Plasticity of postganglionic sympathetic neurons in the rat superior cervical ganglion after axotomy

The neuropeptides galanin (GAL) and vasoactive intestinal polypeptide (VIP) are upregulated in spinal and vagal sensory as well as in cranial motor neurons after axonal transection. In this study an increase of both peptides is demonstrated in axotomized principal ganglionic neurons (PGN) of the rat sympathetic superior cervical ganglion by use of double-labeling immunofluorescence. Compared to control ganglia that do not contain more than 1% GAL- or VIP-positive cells, about 26% of all PGN exhibit GAL immunoreactivity by day 1 after transection of the major postganglionic branches. The proportion of immunoreactive neurons reaches its maximum after 30 days (40%) and decreases to about 27% within the second month after axotomy. The percentage of VIP-positive neurons is much lower than for GAL: 2% of the PGN exhibit VIP immunoreactivity at day 1 and about 7% are observed 30 and 60 days after axotomy. In order to further characterize newly GAL- and VIP-positive PGN, their cell diameters were determined 12 days after axotomy. Compared to the mean overall neuron diameter of 24.8 microns, GAL-immunoreactive neurons are predominantly of small and intermediate size (22.2 microns), whereas VIP occurs mainly in larger neurons (26.1 microns). Besides cell bodies, many intraganglionic nerve fibers stain positive for GAL or VIP, particularly at day 6. Most likely, these fibers represent axons, as indicated by the absence of MAP2, a cytoskeletal protein found in neuronal somata and dendrites. They establish direct membrane contacts with postganglionic perikarya, as revealed by pre-embedding immuno-electron microscopy. Some cell bodies and fibers contain both peptides. Colocalization of GAL or VIP with tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis, reveals a reduced immunoreactivity for TH in intensely GAL- or VIP-positive cells, and vice versa at day 6. However, no difference in staining intensity for VIP or GAL, and TH, is observed after 30 and 60 days. Possible implications of GAL and VIP for peripheral nerve regeneration and their regulation by target-derived factors are discussed.