Mary K. Vaughan

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 daily s.c injection of 25 microgram 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 tests, coagulation of gland and seminal vesicles and depression in pituitary prolactin (Prl) levels. Similar injections of MEL given at 9 a.m. completely failed to cause regression of the sex organs or a depression in pituitary Prl levels. Injections of MEL in the p.m. were completely ineffective in inhibiting either the growth of the gonads and adnexa or the pituitary Prl levels if the animals had been pinealectomized. Likewise, superior cervical ganglionectomy, decentralization of the superior cervical ganglia and anterior hypothalamic deafferetation, procedures which interfere with the sympathetic nerve supply to the pineal gland, negated the ability of p.m. MEL injections to inhibit reproduction in male hamsters. The results indicate that daily MEL injections are capable of suppressing reproductive physiology in male hamsters, but only when the indole is injected late in the light period, in this case, 13 h after light on. The findings also illustrate that daily p.m. MEL injections can inhibit reproduction only in animals that have an intact and sympathetically innervated pineal gland.

Melatonin: Its Inhibition of Pineal Antigonadotrophic Activity in Male Hamsters

Exposure of male hamsters to short daily photoperiods (1 hour of light and 23 hours of darkness daily for 9 weeks led to total involution of the testes and accessory sex organs (seminal vesicles and coagulating glands). Pituitary levels of immunoreactive prolaction also decreased by about 60 percent after dark exposure. The inhibitory effects of darkness on the reproductive organs were prevented either by pinealectomy or by the subcutaneous implantation of a melatonin-beeswax pellet into the animals each week. Both pinealectomy and melatonin treatment also returned pituitary levels of prolactin toward normal. The results suggest that melatonin is not the pineal antigonadotrophic factor in the male golden hamster.

Pineal serotonin-N-acetyltransferase activity in four mammalian species

Abstract The activity of pineal serotonin-N-acetyltransferase (NAT) was compared in four mammalian species: the albino rat, the golden hamster, the Mongolian gerbil and the English short-hair guinea pig. In all species a night-time elevation in pineal NAT activity was apparent. In the rat, pineal NAT activity exhibited a dark:light ratio of 60:1; in the hamster and gerbil the ratio was 3:1 while in the guinea pig the dark:light ratio was 1.5:1. With the exception of the guinea pig, the greatest pineal NAT activity was recorded at 04.00 h (8 h after onset of darkness). In the guinea pig the maximal NAT activity was attained at 24.00 h (4 h after onset of darkness).

Food restriction retards aging of the pineal gland

The effects of chronic (40%) food restriction from 6 weeks of age were studied in aging male Fisher 344 rats. When compared with 3-month-old, ad libitum fed rats, pineal N-acetyltransferase (NAT) activity had declined to less than 30% and pineal and serum levels of melatonin to 40% after 28 months when feeding had been ad libitum. Food restriction significantly retarded this development (P less than 0.05) giving NAT and melatonin levels which were twice as high as in the ad libitum fed group. Nighttime levels of pineal serotonin (5-HT) were similar in food-restricted and ad libitum fed old rats but were nearly twice as high (P less than 0.05) as in young rats. There was also a tendency for increased production of 5-hydroxyindoleacetic acid (5-HIAA) in the pineal gland with higher levels of 5-HT. It is concluded that aging in the rat (Fisher 344) is accompanied by a reduction of pineal NAT activity, thereby reducing the production of melatonin and causing a buildup of 5-HT in the pineal gland. It is furthermore proposed that food restriction, which markedly increases the life span and reduces age-related physiological deterioration and diseases in many animals, may mediate some of its effects through a sustained pineal activity in old age.

Pineal antigonadotrophic substances: polypeptides and indoles.

Abstract Two categories of compounds, polypeptides and indoles, are discussed in terms of their potential importance as pineal antigonadotrophic substances. One of the polypeptides has been structurally identified as arginine vasotocin. Arginine vasotocin and other peptides of unknown structural identity have been found to be strongly antigonadotrophic in experimental animals. Besides melatonin, the gonad-inhibiting ability of several other pineal indoles including serotonin, 5-hydroxytryptophol, 5-methoxytryptophol and N-acetylserotonin was considered. In addition to its gonad suppressing ability, in three species of rodents melatonin has been shown to negate the antigonadotrophic effects of the pineal gland, i.e., it acts as a counter antigonadotrophic factor. A theory is presented in which the indoles may interact with the polypeptides in the control and/or the synthesis of the latter compounds. A mechanism for the secretion of pineal polypeptides is also considered. It is concluded that it is premature to classify any pineal constituent, be it a polypeptide or an indole, as the antigonadotrophic factor of the mammalian pineal gland.

Variation in pineal melatonin content during the estrous cycle of the rat

Abstract Pineal levels of melatonin exhibit a circadian rhythm in the rat. To determine if this rhythm varies during the estrous cycle, adult rats were sacrificed every 2 hr from 2000 hr (prior to lights off) until 0800 hr (2 hr after lights on) throughout each day of the 4-day estrous cycle. Pineal glands were assayed for melatonin by radioimmunoassay. A significant circadian rhythm in the pineal content of melatonin was evident each day of the cycle with peak levels exhibited during the dark phase. There were significant differences in titers of melatonin present at 0200, 0400, and 0600 hr between various stages of the cycle. In general, highest nighttime levels occurred during the evenings of metestrus and diestrus, with levels at a minimum on the evening of estrus. These data indicate that the circadian pineal melatonin rhythm is a function of estrous cycle stage in the rat, and, thus, important differences exist in the pineal biosynthetic dynamics of this species relative to those of seasonal breeders such as the hamster and sheep. Additionally, the results suggest that hydroxyindole-O-methyltransferase, and not serotonin N-acetyltransferase, is the regulatory enzyme responsible for these estrous stage differences in pineal melatonin content.

Subcutaneous melatonin implants inhibit reproductive atrophy in male hamsters induced by daily melatonin injections.

Daily afternoon (at 7 p.m.) injections of melatonin (25 microng in oil) into adult male hamsters for 50 days led to atrophy of the testes and accessory sex organs (seminal vesicles and coagulating glands) and in a significant depression in pituitary LH and prolactin content and concentration. These actions of melatonin were prevented if the animals had been pinealectomized before the daily melatonin injections were begun. Likewise, if hamsters received a weekly subcutaneous implant of melatonin in beeswax (1 mg melatonin in 24 mg beeswax) the daily melatonin injections failed to inhibit the growth of the reproductive organs and to depress pituitary LH and prolactin levels. Beeswax by itself had no such effect.

Swimming Depresses Nighttime Melatonin Content without Changing N-Acetyltransferase Activity in the Rat Pineal Gland

Recently, it was shown that a 1.5-ml subcutaneous saline injection depressed N-acetyltransferase (NAT) activity and melatonin content in the rat pineal gland at night. The present studies were undertaken to determine if another perturbation, swimming, could duplicate this response. Rats swam at 23.10 h (lights out at 20.00 h) for 10 min and were killed 15 and 30 min after the unset of swimming. Pineal NAT activity was found to be unaffected while melatonin content was depressed dramatically. Hydroxyindole-O-methyltransferase (HIOMT) activity as well as the content of serotonin (5HT), 5-hydroxytryptophan (5HTP) and 5-hydroxyindoleacetic acid (5HIAA) were not changed by this treatment. In a second study, pineal melatonin again was depressed without a concomitant drop in NAT activity. Mean serum melatonin at 15 min after onset of swimming was increased although the rise was not statistically significant. In the final study, it was found that NAT activity was slightly increased in intact rats and unchanged in adrenalectomized rats at 7 min after swimming onset. At 15 min both intact and adrenalectomized animals had NAT activity values similar to those of controls. Pineal melatonin content in intact and adrenalectomized rats plummeted to 50% of control values at 7 min and fell further to 25% at 15 min. While the rate of melatonin synthesis was not directly measured, lack of change in the activities of the enzymes involved in melatonin synthesis and the contents of two melatonin precursors suggests that swimming depresses pineal melatonin content by enhancing melatonin efflux from the gland.

Arginine Vasotocin: Effects on Development of Reproductive Organs

Immature 25-day-old mice were injected daily with 1 microgram of arginine vasotocin for 3 or 4 days and killed 24 hours after the last injection. The ovaries were 30 percent smaller in treated females than in controls. The ventral prostates and accessory organs (seminal vesicles and coagulating glands) were less than half the size of these structures in control males. Similar results were observed when 15-day-old mice were given similar injections and killed 2 weeks after the last injection; furthermore, testis weights were 28 percent smaller than those of controls. It is speculated that arginine vasotocin, which has been found in mammalian pineal glands, might mediate effects of the pineal gland on normal sexual development.

Studies on the minimal dosage of melatonin required to inhibit pineal antigonadotrophic activity in male golden hamsters.

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 mug melatonin. With the exception of the 1-mug 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 mug/day. The effects of chronic melatonin treatment are similar to those of pinealectomy.