Melatonin secretion is not regulated via simple negative feedback inhibition. However some results show that melatonin may influence its own synthesis and secretion as well as other processes in the pineal gland. The present study was undertaken to check the effects of melatonin administered at different times of day on the ultrastructure of pig pinealocytes. The study was performed in summer under natural photoperiod. Gilts, aged 4 months, received lmg or 3 x lmg melatonin (i.m.) daily for four consecutive days, at different times of day. Point count analysis was used in quantitative studies of pinealocyte substructures. The administration of melatonin caused clear changes in the pinealocyte ultrastructure and the effects were closely dependent on the time of hormone injection. The most visible changes were observed in pinealocytes of pigs which received one injection of melatonin per 24 hr, at the end of the light phase. This schedule of melatonin administration resulted in a decrease in the relative volume of mitochondria and dense bodies of type MBB-1 (specific structures of the pig pinealocytes) as well as an increase in the relative volume of Golgi apparatus and numerical density of multivesicular bodies. The melatonin administration three times per 24 hr (in the morning, in the early afternoon and in the evening) caused a decrease in the relative volume of mitochondria and Golgi apparatus as well as an increase in the relative volume of MBB-1. Treatment with melatonin once per 24 hr in the morning resulted in a decrease in the relative volume of mitochondria.
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