Inhibitory cardiovascular function of neurons in the caudal ventrolateral medulla of the rabbit: relationship to the area containing A1 noradrenergic cells.

Experiments were performed to test the hypothesis that A1 noradrenergic neurons, in the caudal ventrolateral medulla, have an inhibitory cardiovascular function. The lateral portion of the caudal medulla was systematically explored, using focal electrical stimulation, in anesthetized, paralyzed rabbits and arterial pressure and heart rate responses were recorded. Since electrical stimulation activates fibers of passage as well as neuronal cell bodies, we also determined cardiovascular responses to microinjections of L-glutamate, a neuroexcitatory amino acid with minimum effects on fibers of passage. Histological studies of stimulation sites were combined with catecholamine fluorescence histochemical studies to localize the A1 cells. A decrease in arterial pressure and heart rate, restricted to the region containing A1 cells, was observed with low frequency stimulation and with microinjections of L-glutamate. In contrast, when GABA, an inhibitory amino acid, was microinjected into the A1 region, we observed an increase in arterial pressure and heart rate. L-Glutamate and GABA responses were dose-related. The fall in pressure was independent of the fall in heart rate. Results support our hypothesis, providing evidence that neurons in the A1 area tonically inhibit sympathetic vasomotor tone.