Diurnal and photoperiodic changes in thyrotrophin‐stimulating hormone β expression and associated regulation of deiodinase enzymes (DIO2, DIO3) in the female juvenile chicken hypothalamus

Increased thyrotrophin‐stimulating hormone β (TSHβ) expression in the pars tuberalis is assumed to be an early step in the neuroendocrine mechanism transducing photoperiodic information. The present study aimed to determine the relationship between long‐photoperiod (LP) and diurnal TSHβ gene expression in the juvenile chicken by comparing LP‐photostimulated birds with groups kept on a short photoperiod (SP) for 1 or 12 days. TSHβ expression increased by 3‐ and 23‐fold after 1 and 12 days of LP‐photostimulation both during the day and at night. Under both SP and LP conditions, TSHβ expression was between 3‐ and 14‐fold higher at night than in the day, suggesting that TSHβ expression cycles in a diurnal pattern irrespective of photoperiod. The ratio of DIO2/3 was decreased on LPs, consequent to changes in DIO3 expression, although there was no evidence of any diurnal effect on DIO2 or DIO3 expression. Plasma prolactin concentrations revealed both an effect of LPs and time‐of‐day. Thus, TSHβ expression changes in a dynamic fashion both diurnally and in response to photoperiod.