Diurnal rhythms of common α-subunit mRNA expression in the pars tuberalis of hamsters and chickens

To clarify whether the common alpha-subunit of glycoprotein hormones is involved in photic signal transduction, alpha-subunit mRNA levels in the pars tuberalis (PT) of both hamsters and chickens were estimated at different time points of the day/night cycle by laser capture microdissection (LCM) and real-time quantitative polymerase chain reaction (PCR). Distinct diurnal rhythms were found for alpha-subunit mRNA expression in both species. In the hamster PT, alpha-subunit mRNA levels gradually increased during the dark phase; the diurnal peak was found at time (ZT) 21. The lowest value was obtained at ZT 5 during the day. In the chicken PT, alpha-subunit mRNA levels were maintained at a low constant level at night between ZT 13 and 21. Thus, alpha-subunit mRNA expression in the PT depends on the light-dark cycle and may be controlled by the pineal hormone melatonin. The effect of various photoperiods on the hamster PT was examined by real-time PCR, immunohistochemistry, and electron microscopy. In hamsters kept under short photoperiod (L/D=8 h:16 h) or complete darkness, a dramatic decrease of alpha-subunit mRNA level was induced, and the PT-specific cells accumulated glycogen-like particles and enlarged secretory granules. Under long photoperiods (L/D=16 h:8 h), however, the alpha-subunit mRNA level was elevated and the PT-specific cells exhibited highly active features, i.e., piles of lamellar cisternae of rough endoplasmic reticulum and well-developed Golgi complexes. The alpha-subunit synthesized by the PT-specific cells may therefore participate in the circadian and seasonal regulation of endocrine activities.