Endogenous modulators of hyperexcitability in epilepsy: electrophysiological and optogenetic delineation of neuropeptide Y mechanisms in interneurons

Abstract: Epilepsy is one of the most common neurological disorders worldwide, affecting 1% of the general population, and is characterized by a predisposition for the generation of epileptic seizures. Despite having several different aetiologies, a common underlying cause of epilepsy seems to be an acquired imbalance between excitatory and inhibitory circuits in the brain, which leads to hyperexcitability and appearance of seizures. Current treatment relies on the use of antiepileptic drugs (AEDs), but these only treat the symptoms, not affect the causes of the disease, and trigger undesirable side effects because of their systemic administration. Recent advancements in drug discovery have led to the development of new AEDs that are better tolerated and with improved pharmacokinetics, but 30-40% of all patients with epilepsy, and particularly those with temporal lobe epilepsy (TLE) remain resistant to the treatment. Thus, there is an urgent need for developing new antiepileptic treatment strategies. In the last years, research on novel antiepileptic treatments has identified several endogenous molecules as potential new targets for therapeutic intervention. Among these, neuropeptide Y (NPY ) seems to be a particularly promising target, as it plays an important role in controlling neuronal excitability in different brain areas, including the hippocampus. Indeed, overexpression of NPY via gene therapy approaches in animal models of epilepsy has profound effects on seizure generation and suppression, providing proof of principle evidence that such approach could be successfully used to reduce and control seizures. The actions of NPY are mediated by various receptors, and their activation predominantly causes suppression of glutamatergic synaptic transmission, which leads to decreased excitability. However, little is known about the effect NPY has on GABAergic inhibitory cell populations, and NPY mechanisms of action have to be carefully determined if such an approach could be used in humans. There are several different subtypes of inhibitory cell populations in all cortical areas, and each of them serve a different function with distinct roles in controlling network activity. Perisomatic-targeting interneurons comprise those inhibitory cell types that form synapses onto the perisomatic region of target cells, an area including the cell soma, proximal dendrites and axon initial segment. Thanks to the strategic location of their targets, perisomatic interneurons are particularly suited to control the output of large numbers of excitatory principal cells, with major impact on the network excitability. Two main subclasses, called basket cells, make up the majority of perisomatic interneurons, and their classification is based on the expression of either the neuropeptide Cholecystokinin (CCK) or the calcium binding protein Parvalbumin (PV). PV-basket cells are thought to be important for the generation of gamma-frequency oscillations, while CCK- basket cells are proposed to modulate this activity. Since gamma oscillations can convert into higher frequency epileptiform activity, and NPY strongly modulates network excitability, this thesis aimed to investigate the effects of NPY on CCK- and PV- basket cells, to understand if actions of NPY on perisomatic interneurons could contribute to its seizure-suppressant effects. Using transgenic mice, electrophysiological and optogenetic techniques, the evidence provided in this thesis demonstrates that NPY strongly modulates excitatory and inhibitory incoming synaptic transmission onto CCK-basket cells, but does not directly affect PV cell output onto principal cells. These effects could alter the way CCK-basket cells react to network activity, and have potential impacts on network excitability. In addition, we show that hyperexcitability enhances GABAergic output from PV cells, uncovering a potential mechanism that could increase principal cell synchrony and contribute to the generation of seizures.