Synthetic opioids are powerful, useful tools to manage pain for one simple reason: Receptors for naturally-occurring opioids
(endorphins, enkephalins) are distributed ubiquitously throughout the body and can be found in both central and peripheral
tissues. Several opioid different receptor types and subtypes have been isolated, each with a variant effect. The historic
categorization of mu, kappa, delta, and sigma opioid receptor subtypes have been re-classed according to a rubric aligned
with gene expression. However for the sake of our discussions, this manuscript to familiar and traditional Greek-letter categories,
and in particular mu and kappa receptors as these are the two that are manipulated in animals to provide analgesia.
Activation of a mu-opioid receptor inhibits presynaptic release (especially in the dorsal horn of the spinal cord) and postsynaptic
response (especially in the dorsal root ganglion) to excitatory neurotransmitters. The proposed mechanism includes opioid
receptor coupling with the membrane-associated G protein; this leads to decreased intracellular formation of cAMP which diminishes
calcium channel phosphorylation (closing off the voltage-gated calcium channel) and opens potassium channels enhancing potassium
influx. The resulting effect is hyperpolarization of the neuron and blockade of Substance P release. Nociceptive transmission
is thus greatly impeded.
Opioid tolerance and resistance occurs when signaling cascades force calcium channels to remain open despite the presence
of opioid. Additionally, glial cells, once thought to merely provide supporting roles in the spinal cord but are now known
to be highly interactive with nociceptors, dysregulate opioids...and unfortunately, opioids activate glial cells. Therefore
in an important sense, the pain we perceive is a balancing act between the activity of the opioids and the activity of glia.
A number of different opioid drugs are available which vary in their relative potency and receptor affinity.
Morphine remains the prototype opioid and the opioid in widest use; it has no ceiling effect on analgesia or respiratory depression,
elicits histamine release, and causes vomiting at low doses (higher doses, IV doses, and chronic use do not elicit vomiting,
presumptively by interaction with mu receptors in the antiemetic center). Cats lack glucoronate metabolism, resulting in
minimal production of the analgesic M6G metabolite, therefore morphine may not be the ideal opioid for use in this species.
Oxymorphone (Numorphan®) and hydromorphone (Dilaudid®) do not elicit histamine release (therefore may be wiser choice in cases
of hypovolemia e.g. trauma, dehydration), and nausea may be less pronounced, but they have a much shorter duration of action
than morphine; also, hydromorphone in particular is implicated in episodes of hyperthermia in cats.
Methadone may also be an attractive opioid alternative in animals, in part due to its additional effect as an NMDA antagonist
and evidence of effectiveness in rodent models of neuropathic pain. The parenteral preparation is favored by some veterinarians
as a pre-medication due to its low AE profile (minimal if any nausea, no histamine release) and prolonged sedative properties.
Orally, however, in contradistinction to humans, it appears to have low oral bioavailability and rapid clearance.
Fentanyl is a short-acting opioid preparation (Sublimaze®) with a potency of 80-100x that of morphine. It is most often used
in as an intravenous constant rate infusion. Fentanyl in a transdermal patch (Duragesic®) remains useful in veterinary medicine
though a number of studies have demonstrated wide kinetic variability in veterinary patients due to species, body condition
score, body temperature, surgical procedure, where and how well the patch is placed, etc.,
Meperidine (Demerol®) is a weak mu-agonist (approx. ¼ the potency of morphine) but more importantly carries a very short duration
of action in the dog (less than 1 hour). These features have limited its use in animals.
Commercial oral opioid preparations are widely available, and while there is often a significant first-pass effect limiting
bioavailability, these drugs are not without their utility. Hydrocodone, codeine (both alone and in combination with acetaminophen),
hydromorphone (Dilaudid®) and sustained-released forms of oral opioids include morphine (MSContin®), oxycodone (Oxycontin®),
and oxymorphone (Opana ER®) are all available by prescription. PK data exists for some of these formulations but PD (efficacy)
data is currently lacking in dogs and cats. Oral methadone in humans has much higher bioavailability (>70%) than does morphine
(<20%), but in dogs the oral bioavailability of methadone appears to be very low. Transmucosal preparations such as fentanyl
buccal tablets and suckers (Actiq®, Fentora®) do exist, but pharmacokinetics and pharmacodynamics in dogs and cats is less
established and the limitation of administration to dogs and cats of this kind of delivery system is self-evident. Rectal
suppository opioid formulations may also be prescribed, but appear to provide little advantage in bioavailability over the
oral route in the dog.
Tramadol has also become a popular adjunct to chronic pain management in both human, and veterinary medicine because of its
effectiveness as a weak opioid, and norepinephrine and serotonin (an inhibitory neurotransmitters) agonist. However, conversion
to the active mu agonist M1 metabolite appears to be minimal in the dog, indicating most of its activity in this species may
be derived from its seritoninergic and noradrenergic activity. Per-rectal administration does not appear to offer an advantage
in this regard. Tapentadol (Nucytna®) is a new centrally acting analgesic with a dual mode of action similar to tramadol:
mu-opioid receptor agonism and inhibition of norepinephrine reuptake. However, it is the parent compound, not a metabolite,
that provides for both of these effects, and thus may offer an alternative superior to tramadol in dogs. Tramadol (and tapentadol)
should not be used with other serotoninergic medications such as tricyclic antidepressants, SNRI's, and amitraz-containing