Prolactin stimulation by intravenous labetalol is mediated inside the central nervous system

We have previously reported that labetalol infusion increases prolactin (PRL) secretion in hypertensive patients. In an attempt to investigate the site where labetalol stimulates PRL, the drug was infused intravenously (100 mg) into healthy subjects, both under basal conditions and after pretreatment with L-dopa plus carbidopa (250 mg and 25 mg respectively every 6 h for 1 day), since this regimen has been reported to blunt the PRL responses to centrally acting stimuli. The effects of oral labetalol administration (100 and 200 mg) on PRL was also evaluated. Serum PRL concentration did not change after oral labetalol, whereas it was increased by intravenous drug administration. This effect was completely abolished by pretreatment with L-dopa plus carbidopa. These findings, though they do not demonstrate the mechanism, suggest that the hyperprolactinaemia induced by labetalol is mediated inside the blood-brain barrier.