Development of the baroreflex in the young rat.

The baroreceptor-heart period reflex was assessed in conscious, freely behaving rat pups on postnatal days 6 and 14. The baroreceptor-heart period reflex was elicited using the alpha1-adrenergic agonist phenylephrine to increase blood pressure and the vasodilator, sodium nitroprusside, to decrease blood pressure. The autonomic effects of the baroreceptor manipulations were determined using pharmacological autonomic blockade. The data demonstrate that vasoconstriction produces a potent baroreflex-mediated bradycardia as early as postnatal day 6, which had previously been demonstrated only in anesthetized pups. In the anesthetized pup, the bradycardia is mediated by vagal activation, while we demonstrate that both vagal activation and sympathetic withdrawal occur in unanesthetized animals. In addition, the results replicate previous findings in rats demonstrating minimal cardiac sympathetic activation or vagal withdrawal following vasodilation during the first week of life, but substantial baroreflex-mediated tachycardia by the second week.