CNS drugs and analgesia
Introduced| English | Meaning |
|---|---|
| tolerance/ˈtɒlərəns/ | tolerance |
| dependence/dɪˈpendəns/ | dependence |
| opioids/ˈəʊpɪɔɪdz/ | opioids |
| NSAIDs | NSAIDs |
| benzodiazepines | benzodiazepines |
The same dose that stopped working
- After weeks of a strong painkiller, a patient needs a bigger dose for the same relief. Their nerve cells have adapted.
- Each brain drug acts at a known synapse, and tolerance and dependence are what that synapse learns.
Opioids
- Opioids 阿片类 act at μ receptors: strong pain relief, with slowed breathing, constipation and tiny pupils.
- Naloxone is a competitive μ antagonist that reverses an overdose, but it wears off faster than many opioids, so breathing can slow again.
Which sign makes an opioid overdose most dangerous?
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Respiratory depression is what kills.
NSAIDs and paracetamol
- NSAIDs 非甾体抗炎药 block cyclo-oxygenase (COX), so less prostaglandin is made: less pain and inflammation, but a cost to the stomach lining and kidneys.
- Paracetamol relieves pain and fever with little anti-inflammatory effect. In overdose it uses up glutathione, and its toxic metabolite kills liver cells.
Sedatives and antidepressants
- Benzodiazepines 苯二氮䓬类 make GABA-A receptors respond more strongly to GABA (positive allosteric modulation): calm, sleep, less anxiety.
- SSRIs block serotonin reuptake at once, but the mood effect takes weeks, as the receptors adapt.
Match each drug class to where it acts.
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Each class sits at a named target.
SSRIs improve mood within a day because reuptake is blocked at once.
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Reuptake is blocked at once, but the mood effect needs weeks of receptor adaptation.
Tolerance and dependence
- Tolerance 耐受性 is receptors down-regulating under constant stimulation, so the same level does less. Dependence 依赖性 shows when the drug stops suddenly and the adapted state is left unopposed.
- Constipation and tiny pupils from opioids show little tolerance, so they persist at stable doses.
Put the steps of tolerance and dependence in order.
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Tolerance is the adaptation; dependence is that adaptation left unopposed.
Which opioid effect shows little tolerance?
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Constipation (and small pupils) persist at stable doses.
Worked example: three pains, three sites
- (Fictional) A sprained ankle with swelling: an NSAID at peripheral COX. Severe pain after surgery: an opioid at μ receptors, watching breathing.
- Burning nerve pain: neither works well alone; drugs that calm nerve excitability are used. Match the site, not just the severity.
A swollen, sprained ankle is painful. Which drug acts at the site that makes the pain?
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Prostaglandins at the injury drive this pain; NSAIDs block their production.
Worked example: naloxone wears off
- (Illustrative) Naloxone's half-life is about 1 hour; a long-acting opioid lasts far longer.
- After 3 hours, 0.5³ = 12.5% of the naloxone is left while the opioid is still there, so the patient may stop breathing well again. Watch and repeat doses.
(Illustrative) Naloxone has a half-life of 1 hour. What percentage is left after 2 hours?
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100 → 50 → 25%, while a long-acting opioid may still be active.
Three common slips
- Ranking painkillers by strength alone. They act at different sites, so they can add together.
- Expecting SSRIs to work in days. Reuptake is blocked at once, but the mood effect takes weeks.
- Thinking one naloxone dose ends the risk. It can wear off before the opioid does.
Recap
- Opioids at μ: pain relief, slowed breathing, constipation, tiny pupils; naloxone reverses them, briefly.
- NSAIDs block COX (stomach and kidney cost); paracetamol overdose damages the liver.
- Benzodiazepines boost GABA-A; SSRIs need weeks; tolerance is down-regulation, dependence shows on withdrawal.