CONTROL OF SEPTAL PACEMAKER OF THETA-RHYT HM BY THE MIDBRAIN RAPHE NUCLEI

Neuronal activity of the medial septum (MS-DB) and hippocampal slow el ectroencephalogram (EEG) were recorded in unanaesthetized rabbits with stimulating electrodes implanted in the median raphe nucleus and reti cular formation of midbrain and pens. In all the animals with raphe el ectrodes th background frequency of theta-rhythm was decreased (mean 4 .7 +/- 0.15 Hz) in comparison with the intact animals and those with s timulating electrodes in the reticular formation only (5.2 +/- 0.19 Hz , p< < 0.5). Stimulation of the raphe had effects of suppression of th eta-bursts, decrease in their frequency and regularity, and appearance or increase of delta-modulation in MS - DB neuronal activity. The num ber of spikes pet burst was decreased while interburst pauses became s ignificantly longer. At the same time, gradual decrease of theta frequ ency and amplitude, theta suppression, appearance, of irregular delta- waves and spindles were observed in the hippocampal EEG. Stimulation o f the reticular formation evoked the opposite effects in MS - DB neuro nal activity, as well as in the hippocampal EEG, The data confirm the role of the midbrain raphe in constraining the reticulo-septohippocamp al system of theta-rhythm generation and do not support the statements on the existence of septum-independent high-frequency serotonergic th eta-rhythm. it is suggested that theta-limiting raphe influences may b e important for suppression and switching of attention.