Journal of Neuroscience, Vol 12, 864-871, Copyright © 1992 by Society for Neuroscience
Demonstration of coexisting catecholamine (dopamine), amino acid (GABA), and peptide (NPY) involved in inhibition of melanotrope cell activity in Xenopus laevis: a quantitative ultrastructural, freeze- substitution immunocytochemical study
EP de Rijk, FJ van Strien and EW Roubos
Department of Animal Physiology, Faculty of Science, University of Nijmegen, The Netherlands.
This quantitative ultrastructural immunocytochemical study demonstrates the
coexistence of a catecholamine [dopamine (DA)], an amino acid (GABA), and a
neuropeptide [neuropeptide Y (NPY)] in axon varicosities innervating the
pars intermedia of Xenopus laevis. The varicosities are assumed to control
the pars intermedia melanotrope cells, which regulate skin color during the
physiological process of background adaptation. Varicosity profiles appear
to abut melanotrope cells and folliculostellate cells, star-shaped cells
that intimately contact the melanotropes. All varicosity profiles contain
two morphological types of vesicle. Monolabeling studies on routinely fixed
and freeze- substituted tissues showed that the small, electron-lucent
vesicles store GABA, whereas DA and NPY occur in larger, electron-dense
ones. Double and triple labeling experiments, in which the degree of
immunoreactivity was quantified per varicosity profile and per vesicle, led
to the conclusion that (1) DA, GABA, and NPY coexist within almost all
varicosity profiles and (2) DA and NPY are costored within electron- dense
vesicles. Varicosity profiles that about melanotrope cells show a much
higher ratio between the numbers of electron-lucent and electron- dense
vesicles than varicosities contacting folliculostellate cells (15.8 and
3.3, respectively). This differential distribution is in line with the
previous demonstration that, in contrast to GABA, NPY does not act directly
on the melanotrope cells but indirectly, by controlling the activity of the
folliculostellate cells.
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