Diurnal Rhythm in Proopiomelanocortin mRNA in the Arcuate Nucleus of the Male Rat

We tested the hypotheses that in the male rat, expression of proopiomelanocortin (POMC) mRNA in cells of the arcuate nucleus displays a diurnal fluctuation and that expression of this rhythm is dependent upon the secretory products of the testis. To accomplish this, we sacrificed groups of testes-intact and castrated adult male rats throughout the day and compared levels of POMC mRNA in individual cells of the arcuate nucleus across time and between groups. Adult male rats were housed on a 12–12 L D cycle with lights on a 0600 h and were divided into groups that were either castrated or left intact. Four days later, pairs from these groups were sacrificed at 0600 h, 1200 h, 1800 h, 2400 h, and again at 0600 h (n = 4 per group at each time point). We used in situ hybridization and a computerized image analysis system to measure cellular levels of POMC mRNA, as reflected by the number of autoradiographic grains over individual cells in the rostral quarter of the arcuate nucleus (counting ～ 30 cells per animal). Using cosinor analysis, we observed that in intact male rats, POMC mRNA levels varied significantly over the 24 h day with a nadir value at 1800 h. In contrast, there was no significant diurnal variation in POMC mRNA levels in castrated animals. POMC mRNA levels were significantly greater in the intact compared with castrated animals at every time point (P<0.01), except at 1800 h, when the groups did not differ significantly from one another. We conclude that adult male rats display at diurnal rhythm in cellular POMC mRNA levels in the arcuate nucleus, and we infer that testosterone or some other secretory product of the testis is a prerequisite for expression of this rhythm.