Effect of apomorphine on melatonin secretion in normal subjects

Study of circadian melatonin secretion pattern at different stages of Parkinson's disease

To explore changes in melatonin secretion patterns and biologic rhythms in Parkinson's disease patients with or without levodopa-related motor complications (LDRMCs), the authors investigated, in an observational study, circadian rhythms of central temperature, motor activity, plasma cortisol, and melatonin in three groups: de novo untreated patients (group I), patients treated with levodopa + dopamine agonist and without LDRMCs (group II), and patients treated with levodopa + dopamine agonist and with LDRMCs (group III). There were no differences among the three groups for the rhythm of temperature, motor activity, or plasma cortisol. There was a significant (p < 0.05) phase advance in plasma melatonin secretion in patients receiving a dopaminergic treatment compared with untreated patients. The daytime area under the curve (AUC) was increased significantly in group III, and the nighttime AUC-to-daytime AUC ratio of melatonin secretion decreased significantly in group III, suggesting that the nychthemeral pattern of melatonin secretion was changed in patients with LDRMCs. Comparison of the three groups suggests a slight but insignificant phase advance and amplitude decrease of circadian melatonin secretion related to both evolution and treatment of Parkinson's disease. Despite the lack of a global desynchronization in other circadian biologic rhythms, the circadian secretion pattern of melatonin is modified in patients with LDRMCs.

Melatonin in relation to physiology in adult humans

The role exerted by melatonin in human physiology has not been completely ascertained. Melatonin levels have been measured in different physiopathological conditions, but the effects induced by melatonin administration or withdrawal have been tested only recently. Some effects have been clearly documented. Melatonin has hypothermic properties, and its nocturnal secretion generates about 40% of the amplitude of the circadian body temperature rhythm. Melatonin has sleep inducing properties, and exerts important activities in the regulation of circadian rhythms. Melatonin is capable of phase shifting human circadian rhythms, of entraining free-running circadian rhythms, and of antagonizing phase shifts induced by nighttime exposure to light. Its effect on human reproduction is not completely clear, but stimulatory effects on gonadotropin secretion have been reported in the follicular phase of the menstrual cycle. Direct actions on ovarian cells and spermatozoa have been also documented. Beside these, new important actions for melatonin may be proved. Melatonin may exert protective effects on the cardiovascular system, by reducing the risk of atherosclerosis and hypertension, and may influence immune responses. Finally, by acting as an antioxidant, melatonin could be important in slowing the processes of ageing.

Lithium decreases retinal melatonin levels in the frog

The effect of the acute i.p. administration of lithium chloride (1 mg/kg) and of haloperidol (1 mg/kg) on retinal melatonin levels was studied in light-adapted and dark-adapted frogs of the species Rana esculenta. Two hours following drug administration, animals were killed by decapitation and a single retina from each frog was collected and homogenized in 1 ml of chilled 0.1 N HCl. After centrifugation, the pH of the supernatant was adjusted to 7.0. Melatonin was extracted by diethylether and assayed by double antibody radioimmunoassay (RIA). Lithium induced a significant decrease of retinal melatonin levels both in light-adapted (P less than 0.006) and in dark-adapted (P less than 0.01) animals, whereas no change was observed after haloperidol treatment. These results suggest that the scleral aggregation of pigment granules induced by lithium in the frog retina is not mediated by a stimulation of melatonin synthesis.