Content and In Vitro Release of Endogenous Amino Acids in the Area of the Nucleus of the Solitary Tract of the Rat

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Abstract
We sought to identify amino acid neurotransmitter candidates within the nucleus of the solitary tract in rats. Twenty endogenous amino acids were quantified by reverse-phase HPLC with fluorescence detection (30-fmol limit). Micropunches (1 mm) of the intermediate area of the solitary nucleus were prepared, and the amino acid content determined. Of all the components measured, the putative transmitters Glu, Gly, .gamma.-aminobutyric acid, taurine, Asp, and Ala appeared in greatest concentrations. Bilateral micropunches reperfused in vitro with buffered medium containing 56 mM potassium released Glu, .gamma.-aminobutyric acid, and Gly in a significant manner (p < 0.05) compared with basal levels. With Glu, 78% was calcium-dependent and, therefore, presumably from nerve endings; 99% of .gamma.-aminobutyric acid and 42% of Gly were dependent on calcium. After removal of the nodose ganglion, a bilateral decrease in the calcium-dependent release of Glu and .gamma.-aminobutyric acid, but not Gly, was observed; decreases were significant ipsilateral to the site of ablation. We conclude that (a) Glu is a transmitter of primary afferents in the nucleus of the solitary tract; (b) glutamatergic afferents may interact with .gamma.-aminobutyric acid system(s) in this regiond: (c) Gly also may participate in the mediation and/or modulation of cardiovascular or other visceral reflexes; and (d) amino acid neurotransmission may play an integral role in the neurogenic control of arterial pressure.

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