[The neuronal background activity of the rabbit hippocampus during functional exclusion of the structures that...

A method of controlled successive short-term (25-30 min) stabilization or suppression of the theta rhythm by local injections of lidocaine into median raphe nucleus or medical septal area (MS-DB) was tested for further analysis of the functional significance of theta-modulation in activity of hippocampal neurons. Extracellular recording of their activity (area CAl) was performed in unanaesthetized rabbits. Injection of lidocaine into MS-DS resulted in total suppression of theta modulation in neuronal activity and EEG of the hippocampus. Mean frequency of the background activity was not changed in the majority of the units but was significantly increased in a limited group of high-frequency neurons (putative inhibitory cells). Injection into medial raphe, the source of serotonergic afferents to MS-DB and hippocampus, was followed by an increase in regularity and frequency of theta modulation, by appearance of additional group of neurons with rhythmic modulation, and by expression of continuous theta in the hippocampal EEG. Mean frequency of discharge and its regularity were significantly increased in the majority of the cells. The data confirm the presence of tonic inhibitory control of theta-generating septo-hippocampal system by the medial raphe nucleus, which can be regarded as an antagonist of the midbrain reticular formation in this respect.