Induction of Inhibitory Synaptic Plasticity Enhances Tonic Current by Increasing the Content of α5-Subunit Containing GABAA Receptors in Hippocampal Pyramidal Neurons

Molecular mechanisms of the GABA type A receptor function

The GABA type A receptor (GABAAR) belongs to the family of pentameric ligand-gated ion channels and plays a key role in inhibition in adult mammalian brains. Dysfunction of this macromolecule may lead to epilepsy, anxiety disorders, autism, depression, and schizophrenia. GABAAR is also a target for multiple physiologically and clinically relevant modulators, such as benzodiazepines (BDZs), general anesthetics, and neurosteroids. The first GABAAR structure appeared in 2014, but the past years have brought a particularly abundant surge in structural data for these receptors with various ligands and modulators. Although the open conformation remains elusive, this novel information has pushed the structure-function studies to an unprecedented level. Electrophysiology, mutagenesis, photolabeling, and in silico simulations, guided by novel structural information, shed new light on the molecular mechanisms of receptor functioning. The main goal of this review is to present the current knowledge of GABAAR functional and structural properties. The review begins with an outline of the functional and structural studies of GABAAR, accompanied by some methodological considerations, especially biophysical methods, enabling the reader to follow how major breakthroughs in characterizing GABAAR features have been achieved. The main section provides a comprehensive analysis of the functional significance of specific structural elements in GABAARs. We additionally summarize the current knowledge on the binding sites for major GABAAR modulators, referring to the molecular underpinnings of their action. The final chapter of the review moves beyond examining GABAAR as an isolated macromolecule and describes the interactions of the receptor with other proteins in a broader context of inhibitory plasticity. In the final section, we propose a general conclusion that agonist binding to the orthosteric binding sites appears to rely on local interactions, whereas conformational transitions of bound macromolecule (gating) and allosteric modulation seem to reflect more global phenomena involving vast portions of the macromolecule.

Reactive astrocytes mediate postoperative surgery-induced anxiety through modulation of GABAergic signalling in the zona incerta of mice

BACKGROUND

Surgery can induce severe neuroinflammation and negative emotional symptoms, such as anxiety-like behaviour. We studied whether reactive astrocytes in the zona incerta (ZI) mediate surgery-induced anxiety in mice.

METHODS

Laparotomy under isoflurane 1.5 vol% was used as a model in adult mice. The role of the ZI in surgery-induced anxiety was evaluated by behavioural tests, optical fibre recordings of neuronal activity, in vivo electrophysiological recordings, chemogenetics, and optogenetics.

RESULTS

Operative mice showed increased anxiety-like behaviour. Immunostaining and optical calcium recording revealed that astrocytes were abnormally activated in the ZI. Pharmacologic (F3, 15=5.837, P=0.044) or genetic manipulation (open field test: t7.41=3.66, P=0.007; elevated plus maze [EPM]: t10=2.70, P=0.022) of astrocyte activation in the ZI relieved anxiety-like behaviour in surgery-treated mice. Compared with the sham group, the surgery group showed increased extrasynaptic GABA concentrations and decreased GABA transporter-3 (GAT-3) expression, and inactivation of GABAergic neurones in the ZI. Upregulating GAT-3 in ZI astrocytes (OFT: t10.83=2.91, P=0.014; EPM: t9.15=3.55, P=0.006) or activating the GABAergic projection from ZI to the median raphe nucleus (ZIGABA→median raphe nucleus) (EPM: entries: F1, 24=3.45, P=0.027; time: F1, 25=4.07, P=0.043) ameliorated surgery-induced anxiety.

CONCLUSIONS

Reactive astrocytes in the zona incerta mediate surgery-induced anxiety, possibly by regulating GAT-3-mediated GABA homeostasis and inactivating ZIGABA→median raphe nucleus projections in mice.

Long-term α5 GABA A receptor negative allosteric modulator treatment reduces NMDAR-mediated neuronal excitation and maintains basal neuronal inhibition

α5 subunit-containing GABA type-A receptors (α5 GABAARs) are enriched in the hippocampus and play critical roles in neurodevelopment, synaptic plasticity, and cognition. α5 GABAAR preferring negative allosteric modulators (α5 NAMs) show promise mitigating cognitive impairment in preclinical studies of conditions characterized by excess GABAergic inhibition, including Down syndrome and memory deficits post-anesthesia. However, previous studies have primarily focused on acute application or single-dose α5 NAM treatment. Here, we measured the effects of chronic (7-day) in vitro treatment with L-655,708 (L6), a highly selective α5 NAM, on glutamatergic and GABAergic synapses in rat hippocampal neurons. We previously showed that 2-day in vitro treatment with L6 enhanced synaptic levels of the glutamate NMDA receptor (NMDAR) GluN2A subunit without modifying surface α5 GABAAR expression, inhibitory synapse function, or L6 sensitivity. We hypothesized that chronic L6 treatment would further increase synaptic GluN2A subunit levels while maintaining GABAergic inhibition and L6 efficacy, thus increasing neuronal excitation and glutamate-evoked intracellular calcium responses. Immunofluorescence experiments revealed that 7-day L6 treatment slightly increased the synaptic levels of gephyrin and surface α5 GABAARs. Functional studies showed that chronic α5 NAM treatment did not alter inhibition or α5 NAM sensitivity. Surprisingly, chronic L6 exposure decreased surface levels of GluN2A and GluN2B subunits, concurrent with reduced NMDAR-mediated neuronal excitation as seen by faster synaptic decay rates and reduced glutamate-evoked calcium responses. Together, these results show that chronic in vitro treatment with an α5 NAM leads to subtle homeostatic changes in inhibitory and excitatory synapses that suggest an overall dampening of excitability.

Bidirectional plasticity of GABAergic tonic inhibition in hippocampal somatostatin- and parvalbumin-containing interneurons

GABA_A receptors present in extrasynaptic areas mediate tonic inhibition in hippocampal neurons regulating the performance of neural networks. In this study, we investigated the effect of NMDA-induced plasticity on tonic inhibition in somatostatin- and parvalbumin-containing interneurons. Using pharmacological methods and transgenic mice (SST-Cre/PV-Cre x Ai14), we induced the plasticity of GABAergic transmission in somatostatin- and parvalbumin-containing interneurons by a brief (3 min) application of NMDA. In the whole-cell patch-clamp configuration, we measured tonic currents enhanced by specific agonists (etomidate or gaboxadol). Furthermore, in both the control and NMDA-treated groups, we examined to what extent these changes depend on the regulation of distinct subtypes of GABA_A receptors. Tonic conductance in the somatostatin-containing (SST+) interneurons is enhanced after NMDA application, and the observed effect is associated with an increased content of α5-containing GABA_ARs. Both fast-spiking and non-fast-spiking parvalbumin-positive (PV+) cells showed a reduction of tonic inhibition after plasticity induction. This effect was accompanied in both PV+ interneuron types by a strongly reduced proportion of δ-subunit-containing GABA_ARs and a relatively small increase in currents mediated by α5-containing GABA_ARs. Both somatostatin- and parvalbumin-containing interneurons show cell type-dependent and opposite sign plasticity of tonic inhibition. The underlying mechanisms depend on the cell-specific balance of plastic changes in the contents of α5 and δ subunit-containing GABA_ARs.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

Distinct regulation of tonic GABAergic inhibition by NMDA receptor subtypes

Tonic inhibition mediated by extrasynaptic GABAARs regulates various brain functions. However, the mechanisms that regulate tonic inhibition remain largely unclear. Here, we report distinct actions of GluN2A- and GluN2B-NMDA receptors (NMDARs) on tonic inhibition in hippocampal neurons under basal and high activity conditions. Specifically, overexpression of GluN2B, but not GluN2A, reduces α5-GABAAR surface expression and tonic currents. Additionally, knockout of GluN2A and GluN2B decreases and increases tonic currents, respectively. Mechanistically, GluN2A-NMDARs inhibit and GluN2B-NMDARs promote α5-GABAAR internalization, resulting in increased and decreased surface α5-GABAAR expression, respectively. Furthermore, GluN2A-NMDARs, but not GluN2B-NMDARs, are required for homeostatic potentiation of tonic inhibition induced by prolonged increase of neuronal activity. Last, tonic inhibition decreases during acute seizures, whereas it increases 24 h later, involving GluN2-NMDAR-dependent signaling. Collectively, these data reveal an NMDAR subunit-specific regulation of tonic inhibition in physiological and pathological conditions and provide mechanistic insight into activity-dependent modulation of tonic inhibition.