Table of Contents
Introduction
Opiates are a class of drugs derived from opium that produce potent analgesic effects. They act primarily on opioid receptors in the central and peripheral nervous system to relieve pain, produce sedation, and induce euphoria. Common examples include morphine, codeine, and heroin. Opiates are widely used for moderate to severe pain but are associated with tolerance and dependence.
Mechanism of Action (Step-wise)
- Opiates bind to opioid receptors, mainly μ (mu), κ (kappa), and δ (delta) receptors.
- μ-opioid receptors are primarily responsible for analgesia and euphoria.
- These receptors are G-protein coupled receptors (Gi/o type).
- Activation inhibits adenylate cyclase activity.
- This reduces intracellular cyclic AMP (cAMP) levels.
- Opiates open potassium channels, causing K⁺ efflux.
- This leads to hyperpolarization of neurons.
- They also inhibit voltage-gated calcium channels (↓ Ca²⁺ influx).
- Reduced calcium entry decreases release of neurotransmitters such as substance P, glutamate, and CGRP.
- These effects inhibit transmission of pain signals in the spinal cord.
- Opiates activate descending inhibitory pain pathways from the brainstem.
- The net effect is decreased perception and transmission of pain.
A key exam point is that opiates act on μ-opioid receptors to inhibit neurotransmitter release and reduce neuronal excitability.
Pharmacokinetics
Opiates can be administered orally, intravenously, intramuscularly, subcutaneously, or via transdermal routes depending on the drug. They undergo hepatic metabolism, often forming active metabolites. Many opiates have variable bioavailability due to first-pass metabolism. They are excreted mainly via the kidneys. Lipophilicity influences onset and duration of action.
Clinical Uses
Opiates are used for the management of moderate to severe pain, including postoperative pain, cancer pain, and trauma. They are also used in palliative care. Some opiates are used as antitussives (e.g., codeine) and antidiarrheal agents (e.g., loperamide).
Adverse Effects
Common adverse effects include respiratory depression, sedation, constipation, nausea, vomiting, and miosis. Tolerance and physical dependence develop with prolonged use. Overdose can lead to respiratory arrest. Other effects include pruritus and hypotension due to histamine release.
Comparative Analysis
| Feature | Opiates | NSAIDs | Nefopam |
|---|---|---|---|
| Mechanism | μ-opioid receptor agonism | COX inhibition | Monoamine reuptake inhibition |
| Site of action | Central | Peripheral + central | Central |
| Analgesic strength | Strong | Mild–moderate | Moderate |
| Anti-inflammatory | No | Yes | No |
| Respiratory depression | Yes | No | No |
| Dependence risk | High | None | Low |
Opiates differ from NSAIDs by acting centrally on opioid receptors rather than inhibiting prostaglandins. Compared to nefopam, opiates produce stronger analgesia but carry a higher risk of respiratory depression and dependence.
MCQs
- Opiates primarily act on which receptor?
a) Dopamine receptor
b) μ-opioid receptor
c) Serotonin receptor
d) Histamine receptor
Answer: b) μ-opioid receptor
- Opioid receptors are:
a) Ion channels
b) GPCRs
c) Enzymes
d) Transporters
Answer: b) GPCRs
- Opiates inhibit adenylate cyclase leading to:
a) Increased cAMP
b) Decreased cAMP
c) Increased ATP
d) No change
Answer: b) Decreased cAMP
- Opiates cause hyperpolarization by:
a) Sodium influx
b) Potassium efflux
c) Calcium influx
d) Chloride influx
Answer: b) Potassium efflux
- Opiates reduce neurotransmitter release by decreasing:
a) Sodium
b) Calcium
c) Potassium
d) Chloride
Answer: b) Calcium
- Opiates inhibit release of:
a) Insulin
b) Substance P
c) Thyroxine
d) Cortisol
Answer: b) Substance P
- Opiates are used for:
a) Mild pain only
b) Severe pain
c) Diabetes
d) Asthma
Answer: b) Severe pain
- A major adverse effect is:
a) Hypertension
b) Respiratory depression
c) Hypercalcemia
d) Hypoglycemia
Answer: b) Respiratory depression
- Opiates cause:
a) Mydriasis
b) Miosis
c) No pupil effect
d) Diplopia
Answer: b) Miosis
- Opiates may lead to:
a) Tolerance
b) Increased insulin
c) Hypercalcemia
d) Bradycardia only
Answer: a) Tolerance
- Opiates are metabolized in the:
a) Kidney
b) Liver
c) Lung
d) Brain
Answer: b) Liver
- Compared to NSAIDs, opiates:
a) Are anti-inflammatory
b) Act centrally
c) Have no analgesic effect
d) Increase prostaglandins
Answer: b) Act centrally
FAQs
What is the mechanism of action of opiates?
They activate opioid receptors, reducing neurotransmitter release and neuronal excitability.
Which receptor is most important for analgesia?
μ-opioid receptor.
Why do opiates cause respiratory depression?
They depress brainstem respiratory centers.
Do opiates cause dependence?
Yes, with prolonged use.
Why do opiates cause constipation?
They decrease gastrointestinal motility.
What is a hallmark sign of overdose?
Miosis and respiratory depression.
References
Goodman & Gilman’s The Pharmacological Basis of Therapeutics – Opioids
https://accessmedicine.mhmedical.com/book.aspx?bookid=3191
Katzung: Basic and Clinical Pharmacology – Opioid Analgesics
https://accessmedicine.mhmedical.com/content.aspx?bookid=3382
Tripathi: Essentials of Medical Pharmacology – Opioids
https://www.jaypeedigital.com
Harrison’s Principles of Internal Medicine – Pain Management
https://accessmedicine.mhmedical.com
