Memory Recall in Horses Tranquilized with Acepromazine Maleate

Abstract

Acepromazine maleate (Ace) is a common tranquilizer employed by horse handlers as a “training aid” because it allows handlers more control of fractious horses in training situations. Tranquilizing horses with Ace is effective at increasing tractability in horses, but little research exists on its effect on learning ability and the ability to recall learning at later dates. Thirty-five mature horses were assigned randomly to tranquilized (n = 18) and non-tranquilized control (n = 17) groups and used to determine if recall abilities differed between tranquilized and non-tranquilized horses. Horses were trained for three consecutive days to lever press in a training stall containing a single lever for a food reward with a buzzer as a secondary reinforcer. Horses were trained to a criterion of 30 independent lever presses for each training day (90 total presses). On the fourth day, each horse was administered either 0.088 mg/kg IM of Ace or a saline control, according to treatment, and allowed to stand for 15 min while the tranquilizer took effect. Then the horse was moved into a second stall containing a white lever and a black lever. Each horse was assigned randomly a correct lever color and was trained to criterion of 30 independent lever presses on the assigned lever on that day. Fourteen days after the training date for the lever color discrimination, horses were returned to the two-lever stall and allowed to lever press. Each lever press was recorded as either correct (pressing the originally assigned lever color), which was rewarded with food and the sound reinforcer, or incorrect, which received no food reward or sound reinforcer. Number of correct lever presses and time (s) for the horse to complete 30 correct lever presses were recorded, and data were analyzed using a t-test. No significant difference was detected in number of correct lever presses between control (29.5 ± 0.3) and tranquilized (28.5 ± 0.3) groups. Similarly, no significant differences in time to complete 30 correct responses (control = 643.9 ± 54.8 s; tranquilized = 583.7 ± 53.2 s) were detected. Six months after the initial two-week testing, 26 of the original horses were retested in the same test stall to determine long-term memory recall ability. Procedures were the same as in the 14-day test except horses were not tranquilized or sham-injected, and they received no reinforcement for the correct response. Horses were allowed to respond for 30 lever presses or until 1 hour passed without a lever press by the horse. Again, no significant difference was detected in percent correct responses between control horses and those previously given Ace during learning sessions (65.5 ± 5.1 and 69.1 ± 5.5, respectively). These results indicate that horses can learn while under the influence of Ace and can retain that learned information to be recalled at a later date. This validates the use of Acepromazine maleate as an appropriate tranquilizer to be used during training exercises of a fractious horse.