Abstract: The effect of exogenous melatonin on the adrenocortical axis was studied by determining serum corticosteroid and adrenocorticotrophic hormone (ACTH) levels under basal and stressed conditions in intact, pinealectomized, and adrenalectomized mature male rats. A direct interrelationship between pineal/melatonin and the adrenal cortex, dissociated from the hypothalamic‐pituitary axis, was established. Intraperitoneal injection of 100 μg melatonin brought about an elevated adrenocortical response recorded in the serum corticosteroid levels 15 hr afterwards. This effect could be demonstrated only at the beginning of the photophase, when physiological levels of corticosteroids and ACTH are at their lowest daily levels. The pattern of this reaction is different from those caused by stressors such as ether or hypoglycemic reagents, and did not occur in pinealectomized animals although the daily rhythm of corticosteroids and ACTH persisted. No effect on ACTH was evident in intact or adrenalectomized rats, where the serum levels remained unchanged. Moreover, the corticotrophin releasing factor (CRF) content of the median eminence was not affected by melatonin.

The distribution of melatonin binding sites in synaptosomal preparations from five brain areas of sham-operated and pinealectomized young male rats (maintained in a 14 h light: 10 h darkness cycle; lights on at 5.00 h) was recorded at 10.00, 18.00 and 24.00 h, 18 days after surgery, using 125I-melatonin as a probe. The densities of 125I-melatonin binding sites in the medulla-pons, hippocampus and hypothalamus of the pinealectomized rats, exhibited clear diurnal variations. However, the densities of binding sites in these brain areas at 18.00 h were lower in the pinealectomized animals than at the other times of day tested, whereas in the sham-operated controls, the binding at 18.00 h was higher than at the other times of day. No diurnal variations were evident in the midbrain and cerebellum of the pinealectomized animals. The apparent affinities of the binding sites toward the ligand in the various brain areas were similar in the pinealectomized and sham-operated animals and did not significantly vary at any of the times recorded. Oral supplementation of melatonin to the rats via drinking water had no effect on the diurnal variations in 125I-melatonin binding in the pinealectomized rat brain. The results indicate that the diurnal variations in 125I-melatonin binding sites in the rat brain are not generated by the pineal but are affected by removal of the gland.

Sham-operated and pinealectomized male rats were maintained at 14 h light:10 h dark cycles (lights-on 5.00 h) and injected daily, for 14 days, with oxazepam or vehicle. 125I-melatonin binding was recorded in synaptosomes prepared at 10.00, 18.00, and 24.00 h from the hypothalamus, hippocampus and medulla-pons of the rats. In the sham-operated, vehicle treated rats, specific 125I-melatonin binding in all brain areas studied was higher at 18.00 h, whereas in the oxazepam-treated animals, binding was higher at 24.00 h than at the other times tested. In the pinealectomized, vehicle-treated rats, the binding recorded at 18.00 h in all three brain areas, was lower than at the other times of day tested. Oxazepam treatment decreased 125I-melatonin binding at 24.00 h in the hippocampus and medulla-pons of the pinealectomized rats and did not significantly affect the binding in the hypothalamus. These results indicate the ability of oxazepam, pinealectomy and their combination, to manipulate the diurnal variations in 125I-melatonin binding sites in the rat brain.

Conflicting results on changes of the diurnal melatonin rhythms of patients with affective disorders have been reported in the literature. The heterogeneous data may derive from the great discrepancy in the diagnostic criteria of different authors. A study of 12 schizoaffective and chronic schizophrenic psychotic patients found a constant pattern of an obliterated nocturnal melatonin rise only in the latter group. The presence or absence of the nocturnal melatonin rise was determined in drug-free hospitalized patients and remained unchanged despite 2 months of drug treatment including large doses of neuroleptics. This finding, when confirmed in a larger number of patients, could possibly serve as a marker for the type of mental disorder, drug to be applied and response expected.

After reversing a 12:12‐h light‐dark regimen of environmental lighting, pineal retinal melatonin levels of white leghorn chicks recorded at the first mid‐darkness were greatly enhanced reached maximum levels after three more days; those recorded during the light periods indicate gradual decline but were far from the nadir of the original light period, even at the end of the experiment. The first mid‐darkness serum melatonin levels recorded after photoperiod reversal were not much different from their original mid‐light values. However, on continuing with the reversed regimen, the next mid‐darkness levels were sharply increased, maximum levels were reached after a further 2 days. Under the same experimental conditions light had much more drastic effects, 6 h on the reversed regimen were sufficient to bring down completely the high value of the original mid‐darkness period to the level of the starting mid‐light nadir. Retinal N‐acetylserotonin (NAS) measured simultaneously had a pattern similar to that of melatonin, but the pineal NAS rhythm did invert completely, albeit gradually. Eye covering did not prevent inversion of pineal serum melatonin rhythms, which were identical in eye‐covered sighted control chicks on the reversed regimen of light. However, retinal melatonin values of the light periods were significantly less depressed in eye‐covered than in sighted control chicks. Moreover, eye covering completely prevented the retinal NAS depression under light but did not affect pineal NAS. During darkness retinal pineal NAS elevation was sluggish in the eye‐covered chicks.

Decreased testicular weight with damaged seminiferous tubules were observed in hamsters pinealectomized and exposed continuously to high ambient temperature. In hamsters with intact pineals, exposure to heat resulted in reduced testicular weight without recognizable histological damage. In both groups, serum testosterone levels were elevated, evidently through a compensatory mechanism in the interstitial cells, which did not appear to be damaged. Concomitantly with these findings, reduced serum LH, and pituitary LH and proclactin concentrations were encountered in the heat-exposed pinealectomized hamsters, compared to those of their heat-exposed sham-operated counterparts.

Exposure of pinealectomized rats to high ambient temperature (35 degrees C; PXH) brought about a diminution in pituitary weight and LH content when compared to their sham-operated peers (35 degrees C) or to pinealectomized controls (22 degrees C). Serum corticosterone level of PXH rats was significantly depressed while heat or pinealectomy alone had no effect. Mean oestrous cycle length was prolonged and blood serum progesterone was increased in the heat-exposed rats. However, the extended oestrous cycles and elevated serum progesterone levels of heat-exposed rats were depressed or abolished by pineal ablation. Thus, the pineal appears to exert a moderating effect on heat-induced endocrine changes in female rats. No changes were noticed in uterine and ovarian weights corrected for body weights either on the day of vaginal opening, at occurrence of the oestrous phase expressed as percentage of total oestrous cycle, or in N-acetyltransferase and hydroxyindole-O-methyltransferase activities.

1. 1. Chronic exposure of golden hamsters ( Mesocricetus auratus ) to a high ambient temperature (34°C) depresses body weight as well as the weights of the interscapular brown adipose tissue and several endocrine glands. 2. 2. In some instances pinealectomy prevented-completely or partially—the effect of heat on the endocrine glands, indicating a role for the pineal in moderating some heat-induced endocrine changes.

Exposure of rats to continuous darkness throughout gestation produced a threefold increase in their serum prolactin and a moderate one in their serum LH levels on the day preceding delivery. Parallel decreases in pituitary prolactin and LH contents were experienced indicating enhanced release of the hormones. In the animals kept in constant light only prolactin release was increased before delivery, their LH levels were unaffected, although diminished pituitary contents due to decreased synthesis were evident. Following parturition the photo-induced prolactin levels in serum and pituitary disappeared, but the LH values of the rats exposed to both extremes of illumination persisted, in contrast to those of the control animals kept in alternating light (12L∶12D) whose values fell abruptly. Serum oestradiol was affected neither by darkness nor light but serum progesterone concentrations were depressed before delivery in the rats kept in darkness even more than in the control animals. Exposure to continuous darkness shortened the duration of gestation. The marked tendency to deliver during the daytime hours was not influenced by exposure to continuous darkness but was completely abolished by constant light. It is suggested that the strongly decreased serum progesterone levels-possibly brought about by the high prolactin concentrations observed before parturitionmay be contributing to the earlier onset of delivery in the rats exposed to continuous darkness.

The effects of the pineal gland on endocrine function (especially those of the gonads) were investigated in male rats chronically exposed to increased temperature. Weanling male rats were either pinealectomized (Px) or sham-operated (Sh). Following one week of recovery, animals were assigned to either control temperature of 21±1°C (PxC and ShC) or a temperature of 35±1°C (PxH and ShH). The animals were kept at their respective temperature for at least 30 days. In both groups (PxH and ShH) exposure to increased temperature resulted in a significant reduction in body and hypophysial weights, and in serum LH and testosterone levels as compared with the respective controls (PxC and ShC). Rectal temperature and serum corticosterone were also significantly increased. No changes were found in pineal hydroxyindole-O-methyltransferase activity of shamoperated rats kept in increased temperature (ShH vs. ShC). Pinealectomy alone (PxC vs. ShC) did not alter any of the measured parameters, except for increased pituitary LH content. Increased temperature plus pinealectomy (PxH vs. PxC; and PxH vs. ShH) caused a significant reduction in pituitary LH content and further accentuated (PxH vs. ShH) the diminished serum LH and testosterone levels evoked by exposure to high temperature. The results suggest that in male rats the pineal gland may play a role in moderating the changes in the reproductive processes that are induced by increased temperature.

Following melatonin implants into pregnant rats throughout the entire period of gestation, the prenatal level of LH was significantly lowered in serum and the pituitary, while that of prolactin was enhanced in serum and diminished in the pituitary. After parturition, most of the melatonin-induced changes in LH and prolactin disappeared, being replaced by compensatory stimulation of release and synthesis of LH and inhibition of release of prolactin. Neither the abrupt physiological decline of serum progesterone, nor the enhanced level of oestrogen present during the period preceding delivery seem to be affected by melatonin. The findings were observed in both sham-operated (control) and pinealectomized animals, thus suggesting that melatonin exerts its inhibitory action on hypothalamic LH-RH and PIF not by utilizing an additional antigonadotropic pineal factor and not by the steroid sex hormones.

Pinealectomy leads to significantly higher levels of progesterone (on days 19 and 20 of gestation) and oestradiol (on days 21 and 22) in the serum of pregnant rats. These results indicate that during the last phase of pregnancy the pineal gland may be modifying the levels of gonadal hormones, although not affecting the sequence of the physiological events.

Hexosemonophosphate shunt and phosphorylase activities determined in the pineal glands of rats during mid-light and mid-darkness periods were found to undergo diurnal rhythms. A high active hexosemonophosphate shunt -- indicated by the more ready conversion of CO 2 of the glucose carbon in position 1 than of that in position 6 -- was found to correlate to the function of endocrine activity of night time/darkness. Phosphorylase activity, indicating glycogenolysis, measured at the same times, was found to be increased during the period of mid-light when basal metabolism is high compared to that during mid-darkness. The significance of these findings is discussed.