GABA induced membrane current noise and the time course of the inhibitory synaptic current in crayfish muscle

Characterization and molecular reaction scheme of a chloride channel expressed after axotomy in crayfish

The nerve to the deep extensor abdominal muscle (DEAM) in crayfish species Astacus astacus, containing four excitatory and one inhibitory motor axons, was cut in the third segment on one side of the animal. The distal axon stump was not subject to phagocytosis but was present for months after the axotomy. The two lateral bundles of the DEAM were prepared 4-6 weeks after the axotomy. The gamma-aminobutyric-acid-(GABA-) activated chloride channel of these bundles was characterized by applying pulses of GABA to outside-out patches of the muscle membrane and measuring the responses. Based on the dose/response relationship of the peak current and of the rise time as well as on single-channel kinetics, a detailed molecular scheme for the reaction of the channel with GABA was derived. This scheme contains four binding steps of the agonist to the receptor and two open states. Simulations of the dose/response relationships with this model resulted in a set of rate constants which generate proper fits. In comparison to the channels present in innervated muscles, the channels of denervated muscles have a higher affinity for GABA, a lower single-channel conductance, four versus five binding steps, and non-cooperative binding. The first three of these adaptations of denervated muscles correspond to similar changes in denervated vertebrate muscles.

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.

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.

The effects of temperature, pH and Cl-pump inhibitors on GABA responses recorded from cat dorsal root ganglia

GABA applied by iontophoresis produced GABA-induced currents (GCs) and GABA-induced depolarizations (GDs) which were recorded intracellularly from cat dorsal root ganglia (DRG). Lowering the temperature (37 to 27 degrees C) of the preparation depressed the amplitude of GCs while prolonging their rise-time and decay time. This depressant action was mainly due to a hyperpolarizing shift in the GABA equilibrium potential (EGABA). GABA responses could also be depressed by alkalinization of the superfusion solution or addition of putative chloride pump inhibitors, e.g. SITS, furosemide or bumetanide. However, the mechanism by which these latter procedures depressed GABA responses was not due to a shift in EGABA as occurred with lowered temperature. Instead we suggest that alkalinization or the putative chloride pump inhibitors affect the chloride channel or some other site associated with the GABA receptor complex and cause the depression we observed. GABA responses could be facilitated by lowering the pH of the superfusion solution or by injecting ammonium ion into a DRG. These results suggest that a temperature-sensitive, inwardly directed chloride pump that is resistant to SITS, furosemide or bumetanide, operates in cat DRG.

Effects of glycine on the crayfish neuromuscular junction. I. Glycine-operated inhibitory postsynaptic channels and a glycine-effected decrease in membrane conductance

Inhibitory postsynaptic membrane channels which are activated by glycine were investigated by means of the noise analysis technique. Dose-response curves were obtained for gamma-aminobutyric acid (GABA) in the presence and in the absence of glycine, and it was concluded that GABA and glycine are likely to activate the same receptors. However, glycine proved to have a very low affinity for the inhibitory postsynaptic receptors; this affinity was smaller than that of GABA by a factor of 1 . 10(3)-2 . 10(3). The mean open time tau of the postsynaptic Cl- channels activated by glycine at E = -100 mV and E = -60 mV membrane potentials were tau = 6.1 ms +/- 1.5 ms and tau = 17.7 ms +/- 2.2 ms, respectively. These values are in agreement with the tau obtained by activation with GABA (Dudel et al. 1980); however, on activation by glycine the potential dependence of tau was larger by a factor of 1.35. At E = -100 mV the conductance gamma of glycine-operated channels was about 3 pS which is a third of the respective conductance elicited by GABA. In the presence of high concentrations of glycine (0.1-0.5 mol/l) spontaneous inhibitory postsynaptic currents (sIPSCs) and 'giant' spontaneous inhibitory postsynaptic currents (gsIPSCs) were observed. Furthermore at high concentrations of glycine an additional glycine-induced noise component was found in the power spectra of current fluctuations at higher frequencies. It was concluded that this spectral component resulted from the closing of otherwise open K+ channels, as has been observed already on application of GABA (Dudel and Finger 1980). The mean duration of the low conductance state was tau- = 2.2 ms +/- 0.9 ms and the conductance decrease gamma- coupled to this process was estimated to be about 3 pS. In Na+ free- and Ca2+-enriched bathing solutions the glycine-induced conductances gamma and gamma- were reduced by a factor of about 1.7 while tau and tau- remained unchanged. The decrease in gamma and gamma- was most likely effected by the increase in concentration of divalent cations.

Effects of glycine on the crayfish neuromuscular junction. II. Release of inhibitory transmitter activated by glycine

Glycine applied in the bathing medium at concentrations exceeding 0.1 mol/l elicited high rates of spontaneous inhibitory postsynaptic currents (sIPSCs) in crayfish neuromuscular junctions. This effect of glycine was reversible within seconds. In several experiments on application of 0.5 mol/l glycine the rate of sIPSCs immediately increased to about 10 kHz and thereafter declined exponentially with time constants of between 10 and 20 s. This resulted in a release of about 140,000-200,000 inhibitory quanta per trial. When the readily releasable pool of transmitter had been so depleted by glycine, it was necessary to superfuse the preparation with normal solution for 5-10 min in order to be able to again evoke a high rate of sIPSCs. A similar effect of glycine on spontaneous release was also observed in some preparations which had been previously bathed in zero Ca2+ solution for up to 45 min. Addition of 25 mmol/l Mg2+ to the bathing fluid did not block the glycine evoked release of transmitter. However, in sodium-free superfusions the increase in the rate of sIPSCs induced by glycine was reduced. In the presence of 0.5 mol/l glycine no excitatory miniature currents (sEPSCs) were observed, in fact, glycine depressed excitatory synaptic transmission. In addition to the increasing the rate of sIPSCs, high concentrations of glycine evoked 'giant' sIPSCs (gsIPSCs). They were about 10-15 times larger than the normal sIPSCs and occurred at rates lower than 3 Hz, irrespective of whether the bathing medium contained sodium or not. However, in sodium-free superfusions the time constants of the decay of gsIPSCs were prolonged by a factor 2-3. These results suggest that glycine elicited sIPSCs and gsIPSCs by different mechanisms. Possible mechanisms which might explain the effects of glycine on release of inhibitory transmitter are discussed.

Glutamate activated postsynaptic channels in crayfish muscle investigated by noise analysis

Excitatory synaptic channels in crayfish muscle were investigated under various experimental conditions. Small muscle fibres of length l less than or equal to 0.6 mm were voltage clamped, spatial control of the voltage being sufficient up to at least 500 Hz. Excitatory synaptic current was induced by superfusion of glutamate. The power density spectra of this current could be fitted by single component Lorentz curves. The analysis revealed a mean open time tau noise = 0.93 ms and a conductance gamma = 32.3 pS of the glutamate operated ion channels (membrane potential E = -60 mV, temperature T = 8 degrees C). Both the conductance gamma and the channel closing rate alpha = tau -1 noise increased significantly with temperature (Q10 approximately 2). The temperature dependence of gamma and alpha could be described by Arrhenius equations with the temperature independent activation energies E gamma = 42.3 kJ/mol and E alpha = 50.2 kJ/mol. alpha also dependent on the membrane potential, increasing about e-fold when the membrane was hyperpolarized by 120 mV. The potential dependence varied considerably from fibre to fibre. The mean channel open time tau noise agreed with the time constant of decay tau (sEPSC) of spontaneous excitatory postsynaptic currents (sEPSCs).

Analysis of miniature spontaneous inhibitory postsynaptic currents (sIPSCs) from current noise in crayfish opener muscle

Deviating from the normal situation, some crayfish muscle fibres showed spontaneous inhibitory activity: discharge of large inhibitory postsynaptic currents, IPSCs, alternating with long lasting bursts of current noise. Analysis of the bursts of current noise revealed that they are composed of spontaneous miniature unit currents, sIPSCs. In the burst periods the sIPSCs occurred with an average rate of 3.5--10 k Hz and had an amplitude of about alpha = 90 pA at a driving force delta E = 10 mV. The peak conductance gamma alpha = alpha/delta E of the sIPSCs was gamma alpha = 9.2 nS +/- 0.5 (S.D., n = 5) for membrane potentials between E = --60 mV and E = --80 mV. gamma alpha seemed to decrease when the membrane was hyperpolarized. The time constants of decay, tau of the sIPSCs were identical with tau of the IPSCs. Further, tau and its potential dependence agreed with the mean lifetimes of inhibitory postsynaptic channels operated by gamma-aminobutyric acid (GABA) [Dudel et al. 1977, 1980]. Synchronized opening of about 750 inhibitory synaptic channels generates a sIPSC. Analysis of this anomalous bursting inhibitory activity thus yields the size of the inhibitory quantum of transmission, which could not be obtained from IPSCs.

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.

GABA and muscimol open ion channels of different lifetimes on cultured mouse spinal cord cells

Intracellular recordings of cultured mouse spinal neurons were used to examine the effects of gamma-aminobutyric acid (GABA) and the GABA analog muscimol on neuronal excitability. Muscimol was about twice as effective as GABA in increasing membrane conductance to C1-ions. Fluctuation analysis of membrane current responses showed that muscimol activated ion channels with the same conductance as GABA but with about the average duration. The results provide an economical explanation for the greater potency of muscimol in depressing excitability.

Efficacy of the two-microelectrode voltage clamp technique in crayfish muscle

Crayfish muscle fibres of different dimensions were voltage clamped and white noise current was injected into the fibres at various distances from the voltage clamp current electrode. The clamp current was measured and power spectral densities were calculated. This method revealed the efficacy of the voltage clamp in these fibres. In large fibres (l = 1.8-2.0 mm; diameter = 100-180 micrometer) a space clamp was achieved only for a band width delta f = 40 Hz. At a distance of 100 micrometer from the clamp electrodes delta f was 250-500 Hz. In fibres of medium size (l = 1.0-1.3 mm; diameter = 60-120 micrometer) delta f was about 80 Hz and about 800 Hz at a distance of 100 micrometer. In experiments with very small muscle fibres (l = 400-600 micrometer; diameter = 30-50 micrometer) delta f was more than 500 Hz. The improvement of the space clamp for the smaller muscle fibres resulted mainly from the reduced total membrane capacity, cm, of these fibres. The limitations of the space clamp could be derived from the impedance properties of the fibres. The band width of the space clamp correlated with the band width for which the square of the absolute impedance, /Zp/2, of the muscle fibre could be described by a simple RC-model. This correlation was demonstrated in a model circuit. Power density spectra of membrane current fluctuations were measured also. To optimize the resolution of these measurements the contribution of instrumental noise was minimized. The effects of instrumental noise are discussed.

Inhibitory synaptic channels activated by gamma-aminobutyric acid (GABA) in crayfish muscle

Small crayfish muscle fibres were voltage clamped and synaptic current elicited by superfused GABA solutions was measured. Analysis of the fluctuations of synaptic current and of relaxations of the current after voltage steps yielded analogous results. The current has two components. The first component is characterized by the opening of synaptic channels with a single channel conductance gamma = 9 pS and an average open time tau = 5 ms, measured at 23 degrees C and - 100 mV. tau depends on the membrane potential, tau E = tau 0 x eE/epsilon, and epsilon was about +100 mV in the average. The channel open time agrees with the time constant of decay of the inhibitory postsynaptic current (IPSC) elicited by a nerve stimulus. The current is carried by chloride ions. The second current component is much slower, the average channel open time was tau s = 33 ms at 23 degrees C and -60 mV. The open time tau s of the slow component also was shortened on hyperpolarization. The reversal potential for the current component was more positive than -50 mV. This slow component also seems to be a synaptic one.

Closing of membrane channels effected by gamma-amino-butyric acid (GABA) in crayfish muscle

In a small crayfish muscle fibers current elicited by superfusion of GABA was studied. In the majority of the fibers the GABA induced synaptic current relaxed after voltage steps in agreement with the known voltage and time dependence of the inhibitory chloride channels. In about 20% of the preparations, however, at low GABA concentrations of 5-50 mumol/l an anomalous i--response occurs, namely a reduced total membrane conductance in response to GABA. This i--response can be described by a closing of membrane channels, in which the average closed time increases on hyperpolarization. Equivalent conductance reductions are also observed in measurements of the power spectral density of the current noise. In addition to the noise spectrum of the inhibitory chloride current (Dudel et al. 1980), a spectrum with a higher value of the corner frequency appeared, which seems to represent the i- component. In contrast to the inhibitory chloride current, the i--response is not blocked by picrotoxin. The receptors and channels which mediate the i--response thus seem to be different from those of the inhibitory iCl. Several lines of evidence indicate that the i--response is due to the closing of potassium channels. The i--response counteracts the inhibitory effect of GABA at low GABA concentrations and can distort the dose-response curve.