(-)-Baclofen activates presynaptic GABAB receptors on GABAergic inhibitory neurons from embryonic rat hippocampus.

GABA-mediated monosynaptic inhibitory postsynaptic currents were recorded at synapses between embryonic rat hippocampal neurons in culture. The GABAB receptor agonist L(-)-baclofen reduced inhibitory postsynaptic current amplitude substantially at 10 microM, while the D(+)-isomer of baclofen was without effect. L(-)-Baclofen did not increase postsynaptic membrane conductance or block postsynaptic membrane responses to GABA. It is concluded that L(-)-baclofen depresses GABA-mediated inhibitory postsynaptic potentials by a presynaptic action at GABAB receptors on or near the terminals of the inhibitory neuron. The possible physiological importance of these presynaptic 'autoreceptors' is discussed.