Nerve Fibres Immunoreactive for Calcitonin Gene-Related Peptide and Substance P in the Rat Superior Cervical Sympathetic Ganglion: Distribution, Incidence and Increase with Synapse Formation Following Preganglionic Denervation

Objective: In rat sympathetic ganglia decentralization by preganglionic denervation leads to new synapse formation. We explored the extent to which these synapses might be derived from peptidergic nerve fibres. Material and Method: The distribution of immunoreactivities for calcitonin gene-related peptide and substance P was determined in superior cervical ganglia of normal rats and of rats following preganglionic denervation with prevented reinnervation. Results: This study has shown that networks of nerve fibres immunoreactive for calcitonin gene-related peptide alone, for both peptides together and for substance P alone become profuse and widespread in the decentralized ganglion in the longer term, in the absence of reinnervation by the preganglionic nerve fibres. No principal neurones of the ganglia showed immunoreactivity for substance P, but a few were weakly immunoreactive for calcitonin gene-related peptide. These were not seen to contribute fibres to the networks. The newly-formed networks persist over at least 12 to 16 months, and their constituent fibres form axo-dendritic synaptic contacts upon ganglionic neurones. To the extent that these fibres originate extrinsically to the ganglion, activity in their parent neurones should be capable of inducing or modulating activity in the ganglionic neurones. Conclusion: It is suggested that activity in such new sensory collateral branches may contribute to the low levels of ganglionic activation observable in the autonomic failure of multiple system atrophy in man. The actions of the released peptides might moreover play an important role in the maintenance of the denervated neurones, pending possible reinnervation by regenerating preganglionic nerve fibres.