Scientists boost power of morphine. The opioid receptor, site of action of morphine and like analgesics,desensitizes with continuing exposure to opiate drugs. It appears now that scientists have isolated the mechanism of that down-regulation, provoking hopes that they can figure out how to defeat desensitization and produce more sustained opiate effects. In other pain control news, researchers have found a way to make kappa-opioids, a class of painkilling substances thought effective only in women, work in men too, and more effective in both.