Strong pituitary and hypothalamic responses to photoperiod but not to 6-methoxy-2-benzoxazolinone in female common voles (Microtus arvalis)

The annual cycle of changing day length (photoperiod) is widely used by animals to synchronise their biology to environmental seasonality. In mammals, melatonin is the key hormonal relay for the photoperiodic message, governing thyroid-stimulating hormone (TSH) production in the pars tuberalis (PT) of the pituitary stalk. TSH acts on neighbouring hypothalamic cells known as tanycytes, which in turn control hypothalamic function through effects on thyroid hormone (TH) signalling, mediated by changes in expression of the type II and III deiodinases (Dio2 and Dio3, respectively). Among seasonally breeding rodents, voles of the genus Microtus are notable for a high degree of sensitivity to nutritional and social cues, which act in concert with photoperiod to control reproductive status. In the present study, we investigated whether the TSH/Dio2/Dio3 signalling pathway of female common voles (M. arvalis) shows a similar degree of photoperiodic sensitivity to that described in other seasonal mammal species. Additionally, we sought to determine whether the plant metabolite 6-methoxy-2-benzoxazolinone (6-MBOA), described previously as promoting reproductive activation in voles, had any influence on the TSH/Dio2/Dio3 system. Our data demonstrate a high degree of photoperiodic sensitivity in this species, with no observable effects of 6-MBOA on upstream pituitary/hypothalamic gene expression. Further studies are required to characterise how photoperiodic and nutritional signals interact to modulate hypothalamic TH signalling pathways in mammals.