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 degree C (PxC and ShC) or a temperature of 35 +/- 1 degrees 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 sham-operated 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.

Exposure of adult male rats to continuously elevated temperature of 32-34 degrees C caused a significant decrease of HIOMT activity involved in the specific metabolic process of production of melatonin, considered an active pineal hormone. The effect was already evident after 24 h exposure and increased further during the next 48 h. The results obtained substantiate previous data that the pineal gland may be involved in the system regulating adaptation to extreme temperature 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 (12 L : 12 D) 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 parturition--may be contributing to the earlier onset of delivery in the rats exposed to continuous darkness.

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