Multiple memory object-drug association task (MODAT) for the study of polydrug associations

Polysubstance use is common among individuals with substance use disorder (SUD). However, the underlying neurobiology of the complex interactions between different drug classes is understudied. Further, drug associations can serve as relapse factors, but current behavioral paradigms commonly used to assess these learned associations are not as easily adapted to administration of multiple drugs. To address this, we developed the multiple memory object-drug association task (MODAT), an object-based associative memory task that allows for the same mouse to learn to associate different objects with different drugs. Four neutral scents were identified, as were four objects that could still be discriminated between after a delay. These were then combined to create four complex object-scent stimuli equally preferred by mice. Using these compound stimuli, we optimized MODAT using a stimulant (methamphetamine [METH]) and an opioid (heroin [HER]). Under the optimized protocol conditions, mice increased locomotion upon presentation of a stimulus paired with METH or HER, but not saline, during retention testing. Moreover, mice spent more time investigating the HER-, but not METH-paired stimulus. This provided two distinct behavioral readouts for assessing drug-associated memory in the same animals. A broad survey of cFos expression in the brain was performed to identify brain regions differentially activated between METH and HER-associated memory retrieval. Interestingly, many brain regions displayed decreased activation with HER-associated memory retrieval relative to METH. These regions included the prelimbic and infralimbic cortices and the nucleus accumbens. The converse was true for regions such as the lateral habenula and hypothalamus. MODAT presents a novel and easily implemented paradigm that requires no special equipment to examine multiple drug-associated memories in the same mouse as a way to examine the neurobiology of polysubstance exposure.