Morphological characterization of a murine model of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE)

ADNFLE is a partial epilepsy characterized by clusters of hyperkinetic seizures, often accompanied by sudden arousals, mostly occurring during stage II of NREM sleep. Attacks arise in the frontal lobe, tend to begin in childhood and cognitive and psychological alterations may be present. In 20-30% of the cases, the disease does not respond to antiepileptic drug treatment. About 10–15% of the ADNFLE families bear mutations on genes coding for nicotinic acetylcholine receptor (nAChR) subunits. We studied how mutant nAChRs can lead to ADNFLE, by using a murine model of the pathology conditionally expressing the ADNFLE-linked nicotinic β2-V287L mutation (TG), for comparison with control mice (CTRL) in prefrontal cortex (PFC), somatosensory cortex (SS) and thalamus (TH) at different developmental stages (P8, P21, P60). In these mice, β2-V287L needs to be expressed throughout brain development, until the end of the second postnatal week, for seizures to develop. First of all, we measured the cortical thickness, number of synapses, neuronal volume and density in both PFC and SS at P60, but no differences were observed between CTRL and TG mice. Moreover, the total and synaptic expression of β2* nAChR subunit was evaluated by using densitometric analysis. β2-V287L mutation did not alter the overall β2* nAChR surface density and no significant differences were observed in the cortex between CTRL and TG neither at the presynaptic sites. In addition, we analysed the GABAergic system carrying out immunofluorescence and densitometric analysis on PFC, SS and TH by using antibodies against GAD67, GABAA receptor α1 subunit and VGAT, but no significant differences were found between CTRL and TG mice. Also parvalbuminpositive cells were counted both by stereological and cell counting methods, but we observed no significant difference. Next, we took into account the expression of K+/Cl- cotransporter KCC2, the responsible of the “switch of GABA” effect from excitatory to inhibitory. Alteration in its expression could lead to an excitation/inhibition imbalance and to the pathogenesis of diseases such as epilepsy. Densitometric analysis revealed that in P8 TG mice KCC2 expression significantly decreased in PFC layer V, where a significant increase was instead observed at P60. At this age we revealed a concomitant decrease of this cotransporter in the thalamic reticular nucleus (RT) in TG mice compared to CTRL. Further analyses on the forebrain and mesopontine cholinergic nuclei are being carried out by using stereological and densitometric analyses. Our results demonstrate that β2-V287L mutation has no effect on cortex morphology and distribution of GABAergic markers, not affecting the GABAergic switch. However, it locally alters the expression of KCC2, which may be attributed to a compensatory effect caused by increased excitability in these sensitive regions and in general related to the hyperexcitability of the thalamocortical circuits in ADNFLE.