ESTROGEN MODULATES THE NOCTURNAL SYNTHESIS OF MELATONIN IN PERIPUBERTAL FEMALE RATS

Our objective was to evaluate the effects of estrogen deficit and of e strogen stimulation on the synthesis of pineal melatonin in female rat s during the peripubertal period. The levels of melatonin and N-acetyl serotonin (NAS) and the activities of N-acetyltransferase (NAT) and hy droxyindole-O-methyltransferase (HIOMT) were determined in homogenates of pineal glands obtained from peripubertal female Sprague-Dawley rat s 4 to 12 weeks of age in the mid-dark during the daily light/dark cyc le. Animals were ovariectomized at 4 weeks of age; daily administratio n of estradiol benzoate (E2B,1.0 mu g/d, s.c.) was initiated at 4 week s of age. A peak In the pineal levels of melatonin and NAS and in NAT activity was observed in untreated (control) rats with intact ovaries at 6 weeks. HIOMT activity increased from Week 4 to 6 and remained unc hanged thereafter. Ovariectomy at Week 4 led to significant increases in the levels of melatonin and of NAS and NAT in activity at Week 8. N AT activity Week 10 resembled that of control animals, but levels of m elatonin and NAS were slightly elevated. Ovariectomy did not affect HI OMT activity. Subcutaneous injection of E2B significantly decreased th e levels of melatonin and NAS and of NAT activity at Week 4, as compar ed with those in control rats. E2B suppressed the ovariectomy-induced elevation of levels of melatonin and NAS and of NAT activity, similar to the effect in control animals. E2B did not affect HIOMT activity. O ur results suggest that estrogen modulates the nocturnal synthesis of melatonin in the pineal gland in peripubertal female rats. The effects of estrogen on melatonin synthesis appeared to be mediated by the mod ulation of NAT activity.