Local inhibitory effects of dynorphin A-(1-17) on capsaicin-induced thermal allodynia in rhesus monkeys.

Although dynorphin A-(1-17) has been characterized in vitro as a high efficacy kappa-opioid receptor agonist, functional studies of dynorphin A-(1-17) following central or systemic administration indicate the involvement of both opioid and non-opioid components. The aim of this study was to investigate whether local administration of dynorphin-related analogs can attenuate capsaicin (8-methyl-N-vanillyl-6-nonenamide)-induced nociception and what type of opioid receptor mediates the local action of dynorphin A-(1-17) in monkeys. Capsaicin (100 microg) was used to evoke a nociceptive response, thermal allodynia, which was manifested as a reduced tail-withdrawal latency in normally innocuous 46 degrees C warm water. Co-administration of dynorphin A-(1-17) (0.3-10 microg) with capsaicin in the tail dose-dependently inhibited thermal allodynia; however, both non-opioid fragments dynorphin A-(2-17) (10-300 microg) and dynorphin A-(2-13) (10-300 microg) were ineffective. Local antiallodynia of dynorphin A-(1-17) was antagonized by a small dose (100 microg) of an opioid receptor antagonist, quadazocine, applied s.c. in the tail. Pretreatment with a selective kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI), s.c. 320 microg in the tail also reversed local antiallodynia of dynorphin A-(1-17). Both locally effective doses of antagonists, when applied s.c. in the back, did not antagonize local dynorphin A-(1-17), indicating that peripheral kappa-opioid receptors selectively mediated the local action of dynorphin A-(1-17) in the tail. In addition, a much larger dose of dynorphin A-(1-17) (1000 microg), when administered s. c. in the back or i.m. in the thigh, did not cause sedative or diuretic effects. These results suggest that in vivo opioid actions of dynorphin-related peptides can be differentiated locally in this procedure. They also indicate that local application of peptidic ligands may be a useful medication for localized pain.