A study of the action of picrotoxin on the inhibitory neuromuscular junction of the crayfish

1. The effect of picrotoxin on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The potential changes were recorded intracellularly and extracellularly with micro-electrodes. The membrane conductance of the muscle fibre was also measured.2. Picrotoxin depressed the amplitudes of the inhibitory junctional potentials and the potential changes produced by iontophoretically applied gamma-aminobutyric acid (GABA), but had no appreciable effect on the excitatory junctional potentials and the potential changes produced by L-glutamate.3. The presynaptic action of GABA and the neural transmitter was depressed by picrotoxin. The presynaptic action of beta-guanidinopropionic acid was also depressed by picrotoxin.4. The increase in the membrane conductance produced by the addition of GABA in the bath fluid was depressed by picrotoxin. The dose-response relation showed that picrotoxin depressed the conductance increase produced by GABA in a non-competitive manner. The action of picrotoxin on the conductance increase produced by GABA was more effective in low Cl- solution.5. The analysis of the dose-response curves showed that the action of picrotoxin was well expressed by the Michaelis-Menten equation, but the slope of the dose-response curve of GABA was steeper than this relation. It is proposed that the conductance of the junctional membrane was increased by the combination of two molecules of GABA with a receptor, and the attachment of one molecule of picrotoxin to a specific site depressed the conductance increase.

Anion permeability of the inhibitory post-synaptic membrane of the crayfish neuromuscular junction

1. The membrane conductance of the crayfish muscle fibre was calculated and the increase in the membrane conductance brought about by the application of gamma-aminobutyric acid (GABA) was investigated under various conditions. The permeability of the activated inhibitory post-synaptic membrane was estimated from the conductance change.2. When the concentration of Cl(-) was changed by substituting Cl(-) with methylsulphate, a linear relation was obtained between the membrane conductance change produced by GABA and the concentration of Cl(-). Cations have no appreciable effect on the GABA induced conductance change (less than 0.4%).3. The conductance change produced by GABA on the inhibitory post-synaptic membrane was measured in various solutions in which the Cl(-) was replaced by various anions. The degree of permeability depended on the anion species in the following order:Br(-) > Cl(-) > SCN(-) > I(-) > NO(3) (-) > HCOO(-) > ClO(3) (-) > ClO(4) (-) > BrO(3) (-)4. At high pH the conductance change produced by GABA was decreased, while at low pH it increased.5. These results show that the activated inhibitory post-synaptic membrane of crayfish muscle fibres has the characteristics of an anion selective membrane, suggesting the fixed positive charges in the inhibitory membrane.

A study of the inhibitory action of gamma-amino-butyric acid on neuromuscular transmission in the crayfish

1. The effect of gamma-aminobutyric acid (GABA) on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The drug was applied iontophoretically to a junction and the potential changes were recorded intracellularly as well as extracellularly with micro-electrodes.2. The amplitude of the potential produced by iontophoretically applied L-glutamate was decreased by simultaneous application of GABA to the same junctional area. When the membrane potential was clamped at the resting level the clamping current for the L-glutamate injection was not changed by GABA application, indicating that there is no appreciable competition between L-glutamate and GABA for the glutamate-sensitive receptor.3. The excitatory nerve was stimulated repetitively and the excitatory junctional potentials (e.j.p.s) were recorded extracellularly with a micro-electrode placed on the surface of the muscle. The average size of the extracellular e.j.p.s was reduced by the application of GABA.4. The number of quanta released per impulse calculated from the extracellular e.j.p.s was decreased during the application of GABA, while the quantum size remained unchanged.5. The size of the excitatory presynaptic nerve spike was decreased by the application of GABA. However, no appreciable change was observed in the inhibitory nerve spike.6. These results indicate that in addition to its post-synaptic action GABA acts on the excitatory presynaptic nerve terminal and reduces the release of the transmitter.