Place fields of rat hippocampal pyramidal cells and spatial learning in the watermaze

To provide a background for studying place-related activity in hippocampal neurons during spatial learning, we compared the activity of hippocampal place cells in an annular watermaze and an analogous land-based task. Complex-spike cells had robust place correlates in both conditions, and a significant proportion of the cells had place fields at the same locations. However, the in-field firing rates were slightly higher in the wet condition. Elevated firing was observed also in an open water task. There was no enhancement when the platform location was varied randomly or when there was no platform at all. Second, the place fields were under stronger directional modulation during swimming. In the annular task, directional sensitivity appeared regardless of whether the animals were trained to find a platform or not. There were directionally modulated units also in the open watermaze, but the number was smaller than in the corridor. Altogether, these observations suggest that place fields in the watermaze are largely controlled by the same factors as on dry land, in spite of the differences in kinaesthetic and vestibular input. Differences in firing rate and directional control may depend on the geometric and cognitive structure of the task rather than the medium on which the rats are moving.