Taste determines palatability and ultimate disposition of potential foodstuffs in the oral cavity. The overall goal of this project is to determine the circuitry and neurotransmitters in brainstem nuclei that process taste information. The primary sensory nucleus for taste in mammals, the nucleus of the solitary tract (NTS), is difficult to study because of its compact, complex nature and diffuse organization. Accordingly, the proposed experiments exploit the distinct laminar pattern and large size of the NTS-equivalent in a non-mammalian vertebrate. This NTS-equivalent, the vagal lobe, is organized in cortex-like fashion providing an anatomical separation of primary gustatory afferents from interneuron systems. Our past work has shown that the vagal lobe is amenable to anatomical and physiological studies examining the role of neurotransmitters and circuitry within brainstem gustatory nuclei. The proposed series of experiments tests whether excitatory amino acids meet the criteria for being neurotransmitters at three levels of the gustatory neuraxis: I) primary afferent terminals, ii) reflex systems to vagal motor neurons, and iii) second-order projections to the pontine secondary gustatory nucleus (pontine taste area). First, immunocytochemistry and high-pressure liquid chromatography will be used to test for the presence and potassium-evoked release of excitatory amino acid neurotransmitters. Second, the identity and function of excitatory amino acid receptors in these areas will be studies with radioligand binding and in vitro electrophysiology of vagal lobe slices. Third, radiolabeled analogs will be used to test for the presence of high-affinity uptake systems commonly associated with excitatory amino acid neurotransmitters. A second group of experiments will test whether previously identified peptidergic and serotonergic systems impacting on the vagal lobe can modulate excitatory amino acid neurotransmission with the brainstem gustatory nuclei. The anatomical relationship between primary gustatory afferent terminals and peptidergic/serotonergic terminals will be examined anatomically to test for potential presynaptic contacts between these systems. Finally, the effects of the neuropeptides and serotonin on release and transmission will be studied by means of HPLC and in vitro electrophysiology. The proposed studies will resolve existing controversies regarding the role of excitatory amino acids in gustatory neurotransmission and processing.
