Pharm - GI

• Vomiting center


  • In the brain, controlled by Ach
  • Many signals can stimulate it
  • Inner ear, GI tract, pain/fear, odor/taste
  • Some signals sent through CRTZ (dopamine)
• Function of emesis


  To remove substances from the body that it perceives as toxic
• Emesis should occur before animal can absorb toxin
• 5 triggers of the vomiting center

  • Direct stimulation
  • CRTZ - chemoreceptor trigger zone
  • Distention/irritation of GI tract and related organs (liver)
  • Motion sickness (inner ear disturbance)
  • Emotional stimuli, brain trauma/swelling
• CRTZ and Vomiting Center


  • 1. Inner ear (CN VIII – vestibulocochlear) directly connects to the CRTZ
  • 2. Distension/irritation of the GI tract - stimulates the vagal nerves connected to emetic center
  • 3. Emotion - or overstimulation of areas within brain
• Induction of vomiting is not always desirable
• Situations where vomiting is not desirable

  • Corrosive substance
  • Cleaning fluids
  • Volatile fluids – gasoline, oil
  • More likely to be aspirated
  • Physically unable to vomit
  • Convulsing
  • Comatose/depressed
  • No gag reflex
  • Bloated, possible GI torsion
  • Horses, rabbits, many rodents
• Induction of vomiting depends on ingested substance and amount of time elapsed
• Emetic drugs


  Induce vomiting
• Centrally acting emetics


  • Apomorphine
  • Xylazine
• Apomorphine


  • Stimulates dopamine receptors
  • Works better in dogs than cats (more receptors)
  • Must stimulate CRTZ before CNS depression of the emetic center sets in
  • Don't give to an animal already in respiratory depression
  • Injectable - IM or IV
  • Should work quickly (minutes), no need to redose
• Xylazine


  • Main use: sedative/tranquilizer
  • Works better in cats (more α2 receptors)
  • 90% effective in cats compared to 50% in dogs
  • Reversed by yohimbine
• Locally acting emetics


  • Hydrogen peroxide
  • Salt solutions
  • Mustard
  • Manual (human fingers down throat)
• Hydrogen peroxide


  • Can induce gastritis
  • Foam production - aspiration risk
  • Dose: 1/2 to 1 ml per pound (max 45 ml)
• Salt solutions are not recommended - risk of salt toxicosis
• Manual (human fingers down throat) - can't reach far enough to stimulate gag reflex, risk of bitten fingers
• Anti-emetic drugs


  Prevent or decrease vomiting
• Anti-emetic drug classes


  • Phenothiazine tranquilizers (e.g. acepromazine)
  • Antihistamines
  • Anticholinergics
  • Metoclopramide
  • Cisapride
  • Serotonin antagonists (5-HT antagonists)
• Phenothiazine tranquilizers (e.g. acepromazine)


  • Lower dose than for sedation
  • Block dopamine receptors in the CRTZ
  • Also block histamine
• Antihistamines


  • Work better in dogs (more H1 receptors)
  • Block info getting to CRTZ from vestibular system
• Anticholinergics


  • Block vagal nerve - decreased urge to vomit
  • Examples: atropine, aminopentamide
• Metoclopramide


  • Has both local and central activity
  • More effective in dogs (more dopamine receptors)
  • Prokinetic drug - promotes GI motility
• Cisapride


  • Similar to metoclopramide without central effects
  • Still available to veterinarians
• Serotonin antagonists (5-HT antagonists)

  • Act on serotonin receptors in brain stem and GI tract
  • New class of anti-emetics - used for chemotherapy in humans
  • Expensive - short term use e.g. parvovirus in dogs
  • Example: ondansetron
• Anti-diarrheal drugs


  Drugs that alter motility or block secretions
• Drugs that alter motility


  • Anticholinergics
  • Narcotics (opioids)
• Anticholinergics


  • Ach is the primary NT responsible for GI motility
  • Sympathetic system allowed to dominate
  • Examples: atropine, aminopentamide
• Narcotics (opioids)


  • Increase segmentation, also anti-secretory
  • Examples: diphenoxylate (Lomotil), loperamide (Immodium)
• Anti-diarrheals aren't used when bacterial infection is responsible for diarrhea
• Drugs that block secretions

  • Narcotics/opioids
  • Anti-inflammatories (e.g. salicylates, flunixin meglamine)
  • Bismuth subsalicylate (Pepto, Kaopectate)
  • Sulfasalazine (Azulfidine)
• Narcotics/opioids


  • Decrease secretions and increase segmentation
  • Result in less content and slower movement through GI tract
• Bismuth subsalicylate (Pepto, Kaopectate)


  • Bismuth - coats GI tract
  • Subsalicylate - aspirin like component, anti-inflammatory (anti-prostaglandin)
  • Use caution in cats
  • Can turn feces darker color (not melena)
• Sulfasalazine (Azulfidine)


  • Antibiotic that gets altered by bacteria in the colon
  • Metabolized into two components: aspirin-like (mesalamine) and antibiotic (sulfapridine)
  • Uses: ulcerative colitis, IBS
  • Not for cats (aspirin like)
• Other GI medications


  • Adsorbents/Protectants (e.g. activated charcoal, bismuth products)
  • Laxatives (emollient, bulk, cathartics)
  • Antacids (systemic, non-systemic)
  • Antisialogues (e.g. atropine, glycopyrrolate)
• Adsorbents


  Allow other substances to "stick" to it so it can't contact GI wall/be absorbed
• Protectants


  Physically coat the GI tract, slow absorption
• Laxative types


  • Emollient (e.g. mineral oil, white petrolatum, cod liver oil, glycerin, docusate)
  • Bulk (e.g. bran, psyllium, lactulose)
  • Cathartics (osmotic, irritant)
• Osmotic cathartics


  Hypertonic salts that draw water into GI tract (e.g. milk of magnesia, epsom salts, phosphate enemas)
• Irritant cathartics


  Increase peristalsis (e.g. castor oil)
• Irritant cathartics not for use in suspected blockage cases