Effect of melatonin on in
vitro gonadotropins and prolactin release from pituitary LHRH stimulated, and median
eminence and on ovarian response to hCG in middle-aged female rats by Elena DÌaz, Carmen Fern·ndez, Patricia O. CastrillÛn, Ana I. Esquifino,
Bernardo MarÌn & Beatriz DÌaz
Abstract
The possible influence of exogenous melatonin upon the
reproductive axis of middle-aged (15-19-month-old) female rats showing irregular estrous
cycle length was analyzed. In vitro pituitary and median eminence (ME) luteinizing hormone
(LH), follicle stimulating hormone (FSH) and prolactin release and responsiveness to
luteinizing hormone releasing hormone (LHRH) were investigated in control (n=12) and
melatonin (150 mg/100
g BW) treated (n=16) rats. In vitro ovarian steroidogenic activity and its response to hCG
were also evaluated. Basal secretion, first incubation after one hour (I1), second incubation after 2 hours
(I2) and after two hours plus LHRH (10-7) (I2+LHRH) or hCG (20 UI) (I2+hCG) were studied. Melatonin
administration to middle-aged rats showed a different effect upon in vitro LH release than
upon FSH since only lower FSH secretion rates were found. In vivo melatonin administration
reduced in vitro LH release after LHRH stimulation and this was impaired to pituitary LH
content, indicating that in vivo melatonin administration alters the mechanism of LH
release but does not affect the LH synthesis. Neither melatonin nor LHRH affected
hemipituitary in vitro prolactin release. In vivo melatonin administration again showed a
different effect upon LH release than upon FSH from ME, showing decreasing FSH secretion
rates. Similarly, melatonin did not affect prolactin release or ME content. Melatonin
reduced estradiol release from hCG stimulated ovaries of middle-aged rats. We concluded
that melatonin may have a physiological role in middle-aged rats since decreased in vitro
basal FSH release and blunted LH and estradiol increase after stimuli.
