Developmental Loss of the Acute Inhibitory Effect of Melatonin on thein VitroPituitary Luteinizing Hormone and Follicle-Stimulating Hormone Responses to Luteinizing Hormone-Releasing Hormone*

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Abstract
The effects of melatonin on the pituitary LH and FSH responses to LHRH were studied in culture with the use of pituitary cells from female rats 5, 10, 15, 20, and 30 days of age and from adult male and female rats, long term ovariectomized rats, and ovariectomized animals pretreated with estradiol benzoate (5 μg/day for 5 days). Dispersed anterior pituitary cells were prepared by enzymatic dissociation with collagenase and hyaluronidase and were cultured in serum-containing medium for 22 h before use. After removal of serum, the cells were incubated for an additional 3 h in either control medium or medium containing LHRH (10-8 M), melatonin (10-6 M), or LHRH (10-8 M) and melatonin (10-10-10-66 M). Medium LH and FSH concentrations were determined by double antibody RIA. Melatonin significantly inhibited LHRH-induced LH and FSH release by pituitary cells from 5-, 10-, and 15-day-old animals. The minimal effective dose of melatonin was generally between 10-10-10-9 M, and maximal suppression to approximately 5% of the full response to LHRH was attained with 10-8 M melatonin in each of these age groups. By contrast, pituitary cells from 20-and 30-day-old animals showed no detectable response to melatonin even at micromolar concentrations. Similarly, melatonin had no apparent inhibitory effect on the LH response to LHRH by cells from any of the adult rats examined in this study. These findings reveal that the acute inhibitory effect of melatonin on the pituitary response to LHRH is lost during development in the rat between 15–20 days of age. Loss of inhibition of gonadotropin secretion by the pineal indole at this time in development may be important in regulating maturation of the reproductive system.