[3H]-nociceptin ligand-binding and nociceptin opioid receptor mrna expression in the human brain.

Following the cloning of the novel nociceptin opioid receptor (NOP(1)) and the identification of its endogenous ligand orphanin FQ/nociceptin the distribution and functional role of the NOP(1) receptor system have been studied mainly in the rodent CNS. In the present study the regional distribution and splice variant expression of the NOP(1) receptor was investigated in the adult human brain using [(3)H]-nociceptin autoradiography, NOP(1) reverse transcriptase PCR and mRNA in situ hybridization. Ligand binding revealed strong expression of functional NOP(1) receptors in the cerebral cortex and moderate signals in hippocampus and cerebellum. Interestingly, the NOP(1) receptor specific ligand was also strongly bound in the human striatum. A matching pattern of mRNA expression was observed with high amounts of NOP(1) mRNA in the prefrontal and cingulate cortex as well as in the dentate gyrus of the hippocampus. mRNA levels in the Ammon's horn and cerebellar cortex were moderate and low in the striatum. A considerable expression of N-terminal NOP(1) splice variant mRNAs was not detectable in the human brain by means of in situ hybridization. This suggests that functional NOP(1) receptors in the human brain are encoded by N-terminal full length NOP(1) transcripts. The present data on the anatomical distribution of nociceptin binding sites and NOP(1) receptor mRNA contribute to the knowledge about opioid receptor systems in the human brain and may promote the understanding of function and pharmacology of the orphanin FQ/nociceptin receptor system in the human CNS.

Opioid receptors in the human cerebellum: evidence from [11C]diprenorphine PET, mRNA expression and autoradiography.

Little is known regarding opioid receptors in the human cerebellum. The present [11C]diprenorphine PET study investigated opioid receptor binding in the human cerebellum in vivo, and showed a differential binding level in cerebellar cortex, vermis and dentate nuclei. The additional study in vitro of opioid receptors in human cerebellar cortex and rat brain corroborated the presence of opioidergic mechanisms in the human cerebellum in contrast to the rat. A differential cellular distribution pattern was detected for the three major opioid receptors investigated. For the mu-receptor, and at a lower level for the kappa-receptor, mRNA expression was mainly observed over granule cells. Binding sites were most prominent in the molecular layer. For the delta-receptor no signal was detected. The consideration of cerebellar opioidergic mechanisms and the distribution patterns of the various opioid receptors may promote the understanding of cerebellar function and of opioidergic pharmacology in the human.

Autoradiographic distribution of mu-, delta- and kappa 1-opioid stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate binding in human frontal cortex and cerebellum.

Opioid receptors are known to couple to G-proteins and to inhibit adenylyl cyclase. Receptor activation of G-proteins can be measured by agonist-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate (GTP gamma S-) binding in brain sections to localize neuroanatomically functional coupling of receptors to intracellular signal transduction mechanisms. In the present study the selective mu-, delta- and kappa 1-opioid agonists DAMGO ([D-Ala2,N-Me-Phe4, Gly-ol5]-enkephalin), DPDPE ([D-Pen2,5]-enkephalin) and enadoline (CI-977) were used to stimulate [35S]GTP gamma S-binding in human brain sections of frontal cortex and cerebellum. In human frontal cortex mu- and delta- opioid stimulated [35S]GTP gamma S-binding was evenly distributed throughout the gray matter, while kappa(1)-opioid stimulated [35S]GTP gamma S-binding was detected predominantly in lamina V and VI. In the cerebellar cortex stimulated [35S]GTP gamma S-binding revealed functional coupling of mu- and kappa 1-opioid receptors in the molecular layer.