The gustatory system is responsible for the conscious awareness of taste quality. In addition, the gustatory system discriminates between palatable and unpalatable substances and initiates a series of stereotypic oromotor and autonomic visceromotor responses that lead to ingestion and rejection. Palatable taste stimuli generally activate the chorda tympani (CT), the gustatory nerve that innervates the taste receptors of the anterior tongue. The glossopharyngeal nerve (GN), which innervates the taste receptors on the caudal aspect of the tongue, is relatively more sensitive to unpalatable taste stimuli. The CT and GN project centrally and terminate in a rostral-caudal sequence in the gustatory zone of the nucleus of the solitary tract (NST). In general, each nerve excites different populations of second-order neurons in the NST. Recent studies have identified a population of dopamine (DA) neurons in the gustatory NST in the hamster. DA neurons are in direct synaptic contact with the fibers of the CT and it is likely that these DA neurons are excited by gustatory stimuli that activate the CT. This population of DA motor responses probably contributes to medullary (caudally-directed) projections of the gustatory NST that initiate the motor responses accompanying ingestion. This multisynaptic pathway is probably distinct from a parallel descending motor pathway that is activated by the response of the GN to unpalatable taste stimuli. The proposed studies test the hypothesis that separate populations of DA neurons in the gustatory NST are involved with efferent pathways that control ingestion and rejection. This is explored first at the electron microscopic level by examining the synaptic relationships between the GN and tyrosine hydroxylase-immunoreactive (reflecting the DA phenotype) dendritic processes. Like the CT, we predict that the GN is in direct synaptic contact with DA neurons. The hypothesis that different populations of DA neurons in the gustatory NST are functionally activated by palatable and unpalatable taste stimuli, respectively, is tested using the method of activity-induced Fos expression after strong sucrose and quinine hydrochloride stimulation. Retrograde tracers are used to test the hypothesis that DA neurons contribute to the medullary projections of the gustatory NST and represent one class of output neuron that is specifically concerned with visceromotor and gustatory integration. Finally, in vivo electrophysiological studies test the hypothesis that DA modulates the activity of taste-responsive neurons in the gustatory NST. The source of DA is the intrinsic projections of DA neurons in the gustatory NST. The proposed studies will provide new information about how the gustatory system diverts activity caudally into multisynaptic motor pathways that control a variety of somatic motor and visceromotor responses accompanying ingestion.
