The human locus coeruleus 3-D stereotactic anatomy

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.

Locus Coeruleus and neurovascular unit: From its role in physiology to its potential role in Alzheimer's disease pathogenesis

Locus coeruleus (LC) is the main noradrenergic (NA) nucleus of the central nervous system. LC degenerates early during Alzheimer's disease (AD) and NA loss might concur to AD pathogenesis. Aside from neurons, LC terminals provide dense innervation of brain intraparenchymal arterioles/capillaries, and NA modulates astrocyte functions. The term neurovascular unit (NVU) defines the strict anatomical/functional interaction occurring between neurons, glial cells, and brain vessels. NVU plays a fundamental role in coupling the energy demand of activated brain regions with regional cerebral blood flow, it includes the blood-brain barrier (BBB), plays an active role in neuroinflammation, and participates also to the glymphatic system. NVU alteration is involved in AD pathophysiology through several mechanisms, mainly related to a relative oligoemia in activated brain regions and impairment of structural and functional BBB integrity, which contributes also to the intracerebral accumulation of insoluble amyloid. We review the existing data on the morphological features of LC-NA innervation of the NVU, as well as its contribution to neurovascular coupling and BBB proper functioning. After introducing the main experimental data linking LC with AD, which have repeatedly shown a key role of neuroinflammation and increased amyloid plaque formation, we discuss the potential mechanisms by which the loss of NVU modulation by LC might contribute to AD pathogenesis. Surprisingly, thus far not so many studies have tested directly these mechanisms in models of AD in which LC has been lesioned experimentally. Clarifying the interaction of LC with NVU in AD pathogenesis may disclose potential therapeutic targets for AD.

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

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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.

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LC was delineated in 29 human subjects including subjects with and without an anxiety or stress-related disorder.

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Patients with an anxiety or stress-related disorder had larger LC compared to controls (Cohen's d = 1.08, p = 0.024).

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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.

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.

Rostral locus coeruleus integrity is associated with better memory performance in older adults

For decades, research into memory decline in human cognitive ageing has focused on neocortical regions, the hippocampus and dopaminergic neuromodulation. Recent findings indicate that the locus coeruleus (LC) and noradrenergic neuromodulation may also play an important role in shaping memory development in later life. However, technical challenges in quantification of LC integrity have hindered the study of LC-cognition associations in humans. Using high-resolution, neuromelanin-sensitive magnetic resonance imaging, we found that individual differences in learning and memory were positively associated with LC integrity across a variety of memory tasks in both younger (n = 66) and older adults (n = 228). Moreover, we observed functionally relevant age differences confined to rostral LC. Older adults with a more 'youth-like' rostral LC also showed higher memory performance. These findings link non-invasive, in vivo indices of LC integrity to memory in ageing and highlight the role of the LC norepinephrine system in the decline of cognition.

Modulation of locus coeruleus activity by novel oddball stimuli

It has long been known from animal literature that the locus coeruleus (LC), the source region of noradrenergic neurons in the brain, is sensitive to unexpected, novel, and other salient events. In humans, however, direct assessment of LC activity has proven to be challenging due to its small size and difficult localization, which is why noradrenergic activity has often been assessed using more indirect measures such as electroencephalography (EEG) and pupil recordings. Here, we combined high-resolution functional magnetic resonance imaging (fMRI) with a special anatomical sequence to assess neural activity in the LC in response to different types of salient stimuli in an oddball paradigm (novel neutral oddballs, novel emotional oddballs, and familiar target oddballs). We found a significant linear increase of LC activity from standard trials, over familiar target oddballs, to novel neutral and novel emotional oddballs. Importantly, when breaking down this linear trend, only novel oddball stimuli led to robust activity increases as compared to standard trials, with no statistical difference between neutral and emotional ones. This pattern suggests that activity modulations in the LC in the present study were mainly driven by stimulus novelty, rather than by emotional saliency, task relevance, or contextual novelty alone. Moreover, the absence of significant activity modulations in response to target oddballs (which were reported in a recent study) suggests that the LC represents relative rather than absolute saliency of a stimulus in its respective context.

Quantifying the contrast of the human locus coeruleus in vivo at 7 Tesla MRI

The locus coeruleus is a small brainstem nucleus which contains neuromelanin cells and is involved in a number of cognitive functions such as attention, arousal and stress, as well as several neurological and psychiatric disorders. Locus coeruleus imaging in vivo is generally performed using a T₁-weighted turbo spin echo MRI sequence at 3 Tesla (T). However, imaging at high magnetic field strength can increase the signal-to-noise ratio and offers the possibility of imaging at higher spatial resolution. Therefore, in the present study we explored the possibility of visualizing the locus coeruleus at 7T. To this end, twelve healthy volunteers participated in three scanning sessions: two with 3T MRI and one with 7T MRI. The volumes of the first 3T session were used to segment the locus coeruleus, whereas the volumes of the second 3T and the 7T session were used to quantify the contrast of the locus coeruleus with several reference regions across eight different structural sequences. The results indicate that several of the 7T sequences provide detectable contrast between the locus coeruleus and surrounding tissue. Of the tested sequences, a T₁-weighted sequence with spectral presaturation inversion recovery (SPIR) seems the most promising method for visualizing the locus coeruleus at ultra-high field MRI. While there is insufficient evidence to prefer the 7T SPIR sequence over the 3T TSE sequence, the isotropic voxels at 7T are an important advantage when visualizing small structures such as the locus coeruleus.

In vivo visualization of age-related differences in the locus coeruleus

The locus coeruleus (LC), the major origin of noradrenergic modulation of the central nervous system, may play an important role in neuropsychiatric disorders including Parkinson's disease and Alzheimer's disease. The pattern of age-related change of the LC across the life span is unclear. We obtained normalized, mean LC signal intensity values, that is, contrast ratios (CRs), from magnetization transfer-weighted images to investigate the relationship between LC CR and age in cognitively normal healthy adults (N = 605, age range 18-88 years). Study participants were part of the Cambridge Centre for Ageing and Neuroscience-an open-access, population-based data set. We found a quadratic relationship between LC CR and age, the peak occurring around 60 years, with no differences between males and females. Subregional analyses revealed that age-related decline in LC CR was confined to the rostral portion of the LC. Older adults showed greater variance in overall LC CR than younger adults, and the functional and clinical implications of these observed age-related differences require further investigation. Visualization of the LC in this study may inform how future scanning parameters can be optimized, and provides insight into how LC integrity changes across the life span.

The Use of Physiological Signals in Brainstem/Midbrain fMRI

Brainstem and midbrain nuclei are closely linked to cognitive performance and autonomic function. To advance the localization in this area, precise functional imaging is fundamental. In this study, we used a sophisticated fMRI technique as well as physiological recordings to investigate the involvement of brainstem/midbrain nuclei in cognitive control during a Stroop task. The temporal signal-to-noise ratio (tSNR) increased due to physiological noise correction (PNC) especially in regions adjacent to arteries and cerebrospinal fluid. Within the brainstem/cerebellum template an average tSNR of 68 ± 16 was achieved after the simultaneous application of a high-resolution fMRI, specialized co-registration, and PNC. The analysis of PNC data revealed an activation of the substantia nigra in the Stroop interference contrast whereas no significant results were obtained in the midbrain or brainstem when analyzing uncorrected data. Additionally, we found that pupil size indicated the level of cognitive effort. The Stroop interference effect on pupillary responses was correlated to the effect on reaction times (R 2 = 0.464, p < 0.05). When Stroop stimuli were modulated by pupillary responses, we observed a significant activation of the LC in the Stroop interference contrast. Thus, we demonstrated the beneficial effect of PNC on data quality and statistical results when analyzing neuronal responses to a cognitive task. Parametric modulation of task events with pupillary responses improved the model of LC BOLD activations in the Stroop interference contrast.

In vivo Brainstem Imaging in Alzheimer's Disease: Potential for Biomarker Development

The dearth of effective treatments for Alzheimer's disease (AD) is one of the largest public health issues worldwide, costing hundreds of billions of dollars per year. From a therapeutic standpoint, research efforts to date have met with strikingly little clinical success. One major issue is that trials begin after substantial pathological change has occurred, and it is increasingly clear that the most effective treatment regimens will need to be administered earlier in the disease process. In order to identify individuals within the long preclinical phase of AD who are likely to progress to dementia, improvements are required in biomarker development. One potential area of research that might prove fruitful in this regard is the in vivo detection of brainstem pathology. The brainstem is known to undergo pathological changes very early and progressively in AD. With an updated and harmonized AD research framework, and emerging advances in neuroimaging technology, the potential to leverage knowledge of brainstem pathology into biomarkers for AD will be discussed.

Resting-state functional connectivity of neurotransmitter producing sites in female patients with borderline personality disorder

Impulsive behavior, difficulties in controlling anger and suicidal behavior are typical patterns of affective/behavioral dysregulation in patients with borderline personality disorder (BPD). Previous functional MRI studies in the resting state condition demonstrated altered functional connectivity (FC) between the anterior cingulate cortex (ACC) and the frontoparietal executive control network (ECN), which was significantly associated with impulsivity in BPD. Impulsivity is often defined as a function of inhibitory control, strongly relying on the proper functioning of the fronto-cingulo-striatal network. Noradrenergic, dopaminergic and serotonergic neurotransmitter systems are assumed to be involved in different forms of impulsive behavior and inhibitory control. In our previous study, we investigated the FC of the main monoamine-producing nuclei within the midbrain and brainstem, which were functionally integrated in specific resting-state networks. In the present study we investigated the resting-state FC of midbrain/brainstem nuclei in 33 unmedicated female patients with BPD and 33 matched healthy controls. We further related altered functional connectivity of these nuclei to the patient's degree of impulsivity. The main finding was that BPD patients showed stronger FC from the noradrenergic locus coeruleus (LC) to the ACC. Functional connectivity between the LC and ACC was positively associated with the degree of motor impulsivity in the total group. Controlling for aggression, a stronger FC was also found between serotonergic nucleus centralis superior (NCS) and the frontopolar cortex (FPC) in patients compared to controls. Furthermore, patients showed a weaker "anti-correlation" from the substantia nigra (SNc) to the left dorsolateral prefrontal cortex (DLPFC). The observed enhanced LC-ACC FC in BPD and its association with the motor impulsivity might be indicative of a noradrenergic dysfunction in the neural inhibitory control network, whereas the significant relationship between NCS-FPC FC and aggression points toward serotonergic contribution to prefrontal control of aggressive reactions.

Magnetic resonance imaging of the human locus coeruleus: A systematic review

The locus coeruleus (LC), the major origin of noradrenergic modulation of the central nervous system, innervates extensive areas throughout the brain and is implicated in a variety of autonomic and cognitive functions. Alterations in the LC-noradrenergic system have been associated with healthy ageing and neuropsychiatric disorders including Parkinson's disease, Alzheimer's disease and depression. The last decade has seen advances in imaging the structure and function of the LC, and this paper systematically reviews the methodology and outcomes of sixty-nine structural and functional MRI studies of the LC in humans. Structural MRI studies consistently showed lower LC signal intensity and volume in clinical groups compared to healthy controls. Within functional studies, the LC was activated by a variety of tasks/stimuli and had functional connectivity to a range of brain regions. However, reported functional LC location coordinates were widely distributed compared to previously published neuroanatomical locations. Methodological and demographic factors potentially contributing to these differences are discussed, together with recommendations to optimize the reliability and validity of future LC imaging studies.

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.

In vivo visualization of the locus coeruleus in humans: quantifying the test-retest reliability

The locus coeruleus (LC) is a brainstem nucleus involved in important cognitive functions. Recent developments in neuroimaging methods and scanning protocols have made it possible to visualize the human LC in vivo by utilizing a T₁-weighted turbo spin echo (TSE) scan. Despite its frequent use and its application as a biomarker for tracking the progress of monoaminergic-related neurodegenerative diseases, no study to date has investigated the reproducibility and inter-observer variability of LC identification using this TSE scan sequence. In this paper, we aim to quantify the test-retest reliability of LC imaging by assessing stability of the TSE contrast of the LC across two independent scan sessions and by quantifying the intra- and inter-rater reliability of the TSE scan. Additionally, we created a probabilistic LC atlas which can facilitate the spatial localization of the LC in standardized (MNI) space. Seventeen healthy volunteers participated in two scanning sessions with a mean intersession interval of 2.8 months. We found that for intra-rater reliability the mean Dice coefficient ranged between 0.65 and 0.74, and inter-rater reliability ranged between 0.54 and 0.64, showing moderate reproducibility. The mean LC contrast was 13.9% (SD 3.8) and showed scan-rescan stability (ROI approach: ICC = 0.63; maximum intensity approach: ICC = 0.53). We conclude that localization and segmentation of the LC in vivo are a challenging but reliable enterprise although clinical or longitudinal studies should be carried out carefully.

Differential involvement of brainstem noradrenergic and midbrain dopaminergic nuclei in cognitive control

Several lines of evidence suggest that the lateral prefrontal cortex (PFC), the dorsal anterior cingulate cortex (dACC), the parietal cortex, and the thalamus are central cortical nodes in a network underlying cognitive control. However, the role of catecholamine producing midbrain and brainstem structures has rarely been addressed by functional magnetic resonance imaging (fMRI). We hypothesized differential activation patterns in the ventral tegmental area (VTA)/substantia nigra (SN) and locus coeruleus (LC) with respect to the degree of cognitive control during a Stroop task in healthy subjects. Forty-five healthy subjects were investigated by the manual version of the Stroop task in an event-related fMRI design. We observed significant BOLD activation of both the SN/VTA and LC during the Stroop interference condition (incongruent vs. congruent condition). LC, but not SN/VTA activation significantly correlated with the Stroop interference. Interestingly, a significant linear decrease in BOLD activation during the incongruent condition during the experiment was mainly observed in the fronto-cingulo-striatal network, but not in SN/VTA and LC. Using psychophysiological (PPI) analyses, a significant functional connectivity during cognitive control was observed between SN/VTA and the nigrostriatal/mesolimbic dopaminergic system. For the LC, distinct functional connectivity pattern was observed mainly to the dorsolateral and ventrolateral PFC. Both regions revealed significant functional connectivity to the dACC, parietal and occipital regions. Thus, we demonstrate for the first time that functional activation patterns in the SN/VTA and the LC are modulated by different demands of cognitive control. In addition, these nuclei exhibit distinguishable functional connectivity patterns to cortical brain networks. Hum Brain Mapp 37:2305-2318, 2016. © 2016 Wiley Periodicals, Inc.

Pupil diameter covaries with BOLD activity in human locus coeruleus

The locus coeruleus-noradrenergic (LC-NA) neuromodulatory system has been implicated in a broad array of cognitive processes, yet scope for investigating this system's function in humans is currently limited by an absence of reliable non-invasive measures of LC activity. Although pupil diameter has been employed as a proxy measure of LC activity in numerous studies, empirical evidence for a relationship between the two is lacking. In the present study, we sought to rigorously probe the relationship between pupil diameter and BOLD activity localized to the human LC. Simultaneous pupillometry and fMRI revealed a relationship between continuous pupil diameter and BOLD activity in a dorsal pontine cluster overlapping with the LC, as localized via neuromelanin-sensitive structural imaging and an LC atlas. This relationship was present both at rest and during performance of a two-stimulus oddball task, with and without spatial smoothing of the fMRI data, and survived retrospective image correction for physiological noise. Furthermore, the spatial extent of this pupil/LC relationship guided a volume-of-interest analysis in which we provide the first demonstration in humans of a fundamental characteristic of animal LC activity: phasic modulation by oddball stimulus relevance. Taken together, these findings highlight the potential for utilizing pupil diameter to achieve a more comprehensive understanding of the role of the LC-NA system in human cognition.

Noradrenergic modulation of cognition: therapeutic implications

The noradrenaline (norepinephrine) system exerts profound influences on cognition via ascending projections to the forebrain, mostly originating from the locus coeruleus. This paper provides an overview of available infrahuman and healthy human studies, exploring the effects of specific noradrenergic manipulations on dissociable cognitive functions, including attention, working memory, cognitive flexibility, response inhibition and emotional memory. Remarkable parallels across species have been reported which may account for the mechanisms by which noradrenergic medications exert their beneficial effects in disorders such as depression and attention-deficit hyperactivity disorder (ADHD). The literature is discussed in relation to prevailing models of noradrenergic influences over cognition and novel therapeutic directions, including in relation to investigating the effects of noradrenergic manipulations on other disorders characterized by impulsivity, and dementias. Unanswered questions are also highlighted, along with key avenues for future research, both proof-of-concept and clinical.