Experiments were performed to determine whether growth hormone-releasing hormone (GHRH) mRNA displays diurnal variations in the hypothalamus and cortex of the rat. Levels of GHRH and beta-actin mRNA were measured from hypothalamic and cortical extracts using the reverse transcriptase polymerase chain reaction method in rats sacrificed at 4 h intervals across a 12:12 h light:dark cycle. Hypothalamic GHRH mRNA peaked around the light onset, declined during the light period, and stayed low in the dark. Variations in hypothalamic beta-actin and cortical GHRH mRNA levels were not observed. beta-Actin mRNA expression in the cortex was higher in the dark than in the light period. The results demonstrate that hypothalamic GHRH mRNA displays diurnal variations.
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Bredow et al. (1996) studied this question.