Blockade of BDNF signaling turns chemically-induced long-term potentiation into long-term depression

Long-term potentiation (LTP) is accompanied by increased spine density and dimensions triggered by signaling cascades involving activation of the neurotrophin brain-derived neurotrophic factor (BDNF) and cytoskeleton remodelling. Chemically-induced long-term potentiation (c-LTP) is a widely used cellular model of plasticity whose effects on spines have been poorly investigated. We induced c-LTP by bath-application of the N-methyl-D-aspartate receptor (NMDAR) co-agonist glycine or by the K+ channel blocker tetraethylammonium (TEA) chloride in cultured hippocampal neurons and compared the changes in dendritic spines induced by the two models of c-LTP and determined if they depend on BDNF/TrkB signaling. We found that both TEA and glycine induced a significant increase in stubby spine density in primary and secondary apical dendrites, whereas a specific increase in mushroom spine density was observed upon TEA application only in primary dendrites. Both TEA and glycine increased BDNF levels and the blockade of tropomyosin-receptor-kinase receptors (TrkRs) by the non selective tyrosine kinase inhibitor K-252a or the selective allosteric TrkB receptor (TrkBR) inhibitor ANA-12, abolished the c-LTP-induced increase in spine density. Surprisingly, a blockade of TrkBRs did not change basal spontaneous glutamatergic transmission but completely changed the synaptic plasticity induced by c-LTP, provoking a shift from a long-term increase to a long-term depression (LTD) in miniature excitatory postsynaptic current (mEPSC) frequency. In conclusion, these results suggest that BDNF/TrkB signaling is necessary for c-LTP-induced plasticity in hippocampal neurons and its blockade leads to a switch of c-LTP into chemical-LTD (c-LTD).