Inhibition of substance P release from spinal cord tissue after pretreatment with capsaicin does not mediate the antinociceptive effect of capsaicin in adult mice Virginia M. Goettl a,1 , Dennis L. Larson b , Philip S. Portoghese b , Alice A. Larson a, * a Neuroscience Graduate Program, University of Minnesota, Department of Veterinary PathoBiology, 1988 Fitch Avenue, 295 Animal Science/Veterinary Medicine Building, St. Paul, MN 55108, USA b University of Minnesota, Department of Medicinal Chemistry, 8–111 Health Science Unit F, 308 Harvard Street SE, Minneapolis, MN 55455, USA Received 2 October 1996; revised version received 29 January 1997; accepted 6 February 1997 Abstract Substance P (SP) is released from primary afferent fibers in response to nociceptive stimuli. Capsaicin, which produces an initial hyperalgesic response followed by persistent antinociception, also elicits release of SP from primary afferent fibers. Capsaicin pretreatment decreases the content and release of SP from primary afferent fibers. This effect on SP has been hypothesized to mediate the antinociceptive effect of capsaicin. To test this hypothesis, mice were injected intrathecally (i.t.) with antinociceptive doses of capsaicin or SP(1–7) before superfusion of spinal cord tissue with 3 mM capsaicin 24, 48, 96 or 168 h later. N-terminal metabolic fragments of SP that accumulate after capsaicin-induced SP release and are involved in the antinociceptive effect of capsaicin, were also tested. Like capsaicin SP(1–3), SP(1–4) and SP(1–7) were each antinociceptive when injected 24 h before nociceptive testing. However, at this time there was no decrease in capsaicin-evoked release of SP in tissue from capsaicin- and SP(1–7)-pretreated animals compared to those injected with vehicle. In contrast, capsaicin-evoked SP release decreased significantly in tissue from mice pretreated with capsaicin or SP(1–7) 48 h prior to testing. D-Substance P(1–7), which prevents antinociception, blocked capsaicin- and SP(1–7)-induced decreases in SP release, indicating that these effects are mediated by SP N-terminal activity. Total spinal cord content of SP did not differ amongst treatment groups. These data indicate that antinociception does not appear to depend on decreases in SP release or content as antinociception precedes decreases in SP release.  1997 International Association for the Study of Pain. Published by Elsevier Science B.V. Keywords: Substance P N-terminus; Capsaicin; Superfusion; Antinociception; Writhing assay; Pain 1. Introduction Capsaicin (8-methyl N-vanillyl-6-noneamide), the pun- gent substance in hot peppers, selectively excites polymodal nociceptive primary afferent fibers in adult animals and all types of C-fibers in neonates. Capsaicin binds at a specific site which opens non-selective cation channels for which Ca + + has the greatest permeability (Bevan and Szolcsa ´nyi, 1990). A single injection in neonates results in permanent loss of primary afferent C-fibers (Jancso et al., 1977) which is thought to mediate antinociception (Gamse, 1982). In adults, a single injection of capsaicin does not result in degeneration of primary afferent C-fibers and antinocicep- tion is temporary, lasting 2–7 days in rodents (Gamse, 1982). The mechanism for capsaicin-induced antinociception in adults is not well understood. Substance P (SP), an eleven amino acid peptide, may play a significant role as SP is found in primary afferent neurons (De Biasi and Rustioni, 1988). Substance P is released from primary afferent fibers in response to capsaicin injected i.t. in the cat (Go and Yaksh, 1987) and capsaicin perfused onto spinal cord slices in the rat (Gamse et al., 1981a). Substance P is also released in response to noxious stimuli in the cat (Duggan et al., 1987; Schaible et al., 1990), rabbit (Kuraishi et al., 1989) and rat (Go and Yaksh, 1987). After i.t. or s.c. injection of Pain 71 (1997) 271–278 0304-3959/97/$17.00  1997 International Association for the Study of Pain. Published by Elsevier Science B.V. PII S0304-3959(97)03376-9 * Corresponding author. Tel.: +1 612 6243650; fax: +1 612 6250204; e-mail: larsoø11@maroon.tc.umn.edu 1 Current address: Department of Pharmacology, The Ohio State Univer- sity School of Medicine, 5072 Graves Hall, 333 West 10th Avenue, Columbus, OH 43210, USA.