Chemical Immobilization Pharmacology: Opioids

The San Diego Zoo Wildlife Alliance (SDZWA) is among the zoological societies that offer education in safe capture chemical immobilization techniques for those who are interested in or who are pursuing careers as a veterinarian, wildlife biologist, wildlife manager or related disciplines. One of the courses offered by SDZWA is Chemical Immobilization Pharmacology: Opioids, which details this group of drugs and their use in chemical immobilization. A description of this course is available on the organization’s website.

Like other veterinary prescription drugs, opiates are among those restricted by US federal law to use by or on the order of a licensed veterinarian. Veterinarians making treatment decisions must use sound clinical judgment, current medical information and must be in compliance with federal, state, and local laws and regulations.

Opioid Origins and Use

Opioids are a group of analgesic agents that are widely used in veterinary medicine. Morphine is commonly considered to be “the archetypal opioid analgesic and the agent to which all other painkillers are compared.”¹ While this drug is the most well-known derivative of Papaver somniferum (the opium poppy), four naturally-occurring alkaloids are isolated from it: morphine, codeine, papaverine and thebaine.¹ Chemical manipulation of these basic opiate alkaloids has yielded a range of semi-synthetic opioids (e.g., diamorphine, dihydrocodeine, buprenorphine, nalbuphine, naloxone and oxycodone). Opioids can be classified according to their mode of synthesis into alkaloids, semi-synthetic and synthetic compounds.²

Synthetic opioids can be divided into four chemical groupings: the morphinan derivatives (e.g., levorphanol, butorphanol), the diphenylheptane derivatives (e.g., methadone, propoxyphene), the benzomorphan derivatives (e.g., pentazocine, phenazocine) and the phenylpiperidine derivatives (e.g., pethidine, alfentanil, fentanyl, sufentanil and remifentanil).²

Opioids cause a mild reduction in level of consciousness and euphoria, frequently making them drugs of abuse in human populations. They also exert effects on the respiratory system, reducing the respiratory rate and airway reflexes, which is considered advantageous during anesthesia.

Method of Action

Opioids can also be classified according to their effect at opioid receptors. There are three classical opioid receptors: DOP, KOP and MOP. The novel NOP receptor is considered to be a non-opioid branch of the opioid receptor family.¹ Opioids can act at these receptors as agonists, antagonists or partial agonists. Most clinically-relevant opioid analgesics bind to MOP receptors in the central and peripheral nervous system agonistically to elicit analgesia. Opioid receptors are distributed throughout the central nervous system and within peripheral tissue of neural and non-neural origin.²

In veterinary medicine, opiates are frequently administered via oral or intravenous routes, although subcutaneous, transdermal, sublingual, intramuscular, epidural, intrathecal and intra-articular routes can also be utilized.

Interested in learning more about safe capture? The San Diego Zoo now offers courses in safe capture techniques and best practices. Learn reliable, safe, and effective techniques for the species you work with and the scenarios you encounter!