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Differential Effects of Dopamine on Two Frog Melanotrope Cell Subpopulations¹

Department of Cell Biology, University of Córdoba (J.L.G.d.A., M.M.M., R.M.V.M., A.J.M.F., F.G.N.), 14004-Córdoba, Spain; and European Institute for Peptide Research, Institut Federatif de Recherches Multidisciplinaires sur les Peptides no. 23, Laboratory of Cellular and Molecular Neuroendocrinology (Institut National de la Santé et de la Recherche Médicale U413), Unité Affiliée au Centre National de la Recherche Scientifique (UA CNRS), University of Rouen (M.C.T., H.V.), 76821-Mont-Saint-Aignan, France

Address all correspondence and requests for reprints to: Dr. Francisco Gracia-Navarro, Department of Cell Biology, Avda. San Alberto Magno s/n, University of Córdoba, E-14004 Córdoba, Spain. E-mail: bc1grnaf{at}uco.es

The frog intermediate lobe consists of a single endocrine cell type, the melanotrope cells, which are under the tonic inhibitory control of dopamine. Separation of dispersed pars intermedia cells in a Percoll density gradient has revealed the existence of two melanotrope cell subpopulations, referred to as high-density (HD) and low-density (LD) cells. The aim of the present study was to investigate the effects of dopamine on each of these melanotrope cell subsets. Increasing doses of dopamine, ranging from 10^-9–10^-6 M, inhibited the release of -melanocyte-stimulating hormone (-MSH) in LD (but not in HD) melanotrope cells. In addition, dopamine provoked a significant reduction of the rate of acetylation of -MSH in LD cells but not in HD cells. Similarly, dopamine significantly decreased the accumulation of POMC messenger RNA in LD cells, whereas it did not affect POMC gene expression in the HD melanotrope subset. On the other hand, microfluorimetric studies revealed that dopamine induced a significant reduction of KCl-stimulated cytosolic free calcium concentration in both LD and HD cells. The present study provides additional evidence for functional heterogeneity of melanotrope cells in the frog pars intermedia. Because dopamine plays a pivotal role in the regulation of -MSH secretion, these data suggest the involvement of cell heterogeneity in the physiological process of background color adaptation in amphibians.

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