Endocannabinoids modulate encoding of sequential memory in the rat hippocampus

Rationale This report investigated the role of endocannabinoids in the encoding of task-relevant information by ensembles of hippocampal neurons under conditions in which the CB1 receptor antagonist, rimonabant, was administered during performance of a short-term memory delayed non-match to sample (DNMS) task in rats. Objective The influence of endocannabinoids on the encoding of task relevant information was determined via examination of the firing patterns of ensembles of CA1/CA3 hippocampal neurons during individual trials while rats performed a DNMS task. Materials and methods Multivariate discriminant analysis of the firing patterns of ensembles of hippocampal neurons was used to extract trial-specific codes for task-relevant information under different types of trial sequences. Results It was discovered that rimonabant blocked an inherent hippocampal memory encoding bias used by all animals. This bias was characterized as the preferential encoding of sample information on individual trials based on the similarity (i.e., same or different) and duration of the delay in the preceding trial. Conclusions The results indicate that endocannabinoids are a major influence on the strategic encoding biases of hippocampal ensembles and that pharmacological blockade of CB1 receptors facilitated performance by eliminating such influences.