Calcium-activated outward current in voltage-clamped hippocampal neurones of the guinea-pig

Potassium currents evoked by brief depolarizations in bull-frog sympathetic ganglion cells

1. Sympathetic neurones of the bull-frog Rana catesbeiana were subjected to a two-electrode voltage-clamp technique in order to investigate the K+ currents which can be elicited by action potentials or similar brief depolarizations. 2. Four separate K+ currents were observed (IC, IK, IAHP and IM). These could be separated on the basis of voltage sensitivity, Ca2+ dependence and deactivation kinetics. 3. Two of these currents, which were clearly activated by an action potential, were Ca2+ dependent. A voltage- and TEA (tetraethylammonium)-sensitive K+ current, IC, was activated within the first 1-2 ms of a depolarizing command. This current decayed on average with a time constant of 2.4 ms at -40 mV. The maximal conductance was outside the range which could be adequately voltage clamped but, as much as 2 muS could be activated by brief (2-3 ms) commands. Activation of IC during an action potential accounts for the Ca2+ dependence of the repolarization. IC did not exhibit a transient component. 4. A second Ca2+-dependent K+ current, IAHP, was also activated after as little as 1 ms depolarization but was not voltage sensitive and was much less sensitive to TEA. The current decayed with a time constant of around 150 ms at -40 mV. The maximal conductance was about 30 nS. 5. The voltage-sensitive delayed rectifying current, IK, made a contribution to the total K+ conductance of the cell similar to IC in magnitude; however, the current is not activated within the normal voltage range or time course of an action potential. The current decayed on average with a time constant of 21 ms at -40 mV. 6. IM, a muscarine- and voltage-sensitive current, is not activated to any significant degree by a single action potential. The data further imply that the rate of opening of the ion channels mediating IM is less voltage sensitive than the rate of closing. 7. Large changes in the K+ reversal potential occur following depolarizing commands which evoke large K+ currents. This is attributed to K+ accumulation within a restricted extracellular space. Extracellular K+ may double or even triple during a single action potential.

Calcium-dependent potassium conductance in guinea-pig olfactory cortex neurones in vitro

1. Guinea-pig olfactory cortex neurones in vitro (23-25 degrees C) were voltage clamped by means of a single-micro-electrode sample-and-hold technique. 2. Under current clamp at the resting potential (approximately -80 mV), brief depolarizing stimuli evoked trains of action potentials with little visible after-potential. However, in 90% of recorded cells held at membrane potentials between -70 and -45 mV, depolarizing current pulses evoked a slow after-hyperpolarization (a.h.p.) (approximately 8 mV) lasting several seconds and accompanied by an increase in input conductance. 3. The outward membrane current underlying the a.h.p. was revealed either by switching rapidly to voltage clamp at the end of a spike train ('hybrid' clamp) or by applying brief depolarizing commands from potentials between -60 to -45 mV. The tail current showed a distinct rising phase (time to peak approximately 1 s) and exponential decay (tau approximately 3 s) and was suppressed by removal of external Ca2+, or adding Co2+ (1-2 mM), Cd2+ (200 microM) or Mg2+ (6 mM). The a.h.p. current reversal potential was -96 mV in 3 mM-K+ medium. 4. Low concentrations (1-2 microM) of muscarine, carbachol, oxotremorine or the muscarinic ganglion stimulant, McN-A-343 (1-10 microM) reduced the a.h.p. current and leak conductance and induced a steady inward current, without affecting M-current (IM) relaxations. IM inhibition generally required higher (greater than 10 microM) agonist concentrations, although oxotremorine remained ineffective at up to 50 microM. 5. The a.h.p. current was reduced by noradrenaline and tetraethylammonium (TEA), but not by apamin or tubocurarine. Apart from TEA, these agents had no effect on IM. 6. Addition of tetrodotoxin (TTX, 1 microM) or removing external Na+ depressed the a.h.p. current amplitude recorded under voltage clamp. The residual tail current could be further reduced by adding Cd2+ or muscarinic agonists. 7. Repolarizing tail currents induced following positive voltage commands consisted mainly of IM and slow a.h.p. current with little evidence of a 'fast' Ca2+-activated K+ current (IC). 8. It is concluded that the slow a.h.p. current that underlies the post-burst after-hyperpolarization of olfactory neurones, is a Ca2+-dependent K+ current distinct from IM. It is suggested that the cholinergic modulation of this current (rather than IM) may provide a more subtle control of cell excitability in cortical neurones.

Interactions between grafted serotonin neurons and adult host rat hippocampus

We have followed the development of physiological and functional properties of serotonin-containing raphe neurons grafted into an adult host hippocampus as a model system for graft-host interactions. These raphe cells have no clear identifying properties on the day of grafting: they develop them while growing in the host. Raphe neurons, recorded 1 month after grafting, possess adult normal physiological properties. These include high input resistance, slow membrane time constant, lack of inward rectification, a transient outward rectification, broad spikes having a Ca2+ component, lack of accommodation, and a large afterhyperpolarization. The graft is first innervated by host fibers and later projects to the host tissue. When stimulated, postsynaptic hyperpolarized responses are recorded in hippocampal neurons. In the freely moving rat, raphe grafts can restore sleep-wakefulness variations in an evoked population response of the hippocampus to afferent stimulation, which is eliminated by depletion of serotonin. These studies illustrate that grafted serotonin neurons develop functional relations with a host brain.

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)

Voltage-clamp analysis of somatic gamma-aminobutyric acid responses in adult rat hippocampal CA1 neurones in vitro

1. The response of CA1 pyramidal neurones to somatic application of gamma-aminobutyric acid (GABA) was studied in adult hippocampal slices using single-electrode voltage-clamp techniques. 2. Small ionophoretic applications of GABA produced a pure outward current at the cell resting potential when recording with potassium-acetate-filled microelectrodes. This response reversed at a membrane potential of -69 +/- 5 mV (mean +/- 1 S.D.; n = 20). In recordings made with caesium-chloride-filled electrodes the GABA response reversed at -24 +/- 12 mV (n = 9). 3. The effect of different holding potentials on the size of the GABA response was examined in the range of -100 to -40 mV in twenty neurones using potassium-acetate-filled electrodes. In every case outward rectification of the response was observed. For twelve neurones the mean ratio (+/- 1 S.D. of the mean) of the conductance increase produced by GABA at -55 mV compared to -85 mV was 1.9 +/- 0.5. 4. Step changes in holding potential resulted in shifts in chloride equilibrium potential (ECl), as determined by time-dependent changes in the size of GABA-induced currents. The new value of ECl was generally reached within a few seconds of altering the membrane potential. Shifts in ECl did not appear to affect the extent of rectification but would cause underestimates of conductance measurements unless these were 'instantaneous'. The mean ratio (+/- 1 S.D. of the mean) of the 'instantaneous' conductance increase produced by GABA at 13 mV positive to that at 13 mV negative to ECl was 1.8 +/- 0.3. 5. The outward rectification was greater than that predicted by the constant-field equation. Possible factors that might contribute towards the rectification and its physiological significance are discussed.

A G protein couples serotonin and GABAB receptors to the same channels in hippocampus

Both serotonin and the selective gamma-aminobutyric acidB (GABAB) agonist, baclofen, increase potassium (K+) conductance in hippocampal pyramidal cells. Although these agonists act on separate receptors, the potassium currents evoked by the agonists are not additive, indicating that the two receptors share the same potassium channels. Experiments with hydrolysis-resistant guanosine triphosphate (GTP) and guanosine diphosphate analogs and pertussis toxin indicate that the opening of the potassium channels by serotonin and GABAB receptors involves a pertussis toxin-sensitive GTP-binding (G) protein, which may directly couple the two receptors to the potassium channel.

Two distinct calcium-activated potassium currents in larval muscle fibres of Drosophila melanogaster

The non-synaptic membrane currents of muscle fibres have been studied in late embryogenesis of Drosophila melanogaster using the voltage-clamp technique in wild-type and Shaker mutant third instar larvae. Five currents were found in the wild type muscle membrane at this embryonic stage: one fast inward Ca current (ICa), two fast outward K currents (IA and IAcd) and two slow outward K currents (IK and IC). IAcd and IC are Ca-dependent. Several procedures were used to separate IAcd from IA: IAcd is present in Shaker mutants which are characterized by the absence of IA (Salkoff and Wyman 1981); IAcd, but not IA, is suppressed by Co2+ (10 mM) or La3+ (1 mM); IAcd shows steady-state inactivation at more positive potentials than IA; IAcd, unlike IA, is 3,4-diaminopyridine (3,4-DAP) resistant. Furthermore, tetraethylammonium (TEA, 20 mM) which is known to be uneffective on IA, blocks IAcd. IAcd could not be triggered by using strontium or barium as calcium substitutes. By partial substitution of Ca by Ba or Sr ions, it was found that Ba, but not Sr, blocks the IAcd channel. A non-inactivating, TEA sensitive, Ca-dependent K current (IC), which gave N-shaped I-V plots, could be separated from IK by using Ca-channel blockers. IC and IK activate at membrane potentials of about -25 mV and -10 mV, respectively. The participation of IAcd and IC to membrane electrophysiology is discussed.

Ionic mechanisms underlying the firing properties of rat neonatal motoneurons studied in vitro

Ionic mechanisms underlying the firing properties of spinal motoneurons of neonatal rats (postnatal days 3-10) have been investigated using a hemisected, in vitro spinal cord preparation. These results demonstrate the presence of a high-threshold voltage-dependent calcium response and partial sodium-dependent spikes. The calcium current is evident during the falling phase of the action potential and is the major component of the after-depolarizing potential. The subsequent increase in intracellular calcium concentration activates a calcium-dependent potassium conductance (gK-Ca), the major component of the after-hyperpolarizing potential. The gCa, by activating gK-Ca, is the primary determinant of firing rate in neonatal motoneurons. For, when gCa was blocked by Cd2+, the interspike interval decreased, the maximum firing rate and the slope of the firing frequency-injected current relation increased. The calcium current is particularly robust during the first few postnatal days; during this period, tetrodotoxin resistant action potentials can be elicited by direct stimulation under control conditions. In animals older than 5 days such calcium spikes could be elicited only after decreasing gK with intracellular Cs+ or extracellular tetraethylammonium. This was the case even when 1 mM of the bath CaCl2 was replaced with BaCl2. The rising phases of calcium spikes recorded from neurons in both age groups demonstrate several components suggesting the calcium spikes comprise several discrete events, which probably originate across the dendritic membrane. When gK was decreased by bath application of tetraethylammonium+ and Cs+, neonatal motoneurons generated prolonged Ca-dependent spikes lasting for up to 6 s. Repolarization of Ca spikes occurred in two stages, the first was rapid (-2.11 +/- 0.8 V/s, n = 6) but incomplete. The second, was slower (-0.01 +/- 0.003 V/s, n = 5) and returned the membrane potential to the resting level after about 1-2 s. It is suggested that accumulation of extracellular potassium may contribute to the slow phase of repolarization. Motoneurons from the younger age group (3-5 days old) demonstrate all-or-none partial spikes rising from the after-depolarization of directly elicited sodium-dependent action potentials. Similar partial spikes were elicited from neurons from older animals during intracellular Cs+ loading. The partial spikes had faster rates of rise than the tetrodotoxin-resistant spikes and were not seen after tetrodotoxin treatment, suggesting that they are sodium-dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Kainate reduces two voltage-dependent potassium conductances in rat hippocampal neurons in vitro

The mechanisms of action of kainate were studied in CA1 hippocampal neurons using the single electrode voltage-clamp technique in vitro. Kainate (100-200 nM) reduced the potassium current which is responsible for the anomalous rectification (IQ). In 30% of the cells the drug reduced the calcium-dependent potassium current (IC) which is responsible for the afterhyperpolarization that follows calcium action potentials. The reduction of IC will contribute to the enhancement of the neural excitability by this drug.

Excitatory action of BAYK8644 on hippocampal pyramidal neurones of the guinea pig in vitro

BAYK8944, a calcium 'agonist' caused a slow depolarization of a proportion of CA1 hippocampal pyramidal cells and increased their input resistance. In voltage-clamped CA1 neurones BAYK8644 caused a decrease in depolarization-activated outward current. Reduction of depolarization-activated outward current was also produced by the Ca ionophore A23187. It is proposed that BAYK8644 and A23187 may cause suppression of outward current secondary to a prolonged increase in cytosolic Ca activity.

Non-synaptic modulation of repetitive firing by adenosine is antagonized by 4-aminopyridine in a rat hippocampal slice

In rat hippocampal slices which were superfused with low calcium (0.2 mM) medium, stimulation of the alvear fibers elicited an extracellularly recorded antidromic population spike in CA1 pyramidal neurons which was followed by 2-4 afterpotentials. Adenosine (20-40 microM) and the A1-adenosine agonist 1-phenylisopropyladenosine (L-PIA) blocked these afterpotentials without affecting the first spike. Addition of up to 5 mM tetraethylammonium to the superfused medium did not interfere with this adenosine action. But the addition of only 50 microM 4-aminopyridine (4-AP) antagonized almost completely the adenosine- or L-PIA-induced depression of antidromically evoked repetitive firing. It is concluded that functioning of 4-AP-sensitive potassium channels is a prerequisite for this 'antiepileptic' adenosine action. Since a similar pharmacological characteristic has been described for the A-current, it is likely that adenosine acts by turning on this particular potassium current.

M-current in human neocortical neurones

Intracellular recordings were made in slices of human neocortex that had been surgically excised from patients in order to remove deep lying brain tumours. In more than half the neurones studied under voltage-clamp (n = 9), a non-inactivating K+-current was detected that was turned on at potentials positive to around -60 mV. This conductance persisted when Ca2+-flux into neurones was blocked with Cd2+ and it was suppressed by muscarine (20 microM). The slow kinetics and voltage sensitivity of this K+ conductance, together with its muscarinic suppression, identified it as the M-current (IM). In addition to IM, evidence for the existence of Ca2+ and Ca2+-activated conductances was obtained in human neurones. These results validate the extrapolation of animal-derived data and identify IM as a target for cholinergic modulation in the human.

Isolation of neurons suitable for patch-clamping from adult mammalian central nervous systems

A method is described for the isolation of neurons from defined regions of the mammalian central nervous system, by a combination of mechanical and enzymatic means. The procedure liberates neurons free of cellular debris and glial investments, allowing the formation of giga-ohm seals with patch clamp electrodes. The characteristic morphology of neurons is maintained, together with the diversity of active channels evident in the intact nervous system.

Conditioning-specific membrane changes of rabbit hippocampal neurons measured in vitro

Intracellular recordings were made from hippocampal CA1 pyramidal neurons within brain slices of nictitating membrane conditioned, pseudoconditioned, and naive adult male albino rabbits. All neurons included (26 conditioned, 26 pseudoconditioned, and 28 naive) had stable penetration and at least 60 mV action potential amplitudes. Mean input resistances were approximately equal to 60 mu omega for the three groups. A marked reduction in the afterhyperpolarization (AHP) following an impulse was apparent for conditioned (x = -0.98 mV) as compared to the pseudoconditioned (x = -1.7 mV) and naive (x = -2.0 mV) neurons. The AHP has been attributed previously to activation of a Ca2+-dependent outward K+ current. The distribution of AHP amplitudes for the conditioned group included a new lower range of values for which there was little overlap with the other groups. The conditioning-specific reduction of AHP may be due to reduction of ICa2+-K+ as shown previously for conditioned Hermissenda neurons. This conditioning-induced biophysical alteration of the CA1 pyramidal cell must be stored by mechanisms intrinsic to the hippocampal slice and cannot be explained as a consequence of changes of presynaptic input arising elsewhere in the brain. Our experiments demonstrate the feasibility of analyzing cellular mechanisms of associative learning in mammalian brain with the in vitro brain slice technique.

gamma-Aminobutyric acid hyperpolarizes rat hippocampal pyramidal cells through a calcium-dependent potassium conductance

Application of gamma-aminobutyric acid (GABA) to the dendrites of CA1 pyramidal cells in hippocampal slices produced depolarizing and hyperpolarizing responses. Picrotoxin (50 microM) blocked the depolarizing response of the dendrites to GABA but not the hyperpolarizing responses of the dendrites. The hyperpolarizing response of the cell body to GABA was reduced but not blocked by picrotoxin, suggesting the presence of a complex response at the cell body. The depolarizing response of the dendrites and the hyperpolarizing response of the cell body appeared to be at least partly Cl- dependent as they were respectively increased and decreased in size in low-Cl- artificial cerebrospinal fluid (ACSF), while the hyperpolarizing response of the dendrites was unaffected. The hyperpolarizing response of the dendrites was increased in amplitude in low-K+ ACSF and the extrapolated reversal potential of the response became more negative, suggesting that the response was K+ dependent. The hyperpolarizing response of the dendrites was decreased in size in high-K+ ACSF and could be readily inverted by current injection. The reversal potential became less negative in high-K+ ACSF in a similar manner to that of the slow after-hyperpolarization following a train of spikes, indicating that the response was a K+ conductance. Perfusion of the slice with normal or 0-Ca2+ ACSF containing Cd2+ or Mn2+ blocked synaptic transmission, increased spike duration and blocked the slow phase of the spike after-hyperpolarization (a.h.p.). This latter potential is thought to be mediated by a Ca2+-dependent K+ conductance. Later, the hyperpolarizing response of the dendrites to GABA was blocked without an effect on the other GABA responses. Pressure application of Cd2+ (0.2-2 mM) onto the surface of the slice rapidly reduced or blocked the slow a.h.p. and the dendritic hyperpolarizing response to GABA. Intracellular injection of EGTA rapidly blocked the slow phase of the a.h.p. and then later blocked or reduced the dendritic hyperpolarizing response to GABA. We conclude that the hyperpolarizing response of the dendrites to GABA is mediated by a Ca2+-dependent K+ conductance.

Characteristics of phasic and tonic sympathetic ganglion cells of the guinea-pig

Intracellular recording techniques have been used to determine the electrophysiological properties of sympathetic neurones in ganglia of the caudal lumbar sympathetic chain (l.s.c.) and in the distal lobes of inferior mesenteric ganglia (i.m.g.) isolated from guinea-pigs. Passage of suprathreshold depolarizing current initiated transient bursts of action potentials in 97% of l.s.c. neurones, but only 13% of i.m.g. cells ('phasic' neurones). Most i.m.g. neurones fired continuously during prolonged depolarizing pulses ('tonic' neurones). Passive membrane properties varied; mean cell input resistance was similar between groups, but phasic neurones had smaller major input time constants on average than had tonic cells. Current-voltage relations determined under both current clamp and voltage clamp were linear around resting membrane potential (approximately 60 mV), where membrane conductance was lowest. Instantaneous and time-dependent rectification varied in the different neurone types. The current underlying the after-hyperpolarization following the action potential was significantly larger on average in tonic i.m.g. cells than in phasic neurones, although its time course (tau = 100 ms) was similar. Phasic neurones fired tonically when depolarized after adding the muscarinic agonist, bethanechol (10(-5) M to 10(-4) M), to the bathing solution. Bethanechol blocked a proportion of the maintained outward current (presumably the M-current, IM, Adams, Brown & Constanti, 1982) in phasic neurones; this current was small or absent in tonic neurones. Transient outward currents resembling the A-current (IA, Connor & Stevens, 1971 a) were evoked in tonic but not in phasic neurones by depolarization from resting membrane potential. IA could only be demonstrated in phasic neurones after a period of conditioning hyperpolarization. After a step depolarization to approximately --50 mV, IA reached peak amplitude at about 7 ms and then decayed with a time constant of about 25 ms in both neurone types. Activation characteristics of IA were similar for phasic and tonic neurones, but inactivation curves, although having the same shape, were shifted to more depolarized voltages in tonic neurones. That is, IA was largely inactivated at resting membrane potential in phasic, but not tonic neurones. It is concluded that the discharge patterns of the two populations of sympathetic neurones result from differences in the voltage-dependent potassium channels present in their membranes. The anatomical occurrence of the different cell types suggests that phasic neurones are vasoconstrictor and tonic neurones are involved with visceral motility.

Effects of histamine on hippocampal pyramidal cells of the rat in vitro

The actions of bath applied histamine on CA1 pyramidal cells were investigated in hippocampal slices of the rat. Histamine caused a) a slight depolarization but no significant change in resting membrane conductance; b) an abbreviation of long afterhyperpolarizations after single action potentials, bursts of action potentials or TTX resistant spikes; c) a loss of accommodation of firing. In the presence of TEA or barium, histamine prolonged and increased the size and number of the slow TTX resistant spikes. A depolarizing plateau which follows such spikes was also increased by histamine, but the population spike was increased. The frequency of spontaneous chloride dependent potentials, which reflect interneurone firing, was also increased. These effects considerably outlasted histamine application and were mimicked by the H2-agonist impromidine but not the H1-agonist thiazolethylamine, and blocked by the H2-antagonists cimetidine and metiamide but not the H1-antagonists mepyramine or the beta-antagonist propranolol. It is concluded that histamine, by activating H2-receptors, antagonizes a calcium mediated potassium conductance in hippocampal pyramidal cells without affecting calcium current. By this mechanism histaminergic afferent fibres could effectively regulate cortical responsiveness by selectively potentiating large excitatory inputs of target neurones.

Actions of noradrenaline recorded intracellularly in rat hippocampal CA1 pyramidal neurones, in vitro

CA1 pyramidal neurones were studied in rat in vitro hippocampal slices using standard intracellular and single-electrode voltage-clamp recording techniques to examine the actions of noradrenaline (NA). NA had two different effects on the resting membrane potential of pyramidal neurones; either a hyperpolarization accompanied by a decrease in membrane input resistance, or less commonly, a depolarization accompanied by an increase in input resistance. In many cells, both effects, a hyperpolarization followed by a depolarization were observed. The depolarization was mediated by a noradrenergic beta-receptor. The hyperpolarization was more difficult to characterize, but may result from alpha-receptor activation. NA reduced the amplitude and duration of the slow calcium-activated potassium after-hyperpolarization (a.h.p.) that follows depolarization-induced action potentials. This action of NA was mediated by beta 1-noradrenergic receptors. NA, in the presence of tetrodotoxin and tetraethylammonium, reduced the a.h.p. without reducing the size of the calcium action potential which preceded it. This was unlike the action of the calcium channel blocker, cadmium, which reduced the calcium action potential and the a.h.p. in parallel. Furthermore, NA did not reduce the amplitude of calcium or barium currents recorded under voltage clamp after blockade of potassium currents. A functional consequence of this blockade of the calcium-activated a.h.p. was a reduction of the accommodation of action potential discharge such that the excitatory responses of the neurone to depolarizing stimuli, such as glutamate application or current passed through the recording electrode, were enhanced. We conclude that the effects of NA on calcium-activated potassium conductance and on resting membrane potential can interact to increase the signal-to-noise ratio of hippocampal pyramidal neurone responsiveness.

Substance P augments a persistent slow inward calcium-sensitive current in voltage-clamped spinal dorsal horn neurons of the rat

Rat spinal dorsal horn neurons in slice preparations perfused with Ringer solution containing 0.5-1 microM TTX and/or 10-20 mM tetraethylammonium at 29 degrees C, were studied by using a single microelectrode voltage-clamp technique. Slow persistent inward currents were recorded during depolarizing voltage commands to membrane potentials positive to about -40 mV. The inward current was depressed by removing external Ca, or by adding 0.1-0.2 mM Cd, 5 mM Co or 0.1 mM verapamil, and was increased by adding Ba or Bay-K 8644. Substance P (SP) augmented a persistent slow inward Ca-sensitive current in a dose-dependent manner. It is suggested that this effect may be instrumental in generating the SP-evoked slow depolarization, increase in membrane excitability, and the 'bursting' behavior in the immature rat dorsal horn neurons. In addition, in some neurons SP reduced the M-like current, which effect may contribute to, but not explain, generation of the SP-induced slow depolarization.

Pre-and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones

Using intracellular recording techniques, the effects of beta-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 microM ( +/- ) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a 'leak' or resting potassium conductance (gK), was added to the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Further studies on the action of baclofen on neurons of the dorsolateral septal nucleus of the rat, in vitro

Baclofen hyperpolarizes rat dorsolateral septal neurons in a concentration-dependent manner. The apparent dissociation constant for this action is 200 nM. The baclofen-induced hyperpolarization is due to an increase in potassium conductance with a reversal potential of -88 mV. Of the various potassium and calcium channel blockers we tested, only 4-aminopyridine blocked baclofen-induced hyperpolarizations. Our results suggest that baclofen does not activate a calcium-dependent potassium conductance.

Effects of noradrenaline on some potassium currents in CA1 neurones in rat hippocampal slices

Pyramidal (CA1) cells in rat hippocampal slices were voltage clamped using a single electrode voltage clamp. In the presence of tetrodotoxin (TTX), depolarizing pulses from holding potentials of -60 to -70 mV elicited a slow inward calcium (Ca2+) current and two outward potassium (K+) currents: an A current and a slower, Ca2+-dependent K+ current. Noradrenaline (NA) (20 microM) depressed the amplitude of the K+ currents without affecting the Ca2+ current. The effect of NA could be blocked with propranolol and was mimicked by isoprenaline, suggesting that NA depresses the K+ currents by binding to beta-receptors.

Possible involvement of K+-conductance in the action of gamma-aminobutyric acid in the guinea-pig hippocampus

The mechanism underlying the action of gamma-aminobutyric acid (GABA) in the hippocampus was investigated using guinea-pig brain slices. GABA either superfused or applied directly by microiontophoresis produced a biphasic response in pyramidal cells, comprising hyperpolarizing and depolarizing components. When different concentrations of GABA were applied to the same neurone, the lower concentrations generally produced a hyperpolarization-predominant response, while higher concentrations resulted in a depolarization-predominant response. The depolarizing component of the response to GABA was augmented in a medium containing a low concentration of Cl-, relatively unaffected by a change in external K+ concentration, and blocked by picrotoxin (2 X 10(-5) M). The depolarizing response to GABA persisted in a Ca2+-free medium in which the concentration of Na+ was reduced to 13 mM. Combined application of low doses of picrotoxin and bicuculline eliminated the major part of the depolarizing component of the biphasic response to GABA and produced a relatively pure hyperpolarizing response. The reversal potential of this pharmacologically 'isolated' hyperpolarizing response to GABA was estimated, from the current-voltage relationships, to be about -90 mV and was the same as that of the hyperpolarization induced by baclofen. When the membrane was successively hyperpolarized by inward direct current (d.c.) injections, the reversal point of the 'pharmacologically isolated' hyperpolarizing response to GABA coincided with that of the post-burst hyperpolarization. Low concentrations of Cl- in the bathing medium had no noticeable effect on the hyperpolarizing component of the response to GABA, whereas it markedly increased the amplitude of the depolarizing component. These results suggest that the action of GABA in the hippocampus may involve an activation of K+ conductance.

Spontaneous muscarinic suppression of the Ca-activated K-current in bullfrog sympathetic neurons

Neurons in bullfrog sympathetic ganglia were voltage-clamped using a single microelectrode. A prolonged outward current which was identified as the Ca-activated K-current secondary to a transient Ca entry through voltage-operated channels was shortened by oxotremorine. An inward Ca-current was not significantly depressed by oxotremorine. It was suggested that muscarinic agonists accelerate the re-closure of K-channels either directly or secondarily via their effects on an intracellular sequestration process of Ca ions. It was also suggested that a small amount of acetylcholine only sufficient to cause a miniature synaptic current via nicotinic receptors could shorten the Ca-activated K-current via muscarinic receptors.

Adenosine actions on CA1 pyramidal neurones in rat hippocampal slices

Intracellular recordings with a bridge amplifier of CA1 pyramidal neurones in vitro were employed to study the mechanisms of action of exogenously applied adenosine in the hippocampal slice preparation of the rat. Adenosine enhanced the calcium-dependent, long-duration after-hyperpolarization (a.h.p.) at least in part by a reduction in the rate of decay of the a.h.p. Both the reduced rate of decay and that of the control can be described with a single exponential. Antagonism of the calcium-dependent potassium current (and as a result, the a.h.p.) by bath application of CdCl2 or intracellular injection of EGTA (ethyleneglycolbis-(beta-aminoethyl ether)N,N'-tetraacetic acid) did not reduce the adenosine-evoked hyperpolarization or decrease in input resistance. Similarly, TEA (tetraethylammonium), which antagonizes both the voltage- and calcium-sensitive, delayed, outward rectification, had no effect on the adenosine-evoked changes in resting membrane properties. Adenosine did not affect the early, transient, outward rectification. During exposure to 4-aminopyridine (4-AP) in concentrations sufficient to antagonize this early rectification, the changes in resting membrane properties evoked by adenosine were unaffected. We conclude that the enhancement of the a.h.p. and accommodation by adenosine may be mediated by a change in the regulation of intracellular calcium. However, the mechanism responsible for the hyperpolarization and decrease in input resistance evoked by adenosine is both calcium and voltage insensitive. Thus, it appears distinct from that mediating the enhancement of the a.h.p. and accommodation.

Repetitive firing of CA1 hippocampal pyramidal cells elicited by dendritic glutamate: slow prepotentials and burst-pause pattern

In order to compare responses to dendritic vs. somatic depolarization, CA1 pyramidal cells in rat hippocampal slices were stimulated by iontophoresis of glutamate to sensitive spots in the dendrites, and by somatic current injection. Low intensities of either stimulus elicited slow repetitive firing. Each action potential was preceded by a slow depolarizing prepotential (SPP), lasting 50-300 ms and was followed by fast (3-5 ms) and slow (more than 100 ms) afterhyperpolarizations (AHPs). The SPPs, and AHPs were indistinguishable for the two types of stimuli. In response to strong depolarizations, most cells showed an initial burst of spikes, followed by a pause before the steady discharge. This pattern was elicited by both glutamate and current. The input resistance usually increased 5-20% during subthreshold depolarizations by glutamate or current. In contrast, large doses of glutamate caused a slow decline in the resistance (up to 40%), which was larger than during comparable current-induced discharge, and the response was followed by a longer AHP. It is concluded that both dendritic and somatic depolarization, induced by glutamate and current, respectively, can elicit sustained repetitive firing with SPPs, fast and slow AHPs and burst-pause pattern, thus, increasing the likelihood that these phenomena play a role during natural activation of CA1 cells.

Three components of active membrane current in the C-neurons of rabbit cervical nodose ganglion under voltage clamp

In the C-neurons of rabbit nodose ganglion there is a persistent slow outward current at Vm levels positive to -80 mV. This current was detectable in Na+-free Ringer and disappeared in Ca2+-free medium. Therefore it may be the Ca2+-activated K+ current. This K+ current shows a unique time and voltage dependency, suggesting that it may have a regulatory role on the excitability of C-neurons. Two other types of current also observed in C-neurons were IQ- and IA-like currents. In A-neurons, however, a Ca2+-activated K+ current was not observed at all.

Intracellular Ca2+-ions inactivate K+-current in bullfrog sympathetic neurons

Neurons from bullfrog sympathetic ganglia were voltage clamped using a single microelectrode, in a sodium-free, calcium-rich solution containing tetraethylammonium. A brief inward calcium current was followed by a long-lasting inward current. The long-lasting inward current corresponded to a depolarizing afterpotential which followed a calcium spike under the current clamp. It was largely due to the M-channel closure. The present study indicates that massive calcium entry can cause inactivation of potassium conductance in vertebrate neurons.

Calcium-dependent inward currents in voltage-clamped guinea-pig olfactory cortex neurones

Guinea-pig olfactory cortex neurones in vitro (23 degrees C--25 degrees C) were voltage clamped by means of a single microelectrode sample-and-hold technique. In most Cs+-loaded neurones (in the presence of tetrodotoxin), membrane depolarization beyond -60 mV elicited inward currents, which had rapid activation kinetics. The steady-state current-voltage relationship was N-shaped with a region of negative slope conductance between - 50 mV and - 20 mV. The rate of inactivation varied according to the holding potential and the command potential. The inward currents were maintained when external Ca2+ was replaced by Ba2+, and were blocked by Cd2+, suggesting that Ca2+ was the principal charge carrier. The results demonstrate the existence of calcium current in olfactory cortex neurones.

Calcium-dependent action potentials in guinea-pig olfactory cortex neurones

Ca2+-dependent action potentials were recorded in guinea pig olfactory neurones in vitro (23 degrees--25 degrees C). In most cells (in the presence of tetrodotoxin: TTX) the current-voltage relationship displayed 'anomalous' rectification (apparent high slope resistance) at potentials approximately 20 mV depolarized to the resting membrane potential (approximately -80 mV) and strong outward rectification at more positive potentials. Intracellular Cs+-loading blocked outward rectification and increased action potential duration. Such spikes were TTX-insensitive and were further prolonged by external addition of tetraethylammonium (TEA) or Ba2+. Spikes recorded from Cs+-loaded, TTX/TEA-treated neurones displayed a prolonged plateau and an after-depolarization. They persisted when Ba2+ or Sr2+ were substituted for external Ca2+, but not when Mg2+ was the sole extracellular divalent cation. The spikes were blocked in the presence of Cd2+ but persisted when 82% of the extracellular Na+ was substituted by choline. A TTX-insensitive, slowly inactivating inward current at depolarized potentials is believed to account for the subthreshold 'anomalous' rectification and prolonged spike plateau.

Calcium-induced inactivation of calcium current causes the inter-burst hyperpolarization of Aplysia bursting neurones

A triphasic series of tail currents which follow depolarizing voltage-clamp pulses in Aplysia neurones L2-L6 was described in the preceding paper (Kramer & Zucker, 1985). In this paper, we examine the nature of the late outward component of the tail current (phase III) which generates the inter-burst hyperpolarization in unclamped cells. The phase III tail current does not reverse between -30 and -90 mV, and is relatively insensitive to the external K+ concentration. In contrast, Ca2+-dependent K+ current (IK(Ca)), elicited by intracellular Ca2+ injection, reverses near -65 mV, and the reversal potential is sensitive to the external K+ concentration. Addition of 50 mM-tetraethylammonium (TEA) to the bathing medium causes a small increase in the phase III tail current. In contrast, IK(Ca) is completely blocked by addition of 50 mM-TEA. The phase III tail current is suppressed by depolarizing pulses which approach ECa, is blocked by Ca2+ current antagonists (Co2+ and Mn2+), and is blocked by intracellular injection of EGTA. The phase III tail current is reduced by less than 10% after complete removal of extracellular Na+. These bursting neurones have a voltage-dependent Ca2+ conductance which exhibits steady-state activation at a membrane potential similar to the average resting potential of the unclamped cell (i.e. -40 mV). The steady-state Ca2+ conductance can be inactivated by Ca2+ injection, or by depolarizing pre-pulses which generate a large influx of Ca2+. The steady-state Ca2+ conductance has a voltage dependence similar to that of the phase III tail current. The Ca2+-dependent inactivation of the steady-state Ca2+ conductance occurs in parallel with the phase III tail current; both have a similar sensitivity to Ca2+ influx, and both processes decay with similar rates after a depolarizing pulse. Hence, we propose that the phase III tail current is due to the Ca2+- dependent inactivation of a steady-state Ca2+ conductance. The decay of IK(Ca) following simulated spikes or bursts of spikes is rapid (less than 1 s) compared to the time course of the phase III tail current and the inter-burst hyperpolarization (tens of seconds). Thus, we conclude that IK(Ca) does not have a major role in terminating bursts or generating the inter-burst hyperpolarization in these cells. We present a qualitative model of the ionic basis of the bursting pace-maker cycle. The central features of the model are the voltage-dependent activation and the Ca2+-dependent inactivation of a Ca2+ current.

Ionic conductance associated with electrical activity of guinea-pig red nucleus neurones in vitro

Intracellular recordings were made from red nucleus (r.n.) neurones in guinea-pig slice preparations in vitro. In the control solution, a fast action potential was elicited by a depolarizing current pulse. This fast action potential was abolished by tetrodotoxin (TTX). When tetraethylammonium (TEA) was added to the perfusing solution, a TTX-resistant slow action potential was elicited by a large depolarizing current pulse. This TTX-resistant slow action potential was abolished by Co2+ or Mn2+. In the control solution, the action potential was followed by a fast and a slow after-hyperpolarization (a.h.p.). The fast a.h.p. was abolished by TEA. The amplitude of the fast a.h.p. was dependent on the extracellular K+ concentration. The slow a.h.p. was reversibly abolished by Co2+ or Mn2+. The reversal potential of the slow a.h.p. was dependent on the extracellular K+ concentration. When the membrane potential was hyperpolarized, a time-dependent inward rectification was observed. This inward rectification was inhibited by Cs+ but not by Ba2+, TTX, TEA or Co2+. It is concluded that the fast action potential is produced by a voltage-dependent Na+ conductance, the TTX-resistant slow action potential is produced by a voltage-dependent Ca2+ conductance, the fast a.h.p. is produced by a voltage-dependent K+ conductance, the slow a.h.p. is produced by a Ca2+-activated K+ conductance and the inward rectification is produced by a time-dependent inward rectifier in r.n. neurones.

Effects of caffeine on hippocampal pyramidal cells in vitro

The effects of caffeine on the electrophysiological properties of CA1 pyramidal neurones were investigated in the rat hippocampal slice preparation in vitro. A concentration-dependent increase in both the extracellularly recorded excitatory postsynaptic potential (e.p.s.p.) and the population spike resulting from stimulation of the stratum radiatum could be evoked by caffeine with a threshold concentration of 10 microM. Intracellular recordings demonstrate a caffeine-evoked decrease in resting membrane potential, an increase in input resistance, a reduction of the long afterhyperpolarization (a.h.p.) and a decrease in accommodation. The interaction between caffeine and adenosine was investigated on the extracellularly recorded e.p.s.p. The maximal response evoked by caffeine was increased in the presence of adenosine and the adenosine concentration-response curve was shifted to the right in a parallel fashion in the presence of caffeine. It is suggested that the effects of caffeine on hippocampal neurones may be mediated by a decrease of one or more potassium conductance(s), and that adenosine and caffeine may compete for the same electrophysiologically active receptor site on these cells.

Baclofen activates voltage-dependent and 4-aminopyridine sensitive K+ conductance in guinea-pig hippocampal pyramidal cells maintained in vitro

The ionic mechanism underlying the effect of (-)-baclofen in the hippocampus was investigated using guinea-pig brain slices. (-)-Baclofen either perfused or applied directly by microiontophoresis hyperpolarized the membrane and decreased the membrane input resistance of pyramidal cells in a dose-dependent manner. The value of the reversal potential for the baclofen-induced hyperpolarization, as estimated from the current-voltage relationships, was about -95mV. The reversal potential of the baclofen-induced hyperpolarization measured directly coincided with that for the post-burst hyperpolarization which is known to result from an activation of Ca2+-activated K+ conductance. The amplitude of the baclofen-induced hyperpolarization was increased in low K+ (1.24 mM) medium whereas the hyperpolarization was decreased or abolished in high K+ (12.4 and 25 mM). Low Cl- (10.2 mM) medium had no noticeable effect on the baclofen-induced hyperpolarization. The effect of baclofen was antagonized by a low dose of 4-aminopyridine (5 X 10(-6) M) whereas it was unaffected by picrotoxin (2 X 10(-5) M). These results strongly suggest that the effect of baclofen is mediated by an increase in K+ conductance.

Noradrenaline hyperpolarization and depolarization in cat vesical parasympathetic neurones

Responses to noradrenaline (NA) applied by superfusion, ionophoresis or pressure pulse were analysed using conventional intracellular recording and voltage-clamp methods in cat vesical parasympathetic ganglia. NA (1 microM) hyperpolarized 60% of the neurones, depolarized 25%, and produced a biphasic potential, which comprised a membrane hyperpolarization followed by a membrane depolarization, in 10%. About 5% of the neurones did not respond to NA. The NA hyperpolarization was blocked by yohimbine (1 microM), an alpha 2-adrenoceptor antagonist, whereas the NA depolarization was blocked by prazosin (0.1-1 microM), an alpha 1-adrenoceptor antagonist. These data indicated that the NA hyperpolarization was mediated through alpha 2-adrenoceptors and the NA depolarization through alpha 1-adrenoceptors. The NA hyperpolarization was accompanied by an increase in conductance, while the NA depolarization was associated with a decrease in conductance measured under manual-clamp conditions. Similar conductance changes were observed under voltage clamp. NA hyperpolarizations became smaller as the membrane was hyperpolarized and reversed polarity beyond -100 mV. NA depolarizations also became smaller at hyperpolarized membrane potentials and reversed polarity around -90 mV. The NA responses were enhanced in low-K media and depressed in high-K Krebs solution. The NA hyperpolarization was blocked by the Ca antagonists, Cd, Mn and Co. Intracellular injection of EGTA caused a slowly developing, progressive block of the NA hyperpolarization. The NA depolarization was not affected by low Ca concentrations, Ca antagonists or intracellular injection of EGTA. In some neurones the NA depolarization was unmasked in solutions containing Ca antagonists and after intracellular EGTA injection. The NA hyperpolarization was depressed by intracellular injection and extracellular superfusion of Cs but not by TEA. Ba (10-100 microM) depressed the NA hyperpolarization by 30%. The NA depolarization persisted in the presence of muscarine (10 microM) and was not blocked by Cs or TEA but was depressed 70% by Ba (10 microM). These data are consistent with the hypotheses that alpha 2-adrenoceptor activation produces a membrane hyperpolarization that is mediated through a Ca-dependent K conductance, and that alpha 1-adrenoceptor activation produces a membrane depolarization through closure of a voltage-insensitive K channel.

GABAB-receptor-activated K+ current in voltage-clamped CA3 pyramidal cells in hippocampal cultures

GABAB receptors are a subclass of receptors for gamma-amino-n-butyric acid (GABA) that are also activated by the antispastic drug beta-p-chlorophenyl-GABA (baclofen). One effect of baclofen is to inhibit excitatory transmission from CA3 to CA1 hippocampal pyramidal cells. To identify the ionic mechanism of GABAB-receptor-mediated depression, we have studied the effect of baclofen and GABA on ionic currents in voltage-clamped CA3 pyramidal cell somata in rat hippocampal slice cultures. Baclofen (10 microM) induced an inwardly rectifying outward current that reversed at -74 +/- 4.3 mV (mean +/- SD). This appeared to be a K+ current since (i) its reversal potential showed the expected shift when extracellular K+ concentration was changed and (ii) it was blocked by external Ba2+ or internal Cs+. The action of baclofen was closely imitated by GABA after the GABAA-mediated Cl- current had been abolished with pitrazepin (10 microM); under these conditions, GABA (100 microM) also produced an inwardly rectifying, Ba2+-sensitive current with a reversal potential identical to that of the baclofen-induced current. When outward currents were blocked with internal Cs+, the residual inward voltage-dependent Ca2+ current was not changed by baclofen. It is concluded that the primary effect of GABAB-receptor activation in these neurones is to increase K+ permeability rather than to reduce Ca2+ permeability.

Voltage and ion dependences of the slow currents which mediate bursting in Aplysia neurone R15

The previous paper described a slow depolarizing tail current, ID, and a slow hyperpolarizing tail current, IH, that are activated by action potentials and by brief depolarizing pulses in Aplysia neurone R15. ID and IH are necessary for the generation of bursting pace-maker activity in this cell. In this paper, the voltage and ion dependence of ID and IH are studied in an effort to determine the charge carriers for the two currents. When the slow currents are activated by brief depolarizing pulses delivered under voltage clamp in normal medium, an increase in the size of the pulse of 5-10 mV is usually sufficient to bring about full activation of ID. The apparent threshold in normal medium is approximately -20 mV. In medium in which K+ channels are blocked, full activation of an inward tail current that resembles ID requires increasing the pulse amplitude by only 1-2 mV. In contrast, IH is activated in a graded fashion over a 40 mV range of pulse amplitudes. After activating the currents with action potentials or with supramaximal pulses, ID remains an inward current and IH an outward current over a range of membrane potentials spanning -20 to -120 mV. In normal medium, ID is dependent on both extracellular Na+ concentration ( [Na+]o) and extracellular Ca2+ concentration ( [Ca2+]o). When K+ channels are blocked, ID can be supported by either [Na+]o or [Ca2+]o. IH depends only on [Ca2+]o as long as [Na+]o is at least 50 mM. Neither ID nor IH is decreased by decreasing the K+ gradient or by application of K+ channel blockers. These treatments increase somewhat the apparent amplitude of ID, probably by unmasking it from the large K+ tail current that follows the depolarizing pulse. A direct comparison in the same cell of the tetraethylammonium sensitivity of IH and of the Ca2+-activated K+ current demonstrates that these two currents flow through separate and distinct populations of channels. We conclude that in R15, ID arises in response to the triggering of an axonal action potential which in turn, through an as yet unknown mechanism, causes an increased influx of Na+ and/or Ca2+. We conclude that the apparent outward current IH, which is responsible for the interburst hyperpolarization in a normally bursting R15, in fact arises from a decrease in a resting inward Ca2+ current, possibly as the result of Ca2+-induced inactivation of Ca2+ channels.

Functional innervation of cultured hippocampal neurones by cholinergic afferents from co-cultured septal explants

The rat hippocampus receives a strong cholinergic innervation from the medial septum; information about the development and function of this pathway could help to elucidate the mechanisms of memory functions. Previous electrophysiological studies have shown that septal stimulation in vivo facilitates commissural and perforant path inputs and that stimulation of intrahippocampal cholinergic fibres in vitro produces a slow depolarization of rat hippocampal CA3 pyramidal neurones and increases their excitability. We describe here a different approach to the investigation of this system, by co-culturing slices of young rat hippocampus and septum, then recording the effects of septal nucleus stimulation on single voltage-clamped hippocampal CA3 pyramidal neurones. Under these conditions acetylcholinesterase-staining (presumed cholinergic) fibres grow out from the septum into the hippocampus. Single septal stimuli produce a short-latency non-cholinergic fast excitatory postsynaptic current, whereas trains of stimuli produce a slow inward current augmented by neostigmine and suppressed by atropine; hence this has a cholinergic origin. Our experiments provide both the first demonstration that functional synapses can be established between explanted cholinergic and cholinoceptive neuronal systems from the mammalian brain in organotypic culture and the first description of cholinergic slow excitatory postsynaptic currents in the mammalian central nervous system.

The inhibitory role of the visually responsive region of the thalamic reticular nucleus in the rat

Two-shock inhibition, a feature of 98 of 100 P cells recorded in the dorsal lateral geniculate nucleus of the normal rat, was not observed in 91 of 140 geniculate cells after an electrolytic lesion had been made in the adjacent visually responsive thalamic reticular nucleus. Nine geniculate cells recorded both before and after a reticular lesion had their initial inhibition abolished or substantially reduced after the lesion. The reticular lesion eliminated the bursts of spikes which normally terminate periods of inhibition following electrical or photic stimulation but caused no other changes in receptive field organization of geniculate cells. We conclude that the visually responsive region of the thalamic reticular nucleus in the rat is responsible for the profound two-shock inhibition and for the post-inhibitory bursts which are normal properties of relay cells of the dorsal lateral geniculate nucleus.

Effects of anesthetic and anticonvulsant drugs on calcium-dependent efflux of potassium from human erythrocytes

Anesthetic (halothane, enflurane, isoflurane, chloroform, diethyl ether, pentobarbital, phenyclidine) and anticonvulsant (ethosuximide, phenytoin, phenobarbital, sodium valproate, carbamazepine, delta 9-tetrahydrocannabinol) drugs were investigated for their effects on the calcium-dependent efflux of 86Rb (used as a marker for potassium) from resealed human red blood cells. Most anesthetic agents produced biphasic effects, having stimulatory actions at low (anesthetic) concentrations and inhibitory actions at higher (toxic) concentrations. The drug concentrations required to increase calcium-dependent 86Rb efflux were closely correlated with plasma concentrations required for anesthesia in vivo. Two anticonvulsant agents (phenytoin and phenobarbital) inhibited the efflux process in a concentration-dependent manner, while all other tested anticonvulsants were without significant effects. Neither the anesthetic nor the anticonvulsant substances significantly altered the calcium-independent release of 86Rb. The possibility that general anesthetic and anticonvulsant drugs act on calcium-dependent potassium conductance in the central nervous system, and the therapeutic importance of such actions, is discussed.

Identification of delayed potassium and calcium currents in the rat sympathetic neurone under voltage clamp

Post-ganglionic neurones of the isolated rat superior cervical ganglion were studied at 37 degrees C under two-electrode voltage-clamp conditions. Membrane depolarization beyond -40 mV from holding levels between -50 and -100 mV produced a delayed outward current which exhibited no inactivation within this voltage range. The current is carried primarily by K+ ions and its instantaneous I-V relation is linear. The total outward current could be separated into two distinct components on the basis of ion-substitution experiments. A voltage-dependent component of the delayed current, termed IK(V), is activated by membrane depolarization beyond -40 mV when Ca2+ fluxes are selectively blocked by Cd2+ or in Ca2+-free solution. IK(V) develops following first-order kinetics and rises to a peak with a voltage-dependent delay (239 ms at -30 mV and 23 ms at +10 mV). GK(V) attains a saturating value of the order of 17 mS/cm2 at about +20 mV and can be described in terms of a simple Boltzmann distribution for a single gating particle with a valency equal to +2.5. A second component of the delayed outward current, termed IK(Ca), depends on Ca2+ entry for its activation and was isolated as difference current before and after block of Ca2+ movements across the membrane. IK(Ca) is larger and faster than IK(V): it is strictly related to Ca2+ influx and also depends on membrane potential depolarization. A distinct Ca2+ current, ICa, was recorded from the neurone exposed to Na+-free or tetrodotoxin solution. ICa was activated by membrane depolarization beyond -30 mV and reached a maximum value near 0 mV. Its activation agrees with fourth-order kinetics and becomes faster with increasing depolarization. The Ca2+ current developed with a voltage-dependent time to peak of 2.9-1.8 ms and thereafter completely inactivated. The relationship between ICa and IK(Ca) is discussed. The Ca2+-k+ repolarizing system is expected to be mainly associated with action potentials arising from a depolarized neurone, whereas the IA current (Belluzzi, Sacchi & Wanke, 1985) dominates the repolarization mechanism at the normal membrane potential. The effect of muscarine was examined. Muscarine (10-50 microM) produced a fall in conductance with a voltage dependence similar to that exhibited by GK(Ca) and was ineffective when removing extracellular Ca2+ or adding Cd2+. A partial suppression of ICa by muscarine is demonstrated. It is suggested that the decrease of the outward current magnitude in the presence of muscarine may be accounted for qualitatively by the reduction in ICa.

Alcohol and the calcium-dependent potassium transport of human erythrocytes

In vitro exposure of human red blood cells to ethanol (100 and 400 mM) was found to increase the initial rate of calcium-dependent potassium efflux through the red cell membrane. This effect of ethanol was apparently not due to an elevation of the intracellular free calcium but rather to a direct action of the drug on the transport process as, (1) intracellular calcium concentrations were tightly buffered with EGTA, (2) ethanol did not alter the efflux of 45Ca from the cells, and (3) dantrolene, which has been proposed to counteract the effect of ethanol on intracellular calcium levels in the erythrocyte, did not inhibit the stimulatory action of ethanol. The efflux of potassium from erythrocytes obtained from chronic alcoholics was not different from that of erythrocytes from non-alcoholic individuals. The relationship of these findings to neuronal potassium transport is discussed.

Adenosine enhances afterhyperpolarization and accommodation in hippocampal pyramidal cells

Adenosine added to the perfusion fluid of rat hippocampal slices at 10 mumol X l-1 enhanced long lasting afterhyperpolarizations after single action potentials, bursts of action potentials or calcium spikes. Accommodation of firing during a depolarizing pulse was potentiated. An increase in calcium dependent potassium conductance is likely to mediate these effects. Adenosine at 50 mumol X l-1 induced a hyperpolarization accompanied by a reduction in input resistance. The hyperpolarization could be reversed at -85 mV. In TTX and TTX-barium treated slices the amplitude of the slow spike was decreased. This may result from a shunting of inward current in the dendrites due to an adenosine induced increase in potassium conductance. It is suggested that adenosine reduces pre- and postsynaptic excitatory signals principally by enhancing one or more potassium conductances. This effect is a powerful means for modulation of neuronal excitability and synaptic efficacy and can explain the antiepileptic activity of adenosine.

Outward currents in voltage-clamped rat sympathetic neurones

Outward membrane currents were studied in neurones of the isolated rat superior cervical ganglion by using a two-micro-electrode or single-micro-electrode voltage-clamp technique. Under current clamp, depolarization elicited electrotonic potentials that displayed marked outward rectification. From negative resting potentials (-70 mV) a short latency, short duration outward rectification was observed. From more positive potentials (-40 mV) a longer latency persistent outward rectification could be demonstrated. Under voltage clamp, four distinct outward currents were observed: a delayed rectifier (IK); a transient outward current (IA); a Ca2+-activated current (IC) and the M-current (IM). The maximum amplitude of IK, IA and IC was 1-2 orders of magnitude greater than IM. Depolarizing from -40 mV to potentials more positive than -20 mV co-activated IK and IC, producing a characteristic N-shaped current voltage curve with a minimum at about +80 mV. Superfusion with Mn2+-containing solutions reduced outward current at all voltages and abolished the N-characteristic; the remaining current (IK) slowly inactivated (tau greater than 1 s). Raising [K+]o from 6 to 36 mmol/l reversed outward tail currents observed in normal solution. Addition of tetraethylammonium ions (1-3 mmol/l) strongly reduced the amplitude of IK and IC. IA was characterized by very rapid activation at potentials more positive than -60 mV and by fast and complete inactivation at potentials in the activation range. The amplitude of IA was dependent on [K+]o and was reduced by external 4-aminopyridine (1-3 mmol/l). The activation appeared to depend on the nature and concentration of divalent cations present in the superfusate. It is concluded that the soma membrane of rat sympathetic neurones, like many other vertebrate and invertebrate neurones, contains multiple populations of K+ channels. The possible functions of these in the control of ganglion cell excitability are discussed.