NGF‐dependent and NGF‐independent recovery of sympathetic function after chemical sympathectomy with 6‐hydroxydopamine

To help clarify the distinction between the nerve growth factor (NGF)-dependent collateral sprouting of sympathetic nerves and their NGF-independent regeneration after crush, we used 6-hydroxydopamine (OHDA) to destroy the sympathetic terminals in adult rats; this leaves the axons damage-free. Would recovery occur by regeneration and/or collateral sprouting? A single 6-OHDA injection abolished the sympathetic pilomotor field revealed by electrical stimulation of a cutaneous nerve. Recovery began within 2 days, and by 20 days the field was reestablished. If the field was "isolated" by adjacent denervations at the time of 6-OHDA treatment, the recovering pilomotor field expanded extensively into the surrounding territory. In the presence of anti-NGF, however, the pilomotor field expansion ceased at about 60% of its former size; if anti-NGF treatment was discontinued, expansion recommended and extended into the surrounding skin. We suggest that the latter, NGF-dependent, growth phase corresponds to collateral sprouting, and the initial NGF-independent one to regeneration. After simple nerve crush, however, such regeneration can triple the normal sympathetic field size. This difference between crush- and 6-OHDA-induced regeneration might relate to the "cell body reaction" (CBR); the CBR is reduced with increasing distance of the lesion and is undetectable after a 6-OHDA lesion. Since the CBR and the vigor of regeneration are both increased by repeated axonal injury, we tested the effects of multiple 6-OHDA treatments; this significantly increased the initial NGF-independent expansion. We hypothesize that regeneration is regulated largely by mechanisms associated with the CBR, and that neurotrophin-dependent collateral sprouting occurs independently of these.