Abstracts

Rationale: Hypoxia is the most common cause of neonatal seizures and leads to increased susceptibility to future seizures and to epilepsy. It has been recently demonstrated that rats subjected to hypoxic insult between postnatal days 10-12 demonstrate electrical and behavioral seizures at the time of the hypoxia and shortly thereafter. In addition, neonatal hypoxia given at postnatal day 10 in rats results in the subsequent appearance of spontaneous recurrent seizures that are evident in a majority of rats by 30 days after the initial hypoxic insult (Rakhade et al. 2011, Epilepsia 52(4):753-65). However, it is not clear what changes are occurring in the brain that may lead to increased seizure susceptibility. In several animal models of epileptogenesis, neuroplastic changes are known to occur in the hippocampus. More specifically, aberrant sprouting of mossy fiber axons and basal dendrites from newborn granule cells are observed. This sprouting results in the formation of recurrent excitatory circuits that may influence the epileptogenic progression. Considering that synapse formation onto hilar basal dendrites has been observed in several animal models where increased seizure susceptibility and/or the development of epilepsy is observed, we hypothesized that a similar anatomical phenomenon might be occurring following neonatal hypoxia. Methods: In order to determine if basal dendrites from newborn granule cells were targeted for synaptogenesis, immunoelectron microscopy of doublecortin-labeled cells was performed as previously described (Shapiro and Ribak, 2006, Epilepsy Res. 69(1): 53-66; Shapiro et al. 2007, Eur. J. Neurosci. 26(3):583-82) . Results: The results show a robust synapse formation on basal dendrites from animals that experienced neonatal hypoxia, regardless of whether or not the animals experienced tonic clonic seizures during the hypoxic event. The axon terminals that synapse onto the basal dendrites were determined to be mossy fiber terminals, based on the appearance of dense core vesicles. Alternatively, no such synapses were observed on sham animals analyzed at the same timepoint. Conclusions: The results suggest that aberrant synaptogenesis onto basal dendrites of newborn granule cells is a common anatomical mechanism in numerous seizure models. This aberrant circuit formation may provide an anatomical substrate for increased seizure susceptibility and the development of epilepsy. Future studies are needed to determine if preventing this formation will ameliorate increased seizure susceptibility and/or the development of epilepsy.