Persistent firing and oscillations in the septo-hippocampal system and their relation to locomotion

The medial septum, diagonal band of Broca has received most attention as a putative pacemaker of the hippocampal theta rhythm. However, due to its high interconnectivity with various cortical and subcortical regions, the medial septum is involved in a variety of neural processes. This thesis focuses on the relation between medial septal spiking activity, hippocampal theta rhythm and locomotion. It was previously demonstrated that theta-periodic optogenetic activation of medial septal glutamatergic neurons entrains hippocampal theta oscillation and initiates persistentlocomotion of the animal. We showed that hippocampal theta oscillation and locomotion, both persisting after the stimulus offset, can be induced by a brief continuous light stimulation of medial septal glutamatergic neurons. The hippocampal theta rhythm is not necessary for inducing persistent locomotion, as locomotion initiation is not affected by blocking synaptic transmission in the medial septum that abolishes the hippocampal theta. Furthermore, we observed persistent spiking activity of the medial septal neurons, lasting for many seconds after the stimulus offset.To test whether the persistent activity is generated locally in the medial septum, we repeated the stimulation experiment in an acute medial septal slice preparation. The persistent activity had a shorter duration than in vivo, but was present both in the intact slice and with blocked synaptic transmission, indicating that the persistent firing is a result of intrinsic dynamics of medial septal glutamatergic neurons. Further analysis of spontaneous spiking activity of neurons in the acute medial septal slice preparation revealed the existence of theta-rhythmic neurons that synchronizetheir firing, suggesting that the medial septum can generate the theta oscillation independently of external feedforward and feedback input. Even though medial septal synaptic connectivity is necessary for the hippocampal theta rhythm, our results suggest that the theta-rhythmic firing is a result of intrinsic cellular dynamics and a low level of synchrony can be achieved without synaptic coupling. It remains an open question how the septal theta-rhythmic input is transformed into a travelling theta wave observed in the hippocampus. The last part of the thesis offers a framework for studying the generation of periodic travelling waves in spiking neural networks. We developed a parameter mapping between a discrete network of neurons and apopulation model that describes the spatio-temporal spread of activity as a continuousprocess. Using this mapping, we derived conditions for the existence of periodictravelling waves in the spiking neural network