Abstracts

RATIONALE: MHC molecules are cell surface glycoproteins that play an important role in the vertebrate immune system by presenting foreign antigens to T lymphocytes. These molecules have been detected in diverse neuronal populations in rodents, including neurons in the hippocampal formation, and have been shown to be potentially involved in brain plasticity (Corriveau et al., Neuron 1998; 21: 505-520; Huh et al., Science 2000; 290: 2155-2159). Previous DNA microarray analysis in our laboratory of rat dentate gyrus gene expression found evidence for regulation of multiple MHC mRNAs during epileptogenesis. The objective of this study is to investigate further the possible involvement of MHC molecules in epilepsy-associated network reorganization. To do so, we have characterized the spatial and temporal patterns of MHC expression in the rat dentate gyrus following pilocarpine-induced SE.
METHODS: Adult male Sprague-Dawley rats (180-200g) were given i.p. atropine methylbromide followed 20 min later by pilocarpine hydrochloride to induce SE. Seizure activity was monitored behaviorally and terminated with diazepam after 2h of convulsive SE. Control rats received saline instead of pilocarpine. Animals were killed 3, 7, 14, and 28 days later, and perfusion-fixed brains were processed for non-radioactive in situ hybridization analysis of rat MHC class I and class II mRNAs. Digoxygenin-labeled in situ probes were transcribed from DNA templates generated by polymerase chain reaction (PCR) from neonatal or adult rat hippocampal cDNA libraries.
RESULTS: Our results show that mRNAs coding for MHC class I and class II molecules are differentially regulated during epileptogenesis. At 14 days after SE, MHC class I RT1.Aa and MHC class I RT1-RT44 were markedly increased when compared with saline-treated control animals, with scattered expression throughout the hippocampus and the highest increase in the dentate gyrus. In contrast, expression of MHC class II RT1.B-1 was downregulated throughout the dentate gyrus and pyramidal cell layers. More extensive in situ analysis of MHC class I RT1.Aa and MHC class II RT1.B-1 mRNAs indicated that these molecules are regulated over different time courses. Expression of MHC class I RT1.Aa peaks at 7 to 14d post-SE, and then returns to basal levels over the next two weeks, whereas MHC class II RT1.B-1 expression declines slowly following SE and remains decreased at 28d post-SE.
CONCLUSIONS: The class I and class II families of MHC molecules consist of at least 20-30 related proteins with similar yet distinct functions in the immune system. Our results suggest that the various members are differentially regulated following SE, with particularly marked differences in expression between class I and class II molecules. These findings support and extend previous evidence that MHC molecules are expressed in areas of ongoing neural plasticity and raise the possibility of a potential role for MHC molecules in mediating SE-induced network reorganization.
[Supported by: NIH RO1NS39950 and FAPESP (Brazil).]