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

Abstract

SUMMARY1. y-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.2. In normal bathing medium the decay time constant of the i.j.c. was ri i, -7-6+0-7 ms (clamp potential, Vm -80 mV; temperature, T = 21 TC).3. 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, y (GABA) = 21P6+0-9 pS (Vm = -80 mV; T = 21 TC).4. The mean 'burst length' of the openings produced by a single receptor activation was Tnoise = 4 0 + 0-8 ms, at Vm = -80 mV. This decreased exponentially with hyperpolarization. On average ri1j, exceeded Tnoise although good agreement was found in some fibres.5. 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 rij c = 7-2+004 ms, and their rise time was 3-3+0-12 ms, at Vm = -80 mV. An e-fold decrease in Ti j.c resulted from a 103 + 7-9 mV hyperpolarization; time to peak showed a smaller dependence on Vm.6. 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 (Veq t -40 mV). The amplitude of the quantal event showed a non-linear dependence on Vm.7. 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.

Modulation of gamma-aminobutyric acid-mediated inhibitory synaptic currents in dissociated cortical cell cultures.

“…From analysis of GABA-induced current noise (10), it seems that the voltage dependence of postsynaptic channel kinetics underlies this phenomenon. The nonlinear current-voltage relationship was also seen in other GABA-mediated IPSCs (6,8,9,11). Sometimes the amplitude of our IPSCs decreased to a plateau without recovery.…”

“…We did not investigate which of these factors was operative, and we included data only from recordings in which a stable amplitude was maintained throughout the experiment. Other preparations (6)(7)(8)(9)(10) showed currents with a single exponential decay faster than the one we measured. Only the rate ofthe current decay ofIPSCs from hippocampal GABAergic neurons in culture compares to the fast phase of the current decay of cortical IPSCs (12).…”

“…While a great deal of information is available on the physiology of nicotinic cholinergic transmission, much less is known about GABA-mediated synaptic transmission. Most information is derived from studies of the crayfish and locust inhibitory synapses (6)(7)(8)(9)(10). In good agreement with data from these preparations and from rat hippocampal slices (11), we observed a voltage-dependent decay of the IPSCs recorded from rat cortical cells.…”

“…Voltage-clamp analysis of GABA-mediated inhibitory postsynaptic currents (IPSCs) has been reported on various synapses. The best characterized is the neuromuscular junction of the crayfish and locust (6)(7)(8)(9)(10). IPSCs were also recorded in hippocampal slices (11) and in primary cultures of hippocampal neurons in which diazepam, a ligand for the allosteric modulatory site of GABA receptors, potentiated the IPSCs (12).…”

Excitable properties of adult skeletal muscle fibres from the honeybeeApis mellifera

“…GABA has been reported to act as inhibitory neurotransmitter at several adult insect neuromuscular synapses. Lack of response in our preparation (muscle fibres from tibia) is, however, not surprising, since it has for example been reported that only less than 20% of the muscle fibres from the locust extensor tibiae (femur muscle) received inhibitory nervous supply (Hoyle, 1978;Usherwood and Grundfest, 1965), even though fibres from specific bundles were also shown to respond to GABA in the absence of inhibitory innervation, thanks to extrajunctional GABA receptors (Cull-Candy, 1986;Cull-Candy and Miledi, 1981). Other insect muscle preparations lacked response to GABA, such as mealworm and fly larva, (Jan and Jan, 1976a;Saito and Kawai, 1985).…”