Activity-dependent regulation of GABAergic bouton turnover: 551.6

In neuronal networks, several plasticity mechanisms act in concert to allow storage of new information, while at the same time maintaining a certain operating level of activity. During all these processes, a correct balance of excitation and inhibition is crucial. Although several forms of plasticity have been observed and described for glutamatergic synapses, the extent of GABAergic plasticity remains largely unexplored. Our group recently demonstrated that GABAergic axons are capable of forming new synaptic contacts at a timescale of several hours. These contacts occur exclusively through the formation of new GABAergic boutons at pre-existing axon-dendrite crossings. Here, we examine how this form of structural GABAergic plasticity is affected by neuronal activity. GAD65-GFP hippocampal slice cultures, in which mostly reelin-expressing, dendritically targeting interneurons are labeled, were used for this study. We imaged GFP-expressing GABAergic axons using two-photon microscopy for four to five hours at 30 min intervals. GABAergic boutons consisted of two sub-populations. Although the majority of boutons was non-intermittently present throughout the imaging period (stable boutons), a significant fraction of boutons (30-40%) were present only part of the imaging period and often showed multiple appearances at the same position (non-stable boutons). Non-stable boutons had >2-fold smaller volumes and showed larger variation in their volumes (measured by the CV). Activity blockade with TTX acutely stabilized GABAergic boutons, mostly by reducing the number and turnover of non-stable boutons. Bouton stabilization was maintained, although less pronounced, after 48 hours in TTX. Enhancing activity with 4-AP acutely reduced the number of stable GABAergic boutons and increased bouton turnover. Bicuculline also increased turnover, but only after 48 hours, suggesting that - at least initially - blockade of GABAA receptors could abolish the effect. These results indicate that activity affects plasticity of GABAergic boutons, and therefore potentially synapse formation. We are currently reducing the area of activity manipulation by applying local superfusion and other techniques in order to shed light on the spatial properties of GABAergic plasticity and its possible interactions with excitatory plasticity.