Pargyline effect on luteinizing hormone secretion throughout the rat estrous cycle: Correlation with serotonin, catecholamines and nitric oxide in the medial preoptic area

The neurons that produce gonadotrophin-releasing hormone (GnRH) are mainly found in the medial preoptic area (MPOA) and constitute a common final pathway to the control of luteinizing hormone (LH) surge on proestrus. The control of GnRH secretion depends on several neurotransmitters, such as serotonin (5-HT), noradrenaline (NA), dopamine (DA) and nitric oxide (NO). The aim of this work was to study the profile of 5-HT, catecholamines and their main metabolites in the MPOA throughout the estrous cycle and their interactions with NO system in this area to control LH surge. For this purpose, the following were evaluated: (I) the effect of pargyline (a monoamine oxidase inhibitor) acute treatment on plasma LH secretion throughout the estrous cycle, correlated with changes of 5-HT, DA and NA content as well as activity and expression of neuronal NO synthase (nNOS) within MPOA; (II) the effect of 5,7-dihydroxitriptamine (a drug that depletes 5-HT) microinjection into MPOA on plasma LH in ovariectomized rats treated with oil, estradiol (E2) or E2 plus progesterone (P4). Pargyline prevented LH surge on proestrus without altering its basal secretion. Throughout the estrous cycle, pargyline augmented both 5-HT and DA contents in approximately 300% and NA content in 50% in the MPOA. During proestrus, pargyline stimulated nNOS activity at 9 h and inhibited it at 11 h. nNOS expression was inhibited by pargyline at 15 h. Depletion of 5-HT content in the MPOA increased LH secretion in ovariectomized rats treated with E2 plus P4, but it did not modify in rats treated with either oil or E2. Therefore, the present data show that pargyline treatment can inhibit proestrus LH surge through a mechanism that may involve 5-HT and NO systems in the MPOA. Moreover, the effect of 5-HT in the MPOA for limiting LH surge seems to depend on plasma levels of E2 and P4.