Dopaminergic Receptors in the Rat Anterior Pituitary Change during the Estrous Cycle*

Publisher Website
Google Scholar
Abstract
We investigated whether rat anterior pituitary dopaminergic receptors change in affinity and/or number of binding sites (Bmax) during the estrous cycle and whether they correlate with circulating PRL levels. Dopamine receptors were quantitated in partially purified pituitary membranes by equilibrium binding using [3H]spiperone. At 0900 h of diestrous day 2 and 0900 h of proestrus, Bmax [316.5 ± 7.7 and 297.9 ± 7.3 fmol/ mg protein (mean ± SE), respectively] and plasma PRL levels (18.2 ± 4.2 and 25.0 ± 7.3 ng/ml, respectively) were low. Bmax remained low (305.6 ± 22.1 fmol/mg) at 1300 h of proestrus, but PRL increased 5-fold. At 1700 h of proestrus, Bmax significantly increased to 453.3 ± 27.2 fmol/mg, coincident with a preovulatory PRL surge (241.5 ± 45.4 ng/ml). By 0900 h of estrus, Bmax decreased to 389.5 ± 20.1 fmol/mg and did not return to basal levels until diestrous day 2. PRL also decreased by 0900 h of estrus to 109.6 ± 13.0 ng/ml and progressively fell to basal level by 0900 h of diestrous day 1. Receptor affinity was unchanged throughout the cycle (range, 0.08–0.15 nM). These data indicate that the number of dopaminergic binding sites changes significantly during the estrous cycle, with the sharpest increase in the afternoon of proestrus. Since a complex relationship is apparent between alterations in dopaminergic receptor number and plasma PRL levels, we postulate that these receptors may be regulated by dopamine, ovarian steroids, and/or PRL itself. Such a change in the number of dopaminergic binding sites may be an important component in pituitary responsiveness to dopamine inhibition of PRL secretion throughout the estrous cycle.