Denervation-induced sprouting of intact peripheral afferents into the cuneate nucleus of adult rats.

In adult monkeys with dorsal rhizotomies extending from the second cervical (C2) to the fifth thoracic (T5) vertebrae, cortex deprived of its normal inputs regained responsiveness to inputs conveyed by intact peripheral afferents from the face [T.P. Pons, P.E. Garraghty, A.K. Ommaya, J.H. Kaas, E. Taub, M. Mishkin, Massive reorganization of the primary somatosensory cortex after peripheral sensory deafferentation, Science 252 (1991) 1857-1860]. It has been suggested that the extent of this massive topographic reorganization may be due to the establishment of novel connections between intact afferents and neurons denervated after dorsal rhizotomy [P.E. Garraghty, D.P. Hanes, S.L. Florence, J.H. Kaas, Pattern of peripheral deafferentation predicts reorganizational limits in adult primate somatosensory cortex, Somatosens. Motor Res. 11 (1994) 109-117]. Using adult rats with comparably extensive dorsal rhizotomies, we employed anatomical tracing techniques to address this possibility. Subcutaneous hindpaw injections of horseradish peroxidase conjugated to either wheat germ agglutinin or cholera toxin subunit B revealed aberrant expansions of gracile projections into the cuneate and, in one case, external cuneate nucleus within three months of the deafferentation. It seems plausible that such modest sprouting of ascending projections at the level of the brainstem may form functional connections which, through divergence, ultimately drive a larger population of neurons in cortex. This new growth may well account for both the substantial cortical reorganization observed in the 'Silver Spring monkeys' [T.P. Pons, P.E. Garraghty, A.K. Ommaya, J.H. Kaas, E. Taub, M. Mishkin, Massive reorganization of the primary somatosensory cortex after peripheral sensory deafferentation, Science 252 (1991) 1857-1860] and the 'referred sensation' phenomena (see J.P. Donoghue, Plasticity of adult sensorimotor representations, Curr. Opin. Neurobiol., 5 (1995) 749-754 for review) reported to follow proximal limb amputations in humans.