MarkramLTPspikeTimes - REPORTS London 298, 227 (1982); P....

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London 298 , 227 (1982); P. R. Montague and T. J. Sejnowski, Learn. Mem. 1 , 1 (1994). 27. G. J. Stuart, H. U. Dodt, B. Sakmann, Pflugers Arch. 423 , 511 (1993). 28. J. C. Magee, R. B. Avery, B. R. Christie, D. Johnston, J. Neurophysiol. 76 , 3460 (1996). 29. N. Lasser-Ross, H. Miyakawa, V. Lev-Ram, S. R. Young, W. N. Ross, J. Neurosci . Methods 36 , 253 (1991). 30. We thank B. Christie, C. Colbert, and P. R. Montague for helpful discussions and comments on the manu- script. Supported by NIH grants NS09482 (J.C.M.) and NS11535, MH44754, and MH48432 (D.J.). 17 September 1996; accepted 5 November 1996 Regulation of Synaptic Efficacy by Coincidence of Postsynaptic APs and EPSPs Henry Markram,* Joachim Lu ¨ bke, Michael Frotscher, Bert Sakmann Activity-driven modifications in synaptic connections between neurons in the neocortex may occur during development and learning. In dual whole-cell voltage recordings from pyramidal neurons, the coincidence of postsynaptic action potentials (APs) and unitary excitatory postsynaptic potentials (EPSPs) was found to induce changes in EPSPs. Their average amplitudes were differentially up- or down-regulated, depending on the precise timing of postsynaptic APs relative to EPSPs. These observations suggest that APs propagating back into dendrites serve to modify single active synaptic connections, depending on the pattern of electrical activity in the pre- and postsynaptic neurons. R epetitive activation of neuronal circuits can induce long - term changes in subse - quent responses generated by synapses in many regions of the brain, and such plastic - ity of synaptic connections is regarded as a cellular basis for developmental and learn - ing - related changes in the central nervous system ( 1, 2 ). The actual triggers for synap - tic modifications between two neurons are, however, unclear ( 3 ). Postsynaptic APs are initiated in the axon and then propagate back into the dendritic arbor of neocortical pyramidal neurons ( 4 ), evoking an activity - dependent dendritic Ca 2 1 influx ( 5 ) that could be a signal to induce modifications at the dendritic synapses that were active around the time of AP initiation. To test this hypothesis, we made dual whole - cell voltage recordings from neighboring, thick, tufted pyramidal neurons in layer 5 of the neocortex (Fig. 1A) for which the dendritic locations of synaptic contacts were known ( 6, 7 ), and we investigated whether the postsynaptic AP could induce changes in unitary EPSP amplitudes. When depolarizing current was injected only into the cell body of a presynaptic neuron to evoke a burst of APs, the result - ing high - frequency train of subthreshold unitary EPSPs (Fig. 1B) failed to trigger changes in the average EPSP amplitudes (Fig. 1D), possibly because EPSP ampli - tudes decreased rapidly and a sufficiently large postsynaptic depolarization was not reached ( 3 ). When the postsynaptic neuron was further depolarized by current injection to produce a burst of APs during the EPSPs,
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MarkramLTPspikeTimes - REPORTS London 298, 227 (1982); P....

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