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

The research experiment investigates the effects of drugs on horses using a specially designed apparatus that involves breaking a light beam for rewards. The response rate of the horses to this setup was monitored under the influence of two different drugs – Reserpine and Cocaine.

Experimental Setup

• The researchers set up an operant conditioning apparatus specifically designed for equine pharmacology studies. The functioning of the apparatus was based on the breaking of a light beam by the horses.
• The reward for interacting with the apparatus was 30ml of oats, thus training the horse to respond to the system.
• The setup used a variable-interval-60 schedule, where the rate of reward was not tied directly to the rate of light beam breaking. This stimulated individual rates of response from the horses.
• The response rates were observed to remain stable over months, ranging between 5 and 35 responses per minute for each horse.

Drug Influence Study

• The research investigated the effects of two centrally acting drugs – Reserpine and Cocaine on the horses’ response rate.
• Reserpine, administered intravenously (IV) at 5 mg per horse, was observed to depress the response rate in all tested horses. Its depressing effect peaked between 3 to 5 days post-treatment, lasting for up to 10 days.
• Cocaine was administered in varying doses. A small dose (0.01 mg/kg of body weight IV) stimulated the response rate in one horse, whereas, a larger quantity (1 mg of Cocaine/kg) was required for maximal stimulation in another horse. Higher cocaine doses resulted in response inhibition.

Conclusion

• The variable-interval response proved to be an effective method for assessing drug effects on horses, enabling accurate quantification of the drugs’ impact.
• The observable effects of the drugs were such that they would not have been detected through mere clinical observation.
• The study thus proves the sensitivity of the variable interval responding method in understanding the impacts of central-acting drugs on horses.