Testicular androgens are integral components of the hormonal feedback loops that regulate circulating levels of LH and FSH. The sites of feedback include hypothalamic areas regulating GnRH neurons and pituitary gonadotropes. To better define the roles of androgen receptor (AR), estrogen receptor-α (ERα), and estrogen receptor-β (ERβ) in mediating feedback effects of sex steroids on reproductive neuroendocrine function, we have determined the effects of castration and steroid replacement therapy on hypothalamic GnRH content, pituitary LHβ and FSHβ messenger RNA (mRNA) levels, and serum gonadotropins in male wild-type (WT) and estrogen receptor-α knockout (ERKO) mice. Hypothalami from intact WT and ERKO males contained similar amounts of GnRH, whereas castration significantly reduced GnRH contents in both genotypes. Replacement therapy with estradiol (E₂), testosterone (T), or dihydrotestosterone (DHT) restored hypothalamic GnRH content in castrated (CAST) WT mice; only the androgens were effective in CAST ERKOs.

Analyses of pituitary function revealed that LHβ mRNA and serum LH levels in intact ERKOs were 2-fold higher than those in intact WT males. Castration increased levels of LHβ mRNA (1.5- to 2-fold) and serum LH (4- to 5-fold) in both genotypes. Both E₂ and T treatments significantly suppressed LHβ mRNA and serum LH levels in CAST WT males. However, E₂ was completely ineffective, and T was only partially effective in suppressing these two indexes in the CAST ERKO males. DHT treatments stimulated a 50% increase in LHβ mRNA and serum LH levels in WT males, whereas serum LH was significantly suppressed in DHT-treated ERKO males.

Although the pituitaries from intact ERKO males contained similar amounts of FSHβ mRNA, serum FSH levels were 20% higher than those in the intact WT males. Castration increased FSHβ mRNA levels only in WT males, but significantly increased serum FSH levels in both genotypes. Both E₂ and T treatments significantly suppressed serum FSH in CAST WT males, whereas only E₂ suppressed FSHβ mRNA. DHT treatments of CAST WT mice stimulated a small increase in serum FSH, but failed to alter FSHβ mRNA levels. None of the steroid treatments exerted any significant effect on FSHβ mRNA or serum FSH levels in CAST ERKOs.

These data suggest that hypothalamic GnRH contents can be maintained solely through AR signaling pathways. However, normal regulation of gonadotrope function requires aromatization of T and activation of ERα signaling pathways in the gonadotrope. In addition, serum FSH levels in male ERKOs appear to be regulated largely by nonsteroidal testicular factors such as inhibin. Finally, these data suggest that hypothalamic ERβ may not be involved in mediating the negative feedback effects of T on serum LH and FSH in male mice.