Inositol hexakisphosphate suppresses excitatory neurotransmission via synaptotagmin-1 C2B domain in the hippocampal neuron

Inositol hexakisphosphate (InsP₆) levels rise and fall with neuronal excitation and silence, respectively, in the hippocampus, suggesting potential signaling functions of this inositol polyphosphate in hippocampal neurons. We now demonstrate that intracellular application of InsP₆ caused a concentration-dependent inhibition of autaptic excitatory postsynaptic currents (EPSCs) in cultured hippocampal neurons. The treatment did not alter the size and replenishment rate of the readily releasable pool in autaptic neurons. Intracellular exposure to InsP₆ did not affect spontaneous EPSCs or excitatory amino acid-activated currents in neurons lacking autapses. The InsP₆-induced inhibition of autaptic EPSCs was effectively abolished by coapplication of an antibody to synaptotagmin-1 C2B domain. Importantly, preabsorption of the antibody with a GST-WT synaptotagmin-1 C2B domain fragment but not with a GST-mutant synaptotagmin-1 C2B domain fragment that poorly reacted with the antibody impaired the activity of the antibody on the InsP₆-induced inhibition of autaptic EPSCs. Furthermore, K ⁺ depolarization significantly elevated endogenous levels of InsP₆ and occluded the inhibition of autaptic EPSCs by exogenous InsP₆. These data reveal that InsP₆ suppresses excitatory neurotransmission via inhibition of the presynaptic synaptotagmin-1 C2B domain-mediated fusion via an interaction with the synaptotagmin Ca ²⁺-binding sites rather than via interference with presynaptic Ca²⁺ levels, synaptic vesicle trafficking, or inactivation of postsynaptic ionotropic glutamate receptors. Therefore, elevated InsP ₆ in activated neurons serves as a unique negative feedback signal to control hippocampal excitatory neurotransmission.