Mechanisms of cardiovascular responses to glycine injected into the dorsal vagal motor nucleus in rat.

Microinjection of glycine into the dorsal vagal motor nucleus of anesthetized rats elicits increases in arterial pressure and heart rate. In the nucleus tractus solitarii, where cardiovascular responses to injection of glycine may be mediated through release of acetylcholine, there is a dense concentration of glycinergic nerve terminals and glycine receptors. In this study, using immunohistochemical methods, we show that glycine terminals and receptors are present in caudal dorsal vagal motor nucleus, although the concentration of both terminal elements is less than in adjacent nucleus tractus solitarii. Responses to glycine microinjected into the dorsal vagal motor nucleus are blocked by the muscarinic antagonist atropine microinjected at the same site; but, unlike responses to glycine in the nucleus tractus solitarii, responses to glycine in the dorsal vagal motor nucleus are not prolonged by physostigmine. These data support the possibility that endogenous glycine may play a role as a transmitter in the dorsal vagal motor nucleus. Responses to glycine may be mediated through actions at muscarinic receptors but not through acetylcholine itself.