Is there a role for corticosterone in expression of abnormal behaviour in restricted-fed fowls?

Growing parent stock (breeders) of meat-type chickens (broilers), subjected routinely to chronic food restriction, show increased pacing before a single daily meal and increased drinking and pecking at non-food objects (oral stereotypies) afterwards. Expression of these activities is correlated positively with the level of restriction imposed, and is thought to be controlled mainly by central dopaminergic mechanisms. There is published evidence that glucocorticoids can amplify dopamine mediated behaviours, and this paper describes four experiments examining the relationship between corticosterone and behaviour in individually caged broiler breeders. In Experiment 1 (with 3 levels of food restriction), plasma corticosterone concentration was correlated positively with the level of restriction imposed when blood samples were taken in the morning, but not when they were taken in the afternoon. This may be because corticosterone level declines from morning to afternoon with mild but not severe restriction. In Experiment 2 (severe restriction only), plasma corticosterone level did not change significantly with time of day, and mean values of individual birds were not correlated with their observed times spent in oral stereotypies. In Experiment 3 (severe restriction), object pecking increased in a dose-related way after systemic injection of 1-4 mg/kg corticosterone (significant) and 7.5-30 IU/kg ACTH (not significant), and 10-40 mg/kg metyrapone (corticosterone synthesis inhibitor) had no effect. In Experiment 4 (severe restriction), 180 mg/day metyrapone administered in food for 5 days reduced the plasma corticosterone response to injection of 15 IU/kg ACTH on the fourth day, but otherwise had no effect. Significant increases in drinker directed activity after injection of ACTH on the fourth day and 4 mg/kg corticosterone on the fifth day coincided with greatly elevated plasma levels of corticosterone. It is concluded that the oral stereotypies of restricted-fed broiler breeders do respond to induced increases in plasma corticosterone concentration that are supra-physiological, but there may be only a weak association between behaviour and corticosterone within the physiological range.