Long-term changes in the distribution of galanin in dorsal root ganglia after sciatic or spinal nerve transection in rats

The neuropeptide galanin is upregulated in primary afferent and sympathetic neurones and might be involved in the development of sympathetic perineuronal baskets ("rings") following nerve injury. Galanin, calcitonin gene-related peptide and tyrosine hydroxylase have been examined immunohistochemically in dorsal root ganglia and associated roots at times up to one year after transection of either sciatic or L5 spinal nerves in adult rats. Small diameter somata containing calcitonin gene-related peptide (with or without galanin) were reduced in number, whereas galanin (and, at later times, calcitonin gene-related peptide) appeared in medium to large diameter cells after both types of lesion. Galanin also appeared in axons in grey rami and somata in lumbar paravertebral ganglia. Within dorsal root ganglia, galanin-positive axons formed perineuronal rings of two types: (i) smooth coiled axons surrounded small (< 30 microm diameter) somata from which they probably arose; these were rare after 12 weeks, particularly after a spinal nerve lesion; and (ii) varicose terminals encircled medium to large galanin-positive somata; some arose from brightly immunofluorescent somata nearby and took nearly a year to disappear. About 30% of varicose galanin-positive rings had associated calcitonin gene-related peptide-positive terminals (partly colocalized) whereas nearly 45% had associated tyrosine hydroxylase-positive terminals (partly colocalized). Synaptophysin was present in swollen axons and in some varicosities of all types.We conclude that, after peripheral nerve lesions, varicose perineuronal rings around large diameter dorsal root ganglion cells may be formed by axotomized primary afferent neurones (some containing calcitonin gene-related peptide) and sympathetic neurones, both of which contain upregulated galanin. Exocytosis from the varicosities may modify the excitability of mechanosensitive somata. Small galanin-positive somata disappear over several months after both lesions as calcitonin gene-related peptide reappears in medium to large neurones.