Cortisol is the principal corticosteriod in teleost fishes and its plasma concentrations rise dramatically during stress. The relationship between this cortisol increase and its metabolic consequences are subject to extensive debate. Much of this debate arises from the different responses of the many species used, the diversity of approaches to manipulate cortisol levels, and the sampling techniques and duration. Given the extreme differences in experimental approach, it is not surprising that inconsistencies exist within the literature. This review attempts to delineate common themes on the physiological and metabolic roles of cortisol in teleost fishes and to suggest new approaches that might overcome some of the inconsistencies on the role of this multifaceted hormone. We detail the dynamics of cortisol, especially the exogenous and endogenous factors modulating production, clearance and tissue availability of the hormone. We focus on the mechanisms of action, the biochemical and physiological impact, and the interaction with other hormones so as to provide a conceptual framework for cortisol under resting and/or stressed states. Interpretation of interactions between cortisol and other glucoregulatory hormones is hampered by the absence of adequate hormone quantification, resulting in correlative rather than causal relationships.

Cortisol in teleosts: dynamics, mechanisms of action, and metabolic regulation

The hypothalamus plays a central role in the integrated regulation of energy homeostasis and body weight, and a number of hypothalamic neuropeptides, such as neuropeptide Y (ref. 1), galanin, CRH (ref. 3) and GLP-1 (ref. 4), have been implicated in the mediation of these effects. To discover new hypothalmic peptides involved in the regulation of body weight, we used differential display polymerase chain reaction to identify messenger RNAs that are differentially expressed in the hypothalamus of ob/+ compared with ob/ob C57B1/6J mice. We show here that one mRNA that is overexpressed in the hypothalamus of ob/ob mice encodes the neuropeptide melanin-concentrating hormone (MCH). Fasting further increased expression of MCH mRNA in both normal and obese animals. Neurons containing MCH are located in the zona incerta and in the lateral hypothalamus. These areas are involved in regulation of ingestive behaviour, but the role of MCH in mammalian physiology is unknown. To determine whether MCH is involved in the regulation of feeding, we injected MCH into the lateral ventricles of rats and found that their food consumption increased. These findings suggest that MCH participates in the hypothalamic regulation of body weight.

A role for melanin-concentrating hormone in the central regulation of feeding behaviour

Adipogenesis and obesity: rounding out the big picture.

Many lower vertebrates exhibit colour change in response to the background. A dual hormonal control of colour change by two antagonistic pituitary melanophorotropic hormones was first postulated in amphibia by Hogben and Slome. It is well established that the melanotropins alpha- and beta-MSH are responsible for pigment dispersion in the integumentary melanophore of lower vertebrates and that these molecules are derived from a common precursor protein, proopiocortin, by specific processing within the intermediate lobe. No evidence has been found for an antagonistic hormone in amphibia, although the existence of such a molecule in the pituitary gland of teleost fishes has long been recognized and was termed the melanophore-concentrating hormone by Enami. Early attempts to separate the two hormones proved unsuccessful. Recently, Baker and Ball re-invoked the dual hormone concept, and it has been suggested that a melanin-concentrating hormone (MCH) is synthesized in the hypothalamus of teleosts and stored and released by the neurohyphophysis. We have now isolated a novel peptide from the pituitary of the salmon (Oncorhynchus keta) possessing an antagonistic function to MSH, and we describe here its chemical and biological characteristics.

Characterization of melanin-concentrating hormone in chum salmon pituitaries.

The endocrinology of stress in fish: an environmental perspective.

The teleost melanin-concentrating hormone -- a pituitary hormone of hypothalamic origin.

Further observations on the distribution and properties of teleost melanin concentrating hormone.

Variations in plasma cortisol concentrations over a 24-hour period in the rainbow trout Salmo gairdneri

When rainbow trout (Salmo gairdneri) and eels (Anguilla anguilla) were kept in black tanks for 3-4 weeks, their plasma cortisol titres were about fourfold higher tha in fish kept in white tanks. In trout, the difference was apparent only under a long photoperiod of 16 h light: 8 h darkness, but in eels the difference was clear under both a long or short photoperiod (9.5 h light: 14.5 darkness). It is suggested that the increase in plasma cortisol seen in black-adapted fish is dependent on either ACTH or MSH secreted by the pars intermedia melanotrophs. No difference was seen in the total cortisol-binding capacity of the plasma nor in interrenal histology in trout from black or white backgrounds.

Theresa A. Rance

Papers

The teleost melanin-concentrating hormone -- a pituitary hormone of hypothalamic origin.

Further observations on the distribution and properties of teleost melanin concentrating hormone.

Variations in plasma cortisol concentrations over a 24-hour period in the rainbow trout Salmo gairdneri

Differences in concentrations of plasma cortisol in the trout and the eel following adaptation to black or white backgrounds.

The in vitro response of the trout interrenal to various fragments of ACTH.