An optimized acetylcholine sensor for monitoring in vivo cholinergic activity

Calcium Imaging in Vivo: How to Correctly Select and Apply Fiber Optic Photometric Indicators

Fiber-photometric is a novel optogenetic method for recording neural activity in vivo, which allows the use of calcium indicators to observe and study the relationship between neural activity and behavior in free-ranging animals. Calcium indicators also convert changes in calcium concentration in cells or tissues into recordable fluorescent signals, which can then be observed using the system of fiber-photometric. To date, there is a paucity of relevant literature on the proper selection and application of fiber-photometric indicators. Therefore, this paper will detail how to correctly select and apply fiber-photometer indicators in four sections: the basic principle of optical fiber photometry, the selection of calcium fluorescent probes and viral vector systems, and the measurement of specific expression of fluorescent proteins in specific tissues. Therefore, the correct use of suitable fiber optic recording indicators will greatly assist researchers in exploring the link between neuronal activity and neuropsychiatric disorders.

Imaging probes for the detection of brain microenvironment

The brain microenvironment (BME) is a highly dynamic system that plays a critical role in neural excitation, signal transmission, development, aging, and neurological disorders. BME consists of three key components: neural cells, extracellular spaces, and physical fields, which provide structures and physicochemical properties to synergistically and antagonistically regulate cell behaviors and functions such as nutrient transport, waste metabolism and intercellular communication. Consequently, monitoring the BME is vital to acquire a better understanding of the maintenance of neural homeostasis and the mechanisms underlying neurological diseases. In recent years, researchers have developed a range of imaging probes designed to detect changes in the microenvironment, enabling precise measurements of structural and biophysical parameters in the brain. This advancement aids in the development of improved diagnostic and therapeutic strategies for brain disorders and in the exploration of cutting-edge mechanisms in neuroscience. This review summarizes and highlights recent advances in the probes for sensing and imaging BME. Also, we discuss the design principles, types, applications, challenges, and future directions of probes.

Liquid/liquid junction microelectrodes for monitoring cholinergic transmitter in live mice brain in vivo

Acetylcholine (ACh) is an important neurotransmitter and biomarker for neurological disorders. The quantitative detection of ACh in vivo is critical but remains a challenge. In this work, we developed a novel micrometer-sized electrode based on interface between two immiscible electrolyte solutions (ITIES) to achieve in vivo measurement of ACh at high spatiotemporal resolution. The fabricated microITIES electrode was tested in vitro for ACh sensing using electrochemical methods including cyclical voltammetry and i-t amperometry in artificial cerebrospinal fluid (ACSF) solution. An increase in current was observed in both CV and i-t at -0.25 V (vs E1/2, TEA). Both CV and i-t showed a high sensitivity and a linear response with the linear range starting from as low as 0.5 μM. Then the electrode was applied for in vivo measurement of ACh in the living mouse brain. The electrode was implanted in the cortex of the mouse brain via stereotaxic surgery. The electrode was tested for exogenously applied ACh in vivo by local injection of ACh (500 nL, 0.5 M) twice. Repeated cyclic voltammograms were recorded before, during, and after both injections; the cyclic voltammograms showed a significant increase in current at ACh detection potential. The electrode was also tested for endogenously released ACh in vivo by the local injection of a high concentration KCl solution (500 nL and 1000 nL, 100 mM) to stimulate ACh release. Similarly, repeated cyclic voltammograms were recorded before, during, and after both injections; a significant increase in current at the ACh detection potential in the cyclic voltammograms was observed following each injection of KCl. These results validated the capability of the introduced microITIES electrode to measure exogenous and endogenous ACh in vivo.

Genetically encoded sensors illuminate in vivo detection for neurotransmission: Development, application, and optimization strategies

Limitations in existing tools have hindered neuroscientists from achieving a deeper understanding of complex behaviors and diseases. The recent development and optimization of genetically encoded sensors offer a powerful solution for investigating intricate dynamics such as calcium influx, membrane potential, and the release of neurotransmitters and neuromodulators. In contrast, traditional methods are constrained by insufficient spatial and/or temporal resolution, low sensitivity, and stringent application conditions. Genetically encoded sensors have gained widespread popularity due to their advantageous features, which stem from their genetic encoding and optical imaging capabilities. These include broad applicability, tissue specificity, and non-invasive operation. When combined with advanced microscopic techniques, optogenetics, and machine learning approaches, these sensors have become versatile tools for studying neuronal circuits in intact living systems, providing millisecond-scale temporal resolution and spatial resolution ranging from nanometers to micrometers. In this review, we highlight the advantages of genetically encoded sensors over traditional methods in the study of neurotransmission. We also discuss their recent advancements, diverse applications, and optimization strategies.

Whole-brain mapping of basal forebrain cholinergic neurons reveals a long-range reciprocal input-output loop between distinct subtypes

Basal forebrain cholinergic neurons (BFCNs) influence cognition and emotion through specific acetylcholine release in various brain regions, including the prefrontal cortices and basolateral amygdala (BLA). Acetylcholine release is controlled by distinct BFCN subtypes, modulated by excitatory and inhibitory inputs. However, the organization of the whole-brain input-output networks of these subtypes remains unclear. Here, we identified two distinct BFCN subtypes-BFCN→ACA and BFCN→BLA-innervating the anterior cingulate cortex (ACA) and BLA, each with unique distributions, electrophysiological properties, and projection patterns. Combining rabies-virus-assisted mapping and triple-plex RNAscope hybridization, we characterized their whole-brain input networks, identifying unique excitatory and shared inhibitory inputs for these subtypes. Moreover, our results reveal a long-range reciprocal input-output loop: BFCN→ACA neurons target the isocortex, their shared excitatory-input source, whereas BFCN→BLA neurons target shared inhibitory-input sources such as the striatum and pallidum, thus enabling dynamic interactions among these BFCN subtypes. Our study deepens understanding of cholinergic modulation in cognition and emotion and provides insights into their functional interactions.

Integrating optical neuroscience tools into touchscreen operant systems

Unlocking the neural regulation of complex behavior is a foundational goal of brain science. Touchscreen-based assessments of behavior have been used extensively in the pursuit of this goal, with traditional pharmacological and neurochemical approaches being employed to provide key insights into underlying neural systems. So far, optically based approaches to measure and manipulate neural function, which have begun to revolutionize our understanding of relatively simple behaviors, have been less widely adopted for more complex cognitive functions of the type assessed with touchscreen-based behavioral tasks. Here we provide guidance and procedural descriptions to enable researchers to integrate optically based manipulation and measurement techniques into their touchscreen experimental systems. We focus primarily on three techniques, optogenetic manipulation, fiber photometry and microendoscopic imaging, describing experimental design adjustments that we have found to be critical to the successful integration of these approaches with extant touchscreen behavior pipelines. These include factors related to surgical procedures and timing, alterations to touchscreen operant environments and approaches to synchronizing light delivery and task design. A detailed protocol is included for each of the three techniques, covering their use from implementation through data analysis. The procedures in this protocol can be conducted in as short a time as a few days or over the course of weeks or months.

Septo-subicular cholinergic circuit promotes seizure development via astrocytic inflammation

The central dogma explaining epileptic seizures largely revolves around the classic theory of "excitability-inhibition" imbalance between glutamatergic and GABAergic transmission. Cholinergic neurons play a significant role in epilepsy; however, these neuronal populations are molecularly and structurally heterogeneous. Here, we show a subpopulation of subiculum-projecting septal cholinergic neurons that promote seizure development. Functionally, this subpopulation is suppressed during seizures. Selective manipulation of the septo-subicular cholinergic circuit bidirectionally regulates the development of hippocampal seizures. Notably, cholinergic signaling enhances subicular astrocytic caspase-1-mediated neuroinflammation via M3 muscarinic receptors, increasing excitatory synaptic transmission and promoting seizure development. Together, these results demonstrate that activation of the septo-subicular cholinergic circuits facilitates seizure development via astrocytic inflammation. Our findings provide insight into the cholinergic mechanism involved in epilepsy and suggest targeted therapeutic strategies for epilepsy treatment, focusing on the specific cholinergic neuronal subpopulation.

Neurotransmitter modulation of sleep-wake States: From molecular mechanisms to therapeutic potential

Sleep is one of the most fundamental physiological activities in humans and animals, and a normal sleep cycle is crucial for maintaining overall health. However, sleep disorders are increasingly becoming a major mental health issue affecting individuals and society, as well as a contributing factor to the onset of other diseases. Consequently, the development of novel therapeutic strategies for sleep disorders has emerged as a significant scientific challenge garnering widespread attention. Based on current research findings, focusing on neurotransmitters remains a promising approach for developing effective treatments. Neurotransmitters play a central role in regulating the sleep-wake cycle by precisely modulating the activity states of different brain regions. This review aims to elucidate the neural mechanisms underlying sleep initiation and function, thereby providing a comprehensive understanding of the complex nature of sleep as a physiological process. Furthermore, it seeks to uncover the potential pathological mechanisms of sleep disorders, offering a theoretical foundation and novel insights for precision medicine and drug development, ultimately reducing the negative impact of sleep disorders on individuals and society.

Cellular Activity Modulation Mediated by Near Infrared-Irradiated Polydopamine Nanoparticles: In Vitro and Ex Vivo Investigation

The precise control of cell activity is crucial for understanding and potentially treating many disorders. Focusing on neurons and myotubes, recent advancements in nanotechnology have introduced photoresponsive nanoparticles as an alternative tool for modulating cell function with high spatial and temporal resolution. This approach offers a noninvasive alternative to traditional stimulation techniques, reducing potential tissue damage and improving the specificity of cell activation. Here, we introduce an approach envisioning fully organic polydopamine nanoparticles (PDNPs) to remotely modulate the activity of differentiated SH-SY5Y cells and differentiated C2C12 cells, via near-infrared (NIR) laser stimulation. Confocal microscopy imaging revealed the possibility of thermally activating individual neuron-like cells, eliciting a significant cellular response characterized by the generation of calcium transients and the subsequent release of the neurotransmitter acetylcholine. Similarly, we demonstrated the possibility of precisely triggering the muscle contraction of single myotubes. Additionally, we investigated the antioxidant properties of PDNPs, demonstrating their capacity to prevent an increase in oxidative stress levels related to an increase in intracellular temperature. Moreover, proteomic analysis revealed that a PDNP treatment could positively affect neuronal plasticity and nervous system maturation, besides promoting muscle growth and preserving its functional integrity, underscoring its potential to support both neural and musculoskeletal development. Eventually, the effect of the NIR laser irradiation in the presence of PDNPs in neuron-like cells was successfully evaluated ex vivo on brains of Drosophila melanogaster, genetically modified to express the fluorescent calcium indicator jGCaMP7c.

Brain-wide presynaptic networks of functionally distinct cortical neurons

Revealing the connectivity of functionally identified individual neurons is necessary to understand how activity patterns emerge and support behaviour. Yet the brain-wide presynaptic wiring rules that lay the foundation for the functional selectivity of individual neurons remain largely unexplored. Cortical neurons, even in primary sensory cortex, are heterogeneous in their selectivity, not only to sensory stimuli but also to multiple aspects of behaviour. Here, to investigate presynaptic connectivity rules underlying the selectivity of pyramidal neurons to behavioural state1-10 in primary somatosensory cortex (S1), we used two-photon calcium imaging, neuropharmacology, single-cell-based monosynaptic input tracing and optogenetics. We show that behavioural state-dependent activity patterns are stable over time. These are minimally affected by direct neuromodulatory inputs and are driven primarily by glutamatergic inputs. Analysis of brain-wide presynaptic networks of individual neurons with distinct behavioural state-dependent activity profiles revealed that although behavioural state-related and behavioural state-unrelated neurons shared a similar pattern of local inputs within S1, their long-range glutamatergic inputs differed. Individual cortical neurons, irrespective of their functional properties, received converging inputs from the main S1-projecting areas. Yet neurons that tracked behavioural state received a smaller proportion of motor cortical inputs and a larger proportion of thalamic inputs. Optogenetic suppression of thalamic inputs reduced behavioural state-dependent activity in S1, but this activity was not externally driven. Our results reveal distinct long-range glutamatergic inputs as a substrate for preconfigured network dynamics associated with behavioural state.

Dual neuromodulatory dynamics underlie birdsong learning

Although learning in response to extrinsic reinforcement is theorized to be driven by dopamine signals that encode the difference between expected and experienced rewards1,2, skills that enable verbal or musical expression can be learned without extrinsic reinforcement. Instead, spontaneous execution of these skills is thought to be intrinsically reinforcing3,4. Whether dopamine signals similarly guide learning of these intrinsically reinforced behaviours is unknown. In juvenile zebra finches learning from an adult tutor, dopamine signalling in a song-specialized basal ganglia region is required for successful song copying, a spontaneous, intrinsically reinforced process5. Here we show that dopamine dynamics in the song basal ganglia faithfully track the learned quality of juvenile song performance on a rendition-by-rendition basis. Furthermore, dopamine release in the basal ganglia is driven not only by inputs from midbrain dopamine neurons classically associated with reinforcement learning but also by song premotor inputs, which act by means of local cholinergic signalling to elevate dopamine during singing. Although both cholinergic and dopaminergic signalling are necessary for juvenile song learning, only dopamine tracks the learned quality of song performance. Therefore, dopamine dynamics in the basal ganglia encode performance quality during self-directed, long-term learning of natural behaviours.

Protein Engineering for Spatiotemporally Resolved Cellular Monitoring

Protein engineering has been extensively applied to the development of genetically encoded reporters for spatiotemporally resolved monitoring of dynamic biochemical activity across cellular compartments in living cells. Genetically encoded reporters facilitate the visualization and recording of cellular processes, including transmission of signaling molecules, protease activity, and protein-protein interactions. In this review, we describe and assess common reporter motifs and protein engineering strategies for designing genetically encoded reporters. We also discuss essential parameters for evaluating genetically encoded reporters, along with future protein engineering opportunities in this field.

Binding mechanism and antagonism of the vesicular acetylcholine transporter VAChT

The vesicular acetylcholine transporter (VAChT) has a pivotal role in packaging and transporting acetylcholine for exocytotic release, serving as a vital component of cholinergic neurotransmission. Dysregulation of its function can result in neurological disorders. It also serves as a target for developing radiotracers to quantify cholinergic neuron deficits in neurodegenerative conditions. Here we unveil the cryo-electron microscopy structures of human VAChT in its apo state, the substrate acetylcholine-bound state and the inhibitor vesamicol-bound state. These structures assume a lumen-facing conformation, offering a clear depiction of architecture of VAChT. The acetylcholine-bound structure provides a detailed understanding of how VAChT recognizes its substrate, shedding light on the coupling mechanism of protonation and substrate binding. Meanwhile, the vesamicol-bound structure reveals the binding mode of vesamicol to VAChT, laying the structural foundation for the design of the next generation of radioligands targeting VAChT.

Acetylcholine in the hippocampus: problems and achievements

Cholinergic septohippocampal projections originating from the medial septal area (MSA) play a critical role in regulating attention, memory formation, stress responses, and synaptic plasticity. Cholinergic axons from the MSA extensively innervate all hippocampal regions, providing a structural basis for the simultaneous release of acetylcholine (ACh) across the entire hippocampus. However, this widespread release appears inconsistent with the specific functional roles that ACh is thought to serve during distinct behaviors. A key unresolved question is how the dynamics of ACh tissue concentrations determine its ability to activate different receptor types and coordinate individual synaptic pathways. Here, we highlight several debated issues, including the potential intrinsic source of ACh within the hippocampus - such as cholinergic interneurons - and the co-release of ACh with GABA. Furthermore, we discuss recent findings on in vivo ACh concentration dynamics, which present a new dilemma for understanding ACh signaling in the hippocampus: the contrast between "global" ACh release, driven by synchronous activation of MSA neurons, and "local" release, which may be influenced by yet unidentified factors.

Accumbal Dopamine and Acetylcholine Dynamics during Psychostimulant Sensitization

Behavioral sensitization to repeated psychostimulant exposure is believed to contribute to the development of addiction. Nucleus accumbens (NAcc) dopamine (DA) is known to be a key substrate in sensitization, though recent work suggests that striatal acetylcholine (ACh) may also play a critical role. However, underlying ACh changes and their relationship to DA signaling have not been characterized. Here, we used dual-color fiber photometry to simultaneously measure DA and ACh in the NAcc shell of mice across repeated injections of cocaine or amphetamine. Repeated exposure progressively elevated locomotor activity and increased slow extracellular DA while attenuating transient DA release. Psychostimulants reduced phasic ACh transient amplitude and frequency, an effect that sensitized with repeated injections. However, the temporal coupling of DA and ACh remained unchanged. To determine whether D2 receptors (D2Rs) on cholinergic interneurons (CINs) drive this effect, we generated CIN-selective D2R knockout (KO) mice. Surprisingly, KOs continued to show an acute decrease in ACh and intact DA-ACh correlations after psychostimulant administration. However, they failed to exhibit sensitization of either DA or ACh in response to repeated psychostimulant administration. Despite this lack of sensitization in underlying neuromodulator signaling, the KO mice nevertheless exhibited behavioral sensitization, though at a slower rate than wild-type. These findings suggest that neural sensitization to psychostimulants is dependent on D2R expressed on CINs, but that behavioral sensitization is not dependent on sensitization of these underlying signals.

Sequential transitions of male sexual behaviors driven by dual acetylcholine-dopamine dynamics

The neural mechanisms underlying the sequential transitions of male sexual behaviors, including mounting, intromission, and ejaculation, remain largely unexplored. Here, we report that acetylcholine (ACh)-dopamine (DA) dynamics in the ventral shell of the nucleus accumbens (vsNAc) regulate these sexual transitions in male mice. During intromission, the vsNAc displays a unique pattern of dual ACh-DA rhythms, generated by reciprocal regulation between ACh and DA signaling via nicotinic ACh receptors (nAChRs) and DA D2 receptors (D2Rs). Knockdown of choline acetyltransferase (ChAT) or D2R in the vsNAc diminishes the occurrence of intromission and ejaculation. Optogenetic manipulations demonstrated that DA signaling maintains sexual behaviors by suppressing D2RvsNAc neurons. Moreover, ACh signaling promotes the initiation of mounting and intromission and facilitates the intromission-ejaculation transition by inducing a slowdown in DA rhythm. Together, these findings reveal that coordinated ACh-DA dynamics in the vsNAc play a critical role in orchestrating the sequential transitions of male sexual behaviors.

Functional dissection of a neuronal brain circuit mediating higher-order associative learning

A central feature characterizing the neural architecture of many species' brains is their capacity to form associative chains through learning. In elementary forms of associative learning, stimuli coinciding with reward or punishment become attractive or repulsive. Notably, stimuli previously learned as attractive or repulsive can themselves serve as reinforcers, establishing a cascading effect whereby they become associated with additional stimuli. When this iterative process is perpetuated, it results in higher-order associations. Here, we use odor conditioning in Drosophila and computational modeling to dissect the architecture of neuronal networks underlying higher-order associative learning. We show that the responsible circuit, situated in the mushroom bodies of the brain, is characterized by parallel processing of odor information and by recurrent excitatory and inhibitory feedback loops that empower odors to gain control over the dopaminergic valence-signaling system. Our findings establish a paradigmatic framework of a neuronal circuit diagram enabling the acquisition of associative chains.

A time window for memory consolidation during NREM sleep revealed by cAMP oscillation

Memory formation requires specific neural activity in coordination with intracellular signaling mediated by second messengers such as cyclic adenosine monophosphate (cAMP). However, the real-time dynamics of cAMP remain largely unknown. Here, using a genetically encoded cAMP sensor with high temporal resolution, we found neural-activity-dependent rapid cAMP elevation during learning. Interestingly, in slow-wave sleep, during which memory consolidation occurs, the cAMP level in mice was anti-correlated with neural activity and exhibited norepinephrine β1 receptor-dependent infra-slow oscillations that were synchronized across the hippocampus and cortex. Furthermore, the hippocampal-cortical interactions increased during the narrow time-window of the peak cAMP level; suppressing hippocampal activity specifically during this window impaired spatial memory consolidation. Thus, hippocampal-dependent memory consolidation occurs within a specific time window of high cAMP activity during slow-wave sleep.

Dynorphin modulates reward-seeking actions through a pallido-amygdala cholinergic circuit

The endogenous opioid peptide dynorphin and its receptor κ-opioid receptor (KOR) have been implicated in divergent behaviors, but the underlying mechanisms remain elusive. Here, we show that dynorphin released from nucleus accumbens dynorphinergic neurons exerts powerful modulation over a ventral pallidum (VP) disinhibitory circuit, thereby controlling cholinergic transmission to the amygdala and reward-seeking behavior in mice. On one hand, dynorphin acts postsynaptically via KORs on VP GABAergic neurons to promote disinhibition of cholinergic neurons, which release acetylcholine into the amygdala to facilitate learning and invigorate actions. On the other hand, dynorphin also acts presynaptically via KORs on dynorphinergic terminals to limit its own release. Such autoinhibition keeps cholinergic neurons from prolonged activation and release of acetylcholine and prevents perseverant reward seeking. Our study reveals how dynorphin exquisitely modulates behavior through the cholinergic system and provides an explanation for why these neuromodulators are involved in motivational disorders, including depression and addiction.

Serotonin modulates infraslow oscillation in the dentate gyrus during non-REM sleep

Synchronous neuronal activity is organized into neuronal oscillations with various frequency and time domains across different brain areas and brain states. For example, hippocampal theta, gamma, and sharp wave oscillations are critical for memory formation and communication between hippocampal subareas and the cortex. In this study, we investigated the neuronal activity of the dentate gyrus (DG) with optical imaging tools during sleep-wake cycles in mice. We found that the activity of major glutamatergic cell populations in the DG is organized into infraslow oscillations (0.01-0.03 Hz) during NREM sleep. Although the DG is considered a sparsely active network during wakefulness, we found that 50% of granule cells and about 25% of mossy cells exhibit increased activity during NREM sleep, compared to that during wakefulness. Further experiments revealed that the infraslow oscillation in the DG was correlated with rhythmic serotonin release during sleep, which oscillates at the same frequency but in an opposite phase. Genetic manipulation of 5-HT receptors revealed that this neuromodulatory regulation is mediated by Htr1a receptors and the knockdown of these receptors leads to memory impairment. Together, our results provide novel mechanistic insights into how the 5-HT system can influence hippocampal activity patterns during sleep.

Frontal noradrenergic and cholinergic transients exhibit distinct spatiotemporal dynamics during competitive decision-making

Norepinephrine (NE) and acetylcholine (ACh) are crucial for learning and decision-making. In the cortex, NE and ACh are released transiently at specific sites along neuromodulatory axons, but how the spatiotemporal patterns of NE and ACh signaling link to behavioral events is unknown. Here, we use two-photon microscopy to visualize neuromodulatory signals in the premotor cortex (medial M2) as mice engage in a competitive matching pennies game. Spatially, NE signals are more segregated with choice and outcome encoded at distinct locations, whereas ACh signals can multiplex and reflect different behavioral correlates at the same site. Temporally, task-driven NE transients were more synchronized and peaked earlier than ACh transients. To test functional relevance, we stimulated neuromodulatory signals using optogenetics to find that NE, but not ACh, increases the animals' propensity to explore alternate options. Together, the results reveal distinct subcellular spatiotemporal patterns of ACh and NE transients during decision-making in mice.

Huperzine A attenuates epileptic seizures via enhancing dCA1-projecting septal cholinergic transmission

Cholinergic transmission, independent of classical glutamatergic and GABAergic signaling, critically plays a crucial role in epilepsy. Huperzine A (Hup A), an acetylcholinesterase (AChE) inhibitor, exerts potent anticonvulsant activity, but its mechanism of action within cholinergic circuits remains unclear. Here, we show that Hup A mitigates epileptic seizures by enhancing hippocampal dorsal CA1 (dCA1)-projecting cholinergic transmission. We found that systemic injection of Hup A not only reduces seizures in acute models, including the maximal-electroshock seizure (MES), pentylenetetrazol (PTZ), and kainic acid (KA) models but also alleviates the seizure severity in chronic epilepsy models induced by kindling and KA, indicating a broad-spectrum anti-seizure efficacy. Interestingly, using immunohistochemistry, viral tracing, and in vivo fiber photometry, we found that Hup A selectively inhibits AChE in the dCA1 rather than in other hippocampal subregions or cortex, enhancing dCA1-projecting septal cholinergic transmission. Significantly, selective ablation of septal ChAT+ neurons reversed the anti-seizure effects of Hup A. We further identified that α7 nicotinic acetylcholine receptors in the dCA1 region mediate the anti-seizures cholinergic circuit modulated by Hup A. Together, our results demonstrate that Hup A exerts broad-spectrum anti-seizure efficacy via modulating dCA1-projecting septal cholinergic transmission, providing potential therapeutic avenues for epilepsy through targeted cholinergic modulation.

Principles and Design of Molecular Tools for Sensing and Perturbing Cell Surface Receptor Activity

Cell-surface receptors are vital for controlling numerous cellular processes with their dysregulation being linked to disease states. Therefore, it is necessary to develop tools to study receptors and the signaling pathways they control. This Review broadly describes molecular approaches that enable 1) the visualization of receptors to determine their localization and distribution; 2) sensing receptor activation with permanent readouts as well as readouts in real time; and 3) perturbing receptor activity and mimicking receptor-controlled processes to learn more about these processes. Together, these tools have provided valuable insight into fundamental receptor biology and helped to characterize therapeutics that target receptors.

Molecular and cellular neurocardiology in heart disease

This paper updates and builds on a previous White Paper in this journal that some of us contributed to concerning the molecular and cellular basis of cardiac neurobiology of heart disease. Here we focus on recent findings that underpin cardiac autonomic development, novel intracellular pathways and neuroplasticity. Throughout we highlight unanswered questions and areas of controversy. Whilst some neurochemical pathways are already demonstrating prognostic viability in patients with heart failure, we also discuss the opportunity to better understand sympathetic impairment by using patient specific stem cells that provides pathophysiological contextualization to study 'disease in a dish'. Novel imaging techniques and spatial transcriptomics are also facilitating a road map for target discovery of molecular pathways that may form a therapeutic opportunity to treat cardiac dysautonomia.

Cortical acetylcholine response to deep brain stimulation of the basal forebrain in mice

Deep brain stimulation (DBS) using electrical stimulation of neuronal tissue in the basal forebrain to enhance release of the neurotransmitter acetylcholine is under consideration to improve executive function in patients with dementia. Although some small studies indicate a positive response in the clinical setting, the relationship between DBS and acetylcholine pharmacokinetics is incompletely understood. We examined the cortical acetylcholine response to different stimulation parameters of the basal forebrain. Two-photon in vivo imaging was combined with deep brain stimulation in C57BL/6J mice. Stimulating electrodes were implanted in the subpallidal basal forebrain, and the ipsilateral somatosensory cortex was imaged. Acetylcholine activity was determined using the GRABACh-3.0 acetylcholine receptor sensor, and blood vessels were visualized with Texas red. Experiments manipulating stimulation frequency demonstrated that integrated acetylcholine-induced fluorescence was insensitive to frequency with the same number of pulses, and that maximum peak levels were achieved with frequencies from 60 to 130 Hz. Altering pulse train length indicated that longer stimulation resulted in higher peaks and more activation with sublinear summation. The acetylcholinesterase inhibitor, donepezil, increased the peak response to 600 pulses of stimulation at 60 Hz, and the integrated response increased by 57% with the 2 mg/kg dose and 126% with the 4 mg/kg dose. Acetylcholine levels returned to baseline with a time constant of 14-18 s. Donepezil increases total acetylcholine receptor activation associated with DBS but does not change temporal kinetics. The long time constants observed in the cerebral cortex add to the evidence supporting volume and synaptic neurotransmission.NEW & NOTEWORTHY Peak acetylcholine responses to deep brain stimulation of the subpallidal basal forebrain increases with increased frequency and number of pulses. Long recovery periods in the 10s of seconds support "volume" versus "phasic" transmission of acetylcholine. Donepezil administration enhances the effect of stimulation on cortical acetylcholine release.

Live MSCs Characterizer Displays Stemness and Differentiation Using Colorful LV-cp Biosensors

Mesenchymal stem cells (MSCs) have garnered significant attention in biomedical research due to their accessibility and remarkable differentiation potential. However, the lack of efficient and convenient living cell monitoring methods limits their widespread application in tissue engineering and stem cell therapy. Therefore, we present progress in the development of a novel series of fluorescent protein (FP) sensors based on turn-on fluorescent protein biosensors (Turn-on FPBs), termed the LV-cp biosensor system (novel live cell permuted fluorescent protein biosensors). Utilizing phage display technology to screen for affinity peptides specifically targeting MSCs and chondrocytes, the LV-cp were engineered by subcloning these peptides into permuted fluorescent proteins, thereby integrating the fluorescence activation mechanism with the affinity peptides and achieving highly accurate detection and identification of these two cell types using living cells as "fluorescence keys." This system provides a simplified, nontoxic method to replace traditional antibody kits, and strong fluorescence signals can be obtained through various fluorescence detection devices. In addition, the LV-cp biosensors enabled dynamic observation of MSCs differentiation into chondrocytes through changes in the cell fluorescence colors.

Norepinephrine-mediated slow vasomotion drives glymphatic clearance during sleep

As the brain transitions from wakefulness to sleep, processing of external information diminishes while restorative processes, such as glymphatic removal of waste products, are activated. Yet, it is not known what drives brain clearance during sleep. We here employed an array of technologies and identified tightly synchronized oscillations in norepinephrine, cerebral blood volume, and cerebrospinal fluid (CSF) as the strongest predictors of glymphatic clearance during NREM sleep. Optogenetic stimulation of the locus coeruleus induced anti-correlated changes in vasomotion and CSF signal. Furthermore, stimulation of arterial oscillations enhanced CSF inflow, demonstrating that vasomotion acts as a pump driving CSF into the brain. On the contrary, the sleep aid zolpidem suppressed norepinephrine oscillations and glymphatic flow, highlighting the critical role of norepinephrine-driven vascular dynamics in brain clearance. Thus, the micro-architectural organization of NREM sleep, driven by norepinephrine fluctuations and vascular dynamics, is a key determinant for glymphatic clearance.

Synaptic modulation of glutamate in striatum of the YAC128 mouse model of Huntington disease

BACKGROUND

Altered balance between striatal direct and indirect pathways contributes to early motor, cognitive and psychiatric symptoms in Huntington disease (HD). While degeneration of striatal D2-type dopamine receptor (D2)-expressing indirect pathway medium spiny neurons (iMSNs) occurs prior to that of D1-type dopamine receptor (D1)-expressing direct pathway neurons, altered corticostriatal synaptic function precedes degeneration. D2-mediated signaling on iMSNs reduces their excitability and promotes endocannabinoid (eCB) synthesis, suppressing glutamate release from cortical afferents. D2 receptors are also expressed on glutamatergic cortical terminals, cholinergic interneurons, and dopaminergic terminals from substantia nigra where they suppress release of glutamate, acetylcholine and dopamine, respectively, and these cell types may contribute to early striatal dysfunction in HD. Thus, we used corticostriatal brain slices and optogenetic probes to directly investigate neuromodulatory signaling in the transgenic YAC128 HD mouse model.

RESULTS

Low-dose D2 agonist quinpirole reduced cortically-evoked glutamate release in dorsal striatum of premanifest YAC128 slices but not WT, and blocking type 1 cannabinoid receptors mitigated this effect. YAC128 corticostriatal brain slices also showed increased evoked dopamine and reduced evoked eCB release compared to WT, while acetylcholine signaling patterns remained relatively intact.

CONCLUSIONS

These findings suggest that YAC128 corticostriatal slices show increased D2 sensitivity that is eCB-dependent, and that dopamine and eCB release are altered at an early disease stage. We provide evidence for impaired neuromodulatory signaling in early HD, guiding therapeutic efforts prior to the onset of overt motor symptoms later on.

Simultaneous, real-time tracking of many neuromodulatory signals with Multiplexed Optical Recording of Sensors on a micro-Endoscope

Dozens of extracellular molecules jointly impact a given neuron, yet we lack methods to simultaneously record many such signals in real time. We developed a probe to track ten or more neuropeptides and neuromodulators using spatial multiplexing of genetically encoded fluorescent sensors. Cultured cells expressing one sensor at a time are immobilized at the front of a gradient refractive index (GRIN) lens for 3D two-photon imaging in vitro and in vivo . The sensor identity and detection sensitivity of each cell are determined via robotic dipping of the probe into wells containing various ligands and concentrations. Using this probe, we detected stimulation-evoked release of multiple neuromodulators in acute brain slices. We also tracked endogenous and drug-evoked changes in cerebrospinal fluid composition in the awake mouse lateral ventricle, which triggered downstream activation of the choroid plexus epithelium. Our approach offers a first step towards quantitative, real-time, high-dimensional tracking of brain fluid composition.

Distinct spatially organized striatum-wide acetylcholine dynamics for the learning and extinction of Pavlovian associations

Striatal acetylcholine (ACh) signaling has been proposed to counteract reinforcement signals to promote extinction and behavioral flexibility. ACh dips to cues and rewards may open a temporal window for associative plasticity to occur, while elevations may promote extinction. Changes in multi-phasic striatal ACh signals have been widely reported during learning, but how and where signals are distributed to enable region-specific plasticity for the learning and degradation of cue-reward associations is poorly understood. We used array fiber photometry in mice to investigate how ACh release across the striatum evolves during learning and extinction of Pavlovian associations. We report a topographic organization of opposing changes in ACh release to cues, rewards, and consummatory actions across distinct striatum regions. We localized reward prediction error encoding in particular phases of the ACh dynamics to a specific region of the anterior dorsal striatum (aDS). Positive prediction errors in the aDS were expressed in ACh dips, and negative prediction errors in long latency ACh elevations. Silencing aDS ACh release impaired behavioral extinction, suggesting a role for ACh elevations in down-regulating cue-reward associations. Dopamine release in aDS dipped for cues during extinction, but glutamate input onto cholinergic interneurons did not change, suggesting an intrastriatal mechanism for the emergence of ACh elevations. Our large scale measurements indicate how and where ACh dynamics can shape region-specific plasticity to gate learning and promote extinction of Pavlovian associations.

A high-performance GRAB sensor reveals differences in the dynamics and molecular regulation between neuropeptide and neurotransmitter release

The co-existence and co-transmission of neuropeptides and small molecule neurotransmitters within individual neuron represent a fundamental characteristic observed across various species. However, the differences regarding their in vivo spatiotemporal dynamics and underlying molecular regulation remain poorly understood. Here, we develop a GPCR-activation-based (GRAB) sensor for detecting short neuropeptide F (sNPF) with high sensitivity and spatiotemporal resolution. Furthermore, we investigate the in vivo dynamics and molecular regulation differences between sNPF and acetylcholine (ACh) from the same neurons. Interestingly, our findings reveal distinct spatiotemporal dynamics in the release of sNPF and ACh. Notably, our results indicate that distinct synaptotagmins (Syt) are involved in these two processes, as Syt7 and Sytα for sNPF release, while Syt1 for ACh release. Thus, this high-performance GRAB sensor provides a robust tool for studying neuropeptide release and shedding insights into the unique release dynamics and molecular regulation that distinguish neuropeptides from small molecule neurotransmitters.

Serotonin modulates infraslow oscillation in the dentate gyrus during Non-REM sleep

Synchronous neuronal activity is organized into neuronal oscillations with various frequency and time domains across different brain areas and brain states. For example, hippocampal theta, gamma and sharp wave oscillations are critical for memory formation and communication between hippocampal subareas and the cortex. In this study, we investigated the neuronal activity of the dentate gyrus (DG) with optical imaging tools during sleep-wake cycles. We found that the activity of major glutamatergic cell populations in the DG is organized into infraslow oscillations (0.01 - 0.03 Hz) during NREM sleep. Although the DG is considered a sparsely active network during wakefulness, we found that 50% of granule cells and about 25% of mossy cells exhibit increased activity during NREM sleep, compared to that during wakefulness. Further experiments revealed that the infraslow oscillation in the DG was correlated with rhythmic serotonin release during sleep, which oscillates at the same frequency but in an opposite phase. Genetic manipulation of 5-HT receptors revealed that this neuromodulatory regulation is mediated by 5-HT1a receptors and the knockdown of these receptors leads to memory impairment. Together, our results provide novel mechanistic insights into how the 5-HT system can influence hippocampal activity patterns during sleep.

Absolute measurement of fast and slow neuronal signals with fluorescence lifetime photometry at high temporal resolution

The concentrations of extracellular and intracellular signaling molecules, such as dopamine and cAMP, change over both fast and slow timescales and impact downstream pathways in a cell-type specific manner. Fluorescence sensors currently used to monitor such signals in vivo are typically optimized to detect fast, relative changes in concentration of the target molecule. They are less well suited to detect slowly-changing signals and rarely provide absolute measurements of either fast and slow signaling components. Here, we developed a system for fluorescence lifetime photometry at high temporal resolution (FLIPR) that utilizes frequency-domain analog processing to measure the absolute fluorescence lifetime of genetically-encoded sensors at high speed but with long-term stability and picosecond precision in freely moving mice. We applied FLIPR to investigate dopamine signaling in two functionally distinct regions in the striatum, the nucleus accumbens core (NAC) and the tail of striatum (TOS). We observed higher tonic dopamine levels at baseline in the TOS compared to the NAC and detected differential and dynamic responses in phasic and tonic dopamine to appetitive and aversive stimuli. Thus, FLIPR enables simple monitoring of fast and slow time-scale neuronal signaling in absolute units, revealing previously unappreciated spatial and temporal variation even in well-studied signaling systems.

State-dependent modulation of spiny projection neurons controls levodopa-induced dyskinesia in a mouse model of Parkinson's disease

In the later stages of Parkinson's disease (PD), patients often manifest levodopa-induced dyskinesia (LID), compromising their quality of life. The pathophysiology underlying LID is poorly understood, and treatment options are limited. To move toward filling this treatment gap, the intrinsic and synaptic changes in striatal spiny projection neurons (SPNs) triggered by the sustained elevation of dopamine (DA) during dyskinesia were characterized using electrophysiological, pharmacological, molecular and behavioral approaches. Our studies revealed that the intrinsic excitability and functional corticostriatal connectivity of SPNs in dyskinetic mice oscillate between the on- and off-states of LID in a cell- and state-specific manner. Although triggered by levodopa, these rapid oscillations in SPN properties depended on both dopaminergic and cholinergic signaling. In a mouse PD model, disrupting M1 muscarinic receptor signaling specifically in iSPNs or deleting its downstream signaling partner CalDAG-GEFI blunted the levodopa-induced oscillation in functional connectivity, enhanced the beneficial effects of levodopa and attenuated LID severity.

Synthetic GPCRs for programmable sensing and control of cell behaviour

Synthetic receptors that mediate antigen-dependent cell responses are transforming therapeutics, drug discovery and basic research1,2. However, established technologies such as chimeric antigen receptors3 can only detect immobilized antigens, have limited output scope and lack built-in drug control3-7. Here we engineer synthetic G-protein-coupled receptors (GPCRs) that are capable of driving a wide range of native or non-native cellular processes in response to a user-defined antigen. We achieve modular antigen gating by engineering and fusing a conditional auto-inhibitory domain onto GPCR scaffolds. Antigen binding to a fused nanobody relieves auto-inhibition and enables receptor activation by drug, thus generating programmable antigen-gated G-protein-coupled engineered receptors (PAGERs). We create PAGERs that are responsive to more than a dozen biologically and therapeutically important soluble and cell-surface antigens in a single step from corresponding nanobody binders. Different PAGER scaffolds allow antigen binding to drive transgene expression, real-time fluorescence or endogenous G-protein activation, enabling control of diverse cellular functions. We demonstrate multiple applications of PAGER, including induction of T cell migration along a soluble antigen gradient, control of macrophage differentiation, secretion of therapeutic antibodies and inhibition of neuronal activity in mouse brain slices. Owing to its modular design and generalizability, we expect PAGERs to have broad utility in discovery and translational science.

Live imaging of paracrine signaling: Advances in visualization and tracking techniques

Live imaging techniques have revolutionized our understanding of paracrine signaling, a crucial form of cell-to-cell communication in biological processes. This review examines recent advances in visualizing and tracking paracrine factors through four key stages: secretion from producing cells, diffusion through extracellular space, binding to target cells, and activation of intracellular signaling within target cells. Paracrine factor secretion can be directly visualized by fluorescent protein tagging to ligand, or indirectly by visualizing the cleavage of the transmembrane pro-ligands or plasma membrane fusion of endosomes comprising the paracrine factors. Diffusion of paracrine factors has been studied using techniques such as fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP), fluorescence decay after photoactivation (FDAP), and single-molecule tracking. Binding of paracrine factors to target cells has been visualized through various biosensors, including GPCR-activation-based (GRAB) sensors and Förster resonance energy transfer (FRET) probes for receptor tyrosine kinases. Finally, activation of intracellular signaling is monitored within the target cells by biosensors for second messengers, transcription factors, and so on. In addition to the imaging tools, the review also highlights emerging optogenetic and chemogenetic tools for triggering the release of paracrine factors, which is essential for associating the paracrine factor secretion to biological outcomes during the bioimaging of paracrine factor signaling.Key words: paracrine signaling, live imaging, biosensors, optogenetics, chemogenetics.

Slow and fast cortical cholinergic arousal is reduced in a mouse model of focal seizures with impaired consciousness

Patients with focal temporal lobe seizures often experience loss of consciousness associated with cortical slow waves, like those in deep sleep. Previous work in rat models suggests that decreased subcortical arousal causes depressed cortical function during focal seizures. However, these studies were performed under light anesthesia, making it impossible to correlate conscious behavior with physiology. We show in an awake mouse model that electrically induced focal seizures in the hippocampus cause impaired behavioral responses to auditory stimuli, cortical slow waves, and reduced mean cortical high-frequency activity. Behavioral responses are related to cortical cholinergic release at two different timescales. Slow state-related decreases in acetylcholine correlate with overall impaired behavior during seizures. Fast phasic acetylcholine release is related to variable spared or impaired behavioral responses with each auditory stimulus. These findings establish a strong relationship between decreased cortical arousal and impaired consciousness in focal seizures, which may help guide future treatment.

Red-shifted GRAB acetylcholine sensors for multiplex imaging in vivo

The neurotransmitter acetylcholine (ACh) is essential in both the central and peripheral nervous systems. Recent studies highlight the significance of interactions between ACh and various neuromodulators in regulating complex behaviors. The ability to simultaneously image ACh and other neuromodulators can provide valuable information regarding the mechanisms underlying these behaviors. Here, we developed a series of red fluorescent G protein-coupled receptor activation-based (GRAB) ACh sensors, with a wide detection range and expanded spectral profile. The high-affinity sensor, rACh1h, reliably detects ACh release in various brain regions, including the nucleus accumbens, amygdala, hippocampus, and cortex. Moreover, rACh1h can be co-expressed with green fluorescent sensors in order to record ACh release together with other neurochemicals in various behavioral contexts using fiber photometry and two-photon imaging, with high spatiotemporal resolution. These new ACh sensors can therefore provide valuable new insights regarding the functional role of the cholinergic system under both physiological and pathological conditions.

Dynamic responses of striatal cholinergic interneurons control behavioral flexibility

Striatal cholinergic interneurons (CINs) are key to regulating behavioral flexibility, involving both extinguishing learned actions and adopting new ones. However, the mechanisms driving these processes remain elusive. In this study, we initially demonstrate that chronic alcohol consumption disrupts the burst-pause dynamics of CINs and impairs behavioral flexibility. We next aimed to elucidate the mechanisms by which CIN dynamics control behavioral flexibility. We found that extinction learning enhances acetylcholine (ACh) release and that mimicking this enhancement through optogenetic induction of CIN burst firing accelerates the extinction process. In addition, we demonstrate that disrupting CIN pauses via continuous optogenetic stimulation reversibly impairs the updating of goal-directed behaviors. Overall, we demonstrate that CIN burst firing, which increases ACh release, promotes extinction learning, aiding the extinguishment of learned behaviors. Conversely, CIN firing pauses, which lead to ACh dips, are crucial for reversal learning, facilitating the adaptation of new actions. These findings shed light on how CIN dynamics regulate behavioral flexibility.

Adaptation to visual sparsity enhances responses to isolated stimuli

Sensory systems adapt their response properties to the statistics of their inputs. For instance, visual systems adapt to low-order statistics like mean and variance to encode stimuli efficiently or to facilitate specific downstream computations. However, it remains unclear how other statistical features affect sensory adaptation. Here, we explore how Drosophila's visual motion circuits adapt to stimulus sparsity, a measure of the signal's intermittency not captured by low-order statistics alone. Early visual neurons in both ON and OFF pathways alter their responses dramatically with stimulus sparsity, responding positively to both light and dark sparse stimuli but linearly to dense stimuli. These changes extend to downstream ON and OFF direction-selective neurons, which are activated by sparse stimuli of both polarities but respond with opposite signs to light and dark regions of dense stimuli. Thus, sparse stimuli activate both ON and OFF pathways, recruiting a larger fraction of the circuit and potentially enhancing the salience of isolated stimuli. Overall, our results reveal visual response properties that increase the fraction of the circuit responding to sparse, isolated stimuli.

Enhancing striatal acetylcholine facilitates dopamine release and striatal output in parkinsonian mice

BACKGROUND

L-DOPA has been considered the first-line therapy for treating Parkinson's disease (PD) via restoring striatal dopamine (DA) to normalize the activity of local spiny projection neurons (SPNs) in the direct (dSPNs) pathway and the indirect (iSPNs) pathway. While the changes in striatal acetylcholine (ACh) induced by increasing DA have been extensively discussed, their validity remains controversial. Inhibition of striatal cholinergic signaling attenuates PD motor deficits. Interestingly, enhancing striatal ACh triggers local DA release, suggesting the pro-kinetic effects of ACh in movement control. Here, we investigated the in-vivo dynamics of ACh in the dorsolateral striatum (DLS) of the 6-OHDA-lesioned mouse model after L-DOPA administration, as well as its underlying mechanism, and to explore its modulatory role and mechanism in parkinsonian symptoms.

RESULTS

Using in vivo fiber photometry recordings with genetically encoded fluorescent DA or ACh indicator, we found L-DOPA selectively decreased DLS ACh levels in parkinsonian conditions. DA inhibited ACh release via dopamine D2 receptors and dSPNs-mediated activation of type-A γ-aminobutyric acid receptors on cholinergic interneurons. Restoring DLS ACh levels during L-DOPA treatment induced additional DA release by activating nicotinic acetylcholine receptors, thereby promoting the activity of dSPNs and iSPNs. Enhancing DLS ACh facilitated L-DOPA-induced turning behavior but not dyskinesia in parkinsonian mice.

CONCLUSIONS

Our results demonstrated that enhancing striatal ACh facilitated the effect of L-DOPA by modulating DA tone. It may challenge the classical hypothesis of a purely competitive interaction between dopaminergic and cholinergic neuromodulation in improving PD motor deficits. Modulating ACh levels within the dopaminergic system may improve striatal DA availability in PD patients.

Medial prefrontal cortex acetylcholine signaling mediates the ability to learn an active avoidance response following learned helplessness training

Increased brain levels of acetylcholine (ACh) have been observed in patients with depression, and increasing ACh levels pharmacologically can precipitate stress-related behaviors in humans and animals. Conversely, optimal ACh levels are required for cognition and memory. We hypothesize that excessive ACh signaling results in strengthening of negative encoding in which memory formation is aberrantly strengthened for stressful events. The medial prefrontal cortex (mPFC) is critical for both top-down control of stress-related circuits, and for encoding of sensory experiences. We therefore evaluated the role of ACh signaling in the mPFC in a learned helplessness task in which mice were exposed to repeated inescapable stressors followed by an active avoidance task. Using fiber photometry with a genetically-encoded ACh sensor, we found that ACh levels in the mPFC during exposure to inescapable stressors were positively correlated with later escape deficits in an active avoidance test in males, but not females. Consistent with these measurements, we found that both pharmacologically- and chemogenetically-induced increases in mPFC ACh levels resulted in escape deficits in both male and female mice, whereas chemogenetic inhibition of ACh neurons projecting to the mPFC improved escape performance in males, but impaired escape performance in females. These results highlight the adaptive role of ACh release in stress response, but also support the idea that sustained elevation of ACh contributes to maladaptive behaviors. Furthermore, mPFC ACh signaling may contribute to stress-based learning differentially in males and females.

The role of cholinergic signaling in multi-sensory gamma stimulation induced perivascular clearance of amyloid

Modulatory neurotransmitters exert powerful control over neurons and the brain vasculature. Gamma Entrainment Using Sensory Stimuli (GENUS) promotes amyloid clearance via increased perivascular cerebral spinal fluid (CSF) flux in mouse models of Alzheimer's Disease. Here we use whole-brain activity mapping to identify the cholinergic basal forebrain as a key region responding to GENUS. In line with this, GENUS promoted cortical acetylcholine release, vascular dilation, vasomotion and perivascular clearance. Inhibiting cholinergic signaling abolished the effects of GENUS, including the promotion of arterial pulsatility, periarterial CSF influx, and the reduction of cortical amyloid levels. Our findings establish cholinergic signaling as an essential component of the brain's ability to promote perivascular amyloid clearance via non-invasive sensory stimulation.

Molecular Spies in Action: Genetically Encoded Fluorescent Biosensors Light up Cellular Signals

Cellular function is controlled through intricate networks of signals, which lead to the myriad pathways governing cell fate. Fluorescent biosensors have enabled the study of these signaling pathways in living systems across temporal and spatial scales. Over the years there has been an explosion in the number of fluorescent biosensors, as they have become available for numerous targets, utilized across spectral space, and suited for various imaging techniques. To guide users through this extensive biosensor landscape, we discuss critical aspects of fluorescent proteins for consideration in biosensor development, smart tagging strategies, and the historical and recent biosensors of various types, grouped by target, and with a focus on the design and recent applications of these sensors in living systems.

Rapid modulation of striatal cholinergic interneurons and dopamine release by satellite astrocytes

Astrocytes are increasingly appreciated to possess underestimated and important roles in modulating neuronal circuits. Astrocytes in striatum can regulate dopamine transmission by governing the extracellular tone of axonal neuromodulators, including GABA and adenosine. However, here we reveal that striatal astrocytes occupy a cell type-specific anatomical and functional relationship with cholinergic interneurons (ChIs), through which they rapidly excite ChIs and govern dopamine release via nicotinic acetylcholine receptors on subsecond timescales. We identify that ChI somata are in unexpectedly close proximity to astrocyte somata, in mouse and human, forming a "soma-to-soma" satellite-like configuration not typically observed for other striatal neurons. We find that transient depolarization of astrocytes in mouse striatum reversibly regulates ChI excitability by decreasing extracellular calcium. These findings reveal a privileged satellite astrocyte-interneuron interaction for striatal ChIs operating on subsecond timescales via regulation of extracellular calcium dynamics to shape downstream striatal circuit activity and dopamine signaling.

Applications of functional neurotransmitter release imaging with genetically encoded sensors in psychiatric research

Psychiatric research encompasses diverse methodologies to understand the complex interplay between neurochemistry and behavior in mental health disorders. Despite significant advancements in pharmacological interventions, there remains a critical gap in understanding the precise functional changes underlying psychiatric conditions and the mechanisms of action of therapeutic agents. Genetically encoded sensors have emerged as powerful tools to address these challenges by enabling real-time monitoring of neurochemical dynamics in specific neuronal populations. This prospective explores the utility of neurotransmitter binding genetically encoded sensors in uncovering the nature of neuronal dysregulation underpinning mental illness, assessing the impact of pharmaceutical interventions, and facilitating the discovery of novel treatments.

Cortical acetylcholine dynamics are predicted by cholinergic axon activity and behavior state

Acetylcholine (ACh) is thought to play a role in driving the rapid, spontaneous brain-state transitions that occur during wakefulness; however, the spatiotemporal properties of cortical ACh activity during these state changes are still unclear. We perform simultaneous imaging of GRAB-ACh sensors, GCaMP-expressing basal forebrain axons, and behavior to address this question. We observed a high correlation between axon and GRAB-ACh activity around periods of locomotion and pupil dilation. GRAB-ACh fluorescence could be accurately predicted from axonal activity alone, and local ACh activity decreased at farther distances from an axon. Deconvolution of GRAB-ACh traces allowed us to account for sensor kinetics and emphasized rapid clearance of small ACh transients. We trained a model to predict ACh from pupil size and running speed, which generalized well to unseen data. These results contribute to a growing understanding of the precise timing and spatial characteristics of cortical ACh during fast brain-state transitions.

Striatal cholinergic transmission in an inducible transgenic mouse model of paroxysmal non-kinesiogenic dyskinesia

Altered interaction between striatonigral dopaminergic (DA) inputs and local acetylcholine (ACh) in striatum has long been hypothesized to play a central role in the pathophysiology of dystonia and dyskinesia. Indeed, previous research using several genetic mouse models of human isolated dystonia identified a shared endophenotype with paradoxical excitation of striatal cholinergic interneuron (ChIs) activity in response to activation of dopamine D2 receptors (D2R). These mouse models lack a dystonic motor phenotype, which leaves a critical gap in comprehending the role of DA and ACh transmission in the manifestations of dystonia. To tackle this question, we used a combination of ex vivo slice physiology and in vivo monitoring of striatal ACh dynamics in the inducible, phenotypically penetrant, transgenic mouse model of paroxysmal non-kinesiogenic dyskinesia (PNKD), an animal with both dystonic and dyskinetic features. We found that, similarly to genetic models of isolated dystonia, the PNKD mouse displays D2R-induced paradoxical excitation of ChI firing in ex vivo striatal brain slices. In vivo, caffeine triggers dystonic symptoms while reversing the D2R-mediated excitation of ChIs and desynchronizing ACh release in PNKD mice. In WT littermate controls, caffeine stimulates spontaneous locomotion through a similar but reversed mechanism involving an excitatory switch of the D2R control of ChI activity, associated with enhanced synchronization of ACh release. These observations suggest that the "paradoxical excitation" of cholinergic interneurons described in isolated dystonia models could represent a compensatory or protective mechanism that prevents manifestation of movement abnormalities and that phenotypic dystonia is possible only when this is absent. These findings also suggest that D2Rs may play an important role in synchronizing the ChI network leading to rhythmic ACh release during heightened movement states. Dysfunction of this interaction and corresponding desynchrony of ACh release may contribute to aberrant movements.

Dopamine dynamics in nucleus accumbens across reward-based learning of goal-directed whisker-to-lick sensorimotor transformations in mice

The synaptic and neuronal circuit mechanisms underlying reward-based learning remain to be fully determined. In the mammalian brain, dopamine release in nucleus accumbens is thought to contribute importantly to reward signals for learning and promoting synaptic plasticity. Here, through longitudinal fiber photometry of a genetically-encoded fluorescent sensor, we investigated dopamine signals in the nucleus accumbens of thirsty head-restrained mice as they learned to lick a liquid reward spout in response to single deflections of the C2 whisker across varying reward contingencies. Reward delivery triggered by well-timed licking drove fast transient dopamine increases in nucleus accumbens. On the other hand, unrewarded licking was overall associated with reduced dopamine sensor fluorescence, especially in trials where reward was unexpectedly omitted. The dopamine reward signal upon liquid delivery decreased within individual sessions as mice became sated. As mice learned to lick the reward spout in response to whisker deflection, a fast transient sensory-evoked dopamine signal developed, correlating with the learning of the whisker detection task across consecutive training days, as well as within single learning sessions. The well-defined behavioral paradigm involving a unitary stimulus of a single whisker as a reward-predicting cue along with precisely quantified licking allows untangling of sensory, motor and reward-related dopamine signals and how they evolve across the learning of whisker-dependent goal-directed sensorimotor transformations. Our longitudinal measurements of dopamine dynamics across reward-based learning are overall consistent with the hypothesis that dopamine could play an important role as a reward signal for reinforcement learning.

Neuromodulator and neuropeptide sensors and probes for precise circuit interrogation in vivo

To determine how neuronal circuits encode and drive behavior, it is often necessary to measure and manipulate different aspects of neurochemical signaling in awake animals. Optogenetics and calcium sensors have paved the way for these types of studies, allowing for the perturbation and readout of spiking activity within genetically defined cell types. However, these methods lack the ability to further disentangle the roles of individual neuromodulator and neuropeptides on circuits and behavior. We review recent advances in chemical biology tools that enable precise spatiotemporal monitoring and control over individual neuroeffectors and their receptors in vivo. We also highlight discoveries enabled by such tools, revealing how these molecules signal across different timescales to drive learning, orchestrate behavioral changes, and modulate circuit activity.