Modulation of separation distress by alpha-MSH.

The effects of centrally administered alpha-MSH on separation-induced distress vocalizations (DVs) and squatting were evaluated in domestic chicks for dose-response, time course, and interactions with peripheral naloxone and both peripheral and central morphine. Some of the tests were conducted in both the presence and absence of social stimuli (mirrors or a conspecific). Doses of 0.04 microgram of alpha-MSH or greater eliminated the usual suppression of DVs produced by mirrors or conspecifics. This effect lasted 10-15 minutes and was followed by inhibition of DVs, accompanied by a dose-dependent vigilant squatting posture, that lasted about one hour. These effects showed no development of tolerance to repeated alpha-MSH injections over a six-day period, and no apparent interaction with the effects of peripherally injected naloxone or either peripherally or centrally injected morphine. It is suggested that, in keeping with its role in defensive camouflage in amphibians, alpha-MSH in chicks may activate a central state akin to fear to adaptively modulate DVs and defensive hiding.