Flat-floored air-lifted platform: a new method for combining behavior with microscopy or electrophysiology on awake freely moving rodents

Aberrant cortical activity, functional connectivity, and neural assembly architecture after photothrombotic stroke in mice

Despite substantial progress in mapping the trajectory of network plasticity resulting from focal ischemic stroke, the extent and nature of changes in neuronal excitability and activity within the peri-infarct cortex of mice remains poorly defined. Most of the available data have been acquired from anesthetized animals, acute tissue slices, or infer changes in excitability from immunoassays on extracted tissue, and thus may not reflect cortical activity dynamics in the intact cortex of an awake animal. Here, in vivo two-photon calcium imaging in awake, behaving mice was used to longitudinally track cortical activity, network functional connectivity, and neural assembly architecture for 2 months following photothrombotic stroke targeting the forelimb somatosensory cortex. Sensorimotor recovery was tracked over the weeks following stroke, allowing us to relate network changes to behavior. Our data revealed spatially restricted but long-lasting alterations in somatosensory neural network function and connectivity. Specifically, we demonstrate significant and long-lasting disruptions in neural assembly architecture concurrent with a deficit in functional connectivity between individual neurons. Reductions in neuronal spiking in peri-infarct cortex were transient but predictive of impairment in skilled locomotion measured in the tapered beam task. Notably, altered neural networks were highly localized, with assembly architecture and neural connectivity relatively unaltered a short distance from the peri-infarct cortex, even in regions within 'remapped' forelimb functional representations identified using mesoscale imaging with anaesthetized preparations 8 weeks after stroke. Thus, using longitudinal two-photon microscopy in awake animals, these data show a complex spatiotemporal relationship between peri-infarct neuronal network function and behavioral recovery. Moreover, the data highlight an apparent disconnect between dramatic functional remapping identified using strong sensory stimulation in anaesthetized mice compared to more subtle and spatially restricted changes in individual neuron and local network function in awake mice during stroke recovery.

Norepinephrine regulates calcium signals and fate of oligodendrocyte precursor cells in the mouse cerebral cortex

Oligodendrocyte precursor cells (OPCs) generate oligodendrocytes, contributing to myelination and myelin repair. OPCs contact axons and respond to neuronal activity, but how the information relayed by the neuronal activity translates into OPC Ca2+ signals, which in turn influence their fate, remains unknown. We generated transgenic mice for concomitant monitoring of OPCs Ca2+ signals and cell fate using 2-photon microscopy in the somatosensory cortex of awake-behaving mice. Ca2+ signals in OPCs mainly occur within processes and confine to Ca2+ microdomains. A subpopulation of OPCs enhances Ca2+ transients while mice engaged in exploratory locomotion. We found that OPCs responsive to locomotion preferentially differentiate into oligodendrocytes, and locomotion-non-responsive OPCs divide. Norepinephrine mediates locomotion-evoked Ca2+ increases in OPCs by activating α1 adrenergic receptors, and chemogenetic activation of OPCs or noradrenergic neurons promotes OPC differentiation. Hence, we uncovered that for fate decisions OPCs integrate Ca2+ signals, and norepinephrine is a potent regulator of OPC fate.

Dynamic 3D imaging of cerebral blood flow in awake mice using self-supervised-learning-enhanced optical coherence Doppler tomography

Cerebral blood flow (CBF) is widely used to assess brain function. However, most preclinical CBF studies have been performed under anesthesia, which confounds findings. High spatiotemporal-resolution CBF imaging of awake animals is challenging due to motion artifacts and background noise, particularly for Doppler-based flow imaging. Here, we report ultrahigh-resolution optical coherence Doppler tomography (µODT) for 3D imaging of CBF velocity (CBFv) dynamics in awake mice by developing self-supervised deep-learning for effective image denoising and motion-artifact removal. We compare cortical CBFv in awake vs. anesthetized mice and their dynamic responses in arteriolar, venular and capillary networks to acute cocaine (1 mg/kg, i.v.), a highly addictive drug associated with neurovascular toxicity. Compared with awake, isoflurane (2-2.5%) induces vasodilation and increases CBFv within 2-4 min, whereas dexmedetomidine (0.025 mg/kg, i.p.) does not change vessel diameters nor flow. Acute cocaine decreases CBFv to the same extent in dexmedetomidine and awake states, whereas decreases are larger under isoflurane, suggesting that isoflurane-induced vasodilation might have facilitated detection of cocaine-induced vasoconstriction. Awake mice after chronic cocaine show severe vasoconstriction, CBFv decreases and vascular adaptations with extended diving arteriolar/venular vessels that prioritize blood supply to deeper cortical capillaries. The 3D imaging platform we present provides a powerful tool to study dynamic changes in vessel diameters and morphology alongside CBFv networks in the brain of awake animals that can advance our understanding of the effects of drugs and disease conditions (ischemia, tumors, wound healing).

An open-source head-fixation and implant-protection system for mice

Mice are widely used in neuroscience experiments, which often require head-fixation and attachment of skull-mounted hardware. For many experiments, these components must remain intact over weeks to months, ideally with animals group housed. Many labs have designed ad-hoc head-fixation systems, which is an inefficient process. For example, when reinventing these solutions in our lab, we faced challenges with group housing, wherein mice would chew and damage implanted cannulas and electrodes of their cage mates. We performed several non-trivial design iterations to solve this problem, and present the most successful designs as an open-source collection. The designs include a standard mounting headbar compatible with most skull-mounted hardware, a snap-on protective mouse hat (headhat) to prevent mice from chewing the hardware, and a head-fixation station to facilitate common experimental procedures. We provide 3D-printing files, detail vendors and software used to make the components of the system, and provide editable design files for maximum flexibility to individual lab requirements.

Imaging the brain in action: a motorized optical rotary joint for wide field fibroscopy in freely moving animals

SIGNIFICANCE

The study of neuronal processes governing behavior in awake behaving mice is constantly boosted by the development of technological strategies, such as miniaturized microscopes and closed-loop virtual reality systems. However, the former limits the quality of recorded signals due to constrains in size and weight and the latter suffers from the restriction of the movement repertoire of the animal, therefore, hardly reproducing the complexity of natural multisensory scenes.

AIM

Another strategy that takes advantage of both approaches consists of the use of a fiber-bundle interface to carry optical signals from a moving animal to a conventional imaging system. However, as the bundle is usually fixed below the optics, its torsion resulting from rotations of the animal inevitably constrains the behavior over long recordings. Our aim was to overcome this major limitation of fibroscopic imaging.

APPROACH

We developed a motorized optical rotary joint controlled by an inertial measurement unit at the animal's head.

RESULTS

We show its principle of operation, demonstrate its efficacy in a locomotion task, and propose several modes of operation for a wide range of experimental designs.

CONCLUSIONS

Combined with an optical rotary joint, fibroscopic approaches represent an outstanding tool to link neuronal activity with behavior in mice at the millisecond timescale.

Calcium imaging: A versatile tool to examine Huntington's disease mechanisms and progression

Huntington's disease (HD) is a fatal, hereditary neurodegenerative disorder that causes chorea, cognitive deficits, and psychiatric symptoms. It is characterized by accumulation of mutant Htt protein, which primarily impacts striatal medium-sized spiny neurons (MSNs), as well as cortical pyramidal neurons (CPNs), causing synapse loss and eventually cell death. Perturbed Ca²⁺ homeostasis is believed to play a major role in HD, as altered Ca²⁺ homeostasis often precedes striatal dysfunction and manifestation of HD symptoms. In addition, dysregulation of Ca²⁺ can cause morphological and functional changes in MSNs and CPNs. Therefore, Ca²⁺ imaging techniques have the potential of visualizing changes in Ca²⁺ dynamics and neuronal activity in HD animal models. This minireview focuses on studies using diverse Ca²⁺ imaging techniques, including two-photon microscopy, fiber photometry, and miniscopes, in combination of Ca²⁺ indicators to monitor activity of neurons in HD models as the disease progresses. We then discuss the future applications of Ca²⁺ imaging to visualize disease mechanisms and alterations associated with HD, as well as studies showing how, as a proof-of-concept, Ca²⁺imaging using miniscopes in freely-behaving animals can help elucidate the differential role of direct and indirect pathway MSNs in HD symptoms.

Electrophysiological Studies Support Utility of Positive Modulators of SK Channels for the Treatment of Spinocerebellar Ataxia Type 2

Spinocerebellar ataxia type 2 (SCA2) is an incurable hereditary disorder accompanied by cerebellar degeneration following ataxic symptoms. The causative gene for SCA2 is ATXN2. The ataxin-2 protein is involved in RNA metabolism; the polyQ expansion may interrupt ataxin-2 interaction with its molecular targets, thus representing a loss-of-function mutation. However, mutant ataxin-2 protein also displays the features of gain-of-function mutation since it forms the aggregates in SCA2 cells and also enhances the IP3-induced calcium release in affected neurons. The cerebellar Purkinje cells (PCs) are primarily affected in SCA2. Their tonic pacemaker activity is crucial for the proper cerebellar functioning. Disturbances in PC pacemaking are observed in many ataxic disorders. The abnormal intrinsic pacemaking was reported in mouse models of episodic ataxia type 2 (EA2), SCA1, SCA2, SCA3, SCA6, Huntington's disease (HD), and in some other murine models of the disorders associated with the cerebellar degeneration. In our studies using SCA2-58Q transgenic mice via cerebellar slice recording and in vivo recording from urethane-anesthetized mice and awake head-fixed mice, we have demonstrated the impaired firing frequency and irregularity of PCs in these mice. PC pacemaker activity is regulated by SK channels. The pharmacological activation of SK channels has demonstrated some promising results in the electrophysiological experiments on EA2, SCA1, SCA2, SCA3, SCA6, HD mice, and also on mutant CACNA1A mice. In our studies, we have reported that the SK activators CyPPA and NS309 converted bursting activity into tonic, while oral treatment with CyPPA and NS13001 significantly improved motor performance and PC morphology in SCA2 mice. The i.p. injections of chlorzoxazone (CHZ) during in vivo recording sessions converted bursting cells into tonic in anesthetized SCA2 mice. And, finally, long-term injections of CHZ recovered the precision of PC pacemaking activity in awake SCA2 mice and alleviated their motor decline. Thus, the SK activation can be used as a potential way to treat SCA2 and other diseases accompanied by cerebellar degeneration.

Long-term transverse imaging of the hippocampus with glass microperiscopes

The hippocampus consists of a stereotyped neuronal circuit repeated along the septal-temporal axis. This transverse circuit contains distinct subfields with stereotyped connectivity that support crucial cognitive processes, including episodic and spatial memory. However, comprehensive measurements across the transverse hippocampal circuit in vivo are intractable with existing techniques. Here, we developed an approach for two-photon imaging of the transverse hippocampal plane in awake mice via implanted glass microperiscopes, allowing optical access to the major hippocampal subfields and to the dendritic arbor of pyramidal neurons. Using this approach, we tracked dendritic morphological dynamics on CA1 apical dendrites and characterized spine turnover. We then used calcium imaging to quantify the prevalence of place and speed cells across subfields. Finally, we measured the anatomical distribution of spatial information, finding a non-uniform distribution of spatial selectivity along the DG-to-CA1 axis. This approach extends the existing toolbox for structural and functional measurements of hippocampal circuitry.

Cranial and Spinal Window Preparation for in vivo Optical Neuroimaging in Rodents and Related Experimental Techniques

Optical neuroimaging provides an effective neuroscience tool for multi-scale investigation of the neural structures and functions, ranging from molecular, cellular activities to the inter-regional connectivity assessment. Amongst experimental preparations, the implementation of an artificial window to the central nervous system (CNS) is primarily required for optical visualization of the CNS and associated brain activities through the opaque skin and bone. Either thinning down or removing portions of the skull or spine is necessary for unobstructed long-term in vivo observations, for which types of the cranial and spinal window and applied materials vary depending on the study objectives. As diversely useful, a window can be designed to accommodate other experimental methods such as electrophysiology or optogenetics. Moreover, auxiliary apparatuses would allow the recording in synchrony with behavior of large-scale brain connectivity signals across the CNS, such as olfactory bulb, cerebral cortex, cerebellum, and spinal cord. Such advancements in the cranial and spinal window have resulted in a paradigm shift in neuroscience, enabling in vivo investigation of the brain function and dysfunction at the microscopic, cellular level. This Review addresses the types and classifications of windows used in optical neuroimaging while describing how to perform in vivo studies using rodent models in combination with other experimental modalities during behavioral tests. The cranial and spinal window has enabled longitudinal examination of evolving neural mechanisms via in situ visualization of the brain. We expect transformable and multi-functional cranial and spinal windows to become commonplace in neuroscience laboratories, further facilitating advances in optical neuroimaging systems.

Versatile Surface Electrodes for Combined Electrophysiology and Two-Photon Imaging of the Mouse Central Nervous System

Understanding and modulating CNS function in physiological as well as pathophysiological contexts remains a significant ambition in research and clinical applications. The investigation of the multifaceted CNS cell types including their interactions and contributions to neural function requires a combination of the state-of-the-art in vivo electrophysiology and imaging techniques. We developed a novel type of liquid crystal polymer (LCP) surface micro-electrode manufactured in three customized designs with up to 16 channels for recording and stimulation of brain activity. All designs include spare central spaces for simultaneous 2P-imaging. Nanoporous platinum-plated contact sites ensure a low impedance and high current transfer. The epidural implantation of the LCP micro-electrodes could be combined with standard cranial window surgery. The epidurally positioned electrodes did not only display long-term biocompatibility, but we also observed an additional stabilization of the underlying CNS tissue. We demonstrate the electrode’s versatility in combination with in vivo 2P-imaging by monitoring anesthesia-awake cycles of transgenic mice with GCaMP3 expression in neurons or astrocytes. Cortical stimulation and simultaneous 2P Ca²⁺ imaging in neurons or astrocytes highlighted the astrocytes’ integrative character in neuronal activity processing. Furthermore, we confirmed that spontaneous astroglial Ca²⁺ signals are dampened under anesthesia, while evoked signals in neurons and astrocytes showed stronger dependency on stimulation intensity rather than on various levels of anesthesia. Finally, we show that the electrodes provide recordings of the electrocorticogram (ECoG) with a high signal-to noise ratio and spatial signal differences which help to decipher brain activity states during experimental procedures. Summarizing, the novel LCP surface micro-electrode is a versatile, convenient, and reliable tool to investigate brain function in vivo.

Imaging of C-fos Activity in Neurons of the Mouse Parietal Association Cortex during Acquisition and Retrieval of Associative Fear Memory

The parietal cortex of rodents participates in sensory and spatial processing, movement planning, and decision-making, but much less is known about its role in associative learning and memory formation. The present study aims to examine the involvement of the parietal association cortex (PtA) in associative fear memory acquisition and retrieval in mice. Using ex vivo c-Fos immunohistochemical mapping and in vivo Fos-EGFP two-photon imaging, we show that PtA neurons were specifically activated both during acquisition and retrieval of cued fear memory. Fos immunohistochemistry revealed specific activation of the PtA neurons during retrieval of the 1-day-old fear memory. In vivo two-photon Fos-EGFP imaging confirmed this result and in addition detected specific c-Fos responses of the PtA neurons during acquisition of cued fear memory. To allow a more detailed study of the long-term activity of such PtA engram neurons, we generated a Fos-Cre-GCaMP transgenic mouse line that employs the Targeted Recombination in Active Populations (TRAP) technique to detect calcium events specifically in cells that were Fos-active during conditioning. We show that gradual accumulation of GCaMP3 in the PtA neurons of Fos-Cre-GCaMP mice peaks at the 4th day after fear learning. We also describe calcium transients in the cell bodies and dendrites of the TRAPed neurons. This provides a proof-of-principle for TRAP-based calcium imaging of PtA functions during memory processes as well as in experimental models of fear- and anxiety-related psychiatric disorders and their specific therapies.

Place Cells in Head-Fixed Mice Navigating a Floating Real-World Environment

The hippocampal place cell system in rodents has provided a major paradigm for the scientific investigation of memory function and dysfunction. Place cells have been observed in area CA1 of the hippocampus of both freely moving animals, and of head-fixed animals navigating in virtual reality environments. However, spatial coding in virtual reality preparations has been observed to be impaired. Here we show that the use of a real-world environment system for head-fixed mice, consisting of an air-floating track with proximal cues, provides some advantages over virtual reality systems for the study of spatial memory. We imaged the hippocampus of head-fixed mice injected with the genetically encoded calcium indicator GCaMP6s while they navigated circularly constrained or open environments on the floating platform. We observed consistent place tuning in a substantial fraction of cells despite the absence of distal visual cues. Place fields remapped when animals entered a different environment. When animals re-entered the same environment, place fields typically remapped over a time period of multiple days, faster than in freely moving preparations, but comparable with virtual reality. Spatial information rates were within the range observed in freely moving mice. Manifold analysis indicated that spatial information could be extracted from a low-dimensional subspace of the neural population dynamics. This is the first demonstration of place cells in head-fixed mice navigating on an air-lifted real-world platform, validating its use for the study of brain circuits involved in memory and affected by neurodegenerative disorders.

In vivo analysis of the spontaneous firing of cerebellar Purkinje cells in awake transgenic mice that model spinocerebellar ataxia type 2

Cerebellar Purkinje cells (PCs) fire spontaneously in a tonic mode, although the precision of this pacemaking activity is disturbed in many abnormal conditions involving cerebellar atrophy, such as many spinocerebellar ataxias (SCAs). In our previous studies we used the single-unit extracellular recording method to analyze spontaneous PC firing in vivo in the anesthetized SCA2-58Q transgenic mice. We realized that PCs from aging SCA2-58Q mice fire much less regularly compared to PCs from their wild type (WT) littermates and this abnormal activity can be reversed with an intraperitoneal (i. p.) injection of SK channel-positive modulator chlorzoxazone (CHZ). Here we used the same single-unit extracellular recording method to analyze the spontaneous firing in vivo in awake SCA2-58Q transgenic mice. For this purpose, we used the Mobile HomeCage (Neurotar, Finland) floating platform to immobilize the experimental animal's head during the recording sessions. We discovered that generally PCs from awake animals fired much more frequently and much less regularly than previously observed PCs from anesthetized animals. In vivo recordings from awake SCA2/WT mice revealed that complex spikes, which are generated by PCs in reply to the excitation coming by climbing fibers, as well as simple spikes, were much less frequent in SCA2 mice compared to their WT littermates. To test the effect of the SK channel positive modulation on the PCs firing activity in awake SCA2 mice and also the effect on their motor coordination, we started the CHZ trial in these mice. We discovered that the long-term i. p. injections of CHZ did not affect the spike generation in SCA2-58Q mice, however, they did recover the precision of this spontaneous pacemaking activity. Furthermore, we also showed that treatment with CHZ alleviated the age-dependent motor impairment in SCA2-58Q mice. We propose that the lack of precision in PC spike generation might be a key cause for the progression of ataxic symptoms in different SCAs and that the activation of calcium-activated potassium channels, including SK channels, can be used as a potential way to treat SCAs on the physiological level of the disease.

Stress and behavioral correlates in the head-fixed method: stress measurements, habituation dynamics, locomotion, and motor-skill learning in mice

Manual restriction of head movement, or head-fixation, of awake rodents allows for sophisticated investigation of neural circuits in vivo, that would otherwise be impossible in completely freely moving animals. While it is known that head-fixation induces stress, the scale of this stress and habituation dynamics remain unclear. We used the Mobile HomeCage system (Neurotar Ltd, Finland) where animals have their heads fixed to an aluminum frame but are otherwise freely moving in an ultralight carbon container floating above an air-dispensing base. For 25 consecutive days, mice were head-fixed while standing on the air-lifted platform for 2 h per day and blood samples were taken periodically to measure variation in the stress-related hormone, corticosterone. We showed that the initial increase in corticosterone concentration is followed by a return to control level throughout the days of head-fixed training. We also found a locomotor correlate of this drop. We conducted a battery of stress-sensitive behavioral paradigms in freely-moving mice that revealed minor differences following chronic head-fixation. Finally, we analyzed motor-skill learning in the head-fixed setup with a floating container. We believe that our results may contribute to better interpretation of past literature and future in vivo experiments using head-fixed animals.

Functional imaging evidence for task-induced deactivation and disconnection of a major default mode network hub in the mouse brain

Our report first validates an imaging technique, functional ultrasound, an experimental approach providing significantly higher sensitivity and spatiotemporal resolution than fMRI, for the study of sensory stimulation-induced changes in activation and connectivity patterns in the awake mouse brain. Next, by using this approach in lightly sedated conscious mice, we provide functional evidence supporting an evolutionary preserved function of the default mode network, a set of highly connected brain areas, which is characteristically impaired in major neuropsychiatric diseases in humans. Uncovering the existence and function of the default mode network in mice, the main preclinical model organism for neuropsychiatric diseases, opens new avenues of high translational relevance for brain research and drug development.

The default mode network (DMN) has been defined in functional brain imaging studies as a set of highly connected brain areas, which are active during wakeful rest and inactivated during task-based stimulation. DMN function is characteristically impaired in major neuropsychiatric diseases, emphasizing its interest for translational research. However, in the mouse, a major preclinical rodent model, there is still no functional imaging evidence supporting DMN deactivation and deconnection during high-demanding cognitive/sensory tasks. Here we have developed functional ultrasound (fUS) imaging to properly visualize both activation levels and functional connectivity patterns, in head-restrained awake and behaving mice, and investigated their modulation during a sensory-task, whisker stimulation. We identified reproducible and highly symmetric resting-state networks, with overall connectivity strength directly proportional to the wakefulness level of the animal. We show that unilateral whisker stimulation leads to the expected activation of the contralateral barrel cortex in lightly sedated mice, while interhemispheric inhibition reduces activity in the ipsilateral barrel cortex. Whisker stimulation also leads to elevated bilateral connectivity in the hippocampus. Importantly, in addition to functional changes in these major hubs of tactile information processing, whisker stimulation during genuine awake resting-state periods leads to highly specific reductions both in activation and interhemispheric correlation within the restrosplenial cortex, a major hub of the DMN. These results validate an imaging technique for the study of activation and connectivity in the lightly sedated awake mouse brain and provide evidence supporting an evolutionary preserved function of the DMN, putatively improving translational relevance of preclinical models of neuropsychiatric diseases.

Procedure and protocols for optical imaging of cerebral blood flow and hemodynamics in awake mice

We describe a method and procedure that allows for the optical coherence tomography angiography (OCTA) and intrinsic optical signal imaging (IOSI) of cerebral blood flow and hemodynamics in fully awake mice. We detail the procedure of chronic cranial window preparation, the use of an air-lift mobile homecage to achieve stable optical recording in the head-restrained awake mouse, and the imaging methods to achieve multiparametric hemodynamic measurements. The results show that by using a collection of OCTA algorithms, the high-resolution cerebral vasculature can be reliably mapped at a fully awake state, including flow velocity measurements in penetrating arterioles and capillary bed. Lastly, we demonstrate how the awake imaging paradigm is used to study cortical hemodynamics in the mouse barrel cortex during whisker stimulation. The method presented here will facilitate optical recording in the awake, active mice and open the door to many projects that can bridge the hemodynamics in neurovascular units to naturalistic behavior.

Probing Cortical Activity During Head-Fixed Behavior

The cortex is crucial for many behaviors, ranging from sensory-based behaviors to working memory and social behaviors. To gain an in-depth understanding of the contribution to these behaviors, cellular and sub-cellular recordings from both individual and populations of cortical neurons are vital. However, techniques allowing such recordings, such as two-photon imaging and whole-cell electrophysiology, require absolute stability of the head, a requirement not often fulfilled in freely moving animals. Here, we review and compare behavioral paradigms that have been developed and adapted for the head-fixed preparation, which together offer the needed stability for live recordings of neural activity in behaving animals. We also review how the head-fixed preparation has been used to explore the function of primary sensory cortices, posterior parietal cortex (PPC) and anterior lateral motor (ALM) cortex in sensory-based behavioral tasks, while also discussing the considerations of performing such recordings. Overall, this review highlights the head-fixed preparation as allowing in-depth investigation into the neural activity underlying behaviors by providing highly controllable settings for precise stimuli presentation which can be combined with behavioral paradigms ranging from simple sensory detection tasks to complex, cross-modal, memory-guided decision-making tasks.

Somatic and Dendritic Encoding of Spatial Variables in Retrosplenial Cortex Differs during 2D Navigation

Active amplification of organized synaptic inputs in dendrites can endow individual neurons with the ability to perform complex computations. However, whether dendrites in behaving animals perform independent local computations is not known. Such activity may be particularly important for complex behavior, where neurons integrate multiple streams of information. Head-restrained imaging has yielded important insights into cellular and circuit function, but this approach limits behavior and the underlying computations. We describe a method for full-featured 2-photon imaging in awake mice during free locomotion with volitional head rotation. We examine head direction and position encoding in simultaneously imaged apical tuft dendrites and their respective cell bodies in retrosplenial cortex, an area that encodes multi-modal navigational information. Activity in dendrites was not determined solely by somatic activity but reflected distinct navigational variables, fulfilling the requirements for dendritic computation. Our approach provides a foundation for studying sub-cellular processes during complex behaviors.

Pharmaco-electroencephalographic responses in the rat differ between active and inactive locomotor states

Quantitative electroencephalography from freely moving rats is commonly used as a translational tool for predicting drug‐effects in humans. We hypothesized that drug‐effects may be expressed differently depending on whether the rat is in active locomotion or sitting still during recording sessions, and proposed automatic state‐detection as a viable tool for estimating drug‐effects free of hypo‐/hyperlocomotion‐induced effects. We aimed at developing a fully automatic and validated method for detecting two behavioural states: active and inactive, in one‐second intervals and to use the method for evaluating ketamine, DOI, d‐cycloserine, d‐amphetamine, and diazepam effects specifically within each state. The developed state‐detector attained high precision with more than 90% of the detected time correctly classified, and multiple differences between the two detected states were discovered. Ketamine‐induced delta activity was found specifically related to locomotion. Ketamine and DOI suppressed theta and beta oscillations exclusively during inactivity. Characteristic gamma and high‐frequency oscillations (HFO) enhancements of the NMDAR and 5HT_2A modulators, speculated associated with locomotion, were profound and often largest during the inactive state. State‐specific analyses, theoretically eliminating biases from altered occurrence of locomotion, revealed only few effects of d‐amphetamine and diazepam. Overall, drug‐effects were most abundant in the inactive state. In conclusion, this new validated and automatic locomotion state‐detection method enables fast and reliable state‐specific analysis facilitating discovery of state‐dependent drug‐effects and control for altered occurrence of locomotion. This may ultimately lead to better cross‐species translation of electrophysiological effects of pharmacological modulations.

Longitudinal two-photon imaging in somatosensory cortex of behaving mice reveals dendritic spine formation enhancement by subchronic administration of low-dose ketamine

Ketamine, a well-known anesthetic, has recently attracted renewed attention as a fast-acting antidepressant. A single dose of ketamine induces rapid synaptogenesis, which may underlie its antidepressant effect. To test whether repeated exposure to ketamine triggers sustained synaptogenesis, we administered a sub-anesthetic dose of ketamine (10 mg/kg i.p.) once-daily for 5 days, and repeatedly imaged dendritic spines of the YFP-expressing pyramidal neurons in somatosensory cortex of awake female mice using in vivo two-photon microscopy. We found that the spine formation rate became significantly higher at 72–132 h after the first ketamine injection (but not at 6–24 h), while the rate of elimination of pre-existing spines remained unchanged. In contrast to the net gain of spines observed in ketamine-treated mice, the vehicle-injected control mice exhibited a net loss typical for young-adult animals undergoing synapse pruning. Ketamine-induced spinogenesis was correlated with increased PSD-95 and phosphorylated actin, consistent with formation of new synapses. Moreover, structural synaptic plasticity caused by ketamine was paralleled by a significant improvement in the nest building behavioral assay. Taken together, our data show that subchronic low-dose ketamine induces a sustained shift towards spine formation.

Isoflurane produces antidepressant effects and induces TrkB signaling in rodents

A brief burst-suppressing isoflurane anesthesia has been shown to rapidly alleviate symptoms of depression in a subset of patients, but the neurobiological basis of these observations remains obscure. We show that a single isoflurane anesthesia produces antidepressant-like behavioural effects in the learned helplessness paradigm and regulates molecular events implicated in the mechanism of action of rapid-acting antidepressant ketamine: activation of brain-derived neurotrophic factor (BDNF) receptor TrkB, facilitation of mammalian target of rapamycin (mTOR) signaling pathway and inhibition of glycogen synthase kinase 3β (GSK3β). Moreover, isoflurane affected neuronal plasticity by facilitating long-term potentiation in the hippocampus. We also found that isoflurane increased activity of the parvalbumin interneurons, and facilitated GABAergic transmission in wild type mice but not in transgenic mice with reduced TrkB expression in parvalbumin interneurons. Our findings strengthen the role of TrkB signaling in the antidepressant responses and encourage further evaluation of isoflurane as a rapid-acting antidepressant devoid of the psychotomimetic effects and abuse potential of ketamine.

Simple platform for chronic imaging of hippocampal activity during spontaneous behaviour in an awake mouse

Chronic electrophysiological recordings of neuronal activity combined with two-photon Ca²⁺ imaging give access to high resolution and cellular specificity. In addition, awake drug-free experimentation is required for investigating the physiological mechanisms that operate in the brain. Here, we developed a simple head fixation platform, which allows simultaneous chronic imaging and electrophysiological recordings to be obtained from the hippocampus of awake mice. We performed quantitative analyses of spontaneous animal behaviour, the associated network states and the cellular activities in the dorsal hippocampus as well as estimated the brain stability limits to image dendritic processes and individual axonal boutons. Ca²⁺ imaging recordings revealed a relatively stereotyped hippocampal activity despite a high inter-animal and inter-day variability in the mouse behavior. In addition to quiet state and locomotion behavioural patterns, the platform allowed the reliable detection of walking steps and fine speed variations. The brain motion during locomotion was limited to ~1.8 μm, thus allowing for imaging of small sub-cellular structures to be performed in parallel with recordings of network and behavioural states. This simple device extends the drug-free experimentation in vivo, enabling high-stability optophysiological experiments with single-bouton resolution in the mouse awake brain.

Virtual reality systems for rodents

Over the last decade virtual reality (VR) setups for rodents have been developed and utilized to investigate the neural foundations of behavior. Such VR systems became very popular since they allow the use of state-of-the-art techniques to measure neural activity in behaving rodents that cannot be easily used with classical behavior setups. Here, we provide an overview of rodent VR technologies and review recent results from related research. We discuss commonalities and differences as well as merits and issues of different approaches. A special focus is given to experimental (behavioral) paradigms in use. Finally we comment on possible use cases that may further exploit the potential of VR in rodent research and hence inspire future studies.

Social Ultrasonic Vocalization in Awake Head-Restrained Mouse

Numerous animal species emit vocalizations in response to various social stimuli. The neural basis of vocal communication has been investigated in monkeys, songbirds, rats, bats, and invertebrates resulting in deep insights into motor control, neural coding, and learning. Mice, which recently became very popular as a model system for mammalian neuroscience, also utilize ultrasonic vocalizations (USVs) during mating behavior. However, our knowledge is lacking of both the behavior and its underlying neural mechanism. We developed a novel method for head-restrained male mice (HRMM) to interact with non-restrained female mice (NRFM) and show that mice can emit USVs in this context. We first recorded USVs in a free arena with non-restrained male mice (NRMM) and NRFM. Of the NRMM, which vocalized in the free arena, the majority could be habituated to also vocalize while head-restrained but only when a female mouse was present in proximity. The USVs emitted by HRMM are similar to the USVs of NRMM in the presence of a female mouse in their spectral structure, inter-syllable interval distribution, and USV sequence length, and therefore are interpreted as social USVs. By analyzing the vocalizations of NRMM, we established criteria to predict which individuals are likely to vocalize while head fixed based on the USV rate and average syllable duration. To characterize the USVs emitted by HRMM, we analyzed the syllable composition of HRMM and NRMM and found that USVs emitted by HRMM have a higher proportion of USVs with complex spectral representation, supporting previous studies showing that mice social USVs are context dependent. Our results suggest a way to study the neural mechanisms of production and control of social vocalization in mice using advanced methods requiring head fixation.

Air-Track: a real-world floating environment for active sensing in head-fixed mice

Natural behavior occurs in multiple sensory and motor modalities and in particular is dependent on sensory feedback that constantly adjusts behavior. To investigate the underlying neuronal correlates of natural behavior, it is useful to have access to state-of-the-art recording equipment (e.g., 2-photon imaging, patch recordings, etc.) that frequently requires head fixation. This limitation has been addressed with various approaches such as virtual reality/air ball or treadmill systems. However, achieving multimodal realistic behavior in these systems can be challenging. These systems are often also complex and expensive to implement. Here we present "Air-Track," an easy-to-build head-fixed behavioral environment that requires only minimal computational processing. The Air-Track is a lightweight physical maze floating on an air table that has all the properties of the "real" world, including multiple sensory modalities tightly coupled to motor actions. To test this system, we trained mice in Go/No-Go and two-alternative forced choice tasks in a plus maze. Mice chose lanes and discriminated apertures or textures by moving the Air-Track back and forth and rotating it around themselves. Mice rapidly adapted to moving the track and used visual, auditory, and tactile cues to guide them in performing the tasks. A custom-controlled camera system monitored animal location and generated data that could be used to calculate reaction times in the visual and somatosensory discrimination tasks. We conclude that the Air-Track system is ideal for eliciting natural behavior in concert with virtually any system for monitoring or manipulating brain activity.