Chemoreceptor modulation of endogenous respiratory rhythms in vertebrates

Abstract

The relative contributions of O2- and CO2-sensitive chemoreceptor information to centrally generated respiratory patterns have changed dramatically during vertebrate evolution. Chemoafferent input from branchial O2 chemoreceptors modulates centrally generated respiratory patterns but is not critical for respiratory rhythmogenesis in fishes. In air-breathing fishes, branchial O2 chemoreceptors monitoring internal and external stimuli control the relative contributions of the gills and air-breathing organ to net ventilation, and chemoafferent input is necessary for initiating air breathing. In the transition from water to air breathing by amphibious vertebrates, rhythmic patterns of branchial ventilation are completely replaced by arrhythmic and intermittent patterns of air breathing, and there is progressive dependence on CO2 as a source of respiratory drive. Periodic initiation of air breathing in resting animals appears to depend on attaining a threshold level of afferent activity from O2- and CO2/pH-sensitive chemoreceptors, since hyperoxia and/or hypocapnia can abolish air breathing in all air-breathing vertebrates. Conversely, chemoreceptor stimulation in amphibians and reptiles converts intermittent to more continuous air breathing patterns, suggesting that adequate biasing input from chemoreceptors activates a central rhythm generator. Chemoafferent input in homeotherms serves as one of several sources of drive for rhythmic breathing and supplies feedback for blood gas homeostasis in the face of metabolic or environmental change.

N-methyl-D-aspartate receptors mediate chemoreflexes in the shorthorn sculpinMyoxocephalus scorpius

“…From being quite regular and continuous, the animals displayed more irregular patterns, including frequency cycling and episodic breathing. Breathing in vertebrates originates from a central respiratory pattern generator, which is dependent on numerous afferent inputs for initiation of breathing (Feldman et al, 1990;Smatresk, 1990). Removal of the afferent input by selective denervation in the neotropical fish tambaqui gave rise to similar breathing patterns (as seen in this study), such as frequency cycling (denervation of the whole oro-branchial cavity) or episodic breathing (denervation of only the branchial nerves) (Reid et al, in press).…”

Effect of aerial O2 partial pressure on bimodal gas exchange and air-breathing behaviour inTrichogaster leeri

“…They suggested that an ABO-O 2 chemoreceptor would be advantageous in the regulation of cardiac responses to an air-breathing event. A common pattern of cardiac arrhythmia found in fishes during an air-breathing event is inspiration-induced tachycardia followed by the gradual onset of bradycardia as the ABO-O 2 content falls, leading to pronounced bradycardia with exhalation (Farrell, 1978;Johansen et al, 1968a;Singh and Hughes, 1973;Smatresk, 1988;Smatresk, 1990). Therefore, an ABO-O 2 chemoreceptor may be important in the modulation of mechanoreceptor and other stimuli affecting air-breathing tachycardia, in attenuating tachycardia as ABO-O 2 declines and in terminating the breath when ABO-P O 2 becomes too low to promote O 2 uptake (Graham and Baird, 1984).…”

Ventilatory roll off during sustained hypercapnia is gender specific in pekin ducks

“…This receptor group is typically considered to be important for the immediate ventilatory responses to changes in ambient or blood gas levels, but may also be important for regulating breathing at rest (Smatresk, 1990;Milsom, 1998). In this study, denervation of the carotid bodies appeared to reduce total ventilation by 15% when breathing control gas (0% CO 2 ), as observed previously (Bouverot et al, 1974).…”