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Keywords:

  • regeneration;
  • neurotrophism;
  • neuronal plasticity;
  • rat;
  • sensory neuron

Abstract

Some properties of primary sensory neurons change when they reinnervate new peripheral targets (McMahon et al., Neuroscience, 33, 67–75, 1989). We ask here if such influences can extend to the central connectivity of sensory neurons. In adult rats the nerve to the gastrocnemius muscle (GN) and the cutaneous sural nerve (SN) were self- and cross-anastomosed on left- and right-hand sides, respectively, so that they regenerated to either appropriate or inappropriate targets. Ten to 14 weeks later, the distribution and strength of spinal connections of the SN and GN were determined. The unmyelinated afferents in the GN innervating skin increased their connectivity to 286% of that seen for the GN innervating muscle (P < 0.005), and came to resemble normal cutaneous afferents. However, for the SN there was no significant difference between appropriately and inappropriately regenerated nerves by this measure. The ability of myelinated fibres to produce inhibitions and facilitations in dorsal horn cells was also assessed. The intact or self-anastomosed SN produced predominantly inhibitory effects, whilst the GN produced predominantly facilitatory effects. After the SN had regenerated to muscle its central effects became predominantly facilitatory, whilst those of the GN innervating skin became inhibitory. These changes were statistically significant. In conclusion, we have found that major changes in the physiology of central connections in the dorsal horn may occur following peripheral reinnervation of foreign targets. The changes that were seen were appropriate to the new target, and could not easily be explained by non-specific changes due to axotomy, or changes in A-fibre-mediated inhibitions. We suggest that these effects might arise because of trophic influences arising in and specific to different peripheral targets.