Dopaminergic involvement in the estrogen-induced suppression of frequency of pulsatile luteinizing hormone secretion in the ovariectomized rat.

This experiment examined whether various catecholaminergic synthesis inhibitors and receptor blockers affect the inhibitory effect on the frequency of pulsatile luteinizing hormone (LH) secretion induced by local application of estradiol benzoate (EB) into the preoptic suprachiasmatic area (POSC) of ovariectomized rats. Ovariectomized rats were pretreated with either alpha-methyl-p-tyrosine (AMPT; tyrosine hydroxylase inhibitor), AMPT plus threo-dihydroxyphenylserine (DOPS; norepinephrine precursor), diethyldithiocarbamate (DDC; dopamine-beta-hydroxylase inhibitor), pimozide (dopaminergic receptor blocker), phenoxybenzamine (alpha-adrenergic receptor blocker), propranolol (beta-adrenergic receptor blocker), or their vehicles. EB implantation into the POSC reduced the frequency of existing pulsatile LH secretion in vehicle-treated rats. Pretreatment of the rat with AMPT, AMPT plus DOPS, or pimozide did not affect the basal pulsatile LH secretion but eliminated the suppressive effect of EB implantation on the LH pulse frequency. In rats pretreated with DDC or phenoxybenzamine, basal pulsatile LH secretion was significantly inhibited, and EB implantation did not lower serum LH further in these rats. Propranolol had no obvious effect on either basal pulsatile LH secretion or EB-induced suppression of LH pulse frequency. These findings suggest that, in addition to the alpha-adrenergic involvement in maintaining the basal pulsatile LH secretion, the intact dopaminergic system is required for the suppression of the frequency of LH pulses induced by estrogen.