“…The ability of electrochemical stimulation of the MRn to in hibit the preovulatory surge of LH and ovulation observed in the present work is in agreement with previously reported re sults [5,8,33,52], These findings suggest that activation of MRn neurons inhibits the release of LH on the day of proest rus. That this effect is due to the activation of a serotonergic pathway is supported by several facts, namely: (a) the MRn is rich in neurons that contain and synthezise 5-HT (12,53] and send ascending axons to the preoptic area-medial basal hypo thalamus [2,3,11,32,38,43,54]; (b) local injections of the 5-HT synthesis inhibitor, /;-chlorophenylalanine (PCPA), pre vented the inhibitory effect of MRn stimulation on LH release [33]; (c) systemic injection of the 5-HT antagonist, methyser gide, in the present work also prevented the inhibition of LH release evoked by MRn stimulation; and (d) lesions in the MRn produced a significant decrease of 5-HT content [21,27] and tryptophan hydroxylase activity [18,23] in the hypothalamus, and electrochemical stimulation of this nucleus has been shown to produce an increase in multiunit spike activity in the medial preoptic area [7].…”