Spatial And Temporal Specificity In Activity-induced Plasticity Of Dendritic Integration

Most neurons in the mammalian brain receive thousands of excitatory synaptic inputs thatare widely distributed along the dendritic arbor and activated with varying degrees ofsynchrony. Dendritic integration of these inputs is thus a key step for neuronal informationprocessing, depending on both passive membrane properties and active dendritic conductances.Patterned neuronal activity, which is known to modify synaptic transmission, has been shownto modulate dendritic structure and ion channel properties. However, whether and howneuronal activity can directly modify dendritic integrative function is largely unknown.To answer this question, we examined the efficacy of EPSP summation in hippocampalCA1 pyramidal neuron before and after the induction of long-term synaptic potentiation (LTP)by a paired theta burst stimulation (pTBS). Summation of two EPSPs was usually sublinear,and the linearity depended on the amplitude of summed EPSPs. We first found that, thelinearity of both spatial summation (summation of synchronous EPSPs from two independentsynaptic inputs) and temporal summation (consecutive EPSPs from the same input) increased,following LTP induction, in an input-specific manner. Such enhancement of summationefficacy was attributed to a local modulation of dendritic channels following LTP induction.Furthermore, we found that the enhancement of EPSP summation depends both on thetemporal interval between summed inputs and their dendritic locations. At the distal dendrite,LTP induction leads to an increased summation linearity only for inputs arriving within 5 ms,which may facilitate spike generation for coincident inputs. In contrast, at the proximaldendrite, LTP induction leads to an increased summation linearity for inputs arriving within aninterval over 20 ms. These results pointed to the possibility of a location-dependentdifferential modification of dendritic function in its two fundamental modes – coincidencedetection and temporal integration.To test this possibility, we further examined the spiking output by the summed EPSPs,before and after LTP induction at different dendritic locations. Interestingly, we found anincreased spiking probability for summation of inputs only arriving coincidently at the distaldendrite, but for summation of inputs arriving over 18 ms at the proximal dendrite. Thisindicates an activity-induced enhancement of coincidence detection at the distal dendrite, andthat of temporal integration at the proximal dendrite.Taken together, our study underscores an activity-induced modification of dendriticfunctions, whereby the capacity for coincidence detection and temporal integration at differentdendritic location is differentially regulated by neuronal activity that is known to modifysynaptic efficacy. Together with synaptic plasticity, the plasticity of dendritic functionsconstitutes an integral part of activity-dependent information processing and storage in neuralcircuits.