Effect of Haloperidol and L-Dopa on Plasma Prolactin in Stalk-Sectioned and Intact Monkeys*

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Abstract
To investigate further possible direct effects of dopamine agonists and antagonists on the lactotropic cell in vivo, the PRL [prolactin] response to L-dopa and haloperidol was studied in stalk-sectioned monkeys. Six rhesus monkeys, 4 of which had undergone pituitary stalk-section, were tested twice over a 10-day period. In the stalk-sectioned monkeys baseline PRL levels were elevated as compared to those of controls. L-Dopa (20 mg), the precursor of DA [dopamine], significantly (P < 0.01) depressed these elevated baseline PRL levels within 30 min; haloperidol (0.5 mg), a DA antagonist, injected after L-dopa, significantly (P < 0.01) elevated plasma PRL concentrations towards the original baseline within 45 min. In the 2nd session, drugs were administered in reverse order; haloperidol did not increase further the elevated baseline PRL levels; L-dopa, this time injected after haloperidol, failed to decrease PRL concentrations significantly. These findings indicate a direct effect of haloperidol and DA on the lactotropic cell. A more rapid PRL response in the 2 monkeys with intact stalks in contrast to the relatively slow PRL response in the stalk-sectioned monkeys might be indicative of an additional, brain-mediated effect of haloperidol on PRL in the intact rhesus monkey.