Previous work has shown that the dorsal hippocampus has greater activity than ventral regions during place navigation. Exposure to a novel context has also been found to increase hippocampal activation, possibly due to increased spatial demands. However, activation patterns in dorsal and ventral regions have not been investigated in the Morris water task (MWT), which remains the most popular assay of place memory in rodents. We measured activity in a large population of neurons across the CA1 dorsal–ventral axis by estimating nuclear Arc mRNA with stereologic systematic‐random sampling procedures following changes to goal location or spatial context in the MWT in rats. Following changes to goal location or spatial context in the MWT, we did not find an effect on Arc mRNA expression in CA1. However, Arc expression was greater in the dorsal compared to the ventral aspect of CA1 during task performance. Several views might account for these observed differences in dorsal–ventral Arc mRNA expression, including task parameters or the granularity of representation that differs along the dorsal–ventral hippocampal axis. Future work should determine the effects of task differences and required memory precision in relation to dorsal–ventral hippocampal neuronal activity.

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Evidence from genetic, behavioural, anatomical, and physiological study suggests that the hippocampus functionally differs across its longitudinal (dorsoventral or septotemporal) axis. Although, how to best characterize functional and representational differences in the hippocampus across its long axis remains unclear. While some suggest that the hippocampus can be divided into dorsal and ventral subregions that support distinct cognitive functions, others posit that these regions vary in their granularity of representation, wherein spatial-temporal resolution decreases in the ventral (temporal) direction. Importantly, the cognitive and granular hypotheses make distinct predictions on cellular recruitment dynamics under conditions when animals perform tasks with qualitatively different cognitive-behavioural demands. The cognitive function account implies that dorsal and ventral cellular recruitment differs depending on relevant behavioural demands, while the granularity account suggests similar recruitment dynamics regardless of the nature of the task performed. Here, we quantified cellular recruitment with the immediate early gene (IEG) Arc across the entire longitudinal CA1 axis in female and male rats performing spatial-and fear-guided memory tasks. Our results show that recruitment is greater in dorsal than ventral CA1 regardless of task or sex. This experimentum crucis leads to the strong inference that the granularity hypothesis for functional differences across the longitudinal axis in the rodent hippocampus is correct.