The Mammalian Locus Coeruleus Complex-Consistencies and Variances in Nuclear Organization

Nuclear parcellation of pontine catecholaminergic and cholinergic neurons in gray parrots and pied crow brains

Employing immunohistochemical procedures with antibodies raised against tyrosine hydroxylase (TH) and choline acetyltransferase we identified and mapped the locus coeruleus complex (LoC) and the pontine laterodorsal tegmental (LDT) and pedunculopontine tegmental (PPN) cholinergic nuclei in the brains of a Congo gray parrot, a timneh gray parrot, and a pied crow. The LoC and LDT/PPN are centrally involved in the regulation and generation of different sleep states, and as all birds studied to date show both REM and non-REM sleep states, like mammals, we investigated whether these noradrenergic and cholinergic nuclei in the avian pons shared anatomical features with those in the mammalian pons. The LoC was parcellated into 3 distinct nuclei, including the locus coeruleus (A6), subcoeruleus (A7), and the fifth arcuate nucleus (A5), while distinct LDT and PPN nuclei were revealed. Several similarities that allow the assumption of homology of these nuclei between birds and mammals were revealed, including their location relative to each other and other structures within the pontine region, as well as a specific degree of topographical overlap of the noradrenergic and cholinergic neurons. Despite this, some differences were noted that may be of interest in understanding the differences in sleep between birds and mammals. Further anatomical and physiological studies are needed to determine whether these pontine nuclei in birds play the same role as in mammals, as while the homology is apparent, the functional analogy needs to be revealed.

Role of the locus coeruleus-noradrenergic system in stress-related psychopathology and resilience: Clinical and pre-clinical evidences

Stressful events, from daily stressors to traumatic experiences, are common and occur at any age. Despite the high prevalence of trauma, not everyone develops stress-related disorders like major depressive disorder (MDD) and post-traumatic stress disorder (PTSD), a variation attributed to resilience, the ability to adapt and avoid negative consequences of significant stress. This review examines the locus coeruleus-norepinephrine (LC-NE) system, a critical component in the brain's stress response. It discusses the LC-NE system's anatomical and functional complexity and its role in individual variability in stress responses. How different etiological factors and stress modalities affect the LC-NE system, influencing both adaptive stress responses and psychopathologies, are discussed and supported by evidence from human and animal studies. It also explores molecular and cellular adaptations in the LC that contribute to resilience, including roles of neuropeptide, inflammatory cytokines, and genetic modulation, and addresses developmental and sex differences in stress vulnerability. The need for a multifaceted approach to understand stress-induced psychopathologies is emphasized and pave the way for more personalized interventions for stress-related disorders.

Hovering flight regulation of pigeon robots in laboratory and field

Compared to traditional bio-mimic robots, animal robots show superior locomotion, energy efficiency, and adaptability to complex environments but most remained in laboratory stage, needing further development for practical applications like exploration and inspection. Our pigeon robots validated in both laboratory and field, tested with an electrical stimulus unit (2-s duration, 0.5 ms pulse width, 80 Hz frequency). In a fixed stimulus procedure, hovering flight was conducted with 8 stimulus units applied every 2 s after flew over the trigger boundary. In a flexible procedure, stimulus was applied whenever they deviated from a virtual circle, with pulse width gains of 0.1 ms or 0.2 ms according to the trajectory angle. These optimized protocols achieved a success hovering rate of 87.5% and circle curvatures of 0.008 m-1-0.024 m-1, largely advancing the practical application of animal robots.

Diversity of ancestral brainstem noradrenergic neurons across species and multiple biological factors

The brainstem region, locus coeruleus (LC), has been remarkably conserved across vertebrates. Evolution has woven the LC into wide-ranging neural circuits that influence functions as broad as autonomic systems, the stress response, nociception, sleep, and high-level cognition among others. Given this conservation, there is a strong possibility that LC activity is inherently similar across species, and furthermore that age, sex, and brain state influence LC activity similarly across species. The degree to which LC activity is homogenous across these factors, however, has never been assessed due to the small sample size of individual studies. Here, we pool data from 20 laboratories (1,855 neurons) and show diversity across both intrinsic and extrinsic factors such as species, age, sex and brain state. We use a negative binomial regression model to compare activity from male monkeys, and rats and mice of both sexes that were recorded across brain states from brain slices ex vivo or under different anesthetics or during wakefulness in vivo. LC activity differed due to complex interactions of species, sex, and brain state. The LC became more active during aging, independent of sex. Finally, in contrast to the foundational principle that all species express two distinct LC firing modes ("tonic" or "phasic"), we discovered great diversity within spontaneous LC firing patterns. Different factors were associated with higher incidence of some firing modes. We conclude that the activity of the evolutionarily-ancient LC is not conserved. Inherent differences due to age and species-sex-brain state interactions have implications for understanding the role of LC in species-specific naturalistic behavior, as well as in psychiatric disorders, cardiovascular disease, immunology, and metabolic disorders.

A combined MRI, histological and immunohistochemical rendering of the rhesus macaque locus coeruleus (LC) enables the differentiation of three distinct LC subcompartments

Locus coeruleus (LC) neurons send their noradrenergic axons across multiple brain regions, including neocortex, subcortical regions, and spinal cord. Many aspects of cognition are known to be dependent on the noradrenergic system, and it has been suggested that dysfunction in this system may play central roles in cognitive decline associated with both normative aging and neurodegenerative disease. While basic anatomical and biochemical features of the LC have been examined in many species, detailed characterizations of the structure and function of the LC across the lifespan are not currently available. This includes the rhesus macaque, which is an important model of human brain function because of their striking similarities in brain architecture and behavioral capacities. In the present study, we describe a method to combine structural MRI, Nissl, and immunofluorescent histology from individual monkeys to reconstruct, in 3 dimensions, the entire macaque LC nucleus. Using these combined methods, a standardized volume of the LC was determined, and high-resolution confocal images of tyrosine hydroxylase-positive neurons were mapped into this volume. This detailed representation of the LC allows definitions to be proposed for three distinct subnuclei, including a medial region and a lateral region (based on location with respect to the central gray, inside or outside, respectively), and a compact region (defined by densely packed neurons within the medial compartment). This enabled the volume to be estimated and cell density to be calculated independently in each LC subnucleus for the first time. This combination of methods should allow precise characterization of the LC and has the potential to do the same for other nuclei with distinct molecular features.

The Locus Coeruleus in Chronic Pain

Pain perception is the consequence of a complex interplay between activation and inhibition. Noradrenergic pain modulation inhibits nociceptive transmission and pain perception. The main source of norepinephrine (NE) in the central nervous system is the Locus Coeruleus (LC), a small but complex cluster of cells in the pons. The aim of this study is to review the literature on the LC-NE inhibitory system, its influence on chronic pain pathways and its frequent comorbidities. The literature research showed that pain perception is the consequence of nociceptive and environmental processing and is modulated by the LC-NE system. If perpetuated in time, nociceptive inputs can generate neuroplastic changes in the central nervous system that reduce the inhibitory effects of the LC-NE complex and facilitate the development of chronic pain and frequent comorbidities, such as anxiety, depression or sleeping disturbances. The exact mechanisms involved in the LC functional shift remain unknown, but there is some evidence that they occur through plastic changes in the medial and lateral pathways and their brain projections. Additionally, there are other influencing factors, like developmental issues, neuroinflammatory glial changes, NE receptor affinity and changes in LC neuronal firing rates.

Juxtacellular recordings from identified neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the primary source of noradrenergic transmission in the mammalian central nervous system. This small pontine nucleus consists of a densely packed nuclear core-which contains the highest density of noradrenergic neurons-embedded within a heterogeneous surround of non-noradrenergic cells. This local heterogeneity, together with the small size of the LC, has made it particularly difficult to infer noradrenergic cell identity based on extracellular sampling of in vivo spiking activity. Moreover, the relatively high cell density, background activity and synchronicity of LC neurons have made spike identification and unit isolation notoriously challenging. In this study, we aimed at bridging these gaps by performing juxtacellular recordings from single identified neurons within the mouse LC complex. We found that noradrenergic neurons (identified by tyrosine hydroxylase, TH, expression; TH-positive) and intermingled putatively non-noradrenergic (TH-negative) cells displayed similar morphologies and responded to foot shock stimuli with excitatory responses; however, on average, TH-positive neurons exhibited more prominent foot shock responses and post-activation firing suppression. The two cell classes also displayed different spontaneous firing rates, spike waveforms and temporal spiking properties. A logistic regression classifier trained on spontaneous electrophysiological features could separate the two cell classes with 76% accuracy. Altogether, our results reveal in vivo electrophysiological correlates of TH-positive neurons, which can be useful for refining current approaches for the classification of LC unit activity.

In search of the locus coeruleus: guidelines for identifying anatomical boundaries and electrophysiological properties of the blue spot in mice, fish, finches, and beyond

Our understanding of human brain function can be greatly aided by studying analogous brain structures in other organisms. One brain structure with neurochemical and anatomical homology throughout vertebrate species is the locus coeruleus (LC), a small collection of norepinephrine (NE)-containing neurons in the brainstem that project throughout the central nervous system. The LC is involved in nearly every aspect of brain function, including arousal and learning, which has been extensively examined in rats and nonhuman primates using single-unit recordings. Recent work has expanded into putative LC single-unit electrophysiological recordings in a nonmodel species, the zebra finch. Given the importance of correctly identifying analogous structures as research efforts expand to other vertebrates, we suggest adoption of consensus anatomical and electrophysiological guidelines for identifying LC neurons across species when evaluating brainstem single-unit spiking or calcium imaging. Such consensus criteria will allow for confident cross-species understanding of the roles of the LC in brain function and behavior.

Cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations in the brain of the lesser hedgehog tenrec (Echinops telfairi)

The current study provides an analysis of the cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations, or nuclei, in the brain of the lesser hedgehog tenrec, as revealed with immunohistochemical techniques. For all four of these neuromodulatory systems, the nuclear organization was very similar to that observed in other Afrotherian species and is broadly similar to that observed in other mammals. The cholinergic system shows the most variation, with the lesser hedgehog tenrec exhibiting palely immunopositive cholinergic neurons in the ventral portion of the lateral septal nucleus, and the possible absence of cholinergic neurons in the parabigeminal nucleus and the medullary tegmental field. The nuclear complement of the catecholaminergic, serotonergic and orexinergic systems showed no specific variances in the lesser hedgehog tenrec when compared to other Afrotherians, or broadly with other mammals. A striking feature of the lesser hedgehog tenrec brain is a significant mesencephalic flexure that is observed in most members of the Tenrecoidea, as well as the closely related Chrysochlorinae (golden moles), but is not present in the greater otter shrew, a species of the Potomogalidae lineage currently incorporated into the Tenrecoidea. In addition, the cholinergic neurons of the ventral portion of the lateral septal nucleus are observed in the golden moles, but not in the greater otter shrew. This indicates that either complex parallel evolution of these features occurred in the Tenrecoidea and Chrysochlorinae lineages, or that the placement of the Potomogalidae within the Tenrecoidea needs to be re-examined.

Locus coeruleus and the defensive activation theory of rapid eye movement sleep: A mechanistic perspective

The defensive activation theory (DAT) was recently proposed to explain the biological function of dreaming. Briefly, DAT states that dreams are primarily visual to prevent plastic take-over of an otherwise inactive visual cortex during sleep. Evidence to support the DAT revolve around the interplay between dream activity (REM%) and cortical plasticity found in evolutionary history, primate studies, and coinciding decline in human cortical plasticity and REM% with age. As the DAT may prove difficult to test experimentally, we investigate whether further support for the DAT can be found in the literature. Plasticity and REM sleep are closely linked to functions of the Locus Coeruleus (LC). We therefore review existing knowledge about the LC covering LC stability with age, and the role of the LC in the plasticity of the visual cortex. Recent studies show the LC to be more stable than previously believed and therefore, the LC likely supports the REM% and plasticity in the same manner throughout life. Based on this finding, we review the effect of aging on REM% and visual cortex plasticity. Here, we find that recent, weighty studies are not in complete agreement with the data originally provided as support for DAT. Results from these studies, however, are not in themselves irreconcilable with the DAT. Our findings therefore do not disprove the DAT. Importantly, we show that the LC is involved in all mechanisms central to the DAT. The LC may therefore provide an experimental window to further explore and test the DAT.

Systems Neuroscience 2021 Top Papers: An Editorial Summary

Many years ago, before the Internet and the introduction of the electronic publications, bibliographical research was conducted in physical libraries, and the most commonly used source of information was the regularly updated Index Medicus, a multi-volume treatise that for 125 years summarized and indexed all published medical literature, classifying it by keywords and subject headings [...].

Chemogenetic activation of noradrenergic A5 neurons increases blood pressure and visceral sympathetic activity in adult rats

In mammals, the pontine noradrenergic system influences nearly every aspect of central nervous system function. A subpopulation of pontine noradrenergic neurons, called A5, are thought to be important in the cardiovascular response to physical stressors, yet their function is poorly defined. We hypothesized that activation of A5 neurons drives a sympathetically mediated increase in blood pressure (BP). To test this hypothesis, we conducted a comprehensive assessment of the cardiovascular effects of chemogenetic stimulation of A5 neurons in male and female adult rats using intersectional genetic and anatomical targeting approaches. Chemogenetic stimulation of A5 neurons in freely behaving rats elevated BP by 15 mmHg and increased cardiac baroreflex sensitivity with a negligible effect on resting HR. Importantly, A5 stimulation had no detectable effect on locomotor activity, metabolic rate, or respiration. Under anesthesia, stimulation of A5 neurons produced a marked elevation in visceral sympathetic nerve activity (SNA) and no change in skeletal muscle SNA, showing that A5 neurons preferentially stimulate visceral SNA. Interestingly, projection mapping indicates that A5 neurons target sympathetic preganglionic neurons throughout the spinal cord and parasympathetic preganglionic neurons throughout in the brainstem, as well as the nucleus of the solitary tract, and ventrolateral medulla. Moreover, in situ hybridization and immunohistochemistry indicate that a subpopulation of A5 neurons coreleases glutamate and monoamines. Collectively, this study suggests A5 neurons are a central modulator of autonomic function with a potentially important role in sympathetically driven redistribution of blood flow from the visceral circulation to critical organs and skeletal muscle.

Molecular Organization and Patterning of the Medulla Oblongata in Health and Disease

The medulla oblongata, located in the hindbrain between the pons and the spinal cord, is an important relay center for critical sensory, proprioceptive, and motoric information. It is an evolutionarily highly conserved brain region, both structural and functional, and consists of a multitude of nuclei all involved in different aspects of basic but vital functions. Understanding the functional anatomy and developmental program of this structure can help elucidate potential role(s) of the medulla in neurological disorders. Here, we have described the early molecular patterning of the medulla during murine development, from the fundamental units that structure the very early medullary region into 5 rhombomeres (r7–r11) and 13 different longitudinal progenitor domains, to the neuronal clusters derived from these progenitors that ultimately make-up the different medullary nuclei. By doing so, we developed a schematic overview that can be used to predict the cell-fate of a progenitor group, or pinpoint the progenitor domain of origin of medullary nuclei. This schematic overview can further be used to help in the explanation of medulla-related symptoms of neurodevelopmental disorders, e.g., congenital central hypoventilation syndrome, Wold–Hirschhorn syndrome, Rett syndrome, and Pitt–Hopkins syndrome. Based on the genetic defects seen in these syndromes, we can use our model to predict which medullary nuclei might be affected, which can be used to quickly direct the research into these diseases to the likely affected nuclei.

Feasibility of Canine Adenovirus Type 2 (CAV2) Based Vector for the Locus Coeruleus Optogenetic Activation in Non-Transgenic Rats: Implications for Functional Studies

The locus coeruleus norepinephrine (LC-NE) system modulates many visceral and cognitive functions, while LC-NE dysfunction leads to neurological and neurodegenerative conditions such as sleep disorders, depression, ADHD, or Alzheimer's disease. Innovative viral-vector and gene-engineering technology combined with the availability of cell-specific promoters enabled regional targeting and selective control over phenotypically specific populations of neurons. We transduced the LC-NE neurons in adult male rats by delivering the canine adenovirus type 2-based vector carrying the NE-specific promoter PRSx8 and a light-sensitive channelrhodopsin-2 receptor (ChR2) directly in the LC or retrogradely from the LC targets. The highest ChR2 expression level was achieved when the virus was delivered medially to the trigeminal pathway and ~100 μm lateral to the LC. The injections close or directly in the LC compromised the tissue integrity and NE cell phenotype. Retrograde labeling was more optimal given the transduction of projection-selective subpopulations. Our results highlight a limited inference of ChR2 expression from representative cases to the entire population of targeted cells. The actual fraction of manipulated neurons appears most essential for an adequate interpretation of the study outcome. The actual fraction of manipulated neurons appears most essential for an adequate interpretation of the study outcome. Thus, besides the cell-type specificity and the transduction efficiency, the between-subject variability in the proportion of the remaining viral-transduced targeted cell population must be considered in any functional connectivity study.

When the Locus Coeruleus Speaks Up in Sleep: Recent Insights, Emerging Perspectives

For decades, numerous seminal studies have built our understanding of the locus coeruleus (LC), the vertebrate brain’s principal noradrenergic system. Containing a numerically small but broadly efferent cell population, the LC provides brain-wide noradrenergic modulation that optimizes network function in the context of attentive and flexible interaction with the sensory environment. This review turns attention to the LC’s roles during sleep. We show that these roles go beyond down-scaled versions of the ones in wakefulness. Novel dynamic assessments of noradrenaline signaling and LC activity uncover a rich diversity of activity patterns that establish the LC as an integral portion of sleep regulation and function. The LC could be involved in beneficial functions for the sleeping brain, and even minute alterations in its functionality may prove quintessential in sleep disorders.

Functional Coupling of the Locus Coeruleus Is Linked to Successful Cognitive Control

The locus coeruleus (LC) is a brainstem structure that sends widespread efferent projections throughout the mammalian brain. The LC constitutes the major source of noradrenaline (NE), a modulatory neurotransmitter that is crucial for fundamental brain functions such as arousal, attention, and cognitive control. This role of the LC-NE is traditionally not believed to reflect functional influences on the frontoparietal network or the striatum, but recent advances in chemogenetic manipulations of the rodent brain have challenged this notion. However, demonstrations of LC-NE functional connectivity with these areas in the human brain are surprisingly sparse. Here, we close this gap. Using an established emotional stroop task, we directly compared trials requiring response conflict control with trials that did not require this, but were matched for visual stimulus properties, response modality, and controlled for pupil dilation differences across both trial types. We found that LC-NE functional coupling with the parietal cortex and regions of the striatum is substantially enhanced during trials requiring response conflict control. Crucially, the strength of this functional coupling was directly related to individual reaction time differences incurred by conflict resolution. Our data concur with recent rodent findings and highlight the importance of converging evidence between human and nonhuman neurophysiology to further understand the neural systems supporting adaptive and maladaptive behavior in health and disease.

Using Inhibitory DREADDs to Silence LC Neurons in Monkeys

Understanding the role of the noradrenergic nucleus locus coeruleus (LC) in cognition and behavior is critical: It is involved in several key behavioral functions such as stress and vigilance, as well as in cognitive processes such as attention and decision making. In recent years, the development of viral tools has provided a clear insight into numerous aspects of brain functions in rodents. However, given the specificity of primate brains and the key benefit of monkey research for translational applications, developing viral tools to study the LC in monkeys is essential for understanding its function and exploring potential clinical strategies. Here, we describe a pharmacogenetics approach that allows to selectively and reversibly inactivate LC neurons using Designer Receptors Exclusively Activated by Designer Drugs (DREADD). We show that the expression of the hM4Di DREADD can be restricted to noradrenergic LC neurons and that the amount of LC inhibition can be adjusted by adapting the dose of the specific DREADD activator deschloroclozapine (DCZ). Indeed, even if high doses (>0.3 mg/kg) induce a massive inhibition of LC neurons and a clear decrease in vigilance, smaller doses (<0.3 mg/kg) induce a more moderate decrease in LC activity, but it does not affect vigilance, which is more compatible with an assessment of subtle cognitive functions such as decision making and attention.

Locus Coeruleus in Non-Mammalian Vertebrates

The locus coeruleus (LC) is a vertebrate-specific nucleus and the primary source of norepinephrine (NE) in the brain. This nucleus has conserved properties across species: highly homogeneous cell types, a small number of cells but extensive axonal projections, and potent influence on brain states. Comparative studies on LC benefit greatly from its homogeneity in cell types and modularity in projection patterns, and thoroughly understanding the LC-NE system could shed new light on the organization principles of other more complex modulatory systems. Although studies on LC are mainly focused on mammals, many of the fundamental properties and functions of LC are readily observable in other vertebrate models and could inform mammalian studies. Here, we summarize anatomical and functional studies of LC in non-mammalian vertebrate classes, fish, amphibians, reptiles, and birds, on topics including axonal projections, gene expressions, homeostatic control, and modulation of sensorimotor transformation. Thus, this review complements mammalian studies on the role of LC in the brain.