Journal of Neuroscience, Vol 13, 1106-1119, Copyright © 1993 by Society for Neuroscience
Anatomical and functional changes in the organization of the cuneate nucleus of adult rats after fetal forelimb amputation
RW Rhoades, JT Wall, NL Chiaia, CA Bennett-Clarke and HP Killackey
Department of Anatomy, Medical College of Ohio, Toledo 43699-0008.
A previous study has shown that fetal forelimb removal in the rat results
in an increase in the size of the hindlimb representation in primary
somatosensory cortex and suggested that this anomalous cortical
organization may have resulted from alterations in the primary afferent
innervation of the dorsal column nuclei (Killackey and Dawson, 1989). The
present study used both anatomical and electrophysiological techniques to
examine the effects of fetal forelimb amputation on the dorsal column
nuclei. Rats sustained forelimb removals on embryonic day 16 and were used
in terminal experiments when they reached adulthood (> 60 d of age).
Analysis of cytochrome oxidase-stained sections demonstrated that the
cuneate nucleus ipsilateral to the lesion decreased in volume by an average
of 36.7% (N = 7, p < 0.001, paired t test), but there was no
corresponding increase in the volume of the gracile fasciculus and nucleus.
Bilateral application of HRP to the sciatic nerves demonstrated that axons
that innervate only the gracile nucleus on the intact side of the brainstem
were present in the cuneate nucleus on the deafferented side. Injection of
HRP into the skin overlying the point of the amputation (the stump)
indicated that axons innervating this region filled most of the dorsal
one-half of the shrunken cuneate nucleus and overlapped with the sciatic
nerve afferents innervating the cuneate on this side. Mapping the receptive
fields of multiple unit clusters demonstrated that most recording sites in
the shrunken cuneate nucleus were activated by inputs from the stump and
adjacent skin. In addition, 9.1% (N = 30) of such unit clusters (N = 328)
could also be excited by stimulation of the hindlimb. These were observed
in only three of the nine experiments. Unit clusters with split receptive
fields including the skin overlying the stump and the hindlimb were located
throughout the rostrocaudal extent of the cuneate nucleus. These results
indicate that fetal forelimb amputation results in anatomical expansion of
the central projections of hindlimb afferents into the cuneate nucleus.
This anatomical organization appears weakly expressed in the receptive
fields of cuneate neurons.
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