Plasma corticosterone and meal expectancy in rats: effects of low probability cues.

The plasma corticosterone levels of rats were examined prior to their morning meal on Days 1, 7 and 20 of a regimen of 1 hr access daily to food mash and water. The relationship between external cues and meal provision was varied by feeding some groups immediately upon room entry each morning, and others with a variable interval 90-min delay. On Day 1, corticosterone levels of the hungry rats were elevated around the time of light onset, and until 90 min following room entry. On Day 7, corticosterone levels increased in response to room entry in the delay-fed rats, but did not increase further in immediately fed rats. These results were interpreted as indicating that low, but nonzero, expectancy of meal availability elevates corticosterone levels. On Day 20, the immediately fed rats showed a dip in corticosterone levels after room entry, responding to cues highly predictive of imminent meal availability. Delay-fed rats no longer showed an elevation to room entry. The relationship between corticosterone level and meal expectancy is essentially curvilinear in the hungry rat. The corticosterone levels are high when there is uncertainty about whether food is coming or not, and low when there is a very high or very low probability that food is coming.