CA1 inactivation impairs episodic-like memory in rats.

Episodic memory was initially believed to be unique to humans. However, studies demonstrate that nonhuman species discriminate items based on the triad what, where and when. Here we addressed the role of the dorsal hippocampal subfield CA1 in an integrative what-where-when task in Wistar rats. We performed bilateral inactivation of dorsal CA1 with the GABAA agonist muscimol previously to the task. As expected, sham-operated animals recollected an integrative memory for objects (what), their places (where) and temporal order (when). However, the inactivation of CA1 impaired the performance of the three components of episodic-like memory. In addition, total time of objects exploration and distance traveled were not different between groups, indicating that rats had similar levels of motivation, thus, alterations in exploration does not account for impaired locomotor performance. Altogether, our data provides evidence that CA1 plays an important role in episodic-like memory.

Episodic-like memory impairment induced by sub-anaesthetic doses of ketamine.

Episodic-like memory refers to integration of where and when a certain event (what) happened. The glutamatergic neurotransmission, particularly AMPA and NMDA receptors, in the dorsal hippocampus mediates episodic recall. Ketamine is a non-competitive NMDA antagonist with effect on cognitive performance and plasticity. The goal of this study was to evaluate the acute action of ketamine on behavioural and neurochemical aspects of episodic-like memory (WWWhen/ELM task) through immediate-early gene expression (IEG), c-Fos, in the dorsal hippocampus. Animals received saline 0.9% or ketamine at 8 mg/kg or 15 mg/kg (i.p.) immediately after the second sample. Our data indicate that untreated and saline rats integrate the three elements of episodic-like memory. Conversely, animals treated with ketamine showed impairment of ELM formation. In addition, the highest dose of ketamine increased c-Fos expression in dorsal CA1 subregion when compared to saline rats. Our results indicate that the antagonism of NMDA concurrently impair ELM formation of all three aspects of ELM and increase neuronal activation in dorsal CA1.