AbstractAs neuronal subtypes are increasingly categorized, delineating their functional role is paramount. The preBötzinger Complex (preBötC) subpopulation expressing the neuropeptide somatostatin (SST) is classified as mostly excitatory, inspiratory-modulated and not rhythmogenic. We further characterized their phenotypic identity; 87% were glutamatergic and the balance were glycinergic and/or GABAergic. We then used optogenetics to investigate their modulatory role in both anesthetized and freely moving mice. In anesthetized mice, short photostimulation (100 ms) of preBötC SST+ neurons modulated breathing-related variables in a combinatory phase- and state-dependent manner; changes in inspiratory duration, inspiratory peak amplitude (Amp), and phase were different at higher (≥2.5 Hz) vs. lower (<2.5 Hz) breathing frequency. Moreover, we observed a biphasic effect of photostimulation during expiration that is probabilistic, i.e., photostimulation given at the same phase in consecutive cycles can evoke opposite responses (lengthening vs. shortening of the phase). This unexpected probabilistic state- and phase-dependent responses to photostimulation exposed properties of the preBötC that were not predicted and cannot be readily accounted for in current models of preBötC pattern generation. In freely moving mice, prolonged photostimulation decreased f in normoxia, hypoxia, or hypercapnia, and increased Amp and produced a phase advance, which was similar to the results in anesthetized mice when f≥2.5 Hz. We conclude that preBötC SST+ neurons are a key mediator of the extraordinary and essential lability of breathing pattern.Key points summaryWe transfected preBötzinger Complex (preBötC) somatostatin-expressing (SST+) neurons, which modulate respiratory pattern, but are not rhythmogenic, with channelrhodopsin to investigate phase- and state-dependent modulation of breathing pattern in anesthetized and freely behaving mice in normoxia, hypoxia, and hypercapnia.In anesthetized mice, photostimulation of preBötC SST+ neurons during inspiration increased inspiratory duration and amplitude regardless of baseline breathing frequency, f.In anesthetized mice with low f (<2.5 Hz), photostimulation of preBötC SST+ neurons during expiration evoked either phase advance or phase delay, whereas in anesthetized mice with high f (≥2.5 Hz) and in freely behaving mice in normoxia, hypoxia, or hypercapnia, photostimulation always evoked phase advance.Phase- and state-dependency is a function of overall breathing network excitability.The f-dependent probabilistic modulation of breathing pattern by preBötC SST+ neurons was unexpected, requiring reconsideration of current models of preBötC function, which neither predict nor can readily account for such responses.
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