A comparative study of the action of gamma-aminobutyric acid and piperazine on the lobster muscle fibre and the frog spinal cord.

1 The effects of gamma-aminobutyric acid (GABA) and piperazine were compared on two in vitro preparations, the lobster muscle fibre and the frog spinal cord. 2 Both GABA and piperazine increased the membrane conductance of single lobster muscle fibres without changing the membrane potential; sigmoidal log dose-conductance curves for these agents were obtained and a similar model expressed the receptor interaction of both substances. 3 The actions of GABA and piperazine on lobster muscle were antagonized by picrotoxin and were Cl-dependent. 4 In the frog spinal cord GABA depolarized the dorsal roots presumably by mimicking the activity of the transmitter depolarizing the primary afferents; sigmoidal log dose-response curves for GABA were obtained. 5 On the dorsal roots piperazine produced either depolarizations or biphasic responses; these were mainly indirect effects as was shown by experiments in the presence of tetrodotoxin (TTX). 6 The effects of GABA on the dorsal root (in TTX-treated cords) were antagonized by picrotoxin whereas those of piperazine were more resistant to this alkaloid. The GABA-induced responses appeared to be largely Na+-dependent while both Na+ and Cl- seemed to mediate the effects of piperazine. 7 It is proposed that piperazine has GABA-agonist activity on lobster muscle but little GABA-like activity on the frog spinal cord.