NEUROPEPTIDES IN THE SYMPATHETIC SYSTEM - PRESENCE, PLASTICITY, MODULATION, AND IMPLICATIONS

Neuropeptides are ubiquitous in the sympathetic system and modulate tr ansmission at the levels of the intermediolateral cell column, sympath etic ganglia, and neuroeffector junctions. Several neuropeptide-contai ning pathways from the hypothalamus and medulla modulate excitability of preganglionic neurons. Neuropeptides coexist with norepinephrine or acetylcholine in subpopulations of chemically coded, target-specific sympathetic ganglion neurons. Neuropeptide Y is colocalized in adrener gic vasoconstrictor neurons, whereas vasoactive intestinal polypeptide is colocalized in cholinergic sudomotor neurons. Neuropeptide express ion is plastic; during development, neurons that switch from a noradre nergic to a cholinergic phenotype increase expression of vasoactive in testinal polypeptide, somatostatin, and substance P. Preganglionic inp uts increase neuropeptide Y and inhibit substance P expression. Sympat hetic denervation produces sprouting of sensory fibers containing subs tance P and calcitonin gene-related peptide in target tissues. Neurope ptides from preganglionic fibers (e.g., enkephalin) and primary affere nts (e.g., substance P, vasoactive intestinal polypeptide) modulate tr ansmission in sympathetic ganglia. Neuropeptide Y produces vasoconstri ction, prejunctional inhibition of norepinephrine release, and postjun ctional potentiation of norepinephrine effects. Plasma neuropeptide Y increases during intense sympathoexcitation, hypertension, and pheochr omocytoma. Dystrophic neurites containing neuropeptide Y occur in huma n sympathetic ganglia during aging, diabetes, and dysautonomia. Sympat hetic neuropeptides may thus have important clinical implications.