ONTOGENY OF THE CIRCADIAN-RHYTHM IN MEDIAL BASAL HYPOTHALAMIC BETA-ENDORPHIN CONTENT IN FEMALE RAT

The present study evaluated the possible role of estrogens in generati ng the circadian rhythm of medial basal hypothalamus content at the ti me of puberty in female rats. Accordingly, changes in medial basal hyp othalamus beta-endorphin (beta-EP) content were investigated in female rats, before and at puberty. Groups of intact or ovariectomized rats were studied after estradiol-benzoate or placebo treatment. The result s showed that circadian rhythm of beta-EP content of medial basal hypo thalamus is absent in prepubertal rats, while it appears at puberty, a ssociated to a significant increase of beta-EP concentration. The prim ary involvement of steroids in generating this circadian rhythm was su pported by the finding that estradiol-benzoate treatment caused a prec ocious appearance of beta-EP hypothalamic diurnal changes in prepubert al rats. Moreover, estradiol-benzoate replacement restored the loss of beta-EP nocturnal increase induced by ovariectomy in pubertal animals . Therefore, these data support the significant role of estrogen in in ducing the circadian rhythm of beta-EP content in medial basal hypotha lamus at the time of puberty in female rats.