DIURNAL PATTERN OF PROOPIOMELANOCORTIN GENE-EXPRESSION IN THE ARCUATE NUCLEUS OF PROESTROUS, OVARIECTOMIZED, AND STEROID-TREATED RATS - A POSSIBLE ROLE IN CYCLIC LUTEINIZING-HORMONE SECRETION

Opiate peptides are thought to modulate the pattern of LH release in female rats. We tested the hypothesis that changes in proopiomelanocortin (POMC) gene expression occur in proestrous (PRO) and ovariectomized (OVX) steroid-treated rats which may explain their unique patterns of LH secretion. Using in situ hybridization, we examined whether diurnal changes in POMC gene expression occur in the arcuate nucleus. Four groups of rats were used in this study. 1) PRO rats were used after exhibiting at least two consecutive 4-day estrous cycles; 2) OVX rats were killed 9 days after ovariectomy; 3) estradiol (E2)-treated rats were OVX for 7 days and then treated for 2 days; and 4) E2-progesterone (P4)-treated rats were treated with E2 as described above, and on day 9 at 1030 h, P4 was administered. Rats were killed at 2300, 0300, 1000, 1300, 1500, 1800, or 2300 h, beginning on the evening of dies-trous day 2 or day 8 after ovariectomy. POMC gene expression exhibited a diurnal rhythm on PRO. Levels of mRNA rose during the morning, peaked between 0300-1000 h, and decreased by 2300 h. In Entreated rats, which exhibited a LH surge similar in timing to the PRO surge, POMC mRNA levels exhibited a diurnal rhythm strikingly similar to that observed in PRO animals. OVX abolished the rhythm; however, average POMC mRNA levels across the 24-h period were not significantly different from those in PRO or Entreated rats. P4 treatment increased POMC mRNA levels by 2300 h compared to those in all other experimental groups. These results support the concept that the pattern of POMC gene expression influences the pattern of secretion of LH. Furthermore, they demonstrate that 1) E2 imposes a diurnal rhythm on POMC gene expression, and 2) P4 alters this rhythm and enhances POMC mRNA levels. Finally, these results emphasize the importance of considering the presence of diurnal rhythms in gene expression when designing experiments and interpreting data relative to the effects of endocrine manipulations. © 1990 by The Endocrine Society.