Analgesia in veterinary patients — opioids part two
Abstract
As discussed in the previous article, opioids form the basis of pain management in veterinary patients alongside non-steroidal anti-inflammatory drugs (NSAIDs) and analgesic adjuvants such as lidocaine, ketamine and alpha-2 adrenergic agonists. This second article will focus on commonly used opioids and their relevance to veterinary analgesia.
The term opioid is used broadly to refer to any naturally occurring, semisynthetic or synthetic substance with morphine-like activity which acts at opioid receptors (Covin, 2007; Hellyer et al, 2007a; Kerr, 2007; Gupta et al, 2011). Drugs which act at opioid receptors may be classified as full agonists, partial agonists, agonist-antagonists or antagonists. Opioid receptor types are classified as mu (MOP, MOR or OP3), kappa (KOP, KOR or OP2) and delta (DOP, DOR or OP1) (Kerr, 2007; Lamont and Mathews, 2007; Taylor and Clarke, 2007; Dugdale, 2010).
Opioid agonists are drugs that bind to a receptor and trigger an effect in the associated cell, whereas opioid antagonists prevent such an effect. Partial agonists are substances which activate the receptor but only produce a partial physiological response compared with a full agonist — this is due to a lower intrinsic activity (Dugdale, 2010; Hsu and Riedesel, 2008; Vaughn et al, 2012).
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