Opioids

1. Q. What is the nomenclature for opioid receptors?
   
2. A. Four opioid receptors are currently included in the opioid-related receptor group and several different naming schemes have been proposed (see Table 6.1). In general, opioid nomenclature is somewhat controversial and the literature can be confusing. The International Union of Basic and Clinical Pharmacology currently recommends conservation of the traditional Greek symbols with the added identification of an opioid peptide (NOP) receptor. The NOP receptor is considered opioid-like because it is structurally very similar to opioid receptors but displays a different pharmacologic profile.

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Greek Symbolsa	Opioid Peptidea	Previous Recommendationb
   μ or mu	MOP	OP3
   κ or kappa	KOP	OP1
   δ or delta	DOP	OP2
   —	NOP	OP4

   
   

   ^a Nomenclature scheme currently recommended by IUPHAR.

   
   

   ^b This nomenclature may appear in some literature.

   
   
3. Q. How do opioid receptors function?
   
4. A. Located centrally and peripherally in the nervous system, opioid receptors are inhibitory G protein coupled receptors, membrane-spanning signal transducers that activate second messenger systems within cells. The overall result is to hyperpolarize the cell, decreasing neuronal excitability and reducing transmission of nerve impulses and neurotransmitter release. Inhibitory G proteins interact with various other intracellular signaling cascades with the overall clinical result of analgesia, sedation, respiratory depression, euphoria, and ileus.
   
5. Q. What is meant by full μ agonist, partial agonist, antagonist, and mixed agonist–antagonist?
   
6. A. A full μ agonist is an opioid that binds efficiently to the μ receptor, resulting in cellular and clinical effects that are dose-dependent. Increased doses do not exhibit a ceiling effect but are limited by the development of adverse clinical side effects. A partial agonist opioid exhibits relatively low intrinsic activity at its receptor and a ceiling effect is demonstrated where escalating doses do not result in intensifying effects. An opioid antagonist has no intrinsic activity at the receptor and available compounds are competitive antagonists that bind to all opioid receptors. A mixed agonist–antagonist has agonistic effects at one receptor and antagonistic effects at another (see Table 6.2).

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Receptor activity	Drugs	Dosing rangesb
   Full μ agonists	Fentanyl Hydromorphone Meperidine Methadone Morphine Oxymorphone	2–10 mcg/kg IM or IV 0.05–0.2 mg/kg IM or IV 2–5 mg/kg SC or IM 0.05–1.0 mg/kg IM or IV 0.25–1 mg/kg IM or IV 0.025–0.1 mg/kg IM or IV
   Partial μ agonist	Buprenorphinea	0.02–0.05 mg/kg IM or IV
   Mixed agonist–antagonists	Butorphanol Nalbuphine	0.2–0.8 mg/kg SC, IM, or IV 0.2–0.8 mg/kg SC, IM, or IV
   Antagonist	Naloxone	0.005–0.04 mg/kg

   
   

   ^a Although commonly classified as a partial μ agonist, buprenorphine is also a partial NOP agonist and κ antagonist.

   
   

   ^b In general, the lower end of the dose ranges presented are administered to cats.

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