The binding of [3H]-spiperone to partially purified rat anterior pituitary plasma membranes was quantified throughout the estrous cycle in relation with the serum prolactin (PRL) levels. Receptor affinity remained unchanged throughout the cycle (Kd: 0.08–0.16 nM). The number of receptors was constantly high from diestrus I 10.30 h to proestrus 10.30 h, as long as serum PRL remained low. Between 10.30 and 17.30 h on proestrus, there was a rapid and marked decrease in receptor numbers (Bmax, from 180 ± 50 to 48 ± 10 fmol/mg protein: means ± SEM of three independent determinations), which coincided with the preovulatory PRL surge. Subsequently, [3H]-spiperone binding gradually increased (from 48 ± 10 to 110 ± 21 fmol/mg protein, at 21.30 h), while PRL returned to basal levels. On the afternoon of estrus, the number of dopamine receptors was also negatively correlated with the increase in serum PRL. These results show that, while receptor affinity is constant, the number of dopamine binding sites changes significantly and rapidly on the afternoon of proestrus. A rapid decrease in receptor content is temporally correlated with the onset of the preovulatory PRL surge. Therefore, the number of [3H]-spiperone binding sites may be regulated during the estrous cycle. In addition, the proestrus decrease in the number of these receptors might be a decisive component in the initiation or maintenance of the preovulatory PRL surge in the rat.

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