Variable-interval responding in the horse: a sensitive method of quantitating effects of centrally acting drugs

An operant conditioning apparatus for studies in equine pharmacology was constructed. Horses interacted with this apparatus by breaking a light beam and were rewarded with 30 ml of oats. Horses readily learned to use this apparatus and were trained to respond on a variable-interval-60 schedule. With this schedule, there was no direct relationship between the rate of light beam breaking and the reward. Horses thus developed their own individual response rates (ie, light-beam breaking rates), and these rates remained stable at between 5 and 35 responses/min for each horse over a period of months. The effects of 2 drugs on this paradigm were tested. Reserpine (5 mg/horse, IV) depressed the response rate in all horses tested. This depression was maximal between 3 and 5 days after treatment and lasted for up to 10 days. After small doses of cocaine (0.01 mg/kg of body weight IV), the response rate of 1 horse was stimulated, whereas 1 mg of cocaine/kg was required for maximal stimulation of response rate in another horse. Larger doses of cocaine inhibited response. Variable-interval response was a sensitive method of measuring drug effects in the horse and allowed accurate quantitation of drug effects that were not detectable by clinical observation.