An Optogenetic Approach to Studies of the Mechanisms of Heterosynaptic Plasticity in Neocortical Neurons

Heterosynaptic plasticity is a poorly studied type of synaptic plasticity in which synapses not active during the plasticity induction process are modified. The main methodological problem in studying the mechanisms of heterosynaptic plasticity using conventional techniques are the need for large numbers of experiments because of the probabilistic nature of the development of plastic changes in the synaptic inputs to neurons after intracellular tetanization – the most widely used approach to inducing heterosynaptic plasticity. To overcome this problem, we report here use of a method of delivering optogenetic stimulation to a multitude of presynaptic neurons converging on a single postsynaptic neuron. Our studies using rat brain slices showed that intracellular tetanization of a pyramidal neuron in layer 5 of the neocortex leads to the development of depression or potentiation of a proportion of the synaptic inputs from above-lying pyramidal cells in layers 2/3 expressing light-activated protein channelrhodopsin-2. Use of this stimulation method was found to allow a single experiment to yield sufficient data for statistical evaluation of the correlation between the paired pulse ratio and the amplitude of changes in EPSP after intracellular tetanization. We also showed that so-called "silent" synapses were not activated on development of long-term potentiation induced by intracellular tetanization.

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