The regulation of alpha-MSH release by GABA is mediated by a chloride-dependent [Ca2+]c increase in frog melanotrope cells

Distinct type II opsins in the eye decode light properties for background adaptation and behavioural background preference

Crypsis increases survival by reducing predator detection. Xenopus laevis tadpoles decode light properties from the substrate to induce two responses: a cryptic coloration response where dorsal skin pigmentation is adjusted to the colour of the substrate (background adaptation) and a behavioural crypsis where organisms move to align with a specific colour surface (background preference). Both processes require organisms to detect reflected light from the substrate. We explored the relationship between background adaptation and preference and the light properties able to trigger both responses. We also analysed which retinal photosensor (type II opsin) is involved. Our results showed that these two processes are segregated mechanistically, as there is no correlation between the preference for a specific background with the level of skin pigmentation, and different dorsal retina-localized type II opsins appear to underlie the two crypsis modes. Indeed, inhibition of melanopsin affects background adaptation but not background preference. Instead, we propose pinopsin is the photosensor involved in background preference. pinopsin mRNA is co-expressed with mRNA for the sws1 cone photopigment in dorsally located photoreceptors. Importantly, the developmental onset of pinopsin expression aligns with the emergence of the preference for a white background, but after the background adaptation phenotype appears. Furthermore, white background preference of tadpoles is associated with increased pinopsin expression, a feature that is lost in premetamorphic froglets along with a preference for a white background. Thus, our data show a mechanistic dissociation between background adaptation and background preference, and we suggest melanopsin and pinopsin, respectively, initiate the two responses.

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light-mediated skin pigmentation

Melanopsin photopigments, Opn4x and Opn4m, were evolutionary selected to "see the light" in systems that regulate skin colour change. In this review, we analyse the roles of melanopsins, and how critical evolutionary developments, including the requirement for thermoregulation and ultraviolet protection, the emergence of a background adaptation mechanism in land-dwelling amphibian ancestors and the loss of a photosensitive pineal gland in mammals, may have helped sculpt the mechanisms that regulate light-controlled skin pigmentation. These mechanisms include melanopsin in skin pigment cells directly inducing skin darkening for thermoregulation/ultraviolet protection; melanopsin-expressing eye cells controlling neuroendocrine circuits to mediate background adaptation in amphibians in response to surface-reflected light; and pineal gland secretion of melatonin phased to environmental illuminance to regulate circadian and seasonal variation in skin colour, a process initiated by melanopsin-expressing eye cells in mammals, and by as yet unknown non-visual opsins in the pineal gland of non-mammals.

Pharmacological induction of skin pigmentation unveils the neuroendocrine circuit regulated by light

Light-regulated skin colour change is an important physiological process in invertebrates and lower vertebrates, and includes daily circadian variation and camouflage (i.e. background adaptation). The photoactivation of melanopsin-expressing retinal ganglion cells (mRGCs) in the eye initiates an uncharacterized neuroendocrine circuit that regulates melanin dispersion/aggregation through the secretion of alpha-melanocyte-stimulating hormone (α-MSH). We developed experimental models of normal or enucleated Xenopus embryos, as well as in situ cultures of skin of isolated dorsal head and tails, to analyse pharmacological induction of skin pigmentation and α-MSH synthesis. Both processes are triggered by a melanopsin inhibitor, AA92593, as well as chloride channel modulators. The AA9253 effect is eye-dependent, while functional data in vivo point to GABAA receptors expressed on pituitary melanotrope cells as the chloride channel blocker target. Based on the pharmacological data, we suggest a neuroendocrine circuit linking mRGCs with α-MSH secretion, which is used normally during background adaptation.

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.