The pharmacology of endorphin modulation of chick distress vocalization

Intraventricular injections of beta-endorphin, gamma-endorphin and alpha-endorphin were demonstrated to reduce isolation-induced distress vocalization on 2-4 day old chicks in a dose response manner at doses as small as 12.5 picomoles (pmol). beta-Endorphin was more potent than the other peptides and morphine, while Met-enkephalin was without effect. However, the D-Ala2 substituted form of Met-enkephalin was as potent as morphine. None of the opioid peptides was effective when injected peripherally in doses of 400 pmol/g body weight. Extension of the interval between injection and behavioral observation from 4 minutes eliminated the ability of alpha- and gamma-endorphin to reduce the peeps. Specificity of the opioid effect was determined by testing intraventricular injections (200 pmol) of 9 other endogenously found peptides. Somatostatin, vasoactive intestinal peptide, and human pancreatic peptide reduced the vocalizations modestly, while alpha-MSH reliably increased them.