A1 noradrenergic neurons tonically inhibit sympathoexcitatory neurons of C1 area in rat brainstem

In rats anesthetized with urethane and paralyzed, bilateral microinjections of kainic acid (KA) into the region of caudal ventrolateral medulla (CVL) containing noradrenergic neurons of the A1 group (A1 area) elicited a decrease followed by an increase in arterial pressure (AP), heart rate (HR) and sympathetic renal nerve activity (RNA). The sympathoinhibitory and sympathoexcitatory effects of KA were prevented by bilateral microinjection of tetrodotoxin into an area of the rostral ventrolateral medulla (RVL) containing C1 adrenergic neurons (the C1 area). In contrast, the autonomic responses were not altered by interruption of the two other principal projections of A1 area neurons, namely to the hypothalamus or to the nucleus tractus solitarii. Bilateral microinjections of tyramine, clonidine, alpha-methylnoradrenaline or histamine into the C1 area elicited a dose-dependent, anatomically specific and reversible decrease in AP, HR and RNA. The effect of tyramine was blocked by previous microinjection of reserpine, 6-hydroxydopamine (6-OHDA), or phentolamine into the C1 area. Pretreatment with phentolamine unveiled a hypertensive effect of alpha-methylnoradrenaline. All effects of alpha-methylnoradrenaline were blocked by pretreatment of the C1 area with phentolamine plus DL-propranolol, whereas those elicited by histamine prevailed. Pretreatment of the C1 area with 6-OHDA abolished all changes in AP and HR elicited by microinjections of KA into the A1 area. We conclude that (1) neurons of the CVL tonically inhibit sympathetic activity, (2) this effect is mediated by an action upon vasomotor neurons of the C1 area of RVL, (3) the inhibition is mediated by noradrenergic projections from A1 neurons into the C1 area, and (4) this tonic sympathoinhibitory effect is independent of the baroreceptor reflex.