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.

A potentiated production of [14C]melatonin ([14C]Mt) with a simultaneous decline in [14C]serotonin ([14C]5-HT) conversion to its final metabolite [14C]5-hydroxyindole acetic acid ([14C]5-HIAA) was recorded in a rat pineal medium cultured with [14C]tryptophan and either a neuroleptic [haloperidol (HAL) or chloropromazine (CPZ)] or an antidepressant [desipramine (DIP)] drug. A difference between the pharmacological groups of drugs was evident only in [14C]5-HT levels, which were found significantly elevated in the presence of the neuroleptic HAL and reduced in the presence of the tricyclic antidepressant DIP. A parallel trend was observed in norepinephrine (NE)-induced [14C]tryptophan metabolism in the pineal culture medium. All three drugs tested brought about further enhancement of NE-induced synthesis of MT; HAL produced an elevated, but DIP a reduced, level of [14C]5-HT, compared to the NE-induced value.

Serum and pituitary LH, prolactin and progesterone changes recorded in parturient rats exposed to continuous darkness throughout pregnancy were not prevented by early ablation of the pineal gland. Similarly, the shorter gestation period observed in rats kept in continuous darkness and the overwhelming tendency of rats in general to deliver during the light period (real or subjective), persisted in the pinealectomized animals. Darkness-induced endocrine changes in rat dams during the perinatal period appear not to be mediated by the pineal gland.

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.

Serum LH levels were higher in pinealectomized than in sham-operated control rats during all 4 of the last days of pregnancy studied, although reaching significance only during the final 2 days, 21 and 22. Prolactin (Prl) levels in the serum on the final day of pregnancy, and its contents in the pituitary throughout the entire study period, were significantly lower in pinealectomized rats. Pituitary weights were lower in pinealectomized than in control animals during days 21 and 22 of pregnancy, but no differences were found between the 2 groups in the average number of living foetuses and resorptions. The results presented would seem to indicate that during the last phase of pregnancy the pineal gland plays a role in the modification of gonadotropin synthesis and release. It appears that with LH the release, and with Prl mainly the synthesis, is being affected by the pineal, although the release of Prl may be influenced as well.