Phenothiazine tranquilizers antagonize the stimulatory effects of dopamine. These drugs are reported to have weak anticholinergic
and antihistaminic action. The potential side effects are hypotension from peripheral alpha-adrenergic blockade. Other effects
include sedation and possibly seizures by lowering the seizure threshold. Chlorpromazine (0.5 mg/kg TID to QID given IV, IM,
SC) is the most common anti-emetic from this class.
Antihistaminic drugs block both cholinergic and histamine receptor mediated transmission to the vomiting center arising from
the vestibular system. These drugs generally are used for management of motion sickness or vestibular disease, but will cause
some sedation. A common antihistamine used for this purpose is diphenhydramine (2-4 mg/kg TID given PO).
Anticholinergic drugs block cholinergic afferent pathways peripherally and pathways from the vestibular system leading to
the emetic center. Drugs in this class tend to be poor anti-emetics and have the negative effect of altering gastrointestinal
motility. Common drugs used in small animals include isopropamide (0.2-1.0 mg/kg BID given PO) and propantheline (0.25 mg/kg
TID given PO).
Metoclopramide has anti-emetic effects through several mechanisms. At low doses it inhibits dopamine receptors and at higher
doses inhibits serotonin receptors in the CRTZ. Cats are reported to have few CNS dopamine receptors and consequently metoclopramide
may be a poor anti-emetic choice for them.
Anti-emetic drugs and effects
Metoclopramide is rapidly metabolized through the liver, and the best results are observed when given at continuous-rate infusion
(0.01 to 0.02 mg/kg every hour or 1 to 2 mg/kg every 24 hours). At high doses it causes CNS excitement from dopamine antagonism.
It also has effects on GI motility. Metoclopramide should be avoided in epileptics or in animals receiving other drugs that
are likely to cause extrapyramidal reactions and should not be used with phenothiazine tranquilizers that can cause an additive
Serotonin antagonists are effective at blocking serotonin receptors found peripherally, in the CRTZ and in the vomiting center.
They are not very effective in relieving motion sickness. Common drugs used include ondansetron (0.1-1.0 mg/kg SID-BID given
PO) and dolasetron (0.6 mg/kg given IV q 24 hours). They also are used for prevention of chemotherapy-induced vomiting. Another
serotonin antagonist sometimes used in cats is mirtazepine (1.25-2.5 mg per cat PO once every three days). This drug not only
is a serotonin antagonist, but also has an agonist effect that functions as an appetite stimulant.
Maropitant (1 mg/kg Q 24 hours given SQ or 2 mg/kg Q 24 hours given PO) is a neurokinin-1 antagonist that blocks receptors
found in the emetic center, CRTZ and in peripheral afferent nerves.
Maropitant appears to be a good broad-spectrum anti-emetic approved for use in the dog. At higher doses (8 mg/kg given orally
up to two consecutive days) it also prevents motion sickness. It is effective in preventing vomiting and nausea associated
with chemotherapy and in management of parvovirus enteropathy and pancreatitis, as well as many other causes of vomiting.
It is not yet approved for use in cats, but some have used it in cats. Maropitant does not appear to effect GI motility. Because
of its hepatic metabolism, it should be used with caution in animals that have significant hepatic dysfunction.
Prokinetic disorders and their management
Abnormal gastrointestinal motility generally is associated with megaesophagus, gastroesophageal reflux, gastric hypomotility,
food hypersensitivity problems and chronic constipation. Dogs with megaesophagus do not respond to prokinetic agents because
the esophagus in the dog is predominately skeletal muscle and does not respond to agents that effect visceral smooth muscle.
However, gastric prokinetic agents are used to prevent gastroesophageal reflux.
Metoclopramide acts on both dopaminergic and serotoninergic receptors. The prokinetic effects are through cholinergic stimulation
of the upper gastrointestinal tract mediated by antagonism at the receptors and activation of the serotonin receptors located
in neural and non-neural tissues of the GI tract.
Metoclopramide increases the tone and amplitude of gastric contractions, relaxes the pyloric sphincter and increases peristalsis
of the duodenum and proximal jejunum, resulting in accelerated gastric emptying and intestinal transit. It also increases
the resting tone of the lower esophageal sphincter.
It is given orally at a dosage of 0.2-0.4 mg/kg a half-hour before meals three or four times a day. This drug is contraindicated
in gastric-outflow obstructions, with concurrent phenothiazine or narcotic therapy, or if the animal has epilepsy.
Side-effects with metoclopramide are common, and include central nervous system abnormalities associated with hyperexcitability
or depression. If side effects occur, the drug should be discontinued for 48 hours and then given at a lower dosage. This
drug also has an anti-emetic effect attributed to its central anti-dopaminergic activity through actions at the CRTZ.
Cisapride is a promotility agent that is more potent than metoclopramide. The drug binds to serotonin receptors on enteric
postganglionic cholinergic neurons and stimulates contraction of gastrointestinal smooth muscle. Cisapride increases lower
esophageal sphincter pressure, improves gastric emptying and promotes increased motility of both the small and large intestine.
In cats, cisapride increases distal esophageal motility and has become an important prokinetic agent for management of feline
megacolon. Cisapride was marketed as Propulsid, but was taken off the market because of induced cardiac arrhythmias referred
to as "torsades de pointes" that resulted in sudden death in some people. Cisapride caused QT interval prolongation and slowing
of cardiac repolarization as an explanation for the fatal arrhythmia. This abnormality has not been observed in dogs or cats.
Cisapride is now available only through compounding pharmacies. A recommended dose is 0.1 to 0.5 mg/kg BID to TID given 30
minutes before meals. Higher doses of 1 mg/kg may be required in some cases. Cats appear to tolerate a 2.5-mg dose without
problems. Side effects include vomiting, diarrhea and abdominal discomfort. Cisapride should not be used with anticholinergic
drugs because they will counteract cisapride's effect on motility. Cisapride may affect absorption of some drugs through increased
GI motility. It has few, if any, effects as an anti-emetic.
Erythromycin is a novel prokinetic associated with frequent gastrointestinal side effects, including nausea and vomiting.
The gastrointestinal effects occur from the drugs binding to motilin receptors in both cholinergic nerves and smooth muscle.
In the dog, antral contractions are due to a cholinergic mechanism and in cats it is a smooth-muscle motilin agonist.
When given at very low sub-microbiological doses, erythromycin stimulates migrating motor complexes in the stomach. This motility
occurs during the fasting state with an empty stomach, but studies have shown that, if given during feeding, it accelerates
gastric emptying of food. It also increases lower esophageal sphincter pressure and may be useful in management of gastroesophageal
Erythromycin will increase colonic activity in the dog but not the cat. In dogs, a dose of 0.5 to 1 mg/kg given TID is the
recommended prokinetic dose. Erythromycin also appears to be a better prokinetic drug than metoclopramide.
H2 receptor antagonists ranitidine (Zantac) and nizatidine (Axid) stimulate gastrointestinal motility in addition to their
ability to inhibit gastric acid secretion. They function by inhibiting acetylcholinesterase activity and increasing the amount
of acetylcholine available to smooth-muscle muscarinic-cholinergic receptors in the gastrointestinal tract.
When given at the normal anti-secretory doses, they also stimulate gastric contractions and accelerate gastric emptying. These
drugs could be useful in the management of gastric ulceration with concurrent gastric hypomotility. Their relative potency
as a prokinetic compared to the other drugs discussed is unknown but believed to be weaker.
Domperidone is both an anti-emetic and prokinetic agent not yet available in the United States. Domperidone is a peripheral
and CRTZ dopamine receptor antagonist. It appears to be a better anti-emetic than prokinetic agent, having weak effects on
GI motility. It has little effect on sustaining lower esophageal sphincter pressure, and gastric motility does not appear
to be significantly enhanced during the fed state.