Inhibition of a tonic inhibitory conductance in mouse hippocampal neurones by negative allosteric modulators of α5 subunit-containing γ-aminobutyric acid type A receptors: implications for treating cognitive deficits

Discovering the Intriguing Properties of Extrasynaptic γ-Aminobutyric Acid Type A Receptors

Tonic inhibition in mouse hippocampal CA1 pyramidal neurons is mediated by α5 subunit-containing γ-aminobutyric acid type A receptors. By Caraiscos VB, Elliott EM, You-Ten KE, Cheng VY, Belelli D, Newell JG, Jackson MF, Lambert JJ, Rosahl TW, Wafford KA, MacDonald JF, Orser BA. Proc Natl Acad Sci U S A 2004; 101:3662-7. Reprinted with permission. In this Classic Paper Revisited, the author recounts the scientific journey leading to a report published in the Proceedings of the National Academy of Sciences (PNAS) and shares several personal stories from her formative years and "research truths" that she has learned along the way. Briefly, the principal inhibitory neurotransmitter in the brain, γ-aminobutyric acid (GABA), was conventionally thought to regulate cognitive processes by activating synaptic GABA type A (GABAA) receptors and generating transient inhibitory synaptic currents. However, the author's laboratory team discovered a novel nonsynaptic form of tonic inhibition in hippocampal pyramidal neurons, mediated by extrasynaptic GABAA receptors that are pharmacologically distinct from synaptic GABAA receptors. This tonic current is highly sensitive to most general anesthetics, including sevoflurane and propofol, and likely contributes to the memory-blocking properties of these drugs. Before the publication in PNAS, the subunit composition of GABAA receptors that generate the tonic current was unknown. The team's research showed that GABAA receptors containing the α5 subunit (α5GABAARs) generated the tonic inhibitory current in hippocampal neurons. α5GABAARs are highly sensitive to GABA, desensitize slowly, and are thus well suited for detecting low, persistent, ambient concentrations of GABA in the extracellular space. Interest in α5GABAARs has surged since the PNAS report, driven by their pivotal roles in cognitive processes and their potential as therapeutic targets for treating various neurologic disorders.

Peripheral and central neurobiological effects of botulinum toxin A (BoNT/A) in neuropathic pain: a systematic review

ABSTRACT

Botulinum toxin (BoNT), a presynaptic inhibitor of acetylcholine (Ach) release at the neuromuscular junction (NMJ), is a successful and safe drug for the treatment of several neurological disorders. However, a wide and recent literature review has demonstrated that BoNT exerts its effects not only at the "periphery" but also within the central nervous system (CNS). Studies from animal models, in fact, have shown a retrograde transport to the CNS, thus modulating synaptic function. The increasing number of articles reporting efficacy of BoNT on chronic neuropathic pain (CNP), a complex disease of the CNS, demonstrates that the central mechanisms of BoNT are far from being completely elucidated. In this new light, BoNT might interfere with the activity of spinal, brain stem, and cortical circuitry, modulating excitability and the functional organization of CNS in healthy conditions. Botulinum toxins efficacy on CNP is the result of a wide and complex action on many and diverse mechanisms at the basis of the maladaptive plasticity, the core of the pathogenesis of CNP. This systematic review aims to discuss in detail the BoNT's mechanisms and effects on peripheral and central neuroplasticity, at the basis for the clinical efficacy in CNP syndromes.

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.

Negative Allosteric Modulation of Gamma-Aminobutyric Acid A Receptors at α5 Subunit-Containing Benzodiazepine Sites Reverses Stress-Induced Anhedonia and Weakened Synaptic Function in Mice

BACKGROUND

Abnormal reward processing, typically anhedonia, is a hallmark of human depression and is accompanied by altered functional connectivity in reward circuits. Negative allosteric modulators of GABA_A (gamma-aminobutyric acid A) receptors (GABA-NAMs) have rapid antidepressant-like properties in rodents and exert few adverse effects, but molecular targets underlying their behavioral and synaptic effects remain undetermined. We hypothesized that GABA-NAMs act at the benzodiazepine site of GABA_A receptors containing α5 subunits to increase gamma oscillatory activity, strengthen synapses in reward circuits, and reverse anhedonia.

METHODS

Anhedonia was induced by chronic stress in male mice and assayed by preferences for sucrose and female urine (n = 5-7 mice/group). Hippocampal slices were then prepared for electrophysiological recording (n = 1-6 slices/mouse, 4-6 mice/group). Electroencephalography power was quantified in response to GABA-NAM and ketamine administration (n = 7-9 mice/group).

RESULTS

Chronic stress reduced sucrose and female urine preferences and hippocampal temporoammonic-CA1 synaptic strength. A peripheral injection of the GABA-NAM MRK-016 restored hedonic behavior and AMPA-to-NMDA ratios in wild-type mice. These actions were prevented by pretreatment with the benzodiazepine site antagonist flumazenil. MRK-016 administration increased gamma power over the prefrontal cortex in wild-type mice but not α5 knockout mice, whereas ketamine promoted gamma power in both genotypes. Hedonic behavior and AMPA-to-NMDA ratios were only restored by MRK-016 in stressed wild-type mice but not α5 knockout mice.

CONCLUSIONS

α5-Selective GABA-NAMs exert rapid anti-anhedonic actions and restore the strength of synapses in reward regions by acting at the benzodiazepine site of α5-containing GABA_A receptors. These results encourage human studies using GABA-NAMs to treat depression by providing readily translatable measures of target engagement.

Hippocampal Inhibitory Synapsis Deficits Induced by α5-Containing GABAA Receptors Mediate Chronic Neuropathic Pain-Related Cognitive Impairment

Chronic neuropathic pain often leads to cognitive impairment, but the exact mechanism remains unclear. Gamma-aminobutyric acid A receptors (GABA_ARs) are the major inhibitory receptors in the brain, of which the α5-containing GABA_ARs (GABA_ARs-α5) are implicated in a range of neuropsychiatric disorders with cognitive deficits. However, whether GABA_ARs-α5 are involved in chronic neuropathic pain-related cognitive impairment remains unknown. In this study, the rats with chronic neuropathic pain induced by right sciatic nerve ligation injury (SNI) exhibited cognitive impairment with declined spontaneous alternation in Y-maze test and discrimination index in novel object recognition test. The GABA_ARs-α5 expressing on parvalbumin and somatostatin interneurons increased remarkably in hippocampus, resulting in decreased mean frequency of spontaneous inhibitory postsynaptic currents in hippocampal pyramidal neurons. Significantly, antagonizing the GABA_ARs-α5 by L655708 rescued weakened inhibitory synaptic transmission and cognitive impairment induced by chronic neuropathic pain. Taken together, these data suggest that the GABA_ARs-α5 play a crucial role in chronic neuropathic pain-induced cognitive impairment by weakening inhibitory synaptic transmission, which may provide insights into the pharmacologic treatment of chronic neuropathic pain-related cognitive impairment.

Effect of Anesthesia on Oligodendrocyte Development in the Brain

Oligodendrocytes (OLs) participate in the formation of myelin, promoting the propagation of action potentials, and disruption of their proliferation and differentiation leads to central nervous system (CNS) damage. As surgical techniques have advanced, there is an increasing number of children who undergo multiple procedures early in life, and recent experiments have demonstrated effects on brain development after a single or multiple anesthetics. An increasing number of clinical studies showing the effects of anesthetic drugs on the development of the nervous system may mainly reside in the connections between neurons, where myelin development will receive more research attention. In this article, we review the relationship between anesthesia exposure and the brain and OLs, provide new insights into the development of the relationship between anesthesia exposure and OLs, and provide a theoretical basis for clinical prevention of neurodevelopmental risks of general anesthesia drugs.

Sleep Deprivation Exacerbates Seizures and Diminishes GABAergic Tonic Inhibition

Patients with epilepsy report that sleep deprivation is a common trigger for breakthrough seizures. The basic mechanism of this phenomenon is unknown. In the Kv1.1-/- mouse model of epilepsy, daily sleep deprivation indeed exacerbated seizures though these effects were lost after the third day. Sleep deprivation also accelerated mortality in ~ 52% of Kv1.1-/- mice, not observed in controls. Voltage-clamp experiments on the day after recovery from sleep deprivation showed reductions in GABAergic tonic inhibition in dentate granule cells in epileptic Kv1.1-/- mice. Our results suggest that sleep deprivation is detrimental to seizures and survival, possibly due to reductions in GABAergic tonic inhibition. ANN NEUROL 2021;90:840-844.

Anesthesiology: Resetting Our Sights on Long-term Outcomes: The 2020 John W. Severinghaus Lecture on Translational Science

Anesthesiologists have worked relentlessly to improve intraoperative anesthesia care. They are now well positioned to expand their horizons and address many of the longer-term adverse consequences of anesthesia and surgery. Perioperative neurocognitive disorders, chronic postoperative pain, and opioid misuse are not inevitable adverse outcomes; rather, they are preventable and treatable conditions that deserve attention. The author's research team has investigated why patients experience new cognitive deficits after anesthesia and surgery. Their animal studies have shown that anesthetic drugs trigger overactivity of "memory-blocking receptors" that persists after the drugs are eliminated, and they have discovered new strategies to preserve brain function by repurposing available drugs and developing novel therapeutics that inhibit these receptors. Clinical trials are in progress to examine the cognitive outcomes of such strategies. This work is just one example of how anesthesiologists are advancing science with the goal of improving the lives of patients.

Sustained treatment with an α5 GABA A receptor negative allosteric modulator delays excitatory circuit development while maintaining GABAergic neurotransmission

α5 subunit GABA type A receptor (GABA_AR) preferring negative allosteric modulators (NAMs) are cognitive enhancers with antidepressant-like effects. α5-NAM success in treating mouse models of neurodevelopmental disorders with excessive inhibition have led to Phase 2 clinical trials for Down syndrome. Despite in vivo efficacy, no study has examined the effects of continued α5-NAM treatment on inhibitory and excitatory synapse plasticity to identify mechanisms of action. Here we used L-655,708, an imidazobenzodiazepine that acts as a highly selective but weak α5-NAM, to investigate the impact of sustained treatment on hippocampal neuron synapse and dendrite development. We show that 2-day pharmacological reduction of α5-GABA_AR signaling from DIV12-14, when GABA_ARs contribute to depolarization, delays dendritic spine maturation and the NMDA receptor (NMDAR) GluN2B/GluN2A developmental shift. In contrast, α5-NAM treatment from DIV19-21, when hyperpolarizing GABA_AR signaling predominates, enhances surface synaptic GluN2A while decreasing GluN2B. Despite changes in NMDAR subtype surface levels and localization, total levels of key excitatory synapse proteins were largely unchanged, and mEPSCs were unaltered. Importantly, 2-day α5-NAM treatment does not alter the total surface levels or distribution of α5-GABA_ARs, reduce the gephyrin inhibitory synaptic scaffold, or impair phasic or tonic inhibition. Furthermore, α5-NAM inhibition of the GABA_AR tonic current in mature neurons is maintained after 2-day α5-NAM treatment, suggesting reduced tolerance liability, in contrast to other clinically relevant GABA_AR-targeting drugs such as benzodiazepines. Together, these results show that α5-GABA_ARs contribute to dendritic spine maturation and excitatory synapse development via a NMDAR dependent mechanism without perturbing overall neuronal excitability.