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    • Gamma oscillations appear to participate in sensory percepti

    2018-11-07

    Gamma oscillations appear to participate in sensory perception, problem solving, and memory [24–29], and coherence at these frequencies may occur at cortical or thalamocortical levels [30,31]. Indeed, synchronous gamma band activation among thalamocortical networks [32], is thought to contribute to the merger, or “binding”, of information originating from separate regions that leads to perception [33]. Conversely, gamma oscillation deficits have been suggested as a pathophysiologic feature of diseases like schizophrenia [34–37]. In addition, aberrant gamma band activity and coherence during cognitive tasks or attentional load have been reported in schizophrenic patients reviewed in Ref. [38]. These results suggest that the generation and maintenance of gamma band activity may be abnormal in schizophrenia. In addition, schizophrenic patients suffer from hypofrontality, or low sulfanilamide blood flow [39] which may contribute to the sensory gating deficits, but also to the lack of critical judgment. The results of electroencephalographic (EEG), reflex, and P50 potential testing all point to increased arousal and increased REM sleep drive. That is, the PPN, as the cholinergic arm of the RAS, is overactive in schizophrenia, but it is overactive in a specific manner. Responses to repetitive stimuli are increased and reflexes are exaggerated suggesting that phasic responses to brief stimuli are dysregulated. For example, exaggerated fight-or-flight responses in response to sudden stimuli are present in schizophrenia with devastating consequences [40]. However, the decreased and interrupted gamma band activity also suggests that gamma oscillations are not properly maintained on a tonic basis. This may have the effect of disturbing processes that depend on continuous gamma oscillations such as sensory perception, problem solving, and memory. This combination of short-term hyperexcitability and long-term diminution of RAS activity is functionally devastating. This is in agreement with findings described above showing that anticholinergic agents appear to alleviate some of the negative symptoms of schizophrenia [10,11], although further work is needed in this area.
    Role of the reticular activating system (RAS) The two most important advances on the physiology of the RAS in the last 10 years were, (a) the discovery of electrical coupling in some cells of certain RAS nuclei [41], and (b) the finding that every cell in the same nuclei manifests intrinsic membrane gamma oscillations [17]. The first advance helps explain how these brain centers maintain the coherence necessary to maintain neuronal membrane oscillations at both low and high frequencies. The second advance helps explain how these nuclei induce and maintain gamma band activity necessary for the process of remaining awake and maintaining REM sleep. We will briefly touch on the former, and discuss the latter discovery at length. Two major elements determining the activity of large assemblies of neurons such as in the EEG are coherence and frequency. Coherence is the term for how groups of neurons, firing in coordination, can create a signal that is mirrored instantaneously and precisely by other groups of neurons across the brain. These transient episodes of coherence across different parts of the brain may be an electrical signature of thought and action. Our recent discovery demonstrated the presence of electrical coupling in three nuclei of the RAS, a mechanism that allows groups of neurons to fire synchronously [41–43]. Briefly, the stimulant modafinil is used for the treatment of narcolepsy. Modafinil increases electrical coupling and, since most coupled neurons in the RAS are GABAergic, the coupling decreases input resistance, decreasing activity in these cells and reducing GABA release, thus disinhibiting other cell types. This disinhibition leads to overall higher frequency in activity, i.e. during sleep and arousal, in the RAS [41,42,44], and thalamocortical systems [45]. In other words, because increased coupling in GABAergic neurons will lead to decreased GABA release, the tendency will be to increase coherence and also disinhibit other transmitter systems, leading to increased excitation, especially during waking. That is, since modafinil increases electrical coupling, it should enable better coherence at all frequencies, during waking and even after its effects are waning, during sleeping. That is why modafinil is also useful in regulating coherence during sleep. Conversely, the most fast-acting anesthetics known, inhaled halothane or injected propofol, both block gap junctions, and both put us to sleep very rapidly [41,42]. That is, the control of gap junctions can determine if we are asleep or awake.