CHANGES IN PINEAL INDOLEAMINES IN RATS AFTER SINGLE MELATONIN INJECTIONS - EVIDENCE FOR A DIURNAL SENSITIVITY TO MELATONIN

We recently determined that melatonin stimulated serotonin (5-HT) secr etion from rat pineal glands by increasing 5-HT release from the pinea locytes (mu M melatonin concentrations) and by inhibiting 5-HT uptake in the pineal sympathetic nerve endings (mM melatonin concentrations), The present study investigated whether a single melatonin injection c ould alter the content of indoleamines in the rat pineal gland, as wel l as its possible dependence on the daytime of administration, Melaton in (150 mu g/kg) was i.p. injected at 8 time points (11.00 h, 14.00 h, 17.00 h, 20.00 h, 23.00 h, 02.00 h, 05.00 h and 08.00 h) to rats kept in 12:12 h light:dark cycle (lights on at 07.00 h), Melatonin injecti ons in the afternoon (17:00 h) and late in the nighttime (02.00 h and 05.00 h) decreased pineal 5-HT content 90 min later. The levels of 5-h ydroxyindoleacetic acid (5-HIAA) were also decreased 90 min after the melatonin treatment at 14.00 h, 17.00 h and 02.00 h. The effect of mel atonin on 5-HT content was a long-lasting effect (still evident after 180 min) only when injected at 02.00 h, whereas 5-HIAA levels were fou nd to be decreased 180 min after melatonin treatment at 14.00 h and 23 .00 h. No changes in these compounds were detected 240 min after melat onin treatment, Moreover, melatonin did not change 5-hydroxytryptophan levels at any of the daytime points studied. By contrast, 90 min afte r the injection of melatonin at 20.00 h, an increased content of pinea l N-acetylserotonin was observed, This effect of melatonin could be me diated through a phase alteration of the pineal N-acetyltransferase ac tivity rhythm by acting on the suprachiasmatic clock, althought a dire ct melatonin effect on the pineal rhythmic function cannot be excluded . The effects of the hormone on 5-HT acid 5-HIAA contents agree with p revious findings on the inhibitory effect of pharmacological doses of melatonin on pineal 5-HT uptake, which presumably would result in a de creased intraneuronal content of 5-HT and its acid metabolite. These d ata point to an acute regulatory action of exogenous melatonin on the pineal melatonin synthesis pathway which seems to be limited to two da ytime phases: the afternoon-early evening period and the second half o f the night.