Abstracts

RATIONALE: Hippocampal sclerosis, the substrate of mesial temporal lobe epilepsy, involves neuronal loss of a subset of dentate hilar interneurons. Interestingly, GluR5 kainate receptors appear to be selectively expressed in discrete subsets of neurons, including hilar interneurons, that are preferentially lost in epileptic tissue. Furthermore, loss of GluR5 immunoreactivity has been found in human epileptic hilus. ATPA is a selective GluR5 agonist, although at high concentrations it also acts as an AMPA receptor agonist. We hypothesized that systemic administration of ATPA in the immature rat might lead to excitotoxic injury of GluR5-expressing neurons, whose selective loss may be critical in epileptogenesis, and to the subsequent development of other long-term epilepsy-related changes.
METHODS: Rats aged PN5 to 20 were injected i.p. with age-specific doses of ATPA [(RS)-2-amino-3-(3-hydroxy-5-tetr-butylisoxazol-4-yl)propanoic acid] sufficient to induce limbic and tonic-clonic seizures and compared to vehicle controls. Two to 3 weeks later rats were assessed in 3 ways. (1) Dentate hyperexcitability assessment by paired pulse inhibition measured extracellularly in the granule cell layer while stimulating in the outer molecular layer. The ratio of the second to the first population spike amplitude was calculated. (2) Latency for the induction of high frequency clonus and generalized tonic-clonic seizures with the gaseous convulsant fluorothyl. (3) Nissl staining to determine the extent of neuronal loss.
RESULTS: Limbic followed by tonic-clonic seizures lasting for over 4 hours with less than 25% mortality were produced by the following doses of ATPA (mg/kg): 20 for PN5-6, 15 for PN10-11 and 15 for PN14-15. Older rats required doses above 30mg/kg and were not further studied. Behavioral features following ATPA administration were similar to those previously described for kainic acid: Initially immobility, scratching and ataxia, followed (only in pups younger than PN14) by [dsquote]cycling[dsquote] and head wagging. Tonic-clonic seizures began as a running fit and continued as tonic seizures with additional running fits alternating with scratching and immobility. PN5-6 rats, however, did not exhibit running fts. Latency to onset of fluorothyl-induced seizure was not significantly different between ATPA-treated rats at any one of the 3 age groups (n=16) and controls (n=9). There was no significant neuronal loss in the hippocampus in rats treated with ATPA at PN10 or PN14-15 (n=7) compared to controls (n=6). Paired pulse inhibition at the dentate gyrus was not significantly different between rats treated at PN10 or PN14-15 (n=4) and controls (n=4).
CONCLUSIONS: Systemic injection of the partially selective GluR5 kainate receptor agonist ATPA reliably induces limbic and secondary tonic-clonic seizures in rats younger than PN16. Despite the selective excitatory action of the drug on a subgroup of neurons selectively lost in mesial temporal lobe epilepsy, no long-term effects were induced in the model. Failure to induce long-term effects despite severe seizures could be related to the well documented resistance of immature rats to convulsant-induced neuronal loss and epilepsy and suggests that additional pathophysiologic factors are required for damage to occur.
[Supported by: NS-20253, NS/HD-41366, Grass, EFA, Onassis]