Abstracts

Rationale: Focal cortical dysplasia (FCD) is a leading cause of pharmacoresistant epilepsy and is often associated with cognitive impairments and autistic features. Characterised by cortical dyslamination and the presence of cytomegalic/dysmorphic neurons, FCD is caused in most cases by somatic mutations of genes encoding members of the mTOR signalling cascade. FCD-associated epilepsy is also often a poor candidate for resective surgery because the dysplastic region typically occurs in an eloquent cortical territory.

The need for an improved understanding of epileptogenic mechanisms in FCD, and for novel therapeutic strategies is therefore high. Despite the overt structural abnormality, whether seizures arise from dysplastic neurons and their connections remains unclear. Here we present electrophysiological and imaging data dissecting the contribution of heterotopic/dysmorphic and morphologically normal-appearing neurons in the dysplastic region both in vitro and in the behaving mouse.


Methods: We use a mouse model of FCD generated by in utero electroporation of a constitutive activator of mTOR (Rheb_S16H), which recapitulates many molecular, morphological, electrographic, and behavioural features of FCD type II in patients, including subclinical epileptic features.

Here we combine whole-cell patch clamp recordings in acute brain slices with miniscope calcium imaging of dysplastic neurons in the freely behaving animal to characterise the properties of neurons in the dysplastic region and their activity during animal behaviour to compare their network behaviour to healthy neurons.


Results: Patch-clamp recordings of dysplastic neurons obtained in acute brain slices from FCD mice show a profound intrinsic hypoexcitabilty, while neighbouring, morphologically normal cells show reduced rheobase, increased membrane resistance, and higher stable firing rates than control cells. Recordings of spontaneous excitatory and inhibitory inputs onto dysplastic neurons suggest that they do functionally integrate into the cortical network. In contrast to reduced excitability at the soma, dysplastic neurons show normal action potential output in response to dendritic stimulation when compared to control cells, suggesting that in response to synaptic excitation, their output may be normal.

Implantable miniscope recordings of spontaneous activity in both dysplastic and non-dysplastic neurons with single-cell resolution revealed that dysplastic neurons show much larger calcium transients during wakefulness than control cells, suggesting a tendency to burst firing in the active network. While these neurons do not apparently show greater internal synchrony than their non-dysplastic neighbours, this tendency to burst firing suggests a role in seizure generation.


Conclusions: In contradiction with intrinsic firing properties recorded in brain slices, the behaviour of dysplastic neurons in the behaving animal are strongly hyperexcitable. The tendency to burst firing allows these neurons to drive strong excitation in the network, and act as a synchronising force ahead of seizure onset.


Funding: BBSRC, ERUK, Wellcome Trust