Abstracts

Rationale: Fragile X is the most common form of mental retardation and it is caused by the absence of 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 (KO) mice. The subiculum plays a crucial role in initiating epileptiform discharges and in gating hippocampal outputs; this function depends on GABA_A receptor-mediated inhibition. GABA is the major inhibitory neurotransmitter in the central nervous system. Altered expression of subunits of GABA_A receptors has been shown in several epileptic models as well as in fmr1 KO mice. In our study we investigated if a different expression of the GABA_A currents can cause a hyperecitable background and epilepsy in fragile X patients.Methods: We performed electrophysiological recordings (field potential and patch clamp in whole cell configuration) in subiculum from age-matched fragile X and wild type (WT) mice to evaluate possible differences in expression of the GABA_A current. In addition, we carried out real time RT-PCR and western blot to investigate the level of expression of α5 and δ subunits responsible for the tonic component of the GABA_A receptor-mediated current at an mRNA and protein level. Results: We observed that selective blockade of the GABA_A phasic and tonic components evoked different responses in subicular network of fmr1 KO compare to WT mice, suggesting that an altered GABAergic system is present in fragile X animals. Inhibitory postsynaptic currents (IPSCs) presented in fragile X with a smaller amplitude respect to WT tissue (WT: peak amplitude = 49.46±6.52 pA, n = 13; fragile X: peak amplitude = 34.58±2.61 pA, n = 25). In addition, our data indicate an underexpression of subunits involved in GABAergic tonic inhibition in the subiculum of fragile X mice. α5 subunit gene expression was decreased by 22% (GAPDH) to 27% (TBP), and the δ subunit gene expression was decreased by 17% (GAPDH) to 20% (TBP) in fragile X versus WT mice.Conclusions: Our data demonstrate that GABAergic system is altered in fragile X mice. In particular, a down-regulation of the phasic component and an underexpression of the subunits responsible for the tonic component of GABA_A current occur in the subiculum of fmr1 KO mice. [Supported by : CIHR, FXRFC, CURE and Savoy Foundation]