Opioid Analgesics
Opioid Analgesics in Vet Surgery Refers to exogenous and synthetic compounds that bind to specific subpopulation of opioid receptor.
The analgesic action of opioids are not accompanied by loss of proprioception or consciousness unless excessive doses are given. Mu, Kappa & Sigma are the opioid receptor subtypes.
| Opiod Receptors | Action of Receptor |
| Mu receptor | Supraspinal analgesia, Respiratory depression, Duphorpia and physical dependence |
| Kappa receptors | Spinal analgesia, Miosis, sedation & respiratory |
| Sigma receptors | Psychomimetic activity, Hallucinations, respiratory and vasomotor stimulation |
Examples of Opioid agonist – antagonist are Butorphanol, buprenorphine, pentozocine and Tramadol.
Butorphanol
Butorphanol is a Synthetic opioid analgesic. Analgesic potency is 3-5 times that of morphine
Respiratory depressant effects of butorphanol is less than those of morphine and has a “ceiling effect”
The sedation achieved with butorphanol in dogs are mild. Butorphanol is 115 times more effective than codiene or dextromethorphan in supposing cough in dogs.
Butorphanol at 0.22 mg/kg I/M and 0.1 mg/kg I/V provided visceral analgesia in horses suffering with colic for 4 hrs & 30 min. respectively.
A combination of butorphanol (0.1 mg) plus Xylazine (1.1 mg/kg I/V) will allow standing surgical procedure in horses for approximately 30 min.
Butorphanol readily crosses placental barrier and excretion occurs primarily in urine (70%) with some biliary elimination (11-14%).
Butorphanol is an excellent analgesic to combine with Xylazine or detomidine in animals.
Dose of Butorphanol in dogs is 0.1 – 0.3 mg/kg Body weight and in Cattle is 8–10 mg Butorphenol as total dose.
Buprenorphine
Buprenorphine is a partial Mu opioid agonist – antogonist. It is 30 times more potent than morphine and Onset of action is relatively slow requiring 20-30 minutes to reach full effect.
Analgesic action of Buprenorphine may last as long as 8-12 hrs. It may causes respiratory depression and can be reversed by naloxone or nalthexone.
Buprenorphine is best used with a tranquillisers in horses.
Dose of Buprenorphine in Dogs is 6-10 mcg/kg Body weight I/M or I/V administration gives analgesia for 6-8 hours.
Epidural buprenorphine also used for analgesia for 18-24 hours suited for real limb orthopaedic procedures. When it is placed in an epidural space, the high lipid solubility and affinity of buprenorphine for Mu receptors limits its cephalad spread and the likelihood of respiratory depression.
Pentazocine
Pentazocine have 1/3rd potency as morphine in humans. It has minimal effect of cardiovascular system and is milk respiratory depressant.
Pentazocine is a example of effective analgesic in horses, dogs, pigs, primates, rabbits & rats. Effects are observed with in 15 minutes and last upto 2 hours.
Dose rate of Pentazocine in Horses is 0.5-3 mg/kg body weight by Intravenous or 0.5 – 6 mg/kg body weight Intramuscular route of administration.
In case of Dogs & pigs, dose rate is 2 mg/kg body weight I/M.
Tramadol
Tramadol is a Centrally acting Opioid Analgesic. but chemically it is not a Opioid drug. It is used in dog, cats, horses, zoo and exotic animals to relieve from moderate to moderate severe pain.
Adverse effects of Tramadol include constipation, stomach upset and decreased heart rate. Over dosage causes mental alteration pin point pupil & seizures.