VPT 321: Veterinary Neuropharmacology (2+1)

α₂ AGONISTS

• These are drugs that depress the CNS activity sufficiently to cause lethargy, drowsiness and indifference to the surroundings. They decrease loco motor activity allay fear and apprehension, but the animal remains conscious when normal clinical dose rates are used. The term hypnotic implies loss of consciousness and is used to indicate the effect in higher doses.
• α₂ agonists like xylazine, detomidine and metomidine are used as sedative hypnotics. Xylazine is used as sedative analgesic and muscle relaxant in horses and all farm animals except pigs. In ruminants the drug may abolish ruminal movements leading to tympany and ruminal distension.
• Detomidine is used in horses as sedative analgesic and as preanaesthetic.
• Metomidine is more potent than xylazine and is used in dogs and cats.
• Xylazine - This drug is α₂ an  agonist very similar in structure to clonidine. In cats, horses, and cattle, xylazine will cause sedation and CNS depression. In horses, xylazine produces visceral analgesia better than meperidine, butorphanol, or pentazocine. In horses, xylazine may also cause penile prolapse, but no permanent paralysis is produced.
• The sedation in horses is accompanied by head-lowering, relaxed facial muscles, and a pendulous lower lip. Occasionally, xylazine may cause a paradoxical increase startle reflex.
• The caregiver should be wary of kicks. In cats (and rarely in dogs) xylazine may cause emesis. It is used as an emetic agent in feline in poisonings (producing emesis within about 3-5 minutes).
• In cattle (and less frequently in cats and horses), xylazine may cause polyuria, thought to be mediated by a decrease in ADH release or activity. Species differences occur in the potency of xylazine.
• Ruminants are extremely sensitive to its effects, requiring only 1/10 the equine dose. Alternately, swine are very resistant and must be given 20-30 times the ruminant dose (for this reason, xylazine is rarely used in swine).