Topical capsaicin-induced allodynia in unanesthetized primates: pharmacological modulation

Topically administered capsaicin produces thermal allodynia, and this effect has been used to investigate pain transduction and its pharmacological modulation. This study investigated the parameters of topical capsaicin-induced thermal allodynia in unanesthetized rhesus monkeys and its pharmacological mod-ulation by centrally acting compounds [a -opioid agonist: (5,7,8)-()-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro [4.5]dec-8-yl)-benzeneacetamide (U69,593); and noncom-petitive N-methyl-D-aspartate (NMDA) antagonists: ketamine and MK-801 (dizocilpine)]. Rhesus monkeys (n 4) were studied within the warm water tail withdrawal assay (20-s maximum latency), using thermal stimuli that are normally not noxious (38 and 42°C). Capsaicin was applied topically on the tail (0.0013 and 0.004 M capsaicin solution on a 1-cm