Abstracts

Excessive NMDAR activation is thought to contribute to pathophysiology in many neurological disorders, including limbic epilepsy. We showed previously that chronic treatment with distinct classes of NMDAR antagonists had differential effects on seizures in hippocampal slice cultures (Wang and Bausch, 2004). The NR2B selective antagonist, Ro25,6981, reduced bicuculline (BMI)-induced electrographic granule cell seizures; the high-affinity competitive antagonist, APV, and moderate-affinity uncompetitive antagonist, memantine, elicited trends toward seizure exacerbation. We hypothesized that the selectivity of Ro25,6981 for the NR2B subunit mediated its distinct effects. However differential effects on excitatory and inhibitory transmission or the degree of NMDAR blockade are plausible alternative mechanisms. We investigated each possibility using a seizure model that retains GABAergic inhibition. Organotypic hippocampal slice cultures were treated with the high-affinity competitive antagonist, APV, the NR2B-selective antagonist, Ro25,6981, and/or the NR2A-selective antagonist, NVP-AAM077 for 17-21 days in vitro. Spontaneous electrographic seizures were recorded in the granule cell layer in nominal Mg²⁺ recording buffer using extracellular field potential recordings. In nominal Mg²⁺ buffer, Ro25,6981-treated cultures exhibited an 84% decrease in seizure duration compared to vehicle; results were almost identical to our previous data from the acute BMI model. However, treatment with a maximal concentration of APV (50[mu]M) caused a 431% increase in seizure duration compared to vehicle and 3-fold increase compared to our previous results in BMI. These data reveal profound differences in the effects of APV and Ro25,6981, even when inhibition is intact. To examine the idea that degree of block contributed to differential effects, concentration-response analyses for APV were conducted. Effects of APV treatment were concentration-dependant. Interestingly at 0.1 [mu]M, APV caused a modest trend toward reduced seizure duration compared to vehicle. Lower concentrations of APV are under investigation. To test the contribution of subunit selectivity, we compared the effects of treatment with Ro25,6981 (NR2B) and NVP-AAM077 (NR2A). In contrast to the reduction in seizure duration seen in Ro25,6981-treated cultures, a moderate trend toward increased seizure duration was observed in NVP-AAM077-treated cultures. Interestingly, when cultures were treated with both Ro25,6981 and NVP-AAM077, seizure duration was decreased by 35% compared to vehicle and blunted compared to Ro25,6981 alone. Taken together, these data suggest that subunit selectivity contributes to the differential effects of chronic treatment with distinct classes of NMDAR antagonists on seizures. Degree of NMDAR blockade also may play a role. (Supported by NIH grant NS045964 and CDMRP grant PR030035.)