Substance P and somatostatin content and transport in the vagus and sciatic nerves of the aging Fischer 344 rat.

The two widely distributed neuropeptides, substance P (SP) and somatostatin (SS), are synthesized in cell bodies of the sensory ganglia of the vagus and sciatic nerves and transported bidirectionally toward the central nervous system and sites of sensory innervation. In this study, the content of both peptides was measured in the vagus and sciatic nerves of Fischer 344 rats aged 4, 12 and 25 months. In addition, as an indicator of biosynthesis within the sensory ganglia, the quantity of neuropeptide transported during 22 hours in a peripheral orthograde direction was measured using the ligation technique in animals age 12, 18 and 25 months. The content and transported quantity of SP was unchanged or slightly increased in both nerves as a function of age. Somatostatin content was unchanged and transport increased in the vagus of older rats. In contrast, in the sciatic nerve, SS content was reduced by more than 30% in older rats (p less than 0.01); transported somatostatin was proportionately reduced (p less than 0.05). These findings demonstrate that neuropeptide levels in the sensory vagus are not reduced as a function of age. Somatostatinergic dorsal root ganglion neurons may be selectively vulnerable in the aging Fischer rat.