Oscillating wildly: Turning the evolution of respiratory rhythm generation on its head

Abstract

How the neuronal control of air breathing evolved is unknown. Brainstem circuits controlling breathing have only been investigated in a small handful of extant vertebrates. This makes an evolutionary analysis challenging.In the frog, we have found evidence for three areas involved in respiratory rhythm generation, each with distinct pharmacologies: the buccal area in rhombomere 7 as well as both lung burst priming and lung burst power stroke regions situated in rhombomeres 4‐5. We have long assumed that circuits generating air breathing in mammals and frogs are homologous, evolving from a common ancestry. However, there are some inconsistencies with this assumption. The primary inspiratory oscillator in mammals, the PreBötzinger Complex (PreBötC), is located in rhombomere 7‐‐the same rhombomere as the frog buccal area‐‐and the rhythms produced by buccal and PreBötC are stubbornly independent of CO2. In comparison, like pre‐inspiratory/expiratory activity in mammals, the lung bursts in the frog are highly CO2 sensitive, and both originate in rhombomere 4‐5.These data suggest that we may have to turn our previous hypothesis that the frog lung area and PreBötC are homologous on its head and consider the possibility that the mammalian PreBötC evolved from a fish oscillator used for water‐breathing. Until more studies on non‐mammalian vertebrates are performed any hypothesis on the evolution of air breathing is but a work in progress, and with so little data currently available, further oscillations are likely.This work was funded by NSERC.