A paradoxical switch: the implications of excitatory GABAergic signaling in neurological disorders

Abstract

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. In the mature brain, inhibitory GABAergic signaling is critical in maintaining neuronal homeostasis and vital human behaviors such as cognition, emotion, and motivation. While classically known to inhibit neuronal function under physiological conditions, previous research indicates a paradoxical switch from inhibitory to excitatory GABAergic signaling that is implicated in several neurological disorders. Various mechanisms have been proposed to contribute to the excitatory switch such as chloride ion dyshomeostasis, alterations in inhibitory receptor expression, and modifications in GABAergic synaptic plasticity. Of note, the hypothesized mechanisms underlying excitatory GABAergic signaling are highlighted in a number of neurodevelopmental, substance use, stress, and neurodegenerative disorders. Herein, we present an updated review discussing the presence of excitatory GABAergic signaling in various neurological disorders, and their potential contributions towards disease pathology.

Keywords: GABA; GABAARs; depolarizing; diseases; excitatory; neurological.

Figure 1

The excitatory/inhibitory transition of GABA during neurodevelopment and maturation. Immature neurons express elevated levels of Na-K-2Cl cotransporter 1 (NCKK1 – green) which allows for a greater intracellular chloride concentration while the neuron is at rest. When GABA binds to GABA_A receptors (GABA_ARs – purple), chloride flows down its respective concentration gradient, resulting in an efflux of chloride ions. The elevation in membrane potential furthermore causes the neuron to depolarize and elicit an action potential. Throughout neuronal maturation, NKCC1 expression is diminished while K-Cl cotransporter 2 (KCC2 – red) is elevated. Heighted KCC2 expression in mature neurons allow for a lower intracellular chloride concentration while the neuron is at rest. When GABA binds to GABA_ARs, the result is an influx of chloride ions. The decrease in membrane potential moreover causes the neuron to hyperpolarize and prevent action potential propagation. Figure created with BioRender.com.

Figure 2

Theoretical effects of GABA transsynaptic signaling on excitatory GABAergic responses. GABA itself is critical in transitioning GABA_A receptor (GABA_AR) responses from depolarizing to hyperpolarizing throughout neuronal maturation. In immature neurons, GABA binding to excitatory GABA_ARs promotes the expression of K-Cl cotransporter 2 (KCC2), which decreases intracellular chloride concentration, allowing future GABA_AR-mediated responses to become inhibitory in mature neurons. Stress exposure diminishes GABA transsynaptic signaling by reducing glutamic acid decarboxylase 67 (GAD67) and vesicular GABA transporter (vGAT) expression, while elevating GABA transporter type 1 (GAT-1) expression. This results in lowered extracellular GABA to freely bind to its respective receptors. Stress also reduces the expression of GABA_ARs via downregulation of several GABA_AR subunits. The lack of available GABA to bind to GABA_ARs following stress hypothetically reduces KCC2 expression, which disturbs chloride homeostasis and promotes an excitatory switch in GABAergic signaling. Figure adapted from “Mechanism of action of Selective Serotonin Reuptake Inhibitors,” by BioRender.com (2023). Retrieved from https://app.biorender.com/biorender-templates.

Figure 3

Bumetanide alleviates pathological, electrophysiological, and behavioral hallmarks in an AD-related pathology mouse model. Treatment with NKCC1 inhibitor, bumetanide, in an aged APOE4-KI mouse corrected the following pathological phenotypes in a model of AD-related pathology: (1) normalized neuronal excitability in CA1 pyramidal neurons. (2) reduced amyloid-beta plaque size and number in the hippocampus and cortex. (3) downregulated the expression of AD signature genes that promote disease pathology. (4) rescued deficits in long-term potentiation (LTP) within the CA1 of the hippocampus. (5) Alleviated deficits in spatial memory through a Morris water maze. Figure created with BioRender.com.

Figure 4

Aberrant mTORC1 activity represses KCC2 expression in a model of Tuberous Sclerosis Complex (TSC). Under normal conditions (left), mammalian target of rapamycin complex 1 (mTORC1) equally activates the translation of 155 target mRNAs (mTOR ON proteins) and represses the translations of 166 target mRNAs (mTOR OFF proteins). Tuberous sclerosis proteins 1 and 2 (TSC1/2) acts upstream of mTORC1 and represses its activity. One particular mTOR OFF protein is KCC2, which is synthesized when mTORC1 is inactive. KCC2 is responsible for maintaining a low intracellular chloride concentration to maintain inhibitory GABA_AR-mediated responses. In TSC1 KO neurons, a model of Tuberous Sclerosis Complex (TSC) (right), TSC1/2 are absent which results in aberrant mTORC1 activity. The population of mTOR ON proteins are upregulated while the population of mTOR-OFF proteins are downregulated. Furthermore, KCC2 downregulation from elevated mTORC1 activity results in an accumulation of intracellular chloride, resulting in depolarizing GABA_AR-mediated responses. Figure created with Biorender.com.

Figure 5

Ethanol elicits anti-depressant behavioral effects in mice. (A–F) Depressive-like (self-care and behavioral despair) and anxiety-like behaviors were measured 24 h following i.p. injections with vehicle (Veh; saline) or ethanol (ETOH; 2.5 g kg⁻¹) in mice. (A) Ethanol elicited anti-depressant behavioral effects by decreasing immobility during a forced swim test (FST) and increasing self-grooming behaviors during a splash test (B,C). (D–F) An open field test was used to measure anxiety-like behaviors following ethanol treatment. (D) Ethanol induced anxiolytic behaviors by increasing the time spent in the center of the open field. Additionally, ethanol did not affect mobility in mice (E,F). Figure reused from (166).

Figure 6

Molecular mechanisms underlying the rapid antidepressant properties of ethanol. Acute ethanol exposure antagonizes NMDAR signaling. GABA_BRs are then uncoupled from GIRK channels and endocytosed via stabilization of adaptor protein, 14-3-3-η. GABA_B2R mRNA, a target of RNA-binding protein FMRP, is released from translational repression, and new GABA_B2R protein is synthesized. A new population of surface GABA_BRs activate L-type voltage-gated calcium channels (L-type VGCC), depolarizing the membrane via calcium influx and activating mTORC1 signaling. Furthermore, mTORC1-dependent transcripts are released from FMRP suppression, upregulating transsynaptic proteins required for anti-depressant behavioral effects. Figure created with BioRender.com.

Figure 7

Combinational treatment between Ro-25-6981 and CGP35348 increases hippocampal synapse number in an Fmr1 KO mouse. (A) Representative images of CA1 hippocampal sections from Fmr1 KO mice. MAP 2, a marker of dendrites, is denoted in the first row, while synapses are denoted by the yellow arrows in the second row (PSD95/SYN1 PLA puncta). Synapses are detected in slice using a novel method, DetectSyn, which uses proximity ligation assay (PLA) technology and immunofluorescence to note the presence of synaptic engagement. Bottom row includes a merged image with MAP2 in green, PLA puncta (synapses) in white, and DAPI in blue. (B) Quantification of synapse number following treatments in Fmr1 KO hippocampal slice. Compared to control (red), treatment with Ro-25-6981 (light red), NR2B antagonist, reduced the number of synapses in Fmr1 KO slice. Treatment with Ro-25-6981 and CGP35348, GABA_BR antagonist, increases synapse number as compared to control (checkered red). Figure reused from (183).