Synaptic relations of GABAergic neurons in the area postrema

gamma-Aminobutyric acid (GABA)-like immunoreactive neuronal profiles and their synaptic relations in the area postrema of the rat were studied by ultrastructural immunocytochemistry using an antibody directed against GABA. GABA-like immunoreactive (GABA-LI) perikarya and dendrites usually received synaptic inputs from nonimmunoreactive axon terminals. These synaptic contact zones were mostly asymmetrical. Conversely, GABA-LI axon terminals were found to make synapses with nonimmunoreactive perikarya and dendrites, usually of the symmetrical type. Furthermore, GABA-LI axon terminals in synaptic contact with GABA-LI perikarya and dendrites were sometimes found. Although some GABA-LI axon terminals were in close vicinity of other nonimmunoreactive axon terminals, no clear synaptic contacts could be found. On the other hand, GABA-LI profiles could be found close to the capillaries, although they were always separated by a thin layer of glial processes. The above results suggest that the GABAergic neuron receives information from both the circulating blood and other neurons and then performs its depressor function through other neurons.