Opiates, among the oldest drugs known to mankind, have been used medically for thousands of years for the relief of pain and sedation. They are natural extracts of the poppy plant, Papaver somniferum. In particular, morphine (named after Morpheus the Greek God of dreams) binds to opioid receptors in the central and peripheral nervous system. The search for endogenous ligands for these receptors led to the discovery of two closely related pentapeptides (enkephalins) by Hans Kosterlitz et al. in 1975: methionine-enkephalin (met-enkephalin) and leucine-enkephalin (leu-enkephalin). Subsequently, a plethora of other endogenous opioid peptides were identified. The endogenous opioid system plays a vital role in regulating myriad physiologic functions like pain relief (analgesia), euphoria induction, stress resilience, cardiovascular protection, food intake control, and many more. Three genetically distinct opioid peptides families are considered classical members of the endogenous opioid system:
Issues of Concern
Tolerance is a phenomenon in which an increasing dose of the medication is required to produce the same pharmacologic effect. The body develops tolerance after repeated administration, and higher doses of opioids are required to maintain the same level of analgesia. Tolerance develops quickly to sedation, respiratory depression, analgesia, and gastrointestinal distress. However, there is minimal tolerance to constipation and miosis. Tolerance develops by various mechanisms such as desensitization, downregulation, receptor internalization, and mu/delta heterodimer formation. Animal studies also show that tolerance may develop due to the cellular changes in the laminae of the dorsal horn of the spinal cord.
Different brain areas contain different opioid peptide-producing neurons. Neurons producing enkephalins are spread out and are found in different brain regions. Enkephalins are involved in several biological processes (cardiovascular system, gastrointestinal functions, respiration, pain perception). Enkephalinergic neurons are present in Lamina I, II (substantia gelatinosa), and V of the spinal cord and in the periaqueductal gray (PAG) through which they mediate pain perception. Emotional responses are regulated by their action on the limbic system mainly amygdala. Other areas of high concentration include hypothalamus and basal ganglia particularly globus pallidus. The respiratory and cardiovascular function is mediated by their action on the autonomic nuclei of the hypothalamus. In rats, they have been found to stimulate the release of several pituitary hormones, in particular, growth hormone, prolactin, and vasopressin. This points towards their neuroendocrine role.
Neurons containing beta-endorphins are predominantly found in the anterior and intermediate lobe of the pituitary and brain stem (the nucleus of tractus solitarius).
The majority of dynorphinergic neurons are found in the posterior lobe of the hypothalamus. Peripherally, opioid peptides are found in the adrenal gland, gastrointestinal tract, heart, pancreas, and many organ tissues.
Endogenous opioids are neurotransmitters or neuromodulators that act by changing the electrical properties of other target neurons, thereby making these neurons difficult to excite. Like other small peptide molecules, endogenous opioids are synthesized as a part of a larger precursor molecule. However, unlike other peptides, opioid peptides have a number of different precursors. Each opioid peptide has a pre-pro and a pro form which cleaves the signal peptide. They are then modified by post-translational events namely acetylation, glycosylation, phosphorylation, and methylation based on the biological program of the cell. These changes result in different potencies, pharmacological profile, receptor affinity, or selectivity and is a critical step in the regulation of the opioid system in a particular region. They are derived from three gene product proteins, namely pro-opiomelanocortin (POMC), pro-enkephalin (PENK) and prodynorphin (PDYN) which are precursors for endorphins, enkephalins, and dynorphins respectively. The opioid peptides share a common amino-terminal sequence called, the opioid motif which is: Tyr-Gly-Gly-Phe- (Met/Leu)
Pro-opiomelanocortin is a polypeptide that is cleaved by the enzyme peptidase into adrenocorticotropin-releasing hormone (ACTH) and beta-Lipotropin (beta-LPH) containing 93 amino acids. Beta-LPH is further cleaved to yield alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin (beta-endorphin), a polypeptide containing 31 amino acids. The sequence is Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val-Thr-Leu-Phe-Lys-Asn-Ala-Ile-Ile-Lys-Asn-Ala-Tyr-Lys-Lys-Gly-Glu. Beta-endorphins are also secreted into the bloodstream by the pituitary gland (hormone) in addition to being neuromodulators, i.e., they express dual functionality.
Pro-enkephalin is a seven-peptide-containing structure, first identified in the adrenal medulla. Each molecule of pro-enkephalin contains four met-enkephalins, one leu-enkephalin, one octapeptide, and one heptapeptide. Enkephalins have a short half-life both in vivo and in vitro. They are metabolized primarily by two peptidases: enkephalinase-A, which splits the Gly-Phe bond, and enkephalinase-B, which splits the Gly-Gly bond. Aminopeptidase N (APN), which splits Tyr-Gly bond, also contributes to its catabolism. Recent studies have developed a mixed peptidase inhibitor, kelatorphan, that completely inhibits the metabolism of exogenous met-enkephalin thereby reducing the intracerebroventricular analgesic dose in mice.
The third precursor molecule prodynorphin is a protein that contains three main leu-enkephalin containing peptides: dynorphin A, dynorphin B, and neoendorphin. The final forms of dynorphin is an area of active research.
The action of endogenous opioids is regulated by their action on the specific opioid receptor. Opioid receptors are involved in different physiologic functions. Following are the effects produced:
- Pain modulation: This is one of the major effects. Studies have shown that the levels of beta-endorphins rise after oral, gynecologic, and abdominal surgeries.
- Neuroprotection: Studies explain that activation of DOR increase the pro-survival signals, decrease the oxidative injury thereby showing a role in neuroprotection.
- Respiratory depression: The degree of respiratory depression depends on the receptor-stimulated. MOR produces a significant decrease in the respiratory rate than DOR and KOR.
- Ionic homeostasis: Activation of DOR decreases the hypoxia-induced ionic imbalance.
- Constipation: This is due to a decrease in the movement of the muscles of the gastrointestinal tract.
The action of opioid compounds is mediated through its action on opioid peptide receptors (OPR). Opioid receptors exist throughout the body, but their expression and distribution vary significantly among different organs. An opioid peptide can interact with more than one type of opioid receptor. The receptor-ligand binding engenders a series of biochemical events and brings about various effects. Three major categories of opioid receptors have been identified and cloned: mu-opioid receptor (MOR), kappa-opioid receptor (KOR), and delta-opioid receptor (DOR). A fourth class of receptor has been identified recently, nociception or orphan FQ receptor (NOP/OFQ). The OFQ receptor does not bind to classic opioid ligands but is still considered a part of the opioid family due to the sequence homology. There is a maximum sequence homology in the cytoplasmic and transmembrane domains and minimum homology in the extracellular domain where the ligand binds.
Opioid receptors belong to a family of 7-transmembrane G-protein-coupled receptors. G-proteins are made of three subunits: alpha, beta, and gamma. When a classical opioid agonist binds to its receptor, it results in the inhibition of adenylyl cyclase, which intern reduces intracellular cAMP levels (cyclic-AMP). This leads to increased potassium conductance out of the cell, causing hyperpolarization of the neurons and decreased calcium conductance. Because of this, the neuronal firing rate and neurotransmitter release are reduced.
Opioid-Mediated Pain Suppression
Opioids mediate both ascending and descending pain pathways. The primary afferent pain fibers are the thinly myelinated A-delta fibers and the unmyelinated C fibers. Stimulation of A-delta fibers releases glutamate, which is responsible for fast pain. Stimulation of C fibers leads to release of glutamate and substance P, responsible for slow pain. These afferents reach the dorsal horn of the spinal cord where they synapse with the neurons of the ascending spinothalamic tract. There are opioid-containing interneurons in the dorsal horn that terminate where the pain afferents terminate. These interneurons have an inhibitory action on the pain afferents. Activation of postsynaptic opioid receptors hyperpolarises the ascending fibers while presynaptic activation inhibits release of glutamate and substance P. Together, they reduce the ascending pain transmission.
The endogenous opioid system also modulates the descending pain suppression pathway by their action on the periaqueductal gray (PAG) in the midbrain. The PAG neurons are under the influence of inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Opioids inhibit the release of GABA, thus activating the PAG. The neurons of the PAG then activate serotonergic neurons in the nucleus raphe Magnus and noradrenergic neurons in the rostral ventromedial medulla. These neurotransmitters stimulate the enkephalinergic interneurons in the spinal cord, thus inhibiting pain perception.
The opioid system is believed to be one of the most complex neurotransmitter systems in the body that plays a critical role in major biological processes in the body. The use of exogenous opioids for analgesia have limitations due to their undesirable adverse effects which include sedation, respiratory depression, and constipation. However, experiments have found that drugs that bind to the delta-opioid receptor (DOR) lack respiratory and gastrointestinal adverse effects. So, development of DOR specific drugs will prove to be a clinical advantage. Research suggests that acupuncture produces analgesia by the release of endogenous opioids. This is supported by the finding that administration of an opioid antagonist reverses the analgesia induced by acupuncture.
Another notable finding is the correlation between alcohol consumption and endogenous opioids. Alcohol induces the activation of the endogenous opioid system. Clinical trials on outpatient alcoholics have shown that administration of opioid antagonist naltrexone decreased the average number of drinking days per week, the desire to drink, and the alcohol-induced high. Another important study is the in vivo studies of immunomodulator activity of enkephalins on rats which revealed a dual dose-dependent effect, i.e., high doses inhibit while low doses enhance the immune response. Recent investigations suggest that enkephalins act as modulators of cardiac function and play a vital role in aging, ischaemic preconditioning, heart failure, and hypertension. Studies on animal models have shown that opioid receptors are widely involved in neuroprotection, epileptic seizures, and obesity, but its clinical significance is undergoing research.
An important hypothesis is that of placebo-induced pain suppression. Placebo induces the release of endogenous opioids in anticipation of pain relief. Functional MRI (fMRI) response shows placebo-enhanced response in the rostral anterior cingulate gyrus, periaqueductal gray, rostral ventromedial medulla, and hypothalamus. Administration of opioid antagonist naloxone, reduces the activity in these areas, thus showing the link between endogenous opioids and placebo-induced analgesia.
A new class of endogenous peptides, endomorphins, has been identified which have the highest affinity and selectively for mu-opioid receptor. Evidence has revealed their remarkable role in neuropathic pain, unlike other known opioid analgesics. Also, a novel discovery of an atypical opioid peptide is hemorphin. These are endogenous peptides generated by enzymatic cleavage of hemoglobin. Hemorphins can inhibit angiotensin-converting enzyme (ACE), thus decreasing the blood pressure observed after strenuous physical exercise. However, further studies have to be done to draw a firm conclusion.