INVOLVEMENT OF RETINOHYPOTHALAMIC INPUT, SUPRACHIASMATIC NUCLEUS, MAGNOCELLULAR NUCLEUS AND LOCUS-COERULEUS IN CONTROL OF MELANOTROPE CELLSOF XENOPUS-LAEVIS - A RETROGRADE AND ANTEROGRADE TRACING STUDY

The amphibian Xenopus laevis is able to adapt the colour of its skin t o the light intensity of the background, by releasing alpha-melanophor e-stimulating hormone from the pars intermedia of the hypophysis. In t his control various inhibitory (dopamine, gamma-aminobutyric acid, neu ropeptide Y, noradrenaline) and stimulatory (thyrotropin-releasing hor mone and corticotropin-releasing hormone) neural factors are involved. Dopamine, gamma-aminobutyric acid and neuropeptide Y are present in s uprachiasmatic neurons and co-exist in synaptic contacts on the melano trope cells in the pars intermedia, whereas noradrenaline occurs in th e locus coeruleus and noradrenaline-containing fibres innervate the pa rs intermedia. Thyrotropin-releasing hormone and corticotropin-releasi ng hormone occur in axon terminals in the pars nervosa. In the present study, the neuronal origins of these factors have been identified usi ng 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, w hich were immunocytochemically identified as the suprachiasmatic nucle us and the locus coeruleus, indicating that these areas are involved i n neural inhibition of the melanotrope cells. Thyrotropin-releasing ho rmone and corticotropin-releasing hormone were demonstrated immunocyto chemically in the magnocellular nucleus. This area appeared to be labe lled upon tracer application into the pars nervosa. This finding is in line with the idea that corticotropin-releasing hormone and thyrotrop in-releasing hormone stimulate melanotrope cell activity after diffusi on from the neural lobe to the pars intermedia. After anterograde. fil ling of the optic nerve with horseradish peroxidase, labelled axons we re traced up to the suprachiasmatic area where they showed to be in co ntact with suprachiasmatic neurons. These neurons showed a positive re action with anti-neuropeptide Y and the same held for staining with an ti-tyrosine hydroxylase. It is suggested that a retino-suprachiasmatic pathway is involved in the control of the melanotrope cells during th e process of background adaptation.