Journal of Neuroscience, Vol 14, 5257-5266, Copyright © 1994 by Society for Neuroscience
Target dependence of adult neurons: pattern of terminal arborizations
S Marty, JM Weinitz and M Peschanski
INSERM CJF 91-02, Faculte de medecine, Creteil, France.
During development, the survival of neurons in the CNS depends critically
on interactions with postsynaptic target cells. The role of target cells on
the maintenance of afferent neurons in the adult, however, is a matter of
controversy. Morphological alterations of target-deprived neurons, such as
axonal retraction or pruning, may occur. We have therefore undertaken an
analysis of target-deprived neurons over time after an excitotoxic lesion
in order to investigate these potential changes. Dorsal column nuclei (DCN)
neurons were deprived of their target neurons in adult rats by the
injection of kainic acid into the ventrobasal thalamic complex.
Anterogradely transported wheat germ agglutinin conjugated to HRP from the
DCN showed a progressive decrease of the density of afferent terminals
during the first month after lesion. Density stabilized thereafter through
the longest time studied (8 months). In contrast, the extent of the area
occupied by DCN projections did not change up to 1 month and then shrank
over time. These results indicated a continuous decrease in the number of
axonal elements in the lesion, which is the strongest during the first
month postlesion. To interpret these data, we then studied axonal
morphology. Diffusion-filled lemniscal axons were labeled by WGA- HRP
injections aimed at the medial lemniscus. There was no conspicuous
alteration of axonal stems in the medial lemniscus. Terminal axonal
arborizations and swellings dramatically decreased in number over the first
month after the kainate injection, and large axonal varicosities were
formed over the same period of time. These morphological data suggest that
the decrease in number of axons in an excitotoxic lesion is related, at
least during the first month, to a loss of axonal terminal arborizations
rather than to a retraction of axonal stems. The pattern of terminal
arborizations in the adult CNS may therefore depend critically on
interactions of afferents with their target neurons, while the maintenance
of the axonal stems does not.