Aging reduces cardiovascular and sympathetic responses to NTS injections of serotonin in rats.

Serotonergic mechanisms for baroreflex modulation could become altered with age. This possibility was explored by comparing cardiovascular and sympathetic effects elicited in 2-month- and 24-month-old rats by injecting serotonin (5-HT) directly into the nucleus tractus solitarius (NTS) which is the primary baroreflex relay station in the medulla. Ensuing decreases in mean pressure, heart rate, and renal nerve firing were significantly smaller in 24-month-old than in 2-month-old rats. By contrast, similar injections into the NTS of the vehicle alone were ineffective in both age groups. Postmortem examination of brain sections showed that NTS injection sites were equally distributed in both age groups, thereby indicating that the brain areas affected by 5-HT were identical regardless of age. Reduced sensitivity of peripheral myocardial beta-adrenergic and vascular alpha-adrenergic receptors was considered partly responsible because 5-HT injected into the NTS lowers blood pressure by decreasing sympathetic vasomotor tone and slows the heart by increasing vagal tone with reciprocal sympatho-inhibition. But since reduced adrenergic sensitivity would not account for the concurrent decrease in renal nerve firing, a more logical explanation is that the sensitivity of serotonergic mechanisms in the NTS for inhibiting blood pressure, heart rate, and renal nerve activity decreases with age.