The effects of aging on somatocardiac reflexes in anesthetized rats

Nociceptive cutaneous stimulation produces a reflex tachycardiac response that is mediated through the activation of cardiac sympathetic efferents. This response includes reflex components of both supraspinal and spinal origin, depending on which segmental afferent area is stimulated (for a review see Sato et al.: Rev Physiol Biochem Pharmacol 130: 1-328, 1997). We herein examined the effects of aging on these supraspinal and spinal reflexes in anesthetized rats that were 4-7 (young adult), 24-27 (old), and 32-36 (very old) months of age. In central nervous system (CNS)-intact animals, we found that the supraspinal tachycardiac response induced by the pinching of a hindpaw was well preserved in old rats but was significantly attenuated in very old rats, while pinching-induced increases in cardiac sympathetic nerve activity were well maintained in both of these rat populations. In spinalized animals, spinal-mediated changes in heart rate (HR) and cardiac sympathetic nerve activity induced by the pinching of the chest skin were well preserved in both old and very old rats. There were no significant differences in resting HR among the 3 age groups studied, though the maximum HR induced by the beta-adrenergic agonist isoproterenol was reduced with age. The beta-receptor-mediated maximum HR was greater than that induced by pinching in young adult and old rats, CNS-intact and spinalized rats, and very old spinalized rats, while the maximum HR was nearly the same as the pinching-induced HR in CNS-intact very old rats. These results suggest that both supraspinal and spinal neural reflex pathways involved in the cardiac sympathetic response to cutaneous pinching are well preserved in older animals. They also suggest that the decline in the responsiveness of the heart to beta-adrenergic stimulation results in a reduced pinching-induced supraspinal tachycardiac response in very old rats.