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

Coding principles of dopaminergic transmission modes

Dopaminergic neurons influence diverse behaviors with varied firing patterns, yet the precise mechanisms remain unclear. We introduce a multiplexed genetically encoded sensor-based imaging and voltammetry method to simultaneously record synaptic, perisynaptic, and extrasynaptic dopaminergic transmission at mouse central neurons. Using this method alongside a genetically encoded sensor-based image analysis program, we found that heterogeneous dopaminergic firing patterns create various transmission modes, encoding frequency, number, and synchrony of firing pulses using neurotransmitter quantity, releasing synapse count, and synaptic and/or volume transmission. Under both tonic and low-frequency phasic activities, transporters effectively reuptake dopamine at perisynaptic sites, confining dopamine within synaptic clefts to mediate synaptic transmission. In contrast, under high-frequency, particularly synchronized firing activity or transporter inhibition, released dopamine may overwhelm transporters, escaping from synaptic clefts via one to three outlet channels, triggering volume transmission. Our study illuminates a collaborative mechanism of synaptic enclosures, properties, and transporters that defines the coding principles of activity pattern-dependent dopaminergic transmission modes.

Neuromodulatory processing in the bi-pathway brain architecture

The brain is inherently a complex and parallel system that processes both external and internal sensory cues to generate adaptive responses. Sensory cues encapsulate not only objective information about their physical and chemical properties but also subjective information related to their ecological significance. Objective information is processed and conveyed through relatively stereotyped sensorimotor pathways to drive behaviors, while subjective information is received and transmitted through relatively flexible neuromodulatory systems. These neuromodulatory pathways influence signal processing of the sensorimotor pathways at multiple stages by modulating neuronal excitability and the efficiency of synaptic transmission, thereby endowing animals with flexibility. This sophisticated neuromodulatory processing is finely tuned by the spatiotemporal dynamics of various neuromodulators released from specialized neuromodulatory neurons that encode sensory, motor as well as cognitive variables. Dysfunctions in neuromodulatory pathways disrupt spatiotemporal patterns of neuromodulators, which in turn compromise sensorimotor transformation and cognitive functions. This review aims to delineate the mechanisms and roles of neuromodulatory processing within the bi-pathway brain architecture and propose prospective research topics along with innovative experimental paradigms.

Cross-Species Framework for Emotional Well-Being and Brain Aging: Lessons From Behavioral Neuroscience

Importance

Emotional well-being (EWB) is an emerging therapeutic target for managing and preventing symptoms associated with Alzheimer disease and related dementias (ADRD). However, more research is needed to establish causal inferences between brain changes, EWB, and behavioral changes observed in typical aging and ADRD.

Observations

This article presents a framework for using a cross-species behavioral neuroscience approach to study EWB and brain aging, adopting a well-established biobehavioral model that highlights the reciprocal roles of brain changes, EWB, and ADRD symptoms. First, the challenges and opportunities in this field are reviewed. Then, a practical solution to improve comparability between animal and human studies is proposed.

Conclusions and Relevance

The goal is to draw comprehensive parallels and distinctions that could enhance the understanding of the mechanisms linking brain aging, EWB, and ADRD symptomatic disturbances across different species.

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.

The brain-heart axis: integrative cooperation of neural, mechanical and biochemical pathways

The neural and cardiovascular systems are pivotal in regulating human physiological, cognitive and emotional states, constantly interacting through anatomical and functional connections referred to as the brain-heart axis. When this axis is dysfunctional, neurological conditions can lead to cardiovascular disorders and, conversely, cardiovascular dysfunction can substantially affect brain health. However, the mechanisms and fundamental physiological components of the brain-heart axis remain largely unknown. In this Review, we elucidate these components and identify three primary pathways: neural, mechanical and biochemical. The neural pathway involves the interaction between the autonomic nervous system and the central autonomic network in the brain. The mechanical pathway involves mechanoreceptors, particularly those expressing mechanosensitive Piezo protein channels, which relay crucial information about blood pressure through peripheral and cerebrovascular connections. The biochemical pathway comprises many endogenous compounds that are important mediators of neural and cardiovascular function. This multisystem perspective calls for the development of integrative approaches, leading to new clinical specialties in neurocardiology.

Cholecystokinin Modulates Corticostriatal Transmission and Plasticity in Rodents

Recent findings have shifted the view of cholecystokinin (CCK) from being a cellular neuronal marker to being recognized as a crucial neuropeptide pivotal in synaptic plasticity and memory processes. Despite its now appreciated importance in various brain regions and abundance in the basal ganglia, its role in the striatum, which is vital for motor control, remains unclear. This study sought to fill this gap by performing a comprehensive investigation of the role of CCK in modulating striatal medium spiny neuron (MSN) membrane properties, as well as the secondary somatosensory cortex S2 to MSN synaptic transmission and plasticity in rodents. Using in vivo optopatch-clamp recording in mice on identified MSNs, we showed that the application of CCK receptor Type 2 (CCK2R) antagonists decreases corticostriatal transmission in both direct and indirect pathway MSNs. Moving to an ex vivo rat preparation to maximize experimental access, we showed that CCK2R inhibition impacts MSN membrane properties by reducing spike threshold and rheobase, suggesting an excitability increase. Moreover, CCK modulates corticostriatal transmission mainly via CCK2R, and CCK2R blockage shifted spike-timing-dependent plasticity from long-term potentiation to long-term depression. Our study advances the understanding of CCK's importance in modulating corticostriatal transmission. By showing how CCK2R blockade influences synaptic function and plasticity, we provide new insights into the mechanisms underlying striatal functions, opening new paths for exploring its potential relevance to neurological disorders involving basal ganglia-related behaviors.

Illuminating the impact of stress: In vivo approaches to track stress-related neural adaptations

Stressful experiences can affect both daily life and long-term health outcomes in a variety of ways. Acute challenges may be adaptive, promoting arousal and enhancing memory and cognitive function. Importantly, however, chronic stress dysregulates the body's physiological regulatory mechanisms consisting of complex hormone interactions throughout the peripheral and central nervous systems. This disrupted signaling consequently alters the balance of synapse formation, maturation and pruning, processes which regulate neural communication, plasticity, learning, cognitive flexibility and adaptive behaviors - hallmarks of a healthy, functional brain. The chronically stressed brain state, therefore, is one which may be uniquely vulnerable. To understand the development of this state, how it is sustained and how behavior and neural function are transiently or indelibly impacted by it, we can turn to a number of advanced approaches in animal models which offer unprecedented insights. This has been the aim of my recent work within the field and the goal of my new independent research program. To achieve this, I have employed methods to uncover how key brain circuits integrate information to support motivated behaviors, how stress impacts their ability to perform this process and how best to operationalize behavioral readouts. Here I present an overview of research contributions that I find most meaningful for advancing our understanding of the impact of stress and propose new avenues which will guide my own framework to address the salient outstanding questions within the field.

Coding principles and mechanisms of serotonergic transmission modes

Serotonin-mediated intercellular communication has been implicated in myriad human behaviors and diseases, yet how serotonin communicates and how the communication is regulated remain unclear due to limitations of available monitoring tools. Here, we report a method multiplexing genetically encoded sensor-based imaging and fast-scan cyclic voltammetry, enabling simultaneous recordings of synaptic, perisynaptic, proximate and distal extrasynaptic serotonergic transmission. Employing this method alongside a genetically encoded sensor-based image analysis program (GESIAP), we discovered that heterogeneous firing patterns of serotonergic neurons create various transmission modes in the mouse raphe nucleus and amygdala, encoding information of firing pulse frequency, number, and synchrony using neurotransmitter quantity, releasing synapse count, and synaptic and/or volume transmission. During tonic and low-frequency phasic activities, serotonin is confined within synaptic clefts due to efficient retrieval by perisynaptic transporters, mediating synaptic transmission modes. Conversely, during high-frequency, especially synchronized phasic activities, or when transporter inhibition, serotonin may surpass transporter capacity, and escape synaptic clefts through 1‒3 outlet channels, leading to volume transmission modes. Our results elucidate a mechanism of how channeled synaptic enclosures, synaptic properties, and transporters collaborate to define the coding principles of activity pattern-dependent serotonergic transmission modes.

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.