Antagonist-induced opiate receptor upregulation in cultures of fetal mouse spinal cord-ganglion explants

Chronic exposure of fetal mouse spinal cord-ganglion explants to the opioid antagonist naloxone (10 μM, 7 days) produced a pronounced upregulation of μ opioid receptors. The antagonist action was stereospecific, as it was produced by (-)-, but not by (+)-naloxone, and was dose-dependent. Half-maximal naloxone-induced receptor upregulation occurred after two days; receptor density was maximal at 5 days. Exposure of the explant cultures to naloxone (10 μM) in the presence of the protein synthesis inhibitor, cycloheximide (1 μM; a concentration which blocks >90% protein synthesis) resulted in receptor density changes that were similar to those observed in cultures exposed to naloxone alone. This finding suggests that antagonist-induced opiate receptor upregulation does not require the synthesis of new receptor molecules.