Daily changes in the expression of the steroid receptor coactivator SRC-1.

Steroid receptor coactivators such as SRC-1 significantly modulate the expression of steroid-dependent physiological and behavioral characteristics in birds and mammals. Changes in coactivator protein expression are therefore likely to affect receptor-mediated transcriptional activity. We previously reported a tissue-dependent regulation of SRC-1 mRNA and protein levels by sex, stress and testosterone in the quail brain. In addition, SRC-1 expression has been shown to vary in mammals during development or in adulthood as a function of seasonal variation in photoperiod. We describe here tissue-specific changes of SRC-1 expression over the course of the day in quail. SRC-1 protein quantified by Western blots in the hindbrain gradually increased in the morning, reached a peak around midday and declined significantly in the afternoon. In contrast, SRC-1 protein levels in the optic lobes progressively decreased in the morning to reach their lowest values around midday before rising in the afternoon. The coactivator concentration in the hippocampus exhibited a progressive increase throughout the day. No change in the SRC-1 protein was detected during the day in the preoptic area and in the cerebellum. The functional significance and the mechanisms of regulation underlying such changes remain to be understood. An important unresolved question is whether this diurnal variation in SRC-1 expression is circadian in nature and if so if SRC-1 is an active player linked to clock genes in the generation of circadian rhythms or if the observed changes in SRC-1 expression are a consequence of the rhythms generated by these genes