Perisynaptic GABA Receptors The Overzealous Protector

Astrocyte Regulation of Neuronal Function and Survival in Stroke Pathophysiology

The interactions between astrocytes and neurons in the context of stroke play crucial roles in the disease's progression and eventual outcomes. After a stroke, astrocytes undergo significant changes in their morphology, molecular profile, and function, together termed reactive astrogliosis. Many of these changes modulate how astrocytes relate to neurons, inducing mechanisms both beneficial and detrimental to stroke recovery. For example, excessive glutamate release and astrocytic malfunction contribute to excitotoxicity in stroke, eventually causing neuronal death. Astrocytes also provide essential metabolic support and neurotrophic signals to neurons after stroke, ensuring homeostatic stability and promoting neuronal survival. Furthermore, several astrocyte-secreted molecules regulate synaptic plasticity in response to stroke, allowing for the rewiring of neural circuits to compensate for damaged areas. In this chapter, we highlight the current understanding of the interactions between astrocytes and neurons in response to stroke, explaining the varied mechanisms contributing to injury progression and the potential implications for future therapeutic interventions.

Oscillatory beta/alpha band modulations: A potential biomarker of functional language and motor recovery in chronic stroke?

Stroke remains one of the leading causes of various disabilities, including debilitating motor and language impairments. Though various treatments exist, post-stroke impairments frequently become chronic, dramatically reducing daily life quality, and requiring specific rehabilitation. A critical goal of chronic stroke rehabilitation is to induce, usually through behavioral training, experience-dependent plasticity processes in order to promote functional recovery. However, the efficiency of such interventions is typically modest, and very little is known regarding the neural dynamics underpinning recovery processes and possible biomarkers of their efficiency. Some studies have emphasized specific alterations of excitatory–inhibitory balance within distributed neural networks as an important recovery correlate. Neural processes sensitive to these alterations, such as task-dependent oscillatory activity in beta as well as alpha bands, may be candidate biomarkers of chronic stroke functional recovery. In this review, we discuss the results of studies on motor and language recovery with a focus on oscillatory processes centered around the beta band and their modulations during functional recovery in chronic stroke. The discussion is based on a framework where task-dependent modulations of beta and alpha oscillatory activity, generated by the deep cortical excitatory–inhibitory microcircuits, serve as a neural mechanism of domain-general top-down control processes. We discuss the findings, their limitations, and possible directions for future research.

Crosstalk Between GABAergic Neurotransmission and Inflammatory Cascades in the Post-ischemic Brain: Relevance for Stroke Recovery

Adaptive plasticity processes are required involving neurons as well as non-neuronal cells to recover lost brain functions after an ischemic stroke. Recent studies show that gamma-Aminobutyric acid (GABA) has profound effects on glial and immune cell functions in addition to its inhibitory actions on neuronal circuits in the post-ischemic brain. Here, we provide an overview of how GABAergic neurotransmission changes during the first weeks after stroke and how GABA affects functions of astroglial and microglial cells as well as peripheral immune cell populations accumulating in the ischemic territory and brain regions remote to the lesion. Moreover, we will summarize recent studies providing data on the immunomodulatory actions of GABA of relevance for stroke recovery. Interestingly, the activation of GABA receptors on immune cells exerts a downregulation of detrimental anti-inflammatory cascades. Conversely, we will discuss studies addressing how specific inflammatory cascades affect GABAergic neurotransmission on the level of GABA receptor composition, GABA synthesis, and release. In particular, the chemokines CXCR4 and CX3CR1 pathways have been demonstrated to modulate receptor composition and synthesis. Together, the actual view on the interactions between GABAergic neurotransmission and inflammatory cascades points towards a specific crosstalk in the post-ischemic brain. Similar to what has been shown in experimental models, specific therapeutic modulation of GABAergic neurotransmission and inflammatory pathways may synergistically promote neuronal plasticity to enhance stroke recovery.

Targeting GABAC Receptors Improves Post-Stroke Motor Recovery

Ischemic stroke remains a leading cause of disability worldwide, with limited treatment options available. This study investigates GABA_C receptors as novel pharmacological targets for stroke recovery. The expression of ρ1 and ρ2 mRNA in mice were determined in peri-infarct tissue following photothrombotic motor cortex stroke. (R)-4-amino-cyclopent-1-enyl butylphosphinic acid (R)-4-ACPBPA and (S)-4-ACPBPA were assessed using 2-elecotrode voltage electrophysiology in Xenopus laevis oocytes. Stroke mice were treated for 4 weeks with either vehicle, the α5-selective negative allosteric modulator, L655,708, or the ρ1/2 antagonists, (R)-4-ACPBPA and (S)-4-ACPBPA respectively from 3 days post-stroke. Infarct size and expression levels of GAT3 and reactive astrogliosis were determined using histochemistry and immunohistochemistry respectively, and motor function was assessed using both the grid-walking and cylinder tasks. After stroke, significant increases in ρ1 and ρ2 mRNAs were observed on day 3, with ρ2 showing a further increase on day 7. (R)- and (S)-4-ACPBPA are both potent antagonists at ρ2 and only weak inhibitors of α5β2γ2 receptors. Treatment with either L655,708, (S)-4-ACPBPA (ρ1/2 antagonist; 5 mM only), or (R)-4-ACPBPA (ρ2 antagonist; 2.5 and 5 mM) from 3 days after stroke resulted in a significant improvement in motor recovery on the grid-walking task, with L655,708 and (R)-4-ACPBPA also showing an improvement in the cylinder task. Infarct size was unaffected, and only (R)-4-ACPBPA significantly increased peri-infarct GAT3 expression and decreased the level of reactive astrogliosis. Importantly, inhibiting GABA_C receptors affords significant improvement in motor function after stroke. Targeting the ρ-subunit could provide a novel delayed treatment option for stroke recovery.

Translating concepts of neural repair after stroke: Structural and functional targets for recovery

Stroke is among the most common causes of adult disability worldwide, and its disease burden is shifting towards that of a long-term condition. Therefore, the development of approaches to enhance recovery and augment neural repair after stroke will be critical. Recovery after stroke involves complex interrelated systems of neural repair. There are changes in both structure (at the molecular, cellular, and tissue levels) and function (in terms of excitability, cortical maps, and networks) that occur spontaneously within the brain. Several approaches to augment neural repair through enhancing these changes are under study. These include identifying novel drug targets, implementing rehabilitation strategies, and developing new neurotechnologies. Each of these approaches has its own array of different proposed mechanisms. Current investigation has emphasized both cellular and circuit-based targets in both gray and white matter, including axon sprouting, dendritic branching, neurogenesis, axon preservation, remyelination, blood brain barrier integrity, blockade of extracellular inhibitory signals, alteration of excitability, and promotion of new brain cortical maps and networks. Herein, we review for clinicians recovery after stroke, basic elements of spontaneous neural repair, and ongoing work to augment neural repair. Future study requires alignment of basic, translational, and clinical research. The field continues to grow while becoming more clearly defined. As thrombolysis changed stroke care in the 1990 s and thrombectomy in the 2010 s, the augmentation of neural repair and recovery after stroke may revolutionize care for these patients in the coming decade.

Therapeutic Potential of Neurotrophins for Repair After Brain Injury: A Helping Hand From Biomaterials

Stroke remains the leading cause of long-term disability with limited options available to aid in recovery. Significant effort has been made to try and minimize neuronal damage following stroke with use of neuroprotective agents, however, these treatments have yet to show clinical efficacy. Regenerative interventions have since become of huge interest as they provide the potential to restore damaged neural tissue without being limited by a narrow therapeutic window. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), and their high affinity receptors are actively produced throughout the brain and are involved in regulating neuronal activity and normal day-to-day function. Furthermore, neurotrophins are known to play a significant role in both protection and recovery of function following neurodegenerative diseases such as stroke and traumatic brain injury (TBI). Unfortunately, exogenous administration of these neurotrophins is limited by a lack of blood-brain-barrier (BBB) permeability, poor half-life, and rapid degradation. Therefore, we have focused this review on approaches that provide a direct and sustained neurotrophic support using pharmacological therapies and mimetics, physical activity, and potential drug delivery systems, including discussion around advantages and limitations for use of each of these systems. Finally, we discuss future directions of biomaterial drug-delivery systems, including the incorporation of heparan sulfate (HS) in conjunction with neurotrophin-based interventions.

The flavonoid, 2'-methoxy-6-methylflavone, affords neuroprotection following focal cerebral ischaemia

Tonic inhibitory currents, mediated by extrasynaptic GABA_A receptors, are elevated at a delay following stroke. Flavonoids minimise the extent of cellular damage following stroke, but little is known about their mode of action. We demonstrate that the flavonoid, 2'-methoxy-6-methylflavone (0.1-10 µM; 2'MeO6MF), increases GABA_A receptor tonic currents presumably via δ-containing GABA_A receptors. Treatment with 2'MeO6MF 1-6 h post focal ischaemia dose dependently decreases infarct volume and improves functional recovery. The effect of 2'MeO6MF was attenuated in δ^-/- mice, indicating that the effects of the flavonoid were mediated via δ-containing GABA_A receptors. Further, as flavonoids have been shown to have multiple modes of action, we investigated the anti-inflammatory effects of 2'MeO6MF. Using a macrophage cell line, we show that 2'MeO6MF can dampen an LPS-induced elevation in NFkB activity. Assessment of vehicle-treated stroke animals revealed a significant increase in circulating IL1β, TNFα and IFγ levels. Treatment with 2'MeO6MF dampened the stroke-induced increase in circulating cytokines, which was blocked in the presence of the pan-AKT inhibitor, GSK690693. These studies support the hypothesis that compounds that potentiate tonic inhibition via δ-containing GABA_A receptors soon after stroke can afford neuroprotection.

Delayed inhibition of tonic inhibition enhances functional recovery following experimental ischemic stroke

The current study focuses on the ability to improve cognitive function after stroke with interventions administered at delayed/chronic time points. In light of recent studies demonstrating delayed GABA antagonists improve motor function, we utilized electrophysiology, biochemistry and neurobehavioral methods to investigate the role of α5 GABAA receptors on hippocampal plasticity and functional recovery following ischemic stroke. Male C57Bl/6 mice were exposed to 45 min transient middle cerebral artery occlusion and analysis of synaptic and functional deficits performed 7 or 30 days after recovery. Our findings indicate that hippocampal long-term potentiation (LTP) is impaired 7 days after stroke and remain impaired for at least 30 days. We demonstrate that ex vivo administration of L655,708 reversed ischemia-induced plasticity deficits and importantly, in vivo administration at delayed time-points reversed stroke-induced memory deficits. Western blot analysis of hippocampal tissue reveals proteins responsible for GABA synthesis are upregulated (GAD65/67 and MAOB), increasing GABA in hippocampal interneurons 30 days after stroke. Thus, our data indicate that both synaptic plasticity and memory impairments observed after stroke are caused by excessive tonic GABA activity, making inhibition of specific GABA activity at delayed timepoints a potential therapeutic approach to improve functional recovery and reverse cognitive impairments after stroke.

GAT3 selective substrate l-isoserine upregulates GAT3 expression and increases functional recovery after a focal ischemic stroke in mice

Ischemic stroke triggers an elevation in tonic GABA inhibition that impairs the ability of the brain to form new structural and functional cortical circuits required for recovery. This stroke-induced increase in tonic inhibition is caused by impaired GABA uptake via the glial GABA transporter GAT3, highlighting GAT3 as a novel target in stroke recovery. Using a photothrombotic stroke mouse model, we show that GAT3 protein levels are decreased in peri-infarct tissue from 6 h to 42 days post-stroke. Prior studies have shown that GAT substrates can increase GAT surface expression. Therefore, we aimed to assess whether the GAT3 substrate, L-isoserine, could increase post-stroke functional recovery. L-Isoserine (38 µM or 380 µM) administered directly into the infarct from day 5 to 32 post-stroke, significantly increased motor performance in the grid-walking and cylinder tasks in a concentration-dependent manner, without affecting infarct volumes. Additionally, L-isoserine induced a lasting increase in GAT3 expression in peri-infarct regions accompanied by a small decrease in GFAP expression. This study is the first to show that a GAT3 substrate can increase GAT3 expression and functional recovery after focal ischemic stroke following a delayed long-term treatment. We propose that enhancing GAT3-mediated uptake dampens tonic inhibition and promotes functional recovery after stroke.

Consensus statement on current and emerging methods for the diagnosis and evaluation of cerebrovascular disease

Cerebrovascular disease (CVD) remains a leading cause of death and the leading cause of adult disability in most developed countries. This work summarizes state-of-the-art, and possible future, diagnostic and evaluation approaches in multiple stages of CVD, including (i) visualization of sub-clinical disease processes, (ii) acute stroke theranostics, and (iii) characterization of post-stroke recovery mechanisms. Underlying pathophysiology as it relates to large vessel steno-occlusive disease and the impact of this macrovascular disease on tissue-level viability, hemodynamics (cerebral blood flow, cerebral blood volume, and mean transit time), and metabolism (cerebral metabolic rate of oxygen consumption and pH) are also discussed in the context of emerging neuroimaging protocols with sensitivity to these factors. The overall purpose is to highlight advancements in stroke care and diagnostics and to provide a general overview of emerging research topics that have potential for reducing morbidity in multiple areas of CVD.

Inhibition of GABA transporters fails to afford significant protection following focal cerebral ischemia

Brain ischemia triggers excitotoxicity and cell death, yet no neuroprotective drugs have made it to the clinic. While enhancing GABAergic signaling to counterbalance excitotoxicity has shown promise in animal models, clinical studies have failed. Blockade of GABA transporters (GATs) offers an indirect approach to increase GABA inhibition to lower the excitation threshold of neurons. Among the GATs, GAT1 is known to promote neuroprotection, while the protective role of the extrasynaptic transporters GAT3 and BGT1 is elusive. A focal lesion was induced in the motor cortex in two to four-month-old C57BL/6 J male mice by photothrombosis. The GAT1 inhibitor, tiagabine (1 and 10 mg/kg), the GAT2/3 inhibitor, ( S)-SNAP-5114 (5 and 30 mg/kg) and the GAT1/BGT1 inhibitor, EF-1502 (1 and 10 mg/kg) were given i.p. 1 and 6 h post-stroke to assess their impact on infarct volume and motor performance seven days post-stroke. One mg/kg tiagabine improved motor performance, while 10 mg/kg tiagabine, ( S)-SNAP-5114 and EF-1502 had no effect. None of the compounds affected infarct volume. Interestingly, treatment with tiagabine induced seizures and ( S)-SNAP-5114 led to increased mortality. Although we show that tiagabine can promote protection, our findings indicate that caution should be had when using GAT1 and GAT3 inhibitors for conditions of brain ischemia.

Aerobic exercise modulates intracortical inhibition and facilitation in a nonexercised upper limb muscle

Background

Despite growing interest in the relationship between exercise and short-term neural plasticity, the effects of exercise on motor cortical (M1) excitability are not well studied. Acute, lower-limb aerobic exercise may potentially modulate M1 excitability in working muscles, but the effects on muscles not involved in the exercise are unknown. Here we examined the excitability changes in an upper limb muscle representation following a single session of lower body aerobic exercise. Investigating the response to exercise in a non-exercised muscle may help to determine the clinical usefulness of lower-body exercise interventions for upper limb neurorehabilitation.

Methods

In this study, transcranial magnetic stimulation was used to assess input–output curves, short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI) and intracortical facilitation (ICF) in the extensor carpi radialis muscle in twelve healthy individuals following a single session of moderate stationary biking. Additionally, we examined whether the presence of a common polymorphism of the brain-derived neurotrophic factor (BDNF) gene would affect the response of these measures to exercise.

Results

We observed significant increases in ICF and decreases in SICI following exercise. No changes in LICI were detected, and no differences were observed in input–output curves following exercise, or between BDNF groups.

Conclusions

The current results demonstrate that the modulation of intracortical excitability following aerobic exercise is not limited to those muscles involved in the exercise, and that while exercise does not directly modulate the excitability of motor neurons, it may facilitate the induction of experience-dependent plasticity via a decrease in intracortical inhibition and increase in intracortical facilitation. These findings indicate that exercise may create favourable conditions for adaptive plasticity in M1 and may be an effective adjunct to traditional training or rehabilitation methods.

Early and moderate sensory stimulation exerts a protective effect on perilesion representations of somatosensory cortex after focal ischemic damage

Previous studies have shown that intensive training within an early critical time window after focal cortical ischemia increases the area of damaged tissue and is detrimental to behavioral recovery. We postulated that moderate stimulation initiated soon after the lesion could have protective effects on peri-infarct cortical somatotopic representations. Therefore, we have assessed the effects of mild cutaneous stimulation delivered in an attention-demanding behavioral context on the functional organization of the perilesion somatosensory cortex using high-density electrophysiological mapping. We compared the effects of 6-day training initiated on the 3rd day postlesion (early training; ET) to those of same-duration training started on the 8th day (delayed training; DT). Our findings confirm previous work showing that the absence of training aggravates representational loss in the perilesion zone. In addition, ET was found to be sufficient to limit expansion of the ischemic lesion and reduce tissue loss, and substantially maintain the neuronal responsiveness to tactile stimulation, thereby preserving somatotopic map arrangement in the peri-infarct cortical territories. By contrast, DT did not prevent tissue loss and only partially reinstated lost representations in a use-dependent manner within the spared peri-infarct cortical area. This study differentiates the effects of early versus delayed training on perilesion tissue and cortical map reorganization, and underscores the neuroprotective influence of mild rehabilitative stimulation on neuronal response properties in the peri-infarct cortex during an early critical period.