Central control of melanotrope cells of Xenopus laevis.

Our research focusses on the role of brain and hypophysis in the control of background adaptation in the clawed toad, Xenopus laevis. This adaptation is regulated by alpha-melanophorestimulating hormone (alpha-MSH). Previously, it was shown that various neurotransmitters influence alpha-MSH release. Here we report about the origin of these factors. Using retrograde labelling techniques combined with immunocytochemistry, it was found that the inhibitory transmitters dopamine and neuropeptide Y coexist in neurons in the suprachiasmatic nucleus. These neurons project to the pars intermedia and synaptically contact the alpha-MSH-producing melanotrope cells. In the synapses also GABA is present. Tracing of the optic nerve indicated the presence of a direct retinosuprachiasmatic tract. Furthermore, locus coeruleus neurons project to the pars intermedia. They contain the inhibitory transmitter noradrenaline. The stimulatory factors corticotropin-releasing hormone and thyrotropin stimulating hormone originate from the magnocellular nucleus which send its processes to the neural lobe of the hypophysis.