Heroin, but not levorphanol, produces explosive motor behavior in naloxone-treated rats

Intraventricular infusions of heroin (an opiate) or levorphanol (an opioid) in rats produced rigidity but not explosive motor behavior (EMB). When preceded by an IP injection of naloxone, infusions of heroin but not levorphanol produced EMB. The results suggest that naloxone-antagonized effects of opiates mask EMB and that certain opioids do not induce EMB.