Novel approaches to explore mechanisms of epileptic seizures - optogenetic and chemogenetic manipulation of hippocampal circuitry

Sammanfattning: Epilepsy comprises a family of neurological disorders characterized by recurrent seizures, which can be highly debilitating. Up to 30% of patients with temporal lobe epilepsy, the most common form of the disorder in adults, arising in the hippocampus, cannot be effectively treated by current pharmaceuticals. Novel treatment strategies are highly needed, as well as increased understanding of the hippocampal components and signaling properties involved in the mechanisms of epileptogenesis and induction of seizures, to guide a rational search for such future treatments. In the present thesis, we have applied optogenetics to study these questions, a technology based on modified microbial membrane channels or pumps that are introduced into target neurons, which can thereafter be activated by light. This is a powerful means of achieving excitatory or inhibitory control over neurons in a target specific manner. In paper I, we have explored the use of inhibitory optogenetics to attenuate seizures (epileptiform bursts) in acute chemical models in vitro and in vivo. GABAergic inhibition was abolished creating a strong excitatory drive among principal neurons in mainly area CA3 of the hippocampus. Under these conditions, we showed that we could inhibit hypersynchronized bursts by activating the inhibitory chloride pump NpHR. Repeated neuronal discharges gradually creating a hyperexcitable state in the hippocampal circuitry and presenting with seizure-like afterdischarges, known as the kindling process, is traditionally induced by electrical stimulation. In paper II & III, we showed that similarly, afterdischarges could be generated by repeated optogenetic train stimulation in anaesthetized transgenic mice expressing the excitatory cation channel ChR2. Additionally, we corroborated the well known role of the dentate gyrus is this type of progressive seizure model. Finally, using the chemogenetic novel hyperpolarizing Gi-DREADD receptor, activated by CNO, we could show that inhibiting the dentate gyrus and CA3 contralateral to optogenetic stimulation effectively halted the afterdischarge progression. The work presented in this thesis reinforces the potential of optogenetic and chemogenetic techniques to explore the mechanistic underpinnings and new treatment options for seizures & epilepsy.