Aging escalates baroreceptor reflex suppression by the posterior hypothalamus in rats

To examine whether baroreceptor reflex regulation by the posterior hypothalamus becomes modified with age, we compared baroreceptor reflex sensitivity and hypothalamic responsiveness in 2- and 10-month-old rats anesthetized with urethane-chloralose. Hypothalamic regulation of baroreceptor reflex sensitivity was assessed by recording responses to intravenously infused phenylephrine and afferent aortic nerve stimulation after sham operation or electrolytic destruction of the posterior hypothalamus. Regardless of age, reflex bradycardia and sympathoinhibition elicited during pressor responses to phenylephrine, as well as all cardiovascular and sympathetic nerve responses to afferent aortic nerve stimulation, were stronger in rats with bilateral hypothalamic lesions than in age-matched, sham-operated controls. Distinctively, because baroreceptor reflex sensitivity differed with age only in sham-operated controls but not in lesioned rats, we concluded that age-related differences in baroreceptor reflex sensitivity had been abolished by posterior hypothalamic lesions. Other experiments were then performed to compare responses to graded electrical stimulation of the posterior hypothalamus in baroreceptor-intact rats. Pressor and sympathoexcitatory responses to hypothalamic stimulation were larger, and stimulus thresholds were lower at 10 than at 2 months of age thereby suggesting that hypothalamic responsiveness had increased with age. Our results are in accord with the interpretation that aging exacerbates the baroreceptor reflex suppression normally exerted by the posterior hypothalamus.