| The opioid family of G-protein coupled receptors comprises four known receptor subtype genes (delta, mu, kappa, ORL1) and further receptor heterogeneity within each opioid receptor subfamily has been proposed. All four genes are expressed throughout the central nervous system and are believed to modulate a variety of behavioural responses including analgesia. Opiate drugs such as morphine that are selective for the mu receptor subtype are effective analgesics, but their chronic use is limited by the appearance of side effects such as respiratory depression, constipation and dependence. Consequently, the analgesic potential of agonists selective for other opioid receptors is under investigation. In this regard, previous studies suggest that delta agonists mediate antinociception, yet produce fewer adverse effects than mu agonists. To further investigate the cloned delta opioid receptor (DOR) as a target for novel analgesics, the pharmacological role of DOR in brain was evaluated in rats.;First, we characterized delta agonist binding sites and receptor activation in rat brain membranes. We also introduced a novel antagonist radioligand, [125I]AR-M100613, to label tissues with low delta opioid receptor expression in order to support follow-up studies where radioligand binding was performed on rat brain membranes following antisense treatment. Second, we examined the behavioural response to delta agonists in rats. Deltorphin II and SNC80 (i.c.v.) were shown to induce antinociception in acute pain assays, and to reverse hyperalgesia following tissue inflammation induced by Freund's adjuvant with even greater potency. These findings indicate that delta receptors play an enhanced role in the modulation of descending pain pathways following tissue injury. Deltorphin II and SNC80 (i.c.v. ) were also shown to induce hyperlocomotor activity. Third we used antisense studies to demonstrate that the antinociceptive and locomotor stimulant effects of delta agonists are modulated by the cloned delta opioid receptor (DOR). In contrast to other delta agonists, the antinociceptive effects of DPDPE were not modulated by DOR antisense treatment but rather were blocked by a selective mu antagonist (CTOP) suggesting that DPDPE may activate mu sites in the brain rather than an alternate delta receptor subtype. Finally, we demonstrated that peptide nucleic acids (PNA, i.c.v.) can act as target-specific and sequence-selective antisense agents. In total, these findings demonstrate that DOR is an appropriate target for the development of novel analgesics and that PNA can serve as effective antisense agents for the determination of gene function for CNS targets. |
