Ten lactating Holstein cows were randomly assigned to receive, on 2 consecutive days, either 80 mg ergocryptine (Sandoz-Basel) in 50% ethanol, or 50% ethanol alone. On the day before treatment, serum prolactin in both groups of cows increased from an average of 14–16 ng/ml before milking to 28 ng/ml within 10 min after the application of milking stimuli. Comparable values on the 1st day of treatment were 20 and 35 for control (p < 0.01) and 1.3 and 1.4 for ergocryptine-treated cows. The decrease in serum prolactin after ergocryptine occurred within 2 h and lasted for at least 5 days. Serum cortisol and growth hormone concentrations and daily milk yields were unaffected by ergocryptine treatment. Ergocryptine at doses ranging from 0.01 to 10 µg/ml medium inhibited prolactin but not growth hormone release from pituitary cells cultured in vitro. These results are interpreted as evidence that ergocryptine can decrease serum prolactin in cattle at least in part by a direct effect on the pituitary. In addition, these results indicate that serum prolactin concentrations probably do not directly reflect lactational events.

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