The role of the medial supramammillary nucleus in the control of hippocampal theta activity and behaviour in rats

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Abstract
The medial supramammillary nucleus (mSUM) controls the frequency of hippocampal theta activity, completely in anaethsetized rats and partially in free‐moving rats. mSUM could therefore influence hippocampal contributions to cognition and emotion. Using chemical lesions of mSUM in rats, we tested whether mSUM is involved in controlling several hippocampal‐dependent functions: (i) defensive behaviour (open field, fear conditioning); (ii) behavioural inhibition (fixed interval schedule, differential reinforcement of low rates schedule); and (iii) spatial learning (water maze). Theta frequency was measured in all these tasks. mSUM lesions produced a pattern of changes in motivated/emotional behaviours (hyperactivity in defensive and operant tasks) similar to the pattern produced by hippocampal lesions, but had no significant effect on spatial learning. mSUM lesion decreased theta frequency modestly (by ≈ 0.4 Hz) in behaving rats if the amount of movement was unchanged. There was not always a parallel between changes in theta frequency and behaviour; behaviours changed despite unchanged theta in defensive tasks and learning changed little despite a lower frequency of theta in the water maze task. This suggests that mSUM function impacts on emotional behaviour more than cognition, and can modulate theta and behaviour independently.