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PSYC62H3 (280)
Chapter 12

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University of Toronto Scarborough
Suzanne Erb

Chapter 12: naturally occurring and synthetic opiates and their antagonists - some of the oldest psychotropic drugs used by humans are morphine and codeine since 7000 years - originally, drugs were used in the form of extracts from the poppy plant, which contains opium, and in purified form they are still used extensively - they belong to class of drugs that includes the most effective pain relievers available so are most commonly used analgesic treatments for moderate to severe pain - because users often experience euphoria, drowsiness, and mental clouding these drugs are also used recreationally and are subject to abuse - most common term for morphine and similar acting drugs is narcotic from Greek word for stupor - unfortunately, the term narcotic has taken on many unwarranted connotations; the word has been used primarily to refer to a class of drugs that promote sleep and induce analgesia - pharmacologically: only a drug with the following qualities can be appropriately classified as a narcotic: - 1. it generally has sedative-hypnotic and analgesic properties - 2. it acts stereospecifically on endorphin/enkephalin receptors - 3. its actions are antagonized by naloxone (Narcan) - In essence, narcotics are restricted to extracts of opium (opiates), opiate derivatives, and synthetic drugs with opiate properties - Because of the confusion surrounding the term narcotic, many authorities now refer to these substances as opioids ENDOGENOUS OPIOID PEPTIDES AND THEIR RECEPTORS - Substances endogenous to the brain and body with opiate properties: endogenous opioid peptides - Three distinct families: enkephalins, endorphins, dynorphins - Each family is derived from different precursor polypeptides: - Pro-opiomelanocortin - Found in pituitary gland and limited areas of CNS - Proenkephalin - Prodynorphin - Two above: distributed widely throughout the CNS, especially on those regions related to the modulation of pain perception (e.g., the spinal cord and midbrain), affective states (amygdala, hippo) and the autonomic nervous system (medulla); also found in stomach and intestines - Endogenous opioid peptides are believed to function as neurotransmitters, neurohormones, Neuromodulators, and their physiological role is not well understood; also co-exist with other hormones and neurotransmitters - All opioid receptors identified so far are g-protein coupled receptors metabotropic - Tthee major classes of opioid receptors: mu, delta, and kappa - 4 class: nociceptin/orphanin FQ (N/OFQ) receptors identified in 1990s - Mu receptors: morphine and naloxone-selective (morphine and naloxone bind to these receptors more readily than enkephalins), mediate the euphoria-inducing properties of opiates; mu receptor agonists: supraspinal and spinal analgesia (relieve pain), respiratory depression, cardiovascular effects, slowing of gastrointestinal motility, and sedation - Delta receptors: enkephalin-selective, and induce spinal analgesia - Kappa receptors: high affinity for dynorphin and mediate spinal analgesia and sedation; mediate aversive, psychosis-mimicking opiate effects may explain why some narcotics that are primarily kappa agonists do not produce drug-seeking behaviour 1 - N/OFQ receptors: involved in array of psychological processes and its pharmacological profile is different/opposite from that of classic opiates it induces hyperalgesia (increased sensitivity to pain), decreases the analgesic actions of opiates, blocks the rewarding effect of morphine this could be why Buprenorphine, an agonist at both mu and N/OFQ receptors induces analgesia at low doses but exhibits diminished analgesic qualities with higher doses TYPICAL OPIATES (narcotics, opioids and opiates used interchangeably) - Not all opiates have identical effects - Some drugs with narcotic-like effects by themselves may actually block the effects of other narcotics - In addition to having different pharmacological effects, these drugs differ with respect to potency, intensity, duration of action, and oral effectiveness, in many cases because of differences in pharmacokinetics - Heroine is one of the most potent of the nonendogenous types of opiates 2-4 times more potent than morphine when injected - Because of this difference in potency, many people want to make heroine a legally available medication in the US for treating severe pain in terminally ill patients - BUT the differential potency is due to pharmacokinetics and not to efficacy at receptors. - (1) The potency of heroine and morphine is equal when taken orally - (2) The heroine molecule is simply a slight modification of morphine this allows heroine to penetrate the BBB much more rapidly than morphine does = this allows heroine to accumulate in the brain much more quickly. Once in the brain, heroine is metabolized into morphine, but because it gets there so much more quickly, it exerts its effects much more rapid and intensely - Methadone: - When administered subcutaneously (under the skin into the tissue between the skin and muscle), it has same potency as morphine, and half the potency of heroine for inducing comparable analgesia - Twice as potent as heroin and 4 times as potent as morphine with regards to suppressing withdrawal symptoms - When administered orally, it is much more effective than either heroin or morphine, and has an action that is 3-4 times longer than h and m - Codeine: - About 12 times less potent than morphine when injected but it more readily absorbed through oral admin than morphine - Neither heroine or morphine are readily absorbed orally (heroine is 100 times less potent than when administered via IV) - The endogenous opioid peptides are far more potent than heroine, but are rapidly inactivated by enzymes throughout the body - Fentanyl: - One of the most potent of the nonendogenous opiates (about 50 times more potent than heroine when injected intramuscularly) Behavioural and Physiological Effects of Opiates - The more prominent behavioural effects of narcotics resemble those effects induced by sedative hypnotics - But there may be a brief stimulant-like effect immediately after administration, particularly if administered via IV - The most prominent clinically useful effect of opiates is to reduce pain which is dependent on several neurotransmitter systems located in the spinal cord and supraspinal structures (areas above the spinal cord such as the locus coeruleus and medulla) - Have little influence on the sharp pain initially induced by a noxious (harmful, injurious) stimulus; their major effectiveness is in reducing moderate to severe dull pain that persists after the noxious stimulus 2why opiates are used chronic pain conditions and in treatment of postoperative pain but are by themselves effective during surgery - The use of narcotics in medicine to control chronic pain is a particularly contentious issue due to the conflict between physicians desire to ease pain and their concern over causing addiction - Most physicians acknowledge opioid therapy to be an invaluable and accepted treatment for acute cancer- related pain and pain caused by terminal disease - General approach in treatment: pain in these patients is typically not constant across the day but ebbs and flows with occasional breakthrough pain that if not controlled quickly may be difficult to manage without administering a very high dose of an opiate (which could cause mental clouding, nausea) - (1) use the lowest effective dose of a controlled-release formulation that satisfies the patients baseline pain management needs - (2) determine the frequency, intensity, and timing of their breakthrough pain - (3) use a rapid-acting opiate to cover breakthrough pain - Because each opiate has a unique pharmacokinetic and pharmacodynamic profile (e.g., some compounds with higher affinity for mu receptors that for delta receptors etc), there is a recognition that rotation among different opiate medications is beneficial for reducing adverse side effects resulting from chronic use - Better method of administering opiates have also been developed to improve their efficacy, reduce their side effects, or lower the cost of treatment = transdermally administered opioid analgesics and patient- controlled opiate analgesia: the patient is allowed to self administer opiates, either orally or by pushing button that delivers small does of morphine through an IV tube - Contrary to popular belief, patients taking opiates solely to control pain generally do not become addicted aka develop an uncontrollable compulsion to use o
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