Substance P (SP) and somatostatin (SS) are two widely distributed neuropeptides that within the vagus and sciatic nerves are localized predominantly in sensory fibers. The effect of diabetes mellitus on their content or transport in sensory nerves is unknown. With the nerve ligation technique, the peripheral orthograde 24- h transport of both peptides was quantified in the vagus nerve 3 days or 1 mo after induction of streptozocin (STZ) diabetes and in both the vagus and sciatic nerves after diabetes of 3 mo duration. In acute (3-day) diabetics, neuropeptide transport in the vagus was unaltered. After 1 mo, SP transport was significantly increased; content in unligated contralateral nerve was unaltered. Transport of SS was unchanged, and content in contralateral nerve was too low to reliably quantitate. After diabetes of 3-mo duration, transport of both peptides in the vagus nerve was increased in STZ-induced diabetic (STZ-D) rats versus both weightand age-matched controls: SP 474 ± 17 (N = 10) vs. 358 ± 32 (N = 13) pg/24 h, STZ-D rats vs. controls, mean ± SE, P < .03; SS 29 ± 4 vs. 20 ± 3 pg/24 h, STZ-D rats vs. controls, P < .02. In the sciatic nerve, SP transport and content were unaltered. SS content was significantly reduced: 17 ± 3 vs. 30 ± 3 pg/3-mm nerve segment, STZ-D rats vs. controls, P < .01. SS transport in the sciatic nerve of diabetic rats was variably reduced (P < .07), and transport rates were increased (1.41 ± 0.13 vs. 0.96 ±0.10 mm/h, STZ-D rats vs. controls, P = .06), suggesting increased SS turnover within diabetic nerves.
This study indicates that the diabetic state has little short-term adverse effect on neuropeptide synthesis and transport within sensory nerves. Decreased SS content in the sciatic nerve may be a useful marker of early diabetic neuropathy.