A possible contribution of the locus coeruleus to arousal enhancement with mild exercise: evidence from pupillometry and neuromelanin imaging

Resting-state blink rate does not increase following very-light-intensity exercise, but individual variation predicts executive function enhancement levels

BACKGROUND

Acute physical exercise, even at a very-light-intensity, potentiates prefrontal cortex activation and improves executive function. The underlying circuit mechanisms in the brain remain poorly understood, though we speculate a potential involvement of arousal-related neuromodulatory systems. Recently, our rodent study demonstrated that exercise, even at light-intensity, activates the midbrain dopaminergic neurons. Resting-state spontaneous eye blink rate is linked to brain-arousal neural circuits, and potentially to those modulated by dopaminergic system. We hypothesized that neural substrates linked to resting-state eye blink rate contribute to the cognitive impact of acute very-light-intensity exercise.

METHOD

We analyzed data from a previous study with a renewed focus on resting-state eye blink rate. Twenty-four healthy young adults completed both 10 min of cycling (very-light-intensity exercise: 30% peak oxygen uptake) and rest conditions. Resting-state eye blink rate and Stroop task performance were measured before and after both exercise and resting control.

RESULTS

Results showed no significant differences in eye blink rate changes between conditions. However, correlation analyses revealed that exercise-induced changes in resting-state eye blink rate were significantly associated with individual variations in Stroop task performance enhancement.

CONCLUSION

Very-light-intensity exercise does not elicit a consistent increase in eye blink rate after exercise. This finding does not support the involvement of a blink increase-linked neural substrate in enhancing executive function through very-light-intensity exercise. However, resting-state eye blink rate that is altered by exercise is predictive of executive function enhancement levels; this may serve as a novel contactless biomarker for predicting exercise benefits for brain health and cognition.

Light-exercise-induced dopaminergic and noradrenergic stimulation in the dorsal hippocampus: Using a rat physiological exercise model

Exercise activates the dorsal hippocampus that triggers synaptic and cellar plasticity and ultimately promotes memory formation. For decades, these benefits have been explored using demanding and stress-response-inducing exercise at moderate-to-vigorous intensities. In contrast, our translational research with animals and humans has focused on light-intensity exercise (light exercise) below the lactate threshold (LT), which almost anyone can safely perform with minimal stress. We found that even light exercise can stimulate hippocampal activity and enhance memory performance. Although the circuit mechanism of this boost remains unclear, arousal promotion even with light exercise implies the involvement of the ascending monoaminergic system that is essential to modulate hippocampal activity and impact memory. To test this hypothesis, we employed our physiological exercise model based on the LT of rats and immunohistochemically assessed the neuronal activation of the dorsal hippocampal sub-regions and brainstem monoaminergic neurons. Also, we monitored the extracellular concentration of monoamines in the dorsal hippocampus using in vivo microdialysis. We found that even light exercise increased neuronal activity in the dorsal hippocampal sub-regions and elevated the extracellular concentrations of noradrenaline and dopamine. Furthermore, we found that tyrosine hydroxylase-positive neurons in the locus coeruleus (LC) and the ventral tegmental area (VTA) were activated even by light exercise and were both positively correlated with the dorsal hippocampal activation. In conclusion, our findings demonstrate that light exercise stimulates dorsal hippocampal neurons, which are associated with LC-noradrenergic and VTA-dopaminergic activation. This shed light on the circuit mechanisms responsible for hippocampal neural activation during exercise, consequently enhancing memory function.

Cerebral Benefits Induced by Electrical Muscle Stimulation: Evidence from a Human and Rat Study

Physical exercise (EX) is well established for its positive impact on brain health. However, conventional EX may not be feasible for certain individuals. In this regard, this study explores electromyostimulation (EMS) as a potential alternative for enhancing cognitive function. Conducted on both human participants and rats, the study involved two sessions of EMS applied to the quadriceps with a duration of 30 min at one-week intervals. The human subjects experienced assessments of cognition and mood, while the rats underwent histological and biochemical analyses on the prefrontal cortex, hippocampus, and quadriceps. Our findings indicated that EMS enhanced executive functions and reduced anxiety in humans. In parallel, our results from the animal studies revealed an elevation in brain-derived neurotrophic factor (BDNF), specifically in the hippocampus. Intriguingly, this increase was not associated with heightened neuronal activity or cerebral hemodynamics; instead, our data point towards a humoral interaction from muscle to brain. While no evidence of increased muscle and circulating BDNF or FNDC5/irisin pathways could be found, our data highlight lactate as a bridging signaling molecule of the muscle-brain crosstalk following EMS. In conclusion, our results suggest that EMS could be an effective alternative to conventional EX for enhancing both brain health and cognitive function.

Pupil Dynamics, Salivary Alpha-Amylase, and Prefrontal Haemodynamics in Response to Very Light Exercise

This study aimed to investigate the effects of exercise on pupil dynamics, prefrontal haemodynamic, and salivary alpha-amylase (sAA) responses in 45 healthy graduate students. These participants were divided into two groups: a resting control group (CTL) and a very-light-intensity exercise group (EX). Participants in the EX-group engaged in a 10-min exercise at 30% O2 peak on a cycle ergometer. Salivary samples and pupillometry assessments were collected before the exercise, at the end of the exercise, and 5 min after the completion to evaluate changes over time. Our analysis showed that exercise induced significant changes in the secretion of salivary alpha-amylase, with elevated levels suggesting increased neuroendocrine activity linked to the arousal state triggered by exercise. In addition, functional oxyHb signals indicated greater fluctuations in the mid-left prefrontal cortex among participants in the EX-group compared to those in the CTL group, pointing to altered prefrontal haemodynamic.

Increased neural differentiation after a single session of aerobic exercise in older adults

Aging is associated with decreased cognitive function. One theory posits that this decline is in part due to multiple neural systems becoming dedifferentiated in older adults. Exercise is known to improve cognition in older adults, even after only a single session. We hypothesized that one mechanism of improvement is a redifferentiation of neural systems. We used a within-participant, cross-over design involving 2 sessions: either 30 minutes of aerobic exercise or 30 minutes of seated rest (n = 32; ages 55-81 years). Both functional Magnetic Resonance Imaging (fMRI) and Stroop performance were acquired soon after exercise and rest. We quantified neural differentiation via general heterogeneity regression. There were 3 prominent findings following the exercise. First, participants were better at reducing Stroop interference. Second, while there was greater neural differentiation within the hippocampal formation and cerebellum, there was lower neural differentiation within frontal cortices. Third, this greater neural differentiation in the cerebellum and temporal lobe was more pronounced in the older ages. These data suggest that exercise can induce greater neural differentiation in healthy aging.

Pupil dynamics during very light exercise predict benefits to prefrontal cognition

Physical exercise, even stress-free very-light-intensity exercise such as yoga and very slow running, can have beneficial effects on executive function, possibly by potentiating prefrontal cortical activity. However, the exact mechanisms underlying this potentiation have not been identified. Evidence from studies using pupillometry demonstrates that pupil changes track the real-time dynamics of activity linked to arousal and attention, including neural circuits from the locus coeruleus to the cortex. This makes it possible to examine whether pupil-linked brain dynamics induced during very-light-intensity exercise mediate benefits to prefrontal executive function in healthy young adults. In this experiment, pupil diameter was measured during 10 min of very-light-intensity exercise (30% V˙o2peak). A Stroop task was used to assess executive function before and after exercise. Prefrontal cortical activation during the task was assessed using multichannel functional near-infrared spectroscopy (fNIRS). We observed that very-light-intensity exercise significantly elicited pupil dilation, reduction of Stroop interference, and task-related left dorsolateral prefrontal cortex activation compared with the resting-control condition. The magnitude of change in pupil dilation predicted the magnitude of improvement in Stroop performance. In addition, causal mediation analysis showed that pupil dilation during very-light-intensity exercise robustly determined subsequent enhancement of Stroop performance. This finding supports our hypothesis that the pupil-linked mechanisms, which may be tied to locus coeruleus activation, are a potential mechanism by which very light exercise enhances prefrontal cortex activation and executive function. It also suggests that pupillometry may be a useful tool to interpret the beneficial impact of exercise on boosting cognition.