Seasonal changes in androgen receptor mRNA in the brain of the white-crowned sparrow

Abstract

In songbirds, neurons that regulate learned song behavior undergo extensive seasonal plasticity in their number and size in relation to the bird's reproductive status. Seasonal plasticity of these brain regions is primarily regulated by changes in circulating concentrations of testosterone. Androgen receptors are present in all of the major song nuclei, but it is unknown whether levels of androgen receptor mRNA in the telencephalic song regions HVC, the robust nucleus of the arcopallium, and the lateral magnocellular nucleus of the anterior nidopallium change as a function of season in white-crowned sparrows. To determine whether seasonal changes in levels of androgen receptor mRNA are specific to the song control system, we also measured levels of androgen receptor mRNA in a limbic nucleus, the lateral division of the bed nucleus of the stria terminalis (the lateral division of the bed nucleus of the stria terminalis). We found that levels of androgen receptor mRNA were higher in HVC and the lateral division of the bed nucleus of the stria terminalis of birds in the breeding condition compared with the nonbreeding condition; however, we observed no seasonal differences in levels of androgen receptor mRNA in either the robust nucleus of the arcopallium or the lateral magnocellular nucleus of the anterior nidopallium. These results are consistent with previous observations that seasonal plasticity of the song nuclei results from testosterone acting directly on HVC, which then exerts transsynaptic trophic effects on its efferent targets. The seasonal change in the expression of androgen receptor in HVC may be one component of the cellular mechanisms underlying androgenic effects on seasonal plasticity of the song control nuclei.

Figure 1

A schematic sagittal drawing of the songbird brain showing the locations and projections of major nuclei in the song control system. The descending motor pathway (black arrows) controls the production of song by the syrinx. The grey arrows indicate the anterior forebrain pathway that is essential for song learning. It indirectly links HVC to the robust nucleus of the arcopallium, via area X in the parolfactory lobe, DLM in the thalamus, and the lateral magnocellular nucleus of the anterior nidopallium in the neostriatum. the lateral magnocellular nucleus of the anterior nidopallium also projects to area X. the lateral division of the bed nucleus of the stria terminalis is a limbic nucleus. The black dots indicate the presence of androgen receptors. Abbreviations: the lateral division of the bed nucleus of the stria terminalis, the lateral division of the bed nucleus of the stria terminalis; DLM, medial portion of the dorsolateral nucleus of the thalamus; the lateral magnocellular nucleus of the anterior nidopallium, lateral portion of the magnocellular nucleus of the anterior nidopallium; the robust nucleus of the arcopallium, the robust nucleus of the arcopallium; V, lateral ventricle; X, area X; nXIIts, tracheosyringeal part of the hypoglossal nucleus.

Figure 2

Dark-field photomicrographs showing the expression of androgen receptor mRNA in the song nuclei the lateral magnocellular nucleus of the anterior nidopallium, HVC, and the robust nucleus of the arcopallium, and in the limbic nucleus the lateral division of the bed nucleus of the stria terminalis, in breeding (long day + testosterone) and nonbreeeding (short day) condition white-crowned sparrows. Sections are in the coronal plane

Figure 3

Quantitative analysis of androgen receptor mRNA expression in the song nuclei HVC, the robust nucleus of the arcopallium, and the lateral magnocellular nucleus of the anterior nidopallium, and the limbic nucleus the lateral division of the bed nucleus of the stria terminalis in breeding (long day + testosterone) and nonbreeding (short day) condition birds. Data are expressed as a ratio of the grain density within the nucleus versus the surrounding tissue. Bars are group means ± SEM. * indicates that the differences between long day + testosterone and short day groups are significant (p < 0.05, t test).