Over 50 million people worldwide suffer from epilepsy. Of these, nearly a third will be refractory to medical therapy, and many will be poor candidates for surgical resection. Thus there is a need for novel targets and therapies, the former of which will require a greater understanding of neural networks involved in epilepsy, and the latter of which demands the development of novel therapeutic techniques. Seizures are less frequent during periods where theta – a 3-12Hz oscillatory rhythm in the hippocampal local field potential – is present. Theta is thought to originate in the medial septum, a basal forebrain structure that projects to the site of origin for the most common form of intractable epilepsy, the hippocampus. As has been demonstrated with pharmacologic and electrical stimulation, theta generation via the medial septum is consequently an ideal target for intervention. However, of the three neuron populations within the medial septum – cholinergic, GABAergic, and glutamatergic – it is unclear which is responsible for theta, or indeed if a single population is driving the oscillation. Optogenetics, a novel technique that enables activation and inhibition of genetically-defined neurons on a millisecond time-scale, provides the means to functionally dissect this septohippocampal axis and leverage the results for seizure therapy. In this thesis, I detail the current state of deep brain stimulation for epilepsy, and describe our motivation for targeting the medial septum and the importance of the hippocampal theta rhythm. I describe new technologies, software, and adaptations to our electrophysiology platform, NeuroRighter, to enable concurrent optogenetic neuromodulation and electrophysiology in awake and behaving animals, and demonstrate how these technologies and techniques can be used in several experimental approaches. I next use this system to show that both the GABAergic and glutamatergic neurons of the medial septum can drive and pace hippocampal oscillatory rhythms, but only the glutamatergic neurons are necessary to maintain phase relationships between successive theta cycles. I also demonstrate that activating and inhibiting the cholinergic neurons of the medial septum does not alter hippocampal local field potential activity, but does alter single-unit firing rates. These results shed light on the function of the medial septum in generating and modulating theta, and provide clear targets for optogenetic modulation of epilepsy.

<p>Forebrain cholinergic projection systems innervate the entire cortex and hippocampus. These cholinergic systems are involved in a wide range of cognitive and behavioral functions, including learning and memory, attention, and sleep-waking modulation. However, the <italic>in vivo</italic> physiological mechanisms of cholinergic functions, particularly their fast dynamics and the consequent modulation on the hippocampus and cortex, are not well understood. In this dissertation, I investigated these issues using a number of convergent approaches.</p><p> First, to study fast acetylcholine (ACh) dynamics and its interaction with field potential theta oscillations, I developed a novel technique to acquire second-by-second electrophysiological and neurochemical information simultaneously with amperometry. Using this technique on anesthetized rats, I discovered for the first time the tight <italic>in vivo</italic> coupling between phasic ACh release and theta oscillations on fine spatiotemporal scales. In addition, with electrophysiological recording, putative cholinergic neurons in medial setpal area (MS) were found with firing rate dynamics matching the phasic ACh release. </p><p> Second, to further elucidate the dynamic activities and physiological functions of cholinergic neurons, putative cholinergic MS neurons were identified in behaving rats. These neurons had much higher firing rates during rapid-eye-movement (REM) sleep, and brief responses to auditory stimuli. Interestingly, their firing promoted theta/gamma oscillations, or small-amplitude irregular activities (SIA) in a state-dependent manner. These results suggest that putative MS cholinergic neurons may be a generalized hippocampal activation/arousal network. </p><p> Third, I investigated the hypothesis that ACh enhances cortical and hippocampal immediate-early gene (IEG) expression induced by novel sensory experience. Cholinergic transmission was manipulated with pharmacology or lesion. The resultant cholinergic impairment suppressed the induction of <italic>arc</italic>, a representative IEG, suggesting that ACh promotes IEG induction. </p><p> In conclusion, my results have revealed that the firing of putative cholinergic neurons promotes hippocampal activation, and the consequent phasic ACh release is tightly coupled to theta oscillations. These fast cholinergic activities may provide exceptional opportunities to dynamically modulate neural activity and plasticity on much finer temporal scales than traditionally assumed. By the subsequent promotion of IEG induction, ACh may further substantiate its function in neural plasticity and memory consolidation.</p> / Dissertation