Melanotropin receptors in the cartilaginous fish, Potamotrygon reticulatus and in the lungfish, Lepidosiren paradoxa

Abstract

1. P. reticulatus skins in vitro exhibit punctate melanophores, and darken in response to the melanotropins in a dose-related manner. 2. The relative potencies (Ac-Nle4-alpha-MSH4–13-NH2< alpha-MSH = Ac-Cys4,Cys10-alpha-MSH4–13-NH2) suggest that the supression of the amino-terminal tripeptide may affect the ideal peptide conformation for interaction with the receptor, which is restored with cyclization of the decapeptide fragment. 3. Our data suggest that hormonal control of P. reticulatus pigment cells is mainly exerted by alpha-MSH, as none of the other agonists (isoproterenol, norepinephrine, MCH and melatonin) exhibited pigment dispersing or aggregating activities. 4. L. paradoxa scales possess non-innervated epidermal and dermal melanophores, which exhibit stellate shape in vitro. 5. Norepinephrine is a weak partial agonist in L. paradoxa epidermal melanophores, in the presence or in the absence of the beta-adrenoceptor blocker propranolol, suggesting that catecholamines do not play a role in the control of pigment cells. 6. Both epidermal and dermal melanophore responses to MCH and to its analogue MCH5–15, and to melatonin were also slow, partial and obtained with high doses of the hormones. 7. The relative insensitivity of L. paradoxa melanophores is in accordance with the animal cryptic behavior, since it lives in muddy dark waters, and does not seem to depend on chromatic adaptation for camouflage.