EFFECTS OF PINEALECTOMY AND MELATONIN TREATMENTS ON SEROTONIN UPTAKE AND RELEASE FROM SYNAPTOSOMES OF RAT HYPOTHALAMIC REGIONS

This study examined the effects induced by long-term pinealectomy, dai ly melatonin treatment to pinealectomized and intact rats, and a singl e melatonin injection on [C-14]-serotonin (5-HT) uptake and release fr om synaptosomes obtained of hypothalamic regions. Pinealectomy inhibit ed the accumulation of labeled 5-HT by synaptosomes of the preoptic ar ea-anterior hypothalamus (POA-AH), but it failed to alter the [K+]-evo ked 5-HT release. Melatonin treatment for 10 consecutive days to pinea lectomized rats restored 5-HT uptake in POA-AH, and also increased 5-H T release in medial and posterior hypothalamus. These results suggest that pineal melatonin plays a stimulatory role on the serotoninergic t erminals of the hypothalamus. Moreover, when daily melatonin treatment was administered to intact rats a significant increase in 5-HT uptake activity by synaptosomes of all the hypothalamic regions was observed , but 5-HT release was unaffected. In contrast, a single melatonin inj ection induced a significant decrease in 5-HT release from synaptosome s of the POA-AH was observed. The results suggest the existence of a d ifferential sensitivity in the mechanisms mediating melatonin actions on 5-HT uptake/release, which depends on the presence of the pineal gl and in the animals and on the frequency of the treatments with the pin eal hormone.