Chemoreceptor plasticity and respiratory acclimation in the zebrafishDanio rerio

Abstract

SUMMARY The goals of this study were to assess the respiratory consequences of exposing adult zebrafish Danio rerio to chronic changes in water gas composition (hypoxia, hyperoxia or hypercapnia) and to determine if any ensuing effects could be related to morphological changes in branchial chemoreceptors. To accomplish these goals, we first modified and validated an established non-invasive technique for continuous monitoring of breathing frequency and relative breathing amplitude in adult fish. Under normal conditions 20% of zebrafish exhibited an episodic breathing pattern that was composed of breathing and non-breathing (pausing/apneic) periods. The pausing frequency was reduced by acute hypoxia (PwO2<130 mmHg)and increased by acute hyperoxia (PwO2>300 mmHg), but was unaltered by acute hypercapnia. Fish were exposed for 28 days to hyperoxia (PwO2>350 mmHg), or hypoxia (PwO2=30 mmHg) or hypercapnia(PwCO2=9 mmHg). Their responses to acute hypoxia or hypercapnia were then compared to the response of control fish kept for 28 days in normoxic and normocapnic water. In control fish, the ventilatory response to acute hypoxia consisted of an increase in breathing frequency while the response to acute hypercapnia was an increase in relative breathing amplitude. The stimulus promoting the hyperventilation during hypercapnia was increased PwCO2 rather than decreased pH. Exposure to prolonged hyperoxia decreased the capacity of fish to increase breathing frequency during hypoxia and prevented the usual increase in breathing amplitude during acute hypercapnia. In fish previously exposed to hyperoxia,episodic breathing continued during acute hypoxia until PwO2 had fallen below 70 mmHg. In fish chronically exposed to hypoxia, resting breathing frequency was significantly reduced (from 191±12 to 165±16 min–1); however, the ventilatory responses to hypoxia and hypercapnia were unaffected. Long-term exposure of fish to hypercapnic water did not markedly modify the breathing response to acute hypoxia and modestly blunted the response to hypercapnia. To determine whether branchial chemoreceptors were being influenced by long-term acclimation, all four groups of fish were acutely exposed to increasing doses of the O2 chemoreceptor stimulant, sodium cyanide,dissolved in inspired water. Consistent with the blunting of the ventilatory response to hypoxia, the fish pre-exposed to hyperoxia also exhibited a blunted response to NaCN. Pre-exposure to hypoxia was without effect whereas prior exposure to hypercapnia increased the ventilatory responses to cyanide. To assess the impact of acclimation to varying gas levels on branchial O2 chemoreceptors, the numbers of neuroepithelial cells (NECs) of the gill filament were quantified using confocal immunofluorescence microscopy. Consistent with the blunting of reflex ventilatory responses, fish exposed to chronic hyperoxia exhibited a significant decrease in the density of NECs from 36.8±2.8 to 22.7±2.3 filament–1.

Characterisation of putative oxygen chemoreceptors in bowfin (Amia calva)

“…Type IV cells found in the lamellae have been described in most species of fish studied to date (Coolidge et al, 2008;Jonz et al, 2004;Saltys et al, 2006;Vulesevic et al, 2006), but not in trout or mangrove rivulus (Coolidge et al, 2008;Regan et al, 2011;Saltys et al, 2006). In other fish species, Type IV cells typically contain synaptic vesicles and are innervated.…”

Ammonia sensing by neuroepithelial cells and ventilatory responses to ammonia in rainbow trout

“…We hypothesized that the loss of the acute hyperventilatory response to ammonia after chronic exposure to HEA would be accompanied by changes in the size, density or distribution of NECs on gill arches if these cells were involved in ammonia sensing. Such changes have been seen in zebrafish subjected to chronic hypoxia (Jonz et al, 2004) and hyperoxia (Vulesevic et al, 2006). By using patch-clamp techniques, Jonz et al (Jonz et al, 2004) and Burleson et al (Burleson et al, 2006) demonstrated that NECs of both zebrafish and catfish act as O 2 chemoreceptors, as hypoxia caused inhibition of their background K + currents.…”

“…There is now abundant evidence that O 2 and CO 2 /pH sensors occur on the gills, with a particular focus on the first pair of gill arches (gill arch I) (Smith and Jones, 1978;Gilmour, 2001;Milsom and Burleson, 2007). Many recent studies indicate that the branchial neuroepithelial cells (NECs) are the actual chemoreceptors for these two respiratory gases in blood and/or water (Jonz et al, 2004;Jonz and Nurse, 2006;Vulesevic et al, 2006;Milsom and Burleson, 2007;Coolidge et al, 2008;Qin et al, 2010). Indeed, the receptors on gill arches I and II (embryonic arches III and IV) are thought to represent the phylogenetic antecedents of the mammalian carotid and aortic bodies, respectively (Milsom and Burleson, 2007).…”

Molecular cloning and gene expression analysis in aquaculture science: a review focusing on respiration and immune responses in European sea bass (Dicentrarchus labrax)

“…Available data regarding the effects of CO 2 on fish show that hypercapnia negatively affects many vital physiological functions in exposed animals, e.g., respiration, circulation, central nervous system function, growth, behaviour, and metabolism. An increase in relative breathing amplitude was observed in D. rerio under hypercapnic conditions (Vulsevic et al 2006), whereas in Cyprinus carpio the ventilation frequency significantly increased at high P CO 2 (Soncini and Glass 2000). The exposure of Seriola quinqueradiata to 5 % CO 2 (38 mmHg) resulted in a significant fall in cardiac output and stroke volume and in a rise in blood pressure (Lee et al 2003).…”