Exercise enhances motor skill learning by neurotransmitter switching in the adult midbrain

Motor skill learning is a complex process involving multiple brain regions, including the cortex, thalamus, basal ganglia, brainstem, cerebellum, and spinal cord. Both neuronal and glial plasticity are essential for this learning, and disruptions in this plasticity lead to motor deficits. Aerobic exercise is known to improve motor skill acquisition and is used therapeutically for various motor disorders, yet the underlying mechanisms remain poorly understood. This study focuses on the role of neurotransmitter switching, a type of plasticity where neurons change their neurotransmitter identity in response to sustained stimuli, in mediating the benefits of exercise on motor skill learning. The researchers hypothesized that chronic running induces neurotransmitter switching within a circuit crucial for motor skill learning, specifically focusing on the midbrain.

The authors reviewed existing literature demonstrating the well-established link between aerobic exercise and improved motor skill learning. They cite studies showing the positive effects of exercise in various populations, including those with neurological disorders. The importance of neuronal and glial plasticity in motor skill learning was also highlighted, along with the known impact of disruptions in this plasticity on motor function. Prior research on running-induced plasticity in the brain was discussed to set the stage for the current study's investigation into the potential role of neurotransmitter switching. Existing studies on neurotransmitter switching, particularly its ability to modify behavior, formed a key part of the theoretical framework.

Adult mice were randomly assigned to either a control group (no running wheel) or a running group (voluntary access to a running wheel for one week). Motor skill learning was assessed using the rotarod and balance beam tests. The researchers employed a range of techniques to investigate neurotransmitter switching in the caudal pedunculopontine nucleus (cPPN) including immunohistochemistry (IHC) to detect the presence of choline acetyltransferase (ChAT) and glutamate decarboxylase 1 (GAD1), in situ hybridization (ISH) to analyze mRNA levels of ChAT and GAD1, and RNAscope to analyze transcript levels of transmitter synthetic enzymes. Viral vectors (AAV) were used to manipulate neurotransmitter expression (overexpressing ChAT or knocking down GAD1) to determine the causal role of the switch in motor skill enhancement. Anterograde and retrograde tracing techniques were used to map the projections of cPPN neurons to other brain regions involved in motor control. Stereological counting was used to quantify neuronal populations. Statistical analyses included paired t-tests, Welch's t-tests, Mann-Whitney U tests, Pearson correlation, ANOVA, and Kruskal-Wallis tests, as appropriate.

Mice in the running group showed significantly enhanced motor skill learning on both the rotarod and balance beam tests compared to controls. This enhanced learning was temporally correlated with a significant switch in neurotransmitter expression within the cPPN: a reduction in ChAT-expressing (cholinergic) neurons and a corresponding increase in GAD1-expressing (GABAergic) neurons. Double immunostaining and RNAscope revealed that this switch occurred within the same neuronal population. Viral vector experiments confirmed the causal role of this neurotransmitter switch in mediating the beneficial effects of running on motor skill learning. Overexpression of ChAT, preventing the loss of cholinergic neurotransmission, and knockdown of GAD1, blocking the increase in GABAergic neurotransmission, both abolished the improvement in motor skill learning. Anterograde and retrograde tracing studies demonstrated that the switching cPPN neurons projected to the substantia nigra (SN), ventral tegmental area (VTA), and ventrolateral-ventromedial nuclei of the thalamus (VL-VM), brain regions known to be involved in motor control. Further analysis revealed a reduction in ChAT transcripts in the cPPN neurons projecting to these targets in the running group. Analysis of presynaptic terminals in the SN also showed an increase in vesicular GABA transporter (VGAT) and a decrease in vesicular acetylcholine transporter (VAChT) in the running group, indicating that the neurotransmitter switch extended to the axon terminals.

The study's findings provide compelling evidence that neurotransmitter switching in the cPPN is a critical mechanism underlying the beneficial effects of sustained running on motor skill learning. The temporal correlation between the neurotransmitter switch and improved motor performance, coupled with the results from the viral vector experiments, strongly supports a causal relationship. The projection of switching cPPN neurons to key motor control areas (SN, VTA, and VL-VM) suggests a potential mechanism for how this switch influences motor learning – potentially altering synaptic transmission within these circuits and optimizing motor control. The results suggest a rewiring of motor circuitry where the change from excitatory (cholinergic) to inhibitory (GABAergic) input could enable finer control of motor function.

This study demonstrates that sustained running enhances motor skill learning in adult mice via a neurotransmitter switch in the cPPN from ACh to GABA. Both the loss of ChAT and gain of GAD1 are necessary for this effect. The identified projection targets of these neurons suggest specific circuit mechanisms for the behavioral benefits. Future research could explore the underlying molecular mechanisms of this switch, including the role of transcription factors and microRNAs. Investigating the translational potential of this finding for treating motor disorders through exercise interventions or pharmacological manipulations targeting neurotransmitter switching in the cPPN warrants further investigation.

The study was conducted exclusively in mice, limiting the generalizability to humans. While the viral vector experiments provide strong evidence for causality, the long-term effects of these manipulations were not fully explored. The study focused on a specific type of exercise (wheel running); it is unclear if other forms of exercise would elicit similar neurotransmitter switching. Finally, the study primarily investigated the cPPN; the potential contribution of other brain regions to exercise-induced motor skill improvement might be investigated in future studies.