Role of voltage sensitive calcium channels (VSCCs) in the maturation of the GABAergic system in the fragile X syndrome

GABAA receptors are considered to be major inhibitory receptors in the brain. A significant down-regulation of the GABAA receptors in Fmr1 knockout (KO) mice has been demonstrated in recent studies, which may underlie the mechanism for the anxiety and hyperactivity found in fragile X syndrome patients. However, during early developmental stages, activation of GABAA receptors exerts excitatory effects on neuronal networks. The excitatory effect of GABAA receptors early on sets the major tune in generating giant depolarizing potentials (GDPs), recurrent synchronized spontaneous network discharges which are features of neuronal activity of developing neurons. GDPs control extension and motility of neurites as well as synthesis and expression of the GABAergic phenotype through the activation of voltage sensitive calcium channels (VSCCs) during early developmental stages up to P14 in mice and rats. The developmental excitatory to inhibitory switch in the GABAA receptors function is mainly dependent upon the expression level of the cotransporters KCC2. In this study, we examined the expression level of GABAA receptors and VSCCs in the brain of FVB/NJ wild type (WT) and Fmr1 knockout (KO) mice, and attempted to determine whether modulation of VSCC could affect the maturation of GABAergic system in the KO mice. First, we found reduced expression of GABAA receptors and VSCCs in KOs, and a shift in the time course of the excitatory to inhibitory functional switch of GABAA receptors in KOs. In addition, we modulated VSCCs activity in cerebellar granule cell culture with KCl and nifedipine to verify whether VSCCs can normalize reduced expression of GABAA receptors in KO mice. Large-scale nuclear translocation and exportation of the VSCC beta3 subunit were detected, suggesting VSCC could play a regulatory role in gene expression. Finally, we pharmacologically manipulated VSCCs during embryogenesis by injecting pregnant mice with Bay K 8644 (a VSCC activator) and performed behavioral analyses on the offspring. Injection of VSCC activator rescued part of the KO behavior phenotype. In conclusion, the VSCCs in KO mice brain may be responsible for the activity-dependent, calcium-mediated gene expression resulting in the altered expression of GABA A receptors. Activating VSCCs during early development could partially normalize KO behavior.