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

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.