Dopamine affects the in vitro basal secretion of rat placenta opioids in an opioid and dopamine receptor type-specific manner

Opioid peptides and their receptors are present in the placenta of many species. Dopamine plays an important role in the regulation of opioid release in the nervous system and it may play a similar role in placenta since dopamine receptors are also present in this tissue. The aim of the present work was to examine the effect of dopamine on the basal release of rat placental opioids. The effect of several dopamine receptor agonists and antagonists was tested on the release of immunoreactive beta-endorphin and immunoreactive dynorphin from perfused rat placenta fragments. We found that dopamine and apomorphine stimulated the secretion of immunoreactive beta-endorphin in a dose-dependent manner. The selective D-1 dopamine receptor agonist (+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride or SKF-38393 reproduced the effect of dopamine while the selective D-1 dopamine receptor antagonist R(+)-7-chloro-8-hpdroxy-3-methyl-1-phenyl 1,2,3,4,5-tetrahydro-1H-benzazepine hydrochloride or SCH-23390, prevented the dopamine- and SKF-38393-induced increase of immunoreactive beta-endorphin secretion. The selective and potent D-2 dopamine receptor agonist (+/-)-2-(N- phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride or PPHT had no effect on immunoreactive beta-endorphin. Finally, none of the agonists tested had any effect on the in vitro secretion of placental immunoreactive dynorphin. Our results suggest that dopamine affects the basal release of placental opioids in an opioid and dopamine receptor-specific manner, its effect being different from the effect it exerts on beta-endorphin in the rat neurointermediate pituitary lobe.