Low-Dose Naltrexone Inhibits Pemoline-Induced Self-Biting Behavior in Prepubertal Rats

Pemoline-induced self-biting behavior has been compared to similar self-injurious behavior (SIB) that occurs in the context of some mental retardation syndromes. The opiate antagonist, naltrexone, has been used successfully in the treatment of SIB in individuals with autism or mental retardation. This is the first report of the effect of naltrexone in an animal model of self-biting behavior. Naltrexone (0.01 mg/kg s.c.) significantly reduced the severity of self-biting behavior, but higher doses (0.10–10 mg/kg s.c.) had no such effect. Consistent with these results in the rat, a review of the clinical use of naltrexone in SIB is also suggestive of a relative loss of efficacy at higher doses (apparent therapeutic window). Naltrexone has higher affinity for μ receptors than other opioid receptor subtypes. Thus, the effect of naltrexone in treating SIB appears to be due to (μ-receptor interactions, with the loss of effect at higher doses due to supervening κ receptor binding. Speculatively, one may ask if dosage reduction would convert naltrexone nonresponders to responders by lowering κ receptor–mediated effects. Given the co-localization of opioids and dopamine inputs in some neurons, opioid–dopamine system interactions may also be important in pemoline-induced self-biting behavior.