Sympatho-adrenal medullary activity and behavior during exposure to footshock stress: A comparison of seven rat strains

Abstract

The effects of footshock stress on circulating levels of norepinephrine (NE) and epinephrine (EPI) and behavioral activation were examined in male rats of seven strains: spontaneously hypertensive (SHR), stroke-prone hypertensive (SP-SHR), Wistar-Kyoto (WKY), Sprague-Dawley (SD), Osborne-Mendel (OM), Brown Norway (BN), and Charles River Wistar (CRW). A catheter was inserted into the ventral caudal artery of each rat to allow for direct measurement of blood pressure and heart rate and for repeated sampling of blood in conscious, undisturbed animals. Two days after insertion of the catheter, resting levels of plasma NE and EPI were higher in SHRs than in CRWs; no other comparisons among strains were significant. No strain differences were noted in the increments in plasma NE and EPI following transfer of rats from the home cage to a shock chamber. There were, however, significant strain differences in the responses of rats to 5 min of footshock (2.5 mA, 0.4 sec duration every 5 sec). Shock-induced increments in plasma NE and EPI were greatest in SHRs and lowest in S-D, O-M, B-N and CRW strains. In addition, there was a significant positive relationship between activity and shock-induced increments in plasma NE (r=0.89, p<0.01) and EPI (r=0.93, p<0.01). These findings indicate an association between activity of the sympatho-adrenal medullary system and behavioral activation during footshock stress. In addition, responsivity to stress was greatest in those strains (SHR and SP-SHR) which are genetically predisposed to increases in blood pressure.