VPT 321: Veterinary Neuropharmacology (2+1) copy 1

USES OF OPIOIDS

• Morphine is the prototype drug. – Morphine from opium – dried latex from the unripe capsule of Papaver somniferum- The ‘gold standard’ for assessing analgesics
  • Analgesia – euphoria – respiratory depression – vomition – constipation by increasing the tone of the gut ( closure of lumen and also the sphincters)
  • Man, dog and rabbits – depression
  • Cats and horses – stimulation
  • Miosis in dogs and mydriasis in cats
  • Little effect on CVS – higher doses may cause bradycardia
  • Use – analgesia before, during and after surgery- eg., Codeine, Diacetylmorphine (Heroin) – acts as agonist: Nalorphine, lavallorphan acts as partial agonist and Naloxone as pure antagonist.
• Pethidine – 1/10^th as potent – spasmolytic, analgesic, sedative
• Methadone – synthetic agonist – equipotent – used in horses and dogs
• Fentanyl – 50 times as potent -Part of neuroleptanalgesia, balanced anaesthesia and for postoperative analgesia (transdermal patches – for prolonged pain relief)
• Pentazocine – analgesic- prescription drug
• Buprenorphine – partial agonist - slower onset and long duration of action (bell shaped curve for analgesia) higher doses poor analgesia
• Etorphine – very potent – needs a very small dose – (0.5 mg / 500 kg)
  • Small volume enough to sedate and anaesthetize large animals using dart guns - very long acting
  • Immobilon for neuroleptanalgesia ( + acepromazine or methotripromazine)
  • Diphenoxylate – antidiarrhoeal
  • Codeine – cough suppressant
  • Naloxone – antagonist – used to reverse toxicity or delayed sedation.