Systemic Blockade of the CB1 Receptor Augments Hippocampal Gene Expression Involved in Synaptic Plasticity but Perturbs Hippocampus-Dependent Learning Task

Abstract

Chronic and acute agonism as well as acute antagonism of CB₁ receptors reveal modulation of learning and memory during stable performance of a delayed-nonmatch-to-sample (DNMS) memory task. However, it remains unclear how chronic blockade of the CB₁ receptor alters acquisition of the behavioral task. We examined the effects of chronic rimonabant exposure during DNMS task acquisition to determine if blockade of the CB₁ receptor with the antagonist rimonabant enhanced acquisition of operant task. Long-Evans rats, trained in the DNMS task before imposition of the trial delay, were surgically implanted with osmotic mini pumps to administer rimonabant (1.0 mg/kg/day) or vehicle (dimethyl sulfoxide/Tween-80/Saline). Following surgical recovery, DNMS training was resumed with the imposition of gradually longer delays (1-30 sec). The number of days required to achieve stable performance with either increasing length of delay or reversal of task contingency was compared between vehicle and rimonabant-treated rats. Following the completion of DNMS training, animals were euthanized, and both hippocampi were harvested for gene expression assay analysis. Rimonabant treatment animals required more time to achieve stable DNMS performance than vehicle-treated controls. Quantitative real-time polymerase chain reaction analysis revealed that the expressions of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit, brain-derived neurotrophic factor, and synapsin 1 (Syn1) were significantly increased. These results are consistent with rimonabant increasing mRNAs for proteins associated with hippocampal synapse remodeling, but that those alterations did not necessarily accelerate the acquisition of an operant behavioral task that required learning new contingencies.

Keywords: brain-derived neurotrophic factor; delayed-nonmatch-to-sample; gene expression; hippocampus; memory; rimonabant.

FIG. 1.

Effect of continuous rimonabant administration on DNMS delay acquisition. Comparison of vehicle treatment (n=5) and rimonabant treatment (n=5) on successive discrete acquisition of the delay component of the DNMS task. Vehicle was dimethyl sulfoxide/Tween-80/saline. Means±SEM; **p<0.01. DNMS, delayed-nonmatch-to-sample; SEM, standard error of the mean.

FIG. 2.

Effect of acute rimonabant administration on DNMS to DMS task contingency reversal. Comparison of vehicle treatment (n=8) and rimonabant treatment (n=8) on mean percentage of correct responses (A), mean rate of correct responses (B), and mean latency to move from the nose-poke to response lever (C) during reversal (DMS) task performance across all test days. Vehicle was pluronic F68/saline. Means±SEM; *p<0.01, **p<0.001. DMS, delayed-match-to-sample.

FIG. 3.

Effect of continuous rimonabant treatment on the expression of neuronal marker and CB₁ receptor. (A) Eno2 mRNA expression levels comparing rimonabant-treated rats (n=5) to vehicle-treated rats (n=5). (B) Cnr1 and Cnrip1 mRNA expression levels (n=5 per group). Values represent relative expression normalized to Eno2 mRNA expression levels. Means±SEM. Eno2, enolase 2; Cnr1, cannabinoid receptor 1; Cnrip1, cannabinoid receptor interacting protein.

FIG. 4.

Effect of continuous rimonabant treatment on the expression of genes that influence learning and memory. (A) AMPA receptor 1 (Gria1), kainate receptor 1 (Grik1), NMDA receptor 1 (Grin1), and NMDA receptor 2a subunit (Grin2a) mRNA expression levels (n=5 per treatment group). (B) GABA-A receptor (Gabra1), GABA-B receptor (Gabbr1), and Gad1 mRNA expression levels (n=5 per treatment group). (C) Synapsin1 (Syn1), Synapsin2 (Syn2), Synapsin3 (Syn3), Growth Associated Protein-43 (Gap43) mRNA expression level (n = 5 per treatment group). (D) BDNF, TrkB, Creb1, and Crem1 mRNA expression levels (n=5 per treatment group). Values represent relative expression normalized to Eno2 mRNA expression levels. Means±SEM; **p<0.01; *p<0.05. TrkB, tyrosine receptor kinase B; Creb1, CRE-binding protein; Crem, CRE-modulating protein; Gad1, glutamic acid decarboxylase; BDNF, brain-derived neurotrophic factor; NMDA, N-methyl D-aspartate; GABA, gamma-aminobutyric acid.