Abstracts

RATIONALE: In rats, the period of maximal seizure susceptibility coincides with the 2nd and the 3rd weeks of postnatal life, a time when NMDA receptors are transiently over expressed. In this period, CA1 pyramidal neurons of the hippocampus undergo a period of enhanced sensitivity to NMDA, which is expressed by large influxes of Ca2+, but the brain is highly resistant to neurodegeneration. Knockdown of the NR1 receptor was used as a strategy to reduce or prevent the propagation of seizures during this time when the brain is still resistant to damage.
METHODS: NR1 antisense oligonucleotides (ODNs) were microinfused into different locations along the hippocampal axis (subiculum, CA1, hilus) and into the parietal cortex of rat pups (P13) for 2-3 days. Seizures were subsequently induced by NMDA (i.c.v) or kainic acid (KA) (i.p.). Electrographic activity was monitored with in-depth bipolar electrodes in hippocampus and cortex. For specificity, NR2 AS-ODNs were administered and NR1 and NR2A/B immunohisotochemistry was carried out in control (e.g. sense, missense, buffer) and experimental groups.
RESULTS: The spread of the NR1 AS-0DN was small so that different areas of the hippocampus or cortex were preferentially labeled. Ipsilateral infused NMDA (2-25 nmol) produced electrographic seizures in all of the missense and sense treated controls (n = 13). In 3 controls, NMDA infusions led to immediate spikes but they quickly died down. In contrast, after NR1 knockdown, no changes in baseline activity were observed in 5 of 12 cases, 4 showed highly reduced spike activity and 3 showed some spikes that quickly disappeared. Following cortical knockdown and KA-induced seizures, long bouts of bilateral body tonus and synchronous activity in the electroencephalogram were reduced. Unexpectedly, NR1-AS ODN infusions into the subiculum revealed selective silver staining of CA1 neurons after KA. Immunohistochemistry confirmed specificity that NR1 receptor protein was preferentially decreased in the ipsilateral hemisphere. There was no effect with similar infused doses of NR2 AS-ODNs.
CONCLUSIONS: The data indicate that NR1 knockdown preferentially reduces NMDA-induced seizures. In addition, NR1 knockdown can reduce the severity of seizures even when another compound such as KA is introduced systemically. However, under certain conditions the CA1 seizure-induced damage occurs prematurely. Therefore, high constituent levels of NR1 subunits in development appear to influence survival of CA1 neurons following status epilepticus. The use of NMDA receptor AS-ODNs to block seizures in childhood epilepsy may prevent occurrence of seizures and damage in adulthood.