EXPRESSION OF NGF RECEPTOR AND GAP-43 MESSENGER-RNA IN DRG NEURONS DURING COLLATERAL SPROUTING AND REGENERATION OF DORSAL CUTANEOUS NERVES

The collateral sprouting of intact sensory axons and the regeneration of damaged ones differ in a number of respects. Regeneration is trigge red by axotomy-induced damage, probably involves the loss of a periphe ral signal, and appears to occur independently of NGF, while collatera l sprouting is evoked and sustained by an increase in a target-derived signal, namely NGF. New findings strengthen the distinction between t hese two phenomena. Nerve growth factor receptor (NGFR) mRNA is increa sed in undamaged DRG neurons whose axons are sprouting into denervated skin. This response is related to an increased availability of target -derived NGF, a proposal supported by a number of findings including i ncreased NGF mRNA in the denervated target. In contrast, we observed l ittle or no change in the NGFR mRNA levels in regenerating neurons, co nsistent with the observations that NGF does not play a role in this p rocess. However, increases in neuronal GAP-43 mRNA are found during bo th regeneration and collateral sprouting, a result in keeping with the proposal that GAP-43 is primarily associated with nerve growth, and t he observation that GAP-43 expression is not especially influenced by NGF. (C) 1994 John Wiley and Sons, Inc.