Neurotensin and cortical arousal : an in vitro study
Abstract: The information processing associated with wakefulness occurs during what is commonly referred to as a cortical desynchronized state. In contrast, deep sleep is accompanied by slow, synchronized electrical activity, perhaps best exemplified by the slow (<0.1Hz) oscillation (SO). The brain switches the cortex from the synchronized to desynchronized state through neuromodulation. Modulators are released in the cortex primarily from ascending systems, mostly signaling through biogenic amines and neuropeptides. One modulator associated with wakefulness through its effects on ascending arousal systems is the neuropeptide, neurotensin (NT). Immunohistochemical studies have revealed the presence of NT-immunoreactive fibers throughout the cortical mantle. Yet, the possibility of direct effects of NT on the cortex and cortical arousal has so far received little attention. Multi-unit activity recordings were performed on cortical slices spontaneously exhibiting the SO. Slices were prepared from rat to investigate the role of NT in modulating cortical global network activity. Single-cell and paired recordings of and between neuronal subgroups were performed to assess NT effects on the microcircuitry in slices from rats and mice. In this thesis a method was first developed to facilitate the investigation of cortical network activity in spontaneously oscillating rat slices (study I). Using this method, a robust SO could be recorded in combination with visually guided whole-cell recordings of single neurons. In study II, the effect of NT on spontaneous and evoked global network activity was assessed finding a depression of the spontaneous and evoked response in agreement with an arousal mediating mechanism. Through recordings of single neurons, we identified a rarely studied group of neurons residing within the cortical white matter as particularly sensitive targets for direct, excitatory actions of NT. To further investigate the role of particular neuronal subgroups transgenic mice were used in study III and IV. Neurotensin was found to excite all major classes of inhibitory interneurons, providing an explanation for the reduced global network activity. Collectively the data presented in this thesis strongly support an arousal mediating role for NT in the cortex, and identify salient components of an arousal-promoting cortical microcircuitry.
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