Comparison of carotid sinus baroreceptor sensitivity in newborn and adult rabbits

The impulse activity in single baroreceptor fibers of the carotid sinus nerve to the carotid sinus was studied in newborn rabbits (9--11 days old) during nonpulsatile perfusion of in vitro carotid sinus preparations. Results were compared with those obtained from adult rabbits. Seventy-eight baroreceptor fibers from 10 newborn rabbits showed lower threshold pressure (54.6 +/- 2.5 mmHg) than that of 59 fibers from adult rabbits (74.2 +/- 4.1). The gain of 14 baroreceptors from newborn rabbits (0.62 +/- 0.07 impulses . s-1 . mmHg-1) was higher than that of 21 baroreceptors from adult rabbits (0.38 +/- 0.03). Results provided direct evidence that carotid sinus baroreceptors in the newborn animals were more sensitive to pressure changes than those in the adult. It is suggested that depressed baroreceptor reflex activity previously observed in most newborn animals is not due to underdeveloped afferent activity of the baroreceptors but to other mechanisms including the efferent control systems.