Presynaptic modulation of transmitter release by the early outward potassium current in Aplysia

Past and Future of Analog-Digital Modulation of Synaptic Transmission

Action potentials (APs) are generally produced in response to complex summation of excitatory and inhibitory synaptic inputs. While it is usually considered as a digital event, both the amplitude and width of the AP are significantly impacted by the context of its emission. In particular, the analog variations in subthreshold membrane potential determine the spike waveform and subsequently affect synaptic strength, leading to the so-called analog-digital modulation of synaptic transmission. We review here the numerous evidence suggesting context-dependent modulation of spike waveform, the discovery analog-digital modulation of synaptic transmission in invertebrates and its recent validation in mammals. We discuss the potential roles of analog-digital transmission in the physiology of neural networks.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Effects of pituitary adenylate cyclase activating polypeptide on lumbosacral preganglionic neurons in the neonatal rat spinal cord

The effects of PACAP-38 on phasic and tonic preganglionic neurons (PGN) in L6 and S1 spinal cord slices from neonatal rats (5--11 days old) were studied using the whole-cell patch clamp technique. PGN were identified by retrograde axonal transport of a fluorescent dye (Fast Blue, 5 microl of 4% solution) injected into the intraperitoneal space 3--7 days prior to the study. Bath application of pituitary adenylate cyclase activating polypeptide (PACAP) (20 nM) increased the frequency of spontaneous excitatory postsynaptic potentials (EPSPs) and spontaneous firing in both types of PGN. PACAP markedly increased the number (200--800%) and frequency of action potentials elicited by depolarizing current pulses in phasic PGN, but had a smaller effect on tonic PGN. PACAP decreased the threshold for action potential generation by approximately 25% in both types of neurons (e.g. -34.0+/-1.5 to -38.4+/-1.7 mV from a holding potential of -50 mV in phasic PGN, P<0.005). PACAP did not affect the duration of the action potential. The amplitude of the spike after hyperpolarization was not changed but the duration was significantly reduced by PACAP from 204.4+/-12.2 to 106.2+/-8.1 ms in tonic but not in phasic PGN. PACAP suppressed a transient outward current that was also suppressed by 4-aminopyridine (0.5 mM). These results coupled with the immunohistochemical identification of a dense collection of PACAP fibers in the region of the PGN, raises the possibility that PACAP may function as an excitatory transmitter in lumbosacral parasympathetic reflex pathways in the neonatal rat.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Appearance and maturation of synaptic plasticity during juvenile development in Aplysia

In the present study we examine the developmental appearance and maturation of synaptic plasticity at the A-B neuron synapse in the cerebral ganglion of Aplysia. In the CNS of juvenile Aplysia 120 days after hatching, the excitatory synaptic connection between A and B cluster neurons is essentially the same as in the adult cerebral ganglion. No differences were observed between the amplitudes of the initial EPSPs in the cerebral ganglia of juveniles and adults. One form of plasticity, low frequency synaptic depression, is also present in juveniles. Another form of activity-dependent plasticity, slow developing potentiation (SDP) appears and matures during the late juvenile stage of development. At 120 days posthatching SDP, evoked by tetanic stimulation, is largely absent. Potentiated EPSPs have a significantly smaller amplitude than in adults. Over the next 80 days SDP undergoes a maturation process. The peak potentiation increases linearly with age from 135 +/- 12% at 125 days to 275 +/- 20% at 188 days. The duration of the potentiation, as measured by the time-constant its decay, also increases linearly from 12.7 +/- 3.4 min to 27.9 +/- 3.9 min. From 120-170 days, < 50% of the A neurons tested exhibited SDP. After 170 days, > 85% exhibited SDP. Changes in the rising phase of the A neuron action potential have been implicated in mediating SDP. At 120 days, the A neuron action potential has a significantly shorter duration (half-width) than in the adult. Between 120 and 200 days, both the duration and rise-time of the A neuron action potential increase linearly. These results confirm findings by other investigators, that different forms of synaptic plasticity develop independently, with depression appearing before potentiation.

Modeling Hermissenda: I. Differential contributions of IA and IC to type-B cell plasticity

We developed a multicompartmental Hodgkin-Huxley model of the Hermissenda type-B photoreceptor and used it to address the relative contributions of reductions of two K+ currents, IA and IC, to changes in cellular excitability and synaptic strength that occur in these cells after associative learning. We found that reductions of [symbol: see text] C, the peak conductance of IC, substantially increased the firing frequency of the type-B cell during the plateau phase of a simulated light response, whereas reductions of [symbol: see text] A had only a modest contribution to the plateau frequency. This can be understood at least in part by the contributions of these currents to the light-induced (nonspiking) generator potential, the plateau of which was enhanced by [symbol: see text] C reductions, but not by [symbol: see text] A reductions. In contrast, however, reductions of [symbol: see text] A broadened the type-B cell action potential, increased Ca2+ influx, and increased the size of the postsynaptic potential produced in a type-A cell, whereas similar reductions of [symbol: see text] C had only negligible contributions to these measures. These results suggest that reductions of IA and IC play important but different roles in type-B cell plasticity.

A-type potassium current modulated by A1 adenosine receptor in frog melanotrophs

1. Transient outward current was recorded in cultured frog melanotrophs with the whole-cell configuration of the patch-clamp technique. The ionic dependence, kinetics and pharmacological properties of the current were studied. The effects of the A1 adenosine receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA) on this current were also investigated. 2. In tetrodotoxin- and cobalt-containing solution, depolarization from -120 mV elicited both transient and delayed outward currents. Pulses from -60 mV activated only a sustained late current. 3. 4-Aminopyridine (4 mM) reduced the transient outward current much more than the delayed outward current. In contrast, tetraethylammonium (10-20 mM) selectively reduced the delayed current. 4. Tail current measurements showed a positive shift in the reversal potential when external K+ concentration was increased, indicating that K+ was the predominant charge carrier. 5. Steady-state inactivation was complete at potentials positive to -10 mV and removed by hyperpolarization. 6. Inactivation of the transient current was slowed and accelerated in oxidizing and reducing conditions, respectively, confirming the involvement of an inactivating 'ball and chain' peptide. 7. R-PIA increased the transient current. The steady-state inactivation curve was shifted towards more positive potentials without changing the activation kinetics. Pretreatment with pertussis toxin (1 microgram ml-1) blocked the response to R-PIA. 8. It is concluded that frog melanotrophs possess an A-type current that is likely to play an important role in excitability. This current, which is directly modulated by A1 adenosine receptors through a Gi/G(o) protein, appears to be responsible for the inhibitory effects of adenosine on electrical activity.

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)

Modulation of type A K+ current in Drosophila larval muscle by internal Ca2+; effects of the overexpression of frequenin

The calcium-dependent modulation of type A K+ current (IA) has been investigated using a two-electrode voltage clamp on larval muscle cells of Drosophila. It was found that the amplitude of IA increases when [Ca2+]o is changed from 0.2 mM to 2 mM. The increase in IA amplitude is not due to overlap with the Ca(2+)-dependent fast K+ current, ICF, since it is observed also in slo1 mutants, which are deficient for this current. This effect is not due to Ca(2+)-dependent shifts in the steady-state activation/inactivation kinetics. The phenomenon is probably due to elevations in internal calcium since it is abolished by Ca2+ channel blockers and promoted by caffeine (5 mM) if added in the absence of external calcium. This calcium effect was dose-dependent since it was not observed in the presence caffeine plus 2 mM calcium in the bath nor for values of [Ca2+]o above 4 mM. The Ca(2+)-dependent modulation of IA is absent in V7, a mutation that causes overexpression of frequenin, a recoverin-like Ca(2+)-binding protein which stimulates guanylyl cyclase [31]. One possible explanation for the loss of IA modulation in the V7 mutation is that the excess of frequenin alters intracellular cGMP-dependent metabolic pathways responsible for the internal calcium homeostasis.

Increase in cell size underlies neuron-specific temperature acclimation in Aplysia

Aplysia californica with similar genetic and environmental backgrounds were raised under mariculture conditions at either 10 or 20 degrees C. Determinations of cell surface area using capacitance and visual measurements indicated that there was an increase in the surface area of identified neurons. However, this increase was cell specific, occurring only in certain identified neurons. The amplitude of the early potassium current (IA) was monitored in identified neurons from the cold- and warm-reared groups during acute changes in temperature. The response to acute temperature change differed in the two groups, indicative of partial temperature acclimation of IA. Cold rearing shifted the acute temperature-response curve for IA amplitude to color temperatures, compared with the temperature-response curve for animals reared at warmer temperatures. This acclimation was related to the increase in surface area previously noted. Cells showing an increase in IA amplitude in cold-reared animals also showed an increase in membrane surface area, presumably associated with an increased number of A-channels, while channel density was unchanged. There was a small but significant shift in the steady-stage voltage dependency of inactivation, whereas kinetic properties were unaffected by rearing temperature.

Enhanced synaptic transmission at identified synaptic connections in the cerebral ganglion of Aplysia

The identified A-B neuron synaptic connections in the cerebral ganglion of Aplysia exhibited a novel form of enhanced synaptic transmission. A brief high-frequency train of action potentials (2 s, 10-30 Hz) in the presynaptic A neurons produced a long-lasting increase in the amplitude of excitatory postsynaptic potentials (EPSPs) in B neurons. The increase in synaptic efficacy was termed slow developing potentiation (SDP) since the EPSP amplitude increased slowly with the peak occurring 5 min after the tetanizing train. Peak EPSP amplitudes increased relative to the initial EPSP by an average of greater than 250%. SDP decayed as a single exponential with a time constant of tau = 24 min. The enhanced transmission was neuron specific. Only the connections made by the tetanized A neuron were potentiated. However, potentiation apparently occurred at all the synapses made by the tetanized A neuron. Tetanizing the postsynaptic B neurons neither induced, nor when paired with A neuron tetanization, increased SDP. SDP appears to be primarily due to increased transmitter release by the presynaptic neuron.

A fast, transient K+ current in neurohypophysial nerve terminals of the rat

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Macroscopic terminal membrane currents and voltages were studied using the whole-cell patch clamp technique. 2. In physiological solutions, depolarizing voltage clamp steps, from a holding potential (-80 mV) similar to the normal terminal resting potential, elicited a fast, inward followed by a fast, transient, outward current. 3. The threshold of activation for the outward current was -60 mV. The outward current quickly reached a peak and then decayed more slowly. The decay was fitted by two exponentials with time constants of 21 +/- 2.9 and 143 +/- 36 ms. These decay constants did not show a dependence on voltage. The time to peak of the outward current decreased and the amplitude increased with increasingly depolarized potential steps. 4. The outward current was blocked by the substitution of K+ with Cs+ and its reversal potential was consistent with a potassium current. 5. The transient outward current showed steady-state inactivation at more depolarized (than -80 mV) holding potentials with 50% inactivation occurring at -47.9 mV. The time course of recovery from inactivation was complex with full recovery taking greater than 16 s. 6. 4-Aminopyridine (4-AP) blocked the transient outward current in a dose-dependent manner (approximately IC50 = 3 mM), while charybdotoxin (4 micrograms/ml) and tetraethylammonium (100 mM) had no effect on the current amplitude. 7. Lowering external [Ca2+] had no effect on the fast, transient outward current nor did the calcium channel blocker Cd2+ (2 mM). 8. The neurohypophysial outward current reported here corresponds most closely to IA, and not to the delayed rectifier or Ca2(+)-activated K+ currents. Neurohypophysial IA, however, appears to be different from the outward currents found in the cell bodies in the hypothalamus which project their axons to the posterior pituitary. 9. Under current clamp, evoked action potential duration increased (122%) upon application of 5 mM-4-AP, indicating that IA is involved in neurohypophysial spike repolarization. 10. The existence of this current could help explain why maximal peptide release only occurs in response to bursts of electrical activity invading the nerve terminals.

Maturation of a transient outward potassium current in mouse fetal hypothalamic neurons in culture

The whole-cell voltage clamp technique was used to record potassium currents in mouse fetal hypothalamic neurons developing in culture medium from days 1 to 17. The neurons were derived from fetuses of IOPS/OF1 mice on the 14th day of gestation. The mature neurons (greater than six days in culture) showed both a transient potassium current and a non-inactivating delayed rectifier potassium current. These were identified pharmacologically by using the potassium channel blockers tetraethyl ammonium chloride and 4-aminopyridine, and on the basis of their kinetics and voltage sensitivities. The delayed rectifier potassium current had a threshold of-20 mV, a slow time-course of activation, and was sustained during the voltage pulse. The 4-aminopyridine-sensitive current was transient, and was activated from a holding potential more negative (-80 mV) than that required for evoking the delayed rectifier potassium current (-40 mV). The delayed rectifier potassium current was detectable from day 1 onwards, while the transient potassium current showed a distinct developmental trend. The time-constant of inactivation became faster with age in culture. The half steady-state inactivation potential showed a shift towards less negative membrane potentials with age, and the relationship was best described by a logarithmic regression equation. The developmental trend of the transient potassium current may relate functionally to the progressive morphological changes, and the appearance of synaptic connections during ontogenesis.

Purinergic modulation of hippocampal acetylcholine release involves alpha-dendrotoxin-sensitive potassium channels

Modulation of acetylcholine (ACh) release from superfused hippocampal slices was examined when the release of ACh was stimulated by exposure of slices to elevated K+ concentration. Evoked release was not sensitive to inhibition by 0.1 microM tetrodotoxin, but it could be inhibited in a dose-dependent manner by a muscarinic agonist (10-100 nM oxotremorine) and a purinergic agonist (10-100 nM 2-chloroadenosine). The alpha-dendrotoxin (100 nM), which selectively blocks voltage-gated inactivating K+ channels in nerve endings, did not affect the release of ACh under resting or depolarized conditions. However, alpha-dendrotoxin reduced the 2-chloroadenosine-induced inhibition of release, but did not alter the oxotremorine-induced inhibition. These results suggest that an alpha-dendrotoxin-sensitive K+ channel may be activated as an obligatory step in the modulation of ACh release by presynaptic purinergic receptor activation, but not in the modulation by presynaptic muscarinic receptors.

Peptidergic and serotonergic facilitation of a neuromuscular synapse in Aplysia

(1) A buccal muscle motor neuron which synthesizes the neuromodulatory small cardioactive peptides (SCPs) was identified in the buccal ganglion of Aplysia by using a combination of electrophysiological and single cell biochemical experiments. This neuron was designated B38. (2) Exogenous SCPb enhanced B38-induced contractions when perfused over the target muscle, the rostral portion of the buccal I3 muscle. SCPB potentiation of muscle contraction was associated with an increase in the excitatory junction potential (EJP) amplitude recorded from the muscle fibers, increased muscle cyclic AMP (cAMP) content, hyperpolarization of the muscle fibers, and an increase in the muscle fiber membrane conductance. Exogenous SCPB also depolarized the cell body of B38 and increased electrical coupling between the symmetrically paired B38 neurons. (3) These results suggest that the SCPs may be co-released from B38 along with an unidentified conventional neurotransmitter to homosynaptically facilitate B38 synaptic transmission by modulating presynaptic and postsynaptic components. (4) Stimulation of the identified serotonergic metacerebral neuron or perfusion of exogenous serotonin (5-HT) over the 13 muscle also potentiated B38-induced muscle contractions and EJP amplitude. Thus the B38 neuromuscular synapse represents a peripheral site of serotonergic heterosynaptic facilitation in Aplysia. (5) Presynaptic and postsynaptic serotonergic effects were qualitatively similar to those of SCPB. Serotonergic effects on muscle fiber hyperpolarization and increase in muscle fiber membrane conductance were similar in magnitude to those of SCPB but 5-HT induced a much larger increase in the EJP amplitude which was additive with that of SCPB. (6) The effect of 5-HT on the EJP amplitude was associated with inhibition of a slowly decaying component of synaptic facilitation. Concentrations of SCPB that increased the EJP were much less effective at inhibiting the slow component of facilitation. These observations indicate that 5-HT also exerted a presynaptic effect on B38 transmitter release. (7) Both 5-HT and SCPB increased muscle cAMP levels and application of forskolin mimicked many of their effects. suggesting that at least some of the postsynaptic effects were mediated by increased cAMP levels in the 13 muscle.

Attributes of an alcohol-sensitive and an alcohol-insensitive transient potassium current in Aplysia neurons

We describe two subtypes of transient potassium current IA, found in two identified cells within the nervous system of Aplysia. These subtypes of IA are differentially sensitive to ethanol, since the decay of current is significantly slowed in the presence of ethanol in one cell, while IA is unaffected by ethanol at the same concentrations in the other cell. In this paper, we examine a number of parameters including temperature sensitivity of these different IAS. While both currents fulfill the criteria for being called IA, they are significantly different with respect to the kinetics of activation and inactivation as well as recovery from inactivation. They do not differ significantly in the temperature sensitivity of a number of measured parameters, suggesting that ethanol and temperature do not act as a common locus; e.g., by affecting a bulk lipid membrane fluidity. In light of recent work describing the molecular biology of channels underlying IA, it is reasonable to assume that these IA subtypes reflect different underlying channels. This system may be useful for understanding the relationship of channel structure and function to ethanol action.

Sequential changes of potassium currents in Hermissenda type B photoreceptor during early stages of classical conditioning

Classical conditioning of the marine snail Hermissenda can be produced in a single session of 50 pairings of light and rotation stimuli. Voltage clamp measurements of two outward K+ currents, IA and ICa2(+)-K+ were obtained from medial Type B photoreceptors that were isolated from the nervous system 1 day after animals were exposed to paired light and rotation stimuli or control procedures (Unpaired, or no exposure to light and rotation), ICa2(+)-K+ was found to be unchanged 18-30 h after 50 training trials. This result is consistent with a previous study where ICa2(+)-K+ was found to be unchanged after 50 light and rotation trials, although significantly reduced by 100 trials. In the present study 50 pairings of light and rotation produced a significant reduction in IA, suggesting an important role for this current in the earliest stages of classical conditioning.

Potassium currents in hippocampal pyramidal cells

The hippocampal pyramidal cells provide an example of how multiple potassium (K) currents co-exist and function in central mammalian neurones. The data come from CA1 and CA3 neurones in hippocampal slices, cell cultures and acutely dissociated cells from rats and guinea-pigs. Six voltage- or calcium(Ca)-dependent K currents have so far been described in CA1 pyramidal cells in slices. Four of them (IA, ID, IK, IM) are activated by depolarization alone; the two others (IC, IAHP) are activated by voltage-dependent influx of Ca ions (IC may be both Ca- and voltage-gated). In addition, a transient Ca-dependent K current (ICT) has been described in certain preparations, but it is not yet clear whether it is distinct from IC and IA. (1) IA activates fast (within 10 ms) and inactivates rapidly (time constant typically 15-50 ms) at potentials positive to -60 mV; it probably contributes to early spike-repolarization, it can delay the first spike for about 0.1 s, and may regulate repetitive firing. (2) ID activates within about 20 ms but inactivates slowly (seconds) below the spike threshold (-90 to -60 mV), causing a long delay (0.5-5 s) in the onset of firing. Due to its slow recovery from inactivation (seconds), separate depolarizing inputs can be "integrated". ID probably also participates in spike repolarization. (3) IK activates slowly (time constant, tau, 20-60 ms) in response to depolarizations positive to -40 mV and inactivates (tau about 5s) at -80 to -40 mV; it probably participates in spike repolarization. (4) IM activates slowly (tau about 50 ms) positive to -60 mV and does not inactivate; it tends to attenuate excitatory inputs, it reduces the firing rate during maintained depolarization (adaptation) and contributes to the medium after-hyperpolarization (mAHP); IM is suppressed by acetylcholine (via muscarinic receptors), but may be enhanced by somatostatin. (5) IC is activated by influx of Ca ions during the action potential and is thought to cause the final spike repolarization and the fast AHP (although ICT may be involved). Like IM, it also contributes to the medium AHP and early adaptation. It differs from IAHP by being sensitive to tetraethylammonium (TEA, 1 mM), but insensitive to noradrenaline and muscarine. Large-conductance (BK; about 200 pS) Ca-activated K channels, which may mediate IC, have been recorded. (6) IAHP is slowly activated by Ca-influx during action potentials, causing spike-frequency adaptation and the slow AHP. Thus, IAHP exerts a strong negative feedback control of discharge activity.(ABSTRACT TRUNCATED AT 400 WORDS)

The transient potassium current, the A-current, is involved in spike frequency adaptation in rat amygdala neurons

The possible functional roles of the transient K+ current, IA, in basolateral amygdala (BLA) neurons were studied using a rat brain slice preparation and conventional intracellular recording techniques. Conditioning depolarization, which inactivates IA, slowed the action potential repolarization while conditioning hyperpolarization accelerated the action potential repolarization. 4-Aminopyridine (4-AP, 100 microM), a specific IA antagonist, also caused a clear delay in spike repolarization similar to the effect of conditioning depolarization suggesting that IA is involved in the action potential repolarization. When BLA neurons were excited by injecting long depolarizing current pulses (500 ms), they responded with an initial rapid discharge of action potentials which slowed or accommodated; an afterhyperpolarization (AHP) followed the depolarizing current pulses. Superfusion of 4-AP (100 microM) blocked accommodation resulting in an increase in action potential discharge in 74% (32 out of 43) neurons tested. The remaining 11 cells responded with an increased frequency of discharge of the first few action potentials. Unlike the effect of cadmium (Cd2+, 100 microM), a calcium channel blocker, 4-AP did not reduce the AHP. In the presence of norepinephrine (NE, 10 microM), a neurotransmitter which has been shown to block calcium-activated potassium conductance, 4-AP caused a further increase in the number and frequency of action potential discharge. In addition, in BLA neurons, spontaneous interictal and ictal-like events were observed at low and high concentrations of 4-AP, respectively. We conclude that IA is involved in the action potential repolarization as well as spike frequency adaptation in BLA neurons and that these actions may contribute to the convulsant effect of 4-AP.

Intracellular injection of a Ca2+ chelator inhibits spike repolarization in hippocampal neurons

The Ca-dependence of spike repolarization and afterhyperpolarizations (AHPs) in Ca1 pyramidal cells, was tested with intracellular electrodes containing the Ca buffers EGTA or 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). EGTA blocked only the slow AHP; but the fast-acting Ca chelator BAPTA also inhibited spike repolarization and the fast AHP. This supports the hypothesis that a fast Ca-activated'K-current contributes to spike repolarization.

Action potential repolarization and a fast after-hyperpolarization in rat hippocampal pyramidal cells

1. The repolarization of the action potential, and a fast after-hyperpolarization (a.h.p.) were studied in CA1 pyramidal cells (n = 76) in rat hippocampal slices (28-37 degrees C). Single spikes were elicited by brief (1-3 ms) current pulses, at membrane potentials close to rest (-60 to -70 mV). 2. Each action potential was followed by four after-potentials: (a) the fast a.h.p., lasting 2-5 ms; (b) an after-depolarization; (c) a medium a.h.p., (50-100 ms); and (d) a slow a.h.p. (1-2 s). Both the fast a.h.p. and the slow a.h.p. (but not the medium a.h.p.) were inhibited by Ca2+-free medium or Ca2+-channel blockers (Co2+, Mn2+ or Cd2+); but tetraethylammonium (TEA; 0.5-2 nM) blocked only the fast a.h.p., and noradrenaline (2-5 microM) only the slow a.h.p. This suggests that two Ca2+-activated K+ currents were involved: a fast, TEA-sensitive one (IC) underlying the fast a.h.p., and a slow noradrenaline-sensitive one (IAHP) underlying the slow a.h.p. 3. Like the fast a.h.p., spike repolarization seems to depend on a Ca2+-dependent K+ current of the fast, TEA-sensitive kind (IC). The repolarization was slowed by Ca2+-free medium, Co2+, Mn2+, Cd2+, or TEA, but not by noradrenaline. Charybdotoxin (CTX; 30 nM), a scorpion toxin which blocks the large-conductance Ca2+-activated K+ channel in muscle, had a similar effect to TEA. The effects of TEA and Cd2+ (or Mn2+) showed mutual occlusion. Raising the external K+ concentration reduced the fast a.h.p. and slowed the spike repolarization, whereas Cl- loading of the cell was ineffective. 4. The transient K+ current, IA, seems also to contribute to spike repolarization, because: (a) 4-aminopyridine (4-AP; 0.1 mM), which blocks IA, slowed the spike repolarization; (b) depolarizing pre-pulses, which inactivate IA, had a similar effect; (c) hyperpolarizing pre-pulses speeded up the spike repolarization; (d) the effects of 4-AP and pre-pulses persisted during Ca2+ blockade (like IA); and (e) depolarizing pre-pulses reduced the effect of 4-AP. 5. Pre-pulses or 4-AP broadened the spike less, and in a different manner, than Ca2+-free medium, Cd2+, Co2+, Mn2+, TEA or CTX. The former broadening was uniform, with little effect on the fast a.h.p., whereas the latter affected mostly the last two-thirds of the spike repolarization and abolished the fast a.h.p.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine raises excitability by inhibiting the fast transient potassium current in cultured hippocampal neurons

The effects of acetylcholine on cultured hippocampal neurons were investigated by using the whole-cell version of the patch-clamp technique. The CA1 region of the hippocampus was excised from brain slices of young rats (12-19 day old), incubated in a papain solution, and dissociated. Neurons were plated on a glial feeder layer. The experiments were conducted mostly on neurons cultured for 2-6 days. Upon depolarization under voltage clamp, these cells exhibited a fast transient outward current (A-current), which was inhibited by 4-aminopyridine (2.5 mM). Acetylcholine (0.1 microM) also inhibited this A-current, as did the muscarinic agonists bethanechol and muscarine. As expected from their inhibition of the A-current, acetylcholine and 4-aminopyridine both increased the amplitude of the action potential and prolonged its duration. We conclude that the inhibition of the A-current constitutes a mechanism by which acetylcholine exerts its excitatory influence on hippocampal neurons.

Autoreceptor-mediated changes in dopaminergic terminal excitability: effects of potassium channel blockers

The effects of the potassium channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA), on autoreceptor-mediated changes in dopaminergic terminal excitability were examined in urethane-anesthetized rats. Local infusions of 4-AP or TEA into neostriatal terminal fields of nigral dopaminergic neurons led to marked decreases in terminal excitability, as measured by the increase in stimulating current required to activate the neurons antidromically from the site of the infusion. The decreased excitability resulting from 4-AP could be reversed by subsequent i.v. injection of haloperidol, and was blocked in rats that had been depleted of endogenous dopamine by prior treatment with alpha-methyl-p-tyrosine (AMpT). Thus, the decrease in excitability elicited by the potassium channel-blockers was indirect, and apparently due to increased autoreceptor stimulation resulting from enhanced transmitter release. In addition, co-infusion of 4-AP and apomorphine in AMpT-treated animals led to decreased terminal excitability that did not differ from the effects of apomorphine alone, indicating that 4-AP did not block the effects of exogenous autoreceptor agonist administration. These results provide in situ electrophysiological evidence that autoreceptor-mediated processes occurring at dopaminergic terminals are not mediated by 4-AP- or TEA-sensitive potassium channels. Furthermore, our findings suggest that, as in other types of presynaptic terminals, blockade of voltage-sensitive potassium channels in dopamine terminals leads to enhanced release of transmitter.

Central action of dendrotoxin: selective reduction of a transient K conductance in hippocampus and binding to localized acceptors

Dendrotoxin, a small single-chain protein from the venom of Dendroaspis angusticeps, is highly toxic following intracerebroventricular injection into rats. Voltage-clamp analysis of CA1 neurons in hippocampal slices, treated with tetrodotoxin, revealed that nanomolar concentrations of dendrotoxin reduce selectively a transient, voltage-dependent K conductance. Epileptiform activity known to be induced by dendrotoxin can be attributed to such an action. Membrane currents not affected directly by the toxin include (i) Ca-activated K conductance; (ii) noninactivating voltage-dependent K conductance; (iii) inactivating and noninactivating Ca conductances; (iv) persistent inward (anomalous) rectifier current. Persistence of the effects of the toxin when Cd was included to suppress spontaneous transmitter release indicates a direct action on the neuronal membrane. Using biologically active, 125I-labeled dendrotoxin, protein acceptor sites of high affinity were detected on cerebrocortical synaptosomal membranes and sections of rat brain. In hippocampus, toxin binding was shown autoradiographically to reside in synapse-rich and white matter regions, with lower levels in cell body layers. This acceptor is implicated in the action of toxin because its affinities for dendrotoxin congeners are proportional to their central neurotoxicities and potencies in reducing the transient, voltage-dependent K conductance.

Actions of potassium channel blockers on guinea-pig lateral olfactory tract axons

Population action potentials were recorded from the guinea-pig isolated lateral olfactory tract. At 30 degrees C, the conduction velocity of the fibres was about 4 m/s and the absolute refractory period was less than or equal to 1.5 ms. The population spike was unaffected by removal of calcium ions from the superfusate but was abolished in tetrodotoxin. Tetraethylammonium ions (10 mmol/l) had no effect on the population spike, however the following potassium channel blocking drugs increased the duration in a concentration-dependent manner (in order of decreasing potency): 3,4-diaminopyridine, 4-aminopyridine, 3-aminopyridine, sparteine, cesium ions and barium ions. In addition to a prolongation, these substances also reduced the amplitude of the conducted spike. It is concluded that the rising phase of the spike is generated by a voltage-dependent increase in sodium conductance and that an increase in potassium conductance contributes to the falling phase. The potassium channels are potently blocked by aminopyridine like drugs.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)