Characterisation of plasmatic B-esterases in bottlenose dolphins (Tursiops truncatus) and their potential as biomarkers of xenobiotic chemical exposures

A total of 164 blood samples from 16 clinically healthy bottlenose dolphins (Tursiops truncatus), were obtained from an aquarium in Spain between 2019 and 2020, as part of their preventive medicine protocol. In addition to conventional haematological and biochemical analyses, plasmatic B-esterase activities were characterised to determine the potential application of such analyses in wild counterparts. The hydrolysis rates for the substrates of acetylcholinesterase (AChE), butyrylcholinesterase (BuChE) and carboxylesterase (CE) activity in plasma were measured, the last using two commercial substrates, p-nitrophenyl acetate (pNPA) and p-nitrophenyl butyrate (pNPB). Activity rates (mean ± SEM in nmol/min/mL plasma) were (in descending order): AChE (125.6 ± 3.8), pNPB-CE (65.0 ± 2.2), pNPA-CE (49.7 ± 1.1) and BuChE (12.8 ± 1.3). These values for dolphins are reported in here for the first time in this species. Additionally, the in vitro sensitivity of two B-esterases (AChE and pNPB-CE) to chemicals of environmental concern was determined, and the protective role of plasmatic albumin assessed. Out of the B-esterases measured in plasma of dolphin, AChE activity was more responsive in vitro to pesticides, while CEs had a low response to plastic additives, likely due to the protective presence of albumin. However, the clear in vitro interaction of these environmental chemicals with purified AChE from electric eels and recombinant human hCEs (hCE1 and hCE2) and albumin, predicts their impact in other tissues that require in vivo validation. A relationship between esterase-like activities and health parameters in terrestrial mammals has already been established. Thus, B-esterase measures could be easily included in marine mammal health assessment protocols for dolphins as well, once the relationship between these measures and the animal's fitness has been established.

Effects of ibuprofen and carbamazepine on the ion transport system and fatty acid metabolism of temperature conditioned juveniles of Solea senegalensis

The increasing presence of pharmaceuticals in aquatic environments in the last decades, derived from human and veterinary use, has become an important environmental problem. Previous studies have shown that ibuprofen (IB) and carbamazepine (CBZ) modify physiological and biochemical processes in Senegalese sole (Solea senegalensis) in a temperature-dependent manner. In other vertebrates, there is evidence that both of these pharmaceuticals interfere with the 'arachidonic acid (AA) cascade', which is responsible for the biosynthesis of numerous enzymes that are involved in the osmoregulatory process. The present work aims to study the temperature-dependent effects of these two pharmaceuticals on several biochemical and molecular parameters in Senegalese sole. Regarding osmoregulation, Na⁺, K⁺ -ATPase enzyme activity was determined in the gills, kidney and intestine, and the expressions of both Na⁺, K⁺ -ATPase 1α-subunit isoforms (ATP1A1a and ATP1A1b) were quantified in gills. Gill prostaglandin-endoperoxide synthase-2 (PTGS2) gene expression and fatty acid composition were selected to determine the interference of both pharmaceuticals with the AA cascade. Senegalese sole juveniles, acclimatised at 15°C or 20°C, were exposed through intraperitoneal injection to IB (10mg/kg) and CBZ (1mg/kg) for 48h. Non-injected fish (Control) and those injected with the carrier (sunflower oil; S.O.), acclimated at each of the two temperatures, were used for comparison. The results show that IB directly affected the osmoregulatory mechanisms that alter gill and intestine Na⁺, K⁺ -ATPase activities. In addition, the copy number of ATP1A1a was higher at 20°C than at 15°C, which could be a direct response to the temperature variation. The gene expression of PTGS2 was affected by neither drug administration nor acclimation temperature. Nevertheless, detailed analysis of AA and eicosapentaenoic acid (EPA) percentages revealed a CBZ-derived effect in the fatty acid composition of the gills.

Vitellogenin, sex steroid levels and gonadal biomarkers in wild Solea solea and Solea senegalensis from NW Mediterranean fishing grounds

Specimens of Solea solea and Solea senegalenesis at different developmental stages were obtained from seven fishing grounds along the NW Mediterranean. Gonad development in males was classified into five stages, from early spermatogenesis to recovery, while four stages were considered in females, from growth to maturation. Vitellogenin (VTG) and sex steroid levels including an estrogen (estradiol, E2), two androgens (testosterone, T and 11-ketotestosterone, 11KT) and a progestin (17,20β-dihydroxy pregn-4-en-3-one, 17,20β-P or maturation inducing steroid, MIS) were analysed in plasma. Their levels were more clearly related to the developmental stage of the gonads than to the sampling site characteristics. In addition, enzyme activities in gonads, such as acetylcholinesterase (AChE) and carboxylesterase (CbE) were gender-dependent and higher in males than in females. Gonadal glutathione S-transferase (GST) activity was enhanced in the most anthropogenic impacted sites. VTG was absent in males and very low or undetectable in immature females, while mature females exhibited high VTG levels, clearly related to the gonado-somatic index. Sex steroid levels (ng/ml) varied in males and females regardless of the species. E2 levels in females ranged from 0.22 to 6.98 while in males ranged from 0.11 to 0.27. T varied from 0.12 to 0.93 in females and from 0.56 to 1.36 in males, while 11KT in females fluctuated from 0.03 to 0.57 and from 0.26 to 6.42 in males. Similarly, MIS in females ranged from 0.75 to 3.71 and from 1.12 to 5.61 in males. The lack of endocrine disturbances was confirmed by histological examination of the gonads. This study informs on basal sex hormone levels and enzyme activities during gonadal maturation of wild Solea spp. that can be useful in the identification and further remediation of possible pollution events.

Effects of selected xenobiotics on hepatic and plasmatic biomarkers in juveniles of Solea senegalensis

In recent years, Solea senegalensis has increasingly been used in pollution monitoring studies. In order to assess its response to some particular widespread pollutants, juveniles of S. senegalensis were administered an intraperitoneal injection of the model aryl hydrocarbon receptor agonist β-naphtoflavone (βNF) and chemicals of environmental concern, such as the fungicide ketoconazole (KETO), the lipid regulator gemfibrozil (GEM), the surfactant nonylphenol (NP) and the synthetic hormone ethinylestradiol (EE2). Two days after injection, the effect of these chemicals was followed up as alterations of hepatic microsomal activities of the cytochrome P450 (CYPs) and associated reductases, carboxylesterases (CbEs) and the conjugation enzyme uridine diphosphate glucuronyltransferase (UDPGT). In the cytosolic fraction of the liver, the effect on CbEs, glutathione S-transferase (GST) and antioxidant activities was also considered. Alterations on the endocrine reproductive system were evaluated by plasma levels of vitellogenin (VTG) and the sex steroids estradiol (E2), testosterone (T), 11-ketotestosterone (11KT) and the progestin 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P). Injection with the model compound βNF induced the hydrolysis rate of the seven CYP substrates assayed. The xenobiotic GEM induced three CYP-related activities (e.g. ECOD) and UDPGT, but depressed antioxidant defenses. EE2 induced four CYPs, more significantly ECOD and BFCOD activities. The xenoestrogens NP and EE2 altered the activities of CbE in microsomes and catalase, and were the only treatments that induced de novo VTG synthesis. In addition, the progestin 17,20β-P, was induced in NP-injected fish. None of the treatments caused statistically significant effects on steroid plasma levels. In conclusion, the CYP substrates assayed responded specifically to treatments and juveniles of S. senegalensis appear good candidates for assessing xenobiotics exposure.

Effects of graded levels of arachidonic acid on the reproductive physiology of Senegalese sole (Solea senegalensis): Fatty acid composition, prostaglandins and steroid levels in the blood of broodstock bred in captivity

Previous studies on Senegalese sole (Solea senegalensis) indicated that cultured broodstock (first generation, G1) have lower tissue levels of arachidonic acid (20:4n-6, ARA) than wild counterparts. ARA is metabolized to form prostaglandins (PGs) that are involved in steroid production and follicle maturation in fish. In the present study the effects of different dietary levels of ARA on blood lipid and fatty acid composition, prostaglandin (PGF2α, PGF3α, PGE2 and PGE3) levels and plasmatic steroid levels (11-ketotestosterone, 11-KT, testosterone, T and estradiol, E2) in G1 Senegalese sole were studied. For this purpose, 12 groups of ten fish (1:1 male and female), were fed six diets (each diets was fed to two groups) with different dietary ARA levels over nine months (diets A=0.7, B=1.6, C=2.3, D=3.2, E=5.0, F=6.0% ARA). ARA and CHOL levels in blood showed a significant increase in an ARA dose related manner (P<0.05) whereas EPA and EPA/ARA ratio were reduced. In males, steroid (11-KT and T) levels increased significantly with increasing dietary ARA in a dose dependent manner, whereas in females E2 did not show any change related to dietary ARA content. Plasma concentration of 3-series PGs (i.e., PGE3 and PGF3α) were reduced in parallel to increased ARA levels in blood (P<0.05) and levels of PGs 3-series were always higher than 2-series PGs (PGE2 and PGF2α). In conclusion there is an effect of dietary ARA on steroid production of Senegalese sole males, which might have important consequences in the reproduction of cultured fish.

Exposure of larvae to daily thermocycles affects gonad development, sex ratio, and sexual steroids in Solea senegalensis, kaup

The effect of water temperature during the development of fish larvae on sex differentiation is well known, but not so well known is the impact of the daily thermocycles. Our aim was to investigate the effect of early exposure of Senegal sole larvae to different temperature cycles on gonad development, sex ratio, and sex steroid (11-ketotestosterone (11-KT); estradiol (E(2) ); and testosterone, (T)) content in muscle extracts of juveniles. From 1 to 97 days posthatching (DPH) fish larvae and post-larvae were subjected to three temperature regimes: Thermophase-Cryophase (TC), Cryophase-Thermophase (CT), and constant temperature. In fish exposed to TC, sex determination occurred earlier, because 90% of soles were males/females at 110 DPH, whereas 45% of fish under CT were undifferentiated at that time. Fish under TC showed the highest growth rates, followed by fish under constant temperature and by fish under CT, the differences being statistically significant between the TC and CT groups. Regarding sex ratio, juveniles exposed to TC showed a higher proportion of females than fish under CT or constant temperature. Under TC, fish showed the highest concentration of E(2) , whereas 11-KT concentration was highest in fish under CT and constant temperature. Fish under constant temperature and CT showed higher T levels than those under TC. These results provide the first insights into the effect of daily thermocycles on sex differentiation in fish, and underline the key role of natural environmental cycles on the control of sex ratios during larval development, which may be applied to the manipulation of sex ratio in aquaculture.

Monthly day/night changes and seasonal daily rhythms of sexual steroids in Senegal sole (Solea senegalensis) under natural fluctuating or controlled environmental conditions

In this paper we attempted to investigate the existence of daily fluctuations on plasma sexual steroids (17beta-estradiol, E(2) and testosterone, T) in Senegal sole (Solea senegalensis) females. We described the monthly day/night concentrations and seasonal daily rhythms in animals reared under natural photo- and thermo-period. In addition, the influence of the natural annual fluctuation of the water temperature on the plasma concentration of these steroids was investigated, using one group of Senegal sole under a natural photoperiod, but with an attenuated thermal cycle (around 17-20 degrees C) for one year. Although no significant day/night differences were detected in monthly samplings, the existence of an annual rhythm of E(2) and T (p<0.01) with an acrophase in February was revealed by COSINOR analysis. Maximum values were reached in March for both steroids (6.1+/-1.7 ng mL(-1) at mid-dark, MD and 4.0+/-0.6 ng mL(-1) at mid-light, ML for E2 and 1.4+/-0.4 ng mL(-1) at MD and 0.8+/-0.1 ng mL(-1) at ML for T) in anticipation of the spawning season (May-June). As regards seasonal daily rhythms, the presence of daily oscillations was revealed. At the spring solstice (21st March) a daily rhythm was observed for both steroids (COSINOR, p<0.01), with an acrophase at 20:00 h (E(2)) and at 21:08 h (T). In summer, autumn and winter no daily rhythms were observed due to the low steroid levels at those seasons. When Senegal sole females were submitted to an attenuated annual thermal cycle, the steroid rhythm disappeared (there was no surge in spring, as in the control group) and these fish did not spawn, despite being subjected to natural photoperiod conditions. This result underlined the importance of the natural annual fluctuation of water temperature and photoperiod on the synchronization of the spawning season and on the onset of steroidogenesis.

Mixtures of estrogenic chemicals enhance vitellogenic response in sea bass

BACKGROUND

The potential impact of natural and synthetic estrogens on aquatic ecosystems has attracted considerable attention because it is currently accepted that their joint effects are more severe when they are present in mixtures. Although it is well-known that they occur as mixtures in the marine environment, there is little information about the combined effects of estrogenic chemicals on marine biota.

OBJECTIVE

In 14-day tests with juvenile sea bass, we analyzed singly and in combination the estrogenic activity of estradiol (E(2)), ethynylestradiol (EE(2)), and bisphenol A (BPA) using vitellogenin induction as an end point.

METHODS

Fish were exposed to each compound, and on the basis of these concentration-response data, we predicted mixture effects by applying the model of concentration addition. The mixtures were tested using a fixed-ratio design, and the resulting mixture effects were compared to the predictions.

RESULTS

EE(2) was the most potent steroid, with an EC(50) (median effective concentration) of 0.029 microg/L, 3.6 times more potent than E(2) (EC(50) = 0.104 microg/L); BPA was the least potent chemical, with an EC(50) of 77.94 microg/L. The comparative assessment yielded a good agreement between observed and predicted mixture effects.

CONCLUSIONS

This study demonstrates the potential hazard of these compounds to seawater life by their ability to act together in an additive manner. It provides evidence that concentration addition can be used as a predictive tool for assessing the combined effects of estrogenic chemicals in marine ecosystems.

Seasonal and daily plasma melatonin rhythms and reproduction in Senegal sole kept under natural photoperiod and natural or controlled water temperature

The melatonin daily rhythm provides the organism with photoperiod-related information and represents a mechanism to transduce information concerning time of day. In addition, the duration and amplitude of the nocturnal elevation gives information about duration and thus the time of year. In this study, we investigate the existence of an annual rhythm of plasma melatonin in the Senegal sole. Differences in plasma melatonin levels between fish kept at a controlled temperature (17-20 degrees C) and those exposed to the environmental temperature cycle (11.5-25 degrees C) were also examined throughout the year. Spawning was registered in both groups to determine the time of year in which reproductive rhythms occurred. Our results pointed to the existence of an annual rhythm of plasma melatonin at mid-darkness (MD), with the highest levels (203 +/- 44 pg/mL) observed when water temperature reached 25 degrees C. Water temperature influenced nocturnal, but not diurnal melatonin. Daily melatonin rhythms showed seasonal differences, with higher mean nocturnal levels during the summer solstice (138 +/- 19 pg/mL) and autumn equinox (149 +/- 49 pg/mL). When animals were kept at a constant temperature throughout the year, plasma melatonin levels differed from those observed in fish exposed to the environmental temperature cycle. Regarding the reproductive rhythms, spawning was observed at the end of spring in sole kept under natural temperature conditions, whereas no spawning at all was registered in sole reared at a constant temperature. In short, both photoperiod and temperature affected melatonin production in the Senegal sole, transducing seasonal information and controlling annual reproductive rhythms.

Temporal profile of brain and pituitary GnRHs, GnRH-R and gonadotropin mRNA expression and content during early development in European sea bass (Dicentrarchus labrax L.)

A likely endocrine control mechanism for sexual differentiation in size-graded populations of European sea bass (Dicentrarchus labrax) is proposed by evaluating the brain expression and pituitary content of two forms of gonadotropin-releasing hormone (GnRH), namely sea bream (sbGnRH) and salmon (sGnRH), the pituitary expression of one subtype of GnRH receptor (dlGnRH-R-2A) and the three gonadotropin (GtH) subunits, namely glycoprotein alpha (GPalpha), follicle-stimulating hormone beta (FSHbeta) and luteinizing hormone beta (LHbeta), as well as the pituitary and plasma LH levels between 50 and 300 days post-hatching (dph). Four gradings were conducted between 2 and 8 months after hatching, resulting in a population of large and small individuals, having 96.5% females (female-dominant population) and 69.2% males (male-dominant population), respectively, after the last grading. The onset of gonadal differentiation was different in the two sexes, and coincided with a peak of expression of sbGnRH or sGnRH. Furthermore, the expression of these GnRHs was correlated with the expression of dlGnRH-R-2A. Sex-related differences in the brain and pituitary content of sbGnRH were also found at the time of sexual differentiation. Moreover, the observed sexual dimorphism at the transcriptional or synthesis level of these GnRH forms suggests that a different neuro-hormonal regulation is operating according to sex. At the onset of sex differentiation, FSHbeta transcriptional activity reached maximal values, which were maintained until the completion of the process. The present study suggests a role for sbGnRH, sGnRH and the dlGnRH-R-2A during gonadal differentiation, possibly through enhancement of FSHbeta gene expression. In males, a different endocrine regulation seems to exist also during spermiogenesis and spermiation, when gene transcription, peptide synthesis and release of LH are of greater importance.

Molecular characterization of sea bass gonadotropin subunits (alpha, FSHbeta, and LHbeta) and their expression during the reproductive cycle

Reproduction is controlled by two pituitary gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This study reports the cloning, sequence analysis, and gene expression of gonadotropin (GTH) subunits from the European sea bass (Dicentrarchus labrax). The GTH subunits were cloned from a sea bass brain-pituitary cDNA library. The nucleotide sequences of the common alpha, the FSHbeta, and the LHbeta subunit cDNAs were 625, 521, and 591 base pair (bp) long, respectively, encoding for mature peptides of 94, 105, and 115 amino acids (aa), respectively. Sequence analysis showed that sea bass FSHbeta is more similar to higher vertebrate FSHbeta's (35-37%) than to LHbeta's (26-30%), whereas sea bass LHbeta is more similar to LHbeta's (40-53%) than to FSHbeta's (26-41%). Phylogenetic analysis of fish GTH sequences grouped the beta subunits into two groups, FSH and LH, distributed into four classes, corresponding to the accepted divisions of Elopomorphs, Ostariophysis, Salmonids, and Percomorphs. A dot-blot technique was developed to analyze GTH pituitary mRNA levels during the reproductive cycle of male sea bass. From October (initiation of gametogenesis) to February (spermiation), the expression of all three subunits in the pituitary increased in parallel, concomitantly with the gonadosomatic index (GSI) and the accumulation of LH protein in the pituitary, all values declining sharply at post-spermiation. This study demonstrates that the pituitary of sea bass contains two gonadotropin hormones and that both gonadotropins are probably involved in the control of gametogenesis, gamete maturation, and spermiation.

Pituitary levels of three forms of GnRH in the male European sea bass (Dicentrarchus labrax, L.) during sex differentiation and first spawning season

In the present study, levels of three GnRH forms [seabream GnRH (sbGnRH), chicken GnRH-II (cGnRH-II), and salmon GnRH (sGnRH)] were analyzed in the pituitary of male sea bass during sex differentiation and the first spawning season. Plasma levels of gonadotropin (GTH-2), testosterone (T), and 11-ketotestosterone (11-KT) were determined during the same periods. All GnRH forms were present in the pituitary. sbGnRH levels were 9-fold higher than cGnRH-II and 17-fold higher than sGnRH levels. The highest GnRHs levels were detected in November 1995, when fish were 9 months old and when the gonads started to differentiate. Levels of the three forms decreased and remained low during the first spawning season, with the exception of sbGnRH, which showed a significant increase in November 1996. Plasma GTH-2 levels were lowest in November 1995, later increasing 2.5 times during the next months. During the first spawning season, plasma GTH-2 levels peaked in December 1996, 1 month after the peak of sbGnRH. During sex differentiation, plasma T levels were high in November 1995 but decreased over the next months, while levels of 11-KT remained low and unchanged. During the first spawning season, both steroids peaked in January 1997. These results suggest a possible role for all three GnRH forms in achieving gonadal differentiation, while sbGnRH may be the most relevant form in the regulation of the first spawning season in male sea bass. Moreover, GTH-2 and 11-KT may play important roles in gonadal maturation, since plasma GTH-2 and 11-KT levels were high throughout the period of spermiation.