Pineal N-acetyltransferase (NAT) activity and radioimmunoassayable levels of melatonin were compared in 2-month-old (young), 12-month-old (middle-aged), and 29-month-old (old) female rats killed at 1600 h (during the light) and at 2300 h (4 h after darkness onset) and 0100 h (6 h after darkness onset). During the light period, NAT levels were equivalent in pineals from each age group. With the onset of darkness NAT levels rose sharply and were again equivalent in all groups at 2300 h. At 0100 h pineal NAT values in the old rats were lower than in the other two groups. Melatonin values were low in pineal glands of all animals killed at 1600 h in light. By 4 h after darkness onset pineal melatonin content in the young rats had increased 12-fold compared to a 6-fold rise in the old animals. Melatonin levels in the middle aged rats were intermediate between the other two groups. Similar relationships were observed in the rats killed at 0100 h. By this time the young rats had melatonin levels 17 times higher than during the day while the increase in the old rats was only 7-fold; 12-month-old rats again had intermediate levels. The finding show a marked reduction in pineal melatonin with aging in female rats.

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 glands 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 lights 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.

In order to determine whether the human pattern of circulating melatonin resembles that previously described in lower animals, men 19-32 years old were exposed to a light-dark cycle with 14 hours of light per day (L:D 14:10). In whites and blacks, nocturnal (dark phase, sleeping) melatonin levels were almost always elevated to 0.05-0.1 ng/ml plasma compared with lower or undetectable levels during the day, measured by the tadpole bioassay. Thin-layer migration of bioactive material was identical to that for melatonin standard. A rhythm with nocturnal elevation of urinary 5-hydroxyindoleacetic acid (5-HIAA) was observed. Nocturnal (sleep phase) rise in blood melatonin (but not urinary 5-HIAA) continued during 21/2 day-night cycle lengths after the onset of constant light. Though the dark phase plasma melatonin rise was less marked after reversal of the sleep-wake cycle (no change in the light cycle), dark phase rise in urinary 5-HIAA continued. Though marked cardiovascular and other effects were produced by intravenous isoproterenol or scopolamine, no definite effect on melatonin levels was observed after either drug during the light phase in waking subjects.

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.

Plasma samples obtained during darkness from 5 male volunteers contained a substance with melanin aggregating and chromatographic properties similar to those of melatonin. Peak levels were apparently inversely related to pineal calcification and/or age. Neither this substance nor an inhibitor against its biologic activity was detected in daytime samples.