Influence of melatonin on the testicular regression induced by subcutaneous testosterone pellets in male rats kept in long or short photoperiod

Abstract

Daily afternoon injections of 25 micrograms melatonin for 12 weeks had no effect on testicular weights of male rats kept in long photoperiod (14L:10D); similarly, exposure of rats to short photoperiod (2L:22D) had no effect on gonadal weight. However, rats maintained in a long or short photoperiod and implanted every 2 weeks with a 15 mm Silastic pellet containing testosterone showed a significant reduction in testicular weight; this effect was more pronounced in rats exposed to a short photoperiod. Melatonin injections in testosterone-treated rats in a long photoperiod exacerbated the inhibitory effects of testosterone alone. Subcutaneous 2-weekly implants of a beeswax pellet containing 1 mg melatonin reversed the effects of the melatonin injections on relative testicular weights but not those due to short photoperiod exposure. Testosterone implants significantly reduced pituitary LH values in long and short photoperiod-exposed animals, more particularly in those exposed to short photoperiod. Melatonin injections alone or in combination with melatonin pellets did not further exaggerate the depression in pituitary LH due to testosterone alone in long photoperiod-exposed animals; similarly melatonin pellets did not reverse the depression in pituitary LH observed. No significant differences in plasma prolactin concentrations or in thyroxine concentrations or free thyroxine index were observed after any combination of treatments. We therefore suggest that the effects observed with short photoperiod may be due to melatonin.

Lighting Conditions Affect Testosterone Feedback Sensitivity in Castrated Rats

“…Testosterone-primed rats acted like hamsters under a short-day lighting schedule [10,11]. The authors concluded that in the rat the mechanism for regulating photoperiodicity exists but that it is masked by other regulatory mechanisms.…”

“…It is possible to sensitize the reproductive system of the rat with different procedures, e.g. early adminis tration of testosterone [8], olfactory bulbectomy plus blinding [9], by implanting testosterone pellets to intact rats [10,11], with the result that under continuous darkness (blinded animals) or short photoperiod the gonads regress. The regression can be prevented by pinealectomy [12].…”

Increase in testosterone sensitivity induced by constant light in relation to melatonin injections in rats

“…In the present study melatonin was introduced as a single injection in the afternoon each day for one week. It has been shown earlier that melatonin injections (25-50 pg) given at 16:00 h can induce the short-day effect on the gonadal axis of hamsters (Tamarkin et al, 1976) and rats (Vaughan et al, 1988), indicating that the physiological effect of melatonin can be achieved by this dose and by this method of administration.…”

“…Under constant light a low daily dose (125 pg) of testosterone could decrease the serum LH concentration of castrated rats to the level of intact rats, whereas under periodic light a high dose (250 pg) was needed (Porkka-Heiskanen et al, 1989). We wanted to determine whether this sensitization was due to lack of melatonin under constant light, since there is evidence that the increase in testosterone sensitivity associated with photoperiodicity is mediated by pineal melatonin (Ando et al, 1986;Vaughan et al, 1988). We have shown earlier that daily melatonin injections of 50 pg, administered at 09:00 h or 16:00 h to intact or castrated rats do not modulate the secretion of gonadotrophins under periodic or constant light (Porkka-Heiskanen et al, 1990), but the possibility remained that melatonin might prevent the increase in testosterone sensitivity induced by constant light.…”

“…Rats, which normally lack the photoperiodic response of gonadal regression, can be rendered photoperiodic by several manipulations. After, for example, olfactory bulbectomy (Nelson & Zucker, 1981), administration of testosterone pellets (Vaughan et al, 1988) or neonatal androgenization, (Hoffmann et al, 1968), the testes of rats become smaller when the animals are kept under short photoperiod or constant darkness. It has been hypothesized that the mechanism of photoperiodicity also exists in rats, but is normally overruled by other regulatory mechanisms (Wallen & Turek, 1981).…”