Homeostatic synaptic plasticity of miniature excitatory postsynaptic currents in mouse cortical cultures requires neuronal Rab3a.

Following prolonged activity blockade, amplitudes of miniature excitatory postsynaptic currents (mEPSCs) increase, a form of plasticity termed 'homeostatic synaptic plasticity'. We previously showed that a presynaptic protein, the small GTPase Rab3a, is required for full expression of the increase in miniature endplate current amplitudes following prolonged blockade of action potential activity at the mouse neuromuscular junction (NMJ) in vivo, where an increase in postsynaptic receptors does not contribute. It is unknown whether this form of Rab3a-dependent homeostatic plasticity at the NMJ shares any characteristics with central synapses. We show here that homeostatic synaptic plasticity of mEPSCs is impaired in mouse cortical neuron cultures prepared from Rab3a-/- and mutant mice expressing a single-point mutation of Rab3a, Rab3a Earlybird mice. To determine if Rab3a is involved in the well-established homeostatic increase in postsynaptic AMPA-type receptors (AMPARs), we performed a series of experiments in which electrophysiological recordings of mEPSCs and confocal imaging of synaptic AMPAR immunofluorescence were assessed within the same cultures. We found that the increase in postsynaptic AMPAR levels in wild-type cultures was more variable than that of mEPSC amplitudes, which might be explained by a presynaptic contribution, but we cannot rule out variability in the measurement. Finally, we demonstrate that Rab3a is acting in neurons because only selective loss of Rab3a in neurons, not glia, disrupted the homeostatic increase in mEPSC amplitudes. This is the first demonstration that a protein thought to function presynaptically is required for homeostatic synaptic plasticity of quantal size in central neurons.