Tissue-specific abolition of Per1 expression in the pars tuberalis by pinealectomy in the Syrian hamster.

Melatonin secretion by the pineal gland transduces photoperiod into a neuroendocrine signal. In the pars tuberalis (PT), we have shown that photoperiod modifies the amplitude of the clock gene Per1. The aim of this study was to test whether the endogenous melatonin signal is required for rhythmic expression of Per1 in the PT. Male Syrian hamsters housed in long days (LD, 16:8h light:dark) were pinealectomized and Per1 mRNA expression studied by in situ hybridization. Pinealectomy abolished the rhythm of Per1 expression in the PT, but had no effect on Per1 expression in the suprachiasmatic nucleus (SCN), or the ventromedial nucleus (VMH) of the hypothalamus. Interestingly, a single night-time injection of melatonin (25 microg), given to pinealectomized animals, failed to restore Per1 expression in the PT. These data demonstrate that Per1 expression in the PT is driven by melatonin, and that the features of the endogenous signal through which the Per1 expression is achieved cannot be reproduced by a single melatonin injection.