Cycloheximide-sensitive synthesis of substance P by isolated dorsal root ganglia

Abstract

Substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) may be used as a neurotransmitter by certain primary afferent neurones1,2, particularly those carrying pain impulses3–7. Substance P-like immunoreactivity has been localised to the cell bodies of one population of dorsal root ganglion neurones by immunocytochemistry8. It is contained in vesicles9,10 in the central terminals of these neurones8, and has also been demonstrated in the peripheral terminals11,12. As axons and terminals have very little capacity for peptide biosynthesis13, it is possible that substance P is synthesised and packaged in the perikaryon and transported to the terminals by an axoplasmic transport process. Consistent with this is the finding that substance P accumulates proximal to a ligature placed on the dorsal root14. There has, however, been no direct demonstration of the biosynthesis of substance P in the nervous system. We report here that rat dorsal root ganglia incorporate 35S-methionine into substance P, characterised as authentic by immunoprecipitation followed by HPLC. There is a delay of 1–2 h between addition of label and its incorporation into substance P. Synthesis is blocked by cycloheximide suggesting that, in dorsal root ganglia, substance P is synthesised by a conventional ribosomal process. Synthesis of substance P is reduced by some 90% in ganglia from rats treated neonatally with capsaicin, a drug which is thought to destroy a population of primary afferent neurones15.