Lights, fiber, action! A primer on in vivo fiber photometry

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.

Prelimbic cortical excitatory overdrive and inhibitory underdrive accompany environmental suppression of food seeking

Cues associated with food, such as fast-food advertising, can provoke food cravings and may lead to unhealthy overeating. Environmental enrichment (EE) that enhances cognitive and physical stimulation can reduce cue-evoked sucrose seeking in mice and recruitment of sucrose cue-reactive neurons or 'neuronal ensembles' in the prelimbic cortex (PL), which regulates appetitive behaviors. Hence, EE provides us with a behavioral model and neuronal targets to identify 'anti-craving' relevant mechanisms. Here, we investigated in the PL how EE modulated neuronal excitability and activity patterns in cue-reactive neuronal populations. Chemogenetic inhibition of cue-reactive neurons in PL blocked cue-evoked sucrose seeking, thereby confirming the function of these neurons in sucrose cue memory. EE boosted the baseline excitability of 'originally', or before EE exposure, cue-reactive, excitatory pyramidal cells in PL. Furthermore, their sucrose cue-specificity was lost - resulting in their persistent activation and non-cue selective activation or 'excitatory overdrive'. Furthermore, EE reduced recruitment of cue-reactive, inhibitory interneurons reflecting 'inhibitory underdrive'. Taken together, impaired neuronal food cue processing due to simultaneous prefrontal cortical excitatory 'overdrive' and inhibitory 'underdrive' likely underlies EE's anti-craving action, thereby serving as potential neurophysiological targets to develop novel medications that help control food cravings.

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.

Prelimbic cortical excitatory overdrive and inhibitory underdrive accompany environmental suppression of food seeking

Cues associated with food, such as fast-food advertising, can provoke food cravings and may lead to unhealthy overeating. Environmental enrichment (EE) that enhances cognitive and physical stimulation can reduce cue-evoked sucrose seeking in mice and recruitment of sucrose cue-reactive neurons or 'neuronal ensembles' in the prelimbic cortex (PL), which regulates appetitive behaviors. Hence, EE provides us with a behavioral model and neuronal targets to identify 'anti-craving' relevant mechanisms. Here, we investigated in the PL how EE modulated neuronal excitability and activity patterns in cue-reactive neuronal populations. Chemogenetic inhibition of cue-reactive neurons in PL blocked cue-evoked sucrose seeking, thereby confirming the function of these neurons in sucrose cue memory. EE boosted the baseline excitability of 'originally', or before EE exposure, cue-reactive, excitatory pyramidal cells in PL. Furthermore, their sucrose cue-specificity was lost - resulting in their persistent activation and non-cue selective activation or 'excitatory overdrive'. Furthermore, EE reduced recruitment of cue-reactive, inhibitory interneurons reflecting 'inhibitory underdrive'. Taken together, impaired neuronal food cue processing due to simultaneous prefrontal cortical excitatory 'overdrive' and inhibitory 'underdrive' likely underlies EE's anti-craving action, thereby serving as potential neurophysiological targets to develop novel medications that help control food cravings.

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.

An integrated microfluidic and fluorescence platform for probing in vivo neuropharmacology

Neurotechnologies and genetic tools for dissecting neural circuit functions have advanced rapidly over the past decade although the development of complementary pharmacological methodologies has comparatively lagged. Understanding the precise pharmacological mechanisms of neuroactive compounds is critical for advancing basic neurobiology and neuropharmacology, as well as for developing more effective treatments for neurological and neuropsychiatric disorders. However, integrating modern tools for assessing neural activity in large-scale neural networks with spatially localized drug delivery remains a major challenge. Here, we present a dual microfluidic-photometry platform that enables simultaneous intracranial drug delivery with neural dynamics recording in the rodent brain. The integrated platform combines a wireless, battery-free, miniaturized fluidic microsystem with optical probes, allowing for spatially and temporally restricted drug delivery while sensing activity-dependent fluorescence using genetically encoded calcium indicators (GECIs), neurotransmitter sensors, and neuropeptide sensors. We demonstrate the performance of this platform for investigating neuropharmacological mechanisms in vivo in behaving mice.

Multi-Frequency Interpolation X-talk Removal Algorithm: Enabling Combinations of Concurrent Optogenetics and Lock-in Amplification Fiber Photometry via Removal of Optogenetic Stimulation Crosstalk

Simultaneous fiber photometry and optogenetics is a powerful emerging technique for precisely studying the interactions of neuronal brain networks. However, spectral overlap between photometry and optogenetic components has severely limited the application of an all-optical approach. Due to spectral overlap, light from optogenetic stimulation saturates the photosensor and occludes photometry fluorescence, which is especially problematic in physically smaller model organism brains like mice. Here, we demonstrate the multi-frequency interpolation X-talk removal algorithm (MuFIX or μFIX) for recovering crosstalk-contaminated photometry responses recorded with lock-in amplification. μFIX exploits multifrequency lock-in amplification by modeling the remaining uncontaminated data to interpolate across crosstalk-affected segments (R2 ≈ 1.0); we found that this approach accurately recovers the original photometry response after demodulation (Pearson's r ≈ 1.0). When applied to crosstalk-contaminated data, μFIX recovered a photometry response closely resembling the dynamics of noncrosstalk photometry recorded simultaneously. Upon further verification using simulated and empirical data, we demonstrated that μFIX reproduces any signal that underwent simulated crosstalk contamination (r ≈ 1.0). We believe adopting μFIX will enable experimental designs using simultaneous fiber photometry and optogenetics that were previously not feasible due to crosstalk.

Estradiol protects against pain-facilitated fentanyl use via suppression of opioid-evoked dopamine activity in males

Pain relief is the most frequently reported motivation for opioid misuse, but it remains unclear how pain alters reward pathway function contributing to maladaptive opioid use and whether these neuroadaptations occur in a sex-specific manner. Here, we show that persistent inflammatory pain leads to augmented fentanyl self-administration in male, not female, rats. Wireless in vivo fiber photometry recordings and chemogenetic manipulations indicate that pain-facilitated fentanyl use is mediated by enhanced ventral tegmental area dopamine (VTADA) neuron responses during self-administration. In females, ovariectomy enhances fentanyl self-administration, but the protective effects of ovarian hormones are not solely mediated by estradiol per se. Instead, pain and high estradiol states-naturally occurring in intact females or artificially produced in males-suppress fentanyl self-administration and associated VTADA activity through VTA estrogen receptor beta signaling. These findings highlight the importance of assessing hormonal factors in opioid misuse liability in the context of pain.

A high-performance fluorescent sensor spatiotemporally reveals cell-type specific regulation of intracellular adenosine in vivo

Adenosine (Ado), a nucleoside bridging intracellular metabolism with intercellular communication, plays an essential role in regulating processes such as sleep and seizure. While the functions of extracellular Ado ("eAdo") are well documented, our knowledge about the distribution and regulatory functions of intracellular Ado ("iAdo") is limited by a lack of methods for detecting iAdo in vivo. Here, we develop HypnoS, a genetically encoded fluorescent sensor for iAdo characterized by its high sensitivity, specificity, spatiotemporal resolution, and rapid response (sub-seconds). HypnoS enables real-time visualization of iAdo dynamics in live cultures, acute brain slices, flies, and freely moving mice. Using HypnoS for dual-color mesoscopic imaging in mice, we show that seizure-induced iAdo waves propagated across the cortex, following calcium signals. Additionally, two-photon imaging reveals that iAdo decays more rapidly in astrocytes than in neurons during seizures. Moreover, by recording iAdo dynamics in the basal forebrain during the sleep-wake cycle, we observe that iAdo signals are present during wakefulness and rapid eye movement (REM) sleep, regulated by equilibrative nucleoside transporters (ENT1/2). Thus, HypnoS is a versatile and powerful tool for investigating the biological functions of iAdo across a range of physiological and pathological states.

Single-cell genomics meets systems neuroscience: Insights from mapping the brain circuitry of stress

Responses to external and internal dangers is essential for survival and homeostatic regulation. Hypothalamic corticotropin-releasing hormone neurons (CRHNs) play a pivotal role in regulating neuroendocrine responses to fear and stress. In recent years, the application of neurogenetic tools, such as fiber photometry, chemogenetics and optogenetics, have provided new insights into the dynamic neuronal responses of CRHNs during stressful events, offering new perspectives into their functional significance in mediating neurobehavioural responses to stress. Transsynaptic viral tracers have facilitated the comprehensive mapping of neuronal inputs to CRHNs. Furthermore, the development and application of innovative single-cell genomic tools combined with viral tracing have begun to pave the way for a deeper understanding of the transcriptional profiles of neural circuit components, enabling molecular-anatomical circuit mapping. Here, I will discuss how these systems neuroscience approaches and novel single-cell genomic methods are advancing the molecular and functional mapping of stress neurocircuits, their associated challenges and future directions.

A roadmap for transformative translational research on gambling disorder in the UK

The UK has one of the highest rates of recreational gambling in the world. Some vulnerable individuals progressively lose control over gambling and develop at-risk gambling or gambling disorder (GD), characterised by the compulsive pursuit of gambling. GD destroys lives and incurs massive costs to societies, yet only a few treatments are available. Failure to develop a wider range of interventions is in part due to a lack of funding that has slowed progress in the translational research necessary to understand the individual vulnerability to switch from controlled to compulsive gambling. Current preclinical models of GD do not operationalise the key clinical features of the human condition. The so-called "gambling tasks" for non-human mammals almost exclusively assess probabilistic decision-making, which is not real-world gambling. While they have provided insights into the psychological and neural mechanisms involved in the processing of gains and losses, these tasks have failed to capture those underlying real-world gambling and its compulsive manifestation in humans. Here, we highlight the strengths and weaknesses of current gambling-like behaviour tasks and suggest how their translational validity may be improved. We then propose a theoretical framework, the incentive habit theory of GD, which may prove useful for the operationalisation of the biobehavioural mechanisms of GD in preclinical models. We conclude with a list of recommendations for the development of next-generation preclinical models of GD and discuss how modern techniques in animal behavioural experimentation can be deployed in the context of GD preclinical research to bolster the translational pipeline.

Obtaining artifact-corrected signals in fiber photometry via isosbestic signals, robust regression, and dF/F calculations

Significance

Fiber photometry is a powerful tool for neuroscience. However, measured biosensor signals are contaminated by various artifacts (photobleaching and movement-related noise) that undermine analysis and interpretation. Currently, no universal pipeline exists to deal with these artifacts.

Aim

We aim to evaluate approaches for obtaining artifact-corrected neural dynamic signals from fiber photometry data and provide recommendations for photometry analysis pipelines.

Approach

Using simulated and real photometry data, we tested the effects of three key analytical decisions: choice of regression for fitting isosbestic control signals onto experimental signals [ordinary least squares (OLS) versus iteratively reweighted least squares (IRLS)], low-pass filtering, and dF/F versus dF calculations.

Results

IRLS surpassed OLS regression for fitting isosbestic control signals to experimental signals. We also demonstrate the efficacy of low-pass filtering signals and baseline normalization via dF/F calculations.

Conclusions

We conclude that artifact-correcting experimental signals via low-pass filter, IRLS regression, and dF/F calculations is a superior approach to commonly used alternatives. We suggest these as a new standard for preprocessing signals across photometry analysis pipelines.

Glutamate indicators with increased sensitivity and tailored deactivation rates

Identifying the input-output operations of neurons requires measurements of synaptic transmission simultaneously at many of a neuron's thousands of inputs in the intact brain. To facilitate this goal, we engineered and screened 3365 variants of the fluorescent protein glutamate indicator iGluSnFR3 in neuron culture, and selected variants in the mouse visual cortex. Two variants have high sensitivity, fast activation (< 2 ms) and deactivation times tailored for recording large populations of synapses (iGluSnFR4s, 153 ms) or rapid dynamics (iGluSnFR4f, 26 ms). By imaging action-potential evoked signals on axons and visually-evoked signals on dendritic spines, we show that iGluSnFR4s/4f primarily detect local synaptic glutamate with single-vesicle sensitivity. The indicators detect a wide range of naturalistic synaptic transmission, including in the vibrissal cortex layer 4 and in hippocampal CA1 dendrites. iGluSnFR4 increases the sensitivity and scale (4s) or speed (4f) of tracking information flow in neural networks in vivo.

Dopamine and acetylcholine correlations in the nucleus accumbens depend on behavioral task states

Dopamine release in the nucleus accumbens (NAcc) changes quickly in response to errors in predicting events like reward delivery1,2,3 but also slowly ramps up when animals are moving toward a goal.4,5,6,7,8,9,10 This ramping has attracted much recent attention, as there is controversy regarding its computational role5,7,9,11 and whether they are driven by dopamine neuron firing7,8,9 or local circuit mechanisms.5,6 If the latter is true, cholinergic transmission would be a prime candidate mechanism,12,13,14 and acetylcholine and dopamine signals should be positively correlated during behavior, particularly during motivated approach. However, in the dorsal striatum, striatal cholinergic interneurons typically "dip" their activity when reward or associated cues are presented, in opposition to dopamine,15,16,17,18 and acetylcholine and dopamine release is generally anti-correlated in vivo.19,20 Furthermore, acetylcholine and dopamine have opposing effects on downstream striatal projection neurons (SPNs),21,22 which suggests that cholinergic dips create a permissive window for dopamine to drive plasticity.23 These studies therefore suggest that dopamine and acetylcholine should be anti-correlated during behavior. We tested between these hypotheses by simultaneously recording accumbal dopamine and acetylcholine signals in rats executing a task involving motivated approach. We found that dopamine ramps were not coincidental with changes in acetylcholine. Instead, acetylcholine was positively, negatively, or uncorrelated with dopamine depending on the task phase. Our results suggest that accumbal dopamine and acetylcholine dynamics are largely independent but may combine to engage different postsynaptic mechanisms depending on task demands.

Rapid fluctuations in histamine associated with intake of nutritive and non-nutritive solutions

The neurotransmitter histamine is involved in control of food intake, yet its dynamics during individual feeding episodes remain unexplored. Therefore, we used the novel genetically-encoded histamine sensor, HisLightG, combined with fiber photometry to measure histamine release in two hypothalamic regions critical for the food-suppressive effects of histamine, the paraventricular nucleus of the hypothalamus (PVH), and the ventromedial hypothalamus (VMH). Male mice were tested under different conditions to assess whether hunger, time of day, or the caloric content of the solution they were given affected histamine fluctuations. We found that histamine levels changed rapidly in response to eating. These histamine fluctuations were influenced by experimental conditions, with slightly smaller responses when the test solution was sucralose (both regions) or during the light cycle (PVH only). Notable regional differences were identified, such that in the PVH histamine rebounded to baseline levels, whereas in the VMH histamine remained lower than baseline for at least 10 seconds after licking ceased. In a separate cohort of male and female mice, enhancing histamine tone via administration of a histamine precursor (L-histidine) reduced the number of licks across multiple sucrose concentrations. Together, these findings indicate that histaminergic activity is modulated rapidly during ingestive episodes, and that understanding these release patterns will give insight into histamine's role in appetite suppression.

Fluorescence detection of dopamine signaling to the primate striatum in relation to stimulus-reward associations

Dopamine (DA) signals to the striatum play critical roles in shaping and sustaining stimulus-reward associations. In primates, however, the dynamics of the DA signals remain unknown since conventional methods are not necessarily appropriate in terms of the spatiotemporal resolution or chemical specificity sufficient for detecting the DA signals. In our study, fiber photometry with a fluorescent DA sensor was employed to identify reward-related DA transients in the monkey striatum. This technique, which directly monitors local DA release, reveals a reward prediction error signal in the anterior putamen originating from midbrain DA neurons. Further, DA transients in the head of the caudate nucleus exhibit a value-based response to reward-predicting stimuli. These signals have been found to arise from two separate groups of DA neurons in the substantia nigra pars compacta. The present results demonstrate that fluorescence DA monitoring is applicable to detect DA signals in the primate striatum for investigating their roles.

A statistical framework for analysis of trial-level temporal dynamics in fiber photometry experiments

Fiber photometry has become a popular technique to measure neural activity in vivo, but common analysis strategies can reduce the detection of effects because they condense within-trial signals into summary measures, and discard trial-level information by averaging across-trials. We propose a novel photometry statistical framework based on functional linear mixed modeling, which enables hypothesis testing of variable effects at every trial time-point, and uses trial-level signals without averaging. This makes it possible to compare the timing and magnitude of signals across conditions while accounting for between-animal differences. Our framework produces a series of plots that illustrate covariate effect estimates and statistical significance at each trial time-point. By exploiting signal autocorrelation, our methodology yields joint 95% confidence intervals that account for inspecting effects across the entire trial and improve the detection of event-related signal changes over common multiple comparisons correction strategies. We reanalyze data from a recent study proposing a theory for the role of mesolimbic dopamine in reward learning, and show the capability of our framework to reveal significant effects obscured by standard analysis approaches. For example, our method identifies two dopamine components with distinct temporal dynamics in response to reward delivery. In simulation experiments, our methodology yields improved statistical power over common analysis approaches. Finally, we provide an open-source package and analysis guide for applying our framework.

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.

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.

Fiber photometry analysis of spontaneous dopamine signals: The z-scored data are not the data

Fluorescent sensors have revolutionized the measurement of molecules in the brain, and the dLight dopamine sensor has been used extensively to examine reward- and cue-evoked dopamine release, but only recently has the field turned its attention to spontaneous release events. Analysis of spontaneous events typically requires evaluation of hundreds of events over minutes to hours, and the most common method of analysis, z-scoring, was not designed for this purpose. Here, we compare the accuracy and reliability of three different analysis methods to identify pharmacologically induced changes in dopamine release and uptake in freely moving C57BL/6J mice. The D1-like receptor antagonist SCH23390 was used to prevent dLight sensors from interacting with dopamine in the extracellular space, while cocaine was used to inhibit uptake and raclopride to increase release of dopamine in the nucleus accumbens. We examined peak-to-peak frequency, peak amplitude, and width, the time spent above an established cutoff. The three methods were 1) the widely-used "Z-Score Method", which automatically smooths baseline drift and normalizes recordings using signal-to-noise ratios, 2) a "Manual Method", in which local baselines were adjusted manually and individual cutoffs were determined for each subject, and 3) the "Prominence Method" that combines z-scoring with prominence assessment to tag individual peaks, then returns to the preprocessed data for kinetic analysis. First, SCH23390 drastically reduced the number of signals detected as expected, but only when the Manual Method was used. Z-scoring failed to identify any changes, due to its amplification of noise when signals were diminished. Cocaine increased signal width as expected using the Manual and Prominence Methods, but not the Z-Score Method. Finally, raclopride-induced increases in amplitude were correctly identified by the Manual and Prominence Methods. The Z-Score Method failed to identify any of the changes in dopamine release and uptake kinetics. Thus, analysis of spontaneous dopamine signals requires assessment of the %ΔF/F values, ideally using the Manual Method, and the use of z-scoring is not appropriate.

Anything but small: Microarousals stand at the crossroad between noradrenaline signaling and key sleep functions

Continuous sleep restores the brain and body, whereas fragmented sleep harms cognition and health. Microarousals (MAs), brief (3- to 15-s-long) wake intrusions into sleep, are clinical markers for various sleep disorders. Recent rodent studies show that MAs during healthy non-rapid eye movement (NREM) sleep are driven by infraslow fluctuations of noradrenaline (NA) in coordination with electrophysiological rhythms, vasomotor activity, cerebral blood volume, and glymphatic flow. MAs are hence part of healthy sleep dynamics, raising questions about their biological roles. We propose that MAs bolster NREM sleep's benefits associated with NA fluctuations, according to an inverted U-shaped curve. Weakened noradrenergic fluctuations, as may occur in neurodegenerative diseases or with sleep aids, reduce MAs, whereas exacerbated fluctuations caused by stress fragment NREM sleep and collapse NA signaling. We suggest that MAs are crucial for the restorative and plasticity-promoting functions of sleep and advance our insight into normal and pathological arousal dynamics from sleep.

Hippocampal P2X7 and A2A purinoceptors mediate cognitive impairment caused by long-lasting epileptic seizures

Rationale: Cognitive impairment and depression are salient comorbidities of mesial temporal lobe epilepsy; it is still unclear whether this frequently drug resistant disease is a cause or consequence of hippocampal damage and its interplay with long-lasting seizure activity (status epilepticus; SE). Thus, a major therapeutic advance in this field is badly needed. Methods: We modeled enduring behavioral and electroencephalographic (EEG) seizures in mice by the intraperitoneal injection of kainic acid (KA), and measured the dynamics of the intracellular Ca2+ signals in the hippocampal CA1 area by fiber photometry. Learning and memory were controlled by the Morris Water-Maze and Novel Object Recognition tests on whole animals and by the induction of long-term potentiation in CA1 pyramidal neurons in brain slices. Depressive-like reactions were evaluated by the Tail Suspension, Forced Swim, and Sucrose Preference tests. Results: The intraperitoneal injection of the blood-brain permeable, highly selective, P2X7 and A2A receptor (R) antagonists, JNJ-47965567, and KW6002/SCH58261, respectively, counteracted the effects of KA-induced SE both on seizure activity and the increase of Ca2+ signals (as a measure of changes in the intracellular Ca2+ concentration) in neurons and astrocytes of the hippocampal CA1 area. In addition, these drugs also prevented the impairment of the hippocampus-dependent spatial and non-spatial learning abilities by KA-SE. The knockdown of P2X7Rs in CA1 astrocytes, but not neurons prevented the cognitive deterioration, suggesting that the release of astrocytic signaling molecules onto neighboring neurons might be the cause of this effect. In accordance with our observations, in hippocampal slices prepared from mice which underwent KA-SE, a selective sensitivity increases to the prototypic P2X7R agonist dibenzoyl-ATP (Bz-ATP) manifested in CA1 neurons. This sensitivity increase appeared to be due to a postsynaptic interference between P2X7Rs and the release of excitatory neurotransmitters during SE. In spite of a P2X7 and A2AR-mediated increase of Ca2+ signaling in the medial prefrontal cortex, no similar change was noted after KA-SE in depressive-like reactions or the open-field behavior. Conclusions: SE induced the release of ATP and adenosine from the hippocampus and in consequence decreased the cognitive abilities of mice. The pharmacological blockade of P2X7 and A2ARs prevented the SE-induced seizure activity and cognitive deterioration, but not depressive-like behavior.

Alcohol drinking is associated with greater calcium activity in mouse central amygdala dynorphin-expressing neurons

Alcohol use disorder (AUD) is a chronic disease that poses significant economic burden and health risks. It is pivotal to better understand brain mechanisms engaged by alcohol that promote misuse. The central amygdala (CeA) has emerged as a key mediator of excessive preclinical alcohol consumption. A dynorphin-expressing subpopulation within the CeA (CeA Dyn ) has been implicated in excessive alcohol drinking, yet how cellular activity of CeA Dyn neurons relates to ongoing alcohol drinking is not well-understood. The current study interrogated the engagement of CeA Dyn neurons in male and female mice during voluntary alcohol consumption using fiber photometry and compared this cellular response with that of other solutions having similar motivational and/or taste characteristics. Activity of a calcium sensor, GCaMP7f, expressed in mouse CeA Dyn neurons was recorded and time-locked to bouts of drinking. Multilevel linear mixed modeling was applied to better resolve focal effects from complex data. These analyses revealed a relatively large increase in CeA Dyn neuron calcium transients after bouts of alcohol drinking compared to water or sucrose drinking, indicating these neurons are uniquely engaged during alcohol consumption. Drinking behavior unique to alcohol (i.e., longer bout durations) did not fully explain signal differences between alcohol and other solutions nor did the relatively increased alcohol response diminish over time. No other conditions or solutions tested reproduced the pronounced change in CeA Dyn activity associated with alcohol drinking. These findings, collectively, support the presence of a unique functional signature for alcohol in a cell population known to control excessive alcohol drinking.

Highlights

Central amygdala dynorphin cells (CeA Dyn ) are firmly implicated in alcohol misuse. CeA Dyn neuron activity was higher when mice drank alcohol versus other solutions. Neither how mice drank alcohol nor motivational states could explain this activity. CeA Dyn neurons having uniquely high alcohol responses may underlie AUD development.

Cholinergic dynamics in the septo-hippocampal system provide phasic multiplexed signals for spatial novelty and correlate with behavioral states

In the hippocampal formation, cholinergic modulation from the medial septum/diagonal band of Broca (MSDB) is known to correlate with the speed of an animal's movements at sub-second timescales and also supports spatial memory formation. Yet, the extent to which sub-second cholinergic dynamics, if at all, align with transient behavioral and cognitive states supporting the encoding of novel spatial information remains unknown. In this study, we used fiber photometry to record the temporal dynamics in the population activity of septo-hippocampal cholinergic neurons at sub-second resolution during a hippocampus-dependent object location memory task using ChAT-Cre mice. Using a general linear model, we quantified the extent to which cholinergic dynamics were explained by changes in movement speed, behavioral states such as locomotion, grooming, and rearing, and hippocampus-dependent cognitive states such as recognizing a novel location of a familiar object. The data show that cholinergic dynamics contain a multiplexed code of fast and slow signals i) coding for the logarithm of movement speed at sub-second timescales, ii) providing a phasic spatial novelty signal during the brief periods of exploring a novel object location, and iii) coding for environmental novelty at a seconds-long timescale. Furthermore, behavioral event-related phasic cholinergic activity around the onset and offset of the behavior demonstrates that fast cholinergic transients help facilitate a switch in cognitive and behavioral state before and during the onset of behavior. These findings enhance understanding of the mechanisms by which cholinergic modulation contributes to the coding of movement speed and encoding of novel spatial information.

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.

PinkyCaMP a mScarlet-based calcium sensor with exceptional brightness, photostability, and multiplexing capabilities

Genetically encoded calcium (Ca2+) indicators (GECIs) are widely used for imaging neuronal activity, yet current limitations of existing red fluorescent GECIs have constrained their applicability. The inherently dim fluorescence and low signal-to-noise ratio of red-shifted GECIs have posed significant challenges. More critically, several red-fluorescent GECIs exhibit photoswitching when exposed to blue light, thereby limiting their applicability in all-optical experimental approaches. Here, we present the development of PinkyCaMP, the first mScarlet-based Ca2+ sensor that outperforms current red fluorescent sensors in brightness, photostability, signal-to-noise ratio, and compatibility with optogenetics and neurotransmitter imaging. PinkyCaMP is well-tolerated by neurons, showing no toxicity or aggregation, both in vitro and in vivo. All imaging approaches, including single-photon excitation methods such as fiber photometry, widefield imaging, miniscope imaging, as well as two-photon imaging in awake mice, are fully compatible with PinkyCaMP.

The role of the prefrontal cortex in modulating aggression in humans and rodents

Accumulating evidence suggests that the prefrontal cortex (PFC) plays an important role in aggression. However, the findings regarding the key neural mechanisms and molecular pathways underlying the modulation of aggression by the PFC are relatively scattered, with many inconsistencies and areas that would benefit from exploration. Here, we highlight the relationship between the PFC and aggression in humans and rodents and describe the anatomy and function of the human PFC, along with homologous regions in rodents. At the molecular level, we detail how the major neuromodulators of the PFC impact aggression. At the circuit level, this review provides an overview of known and potential subcortical projections that regulate aggression in rodents. Finally, at the disease level, we review the correlation between PFC alterations and heightened aggression in specific human psychiatric disorders. Our review provides a framework for PFC modulation of aggression, resolves several intriguing paradoxes from previous studies, and illuminates new avenues for further study.

Interactions between Arousal State and CO2 Determine the Activity of Central Chemoreceptor Neurons That Drive Breathing

The homeostatic regulation of pulmonary ventilation, and ultimately arterial PCO2, depends on interactions between respiratory chemoreflexes and arousal state. The ventilatory response to CO2 is triggered by neurons in the retrotrapezoid nucleus (RTN) that function as sensors of central pH, which can be identified in adulthood by the expression of Phox2b and neuromedin B. Here, we examine the dynamic response of genetically defined RTN neurons to hypercapnia and arousal state in freely behaving adult male and female mice using the calcium indicator jGCaMP7 and fiber photometry. We found that hypercapnia vigorously activates RTN neurons with a low CO2 recruitment threshold and with response kinetics that match respiratory activity whereas hypoxia had little effect. RTN activity increased transiently during wakefulness and respiratory-related arousals and rose persistently during rapid eye movement sleep, and their CO2 response persisted under anesthesia. Complementary studies using inhibitory optogenetics show that RTN activity supports eupneic breathing under anesthesia as well as during states of high arousal, but their activity is redundant for voluntary breathing patterns. Collectively, this study demonstrates that CO2-activated RTN neurons are exquisitely sensitive to the arousal state, which determines their contribution to alveolar ventilation in relation to arterial PCO2.

Protocol for synchronized wireless fiber photometry and video recordings in rodents during behavior

Fiber photometry technique allows investigation of in vivo neural activity during behavior allowing understanding of brain-behavior relationship. Here, we provide a protocol for synchronized wireless fiber photometry and video recordings in rodents during behavior. We explain the detailed steps for stereotaxic virus injection, optic fiber cannula implantation, setup for synchronized fiber photometry and behavioral recording, and analysis of photometry data. These protocol steps can be adapted for various animal models, photometry, and behavioral recording systems. For complete details on the use and execution of this protocol, please refer to Tamboli et al.1 and Amalyan et al.2.

Fiber photometry in neuroscience research: principles, applications, and future directions

In recent years, fluorescent sensors are enjoying a surge of popularity in the field of neuroscience. Through the development of novel genetically encoded sensors as well as improved methods of detection and analysis, fluorescent sensing has risen as a new major technique in neuroscience alongside molecular, electrophysiological, and imaging methods, opening up new avenues for research. Combined with multiphoton microscopy and fiber photometry, these sensors offer unique advantages in terms of cellular specificity, access to multiple targets - from calcium dynamics to neurotransmitter release to intracellular processes - as well as high capability for in vivo interrogation of neurobiological mechanisms underpinning behavior. Here, we provide a brief overview of the method, present examples of its integration with other tools in recent studies ranging from cellular to systems neuroscience, and discuss some of its principles and limitations, with the aim of introducing new potential users to this rapidly developing and potent technique.

Hunger modulates exploration through suppression of dopamine signaling in the tail of striatum

Caloric depletion leads to behavioral changes that help an animal find food and restore its homeostatic balance. Hunger increases exploration and risk-taking behavior, allowing an animal to forage for food despite risks; however, the neural circuitry underlying this change is unknown. Here, we characterize how hunger restructures an animal's spontaneous behavior as well as its directed exploration of a novel object. We show that hunger-induced changes in exploration are accompanied by and result from modulation of dopamine signaling in the tail of the striatum (TOS). Dopamine signaling in the TOS is modulated by internal hunger state through the activity of agouti-related peptide (AgRP) neurons, putative "hunger neurons" in the arcuate nucleus of the hypothalamus. These AgRP neurons are poly-synaptically connected to TOS-projecting dopaminergic neurons through the lateral hypothalamus, the central amygdala, and the periaqueductal grey. We thus delineate a hypothalamic-midbrain circuit that coordinates changes in exploration behavior in the hungry state.

Striatal Serotonin Release Signals Reward Value

Serotonin modulates diverse phenotypes and functions including depressive, aggressive, impulsive, and feeding behaviors, all of which have reward-related components. To date, research has focused on understanding these effects by measuring and manipulating dorsal raphe serotonin neurons and using single-receptor approaches. These studies have led to a better understanding of the heterogeneity of serotonin actions on behavior; however, they leave open many questions about the timing and location of serotonin's actions modulating the neural circuits that drive these behaviors. Recent advances in genetically encoded fluorescent biosensors, including the GPCR activation-based sensor for serotonin (GRAB-5-HT), enable the measurement of serotonin release in mice on a timescale compatible with a single rewarding event without corelease confounds. Given substantial evidence from slice electrophysiology experiments showing that serotonin influences neural activity of the striatal circuitry, and the known role of the dorsal medial striatal (DMS) in reward-directed behavior, we focused on understanding the parameters and timing that govern serotonin release in the DMS in the context of reward consumption, external reward value, internal state, and cued reward. Overall, we found that serotonin release is associated with each of these and encodes reward anticipation, value, approach, and consumption in the DMS.

Emerging Functions of Neuromodulation during Sleep

Neuromodulators act on multiple timescales to affect neuronal activity and behavior. They function as synaptic fine-tuners and master coordinators of neuronal activity across distant brain regions and body organs. While much research on neuromodulation has focused on roles in promoting features of wakefulness and transitions between sleep and wake states, the precise dynamics and functions of neuromodulatory signaling during sleep have received less attention. This review discusses research presented at our minisymposium at the 2024 Society for Neuroscience meeting, highlighting how norepinephrine, dopamine, and acetylcholine orchestrate brain oscillatory activity, control sleep architecture and microarchitecture, regulate responsiveness to sensory stimuli, and facilitate memory consolidation. The potential of each neuromodulator to influence neuronal activity is shaped by the state of the synaptic milieu, which in turn is influenced by the organismal or systemic state. Investigating the effects of neuromodulator release across different sleep substates and synaptic environments offers unique opportunities to deepen our understanding of neuromodulation and explore the distinct computational opportunities that arise during sleep. Moreover, since alterations in neuromodulatory signaling and sleep are implicated in various neuropsychiatric disorders and because existing pharmacological treatments affect neuromodulatory signaling, gaining a deeper understanding of the less-studied aspects of neuromodulators during sleep is of high importance.

Distinct dynamics and intrinsic properties in ventral tegmental area populations mediate reward association and motivation

Ventral tegmental area (VTA) dopamine neurons regulate reward-related associative learning and reward-driven motivated behaviors, but how these processes are coordinated by distinct VTA neuronal subpopulations remains unresolved. Here, we compare the contribution of two primarily dopaminergic and largely non-overlapping VTA subpopulations, all VTA dopamine neurons and VTA GABAergic neurons of the mouse midbrain, to these processes. We find that the dopamine subpopulation that projects to the nucleus accumbens (NAc) core preferentially encodes reward-predictive cues and prediction errors. In contrast, the subpopulation that projects to the NAc shell preferentially encodes goal-directed actions and relative reward anticipation. VTA GABA neuron activity strongly contrasts VTA dopamine population activity and preferentially encodes reward outcome and retrieval. Electrophysiology, targeted optogenetics, and whole-brain input mapping reveal multiple convergent sources that contribute to the heterogeneity among VTA dopamine subpopulations that likely underlies their distinct encoding of reward-related associations and motivation that defines their functions in these contexts.

Activity in the dorsal hippocampus-mPFC circuit modulates stress-coping strategies during inescapable stress

Anatomical connectivity and lesion-deficit studies have shown that the dorsal and ventral hippocampi contribute to cognitive and emotional processes, respectively. However, the role of the dorsal hippocampus (dHP) in emotional or stress-related behaviors remains unclear. Here, we showed that neuronal activity in the dHP affects stress-coping behaviors in mice via excitatory projections to the medial prefrontal cortex (mPFC). The antidepressant ketamine rapidly induced c-Fos expression in both the dorsal and ventral hippocampi. The suppression of GABAergic transmission in the dHP-induced molecular changes similar to those induced by ketamine administration, including eukaryotic elongation factor 2 (eEF2) dephosphorylation, brain-derived neurotrophic factor (BDNF) elevation, and extracellular signal-regulated kinase (ERK) phosphorylation. These synaptic and molecular changes in the dHP induced a reduction in the immobility time of the mice in the tail-suspension and forced swim tests without affecting anxiety-related behavior. Conversely, pharmacological and chemogenetic potentiation of inhibitory neurotransmission in the dHP CA1 region induced passive coping behaviors during the tests. Transneuronal tracing and electrophysiology revealed monosynaptic excitatory connections between dHP CA1 neurons and mPFC neurons. Optogenetic stimulation of dHP CA1 neurons in freely behaving mice produced c-Fos induction and spike firing in the mPFC neurons. Chemogenetic activation of the dHP-recipient mPFC neurons reversed the passive coping behaviors induced by suppression of dHP CA1 neuronal activity. Collectively, these results indicate that neuronal activity in the dHP modulates stress-coping strategies to inescapable stress and contributes to the antidepressant effects of ketamine via the dHP-mPFC circuit.

Event boundaries drive norepinephrine release and distinctive neural representations of space in the rodent hippocampus

Episodic memories are temporally segmented around event boundaries that tend to coincide with moments of environmental change. During these times, the state of the brain should change rapidly, or reset, to ensure that the information encountered before and after an event boundary is encoded in different neuronal populations. Norepinephrine (NE) is thought to facilitate this network reorganization. However, it is unknown whether event boundaries drive NE release in the hippocampus and, if so, how NE release relates to changes in hippocampal firing patterns. The advent of the new GRABNE sensor now allows for the measurement of NE binding with sub-second resolution. Using this tool in mice, we tested whether NE is released into the dorsal hippocampus during event boundaries defined by unexpected transitions between spatial contexts and presentations of novel objections. We found that NE binding dynamics were well explained by the time elapsed after each of these environmental changes, and were not related to conditioned behaviors, exploratory bouts of movement, or reward. Familiarity with a spatial context accelerated the rate in which phasic NE binding decayed to baseline. Knowing when NE is elevated, we tested how hippocampal coding of space differs during these moments. Immediately after context transitions we observed relatively unique patterns of neural spiking which settled into a modal state at a similar rate in which NE returned to baseline. These results are consistent with a model wherein NE release drives hippocampal representations away from a steady-state attractor. We hypothesize that the distinctive neural codes observed after each event boundary may facilitate long-term memory and contribute to the neural basis for the primacy effect.

Convergent direct and indirect cortical streams shape avoidance decisions in mice via the midline thalamus

Current concepts of corticothalamic organization in the mammalian brain are mainly based on sensory systems, with less focus on circuits for higher-order cognitive functions. In sensory systems, first-order thalamic relays are driven by subcortical inputs and modulated by cortical feedback, while higher-order relays receive strong excitatory cortical inputs. The applicability of these principles beyond sensory systems is uncertain. We investigated mouse prefronto-thalamic projections to the midline thalamus, revealing distinct top-down control. Unlike sensory systems, this pathway relies on indirect modulation via the thalamic reticular nucleus (TRN). Specifically, the prelimbic area, which influences emotional and motivated behaviors, impacts instrumental avoidance responses through direct and indirect projections to the paraventricular thalamus. Both pathways promote defensive states, but the indirect pathway via the TRN is essential for organizing avoidance decisions through disinhibition. Our findings highlight intra-thalamic circuit dynamics that integrate cortical cognitive signals and their role in shaping complex behaviors.

Hypothalamic AgRP neurons regulate the hyperphagia of lactation

OBJECTIVE

The lactational period is associated with profound hyperphagia to accommodate the energy demands of nursing. These changes are important for the long-term metabolic health of the mother and children as altered feeding during lactation increases the risk of mothers and offspring developing metabolic disorders later in life. However, the specific behavioral mechanisms and neural circuitry mediating the hyperphagia of lactation are incompletely understood.

METHODS

Here, we utilized home cage feeding devices to characterize the dynamics of feeding behavior in lactating mice. A combination of pharmacological and behavioral assays were utilized to determine how lactation alters meal structure, circadian aspects of feeding, hedonic feeding, and sensitivity to hunger and satiety signals in lactating mice. Finally, we utilized chemogenetic, immunohistochemical, and in vivo imaging approaches to characterize the role of hypothalamic agouti-related peptide (AgRP) neurons in lactational-hyperphagia.

RESULTS

The lactational period is associated with increased meal size, altered circadian patterns of feeding, reduced sensitivity to gut-brain satiety signals, and enhanced sensitivity to negative energy balance. Hypothalamic AgRP neurons display increased sensitivity to negative energy balance and altered in vivo activity during the lactational state. Further, using in vivo imaging approaches we demonstrate that AgRP neurons are directly activated by lactation. Chemogenetic inhibition of AgRP neurons acutely reduces feeding in lactating mice, demonstrating an important role for these neurons in lactational-hyperphagia.

CONCLUSIONS

Together, these results show that lactation collectively alters multiple components of feeding behavior and position AgRP neurons as an important cellular substrate mediating the hyperphagia of lactation.

Cortical Acetylcholine Response to Deep Brain Stimulation of the Basal Forebrain

Background

Deep brain stimulation (DBS), the direct electrical stimulation of neuronal tissue in the basal forebrain to enhance release of the neurotransmitter acetylcholine, is under consideration as a method to improve executive function in patients with dementia. While some small studies indicate a positive response in the clinical setting, the relationship between DBS and acetylcholine pharmacokinetics is incompletely understood.

Objective

We examined the cortical acetylcholine response to different stimulation parameters of the basal forebrain.

Methods

2-photon imaging was combined with deep brain stimulation. 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 muscarinic acetylcholine receptor sensor, and blood vessels were imaged with Texas red.

Results

Experiments manipulating pulse train frequency demonstrated that integrated acetylcholine induced fluorescence was insensitive to frequency, and that 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 10s of stimulation at 60Hz, and the integrated response increased 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 to 18 seconds in all experiments.

Conclusions

These data demonstrate that acetylcholine receptor activation is insensitive to frequency between 60 and 130 Hz. High peak responses are achieved with up to 900 pulses. Donepezil increases total acetylcholine receptor activation associated with DBS but did not change temporal kinetics. The long time constants observed in the cerebral cortex add to the evidence supporting volume in addition to synaptic transmission.

Dopamine neurons drive spatiotemporally heterogeneous striatal dopamine signals during learning

Environmental cues, through Pavlovian learning, become conditioned stimuli that invigorate and guide animals toward rewards. Dopamine (DA) neurons in the ventral tegmental area (VTA) and substantia nigra (SNc) are crucial for this process, via engagement of a reciprocally connected network with their striatal targets. Critically, it remains unknown how dopamine neuron activity itself engages dopamine signals throughout the striatum, across learning. Here, we investigated how optogenetic Pavlovian cue conditioning of VTA or SNc dopamine neurons directs cue-evoked behavior and shapes subregion-specific striatal dopamine dynamics. We used a fluorescent biosensor to monitor dopamine in the nucleus accumbens (NAc) core and shell, dorsomedial striatum (DMS), and dorsolateral striatum (DLS). We demonstrate spatially heterogeneous, learning-dependent dopamine changes across striatal regions. Although VTA stimulation-evoked robust dopamine release in NAc core, shell, and DMS, predictive cues preferentially recruited dopamine release in NAc core, starting early in training, and DMS, late in training. Negative prediction error signals, reflecting a violation in the expectation of dopamine neuron activation, only emerged in the NAc core and DMS. Despite the development of vigorous movement late in training, conditioned dopamine signals did not emerge in the DLS, even during Pavlovian conditioning with SNc dopamine neuron activation, which elicited robust DLS dopamine release. Together, our studies show a broad dissociation in the fundamental prediction and reward-related information generated by VTA and SNc dopamine neuron populations and signaled by dopamine across the striatum. Further, they offer new insight into how larger-scale adaptations across the striatal network emerge during learning to coordinate behavior.

Central amygdala CRF+ neurons promote heightened threat reactivity following early life adversity in mice

Failure to appropriately predict and titrate reactivity to threat is a core feature of fear and anxiety-related disorders and is common following early life adversity (ELA). A population of neurons in the lateral central amygdala (CeAL) expressing corticotropin releasing factor (CRF) have been proposed to be key in processing threat of different intensities to mediate active fear expression. Here, we use in vivo fiber photometry to show that ELA results in sex-specific changes in the activity of CeAL CRF+ neurons, yielding divergent mechanisms underlying the augmented startle in ELA mice, a translationally relevant behavior indicative of heightened threat reactivity and hypervigilance. Further, chemogenic inhibition of CeAL CRF+ neurons selectively diminishes startle and produces a long-lasting suppression of threat reactivity. These findings identify a mechanism for sex-differences in susceptibility for anxiety following ELA and have broad implications for understanding the neural circuitry that encodes and gates the behavioral expression of fear.

Identifying behavioral links to neural dynamics of multifiber photometry recordings in a mouse social behavior network

Objective.Distributed hypothalamic-midbrain neural circuits help orchestrate complex behavioral responses during social interactions. Given rapid advances in optical imaging, it is a fundamental question how population-averaged neural activity measured by multi-fiber photometry (MFP) for calcium fluorescence signals correlates with social behaviors is a fundamental question. This paper aims to investigate the correspondence between MFP data and social behaviors.Approach:We propose a state-space analysis framework to characterize mouse MFP data based on dynamic latent variable models, which include a continuous-state linear dynamical system and a discrete-state hidden semi-Markov model. We validate these models on extensive MFP recordings during aggressive and mating behaviors in male-male and male-female interactions, respectively.Main results:Our results show that these models are capable of capturing both temporal behavioral structure and associated neural states, and produce interpretable latent states. Our approach is also validated in computer simulations in the presence of known ground truth.Significance:Overall, these analysis approaches provide a state-space framework to examine neural dynamics underlying social behaviors and reveals mechanistic insights into the relevant networks.

Protocol for fiber photometry recording from deep brain regions in head-fixed mice

To exclude the influence of motion on in vivo calcium imaging, animals usually need to be fixed. However, the whole-body restraint can cause stress in animals, affecting experimental results. In addition, some brain regions are prone to bleeding during surgery, which lowers the success rate of calcium imaging. Here, we present a protocol for calcium imaging using heparin-treated fiber in head-fixed mice. We describe steps for stereotaxic surgery, including virus injection and optic fiber implantation, fiber photometry, and data analysis. For complete details on the use and execution of this protocol, please refer to Du et al.1.

Cell Signaling in the Circadian Pacemaker: New Insights from in vivo Imaging

BACKGROUND

"One for all, and all for one," the famous rallying cry of the Three Musketeers, in Alexandre Dumas's popular novel, certainly applies to the 20,000 cells composing the suprachiasmatic nuclei (SCN). These cells work together to form the central clock that coordinates body rhythms in tune with the day-night cycle. Like virtually every body cell, individual SCN cells exhibit autonomous circadian oscillations, but this rhythmicity only reaches a high level of precision and robustness when the cells are coupled with their neighbors. Therefore, understanding the functional network organization of SCN cells beyond their core rhythmicity is an important issue in circadian biology.

SUMMARY

The present review summarizes the main results from our recent study demonstrating the feasibility of recording SCN cells in freely moving mice and the significance of variations in intracellular calcium over several timescales.

KEY MESSAGE

We discuss how in vivo imaging at the cell level will be pivotal to interrogate the mammalian master clock, in an integrated context that preserves the SCN network organization, with intact inputs and outputs.

An integrated microfluidic and fluorescence platform for probing in vivo neuropharmacology

Neurotechnologies and genetic tools for dissecting neural circuit functions have advanced rapidly over the past decade, although the development of complementary pharmacological method-ologies has comparatively lagged. Understanding the precise pharmacological mechanisms of neuroactive compounds is critical for advancing basic neurobiology and neuropharmacology, as well as for developing more effective treatments for neurological and neuropsychiatric disorders. However, integrating modern tools for assessing neural activity in large-scale neural networks with spatially localized drug delivery remains a major challenge. Here, we present a dual microfluidic-photometry platform that enables simultaneous intracranial drug delivery with neural dynamics monitoring in the rodent brain. The integrated platform combines a wireless, battery-free, miniaturized fluidic microsystem with optical probes, allowing for spatially and temporally specific drug delivery while recording activity-dependent fluorescence using genetically encoded calcium indicators (GECIs), neurotransmitter sensors GRAB NE and GRAB DA , and neuropeptide sensors. We demonstrate the performance this platform for investigating neuropharmacological mechanisms in vivo and characterize its efficacy in probing precise mechanistic actions of neuroactive compounds across several rapidly evolving neuroscience domains.

Dopamine and acetylcholine correlations in the nucleus accumbens depend on behavioral task states

Dopamine in the nucleus accumbens ramps up as animals approach desired goals. These ramps have received intense scrutiny because they seem to violate long-held hypotheses on dopamine function. Furthermore, it has been proposed that they are driven by local acetylcholine release, i.e., that they are mechanistically separate from dopamine signals related to reward prediction errors. Here, we tested this hypothesis by simultaneously recording accumbal dopamine and acetylcholine signals in rats executing a task involving motivated approach. Contrary to recent reports, we found that dopamine ramps were not coincidental with changes in acetylcholine. Instead, we found that acetylcholine could be positively, negatively, or uncorrelated with dopamine depending on whether the task phase was determined by a salient cue, reward prediction error, or active approach, respectively. Our results suggest that accumbal dopamine and acetylcholine are largely independent but may combine to engage different postsynaptic mechanisms depending on the behavioral task states.

Dopamine neurons drive spatiotemporally heterogeneous striatal dopamine signals during learning

Environmental cues, through Pavlovian learning, become conditioned stimuli that invigorate and guide animals toward acquisition of rewards. Dopamine neurons in the ventral tegmental area (VTA) and substantia nigra (SNC) are crucial for this process. Dopamine neurons are embedded in a reciprocally connected network with their striatal targets, the functional organization of which remains poorly understood. Here, we investigated how learning during optogenetic Pavlovian cue conditioning of VTA or SNC dopamine neurons directs cue-evoked behavior and shapes subregion-specific striatal dopamine dynamics. We used a fluorescent dopamine biosensor to monitor dopamine in the nucleus accumbens (NAc) core and shell, dorsomedial striatum (DMS), and dorsolateral striatum (DLS). We demonstrate spatially heterogeneous, learning-dependent dopamine changes across striatal regions. While VTA stimulation evoked robust dopamine release in NAc core, shell, and DMS, cues predictive of this activation preferentially recruited dopamine release in NAc core, starting early in training, and DMS, late in training. Corresponding negative prediction error signals, reflecting a violation in the expectation of dopamine neuron activation, only emerged in the NAc core and DMS, and not the shell. Despite development of vigorous movement late in training, conditioned dopamine signals did not similarly emerge in the DLS, even during Pavlovian conditioning with SNC dopamine neuron activation, which elicited robust DLS dopamine release. Together, our studies show broad dissociation in the fundamental prediction and reward-related information generated by different dopamine neuron populations and signaled by dopamine across the striatum. Further, they offer new insight into how larger-scale plasticity across the striatal network emerges during Pavlovian learning to coordinate behavior.

Engineering, applications, and future perspectives of GPCR-based genetically encoded fluorescent indicators for neuromodulators

This review explores the evolving landscape of G-protein-coupled receptor (GPCR)-based genetically encoded fluorescent indicators (GEFIs), with a focus on their development, structural components, engineering strategies, and applications. We highlight the unique features of this indicator class, emphasizing the importance of both the sensing domain (GPCR structure and activation mechanism) and the reporting domain (circularly permuted fluorescent protein (cpFP) structure and fluorescence modulation). Further, we discuss indicator engineering approaches, including the selection of suitable cpFPs and expression systems. Additionally, we showcase the diversity and flexibility of their application by presenting a summary of studies where such indicators were used. Along with all the advantages, we also focus on the current limitations as well as common misconceptions that arise when using these indicators. Finally, we discuss future directions in indicator engineering, including strategies for screening with increased throughput, optimization of the ligand-binding properties, structural insights, and spectral diversity.