European Journal of Neuroscience, Vol. 10, pp. 464–477, 1998 © European Neuroscience Association The rhythmicity of cells of the medial septum/diagonal band of Broca in the awake freely moving rat: relationships with behaviour and hippocampal theta C. King, M. Recce and J. O’Keefe Department of Anatomy and Developmental Biology, University College London, Gower St, London WC1E 6BT, UK Keywords: direction, medial septum, rhythmicity, theta Abstract Chronic extracellular recordings were obtained from cells of the medial septum and diagonal band of Broca in rats performing a simple behavioural task. The cells were found to display a variety of bursting patterns phase- locked to hippocampal theta rhythm to a greater or lesser degree. Among phase-locked cells, no systematic distribution in preferential phase could be found, and these cells were shown to maintain their 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 those without (‘non-inflected’: putative GABAergic). Non-inflected cells tended to fire rhythmic bursts while inflected cells mostly fired in an irregular fashion, although still significantly phase-locked to hippocampal theta. In neither population did the phase-locked cells show any coherent distribution of their preferential phase. Sixty-five per cent 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 septohippocampal function and the effects of variable septal rhythmicity on the production of hippocampal theta rhythm. Introduction There is now a wealth of evidence that the hippocampus is vital for spatial cognition in freely moving rats. Pyramidal cells of the hippocampus of the rat fire selectively in response to the animal’s location in two-dimensional space (O’Keefe & Dostrovsky, 1971; O’Keefe & Nadel, 1978). In addition, lesions of the hippocampus have been shown to be accompanied by deficits in the animal’s ability to navigate in space (Morris et al., 1982) and learn spatial discriminations (Jarrard, 1978). Initial electrophysiological investigation of the hippocampus revealed the hippocampal theta rhythm (Green & Arduini, 1954), a large (™ 1 mV), sinusoidal (6–10 Hz) oscillation that, in the awake animal, is related to voluntary movement (Vanderwolf, 1969). Recent examinations of the relationship between place cell activity and the theta rhythm have revealed that the phase of theta rhythm at which a place cell fires varies systematically as the animal moves through the cell’s spatial response field (O’Keefe & Recce, 1993; Skaggs et al., 1996). Following the discovery that lesions of the septal area abolished hippocampal theta rhythm (Green & Arduini, 1954), extracellular recordings showed that cells in the medial septum and diagonal band of Broca (MS/DBB) fired in a rhythmically bursting manner in phase with hippocampal theta (Petsche et al., 1962). The rhythmic activity of MS/DBB cells was also found to persist in the absence of theta oscillations in the hippocampus, leading to the conclusion that these cells acted as a pacemaker. Correspondence: C. King, CMBN, 197 University Avenue, Newark, NJ 07102, USA Received 14 May 1997, revised 7 August 1997, accepted 9 September 1997 The pacemaker model was further developed in the light of later anatomical and physiological findings. These showed that the MS/ DBB is composed of two major populations of cells, one cholinergic and one GABAergic (Baisden et al., 1984; Ko ¨hler et al., 1984). While the cholinergic cells project to both interneurones and pyramidal cells in the hippocampus, the GABAergic population synapses only on the interneurones (Freund & Antal, 1988). Both of the models of septohippocampal function presented in recent years are based on evidence collected in anaesthetized or restrained animals and have assumed that both the cholinergic and GABAergic cell groups fire rhythmic bursts (Stewart & Fox, 1990; Lee et al., 1994). There is, however, evidence showing that anaesthetics affect the firing behaviour of MS/DBB cells. Administration of urethane leads to a decrease in the burst frequency of rhythmically bursting cells, and a drop in the number of spikes fired in each burst (Stumpf et al., 1962). Systemic injection of pentobarbital into restrained rabbits caused the frequency of rhythmically bursting cells to lower, and led to the elimination of theta bursts in cells with unstable rhythmicity (Brazhnik & Vinogradova, 1986). An experiment involving the administration of urethane anaesthesia, however, showed that the number of rhythmically bursting cells in the MS/DBB of restrained rats increased significantly over the unanaesthetized state, and the cells exhibited a higher variability of discharge rate in the latter condition (Lee et al., 1991). This was confirmed in a later study on