A proposal for a coordinated effort for the determination of brainwide neuroanatomical connectivity in model organisms at a mesoscopic scale

In this era of complete genomes, our knowledge of neuroanatomical circuitry remains surprisingly sparse. Such knowledge is however critical both for basic and clinical research into brain function. Here we advocate for a concerted effort to fill this gap, through systematic, experimental mapping of neural circuits at a mesoscopic scale of resolution suitable for comprehensive, brain-wide coverage, using injections of tracers or viral vectors. We detail the scientific and medical rationale and briefly review existing knowledge and experimental techniques. We define a set of desiderata, including brain-wide coverage; validated and extensible experimental techniques suitable for standardization and automation; centralized, open access data repository; compatibility with existing resources, and tractability with current informatics technology. We discuss a hypothetical but tractable plan for mouse, additional efforts for the macaque, and technique development for human. We estimate that the mouse connectivity project could be completed within five years with a comparatively modest budget.Comment: 41 page

Neuronal activity regulates neurotransmitter switching in the adult brain following light-induced stress.

Neurotransmitter switching in the adult mammalian brain occurs following photoperiod-induced stress, but the mechanism of regulation is unknown. Here, we demonstrate that elevated activity of dopaminergic neurons in the paraventricular nucleus of the hypothalamus (PaVN) in the adult rat is required for the loss of dopamine expression after long-day photoperiod exposure. The transmitter switch occurs exclusively in PaVN dopaminergic neurons that coexpress vesicular glutamate transporter 2 (VGLUT2), is accompanied by a loss of dopamine type 2 receptors (D2Rs) on corticotrophin-releasing factor (CRF) neurons, and can lead to increased release of CRF. Suppressing activity of all PaVN glutamatergic neurons decreases the number of inhibitory PaVN dopaminergic neurons, indicating homeostatic regulation of transmitter expression in the PaVN

Exosomes regulate neurogenesis and circuit assembly.

Exosomes are thought to be released by all cells in the body and to be involved in intercellular communication. We tested whether neural exosomes can regulate the development of neural circuits. We show that exosome treatment increases proliferation in developing neural cultures and in vivo in dentate gyrus of P4 mouse brain. We compared the protein cargo and signaling bioactivity of exosomes released by hiPSC-derived neural cultures lacking MECP2, a model of the neurodevelopmental disorder Rett syndrome, with exosomes released by isogenic rescue control neural cultures. Quantitative proteomic analysis indicates that control exosomes contain multiple functional signaling networks known to be important for neuronal circuit development. Treating MECP2-knockdown human primary neural cultures with control exosomes rescues deficits in neuronal proliferation, differentiation, synaptogenesis, and synchronized firing, whereas exosomes from MECP2-deficient hiPSC neural cultures lack this capability. These data indicate that exosomes carry signaling information required to regulate neural circuit development

Imaging fast electrical activity in the brain with electrical impedance tomography.

Imaging of neuronal depolarization in the brain is a major goal in neuroscience, but no technique currently exists that could image neural activity over milliseconds throughout the whole brain. Electrical impedance tomography (EIT) is an emerging medical imaging technique which can produce tomographic images of impedance changes with non-invasive surface electrodes. We report EIT imaging of impedance changes in rat somatosensory cerebral cortex with a resolution of 2ms and <200μm during evoked potentials using epicortical arrays with 30 electrodes. Images were validated with local field potential recordings and current source-sink density analysis. Our results demonstrate that EIT can image neural activity in a volume 7×5×2mm in somatosensory cerebral cortex with reduced invasiveness, greater resolution and imaging volume than other methods. Modeling indicates similar resolutions are feasible throughout the entire brain so this technique, uniquely, has the potential to image functional connectivity of cortical and subcortical structures

Photoacoustic brain imaging: from microscopic to macroscopic scales

Human brain mapping has become one of the most exciting contemporary research areas, with major breakthroughs expected in the coming decades. Modern brain imaging techniques have allowed neuroscientists to gather a wealth of anatomic and functional information about the brain. Among these techniques, by virtue of its rich optical absorption contrast, high spatial and temporal resolutions, and deep penetration, photoacoustic tomography (PAT) has attracted more and more attention, and is playing an increasingly important role in brain studies. In particular, PAT complements other brain imaging modalities by providing high-resolution functional and metabolic imaging. More importantly, PAT’s unique scalability enables scrutinizing the brain at both microscopic and macroscopic scales, using the same imaging contrast. In this review, we present the state-of-the-art PAT techniques for brain imaging, summarize representative neuroscience applications, outline the technical challenges in translating PAT to human brain imaging, and envision potential technological deliverables

High-resolution photoacoustic tomography of resting-state functional connectivity in the mouse brain

The increasing use of mouse models for human brain disease studies presents an emerging need for a new functional imaging modality. Using optical excitation and acoustic detection, we developed a functional connectivity photoacoustic tomography system, which allows noninvasive imaging of resting-state functional connectivity in the mouse brain, with a large field of view and a high spatial resolution. Bilateral correlations were observed in eight functional regions, including the olfactory bulb, limbic, parietal, somatosensory, retrosplenial, visual, motor, and temporal regions, as well as in several subregions. The borders and locations of these regions agreed well with the Paxinos mouse brain atlas. By subjecting the mouse to alternating hyperoxic and hypoxic conditions, strong and weak functional connectivities were observed, respectively. In addition to connectivity images, vascular images were simultaneously acquired. These studies show that functional connectivity photoacoustic tomography is a promising, noninvasive technique for functional imaging of the mouse brain

Rabies screen reveals GPe control of cocaine-triggered plasticity.

Identification of neural circuit changes that contribute to behavioural plasticity has routinely been conducted on candidate circuits that were preselected on the basis of previous results. Here we present an unbiased method for identifying experience-triggered circuit-level changes in neuronal ensembles in mice. Using rabies virus monosynaptic tracing, we mapped cocaine-induced global changes in inputs onto neurons in the ventral tegmental area. Cocaine increased rabies-labelled inputs from the globus pallidus externus (GPe), a basal ganglia nucleus not previously known to participate in behavioural plasticity triggered by drugs of abuse. We demonstrated that cocaine increased GPe neuron activity, which accounted for the increase in GPe labelling. Inhibition of GPe activity revealed that it contributes to two forms of cocaine-triggered behavioural plasticity, at least in part by disinhibiting dopamine neurons in the ventral tegmental area. These results suggest that rabies-based unbiased screening of changes in input populations can identify previously unappreciated circuit elements that critically support behavioural adaptations

Reappraisal of transcallosal neuron organization in mice and evaluation of their dendritic remodeling and circuit integration following traumatic brain injury

Traumatic Brain Injury (TBI) is an enormous global socio-economic burden since, apart from its high death rate, it is the primary cause of coma worldwide and a prevalent cause of long-term disability. Until today there is no established treatment for dealing with the long-term outcomes of TBI despite many years of research. Although a lot is known about the pathophysiology of TBI in the damaged tissue and the surrounding area in case of focal lesion, only few studies have investigated the structural and functional integrity of the contralateral intact cortex. In order to explore this territory, this study employs a well-established and widely used animal model of focal open skull TBI known as the Controlled Cortical Impact (CCI) model. The first aim of this study was to systematically characterize a specific neuronal population, the transcallosal projection neurons, as they are the ones connecting the intact cortex with the lesioned cortex. The description of the organization of transcallosal neurons and their axonal projections at the contralateral hemisphere was carried out in healthy, non-injured C57Bl6 mice. Retrograde and anterograde tracing methods were implemented to label transcallosal cell bodies and their axonal projections, respectively. In addition, different injection coordinates were used in order to label transcallosal connections at distinct brain regions, including the motor cortex (M1), somatosensory cortex (S1), and barrel cortex, rostral and caudal to Bregma. In agreement with previous research, I observed that transcallosal projections are organized homotopically across the various brain regions, with the axonal terminals spanning the entire cortical column. Interestingly my study describes for the first time a non-negligible fraction of heterotopic transcallosal neurons that, in addition, display a slightly less strict layer distribution pattern compared to the homotopic ones. After the initial characterization of transcallosal neuron organization, I proceeded by investigating how these neurons with projections at the injury site are affected at various timepoints following focal TBI. I used GFPM mice to visualize dendrites and spines of transcallosal and non-transcallosal neurons, in order to examine their structural integrity at different timepoints post-injury. I detected significant differences in dendritic spine density and morphology between controls and injured mice, which were time-dependent. More specifically, the dendritic spine density in transcallosal neurons was strongly decreased as soon as 7days following injury. Interestingly, spine density in non-transcallosal neurons was not changed following TBI. In terms of spine shape, I found a morphological shift only for the apical tuft segments. These results point towards a general sensitivity of transcallosal spines to TBI-induced damage, where loss of spines (preferentially mature) seems to take place at 1-2 weeks post-injury and resolve at 3-6 weeks post-injury, indicative of late plasticity processes. As the anatomically connected neuronal population seems to recover overtime I then decided to further explore whether transcallosal circuit remodeling takes place after TBI. To do so I used the retrograde mono-trans-synaptic tracer SADΔG-GFP (EnvA) Rabies virus. In that way, I was able to distinctively label transcallosal neurons and their presynaptic partners and obtain an overview of the presynaptic population throughout the cortex across brain regions at different post-injury timepoints. This study demonstrates that spine plasticity did not result in adaptive circuit plasticity with the recruitment of other brain regions but rather that initial circuits were re-established. In brief, during this thesis I have demonstrated the adaptive plastic capacities of anatomically connected neurons to the brain injury. I believe that this knowledge may help in unraveling further compensatory plastic mechanisms that could then be therapeutically targeted to improve the outcome following brain injury

Variation in early life maternal care predicts later long range frontal cortex synapse development in mice.

Empirical and theoretical work suggests that early postnatal experience may inform later developing synaptic connectivity to adapt the brain to its environment. We hypothesized that early maternal experience may program the development of synaptic density on long range frontal cortex projections. To test this idea, we used maternal separation (MS) to generate environmental variability and examined how MS affected 1) maternal care and 2) synapse density on virally-labeled long range axons of offspring reared in MS or control conditions. We found that MS and variation in maternal care predicted bouton density on dorsal frontal cortex axons that terminated in the basolateral amygdala (BLA) and dorsomedial striatum (DMS) with more, fragmented care associated with higher density. The effects of maternal care on these distinct axonal projections of the frontal cortex were manifest at different ages. Maternal care measures were correlated with frontal cortex → BLA bouton density at mid-adolescence postnatal (P) day 35 and frontal cortex → DMS bouton density in adulthood (P85). Meanwhile, we found no evidence that MS or maternal care affected bouton density on ascending orbitofrontal cortex (OFC) or BLA axons that terminated in the dorsal frontal cortices. Our data show that variation in early experience can alter development in a circuit-specific and age-dependent manner that may be relevant to understanding the effects of early life adversity