The vomiting reflex is a complex mechanism that can be initiated peripherally from the GI tract, pharynx, chemoreceptor trigger
zone (CRTZ), and vestibular systems or centrally at the emetic center in the brain. Direct stimulation of the pharynx can
result in vomiting, although this is a relatively infrequent cause in animals. Vomiting can be initiated from the GIT due
to irritation, inflammation, or distention of the stomach or intestines and is typically mediated by serotonin (5HT3) receptors
which than progress through the CRTZ or the solitary tract nucleus in the brain. The solitary tract nucleus contains enkephalin,
histamine (H1), muscarinic (M1), substance P (neurokinin NK-1), and 5HT3 receptors. The chemoreceptor trigger zone can also
initiate the vomiting reflex due to blood borne drugs or toxins (i.e. morphine induced vomiting in dogs) which is mediated
by opiate, 5HT3, dopamine (D2) and M1, NK1, and norepinephrine (α-2) receptors. Motion sickness or vestibular disease can
stimulate the vomiting reflex primarily through histamine (H1) and muscarinic receptors. The emetic center coordinates the
vomiting reflex and contains high concentrations of 5HT1 and α-2 receptors. The complex nature and multiple pathways that
can stimulate the vomiting reflex can be confusing but offer many targets for symptomatic treatment of vomiting, but no one
treatment is expected to be highly efficacious for all causes of vomiting. Is is also important to reiterate that antiemetics
are treating a symptom and not a disease process, therefore every effort should be made to make a primary diagnosis as some
underlying disease can prove fatal if not treated.
Antihistamines are commonly used for vomiting associated with vestibular stimulation such as motion sickness or vestibular
disease. Most antihistamines have weak anticholinergic effects as well, which is the reason they are typically contraindicated
in glaucoma and cardiovascular disease. Diphenhydramine (Benadryl), dimenhydrinate (Dramamine), and hydroxyzine have all been
used for the treatment of vestibular vomiting. The efficacy of antihistamines to treat vestibular vomiting in cats is less
than that in dogs and may not be an optimal choice in cats.
Aminopentamide (Centrine) is the representative antimuscarinic agent used for vomiting in veterinary medicine. The efficacy
of aminopentamide is essentially limited to vestibular induced vomiting. Aminopentamide has low efficacy for other causes
of vomiting. The adverse effects of anticholinergics (decreased stomach emptying, ileus) are often contraindicated for many
causes of vomiting. Other adverse effects include xerostomia, urine retention, increased intraocular pressure, constipation,
and excitement in cats. The clinical usefulness of this drug class as antiemetics is minimal.