Novelty-induced locomotor behavior predicts heroin addiction vulnerability in male, but not female, rats

RATIONALE

The ongoing rise in opioid use disorder (OUD) has made it imperative to better model the individual variation within the human population that contributes to OUD vulnerability. Using animal models that capture such variation can be a useful tool. Individual variation in novelty-induced locomotion is predictive of substance use disorder (SUD) propensity. In this model, rats are characterized as high-responders (HR) or low-responders (LR) using a median split based on distance travelled during a locomotor test, and HR rats are generally found to exhibit a more SUD vulnerable behavioral phenotype.

OBJECTIVES

The HR/LR model has commonly been used to assess behaviors in male rats using psychostimulants, with limited knowledge of the predictive efficacy of this model in females or the use of an opioid as the reward. In the current study, we assessed several behaviors across the different phases of drug addiction (heroin taking, refraining, and seeking) in over 500 male and female heterogeneous stock rats run at two geographically separate locations. Rats were characterized as HRs or LRs within each sex for analysis.

RESULTS

Overall, females exhibit a more OUD vulnerable phenotype relative to males. Additionally, the HR/LR model was predictive of OUD-like behaviors in male, but not female rats. Furthermore, phenotypes did not differ in anxiety-related behaviors, reacquisition of heroin-taking, or punished heroin-taking behavior in either sex.

CONCLUSIONS

These results emphasize the importance of assessing females in models of individual variation in SUD and highlight limitations in using the HR/LR model to assess OUD propensity.

Antecedents and consequences of drug abuse in rats selectively bred for high and low response to novelty

Human genetic and epidemiological studies provide evidence that only a subset of individuals who experiment with potentially addictive drugs become addicts. What renders some individuals susceptible to addiction remains to be determined, but most would agree that there is no single trait underlying the disorder. However, there is evidence in humans that addiction liability has a genetic component, and that certain personality characteristics related to temperament (e.g. the sensation-seeking trait) are associated with individual differences in addiction liability. Consequently, we have used a selective breeding strategy based on locomotor response to a novel environment to generate two lines of rats with distinct behavioral characteristics. We have found that the resulting phenotypes differ on a number of neurobehavioral dimensions relevant to addiction. Relative to bred low-responder (bLR) rats, bred high-responder (bHR) rats exhibit increased exploratory behavior, are more impulsive, more aggressive, seek stimuli associated with rewards, and show a greater tendency to relapse. We therefore utilize this unique animal model to parse the genetic, neural and environmental factors that contribute to addiction liability. Our work shows that the glucocorticoid receptor (GR), dopaminergic molecules, and members of the fibroblast growth factor family are among the neurotransmitters and neuromodulators that play a role in both the initial susceptibility to addiction as well as the altered neural responses that follow chronic drug exposure. Moreover, our findings suggest that the hippocampus plays a major role in mediating vulnerability to addiction. It is hoped that this work will emphasize the importance of personalized treatment strategies and identify novel therapeutic targets for humans suffering from addictive disorders. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.