Failure of opiates to reverse dopamine inhibition of prolactin secretion in vitro

To identify the site of action of opiate-induced prolactin elevation, in vitro rat hemipituitary incubations were performed in the presence of morphine, met-enkephalin, ala2-met5-enkephalinamide and dopamine (DA) and combinations of opiate with the catecholamine. No opiate stimulated prolactin release directly nor did any opiate block the inhibitory effect of DA. We conclude that this opiate endocrine action is not mediated at a pituitary level but may involve interference with hypothalamic DA release.