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Pharmacology > Opioids > Flashcards

Opioids Flashcards

1
Q

Describe how pain is sensed by the brain

A

Tissue damage causes cells to break down. This releases bradykinin, 5HT and prostaglandins

These stimulate nociceptors for pain within tissues which release substance P and glutamate

Substance P agonises inflammtion

Glutamate stimulates afferent nerves to stimulate action potential in dorsal horn (laminae I and V)

Fibres enter dorsal horn and synapse. Secondary order neurones decussate and asecnd up the spinothalamic track up to the thalamus and then to the cortex

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2
Q

What are the two types of afferent pain fibres

A

A-delta fibres - sharp pain. Are myelinated and fast conducting to cause quick response

C-fibres - dull pain. Unmyelinated. Are stimulated after sharp pain settled

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3
Q

What are the peripheral and contral modulators of pain

A

Peripherally have substantia gelatinosa

Centrally have periaqueductal grey

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4
Q

How does the substantia gelatinosa act to modulate pain

A

Stimulation of A-beta fibres (by told rub it better) causes stimulation of substantia gelatinosa which causes inhibition of pain fibres in the dorsal horn

This inhibits pain signals to the thalamus

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5
Q

How does the periaqueductal grey modulate pain

A

Pain signals to thalamus are sent to cortex which then inhibit periaqueductal grey

Thalamus and cortex can interpret pain and act on periaqueductal grey matter to stimulate it

This sends inhibitory signals down spinal cord - 5HT and endogenous opioids modulate

This reduces pain signal to thalamus and reduces pain felt

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6
Q

What are the three endogenous opioid receptors

A

MOP - mu

DOP - delta

KOP - K

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7
Q

Describe MOP opioid receptors

A

MOP are found in supraspinal and GI tract

Activation causes decreased cAMP causing efflux of K, resulting in hyperpolarisation and decreased substance P release

Endogenous opioids acting on MOP - enkephalins and beta-endorphins

Cause analgesia, depression, euphoria, dependence, respiratory sedation

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8
Q

Describe DOP opioid receptors

A

DOP have wide distribution

DOP activation causes decreased cAMP which causes influx of Ca and hyperpolarisation resulting in decreased substance P release

Enkephalins act on DOP

Receptor activation causes analgesia, inhibition of dopamine and modulation of Mu

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9
Q

Describe KOP opioid receptors

A

KOP are found in spinal cord, brain and periphery

Activation causes decreased cAMP causing influx of Ca and efflux of K

This results in hyperpolarisation and decreased substance P release

Dynorphins act on it

Activation causes analgesia, diuresis and dysphoria

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10
Q

Name some opioids

A

Strong agonist - morphine, fentan​yl (heroin, alfetnanil)

Moderate agonist - codeine

Mixed agonist/antagonist - buprenorphine

Antagonist - naloxone (naltrexone)

Other - tramadol

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11
Q

Describe the pharmacokinetics of morphine and how it acts to modulate pain

A

Given IV, PO, IM, SC, PR - gut absorption erratic and there is significant first pass effect so rarely given PO

Enters all tissues but morphine struggles to cross blood-brain barrier

Metabolised in the liver and excreted via the kidneys

Acts on MOP receptors and causes complete activation of MOP receptors

Causes analgesia and euphoria

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12
Q

What are the metabolites of morphine and what effects do they have

A

Morphine is metabolised to produce M6G and M3G

M6G has analgesic effect

M3G has neuroexcitatory effect and irritates the brain as it crosses the BBB

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13
Q

Name some side effects of morphine

A

Respiratory depression - reduces responsiveness of the medullary centre to CO2

Emesis

GI tract - decreased motility and increased sphincter tone -> constipation, nausea and vomiting

Cardiovascular

Miosis

Histamine release - can cause asthama attack

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14
Q

Describe the pharmacokinetics of fentanyl and how it acts in the body

A

Given IV, epidural intrathecal, nasal

High bioavailability

Highly lipophilic and highly protein bound but has short t1/2

High level of CNS crossing

Metabolised in the liver and excreted via kidneys

Higher potency than morphine - higher affinity for MOP and higher intrinsic activity

Cause analgesia and anaesthetic

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15
Q

Name some side effects of fentanyl

A

Respiratory depression

Constipation

Vomiting

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16
Q

Describe the pharmacokinetics of codeine and name some side effects

A

Given PO or SC

Metabolised via CYP2D6 to morphine - variable expression of CYP2D6 means codeine effects vary in patients

Eliminated via liver and kidneys

Has lower potency than morphine as has lower affinity and lower intrinsic efficacy

Used for mild-moderate pain analgesic

Causes constipation and respiratory depression

Contraindicated in children <12

17
Q

Describe the pharmacokinetics of buprenorphine and how it affects the body

A

Given transdermal, buccal, sublingual

Given for chronic pain, palliative care, moderate to severe pain, opioid addiction

Very lipophilic -> wide body distribution and has long t1/2

Metabolised via CYP3A4 and then excreted via the biliary system (also via kidneys but less so)

Has very high affinity for MOP and will displace other opioids from MOP receptors and will bind tightly for a long time to cause sustained, long effect

18
Q

Name some side effects of buprenorphine

A

Respiratory depression

Low BP

Nausea

Dizziness

19
Q

Describe the pharmacokinetics of naloxone and how it affects the body

A

Naloxone is given IV, IM, intranasal, PO

Has low PO bioavailability due to first pass metabolism

Rapid onset of action and is lipophilic -> readily distributes

Has very high affinity for MOP receptors (greatest apart from buprenorphine)

Metabolised via liver and excreted renally

Short action duration and short t1/2 - given as slow infusion for opioid overdose as otherwise its effects will wear off quickly and patient will suffer overdose again

20
Q

How does a patient develop opioid tolerance

A

If synthetic opioid given, both synthetic and endogenous opioids bind to receptors on a cell to both cause effects - will result in a higher number of receptors being filled than normal

This causes the cell to upregulate their receptor production due to high number of receptor binding

As cell has made more receptors, there is not enough opioid to bind to receptors to cause a response if synthetic opioid dose is not changed

In order to cause a response, will need an increased synthetic opioid dose

21
Q

What is given to treat opioid tolerance and withdrawal

A

Give methadone

Methadone has agonist activity - removes side effects and improves withdrawal side effects but does not cause a high

Slowly dose is titrated down to down-regulate opioid receptor number

22
Q

What are the effects of an overdose

A

Dependence

Vomiting

Constipation

Hypotension and bradycardia

Decreased sex drive

Histamine release

Miosis

Drowsiness

Repiratory depression - apnoea

23
Q

Which patients should you give special consideration to before prescribing opioids

A

Manual labourers

Drivers

Elderly

Bedbound

Asthmatics

Biliary tract obstruction

Respiratory disease

Renally impaired

Pregnant

24
Q

Name some contraindications of opioids

A

Hepatic failure

Acute respiratory distress

Comatome

Head injuries and raised ICP - M3G crosses BBB and causes inflammatory and irritability to make head injuries worse

Pharmacology (28 decks)

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