DESCRIPTION: (Applicant's Abstract) Shortly after the cloning of the three opioid receptors, a fourth receptor from this family was cloned and transfected into mammalian cells. Although this receptor (labeled ORL1) has high homology with the opiate receptors, it was found not to bind opiates at physiological concentrations. Recently, an endogenous ligand for this receptor was purified from mammalian brain, by two separate laboratories, and sequenced. This peptide, called nociceptin and orphanin FQ by the two labs, was shown to decrease tail flick latency in mice when administered into the brain. More recent studies have determined that this neurohormone is able to block the antinociception induced by endogenous enkephalins, as well as exogenously administered morphine and other opiate analgesics. Although very little is yet known about this receptor system, potential health relatedness is profound, ranging from the development of nonaddicting analgesics to compounds that block the craving and reward induced by opiate narcotics and other drugs of abuse. The objectives of this grant are to better understand the in vitro and in vivo actions of nociceptin and ORL1. A first step in accomplishing these tasks will be the development and characterization of new ORL1 agonists and antagonists in collaboration with groups using combinatorial chemistry and traditional medicinal chemistry. These newly identified compounds, in conjunction with the careful characterization of binding parameters in various tissues and brain regions, will be used to identify ORL1 subtypes, if they exist, as some evidence suggests. In vitro experiments using synaptosomal or brain slice preparations will be used to determine whether the inhibition of endogenous enkephalin activity is due to inhibition of enkephalin release. In vivo experiments will be conducted to determine whether nociceptin acts as opiate antagonist, or whether it has a more general function as inhibitor of other analgesic pathways such as those mediated by a2-adrenergic agonists, THC, and adenosine A1 agonists. Newly discovered agonists will be used to characterize the systemic actions of ORL1. Finally, newly discovered antagonists will be used to determine whether there are constitutive actions of ORL1 and to test the hypothesis that ORL1 antagonists can act as analgesics.