Individual differences in vulnerability for self-injurious behavior: studies using an animal model

Self-injurious behavior (SIB) is a debilitating characteristic that is prevalent across a broad spectrum of neurodevelopmental disorders. In most of these disorders, some individuals exhibit SIB, whereas others do not. However, the neurobiological mechanisms that confer vulnerability are virtually unexplored. We examined innate characteristics that contribute to vulnerability or resistance for SIB in an animal model of the behavioral pathology. Eighteen outbred Long-Evans rats were screened for behavioral responsiveness to the mild stress of a novel environment. The rats were then categorized as high responders (HR; those rats that had the highest locomotor counts) or low responders (LR; those rats that had lower locomotor counts) by median split. All the rats were then given daily injections of the indirect monoamine agonist pemoline (150 mg/kg/day) for 10 days, and self-injury was evaluated. All 9 HR rats and 5 of the 9 LR rats exhibited self-injury. The HR rats spent more time self-injuring, injured more body sites, and caused larger areas of tissue damage than the LR rats did. Furthermore, the behavioral responsiveness to novelty stress was significantly correlated with each of these measures of self-injury. The HR rats did not exhibit substantially enhanced responses on other measures of psychostimulant action (stereotypy, grooming, locomotion, rearing). Accordingly, vulnerability to develop pemoline-induced SIB is positively correlated with, and can be predicted based upon, a behavioral measure of innate stress responsiveness. These findings suggest that characteristics that are common in developmental disorders may help predispose afflicted individuals to self-injure. The findings also extend the variety of behavioral pathologies (e.g. drug addiction) for which the HR/LR model predicts vulnerability.