Exploring the links between gut microbiota and excitatory and inhibitory brain processes in alcohol use disorder: A TMS study

While the impact of the gut microbiota on brain and behavior is increasingly recognized, human studies examining this question are still scarce. The primary objective of the current study was to explore the potential relationships between the gut microbiota composition, motor cortical excitability at rest and during inhibitory control, as well as behavioral inhibition, in healthy volunteers and in patients suffering from alcohol use disorder. Motor cortical excitability was examined using a range of transcranial magnetic stimulation (TMS) measures probed at rest, including the recruitment curve, short and long intracortical inhibition, and intracortical facilitation within the primary motor cortex. Moreover, TMS was applied during a choice reaction time task to assess changes in motor excitability associated with inhibitory control. Finally, behavioral inhibition was investigated using a neuropsychological task (anti-saccade). Overall, our results highlight several interesting correlations between microbial composition and brain measures. Hence, higher bacterial diversity, as well as higher relative abundances of UGC-002 and Christensenellaceae R-7 group were correlated with stronger changes in motor excitability associated with inhibitory control. Also, higher abundance of Anaerostipes was associated with higher level of corticospinal excitability. Finally, relative abundances of Bifidobacterium and Faecalibacterium were positively related to performance in the neuropsychological task, suggesting that they might have a positive impact on behavioral inhibition. Although correlation is not causation, the present study suggests that excitatory and inhibitory brain processes might be related to gut microbiota composition. This article is part of the Special Issue on 'Microbiome & the Brain: Mechanisms & Maladies'.

Effects of Three Months of Aerobic Endurance Training on Motor Cortical Excitability in Schizophrenia Patients and Healthy Subjects

BACKGROUND

Aerobic endurance training has been discussed to induce brain plasticity and improve cognitive functions in healthy subjects and patients with neuropsychiatric disorders. For schizophrenia, a motor cortical inhibitory deficit has been established as one aspect of impaired plasticity, especially involving impairments in GABAergic interneuron networks, but the possibility to restore these deficits via exercise-induced plasticity has not been evaluated yet.

METHODS

17 schizophrenia patients and 16 matched healthy controls underwent 3 months of aerobic endurance training (30 min, 3 times a week) on bicycle ergometers. After 6 weeks, computer-assisted cognitive remediation training (30 min, 2 times a week) was added. Transcranial magnetic stimulation of the left and right hemispheres was performed at baseline and at the end of the intervention. We evaluated the intensity to induce a motor-evoked potential of 1 mV (S1mV), the resting motor threshold (RMT), the cortical silent period (CSP) at an intensity of 120 and 150% of the individual RMT, short-latency interval intracortical inhibition (3 ms), and intracortical facilitation (7 and 15 ms). Depending on the variable and hemisphere, follow-up data was available for 7-15 schizophrenia patients and for 10-12 healthy controls.

RESULTS

Repeated measures ANOVA revealed no significant time × group interactions for any of the analyzed variables. A significant increase in S1mV and CSP duration at 150% RMT of the left hemisphere could be observed in both groups over time.

CONCLUSION

Regular ergometer training over 3 months increases motor cortical inhibition as displayed by an increase in CSP. The increase in S1mV may also indicate a higher degree of inhibition after the intervention. We could not establish a difference between schizophrenia patients and healthy controls. Due to the limited sample size, our results have to be considered as preliminary and need to be replicated in future trials.

The effect of the anodal transcranial direct current stimulation over the cerebellum on the motor cortex excitability

This study was designed to investigate whether the cerebellum has an inhibitory effect on motor cortical excitability. Sixteen healthy adults (age range, 25-50 years, five female) participated in the study. Anodal cerebellar transcranial direct current stimulation (a-cTDCS) was used to modulate cerebellar excitability. A-cTDCS was given for 20 min at 1 mA intensity. The automatic threshold tracking method was used to investigate cortical excitability. Resting motor threshold (RMT), short interval intracortical inhibition (SICI), short interval intracortical facilitation (SICF), intracortical facilitation (ICF), and the input output curve (I-O curve) were motor cortical excitability parameters. a-cTDCS caused a reduction in overall SICI and the reduced SICF for interstimulus intervals (ISIs) to 2.4-4.4 ms. a-cTDCS has no effect on ICF, RMT, and the I-O curve. There were no significant changes in any of these cortical excitability parameters after sham cTDCS. Results of the study indicate that a-cTDCS has a dual (both inhibitory and excitatory) effect on motor cortical excitability, rather than a simple inhibitory effect. The cerebellum modulates both the inhibitory and facilitatory activities of motor cortex (M1) and suggest that cerebello-cerebral motor connectivity is more complex than solely inhibitory or facilitatory connections.

Posture interacts with arm weight support to modulate corticomotor excitability to the upper limb

The use of arm weight support (WS) to optimize movement quality may be an avenue for improved upper limb stroke rehabilitation; however, the underlying neurophysiological effects of WS are not well understood. Rehabilitation exercises may be performed when sitting or standing, but the interaction of posture with WS has not been examined until now. We explored the effect of posture with WS on corticomotor excitability (CME) in healthy adults. Thirteen participants performed static shoulder abduction in two postures (sitting and standing) at three levels of WS (0, 45, and 90 % of full support). Transcranial magnetic stimulation of primary motor cortex was used to elicit motor-evoked potentials (MEPs) in eight upper limb muscles. Stimulus-response (SR) curves were fitted to the MEP data using nonlinear regression. Whole-body posture interacted with WS to influence tonic activity and CME in all muscles examined. SR curve parameters revealed greater CME when standing compared to sitting for upper arm muscles, but lower CME to the shoulder, forearm, and hand. Distal to the shoulder, tonic activity and CME were modulated independent of any explicit differences in task requirements. Overall, these results support a model of integrated upper limb control influenced by whole-body posture and WS. These findings have implications for the application of WS in settings such as upper limb rehabilitation after stroke.