Optical responses to micro-application of GABA agonists in the embryonic chick brain stem

We investigated optical responses induced by micro-application of muscimol and baclofen in the embryonic chick brain stem. Muscimol evoked biphasic optical signals which were similar to those induced by GABA. The first component was a dye-dependent absorption change that reflected membrane depolarization, and the second component was an intrinsic optical change coupled with changes in the membrane potential. On the other hand, baclofen did not elicit any optical change. The optical responses induced by muscimol persisted in the presence of picrotoxin and 2-hydroxysaclofen, suggesting that they contain a component which is not mediated by classical GABA receptors.