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      Amphibian Melanotrope Subpopulations Respond Differentially to Hypothalamic Secreto-Inhibitors

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          The melanotrope population of the frog intermediate lobe consists of two subtypes of cells, referred to as high-(HD) and low-density (LD) melanotrope cells, which differ markedly in their basal morphofunctional features as well as their in vitro response to hypothalamic factors, such as the stimulator thyrotropin-releasing hormone (TRH) and the inhibitor dopamine. In this study, we have investigated whether other major hypothalamic regulators of the release of α-melanocyte-stimulating hormone (α-MSH), such as γ-aminobutyric acid (GABA) and neuropeptide Y (NPY), also differentially regulate frog melanotrope subpopulations. Our results show that in LD cells, both factors markedly inhibited proopiomelanocortin (POMC) mRNA accumulation and α-MSH secretion. In contrast, the secretory and biosynthetic activity of HD cells was not modified by GABA. NPY inhibited POMC transcript accumulation and tended to reduce α-MSH secretion in HD cells, yet these effects were less pronounced than those evoked in LD cells. In addition, GABA and NPY inhibited the KCl-induced rise in cytosolic free calcium levels in both subpopulations. Taken together, these results further indicate that frog melanotrope subpopulations are differentially regulated by the hypothalamus and strongly suggest that the intensity of such regulation is directly related to the activity of the cell subset. Thus, the LD subpopulation represents a highly responsive cell subset which is regulated by multiple neuroendocrine factors (TRH, dopamine, GABA and NPY), whereas the hormone storage HD subpopulation shows a moderate response to single stimulatory (TRH) and inhibitory (NPY) inputs.

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          Most cited references 13

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          Involvement of retinohypothalamic input, suprachiasmatic nucleus, magnocellular nucleus and locus coeruleus in control of melanotrope cells of Xenopus laevis: a retrograde and anterograde tracing study.

          The amphibian Xenopus laevis is able to adapt the colour of its skin to the light intensity of the background, by releasing alpha-melanophore-stimulating hormone from the pars intermedia of the hypophysis. In this control various inhibitory (dopamine, gamma-aminobutyric acid, neuropeptide Y, noradrenaline) and stimulatory (thyrotropin-releasing hormone and corticotropin-releasing hormone) neural factors are involved. Dopamine, gamma-aminobutyric acid and neuropeptide Y are present in suprachiasmatic neurons and co-exist in synaptic contacts on the melanotrope cells in the pars intermedia, whereas noradrenaline occurs in the locus coeruleus and noradrenaline-containing fibres innervate the pars intermedia. Thyrotropin-releasing hormone and corticotropin-releasing hormone occur in axon terminals in the pars nervosa. In the present study, the neuronal origins of these factors have been identified using axonal tract tracing. Application of the tracers 1,1'dioctadecyl-3,3,3',3' tetramethyl indocarbocyanine and horseradish peroxidase into the pars intermedia resulted in labelled neurons in two brain areas, which were immunocytochemically identified as the suprachiasmatic nucleus and the locus coeruleus, indicating that these areas are involved in neural inhibition of the melanotrope cells. Thyrotropin-releasing hormone and corticotropin-releasing hormone were demonstrated immunocytochemically in the magnocellular nucleus. This area appeared to be labelled upon tracer application into the pars nervosa. This finding is in line with the idea that corticotropin-releasing hormone and thyrotropin-releasing hormone stimulate melanotrope cell activity after diffusion from the neural lobe to the pars intermedia. After anterograde filling of the optic nerve with horseradish peroxidase, labelled axons were traced up to the suprachiasmatic area where they showed to be in contact with suprachiasmatic neurons. These neurons showed a positive reaction with anti-neuropeptide Y and the same held for staining with anti-tyrosine hydroxylase. It is suggested that a retino-suprachiasmatic pathway is involved in the control of the melanotrope cells during the process of background adaptation.
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            Immunocytochemistry and in situ hybridization of neuropeptide Y in the hypothalamus of Xenopus laevis in relation to background adaptation.

            The amphibian Xenopus laevis is able to adapt to a dark background by releasing melanophore-stimulating hormone from the pars intermedia of the pituitary gland. The inhibition of melanophore-stimulating hormone release is accomplished by neuropeptide Y-containing axons innervating the pars intermedia. To determine the production site of neuropeptide Y involved in this inhibitory control, the distribution of neuropeptide Y in the brain has been investigated by immunocytochemistry and in situ hybridization. Immunoreactive cell bodies were visualized in, among others, the ventromedial and posterior thalamic nuclei, and the suprachiasmatic and ventral infundibular hypothalamic nuclei. A positive hybridization signal with a Xenopus-specific probe for preproneuropeptide Y-RNA was found in the diencephalic ventromedial thalamic nucleus and in the suprachiasmatic nucleus. With both immunocytochemistry and in situ hybridization, suprachiasmatic neurons appeared to be stained only in animals adapted to a white background; animals adapted to a black background showed no staining. Quantitative image analysis revealed that this effect of background adaptation is specific for suprachiasmatic neurons because no effect could be demonstrated of the background light condition on the ventral infundibular nucleus (immunocytochemistry) or the ventromedial thalamic nucleus (in situ hybridization). These results indicate that neurons in the suprachiasmatic nucleus enable the adaptation of X. laevis to a white background, by producing and releasing neuropeptide Y that inhibits the release of melanophore-stimulating hormone from the melanotrope cells in the pars intermedia of the pituitary gland.
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              GABA and neuropeptide Y co-exist in axons innervating the neurointermediate lobe of the pituitary of Xenopus laevis — An immunoelectron microscopic study


                Author and article information

                S. Karger AG
                June 2001
                13 June 2001
                : 73
                : 6
                : 426-434
                aDepartment of Cell Biology, University of Córdoba, Córdoba, Spain; bEuropean Institute for Peptide Research (IFRMP 23), Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U-413, UA CNRS, University of Rouen, Mont-Saint-Aignan, France
                54661 Neuroendocrinology 2001;73:426–434
                © 2001 S. Karger AG, Basel

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                Page count
                Figures: 6, Tables: 1, References: 50, Pages: 9
                Regulation of Pituitary Cells


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