Abstracts

Rationale: Seizures are thought to be due to a breakdown of the GABAergic inhibitory system allowing excessive excitation to spread across the brain. Recent work has shown that dendritic inhibition provided by somatostatin-positive (SOM+) interneurons, via GABA_A receptor activation, is too weak to control hyperexcitability. Furthermore, parvalbumin-positive (PV+) interneurons, similarly acting via GABA_A receptors, can become paradoxically pro-epileptic during seizures due to chloride loading of excitatory neurons. Another class of interneurons could, however, be more powerful for controlling network excitability during seizures. Neurogliaform (NGF) interneurons, which work by ‘volume transmission’, indiscriminately inhibit most neuron types within a 100µm radius. They also act via both GABA_A and GABA_B receptors. Thus, their inhibitory action is both long-lasting and not fully reliant on transmembrane chloride gradients.

Here, we investigate the role of NGF cells in seizure generation and maintenance.


Methods: We used a mouse line (Ndnf-Cre) that enables targeting of NGF cells, and a combination of calcium imaging, electrocorticography and closed-loop optogenetic stimulation or chemogenetic activation in models of acute, focal cortical epilepsy.


Results: In vivo calcium imaging revealed that Ndnf+ NGF cells are recruited a few seconds after the onset of ictal discharges. These findings suggest that Ndnf+ NGF neurons are involved in seizure activity but whether they promote or prevent the spread of overexcitation remains unknown. To answer this question, we used optogenetic manipulation of NGF cells together with local chemoconvulsant application. Our data indicates a reduction of interictal spikes when activating NGF cells, while inhibiting them promotes interictal discharges. Importantly, a strong reduction in seizure duration during light stimulation was observed when NGF cells were activated at the onset of seizures, but also when they were stimulated after seizure onset (more than 2 seconds later). Together, these results provide the first evidence that Ndnf+ NGF cell photo-activation can have significant anti-epileptic effects. We tested whether this persistent NGF cell inhibition during seizures involves GABA_B receptor activation. Our preliminary data indeed suggest that in the presence of CGP (GABA_B antagonist), the anti-seizure effect of Ndnf+ NGF cell photoactivation is suppressed.

We are now testing whether chemogenetics, a more subtle and translatable approach to manipulate NGF cell activity, can be used as an anti-epileptic strategy. Our preliminary data suggest that chemogenetic activation of NGF cells has an anti-epileptic action in ex vivo cortical slices. We are now moving towards testing this approach in a chronic model of temporal lobe epilepsy.


Conclusions: In contrast to PV+ and SOM+ interneurons, Ndnf+ NGF cells retain their inhibitory power during seizures, and this could be at least partially explained by GABA_B signaling. Altogether, our findings suggest that GABA_B mediated NGF cell inhibition could be an interesting therapeutic target to prevent seizures.


Funding: This work was supported by ERIUK, the MRC, the Wellcome, the Royal Society, the Rosetrees Trust.