Melanin‐concentrating hormone like and somatolactin. A teleost‐specific hypothalamic‐hypophyseal axis system linking physiological and morphological pigmentation

Abstract

Plastic adaptation to match the skin colour to the surrounding is key to survival. Two biological responses in skin colour are associated with background adaptation. A fast “physiological response” that aggregates/disperses the pigment organelles of skin chromatophores, and a slow “morphological response” that alters the type and/or density of pigment cells in the skin. Both responses are linked by unknown mechanisms. In this review, we discuss the role in skin colour regulation of two molecules that form part of a hypothalamic‐hypophyseal pathway unique to teleosts, melanin‐concentrating hormone “like” (MCHL) (previously known as MCH), and somatolactin. MCHL neurons project to the neurohypophysis and to the pars intermedia pituitary, where they interact with somatolactin‐expressing cells. With a white background MCHL is released into the circulation to induce rapid melanosome aggregation and skin lightening. Somatolactin is also a fish‐specific peptide whose expression and secretion are altered in organisms adapted chronically to white/black backgrounds, and that regulates morphological pigmentation. We discuss the evidence for a model whereby in teleosts, MCHL and somatolactin provide the previously unknown link between physiological and morphological pigmentation.

Novel Pituitary Actions of TAC4 Gene Products in Teleost

“…In fish models, PRL is involved in a wide range of physiological functions, including reproductive migration [ 33 ], gonadal maturation [ 34 ], reproductive cycling [ 35 , 36 ], nesting and brooding behaviors [ 37 ], and osmoregulation [ 38 ]. As the latest member of the growth hormone (GH)/PRL family, somatolactin was a pituitary hormone unique to fish species and has been reported to play a functional role in the regulation of reproduction [ 39 ], growth [ 40 ] and pigmentation [ 41 ]. These results, as a whole, suggested that HK2 was involved in pituitary regulation of PRL and SLα mRNA expression, which presumably may contribute to the reproduction and growth metabolism in teleost.…”

Estrogen mediates sex differences in preoptic neuropeptide and pituitary hormone production in medaka

“…Considering that pomc-expressing cells in the RPD adjacent to prl cells express adcyap1r1a, Pacap and Vip may stimulate prl expression through their action on these pomc cells. Sl is a fishspecific member of the Gh/Prl family of pituitary hormones, which has been implicated in skin coloration 44 . Across vertebrates, Prl is a key hormonal regulator of ion and water transport in various tissues, including those responsible for whole-organism ion homeostasis 45,46 .…”

Harnessing Hue: Advances and Applications of Fish Skin Pigmentation Genetics in Aquaculture

“…Within Cichlasoma dimerus, Sl has been observed to foster melanophore proliferation, whereas in medaka, it selectively orchestrates the morphogenesis of specific pigment cells [145,146]. Further investigations unveil a close physiological and morphological interdependence between Mch and Sl in effecting body color changes, collectively influencing pigment cell differentiation [147]. Additionally, prolactin has been noted in Nile tilapia (Oreochromis niloticus) to disperse erythrophores and xanthophores, thereby facilitating the development of corresponding skin hues [148,149].…”