In vivo metabolism of substance P in rat striatum utilizing microdialysis/liquid chromatography/micro‐electrospray mass spectrometry

Abstract In vivo microdialysis in combination with liquid chromatography/micro‐electrospray mass spectrometry was used to study the metabolism of substance P in rat brain. A microdialysis probe was used both to introduce substance P into the brain and to collect metabolic products. Unilateral infusions of 25 and 50 pmol μl −1 substance P, or isotopically labeled [D 8 ‐Phe 8 ] substance P, at flow rates of 0.3 μl min −1 , were made through the microdialysis probe implanted in the striatum of anesthetized rats. The metabolic products were analyzed by on‐line mass spectrometry and the results showed that substance P, an 11 amino acid residue polypeptide, was metabolized in the striatum to form N ‐terminal fragments 1–9, 1–8 and 1–7; C ‐terminal fragments 3–11, 5–11, 6–11, 7–11 and 8–11; and internal fragments 4–8, 6–10 and 7–10. The concentrations of these metabolites in dialysates were measured in order to compare the relative importance of several possible metabolic degradative pathways.