Presynaptic gamma-aminobutyric acid receptors in the rat anococcygeus muscle and their antagonism by 5-aminovaleric acid.

1 The effects of gamma-aminobutyric acid (GABA) and related drugs on the isolated anococcygeus muscle of the rat were determined. 2 GABA caused a dose-related inhibition of the electrically-evoked twitch response. 3 The maximum response to GABA was a 56.8% depression of twitch response, with an EC50 of 0.68 microM. 4 (+/-)-Baclofen mimicked the effect of GABA (EC50 0.9 microM). (+)-Baclofen was more than 100 times less active than (--)-baclofen. 5 The response to GABA was unaffected by picrotoxin or bicuculline but was antagonized by 5-aminovaleric acid (0.5) mM). 6 Our results suggest that GABAB receptors are present on motor nerve terminals in the rat anococcygeus muscle and that 5-aminovaleric acid is an antagonist of these receptors.