Capsaicin treatment and stress-induced analgesia

Capsaicin modulates animal pain perception, increasing chemosensitive and pressure thresholds following systemic administration, increasing thermal thresholds following intrathecal administration, and decreasing electric shock thresholds following intracerebroventicular (ICV) administration. Since morphine analgesia is decreased in a dose-dependent manner following ICV capsaicin, the present study examined whether ICV injections of capsaicin (0, 25, 50, 100 micrograms) would alter other analgesic responses as well. Experiment 1 demonstrated that the analgesic response to a 450 mg/kg dose of 2-deoxy-D-glucose was significantly reduced by the 25 and 50, but not the 100 micrograms capsaicin dose. Further, while analgesia induced by cold-water swims (CWS) in a 2 degrees C bath was significantly attenuated by the 25 micrograms capsaicin dose, the entire dose range eliminated analgesia induced by CWS in a 15 degrees C bath. Experiment 2 indicated that the capsaicin-induced alterations in CWS analgesia were not attributable to parallel changes in CWS hypothermia. Experiment 3 demonstrated that capsaicin failed to alter both the non-opioid analgesic response induced by 20 inescapable foot shocks (FS) and the opioid analgesic response induced by 80 FS. These data are discussed in terms of the similarities to and/or dissimilarities from capsaicin-induced effects upon morphine analgesia.