Interaction of dopamine receptor ligands with subtypes of the opiate receptor

There are currently several studies in which individual dopamine receptor ligands have been reported to bind with relatively low affinity to opiate receptors. To extend these studies, and to examine the opiate receptor subclass selectivity of such agents, we have examined the ability of six dopamine receptor ligands (prochlorperazine, chlorpromazine, haloperidol, bromocriptine, pimozide and metoclopramide) to compete with four tritiated tracers - [3H]naloxone, [3H][D-Ala2,D-Leu5]enkephalin, [3H]morphine and [3H]ethylketocyclazocine - for binding to rat brain membrane opiate receptors. The dopamine receptor ligands displaced the labelled opiates in a dose-dependent manner, with ED50 values of 3 microM to 3 mM. Pimozide was consistently the most potent (ED50 3-14 microM), and metoclopramide the least potent (ED50 35 microM to 3.5 mM). Dopamine receptor agonists and antagonists thus interact with opiate receptors with no clear subclass selectivity, and with similar hierarchies of inhibitory potency in each of the various opiate receptor systems.