Apomorphine selectively stimulates opiocortin hormone release from the pars distalis in rats

Administration of apomorphine (0.3-3.0 mg/kg s.c.) evoked a rapid increase (10 min) in circulating levels of immunoreactive beta-endorphin (i beta-END) in rats. As judged by gel filtration chromatography, virtually all of the increase corresponded to i beta-END resembling beta-lipotropin (beta-LPH) in molecular size. The apomorphine response was inhibited by pretreatment with either the dopamine antagonist, haloperidol (0.1 mg/kg i.p., 2 h) or the glucocorticoid, dexamethasone (50 micrograms/kg i.p., 4 h). Together, these results indicate that dopamine-receptor activation stimulates the release of opiocortin hormones from corticotrophs of the pars distalis.