In the Japanese quail gonadal steroids can depress plasma levels of LH and FSH. Since it is now accepted that testosterone metabolites may be metabolized to the tissue-active forms of the hormone, the in-vitro tissue incubation has been combined with steroid replacement therapy in vivo to investigate the physiological roles of various testosterone metabolites as inhibitory feedback agents on gonadotrophin secretion in quail. After the incubation of quail pituitary glands for 3 h with labelled testosterone four metabolites could be identified; androst-4-ene-3,17-dione, 5β-dihydrotestosterone (5β-DHT), 5β-androstane-3α,17β-diol and 5α-DHT. No 5α-androstane-3α,17β-diol was found. Quantitatively, androstenedione was the major metabolite and conversion of testosterone to 5β-metabolites was significantly greater than to 5α-androgens. Hypothalamic and hyperstriatal tissues converted testosterone to androstenedione, 5β-DHT and 5β-androstane-3α,17β-diol but not to 5α-DHT or 5α-androstane-3α,17β-diol.
Gonadotrophin secretion was studied in castrated quail after chronic s.c. implantation of steroid-containing silicone elastomer capsules or acute injection i.m. of steroid in ethanol: saline. Irrespective of the route of administration seven androgens, listed in descending order of potency, reduced the increased levels of plasma LH: testosterone; 5α-DHT; androstenedione; 5α-androstane-3,17-dione; 5α-androstane-3α,17β-diol; 5α-androstan-3α-ol-17-one; 5α-androstan-3β-ol-17-one. No changes in levels of plasma LH were observed after the administration of 5β-DHT, 5α-androstane-3β,17β-diol, 5βandrostane-3α,17β-diol, 5β-androstane-3,17-dione, androst-5-ene-3β,17β-diol or androst-5-en-3β-ol-17-one.
Testosterone and oestradiol-17β were effective in inhibiting secretion of both LH and FSH in young photostimulated quail and completely blocked testicular growth: 5α-DHT inhibited only the release of LH and testicular growth was unaffected.
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