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LCRS 1 - Pharmacology > Opioids > Flashcards

Opioids Flashcards

1
Q

Summarise the pharmacokinetics of opioids

A
  • Oral (20% bioavailability) - ionised in blood, heavily metabolised in liver
  • i.v. (100%)
  • Weak bases (pKa>8) - not ionised in stomach, ionised in SI
  • More lipid soluble = more potent in general

Active metabolites:

  • Heroin/codeine –> morphine –> morphine 3-G glucuronide/morphine 6-G glucuronide
  • Less likely to cause dangerous -ve side effects
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2
Q

What are the pharmacological actions of opiates/opioids?

A
  • Analgesia
  • Euphoria
  • Antitussive (depression of cough centre)
  • Respiratory depression (depression of medulla)
  • N+V (stimulate CTZ)
  • Reduced GI tract motility
  • Pupil constriction
  • Histamine release
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3
Q

What are the different types of opioid receptors and their endogenous ligands?

A

Endorphins - mu, delta

Enkephalins - delta

Dynorphins - kappa

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4
Q

What are the main pathways concerned with pain transmission/perception

A
  • Sensory neurone from periphery to dorsal horn
  • Spinothalamic neurones from dorsal horn to nucleus reticularis paragigantocellularis (NRPG) and thalamus (+)
  • Neurone from NRPG to nucleus raphe magnus (NRM) (+)
  • Neurone from thalamus to periaqueductal grey matter (PAG) (+)
  • Neurones from cortex and hypothalamus to PAG (+/-)
  • Neurone from PAG to NRM (+)
  • Descending motor neurones from NRM to dorsal horn (-) - 5-HT, enkephalin
  • Neruone from locus ceruleus (LC) to dorsal horn (-) - NA
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5
Q

How do opioids impact the pain pathways?

A
  • Stimulatory to PAG and NRPG (switch off inhibitory GABA)
  • Inhibitory to dorsal horn
  • Ca
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6
Q

How do opioids activate the reward pathway in the brain?

A
  • Switch off inhibitory GABAergic neurones to ventral tegmental area
  • Increase dopamine release from nucleus accumbens
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7
Q

How can opiates/opioids produce tolerance?

A

Overstimulation of opioid receptors leads to receptor internalisation via arrestin

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8
Q

How can opiates/opioids produce dependence?

A
  • Overactive cell activity
  • Increases adenylate cyclase
  • Converts ATP to increase cAMP
    ??
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9
Q

What are opiates?

A

Alkaloids derived from opium which is derived from poppy seeds (Papaver somniferum)

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10
Q

What are opioids?

A

Compounds that act like morphine but which do not resemble morphine chemically, e.g. endogenous opioid peptides

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11
Q

How do exogenous opioids produce their biological actions?

A

By acting on opioid receptors and mimicking the actions of endogenous opioid peptides (endorphins, enkephalins and dynorphins)

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12
Q

What is the cellular mechanism of action of opioids?

A
  • Depressant effect
  • Hyperpolarisation (K+ efflux)
  • ↓ Ca2+ inward current ↓ NT release
  • ↓ adenylate cyclase activity
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13
Q

Where are opioid receptors found?

A

Located mainly in the CNS but also found in the periphery

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14
Q

Why are there good and poor metabolisers of codeine?

A
  • Common polymorphisms in CYP2DG (O-dealkylation - activates codeine)
  • CYP3A4 deactivates
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15
Q

How does the clearance of fentanyl and methadone differ?

A

Methadone = 0.5 ml/kg/min - lots of enzymes tf builds up in blood + fat

Fentanyl = 15

  • 1 enzyme
  • Fast effect, quickly lost
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16
Q

Why are fentanyl and methadone more potent than codeine and morphine?

A

More lipid soluble

17
Q

Compare codeine and morphine

A

Codeine:

  • Methyl-morphine
  • More lipid soluble - should cross BBB more easily
  • Must be converted to -OH - e.g. prodrug

Morphine:

  • Tertiary nitrogen = analgesia - permits R anchoring
  • Hydroxyl groups - needed for activity
18
Q

What happens if you oxidise the hydroxyl groups at position 6 on morphine?

A

Increases lipophylicity

19
Q

What is opioid withdrawal associated with?

A

Psychological craving
Physical withdrawal
Like flu

20
Q

How can opioids cause analgesia?

A
  • Decrease pain perception

- Increase pain tolerance

21
Q

How do opioids cause pupil constriction (miosis)?

A
  • GABA disinhibition of occulomotor nerve fibres from Edinger-Westphal nucleus to ciliary ganglion
  • Mui opioid receptor
22
Q

How do opioids cause nausea and vomiting?

A
  • GABA disinhibition of nerve fibres to chemoreceptor trigger zone (mui opioid receptors)
  • CTZ communicates w/medullary vomiting centre
  • VC –> vomiting reflex
23
Q

How do opioids act as anti-tussives?

A
  1. Reduce info relay to brain
  2. Depress cough centre
  3. Inhibit feedback of 5-HTRs
24
Q

How do opioids cause respiratory depression?

A
  1. Inhibit central chemoreceptors that sense PaCO2

2. Decrease rhythm generation in Pre-Botzinger complex

LCRS 1 - Pharmacology (33 decks)

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