In vivo dynamic ME-MRI reveals differential functional responses of RA- and area X-projecting neurons in the HVC of canaries exposed to conspecific song

Reconfiguration of brain-wide neural activity after early life adversity

Early life adversity (ELA) predisposes individuals to both physical and mental disorders lifelong. How ELA affects brain function leading to this vulnerability is under intense investigation. Research has begun to shed light on ELA effects on localized brain regions within defined circuits. However, investigations into brain-wide neural activity that includes multiple localized regions, determines relationships of activity between regions and identifies shifts of activity in response to experiential conditions is necessary. Here, we performed longitudinal manganese-enhanced magnetic resonance imaging (MEMRI) to image the brain in normally reared or ELA-exposed adults. Images were captured in the freely moving home cage condition, and short- and long-term after naturalistic threat. Images were analyzed with new computational methods, including automated segmentation and fractional activation or difference volumes. We found that neural activity was increased after ELA compared to normal rearing in multiple brain regions, some of which are involved in defensive and/or reward circuitry. Widely distributed patterns of neural activity, "brain states", and their dynamics after threat were altered with ELA. Upon acute threat, ELA-mice retained heightened neural activity within many of these regions, and new hyperactive responses emerged in monoaminergic centers of the mid- and hindbrain. Nine days after acute threat, heightened neural activity remained within locus coeruleus and increased within posterior amygdala, ventral hippocampus, and dorso- and ventromedial hypothalamus, while reduced activity emerged within medial prefrontal cortical regions (prelimbic, infralimbic, anterior cingulate). These results reveal that functional imbalances arise between multiple brain-systems which are dependent upon context and cumulative experiences after ELA.

Studying Axonal Transport in the Brain by Manganese-Enhanced Magnetic Resonance Imaging (MEMRI)

From the earliest notions of dynamic movements within the cell by Leeuwenhoek, intracellular transport in eukaryotes has been primarily explored by optical imaging. The giant axon of the squid became a prime experimental model for imaging transport due to its size, optical transparency, and physiological robustness. Even the biochemical basis of transport was identified using optical assays based on video microscopy of fractionated squid axoplasm. Discoveries about the dynamics and molecular components of the intracellular transport system continued in many model organisms that afforded experimental systems for optical imaging. Yet whether these experimental systems reflected a valid picture of axonal transport in the opaque mammalian brain was unknown.Magnetic resonance imaging (MRI) provides a non-destructive approach to peer into opaque tissues like the brain . The paramagnetic ion, manganese (MnII), gives a hyperintense signal in T₁ weighted MRI that can serve as a marker for axonal transport. Mn(II) enters active neurons via voltage-gated calcium channels and is transported via microtubule motors down their axons by fast axonal transport. Clearance of Mn(II) is slow. Scanning live animals at successive time points reveals the dynamics of Mn(II) transport by detecting Mn(II)-induced intensity increases or accumulations along a known fiber tract, such as the optic nerve or hippocampal-forebrain projections. Mn(II)-based tract tracing also reveals projections even when not in fiber bundles, such as projections in the olfactory system or from medial prefrontal cortex into midbrain and brain stem. The rate of Mn(II) accumulation, detected as increased signal intensity by MR, serves as a proxy for transport rates. Here we describe the method for measuring transport rates and projections by mangeses-enhanced magnetic resonance imaging, MEMRI.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

A three-dimensional digital neurological atlas of the mustached bat (Pteronotus parnellii)

Substantial knowledge of auditory processing within mammalian nervous systems emerged from neurophysiological studies of the mustached bat (Pteronotus parnellii). This highly social and vocal species retrieves precise information about the velocity and range of its targets through echolocation. Such high acoustic processing demands were likely the evolutionary pressures driving the over-development at peripheral (cochlea), metencephalic (cochlear nucleus), mesencephalic (inferior colliculus), diencephalic (medial geniculate body of the thalamus), and telencephalic (auditory cortex) auditory processing levels in this species. Auditory researchers stand to benefit from a three dimensional brain atlas of this species, due to its considerable contribution to auditory neuroscience. Our MRI-based atlas was generated from 2 sets of image data of an ex-vivo male mustached bat's brain: a detailed 3D-T2-weighted-RARE scan [(59 × 63 x 85) μm³] and track density images based on super resolution diffusion tensor images [(78) μm³] reconstructed from a set of low resolution diffusion weighted images using Super-Resolution-Reconstruction (SRR). By surface-rendering these delineations and extrapolating from cortical landmarks and data from previous studies, we generated overlays that estimate the locations of classic functional subregions within mustached bat auditory cortex. This atlas is freely available from our website and can simplify future electrophysiological, microinjection, and neuroimaging studies in this and related species.

Rapid changes in auditory processing in songbirds following acute aromatase inhibition as assessed by fMRI

Contribution to Special Issue on Fast effects of steroids. This review introduces functional MRI (fMRI) as an outstanding tool to assess rapid effects of sex steroids on auditory processing in seasonal songbirds. We emphasize specific advantages of this method as compared to other more conventional and invasive methods used for this purpose and summarize an exemplary auditory fMRI study performed on male starlings exposed to different types of starling song before and immediately after the inhibition of aromatase activity by an i.p. injection of Vorozole™. We describe how most challenges that relate to the necessity to anesthetize subjects and minimize image- and sound-artifacts can be overcome in order to obtain a voxel-based 3D-representation of changes in auditory brain activity to various sound stimuli before and immediately after a pharmacologically-induced depletion of endogenous estrogens. Analysis of the fMRI data by assumption-free statistical methods identified fast specific changes in activity in the auditory brain regions that were stimulus-specific, varying over different seasons, and in several instances lateralized to the left side of the brain. This set of results illustrates the unique features of fMRI that provides opportunities to localize and quantify the brain responses to rapid changes in hormonal status. fMRI offers a new image-guided research strategy in which the spatio-temporal profile of fast neuromodulations can be identified and linked to specific behavioral inputs or outputs. This approach can also be combined with more localized invasive methods to investigate the mechanisms underlying the observed neural changes.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI)

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for in vivo non-invasive whole-brain mapping of neuronal activity. Mn²⁺ enters active neurons via voltage-gated calcium channels and increases local contrast in T₁-weighted images. Given the property of Mn²⁺ of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn²⁺ dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn²⁺ transport. To this end, we injected mice with low dose MnCl₂ 2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl₂ injections. We assessed changes in T₁ contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn²⁺ content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn²⁺ transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn²⁺ transport. The description of this critical property of Mn²⁺ dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

Manganese-enhanced MRI Offers Correlation with Severity of Spinal Cord Injury in Experimental Models

BACKGROUND

Spinal cord injuries (SCI) are clinically challenging, because neural regeneration after cord damage is unknown. In SCI animal models, regeneration is evaluated histologically, requiring animal sacrifice. Noninvasive techniques are needed to detect longitudinal SCI changes.

OBJECTIVE

To compare manganese-enhanced magnetic resonance imaging (MRI [MEMRI]) in hemisection and transection of SCI rat models with diffusion tensor imaging (DTI) and histology.

METHODS

Rats underwent T9 spinal cord transection (n=6), hemisection (n=6), or laminectomy without SCI (controls, n=6). One-half of each group received lateral ventricle MnCl₂ injections 24 hours later. Conventional DTI or T1-weighted MRI was performed 84 hours post-surgery. MEMRI signal intensity ratio above and below the SCI level was calculated. Fractional anisotropy (FA) measurements were taken 1 cm rostral to the SCI. The percentage of FA change was calculated 10 mm rostral to the SCI epicenter, between FA at the dorsal column lesion normalized to a lateral area without FA change. Myelin load (percentage difference) among groups was analyzed by histology.

RESULTS

In transection and hemisection groups, mean MEMRI ratios were 0.62 and 0.87, respectively, versus 0.99 in controls (P<0.001 and P<0.001, respectively); mean FA decreases were 67.5% and 40.1%, respectively, compared with a 6.1% increase in controls (P=0.002 and P=0.019, respectively). Mean myelin load decreased by 38.8% (transection) and 51.8% (hemisection) compared to controls (99.1%) (P<0.001 and P<0.001, respectively). Pearson's correlation coefficients were -0.94 for MEMRI ratio and FA changes and 0.87 for MEMRI and myelin load.

CONCLUSION

MEMERI results correlated to SCI severity measured by FA and myelin load. MEMRI is a useful noninvasive tool to assess neuronal damage after SCI.

Dose response and time course of manganese-enhanced magnetic resonance imaging for visual pathway tracing in vivo

Axonal tracing is useful for detecting optic nerve injury and regeneration, but many commonly used methods cannot be used to observe axoplasmic flow and synaptic transmission in vivo. Manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) can be used for in vivo longitudinal tracing of the visual pathway. Here, we explored the dose response and time course of an intravitreal injection of MnCl2 for tracing the visual pathway in rabbits in vivo using MEMRI. We found that 2 mM MnCl2 enhanced images of the optic nerve but not the lateral geniculate body or superior colliculus, whereas at all other doses tested (5-40 mM), images of the visual pathway from the retina to the contralateral superior colliculus were significantly enhanced. The images were brightest at 24 hours, and then decreased in brightness until the end of the experiment (7 days). No signal enhancement was observed in the visual cortex at any concentration of MnCl2. These results suggest that MEMRI is a viable method for temporospatial tracing of the visual pathway in vivo. Signal enhancement in MEMRI depends on the dose of MnCl2, and the strongest signals appear 24 hours after intravitreal injection.

Manganese enhanced magnetic resonance imaging (MEMRI): a powerful new imaging method to study tinnitus

Manganese enhanced magnetic resonance imaging (MEMRI) is a method used primarily in basic science experiments to advance the understanding of information processing in central nervous system pathways. With this mechanistic approach, manganese (Mn(2+)) acts as a calcium surrogate, whereby voltage-gated calcium channels allow for activity driven entry of Mn(2+) into neurons. The detection and quantification of neuronal activity via Mn(2+) accumulation is facilitated by "hemodynamic-independent contrast" using high resolution MRI scans. This review emphasizes initial efforts to-date in the development and application of MEMRI for evaluating tinnitus (the perception of sound in the absence of overt acoustic stimulation). Perspectives from leaders in the field highlight MEMRI related studies by comparing and contrasting this technique when tinnitus is induced by high-level noise exposure and salicylate administration. Together, these studies underscore the considerable potential of MEMRI for advancing the field of auditory neuroscience in general and tinnitus research in particular. Because of the technical and functional gaps that are filled by this method and the prospect that human studies are on the near horizon, MEMRI should be of considerable interest to the auditory research community. This article is part of a Special Issue entitled <Annual Reviews 2014>.

Manganese-enhanced magnetic resonance imaging (MEMRI)

The use of manganese ions (Mn(2+)) as an MRI contrast agent was introduced over 20 years ago in studies of Mn(2+) toxicity in anesthetized rats (1). Manganese-enhanced MRI (MEMRI) evolved in the late nineties when Koretsky and associates pioneered the use of MEMRI for brain activity measurements (2) as well as neuronal tract tracing (3). Currently, MEMRI has three primary applications in biological systems: (1) contrast enhancement for anatomical detail, (2) activity-dependent assessment and (3) tracing of neuronal connections or tract tracing. MEMRI relies upon the following three main properties of Mn(2+): (1) it is a paramagnetic ion that shortens the spin lattice relaxation time constant (T(1)) of tissues, where it accumulates and hence functions as an excellent T(1) contrast agent; (2) it is a calcium (Ca(2+)) analog that can enter excitable cells, such as neurons and cardiac cells via voltage-gated Ca(2+) channels; and (3) once in the cells Mn(2+) can be transported along axons by microtubule-dependent axonal transport and can also cross synapses trans-synaptically to neighboring neurons. This chapter will emphasize the methodological approaches towards the use of MEMRI in biological systems.

Manganese-enhanced magnetic resonance imaging

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on contrasts that are due to the shortening of the T (1) relaxation time of tissue water protons that become exposed to paramagnetic manganese ions. In experimental animals, the technique combines the high spatial resolution achievable by MRI with the biological information gathered by tissue-specific or functionally induced accumulations of manganese. After in vivo administration, manganese ions may enter cells via voltage-gated calcium channels. In the nervous system, manganese ions are actively transported along the axon. Based on these properties, MEMRI is increasingly used to delineate neuroanatomical structures, assess differences in functional brain activity, and unravel neuronal connectivities in both healthy animals and models of neurological disorders. Because of the cellular toxicity of manganese, a major challenge for a successful MEMRI study is to achieve the lowest possible dose for a particular biological question. Moreover, the interpretation of MEMRI findings requires a profound knowledge of the behavior of manganese in complex organ systems under physiological and pathological conditions. Starting with an overview of manganese pharmacokinetics and mechanisms of toxicity, this chapter covers experimental methods and protocols for applications in neuroscience.

Sex steroid-induced neuroplasticity and behavioral activation in birds

The brain of adult homeothermic vertebrates exhibits a higher degree of morphological neuroplasticity than previously thought, and this plasticity is especially prominent in birds. In particular, incorporation of new neurons is widespread throughout the adult avian forebrain, and the volumes of specific nuclei vary seasonally in a prominent manner. We review here work on steroid-dependent plasticity in birds, based on two cases: the medial preoptic nucleus (POM) of Japanese quail in relation to male sexual behavior, and nucleus HVC in canaries, which regulates song behavior. In male quail, POM volume changes seasonally, and in castrated subjects testosterone almost doubles POM volume within 2 weeks. Significant volume increases are, however, already observable after 1 day. Steroid receptor coactivator-1 is part of the mechanism mediating these effects. Increases in POM volume reflect changes in cell size or spacing and dendritic branching, but are not associated with an increase in neuron number. In contrast, seasonal changes in HVC volume reflect incorporation of newborn neurons in addition to changes in cell size and spacing. These are induced by treatments with exogenous testosterone or its metabolites. Expression of doublecortin, a microtubule-associated protein, is increased by testosterone in the HVC but not in the adjacent nidopallium, suggesting that neuron production in the subventricular zone, the birthplace of newborn neurons, is not affected. Together, these data illustrate the high degree of plasticity that extends into adulthood and is characteristic of avian brain structures. Many questions still remain concerning the regulation and specific function of this plasticity.

Hyperglycemia induces oxidative stress and impairs axonal transport rates in mice

Background

While hyperglycemia-induced oxidative stress damages peripheral neurons, technical limitations have, in part, prevented in vivo studies to determine the effect of hyperglycemia on the neurons in the central nervous system (CNS). While olfactory dysfunction is indicated in diabetes, the effect of hyperglycemia on olfactory receptor neurons (ORNs) remains unknown. In this study, we utilized manganese enhanced MRI (MEMRI) to assess the impact of hyperglycemia on axonal transport rates in ORNs. We hypothesize that (i) hyperglycemia induces oxidative stress and is associated with reduced axonal transport rates in the ORNs and (ii) hyperglycemia-induced oxidative stress activates the p38 MAPK pathway in association with phosphorylation of tau protein leading to the axonal transport deficits.

Research Design and Methods

T₁-weighted MEMRI imaging was used to determine axonal transport rates post-streptozotocin injection in wildtype (WT) and superoxide dismutase 2 (SOD2) overexpressing C57Bl/6 mice. SOD2 overexpression reduces mitochondrial superoxide load. Dihydroethidium staining was used to quantify the reactive oxygen species (ROS), specifically, superoxide (SO). Protein and gene expression levels were determined using western blotting and Q-PCR analysis, respectively.

Results

STZ-treated WT mice exhibited significantly reduced axonal transport rates and significantly higher levels of ROS, phosphorylated p38 MAPK and tau protein as compared to the WT vehicle treated controls and STZ-treated SOD2 mice. The gene expression levels of p38 MAPK and tau remained unchanged.

Conclusion

Increased oxidative stress in STZ-treated WT hyperglycemic mice activates the p38 MAPK pathway in association with phosphorylation of tau and attenuates axonal transport rates in the olfactory system. In STZ-treated SOD-overexpressing hyperglycemic mice in which superoxide levels are reduced, these deficits are reversed.

Convergence of presenilin- and tau-mediated pathways on axonal trafficking and neuronal function

Alzheimer's disease (AD) is a significant and growing health problem in the aging population. Although definitive mechanisms of pathogenesis remain elusive, genetic and histological clues have implicated the proteins presenilin (PS) and tau as key players in AD development. PS mutations lead to familial AD, and although tau is not mutated in AD, tau pathology is a hallmark of the disease. Axonal transport deficits are a common feature of several neurodegenerative disorders and may represent a point of intersection of PS and tau function. To investigate the contribution of wild-type, as opposed to mutant, tau to axonal transport defects in the context of presenilin loss, we used a mouse model postnatally deficient for PS (PS cDKO) and expressing wild-type human tau (WtTau). The resulting PS cDKO;WtTau mice exhibited early tau pathology and axonal transport deficits that preceded development of these phenotypes in WtTau or PS cDKO mice. These deficits were associated with reduced neurotrophin signaling, defective learning and memory and impaired synaptic plasticity. The combination of these effects accelerated neurodegeneration in PS cDKO;WtTau mice. Our results strongly support a convergent role for PS and tau in axonal transport and neuronal survival and function and implicate their misregulation as a contributor to AD pathogenesis.

Love songs, bird brains and diffusion tensor imaging

The song control system of songbirds displays a remarkable seasonal neuroplasticity in species in which song output also changes seasonally. Thus far, this song control system has been extensively analyzed by histological and electrophysiological methods. However, these approaches do not provide a global view of the brain and/or do not allow repeated measurements, which are necessary to establish causal correlations between alterations in neural substrate and behavior. Research has primarily been focused on the song nuclei themselves, largely neglecting their interconnections and other brain regions involved in seasonally changing behavior. In this review, we introduce and explore the song control system of songbirds as a natural model for brain plasticity. At the same time, we point out the added value of the songbird brain model for in vivo diffusion tensor techniques and its derivatives. A compilation of the diffusion tensor imaging (DTI) data obtained thus far in this system demonstrates the usefulness of this in vivo method for studying brain plasticity. In particular, it is shown to be a perfect tool for long-term studies of morphological and cellular changes of specific brain circuits in different endocrine/photoperiod conditions. The method has been successfully applied to obtain quantitative measurements of seasonal changes of fiber tracts and nuclei from the song control system. In addition, outside the song control system, changes have been discerned in the optic chiasm and in an interhemispheric connection. DTI allows the detection of seasonal changes in a region analogous to the mammalian secondary auditory cortex and in regions of the 'social behavior network', an interconnected group of structures that controls multiple social behaviors, including aggression and courtship. DTI allows the demonstration, for the first time, that the songbird brain in its entirety exhibits an extreme seasonal plasticity which is not merely limited to the song control system as was generally believed.

MRI in small brains displaying extensive plasticity

Manganese-enhanced magnetic resonance imaging (ME-MRI), blood oxygen-level-dependent functional MRI (BOLD fMRI) and diffusion tensor imaging (DTI) can now be applied to animal species as small as mice or songbirds. These techniques confirmed previous findings but are also beginning to reveal new phenomena that were difficult or impossible to study previously. These imaging techniques will lead to major technical and conceptual advances in systems neurosciences. We illustrate these new developments with studies of the song control and auditory systems in songbirds, a spatially organized neuronal circuitry that mediates the acquisition, production and perception of complex learned vocalizations. This neural system is an outstanding model for studying vocal learning, brain steroid hormone action, brain plasticity and lateralization of brain function.

Seasonal rewiring of the songbird brain: an in vivo MRI study

The song control system (SCS) of songbirds displays a remarkable plasticity in species where song output changes seasonally. The mechanisms underlying this plasticity are barely understood and research has primarily been focused on the song nuclei themselves, largely neglecting their interconnections and connections with other brain regions. We investigated seasonal changes in the entire brain, including the song nuclei and their connections, of nine male starlings (Sturnus vulgaris). At two times of the year, during the breeding (April) and nonbreeding (July) seasons, we measured in the same subjects cellular attributes of brain regions using in vivo high-resolution diffusion tensor imaging (DTI) at 7 T. An increased fractional anisotropy in the HVC-RA pathway that correlates with an increase in axonal density (and myelination) was found during the breeding season, confirming multiple previous histological reports. Other parts of the SCS, namely the occipitomesencephalic axonal pathway, which contains fiber tracts important for song production, showed increased fractional anisotropy due to myelination during the breeding season and the connection between HVC and Area X showed an increase in axonal connectivity. Beyond the SCS we discerned fractional anisotropy changes that correlate with myelination changes in the optic chiasm and axonal organization changes in an interhemispheric connection, the posterior commissure. These results demonstrate an unexpectedly broad plasticity in the connectivity of the avian brain that might be involved in preparing subjects for the competitive and demanding behavioral tasks that are associated with successful reproduction.

Neuronal projections from ventral tegmental area to forebrain structures in rat studied by manganese-enhanced magnetic resonance imaging

Manganese-enhanced magnetic resonance imaging (MEMRI) has been widely applied to trace neuronal tracts and to monitor morphological and functional responses of specific brain circuits to changes in physiological and/or environmental conditions. In this study, we traced the efferent axonal projections from ventral tegmental area (VTA) to forebrain structures, an integrating part of the reward circuit implicated in drug addiction, in rats using MEMRI. Urethane- and chloral hydrate-anesthetized rats received injection of 100 nl of 200 mM MnCl(2) solution into the right VTA. Mn(2+)-induced signal enhancements were monitored 24 h after injection. The dose of MnCl(2) injection was shown, by histological evaluation, to have minor toxic effects to the neurons in/near the injection site. Dynamic Mn(2+)-induced signal intensity changes in urethane-anesthetized rats during a 24-h period were fit to a sigmoidal function to obtain parameters slope and t(50), which describe the dynamics of apparent Mn(2+)accumulation. The results showed that most of the forebrain structures known to receive neuronal projections from the VTA, including prefrontal cortex, nucleus accumbens, globus pallidus and caudate putaman, were enhanced at 24 h after injection of MnCl(2) into the ipsilateral VTA, and anesthesia seemed have little effects on the amount of Mn(2+)being transported from the VTA to these structures.

Manganese-enhanced MRI: an exceptional tool in translational neuroimaging

The metal manganese is a potent magnetic resonance imaging (MRI) contrast agent that is essential in cell biology. Manganese-enhanced magnetic resonance imaging (MEMRI) is providing unique information in an ever-growing number of applications aimed at understanding the anatomy, the integration, and the function of neural circuits both in normal brain physiology as well as in translational models of brain disease. A major drawback to the use of manganese as a contrast agent, however, is its cellular toxicity. Therefore, paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest using as low a dose as possible while preserving detectability by MRI. In the present work, the different approaches to MEMRI in translational neuroimaging are reviewed and challenges for future identified from a practical standpoint.

A three-dimensional MRI atlas of the zebra finch brain in stereotaxic coordinates

The neurobiology of birdsong, as a model for human speech, is a fast growing area of research in the neurosciences and involves electrophysiological, histological and more recently magnetic resonance imaging (MRI) approaches. Many of these studies require the identification and localization of different brain areas (nuclei) involved in the sensory and motor control of song. Until now, the only published atlases of songbird brains consisted in drawings based on histological slices of the canary and of the zebra finch brain. Taking advantage of high-magnetic field (7 Tesla) MRI technique, we present the first high-resolution (80 x 160 x 160 microm) 3-D digital atlas in stereotaxic coordinates of a male zebra finch brain, the most widely used species in the study of birdsong neurobiology. Image quality allowed us to discern most of the song control, auditory and visual nuclei. The atlas can be freely downloaded from our Web site and can be interactively explored with MRIcro. This zebra finch MRI atlas should become a very useful tool for neuroscientists working on birdsong, especially for co-registrating MRI data but also for determining accurately the optimal coordinates and angular approach for injections or electrophysiological recordings.

Neuronal dysfunction of a long projecting multisynaptic pathway in response to methamphetamine using manganese-enhanced MRI

RATIONALE

Manganese (Mn2+)-enhanced magnetic resonance imaging (MEMRI) is an emerging in vivo MR approach for pharmacological research. One new application of MEMRI in this area is to characterize functional changes of a specific neural circuit that is essential to the central effects of a drug challenge.

OBJECTIVES

To develop and validate such use of MEMRI in neuropharmacology, the current study applied MEMRI to visualize functional changes within a multisynaptic pathway originating from fasciculus retroflexus (FR) that is central to a commonly abused psychostimulant, methamphetamine (MA).

METHODS

Twelve rats were injected intraperitoneally with MA (10 mg/kg) or saline every 2 h for a total of four injections. After 6 days, Mn2+ was injected into the habenular nucleus (FR origin) of all animals, and MEMRI was repeatedly performed at certain points in time over 48 h. The evolution of Mn2+-induced signal enhancement was assessed across the FR tract, the ventral tegmental area (VTA), the striatum, the nucleus accumbens, and the prefrontal cortex (PFC), in both MA-injected animals and controls.

RESULTS

MA treatment was found to affect the complexity and efficiency of Mn2+ uptake in the VTA, via the FR tract, with significantly increased Mn2+ accumulation in the VTA, the dorsomedial part of the striatum, and the PFC.

CONCLUSIONS

MEMRI successfully visualizes disruptions in the multisynaptic pathway as the consequences of repeated MA exposure. MEMRI is potentially an important method in the future to investigate functional changes within a specific pathway under the influences of pharmacological agents, given its excellent functional, in vivo, spatial, and temporal properties.

Magnetic Resonance Imaging of Functional Anatomy: Use for Small Animal Epilepsy Models

Neuroimaging has greatly assisted the diagnosis and treatment of epilepsy. Volumetric analysis, diffusion-weighted imaging, and other magnetic resonance imaging (MRI) modalities provide a clear picture of altered anatomical structures in both focal and nonfocal disease. More recently, advances in novel imaging methodologies have provided unique insights into this disease. Two examples include manganese-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI). MEMRI involves injection of MnCl(2) to evaluate neuronal activity where it is actively transported. Areas of neuronal hyperactivity are expected to have altered uptake and transport. Mapping of activation along preferential uptake pathways can be confirmed by T(1)-weighted imaging. DTI uses the intrinsic preferential mobility of water movement along axonal pathways to map anatomical regions. DTI has been used to investigate white matter disease and is now being applied to clinical and, to a lesser extent, animal investigations of seizure disorders. These two diverse MRI methods can be applied to animal models to provide important information about the functional status of anatomical regions that may be altered by epilepsy.

Current status of functional MRI on small animals: application to physiology, pathophysiology, and cognition

This review aims to make the reader aware of the potential of functional MRI (fMRI) in brain activation studies in small animal models. As small animals generally require anaesthesia for immobilization during MRI protocols, this is believed to be a serious limitation to the type of question that can be addressed with fMRI. We intend to introduce a fresh view with an in-depth overview of the surprising number of fMRI applications in a wide range of important research domains in neuroscience. These include the pathophysiology of brain functioning, the basic science of activity, and functional connectivity of different sensory circuits, including sensory brain mapping, the challenges when studying the hypothalamus as the major control centre in the central nervous system, and the limbic system as neural substrate for emotions and reward. Finally the contribution of small animal fMRI research to cognitive neuroscience is outlined. This review avoids focusing exclusively on traditional small laboratory animals such as rodents, but rather aims to broaden the scope by introducing alternative lissencephalic animal models such as songbirds and fish, as these are not yet well recognized as neuroimaging study subjects. These models are well established in many other neuroscience disciplines, and this review will show that their investigation with in vivo imaging tools will open new doors to cognitive neuroscience and the study of the autonomous nervous system in experimental animals.

Mapping of the habenulo-interpeduncular pathway in living mice using manganese-enhanced 3D MRI

This magnetic resonance imaging (MRI) study describes mapping of the habenulo-interpeduncular pathway in living mice based on manganese-induced contrast. Six hours after intracerebroventricular microinjection of MnCl2, T1-weighted 3D MRI (2.35 T) at 117 mum isotropic resolution revealed a continuous pattern of anterograde labeling from the habenula via the fasciculus retroflexus to the interpeduncular nucleus. Alternatively, the less invasive systemic administration of MnCl2 allowed for monitoring of the dynamic uptake pattern of respective neural components with even higher reproducibility across animals. Time courses covered the range from 42 min to 24 h after injection. In conclusion, manganese-enhanced MRI may open new ways for functional assessments of the habenulo-interpeduncular system in animal models with cognitive impairment.

In vivo diffusion tensor imaging (DTI) of brain subdivisions and vocal pathways in songbirds

The neural substrate for song behavior in songbirds, the song control system (SCS), is thus far the best-documented brain circuit in which to study neuroplasticity and adult neurogenesis. Not only does the volume of the key song control nuclei change in size, but also the density of the connections between them changes as a function of seasonal and hormonal influences. This study explores the potentials of in vivo Diffusion-Tensor MRI (DT-MRI or DTI) to visualize the distinct, concentrated connections of the SCS in the brain of the starling (Sturnus vulgaris). In vivo DTI on starling was performed on a 7T MR system using sagittal and coronal slices. DTI was accomplished with diffusion gradients applied in seven non-collinear directions. Fractional Anisotropy (FA)-maps allowed us to distinguish most of the grey matter and white matter-tracts, including the laminae subdividing the avian telencephalon and the tracts connecting the major song control nuclei (e.g., HVC with RA and X). The FA-maps also allowed us to discern a number of song control, auditory and visual nuclei. Fiber tracking was implemented to illustrate the discrimination of all tracts running from and to RA. Because of the remarkable plasticity inherent to the songbird brain, the successful implementation of DTI in this model could represent a useful tool for the in vivo exploration of fiber degeneration and regeneration and the biological mechanisms involved in brain plasticity.

IR-SE and IR-MEMRI allow in vivo visualization of oscine neuroarchitecture including the main forebrain regions of the song control system

Songbirds share with humans the capacity to produce learned vocalizations (song). Recently, two major regions within the songbird's neural substrate for song learning and production; nucleus robustus arcopallii (RA) and area X (X) are visualized in vivo using Manganese Enhanced MRI (MEMRI). The aim of this study is to extend this to all main interconnected forebrain Song Control Nuclei. The ipsilateral feedback circuits allow Mn2+ to reach all main Song Control Nuclei after stereotaxic injection of very small doses of MnCl2 (10 nl of 10 mM) into HVC of one and MAN (nucleus magnocellularis nidopallii anterioris) of the other hemisphere. Application of a high resolution (80 micron) Spin Echo Inversion Recovery sequence instead of conventional T1-weighted Spin Echo images improves the image contrast dramatically such that some Song Control Nuclei, ventricles, several laminae, fibre tracts and other specific brain regions can be discerned. The combination of this contrast-rich IR-SE sequence with the transsynaptic transport property of Manganese (Inversion Recovery based MEMRI (IR-MEMRI)) enables the visualization of all main interconnected components of the Song Control System in telencephalon and thalamus.

Manganese-enhanced magnetic resonance imaging for in vivo assessment of damage and functional improvement following spinal cord injury in mice

In past decades, much effort has been invested in developing therapies for spinal injuries. Lack of standardization of clinical read-out measures, however, makes direct comparison of experimental therapies difficult. Damage and therapeutic effects in vivo are routinely evaluated using rather subjective behavioral tests. Here we show that manganese-enhanced magnetic resonance imaging (MEMRI) can be used to examine the extent of damage following spinal cord injury (SCI) in mice in vivo. Injection of MnCl2 solution into the cerebrospinal fluid leads to manganese uptake into the spinal cord. Furthermore, after injury MEMRI-derived quantitative measures correlate closely with clinical locomotor scores. Improved locomotion due to treating the detrimental effects of SCI with an established therapy (neutralization of CD95Ligand) is reflected in an increase of manganese uptake into the injured spinal cord. Therefore, we demonstrate that MEMRI is a sensitive and objective tool for in vivo visualization and quantification of damage and functional improvement after SCI. Thus, MEMRI can serve as a reproducible surrogate measure of the clinical status of the spinal cord in mice, potentially becoming a standard approach for evaluating experimental therapies.

In vivo auditory brain mapping in mice with Mn-enhanced MRI

There are currently no noninvasive imaging methods available for auditory brain mapping in mice, despite the increasing use of genetically engineered mice to study auditory brain development and hearing loss. We developed a manganese-enhanced MRI (MEMRI) method to map regions of accumulated sound-evoked activity in awake, normally behaving mice. To demonstrate its utility for high-resolution (100-microm) brain mapping, we used MEMRI to show the tonotopic organization of the mouse inferior colliculus. To test its efficacy in an experimental setting, we acquired data from mice experiencing unilateral conductive hearing loss at different ages. Larger and persistent changes in auditory brainstem activity resulted when hearing loss occurred before the onset of hearing, showing that early hearing loss biases the response toward the functional ear. Thus, MEMRI provides a sensitive and effective method for mapping the mouse auditory brainstem and has great potential for a range of functional neuroimaging studies in normal and mutant mice.

Functional MRI using molecular imaging agents

Contrast agents for magnetic resonance imaging (MRI) have recently been used as cellular-level probes of neural function. New in vivo labeling strategies now enable researchers to follow plasticity of brain activation patterns and cellular structure over time. On the horizon is the prospect that molecular imaging agents specifically designed for functional imaging (fMRI) on a relatively fast timescale could offer an alternative to conventional hemodynamics-based approaches. Development of several MRI sensors has defined principles by which imaging agents for "molecular fMRI" can be constructed; application of engineered sensors for cellular-level correlates of neuronal activity would allow researchers to combine the noninvasiveness of MRI with spatial resolution of tens of microns and temporal resolution of 100ms or less. Facilitated by advances in imaging-agent delivery methods and model systems appropriate for high-resolution neuroimaging, novel molecular imaging strategies continue to potentiate MRI as a tool for mechanistic investigation of neural systems.

Statistical mapping of functional olfactory connections of the rat brain in vivo

The olfactory pathway is a unique route into the brain. To better characterize this system in vivo, rat olfactory functional connections were mapped using magnetic resonance (MR) imaging and manganese ion (Mn2+) as a transport-mediated tracer combined with newly developed statistical brain image analysis. Six rats underwent imaging on a 1.5-T MR scanner at pre-administration, and 6, 12, 24, 36, 48, and 72 h and 5.5, 7.5, 10.5, and 13.5 days post-administration of manganese chloride (MnCl2) into the right nasal cavity. Images were coregistered, pixel-intensity normalized, and stereotactically transformed to the Paxinos and Watson rat brain atlas, then averaged across subjects using automated image analysis software (NEUROSTAT). Images at each time point were compared to pre-administration using a one-sample t statistic on a pixel-by-pixel basis in 3-D and converted to Z statistic maps. Statistical mapping and group averaging improved signal to noise ratios and signal detection sensitivity. Significant transport of Mn2+ was observed in olfactory structures ipsilateral to site of Mn2+ administration including the bulb, lateral olfactory tract (lo) by 12 h and in the tubercle, piriform cortex, ventral pallidum, amygdala, and in smaller structures such as the anterior commissure after 24 h post-administration. MR imaging with group-wise statistical analysis clearly demonstrated bilateral transsynaptic Mn2+ transport to secondary and tertiary neurons of the olfactory system. The method permits in vivo investigations of functional neuronal connections within the brain.

Magnetic Resonance Imaging Contrast Agents in the Study of Development

The use of magnetic resonance imaging (MRI) as a research tool in the study of development has increased in recent years due in part to improvements in spatial resolution, new imaging agents, and increased availability of MRI scanners. This chapter describes how contrast agent-enhanced MRI can be used to study development. In addition, we highlight some novel applications of contrast agent-enhanced MRI in biology that may be useful as tools for the study of development.

Applications of manganese-enhanced magnetic resonance imaging (MEMRI) to image brain plasticity in song birds

The song control system of song birds is an excellent model for studying brain plasticity and has thus far been extensively analyzed by histological and electrophysiological methods. However, these approaches do not provide a global view of the brain and/or do not allow repeated measures, which are necessary to establish correlations between alterations in neural substrate and behavior. Application of in vivo manganese-enhanced MRI enabled us for the first time to visualize the song control system repeatedly in the same bird, making it possible to quantify dynamically the volume changes in this circuit as a function of seasonal and hormonal influences. In this review, we introduce and explore the song control system of song birds as a natural model for brain plasticity to validate a new cutting edge technique, which we called 'repeated dynamic manganese enhanced MRI' or D-MEMRI. This technique is based on the use of implanted permanent cannulae--for accurate repeated manganese injections in a defined target area--and the subsequent MRI acquisition of the dynamics of the accumulation of manganese in projection brain targets. A compilation of the D-MEMRI data obtained thus far in this system demonstrates the usefulness of this new method for studying brain plasticity. In particular it is shown to be a perfect tool for long-term studies of morphological and functional responses of specific brain circuits to changes in endocrine conditions. The method was also successfully applied to obtain quantitative measures of changes in activity as a function of auditory stimuli in different neuronal populations of a same nucleus that project to different targets. D-MEMRI, combined with other MRI techniques, clearly harbors potential for unraveling seasonal, hormonal, pharmacological or even genetically driven changes in a neuronal circuit, by simultaneously measuring changes in morphology, activity and connectivity.

Functional mapping of neural pathways in rodent brain in vivo using manganese-enhanced three-dimensional magnetic resonance imaging

This work presents three-dimensional MRI studies of rodent brain in vivo after focal and systemic administration of MnCl2. Particular emphasis is paid to the morphology and dynamics of Mn2+-induced MRI signal enhancements, and the physiological mechanisms underlying cerebral Mn2+ uptake and distribution. It turns out that intravitreal and intrahippocampal injections of MnCl2 emerge as useful tools for a delineation of major axonal connections in the intact central nervous system. Subcutaneous administrations may be exploited to highlight regions involved in fundamental brain functions such as the olfactory bulb, inferior colliculus, cerebellum and hippocampal formation. Specific insights into the processes supporting cerebral Mn2+ accumulation may be obtained by intraventricular MnCl2 injection as well as by pharmacologic modulation of, for example, hippocampal function. Taken together, Mn2+-enhanced MRI opens new ways for mapping functioning pathways in animal brain in vivo with applications ranging from assessments of transgenic animals to follow-up studies of animal models of human brain disorders.

Manganese-enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations

Manganese-enhanced MRI (MEMRI) is being increasingly used for MRI in animals due to the unique T1 contrast that is sensitive to a number of biological processes. Three specific uses of MEMRI have been demonstrated: to visualize activity in the brain and the heart; to trace neuronal specific connections in the brain; and to enhance the brain cytoarchitecture after a systemic dose. Based on an ever-growing number of applications, MEMRI is proving useful as a new molecular imaging method to visualize functional neural circuits and anatomy as well as function in the brain in vivo. Paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest at an appropriate dose and in a time-efficient manner. A major drawback to the use of Mn2+ as a contrast agent is its cellular toxicity. Therefore, it is critical to use as low a dose as possible. In the present work the different approaches to MEMRI are reviewed from a practical standpoint. Emphasis is given to the experimental methodology of how to achieve significant, yet safe, amounts of Mn2+ to the target areas of interest.

Differential effects of testosterone on neuronal populations and their connections in a sensorimotor brain nucleus controlling song production in songbirds: a manganese enhanced-magnetic resonance imaging study

Nucleus HVC (formerly called high vocal center) of songbirds contains two types of projecting neurons connecting HVC respectively to the nucleus robustus archistriatalis, RA, or to area X. These two neuron classes exhibit multiple neurochemical differences and are differentially replaced by new neurons during adult life: high rates of neuronal replacement are observed in RA-projecting neurons only. The activity of these two types of neurons may also be modulated differentially by steroids. We analyzed by magnetic resonance imaging the effect of testosterone on the volume of RA and area X and on the dynamics of Mn(2+) accumulation in RA and area X of female starlings that had been injected with MnCl(2) through a permanent cannula implanted in HVC. Repeated visualization 6 weeks apart (before and after testosterone treatment) identified a volume increase of both nuclei in testosterone-treated birds associated with a concomitant decrease in controls. Following testosterone treatment, the total amount of Mn(2+) transported to RA and area X increased but the dynamics of accumulation, reflecting in part the activity of HVC neurons, was specifically altered in area X but not in RA. These data indicate that testosterone differentially affects the RA- and area X-projecting neurons in HVC. Manganese-enhanced magnetic resonance imaging (ME-MRI) thus provides repeated measures of connected brain areas and demonstrates testosterone-dependent regionally specific changes in brain activity and functional connectivity. The slow time scales investigated by this technique (compared to functional MRI) appear ideally suited for characterizing slow processes such as those involved in brain plasticity and learning.