Tyrosine phosphorylation-dependent inhibition of hippocampal synaptic plasticity

We examined the effects of two protein tyrosine phosphatase inhibitors on the induction of synaptic plasticity in CA1 slices of rat hippocampus. Field potential recordings were made in stratum radiatum in response to stimulation of the Schaffer collateral afferents. Bath application of the tyrosine phosphatase inhibitors sodium orthovanadate or phenylarsine oxide for 30 min had little effect on basal synaptic transmission but blocked the induction of both long-term potentiation (LTP) and homosynaptic long-term depression (LTD). LTP could be partially recovered, and LTD fully recovered, when conditioning stimulation was given in conditions of reduced synaptic inhibition. The block of both forms of synaptic plasticity by the phosphatase inhibitors correlated with a concurrent depression of the N-methyl-D-aspartate (NMDA) receptor-mediated potential, as measured both extracellularly and intracellularly. This depression, which was also induced by peroxyvanadate, required synaptic stimulation to be induced, and was tyrosine kinase-dependent. Our results suggest that tyrosine phosphorylation of as yet unidentified proteins is responsible for a novel activity-dependent depression of NMDA receptor function that inhibits synaptic plasticity.