Influence of pharmacological manipulation of dopamine and opioid receptor subtypes on stereotyped behaviour of restricted-fed fowls.

Effects on environmentally induced oral stereotypes (object pecking and drinker directed activity) of antagonists and agonists of dopamine and opioid receptor subtypes were examined in individually caged broiler breeder fowls subjected to chronic food restriction. Three drugs in each category were injected intravenously at three doses, and their effects compared with those of a saline control treatment. With dopamine antagonists, inhibition of both stereotypes was most marked with haloperidol (D2), intermediate with clozapine (D4), and lowest with SCH 23390 (D1). Increased sitting with the high doses of these three drugs may reflect sedation. With dopamine agonists, SKF 38393 (D1) suppressed both stereotypes slightly, quinpirole (D3) did so consistently and potently, possibly reflecting preferential presynaptic action, while bromocriptine (D2) inhibited drinker-directed activity consistently, but its initial suppression of object pecking changed to delayed stimulation with the high dose. This biphasic effect of bromocriptine may reflect change from pre- to postsynaptic action. Two of the opioid antagonists, naltrexone (mu) and MR 2266 (kappa, but also mu), inhibited object pecking partially, while naltrindole (delta) and the opioid agonists fentanyl (mu), BUBU (delta), and PD 117302 (kappa) had delayed and minor effects. These results suggest that expression of object pecking, but not necessarily drinker-directed activity, depends more on activation of D2 dopamine receptors than D1 receptors, the role of D3 and D4 receptors is less clear, and activation of mu and possibly kappa opioid receptors may play a contributory role.