NMDA receptor binding is reduced within mesocorticolimbic regions following chronic inhalation of toluene in adolescent rats.

The purposeful inhalation of volatile solvents, such as toluene, to induce self-intoxication is prevalent, particularly within adolescent populations. Chronic misuse results in cognitive and neurobiological impairments, as well as an increased risk for addictive behaviours in adulthood. Toluene-induced neuroadaptations within mesocorticolimbic circuitry are thought, in part, to mediate some of the adverse outcomes of toluene misuse, however our understanding of the neuroadaptive processes remains equivocal. An understanding of these processes is particularly important relative to exposure that occurs during adolescence and at concentrations that reflect various patterns of use. Therefore, we exposed male adolescent Wistar rats (postnatal day [PN] 27) to either air or low or high concentrations of inhaled toluene in a chronic and intermittent fashion (CIT, 3,000 or 10,000ppm) for 1 h/day, 3-5 times per week for 4 weeks to model different patterns of human inhalant abuse. Brains were subsequently analysed using autoradiography, qPCR and immunohistochemistry 3 days following the exposure period to investigate toluene-induced neuroadaptations within mesocorticolimbic circuitry. In CIT-exposed rats binding to N-methyl-D-aspartate (NMDA) receptors containing the GluN2B subunit, as determined using [(3)H]-ifenprodil, was decreased in a concentration-related manner in the caudal cingulate cortex, dorsal striatum and accumbens; however, this was not associated with changes in GluN2B protein expression. There were no differences in [(3)H]-epibatidine binding to heteromeric neuronal nicotinic acetylcholine (nACh) receptors. Relative expression of mRNA transcripts encoding NMDA, nACh, γ-aminobutyric acid type-A (GABAA) and dopamine receptor subunits was unchanged in all regions assessed following CIT. Our data suggest that adolescent CIT exposure impacts NMDA receptors within regions of corticostriatal circuitry, possibly via post-translational mechanisms. Dysfunctional glutamatergic signalling within corticostriatal regions may contribute to the adverse outcomes observed following adolescent toluene abuse.

Spatial Learning Requires mGlu5 Signalling in the Dorsal Hippocampus.

We examined the role of hippocampal metabotropic glutamate receptor 5 (mGlu5) in spatial learning and memory. Although it has been shown that mGlu5 signalling is required for certain forms of learning and memory, its role in spatial learning is unclear since studies using pharmacological or knockout mice models provide inconsistent findings. Additionally, the location in the brain where mGlu5 signalling may modulate such learning is yet to be precisely delineated. We stereotaxically injected rAAV-Cre into the dorsal hippocampus of mGlu5(loxP/loxP) mice to knockdown mGlu5 in that region. We show for the first time that knockdown of mGlu5 in the dorsal hippocampus is sufficient to impair spatial learning in Morris Water Maze. Locomotor activity and memory retrieval were unaffected by the mGlu5 knockdown. Taken together, these findings support a key role for dorsal hippocampal mGlu5 signalling in spatial learning.

Adolescent toluene inhalation in rats affects white matter maturation with the potential for recovery following abstinence.

Inhalant misuse is common during adolescence, with ongoing chronic misuse associated with neurobiological and cognitive abnormalities. While human imaging studies consistently report white matter abnormalities among long-term inhalant users, longitudinal studies have been lacking with limited data available regarding the progressive nature of such abnormalities, including the potential for recovery following periods of sustained abstinence. We exposed adolescent male Wistar rats (postnatal day 27) to chronic intermittent inhaled toluene (3,000 ppm) for 1 hour/day, 3 times/week for 8 weeks to model abuse patterns observed in adolescent and young adult human users. This dosing regimen resulted in a significant retardation in weight gain during the exposure period (p<0.05). In parallel, we performed longitudinal magnetic resonance imaging (T2-weighted) and diffusion tensor imaging prior to exposure, and after 4 and 8 weeks, to examine the integrity of white matter tracts, including the anterior commissure and corpus callosum. We also conducted imaging after 8 weeks of abstinence to assess for potential recovery. Chronic intermittent toluene exposure during adolescence and early adulthood resulted in white matter abnormalities, including a decrease in axial (p<0.05) and radial (p<0.05) diffusivity. These abnormalities appeared region-specific, occurring in the anterior commissure but not the corpus callosum and were not present until after at least 4 weeks of exposure. Toluene-induced effects on both body weight and white matter parameters recovered following abstinence. Behaviourally, we observed a progressive decrease in rearing activity following toluene exposure but no difference in motor function, suggesting cognitive function may be more sensitive to the effects of toluene. Furthermore, deficits in rearing were present by 4 weeks suggesting that toluene may affect behaviour prior to detectable white matter abnormalities. Consequently, exposure to inhalants that contain toluene during adolescence and early adulthood appear to differentially affect white matter maturation and behavioural outcomes, although recovery can occur following abstinence.