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

Exercise activates the dorsal hippocampus, which triggers the 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 exercise below the lactate threshold (LT), which almost anyone can safely perform with minimal stress, and 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, which is essential to modulate hippocampal activity and impact memory. To examine this hypothesis, we employed our physiological exercise model based on the LT of rats that can be applied to human and immunohistochemically assessed the neuronal activation of the dorsal hippocampal sub-regions and brainstem monoaminergic neurons. Also, we monitored the dynamics of monoamine release at the dorsal hippocampus using in vivo microdialysis. We found that even light exercise increased neuronal activity in the dorsal hippocampal sub-regions and induced noradrenaline and dopamine release. 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 hippocampal neurons, possibly through the LC-noradrenergic and/or VTA-dopaminergic neurons. This sheds light on the circuit mechanisms responsible for hippocampal neural activation during exercise, consequently enhancing memory function. ### Competing Interest Statement The authors have declared no competing interest.