Kappa Opioid Receptor and Endogenous Opioid Ligands
Receptors are chemical structures made up of proteins that receive and transduce chemical messengers integrated into the biological system, causing cellular and tissue response. According to Aldrich & McLaughlin (2021), ligands are substances that produce physical responses by forming complexes with biomolecules. The Kappa-opioid receptor (KOR) or Nociception receptor (N.O.P.) is encoded by the human Opioid receptor kappa one (OPRK1) gene (Lalanne et al., 2014). It is G-protein coupled. The receptors are localized throughout the central Nervous System (C.N.S.) and Peripheral Nervous System (PNS): the spinal cord, the brain, and the peripheral tissues. The receptors bind opioid and the like compounds and mediate nociception, mood, motor control and consciousness. They attach to the endogenous ligand, dynorphin, which is an opioid peptide. Other ligands binding to these receptors are the synthetic ligands, terpenes, and natural alkaloids. Three variants of kappa-opioid receptors have been characterized, and they are k1, k2, and k3 (Aldrich, & McLaughlin, 2021).
The stimulation of Kappa opioid receptors is aversive, which is referred to as anti-reward or stress systems. Recent research show evidence that the distribution of KOR may vary between sexes. Endogenous opioid ligands are peptides packaged in the soma within the dense core vesicles and bind to the orthosteric site on receptors. Their production is in the central nervous system and the peripheral tissues like the adrenal glands and travel down axon terminals to exert their effects. Endogenous opioid ligands consist of four families; beta-endorphins, Leu and Met-enkephalins, dynorphins, and nociception. These dynorphins, enkephalins, and β-endorphin are formed from enzymatically split large molecules forming specific peptide transmitters.
The β-endorphin precursor is Proopiomelanocortin (POMC). Leu and met-enkephalin are cleaved from preproenkephalin while prodynorphin is cleaved into kappa-opioid receptor ligands α-neoendorphin, dynorphin-B [1–13] and dynorphin-A [1–17]. Nociceptin is derived from prepronociceptin. The peptide transmitters formed through splicing of prepropeptides by enzymes are diverse and receptor-specific.
These peptides bind to μ, k and δ opioid receptors. The ligands of endogenous opioids have varying affinities for the different receptors. Dynorphins mainly bind with higher affinity κ-opioid Receptor, β-Endorphin mainly binds to μ- receptors, while met- and leu-enkephalin binds mainly to δ –Receptor. Nociceptin has a significantly higher affinity for nociceptin opioid receptors.
Nociceptin or orphanin is an endogenous ligand for the nociceptin receptor or opioid receptor-like Receptor. In comparison with a novel non-peptide nociceptin, opioid receptor agonist in a series of GTPgamma35S binding assays performed using ovary cell membranes of the Chinese hamster, which express the human recombinant N.O.P. (CHOhNOP), they both stimulate GTPgamma35S binding (Inan et al., 2021). Nociceptin results are pEC50 values (95%CL) of 9.28(028), and the Ro64-6198 values are 7.61(0.18). The whole CHOhNOP cells were performed a cAMP inhibition study. They both inhibit cAMP formation with a nociceptin value of 8.45(0.9) and Ro64-6198 value of pEC50.