Opioid receptor densities analyzed across seasons in the POM and VTA of the dark-eyed junco, Junco hyemalis

The motivation of songbirds to sing is influenced by two brain regions, the medial preoptic area (POM) and ventral tegmental area (VTA), which are located outside the song control system itself. These areas receive opioidergic innervation. Furthermore, the opioid enkephalin has been proposed to play a role in the reward for singing. In order to determine whether seasonal changes in song output relate to seasonal changes in opioid receptor (OR) densities in the POM and VTA, we measured the densities of μ, δ, and κ subtypes in these brain regions in adult male dark-eyed juncos (Junco hyemalis) sampled in spring (singing), summer (singing and breeding), and fall (no singing). Receptor densities in the rostral and caudal portions of the POM were measured separately because these subregions are thought to predominantly influence appetitive and consumatory sexual behaviors, respectively. δ ORs were generally denser than μ or κ in both parts of the POM and μ ORs were denser than the other subtypes in the VTA. Densities of μ ORs in the POM were higher in the summer than in spring or fall, although this difference was statistically significant only for cPOM (p = 0.002). In rPOM, κ OR densities tended to be higher in spring and summer than fall, although this pattern did not reach statistical significance (p = 0.057). In contrast, κ OR densities were lowest in the VTA during the summer compared to spring and fall, although this pattern did not reach statistical significance, either (p = 0.094). Results obtained for cPOM μ ORs suggest a heightened reward potential for sexual behavior during the breeding season.