Previous studies have found opioid peptide-like immunoreactivity in avian vocal control regions, but whether these regions contain receptors for opioid peptides has not been examined. To address this question, we used quantitative in vitro autoradiography to determine the anatomical distribution and to measure the densities of mu, delta, and kappa opioid receptors in vocal control regions (area X, higher vocal center, and nucleus intercollicularis) of adult male dark-eyed juncos (Junco hyemalis). To evaluate whether opioid receptor densities in these regions depend on the activity of the reproductive system, we also measured these densities in birds collected during the spring, summer, and fall. We found area X, the higher vocal center, and nucleus intercollicularis to contain the three receptor types under study, but opioid receptor densities did not vary seasonally in any of these regions. The presence of specific opioid receptors in avian vocal control regions suggests the participation of opioids in the control of vocal behavior. This participation may consist of short-term (e.g., auditory processing) and/or long-term (e.g., neuronal plasticity) influences.
Previously, we found that, unlike adults, adolescent male dark-eyed juncos (Junco hyemalis) maintained large Area X volumes despite having low plasma testosterone concentrations. Other studies indicate that photoperiod may act independently of testosterone to modulate vocal control region (VCR) volumes in adult songbirds. In the present study, we investigated the effects of testosterone and photoperiod on the volumes of four VCRs in adolescent male juncos. To test the hypothesis that VCR volumes in these males are testosterone independent, we treated birds exposed to short days with testosterone and later compared their VCR volumes with those of birds exposed to short days without testosterone. To examine whether photoperiod alone could affect VCR volumes independent of testosterone, we measured these volumes in photorefractory birds exposed to long photoperiod without testosterone. Administering testosterone induced singing, yet increased the volume of only one VCR, the robust nucleus of the anterior neostriatum (RA). In contrast, long photoperiod increased several VCR volumes (Area X, higher vocal center, and RA) despite low testosterone levels, but did not induce singing. Our results suggest a limited role for testosterone, but an important role for photoperiod, in controlling VCR volumes in adolescent male juncos. In addition, the results demonstrate that singing behavior can be induced in adolescent males without a concomitant increase in most VCR volumes.
Previous research established that in several species of seasonally breeding oscine birds, brain areas [vocal control regions (VCRs)] that control vocal behavior learning and expression exhibit seasonal plasticity, being larger during than outside the reproductive period. In adult males, this seasonal decrease correlates with circulating testosterone (T) concentrations. VCRs contain androgen receptors and T plays an important role in neural plasticity and in the control of singing behavior. In behaviorally dimorphic species, VCRs are larger in males than females and change seasonally also in females, but the dependency of these changes on circulating T levels in females has not been established. In free-living adult dark-eyed juncos (Junco hyemalis), a species in which females do not normally sing, the sizes of three VCRs (high vocal center, robust nucleus of the archistriatum, and Area X) were larger in males than females and decreased between summer and fall in both sexes. In males, this decrease was associated with changes in circulating T concentrations. Females, however, had on average undetectable T levels throughout the breeding season. Seasonal changes in VCR volumes in adult females may depend on very low (below detection limit) circulating T concentrations, on nonandrogenic plasma steroids, on androgen (or androgen metabolites) produced in brain tissues, and/or on nonsteroidal factors such as photoperiod or social interactions with conspecific birds.
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