Neurophysiological Mechanisms of Respiratory Activity in Cyclostomes and Fish during Aquatic Breathing

The review addresses the features of the establishment of respiratory activity in the ancient taxa, cyclostomes and fish, which allowed them to adapt to aquatic habitats with a low oxygen level. The brainstem of cyclostomes and fish contains a basic set of nuclei that provides the formation of adequate respiratory activity. The latter is mediated by the universal excitatory and inhibitory neurotransmitters (glutamate, GABA, glycine), suggesting the existence of pivotal, evolutionarily conserved mechanisms to reproduce respiratory oscillations. The qualities of water as a habitat with a reduced oxygen tension determine a high significance of branchial and extrabranchial oxygen-sensitive chemoreceptors which share similar features with highly specialized mammalian oxygen receptors. Neurophysiological details of the respiratory rhythm generator machinery as well as peculiarities of respiratory adaptation to fluctuations in water oxygen tension (PwO 2 ) in agnathans (cyclostomes) and gnathostome fish support the concept of a close relationship between the evolutionarily “verified” adaptive mechanisms regardless of the level of organization of individual vertebrate classes.