Naloxone's antagonism of rigidity but not explosive motor behavior: Possible evidence for two types of mechanisms underlying the actions of opiates and opioids

Rats received an intraperitoneal injection of naloxone prior to an intraventricular infusion of etonitazene or morphine. The rigidity produced by etonitazene was antagonized by naloxone (10 mg/kg). In contrast to the results of a previous study, all doses of naloxone up to 160 mg/kg failed to attenuate the explosive motor behavior resulting from an intraventricular infusion of morphine. The results of the present study indicate that all actions of opiates and opioids are not mediated by the naloxone-blocked opiate receptor.