Chapter 194 - Opioids

Question 1

Mechanism of Action of Opioids in the brain and in the spinal cord?

Answer 1

Receptors in the thalamus
- they dull chronic and poorly localized pain.

Opioid receptors in the limbic system, amygdala, corpus striatum, and hypothalamus (parts of the brain that modulate the emotional response
to pain) are involved in euphoric responses to opioid
administration.

In the spinal cord, receptors in the substantia gelatinosa.

Receptors on synaptic membranes where they regulate
the transmission of information from primary sensory
pain afferents. Opioids also block the release of substance P, an important neurotransmitter of painful stimuli, in the dorsal horn of the spinal cord.

Question 2

How do opioids affect respiration?

Answer 2

Opioids depress the respiratory center’s responsiveness to CO2. Patients have a dose-dependent increase in
PaCO2 due to a decrease in minute volume.

Respiratory depression is well tolerated in animals with no respiratory disease but in animals with upper airway or lung disease, the respiratory depression can cause significant hypoxemia

Question 3

Why do opioids suppress the cough reflex?

Answer 3

There are opioid receptors on A delta fibers located in
or beneath the respiratory epithelium. These are independent of their action on the respiratory center.

Opioids should be avoided in animals that are at risk for regurgitation and aspiration, particularly brachycephalic
breeds with pre-existing evidence of aspiration pneumonia and/or dyspnea. Opioids should also be avoided in dogs that have undergone an arytenoidectomy or laryngeal tieback surgery because they are more likely to aspirate food or regurgitated material that enters their oral cavity.