Sensitivity of ventilation and brain metabolism to ammonia exposure in rainbow trout,Oncorhynchus mykiss

Abstract

SUMMARYAmmonia has been documented as a respiratory gas that stimulates ventilation, and is sensed by peripheral neuroepithelial cells (NECs) in the gills in ammoniotelic rainbow trout. However, the hyperventilatory response is abolished in trout chronically exposed (1+ months) to high environmental ammonia [HEA; 250μmoll −1 (NH 4 ) 2 SO 4 ]. This study investigates whether the brain is involved in the acute sensitivity of ventilation to ammonia, and whether changes in brain metabolism are related to the loss of hyperventilatory responses in trout chronically exposed to HEA ('HEA trout'). Hyperventilation (via increased ventilatory amplitude rather than rate) and increased total ammonia concentration ( These experiments revealed that GSase plays an important role in transferring ammonia to glutamate to make glutamine in trout brain, thereby attenuating the elevation of brain [T Amm ] following HEA exposure, and that glutamate concentration is reduced in HEA trout. The mRNAs for the ammonia channel proteins Rhbg, Rhcg1 and Rhcg2 were expressed in trout brain, and the expression of Rhbg and Rhcg2 increased in HEA trout, potentially as a mechanism to facilitate the efflux of ammonia. In summary, the brain appears to be involved in the sensitivity of ventilation to ammonia, and brain ammonia levels are regulated metabolically in trout.

Rh protein expression in branchial neuroepithelial cells, and the role of ammonia in ventilatory control in fish

“…All fish were fasted for at least 3 days prior to experimentation to minimize the influence of feeding on ammonia metabolism. For methodological details, the reader is referred to our published studies on rainbow trout (Zhang and Wood, 2009;Zhang et al, 2011Zhang et al, , 2013 and dogfish sharks (De Boeck and Wood, submitted for publication). Detailed methods are described below only for the new studies reported in the present paper.…”

“…At present, we favor the idea that internal receptors are responsible for the bulk of the responsiveness, for two reasons. Firstly, the hyperventilatory response to internal ammonia injections is essentially immediate (McKenzie et al, 1993;Zhang and Wood, 2009), whereas the response to waterborne HEA takes 5-10 min and increases progressively thereafter for some time (Zhang et al, 2011(Zhang et al, , 2013. Secondly, as discussed in Section 3.4, it is difficult to see how ventilatory responsiveness to waterborne ammonia alone would be adaptive.…”

“…This raises the question whether the same might be true for ammonia in fish under normal as well as abnormal circumstances, because ammonia readily crosses the blood-brain barrier, and builds up in cerebral tissue in response to internal or external loading (Wright et al, 1988Ip et al, 2001;Chew et al, 2005;Sanderson et al, 2010). Zhang et al (2013) addressed this question in the rainbow trout using several different approaches. After acute exposure to HEA (1000 or 2000 μmol L −1 as (NH 4 )SO 4 ), [T Amm ] in arterial blood plasma, cerebrospinal fluid (CSF), and brain tissue were measured; all increased, but by far the strongest correlation of the increased ventilation was with brain [T Amm ] (Fig.…”

“…A somewhat different approach to this same question employed methionine sulfoxamine (MSOX), a specific inhibitor of glutamine synthetase (GS), to manipulate brain [T Amm ] levels (Zhang et al, 2013).…”

Feeding through your gills and turning a toxicant into a resource: how the dogfish shark scavenges ammonia from its environment

“…Different animals were used in the two treatments, and standard 6 h flux tests were run with 200 µmol l −1 ammonia (added as NH 4 HCO 3 ) in the water. The MSOX dose (60 mg kg −1 ) was approximately 10-fold greater than routinely used in teleosts (Sanderson et al, 2010;Zhang et al, 2013), but glutamine synthetase is more abundant in elasmobranchs (Kajimura et al, 2006). In a preliminary experiment with one animal, this dose proved not to cause mortality but did cause a slow decline in urea-N excretion rate after 48 h. Therefore, the 6 h tests were run at 35-41 h after injection, a time selected to avoid any secondary consequences due to internal urea declines.…”