Inhibitory postsynaptic current in voltage-clamped crayfish muscle

Development of GABA-mediated, chloride-dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices

1. gamma-Aminobutyric acid (GABA)-mediated, Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole-cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2- to 5- (PN2-5), 8- to 13-(PN8-13) and 15- to 20-(PN15-20)day-old CA1 neurones. In the presence of glutamate receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2-5 neurones, but 13 and 25 mV more negative than the RMP in PN8-13 and PN15-20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl-] ([Cl-]p). indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, EGABA agreed with the value predicted by the Goldman-Hodgkin-Katz equation at given concentrations of internal and external anions permeable through GABA-activated Cl- channels, whereas EGABA in older neurones was 8-18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl-]p and resting conductance indicated that the membrane of the PN2-5 neurones was readily permeable to Cl- which followed a passive Donnan equilibrium. Passive distribution of Cl- played a decreasing role in PN8-13 neurones and in PN15-20 neurones. 5. To assess the contribution of outward Cl- co-transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15-20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABAA-mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl- ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic plasticity at crayfish neuromuscular junctions: presynaptic inhibition

Intracellular recordings at sites electronically near terminals of the opener excitor axon in the claw of crayfish (Procambarus simulans) show that stimulation of the inhibitor neuron produces hyperpolarizing or depolarizing presynaptic inhibitory potentials (PIPs). GABA applied anywhere along the length of the opener excitor or inhibitor axons also produces hyperpolarizing or depolarizing potentials. The amplitude of action potentials (APs) at recording sites near some excitor terminals is reduced by an average of 6 mV during presynaptic inhibition, which also reduces excitatory postsynaptic potentials (EPSPs) by 50-70%. The time course of AP reduction equals the time course of EPSP reduction and the amount of AP reduction is independent of the sign or amplitude of the PIPs. All these data are consistent with a hypothesis that a conductance increase produced by GABA in these presynaptic terminals of the excitor axon is responsible for presynaptic inhibition. However, the effect of presynaptic inhibition upon the accumulation of short-term facilitation of excitatory transmitter release is not the same in all muscle fibers. In some terminals, the accumulation of short-term facilitation during short, high-frequency trains of action potentials which are presynaptically inhibited often equals the accumulation of facilitation without inhibition. In other terminals, short-term facilitation accumulated during presynaptic inhibition often does not equal facilitation accumulated in the absence of presynaptic inhibition. These data suggest that some other factor which may contribute to presynaptic inhibition, such as a direct effect to decrease calcium currents, may also affect short-term facilitation in some terminals.

The inhibitory chloride channel of the lobster Panulirus penicillatus neuromuscular junction

1. Single channel activity was recorded from muscle membranes of the lobster Panulirus penicillatus using the patch-clamp technique. 2. Cell-attached, outside-out and inside-out patches were prepared from the deep abdominal extensor muscle. 3. Low amplitude single channel currents were observed in most patches, and were identified as being chloride-currents. 4. The chloride channel was active spontaneously, and tended to desensitize when outside-out patches were exposed to a small jet of glutamate. 5. Amplitude histograms of single channel currents presented a well defined peak of 8 pA at a membrane potential of -160 mV, while open and closed time histograms were fit to single exponential functions with tau open of 3.27 msec and tau closed of 31.58 msec.

Voltage dependence of Ia reciprocal inhibitory currents in cat spinal motoneurones

1. Inhibitory postsynaptic currents (IPSCs) were recorded in voltage clamped posterior biceps or semitendinosus motoneurones of the cat during reciprocal inhibition. 2. Population IPSCs, recorded following stimulation of the whole quadriceps muscle nerve, had an average time-to-peak of 0.51 +/- 0.02 ms (+/- S.E.M., n = 22) and decayed exponentially, with an average time constant of 0.99 +/- 0.04 ms (at 37 degrees C) at resting membrane potentials. 3. Unitary IPSCs, recorded following spike-triggered averaging from an identified reciprocal inhibitory interneurone, had amplitudes of 120-220 pA with an average time-to-peak of 0.40 +/- 0.06 ms (n = 5). The decay of these unitary currents was exponential, with an average time constant of 0.82 +/- 0.07 ms (at 37 degrees C) at resting membrane potentials. 4. The time course of IPSCs was unaffected by either alpha-chloralose or pentobarbitone at concentrations necessary for deep anaesthesia. 5. The peak synaptic current varied linearly with the membrane potential over the range -90 to -30 mV, and had an average reversal potential of -80.7 +/- 1.5 mV (+/- S.E.M., n = 6) when measured using KCH3SO4-filled electrodes. 6. The reversal potential for the IPSC was used to calculate [Cl-]i. This was estimated to be 6.5 mM assuming that the inhibitory synaptic current was mediated purely by Cl- ions. 7. The rate at which synaptic currents decayed was exponentially dependent on the postsynaptic membrane potential, the decay time constant increasing e-fold for a 91 mV depolarization. This result was independent of [Cl-]i or of the magnitude of the synaptic conductance and was interpreted as a voltage dependence of the glycine channel open time. 8. The average unitary peak conductance was 9.1 +/- 1.7 nS (+/- S.E.M., n = 5), corresponding to the opening of approximately 200 glycine-activated postsynaptic channels following neurotransmitter release from a single Ia reciprocal interneurone.

The action of GABA receptor agonists and antagonists on muscle membrane conductance in Schistocerca gregaria

1. The properties of postsynaptic gamma-aminobutyric acid (GABA) receptors in the extensor tibiae muscle of Schistocerca gregaria were studied by conventional electrophysiological recording techniques. 2. GABA and other active GABA receptor agonists produced rapid, dose-dependent, reversible increases in membrane conductance. 3. In two microelectrode experiments the ED50 for GABA was approximately 1 mM. In three microelectrode experiments (assuming short cable theory conditions) the ED50 for GABA was 2.3 mM. The Hill coefficient for GABA estimated from the latter experiments was 1.4. 4. The relative potency of muscimol/GABA at the ED50 for GABA was 1.36. 3-Aminopropane sulphonic acid (3-APS) and isonipecotic acid were weakly active, baclofen and piperidine-4-sulphonic acid (P4S) were inactive. Isoguvacine produced depolarizations and increases in conductance in preparations which hyperpolarized in response to GABA. These depolarizations were enhanced by both picrotoxin and pitrazepin although the increases in input conductance were depressed. 5. Picrotoxin (20 microM), (+)-bicuculline (20-100 microM) and pitrazepin (1-10 microM) all reversibly antagonized GABA-induced responses. Such antagonism was not competitive in the case of picrotoxin and (+)-bicuculline but was competitive for pitrazepin. Schild plot analysis gave an average pA2 value of 5.5 for pitrazepin. 6. The significance of these results is briefly discussed.

Biphasic responses of thalamic neurons to GABA in isolated rat brain slices--II

In isolated thalamic slices, responses of relay neurons to electrophoretically applied GABA were recorded intracellularly and compared with inhibitory postsynaptic potentials evoked by electrical stimulation of the reticularis nucleus of the thalamus. Both reduced the excitability of thalamic neurons and were biphasic in the majority of neurons studied, consisting of an early, negative-going and a later, positive-going component, when recorded close to reversal potential (mean reversal potentials -66.6 and -57.7 mV). Bicuculline and picrotoxin applied electrophoretically reduced conductance increases evoked by GABA in all neurons. The later, positive-going component was more sensitive to these antagonists (applied with submaximal doses) than the early component. Current-voltage relations for responses to GABA, like those for inhibitory postsynaptic potentials, were non-linear in the majority of neurons. In particular, there was a region of reduced slope resistance close to the reversal potential. Holding the membrane at a conditioning potential was found to change the subsequent response and its reversal potential. Positive holding potentials shifted reversal potentials in the positive direction only when GABA was applied during the conditioning period. Negative holding potentials were effective whether GABA was applied during the conditioning period or not. Recovery from these effects followed a similar time course at all membrane potentials tested. Injection of Cl- produced a positive shift in the reversal potential for both components of the response to GABA and of the evoked inhibitory postsynaptic potential. Inhibitory postsynaptic potentials evoked in thalamic relay neurons by stimulation of the nucleus reticularis resembled responses to GABA in their biphasic nature, reversal potentials and sensitivity to antagonists and to changes in intracellular chloride.

'Concentration-clamp' study of gamma-aminobutyric-acid-induced chloride current kinetics in frog sensory neurones

Kinetics of the activation and desensitization phases of gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) were studied in single frog sensory neurones using the 'concentration-clamp' technique which enables perfusion of drugs with the time constant of about 3 ms. Both activation and desensitization phases of GABA response consisted of a single exponential at low concentrations and a double exponential at high concentrations. The time constant of the fast kinetic component in each phase was relatively stable, about 5 ms for activation and 3 s for desensitization over concentrations from 3 X 10(-5) to 3 X 10(-4) M, whereas those of the slow kinetic component decreased with increasing concentrations. The two kinetic components in both phases showed the same reversal potential. The slow and fast activation components recovered sensitivity from desensitization with different time courses: the recovery rate of the fast activation component was slow and that of the slow one, rapid. The peak ICl elicited at GABA concentrations below 10(-5) M increased disproportionally at more negative membrane potentials, thereby suggesting that the activation kinetics is voltage dependent. The steady-state ICl-voltage relationship obtained with less than 10(-5) M-GABA showed a non-linearity, probably due to voltage dependence of activation rather than that of desensitization kinetics. These results suggest the presence of at least two different GABA receptor-Cl- ionophore complexes with a different affinity and kinetics.

Miniature and evoked inhibitory junctional currents and gamma-aminobutyric acid-activated current noise in locust muscle fibres

gamma-Aminobutyric acid (GABA) current noise and inhibitory junctional currents (i.j.c.s) have been examined to give properties of the GABA receptor and its associated synaptic channel. Various procedures were used to identify muscle bundles receiving inhibitory innervation. In normal bathing medium the decay time constant of the i.j.c. was tau i.j.c. = 7.6 +/- 0.7 ms (clamp potential, Vm = -80 mV; temperature, T = 21 degrees C). Most muscle fibres were sensitive to ionophoretically applied GABA, irrespective of the presence of inhibitory innervation. GABA current noise obtained at junctional sites gave spectra which were fitted usually with a single Lorentzian component, or occasionally with the sum of two Lorentzians. The conductance of the single inhibitory channel was, gamma (GABA) = 21.6 +/- 0.9 pS (Vm = -80 mV; T = 21 degrees C). The mean 'burst length' of the openings produced by a single receptor activation was tau noise = 4.0 +/- 0.8 ms, at Vm = -80 mV. This decreased exponentially with hyperpolarization. On average tau i.j.c. exceeded tau noise although good agreement was found in some fibres. I.j.c.s were examined in greater detail after excitatory synaptic receptors had been desensitized with 10(-3) M-L-glutamate to abolish all excitatory synaptic activity. Their decay time constant was tau i.j.c. = 7.2 +/- 0.4 ms, and their rise time was 3.3 +/- 0.12 ms, at Vm = -80 mV. An e-fold decrease in tau i.j.c. resulted from a 103 +/- 7.9 mV hyperpolarization; time to peak showed a smaller dependence on Vm. The mean size of the inhibitory quantal event (i.e. response to a single transmitter packet) was estimated from fluctuations in i.j.c. amplitude. Mean quantal content of the i.j.c. was about 30 at normal levels of release. Mean amplitude of the directly measured miniature i.j.c. = 0.65 +/- 0.08 nA at Vm = -80 mV (V eq approximately equal to -40 mV). The amplitude of the quantal event showed a non-linear dependence on Vm. The burst length of the inhibitory channel, produced by a single receptor activation, is longer in duration (at -80 mV) and exhibits greater voltage dependence than the burst length of the excitatory glutamate-activated channel in these fibres. It is estimated that a single quantum of GABA opens about 600-1000 post-synaptic chloride channels.

Inhibitory synaptic currents in voltage-clamped locust muscle fibres desensitized to their excitatory transmitter

Inhibitory junctional currents (i.j.c.s) have been examined in locust muscle fibres to give properties of GABA-channels activated by the neurally released transmitter. A nerve-muscle preparation is described which has proved suitable for voltage-clamp analysis of inhibitory transmission. I.j.c.s were recorded from fibres in which excitatory synapses had been desensitized with glutamate, to abolish excitatory junctional currents. This procedure had no apparent effect on inhibitory channel properties. The time constant of decay of the i.j.c. was 7.7 +/- 0.3 ms, slightly exceeding the time constant of the membrane noise induced by externally applied GABA. Peak i.j.c. conductance decreased with hyperpolarization. I.j.c.s showed measurable fluctuations permitting an estimate of the mean size of the quantal events composing the i.j.c. Their mean size coincided with the spontaneously occurring miniature inhibitory junctional currents that could be directly recorded in some fibres. The inhibitory nerve-impulse released an average of 35 transmitter packets at sites distributed along the muscle fibre length. Since each m.i.j.c. produced a current of about 0.6 nA (at Vm = -80 mV, ECl = -40 mV) the single quantum of inhibitory transmitter opens 600-1000 postsynaptic chloride channels. This is roughly three to four times the number of channels opened by the excitatory transmitter packet at glutamate synapses in the same fibres.

Intracellular recordings from crustacean motor axons during presynaptic inhibition

Action potentials intracellularly recorded from near the nerve terminals of the opener excitor motor axon in crayfish are reduced in amplitude during presynaptic inhibition. The amplitude and sign (hyperpolarizing or depolarizing) of presynaptic inhibitor potentials (PIPs) depends upon the relationship between the resting membrane potential of each excitor axon and its PIP equilibrium potential (which equals the equilibrium potential for gamma-aminobutyric acid).

Junctional and extrajunctional membrane channels activated by GABA in locust muscle fibres

Iontophoretic application of GABA to voltage-clamped locust muscle fibres has demonstrated the presence of both extrajunctional and junctional GABA receptors. Extrajunctional GABA receptors are distinct from extrajunctional glutamate receptors which also occur in these muscle fibres. Inward GABA currents are nonlinearly dependent on membrane potential. Analysis of membrane current noise produced by iontophoretic GABA application shows that for junctional and extrajunctional GABA receptors the mean channel lifetime is 3-4 ms and the single-channel conductance is approximately 22 pS at - 80 mV (T = 21 degrees C). The mean lifetime as previously demonstrated for glutamate-sensitive excitatory channels in locust muscle fibres.

Membrane-potential effects on an inhibitory post-synaptic conductance in Aplysia buccal ganglia

1. Inhibitory post-synaptic currents (i.p.s.c.s) were recorded under voltage clamp using two electrodes placed in neuronal somas of the buccal ganglia of Aplysia, in order to study the effects of membrane potential (Vm) on decay time constant (tau). 2. From -175 to -40 mV, tau did not vary with Vm. At Vm more depolarized than -40 mV, tau decreased. Also at depolarized Vm, cell input resistance (Rin) decreased and many cells showed non-exponential i.p.s.c. decay, including undershoot. These results suggest that the apparently faster tau is an artifact of remote membrane poorly clamped at the low Rin of depolarized levels, rather than a Vm-dependent i.p.s.c. relaxation. 3. Injected current pulses produced voltage relaxations which decayed faster including undershoot, when Vm was depolarized beyond -40 mV. 4. Step commands across the reversal potential were delivered during i.p.s.c.s. Currents reversed direction, relaxing consistent with the new Vm, thus showing that recorded current decay repressants the true time course of i.p.s.c. relaxation, rather than uncontrolled slow axonal charging from a fast remote synaptic current. 5. I conclude that clamp control is poor at depolarized Vm, due to decreased Rin, and the faster, non-exponential decay seen includes a superimposed nonsynaptic current. Tetraethylammonium injected into the presynaptic neurone produces only slight effect on the i.p.s.c. decay time constant, suggesting that the non-synaptic current is unlikely to be due to a voltage-dependent K conductance.

Interaction of permeant ions with channels activated by acetylcholine in Aplysia neurones

1. Aplysia neurones with an excitatory response to acetylcholine (ACh) were voltage-clamped, and the ACh-induced currents were studied using noise and relaxation techniques. The mean channel open time, tau, and the amplitude of the elementary current, iel, were determined from these experiments, and the variation of these parameters with the ionic content of the extracellular solution was analysed. The goal of this work was to test whether permeant ions may bind in a voltage-dependent manner to channel sites and thereby hinder channel closing, as has been proposed before (Ascher, Marty & Neild, 1978a). 2. The relation between tau and the membrane potential V has a similar shape in normal sea water and after total replacement of Na ions with Li or Cs. In contrast, the shape of the tau(V) relation is modified if Na is replaced by Mg, Sr, or Ba. 3. Replacing the divalent cations (Mg and Ca) present in normal sea water with Na results in a decrease of tau and an increase of iel. Both effects are enhanced by cell hyperpolarization. 4. Similarly partial replacement of Na by Sr causes a voltage-dependent decrease of iel. 5. Experiments were performed in solutions containing Na and sucrose, or Mg and mannitol. In both cases tau was smaller than in an isotonic Na or Mg solution. 6. None of the above observations can be accounted for on the sole basis of outer surface potential changes. 7. A quantitative model of the interaction between permeant ions and ACh-sensitive channels is proposed. The possible relevance of this model for the interpretation of tau(V) curves in other systems is discussed.

Effects of membrane potential and temperature on the excitatory post-synaptic current in the crayfish muscle

1. Effects of membrane potential and temperature on the excitatory post-synaptic current (e.p.s.c.) were studied in the voltage-clamped crayfish muscle. E.p.c. was recorded either by measuring the feedback current through an intracellular wire electrode or by focal recording with an extracellular micro-electrode. 2. The amplitude of the e.p.s.c. obtained by the voltage clamp method varied almost linearly with membrane potential between -100 mV and +70 mV, whilst the reversal potential was +23.8 +/- 3.9 mV (S.E. of mean). 3. The declining phase of the extracellular e.p.s.c. was slightly prolonged by depolarization and shortened by hyperpolarization. Potential dependence of the decay time constant was expressed by tau = a exp (AV), with a = 2.78 msec and A = 0.0037 mV-1. 4. The decay time constant had a Q10 of 2.3 and the growth time had a Q10 of 1.5. 5. The voltage dependence of the decay phase of the e.p.s. was the reverse of that found in frog end-plate. It is concluded that the voltage dependence of the time course is not related either to the charge of ions which carry the synaptic current or to the charge of the transmitter.

Voltage dependence of amplitude and time course of inhibitory synaptic current in crayfish muscle

The membrane of small crayfish muscle fibers was clamped to potentials between-150 and -20 mV and amplitude and time course of inhibitory postsynaptic currents (IPSCs) were studied. The IPSCs were recorded extracellularly by means of a focal microelectrode and also as total clamp current. The IPSCs lasted about 40 ms and were slowed by depolarization. The rate constant alpha of decay of the IPSC depended on membrane potential E according to the relation alpha= 44 s-1 -e-8.7 V E at 13.5 degrees C. alpha increased with temperature with a Q10 of 1.9 to 2.5. The amplitude iI of the IPSC depended nonlinearly on E and decreased with time after a potential shift. This was partly due to movement of Cl--ions, the difference (E--EI) between clamp potential and reversal potential for the IPSC decreasing to a few mV within several minutes after a shift in E. The inhibitory conductance gI increased up to 30-fold for 100 mV depolarization also changed with time. However, the the inhibitory permeability PI proved to be independent of membrane potential and time. The potential dependence of gI is thus largely due to changes in the internal Cl--concentration.