Locus coeruleus: From global projection system to adaptive regulation of behavior

Neonatal immune challenge followed by adult immune challenge induces epigenetic-susceptibility to aggravated visceral hypersensitivity

BACKGROUND

Abdominal pain is one of the major symptoms of inflammatory Bowel Disease (IBD). The inflammatory mediators released by colon inflammation are known to sensitize the afferent neurons, which is one of the contributors to abdominal pain. However, not all IBD patients have abdominal pain, and some patients report abdominal pain during remission, suggesting contributions of other pathological factors to abdominal pain in IBD. Epidemiological studies found early-life gastrointestinal infections a risk factor for IBD symptoms and adult-life gastrointestinal infections may trigger the onset of IBD. We investigated the hypothesis that neonatal colon immune challenge followed by an adult colon immune challenge upregulates spinal cord BDNF that aggravates visceral sensitivity over and above that induced by adult colon immune challenge alone.

METHODS

We induced neonatal and adult colon immune challenges by intraluminal administration of trinitrobenzene sulfonic acid to the rat colon.

KEY RESULTS

We found that neonatal immune challenge triggers epigenetic programming that upregulates tyrosine hydroxylase in the locus ceruleus when these rats are subjected to an adult colon immune challenge. The upregulation of locus ceruleus tyrosine hydroxylase, upregulates norepinephrine in the cerebrospinal fluid that acts on adrenergic receptors to enhance pCREB binding to the cAMP response element, which recruits histone acetylene transferase (HAT) to the BDNF gene to enhance its transcription resulting in aggravated visceromotor response to colorectal distension. HAT and adrenergic receptor antagonists block the aggravation of visceral sensitivity.

CONCLUSION & INFERENCES

HAT and adrenergic receptor inhibitors may serve as alternates to opioids and NSAIDS in suppressing abdominal pain in IBD.

Calcium, mitochondrial dysfunction and slowing the progression of Parkinson's disease

Parkinson's disease is characterized by progressively distributed Lewy pathology and neurodegeneration. The motor symptoms of clinical Parkinson's disease (cPD) are unequivocally linked to the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). Several features of these neurons appear to make them selectively vulnerable to factors thought to cause cPD, like aging, genetic mutations and environmental toxins. Among these features, Ca²⁺ entry through Cav1 channels is particularly amenable to pharmacotherapy in early stage cPD patients. This review outlines the linkage between these channels, mitochondrial oxidant stress and cPD pathogenesis. It also summarizes considerations that went into the design and execution of the ongoing Phase 3 clinical trial with an inhibitor of these channels - isradipine.

The pattern of thalamocortical and brain stem projections to the vibrissae-related sensory and motor cortices in de-whiskered congenital hypothyroid rats

The present study is designed to investigate the plastic organization of the thalamo-cortical (TC) and brain stem afferents of whisker primary sensory (wS1) and motor (wM1) cortical areas in congenital hypothyroid (CH) pups following whisker deprivation (WD) from neonatal to adolescence period. Maternal hypothyroidism was induced by adding propylthiouracil (PTU) to the drinking water from early embryonic day 16 to postnatal day (PND) 60. Pregnant rats were divided into intact and CH groups (n = 8). In each group, the total whiskers of pups (4 of 8) were trimmed continuously from PND 1 to PND 60. Retrograde tracing technique with WGA-HRP was performed in the present study. Retrogradely labeled neurons were observed in the specific thalamic nuclei (VPM and VL) following separately WGA-HRP injections into wS1/M1 cortical areas. The number of labeled cells in the VPM, VL, VM and PO nuclei of the thalamus significantly decreased in CH offsprings rats (P < 0.05). Neonatal WD did not show any significant effects on the number of VPM, VL, VM and PO labeled projection neurons to wS1 and wM1 cortical areas. In addition, retrogradely labeled neurons in dorsal raphe (DR) and locus coeruleus (LC) nuclei were observed in all experimental groups. The number of DR and LC labeled neurons were higher in the CH and whisker deprived groups compared to their matching controls (P < 0.05). Upon our results, CH and WD had no synergic or additive effects on the TC and brain stem afferent patterns of barrel sensory and motor cortices.

Barrington's nucleus: Neuroanatomic landscape of the mouse "pontine micturition center"

Barrington's nucleus (Bar) is thought to contain neurons that trigger voiding and thereby function as the "pontine micturition center." Lacking detailed information on this region in mice, we examined gene and protein markers to characterize Bar and the neurons surrounding it. Like rats and cats, mice have an ovoid core of medium-sized Bar neurons located medial to the locus coeruleus (LC). Bar neurons express a GFP reporter for Vglut2, develop from a Math1/Atoh1 lineage, and exhibit immunoreactivity for NeuN. Many neurons in and around this core cluster express a reporter for corticotrophin-releasing hormone (BarCRH ). Axons from BarCRH neurons project to the lumbosacral spinal cord and ramify extensively in two regions: the dorsal gray commissural and intermediolateral nuclei. BarCRH neurons have unexpectedly long dendrites, which may receive synaptic input from the cerebral cortex and other brain regions beyond the core afferents identified previously. Finally, at least five populations of neurons surround Bar: rostral-dorsomedial cholinergic neurons in the laterodorsal tegmental nucleus; lateral noradrenergic neurons in the LC; medial GABAergic neurons in the pontine central gray; ventromedial, small GABAergic neurons that express FoxP2; and dorsolateral glutamatergic neurons that express FoxP2 in the pLC and form a wedge dividing Bar from the dorsal LC. We discuss the implications of this new information for interpreting existing data and future experiments targeting BarCRH neurons and their synaptic afferents to study micturition and other pelvic functions.

Pupil size as a measure of within-task learning

Pupillometry is commonly used in research to determine how much mental effort an individual is exerting while completing tasks. Traditionally, larger pupils are associated with increased mental effort when completing more difficult tasks. However, little research has investigated how pupils change as individuals learn a new task. In theory, as one repeatedly completes a task, the task demands should reduce, reliance on working memory should decrease, and the task should become more automatic. This should translate to faster completion times and smaller peak pupil dilations. We tested this hypothesis by having participants complete multiple trials of a cognitive task that requires individuals to orient themselves in space relative to a target. We found that trial completion times and maximum pupil size significantly reduced across trials. These data suggest that measuring changes in pupil dilation may help researchers determine whether individuals have shifted from a learned procedure to an automatic processing of information when learning a new task.

Selective neuronal vulnerability in Parkinson disease

Intracellular α-synuclein (α-syn)-rich protein aggregates called Lewy pathology (LP) and neuronal death are commonly found in the brains of patients with clinical Parkinson disease (cPD). It is widely believed that LP appears early in the disease and spreads in synaptically coupled brain networks, driving neuronal dysfunction and death. However, post-mortem analysis of human brains and connectome-mapping studies show that the pattern of LP in cPD is not consistent with this simple model, arguing that, if LP propagates in cPD, it must be gated by cell- or region-autonomous mechanisms. Moreover, the correlation between LP and neuronal death is weak. In this Review, we briefly discuss the evidence for and against the spreading LP model, as well as evidence that cell-autonomous factors govern both α-syn pathology and neuronal death.

The Brainstem in Emotion: A Review

Emotions depend upon the integrated activity of neural networks that modulate arousal, autonomic function, motor control, and somatosensation. Brainstem nodes play critical roles in each of these networks, but prior studies of the neuroanatomic basis of emotion, particularly in the human neuropsychological literature, have mostly focused on the contributions of cortical rather than subcortical structures. Given the size and complexity of brainstem circuits, elucidating their structural and functional properties involves technical challenges. However, recent advances in neuroimaging have begun to accelerate research into the brainstem’s role in emotion. In this review, we provide a conceptual framework for neuroscience, psychology and behavioral science researchers to study brainstem involvement in human emotions. The “emotional brainstem” is comprised of three major networks – Ascending, Descending and Modulatory. The Ascending network is composed chiefly of the spinothalamic tracts and their projections to brainstem nuclei, which transmit sensory information from the body to rostral structures. The Descending motor network is subdivided into medial projections from the reticular formation that modulate the gain of inputs impacting emotional salience, and lateral projections from the periaqueductal gray, hypothalamus and amygdala that activate characteristic emotional behaviors. Finally, the brainstem is home to a group of modulatory neurotransmitter pathways, such as those arising from the raphe nuclei (serotonergic), ventral tegmental area (dopaminergic) and locus coeruleus (noradrenergic), which form a Modulatory network that coordinates interactions between the Ascending and Descending networks. Integration of signaling within these three networks occurs at all levels of the brainstem, with progressively more complex forms of integration occurring in the hypothalamus and thalamus. These intermediary structures, in turn, provide input for the most complex integrations, which occur in the frontal, insular, cingulate and other regions of the cerebral cortex. Phylogenetically older brainstem networks inform the functioning of evolutionarily newer rostral regions, which in turn regulate and modulate the older structures. Via these bidirectional interactions, the human brainstem contributes to the evaluation of sensory information and triggers fixed-action pattern responses that together constitute the finely differentiated spectrum of possible emotions.

Zinc in the Monoaminergic Theory of Depression: Its Relationship to Neural Plasticity

Preclinical and clinical studies have demonstrated that zinc possesses antidepressant properties and that it may augment the therapy with conventional, that is, monoamine-based, antidepressants. In this review we aim to discuss the role of zinc in the pathophysiology and treatment of depression with regard to the monoamine hypothesis of the disease. Particular attention will be paid to the recently described zinc-sensing GPR39 receptor as well as aspects of zinc deficiency. Furthermore, an attempt will be made to give a possible explanation of the mechanisms by which zinc interacts with the monoamine system in the context of depression and neural plasticity.

Stress Response, Brain Noradrenergic System and Cognition

Locus coeruleus is a critical component of the brain noradrenergic system. The brain noradrenergic system provides the neural substrate for the architecture supporting the interaction with, and navigation through, an external world complexity. Changes in locus coeruleus tonic and phasic activity and the interplay between norepinephrine and α₁- and α₂-adrenoceptors in the prefrontal cortex are the key elements of this sophisticated architecture. In this narrative review we discuss how the brain noradrenergic system is affected by increased exposure to corticotropin-releasing hormone triggered by stress response. In particular, we present the mechanisms responsible for thinking inflexibility often observed under highly stressful conditions. Finally, the main directions for future research are highlighted.

Exercise Strategies to Counteract Brain Aging Effects

Stimulating structural and functional adaptation that improves cognitive performance in specific tasks is the major objective of therapeutic exercise training. In this review we briefly summarize central physiological mechanisms activated by exercise. We further discuss the influence of different kinds of exercise on cognitive improvement. In particular, the effects on cognitive function of aerobic endurance, resistance and respiratory exercise, and combinations thereof are presented. The accumulating evidence reinforces the position that regular aerobic, and possibly also resistance training, offers a powerful tool to cope with biologic aging of central nervous system functions. Nevertheless, the potential magnitude of cognition improvement or restrain of age-related cognition deterioration and the quantity of physical activity required to induce meaningful responses remain to be clarified.

Reboxetine Improves Auditory Attention and Increases Norepinephrine Levels in the Auditory Cortex of Chronically Stressed Rats

Chronic stress impairs auditory attention in rats and monoamines regulate neurotransmission in the primary auditory cortex (A1), a brain area that modulates auditory attention. In this context, we hypothesized that norepinephrine (NE) levels in A1 correlate with the auditory attention performance of chronically stressed rats. The first objective of this research was to evaluate whether chronic stress affects monoamines levels in A1. Male Sprague-Dawley rats were subjected to chronic stress (restraint stress) and monoamines levels were measured by high performance liquid chromatographer (HPLC)-electrochemical detection. Chronically stressed rats had lower levels of NE in A1 than did controls, while chronic stress did not affect serotonin (5-HT) and dopamine (DA) levels. The second aim was to determine the effects of reboxetine (a selective inhibitor of NE reuptake) on auditory attention and NE levels in A1. Rats were trained to discriminate between two tones of different frequencies in a two-alternative choice task (2-ACT), a behavioral paradigm to study auditory attention in rats. Trained animals that reached a performance of ≥80% correct trials in the 2-ACT were randomly assigned to control and stress experimental groups. To analyze the effects of chronic stress on the auditory task, trained rats of both groups were subjected to 50 2-ACT trials 1 day before and 1 day after of the chronic stress period. A difference score (DS) was determined by subtracting the number of correct trials after the chronic stress protocol from those before. An unexpected result was that vehicle-treated control rats and vehicle-treated chronically stressed rats had similar performances in the attentional task, suggesting that repeated injections with vehicle were stressful for control animals and deteriorated their auditory attention. In this regard, both auditory attention and NE levels in A1 were higher in chronically stressed rats treated with reboxetine than in vehicle-treated animals. These results indicate that NE has a key role in A1 and attention of stressed rats during tone discrimination.

The impact of luminance on tonic and phasic pupillary responses to sustained cognitive load

Pupillary reactions independent of light conditions have been linked to cognition for a long time. However, the light conditions can impact the cognitive pupillary reaction. Previous studies underlined the impact of luminance on pupillary reaction, but it is still unclear how luminance modulates the sustained and transient components of pupillary reaction - tonic pupil diameter and phasic pupil response. In the present study, we investigated the impact of the luminance on these two components under sustained cognitive load. Fourteen participants performed a novel working memory task combining mathematical computations with a classic n-back task. We studied both tonic pupil diameter and phasic pupil response under low (1-back) and high (2-back) working memory load and two luminance levels (gray and white). We found that the impact of working memory load on the tonic pupil diameter was modulated by the level of luminance, the increase in tonic pupil diameter with the load being larger under lower luminance. In contrast, the smaller phasic pupil response found under high load remained unaffected by luminance. These results showed that luminance impacts the cognitive pupillary reaction - tonic pupil diameter (phasic pupil response) being modulated under sustained (respectively, transient) cognitive load. These findings also support the relationship between the locus-coeruleus system, presumably functioning in two firing modes - tonic and phasic - and the pupil diameter. We suggest that the tonic pupil diameter tracks the tonic activity of the locus-coeruleus while phasic pupil response reflects its phasic activity. Besides, the designed novel cognitive paradigm allows the simultaneous manipulation of sustained and transient components of the cognitive load and is useful for dissociating the effects on the tonic pupil diameter and phasic pupil response.

Activity-Based Anorexia Reduces Body Weight without Inducing a Separate Food Intake Microstructure or Activity Phenotype in Female Rats-Mediation via an Activation of Distinct Brain Nuclei

Anorexia nervosa (AN) is accompanied by severe somatic and psychosocial complications. However, the underlying pathogenesis is poorly understood, treatment is challenging and often hampered by high relapse. Therefore, more basic research is needed to better understand the disease. Since hyperactivity often plays a role in AN, we characterized an animal model to mimic AN using restricted feeding and hyperactivity. Female Sprague-Dawley rats were divided into four groups: no activity/ad libitum feeding (ad libitum, AL, n = 9), activity/ad libitum feeding (activity, AC, n = 9), no activity/restricted feeding (RF, n = 12) and activity/restricted feeding (activity-based anorexia, ABA, n = 11). During the first week all rats were fed ad libitum, ABA and AC had access to a running wheel for 24 h/day. From week two ABA and RF only had access to food from 9:00 to 10:30 a.m. Body weight was assessed daily, activity and food intake monitored electronically, brain activation assessed using Fos immunohistochemistry at the end of the experiment. While during the first week no body weight differences were observed (p > 0.05), after food restriction RF rats showed a body weight decrease: −13% vs. day eight (p < 0.001) and vs. AC (−22%, p < 0.001) and AL (−26%, p < 0.001) that gained body weight (+10% and +13%, respectively; p < 0.001). ABA showed an additional body weight loss (−9%) compared to RF (p < 0.001) reaching a body weight loss of −22% during the 2-week restricted feeding period (p < 0.001). Food intake was greatly reduced in RF (−38%) and ABA (−41%) compared to AL (p < 0.001). Interestingly, no difference in 1.5-h food intake microstructure was observed between RF and ABA (p > 0.05). Similarly, the daily physical activity was not different between AC and ABA (p > 0.05). The investigation of Fos expression in the brain showed neuronal activation in several brain nuclei such as the supraoptic nucleus, arcuate nucleus, locus coeruleus and nucleus of the solitary tract of ABA compared to AL rats. In conclusion, ABA combining physical activity and restricted feeding likely represents a suited animal model for AN to study pathophysiological alterations and pharmacological treatment options. Nonetheless, cautious interpretation of the data is necessary since rats do not voluntarily reduce their body weight as observed in human AN.

Melanin-concentrating hormone axons, but not orexin or tyrosine hydroxylase axons, innervate the claustrum in the rat: An immunohistochemical study

The claustrum is a small, elongated nucleus close to the external capsule and deep in the insular cortex. In rodents, this nucleus is characterized by a dense cluster of parvalbumin labeling. The claustrum is connected with the cerebral cortex. It does not project to the brainstem, but brainstem structures can influence this nucleus. To identify some specific projections from the lateral hypothalamus and midbrain, we analyzed the distribution of projections labeled with antibodies against tyrosine hydroxylase (TH), melanin-concentrating hormone (MCH), and hypocretin (Hcrt) in the region of the claustrum. The claustrum contains a significant projection by MCH axons, whereas it is devoid of TH projections. Unlike TH and MCH axons, Hcrt axons are scattered throughout the region. This observation is discussed mainly with regard to the role of the claustrum in cognitive functions and that of MCH in REM sleep. J. Comp. Neurol. 525:1489-1498, 2017. © 2016 Wiley Periodicals, Inc.

Calcium and Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease in the world. Its causes are poorly understood and there is no proven therapeutic strategy for slowing disease progression. The core motor symptoms of PD are caused by the death of dopaminergic neurons in the substantia nigra pars compacta (SNc). In these neurons, Ca²⁺entry through plasma membrane Cav1 channels drives a sustained feed-forward stimulation of mitochondrial oxidative phosphorylation. Although this design helps prevent bioenergetic failure when activity needs to be sustained, it leads to basal mitochondrial oxidant stress. Over decades, this basal oxidant stress could compromise mitochondrial function and increase mitophagy, resulting in increased vulnerability to other proteostatic stressors, like elevated alpha synuclein expression. Because this feedforward mechanism is no longer demanded by our lifestyle, it could be dispensed with. Indeed, use of dihydropyridines - negative allosteric modulators of Cav1 Ca²⁺ channels - comes with little or no effect on brain function but is associated with decreased risk and progression of PD. An ongoing, NIH sponsored, Phase 3 clinical trial in North America is testing the ability of one member of the dihydropyridine class (isradipine) to slow PD progression in early stage patients. The review summarizes the rationale for the trial and outlines some unanswered questions.

Locus Ceruleus Norepinephrine Release: A Central Regulator of CNS Spatio-Temporal Activation?

Norepinephrine (NE) is synthesized in the Locus Coeruleus (LC) of the brainstem, from where it is released by axonal varicosities throughout the brain via volume transmission. A wealth of data from clinics and from animal models indicates that this catecholamine coordinates the activity of the central nervous system (CNS) and of the whole organism by modulating cell function in a vast number of brain areas in a coordinated manner. The ubiquity of NE receptors, the daunting number of cerebral areas regulated by the catecholamine, as well as the variety of cellular effects and of their timescales have contributed so far to defeat the attempts to integrate central adrenergic function into a unitary and coherent framework. Since three main families of NE receptors are represented-in order of decreasing affinity for the catecholamine-by: α2 adrenoceptors (α2Rs, high affinity), α1 adrenoceptors (α1Rs, intermediate affinity), and β adrenoceptors (βRs, low affinity), on a pharmacological basis, and on the ground of recent studies on cellular and systemic central noradrenergic effects, we propose that an increase in LC tonic activity promotes the emergence of four global states covering the whole spectrum of brain activation: (1) sleep: virtual absence of NE, (2) quiet wake: activation of α2Rs, (3) active wake/physiological stress: activation of α2- and α1-Rs, (4) distress: activation of α2-, α1-, and β-Rs. We postulate that excess intensity and/or duration of states (3) and (4) may lead to maladaptive plasticity, causing-in turn-a variety of neuropsychiatric illnesses including depression, schizophrenic psychoses, anxiety disorders, and attention deficit. The interplay between tonic and phasic LC activity identified in the LC in relationship with behavioral response is of critical importance in defining the short- and long-term biological mechanisms associated with the basic states postulated for the CNS. While the model has the potential to explain a large number of experimental and clinical findings, a major challenge will be to adapt this hypothesis to integrate the role of other neurotransmitters released during stress in a centralized fashion, like serotonin, acetylcholine, and histamine, as well as those released in a non-centralized fashion, like purines and cytokines.