The electrophysiology and pharmacology of lobster neuromuscular synapses

Abstract

Effects of drugs on resting potential, membrane resistance, and excitatory and inhibitory postsynaptic potentials (e.p.s.p.'s and i.p.s.p.'s) of lobster muscle fibers were studied using intracellular microelectrodes Acetylcholine, d-tubocurarine, strychnine, and other drugs of respectively related actions on vertebrate synapses were without effects even in 1 per cent solutions (10(-)w/v). Gamma-aminobutyric acid (GABA) acted powerfully and nearly maximally at 10(-7) to 10(-6)w/v. Membrane resistance fell two- to tenfold, the resting potential usually increasing slightly. This combination of effects, which indicates activation of inhibitory synaptic membrane, was also produced by other short chain omega-amino acids and related compounds that inactivate depolarizing axodendritic synapses of cat. The conductance change, involving increased permeability to Cl(-), by its clamping action on membrane potential shortened as well as decreased individual e.p.s.p.'s. Picrotoxin in low concentration (ca. 10(-7)w/v) and guanidine in higher (ca. 10(-3)w/v) specifically inactivate inhibitory synapses. GABA and picrotoxin are competitive antagonists. The longer chain omega-amino acids which inactivate hyperpolarizing axodendritic synapses of cat are without effect on lobster neuromuscular synapse. However, one member of this group, carnitine (beta-OH-GABA betaine), activated the excitatory synapses, a decreased membrane resistance being associated with depolarzation. The pharmacological properties of lobster neuromuscular synapses and probably also of other crustacean inhibitory synapses appear to stand in a doubly inverted relation to axodendritic synapses of cat.