Development of Leydig cell tumors and onset of changes in the reproductive and endocrine systems of aging F344 rats

The age-dependent onset of spontaneous testicular interstitial cell tumors was examined in F344 male rats. Light microscopy of testes established that nodular interstitial cell hyperplasia was evident in 3 of 5 12-month-old rats and in 5 of 5 rats at 15, 18, 21, and 24 months of age. Involution of the seminiferous epithelium was evident in all testes exhibiting extensive interstitial cell proliferation. Striking increments in serum prolactin and estradiol levels were noted with advancing age, whereas serum levels of follicle-stimulating hormone were unequivocally lower at 21 and 24 months than at 6 months of age. No measurable changes were detected in serum testosterone concentrations between 6 and 18 months of age, but marked increments in this androgen, without any measurable change in circulating luteinizing hormone titers, were apparent in 21- and 24-month-old rats. These findings point to a dynamic relationship between testicular interstitial cell tumorigenesis and age-related changes in the synthesis and/or secretion of gonadal and adenohypophyseal hormones.

Effect of testosterone on the hypothalamic-pituitary-gonadal axis of pinealectomized and pineal-intact male rats

Previous studies indicate that steroid hormones alter pineal biochemistry, and it has been suggested that at least part of the negative feedback effect of steroid hormones on pituitary gonadotropin release may be mediated by the pineal gland. In this study, pinealectomy did not alter the inhibitory effect of testosterone on neuroendocine-gonadal activity in the male rat, suggesting that the pineal gland does not mediate the response of the rat hypothalamic-pituitary axis to testosterone.

The interaction of the photoperiod and testosterone in regulating serum gonadotropin levels in castrated male hamsters

Castrated adult male golden hamsters maintained on either a stimulatory (LD 14:10) or a nonstimulatory (LD 6:18 ) light cycle for 10 weeks following surgery were implanted sc with empty or testosterone-filled Silastic capsules of various lengths. Capsules which maintained circulating androgen levels between 0.5 and 1.7 ng/ml reduced serum levels of LH and FSH in castrated animals exposed to LD 6:18 but not in animals maintained on LD 14:10. When serum androgen levels were maintained at about 3 ng/ml, serum gonadotropin levels were reduced 30--35-fold in nonphotostimulated hamsters but only 2--3-fold in photostimulated animals. Maintenance of serum androgen levels at 14--17 ng/ml induced a maximum decrease in serum LH and FSH in animals exposed to either photoperiod. These results indicate that exposure to short days for 10 weeks renders the hypothalamic-hypophyseal axis of the castrate hamster extremely sensitive to the negative feedback effect of testosterone. This alteration in steroid feedback sensitivity may be one way in which the photoperiod acts to alter neuroendocrine-gonadal activity in seasonally breeding animals.

Pituitary responsiveness to LRF in castrated male hamsters exposed to different photoperiodic conditions

The effect of various doses of LRF on pituitary LH and FSH release was examined in castrated adult male hamsters with different photoperiodic histories. Gonadotropin (Gn) release in response to LRF was independent of whether the animals had been exposed to a photostimulatory (LD 14:10) or a nonstimulatory (LD 6:18) light cycle for 60 days following castration. The lowest dose that caused a significant increase in serum Gns was 10 ng LRF/100 g b.w. for LH and 50 ng LRF/100 g b.w. for FSH. These results indicate that photoperiod, which is well known to exert major effects on the reproductive system of the golden hamster, does not do so by directly altering the responsiveness of the pituitary gland to hypothalamic Gn-releasing factor.

Melatonin: effects on the circadian locomotor rhythm of sparrows

The continuous administration of low levels of melatonin via intraperitoneally placed Silastic capsules either (i) shortened the free-running period of activity or (ii) induced continuous activity in house sparrows (Passer domesticus) maintained in constant darkness. After the melatonin-filled capsules were removed, the period of the circadian rhythm of activity lengthened in rhythmic birds and normal rhythmicity was restored in continuously active birds. The results suggest that melatonin is involved in the physiological control of circadian rhythmicity in sparrows.

Melatonin-induced inhibition of testicular function in adult golden hamsters

Melatonin (12-100 mug/day) administered via subcutaneous Silastic implants prevented or suppressed light-induced testicular recrudescence in adult golden hamsters. In addition, melatonin (150 mug/day) induced marked testicular regression in sexually mature hamsters maintained on photostimulatory long days. These results clearly establish that exogenous melatonin can inhibit gonodal function in adult male hamsters.

Melatonin: antigonadal and progonadal effects in male golden hamsters

Melatonin induced marked testicular regression in hamsters maintained on photostimulatory long days (light-dark 14 : 10). In animals maintained on nonstimulatory short days (light-dark 6 : 18), small amounts of melatonin (50 micrograms per day; 100 millimeters capsule length) prevented testicular regression; but testicular atrophy occurred in hamsters that received larger amounts of melatonin (75 to 100 micrograms per day; 150 to 200 millimeters capsule length) and in control hamsters that received none. The results demonstrate that melatonin can exert either pro- or antigonadal effects and emphasize that the effects of melatonin on the testis cannot be properly assessed unless account is taken of the dosage and mode of melatonin administration and the photoperiod on which experimental animals are maintained.