The Nociceptin Receptor As a Potential Target in Drug Design

There are now four types of opioid receptors. The new designations OP(1), OP(2) and OP(3) correspond, respectively, to the classic delta-, kappa- and micro-nomenclature. OP(4) was previously known as ORL(1), the receptor for the endogenous heptadecapeptide nociceptin/orphanin FQ. Although the cellular effects of nociceptin resemble those of conventional OP(1), OP(2), and OP(3) opioid agonists, its effects on nociceptive processes are quite different. Nociceptin produces spinal analgesia but appears to antagonize the effects of opioids. Following the recent synthesis of the nonpeptide OP(4) agonist Ro-64-6198 by Hoffmann-La Roche and the nonpeptide OP(4) antagonist J-113397 by Banyu, the nociceptin-OP(4) system now represents a viable and intriguing new target for drug design. OP(4) agonists may be of use in the management of neuropathic pain, anxiety, anorexia, epilepsy, drug dependence, male impotence, hypertension, cerebral ischemia and neurogenic bladder. They may also serve as novel diuretics and to help to reduce gastrointestinal motility. OP(4) antagonists may be of use as general analgesics and in the improvement of memory function. This review covers the recent exciting progress in this field, compares the actions of OP(4) agonists and antagonists with those of classic opioids, and seeks to predict some of the untoward effects that may be seen with such drugs.