Catecholamine mechanisms in the feedback effects of estradiol benzoate on the release of LH and prolactin

The role of hypothalamic catecholamines and luteinizing hormone releasing hormone (LHRH) in the negative feedback effect of estradiol benzoate (EB) on luteinizing hormone (LH) release was studied in chronic ovariectomized rats. Administration of 10 micrograms EB decreased plasma LH levels and increased LHRH content in the medial basal hypothalamus (MBH) 1 day after injection. Inhibition of dopamine and norepinephrine synthesis with alpha-methyl-p-tyrosine (alpha-MT) reduced the LHRH content in the MBH in both oil- and EB-treated animals and partially reversed the decrease in plasma LH levels. Inhibition of norepinephrine synthesis with fusaric acid decreased LHRH content in both oil- and EB-treated rats but had no effect on plasma LH levels. The results suggest that at least a portion of the inhibitory effect of EB on LH release is due to the stimulation of an inhibitory dopaminergic mechanism which reduces LHRH release from the MBH. This feedback mechanism is apparently not susceptible to dopaminergic receptor blockade since administration of pimozide had no effect on LH levels. The stimulatory feedback effect of EB on prolactin release was studied in the same animals. alpha-MT and EB produced additive effects on plasma prolactin levels whereas fusaric acid blocked the EB-induced increase in plasma prolactin levels. Pimozide appeared to potentiate the effect of EB on prolactin release. The results reconfirm the possible role of noradrenergic neurons in the release of prolactin induced by EB and also suggest that EB stimulates a dopaminergic mechanism which is inhibitory to prolactin release but is normally masked by increased noradrenergic activity.