EFFECTS OF BOTH A MELATONIN AGONIST AND ANTAGONIST ON SEASONAL-CHANGES IN BODY-MASS AND ENERGY-INTAKE IN THE GARDEN DORMOUSE

BACKGROUND: Many physiological functions including nycthemeral rhythm, reproductive cycles. body temperature and body mass are controlled by photoperiodic changes in different species. In the hibernating garden dormouse, both energy intake and body mass increase with the duration of the night. This seasonal mass gain is spontaneous and reversible. AIM: We have studied the occurrence of the increase of body mass by ta king into account the endogenous variations of melatonin due to changi ng photoperiod or to pharmacological treatment. RESULTS: A single dail y administration of either a melatonin agonist or antagonist just befo re night mimics the short day and long day effects. respectively. Comp ared to the control animals (natural photoperiod), the mass gain was g reater and occurred earlier in animals under short days (6 h light (L) /18 h dark (D)) and in those receiving the melatonin agonist (S 20304) , The animals treated with the antagonist (S 20928) during the same pe riod exhibited no mass gain and their response was similar to that of the long-day group (16L/8D). Solely agonist treatment acted on metabol ic rate. CONCLUSION: These results demonstrate that the duration of me latonin-receptor exposure per day determines the onset of seasonal obe sity in garden dormice and, on the other hand. that restriction of mel atonin-receptor exposure by pharmacological treatment prevents it.