Damage to ceruleo-cortical noradrenergic projections impairs locally cued but enhances spatially cued water maze acquisition

Behavioral Analysis of NMDAR Function in Rodents: Tests of Long-Term Spatial Memory

NMDAR-dependent forms of synaptic plasticity in brain regions like the hippocampus are widely believed to provide the neural substrate for long-term associative memory formation. However, the experimental data are equivocal at best and may suggest a more nuanced role for NMDARs and synaptic plasticity in memory. Much of the experimental data available comes from studies in genetically modified mice in which NMDAR subunits have been deleted or mutated in order to disrupt NMDAR function. Behavioral assessment of long-term memory in these mice has involved tests like the Morris watermaze and the radial arm maze. Here we describe these behavioral tests and some of the different testing protocols that can be used to assess memory performance. We discuss the importance of distinguishing selective effects on learning and memory processes from nonspecific effects on sensorimotor or motivational aspects of performance.

Recurrent stress across life may improve cognitive performance in individual rats, suggesting the induction of resilience

Depressive symptoms are often accompanied by cognitive impairments and recurrent depressive episodes are discussed as a potential risk for dementia. Especially, stressful life events are considered a potent risk factor for depression. Here, we induced recurrent stress-induced depressive episodes over the life span of rats, followed by cognitive assessment in the symptom-free period. Rats exposed to stress-induced depressive episodes learned faster than control rats. A high degree of stress-induced depressive-like behavior early in the paradigm was a predictor of improved cognitive performance, suggesting induction of resilience. Subsequently, exposure to lorazepam prior to stress-induced depressive episodes and cognitive testing in a nonaversive environment prevented the positive effect. This indicates a beneficial effect of the stress-associated situation, with the existence of individual coping abilities. Altogether, stress may in some have a beneficial effect, yet for those individuals unable to tackle these aversive events, consecutive unpleasant episodes may lead to worse cognitive performance later in life.

Contemplating the GANE model using an extreme case paradigm

Early experiences play a crucial role in programming brain function, affecting selective attention, learning, and memory. Infancy literature suggests an extension of the GANE (glutamate amplifies noradrenergic effects) model to conditions with minimal priority-map inputs, yet suggests qualifications by noting that its efficacy is increased when tonic levels of arousal are maintained in an optimal range, in manners that are age and exposure dependent.

A Bias Toward Short-Term Thinking in Threat-Related Negative Emotional States

Some decisions must be made repeatedly and have consequences that change depending on how often each alternative is chosen. Such temporally extended decisions are pervasive and important, and often involve short-term/long-term tradeoffs. Can unrelated emotion and stress influence such decisions? In negative emotional states involving threats, people should favor what is better at the time regardless of possible long-term consequences. Supporting this hypothesis, in Experiment 1, college students randomly assigned to see aversive images repeatedly made choices that had better short-term but poorer long-term effects, and so earned less money than students shown neutral images, effect size r = .71. In Experiment 2, students reporting stress about impending exams showed a similar bias, r=.53.Thehypothesis and results are relevant to decision making, delay of gratification, distributed choice, and self-control.

6-Hydroxydopamine-Induced Dopamine Reductions in the Nucleus Accumbens, but not the Medial Prefrontal Cortex, Impair Cincinnati Water Maze Egocentric and Morris Water Maze Allocentric Navigation in Male Sprague-Dawley Rats

The nucleus accumbens (Nacc) and medial prefrontal cortex (mPFC) receive dopaminergic innervation from the ventral tegmental area and are involved in learning. Male rats with 6-hydroxydopamine (6-OHDA)-induced dopaminergic and noradrenergic reductions in the Nacc or mPFC were tested for allocentric and egocentric learning to determine their role in these forms of neuroplasticity. mPFC dopaminergic and noradrenergic reductions did not result in changes to either type of learning or memory. Nacc dopaminergic and noradrenergic reductions resulted in allocentric learning and memory deficits in the Morris water maze (MWM) on acquisition, reversal, and probe trials. MWM cued performance was also affected, but straight-channel swim times and swim speed during hidden platform trials in the MWM were not affected. Nacc dopaminergic and noradrenergic reductions also impaired egocentric learning in the Cincinnati water maze (CWM). Nacc-lesioned animals tested in the CWM in an alternate path through the maze were not significantly affected. 6-OHDA injections in the Nacc resulted in 63 % dopamine and 62 % norepinephrine reductions in the Nacc and 23 % reductions in adjacent dorsal striatum. 6-OHDA injections in the mPFC resulted in 88 % reductions in dopamine and 59 % reductions in norepinephrine. Hence, Nacc dopamine and/or norepinephrine play a role in egocentric and allocentric learning and memory, while mPFC dopamine and norepinephrine do not.

Accumbal α-adrenoceptors, but not β-adrenoceptors, regulate behaviour that is mediated by reserpine-sensitive storage vesicles

It has previously been demonstrated that mesolimbic α-adrenoceptors, but not β-adrenoceptors, control the release of dopamine that is derived from reserpine-sensitive storage vesicles. The aim of the present study was to investigate whether these storage vesicles also regulate α-adrenoceptor-mediated or β-adrenoceptor-mediated changes in behaviour. Accordingly, rats were pretreated with reserpine before the α-adrenoceptor antagonist phentolamine or the β-adrenoceptor agonist isoproterenol was locally applied to the nucleus accumbens. Both phentolamine and isoproterenol increased the duration of walking, rearing and grooming and decreased the duration of sitting. Reserpine counteracted the behavioural response elicited by phentolamine but not by isoproterenol. The results of the present study demonstrate that mesolimbic α-adrenoceptors, but not β-adrenoceptors, regulate behaviour that is mediated by reserpine-sensitive storage pools. It is hypothesized that the observed α-adrenoceptor-mediated increase in locomotor activity is due to the α-adrenoceptor-mediated increase in the release of accumbal intravesicular dopamine. Our finding that α-adrenoceptors inhibit, whereas β-adrenoceptors stimulate, locomotor activity may help explain why noradrenaline or environmental stressors have previously been found to have opposing effects on the regulation of behaviour.

A novel automated test battery reveals enduring behavioural alterations and cognitive impairments in survivors of murine pneumococcal meningitis

Pneumococcal meningitis, caused by Streptococcus pneumoniae infection, is a major form of lethal bacterial meningitis. Survivors are predisposed to developing lifelong disabling sequelae, including cognitive impairment, psychological problems and motor deficits. In our experimental model, ventricular inoculation of 10(5) colony-forming units of S. pneumoniae type 3 caused 90% of mice to develop life-threatening meningitis within 48 h. Antibiotic treatment with ceftriaxone 20 h post infection reduced the incidence of severe meningitis to <10%. At the time of treatment, upregulation of pro-inflammatory cytokines was detected, including interleukin-1β, interleukin-6 and tumour necrosis factor. We evaluated the long-term behavioural and cognitive sequelae in control mice and those surviving meningitis using an automated system (the IntelliCage) in which mice perform a range of behavioural and spatial tasks to obtain water rewards from conditioning units in their home cage. Surviving mice showed a number of altered behaviours relative to controls, including (i) hypoexploration when first exposed to the IntelliCage, (ii) altered activity patterns (fewer visits to conditioning stations during the light phase and more in the dark phase), (iii) avoidance of light (a constant or flashing LED stimulus), (iv) impaired spatial learning (a complex patrolling task), and (v) impaired discrimination reversal learning. Overall these results suggest photophobia and weakened learning ability in post-meningitic mice, particularly on tasks engaging hippocampal and prefrontal neural substrates. This study also demonstrates a standardised and comprehensive battery of tests that can be readily used to investigate neurological sequelae in undisturbed mice residing in a complex home cage environment.

Noradrenergic modulation of cognition: therapeutic implications

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

Maternal responsiveness moderates the relationship between allostatic load and working memory

A substantial amount of research has demonstrated the deleterious effects of chronic stress on memory. However, much less is known about protective factors. In the current study we test the role of maternal responsiveness in buffering the effects of childhood allostatic load on subsequent adolescent working memory. Allostatic load is a marker of cumulative stress on the body that is caused by mobilization of multiple physiological systems in response to chronic environmental demands. Results of the study suggest that allostatic load negatively affects working memory, but that this effect is significantly attenuated in children with responsive mothers.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Stress effects on working memory, explicit memory, and implicit memory for neutral and emotional stimuli in healthy men

Stress is a strong modulator of memory function. However, memory is not a unitary process and stress seems to exert different effects depending on the memory type under study. Here, we explored the impact of social stress on different aspects of human memory, including tests for explicit memory and working memory (for neutral materials), as well as implicit memory (perceptual priming, contextual priming and classical conditioning for emotional stimuli). A total of 35 young adult male students were randomly assigned to either the stress or the control group, with stress being induced by the Trier Social Stress Test (TSST). Salivary cortisol levels were assessed repeatedly throughout the experiment to validate stress effects. The results support previous evidence indicating complex effects of stress on different types of memory: A pronounced working memory deficit was associated with exposure to stress. No performance differences between groups of stressed and unstressed subjects were observed in verbal explicit memory (but note that learning and recall took place within 1 h and immediately following stress) or in implicit memory for neutral stimuli. Stress enhanced classical conditioning for negative but not positive stimuli. In addition, stress improved spatial explicit memory. These results reinforce the view that acute stress can be highly disruptive for working memory processing. They provide new evidence for the facilitating effects of stress on implicit memory for negative emotional materials. Our findings are discussed with respect to their potential relevance for psychiatric disorders, such as post traumatic stress disorder.

Linear and non-linear dose-response functions reveal a hormetic relationship between stress and learning

Over a century of behavioral research has shown that stress can enhance or impair learning and memory. In the present review, we have explored the complex effects of stress on cognition and propose that they are characterized by linear and non-linear dose-response functions, which together reveal a hormetic relationship between stress and learning. We suggest that stress initially enhances hippocampal function, resulting from amygdala-induced excitation of hippocampal synaptic plasticity, as well as the excitatory effects of several neuromodulators, including corticosteroids, norepinephrine, corticotropin-releasing hormone, acetylcholine and dopamine. We propose that this rapid activation of the amygdala-hippocampus brain memory system results in a linear dose-response relation between emotional strength and memory formation. More prolonged stress, however, leads to an inhibition of hippocampal function, which can be attributed to compensatory cellular responses that protect hippocampal neurons from excitotoxicity. This inhibition of hippocampal functioning in response to prolonged stress is potentially relevant to the well-described curvilinear dose-response relationship between arousal and memory. Our emphasis on the temporal features of stress-brain interactions addresses how stress can activate, as well as impair, hippocampal functioning to produce a hormetic relationship between stress and learning.

Alpha(2)-noradrenergic antagonist administration into the central nucleus of the amygdala blocks stress-induced hypoalgesia in awake behaving rats

Stress-induced hypoalgesia (SIH) is an adaptive behavioral phenomenon mediated in part by the amygdala. Acute stress increases amygdalar noradrenaline levels and focal application of alpha(2)-adrenoceptor agonists in the central nucleus of the amygdala (CeA) is antinociceptive. We hypothesized that alpha(2)-adrenoceptor antagonist administration into the CeA may block SIH. Bilateral microinjections of drug or saline via chronically implanted CeA cannulae were followed by either a period of restraint stress or rest. The nocifensive paw-withdrawal latency (PWL) to a focused beam of light was measured. PWLs were longer in restrained rats, constituting SIH. Microinjection of the alpha(2)-adrenoceptor antagonist idazoxan into the CeA prior to restraint blocked SIH. Idazoxan administration in unrestrained rats had no effect. Microinjection of the alpha(2)-adrenoceptor agonist clonidine in unrestrained rats caused dose dependent hypoalgesia, mimicking the effects of environmental stress. alpha(2)-Adrenoceptor function in the CeA is necessary for restraint-induced SIH.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

Factors that determine the non-linear amygdala influence on hippocampus-dependent memory

Stressful experiences are known to either improve or impair hippocampal-dependent memory tasks and synaptic plasticity. These positive and negative effects of stress on the hippocampus have been largely documented, however little is known about the mechanism involved in the twofold influence of stress on hippocampal functioning and about what factors define an enhancing or inhibitory outcome. We have recently demonstrated that activation of the basolateral amygdala can produce a biphasic effect, enhancement or inhibition, of hippocampal synaptic plasticity, depending on the timing of activation (priming or spaced activation). A key question is under which conditions do the effects of amygdala activation on hippocampus dependent memory functions change from improvement to impairment of learning and memory. In this chapter we suggest that hippocampal outcome of amygdala activation may be critically dependent on four main factors: (1) The intensity of amygdala activation, (2) the temporal relation between the activation of the amygdala and the hippocampus dependent memory function, (3) the duration of amygdala activation, and (4) the contextual input during the processing of the information.

Decision making, the P3, and the locus coeruleus-norepinephrine system

Psychologists and neuroscientists have had a long-standing interest in the P3, a prominent component of the event-related brain potential. This review aims to integrate knowledge regarding the neural basis of the P3 and to elucidate its functional role in information processing. The authors review evidence suggesting that the P3 reflects phasic activity of the neuromodulatory locus coeruleus-norepinephrine (LC-NE) system. They discuss the P3 literature in the light of empirical findings and a recent theory regarding the information-processing function of the LC-NE phasic response. The theoretical framework emerging from this research synthesis suggests that the P3 reflects the response of the LC-NE system to the outcome of internal decision-making processes and the consequent effects of noradrenergic potentiation of information processing.

An integrative theory of locus coeruleus-norepinephrine function: adaptive gain and optimal performance

Historically, the locus coeruleus-norepinephrine (LC-NE) system has been implicated in arousal, but recent findings suggest that this system plays a more complex and specific role in the control of behavior than investigators previously thought. We review neurophysiological and modeling studies in monkey that support a new theory of LC-NE function. LC neurons exhibit two modes of activity, phasic and tonic. Phasic LC activation is driven by the outcome of task-related decision processes and is proposed to facilitate ensuing behaviors and to help optimize task performance (exploitation). When utility in the task wanes, LC neurons exhibit a tonic activity mode, associated with disengagement from the current task and a search for alternative behaviors (exploration). Monkey LC receives prominent, direct inputs from the anterior cingulate (ACC) and orbitofrontal cortices (OFC), both of which are thought to monitor task-related utility. We propose that these frontal areas produce the above patterns of LC activity to optimize utility on both short and long timescales.

Direct and indirect inhibition by catecholamines of hypocretin/orexin neurons

Hypothalamic hypocretin enhances arousal, similar to the actions of norepinephrine (NE). The physiological actions of NE were examined in hypocretin neurons identified by selective green fluorescent protein expression in transgenic mouse hypothalamic slices using whole-cell recording. NE induced an outward current, inhibited spike frequency, and hyperpolarized hypocretin neurons dose dependently. Similar actions were evoked by the selective alpha2 adrenergic agonist clonidine. The alpha2 antagonist idazoxan increased spike frequency, suggesting tonic NE-mediated inhibition. The NE-induced current was inwardly rectified, and the reversal potential was dependent on external potassium concentration; it was blocked by barium in the bath and by GTP-gamma-S in the pipette, suggesting activation of a G-protein inward rectifying K+ (GIRK) current. NE and clonidine decreased calcium currents evoked by depolarizing voltage steps. The selective alpha1 adrenergic agonist phenylephrine had no effect on membrane potential but did increase IPSC frequency; miniature IPSC frequency was also increased, in some cells without any effect on amplitude, suggesting a facilitative presynaptic action at alpha1 receptors on GABAergic axons that innervate hypocretin neurons. NE therefore inhibits hypocretin neurons directly through two mechanisms: activation of a GIRK current, depression of calcium currents, and indirectly through increased inhibitory GABA input. Similar to NE, dopamine and epinephrine reduced or blocked spikes and, in the presence of TTX, showed direct hyperpolarizing actions. The action of dopamine was blocked by the D2 receptor antagonist eticlopride, whereas a D1/5 antagonist had no effect. These data suggest that catecholamines evoke strong inhibitory actions on hypocretin neurons and suggest negative feedback from catecholamine cells that may be excited by hypocretin.

A facilitative role for corticosterone in the acquisition of a spatial task under moderate stress

Emotionally charged experiences alter memory storage via the activation of hormonal systems. Previously, we have shown that compared with rats trained for a massed spatial learning task in the water maze in warm water (25 degrees C), animals that were trained in cold water (19 degrees C) performed better and showed higher levels of the stress hormone corticosterone. Here, we examined whether manipulating the levels of corticosterone can determine the strength of spatial information acquisition and retention. Rats were injected with metyrapone (25, 50, and 75 mg/kg, i.p.) or with corticosterone (10 and 25 mg/kg, i.p.) and trained in a massed spatial task in either cold (19 degrees C) or warm (25 degrees C) water. We found that whereas animals injected with vehicle performed well in the spatial task in cold water (moderate stress), rats injected with the intermediate metyrapone dose showed impairment in performance. Moreover, whereas animals injected with vehicle on average did not perform well in warm water (mild stress), rats injected with the lower corticosterone dose showed improvement in performance in warm water. These two mirror experiments of corticosterone blockade and enhancement strongly suggest that corticosterone is instrumental in the acquisition and retention of the spatial learning task.

High and low responders to novelty and mesolimbic noradrenaline: effects of noradrenergic agents on radial-maze performance

The authors used high and low responders to novelty (HRs and LRs, respectively) to examine the effects of noradrenergic injections into the nucleus accumbens using a special radial-maze task. During the 5 successive test days, solvent-treated HRs acquired this task faster than LRs. Isoproterenol (beta-agonist) combined with phenylephrine (alpha-agonist) improved acquisition in LRs but not in HRs; this effect was counteracted by propranolol (beta-antagonist) and phentolamine (alpha-antagonist). Propranolol combined with phentolamine, as well as phentolamine alone, disrupted acquisition in HRs but not in LRs. Data show that the effects of noradrenergic agents in HRs and LRs are due to differences in acquisition directed by type-specific differences in functional mesolimbic noradrenaline.

Behavioral effects of exposure to endosulfan and methyl parathion in adult rats

Endosulfan (ES) and methyl parathion (MP) are widely used in Latin America, and simultaneous exposure to both products is documented. This exposure may have effects on the nervous system because their targets include the GABAergic and cholinergic systems, which are main modulators of neuronal excitability in the cortex and hippocampus. We tested whether low-level, repeated exposure of adult rats to commercial formulations containing ES and MP disrupts spatial learning in the water maze. Five groups of eight animals received subcutaneously appropriate dilutions of the commercial formulations to yield the following treatments during 10 days: saline, 25 mg/kg ES, 2 mg/kg MP (MP(2)), 25 mg/kg ES plus 1 mg/kg MP (ES+MP(1)) and 25 mg/kg ES plus 2 mg/kg MP (ES+MP(2)). In addition, markers of neurological function, renal and hepatic damage were explored as potential consequences of exposure. In the absence of overt toxicity, the groups exposed to the ES plus MP showed significantly longer escape latencies, higher number of failures to reach the platform and more time in the periphery of the tank than the control and single-exposed groups. This finding shows that commercial formulations of ES and MP have marginal effects when administered individually but can produce behavioral alterations when given in combination.

Urocortin shares the memory modulating effects of corticotropin-releasing factor (CRF): mediation by CRF1 receptors

Intracerebroventricular (i.c.v.) administration of corticotropin-releasing factor (CRF) biphasically affects performance in tests of learning and memory. In the present study, we used CRF, urocortin (Ucn), a recently cloned CRF homologue, and CRF receptor antagonists, to determine which CRF receptor subtype(s) mediate the memory modulating effects of CRF receptor agonists in male Wistar rats. Under difficult learning conditions (massed trials), i.c.v. pretreatment with CRF or Ucn facilitated the acquisition of spatial navigation in the Morris water maze in a non-dose-dependent fashion (optimal doses of 0.1 and 0.03 microg, respectively). Under less difficult learning conditions (spaced trials), both peptides impaired water maze performance. In addition, with i.c.v. posttraining treatment, the peptides were equipotent (1.0 microg) in facilitating the consolidation of passive avoidance learning. The performance-enhancing effects of Ucn in both water maze and passive avoidance paradigms were reversed by i.c.v. pretreatment with D-Phe CRF(12-41) (2.5, 5 microg), a broad CRF(1)/CRF(2) receptor antagonist, or antalarmin (10 microg), a potent, nonpeptide, CRF(1) selective receptor antagonist. Thus, Ucn shares CRF's memory-modulating effects, and these effects appear to be mediated via the CRF(1) receptor. These findings are consistent with the hypothesis that CRF receptor agonists affect performance in tests of learning and memory by increasing arousal.

Noradrenergic regulation of hippocampal place cells

The influence of noradrenergic input to the hippocampus was assessed by recording hippocampal place cells and by modulating the noradrenergic tone with a selective agonist and antagonist of the alpha2-autoreceptors. The rats received intraperitoneal injection of 5 microg/kg of dexmedetomidine (DEX), an alpha2-agonist, 0.2 mg/kg of atipamezole (ATI), an alpha2-antagonist, or saline. Hippocampal place cells were recorded in a familiar rectangular environment and in three types of new environments (rectangle, square, and cylinder). Recordings in the familiar environment were conducted in two phases, either before (early phase) or after (late phase) the exposure to new environments. In the familiar environment, DEX significantly increased the mean firing rate of hippocampal interneurons, while ATI increased the mean firing rate of pyramidal cells. Only ATI in the early phase of the experiment impaired spatial selectivity. Both drugs induced field rotations in the early phase of the study, but in the late phase DEX decreased, while ATI increased field stability in the familiar environment. The drug effects in the new environment were dependent on degree of novelty. No difference between treatments was observed in the new rectangle, but in the square and cylinder, ATI increased the mean firing rate, number of fields, and field area compared to other treatments. Stability of the original firing fields in the familiar rectangle was dependent on type of interfering environment and drug treatment. Exposure to another rectangle had the smallest impact, and exposure to a square the largest impact, on the original field pattern. ATI impaired stability of the original field after exposure to a rectangular and cylinder, while the impairing effect of DEX was only observed after exposure to a cylinder. In conclusion, increased noradrenergic tone increases the firing rate of hippocampal place cells, especially when the experimental situation and environment are new, but this increase is spatially nonselective. Furthermore, manipulation of the noradrenergic tone in either direction leads to instability of firing fields.

Water maze versus radial maze: differential performance of rats in a spatial delayed match-to-position task and response to scopolamine

Studies rarely assess treatment effects across tasks; the present experiments addressed this issue. In Experiments 1 and 2, rats (n=12) were trained and then tested with variable delays on a spatial match-to-position task sequentially in the water and radial mazes (in counterbalanced order). Experiment 1 compared the effect of 0-, 60- and 1440-min delays on performance in both mazes. Rats required fewer (P<0.05) mean (+/-S.E.M.) sessions to reach criterion performance in the water (11.0+/-1.0) versus radial maze (19.3+/-2.2). In test sessions, performance was impaired delay-dependently when scores were averaged across the two tasks (P<0.05) but no significant effect of task or task x delay interaction was found. In the second experiment, the same rats were retrained and tested with 0-, 1-, 3- and 5-min delays in both mazes and testing followed the administration of scopolamine (0, 0.1, 0.4 and 0.8 mg/kg). The mean (+/-S.E.M.) number of acquisition sessions was similar in the radial (6.33+/-0.34) and water maze (6.08+/-0.46). On the sample portion of trials, performance was impaired at the 0.8 mg/kg dose of scopolamine (P<0.02) in the radial maze only. On the recognition portion of trials in the radial maze, the 0.4 and 0.8 mg/kg doses of scopolamine impaired performance whereas in the water maze task the 0.8 mg/kg dose impaired performance. The pattern of results may reflect different natural tendencies of rats to alternate (win-shift) versus not alternate (win-stay) in dry land versus swim tasks.

Effects of lesioning noradrenergic neurones in the locus coeruleus on conditioned and unconditioned aversive behaviour in the rat

1. The brain noradrenergic system may have a role in anxiety disorder. This study has examined the effect of bilateral 6-hydroxydopamine lesions of the noradrenergic neurones in the locus coeruleus (LC) of male Lister hooded rats on behaviour produced by unconditioned and conditioned aversive stimuli. 2. The 6-hydroxydopamine (4 microg) lesions markedly reduced the noradrenaline content of the locus coeruleus hypothalamus, frontal cortex and the periaqueductal grey area without altering the levels of either dopamine or 5-hydroxytryptamine measured 14 days after administration. 3. Exposure to ultrasound (20 kHz at 98 dB for 60 sec), an unconditioned aversive stimulus, induced a defence response in the rats characterised by an increase in activity (running and jumping) followed by a period of inactivity (freezing). 4. Lesioning of the LC significantly attenuated the duration of freezing but was without effect on the active phase of the response. A similar reduction in freezing behaviour was seen with LC lesions when rats were exposed (3 hours after the acquisition) to the contextual cue of the conditioned emotion response paradigm. 5. These findings confirm that the locus coeruleus is involved in the regulation of fear-related behaviour in the rat both in an unconditioned and a conditioned model. Furthermore the results indicate that noradrenaline modifies defence behaviour rather than being the principle activating mechanism.

A behavioral study of alpha-1b adrenergic receptor knockout mice: increased reaction to novelty and selectively reduced learning capacities

Knockout mice lacking the alpha-1b adrenergic receptor were tested in behavioral experiments. Reaction to novelty was first assessed in a simple test in which the time taken by the knockout mice and their littermate controls to enter a second compartment was compared. Then the mice were tested in an open field to which unknown objects were subsequently added. Special novelty was introduced by moving one of the familiar objects to another location in the open field. Spatial behavior and memory were further studied in a homing board test, and in the water maze. The alpha-1b knockout mice showed an enhanced reactivity to new situations. They were faster to enter the new environment, covered longer paths in the open field, and spent more time exploring the new objects. They reacted like controls to modification inducing spatial novelty. In the homing board test, both the knockout mice and the control mice seemed to use a combination of distant visual and proximal olfactory cues, showing place preference only if the two types of cues were redundant. In the water maze the alpha-1b knockout mice were unable to learn the task, which was confirmed in a probe trial without platform. They were perfectly able, however, to escape in a visible platform procedure. These results confirm previous findings showing that the noradrenergic pathway is important for the modulation of behaviors such as reaction to novelty and exploration, and suggest that this is mediated, at least partly, through the alpha-1b adrenergic receptors. The lack of alpha-1b adrenergic receptors in spatial orientation does not seem important in cue-rich tasks but may interfere with orientation in situations providing distant cues only.

Retrieval of spatial information in Nijmegen high and low responders: involvement of beta-adrenergic mechanisms in the nucleus accumbens

Rats selected on the basis of their apomorphine susceptibility or on the basis of their response to novelty were used to investigate the role of mesolimbic adrenoceptors in the retrieval of spatial information in the water maze task. Apomorphine-susceptible rats and high responders to novelty (HR) showed a relatively bad retrieval during the 1st trial of the 2nd day compared with apomorphine-unsusceptible rats and low responders to novelty (LR). Intra-accumbens injections of different doses of the beta-agonist isoproterenol, given just before that trial, disrupted the retrieval in LR rats but not in HR rats. The beta-antagonist propranolol, which itself did not have any direct effect, inhibited the effect of isoproterenol in LR rats. The effect of isoproterenol in LR rats was not modified by alpha-agents. It is concluded that the beta-adrenergic, but not alpha-adrenergic, activity in the nucleus accumbens directs the individual-specific ability to retrieve spatial information under stressful conditions.

Long-term deprivation of oestrogens by ovariectomy potentiates beta-amyloid-induced working memory deficits in rats

1 In the present study, we examined whether deprivation of oestrogens by ovariectomy could modify learning and memory deficits caused by a continuous intracerebroventricular (i.c.v.) infusion of amyloid beta-peptide (Abeta), the major constituent of senile plaques in AD. 2 Neither long-term (3 months) nor short-term (1 month), deprivation of oestrogens by ovariectomy caused a significant impairment in spatial learning and memory in a water maze and spontaneous alternation behaviour in a Y-maze. 3 A continuous i.c.v. infusion of Abeta-(1-42) caused spatial learning and memory deficits in both ovariectomized and sham-operated rats. 4 The Abeta-induced working memory deficits were significantly potentiated in ovariectomized rats compared with sham-operated rats when mnemonic ability was examined 3 months after ovariectomy. 5 These results suggest that long-term deprivation of oestrogens induced by ovariectomy increases susceptibility to memory deficits produced by Abeta-(1-42) in rats.

Stress impairs performance in spatial water maze learning tasks

The water maze task has been developed to test spatial learning abilities in rats or mice, and is widely used. Though it has been reported before that numerous cognitive abilities are of importance for learning this task, poor performance is usually interpreted as an impairment of spatial memory formation. Previous investigations that tried to correlate long-term potentiation (LTP) of synaptic transmission with spatial learning abilities in rats reported that injection of drugs or specific gene deletions which blocked the expression of LTP correlated with learning impairments of spatial tasks in a water maze. Recent studies, however, have shown that pretraining enables these animals to learn such spatial tasks even though LTP was still found to be blocked. I investigated to what degree altered fear condition and stress perception could account for the impaired spatial learning when no pretraining is given. In a fear habituation task, unhandled rats preferred a dark over a well lit chamber more than handled animals did, but unhandled rats favoured the lit chamber more in an active avoidance task. They also performed poorly in a spatial water maze task compared with handled rats. Rats pretrained in a radial arm maze performed better in a water maze than non-pretrained rats. No difference between groups was found in a non-spatial water maze task. On the other hand, when pretrained in a water maze, rats performed only marginally better in a radial arm maze compared to non-pretrained animals. Since animals have to be handled to learn a radial arm maze, the difference in this task was not due to stress but most probably due to getting accustomed to the room dimensions prior to learning the spatial task. The results suggest that impaired learning of spatial tasks in the water maze can be due to increased stress and decreased fear conditioning without actually affecting spatial learning abilities. These results question the interpretations of the results of some previously published results of spatial water maze tasks.

Metabotropic glutamate receptor activation and blockade: their role in long-term potentiation, learning and neurotoxicity

Metabotropic glutamate receptors represent a fairly recent addition to the family of glutamate receptors. These receptors have the distinguishing feature of being coupled to G-proteins rather than ion channels and they appear to have a variety of functional characteristics. These receptors play a vital role, for example, in the induction and maintenance of long-term potentiation, the most popular current model of the biological correlates of learning and memory. Blockade of metabotropic glutamate receptors prevents long-term potentiation induction and learning in a variety of tasks in different species. Chronic metabotropic glutamate receptor activation is also associated with neurodegeneration and selective neuronal loss when agonists of these receptors are injected in high concentrations directly into the brain. Metabotropic glutamate receptors also play a role in the normal development of the nervous system and these sites within the central nervous system offer possible routes for drug therapies; selective receptor antagonists, for example, may prove to have the very desirable feature of endowing neuroprotection during ischaemic episodes whilst allowing normal excitatory neurotransmission to occur.

The role of locus coeruleus in the regulation of cognitive performance

Noradrenergic locus coeruleus (LC) neurons were recorded in monkeys performing a visual discrimination task, and a computational model was developed addressing the role of the LC brain system in cognitive performance. Changes in spontaneous and stimulus-induced patterns of LC activity correlated closely with fluctuations in behavioral performance. The model explains these fluctuations in terms of changes in electrotonic coupling among LC neurons and predicts improved performance during epochs of high coupling and synchronized LC firing. Cross correlations of simultaneously recorded LC neurons confirmed this prediction, indicating that electrotonic coupling in LC may play an important role in attentional modulation and the regulation of goal-directed versus exploratory behaviors.

Protective effects of idebenone and alpha-tocopherol on beta-amyloid-(1-42)-induced learning and memory deficits in rats: implication of oxidative stress in beta-amyloid-induced neurotoxicity in vivo

Amyloid beta-peptide (A beta), the major constituent of the senile plaques in the brains of patients with Alzheimer's disease, is cytotoxic to neurons and has a central role in the pathogenesis of the disease. Previous studies have suggested that oxidative stress is involved in the mechanisms of A beta-induced neurotoxicity in vitro. In the present study, we examined whether oxidative stress contributes to learning and memory deficits caused by continuous intracerebroventricular infusion of A beta-(1-42). In the A beta-(1-42)-infused rats, spontaneous alternation behaviour in a Y-maze and spatial memory in a water maze task were significantly impaired, as compared with A beta-(40-1)-infused control rats. The retention of passive avoidance learning was also significantly impaired by treatment with A beta-(1-42). Potent antioxidants idebenone and alpha-tocopherol prevented the behavioural deficits in Y-maze and water maze, but not passive avoidance, tasks in A beta-(1-42)-infused rats when they were repeatedly administered by mouth once a day from 3 days before the start of A beta infusion to the end of behavioural experiments. Lipid peroxide levels in the hippocampus and cerebral cortex of A beta-(1-42)-infused rats did not differ from those in control animals, and neither idebenone nor alpha-tocopherol affected the lipid peroxide levels. These results suggest that treatment with antioxidants such as idebenone and alpha-tocopherol prevents learning and memory deficits caused by A beta.

Long-term monoamine depletion, differential recovery, and subtle behavioral impairment following methamphetamine-induced neurotoxicity

Squads of rats were assayed at three intervals following MA-induced neurotoxicity to investigate the persistence of monoamine deficits, the potential for monoamine recovery, and spatial task abilities. At 48, 139, and 237 days postinjection, MA animals showed significant monoamine depletions compared with controls. Investigating percent depletions (MA/control) across time showed monoamine recovery in some structures. Initially, 5-HT within medial prefrontal cortex (MPFC), caudate (CdN), and hippocampus (HPC) was reduced to 30% of control levels. By 237 days, MPFC and CdN levels were elevated to 70%. Similarly, initial CdN DA reductions (30% of control levels) showed recovery to 80% by 237 days. These findings support neurochemical recovery following MA neurotoxicity. However, the persistent depression of HPC 5-HT suggests that not all structures recover equally. The HPC did show elevated turnover (metabolite/neurotransmitter) over time, suggesting a unique compensatory response. MA treatment also produced an impairment in the Morris water-maze place task at 65 days postinjection. No impairments were observed in water-maze moving platform or place task at 79 and 165 days postinjection, respectively, or in T-maze alternation. The possibility that partial recovery in tissue monamine levels underlies the sparing of function and behavioral improvement is discussed.

Septal lesions impair the acquisition of a cued place navigation task: attentional or memory deficit?

These experiments were designed to analyze how medial septal lesions reducing the cholinergic innervation in the hippocampus might affect place learning. Rats with quisqualic lesions of the medial septal area (MS) were trained in a water maze and on a homing table where the escape position was located at a spatially fixed position and further indicated by a salient cue suspended above it. The lesioned rats were significantly impaired in reaching the cued escape platform during training. In addition rats, did not show any discrimination of the training sector during a probe trial in which no platform or cue was present. This impairment remained significant during further training in the absence of the cue. When the cued escape platform was located at an unpredictable spatial location, the MS-lesioned rats showed no deficit and spent more time under the cue than control rats during the probe trial. On the homing board, with a salient object in close proximity to the escape hole, the MS rats showed no deficit in escape latencies, although a significant reduction in spatial memory was observed. However, this was overcome by additional training in the absence of the cue. Under these conditions, rats with septal lesions were prone to develop a pure guidance strategy, whereas normal rats combined a guidance strategy with a memory of the escape position relative to more distant landmarks. The presence of a salient cue appeared to decrease attention to environmental landmarks, thus reducing spatial memory. These data confirm the general hypothesis that MS lesions reduce the capacity to rely on a representation of the relation between several landmarks with different salience.

Panic disorder: a theoretical synthesis of medical and psychological approaches

Panic disorder is a common and disabling condition which frequently leads to excessive reliance upon medical facilities. It is also closely associated with the development of agoraphobia. Medical approaches implicate disturbances of ascending brain noradrenergic and serotonergic systems, and support related pharmacotherapies. Contemporary psychological approaches focus upon misinterpretations of bodily sensations and an undue appreciation of the risk of life-threatening illness, and support cognitive/behavioral psychotherapies. A synthesis is possible by developing the view that the implicated ascending aminergic systems normally play a part in "effortful" or context-sensitive behavior. A relative failure of this under conditions of heightened arousal might be responsible for the rigid patterns of fear, belief, and behavior that characterize these patients. Clinical and research implications are discussed.

Conditioned responses of monkey locus coeruleus neurons anticipate acquisition of discriminative behavior in a vigilance task

Impulse activity was recorded extracellularly from noradrenergic neurons in the nucleus locus coeruleus of three cynomolgus monkeys performing a visual discrimination (vigilance) task. For juice reward, the subjects were required to release a lever rapidly in response to an improbable target stimulus (20% of trials) that was randomly intermixed with non-target stimuli presented on a video display. All locus coeruleus neurons examined were phasically and selectively activated by target stimuli in this task. Other task events elicited no consistent response from these neurons (juice reward, lever release, fix spot stimuli, non-target stimuli). With reversal of the task contingency, locus coeruleus neurons ceased responding to the former target stimuli, and began responding instead to the new target (old non-target) stimuli. In addition, the latency of locus coeruleus response to target stimuli increased after reversal (by about 140 ms) in parallel with a similar increase in the latency of the behavioral response. These results indicate that the conditioned locus coeruleus responses reflect stimulus meaning and cognitive processing, and are not driven by physical sensors attributes. Notably, the reversal in locus coeruleus response to stimuli after task reversal occurred rapidly, hundreds of trials before reversal was expressed in behavioral responses. These findings indicate that conditioned responses of locus coeruleus neurons are plastic and easily altered by changes in stimulus meaning, and that the locus coeruleus may play an active role in learning the significance of behaviorally important stimuli.

Experience-dependent facilitating effect of corticosterone on spatial memory formation in the water maze

Stress-related adrenal steroid hormones modulate brain and cognitive function. Electrophysiological studies, including primed burst potentiation and long-term potentiation, have indicated concentration-dependent inverted U-shape effects of corticosterone in hippocampal function and plasticity. Here, we explored the role of corticosterone in the consolidation and long-term retrieval of spatial learning in the Morris water maze task in rats. We postulated that corticosterone actions might be experience-dependent with regard to stimulus intensity, such as differential water temperatures. Indeed, rats trained at 19 degrees C showed a quicker rate of acquisition and better long-term retention than rats trained at 25 degrees C water. In addition, post-training corticosterone levels, on the first training day, were significantly higher in rats in the 19 degrees C group than in the 25 degrees C group. Performance of rats trained at 25 degrees C, but not at 19 degrees C, water was improved by injecting them i.p. with corticosterone immediately after each training session. Thus, the effect of exogenously administered corticosterone appears to be experience-dependent, with the experience-induced corticosterone concentrations as a critical factor determining the cognitive consequences of steroid treatment. Therefore, this work indicates a facilitating corticosterone action, during the post-training period, on the neural mechanisms determining the strength of information storage under acute, physiological conditions.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Dose-dependent suppression of adrenocortical activity with metyrapone: effects on emotion and memory

Different levels of circulating corticosterone are considered to produce different emotional states and effects on learning and memory. The purpose of the present study was to use different doses of the 11-beta-hydroxylase inhibitor metyrapone to produce dose-dependent inhibition of the synthesis of corticosterone and examine the consequences of that on several cognitive and emotional parameters. Systemic (SC) injections of metyrapone (25 or 50 mg/kg) dose-dependently suppressed increases in plasma concentrations of corticosterone induced by spatial training in a water maze, but did not affect plasma corticosterone levels in non-stressed rats. Treatment with the higher and lower dose of metyrapone also differentially affected behavioral measures of emotion and memory. Administration of 50 mg/kg, but not 25 mg/kg, of metyrapone impaired acquisition performance in the spatial water maze task. Both doses of metyrapone impaired retention. The impairment in retention was attenuated by dexamethasone (0.3 mg/kg) given systemically immediately after training, but not by corticosterone (0.3 mg/kg). During the exposure to a conditioned stressor of inescapable footshock, the higher, but not the lower dose of metyrapone attenuated fear-induced immobility. In contrast, the lower, but not the higher dose attenuated the anxiety state in an elevated plus-maze in a novel environment immediately after exposure to the conditioned stressor. It is suggested that emotion, learning, and memory are differentially affected by the different doses of metyrapone due to interference with different types of adrenal steroid receptors and consequent induction of various corticosterone receptor states.

Alpha 2-adrenoceptor antagonists potentiate acetylcholinesterase inhibitor effects on passive avoidance learning in the rat

The cholinergic hypothesis of Alzheimer's disease (AD) has strongly influenced research on learning and memory over the last decade. However, there has been limited success treating AD dementia with cholinomimetics. Furthermore, there are indications that other neurotransmitter systems affected by this disease may be involved in cognitive processes. Animal studies have suggested that norepinephrine and acetylcholine may interact in learning and memory. The current experiments investigate this interaction in a step-down passive avoidance paradigm after coadministration of acetylcholinesterase inhibitors and alpha 2-adrenoceptor antagonists. Administration of acetylcholinesterase inhibitors heptylphysostigmine (0.625-5.0 mg/kg, IP), tacrine (2.5-10.0 mg/kg, PO), velnacrine (0.312-2.5 mg/kg, SC), and galanthamine (0.312-2.5 mg/kg IP) each enhanced retention of a passive avoidance response at selected moderate doses administered 30-60 min prior to training. The alpha 2-adrenoceptor antagonists idazoxan (0.312-2.5 mg/kg, IP), yohimbine (0.078-0.312 mg/kg, IP) and P86 7480 (0.156-0.625 mg/kg, IP) alone failed to enhance learning in this paradigm. Coadministration of a subthreshold dose of heptylphysostigmine (0.625 mg/kg, IP) with doses of idazoxan, yohimbine or P86 7480 enhanced passive avoidance learning. This synergistic interaction may represent effects of antagonism of presynaptic alpha 2-adrenoceptor since coadministration of heptylphysostigmine and the selective postsynaptic alpha 2-adrenoceptor antagonist SKF 104856 did not result in enhanced learning. Taken together these data suggest noradrenergic activation through pre-synaptic alpha 2-adrenoceptor blockade may potentiate cholinergic activity in the formation of a long-term memory trace. These observations may have implications for the treatment of AD with cholinergic and adrenergic agents.

Systemic idazoxan impairs performance in a non-reversal shift test: implications for the role of the central noradrenergic systems in selective attention

Two experiments examined the effect of pharmacological stimulation of the locus coeruleus in a non-reversal shift paradigm to test predictions concerning the role of the ceruleo-cortical noradrenergic system in processes of selective attention. Food-deprived rats were trained to make either visual (experiment 1) or spatial (experiment 2) discriminations in a parallel alley with both sets of cues being present at all times. Two groups of rats received treatments of either 2 mg/kg i.p. of the selective α(2) adrenoceptor antagonist idazoxan or saline control injections before each daily block of trials. Following attainment of criterion, the reinforcement contingencies were altered according to a non-reversal shift design, so that the alternative (i.e. either spatial or visual) set of cues now predicted reward. Rats treated with idazoxan were not impaired in the acquisition of either the spatial or visual discrimination task, but they were impaired in both forms of non-reversal shift. These deficits are interpreted as resulting from narrowed attention in idazoxan-treated rats, thus supporting a selective attention hypothesis of locus coeruleus function.