Injury induced plasticity in primary neuronal culture and the mature brain

The mature central nervous system (CNS) is unable to fully repair after traumatic brain injury (TBI). Following an injury to the adult brain there is a complex sequence of events that take place across a broad time course. The cellular mechanisms evoked by the brain when attempting to respond to injury need to be fully elucidated in order to devise effective therapeutic interventions. The current thesis is based upon the hypotheses that recovery following trauma will require the activation of reactive and compensatory plasticity and that the mechanisms underlying this plasticity are intrinsically different for excitatory and inhibitory cortical networks. This thesis investigated the reactive and regenerative alterations associated with the neuronal response to structural injury. Studies focused on the potential for plasticity following injury, specifically comparing the post-injury characteristics of excitatory and inhibitory neurons. It demonstrated that the neuronal response to injury was subclass specific given that structural injury in vitro induced an axonal regenerative response in excitatory neurons and significant dendritic remodeling in a subpopulation of interneurons. Additionally, results from functional studies indicated that these two mechanisms could both be contributing to the development of post-injury hyperexcitability. This thesis further investigated the interneuron response to injury using a clinically relevant in vivo model of diffuse and focal injury, the lateral fluid percussion injury model. Immunohistochemistry confirmed that interneurons were not lost after mild injury but, consistent with in vitro data, underwent subpopulation specific morphological alterations. Furthermore, electrophysiological studies in vivo demonstrated changes in inhibitory transmission that could lead to overall changes in excitatory/inhibitory balance as demonstrated in in vitro studies. Finally this thesis investigated a potential drug treatment for traumatic injury. Neuronal cytoskeletal alterations, in particular the loss and misalignment of microtubules, are considered a hallmark feature of the degeneration that occurs after TBI. Therefore, microtubule-stabilizing drugs are attractive as potential therapeutics for use following TBI. This thesis characterized the effect of the brain penetrant microtubule-stabilizing agent Epothilone D (Epo D) on post-injury axonal sprouting in an in vitro model of CNS trauma. Epo D was found to modulate axonal sprout number in a dose dependent manner, increasing the number of axonal sprouts generated post-injury. Specific effects on excitatory neurons were also found. This thesis demonstrated that Epo D significantly increases the neuronal regenerative response following structural injury. In summary this thesis demonstrated intrinsic differences in how excitatory neurons and inhibitory interneurons respond to injury. Moreover, it established a possible link between this differential response and alterations in excitatory/inhibitory balance in the cortex after injury. Furthermore, it identified a possible therapeutic intervention for enhancing regeneration following CNS trauma.