Some characteristics of pre- and post-synaptic inhibitory receptors at the hermit crab neuromuscular junction.

1 The effects of gamma-aminobutyric acid (GABA), beta-guanidinopropionic acid (betaGP) and picrotoxin on the pre- and post-synaptic receptors of the hermit crab neuromuscular junction were studied quantitatively usine electrophysiological techniques. Reductions in excitatory junction potential (e.j.p.) amplitude and membrane resistance were measured simultaneously from the same cells. 2 The pre- and post-synaptic receptors were activated by the same order of concentration of GABA, whereas betaGP stimulated the pre-synaptic receptors at a concentration ten times lower than was required to affect the post-synaptic membrane. 3 Picrotoxin appeared to antagonize the pre-synaptic action of betaGP in a competitive manner. The affinity constants (+/- s.e. mean) for picrotoxin 5 times 10(-6)M and 2 times 10(-4)M were 6.80(+/-0.46) times 10(5)M-1 and 6.42(+/-1.8) times 10(5)M-1 respectively. 4 The effect of GABA on e.j.p. amplitude also appeared to be antagonized competitively by picrotoxin whereas the post-synaptic effect was antagonized in a non-competitive manner. 5 Possible differences in the nature of the pre- and post-synaptic receptors are discussed.