Abstracts

RATIONALE: Clinical studies and experimental models of epilepsy have shown that the immature brain is highly susceptible to seizures and status epilepticus compared to the mature brain. However, the immature brain is relatively resistant to seizure-induced damage. NMDA induces seizures and plays a predominant role in neurotoxicity. Maturational effects of NMDA on the hippocampus have not been previously explored. Therefore, we microinjected varied doses of NMDA into the hippocampus of mature and immature rats.
METHODS: Rat pups (P13) and adults were anesthetized with a mixture of ketamine and xylazine and then stereotactically implanted with a cannula/electrode assembly in the CA1 region of the hippocampus. After 24 hr post-operative rest, different doses of NMDA (2.5, 15, 25, 50 nmoles) were delivered with a microinfusion pump and 5[mu]l syringe. EEG recordings were obtained using Datawave acquisition software interfaced to an IBM computer. The animals were sacrificed at 1 and 5 days post intrahippocampal NMDA. Classic histological stains were used to assess the cell injury and the lesion area was quantified with Scion Image software.
RESULTS: At low doses, both adult and rat pups manifested similar behavior within 20 minutes that included scratching, running, hyperactivity, wet dog shakes and body stiffness. At high doses, the adults showed additional behavior that included violent running, shrill vocalization, frothing and occasional fore limb clonus, but the rat pups showed behavior similar as that observed at low doses. In adults, 1 day after intrahippocampal NMDA at 15 nmol, histological damage was preferential to the CA1 subregion. Many cells were lost and the lesion was similar at 1 day and 5 days. In contrast, at higher doses (25-50 nmol), the lesion volume was greater and showed a spread to other hippocampal subfields (DG, hilus and CA3). In rat pups at 1 day, no damage was detected after 2.5 or 25 nmoles of NMDA. However, at 5 days, pups showed a similar pattern of generalized cell loss (15 nmoles: 0.535 mm3; 25 nmoles: 4.34 mm3) but to a lesser degree than adults (15 nmoles: 2.77 mm3; 25 nmoles: 5.16 mm3).
CONCLUSIONS: The NMDA infusion data suggest that the immature hippocampus is more susceptible to seizures but resistant to neuronal damage induced by NMDA relative to the mature brain, somewhat similar to other convulsants (e.g. KA, pilocarpine). However, NMDA is not acutely toxic to the immature hippocampus at doses that induce seizures suggesting differential and age-specific mechanisms of toxicity. Although NMDA produced dose-dependent hippocampal damage in adults and pups, the cell loss was significantly delayed and less extensive in rat pups suggesting that there is an increased window of therapeutic intervention in developmental epilepsy.
[Supported by: NIH-NS-38069 and NJ Neuroscience Institute]