Abstract
Temporal patterns of whole-body α-MSH concentrations and of transcripts of melanocortin receptors during early development as well as the endocrine response (α-MSH, cortisol, MCR mRNAs) to stress at the end of the larval period were characterized in Dicentrarchus labrax. Immunohistochemistry showed α-MSH positive cells in the pituitary pars intermedia in all stages examined. As development proceeds, α-MSH content gradually increases; mRNA levels of mc2r and mc4r remain low until first feeding where peak values are observed. Mc1r expression was constant during development, pomc mRNA levels remain low until the stage of flexion after which a significant increase is observed. At the stage of the formation of all fins, whole-body cortisol and α-MSH concentrations responded with peak values at 2 h post stress. Additionally, the stress challenge resulted in elevated transcript levels of pomc, mc2r and mc4r but not in mc1r, with a pattern characterized by peak values at 1 h post stress and a strong correlation with whole body α-MSH concentrations was found. Our data provide for the first time a view on the importance of the α-MSH stress response in early development of European sea bass, an additional and relatively poorly understood signal involved in the stress response in teleosts.
Highlights
In fish, stress leads to the activation of the hypothalamic-pituitary-interrenal (HPI) axis, and stimulates the pituitary corticotropes in the pars distalis and the melanotropes in the pars intermedia to synthesize and secrete pro-opiomelanocortin (POMC)-derived peptides involved in the mediation and regulation of the stress response[1,2]
The observed increase in whole-body α-MSH concentrations at the advanced stages of early development reported here may reflect the involvement of α-MSH in the formation of melanophores and the coloring of the body[29,30,31,32], which takes place in the period around the formation of the fins
Previous studies of our group have shown that sea bass larvae begin to synthesize cortisol around the stage of first feeding[26,27], which coincides with the expression profile observed for mc2r
Summary
Stress leads to the activation of the hypothalamic-pituitary-interrenal (HPI) axis, and stimulates the pituitary corticotropes in the pars distalis and the melanotropes in the pars intermedia to synthesize and secrete pro-opiomelanocortin (POMC)-derived peptides involved in the mediation and regulation of the stress response[1,2]. In European sea bass (Dicentrarchus labrax, L.) a single form of a functional pomc gene has been cloned and characterized[4] Melanocortins exert their physiological role by binding to a family of specific G protein-coupled receptors (GPCRs) that positively couple to adenylyl cyclase. During HPI axis activation, ACTH secreted by the corticotropes is a key regulator of the acute stress response as it stimulates the interrenal cells via MC2R and results in the synthesis and secretion of cortisol which targets a plethora of tissues, if not all cells in an organism[1,7,8]. More studies have shown that MSH is a corticotrope in Mozabique tilapia[22], in barfin flounder[23] and in rainbow trout[24], but not in carp (Cyprinus carpio)[10] Taken together, these results do suggest a functional role for α-MSH during stress, but whether α-MSH is a corticotrope remains unclear. The search for MCRs other than the MC2R in interrenal tissue is indicated
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