Dopamine Receptors of the Monkey Anterior Pituitary in Various Endocrine States*

Abstract

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]spiperone 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 < 0.05 or 0.01). The postpartum lactating group contained more [³H]spiperone binding activity than the cycling group (P < 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.