GAP-43 AND P75(NGFR) IMMUNOREACTIVITY IN PRESYNAPTIC CELLS FOLLOWING NEUROMUSCULAR BLOCKADE BY BOTULINUM TOXIN IN RAT

Peripheral nerve lesion results in changes in protein epression by neu rons and denervated Schwann cells. In the present study we have addres sed the question whether similar changes take place following function al deneration. Using immunohistochemistry and immunoelectron microscop y we examined changes in growth-associated protein (GAP-43) and low-af finity nerve growth factor receptor (p75(NGFR)) in rat gastrocnemius m uscle following botulinum toxin-induced paralysis. GAP-43 and p75(NGFR ) were selected because they are not expressed by mature intact motor neurons or Schwann cells, but are expressed following nerve lesion in both motor neurons and denervated Schwann cells. In control muscle, GA P-43 and p75(NGFR) immunoreactivity was seen only in nerve fibres near blood vessels. Two weeks after toxin injection, GAP-43 immunoreactivi ty could be seen at the motor endplates and in axons. Intensity of sta ining increased with longer survival and reached a peak between 4 and 8 weeks post-injection. Ultrastructurally, GAP-43 immunoreactivity was confined to nerve terminals and axons, whereas Schwann cells remained negative. Immunostaining for p75(NGFR) also increased following toxin injection and was detected in some terminal Schwann cells and in peri neurial cells of small nerve fascicles near the paralyzed target cells , but not in axons. These results show that changes in expression of G AP-43 in motor neurons following functional denervation closely resemb le the changes following anatomical interruption of nerve-muscle conta ct. GAP-43 was not expressed in Schwann cells, indicating that its upr egulation in these cells is induced by loss of axonal contact or nerve degeneration products. There is no support for a role of p75(NGFR) in incorporation of neurotrophins in axons. The restriction of p75(NGFR) expression to terminal Schwann cells and perineurial cells in close p roximity to the paralyzed target suggests a role for a target-derived signal or, alternatively, macrophages in eliciting this expression.