Portal de Periódicos da CAPES

Effects of Short Days on Aromatization and Accumulation of Nuclear Estrogen Receptors in the Hamster Brain1

1986; Oxford University Press; Volume: 35; Issue: 2 Linguagem: Inglês

10.1095/biolreprod35.2.282

1529-7268

Gloria V. Callard, Paul Mak, Debra J. Solomon,

Neuroendocrine regulation and behavior

Exposure of hamsters to short days increases sensitivity to the negative feedback effects of testosterone (T) but decreases responsiveness to the behavioral effects of the hormone. Since T is metabolized in the brain to 5 α-dihydrotestosterone (DHT) and estradiol, which differentially affect gonadotropin secretion and sex behavior, it is reasonable to postulate that daylength can modulate neural responses by quantitative or qualitative alterations in T metabolism and subsequent receptor binding of active hormone. Experiments reported here focused on aromatization and the nuclear accumulation of estrogen receptors. Adult male hamsters were maintained for 6–12 wk in long (14:10 LD) or short (8:16 LD) daily photoperiods. Both intact and castrated animals were used to assess direct effects of short days versus changes due to short-day-induced testicular regression. Discretely dissected regions of the brain (preoptic area, POA; hypothalamus, HTH; and corticomedial amygdala, CMA) or limbic blocks (LIM) comprised of all three regions were assayed for estrogen-synthesizing activity (aromatase) and estrogen-binding activity (receptors). Aromatase was estimated in vitro by conversion of [7-3H] androstenedione to [3H] estrogen and in vivo by measuring increases in nuclear estrogen receptor levels after injection of aromatizable androgen. Receptor-binding activity was assayed in crude cytosolic and nuclear extracts by incubating samples with [3H] estradiol ± 100-fold excess inert estradiol, and separating free and bound steroids by Sephadex LH-20 gel filtration. When aromatase was assayed in homogenates prepared from discrete brain regions of individual hamsters, significantly lower activity was found in the HTH of short-day animals than in long-day controls. This effect was seen in both intact and castrated animals, which indicates that it was not mediated by the testis. Decreased enzyme activity in the POA and CMA of short-day hamsters was not significant, nor was there an effect of castration independent of short days. Low levels of nuclear estrogen receptors were present in LIM of intact males, but these were reduced after castration or concomitant with testicular regression after short-day exposure. This suggests that the hamster testis normally secretes estrogen or aromatizable androgen. A single injection of estradiol or aromatizable androgen (T or androstenedione) increased nuclear receptors in LIM of castrated animals. Cytosolic receptors were not different in short-day vs. long-day hamsters, nor were there differences in nuclear receptor levels after a single estradiol injection. Thus, photoperiod did not affect the reserve of estrogen receptors or the functional capacity of the system to respond to estradiol per se. By contrast, after injection of androstenedione or T, nuclear estrogen receptors were lower in all short-day groups (intact and castrated) when compared to corresponding long-day controls. Measurement of microsomal aromatase in the same treatment groups verified that aromatization was impaired by short-day exposure. We conclude that aromatization is an important rate-limiting step governing estrogen receptor levels in brain cell nuclei, and that one mechanism by which short daily photoperiods may attenuate certain androgen-dependent responses in the CNS is by reducing aromatization.