title: Role of Aging and Hippocampus in Time-Place Learning: Link to Episodic-Like Memory?
abstract: BACKGROUND
  With time-place learning (TPL), animals link an event with the spatial location and the time of day (TOD). The what-where-when TPL components make the task putatively episodic-like in nature. Animals use an internal sense of time to master TPL, which is circadian system based. Finding indications for a role of the hippocampus and (early) aging-sensitivity in TPL would strengthen the episodic-like memory nature of the paradigm.
  
  
  METHODS
  Previously, we used C57Bl/6 mice for our TPL research. Here, we used CD1 mice which are less hippocampal-driven and age faster compared to C57Bl/6 mice. To demonstrate the low degree of hippocampal-driven performance in CD1 mice, a cross maze was used. The spontaneous alternation test was used to score spatial working memory in CD1 mice at four different age categories (young (3-6 months), middle-aged (7-11 months), aged (12-18 months) and old (>19 months). TPL performance of middle-aged and aged CD1 mice was tested in a setup with either two or three time points per day (2-arm or 3-arm TPL task). Immunostainings were applied on brains of young and middle-aged C57Bl/6 mice that had successfully mastered the 3-arm TPL task.
  
  
  RESULTS
  In contrast to C57Bl/6 mice, middle-aged and aged CD1 mice were less hippocampus-driven and failed to master the 3-arm TPL task. They could, however, master the 2-arm TPL task primarily via an ordinal (non-circadian) timing system. c-Fos, CRY2, vasopressin (AVP), and phosphorylated cAMP response element-binding protein (pCREB) were investigated. We found no differences at the level of the suprachiasmatic nucleus (SCN; circadian master clock), whereas CRY2 expression was increased in the hippocampal dentate gyrus (DG). The most pronounced difference between TPL trained and control mice was found in c-Fos expression in the paraventricular thalamic nucleus, a circadian system relay station.
  
  
  CONCLUSIONS
  These results further indicate a key role of CRY proteins in TPL and confirm the limited role of the SCN in TPL. Based on the poor TPL performance of CD1 mice, the results suggest age-sensitivity and hippocampal involvement in TPL. We suspect that TPL reflects an episodic-like memory task, but due to its functional nature, also entail the translation of experienced episodes into semantic rules acquired by training.
authors.affiliation: Department of Molecular Neurobiology, University of GroningenGroningen, Netherlands; Department of Chronobiology, University of GroningenGroningen, Netherlands.
authors.forename: C K
authors.initials: CK
authors.lastname: Mulder
authors.affiliation: Department of Chronobiology, University of Groningen Groningen, Netherlands.
authors.forename: M P
authors.initials: MP
authors.lastname: Gerkema
authors.affiliation: Department of Molecular Neurobiology, University of Groningen Groningen, Netherlands.
authors.forename: E A
authors.initials: EA
authors.lastname: Van der Zee
keywords: aging; circadian; clock genes; cry; learning; memory; place; time
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