It is known that the hippocampus is important in learning, memory and spatial navigation. How it performs these tasks at the cellular level in response to neurotransmitters and oscillations in neural circuitry is poorly understood. To determine the role of subthreshold oscillations and persistent activity in information processing in cellular circuits within the hippocampus, I chose two different methods. First, I used whole cell recordings in rat hippocampal slices in vitro to elucidate the role of cholinergic modulation on hippocampal cells. Cholinergic modulation causes dynamic changes in neuronal circuits through the release of the neurotransmitter acetylcholine. I found that cholinergic modulation within the dentate gyrus increased the excitability of dentate mossy cells but not local interneurons through activation of an M1 receptor. During cholinergic modulation, plateau potentials and persistent activity could be stabilized or completely turned off by inhibitory potentials generated by activation of local interneurons. Second, I used in vivo whole cell recordings in awake head-fixed mice to investigate subthreshold oscillations in dentate gyrus cells to determine their response during animal movement. I found that dentate gyrus neurons demonstrated subthreshold oscillations in the 8-15 Hz frequency band that were initiated near the onset of spontaneous bouts of motion. These oscillations often preceded the movement onset and their strength in the first 2-seconds was largest in movements along the animals natural forward/backward direction. Lastly, the power of this oscillation would predict the duration of time the animal would run. This data suggest that dynamic changes in network excitability as the result of cholinergic release or subthreshold oscillation functions to control the cellular processes responsible for memory formation and spatial navigation.
