A multi-targeted investigation of Deepwater Horizon crude oil exposure impacts on the marine teleost stress axis.

The toxicity of the polycyclic aromatic hydrocarbons (PAHs) in Deepwater Horizon (DWH) oil is well-established, but a knowledge gap exists regarding how this combination of PAHs affects the vertebrate stress axis. We hypothesized that (1) marine vertebrates exposed to DWH PAHs experience stress axis impairment, and co-exposure to an additional chronic stressor may exacerbate these effects, (2) serotonin (5-hydroxytryptamine; 5-HT) may act as a secondary cortisol secretagogue in DWH PAH-exposed fish to compensate for impairment, and (3) the mechanism of stress axis impairment may involve downregulation of cyclic adenosine monophosphate (cAMP; as proxy for melanocortin 2 receptor (MC2R) functionality), total cholesterol, and/or mRNA expression of CYP1A and steroidogenic proteins StAR, P450scc, and 11ß-h at the level of the kidney. We found that in vivo plasma cortisol and plasma adrenocorticotropic hormone (ACTH) concentrations in Gulf toadfish exposed to an environmentally relevant DWH PAH concentration (ΣPAH50= 4.6 ± 1.6 µg/L) for 7 days were not significantly different from controls, whether fish were chronically stressed or not. However, the rate of cortisol secretion by isolated kidneys after acute stimulation with ACTH was significantly lower in PAH-exposed toadfish compared to clean seawater (SW) controls. 5-HT does not appear to be acting as a secondary cortisol secretagogue, rather, PAH-exposed + stressed toadfish exhibited significantly lower plasma 5-HT concentrations than clean SW + stressed fish as well as a reduced sensitivity to 5-HT at the level of the kidney. There was a tendency for kidney cAMP concentrations to be lower in PAH-exposed fish (p = 0.069); however, mRNA expression of steroidogenic proteins between control and PAH-exposed toadfish were not significantly different and a significant elevation in total cholesterol concentration in PAH-exposed toadfish compared to controls was measured. Future work is needed to establish whether the slower cortisol secretion rate by isolated kidneys of PAH-exposed fish is detrimental, to determine the potential role of other secretagogues in compensating for the impaired kidney interrenal cell function, and to determine whether there is a reduction in MC2R mRNA expression or an impairment in the function of steroidogenic proteins.

Glucocorticoid receptors are involved in the regulation of pulsatile urea excretion in toadfish.

The objectives of this study were to characterize the pattern of pulsatile urea excretion in the gulf toadfish in the wake of exogenous cortisol loading and to determine the receptors involved in the regulation of this mechanism. Toadfish were fitted with indwelling arterial catheters and were infused with isosmotic NaCl for 48 h after which fish were treated with cortisol alone, cortisol + peanut oil, cortisol + RU486 (a glucocorticoid receptor antagonist) or cortisol + spironolactone (a mineralocorticoid receptor antagonist). Upon cortisol loading, fish treated with cortisol alone, cortisol + oil or cortisol + spironolactone experienced a two- to threefold reduction in pulsatile urea excretion. This reduction was due to a decrease in urea pulse size with no effect on pulse frequency compared to values measured during the control NaCl infusion period. In addition, these fish showed an increase in plasma urea concentrations upon treatment. These apparent effects of cortisol treatment were abolished in fish treated with cortisol + RU486. In contrast, these fish showed an increase in pulsatile urea excretion mediated by a twofold increase in pulse size with no change in frequency. Likewise, fish treated with cortisol + RU486 showed a significant decrease in plasma urea concentrations over the course of the experiment. The findings of this study indicate that high levels of cortisol reduce pulsatile urea excretion by decreasing pulse size. In addition, it appears that glucocorticoid receptors and not mineralocorticoid receptors are involved in the regulation of the toadfish pulsatile urea excretion mechanism.