Effects of chronic manipulation of adrenocorticotropic hormone levels in Chinook salmon on expression of interrenal steroidogenic acute regulatory protein and steroidogenic enzymes

The effects of chronic exposure to adrenocorticotropic hormone (ACTH) or the synthetic glucocorticoid dexamethasone (DEX) on the expression of genes involved in cortisol synthesis were examined using quantitative RT-PCR and immunohistochemistry. Juvenile Chinook salmon were treated with either ACTH via micro-osmotic pumps or with DEX via a lipid-based sustained release vehicle. Plasma cortisol levels were significantly elevated in ACTH-treated fish after 1 day, with a significant reduction in this effect with increasing treatment duration. ACTH also appeared to cause progressive hyperplasia of interrenal cells. Steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc) transcripts but not 3β-hydroxysteroid dehydrogenase-isomerase (3β-HSD) or cytochrome P450 11β-hydroxylase (P45011β) transcripts in head kidneys significantly increased after 5 days of ACTH treatment. Significant linear relationships between plasma cortisol levels and transcript levels were identified at day 1 and day 5 for StAR, and day 5 for P450scc. Increased immunoreactivity for P450scc was observed in interrenal cells of ACTH-treated fish after 5 and 10 days. No effect of ACTH on 3β-HSD immunoreactivity was apparent at any time point. P45011β immunoreactivity was more intense after 5 days treatment with ACTH. DEX significantly reduced resting plasma cortisol levels and induced interrenal cell atrophy. Although no significant effect of treatment with DEX was found for any transcript, immunoreactivity for P450scc and P45011β appeared to be reduced. These results indicate that StAR and P450scc are subject to transcriptional regulation by chronic changes in ACTH levels.

Corticotropin-releasing factor and neuropeptide Y mRNA levels are modified by glucocorticoids in rainbow trout, Oncorhynchus mykiss

The primary stress response involves neuronal activation that ultimately leads to the release of glucocorticoids. Circulating glucocorticoids are thought to influence their own synthesis and release through a negative feedback mechanism that inhibits the activity of the hypothalamic and pituitary components of the stress axis. This study was designed to address the hypothesis that glucocorticoids modify corticotropin-releasing factor (CRF) and neuropeptide Y (NPY) mRNA levels in the rainbow trout (Oncorhynchus mykiss) brain. Cortisol implantation significantly reduced CRF1 and NPY mRNA levels in fish exposed to an isolation stress. In contrast, cortisol implantation did not prevent the stress-induced elevation of CRF1 and NPY mRNA levels during confinement. Treatment with the glucocorticoid receptor antagonist RU-486 reduced CRF1 mRNA levels in both isolated and confined fish, but had no effect on NPY mRNA. Although the cytochrome P450 inhibitor metyrapone reduced ACTH-induced cortisol secretion in vitro, plasma cortisol levels were elevated in isolated trout treated with metyrapone. Nevertheless, metyrapone implantation increased CRF1 and NPY mRNA levels in confined fish. Together, these results implicate cortisol as a modulator of CRF and NPY mRNA levels in the preoptic area of the trout brain, but that cortisol is only one such regulating mechanism.

Survey of the Adrenal Homolog in Teleosts

The adrenal homolog of teleosts is not a compact organ as the adrenal glands of most vertebrates but is composed by aminergic chromaffin and interrenal steroidogenic cells located mostly inside the head kidney that, in this taxon, generally has a hematopoietic function. The two tissues can be mixed, adjacent, or completely separated and line the endothelium of the venous vessels or are located in close proximity. The chromaffin cells in some species are also present in the posterior kidney. Histological and ultrastructural work revealed cytological peculiarities of both types of cells as compared to those of other vertebrate species. In particular, the interrenal ones can show some variations in ultrastructure depending on sex, time of the year, and relation to stress events. A periodic renewal of the whole gland tissue is also sustained by some studies. Research regarding development is scanty as compared to mammals and most studies go back to the early years of the past century. The adrenal homolog of teleosts is under hormonal and neuronal control. Moreover, local paracrine interactions may play an important role in modulating a system involved in stress response and osmoregulation. Most previous studies involved a few species with the object of intensive rearing for commercial purposes; in fact cortisol, the main hormone secreted by the interrenal cells, can also influence reproduction and growth. This review summarizes data from morphocytological work and refers to other excellent reviews regarding physiology. Some of the results are compared to data available from other fishes and vertebrate classes with the aim of including them in an evolutionary and environmental framework.

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.

Effects of N-terminal peptide of salmon proopiocortin on interrenal function of the rainbow trout

Effects of N-terminal peptide of salmon proopiocortin (salmon NPP-I) on cortisol secretion was examined in vitro using diced interrenal tissue from the rainbow trout, Salmo gairdneri. ACTH(1-24) at concentrations of 1 to 50 nM stimulated cortisol secretion in dose-dependent manner, whereas salmon NPP-I had no effect over a range of 50 pM to 500 nM. Cortisol secretion in response to various doses of ACTH(1-24) was modified slightly when 1 to 100 nM of salmon NPP-I was added to the incubation medium together with ACTH. An augmentation of in vitro secretion of cortisol in response to ACTH(1-24) was observed when the interrenal was removed from the trout pretreated with one IU of porcine ACTH but not with 10 micrograms of salmon NPP-I. A slight but significant potentiating effect of salmon NPP-I (10 or 100 nM) on the ACTH-induced cortisol secretion was observed when the trout was sensitized to ACTH by porcine ACTH pretreatment. Furthermore, six daily injections of salmon NPP-I into the trout induced hyperplasia of interrenal tissue. These findings suggest that NPP-I, together with ACTH, may be involved in controlling interrenal function in the trout. Such activities could be due to conservative region in the N-terminal portion of NPP.

Effects of natural androgens and corticosteroids on gonad differentiation in the rainbow trout, Salmo gairdneri

Rainbow trout fry were treated with equimolar quantities of cortisol, cortisone, androstenedione, and 11 beta-hydroxyandrostenedione, added to the aquarium water during a 4-week period. All four steroids inhibited ovarian growth. Androstenedione did not influence gonadal sex differentiation; the other steroids pushed the sex ratio in the male direction. However, a near to normal sex ratio was observed 300 days after the 11 beta-hydroxyandrostenedione treatment. When added to the food in two different doses during 8 weeks, 11 beta-hydroxyandrostenedione had a pronounced masculinizing effect, and androstenedione did not modify the gonads at all. It is suggested that the two exogenous corticosteroids were probably converted into 11-oxygenated androstenedione derivatives, and that these 11-oxygenated androstenedione derivatives are particularly important in sustaining the differentiation and early development of the testes in rainbow trout. Testosterone does not seem to be indispensable for these processes, because two different doses of testosterone-blocking cyproterone acetate, added to the food for 8 weeks, failed to affect early gonad development.