Dopamine receptors of the monkey anterior pituitary in various endocrine states

The anterior pituitary of adult Macaca fascicularis monkeys was removed when they were killed and the potent dopamine (DA) antagonist [3H]spiperone was used to label DA receptors. Saturation isotherms of [3H]piperone binding indicated a dissociation constant of 0.09 nM and maximal sites of 170 fmol/mg assay protein (n = 2). Competition studies validated the hypothesis that [3H]spiperone labeled DA receptors; the order of potency of 12 compounds was appropriate for a dopaminergic interaction and was similar to the order found in the anterior pituitary of other mammals. A concentration of [3H]spiperone that would label 90% of the specific binding sites (0.76 nM) was used to estimate the total DA receptor activity in the anterior pituitary of individual monkeys. [3H]spiperone binding in pregnant monkeys was greater than the binding in ovariectomized and lactating females or intact males (P less than 0.05 or 0.01). The postpartum lactating group contained more [3H]spiperone binding activity than the cycling group (P less than 0.05). Determination of the serum concentrations of PRL, estradiol, testosterone, progesterone, and dehydroepiandrosterone confirmed the predicted hormonal status of the individual monkeys at the time of sacrifice. Because DA is a PRL inhibitory hormone, it was of interest that the serum PRL concentration was positively correlated with the degree of [3H]spiperone binding (r2 = 0.74). This observation, combined with the known hypertrophy and/or hyperplasia of the mammotroph during pregnancy, supports the hypothesis that the DA receptor is associated with the PRL cell of the anterior pituitary.