Rapid modulation of micro-opioid receptor signaling in primary sensory neurons.

Management of pain by opioid analgesics is confounded by central adverse effects that limit clinical dosages. Consequently, there is considerable interest to understand peripheral analgesic effects of opioids. The actions of opioids on peripheral sensory neurons have been difficult to study because of a general lack of effect of opioid agonists on nociceptor function in culture despite documented presence of opioid receptors. In this study, the micro-opioid receptor agonist, [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), did not alter guanosine 5'-O-(3-[(35)S]thio)-triphosphate (GTPgamma[(35)S]) binding, adenylyl cyclase activity, or neuropeptide release in primary cultures of rat trigeminal ganglion (TG). However, after brief exposure to bradykinin (BK), DAMGO stimulated GTPgamma[(35)S] binding and inhibited both prostaglandin E(2) (PGE(2))-stimulated adenylyl cyclase activity and BK/PGE(2)-stimulated neuropeptide release. The effect of BK was blocked by the B(2) antagonist HOE 140 [D-Arg[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-bradykinin], but not by the B(1) antagonist, Lys-[Leu8]des-Arg9-BK, and was mimicked by the protease-activated receptor-2 agonist, Ser-Leu-Ile-Gly-Arg-Leu-NH(2), and by activation of protein kinase C (PKC) or by administration of arachidonic acid (AA). The enhanced responsiveness of micro-opioid receptor signaling by BK priming was blocked by both cyclooxygenase and PKC inhibitors; however, the effect of AA was blocked only by a cyclooxygenase inhibitor. The results indicate that micro-opioid receptor signaling in primary sensory TG neurons is enhanced by activation of phospholipase C-coupled receptors via a cyclooxygenase-dependent AA metabolite that is downstream of PKC.

Differential effects of 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) on 5-hydroxytryptamine(2C) receptor-mediated responses.

5-Methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) is a selective, high-affinity 5-hydroxytryptamine (serotonin)(2C) receptor ligand that has been previously characterized as a competitive 5-HT(2C) receptor antagonist that has a long duration of activity in vivo. It is active in two preclinical models of anxiety and has an improved anxiolytic profile compared with benzodiazepines. In this study, we further characterized the pharmacological properties of SB 243213 by measuring its effects on each of multiple responses coupled to the 5-HT(2C) receptor. In Chinese hamster ovary cells, SB 243213 was an inverse agonist for the phospholipase A(2) response, for guanosine 5'-O-(3-[(35)S]thio)triphosphate binding, for reduction of constitutive desensitization, and for enhancement of dopamine release in the rat nucleus accumbens, with relative efficacies of 0.6, 1, 1, and 0.6, respectively. However, for the phospholipase C (PLC) signaling cascade, SB 243213 behaved as an antagonist. Although SB 243213 was previously characterized as a competitive antagonist for the PLC response, the magnitude of the dextral shift of the 5-HT concentration-response curve was time-dependent, and the maximal PLC response to 5-HT was decreased, probably as a result of the slow dissociation rate of SB 243213 (initial dissociation rate was 3.2 times slower than SB206553, a prototypical 5-HT(2C) receptor inverse agonist). Taken together, these data show that the pharmacological characteristics of SB 243213 at the 5-HT(2C) receptor differ depending upon the response measured, and they support the hypothesis that different drugs, acting at the same receptor subtype, can differentially regulate multiple cellular signaling systems.