Synaptic plasticity in the absence of backpropagating spikes of layer II inputs to layer V pyramidal cells in rat visual cortex.

Use-dependent changes of synaptic transmission are thought to depend on the evaluation of temporal correlations between pre- and postsynaptic excitatory activity. Previous studies have demonstrated long-term potentiation (LTP) in layer V pyramidal cells after coincident pairing of presynaptic excitatory input with postsynaptic depolarizations, evoking backpropagating action potentials (BAPs). Here we paired excitatory layer II input with somatic hyperpolarization, which blocked BAPs in layer V pyramidal cells of rat visual cortex and induced reliable long-term depression (LTD). Upon cholinergic receptor activation, this BAP-independent protocol also induced LTP, which was not dependent on N-methyl-d-aspartate receptor activation, but blocked by metabotropic glutamate receptor (mGluR) antagonists.