Stimulation of the nucleus raphe obscurus produces marked serotonin release into the dorsal medulla of fed but not fasted rats--glutamatergic dependence.

Serotonin interacts with TRH at the dorsal vagal complex (DVC) to augment gastric functional parameters. To ascertain physiologic relevance, patterns of stimulated release at the terminal field were characterized. Stimulation of the nucleus raphe obscurus (nRO) by kainic acid (423 pmol/10 nol) produced marked release of serotonin into dorsal medullary dialysates containing the DVC in freely fed, but no 24-h fasted rats. Probe infusion of kynurenic acid (1 mM), but not acute bilateral cervical vagotomy attenuated nRO-stimulated serotonin release in fed animals. The results suggest that the fed state facilitates serotonin release into the dorsal medulla by a mechanism mediated by activation of excitatory amino acid receptors in the dorsal medulla. Enhanced serotonergic neurotransmission at the DVC may comprise a heretofore unrecognized component of the integrated vago-vagal response to a meal.