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This experiment was performed to determine whether a state-dependent learning effect is produced when rats are under the influence of Δ 9 THC. A latent learning procedure utilizing a Lashley III maze was used. Latent learning paradigms offer one a variety of measures not available when using an operant procedure. Forty-five female rats were run; five in each of nine conditions. Each set of nine rats was run as follows: Days 1-5. Each rat received an injection of 0. 0 mg/kg, 0.4 mg/kg, or 1.0 mg/kg of Δ 9 THC. One-half hour later the rat received one-half hour of exposure in a Lashley III maze. The rats received the same doses for each of these five days. Days 6-7. The rats were fed to maintain 80 percent ad lib weight. Days 8-9. The rats received their test condition dosage of Δ9 THC (0.0 mg/kg, 0.4 mg/kg, or 1.0 mg/kg) and were then fed. Day 10. The rats were injected with the test condition dosage and were then placed in the maze. Food was offered as reinforcement. Time-per-trial and errors-per-trial were recorded. One trial was run. Day 11. The rats were given the test condition dose and placed in the maze for four reinforced trials. The expectation was that a state-dependent learning effect would be evidenced by low scores in the 0.0 mg/kg- 0. 0 mg/kg, 0.4 mg/kg-0.4 mg/kg, and the 1.0 mg/kg-1.0 mg/kg conditions. This would result in a significant interaction effect when a three-way analysis of variance was performed on the data. This statistical effect did not happen. An attempt was made to determine why the results were insignificant. The results did not replicate an earlier study done by Burke (personal communication). Burke obtained significant differences between the control group and the drug group. Doses ranged from 0.4 mg/kg to 2.0 mg/kg of 3 Δ9 THC in the Burke experiment. Differences between the present study and the Burke study were explored. They were: (1) Difference in sex of the rats. (2) Ethanol in the solution used in the present study. (3) This study used Δ9 THC and the Burke study used Marijuana Extract Distillate (MED). It was concluded that the sex difference and the presence of ethanol were not factors that differentiated the present study from the Burke study. It was not clear whether MED is effective at lower doses than Δ 9 THC. Literature on the synergistic effect of the components of marijuana other than Δ 9 THC was conflicting. What was clear was that the minimum dose of Δ 9 THC needed to produce a discriminable effect on behavior is 1.3 mg/kg. The maximum dose in the present procedure was 1.0 mg/kg. The suggestion was made that the present study be re-run with higher dose levels.

The purpose of the present investigation is to add to the growing body of knowledge about the behavioral development of rats. It is an enlargement of a study by Roberts (1966) who investigated the effects of quantitative reward shifts on the behavior of immature rats. Roberts' experiment was in turn founded on earlier experiments by Crespi (1942). These experiments tested the effect of different quantitative reward levels on the behavior of adult rats. These studies of Crespi are fundamental to the understanding of the present investigation and so are discussed below in some detail. Crespi trained adult rats, one trail a day, to run a runway for different amounts of food reinforcement. He then measured their resulting running speeds. He was able to show that the rats ran at speeds that were proportional to the amount of food reinforcement that they received. As part of the same group of experiments, Crespi pre trained groups of rats to particular quantitative level of food reinforcement. He then shifted them to a different amount and found that a shift to a larger amount of food led to faster running than did original training at the larger amount. A shift to a smaller amount of food led to slower running than did original training at the smaller amount.

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