Peptide-containing innervation of rat femoral lymphatic vessels

The lymphatic vessels conduct lymph fluid, proteins, and potentially antigenic material from the interstitium back to the bloodstream via lymph nodes, where solids are removed by phagocytic cells and recirculating lymphocytes and immunoglobulins are added. Immunostaining for two general neuronal markers, protein gene product 9.5 (PGP 9.5), a cytoplasmic ubiquitin C-terminal hydrolase, and synaptophysin, a calcium-binding four-span integral synaptic vesicle membrane glycoprotein, disclosed an abundant innervation of the large femoral lymphatic vessels in rats. This confirms and extends earlier findings based on nonspecific intravital methylene blue and silver impregnation staining methods. Nerves containing neuropeptide Y, C-flanking peptide of neuropeptide Y, and tyrosine hydroxylase, markers of noradrenergic postganglionic sympathetic fibers, were frequent whereas immunoreactivity to vasoactive intestinal peptide, a neuropeptide present in many cholinergic parasympathetic nerve fibers, was sparse suggesting possible sympathetic and parasympathetic influences. Furthermore, calcitonin gene-related peptide- and substance P-containing fibers were also present in the walls of lymphatic vessels suggesting a possible sensory influence in the coordinated myogenic responses. By comparison to normal light microscopy, confocal microscopy was found useful to trace the perihilar penetration of blood and afferent lymphatic vessels in lymph nodes. PGP 9.5-immunoreactive fibers were found in and around lymph nodes suggesting that there is a neural regulation of lymphoid node function. Because of their distribution, peptide-containing nerves may participate in regulating the capacity of the lymphatic pumping activity, and may possibly exert paracrine effects on lymphocytes.