Endogenous opioid peptides

There are four families of endogenous opioid peptides: endomorphins, endorphins, enkephalins, and dynorphins. Figure 20-1 illustrates their biologic derivations and structural relationships. The pentapeptide enkephalins—methionine-enkephalin (met-enkephalin) and leucine-enkephalin (leu-enkephalin)—were the first endogenous opioids to be discovered.¹³ These peptides were subsequently shown to be potent opioid receptor agonists in the same biologic systems in which morphine is active. Initially, the enkephalins were thought to be derived from a larger 91-amino acid peptide, β-lipotropin, but it is now clear that β-lipotropin gives rise to a separate group of opioid peptides, the endorphins (see later).

FIGURE 20-1 Derivations and structures of endogenous opioids. A, Endorphins. Proteolysis products of the pituitary hormone β-lipotropin (β-LPH), endorphins are ultimately derived from the precursor molecule pro-opiomelanocortin. Other peptides of biologic importance obtained from pro-opiomelanocortin include adrenocorticotropin (ACTH), γ-lipotropin (γ-LPH), and several melanotropins (not shown). The initial amino acid sequence of β-endorphin is shown (at bottom left) to illustrate its structural relationship to the enkephalins and dynorphins; the numbers refer to amino acid residues of β-LPH. B, Enkephalins. In addition to met-enkephalins and leu-enkephalins, proenkephalin may give rise to at least two other biologically active molecules, a heptapeptide and an octapeptide, both of which contain met-enkephalin as part of their structure. C, Dynorphins. A common precursor, prodynorphin, yields several dynorphins, including dynorphin A(1-17) (shown here), dynorphin A(1-8), dynorphin B(1-13), and at least two other peptides, α-neoendorphin and β-neoendorphin. D, Endomorphins. The precursor for these two endogenous opioid peptides is unknown.

Endomorphin-1 and endomorphin-2 are newly discovered endogenous opioid peptides.⁴⁷ Both are short tetrapeptides (NH₂-Tyr-Pro-Trp/Phe-Phe-CONH₂) and structurally distinct from the other opioid peptides (see later). The endomorphins have been localized to areas in the CNS associated with pain processing (e.g., spinal dorsal horn, trigeminal nucleus, midbrain periaqueductal gray) and in the endings of sensory neurons that terminate in the spinal dorsal horn. Although the genes for the endorphins, enkephalins, and dynorphins are known, the gene for the endomorphins has yet to be isolated.

The precursor of the enkephalins, proenkephalin, is present in the CNS and the adrenal medulla. Although proenkephalin in the brain is similar, if not identical, to proenkephalin in the adrenal medulla, processing of the large precursor polypeptide differs in the two locations. In the brain, cleavage of proenkephalin is generally more complete, and free enkephalins are the predominant products (four copies of met-enkephalin, one copy of leu-enkephalin, and one copy each of a heptapeptide and an octapeptide). In the adrenal medulla, large enkephalin-containing polypeptides predominate.

Prodynorphin is the common precursor for several larger opioid peptides, three dynorphins and two neoendorphins, all of which share with the enkephalins the same N-terminal amino acid sequence: NH₂-Tyr-Gly-Gly-Phe-Met/Leu. The products of prodynorphin are often found in association with the enkephalins within the CNS.

The endorphins are a group of endogenous peptides that are larger in size and are distributed differently in the CNS than the endomorphins, enkephalins, or dynorphins. The precursor for the endorphins, pro-opiomelanocortin, gives rise to several important hormones, including adrenocorticotropic hormone and β-lipotropin, which are further processed to form biologically active products. The most important opioid derived from β-lipotropin is β-endorphin, the 30-amino acid carboxy terminal sequence of β-lipotropin. Shorter cleavage products of β-endorphin, such as α-endorphin and γ-endorphin, have been isolated from the pituitary, but their function is uncertain.

Sites in the CNS where opioid peptides are located differ for the different peptides, confirming that they are involved in different functions. It is a mistaken impression that all opioid peptides in all locations are involved in the modulation of pain. Neurons containing endomorphins are not as widely distributed in the CNS as neurons containing other opioid peptides, but endomorphins are located in pain-processing areas where they are considered to function as neurotransmitters that act at opioid receptors. Neurons containing enkephalins are widely distributed throughout the brain (e.g., striatum, limbic system, midbrain, and medulla) and the spinal cord where they, too, are considered to function principally as neurotransmitters.²² Prodynorphin-derived peptides are abundant in the pituitary, hypothalamus, midbrain, and striatum. Differential processing of proenkephalin and prodynorphin in various brain areas leads to different products having different functions.

The endomorphins, enkephalins, and dynorphins are stored in nerve terminals and are quickly destroyed by peptidases (aminopeptidase N [EC 3.4.11.2] and neutral endopeptidase [EC 3.4.24.11]) when released. β-Endorphin is present in high concentrations in the intermediate lobe of the pituitary and in neurons in the mediobasal hypothalamus, whose axons terminate in the amygdala, periaqueductal gray matter, and brainstem. β-Endorphin coexists with adrenocorticotropic hormone in pituitary secretory granules, and both peptides can be released simultaneously. β-Endorphin is believed to function more as a neurohormone than as a neurotransmitter, mediating diverse autonomic and psychological responses to pain and stress.

Opioid receptors

Three opioid receptors have been cloned: mu (μ), kappa (κ), and delta (δ) (Table 20-1).^15,32 They share considerable structural homology (approximately 60% to 65%), contain seven membrane-spanning α-helical segments, and are coupled to transducing G proteins (Figure 20-2). G proteins couple opioid receptors to intracellular effectors and exist as heterotrimers; there is structural and functional diversity in the three G protein subunits. Each heterotrimer consists of an α subunit isoform (of which there are at least 18) and a dimer of β-γ subunits (which also exist in multiple isoforms) that link with specific (and potentially diverse, given the numbers of isoform combinations that are possible) effector systems. Specifically, opioid receptors couple in this way with adenylyl cyclase and ion channels to reduce neurotransmitter release (see later).

FIGURE 20-2 Diagram of a G protein–coupled opioid receptor. Opioids bind within the hydrophobic membrane-spanning domains of the receptor. Opioid agonist effects at the receptor are mediated by G proteins, an α subunit associated with guanosine diphosphate (GDP), and a β-γ dimer. When an opioid agonist binds, the conformation of the receptor is changed as the membrane-associated G proteins assemble with the receptor. GDP is exchanged with guanosine triphosphate, and this activated G_α complex negatively regulates adenylyl cyclase. The β-γ dimer activates conductance in a G protein inwardly rectifying K⁺ (GIRK) channel and inhibits voltage-gated Ca⁺⁺ channels (VGCC) in the cell membrane. (See Chapters 1 and 5 for more details on signal transduction.) ATP, Adenosine triphosphate; cAMP, cyclic 3′5′-adenosine monophosphate.

The amino acid sequences of the three opioid receptors are most homologous in the transmembrane domains and intracellular loops; the sequences share little homology in the extracellular loops and N- and C-termini. Each receptor is distributed differently in the CNS, peripheral nervous system, and smooth muscle of the gastrointestinal tract. A receptor with high sequence homology to the three cloned opioid receptors, termed ORL-1 (for opioid receptor–like, an “orphan” receptor), has been discovered.^21,33 Despite its structural similarity to opioid receptors, opioids do not bind to ORL-1 with high affinity. A heptadecapeptide termed orphanin FQ (or nociceptin), which is structurally similar to the endogenous opioid peptide dynorphin, seems to be the endogenous ligand for ORL-1 (which has been renamed the N/OFQ receptor [NOP]). Nociceptin does not act at any of the three cloned opioid receptors, however, and the nociceptin–NOP ligand–receptor complex may instead function to facilitate pain at some sites.

Another heptadecapeptide, termed nocistatin, is also derived from the same gene that gives rise to nociceptin. Nocistatin is reported to “block” the effects of nociceptin, but nocistatin does not displace nociceptin from its receptor (ORL-1),²⁶ indicating that the opposing effects of nocistatin are produced at a yet to be identified receptor.⁴⁹ It is believed now that nocistatin exerts an inhibitory effect through a presynaptic G_i/G_o-coupled receptor not related to either the opioid receptors or the NOP receptor.⁵

The μ receptor is the site at which all the currently available pure agonists act to produce analgesia. Morphine and other similar pure agonists have greatest affinity for μ receptors in the CNS and peripheral nervous system and lower affinity for δ and κ receptors. Among the endogenous opioid peptides, the endomorphins have the highest affinity for the μ receptor; β-endorphin and the enkephalins also exert some of their effects at μ receptors. Two subtypes of μ receptor have been characterized by pharmacologic means: μ₁, which is associated with supraspinal analgesia, and μ₂, which subserves spinal analgesia, respiratory depression, and gastrointestinal actions. Pharmacologic studies have also implied the existence of two subtypes of δ receptor, at which the endogenous enkephalins are considered to be the prototypic agonists, although the enkephalins also interact with μ receptors. The δ receptor is also involved in analgesia, mainly through spinal mechanisms, and in causing, along with μ receptors, opioid reinforcement (see Chapter 51).

Dynorphins, representing the third class of endogenous opioid peptides, have been proposed to be ligands for the κ receptor, of which three subtypes have been characterized pharmacologically. Two κ receptors mediate spinal (κ₁) and supraspinal (κ₃) analgesia and are principally responsible for the analgesic effects of the mixed agonist-antagonist group of opioid analgesics currently available. Although the endomorphins selectively bind to the μ opioid receptor,⁴⁸ other endogenous opioids and clinically available drugs do not bind selectively to specific opioid receptors. Although morphine and other pure agonists preferentially bind at μ receptors, they can produce effects at δ and κ opioid receptors as well, particularly as the dosage is increased.

Despite pharmacologic evidence for the existence of multiple subtypes of opioid receptors, only one of each opioid receptor has been cloned. There is no evidence at present for the existence of genes other than those that give rise to the already cloned μ, δ, and κ receptors. It is possible that not all opioid receptor genes have been cloned, but molecular mechanisms and other factors most likely explain the pharmacologic diversity of effects produced by various ligands at opioid receptors. Receptor isoforms may be produced by alternative splicing in the coding regions of the three cloned opioid receptors.⁷ Variant mRNAs have been reported for all three opioid receptors, although their expression is typically very low, and it is not clear that splice variants can be distinguished pharmacologically. It seems more likely that other mechanisms (e.g., post-translational regulation, variable activation of receptors by different ligands, receptor dimerization, intracellular interactions with proteins associated with different effectors) explain the pharmacologic diversity of opioid receptor subtypes. Many G protein–coupled receptors exist as dimers (two receptors linked), and heterodimerization of δ and κ receptors could result in a complex pharmacologic profile.

The sigma (σ) receptor, initially considered to be an opioid receptor, is now known to mediate the dysphoric and psychotomimetic effects of opioids and those of phencyclidine, a nonopioid hallucinogen. This phencyclidine receptor has been identified as an inhibitory component of the N-methyl-d-aspartate receptor complex, which modulates opioid tolerance and dependence.

Physiologic functions

Much remains to be learned about the physiologic roles of the endomorphins, enkephalins, dynorphins, and endorphins. It is established that enkephalins, present principally in local circuits or interneurons in the CNS, have an inhibitory effect on other cells. Endogenous opioids tonically modulate the secretion of gonadotropins from the pituitary. When the opioid receptor antagonist naloxone is administered to normal subjects, the plasma concentrations of luteinizing hormone and follicle-stimulating hormone are increased because naloxone releases hypothalamic neurons from a tonic endogenous opioid inhibition.

Considerable attention has been given to the notion that endogenous opioid peptides tonically modulate nociception (pain). One would expect occupation of opioid receptors by naloxone to prevent any action by endogenous opioid peptides and lower the response threshold for pain if endogenous opioids tonically modulated nociception. A number of studies have shown that the administration of naloxone to normal human volunteers does not affect their resting pain thresholds or responses to experimental pain stimuli. In other situations, naloxone has been reported to attenuate the analgesic effects of acupuncture and of placebo administration after minor oral surgery and to be hyperalgesic in humans after major surgery.³¹ From these and other findings,⁸ it seems that endogenous antinociceptive opioid systems are normally quiescent but can become physiologically active and affect pain processing when significant injury or stress is present. It seems that endogenous opioid activity can be enhanced by the anticipation of pain relief, producing naloxone-reversible placebo effects that diminish the perception of pain (i.e., elicit analgesia).⁶

Sites and mechanism of action

Among the many effects of opioids, analgesia has been most thoroughly studied. We now know that opioids acting at opioid receptors in the periphery and at spinal and supraspinal sites can produce clinically effective analgesia. Knowledge of the central sites of opioid action came first; documentation of direct peripheral analgesic actions of opioids is relatively recent.³⁸

Early investigators attempted to determine the central locus of morphine’s analgesic action by administering morphine directly into selected brain sites. These studies succeeded in identifying an area of the brainstem surrounding the cerebral aqueduct as a site important to morphine analgesia. Corollary studies found that electrical stimulation of this same area in the midbrain in animals and humans produced a potent, long-lasting analgesia partially mediated by endogenous opioids.¹¹ Morphine and other opioid agonists acting at this site produce analgesia by engaging a descending system of pain inhibition.⁸

As illustrated in Figure 20-3, information about pain from nociceptors—that is, peripheral receptors in skin, muscle, joints, and viscera that respond to pain-producing stimuli—can be influenced at the first central synapse (spinal or medullary dorsal horn) by this descending system of pain inhibition. It is important to appreciate two features of this pain-modulating system: the descending pathway from the midbrain is indirect (there is a synaptic relay in the medulla or the dorsolateral pons or both), and although engaged by an opioid acting at opioid receptors in the midbrain, the neurotransmitters in the spinal cord or medullary dorsal horn that ultimately inhibit pain transmission are nonopioids such as 5-hydroxytryptamine (5-HT, serotonin) and norepinephrine.⁸ Engagement of the descending pain inhibitory circuit is also responsible for placebo-induced analgesia.

FIGURE 20-3 Descending pain-modulating pathways from the brainstem to the spinal cord. Descending influences, whether activated in the midbrain periaqueductal gray (PAG) or rostroventral medial medulla (RVM) by endogenous or exogenous opioids (*) or stimulation, are mediated at the level of the spinal cord by noradrenergic and serotonergic receptors.

A second means by which morphine and other opioid agonists modulate pain is by acting at opioid receptors in the brain at sites not associated with activation of the descending pain inhibitory system. Actions of opioids at these brain sites do not affect the response threshold to a painful stimulus but rather influence the interpretation of and emotional reaction to the stimulus.³¹ This so-called motivational-affective component of the response to pain is discussed later in this chapter.

A final means by which morphine and other opioid agonists produce analgesia is by acting at opioid receptors located on the peripheral and central terminals of nociceptors. Opioid receptors are located on the terminals of these specialized nerve fibers, typically Aδ and C fibers, which connect the periphery directly with the CNS. Analgesic effects of opioids can be produced by direct administration of an opioid into a joint or, more commonly, the epidural or intrathecal space.⁴⁶

Opioids can act at several sites, each of which can contribute independently to the analgesia produced. When given systemically, opioids have access to all potential sites of action, and the analgesia produced is likely a product of peripheral, spinal, and supraspinal interactions with opioid receptors. These mechanisms are likely to be synergistic resulting in increased opioid potency and, consequently, reduced toxicity. This knowledge can be used to improve pain control and limit the incidence or severity of undesirable opioid effects. Because pain modulation descending from the brainstem is mediated in the spinal cord by norepinephrine and 5-HT, the direct effects of an opioid given into the epidural space can be enhanced by epidural administration of an α-adrenoceptor agonist such as clonidine.²⁴

Tricyclic antidepressants, which block the reuptake of norepinephrine and 5-HT, are also effective adjuvants (although antidepressants such as amitriptyline also possess analgesic efficacy unrelated to their effects on monoamine reuptake). This strategy allows reduction of the dose of opioid without compromising the analgesia produced. Two drugs, tramadol and tapentadol, produce analgesic actions by engaging multiple mechanisms, including activity at the opioid receptors and inhibition of reuptake of norepinephrine or serotonin or both. By requiring less opioid for adequate pain control, undesirable opioid effects, such as urinary retention, respiratory depression, sedation, and development of analgesic tolerance, can be reduced. It has also been documented that administration of very low doses of morphine directly into the knee joint after arthroscopic knee surgery can control postoperative pain.³⁸

The mechanisms by which opioids produce their effects are best established for direct actions at opioid receptors located on neurons. Actions commonly produced at all three opioid receptors include inhibition of adenylyl cyclase, inhibition of Ca⁺⁺ conductance, activation of K⁺ conductance, and inhibition of neurotransmitter release (see Figure 20-2). Acute inhibition of adenylyl cyclase by an opioid leads to a decrease in intracellular cyclic 3′,5′-adenosine monophosphate, decreasing an inward, nonselective cation current and decreasing cell excitability. All three opioid receptors also activate a G protein inwardly rectifying K⁺ conductance and inhibit voltage-activated Ca⁺⁺ currents, both events mediated by G protein β-γ subunits. Because Ca⁺⁺ influx is required for the stimulus-secretion coupling of neurotransmitter release, opioids decrease the release of excitatory neurotransmitters, such as glutamate, substance P, and calcitonin gene–related peptide, from nociceptor terminals and attenuate the transmission of nociceptive information at the first central synapse. Because opioids activate this G protein–rectifying K⁺ conductance, they produce a relative hyperpolarization of neurons, making them more difficult to excite.

Central pharmacologic effects

The CNS effects of morphine are a combination of stimulation and depression and include analgesia, drowsiness, euphoria-dysphoria, respiratory depression, suppression of the cough reflex, pupillary constriction, suppression of the secretion of some (luteinizing hormone) and enhancement of other (prolactin) pituitary hormones, and initial stimulation of the medullary chemoreceptor trigger zone for emesis followed by depression of vomiting.

Peripheral pharmacologic effects

Morphine exerts important influences on smooth muscle tone that have therapeutic and toxic implications. The drug also affects gastrointestinal activity by reducing glandular secretions and by promoting absorption of fluid from the gastrointestinal lumen.