A role for norepinephrine in arousal, emotion and learning?: limbic modulation by norepinephrine and the Kety hypothesis

Locus Coeruleus and Noradrenergic Pharmacology in Neurodegenerative Disease

Adrenoceptors (ARs) throughout the brain are stimulated by noradrenaline originating mostly from neurons of the locus coeruleus, a brainstem nucleus that is ostensibly the earliest to show detectable pathology in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. The α1-AR, α2-AR, and β-AR subtypes expressed in target brain regions and on a range of cell populations define the physiological responses to noradrenaline, which includes activation of cognitive function in addition to modulation of neurometabolism, cerebral blood flow, and neuroinflammation. As these heterocellular functions are critical for maintaining brain homeostasis and neuronal health, combating the loss of noradrenergic tone from locus coeruleus degeneration may therefore be an effective treatment for both cognitive symptoms and disease modification in neurodegenerative indications. Two pharmacologic approaches are receiving attention in recent clinical studies: preserving noradrenaline levels (e.g., via reuptake inhibition) and direct activation of target adrenoceptors. Here, we review the expression and role of adrenoceptors in the brain, the preclinical studies which demonstrate that adrenergic stimulation can support cognitive function and cerebral health by reversing the effects of noradrenaline depletion, and the human data provided by pharmacoepidemiologic analyses and clinical trials which together identify adrenoceptors as promising targets for the treatment of neurodegenerative disease.

Tauopathy and alcohol consumption interact to alter locus coeruleus excitatory transmission and excitability in male and female mice

Alcohol use disorder is a major public health concern in the United States. Recent work has suggested a link between chronic alcohol consumption and the development of tauopathy disorders, such as Alzheimer's disease and frontotemporal dementia. However, relatively little work has investigated changes in neural circuitry involved in both tauopathy disorders and alcohol use disorder. The locus coeruleus (LC) is the major noradrenergic nucleus in the brain and is one of the earliest sites to be affected by tau lesions. The LC is also implicated in the rewarding effects of ethanol and alcohol withdrawal. In this study we assessed effects of long-term ethanol consumption and tauopathy on the physiology of LC neurons. Male and female P301S mice, a humanized transgenic mouse model of tauopathy, underwent 16 weeks of intermittent access to 20% ethanol from 3 to 7 months of age. We observed higher total alcohol consumption in female mice regardless of genotype. Male P301S mice consumed more ethanol and had a greater preference for ethanol than wild-type (WT) males. At the end of the drinking study, LC function was assessed using ex vivo whole cell electrophysiology. We found significant changes in excitatory inputs to the LC due to both ethanol and genotype. We found significantly increased excitability of the LC due to ethanol with greater effects in female P301S mice than in female WT mice. Our study identifies significant changes in the LC due to interactions between tauopathy and long-term ethanol use. These findings could have important implications regarding LC activity and changes in behavior due to both ethanol- and tauopathy-related dementia.

Construction of complex memories via parallel distributed cortical-subcortical iterative integration

The construction of complex engrams requires hippocampal-cortical interactions. These include both direct interactions and ones via often-overlooked subcortical loops. Here, we review the anatomical organization of a hierarchy of parallel 'Papez' loops through the hypothalamus that are homologous in mammals from rats to humans. These hypothalamic loops supplement direct hippocampal-cortical connections with iterative reprocessing paced by theta rhythmicity. We couple existing anatomy and lesion data with theory to propose that recirculation in these loops progressively enhances desired connections, while reducing interference from competing external goals and internal associations. This increases the signal-to-noise ratio in the distributed engrams (neocortical and cerebellar) necessary for complex learning and memory. The hypothalamic nodes provide key motivational input for engram enhancement during consolidation.

Noradrenaline in Alzheimer's Disease: A New Potential Therapeutic Target

A growing body of evidence demonstrates the important role of the noradrenergic system in the pathogenesis of many neurodegenerative processes, especially Alzheimer’s disease, due to its ability to control glial activation and chemokine production resulting in anti-inflammatory and neuroprotective effects. Noradrenaline involvement in this disease was first proposed after finding deficits of noradrenergic neurons in the locus coeruleus from Alzheimer’s disease patients. Based on this, it has been hypothesized that the early loss of noradrenergic projections and the subsequent reduction of noradrenaline brain levels contribute to cognitive dysfunctions and the progression of neurodegeneration. Several studies have focused on analyzing the role of noradrenaline in the development and progression of Alzheimer’s disease. In this review we summarize some of the most relevant data describing the alterations of the noradrenergic system normally occurring in Alzheimer’s disease as well as experimental studies in which noradrenaline concentration was modified in order to further analyze how these alterations affect the behavior and viability of different nervous cells. The combination of the different studies here presented suggests that the maintenance of adequate noradrenaline levels in the central nervous system constitutes a key factor of the endogenous defense systems that help prevent or delay the development of Alzheimer’s disease. For this reason, the use of noradrenaline modulating drugs is proposed as an interesting alternative therapeutic option for Alzheimer’s disease.

Lateral Habenula and Its Potential Roles in Pain and Related Behaviors

The lateral habenula (LHb) is a tiny structure that acts as a hub, relaying signals from the limbic forebrain structures and basal ganglia to the brainstem modulatory area. Facilitated by updated knowledge and more precise manipulation of circuits, the progress in figuring out the neural circuits and functions of the LHb has increased dramatically over the past decade. Importantly, LHb is found to play an integrative role and has profound effects on a variety of behaviors associated with pain, including depression-like and anxiety-like behaviors, antireward or aversion, aggression, defensive behavior, and substance use disorder. Thus, LHb is a potential target for improving pain management and related disorders. In this review, we focused on the functions, related circuits, and neurotransmissions of the LHb in pain processing and related behaviors. A comprehensive understanding of the relationship between the LHb and pain will help to find new pain treatments.

Worry Modifies the Relationship between Locus Coeruleus Activity and Emotional Mnemonic Discrimination

Background: The locus coeruleus (LC) plays a critical role in modulating emotional memory performance via widespread connections to the medial temporal lobe (MTL). Interestingly, both the LC and MTL are affected during aging. Therefore, we aimed to investigate whether worry during cognitive aging changes the relationship between memory performance and the neural activity patterns during an emotional memory task. Methods: Twenty-eight participants aged 60–83 years from the Maastricht Aging study conducted an emotional mnemonic discrimination task during a 7T fMRI-scan. We performed a robust multiple linear regression to examine the association between worry and mnemonic memory performance under different levels of arousal. Subsequently, we examined if worry modifies the relationship between neuronal activity and mnemonic memory performance. Results: We observed that under low arousal, only participants with low compared to high levels of worry benefitted from additional LC activity. Under high arousal, additional LC activity was associated with lower mnemonic memory performance. Conclusion: Our results suggest there might be an optimal involvement of the NA-system for optimal memory discrimination performance, as we observed that under low levels of worry and with lower levels of arousal, higher LC activity might be needed to achieve similar levels of optimal memory performance as achieved under higher arousal when LC activity remained lower.

Noradrenergic Profile of Hippocampal Formation Theta Rhythm in Anaesthetized Rats

The present study was undertaken to identify the noradrenergic receptors underlying the production of hippocampal formation (HPC) type 2 theta rhythm. The experiments were performed on urethanized rats wherein type 2 theta is the only rhythm present. In three independent stages of experiments, the effects of noradrenaline (NE) and selective noradrenergic α and β agonists and antagonists were tested. We indicate that the selective activation of three HPC noradrenergic receptors, α1, α2 and β1, induced a similar effect (i.e., inhibition) on type 2 theta rhythm. The remaining HPC β2 and β3 noradrenergic receptors do not seem to be directly involved in the pharmacological mechanism responsible for the suppression of theta rhythm in anaesthetized rats. Obtained results provide evidence for the suppressant effect of exogenous NE on HPC type 2 theta rhythm and show the crucial role of α1, α2 and β1 noradrenergic receptors in the modulation of HPC mechanisms of oscillations and synchrony. This finding is in contrast to the effects of endogenous NE produced by electrical stimulation of the locus coeruleus (LC) and procaine injection into the LC (Broncel et al., 2020).

The role of noradrenaline in cognition and cognitive disorders

Many aspects of cognition and behaviour are regulated by noradrenergic projections to the forebrain originating from the locus coeruleus, acting through alpha and beta adrenoreceptors. Loss of these projections is common in neurodegenerative diseases and contributes to their cognitive and behavioural deficits. We review the evidence for a noradrenergic modulation of cognition in its contribution to Alzheimer's disease, Parkinson's disease and other cognitive disorders. We discuss the advances in human imaging and computational methods that quantify the locus coeruleus and its function in humans, and highlight the potential for new noradrenergic treatment strategies.

Ocular measures during associative learning predict recall accuracy

The ability to form associations between stimuli and commit those associations to memory is a cornerstone of human cognition. Dopamine and noradrenaline are critical neuromodulators implicated in a range of cognitive functions, including learning and memory. Eye blink rate (EBR) and pupil diameter have been shown to index dopaminergic and noradrenergic activity. Here, we examined how these ocular measures relate to accuracy in a paired-associate learning task where participants (N = 73) learned consistent object-location associations over eight trials consisting of pre-trial fixation, encoding, delay, and retrieval epochs. In order to examine how within-subject changes and between-subject changes in ocular metrics related to accuracy, we mean centered individual metric values on each trial based on within-person and across-subject means for each epoch. Within-participant variation in EBR was positively related to accuracy in both encoding and delay epochs: faster EBR within the individual predicted better retrieval. Differences in EBR across participants was negatively related to accuracy in the encoding epoch and in early trials of the pre-trial fixation: faster EBR, relative to other subjects, predicted poorer retrieval. Visual scanning behavior in pre-trial fixation and delay epochs was also positively related to accuracy in early trials: more scanning predicted better retrieval. We found no relationship between pupil diameter and accuracy. These results provide novel evidence supporting the utility of ocular metrics in illuminating cognitive and neurobiological mechanisms of paired-associate learning.

Dynamic decision making and value computations in medial frontal cortex

Dynamic decision making requires an intact medial frontal cortex. Recent work has combined theory and single-neuron measurements in frontal cortex to advance models of decision making. We review behavioral tasks that have been used to study dynamic decision making and algorithmic models of these tasks using reinforcement learning theory. We discuss studies linking neurophysiology and quantitative decision variables. We conclude with hypotheses about the role of other cortical and subcortical structures in dynamic decision making, including ascending neuromodulatory systems.

Alterations and interactions of subcortical modulatory systems in Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is not fully understood. Here we summarize current knowledge on the involvement of the serotonergic, noradrenergic, dopaminergic, cholinergic, and opioid systems in AD, emphasizing the importance of interactions between the serotonergic and the other subcortical modulatory systems during the progression of AD. In physiological conditions, all neurotransmitter systems function in concert and are interdependent at both the neuroanatomical and molecular levels. Through their early involvement in AD, cognitive and behavioral abilities that rely on their interactions also become disrupted. Considering that serotonin (5HT) regulates the release of noradrenaline (NA), dopamine (DA) and acetylcholine (ACh), any alteration in 5HT levels leads to disturbance of NA, DA, and ACh homeostasis in the brain. One of the earliest pathological changes during the prodromal phase of AD is a decrease of serotonergic transmission throughout the brain, with serotonergic receptors being also affected. Additionally, serotonergic and noradrenergic as well as serotonergic and dopaminergic nuclei are reciprocally interconnected. As the serotonergic dorsal raphe nucleus (DRN) is affected by pathological changes early in AD, and the noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental area (VTA) exhibit AD-related pathological changes, their connectivity also becomes altered in AD. Such disrupted interactions among neurotransmitter systems in AD can be used in the development of multi-target drugs. Some of the potential AD therapeutics (such as ASS234, RS67333, tropisetron) target multiple neurotransmitter systems to achieve the best possible improvement of cognitive and behavioral deficits observed in AD. Here, we review how serotonergic system interacts with other subcortical modulatory systems (noradrenergic, dopaminergic, cholinergic, and opioid systems) during AD.

The role of the locus coeruleus in the generation of pathological anxiety

This review aims to synthesise a large pre-clinical and clinical literature related to a hypothesised role of the locus coeruleus norepinephrine system in responses to acute and chronic threat, as well as the emergence of pathological anxiety. The locus coeruleus has widespread norepinephrine projections throughout the central nervous system, which act to globally modulate arousal states and adaptive behavior, crucially positioned to play a significant role in modulating both ascending visceral and descending cortical neurocognitive information. In response to threat or a stressor, the locus coeruleus–norepinephrine system globally modulates arousal, alerting and orienting functions and can have a powerful effect on the regulation of multiple memory systems. Chronic stress leads to amplification of locus coeruleus reactivity to subsequent stressors, which is coupled with the emergence of pathological anxiety-like behaviors in rodents. While direct in vivo evidence for locus coeruleus dysfunction in humans with pathological anxiety remains limited, recent advances in high-resolution 7-T magnetic resonance imaging and computational modeling approaches are starting to provide new insights into locus coeruleus characteristics.

Electroacupuncture Ameliorates Acute Myocardial Ischemia: A Potential Role of the Locus Coeruleus

The locus coeruleus (LC) is closely linked with cardiovascular disease. However, whether it mediates the alleviating effect of electroacupuncture (EA) on acute myocardial ischemia (AMI) remains unclear. A rat model of myocardial ischemia was established through occlusion of the left anterior descending coronary artery. Multichannel in vivo recording and other techniques were used to assess neurons in the LC, norepinephrine (NE) and dopamine (DA) levels in central and myocardial tissue, serum levels of inflammatory factors, and cardiac function. After induction of AMI, LC neuron activity increased and the central NE concentrations increased, while those of DA decreased. Moreover, the serum levels of high-sensitivity C-reactive protein (hs-CRP) increased, whereas those of interleukin-10 (IL-10) decreased. However, these effects were reversed by EA. Additionally, LC lesioning affected NE and DA levels in myocardial tissue and weakened the antimyocardial ischemic effect of EA. Collectively, our results indicated that LC is closely related to AMI and plays an important role in the antimyocardial ischemic effect of EA. This mechanism may be related to inhibition of LC neuron activity by EA, which inhibits the release of large amounts of hs-CRP and promotes that of IL-10 in the serum. Besides, after LC lesioning, EA may improve cardiac function by inhibiting the release of large amounts of NE and promoting the release of DA in myocardial tissue.

Distinct temporal integration of noradrenaline signaling by astrocytic second messengers during vigilance

Astrocytes may function as mediators of the impact of noradrenaline on neuronal function. Activation of glial α1-adrenergic receptors triggers rapid astrocytic Ca²⁺ elevation and facilitates synaptic plasticity, while activation of β-adrenergic receptors elevates cAMP levels and modulates memory consolidation. However, the dynamics of these processes in behaving mice remain unexplored, as do the interactions between the distinct second messenger pathways. Here we simultaneously monitored astrocytic Ca²⁺ and cAMP and demonstrate that astrocytic second messengers are regulated in a temporally distinct manner. In behaving mice, we found that while an abrupt facial air puff triggered transient increases in noradrenaline release and large cytosolic astrocytic Ca²⁺ elevations, cAMP changes were not detectable. By contrast, repeated aversive stimuli that lead to prolonged periods of vigilance were accompanied by robust noradrenergic axonal activity and gradual sustained cAMP increases. Our findings suggest distinct astrocytic signaling pathways can integrate noradrenergic activity during vigilance states to mediate distinct functions supporting memory.

Astrocytic GPCRs activate Ca²⁺ and cAMP signaling pathways, however, the in vivo dynamics of the two second messengers have not been fully been characterized. The authors demonstrate distinct noradrenaline-induced astrocytic Ca²⁺ and cAMP dynamics during startle and fear conditioning.

Dopamine and Noradrenaline in the Brain; Overlapping or Dissociate Functions?

Dopamine and noradrenaline are crucial neuromodulators controlling brain states, vigilance, action, reward, learning, and memory processes. Ventral tegmental area (VTA) and Locus Coeruleus (LC) are canonically described as the main sources of dopamine (DA) and noradrenaline (NA) with dissociate functions. A comparison of diverse studies shows that these neuromodulators largely overlap in multiple domains such as shared biosynthetic pathway and co-release from the LC terminals, convergent innervations, non-specificity of receptors and transporters, and shared intracellular signaling pathways. DA–NA interactions are mainly studied in prefrontal cortex and hippocampus, yet it can be extended to the whole brain given the diversity of catecholamine innervations. LC can simultaneously broadcast both dopamine and noradrenaline across the brain. Here, we briefly review the molecular, cellular, and physiological overlaps between DA and NA systems and point to their functional implications. We suggest that DA and NA may function in parallel to facilitate learning and maintain the states required for normal cognitive processes. Various signaling modules of NA and DA have been targeted for developing of therapeutics. Understanding overlaps of the two systems is crucial for more effective interventions in a range of neuropsychiatric conditions.

Effects of precondition α2-adrenoceptor agents on memory- and anxiety-related processes in the transient cerebral ischemic rats

Neurological evidence for the neuroprotective function of α₂-adrenoceptors in the cerebral ischemia is inconsistent. It is not examined how pretreatment with a single dose of α₂-adrenoceptor agents can affect motor function and anxiety- and memory-related responses in the cerebral ischemic animals. The transient forebrain ischemia model was provided, using a bilateral common carotid arterial occlusion (two-vessel occlusion, 2VO) in male Wistar rats. The 2VO rats impaired motor functions in the Rota-rod and wire grip tests and also decreased the step-through latency and the percentage of time spent on the open arms (%OAT), the percentage of entries into the open arms (%OAE) as well as locomotion in the elevated plus maze (EPM), indicating a memory deficit and anxiety-like behavior. Intraperitoneal single administration of yohimbine (0, 0.001, 0.01, and 0.1 mg/kg) before the 2VO did not alter these parameters while the higher and middle doses of clonidine (0.01 and 0.1 mg/kg) prevented the memory deficit and hypo-locomotion and its middle dose abrogated Rota-rod dysfunction and anxiety-like response. Meanwhile, both drugs did not influence on the measured behaviors in the sham groups by themselves. Moreover, yohimbine (0.001 mg/kg) abolished the beneficial effects of clonidine (0.01 and 0.1 mg/kg) on motor function in the Rota-rod and memory retention and also at its middle dose on the %OAT and locomotion in the 2VO rats. Our findings show a neuroprotective role for clonidine in motor function and memory- and anxiety-related behaviors of 2VO rats and the importance of α₂-adrenoceptors in these processes.

Speech-Stimulating Substances in Autism Spectrum Disorders

Autism spectrum disorder (ASD) is characterized by the core domains of persistent deficits in social communication and restricted-repetitive patterns of behaviors, interests, or activities. A heterogeneous and complex set of neurodevelopmental conditions are grouped in the spectrum. Pro-inflammatory events and immune system dysfunctions are cellular and molecular events associated with ASD. Several conditions co-occur with ASD: seizures, gastro-intestinal problems, attention deficit, anxiety and depression, and sleep problems. However, language and speech issues are key components of ASD symptoms current therapies find difficult to face. Several speech-stimulating substances have been shown to be effective in increasing speech ability in ASD subjects. The need for large clinical trials to determine safety and efficacy is recommended.

Cognitive costs of reproduction: life-history trade-offs explain cognitive decline during pregnancy in women

Life-history theory predicts that access to limited resources leads to trade-offs between competing body functions. Women, who face higher costs of reproduction when compared to men, should be especially vulnerable to these trade-offs. We propose the 'cognitive costs of reproduction hypothesis', which states that energy trade-offs imposed by reproduction may lead to a decline in maternal cognitive function during gestation. In particular, we hypothesize that the decline in cognitive function frequently observed during pregnancy is associated with the allocation of resources between the competing energetic requirements of the mother's brain and the developing foetus. Several distinctive anatomical and physiological features including a high metabolic rate of the brain, large infant size, specific anatomical features of the placenta and trophoblast, and the lack of maternal control over glucose flow through the placenta make the occurrence of these trade-offs likely. Herein, we review several lines of evidence for trade-offs between gestation and cognition that are related to: (i) energy metabolism during reproduction; (ii) energy metabolism of the human brain; (iii) links between energy metabolism and cognitive function; and (iv) links between gestation and cognitive function. We also review evidence for the important roles of cortisol, corticotropin-releasing hormone and sex hormones in mediating the effects of gestation on cognition, and we discuss possible neurophysiological mechanisms underlying the observed effects. The evidence supports the view that energy trade-offs between foetal growth and maternal endocrine and brain function lead to changes in maternal cognition, and that this phenomenon is mediated by neuroendocrine mechanisms involving the hypothalamic-pituitary-adrenal axis, brainstem nucleus locus coeruleus and hippocampus.

Drug-free and context-dependent locomotor hyperactivity in DBA/2 J mice previously treated with repeated cocaine: Relationship with behavioral sensitization and role of noradrenergic receptors

Drug-associated contexts and discrete cues can trigger motivational states responsible for drug-seeking behavior and relapse. In preclinical research, drug-free conditioned hyperactivity has been used to investigate the expression of memories associated with psychostimulant drug effects. Addictive drugs can produce long-lasting sensitization to their psychomotor actions, a phenomenon known as behavioral sensitization. The neuroplasticity underlying behavioral sensitization appears to be involved in pathological drug pursuit and abuse. In the present study we evaluated drug-free, context-dependent hyperactivity in DBA/2 J mice previously treated with cocaine and we explored whether this conditioned effect was related to behavioral sensitization. Given the role of noradrenergic (NA) neurotransmission in memory retrieval, consolidation and reconsolidation processes, we also investigated whether conditioned hyperactivity in a drug-free state was mediated by NA receptors. Animals underwent a sensitization protocol with six cocaine injections (0, 5, 10 or 20 mg/kg) paired to a particular floor cue. Three days after this sensitization phase, all animals were exposed to the same familiar floor environment without drug treatment. A second test with an unfamiliar floor was conducted 24 h later. Conditioned hyperactivity was also explored after one or three cocaine pairings and was evaluated for its duration (with repeated familiar vs. unfamiliar floor tests). In a series of pharmacological experiments, we evaluated the effects propranolol (a non-selective antagonist of β1- and β2-receptors) and prazosin (α1-receptor antagonist) on conditioned hyperactivity. Cocaine treatment produced both robust sensitization and drug-free conditioned hyperactivity, an effect that lasted up to 17 days (with cocaine 20 mg/kg). A significant correlation between the magnitude of cocaine sensitization and the level of conditioned hyperactivity was found. Propranolol, but not prazosin, blocked context-dependent hyperlocomotion in a drug-free state. Our data, together with a vast body of literature, indicate that the NA system plays a key role in the retrieval and behavioral expression of drug-associated memories.

Dual contributions of noradrenaline to behavioural flexibility and motivation

INTRODUCTION

While several theories have highlighted the importance of the noradrenergic system for behavioral flexibility, a number of recent studies have also shown a role for noradrenaline in motivation, particularly in effort processing. Here, we designed a novel sequential cost/benefit decision task to test the causal influence of noradrenaline on these two functions in rhesus monkeys.

METHODS

We manipulated noradrenaline using clonidine, an alpha-2 noradrenergic receptor agonist, which reduces central noradrenaline levels and examined how this manipulation influenced performance on the task.

RESULTS

Clonidine had two specific and distinct effects: first, it decreased choice variability, without affecting the cost/benefit trade-off; and second, it reduced force production, without modulating the willingness to work.

CONCLUSIONS

Together, these results support an overarching role for noradrenaline in facing challenging situations in two complementary ways: by modulating behavioral volatility, which would facilitate adaptation depending on the lability of the environment, and by modulating the mobilization of resources to face immediate challenges.

An integrated modelling framework for neural circuits with multiple neuromodulators

Neuromodulators are endogenous neurochemicals that regulate biophysical and biochemical processes, which control brain function and behaviour, and are often the targets of neuropharmacological drugs. Neuromodulator effects are generally complex partly owing to the involvement of broad innervation, co-release of neuromodulators, complex intra- and extrasynaptic mechanism, existence of multiple receptor subtypes and high interconnectivity within the brain. In this work, we propose an efficient yet sufficiently realistic computational neural modelling framework to study some of these complex behaviours. Specifically, we propose a novel dynamical neural circuit model that integrates the effective neuromodulator-induced currents based on various experimental data (e.g. electrophysiology, neuropharmacology and voltammetry). The model can incorporate multiple interacting brain regions, including neuromodulator sources, simulate efficiently and easily extendable to large-scale brain models, e.g. for neuroimaging purposes. As an example, we model a network of mutually interacting neural populations in the lateral hypothalamus, dorsal raphe nucleus and locus coeruleus, which are major sources of neuromodulator orexin/hypocretin, serotonin and norepinephrine/noradrenaline, respectively, and which play significant roles in regulating many physiological functions. We demonstrate that such a model can provide predictions of systemic drug effects of the popular antidepressants (e.g. reuptake inhibitors), neuromodulator antagonists or their combinations. Finally, we developed user-friendly graphical user interface software for model simulation and visualization for both fundamental sciences and pharmacological studies.

The Neuroanatomy of the Reticular Nucleus Locus Coeruleus in Alzheimer's Disease

Alzheimer’s Disease (AD) features the accumulation of β-amyloid and Tau aggregates, which deposit as extracellular plaques and intracellular neurofibrillary tangles (NFTs), respectively. Neuronal Tau aggregates may appear early in life, in the absence of clinical symptoms. This occurs in the brainstem reticular formation and mostly within Locus Coeruleus (LC), which is consistently affected during AD. LC is the main source of forebrain norepinephrine (NE) and it modulates a variety of functions including sleep-waking cycle, alertness, synaptic plasticity, and memory. The iso-dendritic nature of LC neurons allows their axons to spread NE throughout the whole forebrain. Likewise, a prion-like hypothesis suggests that Tau aggregates may travel along LC axons to reach out cortical neurons. Despite this timing is compatible with cross-sectional studies, there is no actual evidence for a causal relationship between these events. In the present mini-review, we dedicate special emphasis to those various mechanisms that may link degeneration of LC neurons to the onset of AD pathology. This includes the hypothesis that a damage to LC neurons contributes to the onset of dementia due to a loss of neuroprotective effects or, even the chance that, LC degenerates independently from cortical pathology. At the same time, since LC neurons are lost in a variety of neuropsychiatric disorders we considered which molecular mechanism may render these brainstem neurons so vulnerable.

Locus coeruleus at asymptomatic early and middle Braak stages of neurofibrillary tangle pathology

AIMS

The present study analyses molecular characteristics of the locus coeruleus (LC) and projections to the amygdala and hippocampus at asymptomatic early and middle Braak stages of neurofibrillary tangle (NFT) pathology.

METHODS

Immunohistochemistry, whole-transcriptome arrays and RT-qPCR in LC and western blotting in hippocampus and amygdala in a cohort of asymptomatic individuals at stages I-IV of NFT pathology were used.

RESULTS

NFTs in the LC increased in parallel with colocalized expression of tau kinases, increased neuroketal adducts and decreased superoxide dismutase 1 in neurons with hyperphosphorylated tau and decreased voltage-dependent anion channel in neurons containing truncated tau were found. These were accompanied by increased microglia and AIF1, CD68, PTGS2, IL1β, IL6 and TNF-α gene expression. Whole-transcriptome arrays revealed upregulation of genes coding for proteins associated with heat shock protein binding and genes associated with ATP metabolism and downregulation of genes coding for DNA-binding proteins and members of the small nucleolar RNAs family, at stage IV when compared with stage I. Tyrosine hydroxylase (TH) immunoreactivity was preserved in neurons of the LC, but decreased TH and increased α_2A adrenergic receptor protein levels were found in the hippocampus and the amygdala.

CONCLUSIONS

Complex alteration of several metabolic pathways occurs in the LC accompanying NFT formation at early and middle asymptomatic stages of NFT pathology. Dopaminergic/noradrenergic denervation and increased expression of α_2A adrenergic receptor in the hippocampus and amygdala occur at first stage of NFT pathology, suggesting compensatory activation in the face of decreased adrenergic input occurring before clinical evidence of cognitive impairment and depression.

Monoaminergic neuropathology in Alzheimer's disease

None of the proposed mechanisms of Alzheimer's disease (AD) fully explains the distribution patterns of the neuropathological changes at the cellular and regional levels, and their clinical correlates. One aspect of this problem lies in the complex genetic, epigenetic, and environmental landscape of AD: early-onset AD is often familial with autosomal dominant inheritance, while the vast majority of AD cases are late-onset, with the ε4 variant of the gene encoding apolipoprotein E (APOE) known to confer a 5-20 fold increased risk with partial penetrance. Mechanisms by which genetic variants and environmental factors influence the development of AD pathological changes, especially neurofibrillary degeneration, are not yet known. Here we review current knowledge of the involvement of the monoaminergic systems in AD. The changes in the serotonergic, noradrenergic, dopaminergic, histaminergic, and melatonergic systems in AD are briefly described. We also summarize the possibilities for monoamine-based treatment in AD. Besides neuropathologic AD criteria that include the noradrenergic locus coeruleus (LC), special emphasis is given to the serotonergic dorsal raphe nucleus (DRN). Both of these brainstem nuclei are among the first to be affected by tau protein abnormalities in the course of sporadic AD, causing behavioral and cognitive symptoms of variable severity. The possibility that most of the tangle-bearing neurons of the LC and DRN may release amyloid β as well as soluble monomeric or oligomeric tau protein trans-synaptically by their diffuse projections to the cerebral cortex emphasizes their selective vulnerability and warrants further investigations of the monoaminergic systems in AD.

Effects of methylphenidate on attention in Wistar rats treated with the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)

The aim of this study was to assess the effects of the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) on attention in rats as measured using the 5-choice-serial-reaction-time task (5CSRTT) and to investigate whether methylphenidate has effects on DSP4-treated rats. Methylphenidate is a noradrenaline and dopamine reuptake inhibitor and commonly used in the pharmacological treatment of individuals with attention deficit/hyperactivity disorder (ADHD). Wistar rats were trained in the 5CSRTT and treated with one of three doses of DSP4 or saline. Following the DSP4 treatment rats were injected with three doses of methylphenidate or saline and again tested in the 5CSRTT. The treatment with DSP4 caused a significant decline of performance in the number of correct responses and a decrease in response accuracy. A reduction in activity could also be observed. Whether or not the cognitive impairments are due to attention deficits or changes in explorative behaviour or activity remains to be investigated. The treatment with methylphenidate had no beneficial effect on the rats' performance regardless of the DSP4 treatment. In the group without DSP4 treatment, methylphenidate led to a reduction in response accuracy and bidirectional effects in regard to parameters related to attention. These findings support the role of noradrenaline in modulating attention and call for further investigations concerning the effects of methylphenidate on attentional processes in rats.

Effects of peripheral tactile nerve stimulation on affective behavior of patients with probable Alzheimer's disease

In the present study, the hypothesis was tested that peripheral tactile nerve stimulation by massage would improve various aspects of affective behavior in patients with probable Alzheimer's disease. It was assumed that peripheral tactile stimulation might activate higher-level brain structures (e.g. the hypothalamus). The present study revealed that patients who were stimulated with tactile stimulation felt less depressed, less anxious, more well tempered, and were more alert. Apart from that, their personal orientation and their environmental orientation in place improved, they were more interested in social contacts and they participated more in activities of daily living. However, the observed effects could not be maintained over a period of six weeks following treatment.

β-Adrenergic Control of Hippocampal Function: Subserving the Choreography of Synaptic Information Storage and Memory

Noradrenaline (NA) is a key neuromodulator for the regulation of behavioral state and cognition. It supports learning by increasing arousal and vigilance, whereby new experiences are “earmarked” for encoding. Within the hippocampus, experience-dependent information storage occurs by means of synaptic plasticity. Furthermore, novel spatial, contextual, or associative learning drives changes in synaptic strength, reflected by the strengthening of long-term potentiation (LTP) or long-term depression (LTD). NA acting on β-adrenergic receptors (β-AR) is a key determinant as to whether new experiences result in persistent hippocampal synaptic plasticity. This can even dictate the direction of change of synaptic strength.

The different hippocampal subfields play different roles in encoding components of a spatial representation through LTP and LTD. Strikingly, the sensitivity of synaptic plasticity in these subfields to β-adrenergic control is very distinct (dentate gyrus > CA3 > CA1). Moreover, NA released from the locus coeruleus that acts on β-AR leads to hippocampal LTD and an enhancement of LTD-related memory processing. We propose that NA acting on hippocampal β-AR, that is graded according to the novelty or saliency of the experience, determines the content and persistency of synaptic information storage in the hippocampal subfields and therefore of spatial memories.

Neuromelanin marks the spot: identifying a locus coeruleus biomarker of cognitive reserve in healthy aging

Leading a mentally stimulating life may build up a reserve of neural and mental resources that preserve cognitive abilities in late life. Recent autopsy evidence links neuronal density in the locus coeruleus (LC), the brain's main source of norepinephrine, to slower cognitive decline before death, inspiring the idea that the noradrenergic system is a key component of reserve (Robertson, I. H. 2013. A noradrenergic theory of cognitive reserve: implications for Alzheimer's disease. Neurobiol. Aging. 34, 298-308). Here, we tested this hypothesis using neuromelanin-sensitive magnetic resonance imaging to visualize and measure LC signal intensity in healthy younger and older adults. Established proxies of reserve, including education, occupational attainment, and verbal intelligence, were linearly correlated with LC signal intensity in both age groups. Results indicated that LC signal intensity was significantly higher in older than younger adults and significantly lower in women than in men. Consistent with the LC-reserve hypothesis, both verbal intelligence and a composite reserve score were positively associated with LC signal intensity in older adults. LC signal intensity was also more strongly associated with attentional shifting ability in older adults with lower cognitive reserve. Together these findings link in vivo estimates of LC neuromelanin signal intensity to cognitive reserve in normal aging.

Effects of a Neonatal Experience Involving Reward Through Maternal Contact on the Noradrenergic System of the Rat Prefrontal Cortex

The noradrenergic system plays an important role in prefrontal cortex (PFC) function. Since early life experiences play a crucial role in programming brain function, we investigated the effects of a neonatal experience involving reward through maternal contact on the noradrenergic system of the rat PFC. Rat pups were exposed during Postnatal days (PNDs) 10-13, to a T-maze in which contact with the mother was used as a reward (RER). RER males had higher norepinephrine levels in the PFC both on PND 13 and in adulthood. The RER experience resulted in adulthood in increased levels of the active demethylase GADD45b, hypomethylation of the β1 adrenergic receptor (ADRB1) gene promoter, and consequent enhanced expression of its mRNA in the PFC. In addition, protein and binding levels of the ADRB1, as well as those of its downstream effector phosphorylated cAMP response element-binding protein were elevated in RER males. The higher activity of the PFC noradrenergic system of the RER males was reflected in their superior performance in the olfactory discrimination and the contextual fear extinction, 2 PFC noradrenergic system-dependent behavioral tasks.

Phasic activation of individual neurons in the locus ceruleus/subceruleus complex of monkeys reflects rewarded decisions to go but not stop

Neurons in the brainstem nucleus locus ceruleus (LC) often exhibit phasic activation in the context of simple sensory-motor tasks. The functional role of this activation, which leads to the release of norepinephrine throughout the brain, is not yet understood in part because the conditions under which it occurs remain in question. Early studies focused on the relationship of LC phasic activation to salient sensory events, whereas more recent work has emphasized its timing relative to goal-directed behavioral responses, possibly representing the end of a sensory-motor decision process. To better understand the relationship between LC phasic activation and sensory, motor, and decision processing, we recorded spiking activity of neurons in the LC+ (LC and the adjacent, norepinephrine-containing subceruleus nucleus) of monkeys performing a countermanding task. The task required the monkeys to occasionally withhold planned, saccadic eye movements to a visual target. We found that many well isolated LC+ units responded to both the onset of the visual cue instructing the monkey to initiate the saccade and again after saccade onset, even when it was initiated erroneously in the presence of a stop signal. Many of these neurons did not respond to saccades made outside of the task context. In contrast, neither the appearance of the stop signal nor the successful withholding of the saccade elicited an LC+ response. Therefore, LC+ phasic activation encodes sensory and motor events related to decisions to execute, but not withhold, movements, implying a functional role in goal-directed actions, but not necessarily more covert forms of processing.

Neurobiological correlates of social anxiety disorder: an update

Social anxiety disorder (SAD) is a condition characterized by pervasiveness and impairment in social functioning, with a prevalence in the general population between 1.9% and 12.1%. The most consistent findings on its neurobiological underpinnings involve a wide range of neurotransmitters (serotonin, norepinephrine, glutamate, and GABA) and neuropeptides (oxytocin), but no comprehensive hypothesis is yet available. In particular, oxytocin is becoming increasingly established as a "prosocial neuropeptide" and, as such, is a major focus of current research, with a great range of therapeutic applications including SAD treatment. Specifically, the amygdala plays a pivotal role in conditioning and processing of fear, and exaggerated amygdala responses in SAD patients have been observed during various social-emotional stimuli. In addition to the amygdala, other brain areas of interest in SAD-related circuitry are represented by the medial prefrontal cortex, dorsal raphe, striatum, locus coeruleus, prefrontal cortex, insular cortex, and anterior cingulate cortex. The aim of this review is to provide an update on neurobiological correlates of SAD, with a special focus on neurotransmitters and brain areas possibly involved, and suggestions for future research that could lead to more specific therapeutic interventions.

Mechanisms of Individual Differences in Impulsive and Risky Choice in Rats

Individual differences in impulsive and risky choice are key risk factors for a variety of maladaptive behaviors such as drug abuse, gambling, and obesity. In our rat model, ordered individual differences are stable across choice parameters, months of testing, and span a broad spectrum, suggesting that rats, like humans, exhibit trait-level impulsive and risky choice behaviors. In addition, impulsive and risky choices are highly correlated, suggesting a degree of correlation between these two traits. An examination of the underlying cognitive mechanisms has suggested an important role for timing processes in impulsive choice. In addition, in an examination of genetic factors in impulsive choice, the Lewis rat strain emerged as a possible animal model for studying disordered impulsive choice, with this strain demonstrating deficient delay processing. Early rearing environment also affected impulsive behaviors, with rearing in an enriched environment promoting adaptable and more self-controlled choices. The combined results with impulsive choice suggest an important role for timing and reward sensitivity in moderating impulsive behaviors. Relative reward valuation also affects risky choice, with manipulation of objective reward value (relative to an alternative reference point) resulting in loss chasing behaviors that predicted overall risky choice behaviors. The combined results are discussed in relation to domain-specific versus domain-general subjective reward valuation processes and the potential neural substrates of impulsive and risky choice.

Pupillometry as a glimpse into the neurochemical basis of human memory encoding

Neurochemical systems are well studied in animal learning; however, ethical issues limit methodologies to explore these systems in humans. Pupillometry provides a glimpse into the brain's neurochemical systems, where pupil dynamics in monkeys have been linked with locus coeruleus (LC) activity, which releases norepinephrine (NE) throughout the brain. Here, we use pupil dynamics as a surrogate measure of neurochemical activity to explore the hypothesis that NE is involved in modulating memory encoding. We examine this using a task-irrelevant learning paradigm in which learning is boosted for stimuli temporally paired with task targets. We show that participants better recognize images that are paired with task targets than distractors and, in correspondence, that pupil size changes more for target-paired than distractor-paired images. To further investigate the hypothesis that NE nonspecifically guides learning for stimuli that are present with its release, a second procedure was used that employed an unexpected sound to activate the LC-NE system and induce pupil-size changes; results indicated a corresponding increase in memorization of images paired with the unexpected sounds. Together, these results suggest a relationship between the LC-NE system, pupil-size changes, and human memory encoding.

A noradrenergic theory of cognitive reserve: implications for Alzheimer's disease

The gap between symptoms and pathology in Alzheimer's disease has been explained by the hypothetical construct of "cognitive reserve"--a set of variables including education, intelligence, and mental stimulation which putatively allow the brain to adapt to-and hence mask--underlying pathologies by maintaining cognitive function despite underlying neural changes. This review proposes a hypothesis that a biological mechanism may mediate between these social/psychological processes on the one hand, and apparently reduced risk of Alzheimer's disease on the other, namely repeated activation of the noradrenergic system over a lifetime by the processes implicated in cognitive reserve. Noradrenaline's neuroprotective effects both in vivo and in vitro, and its key role in mediating the neuroprotective effects of environmental enrichment on the brain, make noradrenaline's key role in mediating cognitive reserve--by disease compensation, disease modification, or a combination of both--a viable hypothesis.

Rational regulation of learning dynamics by pupil-linked arousal systems

The ability to make inferences about the current state of a dynamic process requires ongoing assessments of the stability and reliability of data generated by that process. We found that these assessments, as defined by a normative model, were reflected in nonluminance-mediated changes in pupil diameter of human subjects performing a predictive-inference task. Brief changes in pupil diameter reflected assessed instabilities in a process that generated noisy data. Baseline pupil diameter reflected the reliability with which recent data indicate the current state of the data-generating process and individual differences in expectations about the rate of instabilities. Together these pupil metrics predicted the influence of new data on subsequent inferences. Moreover, a task- and luminance-independent manipulation of pupil diameter predictably altered the influence of new data. Thus, pupil-linked arousal systems can help to regulate the influence of incoming data on existing beliefs in a dynamic environment.

Vagus nerve stimulation enhances perforant path-CA3 synaptic transmission via the activation of β-adrenergic receptors and the locus coeruleus

Vagus nerve stimulation (VNS) is an approved treatment for epilepsy and depression and has cognition-enhancing effects in patients with Alzheimer's disease. The hippocampus is widely recognized to be related to epilepsy, depression, and Alzheimer's disease. One possible mechanism of VNS involves its effect on the hippocampus; i.e. it increases the release of noradrenaline in the hippocampus. However, the effect of VNS on synaptic transmission in the hippocampus is unknown. To determine whether VNS modulates neurotransmission in the hippocampus, we examined the effects of VNS on perforant path (PP)-CA3 synaptic transmission electrophysiologically in anaesthetized rats. VNS induces a persistent enhancement of PP-CA3 field excitatory post-synaptic potentials (fEPSPs). Arc, an immediate early gene, was used to identify active brain regions after VNS. The locus coeruleus (LC), which contains the perikarya of noradrenergic projections, harboured more Arc-positive cells, as measured by in-situ hybridization, after 10-min VNS. In addition, electrical lesions of LC neurons or intraventricular administration of the β-adrenergic receptor antagonist timolol prevented the enhancement of PP-CA3 responses by VNS. In conclusion, the protracted increase in PP-CA3 synaptic transmission that is induced by VNS entails activation of the LC and β-adrenergic receptors. Our novel findings suggest that information from the periphery modulates synaptic transmission in the CA3 region of the hippocampus.

The role of the central noradrenergic system in behavioral inhibition

Although the central noradrenergic system has been shown to be involved in a number of behavioral and neurophysiological processes, the relation of these to its role in depressive illness has been difficult to define. The present review discusses the hypothesis that one of its chief functions that may be related to affective illness is the inhibition of behavioral activation, a prominent symptom of the disorder. This hypothesis is found to be consistent with most previous neuropsychopharmacological and immunohistochemical experiments on active behavior in rodents in a variety of experimental conditions using manipulation of neurotransmission at both locus coeruleus and forebrain adrenergic receptors. The findings support a mechanism in which high rates of noradrenergic neural activity suppress the neural activity of principal neurons in forebrain regions mediating active behavior. The suppression may be mediated through postsynaptic galaninergic and adrenergic receptors, and via the release of corticotrophin-releasing hormone. The hypothesis is consistent with clinical evidence for central noradrenergic system hyperactivity in depressives and with the view that this hyperactivity is a contributing etiological factor in the disorder. A similar mechanism may underlie the ability of the noradrenergic system to suppress seizure activity suggesting that inhibition of the spread of neural activation may be a unifying function.

Changes with aging in the dopaminergic and noradrenergic innervation of rat neocortex

In normal aging, the mammalian cortex undergoes significant remodeling. Although neuromodulation by dopamine and noradrenaline in the cortex is known to be important for proper cognitive function, little is known on how cortical noradrenergic and dopaminergic presynaptic boutons are affected in normal aging. Using rats we investigated whether these two neurotransmitter systems undergo structural reorganization in aging, and if these changes correlated with cognitive loss. Young and aged rats were tested for cognitive performance using the Morris water maze. Following the behavioral characterization, the animals were sacrificed and the cortical tissue was processed for immunofluorescence using antibodies directed against tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) to detect and discriminate noradrenergic and dopaminergic varicosities. We observed a significant increase in dopaminergic varicosities in lamina V of the anterior cingulate cortex (ACC) of aged cognitively unimpaired rats when compared to young and aged-impaired animals. In laminae II and III of the ACC, we observed a significant decrease of dopaminergic varicosities in aged-impaired animals when compared to young or aged cognitively unimpaired animals. Changes in noradrenergic varicosities never reached statistical significance in any group or brain region. The data suggests that the remodeling of mesocortical dopaminergic fibers may participate in age-associated cognitive decline.

Theta bursts in the olfactory nerve paired with beta-adrenoceptor activation induce calcium elevation in mitral cells: a mechanism for odor preference learning in the neonate rat

Odor preference learning in the neonate rat follows pairing of odor input and noradrenergic activation of beta-adrenoceptors. Odor learning is hypothesized to be supported by enhanced mitral cell activation. Here a mechanism for enhanced mitral cell signaling is described. Theta bursts in the olfactory nerve (ON) produce long-term potentiation (LTP) of glomerular excitatory postsynaptic potentials (EPSPs) and of excitatory postsynaptic currents (EPSCs) in the periglomerular (PG) and external tufted (ET) cells. Theta bursts paired with beta-adrenoceptor activation significantly elevate mitral cell (MC) calcium. Juxtaglomerular inhibitory network depression by beta-adrenoceptor activation appears to increase calcium in MCs in response to theta burst stimulation.

Effects of estrogen receptor alpha and beta gene deletion on estrogenic induction of progesterone receptors in the locus coeruleus in female mice

Locus coeruleus (LC) is involved in the LHRH regulation by gonadal steroids. We investigated the expression of progesterone and estrogen receptors (PR; ER) in LC neurons of ERalpha (alphaERKO) or ERbeta (betaERKO) knockout mice, and their wild-type (alphaWT and betaWT). Immunocytochemical studies showed that LC expresses PR and both ERs, although ERbeta was more abundant. Estradiol benzoate (EB) decreased ERalpha-positive cells in WT and betaERKO mice, and progesterone caused a further reduction, whereas none of the steroids influenced ERbeta expression. ERbeta deletion increased ERalpha while ERalpha deletion did not alter ERbeta expression. In both WT mice, EB increased PR expression, which was diminished by progesterone. These steroid effects were also observed in alphaERKO animals but to a lesser extent, suggesting that ERalpha is partially responsible for the estrogenic induction of PR in LC. Steroid effects on PR in betaERKO mice were similar to those in the alphaERKO but to a lesser extent, probably because PR expression was already high in the oil-treated group. This expression seems to be specific of LC neurons, since it was not observed in other areas studied, the preoptic area and ventromedial nucleus of hypothalamus. These findings show that LC in mice expresses alphaER, betaER, and PR, and that a balance between them may be critical for the physiological control of reproductive function.

Arousal facilitates collision avoidance mediated by a looming sensitive visual neuron in a flying locust

Locusts have two large collision-detecting neurons, the descending contralateral movement detectors (DCMDs) that signal object approach and trigger evasive glides during flight. We sought to investigate whether vision for action, when the locust is in an aroused state rather than a passive viewer, significantly alters visual processing in this collision-detecting pathway. To do this we used two different approaches to determine how the arousal state of a locust affects the prolonged periods of high-frequency spikes typical of the DCMD response to approaching objects that trigger evasive glides. First, we manipulated arousal state in the locust by applying a brief mechanical stimulation to the hind leg; this type of change of state occurs when gregarious locusts accumulate in high-density swarms. Second, we examined DCMD responses during flight because flight produces a heightened physiological state of arousal in locusts. When arousal was induced by either method we found that the DCMD response recovered from a previously habituated state; that it followed object motion throughout approach; and--most important--that it was significantly more likely to generate the maintained spike frequencies capable of evoking gliding dives even with extremely short intervals (1.8 s) between approaches. Overall, tethered flying locusts responded to 41% of simulated approaching objects (sets of 6 with 1.8 s ISI). When we injected epinastine, the neuronal octopamine receptor antagonist, into the hemolymph responsiveness declined to 12%, suggesting that octopamine plays a significant role in maintaining responsiveness of the DCMD and the locust to visual stimuli during flight.

Norepinephrine and the dentate gyrus

Norepinephrine's role in the dentate gyrus is assessed based on a review of what is known about its innervation and receptor patterns and its functional effects at both cellular and behavioral levels. The data support seven hypotheses: (1) Norepinephrine's functional actions are primarily mediated by beta adrenoceptors and include electrophysiological enhancement of responses to excitatory input and glycogenolytic metabolic support of excitatory synaptic activity. (2) At the cellular level, locus coeruleus burst release of norepinephrine transiently inhibits feedforward interneurons and either excites or inhibits subpopulations of feedback interneurons. Consistent with reduced feedforward inhibition, granule cell firing is transiently increased. Concomitant EEG effects include transient increases in theta power and decreases in beta and gamma power. (3) Norepinephrine selectively promotes the processing of medial perforant path spatial input. This effect is mediated both through short- and long-term potentiation of cell excitability and through delayed potentiation of synaptic input. A critical level of norepinephrine release is required for long-term effects to norepinephrine alone. Norepinephrine release switches early phase frequency-induced long-term potentiation of perforant path input to an enduring late phase form and can reinstate decayed long-term potentiation. Norepinephrine also promotes frequency-induced potentiation of granule cell output at the mossy fiber to CA3 connection. (4) Local increases in norepinephrine accompany glutamate release and release of other neurotransmitters providing a mechanism for norepinephrine enhancement effects independent of locus coeruleus firing. (5) Stimuli, such as novelty and reward and punishment, which activate locus coeruleus neurons, enhance responses to medial perforant path input and engage late phase frequency-induced long-term potentiation through beta adrenoceptor activation. (6) Behavioral studies are consistent with the mechanistic evidence for a norepinephrine role in promoting learning and memory and assisting retrieval. (7) The overall profile suggests lower levels of norepinephrine may facilitate pattern completion or memory retrieval while higher levels would recruit global remapping and promote long-term episodic memory.

The effects of the mGluR5 antagonist MPEP and the mGluR2/3 antagonist LY341495 on rats' performance in the 5-choice serial reaction time task

Schizophrenia is characterized by attentional deficits possibly associated with glutamate dysfunction. The role of postsynaptic metabotropic glutamate 5 receptors (mGluR5) or presynaptic inhibitory mGluR2/3 on attention is currently unknown. We investigated the effects of the mGluR5 antagonist MPEP (2-methyl-6[phenylethynyl]-pyridine) and the mGluR2/3 antagonist LY341495 on attention in the 5-choice serial reaction time task (5CSRTT), as well as on food intake to evaluate their effects on food motivation. The effects of pre-feeding and the muscle relaxant curare were examined to characterize the effects of alterations in the motivation or ability to perform the task, respectively. MPEP had no effect on accuracy but overall decreased performance in the 5CSRTT, including decreased speed of responding and decreased premature responses. LY341495 had no significant effect on rats' performance in the 5CSRTT. LY341495 decreased food intake in the home cage to a greater extent than MPEP. Curare decreased the speed of correct responding, reflecting motor impairment. Free feeding decreased overall performance, number of trials completed and number of head entries into the feeder, reflecting decreased motivation to perform the task. Thus, blockade of mGluR5, but not mGluR2/3, decreased overall responding without affecting accuracy in the 5CSRTT.

Age-dependent interactive changes in serotonergic and noradrenergic cortical axon terminals in F344 rats

In the frontal cortex of aging rats, we found an increase in sprouting of the noradrenergic (NA) axons originated from the locus coeruleus (LC). The serotonergic (5-HT) axons originating from the dorsal raphe (DR) share the same cortical area and their age-dependent changes and interactions with NA axons were still unclear. To compare quantitatively the extent of axonal sprouting of DR and LC neurons in the frontal cortex, we extracellularly recorded from both DR and LC neurons in the same animals and antidromically stimulated 32 cortical sites (a pair of stimulating electrodes was moved at 100-mum intervals from 500 to 2000 microm in depth). In addition, to examine the effects of degeneration of 5-HT axons on NA axons, and vice versa, we used specific neurotoxins for 5-HT (PCA) or NA (DSP-4) axons. We also used noradrenaline uptake inhibitor (maprotiline) to verify the effects of NA on degeneration of 5-HT axons. Results suggested that 5-HT axons sprouted between 15 and 17 months of age and noradrenaline accelerated the age-dependent change of 5-HT axons.