Juvenile rats rely on olfactory cues to calibrate and develop an abstract spatial framework

To assess when and how spatial abilities based on distant visuo-spatial cues emerge during ontogeny, we used a homing task in which the rats were trained to reach their home cage via an escape hole at a fixed spatial position in a large circular open field (diameter 160 cm). Like the Morris watermaze this is a place learning task, but the rats are free to walk, not forced to swim. Eighteen or fifteen identical holes, equally covered by a small plastic disk easily removed by rats, are regularly distributed on the table surface. All the holes are blocked by a tightly fitted foam rubber stopping masked by the plastic cover. Only the hole at the training position is connected under the table with the home cage (Schenk, 1989).

Intertrial rotations of the walled arena prevents associations between the escape position and local olfactory traces. Location of the escape position is related to distant room cues only. In this task, we observed a slow development of place discrimination until the fifth week postnatal (see Wiener and Schenk, 2005). The development of spatial abilities was synchronous in the Morris watermaze.

If the table remained fixed between trials and the uncontrolled olfactory traces around the holes were undisturbed, rats aged 18 days expressed a marked discrimination of the training position during probe trials with no hole connected. Rotating the table 120 degrees before the test placed the marked hole at a new spatial position and the 18-day old rats were only concentrated on the hole surrounded by the familiar traces. However, in the same experimental design, rats aged 20 days concentrated their movements both on the marked hole and on the spatially defined training position, although the latter was bereft of relevant olfactory traces (unpublished data).

Although they paid attention to the olfactory traces left on the table, rats were also able to integrate distant room cues from the age of 20 days. However, rats of the same age, trained with the local olfactory cues made irrelevant by intertrial table rotation, did not discriminate the training position in a significant manner, as if the absence of olfactory confirmation during training trials had prevented the acquisition of place memory.

This potentiation of place memory by local olfactory marking was still evident in experiments conducted in adult rats with an additional olfactory cue marking the spatially defined training position (Lavenex and Schenk, 1998). Place learning was more rapid in this condition and place discrimination was enhanced during probe trials in spite of the removal of the scented cue. Familiar olfactory cues can thus facilitate the encoding of significant places, but are not necessary for later place recognition.

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