Agonist and antagonist effects of dynorphin A-(1-13) in a thermal antinociception assay in rhesus monkeys

The agonist and antagonist effects of intravenously administered dynorphin A-(1-13) were characterized in the warm water (50 and 55°C) tail withdrawal assay of antinociception in rhesus monkeys. The peptide dose-dependently elevated tail withdrawal latencies in 50°C water, but was ineffective in 55°C water. The antinociceptive effect of dynorphin was surmountably antagonized by quadazocine (0.1 mg/kg) and nor-binaltorphimine (3.2 mg/kg), but was not antagonized by clocinnamox (0.1 mg/kg); this pattern of antagonism is consistent with a κ-opioid receptor mediated effect. Pretreatment with dynorphin A-(1-13) (0.032-3.2 mg/kg) antagonized the antinociceptive effects of U50,488 and U69,593 in 55°C water, suggesting a low efficacy action of the peptide at the receptors activated by these κ agonists. However, dynorphin A-(1-13) (3.2 mg/kg) did not antagonize other κ agonists: bremazocine (0.018-0.056 mg/kg) and enadoline (0.0056-0.018 mg/kg). Taken together, these dynorphin A-(1-13) findings support the notion of functional κ-opioid receptor subtypes, and it appears that dynorphin A-(1- 13) has limited efficacy at one of these sites. Finally, dynorphin A-(1-13) (0.32 mg/kg) also antagonized the antinociceptive effects of the mu-agonist etonitazene (0.0018-0.01 mg/kg).