The levels of immunoreactive somatostatin (IR-somatostatin) were determined in various parts of the rat spinal cord and peripheral nervous system. The axoplasmic transport rate of IR-somatostatin was determined in the sciatic nerve by the ligation technique. The concentration of the peptide (nanograms per mg protein) was 0.8 ± 0.3 in dorsal root ganglia, 22.1 ± 5.1 in dorsal spinal cord, 5.1 ± 1.2 in ventral spinal cord, and 1.1 ± 0.4 in sciatic nerve. IR-somatostatin was present in a higher concentration in the dorsal root (3.9 ± 1.8 ng/mg protein) than in the ventral root (1.3 ± 0.6 ng/mg protein). Column chromatographic studies showed that most of the IR-somatostatin extracted from dorsal root ganglia and sciatic nerve was identical to the synthetic cyclic somatostatin. In addition, 16% of the total immunoreactivity of the dorsal root ganglia, but not of the peripheral nerve, was of a higher molecular weight form, which probably corresponds to a precursor protein of somatostatin. These observations suggest that posttranslational processing of somatostatin is completed before export of the peptide from the neuronal perikarya. IR-somatostatin accumulated proximal to a ligature on the sciatic nerve, thus indicating orthograde transport of the peptide. The estimated apparent transport rate, not corrected for the slow-moving fraction, was 79.8 ± 2.9 mm/24 h. Double ligation studies indicated that only 19% of the peptide in the sciatic nerve was moving at fast rates. The absolute transport rate calculated from these data was 415.6 ± 5.3 mm/24 h. These results confirm earlier histochemical evidence of the presence of somatostatin in sensory ganglia and peripheral nerve and document its orthograde transport toward the peripheral axon terminals. The finding that there are appreciable amounts of IR-somatostatin in ventral roots and the spinal ventral horn support recent evidence of the presence of sensory fibers in the ventral roots.
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