STRUCTURAL AND FUNCTIONAL HETEROGENEITY OF THE OPIOID (MORPHINE) RECEPTOR SYSTEM

G-protein coupled receptors (GPCRs, 7TM receptors) represent an important superfamily of membrane proteins mainly involved in transmembrane signaling. Opioid and antiopiate receptors play a major role in the process of pain relief and are principally involved in the development of opiate addiction. Natural opioid peptides derived from polypeptide macromolecular precursors serve as endogenous ligands of the opioid receptors. Both endogenous opioid peptides and their receptors display significant multiplicity. This group investigates the ligand-receptor interaction and regulation to achieve a better understanding of the molecular basis of heterogeneity. The ultimate goal is to develop new analgesics with reduced side effects.

Phylogenetic diversity of endogenous opioid peptides

Leu- and Met-enkephalin were the first endogenous opioid peptides identified in various mammalian species including the human. Comparative biochemical and bioinformatic evidence indicates that enkephalins are not limited to mammals. Various prodynorphin (PDYN) sequences in lower vertebrates revealed the presence of other enkephalin fingerprints in these precursor polypeptides. Among the novel enkephalins Ile-enkephalin (Tyr-Gly-Gly-Phe-Ile) was primarily observed in the African clawed frog (Xenopus laevis) PDYNs, whereas the structure of Phe-enkephalin (Tyr-Gly-Gly-Phe-Phe) was predicted by analyzing brain cDNA sequences encoding a PDYN of the African lungfish (Protopterus annectens). Ile-enkephalin can also be found in the PDYNs of other fish species including the eel, bichir, zebrafish and tilapia, but no further occurrence for the Phe-enkephalin motif is available as yet. Based on sequencing data, the biological relevance of Phe- and Ile-enkephalin is suggested, because both can arise by regular posttranslational enzymatic processing of the respective neuropeptide precursors. Phylogenetic variability of the endogenous neuropeptides offers an ‘evolutionary established peptide combinatorial library’ that is expectedly increasing as more and more bioinformatic data become available.

Emerging role of opioids and nociceptin in epilepsy

The incidence of epilepsy in the human population is very high, giving a tremendous challenge for the successful medication. For better understanding the mechanisms behind the disease, extensive molecular studies are required. We conducted our studies in animal models with compounds acting in pain relief and playing a role in epilepsy as well. Nociceptin heptadecapeptide influences the threshold for eliciting afterdischarge, generation and spread of seizure activity. The effect is different in partial and generalized seizures. Nociceptin does not modify behavioral changes, spike frequency and duration of discharge. Autoradiographic experiments carried out with radioactive compounds showed a different distribution of the receptors in the brain regions studied, with enhancement of receptor numbers for nociceptin as well as for opioid receptors recognizing morphine. It is shown that nociceptin facilitates the development of seizures in partial, but not in generalized seizures.

Bivalent ligands targeting MOP-NOP receptor heterodimers

Numerous studies have established the presence of GPCRs as dimers in heterologous cell expression systems and also in vivo. In this context, heterodimers of opioid receptors would constitute new targets of specific interest, each entity possessing original properties in terms of function and pharmacology. Mu opioid receptor (MOPr) activation induces analgesia, while NOPr activation would produce hyperalgesia. Thus, in order to fight pain, it would be important to synthesize one molecule combining the agonist effect for the MOP and the antagonist effect for the NOPr receptors, in order to reinforce the synergistic effect on analgesia. The objective of this project is to conceive new analgesic molecules by combining two pharmacophores targeting the MOPr/NOPr heterodimers. This research should therefore speed up comprehension of the physiological roles of pronociceptin-derived neuropeptides and may provide new targets and tools to develop novel, potent, non-opiate and non-addictive analgesic drugs.