Single Particle Tracking of α7 Nicotinic AChR in Hippocampal Neurons Reveals Regulated Confinement at Glutamatergic and GABAergic Perisynaptic Sites

alpha 7 neuronal nicotinic acetylcholine receptors (alpha 7-nAChR) form Ca(2+)-permeable homopentameric channels modulating cortical network activity and cognitive processing. They are located pre- and postsynaptically and are highly abundant in hippocampal GABAergic interneurons. It is unclear how alpha 7-nAChRs are positioned in specific membrane microdomains, particularly in cultured neurons which are devoid of cholinergic synapses. To address this issue, we monitored by single particle tracking the lateral mobility of individual alpha 7-nAChRs labeled with alpha-bungarotoxin linked to quantum dots in live rat cultured hippocampal interneurons. Quantitative analysis revealed different modes of lateral diffusion of alpha 7-nAChR dependent on their subcellular localization. Confined receptors were found in the immediate vicinity of glutamatergic and GABAergic postsynaptic densities, as well as in extrasynaptic clusters of alpha-bungarotoxin labeling on dendrites. alpha 7-nAChRs avoided entering postsynaptic densities, but exhibited reduced mobility and long dwell times at perisynaptic locations, indicative of regulated confinement. Their diffusion coefficient was lower, on average, at glutamatergic than at GABAergic perisynaptic sites, suggesting differential, synapse-specific tethering mechanisms. Disruption of the cytoskeleton affected alpha 7-nAChR mobility and cell surface expression, but not their ability to form clusters. Finally, using tetrodotoxin to silence network activity, as well as exposure to a selective alpha 7-nAChR agonist or antagonist, we observed that alpha 7-nAChRs cell surface dynamics is modulated by chronic changes in neuronal activity. Altogether, given their high Ca(2+)-permeability, our results suggest a possible role of alpha 7-nAChR on interneurons for activating Ca(2+)-dependent signaling in the vicinity of GABAergic and glutamatergic synapses.