CHANGES WITH AGE IN DAYTIME AND NIGHTTIME CONTENTS OF MELATONIN, INDOLEAMINES, AND CATECHOLAMINES IN THE PINEAL-GLAND - A COMPARATIVE-STUDYIN RAT AND SYRIAN-HAMSTER

Previous studies in rodents showed a severe deterioration of pineal ph ysiology with aging. The present study investigated the age-related ch anges in the content of monoamines and metabolites in rat and Syrian h amster pineal gland. In addition to melatonin, the levels of 5-hydroxy tryptophan (5HTP), serotonin (5HT), 5-hydroxyindoleacetic acid (5HIAA) , N-acetylserotonin (N-Ac-5HT), dopamine (DA), 3,4-dihydroxyphenylacet ic acid (DOPAC), and noradrenaline (NA) were measured by HPLC. Pronoun ced reductions were found in 5HT and 5HIAA contents during daytime in rats of 24 months, which had not been observed in animals of 12 months . In addition, nighttime pineal 5HIAA, N-Ac-5HT, and melatonin content s were decreased in the old rats, although a significant day:night var iation persisted. Also a diurnal fluctuation in NA, DA, and DOPAC cont ents was present in young and middle-aged rats but not for NA and DOPA C in the oldest rats due to a decrease in the nighttime levels. Pineal DA levels were also reduced in 24-month-old rats during the night, al though a marked day:night change was still found. In the Syrian hamste r pineal, significant reductions in daytime 5HT and 5HIAA were found r espectively at 12 and 18 months, while nighttime levels of these compo unds were decreased from 18 months. The nocturnal content of N-Ac-5HT dropped gradually from 12 months, and melatonin was reduced by 74% and 86% in hamsters of 18 and 24 months, respectively. In all these compo unds, a significant day:night variation was observed irrespective of a ge. However, neither a day:night variation nor an effect of aging was found in terms of pineal NA content. In contrast, pineal DA and DOPAC levels displayed a diurnal variation in hamsters of 1.5 and 6 months, but not in animals of 12 and 18 months due a reduced nighttime content . These data suggest that the decline of pineal melatonin with age is a consequence of a deficit in the pathway of serotonin utilization. Th is probably is explained by a reduced N-acetyltransferase activity, wh ich may be linked to impaired pineal catecholaminergic neurotransmissi on.