TMS study: reduced GABA-ergic and glutamatergic excitability of the motor cortex in patients with neuro long COVID

This Italian study was conducted to investigate the motor cortex excitability by ‘‘paired-pulse” transcranial magnetic stimulation (ppTMS) and short-latency afferent inhibition (SAI) in a group of long COVID patients with cognitive impairments. Results showed that patients with long COVID had reduced γ-aminobutyric acid (GABA)-ergic and glutamatergic excitability of the motor cortex.

The TMS is a non-invasive research tool to investigate the changes in cortical excitability, connectivity and plasticity in humans. Short-interval intracortical inhibition (SICI) reflects fast inhibitory post-synaptic potentials in corticospinal neurons mediated through γ-aminobutyric acid type A (GABAA) receptors. Long interval intracortical inhibition (LICI) depends on slow inhibitory post-synaptic potentials mediated through GABAB receptors, whereas intracortical facilitation (ICF) reflects glutamatergic signaling. Numerous studies used parameters of ppTMS as early biomarkers for neurodegenerative diseases.

The neurobiochemical circuits studied by the pp TMS protocols (LICI, SICI, ICF) are regulated by GABA receptors and glutamate. Another TMS parameter, short-latency afferent inhibition (SAI) is regulated by the excitatory effect of cholinergic thalamocortical projections on the inhibitory GABA-ergic cortical network. It is thought to be a sensitive biomarker of neurodegenerative diseases and dementia.

Long COVID, or postacute sequelae of COVID-19 (PASC), refers to symptoms that last more than four to twelve weeks after recovery from acute COVID-19 infection. Shortness of breath, fatigue, autonomic dysfunction, neuromuscular disorders, and headaches have been described. The cognitive deficits are mostly related to executive functions, such as working memory, attention, parallel processing, planning, and problem-solving. This group of symptoms has been called ‘‘neuro long COVID”.

γ –aminobutyric acid (GABA), a major inhibitory neurotransmitter in the central nervous system, activates GABAA and GABAC ligand-gated Cl−channels, as well as G protein-coupled GABAB receptors. Activation of GABAB receptors results in generation of slow inhibitory postsynaptic potentials.

SARS-CoV-2 binding to angiotensin-converting enzyme-2 (ACE-2) receptors usurps the neuronal and non-neuronal GABA-ergic systems. GABA depletion caused by SARS-CoV-2 may explain the neuropsychiatric symptoms in COVID-19 and neuro long COVID, such as anxiety, depression, posttraumatic stress disorder, cognitive impairment and seizures.

SARS-CoV-2 depletes host GABA through several mechanisms:

1. ACE-2 is protective for the GABA-ergic signaling in both neuronal and non-neuronal pathways. Binding of SARS-CoV-2 to ACE-2 disrupts the function of the protective renin–angiotensin system branch, including Mas receptor signaling, and lowers GABA.

2. The viral spike protein contains a GABA-mimicking sequence or a short linear motif that can directly usurp host GABA-ergic signaling,

3. The SARS-CoV-2 proteins nonstructural protein 6, open reading frame 8, and open reading frame 3 interact directly with the host mammalian target of rapamycin complex 1, interleukin 17, and transmembrane protein 16F, inducing premature endothelial cell senescence, a phenotype characterized by low GABA,

4. The viral protein ORF3a interacts with toll-like receptor 4, which triggers endothelial cell senescence and lowers GABA,

5. The SARS-CoV-2 viral proteins nonstructural protein 4, nonstructural protein 8, and open reading frame 9c decrease GABA by disrupting the mitochondria, which triggers vascular senescence. Afera A. et al. Neuronal and Non-Neuronal GABA in COVID-19: Relevance for Psychiatry, Reports 2022, 5(2), 22.

In addition, a recent in vivo study showed that enhancement of systemic inflammation by intraperitoneal administration of lipopolysaccharide to mice inoculated with SARS-CoV-2 S1 protein resulted in reduced expression of calbindin, a marker of GABAergic neurons, in the olfactory bulb. 

About the study

The study included 18 right-handed long COVID patients with cognitive impairments (50 ± 11 years, 12/18 females), who were not hospitalized during the acute phase of SARS-CoV-2 infection. For comparison, sixteen healthy controls were recruited.

The patients were examined for the presence of persistent symptoms lasting for more than 12 weeks following the acute SARS-CoV-2 infection, such as fatigue, respiratory symptoms, palpitations, gastrointestinal tract symptoms, myalgia or joint pain, tinnitus, vertigo, visual disturbances and fever. The patients were especially investigated for persistent neurological symptoms, such as paraesthesias, anosmia/ageusia, cognitive deficits, mood disturbances, headaches, hemiparesis and insomnia.

All patients had cognitive and neuropsihological assessments. The Montreal Cognitive Assessment (MoCA) test assessed cognitive decline. A series of psychometrically validated tests that investigate attention and executive functions were used to measure neuropsychological functions. The Fatigue Severity Scale (FSS) was used to evaluate fatigue in daily activities.

The pp TMS protocols were used to investigate inhibitory and excitatory intracortical networks. The resting motor threshold (RMT), the amplitude of the motor evoked potential (MEP), short intracortical inhibition (SICI), intracortical facilitation (ICF), long-interval intracortical inhibition (LICI) and short-afferent inhibition (SAI) were investigated over the motor cortex.

The median MoCA corrected score of the patients was significantly lower than the median score of the healthy controls. Most of the patients performed suboptimally in the neuropsychological assessment of the executive functions. They also displayed high levels of fatigue during the FSS.

Nevertheless, there was no significant difference between the two groups in the resting motor threshold or mean evoked motor potentials. Also, SICI which reflects fast inhibitory post-synaptic potentials mediated through GABAA receptors, and SAI which reflects cholinergic circuits were not significantly different between the two groups.

However, patients with neuro long COVID showed a significant reduction of LICI, which is associated with GABAB receptor-mediated inhibition and a significant reduction of ICF, which is related to glutamatergic regulation, compared to control group.

The researchers noted that these pp-TMS findings suggest that GABAB receptor-mediated inhibition might be changed in long COVID patients. Previous studies have shown that deficit in executive functions, such as working memory and goal-directed behavior, has been linked to impairment of GABAB receptor signaling in the prefrontal cortex.

Also, this study found that long COVID patients have an abnormal pattern of ICF, which is associated with glutamatergic excitatory signaling. The impaired intracortical facilitation was found in dementia patients.

The main conclusion from this study are that LICI and ICF can be significantly reduced in individuals with “neuro-long COVID” compared to healthy controls. Researchers suggested that abnormal GABA-ergic and glutammatergic regulation and normal cholinergic regulation of the motor cortex excitability may reflect a more widespread alteration in the regulatory crosstalk that includes frontal and pre-frontal cortical hubs. They also said that TMS may be a non-invasive electrophysiological tool for assessing cortical excitability in long COVID patients with cognitive impairments. Future studies are needed to address unknown aspects of motor cortex dysfunction in patients with neuro long COVID.

This article was published in Clinical Neurophysiology.

Journal Reference

Manganotti P et al. Deficient GABABergic and glutamatergic excitability in the motor cortex of patients with long-COVID and cognitive impairment. Clinical Neurophysiology 151 (2023) 83–91.

It should be noted here that task-activated BOLD-fMRI study in the post-COVID participants with neuropsychiatric symptoms showed suboptimal functioning in the normal network, but increased brain activation in the contralateral hemisphere during working memory tasks. Several brain regions that showed lower activation in post-COVID participants than in controls included regions in the left hemisphere. At the same time, the post-COVID-19 participants performed worse than controls on the NIHTB battery for motor function, in endurance, locomotion and the dexterity of the dominant hand. In contrast, all brain regions that were more activated in 2-back task in the post-COVID group than in the control group were located in the right hemisphere.