ABSTRACT
The specific involvement of the delta-opioid receptor in the control of nociception was explored by investigating the pharmacological activity in vivo of a selective, orally active, and centrally penetrant delta-opioid agonist. [8R-(4bS*,8aalpha,8abeta,12bbeta)]7,10-dimethyl-1-methoxy-11-(2-methylpropyl)oxycarbonyl 5,6,7,8,12,12b-hexahydro-(9H)-4,8-methanobenzofuro[3,2-e]pyrrolo[2,3-g]isoquinoline hydrochloride (SB-235863) is a new pyrrolomorphinan with high affinity (Ki = 4.81 +/- 0.39 nM) for the delta-opioid receptor, full agonist activity, and binding selectivity versus the mu- and kappa-opioid receptors of 189-fold and 52-fold, respectively. Perorally administered SB-236863 was inactive in the rat tail-flick and hot-plate tests of acute pain response, but potently reversed thermal hyperalgesia in rats resulting from a carrageenan-induced inflammatory response. This activity could be blocked by the delta-opioid antagonist naltrindole (3 mg/kg s.c.), but selective mu- and kappa-opioid antagonists were ineffective. Naltrindole (1 microg i.c.v.) also blocked the activity of 10 mg/kg (p.o.) SB-235863, showing that the compound activates delta-opioid receptor sites in the central nervous system. SB-235863 was additionally effective at reversing chronic hyperalgesia in the Seltzer rat model of partial sciatic nerve ligation after peroral administration. These data show that the delta-opioid receptor plays a selective role in regulating evoked and lasting changes in nociceptive pain signaling. Classical side effects of mu- and kappa-opioid receptor activation (slowing of gastrointestinal transit and motor incoordination, respectively) were not observed after administration of 70 mg/kg (p.o.) SB-235863, nor was evoked seizure activity affected. These results suggest a selective and limited role of delta-opioid receptors in the modulation of nociception.
