Abstracts

Rationale: Hypoxic ischemic encephalopathy (HIE) is one of the leading cause of neonatal seizures. Approximately 35-40% of infants who experience neonatal seizures develop cognitive disabilities and or epilepsy later in life. Using a rodent model of graded global hypoxia-induced seizures in postnatal day (P)10 Long-Evans rats, we have previously shown increased hippocampal hyperexcitability, long-term cognitive deficits and development of spontaneous seizures later in life following early life seizures. To determine whether there are morphologic correlates to the spontaneous seizures in adulthood, we investigated whether hippocampal mossy fiber (MF) or other aberrant axonal sprouting occurs in this model, as other early life seizure models have reported this phenomenon in area CA3 of hippocampus (Huang et al 1999). Methods: Long Evans rat pups were subjected to hypoxia induced seizures at P10 and were allowed to survive to adulthood. Adult rats were subsequently perfused at P100 for Timm silver staining, which labels high concentrations of Zn² found in the axonal terminals at synapses; littermate controls were used for comparison. Using the methods of Holmes et al (1999), both a semiquantitative scale and measurements of the optical density of Timm-stained axonal terminals were measured by a blinded investigator to assess change in amount of sprouting in s. pyramidale of CA3. To test if the increase in sprouting was associated with acute cell death, Fluoro-Jade B immunostaining was performed in rat pups perfused 24 and 48 hours following P10 hypoxia induced seizures. Results: Semiquantitative scoring showed that MF sprouting in s. pyramidale of CA3 increased significantly in hypoxic animals (2.708 0.137; n=7) compared to the control group (2.093 0.114; n=8; p < 0.001). Optical density scores confirmed this significant increase in MF sprouting of the hypoxic animals (184.4 15.3 %; n=4) when normalized to the control group (100.0 9.86 %; n=5; p < 0.001). The aberrant sprouting of mossy fibers was predominantly observed in the CA3 region, no significant difference was elucidated in the dentate gyrus. Fluoro-Jade B staining showed no significant difference between hypoxic animals (n=8) and littermate controls (n=4). Conclusions: These results suggest that hypoxia induced neonatal seizures lead to increased hippocampal connectivity through increased aberrant CA3 s. pyramidale sprouting, with no associated acute cell death. These data suggest that after neonatal seizures and in the absence of cell death, later life spontaneous seizures are associated with an increase in CA3 MF sprouting. Adult models of epilepsy have supported the hypothesis that MF sprouting may enhance positive feedback to excitatory neurons in the hippocampus to induce network hyperexcitability. Similar mechanisms may be involved in promoting hippocampal hyperexcitability and epileptogenesis following early-life seizures. Supported by: P30-HD 18655 NIH R01-NS31718