The Harderian gland is considered as an extrapineal source of melatonin. In
the pineal gland, melatonin is known to present a circadian rhythm with hi
gh 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 t
he hormone synthesizing enzymes, serotonin N-acetyltransferase and hydroxgi
ndole-O-methyltransferase, in young ( 4 months) and old (22 months) animals
at six circadian stages and found that melatonin synthesis in the Harderia
n gland was unaffected by age. We also studied the Wistar rat Harderian gla
nd 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
, bg contrast to the pineal gland, melatonin in Wistar rat Harderian gland
does not exhibit daily variations and that aging does not affect the melato
nin content of the gland. (C) 2001 Academic Press.