I Introduction UNTIL 35 yr ago, most scientists did not take research on the pineal gland seriously The decade beginning in 1956, however, provided several discoveries that laid the foundation for what has become a very active area of investigation These important early observations included the findings that, 1), the physiological activity of the pineal is influenced by the photoperiodic environment (1–5); 2), the gland contains a substance, N-acetyl-5-methoxytryptamine or melatonin, which has obvious endocrine capabilities (6, 7); 3), the function of the reproductive system in photoperiodically dependent rodents is inextricably linked to the physiology of the pineal gland (5, 8, 9); 4), the sympathetic innervation to the pineal is required for the gland to maintain its biosynthetic and endocrine activities (10, 11); and 5), the pineal gland can be rapidly removed from rodents with minimal damage to adjacent neural structures using a specially designed trephine (12) Since the mid 1960s, research on t

Pineal Melatonin: Cell Biology of Its Synthesis and of Its Physiological Interactions*

There is a daily rhythm in the production of the pineal hormone melatonin in all mammalian species. Production is stimulated by darkness and inhibited by light. This provides a signal reflecting the changing environmental lighting cycle. In seasonally breeding mammals that use changes in the photoperiod to time their reproductive cycles, temporal signals to the reproductive system are controlled by the daily rhythm in melatonin production.

https://science.sciencemag.org/content/sci/227/4688/714.full.pdf

Melatonin: A Coordinating Signal for Mammalian Reproduction?

Melatonin (N-acetyl-5-methoxytryptamine) has revealed itself as an ubiquitously distributed and functionally diverse molecule. The mechanisms that control its synthesis within the pineal gland have been well characterized and the retinal and biological clock processes that modulate the circadian production of melatonin in the pineal gland are rapidly being unravelled. A feature that characterizes melatonin is the variety of mechanisms it employs to modulate the physiology and molecular biology of cells. While many of these actions are mediated by well-characterized, G-protein coupled melatonin receptors in cellular membranes, other actions of the indole seem to involve its interaction with orphan nuclear receptors and with molecules, for example calmodulin, in the cytosol. Additionally, by virtue of its ability to detoxify free radicals and related oxygen derivatives, melatonin influences the molecular physiology of cells via receptor-independent means. These uncommonly complex processes often make it difficult to determine specifically how melatonin functions to exert its obvious actions. What is apparent, however, is that the actions of melatonin contribute to improved cellular and organismal physiology. In view of this and its virtual absence of toxicity, melatonin may well find applications in both human and veterinary medicine.

Melatonin: a multitasking molecule.

This study was conducted to determine whether the pineal indoleamine melatonin mediates the effects of photoperiod on the capacity of estradiol to inhibit LH secretion in the ewe. Patterns of serum melatonin were characterized in pineal-intact ovariectomized ewes treated with sc Silastic estradiol implants and exposed to 90-day alternations between long and short photoperiods. High fluctuating levels of serum melatonin were found during the night, with the duration of elevated serum levels corresponding to the length of the dark period. Transfer from long to short photoperiods caused a rapid change in the duration of nightly melatonin secretion and reduced the negative feedback potency of estradiol upon LH secretion during the natural anestrous season. In pinealectomized ewes, the night-time rise of melatonin was absent, and transfer from long to short days failed to reverse the capacity of estradiol to inhibit LH secretion during anestrus. Nightly infusions of melatonin restored patterns of this indoleamine similar to those observed in pineal-intact ewes exposed to the 90-day alternation between long and short days. The melatonin infusions also restored the reproductive response to the inductive photoperiod: in every ewe, the negative feedback effects of estradiol upon LH secretion were diminished after transfer from long to short days. The amplitude and latency of this escape matched those of pineal-intact animals. We conclude that the pineal mediates the reproductive response of the ewe to inductive photoperiods through its daily rhythm of melatonin secretion.

Pineal Melatonin Secretion Drives the Reproductive Response to Daylength in the Ewe

The effects of the pineal hormone, melatonin, and of pinealectomy on the incidence of mammary adenocarcinoma in Sprague-Dawley rats treated with 7,12-dimethylbenz(alpha)-anthracene (DMBA) were investigated. Melatonin (2.5 mg/kg), begun on the same day as DMBA (5 mg) treatment and given daily in the afternoon for 90 days, significantly reduced the incidence of mammary tumors from 79% (control) to 20% (treated) (p less than 0.002). Rats pinealectomized at 20 days of age and treated with 7 mg of DMBA at 50 days of age had a higher incidence of tumors (88%) compared to control animals (22%). Fifteen mg of DMBA, which resulted in a higher incidence of tumors, reduced the difference between pinealectomized and control animals. Melatonin only partially reversed the effects of pinealectomy, reducing the incidence from 87% (pinealectomy alone) to 63% (pinealectomy plus melatonin); however, the tumor incidence was still lower (27%) in nonpinealectomized, melatonin-treated animals. Assessment of plasma prolactin, luteinizing hormone, follicle-stimulating hormone, estradiol, and cortisol in DMBA-treated tumor-free and tumor-bearing animals revealed a significantly lower plasma prolactin concentration [27 +/- 5 (S.E.) ng/ml] in melatonin-treated animals as compared to vehicle-treated animals [65 +/- 8 ng/ml]. The concentration of plasma prolactin was less in melatonin-treated, pinealectomized rats (55 +/- 10 ng/ml) as compared to vehicle-treated, pinealectomized animals (101 +/- 13 ng/ml). Other hormones were not affected by melatonin treatment. These data support the hypothesis that melatonin inhibits the development of DMBA-induced mammary tumors in the rat while removal of the pineal gland stimulates development of such tumors. Additionally, these experiments provide evidence that these effects may be mediated by a suppression of plasma prolactin levels.

https://cancerres.aacrjournals.org/content/canres/41/11_Part_1/4432.full.pdf

Melatonin inhibition and pinealectomy enhancement of 7,12-dimethylbenz(a)anthracene-induced mammary tumors in the rat.

The daily s.c. injection of 25 µg melatonin (MEL) in oil into adult male hamsters at 7 p.m. (lights on 6 a.m. to 8 p.m.) for 50 days caused involution of the testes, coagulation of

Melatonin inhibition of reproduction in the male hamster: its dependency on time of day of administration and on an intact and sympathetically innervated pineal gland

Intact and pinealectomized adult male hamsters kept in light:dark cycles of 14:10 (in hours) were given daily injections of either 25 µg melatonin, 5-methoxy-tryptophol, N-acetylserotonin, or 6-hydroxymelatonin at 19.00 hours (13 h after lights on). The injections were continued for 50 consecutive days. The daily melatonin injections significantly elevated body weights and depressed the growth of the testes and accessory sex organs and pituitary prolactin levels in intact hamsters. By comparison, in pinealectomized animals daily afternoon melatonin injections failed to alter body or testicular weights or pituitary prolactin levels. These findings indicate that for single injections of melatonin to be antigonadotrophic in the male hamster, the pineal must be intact. In intact animals, 5-methoxytryptophol and N-acetylserotonin slightly depressed the growth of the reproductive organs and pituitary prolactin levels, respectively. Again, these effects were negated by removal of the pineal gland. 6-Hydroxymelatonin was without influence on any of the parameters of reproduction that were measured.

The Effects of Pineal Indoles Given Late in the Light Period on Reproductive Organs and Pituitary Prolactin Levels in Male Golden Hamsters

Acute bilateral superior cervical ganglionectomy (SCGX) completely prevents the nocturnal rises in pineal N-acetyltransferase (NAT) activity and melatonin content in male rats kept in light-dark cycles of 14:10 Unilateral SCGX causes the NAT and melatonin levels to be intermediate between those in sham-operated control rats and those in rats from which both ganglia had been removed

Acute effects of unilateral or bilateral superior cervical ganglionectomy on rat pineal N-acetyltransferase activity and melatonin content

Summary. The timing of reproductive events is critically related to the exposure of a photosensitive animal to environmental light. Male hamsters were maintained in long photoperiods, short photoperiods or in darkness interrupted by brief periods (15 min) of light at 6 h intervals for 11 weeks. Gonadal atrophy did not occur in hamsters maintained in long photoperiods or in those maintained in the interrupted photoperiodic cycle, although hamsters maintained in a photoperiod of 2L:22D showed severe involution of the gonads and accessary sex glands (seminal vesicles and coagulating glands). The results indicate that the time of occurrence of environmental light during the photoperiodic cycle in which an animal is maintained is more important in determining the reproductive status of an animal than is the total amount of darkness or the dark to light ratio.

/pdf/influence-of-a-skeleton-photoperiod-on-reproductive-organ-1lecnaj206.pdf

Influence of a skeleton photoperiod on reproductive organ atrophy in the male golden hamster.

Blinding young adult female hamsters was followed by functional involution of the ovaries and uteri and by the cessation of cyclic vaginal phenomena. Light deprivation was also accompanied by elevated plasma and pituitary levels of luteinizing hormone and depressed levels of prolactin in both the blood and the pituitary gland. Only one of 15 blinded hamsters became pregnant when they were exposed to fertile males for 30 days. Both pinealectomy or chronic melatonin treatment (1 mg melatonin implanted subcutaneously per week in beeswax) prevented the changes in the reproductive organs and in pituitary hormone levels attendant on light-deprivation. Both treatment also returned vaginal cycles to normal and restored plasma prolactin titers. Unlike hamsters that were blinded only, light deprived hamsters that were either pinealectomized or melatonin treated were capable of reproducing when they were caged with fertile males. The reproductive capability (i.e., percent of animals that become pregnant and the sizes of their litters) of these animals was equivalent to that of the untreated control hamsters. This is the first report that chronic melatonin treatment restores fertility in blinded female hamsters.

P. K. Rudeen

Papers

Melatonin inhibition of reproduction in the male hamster: its dependency on time of day of administration and on an intact and sympathetically innervated pineal gland

The Effects of Pineal Indoles Given Late in the Light Period on Reproductive Organs and Pituitary Prolactin Levels in Male Golden Hamsters

Acute effects of unilateral or bilateral superior cervical ganglionectomy on rat pineal N-acetyltransferase activity and melatonin content

Influence of a skeleton photoperiod on reproductive organ atrophy in the male golden hamster.

Restoration of fertility in light-deprived female hamsters by chronic melatonin treatment