INTRODUCTION: The mammillary body is a part of so-called extended hippocampal system and its importance for memory is well known. Our previous study has revealed that pharmacological inactivation of the medial mammillary nucleus (MM) attenuates theta rhythm activity in the hippocampus (HP) in urethane-anesthetized rats. In this study, we examined the involvement of GABAergic transmission in the MM in the regulation of hippocampal theta rhythm. AIM(S): The aim of this study was to investigate whether local administration of GABAA receptor agonist (muscimol) into the MM affects theta rhythm in the HP in urethane-anesthetized rats. METHOD(S): Male Wistar rats were implanted with unilateral recording electrodes into the dorsal HP (CA1) and unilateral injection cannula into the lateral part of the MM. Animals received microinjection of either muscimol (n=5) or water (n=5). 1-min tail pinch stimulations were applied at 5- and 10-min intervals to evoke theta rhythm episodes in the HP. Changes in local field potential were assessed on the basis of percent change of total EEG signal power for 1-Hz bands. RESULTS: We found that intra-MM muscimol injection suppressed sensory-elicited theta rhythm in the HP. The infusion decreased the EEG signal power most significantly in 3–4 Hz band, down to 19.1%, and in 7–8 Hz band, down to 48.6%, in comparison to the pre‑injection conditions (100%). Simultaneously, the injection increased total power in delta frequency bands, up to 2415.6% in 0.5–1 Hz band. No significant changes in the hippocampal field activity were found after the water injections in the control water group. CONCLUSIONS: Our study revealed for the first time that GABAergic transmission of the MM is significantly involved in electrophysiological activity of the HP in urethane-anesthetized rats. The obtained results also confirmed the importance of the mammillary body in regulating a theta-rhythm signaling in the extended hippocampal system. FINANCIAL SUPPORT: This research was supported by the National Science Centre (DEC-2014/12/S/NZ3/00621).
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