DIURNAL-VARIATION OF THE PITUITARY-ADRENAL AND AVP RESPONSES TO STRESS IN RATS UNDER FOOD RESTRICTION

In the present study plasma corticosterone (B), ACTH and vasopressin (AVP) were determined in 2 groups of Wistar rats, a control group (C) with free access to food, and a food restricted group (FR) with access to food only from 0900 to 1100 h for 14 days. Blood samples were collected by decapitation under unstressed (basal) conditions and immediately after 30 minutes of immobilization stress applied at AM (0800 h) and at PM (2000 h). The basal plasma B concentrations in C group presented diurnal variation with higher values at PM than at AM. The restricted feeding regimen resulted in a 12 hour shift of peak B values. Basal plasma ACTH rhythm in group C is parallel to diurnal B variation, attaining higher levels at PM. Rats under food restriction presented higher basal plasma ACTH levels at AM than C group, however there was no AM-PM difference. In both groups after stress plasma B concentrations increased in comparison to those in the unstressed rats. There was no significant AM-PM difference in the absolute B values attained in C or FR groups. The stress-evoked ACTH response was higher at AM than at PM in group C. In the FR group the ACTH response was similar in the morning and evening, although the ACTH concentrations achieved in the morning was lower than in group C. Basal plasma AVP levels were similar at AM and PM in both C and FR groups. Immobilization stress evoked no plasma AVP response in neither groups. Our results indicate that long term food restriction is able to modify the diurnal rhythms of both basal secretion and stress responsiveness of pituitary-adrenal axis by an as yet uncharacterized mechanism.