Has the sun set on κ 3-opioid receptors?

Mark Connor*, Ian Kitchen

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review

11 Citations (Scopus)


μ-Opioid receptor agonists are a mainstay of clinical analgesia, despite the significant unwanted effects and dependence liability associated with drugs like morphine. The quest for opioids that produce analgesia with fewer undesirable effects has lead to the putative identification of multiple opioid receptor subtypes, despite the identification of only four opioid-related receptor genes. One such putative receptor subtype is the κ 3 receptor, activation of which supposedly produces analgesia in animals. In the present issue of this Journal, Olianas and co-workers have demonstrated that the prototypic κ 3 agonist naloxone benzoylhydrazone is actually a partial agonist at the cloned μ, δ, and κ opioid receptors and an antagonist at opioid-like NOP receptors. Together with a recent study that showed that high-affinity naloxone benzoylhydrazone binding is abolished in triple μ/δ/κ receptor knockout mice, the present study provides strong evidence that in vivo effects attributed to κ 3 receptor activation probably just reflect the combined actions of a particularly nonselective opioid drug. Indeed, molecular identification of any of the proposed subtypes of μ, δ, and κ opioid receptors has proven elusive, suggesting that it is perhaps time to retire the notion of opioid receptor subtypes until definitive evidence for their existence is provided.

Original languageEnglish
Pages (from-to)349-350
Number of pages2
JournalBritish Journal of Pharmacology
Issue number4
Publication statusPublished - Feb 2006
Externally publishedYes


  • Analgesia
  • Naloxone benzoylhydrazone
  • Pharmacological fantasy
  • Receptor knock out


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