Activity-dependent coordinated mobility of hippocampal inhibitory synapses visualized with presynaptic and postsynaptic tagged-molecular markers

Axonal varicosities and dendritic spines at excitatory synapses are dynamic structures essential for synaptic plasticity, whereas the behavior of inhibitory synapses during development and plasticity remains largely unknown. To investigate the morphology and dynamics of inhibitory synapses, we used two distinct pre- and postsynaptic fluorescent probes: one is a yellow fluorescent protein, Venus, incorporated into vesicular GABA transporter (VGAT) gene as a specific marker of presynaptic inhibitory neurons and the other red fluorescent protein (mCherry)-tagged gephyrin, a postsynaptic scaffolding protein, as a postsynaptic marker. Using primary culture of mouse hippocampal neurons and confocal laser-scanning microscopy, we established a system by which close contacts of Venus-positive axonal varicosities with mCherry-labeled gephyrin clusters in the dendritic shafts of dissociated hippocampal pyramidal neurons could be clearly visualized. Time-lapse imaging revealed that: (1) the presynaptic varicosities actively moved with marked changes in their shapes, and the postsynaptic scaffolding protein gephyrin clusters underwent coordinated movements in a tight association with the presynaptic varicosities, (2) the extents of morphological changes and movements depended on the developmental stages, reaching a stable level as the inhibitory synaptic connections matured, and (3) the motility indexes of the varicosity and its counterpart gephyrin cluster were well correlated. Furthermore, action potential blockade with tetrodotoxin treatment reduced the varicosity size, gephyrin cluster mobility as well as the amplitude of GABAergic synaptic currents in pyramidal neurons. Such a neural activity-dependent dynamic change in GABAergic synaptic morphology is likely to play a critical role in the regulatory mechanism underlying the formation and plasticity of inhibitory synapses.