Setting Treadmill Intensity for Rat Aerobic Training Using Lactate and Gas Exchange Thresholds

Polarized or threshold training: is there a superior training intensity distribution to improve V̇O2max, endurance capacity and mitochondrial function? A study in Wistar Rat models

Conflicting evidence exists regarding the superiority of Polarized Training (POL) vs other training intensity distribution models. Compare POL vs threshold (THR) training on V̇O2max, endurance capacity (EC) and mitochondrial function. Fifteen male Wistar rats (336.1 ± 30.4 g) were divided in: POL (n = 5), THR (n = 5) or control (CON; n = 5) groups. V̇O2max (indirect calorimetry) and EC (treadmill exhaustion test) were determined at baseline four and eight-weeks of training. POL consisted of 80% running volume at 60%V̇O2max and 20% at 90%V̇O2max while THR trained only at 75%V̇O2max. Both protocols were isocaloric and performed 5d/week. All animals were housed in cages with access to running wheel to allow ad libitum activity. After training, animals were sacrificed and left ventricle (LV) myocardium, diaphragm, tibialis anterior and soleus muscles were collected for high-resolution respirometry, biochemical and histological analysis. There were no baseline differences between groups. After training V̇O2max and EC were similar between POL and THR even though THR V̇O2max was higher compared to CON. After training, there were also no significant differences in OXPHOS or any of the other major mitochondrial function markers assessed between POL and THR in any of the tissues analyzed. The expression of MFN1, MFN2, PGC-1α, TFAM, DRP1, OPA1 and TOM20 as well as the activity of citrate synthase were also similar between POL and THR in all tissues. There were no significant differences in endurance performance or markers of bioenergetic function between POL and THR after eight-weeks of training.

Very-light-intensity exercise as minimal intensity threshold for activating dorsal hippocampal neurons: Evidence from rat physiological exercise model

Exercise benefits the brain, particularly the learning and memory center-the dorsal hippocampus (dHPC)-and holds promise for therapeutic applications addressing age-related cognitive deficits. While moderate-to-vigorous-intensity exercise is commonly recommended for health benefits, our translational research proposes the effectiveness of very-light-intensity exercise in enhancing cognitive functions. However, the intensity-dependent characteristics of HPC activation have yet to be fully delineated; therefore, there is no evidence of whether such easily accessible exercises for people of all ages and most fitness levels can activate HPC neurons. Here, we aimed to clarify this question using a physiologically sound rat exercise model. We used a previously established rat treadmill running model within a metabolic chamber and measured maximal oxygen uptake (V˙O2max) during an incremental running test. Referring to the American College of Sports Medicine's V˙O2max-based intensity classification, rats were assigned to one of five groups: resting control, very-light, light, moderate, and vigorous exercise intensity. We immunohistochemically assessed the effects of a single bout of exercise on dHPC neuronal activity and measured V˙O2 and blood lactate as exercise intensity indicators. dHPC neuronal activity increased with exercise intensity, even at light-intensity without blood lactate accumulation, and correlated positively with increasing V˙O2. The dorsal dentate gyrus and CA1 sub-regions were markedly activated even by very-light-intensity exercise. Our findings demonstrate the intensity-dependent activation of dHPC neurons, with very-light-intensity exercise as the minimal intensity threshold. These strongly support our hypothesis that very-light-intensity exercise serves as a viable memory-enhancing strategy, beneficial for various populations including low-fitness individuals and the elderly.

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.