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Updated: Aug 18, 2026
A Novel Procedure for Evaluating the Reinforcing Properties of Tastants in Laboratory Rats: Operant Intraoral Self-administration
Published on: February 6, 2014
Self-administration of morphine in the rat: relative influence of fixed ratio and time-out
Abstract:
Rats were offered 3.2 mg/kg of morphine sulfate on a continuous reinforcement schedule until the daily injection rate had stabilized. The effect of fixed ratio schedules of 4 and 8 were compared to imposing time-out periods of 5 and 10 sec immediately followed each injection. The fixed ratio schedules decreased the injection rate while the time-out schedules had no effect. The hypothesis that the effect of a fixed ratio schedule is a consequence of imposing a time-out period, allowing full effects of the injection to be sensed, is not supported.
Insights
Fixed ratio schedules reduced morphine sulfate injection rates in rats, while time-out periods did not. This suggests fixed ratio effects are not solely due to sensing drug effects during imposed pauses.
Area of Science:
- Pharmacology
- Behavioral Neuroscience
Background:
- Morphine sulfate is a potent opioid analgesic.
- Understanding drug self-administration behavior is crucial for addiction research.
Purpose of the Study:
- To investigate the impact of fixed ratio schedules and time-out periods on morphine self-administration in rats.
- To test the hypothesis that time-out periods mediate the effects of fixed ratio schedules.
Main Methods:
- Rats received daily morphine sulfate injections (3.2 mg/kg) under continuous reinforcement.
- Fixed ratio schedules (FR4, FR8) and time-out periods (5s, 10s) were implemented.
- Injection rates were monitored until stabilization.
Main Results:
- Fixed ratio schedules significantly decreased the rate of morphine injections.
- Time-out periods following injections did not affect the injection rate.
- The data did not support the hypothesis that time-out periods explain fixed ratio effects.
Conclusions:
- Fixed ratio schedules alter drug self-administration patterns independently of simple time-out effects.
- The mechanism by which fixed ratio schedules reduce intake requires further investigation beyond sensory feedback during pauses.
