SHOWN TO LEFT: Crystal structure of the k-opioid receptor in complex with JDTic

PDB file 4DJH

See Also Crystal Structure of µ-opioid receptor.

Protein Structure Home Page

Crystallographic structure of the human ?-opioid receptor homo dimer (4djh) imbedded in a cartoon representation of a lipid bilayer. Each monomer is individually rainbow color-ed (N-terminus = blue, C-terminus = red). The receptor is bound to the ligand JDTic.[1] Image from Wikipedia.com Note: In this new image, the display of lysozyme is hidden.

ABOUT THE K-OPIOID RECEPTOR

The k-opioid receptor (KOR) is a protein that in humans is encoded by the OPRK1 gene. The k-opioid receptor is one of five related receptors that bind opium-like compounds in the brain and are responsible for mediating the effects of these compounds. These effects include altering the perception of pain, consciousness, motor control, and mood.

The k-opioid receptor is a type of opioid receptor that binds the opioid peptide dynorphin as the primary endogenous ligand.^[2] In addition to dynorphin, a variety of natural alkaloids and synthetic ligands bind to the receptor. The k-opioid receptor may provide a natural addiction control mechanism, and consequently selective agonists of this receptor may have therapeutic potential in the treatment of addiction.

Distribution

k-Opioid receptors are widely distributed in the brain (hypothalamus, periaqueductal gray, and claustrum), spinal cord (substantia gelatinosa), and in pain neurons.^[3][4]

Subtypes

Based on receptor binding studies, three variants of the k-opioid receptor designated k₁, k₂, and k₃ have been characterized.^[5][6] However only one cDNA clone has been identified,^[7] hence these receptor subtypes likely arise from interaction of one k-opioid receptor protein with other membrane associated proteins.^[8]

Function

It has long been understood that k-opioid receptor agonists are dysphoric^[9] but dysphoria from k-opioid receptor agonists has been shown to differ between the sexes.^[10][11] More recent studies have shown the aversive properties in a variety of ways^[12] and the k-opioid receptor has been strongly implicated as an integral neurochemical component of addiction and the remission thereof.

It is now widely accepted that k-opioid receptor (partial) agonists have dissociative and deliriant effects, as exemplified by salvinorin A. These effects are generally undesirable in medicinal drugs and could have had frightening or disturbing effects in the tested humans. It is thought that the hallucinogenic effects of drugs such as butorphanol, nalbuphine, and pentazocine serve to limit their opiate abuse potential. In the case of salvinorin A, a structurally novel neoclerodane diterpene κ-opioid receptor agonist, these hallucinogenic, more specifically deliriant and dissociative, effects are sought after, even though the experience is often considered dysphoric by the user. While salvinorin A is considered a hallucinogen, it is not a psychedelic, and its effects are qualitatively different than those produced by the classical psychedelic hallucinogens such as LSD or mescaline.^[13]

The involvement of the k-opioid receptor in stress response has been elucidated.^[9]

Activation of the k-opioid receptor appears to antagonize many of the effects of the k-opioid receptor.^[14]

k-Opioid receptor ligands are also known for their characteristic diuretic effects, due to their negative regulation of antidiuretic hormone (ADH).^[15]

k-Opioid agonism is neuroprotective against hypoxia/ischemia; as such, k-opioid receptors may represent a novel therapeutic target.^[16]

Signal transduction

k-Opioid receptor activation by agonists is coupled to the G protein G_i/G₀, which subsequently increases phosphodiesterase activity. Phosphodiesterases break down cAMP, producing an inhibitory effect in neurons.^[17][18][19] k-Opioid receptors also couple to inward-rectifier potassium^[20] and to N-type calcium ion channels.^[21] Recent studies have also demonstrated that agonist-induced stimulation of the κ-Opioid receptor, like other G-protein coupled receptors, can result in the activation of mitogen-activated protein kinases (MAPK). These include extracellular signal-regulated kinase, p38 MAP kinases, and c-Jun N-terminal kinases.^{[22][23][24][25][26][27]}

Ligands

The synthetic alkaloid ketazocine^[28] and terpenoid natural product salvinorin A^[13] are potent and selective k-opioid receptor agonists. The k-opioid receptor also mediates the action of the hallucinogenic side effects of opioids such as pentazocine.^[29]

Natural agonists

Found in numerous species of mint, (including peppermint, spearmint, and watermint), the naturally-occurring compound Menthol is a weak k-opioid receptor agonist^[33] owing to its antinociceptive effects in rats. In addition, mints can desensitize a region through the activation of TRPM8 receptors (the 'cold'/menthol receptor).^[34]

Role in treatment of drug addiction

k-Opioid agonists have recently been investigated for their therapeutic potential in the treatment of addiction^[38] and evidence points towards dynorphin, the endogenous k-opioid agonist, to be the body's natural addiction control mechanism.^[39] Childhood stress/abuse is a well known predictor of drug abuse and is reflected in alterations of the k- and k-opioid systems.^[40] In experimental "addiction" models the k-opioid receptor 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 k-opioid receptors are required for stress-induced reinstatement of cocaine seeking.^[41][42]

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.^[43] In addition to low NAcc D2 binding,^[44][45] 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 k-opioid agonist produced an opposite effect causing an increase in D2 receptors in the NAcc.^[46]

Additionally, while cocaine overdose victims showed a large increase in k-opioid receptors (doubled) in the NAcc,^[47] k-opioid agonist administration is shown to be effective in decreasing cocaine seeking and self-administration.^[48] Furthermore, while cocaine abuse is associated with lowered prolactin response,^[49] k-opioid activation causes a release in prolactin,^[50] a hormone known for its important role in learning, neuronal plasticity and myelination.^[51]

It has also been reported that the k-opioid 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.^[52][53] These effects are likely caused by stress-induced drug craving that requires activation of the k-opioid 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^[54] The rewarding properties of drug are altered, and it is clear k-opioid 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 k -opioid receptors is likely due to multiple signaling mechanisms. The effects of k-opioid agonism on dopamine systems are well documented, and recent work also implicates the mitogen-activated protein kinase cascade and pCREB in k-opioid dependent behaviors. ^[25][55]

Though cocaine abuse is a frequently used model of addiction, k-opioid agonists have very marked effects on all types of addiction including alcohol and opiate abuse.^[12] Not only are genetic differences in dynorphin receptor expression a marker for alcohol dependence but a single dose of a k-opioid antagonist markedly increased alcohol consumption in lab animals.^[56] There are numerous studies that reflect a reduction in self-administration of alcohol,^[57] and heroin dependence has also been shown to be effectively treated with k-opioid agonism by reducing the immediate rewarding effects^[58] and by causing the curative effect of up-regulation of mu-opioid receptors^[59] that have been down-regulated during opioid abuse.

The anti-rewarding properties of k-opioid agonists are mediated through both long-term and short-term effects. The immediate effect of k-opioid agonism leads to reduction of dopamine release in the NAcc during self administration of cocaine^[60] and over the long term up-regulates receptors that have been down-regulated during substance abuse such as mu-opioid and D₂ receptors. These receptors modulate the release of other neurochemicals such as serotonin in the case of mu-opioid receptor 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 k-opioid agonism (30 minutes or greater) have been linked to k-opioid receptor-dependent stress-induced potentiation and reinstatement of drug seeking. It is hypothesized that these behaviors are mediated by k-opioid-dependent modulation of dopamine, serotonin, or norepinephrine and/or via activation of downstream signal transduction pathways.

Selected Articles

Structure of the human k-opioid receptor in complex with JDTic
Atomic Structure of Molecule That Binds to Opioids in the Brain Discovered
RTI International's JDTic Helps Scientists Uncover Structure of the Kappa Opioid Receptor

References