Convergence of vagal and gustatory afferent input within the parabrachial nucleus of the rat.

Our previous anatomical and electrophysiological studies demonstrated that first-order hepatic and gustatory afferents project to separate regions of the solitary nucleus (NST) and no intra-NST interaction of these two sensory systems could be demonstrated. However, iontophoretic injections of horseradish peroxidase into physiologically identified zones of the NST revealed that both of these regions send overlapping projections to the immediately subjacent parvocellular reticular formation as well as the postero-medial parabrachial nucleus (PBN). The present electrophysiological studies demonstrate that an interstitial zone of neurons in the caudal, medial PBN, indeed, receive convergent input from second-order gustatory and vagal afferents. Co-activation of these PBN units by the simultaneous arrival of both input sources frequently resulted in an additive interaction of evoked activity. PBN units lateral and caudal to this zone responded to vagal stimulation only, while units in the anterior and extreme medial portion of the PBN only responded to gustatory stimulation. By virtue of the efferent projections of the PBN, one might speculate that the convergence of information at this locus may, eventually, play a role in directing long term feeding behavior patterns such as learned taste aversion as well as the more transient changes in taste preference with visceral loading.