Theta oscillation is the largest extracellular synchronous signal that can be recorded from the mammalian brain. It is known to influence information retention in the hippocampus, which plays a key role in declarative memory, recognition memory, working memory, and spatial memory. The theta oscillation field frequency is between 3 and 12 Hz and is present during exploratory behavior and sleep in rodents. Theta rhythm in the hippocampus is postulated to be produced by the rhythmical activity of pacemaking cells in the medial septumvertical limb of the diagonal band of Broca (MS-vDBB). Previous work in our laboratory demonstrated the existence of continuously oscillatory neurons (CONs), the pacemaking cells, and sporadically oscillatory neurons (SONs) in the MS-DB. CONs were found to fire rhythmical action potential bursts within the duration range of a theta wave. The frequency at which they fire correlates with the simultaneously recorded hippocampal theta rhythm. It is believed that inputs from CONs and other ascending neurons are necessary to recruit non-rhythmic neurons to fire along a theta oscillation pattern. Altogether, this initiates a propitious environment for hippocampal theta frequency, which becomes the foundation for memory formation important in neurodegenerative diseases such as Alzheimer’s disease (AD). The MS oscillatory mechanism is believed to lead and recruit theta rhythm generation in the hippocampus. However, the statedependent alterations of the septo-hippocampal connection and the possible imbalance leading to septal or hippocampal dominance are poorly understood. In our investigations, we report that our CON cell recording was immuno-reactive to vi a GABAergic marker, supporting our hypothesis that MS GABAergic neurons are key cells in pacing hippocampal theta. Additionally, we report our findings for one SON cell and one NON-NC cell recorded in the MS
