Rodent Dual Reward-Place Association Task

The formation and transformation of memories into long-lasting forms are supported by coordinated activity between neurons in the cortex and hippocampus. Memory strength is positively influenced by the behavioral relevance of associated experiences.

Michon et al have developed a new paradigm as a variant of the Elevated Plus Maze and Radial Arm Maze to investigate the selective retention of memory in rodents by repeatedly testing the acquisition and retrieval of two food-place associations with different reward values. Using the Duel Reward-Place Association Task, rodents demonstrated better memory performance for experiences associated with larger rewards after 2- or 20-hour delays, depending on the spatial integration required to retrieve the location, making this paradigm suited to study preferential retention of relevant experiences in rats.

This apparatus was successfully used to demonstrate the improved retention of memories associated with larger rewards and the role of post-learning hippocampal replay in enhancing memory consolidation.

The Rodent Duel Reward-Place Association Task is elevated 40 cm off the ground and consists of a home platform that gave access to the left and right side of the room via a short track. The left and right environments are separated by dividers and had a choice platform with up to 6 arms. Food dispensers were positioned at the end of each arm to prevent the use of olfactory cues.

Rodent location can be tracked using a video tracking software package such as Noldus EthoVision, ANY-Maze, or BehaviorCloud.

Habituation

Rodents were handled for 5 minutes a day by the experimenter for 7 days prior to the experimental sessions. Rodents were also pre-trained to run on an elevated linear track to obtain 3 food rewards until they had completed 20 laps within 10 minutes in a row. When a rodent reached one end of the maze, rodents were constrained by a door controlled by the experimenter.

Instruction Phase

Only the two target arms of the maze were present in the two environments. Rodents were given access to one of the environments at a time and rewarded with 3 pellets at the end of the target arm. The order of presentation of the environments was constant within a session but randomized across sessions.

Reinstatement phase

After the instruction phase, rodents were subjected to three reinstatement trials for each environment in the presence of distractor arms. In each reinstatement trial, rats were rewarded for visiting the target arm with 3 pellets and the aim was for the animal to learn to seek the reward at the target arm and ignore the distractor arms. Each reinstatement trial lasted until the subject consumed the reward at the end of the target arm.

The pretraining phase ended when rats visited the target arm during the test for three consecutive days.

Experimental phase

In the experimental phase, reward sizes were changed to 1 and 9 pellets, and longer delays (2h or 20h) were introduced. Rodents were either returned to their home cage or placed in a rest box during the delay phase. A memory probe trial was conducted for large and small reward environments in every other session, and pre-training sessions were held after pauses in training to ensure motivation retention.

The following parameters can be observed using the Rodent Dual Reward-Place Association Task:

• Total number of arm visits 
• Repeat arm visits
• Time taken per trial
• Run speed
• Spontaneous alteration rates

The study investigated whether rats could quickly learn the association between reward magnitude and target locations. During the instruction phase, rats showed a significantly higher running speed towards the reward platform for the large reward amount compared to the small reward amount. Running speed increased in the second trial block for both reward conditions, but subsequently remained elevated for the large reward trials and decreased for the small reward trials. There was no difference between reward conditions for the running speed from the reward platform back home. Rats spent more time consuming the reward on the platform associated with the large reward compared to the small reward.

In this study, 21 rats underwent a probe test after a 2-hour delay, performing a total of 151 sessions. The rats made a median of 8 arm visits and there was a small tendency for them to make more visits in the large reward environment. However, overall, there was a preference for visits to the target arm over the distractor arms in both reward environments. The probability of visiting the target arm was significantly better than chance in both reward conditions

In this study, rats showed a preference for the target arm on their first visit, with a stronger preference in the large reward environment. The probability of visiting the target arm was significantly higher in the large reward environment compared to the small reward environment. During subsequent visits, rats explored non-target arms before establishing a clear preference for revisiting the target arm. The probability of revisiting the target arm was significantly higher than chance and higher in the large reward environment. Rats had a natural tendency to avoid visiting the same arm twice in succession, but showed an increased tendency to immediately return to the target arm without visiting any other arm in between. This tendency was higher in the large reward environment and increased from the fourth visit in both reward conditions, although it was only temporary in the small reward condition.

During the memory probe trial, rats showed a higher tendency to explore non-target arms on their second visit before establishing a clear preference for revisiting the target arm in the remainder of the two-minute test. The probability of revisiting the target arm was significantly higher than chance and higher in the large reward environment compared to the small reward environment. Rats had a natural tendency to alternate maze arms and avoid repeat visits to the same distractor arm. However, rats showed an increased tendency to immediately return to the target arm without visiting any other arm in between. While this probability was lower than chance for both reward conditions, it was significantly higher than the probability of repeat visits to distractor arms. Interestingly, the probability of repeat visits increased from the fourth visit in both reward conditions, but this increase was only temporary in the small reward condition

Strengths

• The Rodent Dual Reward-Place Association Task is a novel paradigm to study the enhanced memory retention of salient experiences in rodents.

Limitations

• Manual application is required to set doors and food rewards.

 The Octopus three-chambered social apparatus is used to study social behaviors in octopus.

•       It consists of a glass aquarium divided into three equal chambers (one central chamber and two lateral chambers) using black plexiglass walls.
•       The subject can move between the chambers through small circular entrances present at the bottom of each partition.
•       One lateral chamber serves as the novel chamber used to place novel objects while the other serves as the social chamber used to place a social object.
•       The time the subject spends in the social chamber compared to the others is used to indicate its preference toward the social object.
•       The apparatus can also be used to study the effects of treatment with drugs or stimulants on social behaviors.
•       The apparatus is large enough to place adult animals.

Michon, F., Sun, J., Kim, C. Y., & Kloosterman, F. (2020). A Dual Reward-Place Association Task to Study the Preferential Retention of Relevant Memories in Rats. Frontiers in Behavioral Neuroscience, 14. https://doi.org/10.3389/fnbeh.2020.00069