CHROMATOGRAPHY OF CORTICOSTEROIDS FROM TELEOST FISHES

Systems approaches to genomic and epigenetic inter-regulation of peptide hormones in stress and reproduction

The evolution of the organismal stress response and fertility are two of the most important aspects that drive the fitness of a species. However, the integrated regulation of the hypothalamic pituitary adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes has been traditionally thwarted by the complexity of these systems. Pepidergic signalling systems have emerged as critical integrating systems for stress and reproduction. Current high throughput systems approaches are now providing a detailed understanding of peptide signalling in stress and reproduction. These approaches were dependent upon a long history of discovery aimed at the structural characterization of the associated molecular components. The combination of comparative genomics, microarray and epigenetic studies has led not only to a much greater understanding of the integration of stress and reproduction but also to the discovery of novel physiological systems. Recent epigenomic approaches have similarly yielded a new level of complexity in the interaction of these physiological systems. Together, such studies have provided a greater understanding of the effects of stress and reproduction.

Neural plasticity and stress coping in teleost fishes

Physiological and behavioural responses to environmental change are individually variable traits, which manifest phenotypically and are subject to natural selection as correlated trait-clusters (coping styles, behavioural syndromes, or personality traits). Comparative research has revealed a range of neuroendocrine-behavioural associations which are conserved throughout the vertebrate subphylum. Regulatory mechanisms universally mediate a switch between proactive (e.g. active/aggressive) and reactive (e.g. conservation/withdrawal) behaviour in response to unpredictable and uncontrollable events. Thresholds for switching from active coping to behavioural inhibition are individually variable, and depend on experience and genetic factors. Such factors affect physiological stress responses as well as perception, learning, and memory. Here we review the role of an important contributor to neural processing, the set of biochemical, molecular, and structural processes collectively referred to as neural plasticity. We will concentrate on work in teleost fishes, while also elucidating conserved aspects. In fishes, environmental and physiological control of brain cell proliferation and neurogenesis has received recent attention. This work has revealed that the expression of genes involved in CNS plasticity is affected by heritable variation in stress coping style, and is also differentially affected by short- and long-term stress. Chronic stress experienced by subordinate fish in social hierarchies leads to a marked suppression of brain cell proliferation. Interestingly, typically routine dependent and inflexible behaviour in proactive individuals is also associated with low transcription of neurogenesis related genes. The potential for these findings to illuminate stress-related neurobiological disorders in other vertebrates is also discussed.

Cortisol receptor expression differs in the brains of rainbow trout selected for divergent cortisol responses

In rainbow trout (Oncorhynchus mykiss), selection for divergent post-stress plasma cortisol levels has yielded low (LR)- and high (HR) responsive lines, differing in behavioural and physiological aspects of stress coping. For instance, LR fish display prolonged retention of a fear response and of previously learnt routines, compared to HR fish. This study aims at investigating putative central nervous system mechanisms controlling behaviour and memory retention. The stress hormone cortisol is known to affect several aspects of cognition, including memory retention. Cortisol acts through glucocorticoid receptors 1 and 2 (GR1 and 2) and a mineralcorticoid receptor (MR), all of which are abundantly expressed in the salmonid brain. We hypothesized that different expressions of MR and GRs in LR and HR trout brains could be involved in the observed differences in cognition. We quantified the mRNA expression of GR1, GR2 and MR in different brain regions of stressed and non-stressed LR and HR trout. The expression of MR was higher in LR than in HR fish in all brain parts investigated. This could be associated with reduced anxiety and enhanced memory retention in LR fish. MR and GR1 expression was also subject to negative regulation by stress in a site-specific manner.

Multiple corticosteroid receptors in a teleost fish: distinct sequences, expression patterns, and transcriptional activities

Corticosteroid hormones, including the mineralocorticoids and the glucocorticoids, regulate diverse physiological functions in vertebrates. These hormones act through two classes of corticosteroid receptors (CR) that are ligand-dependent transcription factors: type I or mineralocorticoid receptor (MR) and type II or glucocorticoid receptor (GR). There is substantial overlap in the binding of these two receptor types to hormones and to DNA. In fish, the overlap in processes controlled by CRs may be different from that in other vertebrates, as fish are thought to synthesize only glucocorticoids, whereas they express both GR and MR. Here we describe the characterization of four CRs in a cichlid fish, Haplochromis burtoni: a previously undescribed GR (HbGR1), another GR expressed in two splice isoforms (HbGR2a and HbGR2b), and an MR (HbMR). Sequence comparison and phylogenetic analysis showed that these CRs sort naturally into GR and MR groups, and that the GR duplication we describe will probably be common to all teleosts. Quantitative PCR revealed differential patterns of CR tissue expression in organs dependent on corticosteroid action. Trans-activation assays demonstrated that the CRs were selective for corticosteroid hormones and showed that the HbMR was similar to mammalian MRs in being more sensitive to both cortisol and aldosterone than the GRs. Additionally, the two HbGR2 isoforms were expressed uniquely in different tissues and were functionally distinct in their actions on classical GR-sensitive promoters. The identification of four CR subtypes in teleosts suggests a more complicated corticosteroid signaling in fish than previously recognized.

Effects of food deprivation and handling stress on head kidney 17alpha-hydroxyprogesterone 21-hydroxylase activity, plasma cortisol and the activities of liver detoxification enzymes in rainbow trout

The 21-hydroxylation of 17alpha-hydroxyprogesterone is one step in the biosynthesis of corticosteroids. Both 7 days of handling-induced stress and 7 weeks of food deprivation significantly elevated head kidney microsomal 17alpha-hydroxyprogesterone 21-hydroxylase activity in juvenile rainbow trout. The increased 21-hydroxylase activity was not paralleled by changes in plasma cortisol levels induced by handling stress whereas food deprivation for 3 and 7 weeks increased both 21-hydroxylase activity and plasma cortisol levels significantly. Food deprivation in rainbow trout affected detoxification enzyme activities, namely glutathione-S-transferase (GST), uridine-di-phosphate glucuronosyltransferase (UGT) and glutathione reductase (GR) activities in the liver. Together our observations suggest that experimental conditions can affect experimental results, especially the values of parameters like GST, UGT and GR. Furthermore, alterations in the metabolic state of the liver caused by stress or food deprivation can alter the balance between detoxification enzymes in rainbow trout liver.

Evidence for ultra-short-loop feedback in ACTH-induced interrenal steroidogenesis in coho salmon: acute self-suppression of cortisol secretion in vitro

Interrenal tissues from coho salmon (Oncorhynchus kisutch) were incubated in a defined medium under blood-gas atmosphere at 17 degrees. Rates of cortisol secretion by tissues incubated in media containing 50 mU/ml porcine-ACTH were initially much greater than those of resting tissues in hormone-free media, but after 3 to 6 hr returned to resting rates. The time course of cortisol accumulation in ACTH-containing media was the same when tissues were incubated in different volumes; the final concentrations of cortisol in these incubations were similar to each other and resembled peak in vivo concentrations in juvenile coho subjected to acute stress. Cortisol secretion rates of tissues sequentially transferred to fresh ACTH-containing media every 6 hr did not return to resting levels but remained elevated for at least 24 hr. Cortisol secretion in response to ACTH was attenuated or completely abolished in tissues incubated in media containing exogenous cortisol; this effect was reversible and dose-dependent. Our results suggest that in coho salmon, cortisol may exert ultra-short-loop negative feedback directly at the level of the interrenal gland to effect self-suppression.

Corticosteroid biosynthesis in the interrenal cells of the teleost fish, Oreochromis mossambicus

Applying high-performance liquid chromatography and thin-layer chromatography to separate corticosteroids, we studied the biosynthesis of steroids by the interrenal cells of the head kidneys (the adrenocortical homolog) of Oreochromis mossambicus. Intact head kidneys converted exogenous 17 alpha-hydroxyprogesterone into mainly cortisol, but 11-deoxycortisol, cortisone, and androstenedione were also recovered from the medium. Incubation of intact tissue with pregnenolone in addition resulted in the formation of large amounts of an unidentified product, which was absent in incubations of tissue homogenates with pregnenolone.

Spontaneous and ACTH-induced interrenal steroidogenesis in juvenile coho salmon (Oncorhynchus kisutch): effects of monovalent ions and osmolality in vitro

We determined the in vitro effects of changes in extracellular monovalent ion levels and osmotic pressure on the spontaneous and adrenocorticotropin (ACTH)-stimulated interrenal activity of coho salmon (Oncorhynchus kisutch). We used a perifusion system of incubation and monitored interrenal activity by measuring the effluent cortisol content with a radioimmunoassay. An increase in the medium osmolality with mannitol, from 206 to 290 or 353 mosmol, caused an increase in the spontaneous release of cortisol only slightly (compared with the much greater increase induced by porcine-ACTH). A similar minor increase was observed when NaCl was elevated from 130 to 180 mM. On the other hand, the spontaneous release of cortisol was not affected by increasing the KCl level from 3.2 to 9.6 mM, but was clearly increased when KCl was raised from 3.2 mM to a supraphysiological level of 27.2 mM. Ionic or osmolality changes, within the physiological range observed in coho salmon plasma, did not affect the characteristics of interrenal secretion of cortisol in response to porcine-ACTH. If our results with interrenal cells in vitro are representative of the basic functioning of the cells in vivo, then one would have to conclude that changes in concentrations of plasma monovalent ions or in osmotic pressure may not play a significant physiological role in the regulation of interrenal steroidogenesis or corticosteroid release in coho salmon.

Interrenal secretion of corticosteroids and plasma cortisol and cortisone concentrations after acute stress and during seawater acclimation in juvenile coho salmon (Oncorhynchus kisutch)

We determined the major corticosteroids secreted by interrenal tissue and those present in plasma of juvenile coho salmon (Oncorhynchus kisutch). Incubation medium of interrenal tissue, unstimulated or stimulated with exogenous ACTH in vitro, and plasma of resting or acutely stressed salmon were extracted and qualitatively analyzed for steroid composition using high-performance liquid chromatography. Concentrations of plasma cortisone and cortisol following an acute handling stress or exposure to seawater were quantitatively measured by radioimmunoassay (RIA). Cortisol was the only corticosteroid detected by HPLC in media after incubation of interrenal tissue in the absence or presence of ACTH in vitro. However, both cortisone and cortisol were detected by HPLC in plasma sampled 1 hr after fish were acutely stressed by handling. Stress and seawater acclimation produced marked elevations in plasma levels of both steroids as determined by RIA and also resulted in long-lasting changes in the plasma cortisone:cortisol ratios. In resting fish, cortisone concentrations were similar or higher than cortisol levels. We concluded that cortisol is the primary steroid secreted by the interrenal tissue of coho salmon, and that plasma cortisone arises primarily from the peripheral conversion of cortisol to cortisone. The relatively high levels of cortisone in resting fish and its increase following stress and seawater acclimation suggest the possibility of a biologically significant role for this hormone.

In vitro release of growth hormone from the pituitary gland of tilapia, Oreochromis mossambicus

The release of growth hormone from the proximal pars distalis of the tilapia, Oreochromis mossambicus, was significantly stimulated by cortisol (1 microgram/ml) in an in vitro system. Growth hormone released into the medium and remaining in the tissue was measured by densitometry after gel electrophoresis. Neither triiodothyronine (6.7 ng/ml) nor equimolar concentrations of thyroxin altered the release of growth hormone. In combination with cortisol, triiodothyronine did not alter the effect of cortisol alone.

Studies on oocyte maturation of the medaka, Oryzias latipes. V. On the structure of steroids that induce maturation in vitro

The response of oocytes within isolated follicles (800-950 micron in diameter) to various steroids was examined with the teleost fish, Oryzias latipes. Continuous exposure of oocytes, which were removed from ovarian investments 17 hours before predicted germinal vesicle breakdown (GVBD), to C19- or C21-steroids brought about maturation in vitro but never triggered ovulation. The steroids effective in inducing maturation have in common a C=0 (or alpha-OH) group at 3C and a beta-OH group at 17C in the C19-steroids, and a C=O (or beta-OH) group at 3C and a C=O (or alpha-OH) group at 20C in the C21-steroids, in addition to an delta4- or delta5-unsaturated for 5alpha-saturated configuration. The orientation of the hydrogen at 5C seems to be critical in determining the ability of a particular steroid to stimulate oocyte maturation. Maturation of oocytes in the ovaries of hypophysectomized females was induced by administering progesterone, but the mature oocytes did not subsequently undergo ovulation. Thus the steroid hormone is capable of inducing oocyte maturation but apparently does not participate directly in the ovulation of Oryzias latipes oocytes.

Teleostean urophysis: stimulation of contractions of bladder of the trout Salmo gairdnerii

A new activity of the teleost caudal neurosecretory system is described. Extracts of the urophysis of the mudsucker Gillichthys mirabilis and of the trout Salmo gairdnerii cause rhythmic contractions of the isolated urinary bladder of the trout. The dose-related response provides the basis for a quantitative bioassay of this urophysial principle.

Hormones and endocrine glands of fishes. Studies of fish endocrinology reveal major physiologic and evolutionary problems

The structures discussed above illustrate some of the evolutionary knowledge to be obtained from studies of the comparative biology of the endocrine system among the fishes (see also 17, 78). The interrenal (adrenocortical) gland is an example of an endocrine structure which shows great morphologic variation in the vertebrate series, and among the fishes themselves. The structural variability implies little selective value for any particular pattern, and, in fact, biochemically the interrenal tissue would seem to be involved in much the same kind of steroidogenesis in the vertebrates generally. However, the hormone aldosterone may be a tetrapod novelty. The caudal neurosecretory system is a good example of a ubiquitous endocrine apparatus, among at least teleostean and elasmo-branch fishes, for which a function has yet to be elucidated and which continues to challenge the comparative physiologist. This system, along with the Stannius corpuscles, is lacking in the tetrapods. The existence of these structures makes it clear that the endocrine biology of fishes cannot be tacitly summarized as being essentially similar to that of the tetrapods, only less well developed. The prolactin situation illustrates the existence of a gland-the pituitary-present among fishes as among tetrapods, which secretes a product only partly related to the tetrapod hormone and having a very different functional significance. In this case it is clear that "the hormone and the uses to which it is put" have undergone evolutionary change during vertebrate phylogeny.