Abstracts

Rationale: Perinatal hypoxia-ischemia (PHI) is common and predisposes the infant to subsequent neurological impairments, such as cognitive deficits, cerebral palsy, and epilepsy. The PHI-induced, anatomical and physiological changes in the neocortex that subsequently contribute to the development of spontaneous recurrent seizures (i.e., chronic epilepsy) remain unknown. Previous data from our group (Kadam et al., 2010 J. Neurosci. 30: 404-15) demonstrate that a commonly used model of hypoxic-ischemic encephalopathy can produce a robust infarct, and rats with PHI-induced brain damage develop epilepsy. We hypothesized that the neocortical pyramidal cells in the peri-infarct region have reduced synaptic innervation, due to local neuronal death, when compared to sham controls. Methods: PHI was induced in rat pups at post-natal day 7 (P7) using the Rice-Vannucci model. Whole-cell recordings were performed on brain slices from PHI-treated animals and sham controls at 24-48 hr (i.e., P8-9) after PHI and at a time hypothesized to be during the latent period (P21-23). Specifically, we recorded miniature inhibitory post-synaptic currents (mIPSCs), miniature excitatory post-synaptic currents (mEPSCs), and tonic inhibition converging onto principal cells within the peri-infarct region that surrounds the damage. Immunohistochemistry for GAD67 and NeuN labeling was used to confirm the loss of interneurons and principal cells in the damaged area and the concentration of cells remaining in the peri-infarct region. Results: The frequency of both mIPSCs and mEPSCs onto superficial cortical pyramidal cells in the PHI-treated animals was significantly decreased within 24-48 hr after PHI. At 2 weeks after PHI, however, the frequency of the mIPSCs and mEPSCs had recovered to control levels. The kinetics, amplitude and decay constant of the mIPSCs and mEPSCs were unchanged at both time points. Likewise, tonic inhibition did not differ between PHI and sham-control animals for either time. At the later time period, isolated groups of cells could clearly be seen in and around the cortical infarct. The GAD67 and NeuN labeling confirmed that these islands of cortex contained both pyramidal cells and interneurons; however, their contribution to normal and abnormal cortical function remains unclear. Conclusions: Our data suggest that one of the first functional changes in the damaged cortex following PHI is a loss of inhibitory and excitatory synaptic innervation, which appears to recover within 2 weeks after PHI. Considering the extensive damage that results from PHI, it is remarkable that so much functional recovery in synaptic innervation occurs within 2 weeks. The initial spontaneous recurrent seizures that begin to occur a few weeks after PHI appear to be generated primarily within the peri-infarct region (Kadam et al., 2010); the present studies provide evidence that substantial reorganization of both excitatory and inhibitory synaptic connections has already occurred by this point, and it is plausible that continued synaptic reorganization contributes to an hyperexcitable network and progressive epileptogensis.