The effects of an inhaled solvent, toluene, on the maturation of white matter in the adolescent rat

© 2014 Katherine Ritli BeringerChronic inhalant abuse causes a pattern of neurological problems, including visuospatial disturbances, impaired cognition, memory loss and ataxia; the deleterious consequences are particularly great for adolescents, who are also the dominant population of inhalant abusers. These disturbances are thought to be strongly associated with abnormalities of the white matter which are frequently observed in neuroimaging studies of inhalant abusers. Little is understood, however, about the underlying mechanisms of injury that induce these white matter changes at the cellular level, although autopsy studies have provided limited evidence that chronic inhalant abuse produces astrogliosis, myelin loss and induces inflammatory responses mediated by microglia. The primary hypothesis of the current study was that exposure to inhalants during adolescence would produce changes to neuroimaging, which would be characterised by white matter pathology, evidenced by astrogliosis, microglial reactivity and myelin loss, but preservation of axonal integrity, in the white matter of adolescent rats. Secondarily, it was hypothesised that recovery would occur following prolonged abstinence. Adolescent male Wistar rats were exposed to 10,000 ppm inhaled toluene for one hour, three times per week for four weeks, from postnatal day 28 (P28) to P54. Volume and diffusivity properties were measured at baseline (P29), immediately post-toluene (P57) and after four weeks of recovery (P87) within the corpus callosum, anterior commissure and cingulum on magnetic resonance and diffusion tensor images. White matter volume increased with age, with exposure to toluene resulting in a significant decrease in the volume of the corpus callosum body relative to controls at P57 (p< 0.05), but not in other regions of white matter. Age, but not treatment, was associated with a decrease in diffusivity over time. Brain tissues were collected and immunohistologically stained with glial fibrillary acidic protein (GFAP) to assess astrogliosis, ionized calcium binding adaptor molecule 1 (IBA-1) to assess microglia reactivity, myelin basic protein (MBP) to examine myelin expression, and neurofilament 200 (NF-200) to examine axonal integrity. The numbers of astrocytes labelled with GFAP, and microglia labelled with IBA-1, were counted, and the optical absorbance of white matter regions stained for MBP and NF-200 were measured at P29, P43, P57 and P87. The body of the corpus callosum contained decreased microglia and increased optical absorbance of MBP in toluene-exposed tissue without changes in astrocytes or neurofilament, which recovered after four weeks of abstinence. These results suggest that the current model of toluene exposure during adolescence does not cause overt pathology, but rather produces subtle delays in white matter maturation in the body, but not the genu, of the corpus callosum