permeability or transmembrane [K+] gradients. This conclusion is supported by calculations showing agreement between estimates of Na+ efflux/impulse based on PTH measurements and estimates of Na+ influx/impulse based on nodal voltage-clamp measurements. Pump activity also appears to contribute to the resting potential. 9. The stimulus intensity required to initiate a propagating action potential increased during PTH but decreased during the posttetanic depolarization recorded in ouabain. Thus changes in axonal excitability after tetanic stimulation correlate with changes in the posttetanic membrane potential. 10. Action potentials that propagated during PTH had a larger peak amplitude and were followed by a larger and longer depolarizing afterpotential than action potentials elicited at the resting potential. This enhancement of the depolarizing afterpotential is consistent with previous reports of an increased superexcitable period after action potentials evoked during PTH.

Effects of aging on the intrinsic membrane properties of medial NTS neurons of Fischer-344 rats.

1. Recent studies have demonstrated that the arterial baroreflex is imparied with aging and have implicated central components of the baroreflex arc in this autonomic dysfunction. Neurons in the medial portion of the nucleus tractus solitarius (mNTS) receive a major input from the arterial baroreceptors. The present study was undertaken to characterize the intrinsic membrane properties of mNTS neurons in young rats and to test the hypothesis that these properties are altered with aging. An in vitro brain stem slice preparation was used to record intracellularly from mNTS neurons; passive membrane properties, action potential characteristics, and repetitive firing properties were examined and compared. 2. Neurons in the mNTS of young (3-5 mo old) Fischer-344 rats (F-344; n = 35) had a resting membrane potential of -57 +/- 6.9 mV (mean +/- SD), a membrane time constant of 18 +/- 9.0 ms, and an input resistance of 110 +/- 60 m omega. Action potential amplitude was 81 +/- 7.5 mV with a duration at half-height of 0.83 +/- 0.15 ms. The spontaneous firing rate in 24 cells was 4.3 +/- 2.9 Hz. The amplitude and duration of the action potential afterhyperpolarization (AHP) were 6.6 +/- 3.0 mV and 64 +/- 34 ms, respectively. All neurons expressed spike frequency adaptation, action potential AHP, and posttetanic hyperpolarization. Delayed excitation and postinhibitory rebound were present in 34 and 14% of neurons tested, respectively. Neurons from adult (10-12 mo old) F-344 rats (n = 34) were similar to the young F-344 rats with respect to all of these variables. 3. Neurons from aged (21-24 mo old) F-344 (n = 32) were similar to those from young and adult rats, but there were two potentially important differences: the mean input resistance of the aged neurons was higher (170 +/- 150 M omega), with a larger proportion (46% of aged neurons vs. 20% of young neurons and 21% of adult neurons) having input resistances > 150 M omega; and there was a tendency for a smaller percentage of aged neurons (16% of aged neurons vs. 34% of young neurons and 29% of adult neurons) to express delayed excitation. 4. The potential significance of a high input resistance was tested by comparing the steady-state current-voltage (I-V) relationships and the frequency-current (f-I) relationships among low-resistance (1-100 M omega), medium-resistance (101-200 M omega).(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic mechanism for heterosynaptic, posttetanic depression in area CA1 of rat hippocampus.

Conditioning stimulation applied to afferent fibers in stratum radiatum or stratum oriens of hippocampal area CA1 produced heterosynaptic, posttetanic depression (PTD) of excitatory postsynaptic potentials (EPSPs). PTD amounted to a 60-80% reduction of EPSPs and recovered over a 5 min period. Conditioning stimulation also induced a posttetanic hyperpolarization (PTH) averaging 4 mV and decaying over a 1-1.5 min period. PTH was accompanied by a large reduction in input resistance. We sought to determine the pre- or postsynaptic locus of heterosynaptic PTD. Our results suggest that PTD reflects a presynaptic mechanism: (1) PTD was observed for both N-methyl-D-aspartate (NMDA) and non-NMDA receptor mediated EPSPs; (2) Direct depolarization of pyramidal cells, substituted for the synaptic depolarization induced by conditioning stimulation, did not elicit PTD; (3) PTD and PTH were differentially affected by pharmacological and postsynaptic manipulations; (4) Conditioning stimulation depressed responses to pressure applied glutamate, but the magnitude and duration were too small to account for PTD. Since afferent fiber volleys were not depressed following conditioning stimulation, while field EPSPs were, we conclude that conditioning stimulation suppresses synaptic release of glutamate.

Functional effects of a hyperpolarization-activated membrane current in the lobster stretch receptor neurone.

The functional effects of a hyperpolarization-activated membrane current (IQ) in the slowly adapting lobster stretch receptor neurone were investigated. From comparisons between changes in membrane excitability due to blockage of IQ by Cs+, in normally impaled and native unimpaled (Edman et al. 1987 b) cells, it could be concluded that the resting voltage of native cells is distinctly more negative than -65 mV (average membrane voltage of impaled cells) and, therefore, under the control of an activated IQ. Starting from this conclusion, impaled cells were polarized to holding (resting) voltages around -75 mV and their polarization and excitability properties studied after tetanic impulse activity and variation of various external influences (K+, pH, temperature), both in control conditions and after blockage of IQ by 2 mM Cs+. It was found that an unblocked IQ (a) greatly accelerates the initial (90%) decay of post-tetanic hyperpolarization, and (b) depresses distinctly any polarization and excitability alterations due to increases in extracellular K+ concentration (from 2.5 to 10 mM), variations in extracellular pH (between 6.4 and 8.6), and changes in temperature (between 14 and 24 degrees C). It was inferred that in well polarized cells, IQ plays a role as a stabilizer of membrane polarization and excitability in conditions of varying external influences. From a model study of IQ it could be concluded that, with its slow dynamic responses, the current is well adapted to its functional purposes and to the rather slow homeostatic effects of the cell's Na-K pump.

Electrophysiological properties of neurons within the nucleus ambiguus of adult guinea pigs

1. The purpose of this study was to determine the electrophysiological properties of neurons within the region of the nucleus ambiguus (NA), an area that contains the ventral respiratory group. By the use of an in vitro brain stem slice preparation, intracellular recordings from neurons in this region (to be referred to as NA neurons, n = 235) revealed the following properties: postinhibitory rebound (PIR), delayed excitation (DE), adaptation, and posttetanic hyperpolarization (PTH). NA neurons were separated into three groups on the basis of their expression of PIR and DE: PIR cells (58%), DE cells (31%), and Non cells (10%). Non cells expressed neither PIR nor DE and no cells expressed both PIR and DE. 2. PIR was a transient depolarization that produced a single action potential or a burst of action potentials when the cell was released from hyperpolarization. In the presence of tetrodotoxin (TTX), the maximum magnitude of PIR was 7-12 mV. Under voltage-clamp conditions, hyperpolarizing voltage steps elicited a small inward current during the hyperpolarization and a small inward tail current on release from hyperpolarization. These currents, which mediate PIR, were most likely due to Q-current because they were blocked with extracellular cesium and were insensitive to barium. 3. DE was a delay in the onset of action potential firing when cells were hyperpolarized before application of depolarizing current. When cells were hyperpolarized to -90 mV for greater than or equal to 300 ms, maximum delays ranged from 150 to 450 ms. The transient outward current underlying DE was presumed to be A-current because of the current's activation and inactivation characteristics and its elimination by 4-aminopyridine (4-AP). 4. Adaptation was examined by applying depolarizing current for 2.0 s and measuring the frequency of evoked action potentials. Although there was a large degree of variability in the degree of adaptation, PIR cells tended to express less adaptation than DE and Non cells. Nearly three-fourths of all NA neurons adapted rapidly (i.e., 50% adaptation in less than 200 ms), but PIR cells tended to adapt faster than DE and Non cells. PTH after a train of action potentials was relatively rare and occurred more often in DE cells (43%) and Non cells (33%) than in PIR cells (13%). PTH had a magnitude of up to 18 mV and time constants that reflected the presence of one (1.7 +/- 1.4 s, mean +/- SD) or two components (0.28 +/- 0.13 and 4.1 +/- 2.2 s).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of nifedipine on neurons of guinea pig celiac ganglion

The effect of nifedipine on electrophysiological membrane properties and nicotinic neurotransmission of guinea pig celiac ganglion neurons was studied using intracellular recordings in vitro. Nifedipine in concentrations of 0.1–10 μM did not affect membrane potential, membrane input resistance or the amplitude and duration of action potentials induced by intracellular current injection. Higher doses of nifedipine (0.1–1 mM) significantly reduced the amplitude and extended the duration of action potentials induced by intracellular current injection. Superfusion of the ganglia with nifedipine in concentrations of 0.1–10 μM significantly inhibited nicotinic fast excitatory postsynaptic potentials (f-EPSPs) and orthodromic action potentials evoked by nerve stimulation. This depressant effect of nifedipine on synaptic transmission was eliminated with high Ca 2+ (12.5 mM). Nifedipine (10 μM) did not affect the postsynaptic effect of exogenous acetylcholine (ACh), but significantly reduced the quantal content but not the quantal size of evoked f-EPSPs in a low Ca 2+ (0.5 mM), high Mg 2+ (5.5 mM) Krebs solution. Nifedipine in concentration of 10 μM did not affect afterspike hyperpolarization (AH) and post-tetanic hyperpolarization (PTH), which have been recognized to be generated mainly by an increase of calcium-dependent potassium conductance. Higher doses of nifedipine (0.1–1 mM) significantly depressed AH and PTH. These experimental results suggest that nifedipine in concentrations of 0.1–10 μM exerts an inhibitory effect on nicotinic neurotransmission without affecting the membrane properties of the guinea pig celiac ganglion neurons. This inhibitory effect of nifedipine on synaptic transmission may result from blocking L-type calcium channels and reducing the quantal release of ACh from the presynaptic nerve terminals.

A prolonged post-tetanic hyperpolarization in rat hippocampal pyramidal cells in vitro

The post-tetanic sequelae of trains of synaptic stimuli (50 pulses at 5 or 10 Hz) were studied with intracellular recordings from rat hippocampal neurons in vitro. In a large proportion of CA1 neurons, stimulation of afferent fibers was followed by a prolonged membrane hyperpolarization (peak amplitude approximately 6 mV) that was associated with a decrease in neuronal input resistance (approximately 33%) that lasted from tens of seconds to over 1 min. Antidromic stimulation or activation of cells with intracellular current injection did not elicit this post-tetanic hyperpolarization (PTH). The PTH could be elicited in chloride (Cl −)-loaded cells, its null potential shifted in response to changes in extracellular potassium ([K +] o), and it was significantly reduced by 5–10 mM extracellular cesium (Cs +). The K +-dependent PTH may also be calcium (Ca 2+) dependent as its amplitude and associated conductanceincrease were sensitive to changes in [Ca 2+] o. The PTH was enhanced by treatments that increase Ca 2+ entry into cells including perfusion with elevated [Ca 2+] o, with picrotoxin or with tetraethylammonium ion (TEA). The K + conductance blocker 4-AP had no consistent effect on the PTH. The PTH was potently blocked by the membrane-permeant forms of cAMP, dibutyryl- and 8-bromo-cAMP. However, phorbol esters that activate protein kinase C and carbachol, which usually block the same potential that is blocked by cAMP, did not depress the PTH. The cardiac glycosides dihydro-ouabain and strophanthidin had only small and variable effects on the PTH. We suggest that prolonged synaptic stimulation activates a novel K + conductance that may be important in regulating the membrane potential during periods of repetitive neuronal firing in many hippocampal neurons.

A novel action of deltamethrin on membrane resistance in mammalian skeletal muscle and non-myelinated nerve fibres

The pyrethroids, deltamethrin and cismethrin, were assessed for their ability to change membrane conductance directly in skeletal muscle and indirectly in non-myelinated nerve fibre preparations from the rat. In diaphragm muscle fibres of the rat, input resistance was significantly increased (35%) by deltamethrin but not by cismethrin, compared with solvent alone. In perfused vagus nerve from the rat, the amplitude of the post-tetanic hyperpolarization was significantly increased (100%) by deltamethrin both in vitro and ex vivo but not by cismethrin or solvent. In both test systems the actions of deltamethrin were abolished by changing the perfusate to a low chloride solution. The enhancement of amplitude of post-tetanic hyperpolarization by deltamethrin was reversed by ivermectin, a compound known to increase the resting chloride flux in neuronal preparations. Depolarizing afterpotentials, indicative of a prolongation in sodium conductance, did not develop until 0.5–1.0 hr after the enhancement of the amplitude of post-tetanic hyperpolarization by deltamethrin in the vagus preparations. The amplitude of post-tetanic hyperpolarization was not enhanced by exposure of the vagus to veratrine. These observations reinforce the conclusion that the enhancement of post-tetanic hyperpolarization by deltamethrin is not the result of intracellular accumulation of sodium. In addition, the chloride-dependent nature of the effects of deltamethrin, in both muscle and non-myelinated nerve, suggests that they are in both cases due to a reduction in resting membrane chloride conductance. This novel action of deltamethrin would be expected to amplify the effect of prolonged sodium current and thus influence the actions on excitable membranes both directly and indirectly.