Synaptic connections and interactions between area postrema and nucleus tractus solitarius.

The purpose of this study was to examine whether there are separate excitatory and inhibitory pathways from the area postrema (AP) to the nucleus tractus solitarius (NTS) and to examine the synaptic interactions between inputs from the AP and solitary tract (ST) on the NTS neurons. The following observations were made: (1) the predominant projections from the AP to the NTS were excitatory. Among the cells that had AP input, 90% of the cells (43/48) were excited by AP stimulation while 10% (5/48) of the cells were inhibited; (2) inputs from the AP and ST mainly summated occlusively on the NTS neurons, but at near threshold of discharge, the input from one source could facilitate the generation of action potentials induced by the other; and (3) single conditioning stimulation of the AP did not significantly inhibit the NTS neuronal response to ST stimulation, but stimulation of the AP with a train of high frequency stimuli inhibited the response of NTS neurons to ST stimulation and inhibited the evoked response to AP stimulation. The results of this study may help in the understanding of the modulatory role of the AP in the baroreflex and the integration process in the NTS.