Corticosterone availability in male rats at rest and during shock avoidance: differences among individuals, temporal...

Repeated measurement of the time-integral of interstitial fluid free corticosterone concentration (corticosterone availability) under home-cage conditions, during exposure to recently unfamiliar apparatus, and during active shock avoidance over 3 days, demonstrated extreme stability within individuals, and consistent differences among individuals, in corticosterone availability under home-cage conditions. These consistent differences among individuals were preserved, while mean levels were doubled, during exposure to apparatus and during shock avoidance after treatment with dexamethasone. However, this persistent pattern of differences among individuals was not apparent during shock avoidance in the absence of dexamethasone. Shock avoidance provoked up to fourfold increases in corticosterone availability. Repeated measurements over 3 days showed consistent hour-to-hour increases within days and a gradual decline over days. Dexamethasone given 4 h before shock avoidance resulted in corticosterone availability intermediate between that in home-cage and that in the previous shock-avoidance session. The situation-dependent loss of consistent interindividual differences is proposed to signal a qualitative change in regulation of corticosterone availability within individuals, and a tactic for using inter-individual differences to study some aspects of complex regulatory systems is suggested. The hour-to-hour increase in corticosterone availability during shock avoidance may be attributable to slow achievement of full adrenal output; the decline over days may be due to behavioural factors.