Research articleInactivation of the medial mammillary nucleus attenuates theta rhythm activity in the hippocampus in urethane-anesthetized rats

- Procaine injection to the medial mammillary nucleus of anesthetized rats was made.
- The pharmacological inactivation suppressed theta rhythm in the hippocampus.
- Procaine injection decreased the EEG signal power of low theta frequency bands.
- The mammillary body may regulate theta-rhythm in the extended hippocampal system.

Although the importance of the mammillary body for memory and learning processes is well known, its exact role has remained vague. The fact, that many neurons in one nucleus of the mammillary body in rats, i.e. the medial mammillary nucleus (MM), fires according with hippocampal theta rhythm, makes this structure crucial for a theta rhythm signaling in so-called extended hippocampal system. These neurons are driven by descending projections from the hippocampal formation, but it is still unknown whether the mammillary body only conveys theta rhythm or may also modulate it. In the present study, we investigated the effect of pharmacological inactivation (local infusion of 0.5 μl of 20% procaine hydrochloride solution) of the MM on hippocampal theta rhythm in urethane-anesthetized rats. We found that intra-MM procaine microinjections suppress sensory-elicited theta rhythm in the hippocampus by reduction of its amplitude, but not the frequency. Procaine infusion decreased the EEG signal power of low theta frequency bands, i.e. 3-5 Hz, down to 9.2% in 3-4 Hz band in comparison to pre-injection conditions. After water infusion (control group) no changes of hippocampal EEG signal power were observed. Our findings showed for the first time that inactivation of the MM leads to a disruption of hippocampal theta rhythm in the rat, which may suggest that the mammillary body can regulate theta rhythm signaling in the extended hippocampal system.