Thalamo-hippocampal pathway regulates incidental memory load in mice

Abstract

Memory can be challenged by increasing both its required duration and the amount of information to be encoded, namely the memory load. The dorsal hippocampus (dHP) has been involved in memory consolidation, which is the stabilization of a trace from short-term (STM) to long-term memory (LTM), as well as in the ability to process high information load. However, how memory load influences memory consolidation, and the underlying neural mechanisms, are yet unknown. To address this question, we used male and female mice that, despite having in our Different Object recognition Task (DOT) the same STM capacity of 6 objects, spontaneously show differences in the number of objects directly transferred to LTM, when tested over longer delays. Males memorize all 6 objects encoded, while females remember only up to 4, both at 1 and 24 h delays. Interestingly, males activate more the dHP (as measured by c-Fos expression), while females the thalamic nucleus reuniens (RE). Optogenetic inhibition of the RE-dHP pathway during off-line memory consolidation favors 6-object LTM retention in females by removing inhibitory control over dHP activation, while chemogenetic RE-activation impairs it in males. Our data represent a first demonstration of a sub-cortical control of dHP recruitment, that might underlie its sex-dependent activation during incidental memory, with potential also for clinical application.