Desensitization or tolerance is a major consequence of long-term opiate exposure. The mechanism of opiate desensitization is only poorly understood. We report that exposure of rat spinal cord-dorsal root ganglion cocultured neurons to kappa-opiate agonist is accompanied by a 60-70% reduction in the level of the alpha i subunit of GTP-binding proteins. Using selective antibodies, which discriminate among the various alpha i subunit forms, it was found that the opiate treatment leads to a reduction in the amount of the alpha i-1 subunit. The levels of alpha s, alpha o, and beta subunits remain unchanged. This molecular event could underlie the development of tolerance and cross-tolerance to opiates.