ResearchPad - optogenetics https://www.researchpad.co Default RSS Feed en-us © 2020 Newgen KnowledgeWorks <![CDATA[Thalamic, cortical, and amygdala involvement in the processing of a natural sound cue of danger]]> https://www.researchpad.co/article/elastic_article_7872 When others stop and silence ensues, animals respond as if threatened. This study highlights the brain areas involved in listening to the dangerous silence.

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<![CDATA[Population dynamics and entrainment of basal ganglia pacemakers are shaped by their dendritic arbors]]> https://www.researchpad.co/article/5c65dcf2d5eed0c484dec628

The theory of phase oscillators is an essential tool for understanding population dynamics of pacemaking neurons. GABAergic pacemakers in the substantia nigra pars reticulata (SNr), a main basal ganglia (BG) output nucleus, receive inputs from the direct and indirect pathways at distal and proximal regions of their dendritic arbors, respectively. We combine theory, optogenetic stimulation and electrophysiological experiments in acute brain slices to ask how dendritic properties impact the propensity of the various inputs, arriving at different locations along the dendrite, to recruit or entrain SNr pacemakers. By combining cable theory with sinusoidally-modulated optogenetic activation of either proximal somatodendritic regions or the entire somatodendritic arbor of SNr neurons, we construct an analytical model that accurately fits the empirically measured somatic current response to inputs arising from illuminating the soma and various portions of the dendritic field. We show that the extent of the dendritic tree that is illuminated generates measurable and systematic differences in the pacemaker’s phase response curve (PRC), causing a shift in its peak. Finally, we show that the divergent PRCs correctly predict differences in two major features of the collective dynamics of SNr neurons: the fidelity of population responses to sudden step-like changes in inputs; and the phase latency at which SNr neurons are entrained by rhythmic stimulation, which can occur in the BG under both physiological and pathophysiological conditions. Our novel method generates measurable and physiologically meaningful spatial effects, and provides the first empirical demonstration of how the collective responses of SNr pacemakers are determined by the transmission properties of their dendrites. SNr dendrites may serve to delay distal striatal inputs so that they impinge on the spike initiation zone simultaneously with pallidal and subthalamic inputs in order to guarantee a fair competition between the influence of the monosynaptic direct- and polysynaptic indirect pathways.

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<![CDATA[Illuminating pathogen–host intimacy through optogenetics]]> https://www.researchpad.co/article/5b5ff78f463d7e28ade495c3

The birth and subsequent evolution of optogenetics has resulted in an unprecedented advancement in our understanding of the brain. Its outstanding success does usher wider applications; however, the tool remains still largely relegated to neuroscience. Here, we introduce selected aspects of optogenetics with potential applications in infection biology that will not only answer long-standing questions about intracellular pathogens (parasites, bacteria, viruses) but also broaden the dimension of current research in entwined models. In this essay, we illustrate how a judicious integration of optogenetics with routine methods can illuminate the host–pathogen interactions in a way that has not been feasible otherwise.

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<![CDATA[Identification of a Group of GABAergic Neurons in the Dorsomedial Area of the Locus Coeruleus]]> https://www.researchpad.co/article/5989dac7ab0ee8fa60bb2fe8

The locus coeruleus (LC)-norepinephrine (NE) system in the brainstem plays a critical role in a variety of behaviors is an important target of pharmacological intervention to several neurological disorders. Although GABA is the major inhibitory neurotransmitter of LC neurons, the modulation of LC neuronal firing activity by local GABAergic interneurons remains poorly understood with respect to their precise location, intrinsic membrane properties and synaptic modulation. Here, we took an optogenetic approach to address these questions. Channelrhodopsin (ChR2) in a tandem with the yellow fluorescent protein (YFP) was expressed in GABAergic neurons under the control of glutamic acid decarboxylase 2 (GAD2) promoter. Immediately dorsomedial to the LC nucleus, a group of GABAergic neurons was observed. They had small soma and were densely packed in a small area, which we named the dorsomedial LC or dmLC nucleus. These GABAergic neurons showed fast firing activity, strong inward rectification and spike frequency adaptation. Lateral inhibition among these GABAergic neurons was observed. Optostimulation of the dmLC area drastically inhibited LC neuronal firing frequency, expanded the spike intervals, and reset their pacemaking activity. Analysis of the light evoked inhibitory postsynaptic currents (IPSCs) indicated that they were monosynaptic. Such light evoked IPSCs were not seen in slices where this group of GABAergic neurons was absent. Thus, an isolated group of GABAergic neurons is demonstrated in the LC area, whose location, somatic morphology and intrinsic membrane properties are clearly distinguishable from adjacent LC neurons. They interact with each and may inhibit LC neurons as well as a part of local neuronal circuitry in the LC.

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<![CDATA[Optogenetically Blocking Sharp Wave Ripple Events in Sleep Does Not Interfere with the Formation of Stable Spatial Representation in the CA1 Area of the Hippocampus]]> https://www.researchpad.co/article/5989da11ab0ee8fa60b79adf

During hippocampal sharp wave/ripple (SWR) events, previously occurring, sensory input-driven neuronal firing patterns are replayed. Such replay is thought to be important for plasticity-related processes and consolidation of memory traces. It has previously been shown that the electrical stimulation-induced disruption of SWR events interferes with learning in rodents in different experimental paradigms. On the other hand, the cognitive map theory posits that the plastic changes of the firing of hippocampal place cells constitute the electrophysiological counterpart of the spatial learning, observable at the behavioral level. Therefore, we tested whether intact SWR events occurring during the sleep/rest session after the first exploration of a novel environment are needed for the stabilization of the CA1 code, which process requires plasticity. We found that the newly-formed representation in the CA1 has the same level of stability with optogenetic SWR blockade as with a control manipulation that delivered the same amount of light into the brain. Therefore our results suggest that at least in the case of passive exploratory behavior, SWR-related plasticity is dispensable for the stability of CA1 ensembles.

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<![CDATA[Optogenetic Stimulation Shifts the Excitability of Cerebral Cortex from Type I to Type II: Oscillation Onset and Wave Propagation]]> https://www.researchpad.co/article/5989db54ab0ee8fa60bdd12b

Constant optogenetic stimulation targeting both pyramidal cells and inhibitory interneurons has recently been shown to elicit propagating waves of gamma-band (40–80 Hz) oscillations in the local field potential of non-human primate motor cortex. The oscillations emerge with non-zero frequency and small amplitude—the hallmark of a type II excitable medium—yet they also propagate far beyond the stimulation site in the manner of a type I excitable medium. How can neural tissue exhibit both type I and type II excitability? We investigated the apparent contradiction by modeling the cortex as a Wilson-Cowan neural field in which optogenetic stimulation was represented by an external current source. In the absence of any external current, the model operated as a type I excitable medium that supported propagating waves of gamma oscillations similar to those observed in vivo. Applying an external current to the population of inhibitory neurons transformed the model into a type II excitable medium. The findings suggest that cortical tissue normally operates as a type I excitable medium but it is locally transformed into a type II medium by optogenetic stimulation which predominantly targets inhibitory neurons. The proposed mechanism accounts for the graded emergence of gamma oscillations at the stimulation site while retaining propagating waves of gamma oscillations in the non-stimulated tissue. It also predicts that gamma waves can be emitted on every second cycle of a 100 Hz oscillation. That prediction was subsequently confirmed by re-analysis of the neurophysiological data. The model thus offers a theoretical account of how optogenetic stimulation alters the excitability of cortical neural fields.

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<![CDATA[A Chimera Na+-Pump Rhodopsin as an Effective Optogenetic Silencer]]> https://www.researchpad.co/article/5989dae8ab0ee8fa60bbe4f3

With the progress of optogenetics, the activities of genetically identified neurons can be optically silenced to determine whether the neurons in question are necessary for the network performance of the behavioral expression. This logical induction is expected to be improved by the application of the Na+ pump rhodopsins (NaRs), which hyperpolarize the membrane potential with negligible influence on the ionic/pH balance. Here, we made several chimeric NaRs between two NaRs, KR2 and IaNaR from Krokinobacter eikastus and Indibacter alkaliphilus, respectively. We found that one of these chimeras, named I1K6NaR, exhibited some improvements in the membrane targeting and photocurrent properties over native NaRs. The I1K6NaR-expressing cortical neurons were stably silenced by green light irradiation for a certain long duration. With its rapid kinetics and voltage dependency, the photoactivation of I1K6NaR would specifically counteract the generation of action potentials with less hyperpolarization of the neuronal membrane potential than KR2.

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<![CDATA[Odorant Responses and Courtship Behaviors Influenced by at4 Neurons in Drosophila]]> https://www.researchpad.co/article/5989db02ab0ee8fa60bc70f3

In insects, pheromones function as triggers to elicit complex behavior programs, such as courtship and mating behavior. In most species, the neurons tuned to pheromones are localized in a specific subset of olfactory sensilla located on the antenna called trichoid sensilla. In Drosophila there are two classes of trichoid sensilla, at1 sensilla that contain the dendrites of a single neuron that is specifically tuned to the male-specific pheromone 11-cis vaccenyl acetate (cVA), and at4 sensilla that contain three neurons with relatively poorly defined chemical specificity and function. Using a combination of odorant receptor mutant analysis, single sensillum electrophysiology and optogenetics, we have examined the chemical tuning and behavioral consequences of the three at4 olfactory neuron classes. Our results indicate that one class, Or65abc neurons, are unresponsive to cVA pheromone, and function to inhibit courtship behaviors in response to an unknown ligand, while the other two neuron classes, Or88a and Or47b neurons, are sensitive to a diverse array of fly and non-fly odors, and activation of these neurons has little direct impact on courtship behaviors.

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<![CDATA[The Timing of Raf/ERK and AKT Activation in Protecting PC12 Cells against Oxidative Stress]]> https://www.researchpad.co/article/5989da84ab0ee8fa60b9ba45

Acute brain injuries such as ischemic stroke or traumatic brain injury often cause massive neural death and irreversible brain damage with grave consequences. Previous studies have established that a key participant in the events leading to neural death is the excessive production of reactive oxygen species. Protecting neuronal cells by activating their endogenous defense mechanisms is an attractive treatment strategy for acute brain injuries. In this work, we investigate how the precise timing of the Raf/ERK and the AKT pathway activation affects their protective effects against oxidative stress. For this purpose, we employed optogenetic systems that use light to precisely and reversibly activate either the Raf/ERK or the AKT pathway. We find that preconditioning activation of the Raf/ERK or the AKT pathway immediately before oxidant exposure provides significant protection to cells. Notably, a 15-minute transient activation of the Raf/ERK pathway is able to protect PC12 cells against oxidant strike that is applied 12 hours later, while the transient activation of the AKT pathway fails to protect PC12 cells in such a scenario. On the other hand, if the pathways are activated after the oxidative insult, i.e. postconditioning, the AKT pathway conveys greater protective effect than the Raf/ERK pathway. We find that postconditioning AKT activation has an optimal delay period of 2 hours. When the AKT pathway is activated 30min after the oxidative insult, it exhibits very little protective effect. Therefore, the precise timing of the pathway activation is crucial in determining its protective effect against oxidative injury. The optogenetic platform, with its precise temporal control and its ability to activate specific pathways, is ideal for the mechanistic dissection of intracellular pathways in protection against oxidative stress.

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<![CDATA[Molecular and Cellular Mechanisms for Trapping and Activating Emotional Memories]]> https://www.researchpad.co/article/5989da9fab0ee8fa60ba511b

Recent findings suggest that memory allocation to specific neurons (i.e., neuronal allocation) in the amygdala is not random, but rather the transcription factor cAMP-response element binding protein (CREB) modulates this process, perhaps by regulating the transcription of channels that control neuronal excitability. Here, optogenetic studies in the mouse lateral amygdala (LA) were used to demonstrate that CREB and neuronal excitability regulate which neurons encode an emotional memory. To test the role of CREB in memory allocation, we overexpressed CREB in the lateral amygdala to recruit the encoding of an auditory-fear conditioning (AFC) memory to a subset of neurons. Then, post-training activation of these neurons with Channelrhodopsin-2 was sufficient to trigger recall of the memory for AFC, suggesting that CREB regulates memory allocation. To test the role of neuronal excitability in memory allocation, we used a step function opsin (SFO) to transiently increase neuronal excitability in a subset of LA neurons during AFC. Post-training activation of these neurons with Volvox Channelrhodopsin-1 was able to trigger recall of that memory. Importantly, our studies show that activation of the SFO did not affect AFC by either increasing anxiety or by strengthening the unconditioned stimulus. Our findings strongly support the hypothesis that CREB regulates memory allocation by modulating neuronal excitability.

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