Estrogen inhibition of dopamine release into hypophysial portal blood

The hypothesis that 17β-estradiol suppresses dopamine secretion into hypophysial portal blood was tested. Portal plasma concentrations of dopamine were significantly lower in proestrous rats (1.0 ± 0.1 ng/ml; mean ± SE) than in estrous rats (1.9 ± 0.38 ng/ml). To deplete the animal of endogenous steroid hormones, proestrous rats were adrenalectomized (Adx) and ovariectomized (Ovx). Twenty-four hours later, hypophysial portal blood was collected for 60 min, and the plasma from this blood was analyzed for dopamine. Arterial plasma from these rats was assayed for 17β-estradiol and progesterone. The concentrations of dopamine in the portal plasma of sham-operated rats and bilaterally Adx-Ovx rats were similar to those in estrous animals. The concentration of dopamine in portal plasma of Adx-Ovx rats injected 24 h earlier with 50 μg 17β-estradiol was 1.0 ± 0.31 ng/ml, which was comparable to that in proestrous animals but less than that in the estrous rats. The concentrations of 17β-estradiol in arterial plasma were as follows: 54 ± 8.3 pg/ml in proestrous rats, 40 ± 2.9 pg/ml in estrous rats, 10 ± 1.3 pg/ml in Adx-Ovx rats, and 96 ± 17.3 pg/ml in Adx-Ovx rats injected with 50 μg 17/8-estradiol. Twenty-four hours after injection of 25 jug 17β-estradiol into Adx-Ovx rats, the plasma 17β-estradiol levels were 51 ± 7.4 pg/ml, and the dopamine concentrations in portal plasma were 1.9 ± 0.57 ng/ml. It is concluded that an acute effect of 17β-estradiol is suppression of hypothalamic secretion of dopamine into hypophysial portal blood.