DENERVATION-INDUCED SPROUTING OF INTACT PERIPHERAL AFFERENTS INTO THECUNEATE NUCLEUS OF ADULT RATS

In adult monkeys with dorsal rhizotomies extending from the second cer vical (C-2) to the fifth thoracic (T-5) vertebrae, cortex deprived of its normal inputs regained responsiveness to inputs conveyed by intact peripheral afferents from the face [T.P. Pens, P.E. Garraghty, A.K. O mmaya, J.H. Kaas, E. Taub, M. Mishkin, Massive reorganization of the p rimary somatosensory cortex after peripheral sensory deafferentation, Science 252 (1991) 1857-1860]. It has been suggested that the extent o f this massive topographic reorganization may be due to the establishm ent of novel connections between intact afferents and neurons denervat ed after dorsal rhizotomy [P.E. Garraghty, D.P. Hanes, S.L. Florence, J.H. Kaas, Pattern of peripheral deafferentation predicts reorganizati onal limits in adult primate somatosensory cortex, Somatosens. Motor R es. 11 (1994) 109-117]. Using adult rats with comparably extensive dor sal rhizotomies, we employed anatomical tracing techniques to address this possibility. Subcutaneous hindpaw injections of horseradish perox idase conjugated to either wheat germ agglutinin or cholera toxin subu nit B revealed aberrant expansions of gracile projections into the cun eate 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 function al connections which, through divergence, ultimately drive a larger po pulation of neurons in cortex. This new growth may well account for bo th the substantial cortical reorganization observed in the 'Silver Spr ing monkeys' [T.P. Pens, P.E. Garraghty, A.K. Ommaya, J.H. Kaas, E. Ta ub, M. Mishkin, Massive reorganization of the primary somatosensory co rtex after peripheral sensory deafferentation, Science 252 (1991) 1857 -1860] and the 'referred sensation' phenomena (see J.P. Donoghue, Plas ticity of adult sensorimotor representations, Curr. Opin. Neurobiol., 5 (1995) 749-754 for review) reported to follow proximal Limb amputati ons in humans. (C) 1997 Elsevier Science B.V.