Antigonadotrophic and Prolactin-Inhibitory Effects of Melatonin in Anosmic Male Rats

Abstract

Immature (25–26 days of age) male rats housed on 14 h of light per day (lights on 06.00–20.00 h) were either olfactory bulbectomized, rendering them anosmic, or left intact. On the day following bulbectomy, both intact and anosmic animals began receiving single, daily afternoon (17.00–18.00 h) s.c. injections of either 25 μg of melatonin (MEL) or diluent for 5 weeks. At the end of this period, intact MEL-treated rats had significantly lower seminal vesicle, ventral prostate, anterior pituitary and body weights compared to those of the intact controls; however, the testes were unaffected by MEL treatment. In the anosmic diluent-treated animals only the seminal vesicle and anterior pituitary weights were significantly lower than those of the intact controls. Neither anosmia nor MEL treatment alone had any significant effect on either pituitary or serum prolactin (Prl) levels, although serum Prl tended to be depressed in these animals. Anosmic rats receiving MEL had significantly decreased body and testicular weights as compared to all other groups. Seminal vesicle and ventral prostate weights in the anosmic MEL-treated group were reduced to 78 and 80% of the intact controls, respectively, and weighed significantly less than those in either the intact MEL-treated or anosmic groups. Furthermore, olfactory deprivation plus MEL treatment resulted in a significant depression in both pituitary and serum Prl when compared with either intact diluent- or MEL-treated animals. These results indicate that anosmic male rats have an increased sensitivity to the antigonadotrophic and prolactin-inhibitory effects of MEL.