Manganese-enhanced 3D MRI of established and disrupted synaptic activity in the developing insect brain in vivo

Longitudinal manganese-enhanced magnetic resonance imaging of neural projections and activity

Manganese-enhanced magnetic resonance imaging (MEMRI) holds exceptional promise for preclinical studies of brain-wide physiology in awake-behaving animals. The objectives of this review are to update the current information regarding MEMRI and to inform new investigators as to its potential. Mn(II) is a powerful contrast agent for two main reasons: (1) high signal intensity at low doses; and (2) biological interactions, such as projection tracing and neural activity mapping via entry into electrically active neurons in the living brain. High-spin Mn(II) reduces the relaxation time of water protons: at Mn(II) concentrations typically encountered in MEMRI, robust hyperintensity is obtained without adverse effects. By selectively entering neurons through voltage-gated calcium channels, Mn(II) highlights active neurons. Safe doses may be repeated over weeks to allow for longitudinal imaging of brain-wide dynamics in the same individual across time. When delivered by stereotactic intracerebral injection, Mn(II) enters active neurons at the injection site and then travels inside axons for long distances, tracing neuronal projection anatomy. Rates of axonal transport within the brain were measured for the first time in "time-lapse" MEMRI. When delivered systemically, Mn(II) enters active neurons throughout the brain via voltage-sensitive calcium channels and clears slowly. Thus behavior can be monitored during Mn(II) uptake and hyperintense signals due to Mn(II) uptake captured retrospectively, allowing pairing of behavior with neural activity maps for the first time. Here we review critical information gained from MEMRI projection mapping about human neuropsychological disorders. We then discuss results from neural activity mapping from systemic Mn(II) imaged longitudinally that have illuminated development of the tonotopic map in the inferior colliculus as well as brain-wide responses to acute threat and how it evolves over time. MEMRI posed specific challenges for image data analysis that have recently been transcended. We predict a bright future for longitudinal MEMRI in pursuit of solutions to the brain-behavior mystery.

In vivo continuous three-dimensional magnetic resonance microscopy: a study of metamorphosis in Carniolan worker honey bees (Apis mellifera carnica)

Three-dimensional (3D) magnetic resonance microscopy (MRM) is a modality of magnetic resonance imaging (MRI) optimized for the best resolution. Metamorphosis of the Carniolan worker honey bee (Apis mellifera carnica) was studied in vivo under controlled temperature and humidity conditions from sealed larvae until the emergence of an adult. The 3D images were analyzed by volume rendering and segmentation, enabling the analysis of the body, tracheal system and gastrointestinal tract through the time course of volume changes. Fat content sensitivity enabled the analysis of flight muscles transformation during the metamorphosis by the signal histogram and gray level co-occurrence matrix (GLCM). Although the transformation during metamorphosis is well known, MRM enables an alternative insight to this process, i.e. 3D in vivo, which has relatively high spatial and temporal resolutions. The developed methodology can easily be adapted for studying the metamorphosis of other insects or any other incremental biological process on a similar spatial and temporal scale.

The use of MEMRI for monitoring central nervous system activity during intact insect walking

BACKGROUND

Monitoring neuronal activity in the intact behaving animal is most desired in neuroethological research, yet it is rarely straightforward or even feasible. Here we present the use of manganese enhanced magnetic resonance imaging (MEMRI), a technique allowing monitoring the activity of an animal's nervous system during specific behavioral patterns. Using MEMRI we were able to show activity in different ganglia of the central nervous system of intact locusts during walking.

RESULTS

We injected two groups of locusts with manganese, which serves as a magnetic contrast agent. One group was forced to walk on a treadmill for two hours, while the other was immobilized and served as a control. Subsequently, all animals were scanned in a T1 MRI protocol, and the accumulation of manganese in the neuronal tissues that were active during walking was demonstrated by comparing the scans of the two groups. Two neuronal sites showed significantly higher T1 signal in the walking locusts compared to the immobilized ones: the prothoracic ganglion, which locally controls the front legs, and the subesophageal ganglion, a head ganglion which takes part in initiation and maintenance of walking.

CONCLUSION

MEMRI is a potent, non-invasive technique for monitoring neuronal activity in intact locusts, and arthropods in general. Specifically, it provides a promising way for revealing the role of central and high-order neuronal structures in motor behaviors such as walking.

Intracellular manganese enhanced MRI signals reflect the frequency of action potentials in Aplysia neurons

BACKGROUND

Manganese-enhanced magnetic resonance imaging (MEMRI) is an increasingly popular alternative to standard functional MRI methods in animal studies. The contrast in MEMRI images is based on the accumulation of Mn²⁺ ions inside neurons, and, since manganese can serve as calcium analogue, this accumulation reflects calcium dynamics providing versatile information about brain neuroarchitecture and functionality. However, despite its use as a functional imaging tool, the exact relationship between the MEMRI signal and neuronal activity remains elusive.

NEW METHOD

In order to better understand the mechanisms underlying Mn²⁺ accumulation resulting in MEMRI signal enhancement we investigated single neuron responses of isolated Aplysia buccal ganglia subjected to chemical (dopamine) or electrical stimulation of an input nerve (oesophageal nerve). The elicited electrical activity that represents a fictive feeding was recorded with electrophysiological methods and the Mn²⁺ uptake in individual neurons was evaluated with MEMRI at 17.2T.

RESULTS & COMPARISON WITH EXISTING METHOD(S)

We show a positive correlation between bursts of electrical activity and MEMRI signal intensity in identified neurons and demonstrate that the MEMRI signal reflects mainly fast and high membrane depolarization processes such as action potentials, and it is not sensitive to slow and small membrane depolarizations, such as post-synaptic potentials.

Magnetic Resonance Imaging of Alimentary Tract Development in Manduca sexta

Non-invasive 3D magnetic resonance imaging techniques were used to investigate metamorphosis of the alimentary tract of Manduca sexta from the larval to the adult stage. The larval midgut contracts in volume immediately following cessation of feeding and then greatly enlarges during the late pharate pupal period. Magnetic resonance imaging revealed that the foregut and hindgut of the pharate pupa undergo ecdysis considerably earlier than the external exoskeleton. Expansion of air sacs in the early pupa and development of flight muscles several days later appear to orient the midgut into its adult position in the abdomen. The crop, an adult auxiliary storage organ, begins development as a dorsal outgrowth of the foregut. This coincides with a reported increase in pupal ecdysteroid titers. An outgrowth of the hindgut, the rectal sac, appears several days later and continues to expand until it nearly fills the dorsal half of the abdominal cavity. This development correlates with a second rise in pupal ecdysteroid titers. In the pharate pupa, the presence of paramagnetic species renders the silk glands hyperintense.

In Vivo Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

This article provides an overview of in vivo magnetic resonance (MR) imaging contrasts obtained for mammalian brain in relation to histological knowledge. Emphasis is paid to the (1) significance of high spatial resolution for the optimization of T1, T2, and magnetization transfer contrast, (2) use of exogenous extra- and intracellular contrast agents for validating endogenous contrast sources, and (3) histological structures and biochemical compounds underlying these contrasts and (4) their relevance to neuroradiology. Comparisons between MR imaging at subnanoliter resolution and histological data indicate that (a) myelin sheaths, (b) nerve cells, and (c) the neuropil are most responsible for observed MR imaging contrasts, while (a) diamagnetic macromolecules, (b) intracellular paramagnetic ions, and (c) extracellular free water, respectively, emerge as the dominant factors. Enhanced relaxation rates due to paramagnetic ions, such as iron and manganese, have been observed for oligodendrocytes, astrocytes, microglia, and blood cells in the brain as well as for nerve cells. Taken together, a plethora of observations suggests that the delineation of specific structures in high-resolution MR imaging of mammalian brain and the absence of corresponding contrasts in MR imaging of the human brain do not necessarily indicate differences between species but may be explained by partial volume effects. Second, paramagnetic ions are required in active cells in vivo which may reduce the magnetization transfer ratio in the brain through accelerated T1 recovery. Third, reductions of the magnetization transfer ratio may be more sensitive to a particular pathological condition, such as astrocytosis, microglial activation, inflammation, and demyelination, than changes in relaxation. This is because the simultaneous occurrence of increased paramagnetic ions (i.e., shorter relaxation times) and increased free water (i.e., longer relaxation times) may cancel T1 or T2 effects, whereas both processes reduce the magnetization transfer ratio.

Cell layers and neuropil: contrast-enhanced MRI of mouse brain in vivo

Contrast-enhanced T₁- and T₂-weighted MRI at 9.4 T and in-plane resolutions of 25 and 30 µm has been demonstrated to differentiate between neural tissues in mouse brain in vivo, including granule cell layers, principal cell layers, general neuropil, specialized neuropil and white matter. In T₁-weighted MRI of the olfactory bulb, hippocampus and cerebellum, contrast obtained by the intracranial administration of gadopentetate dimeglumine (Gd-DTPA) reflects the extra- and intracellular spaces of gray matter in agreement with histological data. General neuropil areas are highlighted, whereas other tissues present with lower signal intensities. The induced contrast is similar to that in plain T₂-weighted MRI, but offers a 16-30-fold higher contrast-to-noise ratio. Systemic administration of manganese chloride increases the signal-to-noise ratio in T₁-weighted MRI to a significantly greater extent in principal cell layers and specialized neuropil than in granule cell layers, whereas gadolinium-enhanced MRI indicates no larger intracellular spaces in these tissues. Granule cell layers are enhanced no more than general neuropil by manganese, whereas gadolinium-enhanced MRI indicates significantly larger intracellular spaces in the cell layers. These discrepancies suggest that the signal increase after manganese administration reflects cellular activity which is disproportionate to the intracellular space. As a result, principal cell layers and specialized neuropil become highlighted, whereas granule cell layers, general neuropil and white matter present with lower signal intensities.

In vivo magnetic resonance microscopy of Drosophilae at 9.4 T

In preclinical research, genetic studies have made considerable progress as a result of the development of transgenic animal models of human diseases. Consequently, there is now a need for higher resolution MRI to provide finer details for studies of small animals (rats, mice) or very small animals (insects). One way to address this issue is to work with high-magnetic-field spectrometers (dedicated to small animal imaging) with strong magnetic field gradients. It is also necessary to develop a complete methodology (transmit/receive coil, pulse sequence, fixing system, air supply, anesthesia capabilities, etc.). In this study, we developed noninvasive protocols, both in vitro and in vivo (from coil construction to image generation), for drosophila MRI at 9.4 T. The 10 10 80-μm resolution makes it possible to visualize whole drosophila (head, thorax, abdomen) and internal organs (ovaries, longitudinal and transverse muscles, bowel, proboscis, antennae and optical lobes). We also provide some results obtained with a Drosophila model of muscle degeneration. This opens the way for new applications of structural genetic modification studies using MRI of drosophila.

Non-invasive imaging of neuroanatomical structures and neural activation with high-resolution MRI

Several years ago, manganese-enhanced magnetic resonance imaging (MEMRI) was introduced as a new powerful tool to image active brain areas and to identify neural connections in living, non-human animals. Primarily restricted to studies in rodents and later adapted for bird species, MEMRI has recently been discovered as a useful technique for neuroimaging of invertebrate animals. Using crayfish as a model system, we highlight the advantages of MEMRI over conventional techniques for imaging of small nervous systems. MEMRI can be applied to image invertebrate nervous systems at relatively high spatial resolution, and permits identification of stimulus-evoked neural activation non-invasively. Since the selection of specific imaging parameters is critical for successful in vivo micro-imaging, we present an overview of different experimental conditions that are best suited for invertebrates. We also compare the effects of hardware and software specifications on image quality, and provide detailed descriptions of the steps necessary to prepare animals for successful imaging sessions. Careful consideration of hardware, software, experiments, and specimen preparation will promote a better understanding of this novel technique and facilitate future MEMRI studies in other laboratories.

Analysis of freshly fixed and museum invertebrate specimens using high-resolution, high-throughput MRI

Magnetic resonance imaging (MRI) is now considered a routine tool for comparative morphological imaging in small vertebrate model organisms. However, the application of high-resolution imaging protocols to visualize the anatomy of invertebrate organisms has not yet become a generally accepted tool among zoologists. Here, we describe MRI protocols that permit visualization of both the internal and the external anatomy of freshly fixed invertebrates and specimens from museum collections. The choice of protocols has been optimized to allow the assembly of the large numbers of datasets that are necessary for comparative morphological analyses. Although the primary focus of our work is on sea urchin internal anatomy, we also present results from a variety of other invertebrate taxa to demonstrate the principal feasibility of MRI studies to obtain anatomical information at high resolutions. Furthermore, we briefly describe procedures suitable for 3D modelling.

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.

Development of a solenoid RF coil for animal imaging in 3 T high-magnetic-field MRI

The purpose of this study was to develop a solenoid coil for use with small animals in a 3 Tesla horizontal magnetic resonance imaging (MRI) system, and to investigate image quality by examination of parameters such as signal-to-noise ratio (SNR) and Q-factor. A receiver solenoid coil was formed by winding three separate coils of copper tape around an acryl cylinder. The cylinder was supported at each end. A euthanized rat weighing 240 g was used as a subject animal for imaging. A cylindrical plastic tube containing a solution of 0.7 g/L CuSO(4) was used as a phantom. Measured SNRs were 985 in the phantom image 995 in the rat. The Q-factor was 89 in the phantom and 84 in the rat, in the loaded condition. The homogeneity of the radiofrequency (RF) field was good and the resolution of the image was sufficient to distinguish internal organs from one another in the abdomen of a rat. This study has demonstrated that a solenoid coil may be used to produce good quality images of small animals.

Systematic comparison and reconstruction of sea urchin (Echinoidea) internal anatomy: a novel approach using magnetic resonance imaging

Background

Traditional comparative morphological analyses and subsequent three-dimensional reconstructions suffer from a number of drawbacks. This is particularly evident in the case of soft tissue studies that are technically demanding, time-consuming, and often prone to produce artefacts. These problems can partly be overcome by employing non-invasive, destruction-free imaging techniques, in particular micro-computed tomography or magnetic resonance imaging.

Results

Here, we employed high-field magnetic resonance imaging techniques to gather numerous data from members of a major marine invertebrate taxon, the sea urchins (Echinoidea). For this model study, 13 of the 14 currently recognized high-ranking subtaxa (orders) of this group of animals were analyzed. Based on the acquired datasets, interactive three-dimensional models were assembled. Our analyses reveal that selected soft tissue characters can even be used for phylogenetic inferences in sea urchins, as exemplified by differences in the size and shape of the gastric caecum found in the Irregularia.

Conclusion

The main focus of our investigation was to explore the possibility to systematically visualize the internal anatomy of echinoids obtained from various museum collections. We show that, in contrast to classical preparative procedures, magnetic resonance imaging can give rapid, destruction-free access to morphological data from numerous specimens, thus extending the range of techniques available for comparative studies of invertebrate morphology.

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.

MRI contrast agents for functional molecular imaging of brain activity

Functional imaging with MRI contrast agents is an emerging experimental approach that can combine the specificity of cellular neural recording techniques with noninvasive whole-brain coverage. A variety of contrast agents sensitive to aspects of brain activity have recently been introduced. These include new probes for calcium and other metal ions that offer high sensitivity and membrane permeability, as well as imaging agents for high-resolution pH and metabolic mapping in living animals. Genetically encoded MRI contrast agents have also been described. Several of the new probes have been validated in the brain; in vivo use of other agents remains a challenge. This review outlines advantages and disadvantages of specific molecular imaging approaches and discusses current or potential applications in neurobiology.