THE EFFECT OF TIME OF DAY ON LEVELS OF HYPOTHALAMIC PROOPIOMELANOCORTIN PRIMARY TRANSCRIPT, PROCESSING INTERMEDIATE AND MESSENGER-RIBONUCLEIC-ACID IN PROESTROUS AND ESTROUS RATS

Several lines of evidence from different laboratories suggest that hyp othalamic beta-endorphinergic activity decreases around the time of in itiation of the LH surge and may increase on estrus to extinguish the expression of the daily neuronal signal for the surge. In several horm one systems, factors that stimulate or suppress hormone release also s timulate or repress transcription of the hormone gene and translation of the messenger RNA encoding the hormone. Therefore, information abou t neurohormone activity may be inferred from data on changes in the le vels of RNA species encoding these neurohormones. We used a solution h ybridization/RNase protection assay to test the hypotheses that 1) the abundance of primary transcript of the hypothalamic POMC gene decreas es at the time of initiation of the proestrous LH surge and 2) levels of POMC primary transcript (and by inference, levels of beta-endorphin neuronal activity and secretion) increase on estrus. 96 rats exhibiti ng at least two consecutive 4-day estrous cycles were killed at either 0600 or 1300 h on proestrus and estrus. Dissections of the medial bas al hypothalamus were pooled into 4 samples at each timepoint (6 rats p er sample) and RNA was extracted from nuclear and cytoplasmic fraction s separately. We measured levels of POMC primary transcript, processin g intermediate and fully spliced mRNA in the nuclear fractions and POM C mRNA in cytoplasmic fractions. Compared to 0600 h, levels of POMC pr imary transcript decreased significantly during the afternoons of both proestrus and estrus (P < 0.05). Levels of nuclear processing interme diate RNA and cytoplasmic mRNA followed the same trend but the afterno on declines did not reach statistical significance. We conclude from t hese data that the afternoon decline in POMC gene expression is not un ique to the day of proestrus and we speculate that an afternoon declin e in beta-endorphinergic neuronal activity may instead be a component of the daily signal for the LH surge.