Brief activation of GABAergic interneurons initiates the transition to ictal events through post-inhibitory rebound excitation

Activation of gamma-aminobutyric acid (GABA(A)) receptors have been associated with the onset of epileptiform events. To investigate if a causal relationship exists between GABA (A)receptor activation and ictal event onset, we activated inhibitory GABAergic networks in the superficial layer (2/3) of the somatosensory cortex during hy-perexcitable conditions using optogenetic techniques in mice expressing channelrhodopsin-2 in all GABAergic interneurons. We found that a brief 30 ms light pulse reliably triggered either an interictal-like event (HE) or ictal-like (ictal) event in the in vitro cortical 4-Aminopyridine (4-AP) slice model. The link between light pulse and epileptiform event onset was lost following blockade of GABA(A) receptors with bicuculline methiodide. Additionally, recording the chronological sequence of events following a light pulse in a variety of configurations (whole-cell, gramicidin-perforated patch, and multi-electrode array) demonstrated an initial hyperpolarization followed by post-inhibitory rebound spiking and a subsequent slow depolarization at the transition to epileptiform activity. Furthermore, the light-triggered ictal events were independent of the duration or intensity of the initiating light pulse, suggesting an underlying regenerative mechanism. Moreover, we demonstrated that brief GABA (A) receptor activation can initiate ictal events in the in vivo 4-AP mouse model, in another common in vitro model of epileptiform activity, and in neocortical tissue resected from epilepsy patients. Our findings reveal that the synchronous activation of GABAergic interneurons is a robust trigger for ictal event onset in hyper-excitable cortical networks.