Pharmacological manipulation of anterior pituitary dopamine content in the male rat: relationship to serum prolactin concentration and lysosomal enzyme activity.

The content of dopamine (DA) and the activity of beta-glucuronidase (a marker for lysosomal enzymes) in the anterior pituitary and the concentration of PRL in serum were determined in male rats that were treated with a variety of drugs that influence the release, storage, or actions of DA. Drugs that increase DA in hypophysial portal blood (amphetamine, methylphenidate and L-dopa) increased DA content, decreased PRL secretion, and had no effect on lysosomal enzyme activity. Drugs that activate DA receptors in the anterior pituitary (apomorphine and bromocriptine) decreased serum PRL but had no effect on DA content or lysosomal enzyme activity. Drugs that decrease DA in the hypophysial portal blood (alpha-methyltyrosine, gamma-butyrolactone, and reserpine) or block DA receptors (haloperidol) increased PRL secretion and decreased DA content and lysosomal enzyme activity. These results suggest that there is no obvious relationship between anterior pituitary DA content and PRL secretion. In addition, lysosomal enzyme activity is not stimulated by increasing the concentration of DA or activating DA receptors in the anterior pituitary, but lysosomal enzyme activity does appear to be tonically stimulated by DA in the control animals since decreasing DA concentrations or receptor activation in the anterior pituitary decreases beta-glucuronidase activity. This latter proposal was confirmed by the demonstration that apomorphine reduced the alpha-methyltyrosine-induced increase in serum PRL and prevented the decrease in anterior pituitary DA content and beta-glucuronidase activity. Furthermore, pretreatment with bromocriptine and L-dopa blocked both the increase in serum PRL concentration and the decrease in anterior pituitary DA content induced by alpha-methyltyrosine and gamma-butyrolactone. On the other hand, bromocriptine and L-dopa blocked the increase in serum PRL concentration but not the reduction in anterior pituitary DA content caused by reserpine, indicating that reserpine has a direct action on DA storage mechanisms in the anterior pituitary. These data suggest that the ability of DA to inhibit PRL secretion and the incorporation of DA into the anterior pituitary are not causally related.