A comprehensive analysis of the effect of DSP4 on the locus coeruleus noradrenergic system in the rat

Tyrosinase-induced neuromelanin accumulation triggers rapid dysregulation and degeneration of the mouse locus coeruleus

Motor symptoms in Parkinson's disease (PD) are caused by degeneration of dopamine (DA) neurons of the substantia nigra (SN), while early non-motor symptoms such as anxiety and sleep disturbances are likely mediated by dysfunction of locus coeruleus (LC) norepinephrine (NE) neurons. The LC develops α-synuclein pathology prior to SN DA neurons in PD, and later undergoes degeneration, but the mechanisms responsible for its vulnerability are unknown. The SN and LC are the only structures in the brain that produces appreciable amounts of neuromelanin (NM), a dark cytoplasmic pigment. It has been proposed that NM initially plays a protective role by sequestering toxic catecholamine metabolites and heavy metals, but may become harmful during aging and PD as they overwhelm cellular machinery and are released during neurodegeneration. Rodents do not naturally produce NM, limiting the study of causal relationships between NM and PD-associated LC pathology. Adapting a viral-mediated approach for expression of human tyrosinase, the enzyme responsible for peripheral melanin production, we successfully promoted pigmentation in mouse LC neurons that recapitulates key features of endogenous NM found in primates, including eumelanin and pheomelanin, lipid droplets, and a double-membrane encasement. Pigment expression results in mild neurodegeneration, reduced NE levels, transcriptional changes, and novelty-induced anxiety phenotypes as early as 1-week post-injection. By 6-weeks, NM accumulation is associated with severe LC neurodegeneration and a robust neuroinflammatory response. These phenotypes are reminiscent of LC dysfunction in PD, validating this model for studying the consequences of pigment accumulation in the LC as it relates to neurodegenerative disease.

Opposing effects of β-2 and β-1 adrenergic receptor signaling on neuroinflammation and dopaminergic neuron survival in α-synuclein-mediated neurotoxicity

BACKGROUND

Noradrenergic neurons in the locus coeruleus (LC) are the primary source of norepinephrine (NE) in the brain and degeneration of these neurons is reported in the early stages of Parkinson's disease (PD), even prior to dopaminergic neuron degeneration in the substantia nigra (SN), which is a hallmark of PD pathology. NE depletion is generally associated with increased PD pathology in neurotoxin-based PD models. The effect of NE depletion in other models of PD-like α-synuclein-based models is largely unexplored. In PD models and in human patients, β-adrenergic receptors' (AR) signaling is associated with a reduction of neuroinflammation and PD pathology. However, the effect of NE depletion in the brain and the extent of NE and β-ARs signaling involvement in neuroinflammation, and dopaminergic neuron survival is poorly understood.

METHODS

Two mouse models of PD, a 6OHDA neurotoxin-based model and a human α-synuclein (hα-SYN) virus-based model of PD, were used. DSP-4 was used to deplete NE levels in the brain and its effect was confirmed by HPLC with electrochemical detection. A pharmacological approach was used to mechanistically understand the impact of DSP-4 in the hα-SYN model of PD using a norepinephrine transporter (NET) and a β-AR blocker. Epifluorescence and confocal imaging were used to study changes in microglia activation and T-cell infiltration after β1-AR and β2-AR agonist treatment in the hα-SYN virus-based model of PD.

RESULTS

Consistent with previous studies, we found that DSP-4 pretreatment increased dopaminergic neuron loss after 6OHDA injection. In contrast, DSP-4 pretreatment protected dopaminergic neurons after hα-SYN overexpression. DSP-4-mediated protection of dopaminergic neurons after hα-SYN overexpression was dependent on β-AR signaling since using a β-AR blocker prevented DSP-4-mediated dopaminergic neuron protection in this model of PD. Finally, we found that the β-2AR agonist, clenbuterol, reduced microglia activation, T-cell infiltration, and dopaminergic neuron degeneration, whereas xamoterol a β-1AR agonist showed increased neuroinflammation, blood brain barrier permeability (BBB), and dopaminergic neuron degeneration in the context of hα-SYN-mediated neurotoxicity.

CONCLUSIONS

Our data demonstrate that the effects of DSP-4 on dopaminergic neuron degeneration are model specific, and suggest that in the context of α-SYN-driven neuropathology, β2-AR specific agonists may have therapeutic benefit in PD.

The Neurotoxin DSP-4 Dysregulates the Locus Coeruleus-Norepinephrine System and Recapitulates Molecular and Behavioral Aspects of Prodromal Neurodegenerative Disease

The noradrenergic locus coeruleus (LC) is among the earliest sites of tau and α-synuclein pathology in Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. The onset of these pathologies coincides with loss of noradrenergic fibers in LC target regions and the emergence of prodromal symptoms including sleep disturbances and anxiety. Paradoxically, these prodromal symptoms are indicative of a noradrenergic hyperactivity phenotype, rather than the predicted loss of norepinephrine (NE) transmission following LC damage, suggesting the engagement of complex compensatory mechanisms. Because current therapeutic efforts are targeting early disease, interest in the LC has grown, and it is critical to identify the links between pathology and dysfunction. We employed the LC-specific neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), which preferentially damages LC axons, to model early changes in the LC-NE system pertinent to AD and PD in male and female mice. DSP-4 (two doses of 50 mg/kg, one week apart) induced LC axon degeneration, triggered neuroinflammation and oxidative stress, and reduced tissue NE levels. There was no LC cell death or changes to LC firing, but transcriptomics revealed reduced expression of genes that define noradrenergic identity and other changes relevant to neurodegenerative disease. Despite the dramatic loss of LC fibers, NE turnover and signaling were elevated in terminal regions and were associated with anxiogenic phenotypes in multiple behavioral tests. These results represent a comprehensive analysis of how the LC-NE system responds to axon/terminal damage reminiscent of early AD and PD at the molecular, cellular, systems, and behavioral levels, and provides potential mechanisms underlying prodromal neuropsychiatric symptoms.

Locus coeruleus ablation in mice: protocol optimization, stereology and behavioral impact

The Locus Coeruleus (LC) is in the brainstem and supplies key brain structures with noradrenaline, including the forebrain and hippocampus. The LC impacts specific behaviors such as anxiety, fear, and motivation, as well as physiological phenomena that impact brain functions in general, including sleep, blood flow regulation, and capillary permeability. Nevertheless, the short- and long-term consequences of LC dysfunction remain unclear. The LC is among the brain structures first affected in patients suffering from neurodegenerative diseases such as Parkinson's disease and Alzheimer's Disease, hinting that LC dysfunction may play a central role in disease development and progression. Animal models with modified or disrupted LC function are essential to further our understanding of LC function in the normal brain, the consequences of LC dysfunction, and its putative roles in disease development. For this, well-characterized animal models of LC dysfunction are needed. Here, we establish the optimal dose of selective neurotoxin N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (DSP-4) for LC ablation. Using histology and stereology, we compare LC volume and neuron number in LC ablated (LCA) mice and controls to assess the efficacy of LC ablation with different numbers of DSP-4 injections. All LCA groups show a consistent decrease in LC cell count and LC volume. We then proceed to characterize the behavior of LCA mice using a light-dark box test, Barnes maze test, and non-invasive sleep-wakefulness monitoring. Behaviorally, LCA mice differ subtly from control mice, with LCA mice generally being more curious and less anxious compared to controls consistent with known LC function and projections. We note an interesting contrast in that control mice have varying LC size and neuron count but consistent behavior whereas LCA mice (as expected) have consistently sized LC but erratic behavior. Our study provides a thorough characterization of an LC ablation model, firmly consolidating it as a valid model system for the study of LC dysfunction.

The Contribution of the Locus Coeruleus-Noradrenaline System Degeneration during the Progression of Alzheimer's Disease

Alzheimer's disease (AD), which is characterized by extracellular accumulation of amyloid-beta peptide and intracellular aggregation of hyperphosphorylated tau, is the most common form of dementia. Memory loss, cognitive decline and disorientation are the ultimate consequences of neuronal death, synapse loss and neuroinflammation in AD. In general, there are many brain regions affected but neuronal loss in the locus coeruleus (LC) is one of the earliest indicators of neurodegeneration in AD. Since the LC is the main source of noradrenaline (NA) in the brain, degeneration of the LC in AD leads to decreased NA levels, causing increased neuroinflammation, enhanced amyloid and tau burden, decreased phagocytosis and impairment in cognition and long-term synaptic plasticity. In this review, we summarized current findings on the locus coeruleus-noradrenaline system and consequences of its dysfunction which is now recognized as an important contributor to AD progression.

Distinct brainstem to spinal cord noradrenergic pathways inversely regulate spinal neuronal activity

Brainstem to spinal cord noradrenergic pathways include a locus coeruleus origin projection and diffuse noxious inhibitory controls. While both pathways are traditionally viewed as exerting an inhibitory effect on spinal neuronal activity, the locus coeruleus was previously shown to have a facilitatory influence on thermal nocioception according to the subpopulation of coerulean neurons activated. Coupled with knowledge of its functional modular organisation and the fact that diffuse noxious inhibitory controls are not expressed in varied animal models of chronicity, we hypothesized a regulatory role for the locus coeruleus on non-coerulean, discrete noradrenergic cell group(s). We implemented locus coeruleus targeting strategies by microinjecting canine adenovirus encoding for channelrhodopsin-2 under a noradrenaline-specific promoter in the spinal cord (retrogradely labelling a coeruleospinal module) or the locus coeruleus itself (labelling the entire coerulean module). Coeruleospinal module optoactivation abolished diffuse noxious inhibitory controls (two-way ANOVA, P < 0.0001), which were still expressed following locus coeruleus neuronal ablation. We propose that the cerulean system interacts with, but does not directly govern, diffuse noxious inhibitory controls. This mechanism may underlie the role of the locus coeruleus as a 'chronic pain generator'. Pinpointing the functionality of discrete top-down pathways is crucial for understanding sensorimotor modulation in health and disease.

Locus Coeruleus Noradrenergic Modulation of Diurnal Corticosterone, Stress Reactivity, and Cardiovascular Homeostasis in Male Rats

INTRODUCTION

Activation of the locus coeruleus-noradrenergic (LC-NA) system during awakening is associated with an increase in plasma corticosterone and cardiovascular tone. These studies evaluate the role of the LC in this corticosterone and cardiovascular response.

METHODS

Male rats, on day 0, were treated intraperitoneally with either DSP4 (50 mg/kg body weight) (DSP), an LC-NA specific neurotoxin, or normal saline (SAL). On day 10, animals were surgically prepared with jugular vein (hypothalamic-pituitary-adrenal [HPA] axis) or carotid artery (hemodynamics) catheters and experiments performed on day 14. HPA axis activity, diurnally (circadian) and after stress (transient hemorrhage [14 mL/kg body weight] or air puff-startle), and basal and post-hemorrhage hemodynamics were evaluated. On day 16, brain regions from a subset of rats were dissected for norepinephrine and corticotropin-releasing factor (CRF) assay.

RESULTS

In DSP rats compared to SAL rats, (1) regional brain norepinephrine was decreased, but there was no change in median eminence or olfactory bulb CRF content; (2) during HPA axis acrophase, the plasma corticosterone response was blunted; (3) after hemorrhage and air puff-startle, the plasma adrenocorticotropic hormone response was attenuated, whereas the corticosterone response was dependent on stressor category; (4) under basal conditions, hemodynamic measures exhibited altered blood flow dynamics and systemic vasodilation; and (5) after hemorrhage, hemodynamics exhibited asynchronous responses.

CONCLUSION

LC-NA modulation of diurnal and stress-induced HPA axis reactivity occurs via distinct neurocircuits. The integrity of the LC-NA system is important to maintain blood flow dynamics. The importance of increases in plasma corticosterone at acrophase to maintain short- and long-term cardiovascular homeostasis is discussed.

Locus Coeruleus Degeneration Induces Forebrain Vascular Pathology in a Transgenic Rat Model of Alzheimer's Disease

Noradrenergic locus coeruleus (LC) neuron loss is a significant feature of mild cognitive impairment and Alzheimer's disease (AD). The LC is the primary source of norepinephrine in the forebrain, where it modulates attention and memory in vulnerable cognitive regions such as prefrontal cortex (PFC) and hippocampus. Furthermore, LC-mediated norepinephrine signaling is thought to play a role in blood-brain barrier (BBB) maintenance and neurovascular coupling, suggesting that LC degeneration may impact the high comorbidity of cerebrovascular disease and AD. However, the extent to which LC projection system degeneration influences vascular pathology is not fully understood. To address this question in vivo, we stereotactically lesioned LC projection neurons innervating the PFC of six-month-old Tg344-19 AD rats using the noradrenergic immunotoxin, dopamine-β-hydroxylase IgG-saporin (DBH-sap), or an untargeted control IgG-saporin (IgG-sap). DBH-sap-lesioned animals performed significantly worse than IgG-sap animals on the Barnes maze task in measures of both spatial and working memory. DBH-sap-lesioned rats also displayed increased amyloid and inflammation pathology compared to IgG-sap controls. However, we also discovered prominent parenchymal albumin extravasation with DBH-sap lesions indicative of BBB breakdown. Moreover, microvessel wall-to-lumen ratios were increased in the PFC of DBH-sap compared to IgG-sap rats, suggesting that LC deafferentation results in vascular remodeling. Finally, we noted an early emergence of amyloid angiopathy in the DBH-sap-lesioned Tg344-19 AD rats. Taken together, these data indicate that LC projection system degeneration is a nexus lesion that compromises both vascular and neuronal function in cognitive brain areas during the prodromal stages of AD.

The role of norepinephrine in the pathophysiology of schizophrenia

Several lines of evidence have suggested for decades a role for norepinephrine (NE) in the pathophysiology and treatment of schizophrenia. Recent experimental findings reveal anatomical and physiological properties of the locus coeruleus-norepinephrine (LC-NE) system and its involvement in brain function and cognition. Here, we integrate these two lines of evidence. First, we review the functional and structural properties of the LC-NE system and its impact on functional brain networks, cognition, and stress, with special emphasis on recent experimental and theoretical advances. Subsequently, we present an update about the role of LC-associated functions for the pathophysiology of schizophrenia, focusing on the cognitive and motivational deficits. We propose that schizophrenia phenomenology, in particular cognitive symptoms, may be explained by an abnormal interaction between genetic susceptibility and stress-initiated LC-NE dysfunction. This in turn, leads to imbalance between LC activity modes, dysfunctional regulation of brain network integration and neural gain, and deficits in cognitive functions. Finally, we suggest how recent development of experimental approaches can be used to characterize LC function in schizophrenia.

Effects of Reducing Norepinephrine Levels via DSP4 Treatment on Amyloid-β Pathology in Female Rhesus Macaques (Macaca Mulatta)

The degeneration in the locus coeruleus associated with Alzheimer's disease suggests an involvement of the noradrenergic system in the disease pathogenesis. The role of depleted norepinephrine was tested in adult and aged rhesus macaques to develop a potential model for testing Alzheimer's disease interventions. Monkeys were injected with the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) or vehicle at 0, 3, and 6 months; brains were harvested at 9 months. Reduced norepinephrine in the locus coeruleus was accompanied by decreased dopamine β-hydroxylase staining and increased amyloid-β load in the aged group, and the proportion of potentially toxic amyloid-β42 peptide was increased. Immunohistochemistry revealed no effects on microglia or astrocytes. DSP4 treatment altered amyloid processing, but these changes were not associated with the induction of chronic neuroinflammation. These findings suggest norepinephrine deregulation is an essential component of a nonhuman primate model of Alzheimer's disease, but further refinement is necessary.

Acute forced exercise increases Bdnf IV mRNA and reduces exploratory behavior in C57BL/6J mice

Acute exercise has been shown to improve memory in humans. Potential mechanisms include increased Bdnf expression, noradrenergic activity and modification of glutamate receptors. Because mice are commonly used to study exercise and brain plasticity, it is important to explore how acute exercise impacts behavior in this model. C57BL/6J mice were assigned to three groups: control, moderate-intensity running, and high-intensity running. Control mice were placed on a stationary treadmill for 30 minutes and moderate- and high-intensity mice ran for 30 minutes at 12 and 15-17 m/min, respectively. Mice were sacrificed immediately after running and the hippocampus removed. Total Bdnf, Bdnf exon IV, and glutamate receptor subunits were quantified with quantitative polymerase chain reaction. Total and phosphorylated GluR1 (Ser845 and Ser831) protein was quantified following immunoblotting. Utilizing the same protocol for control and high-intensity running, object location memory was examined in a separate cohort of mice. Anxiety-like behavior was assessed in the open field task (OFT) in a third cohort of mice that were separated into four groups: control-saline, control-DSP-4, acute exercise-saline, and acute exercise-DSP-4. DSP-4 was used to lesion the central noradrenergic system. We observed higher Bdnf IV mRNA in high-intensity runners compared to controls, but no effects of acute exercise on memory. In the OFT, runners traveled less distance and spent more time grooming than controls. DSP-4 did not attenuate the effects of exercise. A single bout of exercise increases Bdnf IV mRNA in an intensity-dependent manner; however, high-intensity running reduces exploratory behavior in C57BL/6J mice.

Noradrenaline is crucial for the substantia nigra dopaminergic cell maintenance

In Parkinson's disease, degeneration of substantia nigra dopaminergic neurons is accompanied by damage on other neuronal systems. A severe denervation is for example seen in the locus coerulean noradrenergic system. Little is known about the relation between noradrenergic and dopaminergic degeneration, and the effects of noradrenergic denervation on the function of the dopaminergic neurons of substantia nigra are not fully understood. In this study, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) was injected in rats, whereafter behavior, striatal KCl-evoked dopamine and glutamate releases, and immunohistochemistry were monitored at 3 days, 3 months, and 6 months. Quantification of dopamine-beta-hydroxylase-immunoreactive nerve fiber density in the cortex revealed a tendency towards nerve fiber regeneration at 6 months. To sustain a stable noradrenergic denervation throughout the experimental timeline, the animals in the 6-month time point received an additional DSP4 injection (2 months after the first injection). Behavioral examinations utilizing rotarod revealed that DSP4 reduced the time spent on the rotarod at 3 but not at 6 months. KCl-evoked dopamine release was significantly increased at 3 days and 3 months, while the concentrations were normalized at 6 months. DSP4 treatment prolonged both time for onset and reuptake of dopamine release over time. The dopamine degeneration was confirmed by unbiased stereology, demonstrating significant loss of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra. Furthermore, striatal glutamate release was decreased after DSP4. In regards of neuroinflammation, reactive microglia were found over the substantia nigra after DSP4 treatment. In conclusion, long-term noradrenergic denervation reduces the number of dopaminergic neurons in the substantia nigra and affects the functionality of the nigrostriatal system. Thus, locus coeruleus is important for maintenance of nigral dopaminergic neurons.

Central Noradrenergic Agonists in the Treatment of Ischemic Stroke-an Overview

Ischemic stroke is the leading cause of morbidity and mortality with a significant health burden worldwide and few treatment options. Among the short- and long-term effects of ischemic stroke is the cardiovascular sympathetic autonomic dysfunction, presented in part as the by-product of the ischemic damage to the noradrenergic centers of the brain. Unlike high levels in the plasma, the brain may face suboptimal levels of norepinephrine (NE), with adverse effects on the clinical and functional outcomes of ischemic stroke. The intravenous administration of NE and other sympathomimetic agents, in an attempt to increase cerebral perfusion pressure, often aggravates the ischemia-induced rise in blood pressure (BP) with life-threatening consequences for stroke patients, the majority of whom present with hypertension at the time of admission. Unlike the systemic administration, the central administration of NE reduces BP while exerting anti-inflammatory and neuroprotective effects. These characteristics of centrally administered NE, combined with the short latency of response, make it an ideal candidate for use in the acute phase of stroke, followed by the use of centrally acting noradrenergic agonists, such as NE reuptake inhibitors and B2-adrenergic receptor agonists for stroke rehabilitation. In addition, a number of nonpharmacological strategies, such as transcutaneous vagus nerve stimulation (tVNS) and trigeminal nerve stimulation (TNS), have the potential to enhance the central noradrenergic functional activities and improve stroke clinical outcomes. Many factors could influence the efficacy of the noradrenergic treatment in stroke patients. These factors include the type of the noradrenergic agent; the dose, frequency, and duration of administration; the timing of administration in relation to the acute event; and the site and characteristics of the ischemic lesions. Having this knowledge, combined with the better understanding of the regulation of noradrenergic receptors in different parts of the brain, would pave the path for the successful use of the centrally acting noradrenergic agents in the management of ischemic stroke.

The contribution of the locus coeruleus-norepinephrine system in the emergence of defeat-induced inflammatory priming

Exposure to psychosocial stress is known to precipitate the emergence of stress related psychiatric disorders such as depression and anxiety. While mechanisms by which this occurs remain largely unclear, recent evidence points towards a causative role for inflammation. Neurotransmitters, such as norepinephrine (NE), are capable of regulating expression of proinflammatory cytokines and thus may contribute to the emergence of stress-related disorders. The locus coeruleus (LC) is the major source of norepinephrine (NE) to the brain and therefore the current study utilized N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), an LC selective noradrenergic neurotoxin, to determine the discrete involvement of the LC-NE system in social defeat-induced inflammation in LC projection regions including the central amygdala (CeA), dorsal raphe (DR) and plasma. In the current study, rats were exposed to brief social defeat or control manipulations on 5 consecutive days. To determine whether a history of social defeat enhanced or "primed" the inflammatory response to a subsequent defeat exposure, all rats regardless of stress history were exposed to an acute social defeat challenge immediately preceeding tissue collection. As anticipated, prior history of social defeat primed inflammatory responses in the plasma and CeA while neuroinflammation in the DR was markedly reduced. Notably, DSP-4 treatment suppressed stress-induced circulating inflammatory cytokines independent of prior stress history. In contrast, neuroinflammation in the CeA and DR were greatly augmented selectively in DSP-4 treated rats with a history of social defeat. Together these data highlight the dichotomous nature of NE in stress-induced inflammatory priming in the periphery and the brain and directly implicate the LC-NE system in these processes.

Divergent influences of the locus coeruleus on migraine pathophysiology

Migraine is a common disabling neurological condition that is associated with several premonitory symptoms that can occur days before the headache onset. The most commonly reported premonitory symptom is marked fatigue that has been shown to be highly predictive of an ensuing migraine attack. The locus coeruleus (LC) is a key nucleus involved in arousal that has also been shown to impact pain processing. It provides one of the major sources of noradrenaline to the dorsal horn of the spinal cord and neocortex. Given the clinical association between migraine, sleep-wake regulation, and fatigue, we sought to determine whether LC modulation could impact migraine-related phenotypes in several validated preclinical models of migraine. To determine its role in migraine-related pain, we recorded dural nociceptive-evoked responses of neurons in the trigeminocervical complex, which receives trigeminal primary afferents from the durovascular complex. In addition, we explored the susceptibility to cortical spreading depression initiation, the presumed underlying phenomenon of migraine aura. Our experiments reveal a potent role for LC disruption in the differential modulation of migraine-related phenotypes, inhibiting dural-evoked activation of wide dynamic neurons in the trigeminocervical complex while increasing cortical spreading depression susceptibility. This highlights the potential divergent impact of LC disruption in migraine physiology, which may help explain the complex interactions between dysfunctional arousal mechanisms and migraine.

Noradrenergic Hypothesis Linking Neurodegeneration-Based Cognitive Decline and Astroglia

In the past, manipulation of the cholinergic system was seen as the most likely therapeutic for neurodegeneration-based cognitive decline in Alzheimer's disease (AD) (Whitehouse et al., 1982). However, targeting the noradrenergic system also seems a promising strategy, since more recent studies revealed that in post-mortem tissue from patients with AD and other neurodegenerative disorders there is a robust correlation between cognitive decline and loss of neurons from the Locus coeruleus (LC), a system with diffuse noradrenaline (NA) innervation in the central nervous system (CNS). Therefore, the hypothesis has been considered that increasing NA signaling in the CNS will prevent, or at least halt the progression of neurodegeneration and cognitive decline. A hallmark of the age- and neurodegeneration-related cognitive decline is reduced neurogenesis. We here discuss noradrenergic dysfunction in AD-related cognitive decline in humans and its potential involvement in AD pathology and disease progression. We also focus on animal models to allow the validation of the noradrenergic hypothesis of AD, including those based upon the immunotoxin-mediated ablation of LC based on saporin, a protein synthesis interfering agent, which offers selective and graded demise of LC neurons, Finally, we address how astrocytes, an abundant and functionally heterogeneous cell type of neuroglia maintaining homeostasis, may participate in the regulation of neurogenesis, a new strategy for preventing LC neuron loss.

Loss of Brain Norepinephrine Elicits Neuroinflammation-Mediated Oxidative Injury and Selective Caudo-Rostral Neurodegeneration

Environmental toxicant exposure has been strongly implicated in the pathogenesis of Parkinson's disease (PD). Clinical manifestations of non-motor and motor symptoms in PD stem from decades of progressive neurodegeneration selectively afflicting discrete neuronal populations along a caudo-rostral axis. However, recapitulating this spatiotemporal neurodegenerative pattern in rodents has been unsuccessful. The purpose of this study was to generate such animal PD models and delineate mechanism underlying the ascending neurodegeneration. Neuroinflammation, oxidative stress, and neuronal death in mice brains were measured at different times following a single systemic injection of lipopolysaccharide (LPS). We demonstrate that LPS produced an ascending neurodegeneration that temporally afflicted neurons initially in the locus coeruleus (LC), followed by substantia nigra, and lastly the primary motor cortex and hippocampus. To test the hypothesis that LPS-elicited early loss of noradrenergic LC neurons may underlie this ascending pattern, we used a neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to deplete brain norepinephrine. DSP-4 injection resulted in a time-dependent ascending degenerative pattern similar to that generated by the LPS model. Mechanistic studies revealed that increase in nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-2 (NOX2)-dependent superoxide/reactive oxygen species (ROS) production plays a key role in both LPS- and DSP-4-elicited neurotoxicity. We found that toxin-elicited chronic neuroinflammation, oxidative neuronal injuries, and neurodegeneration were greatly suppressed in mice deficient in NOX2 gene or treated with NOX2-specific inhibitor. Our studies document the first rodent PD model recapturing the ascending neurodegenerative pattern of PD patients and provide convincing evidence that the loss of brain norepinephrine is critical in initiating and maintaining chronic neuroinflammation and the discrete neurodegeneration in PD.

Stress modulates cortical excitability via α-2 adrenergic and glucocorticoid receptors: As assessed by spreading depression

An increase in cortical excitability may be one of the factors mediating stress-induced vulnerability to neuropsychiatric disorders. Since stress increases extracellular glutamate and predisposes to migraine with aura attacks, we aimed to study the effect of stress on cortical spreading depression (CSD), the biological substrate of migraine aura and a measure of cortical excitability. CSD was induced by increasing concentrations of KCl applied over the dura with 5-minute intervals and recorded from parieto-occipital cortex to assess the CSD-induction threshold in acutely-stressed, chronically-stressed and naive mice. To study the mechanisms of acute stress-induced decrease in CSD threshold, we systemically administered clonidine, yohimbine, propranolol, CRH1 receptor antagonist NBI27914, mifepristone and spironolactone at doses shown to be effective on stress as well as a central noradrenergic neurotoxin (DSP-4) before acute stress. CSD threshold was significantly lowered by acute and chronic stress as well as central noradrenergic denervation. Clonidine and mifepristone further decreased the CSD threshold below the acute stress-induced levels, whereas yohimbine reversed the acute stress-induced decrease in CSD threshold compared to the saline-injected and stressed control groups. Propranolol, NBI27914 and spironolactone did not modify the effect of acute stress on CSD threshold. Stress increases cortical excitability as illustrated by a decrease in CSD-induction threshold. This action of acute stress is mediated by α2-adrenergic and glucocorticoid receptors. The decrease in CSD threshold may account for the stress-induced susceptibility to migraine. CSD may be used as a tool to study the link between stress-related disorders and cortical excitability.

The Neurotoxin DSP-4 Induces Hyperalgesia in Rats that is Accompanied by Spinal Oxidative Stress and Cytokine Production

Central neuropathic pain (CNP) a significant problem for many people, is not well-understood and difficult to manage. Dysfunction of the central noradrenergic system originating in the locus coeruleus (LC) may be a causative factor in the development of CNP. The LC is the major noradrenergic nucleus of the brain and plays a significant role in central modulation of nociceptive neurotransmission. Here, we examined CNS pathophysiological changes induced by intraperitoneal administration of the neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride). Administration of DSP-4 decreased levels of norepinephrine in spinal tissue and cerebrospinal fluid (CSF) and led to the development of thermal and mechanical hyperalgesia over 21 days, that was reversible with morphine. Hyperalgesia was accompanied by significant increases in noradrenochrome (oxidized norepinephrine) and expression of 4-hydroxynonenal in CSF and spinal cord tissue respectively at day 21, indicative of oxidative stress. In addition, spinal levels of pro-inflammatory cytokines (interleukins 6 and 17A, tumor necrosis factor-α), as well as the anti-inflammatory cytokine interleukin10 were also significantly elevated at day 21, indicating that an inflammatory response occurred. The inflammatory effect of DSP-4 presented in this study that includes oxidative stress may be particularly useful in elucidating mechanisms of CNP in inflammatory disease states.

Treatment with the noradrenaline re-uptake inhibitor atomoxetine alone and in combination with the α2-adrenoceptor antagonist idazoxan attenuates loss of dopamine and associated motor deficits in the LPS inflammatory rat model of Parkinson's disease

The impact of treatment with the noradrenaline (NA) re-uptake inhibitor atomoxetine and the α₂-adrenoceptor (AR) antagonist idazoxan in an animal model of Parkinson's disease (PD) was assessed. Concurrent systemic treatment with atomoxetine and idazoxan, a combination which serves to enhance the extra-synaptic availability of NA, exerts anti-inflammatory and neuroprotective effects following delivery of an inflammatory stimulus, the bacterial endotoxin, lipopolysaccharide (LPS) into the substantia nigra. Lesion-induced deficits in motor function (akinesia, forelimb-use asymmetry) and striatal dopamine (DA) loss were rescued to varying degrees depending on the treatment. Treatment with atomoxetine following LPS-induced lesion to the substantia nigra, yielded a robust anti-inflammatory effect, suppressing microglial activation and expression of the pro-inflammatory cytokine TNF-α whilst increasing the expression of neurotrophic factors. Furthermore atomoxetine treatment prevented loss of tyrosine hydroxylase (TH) positive nigral dopaminergic neurons and resulted in functional improvements in motor behaviours. Atomoxetine alone was sufficient to achieve most of the observed effects. In combination with idazoxan, an additional improvement in the impairment of contralateral limb use 7 days post lesion and a reduction in amphetamine-mediated rotational asymmetry 14 days post-lesion was observed, compared to atomoxetine or idazoxan treatments alone. The results indicate that increases in central NA tone has the propensity to regulate the neuroinflammatory phenotype in vivo and may act as an endogenous neuroprotective mechanism where inflammation contributes to the progression of DA loss. In accordance with this, the clinical use of agents such as NA re-uptake inhibitors and α₂-AR antagonists may prove useful in enhancing the endogenous neuroimmunomodulatory potential of NA in conditions associated with brain inflammation.

Essential role of hippocampal noradrenaline in the regulation of spatial working memory and TDP-43 tissue pathology

Extensive loss of noradrenaline-containing neurons and fibers is a nearly invariant feature of Alzheimer's Disease (AD). However, the exact noradrenergic contribution to cognitive and histopathological changes in AD is still unclear. Here, this issue was addressed following selective lesioning and intrahippocampal implantation of embryonic noradrenergic progenitors in developing rats. Starting from about 3 months and up to 12 months post-surgery, animals underwent behavioral tests to evaluate sensory-motor, as well as spatial learning and memory, followed by post-mortem morphometric analyses. At 9 months, Control, Lesioned and Lesion + Transplant animals exhibited equally efficient sensory-motor and reference memory performance. Interestingly, working memory abilities were seen severely impaired in Lesion-only rats and fully recovered in Transplanted rats, and appeared partly lost again 2 months after ablation of the implanted neuroblasts. Morphological analyses confirmed the almost total lesion-induced noradrenergic neuronal and terminal fiber loss, the near-normal reinnervation of the hippocampus promoted by the transplants, and its complete removal by the second lesion. Notably, the noradrenergic-rich transplants normalized also the nuclear expression of the transactive response DNA-binding protein 43 (TDP-43) in various hippocampal subregions, whose cytoplasmic (i.e., pathological) occurrence appeared dramatically increased as a result of the lesions. Thus, integrity of ascending noradrenergic inputs to the hippocampus may be required for the regulation of specific aspects of learning and memory and to prevent TDP-43 tissue pathology.

Effect of Deep Brain Stimulation of the ventromedial prefrontal cortex on the noradrenergic system in rats

BACKGROUND

Deep Brain Stimulation (DBS) of the subgenual cingulate cortex (SCC) is a promising therapeutic alternative to treat resistant major depressive disorder. In preclinical studies, DBS of the ventromedial prefrontal cortex (vmPFC, the rodent SCC correlate) provokes an antidepressant-like effect, along with changes in noradrenaline levels at the site of stimulation. Hence, DBS appears to activate the noradrenergic-locus coeruleus (LC) system.

OBJECTIVE/HYPOTHESIS

The aim of this study was to evaluate the effect of vmPFC DBS on the electrical activity of noradrenergic LC neurons, cortical oscillations and coherence between both brain areas in male rats.

METHODS

The antidepressant-like effect of vmPFC DBS was evaluated through the forced swimming test. Tonic and evoked activity of LC neurons, LC activity of alpha2-adrenoceptors, local field potentials from LC and electrocorticogram signals were studied after DBS by electrophysiological recordings in anaesthetized rats. The effect of DBS on tyrosine hydroxylase (TH), noradrenaline transporters (NAT), phosphorylation of the extracellular signal-regulated kinase (ERK) and corticotropin releasing factor (CRF) expression in the LC were measured by western blot assays.

RESULTS

DBS induced an antidepressant-like effect increasing climbing behaviour in the FST that was accompanied by a robust increase of TH expression in the rat LC. The tonic and evoked activity of LC neurons was enhanced by DBS, which impaired alpha2-adrenoceptors activity. DBS also promoted an increase in slow LC oscillations, as well as a shift in LC-cortical coherence.

CONCLUSION

DBS of the vmPFC appears to affect the LC, producing changes that may underlie its antidepressant-like effects.

A single bout of exercise increases hippocampal Bdnf: influence of chronic exercise and noradrenaline

Research in human subjects suggests that acute exercise can improve memory performance, but the qualities of the exercise necessary to promote improved memory, and the signaling pathways that mediate these effects are unknown. Brain-derived neurotrophic factor (Bdnf), noradrenergic signaling, and post-translational modifications to AMPA receptors have all been implicated in the enhancement of memory following emotional or physical arousal; however, it is not known if a single bout of exercise is sufficient to engage these pathways. Here we use a rodent model to investigate the effects of acute and chronic exercise on hippocampal transcript-specific Bdnf expression and phosphorylation of the GluR1 subunit of the AMPA-type glutamate receptor. A single bout of treadmill exercise was insufficient to mimic the increased expression of GluR1 protein and phosphorylation at Ser845 observed following 1 month of voluntary wheel running. However, acute exercise was sufficient to increase Bdnf transcript IV messenger RNA (mRNA) expression in sedentary subjects, but not subjects housed for 1 month with a running wheel. High-intensity acute exercise increased total Bdnf mRNA in sedentary mice, but not above levels observed following chronic access to the running wheel. Although depletion of central noradrenergic signaling with DSP-4 reduced Bdnf IV mRNA, the effect of acute exercise on Bdnf mRNA persisted. Our characterization of the effects of acute exercise on Bdnf expression and persistence in the absence of noradrenergic modulation may inform strategies to employ physical activity to combat cognitive aging and mental health disorders.

Cortical adrenoceptor expression, function and adaptation under conditions of cannabinoid receptor deletion

A neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. Acute and repeated administration of a cannabinoid receptor synthetic agonist is capable of increasing multiple indices of noradrenergic activity. This includes cannabinoid-induced 1) increases in norepinephrine (NE) release in the medial prefrontal cortex (mPFC); 2) desensitization of cortical α2-adrenoceptor-mediated effects; 3) activation of c-Fos in brainstem locus coeruleus (LC) noradrenergic neurons; and 4) increases in anxiety-like behaviors. In the present study, we sought to examine adaptations in adrenoceptor expression and function under conditions of cannabinoid receptor type 1 (CB1r) deletion using knockout (KO) mice and compare these to wild type (WT) controls. Electrophysiological analysis of α2-adrenoceptor-mediated responses in mPFC slices in WT mice showed a clonidine-induced α2-adrenoceptor-mediated increase in mPFC cell excitability coupled with an increase in input resistance. In contrast, CB1r KO mice showed an α2-adrenoceptor-mediated decrease in mPFC cell excitability. We then examined protein expression levels of α2- and β1-adrenoceptor subtypes in the mPFC as well as TH expression in the locus coeruleus (LC) of mice deficient in CB1r. Both α2- and β1-adrenoceptors exhibited a significant decrease in expression levels in CB1r KO mice when compared to WT in the mPFC, while a significant increase in TH was observed in the LC. To better define whether the same cortical neurons express α2A-adrenoceptor and CB1r in mPFC, we utilized high-resolution immunoelectron microscopy. We localized α2A-adrenoceptors in a knock-in mouse that expressed a hemoagglutinin (HA) tag downstream of the α2A-adrenoceptor promoter. Although the α2A-adrenoceptor was often identified pre-synaptically, we observed co-localization of CB1r with α2-adrenoceptors post-synaptically in the same mPFC neurons. Finally, using receptor binding, we confirmed prior results showing that α2A-adrenoceptor is unchanged in mPFC following acute or chronic exposure to the synthetic cannabinoid receptor agonist, WIN 55,212-2, but is increased, following chronic treatment followed by a period of abstinence. Taken together, these data provide convergent lines of evidence indicating cannabinoid regulation of the cortical adrenergic system.

Noradrenergic Modulation of Cognition in Health and Disease

Norepinephrine released by the locus coeruleus modulates cellular processes and synaptic transmission in the central nervous system through its actions at a number of pre- and postsynaptic receptors. This transmitter system facilitates sensory signal detection and promotes waking and arousal, processes which are necessary for navigating a complex and dynamic sensory environment. In addition to its effects on sensory processing and waking behavior, norepinephrine is now recognized as a contributor to various aspects of cognition, including attention, behavioral flexibility, working memory, and long-term mnemonic processes. Two areas of dense noradrenergic innervation, the prefrontal cortex and the hippocampus, are particularly important with regard to these functions. Due to its role in mediating normal cognitive function, it is reasonable to expect that noradrenergic transmission becomes dysfunctional in a number of neuropsychiatric and neurodegenerative diseases characterized by cognitive deficits. In this review, we summarize the unique role that norepinephrine plays in prefrontal cortical and hippocampal function and how its interaction with its various receptors contribute to cognitive behaviors. We further assess the changes that occur in the noradrenergic system in Alzheimer's disease, Parkinson's disease, attention-deficit/hyperactivity disorder, and schizophrenia and how these changes contribute to cognitive decline in these pathologies.

Morphological correlates of sex differences in acoustic startle response and prepulse inhibition through projections from locus coeruleus to cochlear root neurons

The noradrenergic locus coeruleus (LC) plays an important role in the promotion and maintenance of arousal and alertness. Our group recently described coerulean projections to cochlear root neurons (CRNs), the first relay of the primary acoustic startle reflex (ASR) circuit. However, the role of the LC in the ASR and its modulation, prepulse inhibition (PPI), is not clear. In this study, we damaged LC neurons and fibers using a highly selective neurotoxin, DSP-4, and then assessed ASR and PPI in male and female rats. Our results showed that ASR amplitude was higher in males at 14 days after DSP-4 injection when compared to pre-administration values and those in the male control group. Such modifications in ASR amplitude did not occur in DSP-4-injected females, which exhibited ASR amplitude within the range of control values. PPI differences between males and females seen in controls were not observed in DSP-4-injected rats for any interstimulus interval tested. DSP-4 injection did not affect ASR and PPI latencies in either the male or the female groups, showing values that were consistent with the sex-related variability observed in control rats. Furthermore, we studied the noradrenergic receptor system in the cochlear nerve root using gene expression analysis. When compared to controls, DSP-4-injected males showed higher levels of expression in all adrenoceptor subtypes; however, DSP-4-injected females showed varied effects depending on the receptor type, with either up-, downregulations, or maintenance of expression levels. Lastly, we determined noradrenaline levels in CRNs and other LC-targeted areas using HPLC assays, and these results correlated with behavioral and adrenoceptor expression changes post DSP-4 injection. Our study supports the participation of LC in ASR and PPI, and contributes toward a better understanding of sex-related differences observed in somatosensory gating paradigms.

Medullary Norepinephrine Projections Release Norepinephrine into the Contralateral Bed Nucleus of the Stria Terminalis

Central norepinephrine signaling influences a wide range of behavioral and physiological processes, and the ventral bed nucleus of the stria terminalis (vBNST) receives some of the densest norepinephrine innervation in the brain. Previous work describes norepinephrine neurons as projecting primarily unilaterally; however, recent evidence for cross-hemispheric catecholamine signaling challenges this idea. Here, we use fast-scan cyclic voltammetry and retrograde tracing to characterize cross-hemispheric norepinephrine signaling in the vBNST. We delivered stimulations to noradrenergic pathways originating in the A1/A2 and locus coeruleus and found hemispherically equivalent norepinephrine release in the vBNST regardless of stimulated hemisphere. Unilateral retrograde tracing revealed that medullary, but not locus coeruleus norepinephrine neurons send cross-hemispheric projections to the vBNST. Further characterization with pharmacological lesions revealed that stimulations of the locus coeruleus and its axon bundles likely elicit vBNST norepinephrine release through indirect activation. These experiments are the first to demonstrate contralateral norepinephrine release and establish that medullary, but not coerulean neurons are responsible for norepinephrine release in the vBNST.

Selective noradrenaline depletion impairs working memory and hippocampal neurogenesis

Noradrenergic neurons in the locus coeruleus play a role in learning and memory, and their loss is an early event in Alzheimer's disease pathogenesis. Moreover, noradrenaline may sustain hippocampal neurogenesis; however, whether are these events related is still unknown. Four to five weeks following the selective immunotoxic ablation of locus coeruleus neurons, young adult rats underwent reference and working memory tests, followed by postmortem quantitative morphological analyses to assess the extent of the lesion, as well as the effects on proliferation and/or survival of neural progenitors in the hippocampus. When tested in the Water Maze task, lesioned animals exhibited no reference memory deficit, whereas working memory abilities were seen significantly impaired, as compared with intact or sham-lesioned controls. Stereological analyses confirmed a dramatic noradrenergic neuron loss associated to reduced proliferation, but not survival or differentiation, of 5-bromo-2'deoxyuridine-positive progenitors in the dentate gyrus. Thus, ascending noradrenergic afferents may be involved in more complex aspects of cognitive performance (i.e., working memory) possibly via newly generated progenitors in the hippocampus.

Evidence for a specialized role of the locus coeruleus noradrenergic system in cortical circuitries and behavioral operations

The brainstem nucleus locus coeruleus (LC) innervates the entire central nervous system and is the primary source of norepinephrine (NE) to the neocortex. While classically considered a homogenous modulator of forebrain activity by virtue of highly widespread and divergent axons, recent behavioral and pharmacological evidence suggest this nucleus may execute distinct operations within functionally distinct terminal fields. Summarized in this review are the anatomical and physiological properties of the nucleus within a historical context that led to the interpretation of the nucleus as a homogeneous entity with uniform and simultaneous actions throughout its terminal fields. Also included are findings from several laboratories which point to a more nuanced model of LC/NE function that parallels that seen in other forebrain-projecting monoaminergic nuclei. Such compartmentalized models of the nucleus promote the idea that specific LC circuits are involved in discrete behavioral operations, and therefore, by identifying the networks that are engaged by LC, the substrates for these behaviors can be identified and manipulated. Perturbations in the functional anatomy and physiology of this system may be related to neuropsychiatric conditions associated with dysregulation of the LC-noradrenergic system such as attention deficit hyperactivity disorder. Recent findings regarding the organization and operation of the LC/NE system collectively challenge the classical view of the nucleus as a relatively homogenous modulator of forebrain activity and provide the basis for a renewed scientific interest in this region of the brain. This article is part of a Special Issue entitled SI: Noradrenergic System.

Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system

Aside from its roles in as a classical neurotransmitter involved in regulation of behavior, noradrenaline (NA) has other functions in the CNS. This includes restricting the development of neuroinflammatory activation, providing neurotrophic support to neurons, and providing neuroprotection against oxidative stress. In recent years, it has become evident that disruption of physiological NA levels or signaling is a contributing factor to a variety of neurological diseases and conditions including Alzheimer's disease (AD) and Multiple Sclerosis. The basis for dysregulation in these diseases is, in many cases, due to damage occurring to noradrenergic neurons present in the locus coeruleus (LC), the major source of NA in the CNS. LC damage is present in AD, multiple sclerosis, and a large number of other diseases and conditions. Studies using animal models have shown that experimentally induced lesion of LC neurons exacerbates neuropathology while treatments to compensate for NA depletion, or to reduce LC neuronal damage, provide benefit. In this review, we will summarize the anti-inflammatory and neuroprotective actions of NA, summarize examples of how LC damage worsens disease, and discuss several approaches taken to treat or prevent reductions in NA levels and LC neuronal damage. Further understanding of these events will be of value for the development of treatments for AD, multiple sclerosis, and other diseases and conditions having a neuroinflammatory component. The classical neurotransmitter noradrenaline (NA) has critical roles in modulating behaviors including those involved in sleep, anxiety, and depression. However, NA can also elicit anti-inflammatory responses in glial cells, can increase neuronal viability by inducing neurotrophic factor expression, and can reduce neuronal damage due to oxidative stress by scavenging free radicals. NA is primarily produced by tyrosine hydroxylase (TH) expressing neurons in the locus coeruleus (LC), a relatively small brainstem nucleus near the IVth ventricle which sends projections throughout the brain and spinal cord. It has been known for close to 50 years that LC neurons are lost during normal aging, and that loss is exacerbated in neurological diseases including Parkinson's disease and Alzheimer's disease. LC neuronal damage and glial activation has now been documented in a variety of other neurological conditions and diseases, however, the causes of LC damage and cell loss remain largely unknown. A number of approaches have been developed to address the loss of NA and increased inflammation associated with LC damage, and several methods are being explored to directly minimize the extent of LC neuronal cell loss or function. In this review, we will summarize some of the consequences of LC loss, consider several factors that likely contribute to that loss, and discuss various ways that have been used to increase NA or to reduce LC damage. This article is part of the 60th Anniversary special issue.

Lesion of the locus coeruleus aggravates dopaminergic neuron degeneration by modulating microglial function in mouse models of Parkinson׳s disease

The degeneration of noradrenergic neurons in the locus coeruleus (LC) commonly occurs in patients with Parkinson's disease (PD), which is characterized by a selective injury of dopaminergic neurons in the substantia nigra (SN). The pathological impact of the LC on the SN in the disease is unknown. In the present study, we used a noradrenergic toxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), to deplete noradrenaline (NA) derived from the LC to explore its influence on degeneration or injury of dopaminergic neurons in the SN in mouse model produced by intraperitoneal injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or lipopolysaccharide (LPS). Our results demonstrated that lesion of the LC could change microglial function in the brain, which led to enhanced or prolonged expression of pro-inflammatory cytokines, diminished neurotrophic factors, and weakened ability of anti-oxidation in the SN. The in vitro experiments further confirmed that NA could reduce the inflammatory reaction of microglia. The selective injury of dopaminergic neurons by inflammation, however, was due to the inflammation in different brain regions rather than the depletion of NA. Our results indicate that the lesion in the LC is an important factor in promoting dopaminergic neuron degeneration by impacting the function of microglia in the midbrain.

Norepinephrine activates dopamine D4 receptors in the rat lateral habenula

The lateral habenula (LHb) is involved in reward and aversion and is reciprocally connected with dopamine (DA)-containing brain regions, including the ventral tegmental area (VTA). We used a multidisciplinary approach to examine the properties of DA afferents to the LHb in the rat. We find that >90% of VTA tyrosine hydroxylase (TH) neurons projecting to the LHb lack vesicular monoamine transporter 2 (VMAT2) mRNA, and there is little coexpression of TH and VMAT2 protein in this mesohabenular pathway. Consistent with this, electrical stimulation of LHb did not evoke DA-like signals, assessed with fast-scan cyclic voltammetry. However, electrophysiological currents that were inhibited by L741,742, a DA-D4-receptor antagonist, were observed in LHb neurons when DA uptake or degradation was blocked. To prevent DA activation of D4 receptors, we repeated this experiment in LHb slices from DA-depleted rats. However, this did not disrupt D4 receptor activation initiated by the dopamine transporter inhibitor, GBR12935. As the LHb is also targeted by noradrenergic afferents, we examined whether GBR12935 activation of DA-D4 receptors occurred in slices depleted of norepinephrine (NE). Unlike DA, NE depletion prevented the activation of DA-D4 receptors. Moreover, direct application of NE elicited currents in LHb neurons that were blocked by L741,742, and GBR12935 was found to be a more effective blocker of NE uptake than the NE-selective transport inhibitor nisoxetine. These findings demonstrate that NE is released in the rat LHb under basal conditions and that it activates DA-D4 receptors. Therefore, NE may be an important regulator of LHb function.

New perspectives on catecholaminergic regulation of executive circuits: evidence for independent modulation of prefrontal functions by midbrain dopaminergic and noradrenergic neurons

Cognitive functions associated with prefrontal cortex (PFC), such as working memory and attention, are strongly influenced by catecholamine [dopamine (DA) and norepinephrine (NE)] release. Midbrain dopaminergic neurons in the ventral tegmental area and noradrenergic neurons in the locus coeruleus are major sources of DA and NE to the PFC. It is traditionally believed that DA and NE neurons are homogeneous with highly divergent axons innervating multiple terminal fields and once released, DA and NE individually or complementarily modulate the prefrontal functions and other brain regions. However, recent studies indicate that both DA and NE neurons in the mammalian brain are heterogeneous with a great degree of diversity, including their developmental lineages, molecular phenotypes, projection targets, afferent inputs, synaptic connectivity, physiological properties, and behavioral functions. These diverse characteristics could potentially endow DA and NE neurons with distinct roles in executive function, and alterations in their responses to genetic and epigenetic risk factors during development may contribute to distinct phenotypic and functional changes in disease states. In this review of recent literature, we discuss how these advances in DA and NE neurons change our thinking of catecholamine influences in cognitive functions in the brain, especially functions related to PFC. We review how the projection-target specific populations of neurons in these two systems execute their functions in both normal and abnormal conditions. Additionally, we explore what open questions remain and suggest where future research needs to move in order to provide a novel insight into the cause of neuropsychiatric disorders related to DA and NE systems.

Effects of DSP4 on the noradrenergic phenotypes and its potential molecular mechanisms in SH-SY5Y cells

Dopamine β-hydroxylase (DBH) and norepinephrine (NE) transporter (NET) are the noradrenergic phenotypes for their functional importance to noradrenergic neurons. It is known that in vivo N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment induces degeneration of noradrenergic terminals by interacting with NET and depleting intracellular NE. However, DSP4's precise mechanism of action remains unclear. In this study various biochemical approaches were employed to test the hypothesis that DSP4 down-regulates the expression of DBH and NET, and to determine molecular mechanisms that may be involved. The results showed that treatment of SH-SY5Y neuroblastoma cells with DSP4 significantly decreased mRNA and protein levels of DBH and NET. DSP4-induced reduction of DBH mRNA and proteins, as well as NET proteins showed a time- and concentration-dependent manner. Flow cytometric analysis demonstrated that DSP4-treated cells were arrested predominantly in the S-phase, which was reversible. The arrest was confirmed by several DNA damage response markers (phosphorylation of H2AX and p53), suggesting that DSP4 causes replication stress which triggers cell cycle arrest via the S-phase checkpoints. Moreover, the comet assay verified that DSP4 induced single-strand DNA breaks. In summary, the present study demonstrated that DSP4 down-regulates the noradrenergic phenotypes, which may be mediated by its actions on DNA replication, leading to replication stress and cell cycle arrest. These action mechanisms of DSP4 may account for its degenerative consequence after systematic administration for animal models.

Identification and distribution of projections from monoaminergic and cholinergic nuclei to functionally differentiated subregions of prefrontal cortex

The prefrontal cortex (PFC) is implicated in a variety of cognitive and executive functions and is composed of several distinct networks, including anterior cingulate cortex (ACC), medial prefrontal cortex (mPFC), and orbitofrontal cortex (OFC). These regions serve dissociable cognitive functions, and are heavily innervated by acetylcholine, dopamine, serotonin and norepinephrine systems. In this study, fluorescently labeled retrograde tracers were injected into the ACC, mPFC, and OFC, and labeled cells were identified in the nucleus basalis (NB), ventral tegmental area (VTA), dorsal raphe nucleus (DRN) and locus coeruleus (LC). DRN and LC showed similar distributions of retrogradely labeled neurons such that most were single labeled and the largest population projected to mPFC. VTA showed a slightly greater proportion of double and triple labeled neurons, with the largest population projecting to OFC. NB, on the other hand, showed mostly double and triple labeled neurons projecting to multiple subregions. Therefore, subsets of VTA, DRN and LC neurons may be capable of modulating individual prefrontal subregions independently, whereas NB cells may exert a more unified influence on the three areas simultaneously. These findings emphasize the unique aspects of the cholinergic and monoaminergic projections to functionally and anatomically distinct subregions of PFC.

Selective loss of noradrenaline exacerbates early cognitive dysfunction and synaptic deficits in APP/PS1 mice

BACKGROUND

Degeneration of the locus coeruleus (LC), the major noradrenergic nucleus in the brain, occurs early and is ubiquitous in Alzheimer's disease (AD). Experimental lesions to the LC exacerbate AD-like neuropathology and cognitive deficits in several transgenic mouse models of AD. Because the LC contains multiple neuromodulators known to affect amyloid β toxicity and cognitive function, the specific role of noradrenaline (NA) in AD is not well understood.

METHODS

To determine the consequences of selective NA deficiency in an AD mouse model, we crossed dopamine β-hydroxylase (DBH) knockout mice with amyloid precursor protein (APP)/presenilin-1 (PS1) mice overexpressing mutant APP and PS1. Dopamine β-hydroxylase (-/-) mice are unable to synthesize NA but otherwise have normal LC neurons and co-transmitters. Spatial memory, hippocampal long-term potentiation, and synaptic protein levels were assessed.

RESULTS

The modest impairments in spatial memory and hippocampal long-term potentiation displayed by young APP/PS1 or DBH (-/-) single mutant mice were augmented in DBH (-/-)/APP/PS1 double mutant mice. Deficits were associated with reduced levels of total calcium/calmodulin-dependent protein kinase II and N-methyl-D-aspartate receptor 2A and increased N-methyl-D-aspartate receptor 2B levels and were independent of amyloid β accumulation. Spatial memory performance was partly improved by treatment with the NA precursor drug L-threo-dihydroxyphenylserine.

CONCLUSIONS

These results indicate that early LC degeneration and subsequent NA deficiency in AD may contribute to cognitive deficits via altered levels of calcium/calmodulin-dependent protein kinase II and N-methyl-D-aspartate receptors and suggest that NA supplementation could be beneficial in early AD.

Release of norepinephrine in the preoptic area activates anteroventral periventricular nucleus neurons and stimulates the surge of luteinizing hormone

The role of norepinephrine (NE) in regulation of LH is still controversial. We investigated the role played by NE in the positive feedback of estradiol and progesterone. Ovarian-steroid control over NE release in the preoptic area (POA) was determined using microdialysis. Compared with ovariectomized (OVX) rats, estradiol-treated OVX (OVX+E) rats displayed lower release of NE in the morning but increased release coincident with the afternoon surge of LH. OVX rats treated with estradiol and progesterone (OVX+EP) exhibited markedly greater NE release than OVX+E rats, and amplification of the LH surge. The effect of NE on LH secretion was confirmed using reverse microdialysis. The LH surge and c-Fos expression in anteroventral periventricular nucleus neurons were significantly increased in OVX+E rats dialyzed with 100 nm NE in the POA. After Fluoro-Gold injection in the POA, c-Fos expression in Fluoro-Gold/tyrosine hydroxylase-immunoreactive neurons increased during the afternoon in the A2 of both OVX+E and OVX+EP rats, in the locus coeruleus (LC) of OVX+EP rats, but was unchanged in the A1. The selective lesion of LC terminals, by intracerebroventricular N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, reduced the surge of LH in OVX+EP but not in OVX+E rats. Thus, estradiol and progesterone activate A2 and LC neurons, respectively, and this is associated with the increased release of NE in the POA and the magnitude of the LH surge. NE stimulates LH secretion, at least in part, through activation of anteroventral periventricular neurons. These findings contribute to elucidation of the role played by NE during the positive feedback of ovarian steroids.

The locus coeruleus-norepinephrine network optimizes coupling of cerebral blood volume with oxygen demand

Given the brain's uniquely high cell density and tissue oxygen levels bordering on hypoxia, the ability to rapidly and precisely match blood flow to constantly changing patterns in neural activity is an essential feature of cerebrovascular regulation. Locus coeruleus-norepinephrine (LC-NE) projections innervate the cerebral vasculature and can mediate vasoconstriction. However, function of the LC-mediated constriction in blood-flow regulation has never been addressed. Here, using intrinsic optical imaging coupled with an anesthesia regimen that only minimally interferes with LC activity, we show that NE enhances spatial and temporal aspects of functional hyperemia in the mouse somatosensory cortex. Increasing NE levels in the cortex using an α(2)-adrenergic receptor antagonist paradoxically reduces the extent of functional hyperemia while enhancing the surround blood-flow reduction. However, the NE-mediated vasoconstriction optimizes spatial and temporal focusing of the hyperemic response resulting in a sixfold decrease in the disparity between blood volume and oxygen demand. In addition, NE-mediated vasoconstriction accelerated redistribution to subsequently active regions, enhancing temporal synchronization of blood delivery. These observations show an important role for NE in optimizing neurovascular coupling. As LC neuron loss is prominent in Alzheimer and Parkinson diseases, the diminished ability to couple blood volume to oxygen demand may contribute to their pathogenesis.

Effects of the noradrenergic neurotoxin DSP-4 on the expression of α1-adrenoceptor subtypes after antidepressant treatment

We have previously reported that chronic imipramine and electroconvulsive treatments increase the α(1A)-adrenoceptor (but not the α(1B) subtype) mRNA level and the receptor density in the rat cerebral cortex. Furthermore, we have also shown that chronic treatment with citalopram does not affect the expression of either the α(1A)- or the α(1B)-adrenoceptor, indicating that the previously observed up-regulation of α(1A)-adrenoceptor may depend on the noradrenergic component of the pharmacological mechanism of action of these antidepressants. Here, we report that previous noradrenergic depletion with DSP-4 (50 mg/kg) (a neurotoxin selective for the noradrenergic nerve terminals) significantly attenuated the increase of α(1A)-adrenoceptor mRNA induced by a 14-day treatment with imipramine (IMI, 20 mg/kg, ip) and abolished the effect of electroconvulsive shock (ECS, 150 mA, 0.5 s) in the prefrontal cortex of the rat brain. The changes in the receptor protein expression (as reflected by its density) that were induced by IMI and ECS treatments were differently modulated by DSP-4 lesioning, and only the ECS-induced increase in α(1A)-adrenoceptor level was abolished. This study provides further evidence corroborating our initial hypothesis that the noradrenergic component of the action of antidepressant agents plays an essential role in the modulation of α(1A)-adrenoceptor in the rat cerebral cortex.

Sequential Loss of LC Noradrenergic and Dopaminergic Neurons Results in a Correlation of Dopaminergic Neuronal Number to Striatal Dopamine Concentration

Noradrenergic neurons in the locus coeruleus (LC) are significantly reduced in Parkinson’s disease (PD) and the LC exhibits neuropathological changes early in the disease process. It has been suggested that a loss of LC neurons can enhance the susceptibility of dopaminergic neurons to damage. To determine if LC noradrenergic innervation protects dopaminergic neurons from damage, the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was administered to adult male C57Bl/6 mice 3 days after bilateral LC administration of 6-hydroxydopamine (6OHDA), a time when there is a significant reduction in LC neuronal number and innervation to forebrain regions. To assess if LC loss can affect dopaminergic loss four groups of animals were studied: control, 6OHDA, MPTP, and 6OHDA + MPTP; animals sacrificed 3 weeks after MPTP administration. The number of dopaminergic neurons in the substantia nigra (SN) and ventral tegmental area (VTA), and noradrenergic neurons in the LC were determined. Catecholamine levels in striatum were measured by high-pressure liquid chromatography. The loss of LC neurons did not affect the number of dopaminergic neurons in the SN and VTA compared to control; however, LC 6OHDA significantly reduced striatal dopamine (DA; 29% reduced) but not norepinephrine (NE) concentration. MPTP significantly reduced SN and VTA neuronal number and DA concentration in the striatum compared to control; however, there was not a correlation of striatal DA concentration with SN or VTA neuronal number. Administration of 6OHDA prior to MPTP did not enhance MPTP-induced damage despite an effect of LC loss on striatal DA concentration. However, the loss of LC neurons before MPTP resulted now in a correlation between SN and VTA neuronal number to striatal DA concentration. These results demonstrate that the loss of either LC or DA neurons can affect the function of each others systems, indicating the importance of both the noradrenergic and dopaminergic system in PD.

Lesioning noradrenergic neurons of the locus coeruleus in C57Bl/6 mice with unilateral 6-hydroxydopamine injection, to assess molecular, electrophysiological and biochemical changes in noradrenergic signaling

The locus coeruleus (LC) is the major loci of noradrenergic innervation to the forebrain. Due to the extensive central nervous system innervation of the LC noradrenergic system, a reduction in the number of LC neurons could result in significant changes in noradrenergic function in many forebrain regions. LC noradrenergic neurons were lesioned in adult male C57Bl/6 mice with the unilateral administration of 6-hydroxydopamine (6OHDA) (vehicle on the alternate side). Noradrenergic markers were measured 3 weeks later to determine the consequence of LC loss in the forebrain. Direct administration of 6OHDA into the LC results in the specific reduction of noradrenergic neurons in the LC (as measured by electrophysiology, immunoreactivity and in situ hybridization), the lateral tegmental neurons and dopaminergic neurons in the substantia nigra (SN) and ventral tegmental region were unaffected. The loss of LC noradrenergic neurons did not result in compensatory changes in the expression of mRNA for norepinephrine (NE)-synthesizing enzymes. The loss of LC noradrenergic neurons is associated with reduced NE tissue concentration and NE transporter (NET) binding sites in the frontal cortex and hippocampus, as well as other forebrain regions such as the amygdala and SN. Adrenoreceptor (AR) binding sites (α(1)- and α(2)-AR) were not significantly affected on the 6OHDA-treated side compared to the vehicle-treated side, although there is a reduction of AR binding sites on both the vehicle- and 6OHDA-treated side in specific forebrain regions. These studies indicate that unilateral stereotaxic injection of 6OHDA into mice reduces noradrenergic LC neurons and reduces noradrenergic innervation to many forebrain regions, including the contralateral side.

Effects of noradrenergic denervation on L-DOPA-induced dyskinesia and its treatment by α- and β-adrenergic receptor antagonists in hemiparkinsonian rats

While L-3,4-dihydroxyphenylalanine (L-DOPA) remains the standard treatment for Parkinson's disease (PD), long-term efficacy is often compromised by L-DOPA-induced dyskinesia (LID). Recent research suggests that targeting the noradrenergic (NE) system may provide relief from both PD and LID, however, most PD patients exhibit NE loss which may modify response to such strategies. Therefore this investigation aimed to characterize the development and expression of LID and the anti-dyskinetic potential of the α2- and β-adrenergic receptor antagonists idazoxan and propranolol, respectively, in rats receiving 6-OHDA lesions with (DA lesion) or without desipramaine protection (DA+NE lesion). Male Sprague-Dawley rats (N=110) received unilateral 6-hydroxydopamine lesions. Fifty-three rats received desipramine to protect NE neurons (DA lesion) and 57 received no desipramine reducing striatal and hippocampal NE content 64% and 86% respectively. In experiment 1, the development and expression of L-DOPA-induced abnormal involuntary movements (AIMs) and rotations were examined. L-DOPA efficacy using the forepaw adjusting steps (FAS) test was also assessed in DA- and DA+NE-lesioned rats. In experiment 2, DA- and DA+NE-lesioned rats received pre-treatments of idazoxan or propranolol followed by L-DOPA after which the effects of these adrenergic compounds were observed. Results demonstrated that moderate NE loss reduced the development and expression of AIMs and rotations but not L-DOPA efficacy while anti-dyskinetic efficacy of α2- and β-adrenergic receptor blockade was maintained. These findings suggest that the NE system modulates LID and support the continued investigation of adrenergic compounds for the improved treatment of PD.

The locus coeruleus is directly implicated in L-DOPA-induced dyskinesia in parkinsonian rats: an electrophysiological and behavioural study

Despite being the most effective treatment for Parkinson’s disease, L-DOPA causes a development of dyskinetic movements in the majority of treated patients. L-DOPA-induced dyskinesia is attributed to a dysregulated dopamine transmission within the basal ganglia, but serotonergic and noradrenergic systems are believed to play an important modulatory role. In this study, we have addressed the role of the locus coeruleus nucleus (LC) in a rat model of L-DOPA-induced dyskinesia. Single-unit extracellular recordings in vivo and behavioural and immunohistochemical approaches were applied in rats rendered dyskinetic by the destruction of the nigrostriatal dopamine neurons followed by chronic treatment with L-DOPA. The results showed that L-DOPA treatment reversed the change induced by 6-hydroxydopamine lesions on LC neuronal activity. The severity of the abnormal involuntary movements induced by L-DOPA correlated with the basal firing parameters of LC neuronal activity. Systemic administration of the LC-selective noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine did not modify axial, limb, and orolingual dyskinesia, whereas chemical destruction of the LC with ibotenic acid significantly increased the abnormal involuntary movement scores. These results are the first to demonstrate altered LC neuronal activity in 6-OHDA lesioned rats treated with L-DOPA, and indicate that an intact noradrenergic system may limit the severity of this movement disorder.

Selective enhancement of mesocortical dopaminergic transmission by noradrenergic drugs: therapeutic opportunities in schizophrenia

The superior efficacy of atypical vs. classical antipsychotic drugs to treat negative symptoms and cognitive deficits in schizophrenia appears related to their ability to enhance mesocortical dopamine (DA) function. Given that noradrenergic (NE) transmission contributes to cortical DA output, we assessed the ability of NE-targeting drugs to modulate DA release in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc), with the aim of selectively increasing mesocortical DA. Extracellular DA was measured using brain microdialysis in rat mPFC and NAc after local/systemic drug administration, electrical stimulation and selective brain lesions. Local GBR12909 [a selective DA transporter (DAT) inhibitor] administration increased DA output more in NAc than in mPFC whereas reboxetine [a selective NE transporter (NET) inhibitor] had an opposite regional profile. DA levels increased comparably in both regions of control rats after local nomifensine (DAT+NET inhibitor) infusion, but this effect was much lower in PFC of NE-lesioned rats (DSP-4) and in NAc of 6-OHDA-lesioned rats. Electrical stimulation of the locus coeruleus preferentially enhanced DA output in mPFC. Consistently, the administration of reboxetine+RX821002 (an α2-adrenoceptor antagonist) dramatically enhanced DA output in mPFC (but not NAc). This effect also occurred when reboxetine+RX821002 were co-administered with haloperidol or clozapine. The preferential contribution of the NE system to PFC DA allows selective enhancement of DA transmission by simultaneously blocking NET and α2-adrenoceptors, thus preventing the autoreceptor-mediated negative feedback on NE activity. Our results highlight the importance of NET and α2-adrenoceptors as targets for treating negative/cognitive symptoms in schizophrenia and related psychiatric disorders.