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Abstract
Neural mechanisms for the abuse-related effects of solvents are poorly understood. Previous studies from this laboratory showed that toluene dose dependently inhibited N-methyl-D-aspartate (NMDA), but not non-NMDA glutamate ionotropic receptors expressed in Xenopus oocytes (Cruz et.al., 1998). We expanded these studies to the nicotinic acetylcholine receptor (nAChR). Toluene (50 μM to 10 mM) produced a reversible, dose-dependent inhibition of acetylcholine-induced currents in Xenopus oocytes expressing various nicotinic receptor subtypes. The α4β2 and α3β2 subunit combinations were significantly more sensitive to toluene inhibition than the α4β4, α3β4 and α7 receptors. Yamakura et al. (2000) found an anesthetic-sensitive site in the second transmembrane domain of the β nAChR subunits. These mutants were examined for toluene sensitivity. Receptors composed of α4 and β2(V253F) subunits showed α4β4-like toluene sensitivity while those containing α4β4(F255V) subunits showed α4β2-like sensitivity. In hippocampal neurons, toluene (50 μM to 10 mM) dose-dependently inhibited ACh-mediated responses with an IC 50 of 1.1 mM.
The effect of chronic exposure of cells to toluene was deterred using hippocampal neurons. Chronic toluene treatment (1 mM; 96 hours) of hippocampal neurons caused an increase in NMDA-mediated responses. Immunoblot studies showed that there was an upregulation in the total-expression of NR2A and NR2B subunits with no change in the NR1 subunit. Further analysis, using immunohistochemical staining and confocal microscopy, showed increases in NR1 subunit expression at the neuron terminal. Chronic toluene treatment also caused a decrease in GABA-mediated responses, but no changes were observed with AMPA, kainate, or acetylcholine responses. The upregulation of NMDA responses coupled with the decrease in GABA-mediated responses from chronic toluene exposure would be predicted to increase synaptic excitability and neuronal excitotoxicity upon withdrawal from toluene. Taken together, these results suggest that toluene significantly alters the function of several classes of ion channels that regulate cellular activity. (Abstract shortened by UMI.)
