Studies of the spatio-temporal distribution of inhibitory postsynaptic potentials (IPSPs) in a neuron have been limited by the spatial information that can be obtained by electrode recordings. We describe a method that overcomes these limitations by imaging IPSPs with voltage-sensitive dyes. CA1 hippocampal pyramidal neurons from brain slices were loaded with the voltage-sensitive dye JPW-1114 from a somatic patch electrode in whole-cell configuration. After removal of the patch electrode, we found that neurons recover their physiological intracellular chloride concentration. Using an improved voltage-imaging technique, dendritic GABAergic IPSPs as small as 1 mV could be resolved optically from multiple sites with spatial averaging. We analyzed the sensitivity of the technique, in relation to its spatial resolution. We monitored the origin and the spread of IPSPs originating in different areas of the apical dendrite and reconstructed their spatial distribution. We achieved a clear discrimination of IPSPs from the dendrites and from the axon. This study indicates that voltage imaging is a uniquely suited approach for the investigation of several fundamental aspects of inhibitory synaptic transmission that require spatial information.
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