Endogenous cortisol production and its relationship with feeding transitions in larval lake sturgeon (Acipenser fulvescens)

Our understanding of the importance of cortisol in the development of fishes largely stems from teleosts and in particular the zebrafish, Danio rerio. However, studies examining the ontogeny of the cortisol endocrine axis in acipenseriformes (sturgeon and paddlefish) have demonstrated similar general patterns during early development. Beginning with maternal deposition of cortisol in the egg, followed by development of de novo synthesis, a hypo-responsive period, and finally the ability of the fish to appropriately increase whole-body levels of cortisol in response to a stressor. In the present study, we demonstrate a similar pattern of ontogeny in the cortisol response in lake sturgeon over two-year classes. Whole-body levels of cortisol were examined over two cohorts and found to be different in both concentration and timing of endogenous production. The 2016 cohort were found to have relatively high levels of cortisol and developed to first feeding approximately six days faster than the 2017 cohort with lower levels of cortisol. In the 2017 cohort, mRNA expression of steroidogenic acute regulatory protein (StAR) and glucocorticoid receptor 1 (GR1) increased just prior to the increase in cortisol and associated onset of exogenous feeding. Treatment in metyrapone, an inhibitor of 11β-hydroxylase, significantly inhibited cortisol production and resulted in the inability of the fish to appropriately transition to exogenous feeding. Data suggest a potential key role for cortisol in lake sturgeon as they transition between diets during early life history.

Do Gulf Toadfish Use Pulsatile Urea Excretion to Chemically Communicate Reproductive Status?

Gulf toadfish (Opsanus beta) are exceptionally capable of switching from excreting ammonia as their primary nitrogenous waste to excreting predominantly urea in distinct pulses across the gill. Previous studies suggest that these urea pulses may be used for intraspecific chemical communication. To determine whether pulsatile urea excretion communicates reproductive status, toadfish were sexed using ultrasound and delivered conspecific-conditioned seawater (CC-SW) that previously housed a conspecific of the opposite sex, a conspecific chemical alarm cue (avoidance control), or a prey cue (attraction control). Swim behavior, attraction to or avoidance of the cues, and changes in the pattern of pulsatile urea excretion were monitored during and after delivery. Gulf toadfish did not spend more time in zones that were delivered CC-SW or prey cue. However, male toadfish spent significantly more time swimming after the delivery of female cues than control seawater (SW). In contrast, toadfish did not appear to have an immediate avoidance response to the conspecific alarm cue. Additionally, significantly more toadfish pulsed within 7 h of CC-SW and prey cue delivery compared to control SW, and pulse frequency was 1.6 times greater in response to CC-SW than control SW. These results, in combination with increased urea production and excretion the during breeding season, suggest that toadfish may use pulsatile urea excretion to communicate with conspecifics when exposed to chemosensory cues from the opposite sex.

A waterborne chemical cue from Gulf toadfish, Opsanus beta, prompts pulsatile urea excretion in conspecifics

The Gulf toadfish (Opsanus beta) has a fully functional ornithine urea cycle (O-UC) that allows it to excrete nitrogenous waste in the form of urea. Interestingly, urea is excreted in a pulse across the gill that lasts 1-3h and occurs once or twice a day. Both the stress hormone, cortisol, and the neurotransmitter, serotonin (5-HT) are involved in the control of pulsatile urea excretion. This and other evidence suggests that urea pulsing may be linked to toadfish social behavior. The hypothesis of the present study was that toadfish urea pulses can be triggered by waterborne chemical cues from conspecifics. Our findings indicate that exposure to seawater that held a donor conspecific for up to 48h (pre-conditioned seawater; PC-SW) induced a urea pulse within 7h in naïve conspecifics compared to a pulse latency of 20h when exposed to seawater alone. Factors such as PC-SW intensity and donor body mass influenced the pulse latency response of naïve conspecifics. Fractionation and heat treatment of PC-SW to narrow possible signal candidates revealed that the active chemical was both water-soluble and heat-stable. Fish exposed to urea, cortisol or 5-HT in seawater did not have a pulse latency that was significantly different than seawater alone; however, ammonia, perhaps in the form of NH₄Cl, was found to be a factor in the pulse latency response of toadfish to PC-SW and could be one component of a multi-component cue used for chemical communication in toadfish. Further studies are needed to fully identify the chemical cue as well as determine its adaptive significance in this marine teleost fish.

Crowding stress inhibits serotonin 1A receptor-mediated increases in corticotropin-releasing factor mRNA expression and adrenocorticotropin hormone secretion in the Gulf toadfish

Stimulation of the serotonin 1A (5-HT1A) receptor subtype by 5-HT has been shown to result in an elevation in plasma corticosteroid levels in both mammals and several species of teleost fish, including the Gulf toadfish (Opsanus beta); however, in the case of teleost fish, it is not clearly known at which level of the hypothalamic-pituitary-interrenal axis the 5-HT1A receptor is stimulated. Additionally, previous investigations have revealed that chronic elevations of plasma cortisol mediate changes in brain 5-HT1A receptor mRNA and protein levels via the glucocorticoid receptor (GR); thus, we hypothesized that the function of centrally activated 5-HT1A receptors is reduced or abolished as a result of chronically elevated plasma cortisol levels and that this response is GR mediated. Our results are the first to demonstrate that intravenous injection of the 5-HT1A receptor agonist, 8-OH-DPAT, stimulates a significant increase in corticotropin-releasing factor (CRF) precursor mRNA expression in the hypothalamic region and the release of adrenocorticotropic hormone (ACTH) from the pituitary of teleost fish compared to saline-injected controls. We also provide evidence that cortisol, acting via GRs, attenuates the 5-HT1A receptor-mediated secretion of both CRF and ACTH.

The effect of stress on gill basolateral membrane binding kinetics of 5-ht2 receptor ligands: potential implications for urea excretion mechanisms

The goal of this study was to determine the relationship between cortisol and the toadfish serotonin 2A (5-HT2A ) receptor, which is believed to be responsible for the activation of the toadfish urea transporter, tUT. We hypothesize that elevations in cortisol would play a role in the regulation of the 5-HT2A receptor at the level of mRNA expression, ligand binding, and/or function. To test this idea, cortisol levels were manipulated by either crowding or through treatment with the cortisol synthesis blocker, metyrapone. Crowded fish had significantly higher circulating cortisol levels compared to uncrowded fish and cortisol levels in metyrapone-treated fish were significantly lower than saline-treated controls. No significant difference was measured in gill 5-HT2A mRNA expression levels between uncrowded and crowded, control- or metyrapone-treated fish. Furthermore, no significant difference was measured in [(3) H]-5-HT binding kinetics or in the competitive binding of the 5-HT2 agonist, α-methyl 5-HT, to isolated gill basolateral membranes of uncrowded or crowded toadfish. However, the binding maximum (Bmax ) of the 5-HT2A receptor antagonist, [(3) H]-ketanserin, was significantly different between all four groups of fish (metyrapone > control > crowded > uncrowded). Furthermore, metyrapone-treated fish excreted approximately twofold more urea compared to controls when injected with α-methyl 5-HT, a 5-HT2 receptor agonist shown to stimulate urea excretion. Our results suggest that cortisol may have differential effects on 5-HT receptor binding, which could have potential implications on the control of pulsatile urea excretion in toadfish.

The toadfish serotonin 2A (5-HT(2A)) receptor: molecular characterization and its potential role in urea excretion

Based on early pharmacological work, the serotonin 2A (5-HT(2A)) receptor subtype is believed to be involved in the regulation of toadfish pulsatile urea excretion. The goal of the following study was to characterize the toadfish 5-HT(2A) receptor at a molecular level, to determine the tissues in which this receptor is predominantly expressed and to further investigate the pharmacological specificity of toadfish pulsatile urea excretion by examining the effect of ketanserin, a 5-HT(2A) receptor antagonist, on resting rates of pulsatile urea excretion. The full-length toadfish 5-HT(2A) receptor encodes a 496 amino acid sequence and shares 57-80% sequence identity to 5-HT(2A) receptors of other organisms, with 100% conservation among important ligand-binding residues. Toadfish 5-HT(2A) receptor mRNA expression was highest in the swim bladder and gonad, followed by the whole brain. All other tissues tested (esophagus, stomach, anterior intestine, posterior intestine, rectum, liver, kidney, heart, muscle and gill) had mRNA expression levels that were significantly less than whole brain. Toadfish 5-HT(2A) receptor mRNA expression within the brain was highest in the hindbrain, telencephalon and midbrain/diencephalon regions. Treatment with the 5-HT(2A) receptor antagonist, ketanserin, resulted in a significant decrease in the pulsatile component of spontaneous urea excretion due to a reduction in urea pulse size with no significant change in pulse frequency. These results lend further support for the 5-HT(2A) receptor in the regulation of pulsatile urea excretion in toadfish.

New insights into the mechanisms controlling urea excretion in fish gills

Not long ago, urea was believed to freely diffuse across plasma membranes. The discovery of specialized proteins to facilitate the movement of urea across the fish gill, similar to those found in mammalian kidney, was exciting, and at the same time, perplexing; especially considering the fact that, aside from elasmobranchs, most fish do not produce urea as their primary nitrogenous waste. Increasingly, it has become apparent that many fish do indeed produce at least a small amount of urea through various processes and continued work on branchial urea transporters in teleost and elasmobranch fishes has led to recent advances in the regulation of these mechanisms. The following review outlines the substantial progress that has been made towards understanding environmental and developmental impacts on fish gill urea transport. This review also outlines the work that has been done regarding endocrine and neural control of urea excretion, most of which has been collected from only a handful of teleost fish. It is evident that more research is needed to establish the endocrine and neural control of urea excretion in fish, including fish representative of more ancient lineages (hagfish and lamprey), and elasmobranch fish.

Consequences of acute stress and cortisol manipulation on the physiology, behavior, and reproductive outcome of female Pacific salmon on spawning grounds

Life-history theory predicts that stress responses should be muted to maximize reproductive fitness. Yet, the relationship between stress and reproduction for semelparous salmon is unusual because successfully spawning individuals have elevated plasma cortisol levels. To tease apart the effects of high baseline cortisol levels and stress-induced elevation of cortisol titers, we determined how varying degrees of cortisol elevation (i.e., acute and chronic) affected behavior, reproductive physiology, and reproductive success of adult female pink salmon (Oncorhynchus gorbuscha) relative to different states of ovulation (i.e., ripe and unripe). Exhaustive exercise and air exposure were applied as acute stressors to manipulate plasma cortisol in salmon either confined to a behavioral arena or free-swimming in a spawning channel. Cortisol (eliciting a cortisol elevation to levels similar to those in post-spawn female salmon) and metyrapone (a corticosteroid synthesis inhibitor) implants were also used to chemically manipulate plasma cortisol. Cortisol implants elevated plasma cortisol, and impaired reproductive success; cortisol-treated fish released fewer eggs and died sooner than fish in other treatment groups. In contrast, acute stressors elevated plasma cortisol and the metyrapone implant suppressed plasma cortisol, but neither treatment significantly altered reproductive success, behavior, or physiology. Our results suggest that acute stressors do not influence behavior or reproductive outcome when experienced upon arrival at spawning grounds. Thus, certain critical aspects of salmonid reproduction can become refractory to various stressful conditions on spawning grounds. However, there is a limit to the ability of these fish to tolerate elevated cortisol levels as revealed by experimental elevation of cortisol.

Interactions between cortisol and Rhesus glycoprotein expression in ureogenic toadfish, Opsanus beta

In their native environment, gulf toadfish excrete equal quantities of ammonia and urea. However, upon exposure to stressful conditions in the laboratory (i.e. crowding, confinement or air exposure), toadfish decrease branchial ammonia excretion and become ureotelic. The objective of this study was to determine the influences of cortisol and ammonia on ammonia excretion relative to expression of Rhesus (Rh) glycoproteins and the ammonia-fixing enzyme, glutamine synthetase (GS). In vivo infusions and/or injections were used to manipulate corticosteroid activity and plasma ammonia concentrations in ureotelic toadfish. Metyrapone treatment to lower circulating cortisol levels resulted in a 3.5-fold elevation of ammonia excretion rates, enhanced mRNA expression of two of the toadfish Rh isoforms (Rhcg1 and Rhcg2), and decreased branchial and hepatic GS activity. Correspondingly, cortisol infusion decreased ammonia excretion 2.5-fold, a change that was accompanied by reduced branchial expression of all toadfish Rh isoforms (Rhag, Rhbg, Rhcg1 and Rhcg2) and a twofold increase in hepatic GS activity. In contrast, maintenance of high circulating ammonia levels by ammonia infusion enhanced ammonia excretion and Rh expression (Rhag, Rhbg and Rhcg2). Toadfish treated with cortisol showed an attenuated response to ammonia infusion with no change in Rh mRNA expression or GS activity. In summary, the evidence suggests that ammonia excretion in toadfish is modulated by cortisol-induced changes in both Rh glycoprotein expression and GS activity.

Evidence for transcriptional regulation of the urea transporter in the gill of the Gulf toadfish, Opsanus beta

Ureotelic Gulf toadfish (Opsanus beta) do not excrete urea continuously; instead, urea is accumulated internally until a branchial urea transport mechanism is activated to facilitate the excretion of urea in distinct pulses. This unusual pulsatile urea excretion pattern is regulated, in part, by permissive declines in circulating cortisol concentrations. The current study examined toadfish urea transporter (tUT) and glucocorticoid receptor (GR) transcript levels in toadfish gill following chronic (days) and acute (hours) changes in corticosteroid activity. Experimentally lowering circulating cortisol did not significantly alter tUT mRNA abundance but increased GR mRNA. On an acute timescale, a 6.2-fold upregulation of tUT mRNA occurred 12 to 18 h following a urea pulse event with no change in GR mRNA. In silico analysis of an isolated 1.2 kb fragment, upstream promoter region of the tUT gene, revealed 6 putative glucocorticoid response element (GRE) half sites. In vivo reporter assays of the tUT promoter fragment demonstrated relative luciferase activity was enhanced 3.4- and 9.8-fold following exposure to moderate (via a 48 h crowding stress) and high (via infusion for 48 h) cortisol. We conclude that a GRE-mediated upregulation of mRNA may be required to maintain tUT activity by offsetting post-transcriptional and/or post-translational changes that may be associated with chronically elevated plasma cortisol.

Higher levels of aggression are observed in socially dominant toadfish treated with the selective serotonin reuptake inhibitor, fluoxetine

The following study set out to test the hypothesis that acute treatment with the selective serotonin reuptake inhibitor, fluoxetine, would result in a rise in circulating 5-HT levels and consequently a decrease in territorial aggression in the Gulf toadfish, Opsanus beta. Size-matched pairs of toadfish were implanted intraperitoneally with the same dose of fluoxetine (0, 10 or 25 μg g⁻¹). After a social interaction between a pair of fish, circulating levels of serotonin (5-HT; 5-hydroxytryptamine) and cortisol were measured and relative mRNA expression of the 5-HT(1A) receptor in the toadfish brain was determined using quantitative (real-time) PCR (qPCR). Behavioral endpoints such as the number of aggressive acts and swimming activity were also quantified so that dominant and subordinate fish could be identified. Fluoxetine treatment resulted in an increase in circulating levels of 5-HT, regardless of social status. Circulating cortisol concentrations were unaffected by fluoxetine, but were significantly higher in subordinate individuals when compared to dominant fish. Toadfish brain 5-HT(1A) receptor mRNA expression was not affected by treatment or social status. Lastly and contrary to our predictions, fluoxetine treatment resulted in an increase in the number of aggressive acts made by dominant individuals, with no differences in the level of aggression or swimming activity of subordinate fish. This study is the first to describe elevated aggression in a teleost fish with elevated circulating levels of 5-HT.

Fluoxetine treatment affects nitrogen waste excretion and osmoregulation in a marine teleost fish

Measurable quantities of the selective serotonin reuptake inhibitor (SSRI), fluoxetine, have been found in surface waters and more recently in the tissues of fish. This highly prescribed pharmaceutical inhibits the reuptake of the monoamine, serotonin (5-HT; 5-hydroxytryptamine), causing a local amplification of 5-HT concentrations. Serotonin is involved in the regulation of many physiological processes in teleost fish including branchial nitrogen excretion and intestinal osmoregulation. Since the gill and intestine are directly exposed to the environment, environmental exposure to fluoxetine has the potential of affecting both these mechanisms. In the present study, we test the potential sensitivity of these processes to fluoxetine by implanting gulf toadfish, Opsanus beta, intraperitoneally with different concentrations of fluoxetine (0 (control), 25, 50, 75 and 100 microgg(-1). Fluoxetine treatments of 25 and 50 microgg(-1) were sublethal and were used in subsequent experiments. Fish treated with both 25 and 50 microgg(-1) fluoxetine had significantly higher circulating levels of 5-HT than control fish, suggesting that any 5-HT sensitive physiological process could potentially be affected by these two fluoxetine doses. However, only fish treated with 25 microgg(-1) fluoxetine showed a significant increase in urea excretion. A similar increase was not measured in fish treated with 50 microgg(-1) fluoxetine, likely because of their high circulating levels of cortisol which inhibits urea excretion in toadfish. Intestinal fluid absorption appeared to be stimulated in fish treated with 25g microgg(-1) fluoxetine but inhibited in 50 microgg(-1) treated fish. Despite these differing responses, both doses of fluoxetine resulted in lowered plasma osmolality values, which was expected based on the stimulation of fluid absorption in the 25 microgg(-1) fluoxetine-treated fish but is surprising with the 50 microgg(-1) treated fish. In the case of the latter, the corresponding stress response invoked by this level of fluoxetine may have resulted in an additional osmoregulatory response which accounts for the lowered plasma osmolality. Our findings suggest that branchial urea excretion and intestinal osmoregulation are responsive to the SSRI, fluoxetine, and further investigation is needed to determine the sensitivity of these processes to chronic waterborne fluoxetine contamination.

Fluoxetine treatment affects nitrogen waste excretion and osmoregulation in a marine teleost fish

Measurable quantities of the selective serotonin reuptake inhibitor (SSRI), fluoxetine, have been found in surface waters and more recently in the tissues of fish. This highly prescribed pharmaceutical inhibits the reuptake of the monoamine, serotonin (5-HT; 5-hydroxytryptamine), causing a local amplification of 5-HT concentrations. Serotonin is involved in the regulation of many physiological processes in teleost fish including branchial nitrogen excretion and intestinal osmoregulation. Since the gill and intestine are directly exposed to the environment, environmental exposure to fluoxetine has the potential of affecting both these mechanisms. In the present study, we test the potential sensitivity of these processes to fluoxetine by implanting gulf toadfish, Opsanus beta, intraperitoneally with different concentrations of fluoxetine (0 (control), 25, 50, 75 and 100 microgg(-1)). Fluoxetine treatments of 25 and 50 microgg(-1) were sub-lethal and were used in subsequent experiments. Fish treated with both 25 and 50 microgg(-1) fluoxetine had significantly higher circulating levels of 5-HT than control fish, suggesting that any 5-HT sensitive physiological process could potentially be affected by these two fluoxetine doses. However, only fish treated with 25 microgg(-1) fluoxetine showed a significant increase in urea excretion. A similar increase was not measured in fish treated with 50 microgg(-1) fluoxetine, likely because of their high circulating levels of cortisol which inhibits urea excretion in toadfish. Intestinal fluid absorption appeared to be stimulated in fish treated with 25 microgg(-1) fluoxetine but inhibited in 50 microgg(-1) treated fish. Despite these differing responses, both doses of fluoxetine resulted in lowered plasma osmolality values, which was expected based on the stimulation of fluid absorption in the 25 microgg(-1) fluoxetine-treated fish but is surprising with the 50 microgg(-1) treated fish. In the case of the latter, the corresponding stress response invoked by this level of fluoxetine may have resulted in an additional osmoregulatory response which accounts for the lowered plasma osmolality. Our findings suggest that branchial urea excretion and intestinal osmoregulation are responsive to the SSRI, fluoxetine, and further investigation is needed to determine the sensitivity of these processes to chronic waterborne fluoxetine contamination.