Pain ,opiate analgesics and antagonists

Dr. Israa

Mechanism of pain and nociception

• Polymodal nociceptors are the main type of peripheral sensory neuron that responds to noxious stimuli; the majority are non-myelinated C fibers whose endings respond to thermal, mechanical and chemical stimuli.

• Chemical stimuli causing pain includes: – Bradykinin – Protons– ATP – Vanilloids (e.g. Capsaicin)

• Stimuli to these receptors (agonist) open cation channels and causing membrane depolarization and action potential initiation .

• Theses receptors are sensitized by prostaglandins which explain the analgesic effect of NSAIDs.

• Nociceptive fibers terminate in the superficial fibers of the dorsal horn, forming synaptic connection with transmission neurons running to the thalamus

• Transmission in the dorsal horn is subjective to various modulatory influence, constituting the “gate control mechanism”

• Descending pathways from the midbrain and brain stem exert strong inhibitory effect on dorsal horn transmission.

• The descending inhibition is mediated mainly by enkephalin, 5-HT from NRM (Neuclus Raphi Magnus) and noradrenaline which is released from the locus coeruleus .

• Opioids analgesics causes analgesia partly by inhibiting transmission in the dorsal horn, partly by activating the descending pathways, partly by inhibiting excitation of the sensory nerve terminals in the periphery

• C-fiber activity facilitates transmission through the dorsal horn through substance P receptors and NMDA receptors.

OPIOID ANALGESICS (NARCOTIC ANALGESICS)

• Analgesia: Relief of pain without loss of consciousness.

• Opium: Natural extract from Poppy plant used for social and medicinal purpose for thousands of years to produce euphoria, analgesia, sleep and to prevent diarrhea

• Opioid drugs: natural + synthetic morphine-like drugs.

Opioids analgesics and antagonists

Strong agonists includes: – Alfentanil– Fentanyl – Heroin– Mepridine– Methadone – Morphine– Oxycodone – Remifentanil – Sufentanil.

Moderate agonists includes:–Codeine –Propoxyphene.

Mixed Agonists and Antagonists•Pentazocine•Nalbuphine•Butorphanol•Buprenorphine

Opioid Antagonists•Naloxone•Naltrexone•Nalmephine

Opioid receptors

• The opioid agonists act at specific receptor sites to produce their pharmacological effects.

• Opioid Receptors are: –μ (μ1, μ 2)–қ (қ1, қ2, қ3)–δ (δ1, δ2)

• They are four endogenous opioid-like substances: (which also stimulate opioid receptors)– [Met]enkephalin: Tyr-Gly-Gly-Phe-Met– [Leu]enkephalin: Tyr-Gly-Gly-Phe-Leu–Beta Endorphin: a 31 amino acid peptide with

[Met]enkephalin at N-terminal sequence–Dynorphin: a 17 amino acid peptide with

[Leu]enkephalin at N-terminal sequence

Endogenous Opioid Peptides (Opiopeptins)

FamiliesPrecursorsPeptides

EnkephalinsProenkephalins(also known as proenkephalin A)

Met-enkephalinLeu-enkephalin

EndorphinsPro-opiomelonocortin(POMC)

MSH*ACTH*β-Lipoprotein*β-Endorphin

DynorphinsProdynorphin(also known as proenkephalin B)

Dynorphin A Dynorphin Bα-NeoendorphinΒ-Neoendorphin

* Non-opioid peptides

Opioid Receptors and their Prototypic Ligands

Receptor typeRepresentative Ligands

EndogenousExogenous

Mu (μ)(μ1, μ2)

Β-endorphinMorphine

Delta (δ)(δ1, δ2)

Met-enkephalinEtorphine

Kappa (κ)(κ1, κ2, κ3)

Dynorphin AEthyl-keto-cycla-zocine

Action and selectivity of some opioids (and opioid antagonists) at various opioid receptors

DrugReceptor type

Mu - μKappa - κDelta - δ

Agonists

Morphine++++

Codeine+

Methadone+++

Meperidine++++

Fentanyl+++

Sufentanyl+++++

Partial and Mixed Agonists

BuprenorphinePAAntagonist (−)

PentazocinePAAntagonist (---)

Antagonists

Nalaxone and NaltrexoneAntagonist (−−−)Antagonist (−−)Antagonist (−)

Endogenous Peptides

Met-enkephalin+++++

Beta-endorphin++++++

Dynorphin A+++++

• Three genes have been identified which code for opioid peptides

• Beta endorphin and ACTH • Enkephalins • Dynorphins• These neuropeptides are released by stress

and appear to modulate the release of other neurotransmitters.

Mechanism of Action of opioids

• Morphine binds opioid receptors and thus impairs the normal sensory pathways through:

Blockade of calcium channels which leads to decreased release of substance P and glutamate from the 1st neuron of the sensory pathway (in substantia gelatinosa in spinal cord).

Decreased c-AMP which leads to opening of K-channels and hyperpolarization of the 2nd neuron of the sensory pathway.

Mechanism of Action of Opioids

A. Effects due to μ-receptor stimulation:

• Supraspinal, spinal & peripheral analgesia

• Euphoria • Respiratory depression• Miosis • Decreased GIT motility, • Sedation • Physical dependence.

B. Effects due to қ-receptor stimulation:

• Spinal and peripheral analgesia,

• Dysphoria • Sedation • Respiratory depression

(less)• Miosis (less)• Decrease GIT motility • Physical dependence

C. Effects due to δ-receptor stimulation:• Spinal analgesia• Respiratory depression• Decrease GIT motility .• They are not true opioid receptors only some

opioids react with them .

I. Morphine

• Effective orally, but is much less effective than when given parenterally due to first-pass metabolism in the liver.

• Metabolism involves glucuronide formation, the product of which is excreted in the urine.

A. Effects of morphine1. Central Nervous System Effects•Morphine has mixed depressant and stimulatory actions on the CNS. - Depressant effects predominate in man. - Excitatory effects predominate in cats and horses.

a) Analgesia:–Drowsiness is common–Continuous dull pain relieved more

effectively than sharp intermittent pain–Most patients indicate that they can still feel

the pain, but that it no longer bothers them–Morphine is an agonist at μ and қ opioid

receptors.

b) Euphoria and sedation – It is mainly due to activation of μ-receptor

c) Emesis–Morphine directly stimulates the chemoreceptor trigger

zone, usually transient and disappear with repeated administration .

d) Antianxietye) Miosis (pinpoint pupil).– It is due to stimulation of the Edenger- Westphal nucleus of

the oculomotor nerve .

f) Cough reflex is inhibited:– This action, surprisingly, does not correlate closely

with analgesic and respiratory depressant effect of opiates, and its mechanism of action at receptor level is unclear

– Chemical modification (Codiene and Pholcodiene ) the antitussive effect can occur at sub- analgesic dose.

g) Respiration depression:– Due to a direct effect on the brain stem

respiratory center.– Death from narcotic overdose is nearly

always due to respiratory arrest.– It occur at therapeutic doses but not

accompanied with cardiac center depression in contrast to other CNS depressant like general anesthetic agents.

h) Other effects:–Morphine is a basic drug causes the release of

histamine causes the body to feel warm and the face, nose to itch, bronchoconstriction and hypotension .– It also abolishes hunger – It dilate the cerebral vessels and increase

intracranial pressure

2. Cardiovascular Effects• Postural orthostatic hypotension due

primarily to decreased V.M.C. activity leading to peripheral vasodilation, which may be also due in part to histamine release.

• In congestive heart failure, morphine decreases the left ventricular workload and myocardial oxygen demand.

3. Endocrine Effects• Increases prolactin secretion• Increases vasopressin (ADH) secretion (oliguria).• Decreases pituitary gonadotropin (LH & FSH)

secretion.• Decreases stress induced ACTH secretion.

4. Gastrointestinal Effects• It decrease the motility and increase the tone

of the intestinal circular muscle and the tone of the anal sphincter , it also causes contraction of the gallbladder and constriction of the biliary sphincter.

• Constipation (tolerance does not develop to this effect).

• Diphenoxylate and Loperamide can be used in the treatment of diarrhea.

• They decrease GIT motility and peristalsis

5. Genitourinary Effects •Morphine prolong the second stage of labor by decreasing the strength, duration and frequency of uterine contraction•Inhibit urinary bladder voiding reflex (sometimes catheterization may be required in some cases )

B. Adverse Reactions•Generally direct extensions of their pharmacological actions.– Respiratory depression, apnea– Nausea and vomiting– Dizziness, orthostatic hypotension, edema– Mental clouding, drowsiness– Constipation, ileus– Biliary spasm (colic)– Dry mouth– Urine retention, urinary hesitancy– Hypersensitivity reactions (contact dermatitis,

urticaria)– Immunosuppression (recurrent infections)

C. Precautions– Respiratory depression, particularly in the newborn and

patients with COPD– Orthostatic hypotension– Histamine release (asthma)– Drug interactions (with other CNS depressants)– Tolerance and cross tolerance to other opioids– Benign prostatic hyperplasia(may precipitate urine

retention – Dependence (psychological & physical)– Liver disease (accumulation of the drug) – Increase intracranial pressure and head injury (it enhances

cerebral ischemia)

D. Therapeutic uses: - Analgesia: myocardial infarction, terminal cancer, surgery, obstetrical procedures - Dyspnea due to pulmonary edema - Severe diarrhea.

II. Other Opioid (Narcotic) Analgesics

A. Heroin (diacetyl morphine)–μ- agonist–Heroin is more lipid soluble than morphine

and about 2½ times more potent– It enters the CNS more readily– It is a schedule I drug and is not used

clinically, but it is a drug of abuse.

B. Codeine– From opium or synthesized by methylation of

morphine– Has a much better oral /parenteral absorption ratio

than morphine.– Effective for mild to moderate pain, cough, diarrhea.– Metabolized in part to morphine by O-demethylation.– μ- receptor agonist.– Has a more potent histamine-releasing action than

morphine.– Dependence liability of codeine is less than that of

morphine, .– It is 1/12 as potent as morphine

C. Dextromethorphan:– Excellent oral antitussive–No analgesic effect–No GI effects–No respiratory depression

D. Meperidine (Pethidine)– Produces analgesia, sedation, euphoria and

respiratory depression.– Less potent than morphine, 80-100 mg meperidine

equals 10 mg morphine.– Shorter duration of action than morphine (2-4 hrs).– Meperidine has greater excitatory activity than does

morphine and toxicity may lead to convulsions.– Meperidine appears to have weak atropine-like

activity. – It does not constrict the pupils to the same extent

as morphine.

– Does not cause as much constipation as morphine– Purely synthetic μ- agonist– Not an effective antitussive agent.– less effect in uterine contraction commonly employed

in obstetrics also causes less urine retentionAdverse reactions to Meperidine– Respiratory depression– Tremors– Delirium and possible convulsions– Dry mouth– Severe reaction if given with MAOI, consists of

excitement, hyperthermia and convulsions; it is due to accumulation of pethidine (norpethidine) metabolite but the details are still unclear.

E. Fentanyl – μ- agonist, related chemically to meperidine.– Approximately 80 times more potent than

morphine, main use in anesthesia .– Duration of action very short (t1/2 = 20 min).

– Can be given IM, IV, transdemally or via patient controlled infusion system and may be given intrathecally.

–Often used during cardiac surgery because of its negligible effect on the myocardial contractility.

F. Sufentanil –A synthetic opioid related to fentanyl.–About 7 times more potent than fentanyl.–Has a slightly more rapid onset of action

than fentanyl.

G. Methadone – Pharmacology and analgesic potency similar

to morphine; μ- receptor agonist.– Very effective following oral administration.– Longer duration of action than morphine due

to plasma protein binding (t1/2 approximately 25 hrs).

– Used in methadone maintenance programs for treatment of opioid addicts and for opiate withdrawal syndrome (it reduces the physical abstinence syndrome) make it possible to wean the addict from opioids.

H. Propoxyphene –A methadone analog.–Used orally to relieve mild to moderate pain,

it is weak analgesic; often combined with Paracetamol.–Has a low addiction potential – The most common adverse effects are:

dizziness, drowsiness, and nausea and vomiting.–CNS depression is additive with other CNS

depressants (alcohol and sedatives).–Can cause cardio- toxicity and pulmonary

edema which can not reversed by naloxone

III. Mixed Narcotic Agonists/Antagonists

• These drugs produce analgesia, but have a lower potential for abuse and do not produce as much respiratory depression.

A. Pentazocine

– μ agonist (analgesia) and - қ- antagonist (less respiratory depression).

–Orally, it has about the same analgesic potency as codeine.

–Adverse reactions: Nausea, vomiting, dizziness, dysphoria, nightmares & visual hallucinations.

–Rarely used nowadays

B. Nalbuphine –Resembles pentazocine pharmacologically.– Like morphine, nalbuphine reduces myocardial

oxygen demand. May be of value following acute myocardial infarction due to both its analgesic properties and reduced myocardial oxygen demand.–Most frequent side effect is sedation.– Less propensity to produce psychotomimetic

side effects

C. Butorphanol –Resembles pentazocine pharmacologically.–3.5 to 7 times more potent than morphine.–Not available for oral administration– They exhibit ceiling of respiratory depression

effect

D. Buprenorphine– A partial agonist at μ-receptor.– 200 times more potent than morphine.– Low potential abuse, but can precipitate

withdrawal in addicts– In naive persons it acts like morphine – Major use is office-based detoxification of

opioids – Causes less sedation, respiratory depression

and hypotension even in high doses.

IV. Opiate Antagonists

Opiate antagonists have no agonist properties. They are utilized to reverse opiate induced respiratory depression and to prevent drug abuse.

A. Naloxone (Narcan)– Pure opiate antagonist at all opioid receptors– Given parenterally -Short duration of action

(1-4 h)– Can precipitate withdrawal in addicts.

B. Naltrexone–Oral pure opioid antagonist– Long duration of action–Contraindicated in liver disease–Used in late stages of opioid addiction

treatment ( also in treatment of alcoholism).C. Nalmephine– Long-acting parenteral opioid antagonist.