Mechanisms of reduced chloride homeostasis in the human epileptogenic neocortex

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GABA is the key inhibitory neurotransmitter in the central nervous system. The efficacy of the fast GABA-mediated inhibition depends upon the amount of GABA being released, density and subtypes of receptor subunits and crucially upon the transmembranal chloride gradient. During the last few years disturbances of chloride regulation were demonstrated in several neurological disorders (temporal lobe epilepsy, schizophrenia, neuropathic pain).Our previous studies on slices from epilepsy surgery tissues revealed a marked impairment of chloride homeostasis of individual neurones. The most prominent transporter, a potassium coupled chloride outward transport mechanism (KCC2), dominates in the healthy rodent cortex (46 % of the total transport capacity). In single cell recordings we found this transporter being reduced in the human epileptogenic neocortex; on average by about 80 %. Further routes of transmembrane chloride movement are considerably reduced as well (on average NKCC1 about 60 %, chloride channels about 80 %). In addition, our data indicate that the functional impairments in different tissues and even within a given tissue exhibit a marked interneuronal variability. The precise molecular mechanisms of these downregulations in the neocortex are unsolved, though. In the submitted grant proposal we will further investigate the pathophysiological alterations of chloride homeostasis is human tissues using complementary methods. In addition to field and intracellular recordings, we will investigate the distribution of intracellular chloride using 2 photon fluorescence lifetime imaging. The physiologically characterized slices will subsequently be processed using RTqPCR, Western blot und immunohistochemical staining. The combination of methods enables a localization of the disturbance between transcription, translation and integration of functional proteins into the membrane. From our studies we expect a marked gain in knowledge regarding pathophysiological alterations in the human epileptogenic neocortex. The initial disturbances after seizures are of particular interest as they may present the common denominator of various epilepsies. The indepth understanding of neuronal chloride regulation and of the molecular mechanisms underlying functional deficits open new venues for designing rational causal therapeutic strategies to alleviate the burden of epilepsy patients.