THE SUPPRESSIVE EFFECTS OF TESTOSTERONE ON GROWTH IN YOUNG CHICKENS APPEARS TO BE MEDIATED VIA A PERIPHERAL ANDROGEN RECEPTOR - STUDIES OF THE ANTIANDROGEN ICI-176,334

ICI 176,334 is a nonsteroidal anti-androgen that has been shown to sel ectively block peripheral androgen receptors in rats and is presumed t o do so in chickens. In chickens, androgens stimulate secondary sexual characteristics (e.g., comb), but inhibit growth and the immune tissu es. The present study examined the effect of dietary ICI 176,334 (5 or 25 mg/kg body weight) on growth in chickens in the presence or absenc e of testosterone treatment (as l-cm long silastic implants). Treatmen ts began at 2 wk of age and continued through 6 wk of age. Testosteron e alone reduced body growth (average daily gain and shank-toe length, together with weights of the body, skeletal muscle, and the bursa of F abricius, an immune tissue), and stimulated comb development. At the l ow dose (5 mg/kg), ICI 176,334 alone had no effect on body growth or o rgan weight with the exception that comb weight was reduced. At the hi gh dose (25 mg/kg), ICI 176,334 decreased growth (body weight, average daily gain, and shank-toe length) and organ weights (breast muscle, b ursa of Fabricius, testis, and comb weights). This effect may represen t a toxicity. As might be expected with an anti-androgen, ICI 176,334 (at either 5 or 25 mg/kg) completely suppressed the stimulation of com b growth evoked by testosterone. Similarly, ICI 176,334 (5 mg/kg) over came, albeit partially, the growth-suppressive effects of testosterone (on body weight, average daily gain, shank-toe length, and breast mus cle weight) and also had inhibitory effects on the weights of the test is and bursa of Fabricius. The anti-androgen, ICI 176,334, did not inf luence the reduction in circulating concentrations of luteinizing horm one occurring after testosterone treatment. The present data are consi stent with the growth-suppressive effects of testosterone in chickens being mediated via a peripheral androgen receptor. No effects of eithe r testosterone or ICI 176,334 were observed on circulating concentrati ons of insulin-like growth factor-I despite the marked changes in grow th rate.