Effects of number of items and interval length on the acquisition of temporal order discrimination in radial maze in rats

Sequential order spatial memory in male rats: Characteristics and impact of medial prefrontal cortex and hippocampus disruption

Remembering an experience entails linking what happened, where the event transpired, and when it occurred. Most rodent hippocampal studies involve tests of spatial memory, but fewer investigate temporal and sequential order memory. Here we provide a demonstration of rats learning an aversive sequential order task using a radial arm water maze. Male rats learned a fixed sequence of up to seven spatial locations, with each decision session separated by a temporal delay. Rats relied on visuospatial cues and the number of times they had entered the maze for a given day in order to successfully perform the task. Behavioral patterns during asymptotic performance showed similarities to the serial-position effect, especially with regards to faster first choice latency. Rats at asymptotic performance were implanted with bilateral cannula in medial prefrontal cortex, dorsal, and ventral hippocampus. After re-training, we injected muscimol to temporarily disrupt targeted brain regions. While control rats made prospective errors, rats with mPFC muscimol exhibited more retrospective errors. Rats with hippocampal muscimol no longer exhibited a prospective bias and were at chance levels in their error choices. Taken together, our results suggest disruption of mPFC, but not the hippocampus, produced an error choice bias during an aversive sequential order spatial processing task.

Temporal order memory of the rat in spontaneous object recognition: effects of number of items, exposure interval, and retention time

Temporal order memory was analyzed using a spontaneous object recognition (SOR) paradigm in rats. In SOR, animals were allowed to explore freely two or five different objects sequentially in the sample phase, and then, two different objects shown in the sample phase were simultaneously presented, and exploration time of object shown earlier compared with that shown later was regarded as a discrimination index. Here we investigated the effects of (1) number of sample items, (2) sample exposure interval, and (3) retention time between the sample and test phases on temporal order memory in SOR. In experiment 1, rats showed significant discrimination between the objects presented earlier and later under five-item, but not two-item, condition. In experiment 2, using the five-item condition, longer retention time (between sample and test phases) induced lower discrimination and longer sample exposure interval produced higher discrimination. In experiment 3, using the two-item condition, longer sample exposure interval produced higher discrimination and longer retention time tended to make rats’ discrimination worse. Taken together, these results indicate that rats’ performance in this test paradigm was better with more sample items and longer exposure intervals as well as shorter retention time, suggesting that the familiarity to items and the temporal distance from the present to the occurrence of events affect temporal order memory.