Melatonin synthesis in the rat harderian gland: age- and time-related effects

The Harderian gland is considered as an extrapineal source of melatonin. In the pineal gland, melatonin is known to present a circadian rhythm with high concentration during nighttime in all species studied. We determined in Wistar male rats the effects of age and time of day on melatonin synthesis in the Harderian gland. We compared Harderian gland melatonin content and the hormone synthesizing enzymes, serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase, in young (4 months) and old (22 months) animals at six circadian stages and found that melatonin synthesis in the Harderian gland was unaffected by age. We also studied the Wistar rat Harderian gland at ten different circadian stages and found that the Harderian gland did not exhibit a daily rhythm in its melatonin content. This study shows that, by contrast to the pineal gland, melatonin in Wistar rat Harderian gland does not exhibit daily variations and that aging does not affect the melatonin content of the gland.