A major impediment to identifying the molecular substrates of addiction and developing effective therapeutics has been the lack of animal models that both recapitulate clinical disease presentation and are amenable to genetic manipulation. Intravenous self-administration is the “gold standard” for modeling cocaine and opioid addiction-associated behaviors in animals, but technical limitations have precluded its widespread use in mice, the mammalian species for which the most genetic tools are available. Here, we describe the establishment and demonstrate the utility of multi-stage cocaine and remifentanil paradigms that overcome classic challenges in murine intravenous self-administration. We evaluated self-administration acquisition, maintenance, extinction, and cue-induced reinstatement of drug seeking longitudinally in large cohorts of mice with indwelling catheters. In short daily sessions, mice without prior operant conditioning engaged in drug-associated lever responding that was fixed ratio- and dose-dependent, extinguished by the withholding of drugs, and reinstated by the reintroduction of paired cues. Multivariate statistics, to which this type of longitudinal data is well-suited, revealed that patterns of cocaine and remifentanil taking were similar while those of cocaine and remifentanil seeking were distinct. Individual performance in both drug paradigms was a function of two latent variables we termed incentive motivation and discriminative control. These latent variables identified drug class-specific self-administration phenotypes and were differentially predicted by a priori novelty- and drug-induced locomotor activity in the open field. Application of this behavioral and statistical analysis approach to genetically engineered mice will facilitate the identification of the cell types and neural circuits driving addictive behaviors and their underlying constructs.

Genetically tractable animal models provide needed strategies to resolve the biological basis of drug addiction. Intravenous self-administration (IVSA) is the gold standard for modeling psychostimulant and opioid addiction in animals, but technical limitations have precluded the widespread use of IVSA in mice. Here, we describe IVSA paradigms for mice that capture the multi-stage nature of the disorder and permit predictive modeling. In these paradigms, C57BL/6J mice with long-standing indwelling jugular catheters engaged in cocaine- or remifentanil-associated lever responding that was fixed ratio-dependent, dose-dependent, extinguished by withholding the drug, and reinstated by the presentation of drug-paired cues. The application of multivariate analysis suggested that drug taking in both paradigms was a function of two latent variables we termed incentive motivation and discriminative control. Machine learning revealed that vulnerability to drug seeking and relapse were predicted by a mouse’s a priori response to novelty, sensitivity to drug-induced locomotion, and drug-taking behavior. The application of these behavioral and statistical-analysis approaches to genetically-engineered mice will facilitate the identification of neural circuits driving addiction susceptibility and relapse and focused therapeutic development.

The United States is experiencing a dramatic increase in maternal opioid misuse and, consequently, the number of individuals exposed to opioids in utero. Prenatal opioid exposure has both acute and long-lasting effects on health and wellbeing. Effects on the brain, often identified at school age, manifest as cognitive impairment, attention deficit, and reduced scholastic achievement. The neurobiological basis for these effects is poorly understood. Here, we examine how in utero exposure to heroin affects brain development into early adolescence in a mouse model. Pregnant C57BL/6J mice received escalating doses of heroin twice daily on gestational days 4-18. The brains of offspring were assessed on postnatal day 28 using 9.4 T diffusion MRI of postmortem specimens at 36 μm resolution. Whole-brain volumes and the volumes of 166 bilateral regions were compared between heroin-exposed and control offspring. We identified a reduction in whole-brain volume in heroin-exposed offspring and heroin-associated volume changes in 29 regions after standardizing for whole-brain volume. Regions with bilaterally reduced standardized volumes in heroin-exposed offspring relative to controls include the ectorhinal and insular cortices. Regions with bilaterally increased standardized volumes in heroin-exposed offspring relative to controls include the periaqueductal gray, septal region, striatum, and hypothalamus. Leveraging microscopic resolution diffusion tensor imaging and precise regional parcellation, we generated whole-brain structural MRI diffusion connectomes. Using a dimension reduction approach with multivariate analysis of variance to assess group differences in the connectome, we found that in utero heroin exposure altered structure-based connectivity of the left septal region and the region that acts as a hub for limbic regulatory actions. Consistent with clinical evidence, our findings suggest that prenatal opioid exposure may have effects on brain morphology, connectivity, and, consequently, function that persist into adolescence.