Acute treatment with cannabinoid (CB) agonists impairs learning in rodents in a number of paradigms. However, it has recently been shown that CB₁ receptor knockout mice are impaired in reversal learning in the Morris water maze (Varvel et al., 2002). The aim of the present study was to investigate the effects of a CB agonist on both acquisition and reversal learning in mice in this paradigm. In addition, the effect of performance of the test on immediate early gene expression, and its alteration by CBs, was studied.

Male C57BL/6 mice (20-30g, n=8/group) were given 4 water maze trials per day for 5 days, in which they swam for up to 60s to find an invisible platform in a fixed location (acquisition phase), followed on the 5th day by a probe test in the absence of the platform. Reversal training (4 trials per day for 4 days) was conducted with the platform in a new location. All sessions began 30 min after ip injection of ⁹ -tetrahydrocannabinol (THC) 0.1 or 10 mg/kg or vehicle (1% Tween 80 in saline). 30 min after the last trial, mice were euthanased and brains dissected. Vehicle-injected behaviour-negative mice were brought into the test room each day with experimental cage-mates, but not exposed to the behavioural test, and were euthanased in parallel with the tested mice. In situ hybridisation was performed on brain sections using a ³⁵S-labelled oligonucleotide probe complementary to mRNA encoding the immediate early gene Arc.

In the acquisition phase, the performance of all groups improved across successive days. 10 mg/kg but not 0.1 mg/kg THC significantly delayed acquisition of the task. In the reversal phase, 10 mg/kg-treated mice failed to learn the new platform location over 4 days (Table 1). Preliminary studies also indicate that 3 mg/kg THC did not affect acquisition, but enhanced reversal learning. Performance of the task increased the expression of Arc mRNA in 19 out of 26 structures examined including regions of the hippocampus, cortex, pre-frontal cortex and striatum. 10 mg/kg THC reduced Arc mRNA expression in somatosensory, posterior cingulate and prefrontal cortical areas, but not in hippocampus, compared with vehicle-treated mice performing the task.

p < 0.05, **p<0.02, ***p<0.001 vs vehicle controls (Repeated measures ANOVA followed by pairwise comparisons (LSD) where appropriate)

These data demonstrate that in mice a high dose of THC impairs water maze performance in a reference memory paradigm, both in acquisition and reversal learning. Performance of the test increases Arc mRNA expression in brain areas which include: sensory and motor cortex and striatum, consistent with the nature of the task; hippocampus, involved in spatial memory processes; but also prefrontal cortex. Interestingly, a THC dose which impaired performance did not alter Arc expression in the hippocampus, but reduced it in the prefrontal cortex. This suggests involvement of prefrontal cortex in aspects of water maze learning and its impairment by cannabinoids.