The existence of opioid receptors (OR) was postulated in 1954 (Beckett and Casy 1954), at a time when evidence for specific drug-responsive substances was evasive and the concept of biochemical receptors was still in its infancy. Unequivocal confirmation of specific binding sites for opioids in the brain was first reported in 1973 (Pert and Snyder 1973; Simon et al. 1973; Terenius 1973). This breakthrough was shortly followed by evidence that opioid ligands bind specifically to different anatomical locations and show different pharmacological activity, suggesting that several types of opioid receptors exist (Martin et al. 1976; Lord et al. 1977). Based on extensive pharmacological and animal model studies that followed, opioid receptors were originally classified into four types: mu (μ), kappa (κ), sigma (σ), and delta (δ). The first three variants were named after specific ligands used for their characterization: μ after morphine, κ after ketocyclazocine, and σ after SKF-10047, whereas the δ-opioid receptor was identified in mice and named after the thick-walled tube in the male reproductive system (called the vas deferens) from which it was characterized (Lord et al. 1977). The ultimate proof for the existence of opioid receptor subtypes was provided by the isolation of mouse δ-opioid receptor mRNA, leading to cDNA synthesis and receptor cloning (Kieffer et al. 1992; Evans et al. 1992). Subsequent studies have led to the identification of receptor sequences for the μ- and κ-opioid receptors, dismissed the σ-opioid receptor as an opioid receptor family member, and revealed the existence of a receptor with a sequence similar to opioid receptors but with distinct pharmacology. This “opioid receptor-like” (ORL), or nociceptin/orphanin (N/OFQ) protein, remained an “orphan” receptor until its corresponding endogenous peptide ligand nociceptin/orphanin FQ was identified in 1995 (Meunier et al. 1995; Reinscheid et al. 1995). It is nowadays commonly regarded that the opioid receptor family contains four types, the classical opioid receptors μ, δ, and κ, and the nonclassical nociceptin opioid receptor. The Committee on Receptor Nomenclature and Drug Classification of the International Union of Basic and Clinical Pharmacology (IUPHAR) has recommended that μ, δ, κ, and nociceptin opioid receptors should be denoted as MOP, DOP, KOP, and NOP, respectively.
Opioid Receptor Relevance
Opioid receptors MOP, DOP, KOP, and NOP, and their respective peptide ligands: endorphins, enkephalins, dynorphins, and nociceptin/orphanin, constitute the endogenous opioid neuromodulatory system. This system plays a critical role in regulating sensory and cognitive system functions and is intricately implicated in emotional self-regulation (Bodnar 2010). For example, the opioid system controls the nociceptive pathways (response to noxious and painful stimuli) and plays a decisive role in substance and behavioral addiction, mental disorders, memory, learning, and response to stress. In addition, the opioid system is implicated in the regulation of neuroendocrine, cardiovascular, gastrointestinal, renal and hepatic functions, hormonal activity, respiration, thermoregulation, and immunological responses.
Opioid receptors are also very important pharmacological targets – natural alkaloid opiates like morphine or synthetic opiates like methadone are potent pain-killers but also highly addictive drugs. Opioid antagonists, such as naltrexone, are clinically used to reduce craving in substance addiction (heroin, alcohol, and amphetamine), food abuse in obesity, and the hedonic responses in nonsubstance-related, i.e., behavioral addictions (gambling and gaming).
Opioid Receptor Evolution
The opioid system is an ancient neuromodulatory system. Chromosomal phylogeny studies suggest that the opioid system with four receptor types and four opioid peptide precursors was already present approximately 450 million years ago (Dreborg et al. 2008). These studies imply that the ancestral opioid peptide precursor and opioid receptor gene were located on the same chromosome and concomitantly duplicated in two whole genome duplication events. The first genome duplication incident yielded two genes DOP/MOP and NOP/KOP and their precursors. A second genome duplication event has led to the development of four opioid receptor types and three opioid precursor molecules. The fourth opioid peptide precursor gene appeared to be created later on, through local gene duplication (Dreborg et al. 2008).
Opioid Receptor Cellular Dynamics and Function
Opioid receptors transmit chemical signals across the cellular plasma membrane. Opioid receptors and/or opioid peptide precursors are expressed in neuronal and nonneuronal cells. The cellular physiology of opioid receptor and opioid peptide production is complex. In brief, the opioid receptors and opioid peptide precursor molecules are assembled by ribosomes, transferred to the endoplasmic reticulum (ER), the Golgi complex, and the trans-Golgi network (TGN) and sorted into transport vesicles. During this process, opioid receptor molecules undergo several posttranslational modifications such as disulfide bond formation, palmitoylation, acetylation, phosphorylation, and glycosylation, whereas the peptide precursor molecules are subjected to proteolytic cleavage by prohormone convertases (PC). At the cell termini, opioid receptors are inserted into the plasma membrane, whereas opioid peptides are secreted by transport vesicles fusing with the cell membrane.
Increasing experimental evidence supports the notion that opioid receptor cellular dynamics and localization in the plasma membrane are central to opioid receptor signaling. Cellular dynamics of surface receptors is a critical factor in determining receptor availability at the plasma membrane (Hanyaloglu and von Zastrow 2008). A number of factors and highly regulated processes are involved in opioid receptor cellular dynamics. For example, endocytic membrane trafficking is an important regulator of opioid receptor density at the membrane surface (Fig. 2b). Through this dynamic process, the surface density of opioid receptors and their availability for ligand activation can be controlled without modifying the overall number of receptors through receptor synthesis (up- or downregulation). Opioid receptor ligands differ in their ability to induce receptor trafficking. For example, opioid peptides such as DAMGO cause internalization of MOP, whereas the alkaloid morphine produces little MOP internalization.
Opioid Receptor Structure
Opioid Receptor Isoforms
Naturally occurring differences in opioid receptor primary structure can alter opioid receptor binding properties and signaling, thereby predisposing a susceptibility to disease and responsiveness to therapy. Missense mutations and alternative splicing are two principle mechanisms through which opioid receptor variants are generated.
Splicing is a regulated modification of the pre-mRNA generated by gene transcription in which introns are removed and exons are joined. Alternative splicing of pre-mRNA may result in the creation of different mRNAs, which may be translated into opioid receptor isoforms differing in several amino acids. So far, at least six alternative splicing variants of hMOP have been described (Anton et al. 2008).
Opioid Receptor Oligomerization
Surface receptor oligomerization is a common way to increase the functional repertoire of the receptor and represents a key regulatory step in the function of several GPCRs (Pan et al. 2003). In addition, receptor oligomerization may be a critical step for rapid “fine-tuning” of receptor density at the plasma membrane. The dynamic equilibrium between monomers and oligomers can be changed quickly in one direction or the other, rendering the receptor available/not available for ligand (Milligan 2010). Opioid receptor homo- and hetero-oligomerization has been inferred from a variety of studies. However, quantitative information on the cellular dynamics of opioid receptor dimers is still limited and many molecular details of the oligomerization process have yet to be resolved.
Opioid Receptor Association with Lipid Rafts
The opioid neuromodulatory system is a dynamic, spatio-temporally orchestrated structure comprising signaling circuitries at all levels of organization – the cellular, organ, and organism levels. The opioid system is implicated in the regulation of physiological, sensory, cognitive, and emotional functions. The importance of the opioid system for normal physiology is long known and drugs targeting this innately complex system have been used for centuries recreationally and for medical purposes. Despite extensive utilization of opioid receptors as drug targets, advances in opiate pharmacotherapy, and intensive research, we still do not fully understand the function of the opioid system at the cellular and molecular level. Moreover, we have yet to learn how best to control the potentially deleterious effects of opioids. Given the manifold functions of the opioid system, it is not surprising that drugs targeting show numerous unwanted side effects, such as inhibition of gastrointestinal motility, respiratory depression, muscle rigidity, altered thermoregulation, sedation, mood disorders, dependence, and abuse.
This review succinctly summarizes our current understanding of the cellular and molecular mechanisms underlying opioid system function, with a focus on the opioid receptor role. Since the previous edition of the Encyclopedia of Signaling Molecules, significant progress has been made and the most pertinent information missing at that time is now available. In particular, the 3D structures of opioid receptors have been resolved (Manglik et al. 2012; Wu et al. 2012; Thompson et al. 2012; Granier et al. 2012), providing a basis for understanding mechanistic details of opioid receptor function at the atomic level (Huang et al. 2015; Sounier et al. 2015) and advancing drug discovery (Manglik et al. 2016). Despite these outstanding progresses, as the available opioid receptor crystal structures include only those forms of the receptors stabilized by high-affinity ligands, it is still unclear how opioid receptors can successfully accommodate and elicit a specific response to over dozens of compounds, differing widely in chemical composition and reactivity.
During the past few years, significant progress was also made in understanding the nanoscale distribution of MOP using super-resolution fluorescence microscopy techniques (Tobin et al. 2014; Jorand et al. 2016; Halls et al. 2016). These studies show that MOP largely organizes into nano-domains and that a fraction of this receptor associates with GPI-enriched domains (Tobin et al. 2014; Jorand et al. 2016). While a complete picture of the physiological importance of cell surface MOP lateral organization is lacking, a number of details are emerging (Halls et al. 2016), such as the chemical composition, size, and lifetime of opioid receptor receptor-harboring domains.
A better understanding of opioid receptor function will offer new insight into how drug-induced signals can be converted into long-term alterations in cellular function. Changes in opioid receptor trafficking and organization at the plasma membrane may represent the first steps in a cascade of events leading to the remodeling of signaling circuits and eventually to altered behavior, including drug-seeking and compulsive opiate use. Such changes at the neuronal level may even underlie the long persistence of addiction and the risk for relapse! Thus, studies on opioid receptor function at the molecular and cellular levels are of great importance for characterizing opioid system function. These studies may therefore deepen our understanding of the role of the opioid system in the development of tolerance, addiction, and adaptive transformations specific for opioid receptor-mediated signaling pathways that have largely restricted the effective use of opiates in clinical applications.
- Anton RF, Oroszi G, O’Malley S, Couper D, Swift R, Pettinati H, Goldman D. An evaluation of mu-opioid receptor (OPRM1) as a predictor of naltrexone response in the treatment of alcohol dependence: results from the Combined Pharmacotherapies and Behavioral Interventions for Alcohol Dependence (COMBINE) study. Arch Gen Psychiatry. 2008;65:135–44.PubMedPubMedCentralCrossRefGoogle Scholar
- Dean R, Bilsky EJ, Stevens Negus S (Eds). Opiate receptors and antagonists. From bench to clinic. New York: Humana Press; 2009. p. 201–26.Google Scholar
- Huang W, Manglik A, Venkatakrishnan AJ, Laeremans T, Feinberg EN, Sanborn AL, Kato HE, Livingston KE, Thorsen TS, Kling RC, Granier S, Gmeiner P, Husbands SM, Traynor JR, Weis WI, Steyaert J, Dror RO, Kobilka BK. Structural insights into μ-opioid receptor activation. Nature. 2015;524(7565):315–21.PubMedPubMedCentralCrossRefGoogle Scholar
- Manglik A, Lin H, Aryal DK, McCorvy JD, Dengler D, Corder G, Levit A, Kling RC, Bernat V, Hübner H, Huang XP, Sassano MF, Giguère PM, Löber S, Duan D, Scherrer G, Kobilka BK, Gmeiner P, Roth BL, Shoichet BK. Structure-based discovery of opioid analgesics with reduced side effects. Nature. 2016;537(7619):185–90.PubMedPubMedCentralCrossRefGoogle Scholar
- Wu H, Wacker D, Mileni M, Katritch V, Han GW, Vardy E, Liu W, Thompson AA, Huang XP, Carroll FI, Mascarella SW, Westkaemper RB, Mosier PD, Roth BL, Cherezov V, Stevens RC. Structure of the human κ-opioid receptor in complex with JDTic. Nature. 2012;485(7398):327–32.PubMedPubMedCentralCrossRefGoogle Scholar