Optogenetic Stimulation of 5-HT Neurons in the Median Raphe Nucleus Affects Anxiety and Respiration

Abstract

Anxiety affects respiration, and in turn perturbs the internal environment, although the neuronal systems controlling anxiety-related respiration remain unclear. Recent reports indicate that serotonin 5-HT neurons in the median raphe nucleus MRN enhance anxiety. In the present study, we aimed to clarify the contribution of 5-HT neurons in the MRN to anxiety and respiratory control using mice expressing a channelrhodopsin-2 variant-enhanced yellow uorescent protein ChR2 C128S-EYFP ; a step-function opsin in the central 5-HT neurons. We applied an optogenetic method to bigenic mice expressing ChR2 C128S-EYFP in 5-HT neurons and to monogenic mice without such expression. Photostimulation of freemoving mice was performed using a wireless system through an optical ber preinserted above the MRN, and respiratory variables were measured using wholebody plethysmography. Anxiety was evaluated using an elevated-plus maze test. In the bigenic mice, we con rmed ChR2 C128S-EYFP expression in tryptophan hydroxylase 2 a brain 5-HT synthase-positive neurons in the raphe nuclei of the mesopontine, such as the MRN and the dorsal raphe nucleus. Blue light illumination to the MRN of the bigenic mice signi cantly increased respiratory rate and minute ventilation without affecting tidal volume, and significantly decreased the time spent in the open arms of the elevated plus maze without changing distance traveled, compared with monogenic mice. These results suggest that 5-HT neuron activity in the MRN increases anxiety-like behavior without affecting locomotor activity, enhances respiratory rhythm and minute ventilation without changing tidal volume, and can mimic anxiety-related respiratory responses in humans.