Suprachiasmatic nucleus as the site of androgen action on circadian rhythms

Abstract

Androgens act widely in the body in both central and peripheral sites. Prior studies indicate that in the mouse, suprachiasmatic nucleus (SCN) cells bear androgen receptors (ARs). The SCN of the hypothalamus in mammals is the locus of a brain clock that regulates circadian rhythms in physiology and behavior. Gonadectomy results in reduced AR expression in the SCN and in marked lengthening of the period of free-running activity rhythms. Both responses are restored by systemic administration of androgens, but the site of action remains unknown. Our goal was to determine whether intracranial androgen implants targeted to the SCN are sufficient to restore the characteristic free-running period in gonadectomized male mice. The results indicate that hypothalamic implants of testosterone propionate in or very near the SCN produce both anatomical and behavioral effects, namely increased AR expression in the SCN and restored period of free-running locomotor activity. The effect of the implant on the period of the free-running locomotor rhythm is positively correlated with the amount of AR expression in the SCN. There is no such correlation of period change with amount of AR expression in other brain regions examined, namely the preoptic area, bed nucleus of the stria terminalis and premammillary nucleus. We conclude that the SCN is the site of action of androgen effects on the period of circadian activity rhythmicity.

Keywords: Androgen receptor; Brain clock; Free-running period; Hypothalamus; Light; Locomotor activity; Sex difference; Sex steroid; Testosterone.

Figure 1

Photomicrographs show AR-ir in the SCN, POA, BNST and PMN of one representative animal from each experimental group, namely Intact, GDX and Implanted with TP. The Intact animal has high AR expression in each nucleus; the GDX, very low or no expression. The GDX-TP implanted animal with the pellet close to the SCN (bottom left panel) has high expression of AR in the SCN on the side of the implant and very little expression in the contralateral SCN. Other regions express some AR-ir, mainly on the side of the implant. Scale bar length is shown in microns.

Figure 2

Bar graphs show average AR expression (mean ± SEM relative optical density, ROD) in SCN, POA, BNST and PMN on the side of the implant in the Intact, GDX, and GDX-TP experimental groups. AR expression is reduced by GDX compared to intact in all brain regions, and this is rescued by the T implant only in the SCN.* p<0.01 GDX compared to Intact; # p<0.001 T-Implanted compared to GDX.

Figure 3

Top panels: Correlation between site of TP implant and amount of AR expression ipsilateral to side of implant, measured for SCN, POA, BNST and PMN for GDX-TP animals. Bottom panels: Correlation between intensity of ipsilateral AR expression SCN, POA, BNST and change in the period of the free-running rhythm. Each point represents one animal.

Figure 4

Double-plotted actograms showing the change in free-running period of wheel-running activity of two representative animals while intact, following GDX, and after implantation of TP. The intact animals have a free-running period of ~24 hours. Following GDX the period of activity onset lengthens in both animals. The animal shown in the left panel has an implant close to the SCN (labeled “a” in Fig. 5). The animal shown in the right panel bears an implant far from the SCN (labeled “b” in Fig. 5).

Figure 5

Schematics of brain sections show TP implant placement. For orientation the numbers to the right of the schematics indicate distance from bregma (millimeters). The SCN is located at bregma −0.25to −0.75. Each implant site is shown by a black oval, and as some sites overlap, the number of implants at each level is also given. Implants that resulted in a period decrease of >15 min are shown on the left side of the schematic, while those that resulted in a period decrease of <15 min on the right. The locations of implants of animals whose actograms are shown in Fig. 4 are indicated by the letters “a” for the left actogram and “b” for the right actogram.

Figure 6