THE RHYTHMICITY OF CELLS OF THE MEDIAL SEPTUM DIAGONAL BAND OF BROCA IN THE AWAKE FREELY MOVING RAT - RELATIONSHIPS WITH BEHAVIOR AND HIPPOCAMPAL THETA

Chronic extracellular recordings were obtained from cells of the media l septum and diagonal band of Broca in rats performing a simple behavi oural task. The cells were found to display a variety of bursting patt erns phase-locked to hippocampal theta rhythm to a greater or lesser d egree. Among phase-locked cells, no systematic distribution in prefere ntial phase could be found, and these cells were shown to maintain the ir preferential phase for extended periods. Cells were classified into those which showed signs of a broadening of the repolarization phase of their action potential ('inflected': putative cholinergic) and thos e without ('non-inflected': putative GABAergic). Non-inflected cells t ended to fire rhythmic bursts while inflected cells mostly fired in an irregular fashion, although still significantly phase-locked to hippo campal theta. In neither population did the phase-locked cells show an y coherent distribution of their preferential phase. Sixty-five per ce nt of the rhythmically bursting cells showed a significant correlation between the interburst frequency and the animal's running speed. Five cells displayed rhythmic activity only when the rat ran in a specific direction. These results have implications for models of septohippoca mpal function and the effects of variable septal rhythmicity on the pr oduction of hippocampal theta rhythm.