Sex differences in nuclear androgen receptors in guinea pig brain and the effects of an alpha 2 noradrenergic blocker on androgen receptors.

The binding of the synthetic androgen, [3H]methyltrienolone (R1881) to nuclear androgen receptors (NAR) was studied in various brain areas of gonadectomized male and female guinea pigs treated for 3 days with 2 mg testosterone propionate. The Scatchard analysis of salt-extracted NAR showed a single, high-affinity receptor with a Kd of 0.152 nM and maximum binding sites (Bmax) of 161.9 fmol/mg DNA. The concentration of NAR was highest in the hypothalamus (HYPO) and preoptic area (POA) in both males and females. Lower receptor levels were detected in the amygdala and cortex. NAR levels were significantly lower in the POA of females than in males. Systemic injection of prazosin, an alpha 1-adrenergic antagonist had no effect on NAR concentrations, but an alpha 2-antagonist, idazoxan, significantly reduced the binding of [3H]R1881 to NAR in both HYPO and POA. The reduction in binding of the ligand to receptor was not due to alterations in affinity of NAR, but rather to the decline in the number of receptors.