Effect of temperature and photoperiod on prolactin transcription in Cyprinus carpio

We studied the effect of temperature and photoperiod on prolactin (PRL) gene expression in the pituitaries of summer-aclimatized carp (C. carpio). To assess the level of PRL gene transcription, we used a 24mer synthetic oligonucleotide probe derived from the known carp PRL gene sequence. Employing in situ hybridization assays, we observed high expression of PRL mRNA in the rostral pars distalis of summer-acclimatized carp in contrast to the almost negligible PRL transcription which occurs in the winter-acclimatized fish. When experimental combinations of long and short photoperiods with 10°C and 20°C environmental temperatures were studied, only a short photoperiod (8L-16D) in summer-acclimatized carp acclimated to a winter temperature (10°C) markedly depressed PRL gene expression. Our observations indicate that photoperiod constitutes a particularly relevant modulator in the neuroendocrine cascade that activates PRL transcription in the carp.