Effects of melatonin on spontaneous and evoked neuronal activity in the mesencephalic reticular formation

The acute effects of melatonin on the spontaneous activity of single cells in the mesencephalic reticular formation were studied in 40 male rats unanesthetized and immobilized with Flaxedil. One hundred and ten neurons were explored. Only 64 modified their spontaneous activity after the intravenous administration of melatonin. This response consisted of an increase in neural firing (6 neurons), decrease (55 neurons) and biphasic response of decrease and increase (3 neurons). When the effect of melatonin on the evoked activity in the mesencephalic neurons by peripheral stimulation (sciatic and photic) was checked an increase of the number of neurons that showed inhibitory responses to photic stimulation was found. No changes in blood pressure and EEG were observed at the doses of melatonin used (200, 400 and 600 micrograms/100 g of body weight). However, with doses of 600 micrograms a tendency toward synchronization was seen in the EEG. The present observations indicate an inhibitory effect of melatonin on the spontaneous neuronal activity of the mesencephalic reticular formation. This effect may contribute to the changes in the sleep-wakefulness cycle and anticonvulsant action attributed to this hormone.