Two time periods of hippocampal mRNA synthesis are required for memory consolidation of fear-motivated learning

Memory extinction requires gene expression in rat hippocampus

Rats with cannulae in the dorsal CA1 region of the hippocampus were trained in one-trial step-down inhibitory avoidance, and submitted to four consecutive daily test sessions without the footshock. This produced extinction of the conditioned response in control animals. The bilateral infusion into the CA1 region of the dorsal hippocampus of two different inhibitors of gene transcription, DRB (80 microg/side) or alpha-amanitin (25 pg/side), or of the protein synthesis inhibitor, anisomycin (80 microg/side) blocked extinction of the CR. The treatments were effective when given 15 min before, but not 1 or 3h after the first test session. Retrieval itself was not affected by the drugs. The treatments did not affect general activity in an open field or anxiety levels measured in an elevated plus maze. The data indicate that gene transcription and protein synthesis are necessary at the time of the first test session in order to generate extinction. These requirements are to be expected from learning that involves new synaptic associations.

Learning-induced arg 3.1/arc mRNA expression in the mouse brain

The effector immediate-early gene (IEG) arg 3.1, also called arc, encodes a protein interacting with the neuronal cytoskeleton. The selective localization of arg 3.1/arc mRNA in activated dendritic segments suggests that the arg 3.1/arc protein may be synthesized at activated post-synaptic sites and that arg 3.1/arc could participate in structural and functional modifications underlying cognitive processes like memory formation. To analyze whether learning itself is sufficient to trigger expression of arg 3.1/arc, we developed a one-trial learning paradigm in which mice learned to enter a dark compartment to escape from an aversively illuminated area. Arg 3.1/arc mRNA expression was analyzed by in situ hybridization in three groups of mice as follows: a control group with no access to the dark compartment, a learning group having access to the dark compartment for one trial, and a retrieval group having access to the dark compartment for two trials on consecutive days. All animals from the learning and retrieval groups escaped the illuminated area, and those tested 24 h later (retrieval group) showed a strongly reduced latency to enter the dark compartment, demonstrating the validity of our learning paradigm to induce long-term memory. Our results show that acquisition of a simple task results in a brain area-specific biphasic increase in arg 3.1/arc mRNA expression 15 min and 4.5 h post-training. This increase was detected specifically in the learning group but neither in the control nor in the retrieval groups. The pattern of arg 3.1/arc mRNA expression corresponds temporally to the two mRNA- and protein-synthesis-dependent periods of long-term memory formation. Our study provides the first unequivocal evidence that arg 3.1/arc expression is induced by a learning task and strongly suggests a role of arg 3.1/arc mRNA in the early and late cellular mechanisms underlying the stabilization of the memory trace.

Inhibition of mRNA and protein synthesis in the CA1 region of the dorsal hippocampus blocks reinstallment of an extinguished conditioned fear response

Memories are extinguished by the repeated presentation of a conditioned stimulus in the absence of an unconditioned stimulus to which it has been associated. It is believed that extinction establishes a new hierarchy of responses rather than an actual forgetting of the original response, which can usually reappear spontaneously after interruption of the extinction process. In this study, our aim was to analyze how profound extinction can be. Rats were trained in a one-trial, step-down inhibitory avoidance paradigm and then were exposed to several extinction sessions in which they were allowed to freely explore the apparatus for 30 sec after having stepped down. Extinction was complete enough so that there was no spontaneous recovery, and test session performance could not be enhanced by pharmacological agents with well known facilitative actions on retrieval. After being submitted to a new training session, control animals reacquired the avoidance response; however, animals failed to do so after receiving bilateral intra-CA1 infusions of either the protein synthesis inhibitor anisomycin or the mRNA synthesis blocker 5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole 15 min before the retraining session. Our results indicate that extinction can be carried to a point at which reinstallment of the conditioned response requires, like the original learning, de novo gene expression and protein synthesis in the CA1 region of the dorsal hippocampus.