Elsevier

Neurobiology of Learning and Memory

Volume 136, December 2016, Pages 236-243
Neurobiology of Learning and Memory

Circadian time-place (or time-route) learning in rats with hippocampal lesions

https://doi.org/10.1016/j.nlm.2016.09.004Get rights and content

Highlights

  • Rats were trained on a novel circadian TPL task.

  • Rats received HPC lesions and exhibited only a transient impairment on the task.

  • The results may reflect a switch from allocentric to egocentric based navigation.

  • The HPC is not crucial for providing circadian time cues to perform this TPL task.

Abstract

Circadian time-place learning (TPL) is the ability to remember both the place and biological time of day that a significant event occurred (e.g., food availability). This ability requires that a circadian clock provide phase information (a time tag) to cognitive systems involved in linking representations of an event with spatial reference memory. To date, it is unclear which neuronal substrates are critical in this process, but one candidate structure is the hippocampus (HPC). The HPC is essential for normal performance on tasks that require allocentric spatial memory and exhibits circadian rhythms of gene expression that are sensitive to meal timing. Using a novel TPL training procedure and enriched, multidimensional environment, we trained rats to locate a food reward that varied between two locations relative to time of day. After rats acquired the task, they received either HPC or SHAM lesions and were re-tested. Rats with HPC lesions were initially impaired on the task relative to SHAM rats, but re-attained high scores with continued testing. Probe tests revealed that the rats were not using an alternation strategy or relying on light-dark transitions to locate the food reward. We hypothesize that transient disruption and recovery reflect a switch from HPC-dependent allocentric navigation (learning places) to dorsal striatum-dependent egocentric spatial navigation (learning routes to a location). Whatever the navigation strategy, these results demonstrate that the HPC is not required for rats to find food in different locations using circadian phase as a discriminative cue.

Introduction

Circadian time-place learning (cTPL) is the ability to remember both the place and biological time of day that a significant event (e.g., food-availability) occurred. This ability is likely to be widely represented in the animal world, where optimal foraging requires attention to both time and place. cTPL has been demonstrated experimentally in a variety of species (e.g., insects, fish, birds, rats and mice), typically by providing a food reward at one time of day in one location, and at another time of day in a different location in a multi-chamber test environment (reviewed in Mulder, Gerkema, & Van der Zee, 2013). After training animals to some level of performance reliably better than chance, test trials are skipped to determine whether animals are learning an alternation rule (if fed in one of two locations in the morning, go to the other place in the evening) or are discriminating and remembering time of day (food is available in one place in the morning, and in the other place in the evening). To determine whether time of day is recognized using environmental cues (e.g., the light-dark cycle, LD) or internal time cues (e.g., the phase of a circadian clock), LD is replaced by constant light or dark for a day or more. These test conditions have confirmed that the phase of a circadian clock can be linked to memories of feeding events and places (Biebach et al., 1991, Boulos and Logothetis, 1990, Mistlberger et al., 1996, Mulder et al., 2014, Mulder et al., 2015, Mulder et al., 2013, Saksida and Wilkie, 1994, Van der Zee et al., 2008).

The location of the circadian clock utilized for cTPL is unknown. The suprachiasmatic nucleus (SCN) is the site of a master circadian clock critical for entrainment of circadian rhythms to daily LD cycles, but is not required for entrainment to daily feeding cycles, or for accurate performance on TPL tasks (Boulos and Logothetis, 1990, Mistlberger et al., 1996, Mulder et al., 2014). A brain region hypothesized to play a critical role is the hippocampus (HPC) (Mulder, Gerkema, & Van der Zee, 2016). The HPC mediates allocentric spatial navigation and memory, and has been shown to express food-entrainable rhythms of circadian clock gene expression and other processes (Loh et al., 2015, Wakamatsu et al., 2001), indicating sensitivity to meal timing. The objective of the present study was to determine whether HPC lesions would disrupt performance on a cTPL task in rats. To test this, we developed a novel procedure and apparatus, in which rats are trained in a large multi-level environment to locate a food reward that varies between two locations according to time of day. To confirm that the rats were using time-of-day cues to correctly locate the food reward, rather than learning to alternate from one location to the other, we conducted probe tests that entailed omitting sessions and recording performance on the subsequent session. Rats that used a circadian strategy rather than simple alternation then received either HPC or SHAM lesions. To rule out the possibility that rats were discriminating intervals between LD transitions and test times to choose correctly, LD was replaced by constant dim red light (DDr) and the rats were tested for an additional two weeks.

Section snippets

Subjects

The subjects were 20 experimentally naïve male Long-Evans rats (Charles River, St. Constant, QC), weighing 275–315 g at the start of the experiment. The rats were housed in polypropylene cages (48 × 25 × 20 cm) in a colony room maintained at 21 °C on a reverse 12:12 light-dark (LD) cycle, with lights off at 8:00 a.m., denoted Zeitgeber Time (ZT) 12, by convention. The rats had continuous access to water, and each received ∼12 g of rat chow (Charles River Rodent Animal Diet, No. 5075) after each trial

Results

At the end of Session 3 of pre-surgery acquisition 5 rats were removed from the study because they failed to reach a consistent level of accuracy on the task. Six of the remaining 15 rats failed the skip-trial probe tests based on the set criterion, and testing of these rats ended after Week 17, prior to surgery. Lastly, there was no observed difference in the mean percent correct choice between the rats tested in Apparatus 1 and rats tested in Apparatus 2, thus these groups were pooled.

Discussion

To evaluate a role for the hippocampus in cTPL, we developed a novel training protocol using an enriched test environment. A potential advantage of the large apparatus is that rats can initially be trained as a group before continued testing as individuals. Using this procedure a majority of rats (75%) successfully acquired the task, and 60% of those rats employed a circadian strategy.

Rats with HPC lesions were impaired on the task relative to the SHAM group during the first session of

Acknowledgements

The researchers thank Frank Mossa, Joëlle Ziadé, and Jonathan Cuthbert for their assistance in the behavioral data collection. We are grateful to Aileen Murray and the Animal Care Facility staff for their assiduous efforts in maintaining the animal colony room environment throughout the experiment and for their care of the rats. This research was supported by operating grants from the Natural Sciences and Engineering Research Council of Canada (NSERC, Canada) to Dave Mumby and Ralph

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      In daily time-place-learning (TPL) tasks, a rewarding or aversive stimulus is available in one location in the morning and in another location in the afternoon and the animal uses this information to modify its search patterns (Mulder et al., 2015; Thorpe and Wilkie, 2006). Rats are able to learn a variety of different TPL paradigms that can vary in the nature of the biologically significant event, or the response required to approach or avoid the goal location (Aragona et al., 2002; Carr & Wilkie, 1997; Cole et al., 2016; Deibel et al., 2014, 2017; Deibel and Thorpe, 2012; Lukoyanov et al., 2002; Mistlberger et al., 1996; Pizzo and Crystal, 2002, 2004; 2006; Thorpe et al., 2012a,b; Widman et al., 2000; 2004). TPL is an adaptive characteristic displayed by a variety of species such as humans (Thorpe et al., 2012a,b), rats (Deibel and Thorpe, 2012; Thorpe and Wilkie, 2002), honeybees (Wahl, 1932; cited in Reebs, 1993), marine iguanas (Wikelski and Hau, 1995), and fish (Reebs, 1996), as well as many avian species such as warblers (Biebach et al., 1989), pigeons (Saksida and Wilkie, 1994), and hummingbirds (Tello-Ramos et al., 2015).

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