Synaptic accumulation of GluN2B-containing NMDA receptors mediates the effects of BDNF-TrkB signalling on synaptic plasticity and in hyperexcitability during status epilepticus.

Background: Brain-derived neurotrophic factor (BDNF) is a key mediator of synaptic plasticity and memory formation in the hippocampus. However, the BDNF-induced alterations in the glutamate receptors coupled to the plasticity of glutamatergic synapses in the hippocampus have not been elucidated. In this work we investigated the putative role of GluN2B-containing NMDA receptors in the plasticity of glutamatergic synapses induced by BDNF.

Methods: The effects of BDNF on the surface expression of GluN2B-containing NMDA receptors was investigated in cultured hippocampal neurons and in hippocampal synaptoneurosomes by immunocytochemistry under non-permeabilizing conditions, using an antibody that binds to an extracellular epitope. Long term potentiation of hippocampal CA1 synapses was induced by using θ-burst stimulation. Epileptic seizures were induced using the Li-pilocarpine model of temporal lobe epilepsy. Pyk2 phosphorylation was assessed by western blot with a phosphospecific antibody.

Results: Stimulation of hippocampal synaptoneurosomes with BDNF led to a significant time-dependent increase in the synaptic surface expression of GluN2B-containing NMDA receptors as determined by immunocytochemistry with colocalization with pre- (vesicular glutamate transporter) and post-synaptic markers (PSD-95). Similarly, BDNF induced the synaptic accumulation of GluN2B-containing NMDA receptors at the synapse in cultured hippocampal neurons, by a mechanism sensitive to the PKC inhibitor GӦ6983. The effects of PKC may be mediated by phosphorylation of Pyk2, as suggested by western blot experiments analyzing the phosphorylation of the kinase on Tyrosine 402. GluN2B-containing NMDA receptors mediated the effects of BDNF in the facilitation of the early phase of long-term potentiation (LTP) of hippocampal CA1 synapses induced by θ-burst stimulation, since the effect of the neurotrophin was abrogated in the presence of the GluN2B inhibitor Co 101244. In the absence of BDNF, the GluN2B inhibitor did not affect LTP. Surface accumulation of GluN2B-containing NMDA receptors was also observed in hippocampal synaptoneurosomes isolated from rats subjected to the pilocarpine model of temporal lobe epilepsy, after reaching Status Epilepticus, an effect that was inhibited by administration of the TrkB receptor inhibitor ANA-12.

Conclusion: Together, these results show that the synaptic accumulation of GluN2B-containing NMDA receptors mediate the effects of BDNF in the plasticity of glutamatergic synapses in the hippocampus.