Pharmacology for Anaesthesia and Intensive Care

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Section IICoredrugs in anaesthetic practice

Table 9.1.Classification of opioid receptors.

Receptor Effects

MOP,μ,mu analgesia, meiosis, euphoria, respiratory depression,

bradycardia, inhibition of gut motility

KOP,κ, kappa analgesia, sedation, meiosis

DOP,δ, delta analgesia, respiratory depression

NOP

Other important agents such as local anaesthetics, antidepressants, anti-epileptics,

guanethidine, ketamine and clonidine are often used to treat pain and are discussed

elsewhere.

Opioids and related drugs

The term ‘opiate’ refers to all naturally occurring substances with morphine like

properties, while ‘opioid’ is a more general term that includes synthetic substances

that have an affinity for opioid receptors. Opioids are basic amines.

Receptor Classification

Classical receptor classification, that is, kappa and delta, was based on either the

name of the agonist that acted at that receptor,mu(μ)–morphine,kapppa (κ)–

ketcyclazocine or the location of the receptor,delta (δ)–vasdeferens. The latest

reclassification is listed in Table9.1. and includes an additional non-classical recep-

tor, NOP, which was discovered at the time of receptor cloning. It is known as the

nociceptin/orphanin FQ peptide receptor.

Both receptor types are serpentine (i.e. span the membrane seven times) and are

linked to inhibitory G-proteins so that when stimulated by an appropriate opioid

(a)

Descending pathways

Aβ fibre C fibre

(ii)

(i)

(b)

Figure 9.1.Principle of the gate theory of pain within the dorsal horn of the spinal cord. (a)

Pain mediated via C fibres passes through the gate centrally; (b) the gate is shut as Aβfibres

stimulate inhibitory interneurones (i) and by descending pathways, preventing the central

passage of pain (ii).