Denise Manahan-Vaughan

A multitude of studies have confirmed the essential role of the hippocampus in the acquisition and updating of associative experience. In rodents, spatial memories are dependent on hippocampal information processing and encoding. The role of the dorsal hippocampal pole is well documented, but information about the roles of the intermediate and ventral hippocampus in spatial learning are few and often contradictory. Here, we used fluorescence in situ hybridization (FISH) to identify nuclear expression of the immediate early (IEG) gene, Homer1a, that was triggered by specific spatial learning events in hippocampal neurons of adult male rats. We compared IEG expression in the cornu ammonis (CA) and dentate gyrus (DG) along the proximodistal and dorsoventral hippocampal axis, triggered by novel learning about spatial constellations of large (macroscale), or partially concealed (microscale) objects within a familiar environment. We observed that the dorsal hippocampus predominates with regard to the neuronal encoding of both forms of spatial content information, whereby the distal CA1 (dCA1) and proximal CA3 (pCA3) preferentially encode microscale spatial location. The infrapyramidal blade of the DG, as well as pCA3, encode macroscale information. The intermediate CA shows increased IEG expression in dCA1 triggered by novel microscale information, but the DG of neither the intermediate, nor ventral, hippocampus encode macroscale information. The ventral hippocampus exhibits IEG expression patterns that are unique in comparison to the other two structures: here, both CA1 (proximal and distal) and pCA3 encode macroscale information. Taken together, these findings suggest that the intermediate and ventral hippocampus support spatial memory encoding, but may do so in support of differentiated and distinct aspects of associative learning.

Animals rely on associative spatial memory to navigate toward previously learned, reward-associated goals. This reward-guided navigation is supported by the hippocampus and its interactions with cortical and subcortical regions: processes which are vital for integrating sensory cues and forming experience-dependent associations. In adulthood, hippocampal-dependent information processing is shaped by aging, reflecting changes in synaptic plasticity and neuromodulatory support. In parallel, mice with distinct genetic backgrounds show systematic differences in behavior, sensory function, and hippocampal plasticity. How mouse strain and age interact to affect spatial appetitive memories has not been well defined. Here, we trained CBA/CaOlaHsd and C57BL/6 mice in early (2-3 months) or late adulthood (7-8 months), in daily training events, to perform a T-maze task with rewards available at a fixed location and decreasing probability in one maze arm. The task consisted of an initial deterministic phase in which a correct response was always rewarded, followed by a probabilistic phase during which reward omissions became increasingly prevalent. We measured correct choices, as well as latencies across training blocks, and combined trial-by-trial metrics with reinforcement learning modeling to assess decision strategies. We observed that CBA/CaOlaHsd mice displayed lower choice latencies than C57BL/6 mice, reached high performance earlier, and maintained better performance when the reward probability decreased. Age was associated with higher latencies and modulated both performance and decision policies in a strain-dependent manner. Moreover, CBA/CaOlaHsd mice displayed higher learning rates from positive outcomes and adopted strategies consistent with more robust context exploitation under uncertainty. C57BL/6 mice, by contrast, exhibited stronger omission-driven strategy switching. Together, these findings reveal robust strain- and age-dependent differences in trial-by-trial decision policies and spatial learning performance.

Hippocampal neuronal ensembles are likely to support the acquisition, stabilization and updating of spatial experience. Spatial learning is typically cumulative, but little is known about how neuronal ensembles are manifested during this process. Here, we used wide-field Ca ²⁺ -imaging and in vivo electrophysiology to monitor CA1 pyramidal cells and synapses, respectively, during cumulative item-place learning in adult male CBA/CaOlaHsd mice. In controls, we observed refinement in neuronal activation patterns along with reduced reactivation in response to novel item-place configuration. Synaptic plasticity responses, place cell-like properties, population burst activity, and functional connectivity were consistent with the encoding and updating of item-place memory. Noradrenaline activity via β-adrenergic receptors (β-AR) is a critical modulator of CA1 function, but its role in regulating cellular dynamics during cumulative item-place learning remains unclear. To examine this, we pharmacologically antagonized β-AR prior to the 1st item-place exposure. This led to reduced cellular recruitment, disrupted ensemble reactivation, impaired synaptic plasticity, reduced spatial tuning, dampened population bursts, and altered functional connectivity within neurons. This was accompanied by impaired spatial learning compared to controls. Our results reveal the population activity of CA1 neurons during item-place learning and show that β-AR support memory function by influencing both neuronal and network-level dynamics.

Acquired spatial representations are not static. Each re-exposure to the spatial environment stimulates retrieval of the stored experience followed by information re-encoding, including updating if the environment has changed. It remains unclear if the same neurons are involved in these three events. Here, we used a multiplexed fluorescence in situ hybridization (FISH) approach that detected “time-locked” nuclear immediate early gene (IEG) expression to identify hippocampal neuronal ensembles that were engaged in the acquisition of a spatial representation, as well as its subsequent stabilization and/or updating. Responses were assessed in distal CA1 (dCA1) and proximal CA1 (pCA1) of the dorsal hippocampus of male rats. Homer1a was used to detect neuronal recruitment triggered by novel learning of a holeboard environment (HB). cFos and Arc expression were used to detect ensemble stability and/or expansion, or ensemble remodeling, respectively, that was triggered by animal exposure to the now familiar HB that included novel objects (HBO) 25 min after the initial HB exposure. Novel HB exposure resulted in nuclear Homer1a expression in both dCA1 and pCA1. Subsequent HBO triggered significant cFos and Arc expression only in dCA1. IEG co-labeling (Homer1a/cFos, Homer1a/Arc and Homer1a/cFos/Arc) was also only evident in dCA1, reflecting both re-iteration and remodeling of dCA1, but not pCA1 ensembles. In sum, we show that the contiguous acquisition and updating of spatial representations recruits distinct populations of CA1-neurons reflecting ensemble selection and stabilization, as well as ensemble remodeling. Moreover, whereas dCA1 and pCA1 are involved in the acquisition of the original spatial representation, only dCA1 is engaged in representation updating related to changes in spatial content information.

Hippocampal neuronal ensembles are likely to support the acquisition, stabilization and updating of spatial experience. Spatial learning is typically cumulative, but little is known about how neuronal ensembles are manifested during this process. Here, we used wide-field Ca ²⁺ -imaging to monitor CA1 pyramidal cells during cumulative item-place learning in adult male CBA/CaOlaHsd mice. In control mice, initial learning prompted activity in a population of CA1 neurons, some of which re-appeared during re-exposure to the same item-place configuration 60 min after 1 ^st exposure. Item-place reconfiguration (60 min later) caused a change in population dynamics as reflected by alterations in neuronal recruitment and reactivation patterns. Place cell-like properties, population burst activity, and functional connectivity were consistent with the encoding and updating of item-place memory. To examine the role of noradrenergic neuromodulation on these processes, we pharmacologically antagonized β-adrenergic receptors(β-AR) prior to the 1 ^st item-place exposure. This led to reduced cellular recruitment, disrupted ensemble reactivation, reduced spatial tuning, dampened population bursts, and altered functional connectivity within neurons. This was accompanied by impaired spatial learning compared to controls. Our results reveal the population activity of CA1 neurons during item-place learning and show that β-AR support memory function by influencing both neuronal and network-level dynamics.

Extinction learning (EL) and the subsequent renewal of learned behaviours are crucial for adaptive responding, yet the underlying neural mechanisms that differentiate successful renewal from its absence remain unclear. Here, we explored the behavioral and neurophysiological basis of spatial appetitive EL, as well as renewal failure and renewal success. We recorded place cell activity from hippocampal area CA1 in male rats that performed a context-dependent spatial appetitive learning task in a T-maze (rewarded context A), followed by EL (unrewarded context B), and subsequent renewal testing (unrewarded context A). Half of the animals exhibited significant renewal (“renewers”) and the other half failed to renew (“non-renewers”). Our findings reveal fundamental differences in learning strategies between groups revealed by differences in both spatial behavior and in place cell activity during both initial acquisition and subsequent EL. Specifically, renewers exhibited a context-based EL strategy, whereas non-renewers followed a goal-directed strategy. The spatial distribution of hippocampal place cell activity differed significantly between groups, indicating the hippocampus’ role in the learning processes. Furthermore, renewers exhibited a greater extent of global remapping of place cells compared to non-renewers, consistent with predictions by our computational modeling. This suggests that global remapping serves as a key neural mechanism underlying effective renewal, allowing for the segregation of distinct memories and preventing the generalization of extinction. Our results highlight how hippocampal place cell dynamics during acquisition and EL predict later behavioral renewal outcomes, providing critical insights into the neural basis of memory updating and contextual control over learned behaviors. Significance Statement: This study reveals that individual differences in associative learning strategies determine the outcome of memory renewal/re-activation. After first learning, and then extinguishing, a spatial learning task in two different contexts, some animals renewed their behavior when returned to the acquisition context (“renewers”), while others did not (“non-renewers”). Intriguingly, hippocampal place cell activity during acquisition and extinction learning predicted the subsequent renewal efficacy in these distinct animal groups. In particular, renewers exhibited more global remapping than non-renewers, a neural mechanism that segregates memories from different contexts and prevents over-generalization of extinction. These findings provide critical insights into how the hippocampus supports context-dependent memory acquisition and updating, providing new insights into the physiological basis of individual differences in behavioral flexibility.

Acquired spatial representations are not static. Each re-exposure to the spatial environment stimulates retrieval of the stored experience followed by information re-encoding, including updating if the environment has changed. It remains unclear if the same neurons are involved in these three events. Here, we used a multiplexed fluorescence in situ hybridization(FISH) approach that detected ‘time-locked’ nuclear immediate early gene(IEG) expression to identify hippocampal neuronal ensembles that were engaged in the acquisition of a spatial representation, as well as its subsequent stabilization and/or updating. Responses were assessed in distal(dCA1) and proximal CA1(pCA1) of male rats. Homer1a was used to detect neuronal recruitment triggered by novel learning of a holeboard environment(HB). cFos and Arc expression was used to detect ensemble stability and/or expansion, or ensemble remodeling, respectively, that was triggered by animal exposure to the now familiar HB that included novel objects(HBO) 25min after the initial HB exposure. Novel HB exposure resulted in nuclear Homer1a expression in both dCA1 and pCA1. Subsequent HBO triggered significant cFos and Arc expression only in dCA1. IEG co-labelling (Homer1a/cFos, Homer1a/Arc and Homer1a/cFos/Arc) was also only evident in dCA1, reflecting both re-iteration and remodeling of dCA1, but not pCA1 ensembles. In sum, we show that the contiguous acquisition and updating of spatial representations recruits distinct populations of CA1-neurons reflecting ensemble selection and stabilization, as well as ensemble remodeling. Moreover, whereas dCA1 and pCA1 are involved in the acquisition of the original spatial representation, only dCA1 is engaged in representation updating related to changes in spatial content information.

During spatial appetitive extinction learning (EL), rodents learn that previously rewarded behavior is no longer rewarded. Renewal of the extinguished behavior is enabled by re-exposure to the context in which rewarded learning occurred. When the renewal response (RR) is unrewarded, it is rapidly followed by response extinction (RE). Although the hippocampus is known to be engaged, whether this dynamic is supported by different brain networks is unclear. To clarify this, male rats engaged in context-dependent spatial memory acquisition, EL and RR testing in a T-Maze. Fluorescence in situ hybridization disambiguated somatic immediate early gene expression in neuronal somata engaged in RR or RE. Graph analysis revealed pronounced hippocampal connectivity with retrosplenial and prefrontal cortex (PFC) during initial RR. By contrast, RE was accompanied by a shift towards elevated coordinated activity within all hippocampal subfields. Given that β-adrenergic receptors (β-AR) regulate spatial memory, we activated β-AR to further scrutinize these network effects. This enhanced RR and prevented RE. Effects were associated with initially increased thalamic-hippocampus activity, followed by a decrease in hippocampal intraconnectivity and the predominance of network activity within PFC. Our findings highlight a critical hippocampal-cortical-thalamic network that underpins renewal behavior, with noradrenergic neuromodulation playing a pivotal role in governing this circuit’s dynamics.