Potentiation of GABAA-receptor-mediated responses by barbiturates in the guinea-pig ileum.

In the isolated ileum of the guinea-pig, gamma-aminobutyric acid (GABA) elicited both GABAA- and GABAB-receptor-mediated responses. Barbiturates significantly potentiated the GABAA receptor-induced contractions over the lower dose range of applied GABA (less than 50 microM), without affecting the GABAB-receptor-induced 'after-relaxation', their relative potencies being thiopentone (ThP) greater than pentobarbitone (PB) greater than barbitone (Bb) greater than phenobarbitone (PhB). Also, higher doses of ThP, PB and Bb elicited bicuculline- and picrotoxinin-sensitive GABA-like contractions. PB shifted the dose-response curve for GABA in the presence of bicuculline methochloride to the left without altering the slope or the dose ratio, and not only displaced the dose-response curve for GABA in the presence of picrotoxinin but also restored the slope and the maximum response towards that of the control GABA-induced response. These results further characterise the GABAA-receptor sites of the guinea-pig enteric nervous system, where GABA is evidently a neurotransmitter.