We have previously demonstrated that neurons in the rostral arcuate nucleus expressing the messenger RNA (mRNA) for pro-opiomelanocortin (POMC) are responsive to modulation by physiological levels of testosterone. It is uncertain, however, whether testosterone's action is mediated through direct activation of androgen receptors or through aromatization to estradiol and subsequent binding to estrogen receptors. We examined this question by evaluating the effectiveness of estradiol and dihydrotestosterone (DHT), a nonaromatizable androgen, in reversing the castration-induced diminution of POMC mRNA in the arcuate nucleus. Using in situ hybridization, we measured POMC mRNA content within arcuate neurons of intact, castrated, castrated testosterone-replaced, castrated estradiol-replaced, and castrated DHT-replaced male rats. Adult male rats were castrated and implanted (s.c.) with a Silastic capsule filled to one of the following specifications: crystalline testosterone (30 mm; n = 4); 17 beta-estradiol (E2) diluted 1:1 with cholesterol (5 mm; n = 4); DHT (40 mm; n = 4); or empty (30 mm; n = 4). Control, sham-operated animals (n = 4) were left intact. Analysis of the results showed that following castration, POMC mRNA content was significantly reduced in cells of the arcuate nucleus (intact: 152 +/- 3 grains/cell vs. castrate: 110 +/- 3 grains/cell). Replacement with physiological levels of testosterone prevented the decline of POMC mRNA levels (castrated testosterone-replaced: 143 +/- 6 grains/cell), as did replacement with physiological levels of estrogen (castrated estrogen-replaced: 149 +/- 8 grains/cell). Treatment with DHT failed to prevent the postcastration decline in POMC mRNA content (castrated DHT-treated: 118 +/- 4 grains/cell).(ABSTRACT TRUNCATED AT 250 WORDS)