Stress-responsivitet hos laksefisk : effekter på gener, organer og organismer

Stress is an inescapable burden for fish like for all other animals. For individuals that frequently encounter stressful situations, repeated or prolonged physiological stress responses, can potentially compromise the physiological and psychological basis of that organism`s health and welfare. In particular, stress has been shown to adversely affect important biological systems such as the immune system, the central nervous system (CNS) and the cardiovascular system (CVS). The impact of stress largely depends on the inherent stress coping style of the affected individual. Stress coping style can be defined by the set of behavioral and physiological responses to stress that is consistently employed by one individual across unrelated and temporally separated situations. One such physiological stress response, which is highly heritable, yet subject to great individual variation, is the cortisol response. Cortisol, an interrenal/adrenal steroid stress hormone, is responsible for most stressrelated diseases in man as well as in fish. High heritability of the cortisol response has allowed for the generation of two strains of rainbow trout (Oncorhynchus mykiss) that differ consistently in stress-induced cortisol production and consequently in morphology, endocrinology, behavior and cognition. Lowresponding (LR) fish were initially selected for low post-stress cortisol levels and subsequently display proactive behaviors and endocrine profiles. High-responding (HR) fish, on the other hand, were selected for high post-stress cortisol levels and display reactive behaviors and endocrine profiles. The phenomenon of contrasting stress coping styles is attracting considerable scientific and public interest. Particularly, the recognition of individual variation in disease vulnerability has encouraged scientists to try to elucidate the biology behind stress coping. In this context, the LR-HR model serves as an excellent model to study the proximate mechanisms behind genetically linked behavioral and physiological trait characteristics. In this work we aimed at investigating CNS plasticity and cardiac remodeling by integrating studies at the molecular (genes), physiological-anatomical (organ) and behavioral (organism) levels in the context of divergent stress coping styles. In particular, we investigated putative CNS mechanisms controlling behavior and memory retention (i.e. mRNA expression of genes involved in the cortisol response, postembryonic neurogenesis and neuronal plasticity) in stressed and non-stressed LR and HR rainbow trout. Further, the association between cortisol responsiveness and cardiac morphology (i.e. size, composition and collagen depositions) and gene expression (i.e. mRNA expression of specific markers of cardiac remodeling) in LR and HR rainbow trout was explored. Finally, we investigated if a trait correlation between cortisol responsiveness and heart size also existed in wild-type European brown trout (Salmo trutta). We discovered that the expression of central cortisol receptors was affected by heritable variation in stress coping (i.e. LR vs. HR) and downregulated by stress in a brain region-specific manner. We also found that the expression of genes involved in CNS plasticity was affected by heritable variation in stress coping and also differentially affected by short – and long-term stress. Lastly, we show that high cortisol responsiveness was associated with cardiac remodeling (i.e. growth) in HR fish and in high cortisol-responding wild-type brown trout. Further, the cardiac growth in HR fish appeared to be caused mainly by hypertrophic growth of the compact myocardium. This growth was accompanied by focal collagen depositions and a high expression of genes involved in hypertrophy, development of fibrosis and the cortisol response. The latter indicates that cortisol is directly mediating the cardiac remodeling in HR trout. Combined, the results showing that the expression of central cortisol receptors and genes involved in CNS plasticity differed between LR and HR trout, can contribute to increased knowledge about the proximate mechanisms behind genetically linked physiological and behavioral trait characteristics. Also, the strong association between cortisol responsiveness and cardiac remodeling in two salmonid species suggests that cortisol might be a general causative factor in salmonid cardiac disease. The presence of this anatomicalphysiological trait correlation in a wild population of salmonids, also suggests an ecologicalevolutionary role for individually variable heart function.Stress er et uunngåelig onde for fisk så vel som for alle andre dyregrupper. For individer som ofte opplever stressende episoder, kan gjentatte og/eller langvarige fysiologiske stressresponser potensielt kompromittere det fysiologiske og psykologiske grunnlaget for helse og velferd. Spesielt har stress vist seg å ha skadelige effekter på viktige biologiske systemer som sentralnervesystemet (SNS) og det kardiovaskulære systemet (KVS). Virkningen av stress avhenger i stor grad av det berørte individets iboende stressmestringsevne. En stressmestringsstrategi kan defineres ut ifra de atferdsmessige og fysiologiske stressresponsene som konsekvent anvendes av et individ på tvers av urelaterte og tidsavskilte situasjoner. Et eksempel på en slik fysiologisk respons, som er svært arvelig, dog gjenstand for betydelig individuell variasjon, er kortisol-responsen. Kortisol, et stresssteroidhormon produsert i interrenale celler/binyrene, er ansvarlig for de fleste stress-relaterte sykdommer hos fisk og hos menneske. Det faktum at kortisol-responsivitet er svært arvelig, har gjort det mulig å utvikle to stammer av regnbueørret (Oncorhynchus mykiss) som konsekvent responderer på stress med forskjellige kortisol-nivåer. De to stammene viser også forkjeller i atferd, morfologi, endokrinologi og kognisjon. Lav-responsive (LR)-fisk ble opprinnelig selektert for en lav kortisol-respons etter stress og viser proaktiv atferd og fysiologi. Høy-responsive (HR)-fisk, derimot, ble selektert for en høy kortisol-respons etter stress og har en reaktiv atferd og fysiologi. Fenomenet ”stressmestringsstrategi” vekker betydelig vitenskapelig og allmenn interesse. Spesielt har anerkjennelsen av individuell variasjon i sykdomssårbarhet oppmuntretforskere til å forøke å belyse biologien bak forkjeller i stressmestring. I denne sammenheng fungerer LR-HR-modellen som en utmerket modell for å studere proksimate mekanismer bak genetisk knyttede atferdsmessige og fysiologiske trekk. I dette arbeidet hadde vi som mål å undersøke SNS-plastisitet og remodellering av hjertet ved å integrere studier på det atferdsmessige (organismer), fysiologisk-anatomiske (organer) og molekylære (genregulering) plan i sammenheng med divergerende stressmestringsstrategier. Mer presist undersøkte vi antatte SNS-mekanismer som kontrollerer atferd og minneretensjon (dvs. mRNA-uttrykk av gener involvert i kortisol-responsen, postembryonal nevrogenese og nevronal plastisitet) i stressede og ustressede LR – og HR-regnbueørret. Vi har også undersøkt sammenhengen mellom kortisol-responsivitet og hjerte-morfologi (dvs. størrelse, komposisjon og kollagen-avsetninger) og gen-uttrykk av spesifikke markører for hjerte-remodellering i LR – og HR-regnbueørret. Til sist undersøkte vi om det eksisterte en korrelasjon mellom kortisol-responsivitet og hjertestørrelse også i vill-type brunørret (Salmo trutta). Vi viser at uttrykket av sentrale kortisol-reseptorer var påvirket av arvelig variasjon i stressmestring og ble nedregulert av stress avhengig av hjernedel. Vi viser også at uttrykket av gener involvert i SNS-plastisitet var påvirket av arvelig variasjon i stressmestring (LR vs. HR), men også ulikt berørt av korttids – og langtidsstress. Vi viser også at en høy kortisolresponsivitet var assosiert med remodellering av hjertet (dvs. vekst) i HR fisk og i vill-type brunørret med en høy kortisol-respons. Videre viser vi at hjerteveksten i HR-fisk hovedsakelig skyldtes hypertrofisk vekst av kompakt myokardium. Hjerteveksten var også ledsaget av fokale kollagenavsetninger og et høyt uttrykk av gener involvert i hypertrofi, utvikling av fibrose og kortisol-responsen. Sistnevnte indikerer at kortisol direkte medierer remodellering av hjertet i HR-ørret. Våre resultater, som viser at uttrykket av sentrale kortisol-reseptorer og gener involvert i SNS-plastisitet varierer mellom LR – og HR- ørret, kan potensielt bidra til økt kunnskap om proksimate mekanismer bak genetisk knyttede fysiologiske og atferdsmessige trekk. Dessuten indikerer en sterk assosiasjon mellom kortisol-responsivitet og remodellering av hjertet, i to arter av laksefisk, at kortisol kan være en medvirkende faktor i utvikling av hjertesykdom hos salmonider. Tilstedeværelsen av denne anatomisk-fysiologiske sammenhengen i en vill populasjon av laksefisk, kan tyde på en økologisk-evolusjonær rolle for individuell variasjon i hjertefunksjon

Kindness to the Final Host and Vice Versa: A Trend for Parasites Providing Easy Prey?

Traditionally the “extended phenotype” concept refers to parasites that manipulate host phenotype to increase parasite fitness. This includes parasites that render intermediate hosts more susceptible to predation by final hosts. We explore here the proposition that an evolutionary driver in such cases is the energetic benefit to the final host, in addition to increased parasite fitness. We will review some well-established host-manipulation models, where such a scenario seems likely. One example is provided by the protozoan Toxoplasma gondii, which conspicuously impairs predator avoidance in rodents. Pathologies in humans that acquire T. gondii are known, but infection in adult feline definitive hosts are most commonly asymptomatic and apparently innocuous. In another well-documented case of parasite-mediated trophic transmission, trematode (Euhaplorchis californiensis) infected killifish (Fundulus parvipinnis) abandon normal caution and exhibit a range of behaviors which makes them more conspicuous to predatory birds. The birds get a free meal, but the presence of adult trematodes in the gut would seem to incur few if any negative consequences for the birds. There are exceptions to this pattern also among cases of parasite mediated trophic transmission, but major pathology in definitive hosts seems for the most part restricted to cases where manipulated intermediate hosts are of minor energetic importance. Current theories for the evolution of reduced pathogenicity in predatory final hosts primarily focus on parasites minimizing pathogenicity to increase their own reproductive output and/or avoid selection on host preference for non-infected prey types. Here we advocate another alternative: If or when the benefit of increased prey acquisition outweighs pathogenicity or resource drain, there should be little or no selection on final hosts to minimize parasite infections. This means that not only will host avoidance of infection not develop, but the molecular arms race for increased immunological defense will also likely be halted in such cases

Behavioural effects of the common brain-infecting parasite Pseudoloma neurophilia in laboratory zebrafish (Danio rerio)

Research conducted on model organisms may be biased due to undetected pathogen infections. Recently, screening studies discovered high prevalence of the microsporidium Pseudoloma neurophilia in zebrafish (Danio rerio) facilities. This spore-forming unicellular parasite aggregates in brain regions associated with motor function and anxiety, and despite its high occurrence little is known about how sub-clinical infection affects behaviour. Here, we assessed how P. neurophilia infection alters the zebrafish´s response to four commonly used neurobehavioral tests, namely: mirror biting, open field, light/dark preference and social preference, used to quantify aggression, exploration, anxiety, and sociability. Although sociability and aggression remained unaltered, infected hosts exhibited reduced activity, elevated rates of freezing behaviour, and sex-specific effects on exploration. These results indicate that caution is warranted in the interpretation of zebrafish behaviour, particularly since in most cases infection status is unknown. This highlights the importance of comprehensive monitoring procedures to detect sub-clinical infections in laboratory animals.publishedVersio

Kindness to the final host and vice versa: A trend for parasites providing easy prey?

Traditionally the “extended phenotype” concept refers to parasites that manipulate host phenotype to increase parasite fitness. This includes parasites that render intermediate hosts more susceptible to predation by final hosts. We explore here the proposition that an evolutionary driver in such cases is the energetic benefit to the final host, in addition to increased parasite fitness. We will review some well-established host-manipulation models, where such a scenario seems likely. One example is provided by the protozoan Toxoplasma gondii, which conspicuously impairs predator avoidance in rodents. Pathologies in humans that acquire T. gondii are known, but infection in adult feline definitive hosts are most commonly asymptomatic and apparently innocuous. In another well-documented case of parasite-mediated trophic transmission, trematode (Euhaplorchis californiensis) infected killifish (Fundulus parvipinnis) abandon normal caution and exhibit a range of behaviors which makes them more conspicuous to predatory birds. The birds get a free meal, but the presence of adult trematodes in the gut would seem to incur few if any negative consequences for the birds. There are exceptions to this pattern also among cases of parasite mediated trophic transmission, but major pathology in definitive hosts seems for the most part restricted to cases where manipulated intermediate hosts are of minor energetic importance. Current theories for the evolution of reduced pathogenicity in predatory final hosts primarily focus on parasites minimizing pathogenicity to increase their own reproductive output and/or avoid selection on host preference for non-infected prey types. Here we advocate another alternative: If or when the benefit of increased prey acquisition outweighs pathogenicity or resource drain, there should be little or no selection on final hosts to minimize parasite infections. This means that not only will host avoidance of infection not develop, but the molecular arms race for increased immunological defense will also likely be halted in such cases

Intensive smolt production is associated with deviating cardiac morphology in atlantic salmon

High pre-slaughter mortality rates of Atlantic salmon (Salmo salar L.) are a reoccurring welfare issue and economic burden in aquaculture. Sudden death immediately prior to slaughter is particularly problematic given the considerable resources invested to reach this stage. Although the underlying causes of such mortality are largely unknown, cardiac deformities and diseases have become increasingly prevalent observations in deceased fish. The factors leading to this pathology remains to be revealed. Thus, we presently examined if intensive smolt production and concordant fast growth rates in young fish is associated with altered cardiac morphology at later production stages in Atlantic salmon. The observed alterations were subsequently related to mortality risk following de-lousing in a facility with a cardiomyopathy syndrome (CMS) outbreak. We observed that intensive smolt production is indeed associated with slower growth rates at sea, distinct pathological cardiac morphological alterations, and expression of cardiac pathology markers. Moreover, the observed cardiac alterations co-occurred with CMS-related cardiac rupture at a different production facility. The present study demonstrates a clear link between pace of growth at early rearing stages and cardiac deformities later in life. Furthermore, these cardiac deformities are associated with cardiac rupture and mortality in individuals with CMS during delousing. We therefore believe that a slower pace of smolt production improves cardiac health and reduces the risk of mortality during CMS outbreaks.publishedVersio

Pigments, Parasites and Personalitiy: Towards a Unifying Role for Steroid Hormones?

A surging interest in the evolution of consistent trait correlations has inspired research on pigment patterns as a correlate of behavioural syndromes, or ‘‘animal personalities’’. Associations between pigmentation, physiology and health status are less investigated as potentially conserved trait clusters. In the current study, lice counts performed on farmed Atlantic salmon Salmo salar naturally infected with ectoparasitic sea lice Lepeophtheirus salmonis showed that individual fish with high incidence of black melanin-based skin spots harboured fewer female sea lice carrying egg sacs, compared to less pigmented fish. There was no significant association between pigmentation and lice at other developmental stages, suggesting that host factors associated with melanin-based pigmentation may modify ectoparasite development to a larger degree than settlement. In a subsequent laboratory experiment a strong negative correlation between skin spots and post-stress cortisol levels was revealed, with less pigmented individuals showing a more pronounced cortisol response to acute stress. The observation that lice prevalence was strongly increased on a fraction of sexually mature male salmon which occurred among the farmed fish further supports a role for steroid hormones as mediators of reduced parasite resistance. The data presented here propose steroid hormones as a proximate cause for the association between melanin-based pigmentatio

Short-term cortisol exposure alters cardiac hypertrophic and non-hypertrophic signalling in a time-dependent manner in rainbow trout

Cardiac disease is a growing concern in farmed animals, and stress has been implicated as a factor for myocardial dysfunction and mortality in commercial fish rearing. We recently showed that the stress hormone cortisol induces pathological cardiac remodelling in rainbow trout. Wild and farmed salmonids are exposed to fluctuations and sometimes prolonged episodes of increased cortisol levels. Thus, studying the timeframe of cortisol-induced cardiac remodelling is necessary to understand its role in the pathogenesis of cardiovascular disease in salmonids. We here establish that 3 weeks of cortisol exposure is sufficient to increase relative ventricular mass (RVM) by 20% in rainbow trout. Moreover, increased RVMs are associated with altered expression of hypertrophic and non-hypertrophic remodelling markers. Further, we characterised the time course of cortisol-induced cardiac remodelling by feeding rainbow trout cortisol-containing feed for 2, 7 and 21 days. We show that the effect of cortisol on expression of hypertrophic and non-hypertrophic remodelling markers is time-dependent and in some cases acute. Our data indicate that short-term stressors and life cycle transitions associated with elevated cortisol levels can potentially influence hypertrophic and non-hypertrophic remodelling of the trout heart

Intensive smolt production is associated with deviating cardiac morphology in atlantic salmon

High pre-slaughter mortality rates of Atlantic salmon (Salmo salar L.) are a reoccurring welfare issue and economic burden in aquaculture. Sudden death immediately prior to slaughter is particularly problematic given the considerable resources invested to reach this stage. Although the underlying causes of such mortality are largely unknown, cardiac deformities and diseases have become increasingly prevalent observations in deceased fish. The factors leading to this pathology remains to be revealed. Thus, we presently examined if intensive smolt production and concordant fast growth rates in young fish is associated with altered cardiac morphology at later production stages in Atlantic salmon. The observed alterations were subsequently related to mortality risk following de-lousing in a facility with a cardiomyopathy syndrome (CMS) outbreak. We observed that intensive smolt production is indeed associated with slower growth rates at sea, distinct pathological cardiac morphological alterations, and expression of cardiac pathology markers. Moreover, the observed cardiac alterations co-occurred with CMS-related cardiac rupture at a different production facility. The present study demonstrates a clear link between pace of growth at early rearing stages and cardiac deformities later in life. Furthermore, these cardiac deformities are associated with cardiac rupture and mortality in individuals with CMS during delousing. We therefore believe that a slower pace of smolt production improves cardiac health and reduces the risk of mortality during CMS outbreaks