Abstracts

Fragile X syndrome is the most common inherited cause of mental impairment resulting from the inactivation of the fragile X mental retardation gene ([italic]fmr1[/italic]) and the subsequent inability to produce the fragile X mental retardation protein (FMRP). The neurologic phenotype of fragile X patients includes epilepsy and it is well reproduced in the fmr1 knockout mice, considered a genetic model of epilepsy. The subiculum belongs to the limbic system and plays a crucial role in initiating epileptiform discharges and in gating hippocampal outputs. This gating function depends on GABA[sub]A[/sub] receptor-mediated inhibition, which is important in controling hyperexcitability both at synaptic (GABA[sub]A[/sub] phasic inhibition) and extrasynaptic (GABA[sub]A[/sub] tonic inhibition) levels. In preliminary electrophysiological studies we have found a loss of GABAergic tonic inhibition in the subiculum of seizure prone fragile X knockout mice. In addition, altered expression of [alpha]5 and [delta] GABA[sub]A[/sub] receptor subunits, which compose receptors mediating tonic inhibition, have been reported in other models of epilepsy. Thereby, in the present study we investigated at the molecular level whether a possible loss of GABAergic tonic inhibition occurs in the subiculum of fragile X versus wild type (wt) mice., We performed real time RT-PCR experiments with RNAs extracted from subiculum of age-matched fragile X and wt mice. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and TATA box binding protein (TBP) housekeeping genes [ndash] reported as suitable for RNA quantification (Radonic [italic]et al.[/italic] 2003) [ndash] were used as reference genes. Using the Pfaffl method (Pfaffl, 2001), we calculated [italic]R values [/italic](level of expression relative to control) for GABA[sub]A [/sub][alpha]5 and [delta] subunits genes., GABA[sub]A [/sub][alpha]5 and [delta] subunits genes were consistantly underexpressed in fragile X mice: for GABA[sub]A[/sub][alpha]5 R=0.779[plusmn]0.022 with GAPDH (n=8) and 0.729[plusmn]0.017 with TBP (n=3); for GABA[sub]A[/sub][delta] R=0.829[plusmn]0.021 with GAPDH (n=8) and 0.808[plusmn]0.033 with TBP (n=3). So the [alpha]5 subunit gene expression was decreased by 22% (GAPDH) to 27% (TBP), and the [delta] subunit gene expression was decreased by 17% (GAPDH) to 20% (TBP) in fragile X versus wt mice. This mild but reproductible decrease was significant: mean R values were different from 1 with a T-test (p values[lt]0.05)., Our data demonstrate that a down-regulation of subunits involved in GABAergic tonic inhibition occurs at the level of RNA in the subiculum of fragile X knockout mice versus control., (Supported by CIHR, FXRFC, CURE and Savoy Foundation.)