The Abused Inhalant Toluene Impairs Medial Prefrontal Cortex Activity and Risk/Reward Decision-Making during a Probabilistic Discounting Task

Abstract

Inhalant (e.g., toluene) misuse is linked to behavioral and cognitive deficits in humans, yet preclinical studies of the effect of inhalants on higher-order cognition are limited. We addressed this gap in the literature by examining the effect of toluene vapor exposure on risk/ reward decision-making in male and female Sprague-Dawley rats using a probabilistic discounting task. In this task, rodents chose a risky/large reward or a safe/small reward, with the odds of risky reinforcement descending or ascending throughout the test session. We observed a dose-dependent, sex-independent deficit in behavioral flexibility during probabilistic discounting caused by acute toluene exposure. Rats exposed to toluene vapor during adolescence and tested as adults performed comparably to air-treated controls and were susceptible to the effects of an acute toluene challenge. These behavioral flexibility deficits observed suggests dysfunctional medial prefrontal cortex (mPFC) activity. To address this hypothesis, we virally expressed the genetically encoded calcium sensor GCaMP6f in glutamatergic mPFC neurons and monitored calcium transients in real-time using in vivo fiber photometry. mPFC activity peaked before either lever press during free-choice trials in toluene-and air-treated animals. During forced-choice trials, GCaMP6f transients shifted from pre-risky to pre-safe choice, an effect mitigated by acute toluene exposure. mPFC activity decreased during rewarded trials, with larger decreases following risky/large wins compared with safe/small wins. Toluene-treated animals also had decreased mPFC activity during rewarded trials, but there was no distinction between risky/large wins and safe/small wins. These results provide physiological evidence for mPFC-dependent behavioral deficits caused by toluene.

Adolescent chronic intermittent toluene inhalation dynamically regulates the transcriptome and neuronal methylome within the rat medial prefrontal cortex

“…Acutely following adolescent CIT exposure, we identified an enrichment of altered gene networks mediating transcriptional regulation within the mPFC. This included several IEGs with critical roles in homeostatic and stimulus‐induced synaptic plasticity 33 that are of particular interest considering the PFC‐dependent cognitive deficits previously observed in our model 23,26 and others following adolescent toluene exposure 27 . For example, the neuronal specific IEG, Npas4 , mediates aspects of homeostatic plasticity and excitation/inhibition balance within the cortex and hippocampus and is critical for learning‐induced plasticity 35 via stimulus‐induced induction of downstream gene networks 36 .…”

“…To address this knowledge gap, the aim of the present study was to comprehensively analyse molecular adaptations within the brain using a preclinical model of chronic adolescent inhalant abuse. We focused our analysis on the medial PFC (mPFC) due to previous reports of PFC‐dependent cognitive deficits and neuroadaptations in this model, 23,26 and others, 27 as well as the vulnerability of the mPFC to toluene‐induced adaptions during adolescence 6,24 . We hypothesised that exposure to chronic intermittent toluene (CIT) during adolescence would result in dynamic regulation of the mPFC transcriptome, in part via epigenetic mechanisms, specifically DNA methylation.…”

Inhibition of the Ventral Hippocampus Increases Alcohol Drinking

“…For the fiber photometry in Experiment 2, mice had access to only a single bottle of ethanol during the 2hr access period, as described previously (Braunscheidel et al 2019;Rinker et al 2018). Additionally, the fiber photometry procedures required that mice were tethered during acquisition of GCaMP6f transients and that lickometers (Med Associates, Inc) are used to capture bottle contact data in real time in order to time-lock licking/consummatory behavior with GCaMP6f activity.…”

“…Photometry data were acquired using acquisition equipment based on that described by the Deisseroth lab with modifications (Braunscheidel et al 2019). Illumination was provided by 405 nm and 470 nm fiber collimated LEDs (Thorlabs; 30 µW per channel) connected to a four-port fluorescence mini-cube (Doric Lenses, Inc).…”

“…Raw data collected from the photoreceiver were low-pass filtered and then the extracted 470 nm and 405 nm signals were normalized and analyzed using custom-written scripts in MATLAB (Mathworks, Inc.), following previously published methods (Braunscheidel et al 2019;Rinker et al 2018). To normalize for differing rates of photobleaching (slope measured over single recordings sessions, mean ± sem; 405 nm -0.056 ± 0.001, 470 nm -0.0042 ± 0.0004, N=45), the 405 nm and 470 nm signals for each channel were first fitted to a polynomial versus time.…”