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

• Published in: Neuroscience Vol. 61; no. 2; pp. 411 - 420
• Main Authors: Tuinhof, R., Artero, C., Fasolo, A., Franzoni, M.F., Ten Donkelaar, H.J., Wismans, P.G.P., Roubos, E.W.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.07.1994; Elsevier
• Subjects: Adaptation, Physiological - physiology; Afferent Pathways - physiology; Afferent Pathways - ultrastructure; alpha-MSH - metabolism; Anatomy; Animals; Axonal Transport; Biological and medical sciences; Brain Mapping; Carbocyanines; Central nervous system; Corticotropin-Releasing Hormone - physiology; Fundamental and applied biological sciences. Psychology; Horseradish Peroxidase; Hypothalamo-Hypophyseal System - physiology; Locus Coeruleus - cytology; Locus Coeruleus - physiology; Melanophores - physiology; Neurotransmitter Agents - metabolism; Optic Nerve - physiology; Pituitary Gland, Anterior - cytology; Pituitary Gland, Anterior - metabolism; Preoptic Area - cytology; Preoptic Area - physiology; Retina - physiology; Skin Pigmentation - physiology; Skin Pigmentation - radiation effects; Suprachiasmatic Nucleus - cytology; Suprachiasmatic Nucleus - physiology; Thyrotropin-Releasing Hormone - physiology; Vertebrates: nervous system and sense organs; Xenopus laevis - physiology; NA; DAB; TH; Dil; NPY; HRP; CRH; PFA; TRH; α-MSH; DA; Suprachiasmatic nucleus; Innervation; Xenopus laevis; Central nervous system; Amphibia; Retina; Salientia; Hypothalamus; Anatomy; Vertebrata; Melanotropic cell; Locus coeruleus; Brain (vertebrata)
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/0306-4522(94)90241-0

The amphibian Xenopus laevis is able to adapt the colour of its skin to the light intensity of the background, by releasing α-melanophore-stimulating hormone from the pars intermedia of the hypophysis. In this control various inhibitory (dopamine, γ-aminobutyric acid, neuropeptide Y, noradrenaline) and stimulatory (thyrotropin-releasing hormone and corticotropin-releasing hormone) neural factors are involved. Dopamine, γ-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.