A comparison of the antinociceptive and adverse effects of the mu-opioid agonist morphine and the delta-opioid agonist SNC80

Basic Clin Pharmacol Toxicol. 2005 Jul;97(1):39-51. doi: 10.1111/j.1742-7843.2005.pto_97107.x.


delta-Opioid receptor agonists have been postulated to induce analgesia without the adverse effects commonly associated with mu-opioids e.g. morphine. In the present study, we evaluated the occurrence of antinociceptive and opioid-like side effects in rats (n=5-7) treated with a single dose of subcutaneous morphine (0.01 to 40 mg/kg) or SNC80 (0.63 to 80 mg/kg). The antinociceptive effects of morphine and SNC80 were compared using a range of nociceptive tests including the tail withdrawal test, the acetic acid-induced abdominal constriction (writhing) assay, the automated formalin test and a model of inflammation-induced thermal hyperalgesia. The adverse effects of both drugs were examined in animal models for gastrointestinal (GI) inhibition (charcoal test; ricinus oil test), respiratory depression (blood-gas analysis), motor disturbances (automated rotarod model) and abuse liability (drug discrimination learning). Morphine displayed significant antinociceptive and adverse effects in all the animal models employed. SNC80 exhibited a significant effect in the writhing test and limited efficacy in a model of inflammation-induced thermal hypersensitivity. A delay in the occurrence of diarrhoea and some limited increases in PCO(2) were observed with the higher doses of SNC80 (> or =40 mg/kg). In conclusion, the delta-opioid agonist SNC80 lacks both the analgesic efficacy and adverse effects of mu-opioids. However, the activity of SNC80 in the inflammatory model suggests delta-opioid agonists may be useful analgesics in the treatment of some forms of inflammatory pain.

Publication types

  • Comparative Study

MeSH terms

  • Animals
  • Benzamides / pharmacology*
  • Benzamides / toxicity
  • Carbon Dioxide / blood
  • Discrimination Learning / drug effects
  • Hyperalgesia / drug therapy
  • Male
  • Morphine / pharmacology*
  • Morphine / toxicity
  • Motor Activity / drug effects
  • Pain / drug therapy*
  • Piperazines / pharmacology*
  • Piperazines / toxicity
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, Opioid, delta / agonists*
  • Receptors, Opioid, mu / agonists*


  • Benzamides
  • Piperazines
  • Receptors, Opioid, delta
  • Receptors, Opioid, mu
  • Carbon Dioxide
  • 4-(alpha-(4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl)-N,N-diethylbenzamide
  • Morphine