Seasonal changes in the densities of α₂‐noradrenergic receptors are inversely related to changes in testosterone and the volumes of song control nuclei in male European starlings

Abstract

The functions of song and the contextual cues that elicit song change seasonally in parallel with testosterone (T) concentrations in male European starlings. T is high in spring when at least one function of male song is that of immediate mate attraction, and low outside the context of breeding, when starlings primarily use song for dominance or flock maintenance. Several brain nuclei that control song contain high densities of alpha(2)adrenergic receptors. T can regulate the density of alpha(2)adrenergic receptors in the avian brain, indicating that the density of alpha(2) adrenergic receptors within the song system might change seasonally. Although the function of seasonal brain variation is not entirely clear, in many songbirds the volumes of song nuclei are largest when T is high and males sing most. Male starlings, however, sing both when T is high and when T is low. Therefore, exploring seasonal changes in T and the volumes of song nuclei could provide insight into the function of these changes. The present study was performed to explore the relationships among T, the volumes of song nuclei, and the densities of alpha(2) adrenergic receptors within the song system of male starlings. Song nuclei (the high vocal center [HVc], robust nucleus of the archistriatum [RA], and Area X) were largest, T was highest, and the density of alpha(2) adrenergic receptors (within HVc and RA) was lowest during the breeding season. The reverse pattern was observed outside of the breeding season. These results suggest that changes in T, volumes of song nuclei, and alpha(2) receptor densities might regulate seasonal changes in song behavior or the context that will elicit song in male starlings.

Structural Changes between Seasons in the Songbird Auditory Forebrain

“…Male starlings sing throughout the year, but during the breeding season (spring), when concentration of plasma testosterone (T) is elevated, singing behavior can be highly sexually motivated (Riters et al, 2000). In the nonbreeding season, when plasma concentration of T is basal (Ball and Wingfield, 1987;Riters et al, 2002), song rather plays a role in social interactions such as maintenance of cohesion and of dominance hierarchies within the flock (Summers et al, 1987;Hausberger et al, 1995;Eens, 1997). As such, male European starlings are an ideal model system to explore seasonal neural plasticity of singing behavior.…”

Evidence that dopamine within motivation and song control brain regions regulates birdsong context-dependently

“…Male starlings sing year-round but the function of song and the social and physiological cues that stimulate song production differ depending upon the context in which it occurs [1]. Within a breeding context (i.e., during the spring breeding season) circulating levels of testosterone (T) are elevated [2,3] and singing behavior is critical for mate attraction. Males increase their rate of song production in response to females [4], and high rates of song may be reinforced through copulation [5][6][7][8][9].…”

ZENK labeling within social behavior brain regions reveals breeding context-dependent patterns of neural activity associated with song in male European starlings (Sturnus vulgaris)

“…Outside of a breeding context, in fall and winter, circulating levels of gonadal steroids (i.e. testosterone (T)) are low [6,56], and male and female starlings are found feeding and roosting in large, mixed-sex flocks [20]. At this time, song appears to play no direct role in mate attraction [57], but rather is thought to be important for flock cohesion and establishing/maintaining social hierarchies within the flock [31,64].…”

“…In late winter and early spring, circulating levels of T become elevated [6,56]. Flocks disperse and males compete over limited numbers of appropriate nest sites [30].…”