Movement-related correlates of single-cell activity in the medial mammillary nucleus of the rat during a pellet-chasing task.

Although the functional role of the mammillary bodies has remained obscure, lesion studies suggest this structure may play a role in memory-in particular, memory for spatial information. Indeed, anatomically, the mammillary bodies are strongly interconnected with limbic system regions, such as the hipppocampal formation, which are also thought to play a role in spatial behavior. Each of these limbic regions so far investigated contains cells that signal either the momentary location and/or directional heading of an animal as it travels through space. In fact, the lateral mammillary nucleus itself contains head direction cells, and is thought to be critical for the initial calculation of this directional signal. Here, we provide an initial report on cell activity in the medial mammillary nucleus. Cells were recorded while rats performed a pellet-chasing task that has been used for much of the work on place and head direction cells. The main findings are 1) approximately 1/3 of the cells showed a temporally precise relationship to angular motion of the head, so that they differentially indicated clockwise versus counterclockwise angular motion, 2) approximately 60% of the cells showed a temporally coarse correlation with translational motion, 3) firing rate for almost all cells was strongly modulated at theta frequency, and 4) no cells showed evidence of either directional or place-related activity. These data suggest that the medial and lateral mammillary nuclei together provide the directional and trajectory information thought to be critical for generation of the spatial signals in the hippocampal region.