Dual coupling of opioid receptor-like (ORL1) receptors to adenylyl cyclase in the different layers of the rat main olfactory bulb

Abstract

The coupling of opioid receptor-like (ORL1) receptors to adenylyl cyclase has been investigated in specific layers of the rat main olfactory bulb. Membranes prepared from the olfactory nerve-glomerular layer (ON-G layer), external plexiform layer (EP layer) and granule cell layer (GR layer) displayed specific binding sites for [3H]-nociceptin/orphanin FQ ([3H]Noc/OFQ). In each layer, the presence of high-and low-affinity binding sites, with KD values in the picomolar and nanomolar range, respectively, was detected. The binding of [3H]Noc/OFQ was displaced by unlabelled Noc/OFQ, but not by opioid antagonists. In each layer, Noc/OFQ significantly stimulated [35S]GTPγS binding with nanomolar potencies. In ON-G layer, Noc/OFQ inhibited basal adenylyl cyclase activity and the enzyme stimulations by corticotropin releasing hormone (CRH), Ca2+/calmodulin (Ca2+/CaM) and forskolin (FSK). In EP layer, Noc/OFQ inhibited Ca2+/CaM-and FSK-stimulated enzyme activities. Conversely, in GR layer the peptide stimulated basal cyclase activity and potentiated the enzyme activation by CRH. The Noc/OFQ stimulation was counteracted by the GDP-bound form of the α subunit of transducin and was mimicked by transducin βγ subunits. In the same tissue layer, Ca2+/CaM-and FSK-stimulated enzyme activities were inhibited. Naloxone failed to antagonize all the actions of Noc/OFQ. Western blot and RT-PCR analysis revealed the expression of Ca2+-insensitive and -sensitive adenylyl cyclases in the three layers. These results demonstrate that in rat main olfactory bulb ORL1 receptors can differentially affect distinct forms of adenylyl cyclase in a layer specific manner.