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

Acetylcholine and GABA mediate opposing actions on neuronal chloride channels in crayfish

A central principle of neural integration is that excitatory and inhibitory neurotransmitters effect the opening of distinct classes of membrane ionic channels and that integration consists of the summation of the opposing ionic currents on the postsynaptic membrane. In tangential cells of crayfish optic lobes, a hyperpolarizing, biphasic synaptic potential is produced by the concurrent action of acetylcholine and gamma aminobutyric acid (GABA). Acetylcholine hyperpolarizes the cell and increases chlorine conductance. GABA depolarizes the cell by closing some of the same chloride channels. Therefore, in this case integration is achieved by the antagonistic actions of two transmitters on the same ionic channel.

Veratridine-induced high-frequency asynchronous release of inhibitory transmitter quanta in crayfish nerve-muscle synapses superfused with normal and low-calcium saline

Crayfish fibres of opener muscles were voltage clamped to E = -80 mV membrane potential (T = 19-22 degrees C), and veratridine (10-100 mumol/l) was added to the superfusate. Within 30-60 s this caused large fluctuations of the clamp current due to vigorous asynchronous quantal release from the inhibitory nerve terminals along the muscle fibre. Excitatory postsynaptic receptors were previously desensitized by application of 5 mmol/l glutamate. Current fluctuations were evaluated by means of the noise analysis technique. Typically, 100 mumol/l veratridine increased instantaneously the quantal release rate n from n less than 1 quantum/s to n congruent to 10,000 quanta/s. Thereafter, n declined exponentially with a time constant of congruent to 70 s. On average, about 500,000 inhibitory quanta could be liberated in this way from the terminals on a single muscle fibre of congruent to 1 mm length. Serotonin (1 mumol/l) facilitated the effect of lower veratridine concentrations (1-10 mumol/l). In opener muscles veratridine-induced asynchronous quantal release showed little dependence on the bath concentration of Ca2+. The opposite was found for fibres of the superficial abdominal extensor muscle. Beside postsynaptic current fluctuations, veratridine elicited slowly changing average postsynaptic DC-currents which could be explained partly by superposition of individual inhibitory quantal currents. These DC-currents suggest that beside inhibitory quantal release another factor activates inhibitory postsynaptic receptors after application of veratridine.

Diazepam action on gamma-aminobutyric acid-activated chloride currents in internally perfused frog sensory neurons

The Cl- current (ICl) in gamma-aminobutyric acid (GABA)-sensitive frog sensory neuron was separated from other Na+, Ca2+, and K+ currents using a suction pipette technique which allows internal perfusion under a single-electrode voltage clamp. Diazepam (DZP) itself evoked no response but facilitated the dose- and time-dependently GABA-induced ICl without changing the GABA equilibrium potential (EGABA) at concentrations ranging widely, from 3 X 10(-9) to 10(-4) M. In the presence of DZP, the GABA dose-response curve shifted to the left without changing the maximum current, indicating that DZP modifies the interaction between GABA and its receptor rather than affecting directly the channel activation step. The enhancement of the GABA-induced ICl by DZP depended neither on the membrane voltage nor on the inward or outward direction of the ICl. DZP also potentiated the ICl elicited by GABA agonists such as beta-alanine, taurine, homotaurine, 5-aminovaleric acid, l-GABOB, d-GABOB, glycine, and muscimol. The GABA response enhanced by pentobarbital (PB) was further enhanced by adding DZP, indicating that DZP and PB do not act in the same way. Ro5-3663, a diazepam analogue, enhanced the GABA-induced ICl only in a narrow range of the concentrations but inhibited the current at concentrations higher than 2 X 10(-6) M.

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.

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).

On the mechanism of action of GABA in pelvic vesical ganglia: biphasic responses evoked by two opposing actions on membrane conductance

Intracellular recording techniques were used to study the response of cat vesical pelvic ganglion neurones loaded with permeable anions to the application of GABA in vitro. In 106/127 neurones GABA evoked a biphasic response, the initial phase of which was depolarizing and associated with a conductance increase; the latter phase was hyperpolarizing and associated with a conductance decrease. The GABA evoked hyperpolarization and conductance decrease were related and behaved as though generated by closure of ion channels open in the resting membrane. The hyperpolarization had a strong inhibitory action on both spontaneous activity, and excitation evoked by depolarizing current injection and pre-synaptic nerve stimulation. Ion substitution experiments suggest that the conductance decrease is primarily to chloride ions, although other ionic species may contribute. Short iontophoretic applications of GABA-evoked monophasic depolarizing excitatory responses, even during the hyperpolarizing response evoked by perfusion of GABA, suggesting no cross-desensitization between the mechanisms generating the initial and late phases of the biphasic response.

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.

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.