Influence of 4-day long treatment with vasoactive intestinal peptide on ultrastructure and function of the rat pinealocytes in organ culture.

Vasoactive intestinal peptide (VIP) is one of neuropeptides involved in the regulation of the pineal gland function. The acute treatment of rat pinealocytes with VIP caused changes in their biochemical parameters. The present study concerns the effects of the chronic treatment with VIP on ultrastructure and function of the rat pinealocytes in organ culture. The pineals of adult male rats were assigned to one of three groups and placed in organ culture for four consecutive days. The pineals of the first group were incubated in the control medium, the pineals of the second group--12 hrs in control medium and 12 hrs in medium with 1 microM VIP (between 20.00 and 8.00) during each day, the pineals of the third group--24 hrs per day in medium with 1 microM VIP. The melatonin concentration was measured using RIA and activity of enzymes using radiochemical methods. Point count method was used in quantitative ultrastructural analysis. Both modes of chronic treatment with VIP increased significantly the level of melatonin secretion during four days of the culture and the content of this hormone in the pineal explants at the end of the experiment. Treatment with the neuropeptide for 12 hrs and 24 hrs per day elevated also the activity of arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase. On the other hand, VIP had no effect on the activity of arylamine-N-acetyltransferase. VIP increased the relative volume of rough endoplasmic reticulum, Golgi apparatus and mitochondria and did not influence the relative volume of lysosomes and lipid droplets as well as the numerical density of dense core vesicles in the examined rat pinealocytes. The obtained results indicate stimulatory effect of chronic treatment with VIP on the synthesis and secretion of melatonin in the rat pinealocytes in vitro. The results of morphological study are in agreement with the obtained biochemical data and point to the increase in secretory and metabolic activity of the rat pinealocytes in response to VIP.