The Effects of Aging on Somatocardiac Reflexes in Anesthetized Rats

Abstract

Cardiac function is homeostatically regulated in response to internal and external environmental changes. It is well known that, in the elderly, cardiac function is well maintained at rest, but its ability to adjust to environmental changes is reduced compared to younger individuals [1]. For example, the cardiac response to changes in blood pressure (baro-reflex) decline with advancing age [2,3] as a result of reduced sensitivity of baroreceptors [4][5][6].Cardiac responses to changes in the external environment, such as the bradycardia that results from exposure of the face to cold temperatures, also decrease with age [7], though the mechanism by which this occurs remains to be elucidated. It has been reported that nociceptive stimulation of the skin in anesthetized adult animals results in a reflexive increase in HR that Japanese Journal of Physiology, 54, 137-141, 2004 Key words: heart rate, cardiac sympathetic nerve, somatocardiac reflex, aging, ␤-adrenergic receptor.Abstract: 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 pinchinginduced 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 ␤-adrenergic agonist isoproterenol was reduced with age. The ␤-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 ␤-adrenergic stimulation results in a reduced pinching-induced supraspinal tachycardiac response in very old rats.

Age-related change in the effect of gentle mechanical cutaneous stimulation on the somato-cardiac sympathetic C-reflex

“…The present study is the first to demonstrate that cardiac sympathetic A-and C-reflexes are evoked by electrical stimulation applied to somatic afferents in aged rats as in adult ones. This result is consistent with a report by Suzuki et al [9] that showed the response of cardiac sympathetic nerve activity to noxious pinch stimulus in 24-27-and 32-36-month-old rats was not different from in 4-7-monthold rats. Further, we examined the effect of touch on the evoked somato-cardiac sympathetic reflexes in aged rats and found that reflex discharges were not inhibited by touch in most of the rats.…”

“…An initial dose of urethane (0.9-1.1 g/kg i.p.) was based on a previous study [9], and experimental methods were as described previously [3]. Arterial blood pressure and heart rate were continuously measured using a catheter placed in the carotid artery.…”

Reflex Inhibition of Heart Rate and Efferent Cardiac Sympathetic Outflow Induced by Colorectal Distension in Anesthetized Rats

“…Resting HR and cardiac sympathetic nerve activity in spinalized rats were lower than those in CNS-intact rats, as we had reported previously [11,13]. CRD stimulations of 60 and 80 mmHg did not induce a significant change in the HR and cardiac sympathetic nerve activity in these animals, suggesting that the reflex center that mediates CRD-induced bradycardia is supraspinal in its location.…”

“…With the rats in a prone position, the cardiac sympathetic nerves were dissected free following the removal of the second costal bone, as described previously [11,12], and their neuronal activity was amplified using a preamplifier (time constant 0.01 s; S-0476, Nihon Kohden, Tokyo) and sampled using an analog-to-digital converter (PowerLab/8, AD Instruments, Australia) at a rate of 2,000 samples/s that was rectified and integrated every 5 s. The signal recorded at the end of the experiment (after the rat had died) was set as the background noise. The value of the nervous discharge was calculated by subtracting the background noise from the recorded value for statistical analysis [13].…”

Heart Rate Changes in Response to Mechanical Pressure Stimulation of Skeletal Muscles Are Mediated by Cardiac Sympathetic Nerve Activity

“…One characteristic of somatocardiovascular reflexes is that evoked responses differ depending on the type of stimulation. For example, a strong pinch of the skin (Kimura et al, 1995; Sato et al, 1997; Suzuki et al, 2004) and noxious thermal stimulation (Kaufman et al, 1977) generally induce tachycardiac and pressor responses. In contrast, brushing and non-noxious thermal stimulation do not lead to heart rate (HR) responses or provide only a small response (Kaufman et al, 1977; Sato et al, 1997).…”