The effects of melatonin on isolation distress in chickens.

Melatonin (0.1-1.0 mg/kg) reduced isolation-induced distress vocalizations (DVs) in young domestic chickens in a dose-dependent manner. This effect was unaffected by the administration of d-amphetamine (1.0 mg/kg) suggesting that melatonin's effects were not merely due to fatigue. The melatonin reduction in DVs was not naloxone reversible, indicating an action independent of the endogenous opioid system. However, chronic pretreatment with naltrexone facilitated the melatonin effect, suggesting a complex relationship between melatonin and the endogenous opioids in regulating distress vocalizations. Chickens exhibited a marked reduction in DVs when isolation chambers were darkened, suggesting endogenous, as well as exogenous, melatonin mediation of isolation distress; however; pinealectomy only partially reversed the darkness effect. Pinealectomized animals, like control animals, exhibited a reduction in DVs following melatonin treatment; however, the melatonin effect was shorter lasting. The implications that these results may have for socialization and emotional distress are discussed.