Description:
The role of inhibitory neurons in the respiratory network is a matter of ongoing debate.Conflicting and contradicting results are manifold and the question whether inhibitory neurons areessential for the generation of the respiratory rhythm as such is controversial. Inhibitory neuronsare required in pulmonary reflexes for adapting the activity of the central respiratory network to thestatus of the lung and it is hypothesized that glycinergic neurons mediate the inspiratory off-switch.Over the years, optogenetic tools have been developed that allow for cell-specific activation ofsubsets of neurons in vitro and in vivo. In this study, we aimed to identify the effect of activationof inhibitory neurons in vivo. Here, we used a conditional transgenic mouse line that expressesChannelrhodopsin 2 in inhibitory neurons. A 200 m multimode optical fiber ferrule was implantedin adult mice using stereotaxic surgery, allowing us to stimulate inhibitory, respiratory neurons withinthe core excitatory network in the preBötzinger complex of the ventrolateral medulla. We show that,in anesthetized mice, activation of inhibitory neurons by blue light (470 nm) continuously or withstimulation frequencies above 10 Hz results in a significant reduction of the respiratory rate, in somecases leading to complete cessation of breathing. However, a lower stimulation frequency (4–5 Hz)could induce a significant increase in the respiratory rate. This phenomenon can be explained bythe resetting of the respiratory cycle, since stimulation during inspiration shortened the associatedbreath and thereby increased the respiratory rate, while stimulation during the expiratory intervalreduced the respiratory rate. Taken together, these results support the concept that activation ofinhibitory neurons mediates phase-switching by inhibiting excitatory rhythmogenic neurons in thepreBötzinger complex.