Light-dependent changes in the chick pineal temperature and the expression of cHsp90α gene:: A potential contribution of in vivo temperature change to the photic-entrainment of the chick pineal circadian clock

The circadian clock is entrained to the diurnal alteration of environmental conditions such as light and temperature, but the molecular mechanism underlying the entrainment is not fully understood. In the present study, we employed a differential display-based screening for a set of genes that are induced by light in the chick pineal gland, a structure of the central clock entrainable to both light and temperature changes. We found that the level of the mRNA encoding chicken heat shock protein 90 alpha (cHSP90alpha) was rapidly elevated in the pineal gland within a 5-min exposure of chicks to light. Furthermore, the pineal cHsp90alpha mRNA was expressed rhythmically under both 12-hr light/12-hr dark (LD) cycles and constant dark (DD) conditions. The total amount of the pineal cHSP90a protein was, however, kept at nearly constant levels under LD cycles, and immunohistochemical analyses of the pineal cHSP90a showed invariable localization at the cytoplasm throughout the day. In vivo measurement of the chick pineal temperature demonstrated its light-dependent and time-of-day-dependent change, and the profile was very similar to that of the pineal cHsp90a mRNA level. These observations suggest that the in vivo temperature change regulates the expression of temperature-responsive genes including cHsp90a in the pineal gland. The temperature change may induce a phase-shift of the pineal clock, thereby facilitating its efficient entrainment to environmental LD cycles.