Multiple Actions of Melanotropic Peptides in the Teleost Oreochromis mossambicus (Tilapia)

In teleosts, two melanotropic peptides regulate the skin melanophores in an antagonistic manner: MCH (melanin-concentrating hormone), and a-MSH (a-melanocyte stimulating hormone^. In addition to their roles in background adaptation, other functions have been postulated for these peptides in both teleosts and mammals. Effects of the peptides have been documented on behavior, immune function, and the neuroendocrine stress response.' Accordingly, influences other than changes in background color have been demonstrated to stimulate or inhibit a-MSH and/or MCH secretion.* Recently, we have shown the remarkable steroidogenic potency of the secretory signal from the neurointermediate lobe of tilapia.3 The presence of a-MSH in this signal could be shown to be essential for this effect, and in particular diacetyl-a-MSH has a strong corticotropic p ~ t e n c y . ~ FIGURE 1 demonstrates that communication between the melanotropes and the interrenal cortisol producing cells is bidirectional: cortisol, when administered in uitro at a concentration measured previously in this species during the initial phase of the stress response, signifi~antly inhibits the sensitivity of the pars intermedia cells to CRH (corticotropinreleasing hormone), without affecting the unstimulated a-MSH release. Areas under the curves (arbitrary units) were 1198 -t311 for the controls, 93 -t59 after cortisol treatment [mean r standard error of the mean (SEM), p < 0.001; n = 6 for both groups]. A control experiment indicated that the cortisol effect could not be attributed to depletion of the melanotropes, since two consecutive CRH pulses, administered at t = 4 h and 1 = 8 h without cortisof pretreatment, elicited statistically indistinguishable responses: 873 -t125 versus 845 It 248 (n = 4; area under the curve). Direct feedback of corticosteroids on melanotropes has to date not been demonstrated in other vertebrate classes, although indirect mechanisms of action have been suggested.s The present results suggest that the melanotropes might be an integral part of the pituitary-inte~enal axis in teleosts. The release of the second melanotropic peptide. MCH, has also been demonstrated to be under the negative influence of corticosteroids. ' However, the authors could not demonstrate a direct in vitro action of MCH on interrenal cortisol release. TABLE 1 illustrates that, in tilapia, immunoads0~tion of endogenously released MCH in uitro reversibly affects the release of a-MSH. MCH, as other hypothalamic neuropeptides in teleosts, is released in the proximity of the melanotropes, and the release of these neuropeptides