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 INVITEE          

 

• pharmacological characterization of a particular compund (ligand): assessment of its effects,

• search for the specific binding site(s)/receptor(s) of the compound (ligand) characterized and assessed,

• specification and typing of the binding site(s)/receptor(s) found.

The publications issued in 1994-1995 covering the results of research that time played crucial role in the discovery of the nociceptinergic system. In the history of pharmacology, this discovery gave the first example of reverse order in search for an unknown endogeneous ligand, preceded by the exploration of the corresponding receptor.
Cloning of the delta-opioid receptor and the related homology tests led to the structural discovery of a new existing, opioid type receptor [Mollerau C. et al.(1994): FEBS Letters 341: 33-38.; Chen Y. et al.(1994): FEBS Letters 347: 279-283.], with no binding ligand known among those for opioid receptors. This „orphan” receptor earned pharmacological/physiological importance when endogeneous agonist to it was isolated by two separate research groups, in the same time, under the name nociceptin [Meunier J.C. et al.(1995): Nature 377: 532-535.] and orphanin FQ [Reinscheid R.K. et al.(1995): Science 270: 792-794.], respectively. In the beginning, the receptor itself was named „opioid receptor-like 1” (ORL1), however, as of the IUPHAR nomenclature, the name is changed to NOP receptor since the year 2000. 

The gene for the human NOP receptor is located on chromosome 20q13.2-13.3. Similar to opioid receptors, the NOP is a seven-transmembrane domain receptor; naloxone does not exert antagonizing effects on it; agonists to classical mu-, kappa-, delta-opioid receptors are not bound by it, and the post-receptor effects of NOP are conveyed by Gi/G0 proteins.

Activation of NOP receptor results in the inhibition of cAMP accumulation [Reinscheid R.K. et al.(1995): Science 270: 792-794.], in the enhancement of phospholipase C activity and of mitogen activated protein kinase activity [New D.C., Wong Y.H.(2002): Neurosignals 11: 197-212.]; further, inhibition of the activity of voltage-gate Ca2+-channels can be found [Connor M. et al.(1996): Br.J.Pharmacol.118: 205-207.], and the intensity of inward K+ current into the cell is elevated [Nicol B. et al.(1996): Br.J.Pharmacol.119: 1080-1083].

¹²⁵I-[Tyr14]NC (labelled nociceptin) binding assays suggested the probable existence of the NOP receptor in two conformational states, or, two distinct receptor manifestations [Mogil J.S., Pasternak G.W.(2001): Pharmacol.Rev. 53: 381-415]. The majority of NOP receptors are shown in postsynaptic locations, although, a lot of experiments described the presence of NOP receptors in the presynaptic areas [Yamazaki T. et al.(2001): Brain Res. 904: 153-156.; Tallent M.K. et al.(2001): J.Neurosci.21: 6940-6948.; Hou M. et al.(2003): Brain Res.964: 179-186.; Pennock R.L. et al.(2012): J.Neurosci.32: 10192-10200.].

By using PET technique  and  ¹¹C labelled NOP-1A  receptor antagonist, the distribution of NOP receptors in healthy human brain was published by Lohith and colleagues [Lohith T.G. et al.(2012): J.Nucl.Med. 53: 385-392.]. Also, the same year, the crystal structure of NOP receptor was explored [Thompson A.A. et al.(2012): Nature 485: 395-399]. NOP receptors were detected in the peripheral nervous system, also in diverse tissues, such as in rat kidney [Hadrup N. et al.(2007): J.Pharmacol.Exp.Ther. 323: 516-524.], in stomach [Grandi D. et al.(2007): Peptides 28: 1572-1579.]. NOP receptor gene expression was studied in small and large intestines, in liver, in ovarium, in testis [Wei Y.et al.(1999): Chin.Med.Sci.J. 14:67-70], in epidermis [Andoh T. et al.(2004): J.Invest.Dermatol. 123: 196-201], in spleen, in vascular endothelial cells, further, in lymphocytes [Nothacker H.P. et al.(1996): Proc.Natl.Acad.Sci.USA 93: 8677-8682.; Arjomand J. et al.(2002): J.Neuroimmunol.130: 100-108.].

Activation of NOP receptor ends up in wide range of effects; i.c.v. (intracerebroventricular) delivery of agonist gives rise to hyperalgesia, to central and peripheral pain relief, to modification in stress related behaviour patterns, to bradycardia, to decrease in blood pressure, to increased diuresis, to decrease in sodium secretion, to increase in bladder capacity, to inhibition of micturition reflex, to inhibition in rewarding effects evoked by morphin, cocain, and alcohol, to immunodepression, to inhibition of bronchus contraction, to inhibition of gastroduodenal motility, to increase in food intake, to inhibition in learning and memory functions, and to inhibition in erectile functions [Lambert D.G.(2008): Nat.Rev.Drug Discovery 7: 694-710.; Mustazza C., Bastanzio G.(2011): Med.Res.Rev. 31: 605-648.]

Nociceptin, the natural agonist to NOP receptor is a 17 aminoacid peptide (heptadecapeptide), differing from the classical opioid peptides in the N terminus, where the first aminoacid among the four ("message domain") is phenylalanine instead of tyramine, determining receptor binding and sequential physiological effects.  Nociceptin (NC) is generated by the enzymatic cleavage of the 176 aminoacid long prepronociceptin (ppNC / ppOFQ). In humans, the gene coding for ppNC is located on chromosome 8p21 [Mollereau C. et al.(1996): Proc.Natl.Acad.Sci.USA 93: 8666-8670.], together with sequences determining other neuropeptides of biological significance (nocistatin, NCII and NCIII). The presence of ppNC and NC could be demonstrated in the developing brain of early gestational period, and the particular representation areas are those touched by pain sensation and stress reactions [Neal C.R. Jr. et al.(2001): J.Chem.Neuroanat.22: 219-249.]. Also. the nociceptin level was demonstrated in some areas of human adult brain [Witta J.et al.(2004): Brain Res. 997: 24-29.].
Nocistatin generated from ppNC by prohormone convertase II acts as if functional antagonist to NC in number of effects [Mogil J.S., Pasternak G.W.(2001): Pharmacol.Rev. 53: 381-415; Chen Y.L. et al.(2010): Neuroscience 168: 226-239.]. According to research proof, it is the conserved C terminus of nocistatin that is responsible for the effects of the peptide exerted in diverse animal species and in humans, too [Crescenzi O. et al.(2000): Biopolymers 53: 257-264.]. Regarding that receptors for nocistatin are not identified so far, unfolding the role of this peptide is strongly retarded even if data inform on the involvement of G protein coupled signalling pathways in nocistatin  downstream actions [Johnson E.E., Connor M.(2007): Br.J.Pharmacol. 152: 315-416.].

Following the burst in knowledge related to NOP receptor, international research cooperations were launched for studies on nociceptinergic system, with synthesis of diverse agonists and antagonists to NOP receptor included. Month to month the number of new publications and patents grew almost exponentially, and the hope for the fast appearance of inventive drugs gave fuel to the ongoing research in the field [Lambert D.G.(2008): Nat.Rev.Drug Discovery 7: 694-710.]

The vehemence of research in the field seems to calm down, still, the fine circumsciptions in publications of our days leave those skilled in the art with the conclusion that, a drug acting on nociceptinergic system is to be expected to reach the market, quite soon [Tekes K. et al.(2013): Mini Rev. Med. Chem.13: 1389-1397.].

Together with my enthusiastic and diligent fellows, we dropped early in the research on nociceptin. We faced the limited facility at work considering them challenge instead of handicap. Soon we decided that the special experimental segment for us to keep step with international research, is the determination, quantification and functional characterization of nociceptin in human tissues, and body fluids.

We succeeded to confirm the functional significance of changes in plasma nociceptin levels confined to patients suffering from primary headaches, notably those suffering from migraine headache and cluster headache; and, also confirmed the functional significance of changes in plasma nociceptin levels confined to patients with chronic ischemic cardiovascular diseases [CEPHALALGIA (2005): 25: 261-266.; CEPHALALGIA (2004): 24: 280-283.; REGULATORY PEPTIDES (2011): 169: 1-5.].

Let me highlight the items of our recent work dealing with nociceptin reactions in uterus tissue, and with chronic diabetes from the point of nociceptin.