Does MCH play a role on establishment or maintenance of social hierarchy in Nile tilapia?

Body coloration has a fundamental role in animal communication by signaling sex, age, reproductive behavior, aggression, etc. Nile-tilapia exhibits dominance hierarchy and the dominants are paler than subordinates. During social interactions in these animals, these color changes occur rapidly, and normally the subordinates become dark. In teleosteans, from the great number of hormones and neurotransmitters involved in color changes, melanocyte hormone stimulates (α-MSH) and melanin concentrates hormone (MCH) are the most remarkable. The aim of this project was to investigate the role of MCH in the establishment of hierarchical dominance of the Nile-tilapia. We analyzed the effect of background coloration in the dominance hierarchy. It was then compared to the melanophore sensibility of dominants and subordinates' fishes to MCH; finally, it was checked if the social rank affects the number of these pigment cells in dominants and subordinated fishes. Fishes which have a social hierarchy established and adjusted individually to the background exhibits paler body coloration when a visual contact was possible, independently of previous social rank and background color. Probably, even recognizing each other, fishes could be defending their new territory. Melanophores of the subordinate fishes were more sensible to MCH than dominants. It suggests that dominants fishes, which are paler than subordinates, could be under a chronic effect of MCH, which could be due a desensitization of melanophores to this hormone. The opposite effect seems to be occurring on subordinate fishes. It was not observed a significant change in the number of melanophores when the fishes were exposed to a prolonged period of agonistic interaction. It is possible that the exposure time for this interaction might not have been sufficient to have any change in the number of these cells of dominants and subordinate fishes.

Chromatic effects of endothelin family peptides in non-innervated fish, Synbranchus marmoratus, melanophores.

The biological activity of endothelins (ETs) in non-innervated Synbranchus marmoratus melanophores was demonstrated. These peptides induced a dose-dependent pigment aggregation (lightening skin) in these cells. However, they presented EC50's (effective concentration required to produce 50% of response) 26, 106 and 35 times higher than, respectively, the melanin concentrating hormone (MCH) EC50, and exhibited a characteristic temporal and dose-dependent autodessensibilization of the aggregative effect on the melanophores of this fish. The receptor characterization suggested the presence of the ET(B) subtype, since BQ-788 (selective antagonist of ET(B)) but not BQ-485 (selective antagonist of ET(A)) blocked the aggregative effect of the hormones. Confirming these data, sarafotoxin (SRTX) S6c, a toxin selective for ET(B), induced maximal aggregation of pigment granules. S6c presented an EC50 6.8 times higher than the MCH EC50, and 3.9, 15.6 and 5.1 times lower than the EC50's ETs, respectively. The melanotropic effect of SRTX S6b and vasoactive intestinal contractor (VIC) were demonstrated for the first time in this work. SRTX S6b induced a dose-dependent pigment aggregation and presented an EC50 2.54 and 17.2 times higher than the S6c and MCH EC50's, respectively. Compared to the ETs it was 1.53, 6.19 and 2.03 times lower, respectively.

The endothelin/sarafotoxin-induced increase of the proliferation of undifferentiated and DMSO-differentiated GEM-81 goldfish erythrophoroma cells is mediated by ETB receptors.

Endothelins (ETs) and sarafotoxins (SRTXs) have been reported to exert ET(B)-mediated effects on vertebrate pigment cells. GEM-81 cell line, a red pigment cell-derived cutaneous tumor of the teleost Carassius auratus, expresses ET(B) receptors and can be differentiated with 1.5% DMSO treatment, thus constituting an useful model to investigate ET and SRTX effects on cultured fish pigment cells. Our aim was to characterize the pharmacology and biological effects mediated by ET receptors in DMSO-differentiated and undifferentiated cells. ET subtype receptors and their respective Ki values in both cell types were determined by competitive binding assays using (125)I ET-1 and BQ-485 (an ET(A) antagonist) or BQ-788 (an ET(B) antagonist). BQ-788, but not BQ-485, significantly reduced (125)I-ET-1 binding in both cell types, with similar low (Ki > nM) affinities. To determine the proliferation effects of ETs/SRTXs, cells were treated for 72 h with the hormones, and counted in a hemocytometer. The proliferation assays were repeated for SRTX S6c in the presence or absence of BQ-788. The results demonstrated that, with the exception of ET-1 (biphasic effect) and ET-3 (no significant effect) in undifferentiated GEM-81 cells, all the tested hormones induced increases in the proliferation of both types of cells. The hormones were equipotent in DMSO-differentiated cells, which exhibited increased sensitivity to ETs, but not to SRTXs, as compared with undifferentiated cells. The BQ-788 antagonistic effect was also exerted on the proliferation responses to SRTX S6c. These results corroborate the long and important evolutionary history of the ET/SRTX receptor system in vertebrate pigment cells.