BACKGROUND ACTIVITY OF HIPPOCAMPAL-NEURONS IN RABBIT DURING FUNCTIONAL SUPPRESSION OF THE THETA-CONTROLLING BRAIN STRUCTURES

A method of controlled successive short-term (25-30 min) stabilization or suppression of the theta-rhythm by local injections of Lidocaine i nto median raphe nucleus or medial 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 act ivity (area CAI) was performed in unanaesthetized rabbits. Injection o f Lidocaine into MS-DS resulted in total suppression of theta-modulati on in neuronal activity and EEG of the hippocampus. Mean frequency of the background activity was not changed in the majority of the units b ut was significantly increased in a limited group of high-frequency ne urons (putative inhibitory cells). Injection into medial raphe, the so urce of serotonergic afferents to MS-DB and hippocampus, was followed by an increase in regularity and frequency of theta-modulation, by app earance of additional group of neurons with rhythmic modulation, and b y expression of continuous theta in the hippocampal EEG. Mean frequenc y of discharge and its regularity were significantly increased in the majority of the cells. The data confirm the presence of tonic inhibito ry 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.