CONTRIBUTION OF SYNAPSES IN THE MEDIAL SUPRAMAMMILLARY NUCLEUS TO THEFREQUENCY OF HIPPOCAMPAL-THETA RHYTHM IN FREELY MOVING RATS

We have previously shown that in urethane-anesthetized rats the freque ncy of rhythmical slow activity in the hippocampus (''theta'') is cont rolled by the medial supramammillary nucleus (SUM). In particular, inj ections of procaine into SuM in urethane-anesthetized animals reduce t he frequency of theta. However, it has been reported that, in freely m oving animals, lesions of SuM do not affect theta. The present experim ents were designed to resolve this anomaly. Injections of procaine or chlordiazepoxide into SuM in urethane-anesthetized animals reduced the frequency of theta elicited by reticular stimulation, Mapping showed that procaine injections in freely moving animals were effective in th e same locations as under urethane anesthesia. Injections of chlordiaz epoxide were effective in a more restricted area than procaine, consis tent with an action on synapses in SuM and sparing fibers afferent to SuM. Analysis of the functional spread indicated an effective radius o f diffusion of the drugs of 500 mu m. With optimal placements, this im plied an action on at least 80% of SuM. However, in contrast to the re sults under urethane, the maximal frequency reductions obtained were l ess than 50% of the theoretical maximum. In a number of animals receiv ing repeated injections into SuM, lesions developed which encompassed the whole of SUM. As previously reported, theta was largely intact in SUM-lesioned animals. However, the frequency of theta produced by reti cular stimulation was reduced after lesion by approximately the same a mount as by procaine injections before lesion. These results suggest t hat in freely moving animals SuM is only one of two or more nuclei whi ch jointly control the frequency of reticular-elicited theta. (C) 1995 Wiley-Liss, Inc.