Dorsal hippocampal administration of triiodothyronine enhances long-term memory for trace cued and delay contextual fear conditioning in rats.

Thyroid hormones play critical roles in brain maturation and cognitive functions. The present study investigated the role of thyroid hormone in emotional learning and memory using trace and delay contextual and cued fear conditioning tasks, respectively. Rats were administered triiodothyronine (T3) into the dorsal hippocampal area 10 min before training or immediately after training, and were scored for freezing behaviour in the same context and in a novel context with and without an auditory cue that had been paired previously with an aversive stimulus, a foot shock. Rats administered T3 before and after training both exhibited significantly increased long-term fear memory in the trace cued and the delay contextual fear conditioning procedures compared to their control groups. The T3-administered rats were not significantly different from their respective controls on the acquisition and short-term fear memory in the trace and delay fear conditioning tasks. No significant difference on long-term trace contextual and delay cued fear memory, respectively, was found. These results indicate that the observed T3-induced enhancement of long-term contextual and cued fear memory was specific to the hippocampus-dependent conditioning tasks. These findings are the first to demonstrate that infusion of T3 into the dorsal hippocampus can improve performance on an emotional memory task.