Increased content and transport of substance P and calcitonin gene-related peptide in sensory nerves innervating inflamed tissue: Evidence for a regulatory function of nerve growth factor in vivo

The responses of sensory neuropeptides during unilateral, Freund's adjuvant-induced, paw inflammation in the rat were examined. After five days of inflammation, the substance P and calcitonin gene-related peptide content in the sciatic nerve supplying the inflamed paw were increased by 60–75% when compared with the contralateral side. At this time-point, there was also a 30–40% increase in the substance P and calcitonin gene-related peptide content of the dorsal root ganglia (L4–L6), and a 40% increase in the calcitonin gene-related peptide content of the L4–L6 segments of the dorsal spinal cord on the inflammation side. In the dorsal root ganglia, calcitonin gene-related peptide content was also increased as early as 12 h and 48 h after induction of paw inflammation. On day 5 of inflammation, the axonal transport of both sensory neuropeptides towards the inflamed paw, as determined after sciatic nerve ligation, was also markedly increased as compared with the control side. Despite this increased transport, the amount of substance P and calcitonin gene-related peptide present in the inflamed paw itself was either reduced or remained unchanged from day 1 through to day 5 of inflammation pointing towards reduced storage and increased release of the peptides in the inflamed tissue. Nerve growth factor content was markedly increased in the sciatic nerve of the inflamed paw with a peak of +136% at time-point 24 h after induction of inflammation. When rats were systemically treated with anti-nerve growth factor serum, the increase in neuropeptide content in the sciatic nerve of the inflamed paw (day 5) was prevented. On the other hand, local injections of nerve growth factor for 5 days into a noninflamed paw were able to induce an increase in substance P and calcitonin gene-related peptide content in the supplying sciatic nerve. These findings point towards a regulatory function for nerve growth factor in vivo in the stimulation of sensory neuropeptide synthesis during prolonged inflammatory processes.