Corticotropin-releasing hormone (CRH) in the teleost fish Oreochromis mossambicus (tilapia): in vitro release and brain distribution determined by a novel radioimmunoassay

Stressor timing, not cortisol, is an important embryo viability determinant in female rainbow trout Oncorhynchus mykiss

The objective of this study was to determine whether the timing of elevated cortisol levels in adult female rainbow trout Oncorhynchus mykiss, achieved by silastic implants within the body cavity, had an effect on embryo viability. Cortisol-implanted fish experienced a significant increase in circulating levels of plasma cortisol compared with sham-implanted fish, and plasma cortisol remained elevated relative to sham-implanted fish for 4 months in each of the three treatment groups (0-4, 4-8 and 8-12 months). There were no significant differences in embryo viability, egg diameters or plasma 17β-oestradiol levels between the cortisol and sham-implanted treatments in any of the groups. There was a significant difference in the number of subfertile females (<80% embryo viability) when the three treatments were compared. The majority of the females (75%) implanted immediately postspawn (0-4 months) produced subfertile eggs, which was significantly higher than those treated 4-8 (33%) or 8-12 (17%) months postspawn. These results imply that silastic implantation can affect oocyte development, independent of plasma cortisol levels, leading to a reduction in embryo viability.

Acute stress inhibits food intake and alters ghrelin signaling in the brain of tilapia (Oreochromis mossambicus)

This study investigated the effect of an acute stress on food intake and on the expression of neuropeptide Y (NPY), corticotropin-releasing hormone (CRH), and ghrelin and its receptors, growth hormone secretagogue receptors (GHSRs) in the tilapia (Oreochromis mossambicus). Food intake was significantly (P < 0.01) reduced in fish after a 30-min crowding and handling stress. In a second group of animals exposed to the same 30-min stressor, tissue samples were collected immediately after the stressor to determine changes in the neuroendocrine regulators of food intake. Although CRH and NPY are considered the major mediators of appetite during stress, both mRNA levels were unaltered in the telencephalon/pre-optic area and in the hypothalamic/optic tectum. Interestingly, there was an elevation in the ghrelin transcript (P < 0.05) in the telencephalon/pre-optic area and elevation of its functional receptor (GHSR1a-LR) (P < 0.001) in the hypothalamic/optic tectum. Elevation of GHSR-LR heteronuclear RNA (P < 0.01) in the telencephalon/pre-optic area and suppression in the hypothalamic/optic tectum (P < 0.001) suggest rapid control of the ghrelin regulatory system in response to acute stress. These results suggest that ghrelin signaling is altered during acute stress. It is not clear if these changes result in altered feeding behavior because no changes in CRH or NPY mRNA expression were observed or if ghrelin is playing a role in regulating overall metabolic changes after acute stress.

Cortisol treatment reduces ghrelin signaling and food intake in tilapia, Oreochromis mossambicus

It is well known that after a stressor, levels of plasma cortisol rise, inducing physiological changes within the animal that are directed toward maintaining homeostasis. Less well understood is the role of cortisol in regulating food intake in teleosts. This study investigated the effect of cortisol on food intake and regulation of the neuroendocrine appetite-stimulating hormones, neuropeptide Y (NPY) and ghrelin, in tilapia (Oreochromis mossambicus). Male and female tilapia were randomly assigned to one of the following treatments: unhandled control, vehicle-injected control, or cortisol (2 μg/g BW). Food intake was determined 24 h after injection during a 1-h feeding trial. Cortisol reduced food intake (P<0.001). An identical study was conducted to measure the effects of 24-h cortisol treatment on the endocrine regulators of food intake. Cortisol reduced stomach expression of ghrelin mRNA (P<0.05) and plasma concentrations of ghrelin (P<0.05). In the hypothalamus/optic tectum cortisol reduced levels of GHSR1a-LR (biologically active ghrelin receptor) mRNA. In the telencephalon/preoptic area cortisol significantly reduced levels of NPY and GHSR1b-LR (biologically inactive ghrelin receptor) mRNA. These findings suggest that anorexigenic actions of cortisol may be mediated via two separate pathways: (1) reducing circulating ghrelin levels as well as GHSR1a-LR expression in the hypothalamus/optic tectum and/or (2) suppressing NPY expression in the telencephalon/preoptic area.

A three-dimensional stereotaxic MRI brain atlas of the cichlid fish Oreochromis mossambicus

The African cichlid Oreochromis mossambicus (Mozambique tilapia) has been used as a model system in a wide range of behavioural and neurobiological studies. The increasing number of genetic tools available for this species, together with the emerging interest in its use for neurobiological studies, increased the need for an accurate hodological mapping of the tilapia brain to supplement the available histological data. The goal of our study was to elaborate a three-dimensional, high-resolution digital atlas using magnetic resonance imaging, supported by Nissl staining. Resulting images were viewed and analysed in all orientations (transverse, sagittal, and horizontal) and manually labelled to reveal structures in the olfactory bulb, telencephalon, diencephalon, optic tectum, and cerebellum. This high resolution tilapia brain atlas is expected to become a very useful tool for neuroscientists using this fish model and will certainly expand their use in future studies regarding the central nervous system.

Stressors, glucocorticoids and ovarian function in teleosts

The purpose of this overview is to re-examine the postulated direct and indirect actions of glucocorticoids on ovarian function in teleosts. The re-examination is undertaken in light of recent advances in the understanding of the stress response itself, the mode of action of the hypothalamus-pituitary gland-ovarian (HPO) axis, the mechanisms of control of oestrogen-dependent hepatic vitellogenin (VtG) secretion and the apparent roles of corticotrophin-releasing hormone (CRH) and CRH-related factors in the regulation of feeding activity. Many of the results of different studies, particularly whole-animal studies, are conflicting, and little is known as to whether the hormone acts directly on various components of the HPO axis or indirectly by virtue of redirection of energy resources away from ovarian growth to provide a source of metabolic resources for other organ systems involved in the physiological stress response. In vitro studies provide some new insights into the direct actions of glucocorticoid on hepatic VtG synthesis and ovarian follicle steroidogenesis, but even here, in some studies the cellular sites of action of these hormones is not altogether clear. The overview emphasizes the complexity of the stress response, the complexity of the regulation of glucocorticoid-dependent gene expression and the extensive interactive nature of the HPO with other hypothalamus-pituitary gland-peripheral endocrine gland axes, such as the thyroid (HPT), 'somatic' (GH-IGF) and interrenal tissue (HPI) axes.

Bacterial lipopolysaccharide (LPS) modulates corticotropin-releasing hormone (CRH) content and release in the brain of juvenile and adult tilapia (Oreochromis mossambicus; Teleostei)

Although immune endocrine interactions in teleost fish have been shown to involve adrenocorticotropin hormone (ACTH) and cortisol, the involvement of corticotropin-releasing hormone (CRH) has not been demonstrated. The present study investigates whether treatment with bacterial endotoxin(lipopolysaccharide, LPS) modulates brain CRH contents or in vitroCRH release in tilapia (Oreochromis mossambicus). 10 days LPS(Escherichia coli) exposure of juvenile tilapia (4.5 weeks post hatch) via the ambient water increased brain CRH and α-MSH content,whereas cortisol contents were not increased. This indicates that the elevation of brain CRH levels were not secondary to activation of HPI-axis. Adult tilapia were treated for 6 days with LPS (intraperitoneally) and were sampled before and after 24 h of confinement. Overall LPS pre-treatment modified the reaction of tilapia to the additional stressor of 24 h confinement, as interactions between LPS treatment and confinement were observed at the level of the hypothalamus (diencephalic CRH content), the pituitary (CRH and α-MSH content) and in plasma glucose levels. In vitro, LPS pre-treatment abolished CRH release from telencephalic tissues induced by norepinephrine, one of the CRH secretagogues released during stress in vivo. This effect might be a mechanism of action through which LPS in vivo abolished the up-regulation of telencephalic CRH induced by confinement stress. Our results provide evidence that the role of CRH in immune–endocrine interactions is a phylogenetically old mechanism, and we here demonstrate that LPS molecules are able to locally modulate CRH release in the central nervous system.

Ontogeny of corticotropin-releasing factor and of hypothalamic-pituitary-interrenal axis responsiveness to stress in tilapia (Oreochromis mossambicus; Teleostei)

The ontogeny of the corticotropin-releasing factor (CRF) system and of the ability of the hypothalamic-pituitary-interrenal (HPI) axis to respond to stressors (capture or confinement), or to cortisol treatment was investigated in tilapia (Oreochromis mossambicus). In 2 days post hatching (dph) larvae, the first developmental stage used for immunohistochemistry, CRF-immunoreactivity (ir) was observed in the nucleus preopticus (npo), and in two hypothalamic nuclei (nlt and nrl). In this stage, CRF- and AVT-ir was found in the neural part of the pituitary, and endocrine cells in the pars distalis and pars intermedia contained POMC-derived peptides. In the ventral telencephalon, CRF-ir cells were first observed 5 dph, whereas projections from these cells into the anterior part of the latero-dorsal telencephalon (Dla) from 7 dph onwards. CRF, ACTH, alpha-MSH, and cortisol were quantified by radioimmunoassays in homogenates of the anterior-cranial region of the larvae containing brain, pituitary, and headkidneys. CRF contents increased from 43 +/- 3 to 1070 +/- 70 pg/larvae between 5 and 110 dph. Larvae of age 5, 12, 24, and 42 dph were captured sequentially from a group. All life stages were able to rapidly increase their cortisol content in response to this stressor (ANOVA: P < 0.001). Overall, the developmental stage affected cortisol content (ANOVA: P < 0.001), but developmental stage did not influence the cortisol reaction to stress (ANOVA: P > 0.162). Whole brain CRF content did not change during the 20 min stress period and the relationship between CRF-producing neurons and the initial HPI stress response in early life stages remains to be established. Cortisol feeding of 18 and 29 dph larvae for periods ranging from 2 to 24 days resulted in elevations of the CRF content (P < 0.003) in comparison to controls. In 18 dph larvae cortisol feeding abolished the cortisol response to capture stress as observed in control fed larvae (P < 0.008). We propose that cortisol induced upregulation of CRF takes place in the telencephalon and is restricted to a time period during larval development, characterised by the absence of glucocortoid receptor (GR) expression in the telencephalic Dm region in these larvae. Finally, the stress response to 24 h confinement was compared between saltwater adapted and freshwater adapted juveniles (age 77 dph). Confinement stress (24 h) affected cortisol and CRF content (ANOVA: P < 0.001, P < 0.008, respectively), but not ACTH content. Interactions were observed between salinity and confinement regarding cortisol and alpha-MSH contents (ANOVA: P < 0.02), but not regarding CRF and ACTH contents. The increase in cortisol levels induced by confinement was remarkably high in freshwater adapted larvae (five times higher than in saltwater adapted larvae). Regarding the cortisol response it is concluded that during and after the period of mouth breeding tilapia larvae respond to capture stress in a similar fashion (onset and height) as adults. Previously, we reported that the initial plasma cortisol response to capture stress in adult tilapia occurred independently from changes in plasma ACTH levels. The current finding that also brain CRF contents do not alter during the initial cortisol response in larvae further indicates that the initial cortisol response in this species may be regulated independently from CRF and ACTH.

Expression and hypophysiotropic actions of corticotropin-releasing factor in Xenopus laevis

Members of the corticotropin-releasing factor (CRF) family of peptides play pivotal roles in the regulation of neuroendocrine, autonomic, and behavioral responses to physical and emotional stress. In amphibian tadpoles, CRF-like peptides stimulate both thyroid and interrenal (adrenal) hormone secretion, and can thereby modulate the rate of metamorphosis. To better understand the regulation of expression and actions of CRF in amphibians we developed a homologous radioimmunoassay (RIA) for Xenopus laevis CRF (xCRF). We validated this RIA and tissue extraction procedure for the measurement of brain CRF content in tadpoles and juveniles. We show that the CRF-binding protein, which is highly expressed in X. laevis brain, is largely removed by acid extraction and does not interfere in the RIA. We analyzed CRF peptide content in five microdissected brain regions in prometamorphic tadpoles and juveniles. CRF was detected throughout the brain, consistent with its role as both a hypophysiotropin and a neurotransmitter/neuromodulator. CRF content was highest in the region of the preoptic area (POa) and increased in all brain regions after metamorphosis. Exposure to 4h of handling/shaking stress resulted in increased CRF peptide content in the POa in juvenile frogs. Injections of xCRF into prometamorphic tadpoles increased whole body corticosterone and thyroxine content, thus supporting findings in other anuran species that this peptide functions as both a corticotropin- and a thyrotropin (TSH)-releasing factor. Furthermore, treatment of cultured tadpole pituitaries with xCRF (100nM for 24h) resulted in increased medium content, but decreased pituitary content of TSHbeta-immunoreactivity. Our results support the view that CRF functions as a stress neuropeptide in X. laevis as in other vertebrates. Furthermore, we provide evidence for a dual hypophysiotropic action of CRF on the thyroid and interrenal axes in X. laevis as has been shown previously in other amphibian species.

Distribution and quantification of corticotropin-releasing hormone (CRH) in the brain of the teleost fish Oreochromis mossambicus (tilapia)

The recent characterization of the corticotropin-releasing hormone (CRH) prehormone of the fish tilapia (Oreochromis mossambicus) showed that more variation exists between vertebrate CRH amino acid sequences than recognized before. The present study investigates whether the deviating composition of tilapia CRH coincides with an atypical distribution of CRH in the brain. For this purpose we applied immunohistochemistry, as well as radioimmunoassay (RIA) quantification in brain slices. The results are plotted in a new atlas and reconstruction of the tilapia brain. The largest population of CRH-immunoreactive (ir) neurons is present in the lateral part of the ventral telencephalon (Vl). Approximately tenfold less CRH-ir neurons are observed in the preoptic and tuberal region. The CRH-ir neurons observed in the preoptic region are parvocellular and do not, or hardly, display arginine-vasotocin (AVT) immunoreactivity. CRH-ir neurons are also present in the glomerular layer of the olfactory bulb, in the periventricular layer of the optic tectum, and caudal to the glomerular nucleus. A very dense plexus of CRH-ir terminals is located in the most rostral part of the dorsal telencephalon. This region has not been described in other teleosts and is in the present study subdivided into the anterior part of the dorsal telencephalon (Da) and the anterior part of the laterodorsal telencephalon (Dla). High densities of CRH-ir terminals were observed in and around Vl, in the tuberal region, around the rostral part of the lateral recess, and in the caudal part of the vagal lobe. In the pituitary, CRH-ir terminals are concentrated in the neuro-intermediate lobe. Overall, the immunohistochemical and quantitative data correlated well, as the RIA CRH profile in serial 160-microm slices revealed four peaks, which corresponded with major ir-cell groups and terminal fields. Our results strongly suggest that the CRH-ir cells of Vl project to the rostro-dorsal telencephalon. Consequently, they may not be primarily involved in regulation of pituitary cell types but may subserve other functions. The presence of a CRH-containing Vl-Da/Dla projection seems to be restricted to the most modern group of teleosts, i.e., the Acanthopterygians. Further anatomic indications for non-pituitary-related functions of CRH are found in the vagal lobe and the optic tectum of tilapia. Although the low CRH content of the preoptic region reported here for tilapia may be typical for unstressed fish, the fact remains that remarkably few CRH-ir neurons are involved in regulating the pituitary. Overall, the CRH distribution in the brain of tilapia is more widespread than previously reported for other teleosts.