Local and diffuse synaptic actions of GABA in the hippocampus.

In the CNS, gamma-aminobutyric acid (GABA) acts as an inhibitory transmitter via ligand-gated GABAA receptor channels and G protein-coupled GABAB receptors. Both of these receptor types mediate inhibitory postsynaptic transmission in the hippocampus. In addition to these direct postsynaptic actions, GABAB receptor agonists inhibit excitatory transmission through presynaptic receptors on excitatory afferent terminals. However, a physiological role for the GABAB receptors on excitatory nerve endings has not been established. In this study, we have found a brief, heterosynaptic depression of excitatory synaptic transmission in the CA1 region of the hippocampal slice following short-lasting repetitive stimulation and determined that this inhibition is mediated by presynaptic GABAB receptors. The inhibition of GABA uptake greatly enhanced both the presynaptic action of GABA and the slow GABAB-mediated inhibitory postsynaptic current. Transmitter uptake was also found to regulate the "spill-over" of GABA at conventional GABAA synapses. These results suggest that uptake mechanisms restrict the spatial range of both point-to-point synaptic transmission mediated by GABA and its action at a distance.