Catecholaminergic nucleus tractus solitarius lesions in anesthetized rats alter baroreflexes differently with age

To explore whether catecholaminergic lesions produced chemically in the nucleus tractus solitarius (NTS) would alter the baroreflex impairment that normally occurs with age, we compared baroreflex responses in 3- and 14-month-old rats given bilateral microinjections of 6-hydroxydopamine (6-OHDA) or its vehicle into the NTS. After 2 weeks, basal pressures were unaffected but heart rates were lower in 14- than in 3-month-old rats. Sympathetic nerve inhibition elicited reflexly during intravenous infusions of sodium nitroprusside was reduced in 3-month-old rats, but increased in 14-month-old rats. Depressor, bradycardiac, and sympatho-inhibitory responses to afferent aortic nerve stimulation were likewise reduced in 3-month-old rats and oppositely enhanced in 14-month-old rats. Enhanced bradycardia probably does not depend on increased parasympathetic sensitivity because responses to vagal stimulation did not differ between age groups. Although underlying mechanisms remain uncertain, by showing that baroreflex responses become altered differently in 3- and 14-month-old rats by 6-OHDA these results suggest that the baroreflex changes normally occurring with age may be due, at least in part, to modified catecholaminergic mechanisms in the NTS.