Behavioral alterations produced by chronic naloxone injections

Repeated blockade of the endorphin receptors eventually induces symptoms resembling an opiate abstinence syndrome, despite the complete absence of opiate narcotics. Rats were injected with 0.6 mg/kg naloxone or with injection vehicle alone twice a day for six days. They were observed twice a day for four subsequent days. Body shakes, head shakes, scratches and total symptoms were significantly elevated in the naloxone treated group over controls. Symptoms were completely reversed by a small dose of morphine but not by naloxone. In a second experiment, rats were injected for ten days with the same dosage of naloxone. The abstinence-like syndrome began after six days of naloxone and continued for several days after cessation of injections. Total symptoms, body shakes, scratches and aggression were significantly elevated over controls and were reversed by morphine but not by naloxone.