Direct action of opiates on bromocriptine-inhibited prolactin release by human prolactinoma cells in primary culture.
The present study was undertaken in order to examine the existence of opioid binding sites on cell membranes of human PRL-secreting tumors. Determination of opioid binding sites using different opiate ligands revealed one class of high affinity (KD, 1.3 nM) binding sites. Pharmacological characterization revealed kappa-1 selectivity (high affinity ethylketocyclazocine (EKC) binding, insensitive to 5 microM (D-Ala2, D-Leu5]enkephalin). Subsequently EKC was added to hPRL-secreting tumor cells in primary culture, alone or in combination with the dopaminergic agonist bromocriptine, and PRL release was measured. Opiates had no direct effect on PRL release by prolactinoma cells. When cells were preincubated with bromocriptine [6.6 +/- 4.8 (SD) X 10(-11) M], EKC (10(-11) to 10(-9) M) antagonized, in a dose-dependent manner, the dopaminergic inhibition of PRL release. The opiate effect was reversed by the opiate antagonist diprenorphine (10(-7) M). Cross-competition studies indicated that this effect was not due to the interaction of opiates with the dopaminergic receptor. In conclusion, opioid binding sites are found on prolactinoma cells. The binding of kappa-1 type opioid ligands modulates the inhibitory effect of dopamine upon PRL release.
D000236:Adenoma / Q000378:metabolism*; D001971:Bromocriptine / Q000494:pharmacology*; D002478:Cells, Cultured; D003496:Cyclazocine / Q000031:analogs & derivatives / Q000494:pharmacology; D004298:Dopamine / Q000502:physiology; D004347:Drug Interactions; D016314:Ethylketocyclazocine; D005260:Female; D006801:Humans; D008297:Male; D009294:Narcotics / Q000494:pharmacology*; D010911:Pituitary Neoplasms / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D011957:Receptors, Opioid / Q000378:metabolism
D009294:Narcotics; D011957:Receptors, Opioid; D001971:Bromocriptine; D016314:Ethylketocyclazocine; D011388:Prolactin; D003496:Cyclazocine; D004298:Dopamine