Plasma corticosterone and renin activity during two-way active avoidance learning in spontaneously hypertensive and Wistar-Kyoto rats

Abstract

The spontaneously hypertensive rat (SHR) exhibits increased sympathetic and behavioral responses to several types of environmental stress compared to its normotensive progenitor, the Wistar-Kyoto rat (WKY). The aim of the present study was to investigate response-dampening mechanisms in the SHR. This study examines whether learning to control a stressful environment reduces behavioral and neuroendocrine activation in the SHR. Twelve SHR and 12 WKY were subjected to daily sessions (S) of 20 trials of signaled two-way active avoidance. Another 12 SHR served as maturation controls. Blood samples were collected immediately after S 1, S 5, and S 14 and plasma level of corticosterone (p-CS) and plasma renin activity (PRA) were measured. There was an insignificant tendency for the SHR to exhibit larger increases in p-CS in response to stress than the WKY did. By S 5 both groups had attained the avoidance task. However, the level of p-CS was the same as after S 1, showing that activation is independent of the number of shock pulses received and performance. Postsession p-CS decreased between S 5 and 14, slightly more so in the SHR than in the WKY. The SHR exhibited lower PRA than the WKY, but there were no significant effects of exposure to the avoidance schedule. Behaviorally, the SHR exhibited shorter latency to escape in the first trial than the WKY did. The SHRs were hyperactive compared to the WKY, showing more ambulation and rearing behavior. The maturation control SHR reached the same mean arterial pressure as the SHRs which were tested. After S 14, signal duration was shortened from 10 to 3 s for six additional sessions. Both p-CS and PRA increased, while at the same time ambulation and rearing behavior decreased. These data show that the relationship between behavioral control and adrenocortical activation is normal in the SHR in spite of differences in behavior. The present results also confirm previous data on hyperreactivity to footshocks in two-way active avoidance tasks in the SHR.