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The efficacy and short-term modification of neocortical synaptic connections vary with the type of target neuron. We investigated presynaptic Ca2+ and release probability at single synaptic contacts between pairs of neurons in layer 2/3 of the rat neocortex. The amplitude of Ca2+ signals in boutons of pyramids contacting bitufted or multipolar interneurons or other pyramids was dependent on the target cell type. Optical quantal analysis at single synaptic contacts suggested that release probabilities are also target cell-specific. Both the Ca2+ signal and the release probability of different boutons of a pyramid contacting the same target cell varied little. We propose that the mechanisms that regulate the functional properties of boutons of a pyramid normalize the presynaptic Ca2+ influx and release probability for all those boutons that innervate the same target cell.
