In mammalian brain the main inhibitory drive is determined by GABAergic synaptic transmission. Released from presynaptic terminals GABA activate postsynaptic Cl-selective GABAA receptors, which results in hyperpolarization of cells and inhibition of neuronal network. Dysfunction of GABAA receptors may lead to a number of pathologies, including epilepsy, Alzheimer’s disease, sleep disorders and schizophrenia. The main approach for functional analysis of GABAA receptors is represented by electrophysiological methods. An alternative approach would be to use non-invasive fluorescent analysis of GABAA receptors activation, which requires creation of new probes. We propose to design genetically encoded Cl-sensitive sensors capable to monitor activation of Cl-selective GABAA receptors. We will develop the probe “BioSensor-GABAR”: a chimeric GABAA receptor subunit with Cl-Sensor inserted to cytoplasmic domain. Development of “GABAR-Sensors” would allow us (i) to monitor activation of the GABAA receptors (ii) to estimate [Cl-]i in the local areas in close vicinity of the GABAA receptor clusters and (iii) to visualize GABAergic synaptic networks during different patterns of neuronal activity.Given the diverse roles of GABAergic signaling in the brain including; hyperpolarisation, decreasing excitability, shunting inhibition, tonic inhibition, disinhibition, and modulation of oscillatory activity it is not clear a priori what the effects of the change in GABAA receptor activity will be on a particular neuronal network. Creation of BioSensor-GABAR will make an important step to the understanding of the physiology of inhibitory neurons and neuronal networks functioning. Results to be obtained in this project will shed light on the functional role of particular classes of GABAergic interneurons. These data will have important clinical implications as they provide an assay for fast screening of neuroleptics and other modulators of GABAergic neurotransmission.