Suppression by dexamethasone of interrenal activity in adult sockeye salmon (Oncorhynchus nerka)

Reproduction and resistance to stress: when and how

Environmental and social stresses have deleterious effects on reproductive function in vertebrates. Global climate change, human disturbance and endocrine disruption from pollutants are increasingly likely to pose additional stresses that could have a major impact on human society. Nonetheless, some populations of vertebrates (from fish to mammals) are able to temporarily resist environmental and social stresses, and breed successfully. A classical trade-off of reproductive success for potential survival is involved. We define five examples. (i) Aged individuals with minimal future reproductive success that should attempt to breed despite potential acute stressors. (ii) Seasonal breeders when time for actual breeding is so short that acute stress should be resisted in favour of reproductive success. (iii) If both members of a breeding pair provide parental care, then loss of a mate should be compensated for by the remaining individual. (iv) Semelparous species in which there is only one breeding period followed by programmed death. (v) Species where, because of the transience of dominance status in a social group, individuals may only have a short window of opportunity for mating. We suggest four mechanisms underlying resistance of the gonadal axis to stress. (i) Blockade at the central nervous system level, i.e. an individual no longer perceives the perturbation as stressful. (ii) Blockade at the level of the hypothalamic-pituitary-adrenal axis (i.e. failure to increase secretion of glucocorticosteroids). (iii) Blockade at the level of the hypothalamic-pituitary-gonad axis (i.e. resistance of the reproductive system to the actions of glucocorticosteroids). (iv) Compensatory stimulation of the gonadal axis to counteract inhibitory glucocorticosteroid actions. Although these mechanisms are likely genetically determined, their expression may depend upon a complex interaction with environmental factors. Future research will provide valuable information on the biology of stress and how organisms cope. Such mechanisms would be particularly insightful as the spectre of global change continues to unfold.

Elevation of plasma cortisol during the spawning migration of landlocked kokanee salmon (Oncorhynchus nerka kennerlyi)

Kokanee salmon (Oncorhynchus nerka kennerlyi ), a landlocked subspecies of sockeye salmon, exhibited hypothalamic-pituitary interrenal (HPI, adrenal homologue) axis activation and an increase in plasma cortisol concentration up to 639 +/- 55.9 ng/ml in association with upstream migration in the upper Colorado River even though they were not exposed to a change in salinity and lengthy migration. Kokanee salmon were collected at various stages of migration and concomitant sexual maturation. The pattern of cortisol elevation in kokanee is similar to that in ocean-run sockeye salmon (O. nerka nerka). The presence of plasma cortisol elevation in an upstream migrating, landlocked Pacific salmon suggests that stressors previously considered to cause the cortisol increase, such as long-distance migration and changes in salinity, may not be primary causes of the HPI axis activation.

Immunocytochemical and histological differences in the interrenal axis of feral brown trout, Salmo trutta, in metal-contaminated waters

There are more CRH-like immunoreactive neurons in the preoptic nucleus and nucleus lateralis tuberis in the brain of feral brown trout, Salmo trutta, living in cadmium- and zinc-contaminated regions of the Eagle River than in fish from an uncontaminated control site. Histological analyses revealed that interrenal cells are more stimulated (exhibiting both hypertrophy and hyperplasia) in fish living in contaminated sites than interrenal cells of fish at the control site. These results suggest that the hypothalamo-pituitary-interrenal (HPI) axis of fish living in the metal-contaminated water shows evidence of chronic stimulation. We suggest that assessment of these parameters of the HPI axis may be useful indices of chronic environmental stress in trout.

On the use of dexamethasone to block the pituitary-interrenal axis in the brown trout, Salmo trutta L

Oral administration of dexamethasone in a single meal at a dose of 3 micrograms g-1 body wt was sufficient to block the secretion of ACTH and cortisol in brown trout subjected to a handling stress followed by 30 min confinement. This dose, however, also had a marked effect on several of the circulating blood cell types. Lymphocyte and neutrophil counts were reduced in dexamethasone-treated fish whereas erythrocyte counts increased. The value of dexamethasone as a tool for investigating the role of interrenal tissue during stress responses in teleost fish is limited by its cortisol-like effects on other steroid-sensitive tissues.

The development and validation of a radioimmunoassay to measure plasma ACTH levels in salmonid fishes

A radioimmunoassay (RIA) capable of determining blood ACTH levels in salmonid fishes was developed and validated. The RIA used an antibody raised against mammalian ACTH, iodinated human ACTH as tracer, and human 1-39 ACTH as standard. Incubation of the standard or unknown with antibody for 3 days before addition of as little high-specific activity tracer as practicable (1500 cpm; equivalent to 5 pg ACTH) produced a very sensitive RIA; the operating range was 5 to 200 pg ACTH/ml. Extracts of both pars distalis and neurointermediate lobe of the pituitary glands from a range of salmonid species diluted parallel to the ACTH standard in the RIA. There was always considerably more ACTH-immunoreactivity (ACTH-IR) in the pars distalis extracts than in the neurointermediate lobe. Generally plasmas also diluted parallel to the ACTH standard, with the exception only of the plasma from sexually mature female salmonids, which diluted very non-parallel to the standard, leading to unrealistically low estimates of the ACTH-IR level. The use of heparin as an anticoagulant during collection of samples caused problems when these plasmas were immunoassayed; instead EDTA was found to be a suitable anticoagulant. When the ACTH-IR was extracted from a pool of plasma obtained from acutely stressed salmon and chromatographed on a column of BioGel P6, followed by subsequent ACTH RIA of the fractions, only a single sharp peak of ACTH-IR was detected, which eluted in the position of authentic 1-39 ACTH. The plasma ACTH-IR level in unstressed fish was low, and near the detection limit of the RIA. An acute stress, produced by crowding and confinement for 30 min, increased ACTH-IR approximately 10-fold, and plasma cortisol levels 50-fold, but the plasma alpha-MSH level was not affected. Dexamethasone-treated fish did not respond to this stressor with any increase in either ACTH or cortisol levels.

Radioimmunoassay of serum cortisol in the plaice (Pleuronectes platessa L.)

The validity of a radioimmunoassay kit for direct measurement of cortisol in the plaice is described. This technique was used to determine the cortisol concentration in adult male and female plaice in 100 microliter of either plasma or serum. There was no significant difference in the serum cortisol values determined by radioimmunoassay either with or without chromatography. Storage of whole blood at 4 degrees for varying periods up to 24 hr, before removal of the serum, did not affect the cortisol concentration. Decreases also did not occur in serum samples kept at room temperature for up to 5 days, although losses of 1-12% occurred after 7 days and 11-39% after 8 days. The daily variations in serum cortisol levels were examined in fish exposed at 11 degrees to 12 hr light: 12 hr dark over a 32-hr period and the existence of a 24-hr cortisol rhythm was observed. Blood samples were taken from the same six fish every 4 hr, each fish being bled nine times during the experiment. Blood samples were also taken every 4 hr from groups of seven fish, each fish during this experiment being bled only once. Measurement of monthly serum samples throughout 1 year showed cortisol concentrations at a maximum in April, during the peak spawning period. There was no significant difference between cortisol levels in male and female plaice, except in January, and no difference between serum and plasma values in either sex.