Unresponsiveness of prolactin to repeated injection of sulpiride in man

The prolactin (PRL) response to repeated injections of sulpiride, a dopamine antagonist, was evaluated in 18 healthy young men. Each subject received one or two injections of sulpiride 1 mg/kg i.m. The first injection was always given at 9 a.m. and the second after 2, 8, 24 or 48 h. Blood samples were taken through an indwelling venous catheter every 20 min during the first hour following the injection and then hourly for 12 h. The first sulpiride injection increased serum PRL level 10-fold the peak being reached after 20 or 40 min. The second injection up to 24 h had no effect on serum PRL, but when two sulpiride injections were given at a 48 h-interval, both caused a sharp increase in PRL. To conclude, PRL cells in man, after a sulpiride injection, may therefore be unresponsive to a second sulpiride injection for more than 24 h.