Inhibitory synapse dynamics: coordinated presynaptic and postsynaptic mobility and the major contribution of recycled vesicles to new synapse formation

Dynamics of GABAergic synaptic components have been studied previously over milliseconds to minutes, revealing mobility of postsynaptic scaffolds and receptors. Here we image inhibitory synapses containing fluorescently tagged postsynaptic scaffold Gephyrin, together with presynaptic vesicular GABA transporter (VGAT) or postsynaptic GABA(A) receptor γ2 subunit (GABA(A)Rγ2), over seconds to days in cultured rat hippocampal neurons, revealing modes of inhibitory synapse formation and remodeling. Entire synapses were mobile, translocating rapidly within a confined region and exhibiting greater nonstochastic motion over multihour periods. Presynaptic and postsynaptic components moved in unison, maintaining close apposition while translocating distances of several micrometers. An observed flux in the density of synaptic puncta partially resulted from the apparent merging and splitting of preexisting clusters. De novo formation of inhibitory synapses was observed, marked by the appearance of stably apposed Gephyrin and VGAT clusters at sites previously lacking either component. Coclustering of GABA(A)Rγ2 supports the identification of such new clusters as synapses. Nascent synapse formation occurred by gradual accumulation of components over several hours, with VGAT clustering preceding that of Gephyrin and GABA(A)Rγ2. Comparing VGAT labeling by active uptake of a luminal domain antibody with post hoc immunocytochemistry indicated that recycling vesicles from preexisting boutons significantly contribute to vesicle pools at the majority of new inhibitory synapses. Although new synapses formed primarily on dendrite shafts, some also formed on dendritic protrusions, without apparent interconversion. Altogether, the long-term imaging of GABAergic presynaptic and postsynaptic components reveals complex dynamics and perpetual remodeling with implications for mechanisms of assembly and synaptic integration.