Plasma catecholamines in rats: Daily variations in basal levels and increments in response to stress

Abstract

Chronic tail artery catheters were surgically implanted into male Sprague-Dawley rats to allow for repeated sampling of blood. Animals were housed individually and maintained on a controlled photoperiod (lights on 0600–1800 hours). In the first experiment, blood samples were obtained 2 days after surgery from animals at 0800, 1600, and 2400 hours and again at 0800 hours on the next day. Plasma levels of the sympathetic transmitter norepinephrine (NE) were higher at 2400 hours compared to all other sampling times. In contrast, plasma levels of the adrenal medullary hormone, epinephrine (EPI), were similar across sampling times. In a second experiment, individual rats were transferred to a shock chamber and after 3 minutes received 10 footshocks (2.0 mA, 0.5 sec duration, every 6 sec). Separate groups of rats were stressed in this manner at 0800, 1600, or 2400 hours. Basal and transfer-induced increments in plasma levels of NE and EPI did not vary with time of day. However, footshock-induced increments in plasma levels of EPI were greater at 0800 hours than at 1600 or 2400 hours. Footshock-induced alterations in plasma NE and behavior of rats did not vary with time of day. These results suggest that basal plasma levels of NE are elevated at 2400 hours due to postural changes associated with increased locomotor activity and not to an endogenous circadian rhythm. Furthermore, this effect is consistent within but not across groups of rats. The responsiveness of the adrenal medulla to the intense stress of footshock appears to be greatest in early morning compared to later afternoon and midnight.