WIBR, UCL, London, UK
Dendritic integration is thought to be strongly dependent on the spatiotemporal pattern of synaptic input. Indeed, it has been shown that highly synchronous inputs delivered to a spatially restricted segment of a dendrite can produce local dendritic spikes, while temporally distributed input does not. However, it is not known if such extreme degrees of spatiotemporal synchrony exist in vivo, nor whether more asynchronous input patterns can be distinguished from each other. We have therefore tested whether the dendritic response to a particular spatial pattern of asynchronous synaptic inputs is sensitive to the temporal sequence of their activation. We have made whole-cell recordings from layer 2/3 pyramidal cells and activated synaptic input onto the basal dendrites using 2-photon glutamate uncaging in acute slices of the rat somatosensory cortex. We found that sequential activation of synapses along a basal dendritic branch, from the end of the dendrite towards the soma, produces a significantly larger somatic voltage change than activation in the reverse order. This effect is dependent on the membrane potential, and on both the spatial and the temporal distribution of the activated synapses. Furthermore, we find that NMDA receptor activation is essential for the observed directional selectivity, and compartmental modelling shows that this can be explained by a combination of passive cable properties and NMDA receptor kinetics. We conclude that basal dendrites of layer 2/3 cortical neurons can detect the sequence of synaptic activation, and suggest that this may be an important property of dendrites for information coding and processing.