Optogenetics: implications for Alzheimer's disease research and therapy

Neuronal plasticity and its role in Alzheimer's disease and Parkinson's disease

Neuronal plasticity, the brain's ability to adapt structurally and functionally, is essential for learning, memory, and recovery from injuries. In neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease, this plasticity is disrupted, leading to cognitive and motor deficits. This review explores the mechanisms of neuronal plasticity and its effect on Alzheimer's disease and Parkinson's disease. Alzheimer's disease features amyloid-beta plaques and tau tangles that impair synaptic function, while Parkinson's disease involves the loss of dopaminergic neurons affecting motor control. Enhancing neuronal plasticity offers therapeutic potential for these diseases. A systematic literature review was conducted using databases such as PubMed, Scopus, and Google Scholar, focusing on studies of neuronal plasticity in Alzheimer's disease and Parkinson's disease. Data synthesis identified key themes such as synaptic mechanisms, neurogenesis, and therapeutic strategies, linking molecular insights to clinical applications. Results highlight that targeting synaptic plasticity mechanisms, such as long-term potentiation and long-term depression, shows promise. Neurotrophic factors, advanced imaging techniques, and molecular tools (e.g., clustered regularly interspaced short palindromic repeats and optogenetics) are crucial in understanding and enhancing plasticity. Current therapies, including dopamine replacement, deep brain stimulation, and lifestyle interventions, demonstrate the potential to alleviate symptoms and improve outcomes. In conclusion, enhancing neuronal plasticity through targeted therapies holds significant promise for treating neurodegenerative diseases. Future research should integrate multidisciplinary approaches to fully harness the therapeutic potential of neuronal plasticity in Alzheimer's disease and Parkinson's disease.

Noninvasive Optogenetics Realized by iPSC-Derived Tentacled Carrier in Alzheimer's Disease Treatment

Neural-activated optogenetics technique contributing to the "restart" of degenerative neurons offers hope for the treatment of several neurodegenerative diseases. However, the limitations of persistent invasiveness and inadequate repair of the pathological environment strongly hinder its further application. Here, a concept of differentiating stem cells is proposed to produce functional materials to enhance the therapeutic applicability of optogenetics. Induced pluripotent stem cells (iPSCs) are differentiated to generate the "tentacled" stem cells TenSCs. Their "tentacled" vesicles TenSCev, upon inheriting the biological functions of the parent cell, will possess both neural targeting capacity and pathological environment repair ability. Hence, TenSCev are utilized as functional carrier to deliver optogenetics elements for completely non-traumatic and controllable neuron activation, while also facilitating the comprehensive restoration of the pathological environment, thus effectively halted disease progression and significantly improved cognitive function in Alzheimer's disease or aged mice. Further, the concept of generating specialized biomaterials from differentiated stem cells as functional carriers holds the potential to broaden the applicability of various neuroregulatory techniques in the treatment of neurological disorders.

Neurostimulation devices to treat Alzheimer's disease

The use of neurostimulation devices for the treatment of Alzheimer's disease (AD) is a growing field. In this review, we examine the mechanism of action and therapeutic indications of these neurostimulation devices in the AD process. Rapid advancements in neurostimulation technologies are providing non-pharmacological relief to patients affected by AD pathology. Neurostimulation therapies include electrical stimulation that targets the circuitry-level connection in important brain areas such as the hippocampus to induce therapeutic neuromodulation of dysfunctional neural circuitry and electromagnetic field (EMF) stimulation that targets anti-amyloid molecular pathways to promote the degradation of beta-amyloid (Aβ). These devices target specific or diffuse cortical and subcortical brain areas to modulate neuronal activity at the electrophysiological or molecular pathway level, providing therapeutic effects for AD. This review attempts to determine the most effective and safe neurostimulation device for AD and provides an overview of potential and current clinical indications. Several EMF devices have shown a beneficial or harmful effect in cell cultures and animal models but not in AD human studies. These contradictory results may be related to the stimulation parameters of these devices, such as frequency, penetration depth, power deposition measured by specific absorption rate, time of exposure, type of cell, and tissue dielectric properties. Based on this, determining the optimal stimulation parameters for EMF devices in AD and understanding their mechanism of action is essential to promote their clinical application, our review suggests that repeated EMF stimulation (REMFS) is the most appropriate device for human AD treatments. Before its clinical application, it is necessary to consider the complicated and interconnected genetic and epigenetic effects of REMFS-biological system interaction. This will move forward the urgently needed therapy of EMF in human AD.

Advances in Photonic Materials and Integrated Devices for Smart and Digital Healthcare: Bridging the Gap Between Materials and Systems

Recent advances in developing photonic technologies using various materials offer enhanced biosensing, therapeutic intervention, and non-invasive imaging in healthcare. Here, this article summarizes significant technological advancements in materials, photonic devices, and bio-interfaced systems, which demonstrate successful applications for impacting human healthcare via improved therapies, advanced diagnostics, and on-skin health monitoring. The details of required materials, necessary properties, and device configurations are described for next-generation healthcare systems, followed by an explanation of the working principles of light-based therapeutics and diagnostics. Next, this paper shares the recent examples of integrated photonic systems focusing on translation and immediate applications for clinical studies. In addition, the limitations of existing materials and devices and future directions for smart photonic systems are discussed. Collectively, this review article summarizes the recent focus and trends of technological advancements in developing new nanomaterials, light delivery methods, system designs, mechanical structures, material functionalization, and integrated photonic systems to advance human healthcare and digital healthcare.

Emerging biophysical techniques for probing synaptic transmission in neurodegenerative disorders

Plethora of research has shed light on the critical role of synaptic dysfunction in various neurodegenerative disorders (NDDs), including Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). Synapses, the fundamental units for neural communication in the brain, are highly vulnerable to pathological conditions and are central to the progression of neurological diseases. The presynaptic terminal, a key component of synapses responsible for neurotransmitter release and synaptic communication, undergoes structural and functional alterations in these disorders. Understanding synaptic transmission abnormalities is crucial for unravelling the pathophysiological mechanisms underlying neurodegeneration. In the quest to probe synaptic transmission in NDDs, emerging biophysical techniques play a pivotal role. These advanced methods offer insights into the structural and functional changes occurring at nerve terminals in conditions like AD, PD, HD & ALS. By investigating synaptic plasticity and alterations in neurotransmitter release dynamics, researchers can uncover valuable information about disease progression and potential therapeutic targets. The review articles highlighted provide a comprehensive overview of how synaptic vulnerability and pathology are shared mechanisms across a spectrum of neurological disorders. In major neurodegenerative diseases, synaptic dysfunction is a common thread linking these conditions. The intricate molecular machinery involved in neurotransmitter release, synaptic vesicle dynamics, and presynaptic protein regulation are key areas of focus for understanding synaptic alterations in neurodegenerative diseases.

Mystery of gamma wave stimulation in brain disorders

Neuronal oscillations refer to rhythmic and periodic fluctuations of electrical activity in the central nervous system that arise from the cellular properties of diverse neuronal populations and their interactions. Specifically, gamma oscillations play a crucial role in governing the connectivity between distinct brain regions, which are essential in perception, motor control, memory, and emotions. In this context, we recapitulate various current stimulation methods to induce gamma entrainment. These methods include sensory stimulation, optogenetic modulation, photobiomodulation, and transcranial electrical or magnetic stimulation. Simultaneously, we explore the association between abnormal gamma oscillations and central nervous system disorders such as Alzheimer's disease, Parkinson's disease, stroke, schizophrenia, and autism spectrum disorders. Evidence suggests that gamma entrainment-inducing stimulation methods offer notable neuroprotection, although somewhat controversial. This review comprehensively discusses the functional role of gamma oscillations in higher-order brain activities from both physiological and pathological perspectives, emphasizing gamma entrainment as a potential therapeutic approach for neuropsychiatric disorders. Additionally, we discuss future opportunities and challenges in implementing such strategies.

Transition Metal Complexes as Therapeutics: A New Frontier in Combatting Neurodegenerative Disorders through Protein Aggregation Modulation

Neurodegenerative disorders (NDDs) are a class of debilitating diseases that progressively impair the protein structure and result in neurological dysfunction in the nervous system. Among these disorders, Alzheimer's disease (AD), prion diseases such as Creutzfeldt-Jakob disease (CJD), and Parkinson's disease (PD) are caused by protein misfolding and aggregation at the cellular level. In recent years, transition metal complexes have gained significant attention for their potential applications in diagnosing, imaging, and curing these NDDs. These complexes have intriguing possibilities as therapeutics due to their diverse ligand systems and chemical properties and can interact with biological systems with minimal detrimental effects. This review focuses on the recent progress in transition metal therapeutics as a new era of hope in the battle against AD, CJD, and PD by modulating protein aggregation in vitro and in vivo. It may shed revolutionary insights into unlocking new opportunities for researchers to develop metal-based drugs to combat NDDs.

Treating Alzheimer's disease with brain stimulation: From preclinical models to non-invasive stimulation in humans

Alzheimer's disease (AD) is a severe and progressive neurodegenerative condition that exerts detrimental effects on brain function. As of now, there is no effective treatment for AD patients. This review explores two distinct avenues of research. The first revolves around the use of animal studies and preclinical models to gain insights into AD's underlying mechanisms and potential treatment strategies. Specifically, it delves into the effectiveness of interventions such as Optogenetics and Chemogenetics, shedding light on their implications for understanding pathophysiological mechanisms and potential therapeutic applications. The second avenue focuses on non-invasive brain stimulation (NiBS) techniques in the context of AD. Evidence suggests that NiBS can successfully modulate cognitive functions associated with various neurological and neuropsychiatric disorders, including AD, as demonstrated by promising findings. Here, we critically assessed recent findings in AD research belonging to these lines of research and discuss their potential impact on the clinical horizon of AD treatment. These multifaceted approaches offer hope for advancing our comprehension of AD pathology and developing novel therapeutic interventions.

The application of optogenetics in traumatic brain injury research: A narrative review

Optogenetics has revolutionized the landscape of research on neurological disorders by enabling high spatial specificity and millisecond-level temporal precision in neuroscience studies. In the field of traumatic brain injury (TBI), optogenetic techniques have greatly advanced our understanding of the pathological and physiological processes involved, providing valuable guidance for both monitoring and therapeutic interventions. This article offers a review of the latest research applications of optogenetics in the study of TBI.

Mechanistic insights and emerging therapeutic stratagems for Alzheimer's disease

Alzheimer's disease (AD), a multi-factorial neurodegenerative disorder has affected over 30 million individuals globally and these numbers are expected to increase in the coming decades. Current therapeutic interventions are largely ineffective as they focus on a single target. Development of an effective drug therapy requires a deep understanding of the various factors influencing the onset and progression of the disease. Aging and genetic factors exert a major influence on the development of AD. Other factors like post-viral infections, iron overload, gut dysbiosis, and vascular dysfunction also exacerbate the onset and progression of AD. Further, post-translational modifications in tau, DRP1, CREB, and p65 proteins increase the disease severity through triggering mitochondrial dysfunction, synaptic loss, and differential interaction of amyloid beta with different receptors leading to impaired intracellular signalling. With advancements in neuroscience tools, new inter-relations that aggravate AD are being discovered including pre-existing diseases and exposure to other pathogens. Simultaneously, new therapeutic strategies involving modulation of gene expression through targeted delivery or modulation with light, harnessing the immune response to promote clearance of amyloid deposits, introduction of stem cells and extracellular vesicles to replace the destroyed neurons, exploring new therapeutic molecules from plant, marine and biological sources delivered in the free state or through nanoparticles and use of non-pharmacological interventions like music, transcranial stimulation and yoga. Polypharmacology approaches involving combination of therapeutic agents are also under active investigation for superior therapeutic outcomes. This review elaborates on various disease-causing factors, their underlying mechanisms, the inter-play between different disease-causing players, and emerging therapeutic options including those under clinical trials, for treatment of AD. The challenges involved in AD therapy and the way forward have also been discussed.

Intracellular microbial rhodopsin-based optogenetics to control metabolism and cell signaling

Microbial rhodopsin (MRs) ion channels and pumps have become invaluable optogenetic tools for neuroscience as well as biomedical applications. Recently, MR-optogenetics expanded towards subcellular organelles opening principally new opportunities in optogenetic control of intracellular metabolism and signaling via precise manipulations of organelle ion gradients using light. This new optogenetic field expands the opportunities for basic and medical studies of cancer, cardiovascular, and metabolic disorders, providing more detailed and accurate control of cell physiology. This review summarizes recent advances in studies of the cellular metabolic processes and signaling mediated by optogenetic tools targeting mitochondria, endoplasmic reticulum (ER), lysosomes, and synaptic vesicles. Finally, we discuss perspectives of such an optogenetic approach in both fundamental and applied research.

Reduced Prefrontal-Thalamic Theta Flow During Working Memory Retrieval in APP/PS1 Mice

Background

Working memory deficits in Alzheimer's disease (AD) are linked to impairments in the retrieval of stored memory information. However, research on the mechanism of impaired working memory retrieval in Alzheimer's disease is still lacking.

Objective

The medial prefrontal cortex (mPFC) and mediodorsal thalamus (MD) are involved in memory retrieval. The purpose of this study is to investigate the functional interactions and information transmission between mPFC and MD in the AD model.

Methods

We recorded local field potentials from mPFC and MD while the mice (APP/PS1 transgenic model and control) performed a T-maze spatial working memory task. The temporal dynamics of oscillatory activity and bidirectional information flow between mPFC and MD were assessed during the task phases.

Results

We mainly found a significant decrease in theta flow from mPFC to MD in APP/PS1 mice during retrieval.

Conclusions

Our results indicate an important role of the mPFC-MD input for retrieval and the disrupted information transfer from mPFC to MD may be the underlying mechanism of working memory deficits in APP/PS1 mice.

Genetically encoded mediators for sonogenetics and their applications in neuromodulation

Sonogenetics is an emerging approach that harnesses ultrasound for the manipulation of genetically modified cells. The great penetrability of ultrasound waves enables the non-invasive application of external stimuli to deep tissues, particularly advantageous for brain stimulation. Genetically encoded ultrasound mediators, a set of proteins that respond to ultrasound-induced bio-effects, play a critical role in determining the effectiveness and applications of sonogenetics. In this context, we will provide an overview of these ultrasound-responsive mediators, delve into the molecular mechanisms governing their response to ultrasound stimulation, and summarize their applications in neuromodulation.

Bibliometric analysis of hot literature on neural circuit research

Numerous brain diseases have been attributed to abnormalities in the connections of neural circuits. Exploration of neural circuits may give enlightenment in treating some intractable brain diseases. Here, we screened all publications on neural circuits in the Web of Science database from 2007 to 2022 and analyzed the research trends through VOSviewer, CiteSpace, Microsoft Excel 2019, and Origin. The findings revealed a consistent upward trend in research on neural circuits during this period. The United States emerged as the leading contributor, followed by China and Japan. Among the top 10 institutions with the largest number of publications, both the United States and China have a strong presence. Notably, the Chinese Academy of Sciences demonstrated the highest publication output, closely followed by Stanford University. In terms of influential authors, Karl Deisseroth stood out as one of the most prominent investigators. During this period, the majority of publications and citations on neural circuit research were found in highly influential journals including NEURON, NATURE JOURNAL OF NEUROSCIENCE, and so forth. Keyword clustering analysis highlighted the increasing focus on neural circuits and photogenetics in neuroscience research, and the reconstruction of neural circuits has emerged as a crucial research direction in brain science. In conclusion, over the past 15 years, the increasing high-quality publications have facilitated research development of neural circuits, indicating a promising prospect for investigations on neurological and psychiatric diseases.

Therapy and diagnosis of Alzheimer's disease: from discrete metal complexes to metal-organic frameworks

Alzheimer's disease (AD) is a neurodegenerative disorder affecting 44 million people worldwide. Although many issues (pathogenesis, genetics, clinical features, and pathological aspects) are still unknown, this disease is characterized by noticeable hallmarks such as the formation of β-amyloid plaques, hyperphosphorylation of tau proteins, the overproduction of reactive oxygen species, and the reduction of acetylcholine levels. There is still no cure for AD and the current treatments are aimed at regulating the cholinesterase levels, attenuating symptoms temporarily rather than preventing the AD progression. In this context, coordination compounds are regarded as a promissing tool in AD treatment and/or diagnosis. Coordination compounds (discrete or polymeric) possess several features that make them an interesting option for developing new drugs for AD (good biocompatibility, porosity, synergetic effects of ligand-metal, fluorescence, particle size, homogeneity, monodispersity, etc.). This review discusses the recent progress in the development of novel discrete metal complexes and metal-organic frameworks (MOFs) for the treatment, diagnosis and theragnosis of AD. These advanced therapies for AD treatment are organized according to the target: Aβ peptides, hyperphosphorylated tau proteins, synaptic dysfunction, and mitochondrial failure with subsequent oxidative stress.

Alzheimer's disease as a synaptopathy: Evidence for dysfunction of synapses during disease progression

The synapse has consistently been considered a vulnerable and critical target within Alzheimer's disease, and synapse loss is, to date, one of the main biological correlates of cognitive decline within Alzheimer's disease. This occurs prior to neuronal loss with ample evidence that synaptic dysfunction precedes this, in support of the idea that synaptic failure is a crucial stage within disease pathogenesis. The two main pathological hallmarks of Alzheimer's disease, abnormal aggregates of amyloid or tau proteins, have had demonstrable effects on synaptic physiology in animal and cellular models of Alzheimer's disease. There is also growing evidence that these two proteins may have a synergistic effect on neurophysiological dysfunction. Here, we review some of the main findings of synaptic alterations in Alzheimer's disease, and what we know from Alzheimer's disease animal and cellular models. First, we briefly summarize some of the human evidence to suggest that synapses are altered, including how this relates to network activity. Subsequently, animal and cellular models of Alzheimer's disease are considered, highlighting mouse models of amyloid and tau pathology and the role these proteins may play in synaptic dysfunction, either in isolation or examining how the two pathologies may interact in dysfunction. This specifically focuses on neurophysiological function and dysfunction observed within these animal models, typically measured using electrophysiology or calcium imaging. Following synaptic dysfunction and loss, it would be impossible to imagine that this would not alter oscillatory activity within the brain. Therefore, this review also discusses how this may underpin some of the aberrant oscillatory patterns seen in animal models of Alzheimer's disease and human patients. Finally, an overview of some key directions and considerations in the field of synaptic dysfunction in Alzheimer's disease is covered. This includes current therapeutics that are targeted specifically at synaptic dysfunction, but also methods that modulate activity to rescue aberrant oscillatory patterns. Other important future avenues of note in this field include the role of non-neuronal cell types such as astrocytes and microglia, and mechanisms of dysfunction independent of amyloid and tau in Alzheimer's disease. The synapse will certainly continue to be an important target within Alzheimer's disease for the foreseeable future.

Neuromodulation of Neural Oscillations in Health and Disease

Using EEG and local field potentials (LFPs) as an index of large-scale neural activities, research has been able to associate neural oscillations in different frequency bands with markers of cognitive functions, goal-directed behavior, and various neurological disorders. While this gives us a glimpse into how neurons communicate throughout the brain, the causality of these synchronized network activities remains poorly understood. Moreover, the effect of the major neuromodulatory systems (e.g., noradrenergic, cholinergic, and dopaminergic) on brain oscillations has drawn much attention. More recent studies have suggested that cross-frequency coupling (CFC) is heavily responsible for mediating network-wide communication across subcortical and cortical brain structures, implicating the importance of neurotransmitters in shaping coordinated actions. By bringing to light the role each neuromodulatory system plays in regulating brain-wide neural oscillations, we hope to paint a clearer picture of the pivotal role neural oscillations play in a variety of cognitive functions and neurological disorders, and how neuromodulation techniques can be optimized as a means of controlling neural network dynamics. The aim of this review is to showcase the important role that neuromodulatory systems play in large-scale neural network dynamics, informing future studies to pay close attention to their involvement in specific features of neural oscillations and associated behaviors.

Optogenetics Sheds Light on Brown and Beige Adipocytes

Excessive food intake leads to lipid accumulation in white adipose tissue, triggering inflammation, cellular stress, insulin resistance, and metabolic syndrome. In contrast, the dynamic energy expenditure and heat generation of brown and beige adipose tissue, driven by specialized mitochondria, render it an appealing candidate for therapeutic strategies aimed at addressing metabolic disorders. This review examines the therapeutic potential of brown and beige adipocytes for obesity and metabolic disorders, focusing on recent studies that employ optogenetics for thermogenesis control in these cells. The findings delve into the mechanisms underlying UCP1-dependent and UCP1-independent thermogenesis and how optogenetic approaches can be used to precisely modulate energy expenditure and induce thermogenesis. The convergence of adipocyte biology and optogenetics presents an exciting frontier in combating metabolic disorders and advancing our understanding of cellular regulation and energy balance.

The Role of Aberrant Neural Oscillations in the Hippocampal-Medial Prefrontal Cortex Circuit in Neurodevelopmental and Neurological Disorders

The hippocampus (HPC) and medial prefrontal cortex (mPFC) have well-established roles in cognition, emotion, and sensory processing. In recent years, interests have shifted towards developing a deeper understanding of the mechanisms underlying interactions between the HPC and mPFC in achieving these functions. Considerable research supports the idea that synchronized activity between the HPC and the mPFC is a general mechanism by which brain functions are regulated. In this review, we summarize current knowledge on the hippocampal-medial prefrontal cortex (HPC-mPFC) circuit in normal brain function with a focus on oscillations and highlight several neurodevelopmental and neurological disorders associated with aberrant HPC-mPFC circuitry. We further discuss oscillatory dynamics across the HPC-mPFC circuit as potentially useful biomarkers to assess interventions for neurodevelopmental and neurological disorders. Finally, advancements in brain stimulation, gene therapy and pharmacotherapy are explored as promising therapies for disorders with aberrant HPC-mPFC circuit dynamics.

Chemogenetic and Optogenetic Manipulations of Microglia in Chronic Pain

Chronic pain relief remains an unmet medical need. Current research points to a substantial contribution of glia-neuron interaction in its pathogenesis. Particularly, microglia play a crucial role in the development of chronic pain. To better understand the microglial contribution to chronic pain, specific regional and temporal manipulations of microglia are necessary. Recently, two new approaches have emerged that meet these demands. Chemogenetic tools allow the expression of designer receptors exclusively activated by designer drugs (DREADDs) specifically in microglia. Similarly, optogenetic tools allow for microglial manipulation via the activation of artificially expressed, light-sensitive proteins. Chemo- and optogenetic manipulations of microglia in vivo are powerful in interrogating microglial function in chronic pain. This review summarizes these emerging tools in studying the role of microglia in chronic pain and highlights their potential applications in microglia-related neurological disorders.

Not All Paths are Equal: Stimulating Specific Cerebellar Outputs Inhibits Hippocampal Seizures

Distinct Fastigial Output Channels and Their Impact on Temporal Lobe Seizures

Streng ML, Tetzlaff MR, Krook-Magnuson E. 2021. J. Neurosci. 41(49):10091-10107. doi:10.1523/JNEUROSCI.0683-21.2021.

Despite being canonically considered a motor control structure, the cerebellum is increasingly recognized for important roles in processes beyond this traditional framework, including seizure suppression. Excitatory fastigial neurons project to a large number of downstream targets, and it is unclear whether this broad targeting underlies seizure suppression, or whether a specific output may be sufficient. To address this question, we used the intrahippocampal kainic acid mouse model of temporal lobe epilepsy, male and female animals, and a dual-virus approach to selectively label and manipulate fastigial outputs. We examined fastigial neurons projecting to the superior colliculus, medullary reticular formation, and central lateral nucleus of the thalamus, and found that these comprise largely nonoverlapping populations of neurons that send collaterals to unique sets of additional, somewhat overlapping, thalamic and brainstem regions. We found that neither optogenetic stimulation of superior colliculus nor reticular formation output channels attenuated hippocampal seizures. In contrast, on-demand stimulation of fastigial neurons targeting the central lateral nucleus robustly inhibited seizures. Our results indicate that fastigial control of hippocampal seizures does not require simultaneous modulation of many fastigial output channels. Rather, selective modulation of the fastigial output channel to the central lateral thalamus, specifically, is sufficient for seizure control. More broadly, our data highlight the concept of specific cerebellar output channels, whereby discrete cerebellar nucleus neurons project to specific aggregates of downstream targets, with important consequences for therapeutic interventions.

Feasibility of Canine Adenovirus Type 2 (CAV2) Based Vector for the Locus Coeruleus Optogenetic Activation in Non-Transgenic Rats: Implications for Functional Studies

The locus coeruleus norepinephrine (LC-NE) system modulates many visceral and cognitive functions, while LC-NE dysfunction leads to neurological and neurodegenerative conditions such as sleep disorders, depression, ADHD, or Alzheimer's disease. Innovative viral-vector and gene-engineering technology combined with the availability of cell-specific promoters enabled regional targeting and selective control over phenotypically specific populations of neurons. We transduced the LC-NE neurons in adult male rats by delivering the canine adenovirus type 2-based vector carrying the NE-specific promoter PRSx8 and a light-sensitive channelrhodopsin-2 receptor (ChR2) directly in the LC or retrogradely from the LC targets. The highest ChR2 expression level was achieved when the virus was delivered medially to the trigeminal pathway and ~100 μm lateral to the LC. The injections close or directly in the LC compromised the tissue integrity and NE cell phenotype. Retrograde labeling was more optimal given the transduction of projection-selective subpopulations. Our results highlight a limited inference of ChR2 expression from representative cases to the entire population of targeted cells. The actual fraction of manipulated neurons appears most essential for an adequate interpretation of the study outcome. The actual fraction of manipulated neurons appears most essential for an adequate interpretation of the study outcome. Thus, besides the cell-type specificity and the transduction efficiency, the between-subject variability in the proportion of the remaining viral-transduced targeted cell population must be considered in any functional connectivity study.