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2. Transmission of pain info from periphery to dorsal horn of spinal cord (gate control mechanism)

3. Passage of pain info from dorsal horn to brain via spinothalamic tract

Opioid peptide release in the spinal cord and brainstem can reduce the activity of the dorsal horn relay neurones and cause analgesia (shutting the gate).

Mode of Action

Opioids such as diamorphine mimic endogenous opioid peptides (the enkephalins, endorphins and dynorphins) by acting as agonists on opioid receptors. These receptors are found throughout the CNS and PNS and in organs such as the GI tract and heart, there are three main types:

1.

Mu receptors

2.

Delta receptors

3.

Kappa receptors

In the CNS opioid receptors are found in areas involved in pain modulation, binding of the opioid to the opioid receptor reduces the transmission of pain signals by:

1)

Inhibiting adenylate cyclase which produces a intracellular camp. This leads to an increased K channel opening = hyperpolarsation and therefore reduces likelihood of firing

2)

Inhibition of transmitter release by decreasing ca channel opening

3)

Decrease in exocytosis and therefore release in neurotransmitters

and therefore produces an analgesic effect.

Useful diagram - http://www.fleshandbones.com/readingroom/viewchapter.cfm?ID=445

Side effects:

Large doses: Respiratory Depression, miosis, hypotension and muscle rigidity

Nausea

Vomiting

Constipation

Drowsiness

Mood alterations: variable -quiet sedation to euphoria to psychotic like behaviour

Other Side effects:

difficulty with micturition

ureteric or biliary spasm

dry mouth

sweating

headache

facial flushing

vertigo

bradycardia/tachycardia

palpitations

hypothermia

hallucinations

dependence

decreased libido or potency

rashes

urticaria

pruritus

18

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