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

Publisher Website
Google Scholar
Abstract
.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.degree. 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 Lorenztzian 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.degree. 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 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 (Veq .apprxeq. -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 600-100 post-synaptic chloride channels.

This publication has 54 references indexed in Scilit: