The role of extracellular signal-regulated kinase in cognitive and motor deficits following experimental traumatic brain injury

Abstract

Traumatic brain injury (TBI) causes neuronal death and alters the plasticity (e.g. morphology) of surviving neurons. Both of these events contribute to TBI-associated neurological deficits, such as memory dysfunction. Although a majority of current research is directed towards identifying biochemical cascades responsible for cell death, little is known about mechanisms of altered neuronal plasticity following TBI. Extracellular signal-regulated kinases (Erk1 and 2) play a critical role in growth and have been implicated in long-lasting neuronal plasticity and memory storage. The activation of Erk following TBI was investigated utilizing an antibody that specifically binds to dually phosphorylated Erk. Using this antibody, we report that lateral cortical impact injury in rats increases Erk phosphorylation both in the cortex and the hippocampus as early as 10 min post-injury. Double immunostaining experiments using either a neuron-specific or an astroglial-specific marker show that the active Erk is localized almost exclusively in neuronal cells. Furthermore, the increase in phospho-Erk immunoreactivity was initially localized to axons and at later time points was observed to be predominantly in the cell soma. This suggests that Erk redistributed over time and may play a role in retrograde signaling. Administration of inhibitors of the Erk cascade worsened retrograde amnesia, impaired performances in hippocampus- and amygdala-dependent memory tasks, and exacerbated motor deficits following TBI. Furthermore, inhibition of this cascade did not have any overt effects on cell survival, but altered neuronal morphology as detected by a dendritic-specific marker. These findings suggest that the Erk cascade plays an essential role for the maintenance of neuronal function and plasticity following TBI.