Age- and sex-related differences in opioid receptor densities in the songbird vocal control system

Avian vocal control regions of adult male songbirds contain opioid peptides and receptors, suggesting that opioids play a role in avian vocal behavior control. In a previous study, we found no difference in opioid receptor de nsities in singing versus nonsinging adult male dark-eyed juncos (Junco hye malis), leading us to hypothesize that opioids are not involved in controll ing song production. To assess whether opioids may be involved in other asp ects of vocal behavior, we used quantitative in vitro autoradiography to co mpare mu and delta opioid receptor densities in vocal control regions of si nging adult males with those of adult females and adolescent (about 3 month s old) males and females. We found mu and delta receptors in all vocal cont rol regions measured. Adolescents had significantly higher opioid receptor densities than did adults in area X (delta), robust n. of the archistriatum (delta and mu), and n. intercollicularis (mu), suggesting a developmental role for opioids in the vocal control system. Based on opioid roles in othe r animal models, we propose that opioids may be involved in song learning, auditory processing, and/or vocal control system development. (C) 1999 Wile y-Liss, Inc.