Evidence for GABAB autoreceptors in median eminence

The effect of the selective GABAB receptor agonist baclofen was examined on stimulus-induced release of [3H]GABA from crude synaptosomal preparations of median eminence (ME) and pituitary neurointermediate lobe (NI). Baclofen stereospecifically inhibited release of [3H]GABA in a concentration-dependent manner from ME but had no effect in NI. The effect of (+/-) baclofen was partly antagonized by the putative GABAB receptor antagonist delta-aminovaleric acid, but these experiments were complicated by a degree of heteroexchange. These results provide the first evidence for GABAB autoreceptors in the CNS.