Differential expression of the K-2 opioid receptor protein under the effect of opioid agonists in embryonic chick neurons in vitro.

Neuronal cells cultured from 7-day-old chick embryos and differentiated under the chronic effect of opioid drugs were studied for kappa-opioid receptor expression. Plasma-membrane integrated receptors were measured by radioligand ([3H]-naloxone 1 nM, [3H]-ethylketocyclazocine, 4 nM) binding to intact neurons. These data were compared to the results of k-opioid receptor immunostaining (mAb KA8, Maderspach et al., 1991). The chronic presence of kappa-selective opioid agonist dynorphin1-13 (10(-6)-10(-7) M) or bremazocine (10(-7)-10(-8) M) between cultivation days 2-4 resulted in the significant (80%) down-regulation of the membrane integrated binding sites in concentration dependent fasion. Antagonists naloxone (10(-5) M) and norbinaltorphimine (10(-8) M) alone were ineffective, however, they essentially balanced the changes caused by the agonists. Mu ligand morphine was without effect while kappa-1 selective U 50488 H caused moderate decrease only at high concentrations. Interestingly, mAb KAB also caused down-regulation indicating that it has some agonist character. The down-regulation became measurable in short time (58% within 24 hours) and persisted during the three-day presence of the agonists. Kappa-receptor immunostaining of the neurons was not significantly influenced by the agonists. We suppose that the chronic opioid treatment may influence the intracellular traffic of the receptor protein.