Dopaminergic receptors in the rat anterior pituitary change during the estrous cycle.
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 106.6 +/- 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 sides 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 e an important component in pituitary responsiveness to dopamine inhibition of PRL secretion throughout the estrous cycle.