MODE OF ACTION OF INHIBIN-LIKE PINEAL ANTIGONADOTROPIN IS DIFFERENT FROM MELATONIN DURING COMPENSATORY OVARIAN HYPERTROPHY

Both melatonin and pineal antigonadotropic peptides have the same end effect, i.e., prevention of the hypertrophic response when tested in t he conventional compensatory ovarian hypertrophy (COH) model. The pres ent work was undertaken to study the effect of melatonin and a melaton in- and steroid-free inhibin-like ovine pineal antigonadotropin (PI) o n serum follicle stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL) following hemiovariectomy in adult Holtzman rats a nd also to ascertain if any similarity exists in their mode of action during COH. While melatonin prevented the transient rise in FSH at 12 hr after unilateral ovariectomy (ULO), thus retaining the basal preope rative level, PI depressed basal levels of FSH too. In addition, melat onin suppressed PRL and LH levels at 12 hr and 120 hr after ULO, respe ctively. PI, on the other hand, had no effect on serum LH and PRL leve ls. In light of our earlier in vitro results, which showed a direct in hibitory effect of PI and not of melatonin on pituitary FSH synthesis and release, the present results indicate a dichotomy in the mode of a ction of PI and melatonin. PI acts directly at the level of the pituit ary while melatonin may act at the level of the hypothalamus or higher brain centers to suppress the FSH surge and the ensuing compensatory response.