1. Pharmacology-1 PHL 211
2nd Term 8th Lecture By Abdelkader Ashour, Ph.D. Phone:

2. Narcotic analgesics

3. Narcotic analgesics (Opioid Analgesics), Introduction
The term narcotic was derived from the Greek word for "stupor." At one time, the term referred to any drug that induced sleep, but then it became associated with opioids. It often is used in a legal context to refer to a variety of substances with abuse or addictive potential state of unconsciousness
The term opioid refers broadly to all compounds related to opium and applies to any substance, whether endogenous (e.g., endorphins), synthetic (e.g., Pethidine “known as meperidine in the USA”, the first fully synthetic morphine-like drug) or semisynthetic (e.g., buprenorphine; a highly lipophilic opioid derived from thebaine) that produces morphine-like effects that are blocked by antagonists such as naloxone
Opium is an extract of the juice of Papaver somniferum, which has been used for social and medicinal purposes for thousands of years as an agent to produce euphoria, analgesia and sleep, and to prevent diarrhoea
Opiates are drugs derived from opium, and they include the natural products morphine, codeine, and thebaine, and many semisynthetic derivatives such as buprenorphine. Endogenous opioid peptides are the naturally occurring ligands for opioid receptors. The term endorphin is used synonymously with endogenous opioid peptides
Opioid analgesics are used for moderate to severe pain
Opioids such as heroin and morphine exert their effects by mimicking naturally occurring substances, called endogenous opioid peptides or endorphins

4. Narcotic analgesics (Opioid Analgesics), Receptors
Three classical types of opioid receptor, termed μ, δ and κ (all of them typical G-protein-coupled receptors), mediate the main pharmacological effects of opiates
μ-Receptors are responsible for most of the analgesic effects of opioids, and for some major unwanted effects (e.g. respiratory depression, euphoria, sedation and dependence)
δ-Receptors are probably more important in the periphery but may also contribute to analgesia
κ-Receptors contribute to analgesia at the spinal level and may elicit sedation and dysphoria, but produce relatively few unwanted effects and do not contribute to dependence. Some analgesics are relatively κ-selective
All opioid receptors are linked through G-proteins to inhibition of adenylate cyclase. They also facilitate opening of potassium channels (causing hyperpolarisation) and inhibit opening of calcium channels (inhibiting transmitter release)
Most of the clinically used opioids are relatively selective for μ-receptors, reflecting their similarity to morphine. However, drugs that are relatively selective at standard doses will interact with additional receptor subtypes when given at sufficiently high doses, leading to possible changes in their pharmacological profile

5. Narcotic analgesics (Opioid Analgesics), Classification & MOA
The main pharmacological categories of opioids are:
Pure agonists. This group includes most of the typical morphine-like drugs such as morphine, pethidine, methadone (long acting) and fentanyl. They all have high affinity for μ receptors and generally lower affinity for δ and κ sites
Partial agonists and mixed agonist-antagonists. These drugs combine a degree of agonist and antagonist activity on different receptors. For example, pentazocine and cyclazocine are antagonists at μ-receptors but partial agonists on δ and κ-receptors. Most of the drugs in this group tend to cause dysphoria rather than euphoria, probably by acting on the κ-receptor
Antagonists. These drugs produce very little or no effect when given on their own but block the effects of opiates. The most important examples are naloxone and naltrexone
MECHANISM OF ACTION:
Opioid receptors belong to the family of GiPCRs, and all three receptor subtypes inhibit adenylyl cyclase, so reducing the intracellular cAMP content, secondarily affecting protein phosphorylation pathways and hence cell function
They also exert effects on ion channels through a direct G-protein coupling to the channel. By these means, opiates promote the opening of K+ channels and inhibit the opening of voltage-gated Ca2+ channels, which are the main effects seen at the membrane level:
These membrane effects reduce both neuronal excitability (because the increased K+ conductance causes hyperpolarisation of the membrane) and transmitter release (due to inhibition of Ca2+ entry). The overall effect is therefore inhibitory at the cellular level

6. Narcotic analgesics (Opioid Analgesics), Actions
Morphine is typical of many opiate analgesics and will be taken as the reference compound
The most important effects of morphine are on the CNS and the GIT, although numerous effects of lesser significance on many other systems have been described
Effects on the CNS:
Analgesia. Morphine is effective in most kinds of acute and chronic pain, such as pain associated with tissue injury, inflammation and tumour growth
Euphoria. Morphine causes a powerful sense of contentment and well-being. This is an important component of its analgesic effect, because the agitation and anxiety associated with a painful illness or injury are thereby reduced
Euphoria is mediated through μ receptors, and to be balanced by the dysphoria associated with κ-receptor activation. Thus, different opiate drugs vary greatly in the amount of euphoria that they produce. It does not occur with codeine or with pentazocine to any marked extent, and nalorphine (mixed agonist-antagonist), in doses sufficient to cause analgesia, produces dysphoria
Respiratory depression. It occurs with a normal analgesic dose of morphine or related compounds. It is associated with a decrease in the sensitivity of the respiratory centre to Pco2. Analgesia and respiratory depression are both mediated by μ-receptors
Respiratory depression is the most troublesome unwanted effect of these drugs and, unlike that due to general CNS depressant drugs, it occurs at therapeutic doses. It is the commonest cause of death in acute opiate poisoning

7. Narcotic analgesics (Opioid Analgesics), Actions
Depression of cough reflex. Cough suppression does not correlate closely with the analgesic and respiratory depressant actions of opiates. Codeine suppresses cough in subanalgesic doses and is often used in cough medicines. Pholcodine is even more selective, although these agents cause constipation as an unwanted effect
Nausea and vomiting. They occur in up to 40% of patients to whom morphine is given, and do not seem to be separable from the analgesic effect among a range of opiate analgesics. The site of action is the chemoreceptor trigger zone (a region of the medulla where chemical stimuli of many kinds may initiate vomiting). Nausea and vomiting following morphine injection are usually transient and disappear with repeated administration
Pupillary constriction. It is caused by μ and κ receptor-mediated stimulation of the oculomotor nucleus. Pinpoint pupils are an important diagnostic feature in opiate poisoning, because most other causes of coma and respiratory depression produce pupillary dilatation
Effects on the GIT:
Morphine increases tone and reduces motility in many parts of the GI system, resulting in constipation. Pressure in the biliary tract increases because of contraction of the gall bladder and constriction of the biliary sphincter. Opiates should be avoided in patients suffering from biliary colic due to gallstones, in whom pain may be increased rather than relieved
The receptors involved in these effects are of the μ, κ and δ type

8. Narcotic analgesics (Opioid Analgesics), Actions
Other actions of opiates:
Morphine releases histamine from mast cells by an action unrelated to opioid receptors. This release of histamine can cause local effects, such as urticaria and itching at the site of the injection, or systemic effects, namely bronchoconstriction and hypotension. The bronchoconstrictor effect can have serious consequences for asthmatic patients, to whom morphine should not be given. Pethidine does not produce this effect
Hypotension and bradycardia occur with large doses of most opiates, due to an action on the medulla. With morphine and similar drugs, histamine release may contribute to the hypotension
Effects on smooth muscle other than that of the GIT and bronchi are slight, although spasms of the ureters, bladder and uterus sometimes occur
Opiates also exert complex immunosuppressant effects. The immune system is depressed by long-term opiate abuse, leading to increased susceptibility to infections

9. Narcotic analgesics (Opioid Analgesics), Uses & Side Effects
Analgesia. Severe, constant pain is usually relieved with opioid analgesics with high intrinsic activity. Example: morphine
In anesthesia. The opioids such as fentanyl are frequently used as premedicant drugs before anesthesia and surgery because of their sedative, anxiolytic, and analgesic properties. They are also used intraoperatively both as adjuncts to other anesthetic agents and, in high doses, as a primary component of the anesthetic regimen
Cough suppression. Example: codeine
Diarrhoea: Example: diphenoxylate
SIDE EFFECTS
Morphine and related opioids produce a wide spectrum of unwanted effects, including:
respiratory depression,
nausea, vomiting, constipation, increased pressure in the biliary tract
urinary retention
dizziness, mental clouding, dysphoria
pruritus
Increased sensitivity to pain after analgesia has worn off also may occur

10. Narcotic analgesics (Opioid Analgesics), Tolerance & Dependence
Tolerance to opiates (i.e. an increase in the dose needed to produce a given pharmacological effect) develops within a few days. To reproduce the original response, a larger dose must be administered. Tolerance includes analgesia, emesis, euphoria and respiratory depression, but affects the constipating and pupil-constricting actions much less
Physical dependence refers to a state in which withdrawal of the drug causes adverse physiological effects, i.e. the abstinence syndrome. These phenomena occur to some degree whenever opiates are administered for more than a few days. Weak, long-acting μ-receptor agonists such as methadone may be used to relieve withdrawal symptoms
These phenomena must not be confused with addiction, in which physical dependence is much more pronounced and psychological dependence (or 'craving') is the main driving force. Addiction is rare in patients receiving opiates to control pain
Certain opioid analgesics, such as codeine, pentazocine, buprenorphine and tramadol, are much less likely to cause physical or psychological dependence