Postsynaptic long-term potentiation of inhibition (iLTP) can rely on increased GABAA receptors (GABAARs) at synapses by promoted exocytosis. However, the molecular mechanisms that enhance the clustering of postsynaptic GABAARs during iLTP remain obscure. Here we demonstrate that during chemically induced iLTP (chem-iLTP), GABAARs are immobilized and confined at synapses, as revealed by single-particle tracking of individual GABAARs in cultured hippocampal neurons. Chem-iLTP expression requires synaptic recruitment of the scaffold protein gephyrin from extrasynaptic areas, which in turn is promoted by CaMKII-dependent phosphorylation of GABAAR-β3-Ser383. Impairment of gephyrin assembly prevents chem-iLTP and, in parallel, blocks the accumulation and immobilization of GABAARs at synapses. Importantly, an increase of gephyrin and GABAAR similar to those observed during chem-iLTP in cultures were found in the rat visual cortex following an experience-dependent plasticity protocol that potentiates inhibitory transmission in vivo. Thus, phospho-GABAAR-β3-dependent accumulation of gephyrin at synapses and receptor immobilization are crucial for iLTP expression and are likely to modulate network excitability.

Nanoparticles (NPs) are increasingly used in biomedical applications, but the factors that influence their interactions with living cells need to be elucidated. Here, we reveal the role of NP surface charge in determining their neuronal interactions and electrical responses. We discovered that negatively charged NPs administered at low concentration (10 nM) interact with the neuronal membrane and at the synaptic cleft, whereas positively and neutrally charged NPs never localize on neurons. This effect is shape and material independent. The presence of negatively charged NPs on neuronal cell membranes influences the excitability of neurons by causing an increase in the amplitude and frequency of spontaneous postsynaptic currents at the single cell level and an increase of both the spiking activity and synchronous firing at neural network level. The negatively charged NPs exclusively bind to excitable neuronal cells, and never to nonexcitable glial cells. This specific interaction was also confirmed by manipulating the electrophysiological activity of neuronal cells. Indeed, the interaction of negatively charged NPs with neurons is either promoted or hindered by pharmacological suppression or enhancement of the neuronal activity with tetrodotoxin or bicuculline, respectively. We further support our main experimental conclusions by using numerical simulations. This study demonstrates that negatively charged NPs modulate the excitability of neurons, revealing the potential use of NPs for controlling neuron activity.

To reach the open state, the GABA(A) receptor (GABA(A)R) is assumed to bind two agonist molecules. Although it is currently believed that GABA(A)R could also operate in the monoliganded state, the gating properties of singly bound GABA(A)R are poorly understood and their physiological role is still obscure. In the present study, we characterize for the first time the gating properties of singly bound GABA(A)Rs by using a mutagenesis approach and we propose that monoliganded GABA(A)R contribute in shaping synaptic responses. At saturating GABA concentrations, currents mediated by recombinant GABA(A)Rs with a single functional binding site display slow onset, fast deactivation kinetics, and slow rate of desensitization-resensitization. GABA(A)Rs with two binding sites activated by brief pulses of subsaturating GABA concentrations (in the range of the GABA concentration profile in the synaptic cleft) could also mediate fast deactivating currents, displaying deactivation kinetics similar to those mediated by GABA(A)Rs with a single functional binding site. Model simulations of receptors activated by realistic synaptic GABA waves revealed that a considerable proportion of GABA(A) receptors open in the monoliganded state during synaptic transmission, therefore contributing in shaping IPSCs.

SummaryThe lateral mobility of neurotransmitter receptors has been shown to tune synaptic signals. Here we report that GABAA receptors (GABAARs) can diffuse between adjacent dendritic GABAergic synapses in long-living desensitized states, thus laterally spreading “activation memories” between inhibitory synapses. Glutamatergic activity limits this inter-synaptic diffusion by trapping GABAARs at excitatory synapses. This novel form of activity-dependent hetero-synaptic interplay is likely to modulate dendritic synaptic signaling.

Highlights d LTP of individual dendritic spines causes iLTD at neighboring GABAergic synapses d Interaction between single-spine LTP and iLTD occurs in the spatial range of ±3 mm d This iLTD depends on the local dendritic calcium increase and calpain activation d iLTD is associated with reduced gephyrin clustering and increased GABAAR mobility