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 densities in singing versus nonsinging adult male dark-eyed juncos (Junco hyemalis), leading us to hypothesize that opioids are not involved in controlling song production. To assess whether opioids may be involved in other aspects of vocal behavior, we used quantitative in vitro autoradiography to compare mu and delta opioid receptor densities in vocal control regions of singing adult males with those of adult females and adolescent (about 3 months old) males and females. We found mu and delta receptors in all vocal control 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 other animal models, we propose that opioids may be involved in song learning, auditory processing, and/or vocal control system development.