Studies on the Minimal Dosage of Melatonin Required to Inhibit Pineal Antigonadotrophic Activity in Male Golden Hamsters

Abstract

Blinding adult male golden hamsters was followed by atrophy, within 12 weeks, of the testes and accessory sex organs (seminal vesicles and coagulating glands) and by a significant reduction in pituitary prolactin levels. In experiment 1 blind hamsters received subcutaneously implanted melatonin-beeswax (1:24 mg) pellets at the following intervals: once per week, per 2, 3, 4, 6 weeks, or only one pellet during the 12-week experimental period. The melatonin-beeswax pellets, regardless of the frequency of implantation, overcame completely the inhibitory effects of blinding on reproduction and nearly completely the depressant action of light deprivation on pituitary prolactin levels. In the second study the melatonin-beeswax pellets were implanted subcutaneously into blind hamsters every 2 weeks. The pellets contained either 1 mg, 500, 100, 50, or 1 µg melatonin. With the exception of the 1-µg dosage, melatonin again negated almost totally the inhibitory action of darkness on the gonads and accessory organs and also, for the most part, prevented the drop in pituitary prolactin levels. Based on these studies, when melatonin is chronically administered subcutaneously in a beeswax pellet the minimal dosage of melatonin required to counteract the inhibitory effect of darkness on reproduction seems to be less than 3.6 µg/day. The effects of chronic melatonin treatment are similar to those of pinealectomy.

Influence of Subcutaneous Deposits of Melatonin on the Antigonadotrophic Effects of Blinding and Anosmia in Male Rats

“…This is consistent with findings in hamsters where subcutaneous implants of mela tonin also prevent gonadal regression induced by light de privation [14], It is interesting to note that, in rats, dual sensory de privation is required to demonstrate the maximal anti gonadotrophic effects of the pineal gland while light de privation or merely exposure to short photoperiods pro duces the same effects in the hamster [15], In both species, pinealectomy totally reverses the antigonadotrophic effects on reproductive function. The pineal hormone melatonin was believed to account for the ability of the pineal to sup press reproductive physiology; yet, when administered continually (such as in the present study) it obviously acted as a counter antigonadotrophic agent.…”

“…The concept of melatonin being solely anti gonadotrophic in nature has recently been challenged by the discovery that chronic implantation of melatonin pel lets can actually prevent regression of the testes and the accessory sex organs induced by artificially shortened pho toperiods [15], by natural photoperiods during winter months [17], or by daily injection of melatonin [16]. The minimal effective dose of melatonin required to reverse the effects of light deprivation have been studied in male hamsters [14]. The purpose of the present study was to…”

Influence of Morning Melatonin Injections on the Antigonadotrophic Effects of Afternoon Melatonin Administration in Male and Female Hamsters

“…With respect to the lack of a graded response of accesso ry organ weights to different a.m. doses of melatonin, the only other dose-response study on the counterantigonado tropic effects of melatonin in male hamsters was conducted by Reiter et al [23]. They examined the influence of varying the amounts of melatonin in beeswax implants and the fre quency of implants on the reproductive system of blinded hamsters and found that both the testes and accessory or gans responded in a similar manner.…”

Serum Melatonin Response to Melatonin Administration in the Syrian Hamster

“…These effects are similar to those observed in animals maintained in short days. On the other hand, administration of MT as a subcu taneous depot prevents short daily photoperiods from producing gonadal regression [17], Daily infusions of mela tonin for 12 h in the Djungarian hamster will produce go nadal regression while shorter infusions are progonadal [5,6]. Indeed MT can produce either progonadotropic or antigonadotropic effects [ 18] depending upon conditions of the experiment.…”