Melatonin stimulates thyroid-stimulating hormone accumulation in the thyrotropes of the rat pars tuberalis

We have reported that the unique thyroid-stimulating hormone-immunoreactive cells (TSH cells) in the intact adult and fetal rat pars tuberalis (PT) show an intense spot-like TSH immunoreaction in the perinuclear region. The present study was designed to investigate the relationship between melatonin and these unique TSH cells. We classified TSH cells in the PT (PT-TSH cells), on the basis of immunoreactivity, into spot-like stained cells (SC) and whole cytoplasm stained cells (WC), and estimated the proportion of each TSH cell type to total cells in the experimental rats by morphometry. Chronic administration of melatonin to control rats leads to an increase of WC in number but a decrease of SC. On the other hand, the intensity of TSH immunoreactivity and the number of rat PT-TSH cells significantly decreased after pinealectomy and recovered by melatonin administration. Radioimmunoassay showed that melatonin treatment increased the TSH content in the PT. Moreover, electron microscopy showed that the number of TSH secretory granules in the PT-TSH cells increased in the melatonin-replaced rats. These results demonstrated that melatonin stimulates the accumulation of TSH in the rat PT-TSH cells via secretory granule formation and suggest that melatonin regulates TSH release from PT-TSH cells.