Medial septal cell interactions in relation to hippocampal field activity and the effects of atropine
Authors: Colom Luis V. , Bland Brian H. 
Published: January 2005

Simultaneously recorded pairs (from a single electrode) of cells in the medial septum (MS) vertical limb of the diagonal band of Broca (vDBB) were studied during the simultaneous occurrence of field activity in the hippocampal formation of urethane‐anesthetized rats and following the systemic administration of atropine sulphate (ATSO4). The 2 types of field activities recorded were cholinergically mediated type 2 theta (θ) and large‐amplitude irregular activity (LIA). The study had 3 objectives: (1) to determine if cells in close proximity in the MS/vDBB had a high probability of having similar discharge properties; (2) to determine the possible functional relationships occurring between pairs of MS/vDBB cells using standard cross correlational analyses; and (3) to determine the effects of ATSO4 on both the discharge properties of MS/vDBB cells and the interactions between these cells. Of the 143 cells recorded, 133 (93%) were θ‐related and classified as θ‐on or θ‐off cells. θ‐on cells had a significant increase in mean discharge rate during θ compared to their mean discharge rate during LIA and/or had a linear increase in discharge rate in relation to increases in θ frequency. θ‐off cells had a significant increase in mean discharge rate during LIA compared to their mean discharge rate during θ and/or had a linear decrease in discharge rate in relation to increases in θ frequency. The remaining 10 (7%) cells were classified as nonrelated to θ. A probability analysis carried out on each of the separate criteria for classification revealed that several of these could predict whether or not cells were in close proximity of one another. Cells occurred in close proximity according to whether they had higher discharge rate during θ or LIA and according to whether or not they varied their discharge rates as θ frequency varied (linear‐nonlinear). Cell discharge patterns of rhythmicity (phasic), or nonrhythmicity (tonic) on the other hand, were not good predictors of whether cells were in close proximity since there was an equal probability of a phasic cell being paired with either a phasic or a tonic cell. The presence of a phasic discharge pattern was, however, the main determinant of whether a correlation occurred between cell pairs. Cross correlations of the cell pair spike trains revealed that 50% of the 36 phasic‐phasic cell pairs were correlated during both θ and LIA field activities. The characteristics of the cross correlation functions of these cell pairs (strong symmetrical correlations with sharp primary peaks or troughs occurring around the origin) suggested that they received shared excitatory and inhibitory inputs. The remaining 50% of the phasic‐phasic pairs were cross correlated during θ only. Seventeen (68%) of 25 cell pairs consisting of one phasic and one tonic cell were cross correlated during θ only, and the 12 tonic‐tonic cell pairs were not cross correlated during θ or LIA. The systemic administration of ATSO4 abolished all 6 field activity in the hippocampal formation as analyzed visually and by fourier analysis. Twenty‐three out of 32 (71.9%) phasic cells continued to discharge in a phasic pattern, while the remaining 9 cells (28.1%) became irregular following the administration of ATSO4 The effects of ATSO4 on the interaction between cell pairs appeared to be dependent on two factors: (1) the nature of their interactions in the pre‐drug condition, during θ and LIA; and (2) whether or not the cell(s) were phasic during ATSO4, Cell pairs that were cross correlated during both θ and LIA were always cross correlated during ATSO4, regardless of what effect ATSO4 had on each cell. Cell pairs that were cross correlated only during θ were cross correlated during ATSO4 only if at least 1 of the cells in the pair remained phasic. These data suggest that there is a critical subpopulation of phasic θ‐on and θoff MSivDBB cells (approximately 50%) that share excitatory and inhibitory inputs, possibly from ascending synchronizing and desynchronizing pathways. The exact nature of the control that the output of these and other MS/vDBB cells exerts on hippocampal field and cellular activity, remains to be determined.