Heterogeneous regulation of individual lactotroph cells by photoperiod in the Syrian hamster (Mesocricetus auratus)

In many mammals, changes in daylength (photoperiod) regulate multiple aspects of physiology, including the synthesis and secretion of the anterior pituitary hormone prolactin. Here, we tested the hypothesis that individual lactotroph cells exhibit a heterogeneous response to changes in photoperiod, by exploiting a recently developed assay for prolactin gene expression in single pars distalis (PD) cells. Male Syrian hamsters were exposed to either long (LD; 16 h light: 8 h dark) or short (SD; 8 h light: 16 h dark) photoperiods for 12 weeks. Response of the lactotrophic axis to photoperiod was confirmed by the significantly (P0.01) lower plasma prolactin concentrations in SD than LD hamsters. Analysis of freshly dispersed PD cells by in situ hybridisation demonstrated that photoperiod has no effect (P0.05) on the proportion of PD cells (approximately 25%) that expressed prolactin mRNA. Heterogeneity of prolactin mRNA expression was observed in both LD and SD. A similar proportion of cells expressed low levels of prolactin mRNA in both photoperiods, suggesting that they may be unresponsive to photoperiod change. In contrast, the remaining PD cells that expressed prolactin mRNA exhibited markedly increased gene expression in LD, consistent with the selective recruitment of a lactotroph subpopulation to a more transcriptionally active state in this photoperiod.