Introduction
The mu (μ) receptors are a class involved in neuromodulating different physiological functions. These receptors primarily affect nociception but also stress, temperature, respiration, endocrine activity, gastrointestinal activity, memory, mood, and motivation. Because these receptors bind opioids, they are also commonly referred to as mu-opioid receptors (MORs). However, opioid receptors are a large family of receptors that also includes delta (δ)-opioid receptors (DORs), kappa (κ)-opioid receptors (KORs), and nociceptin receptors (NORs), also referred to as opioid-receptor-like receptor 1 (ORL1), which appear to have a role in the development of tolerance to mu-opioid agonists used as analgesics.
Other opioid receptors include the zeta (ζ), epsilon (ε), lambda (λ), and iota (ι) opioid receptors. Sigma (σ) receptors are no longer considered opioid receptors as the opioid antagonist naloxone does not reverse their activation or associated effects. According to the International Union of Basic and Clinical Pharmacology (IUPHAR) recommendation, the appropriate terminology for the 3 classical opioid receptors and the nociceptin receptor should be MOP ("Mu OPioid receptor"), DOP, KOP, and NOP, respectively, in this chapter we will refer to the acronym MOR for indicating mu-opioid receptors as it is the most used abbreviation in the scientific literature.
Opioid receptors are part of the G-protein-coupled receptors (GPCRs) family, characterized by crystallographic studies. This important superfamily of receptors controls different aspects of cellular function and is implicated in various neurotransmitter processes. The basic GPCR structure consists of a single polypeptide chain that crosses the cell membrane 7 times (giving the alternate name "7-transmembrane domain receptor"), has an extracellular N-terminal domain of variable length and an intracellular C-terminal domain, and interacts with heterotrimeric G-proteins.[1]
The GPCRs are divided into 3 distinct families (types A, B, and C) that share a similar heptahelical structure but differ in various aspects, mainly due to the length of the N-terminal sequence and the location of the agonist binding site. The connection between the receptor and the first stage of signal transduction is established by the heterotrimeric G-protein consisting of the alpha (Gα), beta (Gβ), and gamma (Gγ) subunits.
The main targets of G-protein signaling include adenyl cyclase, the enzyme responsible for the formation of cyclic adenosine monophosphate (cAMP); phospholipase C, the enzyme responsible for the formation of inositol triphosphate (IP3) and diacylglycerol (DAG); and several ion channels including calcium and potassium channels. Cyclic AMP, IP3, and DAG are second messenger molecules that carry out intracellular signal transduction. GPCRs can directly control the activity of ion channels through mechanisms that do not involve second messengers. For example, opioids reduce neuronal excitability by opening the G-protein-dependent inward rectifying potassium channels (GIRK), causing cell membrane hyperpolarization.
The opening of an ion channel occurs via direct interaction of the β-γ subunit complex on the channel. Several GIRK subtypes have been isolated, including the GIRK1, GIRK2, and GIRK3 types (distributed broadly in the brain) and GIRK4 (heart). This channel type is the subject of active study as a potential target for new drugs.[2]
Endogenous and exogenous opioids operate through inhibitory and excitatory action at the presynaptic and postsynaptic sites. In particular, the MORs interact with members of the G-alpha-iota (ι)/-omicron (ο) class of adenylate cyclase, a group of inhibitory G-proteins. Given the structure of the α subunit, there are various potential subunit and binding site combinations, including the G-α-ι forms (1, 2, and 3), G-α-ο types (A and B), and G-α-zeta (ζ) types.
Additionally, the β-γ heterodimer forms from 1 of the 5 different β and 1 of the 12 different γ subtypes. In the resting state, the Gαβγ complex is wholly present, and the α subunit binds guanosine diphosphate (GDP). The binding of the opioid agonist (endogenous or exogenous) to the extracellular N-terminus of the MOR induces dissociation of GDP from the Gα subunit, which is replaced by guanosine triphosphate (GTP). Subsequently, Gα-GTP dissociates from the β-γ heterodimer.
The now active Gα-GTP and β-γ complex interact with different intracellular signaling pathways, such as the phospholipase C and the mitogen-activated protein kinase (MAPK) pathway, as well as IRK-mediated hyperpolarization mechanisms and calcium channel processes. The intracellular signaling pathway ends with the action of the GTPase, which hydrolyzes the Gα-bound GTP to GDP. Gα-GDP cannot activate effector proteins and subsequently re-associates with the β-γ heterodimer to restore the inactive GDP-bound Gαβγ heterotrimer.
Because the enzymatic GTP turnover lasts approximately 2 to 5 minutes, a new signal attempt may not cause a response. However, a regulator of G-protein signaling (RGS) protein accelerates GTP hydrolysis up to 100-fold. A RGS protein acts as a negative regulator of GPCR signaling by binding the Gα subunit and removing the active Gα-GTP and β-γ heterodimer. The RGS protein family represents another potential therapeutic target, as their specific pharmacological inhibitors could potentiate opioid effects.[3]
Several subtypes of MOR are splice variant forms designated MOR-1A through MOR-1X; some variants express truncated forms of the receptor. The B, C, and D variants differ in the amino acid composition at the C-terminus. All variants are transcribed from a single gene (OPRM1 gene, chromosomal location 6q24-q25).[4] Because different variants have undergone isolation in both human and invertebrate tissues, these subtypes are conserved during evolution.[5] Studies have identified several single nucleotide polymorphisms in the human receptor. For instance, the variant receptor Ser268 -> Pro significantly reduces coupling efficiency and is less desensitized upon agonist exposure.[6]
MORs are present in the central nervous system (CNS) and are the most commonly expressed opioid receptors. These receptors are expressed in the dorsal horn of the spinal cord and different brain regions (primarily the somatosensorial cerebral cortex) that are involved in processing nociceptive (pain) signals. In the spinal cord, MORs are localized (presynaptic and postsynaptic) to the substantia gelatinosa of Rolando (laminae I and II), which receives sensory information from primary afferent nerve fibers innervating the skin and deeper tissues of the body. The activation of presynaptic MORs inhibits the release of excitatory neurotransmitters (eg, substance P and glutamate), whereas activating postsynaptic MORs results in the direct hyperpolarization of postsynaptic neurons, causing inhibition of the afferent neural transmission of pain signals and other types of information.
Apart from the somatosensory system, MORs are localized in the extrapyramidal system and the limbic system, including the limbic lobe, orbitofrontal cortex (decision-making), piriform cortex, entorhinal cortex (memory and associative functions), hippocampus (opioid-induced consolidation of new memories by increasing LTP in CA3 neurons), fornix, septal nuclei, amygdala (emotional processes), nucleus accumbens (reward, pleasure, and addiction), diencephalic structures that regulate many autonomic processes (eg, hypothalamus), and the mammillary bodies. Immunohistochemistry, in situ hybridization, and radioligand binding have demonstrated that MORs are distributed in the mesencephalon (ventral tegmental area, interpeduncular nucleus, pars reticulata of the substantia nigra, superior colliculus), pons (locus coeruleus), thalamus, and the caudate putamen.[7]
MORs are expressed in the gastrointestinal tract (and are responsible for opioid-induced constipation), the pupil (miosis), and in immune cells (eg, CEM x174 T/B lymphocytes, Raji B cells, CD4+, monocytes/macrophages, neutrophils) that regulate interleukin-4 activity in T-lymphocytes and modulate macrophage phagocytosis and macrophage secretion of TNF-α.[8] Numerous preclinical studies have investigated the effects of opioids on cancer growth and progression.[9][10][11]
Endogenous opioids are the natural ligands of opioid receptors that play a role in neurotransmission, pain modulation, and other homeostatic and functional pathways of the brain and peripherally. Beta-endorphin serves primarily as an agonist for MORs and minorly for DORs. This peptide is derived from the larger proopiomelanocortin (POMC) peptide and is secreted by the arcuate nucleus of the hypothalamus (via the anterior lobe of the pituitary gland) during stress and exercise. This peptide stimulates glucose uptake, induces euphoria, and inhibits post-exercise pain and muscular fatigue. Because β-endorphin exerts a tonic inhibitory influence upon the secretion of gonadotropin-releasing hormone (GnRH), it also regulates reproductive function.
Other endogenous opioids include the enkephalins that bind DORs and MORs and the dynorphins that bind mainly the KORs.
Enkephalins are short (5-amino acid) polypeptides, including Met-enkephalin (YGGFM) and Leu-enkephalin (YGGFL). These pentapeptides are generated from a precursor protein called proenkephalin and are found primarily in the amygdala, brainstem, dorsal horn of the spinal cord, adrenal medulla, and other peripheral tissues.
The dynorphins include dynorphin A (17 amino acids long; the first 5 are Leu-enkephalin), dynorphin B (rimorphin), and dynorphin 1-8. They are secreted in the hippocampus, amygdala, hypothalamus, striatum, and spinal cord and are involved in numerous functions related to learning and memory, emotional control, stress response, and pain.[12] The effects of both endogenous and exogenous opioids are characteristically reversed by naloxone.
Drugs that activate MORs are used to provide pain relief. These opioid drugs include the "weak" opioids codeine and tramadol and the "strong" opioids oxycodone, morphine, hydromorphone, meperidine, tapentadol, methadone, fentanyl, sufentanil, and remifentanil.[13][14]
Experiments conducted on MOR knockout mice revealed that, in addition to pain management, the binding of opioids to MORs can also induce various effects in multiple organ systems. These effects can appear similar to adverse effects and are associated with acute and chronic opioid use. Acute effects include, but are not limited to, respiratory depression, slowed gastrointestinal motility, nausea, vomiting, constipation, dizziness, pruritus, cough suppression, miosis, hallucinations, dysphoria, and sedation.[15][16]
Furthermore, chronic opioid use induces continued activation of the MOR-related signaling pathways (G-protein signaling) and can lead to homeostatic changes (eg, tolerance, hyperalgesia, physical dependence). MORs mediate opioid rewarding and euphoric effects.[17]
The misuse (or abuse) of prescribed opioid drugs after an initial therapeutic use or during self-medication is the root cause of the North American opioid crisis that began in the early 2000s.[18] In 2016, over 20,000 deaths in the United States occurred due to an overdose of prescription opioids; another 13,000 deaths occurred due to a heroin overdose. U.S. epidemiological data indicate that drug overdoses are the leading cause of death in adults younger than 50, and opioids cause more than half of all overdose deaths.