BACKGROUND ADAPTATION AND SYNAPSE PLASTICITY IN THE PARS-INTERMEDIA OF XENOPUS-LAEVIS

The amphibian Xenopus laevis adapts the colour of its skin to the colo ur of its background, by the release of the pro-opiomelanocortin-deriv ed peptide alpha-melanophore-stimulating hormone from the pars interme dia of the pituitary gland. Suprachiasmatic neurons play an important role in adaptation to a light background as they produce the neurotran smitters GABA, dopamine and neuropeptide Y, which inhibit the release of alpha-melanophore-stimulating hormone. These factors are transporte d to axon varicosities contacting the melanotrope cells. In these vari cosities GABA resides in electron-lucent vesicles and neuropeptide Y a nd dopamine coexist in electron-dense vesicles. In this study the effe cts of background adaptation on the morphology of the varicosities in the pars intermedia were studied, using immunoelectron microscopy and morphometry with freeze-substitution-fixed material. Varicosities were found singly and in clusters throughout the pars intermedia. Varicosi ties were identified by the presence of electron-dense and electron-lu cent secretory vesicles, the latter being immunopositive for anti-GABA . Both varicosity types revealed active zones with exclusively GABA-co ntaining vesicles in contact with the presynaptic membrane. When white - and black-adapted animals were compared, significant background effe cts were found with respect to the organization of the varicosities: t he density of varicosity profiles was twice as high in white-adapted a nimals as in black-adapted ones, due to an increase in density of the clustered varicosities. Furthermore, in white-adapted animals varicosi ties were about twice as large as in black-adapted animals. With respe ct to vesicle types, single and clustered varicosities showed a differ ential effect. For both the population of electron-lucent and electron -dense vesicles, single varicosities showed equal numbers in white- an d black-adapted animals, but clustered varicosities showed higher numb ers of electron-lucent and electron-dense vesicles in black-adapted an imals, indicating storage of neurotransmitters. Finally, in varicositi es of white-adapted animals the number and size of the active zones an d the number of electron-lucent vesicles attached to the active zones, were about twice as high as in black-adapted animals, indicating a st ronger GABA release. It is concluded that the profound effects of envi ronmental light conditions on synaptic structure and substructure in t he Xenopus pars intermedia are related to a changed release activity o f neurotransmitters inhibiting the activity of the melanotrope cells.