Abstracts

Rationale: There is a strong correlation between prolonged (>15 minutes) febrile seizures (FS) in young children and the subsequent development of mesial temporal lobe epilepsy (mTLE). Clinical evidence suggests that neuronal migration disorders, such as focal cortical dysplasia (FCD), facilitate FS generation as well as successive progression to mTLE (reviewed in Sanon N.T. et al., Epilepsy Res. Treat. 2012:342928). Yet, it is unclear how FCD renders the brain vulnerable to ictogenesis. More recently, intralesional expression of the pro-inflammatory cytokine interleukin-1 beta (IL-1ß) was reported in FCD specimens from mTLE patients (Iyer A. et al., Epilepsia. 2010; 51:1763-73). Therefore, we set out to investigate whether FCD-induced activation of IL-1ß is sufficient to elevate seizure susceptibility in the immature brain. Methods: To recapitulate FCD pathology, we used a frozen copper probe to induce a localized cortical freeze-lesion (CFL) in the right frontoparietal cortex of postnatal day 1 (P1) Sprague Dawley rats. For sham-operated rats, the probe was kept at room temperature. At P10, we provoked hyperthermic seizures (HS), i.e. an experimental model for FS, by exposing CFL and sham control rats to 45-50°C heated air until the manifestation of generalized convulsions. Markers of IL-1β signaling pathway were detected via Western blot (WB) and immunohistochemistry (IHC). Whole-cell patch-clamp electrophysiological recordings of hippocampal CA1 pyramidal neurons during Schaffer collaterals stimulation were used to assess network excitability around P20. Furthermore, small-molecule inhibitors were dissolved in corn oil and delivered to rats through subcutaneous injections. Results: Upstream regulators of IL-1ß, such as IL-1ß-converting enzyme Caspase-1, were aberrantly elevated from P3 up until P6 in the hippocampus ipsilateral to the lesion in CFL rats (n = 8, WB; n = 12, IHC), but not sham control rats (n = 7, WB; n = 12, IHC). Hence, we daily administered CFL rats with the selective Caspase-1 inhibitor VX-765 between P3 and P7 in order to suppress IL-1ß maturation. During HS, CFL rats injected with corn oil only, i.e. the vehicle, (designated as CFL+vehicle; n = 8) displayed shorter seizure latency (44-50%) and lower temperature threshold (44-46%) of the first behavioral HS manifestation, i.e. jaw myoclonus (JMC), than CFL rats injected with VX-765 (designated as CFL+VX765; n = 8; p < 0.05) and sham controls (n = 7; p < 0.05). Of note, JMC occurrence suggests seizures of limbic origin. Moreover, patch-clamp recordings of hippocampal neurons revealed 279-354% enhancement of the amplitude of NMDA-mediated evoked excitatory postsynaptic currents (NMDA-eEPSCs) in the CFL+vehicle group (n = 6) compared to the CFL+VX765 (n = 6; p < 0.05) and sham control groups (n = 6; p < 0.05). Conclusions: Our results demonstrate that inflammation corollary to cortical insults in the immature brain, like FCD, can remotely predispose the limbic formation, namely the hippocampus, to seizures, through transduction of Caspase-1-mediated inflammation, where Caspase-1 activation in the hippocampus seems to elicit impairment of CA3-CA1 synaptic transmission during network maturation, consequently, leading to long-lasting deleterious modifications of the hippocampal circuitry. Thus, targeting Caspase-1 could potentially reduce FS severity, thereby, providing therapeutic benefits for children with preexisting congenital cortical malformations.  Funding: Canadian Institutes of Health Research (CIHR) grant MOP-123538