Abstract
Rodent aging research often utilizes spatial mazes, such as the water radial-arm-maze (WRAM), to evaluate cognition. The WRAM can simultaneously measure spatial working and reference memory, wherein these two memory types are often represented as orthogonal. There is evidence, however, that these two memory forms yield interference at a high working memory load. The current study systematically evaluated whether the presence of a reference memory component impacts handling of an increasing working memory load. Young and aged female rats were tested to assess whether aging impacts this relationship. Cholinergic projections from the basal forebrain to the hippocampus and cortex can affect cognitive outcomes, and are negatively impacted by aging. To evaluate whether age-related changes in working and reference memory profiles are associated with cholinergic functioning, we assessed choline acetyltransferase activity in these behaviorally-tested rats. Results showed that young rats outperformed aged rats on a task testing solely working memory. The addition of a reference memory component deteriorated the ability to handle an increasing working memory load, such that young rats performed similar to their aged counterparts. Aged rats also had challenges when reference memory was present, but in a different context. Specifically, aged rats had difficulty remembering which reference memory arms they had entered within a session, compared to young rats. Further, aged rats that excelled in reference memory also excelled in working memory when working memory demand was high, a relationship not seen in young rats. Relationships between cholinergic activity and maze performance differed by age in direction and brain region, reflecting the complex role that the cholinergic system plays in memory and attentional processes across the female lifespan. Overall, the addition of a reference memory requirement detrimentally impacted the ability to handle working memory information across young and aged timepoints, especially when the working memory challenge was high; these age-related deficits manifested differently with the addition of a reference memory component. This interplay between working and reference memory provides insight into the multiple domains necessary to solve complex cognitive tasks, potentially improving the understanding of complexities of age- and disease- related memory failures and optimizing their respective treatments.
Highlights
The field of learning and memory has historically utilized a variety of methods to assess spatial navigation during learning and memory tasks, ranging from Tolman’s sunburst maze to test rodents (Tolman, 1948) to the virtual reality techniques used in more recent human neuroimaging studies (Spiers et al, 2001; see review: Burgess et al, 2002)
To aid the collective interpretation, we visually represent the proportion of Working Memory Correct (WMC) errors that contribute to Total errors from the 8-Arm water radial-arm maze (WRAM) and the 12-Arm WRAM; we did this for each age group separately (Figure 2)
Analysis of maze performance revealed a main effect of Day for Total errors [F(10, 350) = 2.28, p < 0.05], with errors decreasing across Days 2–12, indicating that rats did learn the WRAM task
Summary
The field of learning and memory has historically utilized a variety of methods to assess spatial navigation during learning and memory tasks, ranging from Tolman’s sunburst maze to test rodents (Tolman, 1948) to the virtual reality techniques used in more recent human neuroimaging studies (Spiers et al, 2001; see review: Burgess et al, 2002). Spatial learning and memory in rodents can be categorized into multiple domains, most commonly spatial working memory, which is a form of short-term memory that must be constantly updated, and spatial reference memory, which is a form of long-term memory that remains fixed across time (Frick et al, 1995; Bizon et al, 2009; Foster, 2012; BimonteNelson et al, 2015; for origin of definitions see: Olton, 1979; Olton and Papas, 1979; Jarrard et al, 1984). In the preclinical literature, working, and reference memory are often described as representing two separate memory systems, and are quantified as such (Olton, 1979; Jarrard et al, 1984, 2012; Dennes and Barnes, 1993; Bimonte-Nelson et al, 2003, 2015; Braden et al, 2011; Camp et al, 2012; Auger and Floresco, 2014; Bimonte-Nelson, 2015c; Hiroi et al, 2016)
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