Complex effects of baclofen on synaptic transmission of the frog optic tectum in vitro.

In the frog optic tectum in vitro, low concentrations of the GABAB agonist (-)-baclofen reversibly depressed field potentials evoked by optic nerve stimulation. This effect, which was dose-dependent and stereo-specific, differed from the potent facilitatory action of bath-applied GABA. A transient enhancement in field potential amplitude often preceded the more consistently observed depression. Both phases of the baclofen response were insensitive to the GABAA antagonist picrotoxin. Since the effects of baclofen and GABA were different, it is suggested that GABAB receptors were unlikely to mediate the action of GABA on synaptic transmission in the optic tectum.