Abstracts

Rationale: One of the most common etiologies for acquired epilepsy worldwide is traumatic brain injury. Despite medical advances in controlling acute seizures with anticonvulsants, there is currently no strategy that prevents the development of post-traumatic epilepsy; currently available drugs are anti-epileptic but not anti-epileptogenic. Better prophylactic treatments for post-traumatic epilepsy, therefore, require a more thorough understanding of the sequence of events that are initiated with cortical injury. One common finding in both animal models and human brain following injury is a robust sprouting response of axons from surviving neurons. However, little is known about when sprouting is initiated in vivo and how persistent newly formed synapses are following injury.Methods: Partial cortical isolations, a model of injury-induced neocortical epileptogenesis, were placed in Sprague-Dawley rats at P21. Immunohistochemistry for GAP-43, a protein found in newly formed axons, was performed 3 and 10 days after injury, in combination with VGLUT1 and VGAT, markers of excitatory and inhibitory terminals, respectively. To obtain functional correlates, voltage clamp recordings of layer V pyramidal neurons were made 3 and 10 days after injury, and the frequency and kinetics of spontaneous and miniature excitatory and inhibitory post synaptic currents (PSCs) were analyzed.Results: Anatomical studies suggest a robust sprouting response of both excitatory and inhibitory terminals 3 days after injury. GAP43 immunoreactivity colocalized with both VGLUT1 (34.3% colocalization, n = 30 cells from 4 animals) and VGAT (78.4% colocalization, n = 13 cells from 4 animals). Newly formed terminals were either excitatory or inhibitory, but not both, as evidenced by a lack of colocalization for VGLUT1 and VGAT. In the perisomatic region of layer V pyramidal neurons, VGLUT1 expression was increased while VGAT expression was decreased. Recordings from layer V pyramidal neurons revealed an increase in the frequency of both spontaneous and miniature inhibitory PSCs at 3 days post injury, concomitant with an increase in the frequency of spontaneous excitatory PSCs. Analysis of the kinetics of excitatory PSCs suggested the presence of faster and larger events in neurons from undercut slices, while inhibitory currents were slower and larger. Interestingly, 10 days after injury, the frequency of both excitatory and inhibitory spontaneous events was not significantly different from controls.Conclusions: Together, these findings suggest that a robust sprouting response of both excitatory and inhibitory terminals early after injury leads to an increase in the frequency of both excitatory and inhibitory inputs to surviving pyramidal neurons in layer V of cortex. However, 10 days after injury, these newly formed synaptic innervations are either eliminated or are no longer functional, as the frequency of both spontaneous and inhibitory PSCs return to control levels.