Immunohistochemical evidence for the presence of tryptophan hydroxylase and serotonin in the rodent Harderian gland

The Harderian gland is considered as being an extrapineal source of melatonin. In most rodents, the Harderian gland contains two epithelial cell types (I and II). The aim of this study has been to define which cell type is involved in indoleamine synthesis. The presence and localization of serotonin (melatonin precursor) and tryptophan hydroxylase (the rate-limiting enzyme for serotonin synthesis) have been investigated by immunohistochemistry in male Wistar rats, Syrian hamsters and Djungarian hamsters. The results of the present study show that immunoreactivity for tryptophan hydroxylase and serotonin is confined to the type I cell, suggesting that this cell type is involved in indoleamine synthesis in the rodent Harderian gland.