Taste information is channeled to the CNS to provide sensory-discriminative information for analyzing the chemical composition of ingesta, motivational signals to impact appetitive behavior, and triggering signals that drive cephalic phase and somatic oromotor reflexes. The 1storder gustatory relay, the rostral nucleus of the solitary tract rNST), is spatially organized along multiple dimensions: a rostrocaudal axis reflecting peripheral innervation, a subnuclear representation reflecting cellular morphology and projections, and a regional specialization reflecting immediate-early gene expression following taste stimulation. Experiments in the current proposal will begin to determine whether this organization is related to the varied tasks the taste system performs. Aim 1 will use antidromic stimulation from the parabrachial nucleus (PBN) to test the hypothesis that there are distinct populations of NST neurons, suggestive of different functions. In particular, we predict that anterior and posterior oral cavity bitter-responsive neurons will have response profiles and projections indicating respective roles in disc/m/native versus reflex functions. Aim 2 will use double-labeling to determine whether the different populations of NST output neurons that project to the PBN, reticular formation (RF) or visceral NST, are neurochemically specialized. It is hypothesized that substantial proportions of each population will contain a marker for glutamate neurotransmission, the mRNA for recently described glutamate transporter, DNIP/VGLUT2, but that nitrergic, catecholaminergic, and peptidergic phenotypes be more segregated. Aim 3 will reversibly inactivate the NST using the GABAA agonist, muscimol, to determine whether the bitter-elicited oromotor rejection reflex has a regional spatial topography, as suggested by the circumscribed distribution of Fos-activated neurons in response to these tastants. Aim 4 will begin to define neurotransmitters in neurons that express Fos-like immunoreactivity in response to bitter tastants. The mammalian gustatory system plays critical roles in regulating food intake and homeostasis. Experiments in the present proposal will begin to unravel the parallel pathways through which the taste system achieves these complex interactions. The emphasis on bitter stimuli will clarify protective mechanisms of the taste system in avoiding noxious, potentially toxic food. [unreadable] [unreadable] [unreadable]