The κ-opioid receptor or kappa opioid receptor, abbreviated KOR or KOP for its ligand ketazocine, is a G protein-coupled receptor that in humans is encoded by the OPRK1gene. The KOR is coupled to the G proteinGi/G0 and is one of four related receptors that bind opioid-like compounds in the brain and are responsible for mediating the effects of these compounds. These effects include altering nociception, consciousness, motor control, and mood. Dysregulation of this receptor system has been implicated in alcohol and drug addiction.[5][6]
The KOR is a type of opioid receptor that binds the opioid peptidedynorphin as the primary endogenous ligand (substrate naturally occurring in the body).[7] In addition to dynorphin, a variety of natural alkaloids, terpenes and synthetic ligands bind to the receptor. The KOR may provide a natural addiction control mechanism, and therefore, drugs that target this receptor may have therapeutic potential in the treatment of addiction [citation needed].
There is evidence that distribution and/or function of this receptor may differ between sexes.[8][9][10][11]
Based on receptor binding studies, three variants of the KOR designated κ1, κ2, and κ3 have been characterized.[14][15] However, only one cDNA clone has been identified,[16] hence these receptor subtypes likely arise from interaction of one KOR protein with other membrane associated proteins.[17]
Centrally active KOR agonists have hallucinogenic or dissociative effects, as exemplified by salvinorin A (the active constituent in Salvia divinorum). These effects are generally undesirable in medicinal drugs. It is thought that the hallucinogenic and dysphoric effects of opioids such as butorphanol, nalbuphine, and pentazocine serve to limit their abuse potential. In the case of salvinorin A, a structurally novel neoclerodanediterpene KOR agonist, these hallucinogenic effects are sought by recreational users, despite the dysphoria experienced by some users. Another KOR agonist with comparable effects is ibogaine, which has possible medical application in addiction treatment. While these KOR agonists possess hallucinogenic and dissociative effects, they are mechanistically and qualitatively different from those of the 5HT2AR agonist psychedelic hallucinogens such as lysergic acid diethylamide (LSD) or psilocybin and those of NMDAR antagonist dissociatives/anesthetics ketamine and phencyclidine.[19]
The claustrum is the region of the brain in which the KOR is most densely expressed.[20][21][22] It has been proposed that this area, based on its structure and connectivity, has "a role in coordinating a set of diverse brain functions", and the claustrum has been elucidated as playing a crucial role in consciousness.[21][22] As examples, lesions of the claustrum in humans are associated with disruption of consciousness and cognition, and electrical stimulation of the area between the insula and the claustrum has been found to produce an immediate loss of consciousness in humans along with recovery of consciousness upon cessation of the stimulation.[22][23] On the basis of the preceding knowledge, it has been proposed that inhibition of the claustrum (as well as, "additionally, the deep layers of the cortex, mainly in prefrontal areas") by activation of KORs in these areas is primarily responsible for the profound consciousness-altering/dissociative hallucinogen effects of salvinorin A and other KOR agonists.[21][22] In addition, it has been stated that "the subjective effects of S. divinorum indicate that salvia disrupts certain facets of consciousness much more than the largely serotonergic hallucinogen [LSD]", and it has been postulated that inhibition of a brain area that is apparently as fundamentally involved in consciousness and higher cognitive function as the claustrum may explain this.[21] However, these conclusions are merely tentative, as "[KORs] are not exclusive to the claustrum; there is also a fairly high density of receptors located in the prefrontal cortex, hippocampus, nucleus accumbens and putamen", and "disruptions to other brain regions could also explain the consciousness-altering effects [of salvinorin A]".[22]
In supplementation of the above, according to Addy et al.:[20]
Theories suggest the claustrum may act to bind and integrate multisensory information, or else to encode sensory stimuli as salient or nonsalient (Mathur, 2014). One theory suggests the claustrum harmonizes and coordinates activity in various parts of the cortex, leading to the seamless integrated nature of subjective conscious experience (Crick and Koch, 2005; Stiefel et al., 2014). Disrupting claustral activity may lead to conscious experiences of disintegrated or unusually bound sensory information, perhaps including synesthesia. Such theories are in part corroborated by the fact that [salvia divinorum], which functions almost exclusively on the KOR system, can cause consciousness to be decoupled from external sensory input, leading to experiencing other environments and locations, perceiving other "beings" besides those actually in the room, and forgetting oneself and one's body in the experience.[20]
The depressive-like behaviors following prolonged morphine abstinence appear to be mediated by upregulation of the KOR/dynorphin system in the nucleus accumbens, as the local application of a KOR antagonist prevented the behaviors.[26] As such, KOR antagonists might be useful for the treatment of depressive symptoms associated with opioid withdrawal.[26]
A variety of other effects of KOR activation are known:
Activation of the KOR appears to antagonize many of the effects of the MOR, including analgesia, tolerance, euphoria, and memory regulation.[27]Nalorphine and nalmefene are dual MOR antagonists and KOR agonists that have been used clinically as antidotes for opioid overdose, although the specific role and significance of KOR activation in this indication, if any, is uncertain. In any case however, KOR agonists notably do not affect respiratory drive, and hence do not reverse MOR activation-induced respiratory depression.[28]
KOR agonists suppress itching, and the selective KOR agonist nalfurafine is used clinically as an antipruritic (anti-itch drug).
Eluxadoline is a peripherally restricted KOR agonist as well as MOR agonist and DOR antagonist that has been approved for the treatment of diarrhea-predominant irritable bowel syndrome. Asimadoline and fedotozine are selective and similarly peripherally restricted KOR agonists that were also investigated for the treatment of irritable bowel syndrome and reportedly demonstrated at least some efficacy for this indication but were ultimately never marketed.
KOR agonists are known for their characteristic diuretic effects, due to their negative regulation of vasopressin, also known as antidiuretic hormone (ADH).[29]
Found in numerous species of mint, (including peppermint, spearmint, and watermint), the naturally-occurring compound menthol is a weak KOR agonist[56] owing to its antinociceptive, or pain blocking, effects in rats. In addition, mints can desensitize a region through the activation of TRPM8 receptors (the 'cold'/menthol receptor).[57]
Used for the treatment of addiction in limited countries, ibogaine has become an icon of addiction management among certain underground circles. Despite its lack of addictive properties, ibogaine is listed as a Schedule I compound in the US because it is a psychoactive substance, hence it is considered illegal to possess under any circumstances. Ibogaine is also a KOR agonist[60] and this property may contribute to the drug's anti-addictive efficacy.[61]
KOR agonists have been investigated for their therapeutic potential in the treatment of addiction[64] and evidence points towards dynorphin, the endogenous KOR agonist, to be the body's natural addiction control mechanism.[65] Childhood stress/abuse is a well known predictor of drug abuse and is reflected in alterations of the MOR and KOR systems.[66] In experimental "addiction" models the KOR has also been shown to influence stress-induced relapse to drug seeking behavior. For the drug-dependent individual, risk of relapse is a major obstacle to becoming drug-free. Recent reports demonstrated that KORs are required for stress-induced reinstatement of cocaine seeking.[67][68]
One area of the brain most strongly associated with addiction is the nucleus accumbens (NAcc) and striatum while other structures that project to and from the NAcc also play a critical role. Though many other changes occur, addiction is often characterized by the reduction of dopamine D2 receptors in the NAcc.[69] In addition to low NAcc D2 binding,[70][71] cocaine is also known to produce a variety of changes to the primate brain such as increases prodynorphin mRNA in caudate putamen (striatum) and decreases of the same in the hypothalamus while the administration of a KOR agonist produced an opposite effect causing an increase in D2 receptors in the NAcc.[72]
Additionally, while cocaine overdose victims showed a large increase in KORs (doubled) in the NAcc,[73] KOR agonist administration is shown to be effective in decreasing cocaine seeking and self-administration.[74] Furthermore, while cocaine abuse is associated with lowered prolactin response,[75] KOR activation causes a release in prolactin,[76] a hormone known for its important role in learning, neuronal plasticity and myelination.[77]
It has also been reported that the KOR system is critical for stress-induced drug-seeking. In animal models, stress has been demonstrated to potentiate cocaine reward behavior in a kappa opioid-dependent manner.[78][79] These effects are likely caused by stress-induced drug craving that requires activation of the KOR system. Although seemingly paradoxical, it is well known that drug taking results in a change from homeostasis to allostasis. It has been suggested that withdrawal-induced dysphoria or stress-induced dysphoria may act as a driving force by which the individual seeks alleviation via drug taking.[80] The rewarding properties of drug are altered, and it is clear KOR activation following stress modulates the valence of drug to increase its rewarding properties and cause potentiation of reward behavior, or reinstatement to drug seeking. The stress-induced activation of KORs is likely due to multiple signaling mechanisms. The effects of KOR agonism on dopamine systems are well documented, and recent work also implicates the mitogen-activated protein kinase cascade and pCREB in KOR-dependent behaviors.[40][81]
While the predominant drugs of abuse examined have been cocaine (44%), ethanol (35%), and opioids (24%).[82] As these are different classes of drugs of abuse working through different receptors (increasing dopamine directly and indirectly, respectively) albeit in the same systems produce functionally different responses. Conceptually then pharmacological activation of KOR can have marked effects in any of the psychiatric disorders (depression, bipolar disorder, anxiety, etc.) as well as various neurological disorders (i.e. Parkinson's disease and Huntington's disease).[6][83] Not only are genetic differences in dynorphin receptor expression a marker for alcohol dependence but a single dose of a KOR antagonist markedly increased alcohol consumption in lab animals.[84] There are numerous studies that reflect a reduction in self-administration of alcohol,[85] and heroin dependence has also been shown to be effectively treated with KOR agonism by reducing the immediate rewarding effects[86] and by causing the curative effect of up-regulation (increased production) of MORs[87] that have been down-regulated during opioid abuse.
The anti-rewarding properties of KOR agonists are mediated through both long-term and short-term effects. The immediate effect of KOR agonism leads to reduction of dopamine release in the NAcc during self-administration of cocaine[88] and over the long term up-regulates receptors that have been down-regulated during substance abuse such as the MOR and the D2 receptor. These receptors modulate the release of other neurochemicals such as serotonin in the case of MOR agonists and acetylcholine in the case of D2. These changes can account for the physical and psychological remission of the pathology of addiction. The longer effects of KOR agonism (30 minutes or greater) have been linked to KOR-dependent stress-induced potentiation and reinstatement of drug seeking. It is hypothesized that these behaviors are mediated by KOR-dependent modulation of dopamine, serotonin, or norepinephrine and/or via activation of downstream signal transduction pathways.
Of significant note, while KOR activation blocks many of the behavioral and neurochemical responses elicited by drugs of abuse as stated above. These results are indicative of the KOR induced negative affective states counteracting the rewarding effects of drugs of abuse. Implicating the KOR/dynorphin system as an anti-reward system, supported by the role of KOR signaling and stress, mediating both stress-induced potentiation of drug reward and stress-induced reinstatement of seeking behavior.[6][83] This in turn addresses what was thought to be paradoxical above. That is, rather, KOR signaling is activated/upregulated by stress, drugs of abuse and agonist administration - resulting in negative affective state. As such drug addiction is maintained by avoidance of negative affective states manifest in stress, craving, and drug withdrawal.[89] Consistent with KOR induced negative affective states and role in drug addiction, KOR antagonists are efficacious at blocking negative affect induced by drug withdrawal and at decreasing escalated drug intake in pre-clinical trial involving extended drug access.[6][83][82] Clinically there has been little advancement to evaluate the effects of KOR antagonists due to adverse effects and undesirable pharmacological profiles for clinical testing (i.e. long half-life, poor bioavailability). More recently, a selective, high-affinity KOR antagonist LY2456302 was well-tolerated in CUD patients.[90] Showing feasibility a subsequent proof-of-mechanism trial evaluated JNJ-67953964 (previously LY2456302) potential for treating anhedonia in a double-blind, placebo-controlled, randomized trial in patients with anhedonia and a mood or anxiety disorder.[91] The KOR antagonist significantly increased fMRI ventral striatum activation during reward anticipation while accompanied by therapeutic effects on clinical measures of anhedonia, further reinforces the promise of KOR antagonism and proceeding assessment of clinical impact.[91] Additionally a positron emission tomography (PET) study in cocaine use disorder (CUD) patients utilizing a KOR selective agonist [11C]GR103545 radioligand showed CUD individuals with higher KOR availability were more prone to stress-induced relapse.[92] A subsequent PET scan following a three-day cocaine binge showed a decrease in KOR availability, interpreted as increased endogenous dynorphin competing with the radioligand at the KOR binding sites.[92] Taken together these findings are in support of the negative affect state and further implicate the KOR/dynorphin system clinically and therapeutically relevant in humans with CUD.
Taken together, in drug addiction the KOR/dynorphin system is implicated as a homeostatic mechanism to counteract the acute effects of drugs of abuse. Chronic drug use and stress up-regulate the system in turn leading to a dysregulated state which induces negative affective states and stress reactivity.[83]
Traditional models of KOR function in drug addiction have postulated that KOR signaling is associated with dysphoria and aversion, thought to underlie the stress-induced exacerbation of addiction. However, recent research in animal models has proposed alternative models, suggesting that KOR-mediated responses may not act directly on negative valence systems but modulate related processes such as novelty processing.[93][94] Studies in humans same to similar conclusions that KORs may modulate various aspects of reward processing in a manner that is independent of the hedonic valence traditionally ascribed to them.[95][96] This broadens the potential understanding of KORs in addiction beyond a unidimensional framework, implicating their role in complex behaviors and treatment approaches that do not align strictly with stress or aversion. These emerging perspectives may inform the development of novel pharmacotherapies targeting KORs for the treatment of substance use disorders, as they highlight the receptor's multifaceted role in addiction.
^ abcdKarkhanis A, Holleran KM, Jones SR (2017). "Dynorphin/Kappa Opioid Receptor Signaling in Preclinical Models of Alcohol, Drug, and Food Addiction". International Review of Neurobiology. 136: 53–88. doi:10.1016/bs.irn.2017.08.001. ISBN9780128124734. PMID29056156.
^James IF, Chavkin C, Goldstein A (1982). "Selectivity of dynorphin for kappa opioid receptors". Life Sciences. 31 (12–13): 1331–4. doi:10.1016/0024-3205(82)90374-5. PMID6128656.
^Mansour A, Fox CA, Akil H, Watson SJ (January 1995). "Opioid-receptor mRNA expression in the rat CNS: anatomical and functional implications". Trends in Neurosciences. 18 (1): 22–9. doi:10.1016/0166-2236(95)93946-U. PMID7535487. S2CID300974.
^de Costa BR, Rothman RB, Bykov V, Jacobson AE, Rice KC (February 1989). "Selective and enantiospecific acylation of kappa opioid receptors by (1S,2S)-trans-2-isothiocyanato-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexy l] benzeneacetamide. Demonstration of kappa receptor heterogeneity". Journal of Medicinal Chemistry. 32 (2): 281–3. doi:10.1021/jm00122a001. PMID2536435.
^Mansson E, Bare L, Yang D (August 1994). "Isolation of a human kappa opioid receptor cDNA from placenta". Biochemical and Biophysical Research Communications. 202 (3): 1431–7. doi:10.1006/bbrc.1994.2091. PMID8060324.
^ abcAddy PH, Garcia-Romeu A, Metzger M, Wade J (April 2015). "The subjective experience of acute, experimentally-induced Salvia divinorum inebriation". Journal of Psychopharmacology. 29 (4): 426–35. doi:10.1177/0269881115570081. PMID25691501. S2CID34171297.
^ abcdeChau A, Salazar AM, Krueger F, Cristofori I, Grafman J (November 2015). "The effect of claustrum lesions on human consciousness and recovery of function". Consciousness and Cognition. 36: 256–64. doi:10.1016/j.concog.2015.06.017. PMID26186439. S2CID46139982.
^Koubeissi MZ, Bartolomei F, Beltagy A, Picard F (August 2014). "Electrical stimulation of a small brain area reversibly disrupts consciousness". Epilepsy & Behavior. 37: 32–5. doi:10.1016/j.yebeh.2014.05.027. PMID24967698. S2CID8368944.
^ abUrbano M, Guerrero M, Rosen H, Roberts E (May 2014). "Antagonists of the kappa opioid receptor". Bioorganic & Medicinal Chemistry Letters. 24 (9): 2021–32. doi:10.1016/j.bmcl.2014.03.040. PMID24690494.
^ abZan GY, Wang Q, Wang YJ, Liu Y, Hang A, Shu XH, Liu JG (September 2015). "Antagonism of κ opioid receptor in the nucleus accumbens prevents the depressive-like behaviors following prolonged morphine abstinence". Behavioural Brain Research. 291: 334–41. doi:10.1016/j.bbr.2015.05.053. PMID26049060. S2CID32817749.
^Yamada K, Imai M, Yoshida S (January 1989). "Mechanism of diuretic action of U-62,066E, a kappa opioid receptor agonist". European Journal of Pharmacology. 160 (2): 229–37. doi:10.1016/0014-2999(89)90495-0. PMID2547626.
^Tortella FC, Robles L, Holaday JW (April 1986). "U50,488, a highly selective kappa opioid: anticonvulsant profile in rats". The Journal of Pharmacology and Experimental Therapeutics. 237 (1): 49–53. PMID3007743.
^Lawrence DM, Bidlack JM (September 1993). "The kappa opioid receptor expressed on the mouse R1.1 thymoma cell line is coupled to adenylyl cyclase through a pertussis toxin-sensitive guanine nucleotide-binding regulatory protein". The Journal of Pharmacology and Experimental Therapeutics. 266 (3): 1678–83. PMID8103800.
^Konkoy CS, Childers SR (January 1993). "Relationship between kappa 1 opioid receptor binding and inhibition of adenylyl cyclase in guinea pig brain membranes". Biochemical Pharmacology. 45 (1): 207–16. doi:10.1016/0006-2952(93)90394-C. PMID8381004.
^Henry DJ, Grandy DK, Lester HA, Davidson N, Chavkin C (March 1995). "Kappa-opioid receptors couple to inwardly rectifying potassium channels when coexpressed by Xenopus oocytes". Molecular Pharmacology. 47 (3): 551–7. PMID7700253.
^Tallent M, Dichter MA, Bell GI, Reisine T (December 1994). "The cloned kappa opioid receptor couples to an N-type calcium current in undifferentiated PC-12 cells". Neuroscience. 63 (4): 1033–40. doi:10.1016/0306-4522(94)90570-3. PMID7700508. S2CID22003522.
^Kam AY, Chan AS, Wong YH (July 2004). "Kappa-opioid receptor signals through Src and focal adhesion kinase to stimulate c-Jun N-terminal kinases in transfected COS-7 cells and human monocytic THP-1 cells". The Journal of Pharmacology and Experimental Therapeutics. 310 (1): 301–10. doi:10.1124/jpet.104.065078. PMID14996948. S2CID39445016.
^Nielsen CK, Ross FB, Lotfipour S, Saini KS, Edwards SR, Smith MT (December 2007). "Oxycodone and morphine have distinctly different pharmacological profiles: radioligand binding and behavioural studies in two rat models of neuropathic pain". Pain. 132 (3): 289–300. doi:10.1016/j.pain.2007.03.022. PMID17467904. S2CID19872213.
^Chavkin C, Sud S, Jin W, Stewart J, Zjawiony JK, Siebert DJ, Toth BA, Hufeisen SJ, Roth BL (March 2004). "Salvinorin A, an active component of the hallucinogenic sage salvia divinorum is a highly efficacious kappa-opioid receptor agonist: structural and functional considerations". The Journal of Pharmacology and Experimental Therapeutics. 308 (3): 1197–203. doi:10.1124/jpet.103.059394. PMID14718611. S2CID2398097.
^Hasebe K, Kawai K, Suzuki T, Kawamura K, Tanaka T, Narita M, Nagase H, Suzuki T (October 2004). "Possible pharmacotherapy of the opioid kappa receptor agonist for drug dependence". Annals of the New York Academy of Sciences. 1025 (1): 404–13. Bibcode:2004NYASA1025..404H. doi:10.1196/annals.1316.050. PMID15542743. S2CID85031737.
^Michaels CC, Holtzman SG (April 2008). "Early postnatal stress alters place conditioning to both mu- and kappa-opioid agonists". The Journal of Pharmacology and Experimental Therapeutics. 325 (1): 313–8. doi:10.1124/jpet.107.129908. PMID18203949. S2CID30383220.
^Beardsley PM, Howard JL, Shelton KL, Carroll FI (November 2005). "Differential effects of the novel kappa opioid receptor antagonist, JDTic, on reinstatement of cocaine-seeking induced by footshock stressors vs cocaine primes and its antidepressant-like effects in rats". Psychopharmacology. 183 (1): 118–26. doi:10.1007/s00213-005-0167-4. PMID16184376. S2CID31140425.
^Blum K, Braverman ER, Holder JM, Lubar JF, Monastra VJ, Miller D, Lubar JO, Chen TJ, Comings DE (November 2000). "Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors". Journal of Psychoactive Drugs. 32 (Suppl): i–iv, 1–112. doi:10.1080/02791072.2000.10736099. PMID11280926. S2CID22497665.
^Stefański R, Ziółkowska B, Kuśmider M, Mierzejewski P, Wyszogrodzka E, Kołomańska P, Dziedzicka-Wasylewska M, Przewłocki R, Kostowski W (July 2007). "Active versus passive cocaine administration: differences in the neuroadaptive changes in the brain dopaminergic system". Brain Research. 1157: 1–10. doi:10.1016/j.brainres.2007.04.074. PMID17544385. S2CID42090922.
^D'Addario C, Di Benedetto M, Izenwasser S, Candeletti S, Romualdi P (January 2007). "Role of serotonin in the regulation of the dynorphinergic system by a kappa-opioid agonist and cocaine treatment in rat CNS". Neuroscience. 144 (1): 157–64. doi:10.1016/j.neuroscience.2006.09.008. PMID17055175. S2CID34243587.
^Butelman ER, Kreek MJ (July 2001). "kappa-Opioid receptor agonist-induced prolactin release in primates is blocked by dopamine D(2)-like receptor agonists". European Journal of Pharmacology. 423 (2–3): 243–9. doi:10.1016/S0014-2999(01)01121-9. PMID11448491.
^ abcdTejeda HA, Bonci A (June 2019). "Dynorphin/kappa-opioid receptor control of dopamine dynamics: Implications for negative affective states and psychiatric disorders". Brain Research. 1713: 91–101. doi:10.1016/j.brainres.2018.09.023. PMID30244022. S2CID52339964.
^Mitchell JM, Liang MT, Fields HL (November 2005). "A single injection of the kappa opioid antagonist norbinaltorphimine increases ethanol consumption in rats". Psychopharmacology. 182 (3): 384–92. doi:10.1007/s00213-005-0067-7. PMID16001119. S2CID38011973.
^Xi ZX, Fuller SA, Stein EA (January 1998). "Dopamine release in the nucleus accumbens during heroin self-administration is modulated by kappa opioid receptors: an in vivo fast-cyclic voltammetry study". The Journal of Pharmacology and Experimental Therapeutics. 284 (1): 151–61. PMID9435173.
^Narita M, Khotib J, Suzuki M, Ozaki S, Yajima Y, Suzuki T (June 2003). "Heterologous mu-opioid receptor adaptation by repeated stimulation of kappa-opioid receptor: up-regulation of G-protein activation and antinociception". Journal of Neurochemistry. 85 (5): 1171–9. doi:10.1046/j.1471-4159.2003.01754.x. PMID12753076. S2CID26034314.
^Maisonneuve IM, Archer S, Glick SD (November 1994). "U50,488, a kappa opioid receptor agonist, attenuates cocaine-induced increases in extracellular dopamine in the nucleus accumbens of rats". Neuroscience Letters. 181 (1–2): 57–60. doi:10.1016/0304-3940(94)90559-2. PMID7898771. S2CID25258989.
"Opioid Receptors: κ". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology. Archived from the original on 2014-02-23. Retrieved 2007-07-23.