OBJECTIVES: The purpose of this study was to investigate and compare the effects of the beta-adrenergic agonist, isoproterenol, on induced increase in melatonin production in the pineal gland of young and old rats, at different circadian stages.
MATERIALS AND METHODS: We report here the effects of 10(-6) M isoproterenol-stimulated melatonin production by perifused pineal glands obtained from young (55 day old) and old (21 month old) male Wistar rats acclimatised to light:dark cycles regimen of 12:12 for 3 weeks. Pineal glands were collected at different circadian stages: 3, 7, 11, 15, 19 and 23 hours after light onset (HALO), and perifused for 510 min.
RESULTS: The basal levels of melatonin production in the young rats were approximately twice greater than those of the old rats. Isoproterenol stimulated melatonin production in both young and old rat pineal glands, whatever the circadian stage. The intensity of the response to 10(-6) M isoproterenol infusion was greater in young than in old rat pineal glands (P<0.001), with a trend towards an increase during the light phase, at 7 HALO, in both young and old rat pineal glands, although this trend towards increased melatonin response did not reach statistical significance (P>0.05).
CONCLUSIONS: These results show that isoproterenol is able to stimulate both young and old rat pineal glands whatever the circadian stage. The magnitude of isoproterenol stimulation is greater in young than in old glands. Our results also suggest that the pineal gland response to isoproterenol is not dependent on circadian stage, at least, under our experimental conditions.