A cytoplasmic progestin receptor has been characterized in the brain of castrated ring doves using an in-vitro assay that measures the binding of a synthetic progestin, [3H]17α,21-dimethyl-19-nor-pregna-4,9-diene-3,20-dione(promegestone; R5020). The affinity of the receptor was similar in both the hyperstriatum and the hypothalamus (Kd≃4 × 10−10 mol/l). Its concentration was higher in the anterior hypothalamus–preoptic area (63 ± 4 fmol/mg (s.e.m.) protein) than in other brain regions (posterior hypothalamus, 33 ± 5; hyperstriatum, 28 ± 3; midbrain, 17 ± 4 fmol/mg protein; n = 7). Progesterone and R5020 competed well for binding but oestradiol and 5β-dihydrotestosterone did not. Corticosterone and, to a lesser extent, testosterone and 5α-dihydrotestosterone competed for binding but much higher concentrations were required than for progestins. Injections of testosterone (200 pg testosterone propionate daily for 7 days) significantly increased the concentration of progestin receptors in the anterior and posterior hypothalamus without having any significant effect on other brain areas. Shorter treatment, lasting for 2 days, with testosterone propionate (200 μg daily), 5α-dihydrotestosterone (200 μg daily) or oestradiol benzoate (50 μg daily) did not always cause this increase but seven injections of oestradiol benzoate (50 pg daily for 7 days) were even more effective than seven injections of testosterone propionate (200 μg daily for 7 days). These data suggested that the sensitivity to progesterone of the brain of the bird changes as a consequence of increases in the level of testosterone in the circulation.
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