The pathophysiology of traumatic brain injury in α7 nicotinic cholinergic receptor knockout mice

The α7 nicotinic cholinergic receptor is a ligand-gated ion channel with calcium permeability similar to that of ionotrophic glutamate receptors. Previous studies from our laboratory have implicated changes in expression α7 nicotinic cholinergic receptors in the pathophysiology of traumatic brain injury (TBI). In rats, TBI causes a time-dependent and significant decrease in cortical and hippocampal α-[¹²⁵I]-bungarotoxin (BTX) binding. We have postulated that deficits in α7 expression may contribute to TBI-induced cognitive impairment and that nicotinic receptor agonists can reverse α7 binding deficits and result in significant cognitive improvement compared to saline-treated controls. Thus, α7 nAChRs could be involved in a form of cholinergically mediated excitotoxicity following brain injury. In the current study, wild-type, heterozygous and null mutant mice were employed to test the hypothesis that genotypic depletion of the α7 receptor would render animals less sensitive to tissue loss and brain inflammation following experimental brain injury. Mice were anesthetized and subjected to a 0.5-mm cortical contusion injury of the somatosensory cortex. Brain inflammation, changes in nicotinic receptor expression and cortical tissue sparing were evaluated in wild-type, heterozygous and homozygous mice 1 week following TBI. In wild-type mice, brain injury caused a significant decrease in BTX binding in several hippocampal regions, consistent with what we have measured in rat brain following TBI. However, there were no genotypic differences in cortical tissue sparing or brain inflammation in this experiment. Although the results of this study were largely negative, it is still plausible that changes in the activity/expression of native α7 receptors contribute to pathophysiology following TBI. However, when null mutant mice develop in the absence of central α7 expression, it is possible that compensatory changes occur that confound the results obtained.