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.