Locus coeruleus: a new look at the blue spot.

The locus coeruleus (LC), or 'blue spot', is a small nucleus located deep in the brainstem that provides the far-reaching noradrenergic neurotransmitter system of the brain. This phylogenetically conserved nucleus has proved relatively intractable to full characterization, despite more than 60 years of concerted efforts by investigators. Recently, an array of powerful new neuroscience tools have provided unprecedented access to this elusive nucleus, revealing new levels of organization and function. We are currently at the threshold of major discoveries regarding how this tiny brainstem structure exerts such varied and significant influences over brain function and behaviour. All LC neurons receive inputs related to autonomic arousal, but distinct subpopulations of those neurons can encode specific cognitive processes, presumably through more specific inputs from the forebrain areas. This ability, combined with specific patterns of innervation of target areas and heterogeneity in receptor distributions, suggests that activation of the LC has more specific influences on target networks than had initially been imagined.

An in vivo probabilistic atlas of the human locus coeruleus at ultra-high field.

Early and profound pathological changes are evident in the locus coeruleus (LC) in dementia and Parkinson's disease, with effects on arousal, attention, cognitive and motor control. The LC can be identified in vivo using non-invasive magnetic resonance imaging techniques which have potential as biomarkers for detecting and monitoring disease progression. Technical limitations of existing imaging protocols have impaired the sensitivity to regional contrast variance or the spatial variability on the rostrocaudal extent of the LC, with spatial mapping consistent with post mortem findings. The current study employs a sensitive magnetisation transfer sequence using ultrahigh field 7T MRI to investigate the LC structure in vivo at high-resolution (0.4 × 0.4 × 0.5 mm). Magnetisation transfer images from 53 healthy older volunteers (52 - 84 years) clearly revealed the spatial features of the LC and were used to create a probabilistic LC atlas for older adults. This atlas may be especially relevant for studying disorders associated with older age. To use the atlas does not require use of the same MT sequence of 7T MRI, provided good co-registration and normalisation is achieved. Consistent rostrocaudal gradients of slice-wise volume, contrast and variance along the LC were observed, mirroring distinctive ex vivo spatial distributions of LC cells in its subregions. The contrast-to-noise ratios were calculated for the peak voxels, and for the averaged signals within the atlas, to accommodate the volumetric differences in estimated contrast. The probabilistic atlas is freely available, and the MRI dataset will be made available for non-commercial research, for replication or to facilitate accurate LC localisation and unbiased contrast extraction in future studies.

Uncovering the locus coeruleus: Comparison of localization methods for functional analysis.

Functional neuroimaging of small brainstem structures in humans is gaining interest due to their potential importance in aging and many clinical conditions. Researchers have used different methods to measure activity in the locus coeruleus (LC), the main noradrenergic nucleus in the brain. However, the extent to which these different LC localization methods yield similar results is unclear. In the present article, we compared four different approaches to estimate localization of the LC in a large sample (N = 98): 1) a probabilistic map from a previous study, 2) masks segmented from neuromelanin-sensitive scans, both manually and semi-automatically, 3) components from a masked-independent components analysis of the functional data, and 4) a mask from pupil regression of the functional data. The four methods have all been used previously in the imaging community to localize the LC in vivo in humans. We report several measures of similarity between the LC masks obtained from the different methods. In addition, we compare functional connectivity maps obtained from the different masks. We conclude that sample-specific masks appear more suitable than masks obtained from an independent sample, that masks based on structural versus functional methods may capture different portions of LC, and that, at the group level, the creation of a "consensus" mask using more than one approach may give a better estimate of LC localization.

The rostromedial tegmental nucleus is essential for non-rapid eye movement sleep.

The rostromedial tegmental nucleus (RMTg), also called the GABAergic tail of the ventral tegmental area, projects to the midbrain dopaminergic system, dorsal raphe nucleus, locus coeruleus, and other regions. Whether the RMTg is involved in sleep-wake regulation is unknown. In the present study, pharmacogenetic activation of rat RMTg neurons promoted non-rapid eye movement (NREM) sleep with increased slow-wave activity (SWA). Conversely, rats after neurotoxic lesions of 8 or 16 days showed decreased NREM sleep with reduced SWA at lights on. The reduced SWA persisted at least 25 days after lesions. Similarly, pharmacological and pharmacogenetic inactivation of rat RMTg neurons decreased NREM sleep. Electrophysiological experiments combined with optogenetics showed a direct inhibitory connection between the terminals of RMTg neurons and midbrain dopaminergic neurons. The bidirectional effects of the RMTg on the sleep-wake cycle were mimicked by the modulation of ventral tegmental area (VTA)/substantia nigra compacta (SNc) dopaminergic neuronal activity using a pharmacogenetic approach. Furthermore, during the 2-hour recovery period following 6-hour sleep deprivation, the amount of NREM sleep in both the lesion and control rats was significantly increased compared with baseline levels; however, only the control rats showed a significant increase in SWA compared with baseline levels. Collectively, our findings reveal an essential role of the RMTg in the promotion of NREM sleep and homeostatic regulation.

Groupwise track filtering via iterative message passing and pruning.

Tractography is an important tool for the in vivo analysis of brain connectivity based on diffusion MRI data, but it also has well-known limitations in false positives and negatives for the faithful reconstruction of neuroanatomy. These problems persist even in the presence of strong anatomical priors in the form of multiple region of interests (ROIs) to constrain the trajectories of fiber tractography. In this work, we propose a novel track filtering method by leveraging the groupwise consistency of fiber bundles that naturally exists across subjects. We first formalize our groupwise concept with a flexible definition that characterizes the consistency of a track with respect to other group members based on three important aspects: degree, affinity, and proximity. An iterative algorithm is then developed to dynamically update the localized consistency measure of all streamlines via message passing from a reference set, which then informs the pruning of outlier points from each streamline. In our experiments, we successfully applied our method to diffusion imaging data of varying resolutions from the Alzheimer's Disease Neuroimaging Initiative (ADNI) and Human Connectome Project (HCP) for the consistent reconstruction of three important fiber bundles in human brain: the fornix, locus coeruleus pathways, and corticospinal tract. Both qualitative evaluations and quantitative comparisons showed that our method achieved significant improvement in enhancing the anatomical fidelity of fiber bundles.

Stretchable multichannel antennas in soft wireless optoelectronic implants for optogenetics.

Optogenetic methods to modulate cells and signaling pathways via targeted expression and activation of light-sensitive proteins have greatly accelerated the process of mapping complex neural circuits and defining their roles in physiological and pathological contexts. Recently demonstrated technologies based on injectable, microscale inorganic light-emitting diodes (µ-ILEDs) with wireless control and power delivery strategies offer important functionality in such experiments, by eliminating the external tethers associated with traditional fiber optic approaches. Existing wireless µ-ILED embodiments allow, however, illumination only at a single targeted region of the brain with a single optical wavelength and over spatial ranges of operation that are constrained by the radio frequency power transmission hardware. Here we report stretchable, multiresonance antennas and battery-free schemes for multichannel wireless operation of independently addressable, multicolor µ-ILEDs with fully implantable, miniaturized platforms. This advance, as demonstrated through in vitro and in vivo studies using thin, mechanically soft systems that separately control as many as three different µ-ILEDs, relies on specially designed stretchable antennas in which parallel capacitive coupling circuits yield several independent, well-separated operating frequencies, as verified through experimental and modeling results. When used in combination with active motion-tracking antenna arrays, these devices enable multichannel optogenetic research on complex behavioral responses in groups of animals over large areas at low levels of radio frequency power (<1 W). Studies of the regions of the brain that are involved in sleep arousal (locus coeruleus) and preference/aversion (nucleus accumbens) demonstrate the unique capabilities of these technologies.

High-resolution in vivo imaging of human locus coeruleus by magnetization transfer MRI at 3T and 7T.

Locus Coeruleus (LC) is a neuromelanin-rich brainstem structure that is the source of noradrenaline in the cortex and is thought to modulate attention and memory. LC imaging in vivo is commonly performed with a 2D T1-weighted Turbo Spin Echo (TSE) MRI sequence, an approach that suffers from several drawbacks at 3T, including long acquisition times and highly anisotropic spatial resolution. In this study, we developed a high-resolution Magnetization Transfer (MT) sequence for LC imaging at both 7T and 3T and compared its performance to a TSE sequence. Results indicate that LC imaging can be achieved with an MT sequence at both 7 and 3T at higher spatial resolution than the 3T TSE. Furthermore, we investigated whether the currently disputed source of contrast in the LC region with a TSE sequence relates to MT effects or shortened T1 and T2* due to increased iron concentration. Our results suggest that the contrast in the LC area relates to MT effects. To conclude, in this study we managed to image the LC, for the first time, at 7T and at an increased resolution compared to the current state-of-the-art. Imaging the LC is highly relevant for clinical diagnostics as structural tissue properties of the LC may hold promise as a biomarker in neurodegenerative diseases.

Heterogeneous organization of the locus coeruleus projections to prefrontal and motor cortices.

The brainstem nucleus locus coeruleus (LC) is the primary source of norepinephrine (NE) to the mammalian neocortex. It is believed to operate as a homogeneous syncytium of transmitter-specific cells that regulate brain function and behavior via an extensive network of axonal projections and global transmitter-mediated modulatory influences on a diverse assembly of neural targets within the CNS. The data presented here challenge this longstanding notion and argue instead for segregated operation of the LC-NE system with respect to the functions of the circuits within its efferent domain. Anatomical, molecular, and electrophysiological approaches were used in conjunction with a rat model to show that LC cells innervating discrete cortical regions are biochemically and electrophysiologically distinct from one another so as to elicit greater release of norepinephrine in prefrontal versus motor cortex. These findings challenge the consensus view of LC as a relatively homogeneous modulator of forebrain activity and have important implications for understanding the impact of the system on the generation and maintenance of adaptive and maladaptive behaviors.

Projection specificity in heterogeneous locus coeruleus cell populations: implications for learning and memory.

Noradrenergic neurons in the locus coeruleus (LC) play a critical role in many functions including learning and memory. This relatively small population of cells sends widespread projections throughout the brain including to a number of regions such as the amygdala which is involved in emotional associative learning and the medial prefrontal cortex which is important for facilitating flexibility when learning rules change. LC noradrenergic cells participate in both of these functions, but it is not clear how this small population of neurons modulates these partially distinct processes. Here we review anatomical, behavioral, and electrophysiological studies to assess how LC noradrenergic neurons regulate these different aspects of learning and memory. Previous work has demonstrated that subpopulations of LC noradrenergic cells innervate specific brain regions suggesting heterogeneity of function in LC neurons. Furthermore, noradrenaline in mPFC and amygdala has distinct effects on emotional learning and cognitive flexibility. Finally, neural recording data show that LC neurons respond during associative learning and when previously learned task contingencies change. Together, these studies suggest a working model in which distinct and potentially opposing subsets of LC neurons modulate particular learning functions through restricted efferent connectivity with amygdala or mPFC. This type of model may provide a general framework for understanding other neuromodulatory systems, which also exhibit cell type heterogeneity and projection specificity.

Histologic validation of locus coeruleus MRI contrast in post-mortem tissue.

The locus coeruleus (LC) noradrenergic system regulates arousal and modulates attention through its extensive projections across the brain. LC dysfunction has been implicated in a broad range of neurodevelopmental, neurodegenerative and psychiatric disorders, as well as in the cognitive changes observed during normal aging. Magnetic resonance imaging (MRI) has been used to characterize the human LC (elevated contrast relative to surrounding structures), but there is limited understanding of the factors underlying putative LC contrast that are critical to successful biomarker development and confidence in localizing nucleus LC. We used ultra-high-field 7 T magnetic resonance imaging (MRI) to acquire T1-weighted microscopy resolution images (78 µm in-plane resolution) of the LC from post-mortem tissue samples. Histological analyses were performed to characterize the distribution of tyrosine hydroxylase (TH) and neuromelanin in the scanned tissue, which allowed for direct comparison with MR microscopy images. Our results indicate that LC-MRI contrast corresponds to the location of neuromelanin cells in LC; these also correspond to norepinephrine neurons. Thus, neuromelanin appears to serve as a natural contrast agent for nucleus LC that can be used to localize nucleus LC and may have the potential to characterize neurodegenerative disease.

The locus coeruleus and noradrenergic modulation of cognition.

Mood, attention and motivation co-vary with activity in the neuromodulatory systems of the brain to influence behaviour. These psychological states, mediated by neuromodulators, have a profound influence on the cognitive processes of attention, perception and, particularly, our ability to retrieve memories from the past and make new ones. Moreover, many psychiatric and neurodegenerative disorders are related to dysfunction of these neuromodulatory systems. Neurons of the brainstem nucleus locus coeruleus are the sole source of noradrenaline, a neuromodulator that has a key role in all of these forebrain activities. Elucidating the factors that control the activity of these neurons and the effect of noradrenaline in target regions is key to understanding how the brain allocates attention and apprehends the environment to select, store and retrieve information for generating adaptive behaviour.

Lower fractional dimension in Alzheimer's disease correlates with reduced locus coeruleus signal intensity.

This study aimed to determine the pattern of fractional dimension (FD) in Alzheimer's disease (AD) patients, and investigate the relationship between FD and the locus coeruleus (LC) signal intensity.A total of 27 patients with AD and 25 healthy controls (HC) were collected to estimate the pattern of fractional dimension (FD) and cortical thickness (CT) using the Computational Anatomy Toolbox (CAT12), and statistically analyze between groups on a vertex level using statistical parametric mapping 12. In addition, they were examined by neuromelanin sensitive MRI(NM-MRI) technique to calculate the locus coeruleus signal contrast ratios (LC-CRs). Additionally, correlations between the pattern of FD and LC-CRs were further examined.Compared to HC, AD patients showed widespread lower CT and FD Furthermore, significant positive correlation was found between local fractional dimension (LFD) of the left rostral middle frontal cortex and LC-CRs. Results suggest lower cortical LFD is associated with LCCRs that may reflect a reduction due to broader neurodegenerative processes. This finding may highlight the potential utility for advanced measures of cortical complexity in assessing brain health and early identification of neurodegenerative processes.

Locus coeruleus contrast and diffusivity metrics differentially relate to age and memory performance.

Neurocognitive aging researchers are increasingly focused on the locus coeruleus, a neuromodulatory brainstem structure that degrades with age. With this rapid growth, the field will benefit from consensus regarding which magnetic resonance imaging (MRI) metrics of locus coeruleus structure are most sensitive to age and cognition. To address this need, the current study acquired magnetization transfer- and diffusion-weighted MRI images in younger and older adults who also completed a free recall memory task. Results revealed significantly larger differences between younger and older adults for maximum than average magnetization transfer-weighted contrast (MTC), axial than mean or radial single-tensor diffusivity (DTI), and free than restricted multi-compartment diffusion (NODDI) metrics in the locus coeruleus; with maximum MTC being the best predictor of age group. Age effects for all imaging modalities interacted with sex, with larger age group differences in males than females for MTC and NODDI metrics. Age group differences also varied across locus coeruleus subdivision for DTI and NODDI metrics, and across locus coeruleus hemispheres for MTC. Within older adults, however, there were no significant effects of age on MTC or DTI metrics, only an interaction between age and sex for free diffusion. Finally, independent of age and sex, higher restricted diffusion in the locus coeruleus was significantly related to better (lower) recall variability, but not mean recall. Whereas MTC has been widely used in the literature, our comparison between the average and maximum MTC metrics, inclusion of DTI and NODDI metrics, and breakdowns by locus coeruleus subdivision and hemisphere make important and novel contributions to our understanding of the aging of locus coeruleus structure.

Heterogeneous organization of Locus coeruleus: An intrinsic mechanism for functional complexity.

Locus coeruleus (LC) is a small nucleus located deep in the brainstem that contains the majority of central noradrenergic neurons, which provide the primary source of noradrenaline (NA) throughout the entire central nervous system (CNS).The release of neurotransmitter NA is considered to modulate arousal, sensory processing, attention, aversive and adaptive stress responses as well as high-order cognitive function and memory, with the highly ramified axonal arborizations of LC-NA neurons sending wide projections to the targeted brain areas. For over 30 years, LC was thought to be a homogeneous nucleus in structure and function due to the widespread uniform release of NA by LC-NA neurons and simultaneous action in several CNS regions, such as the prefrontal cortex, hippocampus, cerebellum, and spinal cord. However, recent advances in neuroscience tools have revealed that LC is probably not so homogeneous as we previous thought and exhibits heterogeneity in various aspects. Accumulating studies have shown that the functional complexity of LC may be attributed to its heterogeneity in developmental origin, projection patterns, topography distribution, morphology and molecular organization, electrophysiological properties and sex differences. This review will highlight the heterogeneity of LC and its critical role in modulating diverse behavioral outcomes.

Tracing of noradrenergic projections using manganese-enhanced MRI.

We examined the applicability of manganese-enhanced MRI (MEMRI) to the in vivo tracing of diffuse neuromodulatory projections by means of simultaneous iontophoretic injections of an extremely low, non-toxic concentration of MnCl(2) (10mM) and fluorescent dextran in the locus coeruleus (LC) in the rat. We validated the use of the iontophoretic injection by reproducing previously reported results from pressure injections of MnCl(2) in primary somatosensory cortex. Twenty fourhours after injection in LC, Mn(2+) labeling was detected in major cortical and subcortical targets of LC projections including predominantly ipsilateral primary motor and somatosensory cortices, hippocampus and amygdala. Although the injections were in most cases centered in the core of LC, the pattern of Mn(2+) labeling greatly varied across rats. In addition, despite a certain degree of overlap of the labeling obtained with both MEMRI and classical tracing, MEMRI tracing consistently failed to reliably label not only several minor but also major targets of LC, notably the thalamus. The lack of Mn(2+) labeling in thalamus possibly reflected a weaker functional connectivity within coeruleothalamic projections that could not be predicted by anatomical tracing. Inversely, a number of brain regions, particularly contralateral motor cortex, that were not or only sparsely labeled with fluorescent dextran were strongly labeled by Mn(2+). This discrepancy could be partly due to both the activity-dependent and transsynaptic nature of Mn(2+) transport. The overall labeling produced using MEMRI with iontophoretic injections in LC indicates that the Mn(2+) imaging of highly diffuse projections is in principle feasible. However, the labeling pattern of each individual case needs to be carefully interpreted particularly before submitting data for group analysis or in the case of longitudinal examination of discrete changes in functional connectivity under various physiological or behavioral conditions.

The human locus coeruleus 3-D stereotactic anatomy.

PURPOSE: The main goal of this work was to study the stereotactic anatomy of the human locus coeruleus (LC), important relay of adrenergic and dopaminergic human brainstem (HB) circuitry, to allow its easy localization on MRI and in microsurgical procedures. METHODS: Forty LC were studied from 20 adult HB of both sexes. The melanin pigmentation of its cells was used to identify and localize them and so to define the 2-D and 3-D LC contours. These HB were cut on a cryomicrotome with 3-D referenciation. The slices were coloured with haematoxyline-eosin. On the slices, digitized images of the cells were referenced to the midline, the fourth ventricle floor plane and the pontomedullary junction plane with an appropriate computer program. RESULTS: The LC revealed to be a symmetric, thin and elongated nucleus, divergent caudally except in its superior part, with a sub-ependymal location on the superior dorsal lateral pons. The main LC dimensions are: length 12.0-17.0 mm (m 14.5); width 2.5 mm; height 2.0 mm. The 3-D references of the LC center are: 3.2 ± 0.3 mm to the midline; 1.1 ± 0.2 mm to the IV ventricle floor and 18.5 ± 1.5 mm to the ponto-medullary junction. CONCLUSIONS: The human LC is a nucleus thinner and longer than previously described (in average 14.5 mm long and 2-2.5 mm thick), localized 1 mm under the IV ventricle, 3 mm apart from the midline and centered 14-21 mm above de ponto-medullary junction. No correlation was found between LC and pons dimensions, the gender or the age.

The role of locus coeruleus in the antiepileptic activity induced by vagus nerve stimulation.

Stimulation of the vagus nerve produces antiepileptic effects. This is used clinically to treat drug-refractory epilepsies. The mechanisms responsible for these effects depend on the activation of vagal afferents reaching the nucleus of the solitary tract. This review focuses on the neuroanatomy of the nucleus of the solitary tract and its relation with the nucleus locus coeruleus as a preferential anatomical substrate in producing antiepileptic effects. In fact, following the transient or permanent inactivation of locus coeruleus neurons, some antiepileptic effects of vagus nerve stimulation are lost. The activation of locus coeruleus per se is known to limit the spread of a seizure and the duration of a variety of seizure types. This is due to the fine chemical neuroanatomy of norepinephrine pathways that arise from the locus coeruleus, which produce widespread changes in cortical areas. These changes may be sustained by norepinephrine alone, or in combination with its co-transmitters. In addition, vagus nerve stimulation may prevent seizures by activating the serotonin-containing dorsal raphe neurons.

Unraveling the contributions to the neuromelanin-MRI contrast.

The Locus Coeruleus (LC) and the Substantia Nigra (SN) are small brainstem nuclei that change with aging and may be involved in the development of various neurodegenerative and psychiatric diseases. Magnetization Transfer (MT) MRI has been shown to facilitate LC and the SN visualization, and the observed contrast is assumed to be related to neuromelanin accumulation. Imaging these nuclei may have predictive value for the progression of various diseases, but interpretation of previous studies is hindered by the fact that the precise biological source of the contrast remains unclear, though several hypotheses have been put forward. To inform clinical studies on the possible biological interpretation of the LC- and SN contrast, we examined an agar-based phantom containing samples of natural Sepia melanin and synthetic Cys-Dopa-Melanin and compared this to the in vivo human LC and SN. T1 and T2* maps, MT spectra and relaxation times of the phantom, the LC and the SN were measured, and a two-pool MT model was fitted. Additionally, Bloch simulations and a transient MT experiment were conducted to confirm the findings. Overall, our results indicate that Neuromelanin-MRI contrast in the LC likely results from a lower macromolecular fraction, thus facilitating interpretation of results in clinical populations. We further demonstrate that in older individuals T1 lengthening occurs in the LC.

Sub-millimeter variation in human locus coeruleus is associated with dimensional measures of psychopathology: An in vivo ultra-high field 7-Tesla MRI study.

The locus coeruleus (LC) has a long-established role in the attentional and arousal response to threat, and in the emergence of pathological anxiety in pre-clinical models. However, human evidence of links between LC function and pathological anxiety has been restricted by limitations in discerning LC with current neuroimaging techniques. We combined ultra-high field 7-Tesla and 0.4 × 0.4 × 0.5 mm quantitative MR imaging with a computational LC localization and segmentation algorithm to delineate the LC in 29 human subjects including subjects with and without an anxiety or stress-related disorder. Our automated, data-driven LC segmentation algorithm provided LC delineations that corresponded well with postmortem anatomic definitions of the LC. There was variation of LC size in healthy subjects (125.7 +/- 59.3 mm3), which recapitulates histological reports. Patients with an anxiety or stress-related disorder had larger LC compared to controls (Cohen's d = 1.08, p = 0.024). Larger LC was additionally associated with poorer attentional and inhibitory control and higher anxious arousal (FDR-corrected p's<0.025), trans-diagnostically across the full sample. This study combined high-resolution and quantitative MR with a mixture of supervised and unsupervised computational techniques to provide robust, sub-millimeter measurements of the LC in vivo, which were additionally related to common psychopathology. This work has wide-reaching applications for a range of neurological and psychiatric disorders characterized by expected LC dysfunction.

The brain nucleus locus coeruleus: restricted afferent control of a broad efferent network.

Dense, focal injections of wheat germ agglutinin conjugated-horseradish peroxidase in the locus coeruleus of rats labeled afferent neurons in unexpectedly few brain regions. Major inputs emanate from only two nuclei--the paragigantocellularis and the prepositus hypoglossi, both in the rostral medulla. The dorsal cap of the paraventricular hypothalamic nucleus and the spinal intermediate gray are possible minor afferents to locus coeruleus. Other areas reported to project to locus coeruleus (for example, amygdala, nucleus tractus solitarius, and spinal dorsal horn) did not exhibit consistent retrograde labeling. Anterograde tracing and electrophysiologic experiments confirmed the absence of input to locus coeruleus from these areas, which instead terminate in targets adjacent to locus coeruleus. These findings redefine the anatomic organization of the locus coeruleus, and have implications for hypotheses concerning the functions of this noradrenergic brain nucleus.