Single cell responses in cat visual cortex to visual stimulation during iontophoresis of noradrenaline

Probing the structure and function of locus coeruleus projections to CNS motor centers

The brainstem nucleus locus coeruleus (LC) sends projections to the forebrain, brainstem, cerebellum and spinal cord and is a source of the neurotransmitter norepinephrine (NE) in these areas. For more than 50 years, LC was considered to be homogeneous in structure and function such that NE would be released uniformly and act simultaneously on the cells and circuits that receive LC projections. However, recent studies have provided evidence that LC is modular in design, with segregated output channels and the potential for differential release and action of NE in its projection fields. These new findings have prompted a radical shift in our thinking about LC operations and demand revision of theoretical constructs regarding impact of the LC-NE system on behavioral outcomes in health and disease. Within this context, a major gap in our knowledge is the relationship between the LC-NE system and CNS motor control centers. While we know much about the organization of the LC-NE system with respect to sensory and cognitive circuitries and the impact of LC output on sensory guided behaviors and executive function, much less is known about the role of the LC-NE pathway in motor network operations and movement control. As a starting point for closing this gap in understanding, we propose using an intersectional recombinase-based viral-genetic strategy TrAC (Tracing Axon Collaterals) as well as established ex vivo electrophysiological assays to characterize efferent connectivity and physiological attributes of mouse LC-motor network projection neurons. The novel hypothesis to be tested is that LC cells with projections to CNS motor centers are scattered throughout the rostral-caudal extent of the nucleus but collectively display a common set of electrophysiological properties. Additionally, we expect to find these LC projection neurons maintain an organized network of axon collaterals capable of supporting selective, synchronous release of NE in motor circuitries for the purpose of coordinately regulating operations across networks that are responsible for balance and movement dynamics. Investigation of this hypothesis will advance our knowledge of the role of the LC-NE system in motor control and provide a basis for treating movement disorders resulting from disease, injury, or normal aging.

Effect of the Trigeminal Nerve Stimulation on Auditory Event-Related Potentials

Trigeminal sensorimotor activity stimulates arousal and cognitive performance, likely through activation of the locus coeruleus (LC). In this study we investigated, in normal subjects, the effects of bilateral trigeminal nerve stimulation (TNS) on the LC-dependent P300 wave, elicited by an acoustic oddball paradigm. Pupil size, a proxy of LC activity, and electroencephalographic power changes were also investigated. Before TNS/sham-TNS, pupil size did not correlate with P300 amplitude across subjects. After TNS but not sham-TNS, a positive correlation emerged between P300 amplitude and pupil size within frontal and median cortical regions. TNS also reduced P300 amplitude in several cortical areas. In both groups, before and after TNS/sham-TNS, subjects correctly indicated all the target stimuli. We propose that TNS activates LC, increasing the cortical norepinephrine release and the dependence of the P300 upon basal LC activity. Enhancing the signal-to-noise ratio of cortical neurons, norepinephrine may improve the sensory processing, allowing the subject to reach the best discriminative performance with a lower level of neural activation (i.e., a lower P300 amplitude). The study suggests that TNS could be used for improving cognitive performance in patients affected by cognitive disorders or arousal dysfunctions.

Ocular dominance plasticity: Molecular mechanisms revisited

Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition-driven disparity. Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP-promoting effect of NA is extended to the adult with age-related limitations. Following the enhanced NA availability, the chain events downstream lead to the β-adrenoreceptor-induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long-term memory. In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: (a) Are we testing ODP per se? (b) What does monocular deprivation deprive of the immature cortex? (c) The critical importance of binocular competition, (d) What is the adult plasticity? (e) Excitation-Inhibition balance in local circuits, and (f) Species differences in the animal models.

Bidirectional pharmacological perturbations of the noradrenergic system differentially affect tactile detection

The brain neuromodulatory systems heavily influence behavioral and cognitive processes. Previous work has shown that norepinephrine (NE), a classic neuromodulator mainly derived from the locus coeruleus (LC), enhances neuronal responses to sensory stimuli. However, the role of the LC-NE system in modulating perceptual task performance is not well understood. In addition, systemic perturbation of NE signaling has often been proposed to specifically target the LC in functional studies, yet the assumption that localized (specific) and systemic (nonspecific) perturbations of LC-NE have the same behavioral impact remains largely untested. In this study, we trained mice to perform a head-fixed, quantitative tactile detection task, and administered an α2 adrenergic receptor agonist or antagonist to pharmacologically down- or up-regulate LC-NE activity, respectively. We addressed the outstanding question of how bidirectional perturbations of LC-NE activity affect tactile detection, and tested whether localized and systemic drug treatments exert the same behavioral effects. We found that both localized and systemic suppression of LC-NE impaired tactile detection by reducing motivation. Surprisingly, while locally activating LC-NE enabled mice to perform in a near-optimal regime, systemic activation impaired behavior by promoting impulsivity. Our results demonstrate that localized silencing and activation of LC-NE differentially affect tactile detection, and that localized and systemic NE activation induce distinct behavioral changes.

Locus coeruleus-norepinephrine modulation of sensory processing and perception: A focused review

The locus coeruleus-norepinephrine (LC-NE) system is involved in many brain functions and neurological disorders. In this review we discuss how LC-NE signaling affects the activity of cortical and subcortical sensory neurons, and how it influences perception-driven behaviors associated with mammalian somatosensory, visual, auditory, and olfactory systems. We summarize the consistent as well as seemingly inconsistent findings across brain areas and sensory modalities and propose a framework to understand these phenomena from the perspective of adrenergic receptor expression, dose-dependent physiology and excitation-inhibition balance. We also discuss potential future research directions in this field.

Adrenergic Modulation of Visually-Guided Behavior

Iontophoretic application of norepinephrine (NE) into the primary visual cortex (V1) in vivo reduces spontaneous and evoked activity, without changing the functional selectivity of cortical units. One possible consequence of this phenomenon is that adrenergic receptors (ARs) regulate the signal-to-noise ratio (SNR) of neural responses in this circuit. However, despite such strong inhibitory action of NE on neuronal firing patterns in V1, its specific action on visual behavior has not been studied. Furthermore, the majority of observations regarding cortical NE from in vivo recordings have been performed in anesthetized animals and have not been tested behaviorally. Here, we describe how micro-infusion of AR agonists/antagonists into mouse V1 influences visually-guided behavior at different contrasts and spatial frequencies. We found that cortical activation of α₁- and β-AR produced a substantial reduction in visual discrimination performance at high contrasts and low spatial frequencies, consistent with a divisive effect. This reduction was reversible and was accompanied by a rise in escape latencies as well as an increase in the group averaged choice variance as a function of stimulus contrast. We conclude that pharmacological activation of cortical AR regulates visual perception and adaptive behavior through a divisive gain control of visual responses.

The homotopic connectivity of the functional brain: a meta-analytic approach

Homotopic connectivity (HC) is the connectivity between mirror areas of the brain hemispheres. It can exhibit a marked and functionally relevant spatial variability, and can be perturbed by several pathological conditions. The voxel-mirrored homotopic connectivity (VMHC) is a technique devised to enquire this pattern of brain organization, based on resting state functional connectivity. Since functional connectivity can be revealed also in a meta-analytical fashion using co-activations, here we propose to calculate the meta-analytic homotopic connectivity (MHC) as the meta-analytic counterpart of the VMHC. The comparison between the two techniques reveals their general similarity, but also highlights regional differences associated with how HC varies from task to rest. Two main differences were found from rest to task: (i) regions known to be characterized by global hubness are more similar than regions displaying local hubness; and (ii) medial areas are characterized by a higher degree of homotopic connectivity, while lateral areas appear to decrease their degree of homotopic connectivity during task performance. These findings show that MHC can be an insightful tool to study how the hemispheres functionally interact during task and rest conditions.

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

Noradrenaline Modulates Visual Perception and Late Visually Evoked Activity

An identical sensory stimulus may or may not be incorporated into perceptual experience, depending on the behavioral and cognitive state of the organism. What determines whether a sensory stimulus will be perceived? While different behavioral and cognitive states may share a similar profile of electrophysiology, metabolism, and early sensory responses, neuromodulation is often different and therefore may constitute a key mechanism enabling perceptual awareness. Specifically, noradrenaline improves sensory responses, correlates with orienting toward behaviorally relevant stimuli, and is markedly reduced during sleep, while experience is largely "disconnected" from external events. Despite correlative evidence hinting at a relationship between noradrenaline and perception, causal evidence remains absent. Here, we pharmacologically down- and upregulated noradrenaline signaling in healthy volunteers using clonidine and reboxetine in double-blind placebo-controlled experiments, testing the effects on perceptual abilities and visually evoked electroencephalography (EEG) and fMRI responses. We found that detection sensitivity, discrimination accuracy, and subjective visibility change in accordance with noradrenaline (NE) levels, whereas decision bias (criterion) is not affected. Similarly, noradrenaline increases the consistency of EEG visually evoked potentials, while lower noradrenaline levels delay response components around 200 ms. Furthermore, blood-oxygen-level-dependent (BOLD) fMRI activations in high-order visual cortex selectively vary along with noradrenaline signaling. Taken together, these results point to noradrenaline as a key factor causally linking visual awareness to external world events. VIDEO ABSTRACT.

Consequences of tuning network function by tonic and phasic locus coeruleus output and stress: Regulating detection and discrimination of peripheral stimuli

Flexible and adaptive behaviors have evolved with increasing complexity and numbers of neuromodulator systems. The neuromodulatory locus coeruleus-norepinephrine (LC-NE) system is central to regulating cognitive function in a behaviorally-relevant and arousal-dependent manner. Through its nearly ubiquitous efferent projections, the LC-NE system acts to modulate neuron function on a cell-by-cell basis and exert a spectrum of actions across different brain regions to optimize target circuit function. As LC neuron activity, NE signaling, and arousal level increases, cognitive performance improves over an inverted-U shaped curve. Additionally, LC neurons burst phasically in relation to novel or salient sensory stimuli and top-down decision- or response-related processes. Together, the variety of LC activity patterns and complex actions of the LC-NE system indicate that the LC-NE system may dynamically regulate the function of target neural circuits. The manner in which neural networks encode, represent, and perform neurocomputations continue to be revealed. This has improved our ability to understand the optimization of neural circuits by NE and generation of flexible and adaptive goal-directed behaviors. In this review, the rat vibrissa somatosensory system is explored as a model neural circuit to bridge known modulatory actions of NE and changes in cognitive function. It is argued that fluid transitions between neural computational states reflect the ability of this sensory system to shift between two principal functions: detection of novel or salient sensory information and detailed descriptions of sensory information. Such flexibility in circuit function is likely critical for producing context-appropriate sensory signal processing. Nonetheless, many challenges remain including providing a causal link between NE mediated changes in sensory neural coding and perceptual changes, as well as extending these principles to higher cognitive functions including behavioral flexibility and decision making.

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

The brainstem nucleus locus coeruleus (LC) is a major source of norepinephrine (NE) projections throughout the CNS. This important property was masked in very early studies by the inability to visualize endogenous monoamines. The development of monoamine histofluorescence methods by Swedish scientists led to a plethora of studies, including a paper published in Brain Research by Loizou in 1969. That paper was highly cited (making it a focal point for the 50th anniversary issue of this journal), and helped to spark a large and continuing set of investigations to further refine our understating of the LC-NE system and its contribution to brain function and behavior. This paper very briefly reviews the ensuing advances in anatomical, physiological and behavioral aspects of the LC-NE system. Although its projections are ubiquitously present throughout the CNS, recent studies find surprising specificity within the organizational and operational domains of LC neurons. These and other findings lead us to expect that future work will unmask additional features of the LC-NE system and its roles in normative and pathological brain and behavioral processes. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Stimulus-specific effects of noradrenaline in auditory cortex: implications for the discrimination of communication sounds

KEY POINTS

Many studies have described the action of Noradrenaline (NA) on the properties of cortical receptive fields, but none has assessed how NA affects the discrimination abilities of cortical cells between natural stimuli. In the present study, we compared the consequences of NA topical application on spectro-temporal receptive fields (STRFs) and responses to communication sounds in the primary auditory cortex. NA application reduced the STRFs (an effect replicated by the alpha1 agonist Phenylephrine) but did not change, on average, the responses to communication sounds. For cells exhibiting increased evoked responses during NA application, the discrimination abilities were enhanced as quantified by Mutual Information. The changes induced by NA on parameters extracted from the STRFs and from responses to communication sounds were not related.

ABSTRACT

The alterations exerted by neuromodulators on neuronal selectivity have been the topic of a vast literature in the visual, somatosensory, auditory and olfactory cortices. However, very few studies have investigated to what extent the effects observed when testing these functional properties with artificial stimuli can be transferred to responses evoked by natural stimuli. Here, we tested the effect of noradrenaline (NA) application on the responses to pure tones and communication sounds in the guinea-pig primary auditory cortex. When pure tones were used to assess the spectro-temporal receptive field (STRF) of cortical cells, NA triggered a transient reduction of the STRFs in both the spectral and the temporal domain, an effect replicated by the α1 agonist phenylephrine whereas α2 and β agonists induced STRF expansion. When tested with communication sounds, NA application did not produce significant effects on the firing rate and spike timing reliability, despite the fact that α1, α2 and β agonists by themselves had significant effects on these measures. However, the cells whose evoked responses were increased by NA application displayed enhanced discriminative abilities. These cells had initially smaller STRFs than the rest of the population. A principal component analysis revealed that the variations of parameters extracted from the STRF and those extracted from the responses to natural stimuli were not correlated. These results suggest that probing the action of neuromodulators on cortical cells with artificial stimuli does not allow us to predict their action on responses to natural stimuli.

Catecholamine modulation of prefrontal cortical cognitive function

The prefrontal cortex (PFC) utilizes working memory to guide behavior and to release the organism from dependence on environmental cues and is commonly disrupted in neuropsychiatric disorders, normal aging, or exposure to uncontrollable stress. This review posits that the PFC is very sensitive to changes in the neuromodulatory inputs it receives from norepinephrine (NE) and dopamine (DA) systems and that this sensitivity can lead to marked changes in the working-memory functions of the PFC. While NE and DA have important beneficial influences on processing in this area, very high levels of catecholamine release, for example, during exposure to uncontrollable stress, disrupt the cognitive functions of the PFC. This fresh understanding of the neurochemical influences on PFC function has led to new treatments for cognitive disorders such as Attention Deficit Hyperactivity Disorder (ADHD), and may help to elucidate the prevalence of PFC dysfunction in other mental disorders.

Interacting brain systems modulate memory consolidation

Emotional arousal influences the consolidation of long-term memory. This review discusses experimental approaches and relevant findings that provide the foundation for current understanding of coordinated interactions between arousal activated peripheral hormones and the brain processes that modulate memory formation. Rewarding or aversive experiences release the stress hormones epinephrine (adrenalin) and glucocorticoids from the adrenal glands into the bloodstream. The effect of these hormones on memory consolidation depends upon binding of norepinephrine to beta-adrenergic receptors in the basolateral complex of the amygdala (BLA). Much evidence indicates that the stress hormones influence release of norepinephrine in the BLA through peripheral actions on the vagus nerve which stimulates, through polysynaptic connections, cells of the locus coeruleus to release norepinephrine. The BLA influences memory storage by actions on synapses, distributed throughout the brain, that are engaged in sensory and cognitive processing at the time of amygdala activation. The implications of the activation of these stress-activated memory processes are discussed in relation to stress-related memory disorders.

Experimental strategies for investigating psychostimulant drug actions and prefrontal cortical function in ADHD and related attention disorders

Amphetamine-like psychostimulant drugs have been used for decades to treat a variety of clinical conditions. Methylphenidate (MPH)-Ritalin(R) , a compound that blocks reuptake of synaptically released norepinephrine (NE) and dopamine (DA) in the brain, has been used for more than 30 years in low dose, long-term regimens to treat attention deficit-hyperactive disorder (ADHD) in juveniles, adolescents, and adults. Now, these agents are also becoming increasingly popular among healthy individuals from all walks of life (e.g., military, students) and age groups (teenagers thru senior citizens) to promote wakefulness and improve attention. Although there is agreement regarding the primary biochemical action of MPH, the physiological basis for its efficacy in normal individuals and ADHD patients is lacking. Study of the behavioral and physiological actions of clinically and behaviorally relevant doses of MPH in normal animals provides an opportunity to explore the role of catecholamine transmitters in prefrontal cortical function and attentional processes as they relate to normal operation of brain circuits and ADHD pathology. The goal of ongoing studies has been to: (1) assess the effects of low dose MPH on rodent performance in a well characterized sensory-guided sustained attention task, (2) examine the effects of the same low-dose chronic MPH administration on task-related discharge of prefrontal cortical (PFC) neurons, and (3) investigate the effects of NE and DA on membrane response properties and synaptic transmission in identified subsets of PFC neurons. Combinations of these approaches can be used in adolescent, adult, and aged animals to identify the parameters of cell and neural circuit function that are regulated by MPH and to establish an overarching explanation of how MPH impacts PFC operations from cellular through behavioral functional domains.

State-dependent performance of optic-flow processing interneurons

Active locomotive states are metabolically expensive and require efficient sensory processing both to avoid wasteful movements and to cope with an extended bandwidth of sensory stimuli. This is particularly true for flying animals because flight, as opposed to walking or resting, imposes a steplike increase in metabolism for the precise execution and control of movements. Sensory processing itself carries a significant metabolic cost, but the principles governing the adjustment of sensory processing to different locomotor states are not well understood. We use the blowfly as a model system to study the impact on visual processing of a neuromodulator, octopamine, which is known to be involved in the regulation of flight physiology. We applied an octopamine agonist and recorded the directional motion responses of identified visual interneurons known to process self-motion-induced optic flow to directional motion stimuli. The neural response range of these neurons is increased and the response latency is reduced. We also found that, due to an elevated spontaneous spike rate, the cells' negative signaling range is increased. Meanwhile, the preferred self-motion parameters the cells encode were state independent. Our results indicate that in the blowfly energetically expensive sensory coding strategies, such as rapid, large responses, and high spontaneous spike activity could be adjusted by the neuromodulator octopamine, likely to save energy during quiet locomotor states.

Defining Meyer's loop-temporal lobe resections, visual field deficits and diffusion tensor tractography

Anterior temporal lobe resection is often complicated by superior quadrantic visual field deficits (VFDs). In some cases this can be severe enough to prohibit driving, even if a patient is free of seizures. These deficits are caused by damage to Meyer's loop of the optic radiation, which shows considerable heterogeneity in its anterior extent. This structure cannot be distinguished using clinical magnetic resonance imaging sequences. Diffusion tensor tractography is an advanced magnetic resonance imaging technique that enables the parcellation of white matter. Using seed voxels antero-lateral to the lateral geniculate nucleus, we applied this technique to 20 control subjects, and 21 postoperative patients. All patients had visual fields assessed with Goldmann perimetry at least three months after surgery. We measured the distance from the tip of Meyer's loop to the temporal pole and horn in all subjects. In addition, we measured the size of temporal lobe resection using postoperative T₁-weighted images, and quantified VFDs. Nine patients suffered VFDs ranging from 22% to 87% of the contralateral superior quadrant. In patients, the range of distance from the tip of Meyer's loop to the temporal pole was 24–43 mm (mean 34 mm), and the range of distance from the tip of Meyer's loop to the temporal horn was −15 to +9 mm (mean 0 mm). In controls the range of distance from the tip of Meyer's loop to the temporal pole was 24–47 mm (mean 35 mm), and the range of distance from the tip of Meyer's loop to the temporal horn was −11 to +9 mm (mean 0 mm). Both quantitative and qualitative results were in accord with recent dissections of cadaveric brains, and analysis of postoperative VFDs and resection volumes. By applying a linear regression analysis we showed that both distance from the tip of Meyer's loop to the temporal pole and the size of resection were significant predictors of the postoperative VFDs. We conclude that there is considerable variation in the anterior extent of Meyer's loop. In view of this, diffusion tensor tractography of the optic radiation is a potentially useful method to assess an individual patient's risk of postoperative VFDs following anterior temporal lobe resection.

Noradrenergic modulation of activity in a vocal control nucleus in vitro

Norepinephrine (NE) can profoundly modulate sensory processing, but its effect on motor function is less well understood. Birdsong is a learned behavior involving sensory and motor processes that are influenced by NE. A potential site of NE action is the robust nucleus of the arcopallium (RA): RA receives noradrenergic inputs and has adrenergic receptors, and it is a sensorimotor area instrumental to song production. We hypothesized that NE modulates RA neurons, and as a first test, we examined the effect of NE on RA activity in vitro. We recorded spontaneous activity extracellularly from isolated RA neurons in brain slices made from adult male zebra finches. These neurons exhibited regular tonic activity with firing rates averaging 5.5 Hz. Bath application of NE rapidly and reversibly decreased firing for the majority of neurons, to the extent that spontaneous activity was often abolished. This was likely a direct effect on the cell recorded, because it occurred with blockade of fast excitatory and inhibitory synaptic transmission or of all synaptic transmission. The NE-induced suppression involved alpha2-adrenergic receptors: yohimbine, an antagonist, completely reversed the suppression, and clonidine, an agonist, partially mimicked it. Perforated patch recordings revealed that NE induced a conductance increase in RA neurons; however, this did not prevent cells from firing when stimulated by afferents in HVC. For some neurons, NE application resulted in an increase in signal-to-noise ratio for spikes evoked by HVC stimulation. Thus NE could strongly modulate the spontaneous activity of RA cells, potentially enhancing signals relayed through RA.

Cognitive enhancement mediated through postsynaptic actions of norepinephrine on ongoing cortical activity

We propose two distinct types of norepinephrine (NE)-neuromodulatory systems: an enhanced-excitatory and enhanced-inhibitory (E-E/E-I) system and a depressed-excitatory and enhanced-inhibitory (D-E/E-I) system. In both systems, inhibitory synaptic efficacies are enhanced, but excitatory ones are modified in a contradictory manner: the E-E/E-I system enhances excitatory synaptic efficacies, whereas the D-E/E-I system depresses them. The E-E/E-I and D-E/E-I systems altered the dynamic property of ongoing (background) neuronal activity and greatly influenced the cognitive performance (S/N ratio) of a cortical neural network. The E-E/E-I system effectively enhanced S/N ratio for weaker stimuli with lower doses of NE, whereas the D-E/E-I system enhanced stronger stimuli with higher doses of NE. The neural network effectively responded to weaker stimuli if brief gamma-bursts were involved in ongoing neuronal activity that is controlled under the E-E/E-I neuromodulatory system. If the E-E/E-I and the D-E/E-I systems interact within the neural network, depressed neurons whose activity is depressed by NE application have bimodal property. That is, S/N ratio can be enhanced not only for stronger stimuli as its original property but also for weaker stimuli, for which coincidental neuronal firings among enhanced neurons whose activity is enhanced by NE application are essential. We suggest that the recruitment of the depressed neurons for the detection of weaker (subthreshold) stimuli might be advantageous for the brain to cope with a variety of sensory stimuli.

Noradrenergic inputs mediate state dependence of auditory responses in the avian song system

Norepinephrine (NE) plays a complex role in the behavioral state-dependent regulation of sensory processing. However, the role of forebrain NE action in modulating high-order sensory activity has not been directly addressed. In this study, we take advantage of the discrete, feedforward organization of the avian song system to identify a site and mechanism of NE action underlying state-dependent modulation of sensory processing. We have developed an experimental paradigm in which brief arousal repeatedly suppresses song system auditory responsiveness. Using pharmacological manipulations in vivo, we show that infusion of alpha-adrenergic antagonists into the NIf (nucleus interfacialis of the nidopallium), an auditory forebrain area, blocks this state-dependent modulation. We also demonstrate dose-dependent enhancement and suppression of song system auditory response properties by NE and adrenergic agonists. Our results demonstrate that noradrenergic release in a single forebrain area is a mechanism underlying behavioral state-dependent regulation of auditory processing in a neural system specialized for vocal learning.

The α2-adrenergic agonist guanfacine reduces excitability of human motor cortex through disfacilitation and increase of inhibition

OBJECTIVE

To test the acute effects of the alpha2-adrenoceptor agonist guanfacine (GFC) on motor excitability in intact humans.

METHODS

Eight healthy right-handed adults received a single oral dose of 2 mg of GFC. Motor cortex excitability was tested by focal transcranial magnetic stimulation of the hand area of the left motor cortex. Motor evoked potentials (MEP) were recorded from the right abductor pollicis brevis muscle. In addition, spinal and neuromuscular excitability were tested. All measures were obtained immediately before GFC intake (baseline), and 2, 6, and 24 h later.

RESULTS

GFC decreased the slope of the MEP intensity curve, increased paired-pulse short-interval intracortical inhibition, and decreased paired-pulse intracortical facilitation and I-wave facilitation. These effects were maximal at 2-6 h and returned to baseline at 24 h. Motor threshold, cortical silent period, and the measures of spinal (peripheral silent period, F waves) and neuromuscular excitability (maximum M wave) remained unaffected.

CONCLUSIONS

This is the first study on the effects of an anti-noradrenergic drug on human motor cortex excitability. GFC reduced cortical excitability by disfacilitation and increased inhibition. These findings support the idea that anti-noradrenergic drugs are detrimental for cortical plasticity and learning which are down-regulated by disfacilitation or increased inhibition.

GABA and its agonists improved visual cortical function in senescent monkeys

Human cerebral cortical function degrades during old age. Much of this change may result from a degradation of intracortical inhibition during senescence. We used multibarreled microelectrodes to study the effects of electrophoretic application of gamma-aminobutyric acid (GABA), the GABA type a (GABAa) receptor agonist muscimol, and the GABAa receptor antagonist bicuculline, respectively, on the properties of individual V1 cells in old monkeys. Bicuculline exerted a much weaker effect on neuronal responses in old than in young animals, confirming a degradation of GABA-mediated inhibition. On the other hand, the administration of GABA and muscimol resulted in improved visual function. Many treated cells in area V1 of old animals displayed responses typical of young cells. The present results have important implications for the treatment of the sensory, motor, and cognitive declines that accompany old age.

Methylphenidate facilitates and disinhibits the motor cortex in intact humans

Animal experiments show that motor recovery after focal brain injury is accelerated by the indirect norepinephrine agonist methylphenidate (MPH). The underlying mechanisms are unknown, but an MPH-induced increase in cortical excitability has been advocated. Here, we tested the acute effects of a single oral dose of 40 mg MPH (Ritalin) on motor cortical excitability in eight healthy subjects using focal transcranial magnetic stimulation. MPH increased the slope of the motor evoked potentials (MEP) intensity curve in a hand muscle, reduced short-interval intracortical inhibition, and increased I-wave facilitation. MEP threshold, cortical silent period and measures of spinal and neuromuscular excitability remained unaffected. Findings support the idea that MPH promotes accelerated motor recovery after lesion through facilitation and disinhibition.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Noradrenergic modulation of functional selectivity in the cat visual cortex: an in vivo extracellular and intracellular study

In vitro intracellular studies have shown that norepinephrine modulates cellular excitability and synaptic transmission in the cortex. Based on these effects, norepinephrine has been proposed to enhance the signal-to-noise ratio and to improve functional selectivity by potentiating strong synaptic responses and reducing weak ones. Here we have studied the functional effects of iontophoretic applications of norepinephrine during in vivo extracellular and intracellular recordings from neurons of the primary visual cortex of kittens and adult cats. Analysis of extracellular data concentrated on norepinephrine-induced changes in spontaneous and evoked activities, in signal-to-noise ratio, and in orientation and direction selectivity. Analysis of the intracellular data concentrated on actions of norepinephrine on spike firing accommodation, which has been shown to be reduced by norepinephrine in vitro, and on synaptic responses. Application of norepinephrine resulted in a depression of both spontaneous and evoked spiking activity. However, no systematic change in signal-to-noise ratio was observed. The suppressive effect of norepinephrine was exerted with no significant sharpening of direction or orientation selectivity tuning. The overall reduction in visual activity by norepinephrine affected the orientation tuning curves in a way compatible with a divisive effect, that is a normalization or gain control with no change in tuning width. Norepinephrine applied during intracellular recordings reduced the visually evoked depolarizing potentials whereas no change in the responsiveness of the cell to current-induced depolarizations was observed. In conditions of optimal visual stimulation which produced large depolarizations of several hundreds of milliseconds and sustained repetitive firing comparable to that obtained by direct current injection, we were unable to observe a facilitation of the evoked responses by norepinephrine as it would be expected from the well-documented increase in excitability induced by norepinephrine in vitro. In conclusion, from these results we suggest that norepinephrine released in the primary visual cortex primarily reduces the level of cortical activation by afferent signals, without affecting the cortical functional selectivity nor increasing the signal-to-noise ratio.

Noradrenergic innervation of the developing and mature visual and motor cortex of the rat brain: a light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation of the developing and adult visual and motor cortex of the rat was examined with light and electron microscopic immunocytochemistry by using antibodies against dopamine-beta-hydroxylase. At birth, NA fibers were present in both cortical areas, appearing as two tangential streams, one above and the other below the cortical plate. During the subsequent weeks, these two streams arborized gradually innervating all cortical layers. The adult pattern of distribution was attained by postnatal day 14, but the density of innervation, which was higher in the motor than in the visual cortex, appeared similar to the adult by the end of the third postnatal week. Electron microscopic analysis revealed that a low proportion of NA varicosities (the highest value was 12% in the adult motor cortex in single sections) were engaged in synaptic contact, throughout development, in both areas examined. The overwhelming majority of these synapses were symmetrical, involving predominantly small or medium dendrites. This evidence suggests that transmission by diffusion is the major mode of NA action in the developing and adult cerebral cortex. Noradrenaline released in the rare synaptic junctions may act mainly to reduce the activity of its cortical targets. The results altogether provide morphologic evidence for an involvement of noradrenaline in the development of the neocortex and, along with earlier data on the serotonergic system, indicate that the monoaminergic systems are endowed with a specific anatomic organization in various areas of the brain.

Effect of partial sensory deprivation on monoaminergic neuromodulators in striate cortex of adult cat

The role of monoaminergic neuromodulators in the reorganization of cortical topography following limited sensory deprivation in the adult cat was investigated. The total concentrations of dopamine, noradrenaline, serotonin and their major metabolites were measured in the visual cortex of both normal control and experimental animals using microbore high-performance liquid chromatography coupled with electrochemical detection. The experimental animals were subjected to a binocular retinal lesion corresponding to the central 10 degrees of vision and killed two weeks post-lesion. The sensory deprivation was confirmed in area 17 by measuring immediate-early gene zif-268 messenger RNA expression. Following the retinal lesion, the total concentrations of noradrenaline and dopamine were significantly higher in the non-deprived cortex of retinal lesion cats than in the deprived cortex of retinal lesion cats and the cortex of normal animals. This pattern follows the release of the excitatory neurotransmitter glutamate under the same conditions. Serotonin levels were significantly lower in the deprived cortex, and its metabolite 5-hydroxyindole-3-acetic acid was significantly higher in the non-deprived cortex than in deprived cortex and normal cortex. From these results, we suggest that the modulation of noradrenaline, dopamine and serotonin is regulated by visual afferent activity.

Noradrenergic modulation of tactile responses in rat cortex. Current source-density and unit analyses

This study describes the noradrenergic modulation of tactile afferent information in the sensorimotor cortex of urethane-anesthetized rats. Synaptic and spike responses to a mechanical stimulation of the hand palm were evaluated by means of current source-density analysis and unit activity recording in all cortical layers. Results showed that activation of the locus coeruleus decreased and shortened afferent synaptic excitation in supragranular, but not in deep layers. On the average, unit responses exhibited facilitated latency, moderately increased amplitude, enhanced postexcitatory inhibition and synchronization of responses across layers. The apparent paradox of this global phasic facilitation correlated with a decrease in input synaptic currents was discussed according to hypotheses which might explain its functional significance.

Selective suppression of horizontal propagation in rat visual cortex by norepinephrine

The release of norepinephrine in the cerebral cortex from axon terminals of locus coeruleus neurons was suggested to be involved in the control of attention. Accumulating data indicate that the responses of cortical neurons are varied when norepinephrine is applied iontophoretically in the vicinity of the cells being recorded. However, it is not known how the pattern of excitatory propagation is modified when norepinephrine is applied over a wide area in the visual cortex. By applying optical imaging to rat visuocortical slices, we found a new mode of norepinephrine action; a prominent suppression of the horizontal propagation in layers II/III. This action of norepinephrine was confirmed by the simultaneous recording of field potentials from multiple sites by use of a multi-electrode dish. Furthermore, our electrophysiological recordings showed that this norepinephrine action is exerted through suppression of excitatory neural transmission and enhancement of inhibitory transmission to the pyramidal neurons in these layers. Because the release of norepinephrine in the visual cortex is regulated by the level of attention, the neural basis of visual attention may relate partially to the suppression of the integration of visual information by norepinephrine resulting in a state-dependent restructuring of the receptive field.

The effect of increasing doses of levodopa on children with strabismic amblyopia

PURPOSE

The purpose of this study was to determine the efficacy and tolerance of a levodopa/carbidopa combination and to test its dose-effect response when combined with occlusion therapy for children with amblyopia.

METHODS

A 1-week randomized, double-blind, parallel, and placebo-controlled study was performed with 78 children with amblyopia aged between 7 and 17 years. Children who weighed less than 40 kg were randomly assigned to receive 5, 10, or 20 mg of levodopa or placebo (3 times a day), and those who weighed 40 kg or more were randomly assigned to receive 10, 20, or 40 mg of levodopa or placebo. Subjects were instructed to occlude the dominant eye (3 h/d). Snellen visual acuity was measured at baseline and at 2 hours after ingestion of the last capsule. Compliance with occlusion and capsule consumption was verified by a questionnaire and counting capsules. Adverse effects were checked with a physical examination and a questionnaire.

RESULTS

From baseline to the follow-up test trial, the improvement in the logarithm of the Snellen fraction was 0.05+/-0.11 in the placebo group, 0.18 +/-0.21 in group 1, 0.08+/-0.11 in group II, and 0.16+/-0.15 in group III (P = .031). Visual acuity improved from 0.59 to 0.45 in the levodopa/carbidopa group and from 0.69 to 0.63 in the control group (P = .023). The compliance with occlusion was similar in all groups. No one complained of adverse side effects.

CONCLUSION

Levodopa, at an average dose of 0.51 mg/kg per day, is well tolerated and produces a clinical and statistically significant short-term improvement of visual acuity in children with amblyopia.

Differential expression of immediate-early genes, c-fos and zif268, in the visual cortex of young rats: effects of a noradrenergic neurotoxin on their expression

We investigated the expression pattern of two immediate-early genes, zif268 and c-fos, under various visual conditions using immunohistochemical and northern blot analysis in the visual cortex of young rats. The basal expression of c-fos was low and was further reduced by dark rearing that lasted for one week. A marked and transient increase was induced upon visual stimulation applied immediately after dark rearing. Zif268 showed a relatively high basal level. Its expression was reduced by dark rearing of the animals, but returned rapidly to the basal expression level following the introduction of light. Administration of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, a selective noradrenergic neurotoxin, suppressed the basal expression of c-fos messenger RNA. The response of c-fos to photo-stimulation was also significantly lower in the visual cortex of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine-treated young rats. In contrast, no significant change in zif268 expression was detected between normal and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine-treated animals. These findings suggest that differential expression of these immediate-early genes is involved in the activity-dependent regulation of cortical function. One possibility is that the noradrenergic system controls cortical function, including plasticity, by modifying the expression of c-fos.

Effects of noradrenaline on frequency tuning of auditory cortex neurons during wakefulness and slow-wave sleep

This study shows the effects of noradrenaline (NA) on receptive fields of auditory cortex neurons in awake animals; it is the first one to describe the effects of NA on neurons in sensory cortex, in different natural states of vigilance. The frequency receptive field of 250 auditory cortex neurons was determined before, during and after ionophoretic application of NA while recording the state of vigilance of unanaesthetized guinea-pigs. When NA significantly changed the spontaneous activity (85 out of 250 cells), the dominant effect was a decrease (61 out of 85 cells, 72%). When NA significantly changed the evoked activity (107 out of 250 cells), the dominant effect was also a decrease (84 out of 107 cells, 78%). During and after NA application, the signal-to-noise ratio (S/N, i.e. evoked/spontaneous activity) was unchanged, but the selectivity for pure-tone frequencies was enhanced. When the effects occurring in wakefulness and in slow-wave sleep (SWS) were compared, it appeared that the predominantly inhibitory effect of NA on spontaneous and evoked activity was present in both states. The S/N ratio was unchanged and the selectivity was increased in both states. However, during SWS, the percentage of cells inhibited by NA was lower, and the effects on the frequency selectivity were smaller than in wakefulness. In contrast, GABA produced similar inhibitory effects on spontaneous and on evoked activity during wakefulness and SWS. Comparisons with previous data obtained using the same protocol in urethane anaesthetized animals (Manunta & Edeline 1997) indicate that the effects of NA were qualitatively the same. Based on these results, we suggest that any hypothesis concerning the role of NA in cortical plasticity should take into account the fact that the predominantly inhibitory effects of NA lead to decrease the size of the receptive field.

Learning-induced physiological plasticity in the thalamo-cortical sensory systems: a critical evaluation of receptive field plasticity, map changes and their potential mechanisms

The goal of this review is to give a detailed description of the main results obtained in the field of learning-induced plasticity. The review is focused on receptive field and map changes observed in the auditory, somatosensory and visual thalamo-cortical system as a result of an associative training performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the auditory and somatosensory system are reviewed. In the visual system, as there is no RF and map analysis during learning per se, the evidence presented are from increased neuronal responsiveness, and from the effects of perceptual learning in human and non human primates. Across sensory modalities, the re-tuning of neurons to a significant stimulus or map reorganizations in favour of the significant stimuli were observed at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neuronal changes and (ii) of neuromodulators as "gating" factors of the neuronal changes, is evaluated. The weakness of the Hebbian schema to explain learning-induced changes and the need to better define what the word "learning" means are stressed. It is suggested that future research should focus on the dynamic of information processing in sensory systems, and the concept of "effective connectivity" should be useful in that matter.

Occlusion and levodopa-carbidopa treatment for childhood amblyopia

PURPOSE

The purpose of the current study was to compare the effects of levodopa-carbidopa with and without part-time occlusion on visual function in older amblyopic children.

METHODS

Thirteen older amblyopic children were randomly assigned to receive or not receive part-time occlusion (3 h/day) combined with 7 weeks of oral dosing with levodopa-carbidopa (1.02 mg/0.25 mg/kg body weight three times daily). Visual acuity, contrast sensitivity, and fusion were measured at baseline; 1, 3, 5, and 7 weeks during the treatment regimen; and 4 weeks after termination of all treatment. At these same times health status was assessed with standard laboratory blood tests, physical examination, and subjective questionnaire.

RESULTS

From baseline to the follow-up test trial, both groups improved in visual acuity in the amblyopic eyes (occlusion group 20/116 to 20/76, P < .001; no occlusion group 20/90 to 20/73, P < .01) and dominant eyes (occlusion group 20/18 to 20/15, P > .05; no occlusion group 20/20 to 20/16, P < .01). The occlusion group exhibited a significant decrease in the difference in acuity between the dominant and amblyopic eyes of 1.3 lines (P < .02), whereas the no occlusion group revealed no significant effect. A comparison between groups revealed a significantly greater improvement in visual acuity in the amblyopic eye in the occlusion group compared with the no occlusion group (P = .01). In contrast, there was no significant difference between groups in terms of the change in visual acuity in the dominant eye (P = .15). Mean log contrast sensitivity in the amblyopic eye significantly improved in the occlusion group and did not significantly change in the no occlusion group. Fusion changed similarly in both groups. The improvements in visual function were maintained 4 weeks after the termination of all treatment. Adverse side effects were minimal in both groups.

CONCLUSION

The combination of levodopa-carbidopa and occlusion improves visual function more than levodopa-carbidopa alone in older amblyopic children.

Phasic activation of the locus coeruleus enhances responses of primary sensory cortical neurons to peripheral receptive field stimulation

In the present study we examined the effects of phasic activation of the nucleus locus coeruleus (LC) on transmission of somatosensory information to the rat cerebral cortex. The rationale for this investigation was based on earlier findings that local microiontophoretic application of the putative LC transmitter, norepinephrine (NE), had facilitating actions on cortical neuronal responses to excitatory and inhibitory synaptic stimuli and more recent microdialysis experiments that have demonstrated increases in cortical levels of NE following phasic or tonic activation of LC. Glass micropipets were used to record the extracellular activity of single neurons in the somatosensory cortex of halothane-anesthetized rats. Somatosensory afferent pathways were activated by threshold level mechanical stimulation of the glabrous skin on the contralateral forepaw. Poststimulus time histograms were used to quantitate cortical neuronal responses before and at various time intervals after preconditioning burst activation of the ipsilateral LC. Excitatory and postexcitatory inhibitory responses to forepaw stimulation were enhanced when preceded by phasic activation of LC at conditioning intervals of 200-500 ms. These effects were anatomically specific in that they were only observed upon stimulation of brainstem sites close to (>150 micron) or within LC and were pharmacologically specific in that they were not consistently observed in animals where the LC-NE system had been disrupted by 6-OHDA pretreatment. Overall, these data suggest that following phasic activation of the LC efferent system, the efficacy of signal transmission through sensory networks in mammalian brain is enhanced.

Noradrenergic suppression of synaptic transmission may influence cortical signal-to-noise ratio

Norepinephrine has been proposed to influence signal-to-noise ratio within cortical structures, but the exact cellular mechanisms underlying this influence have not been described in detail. Here we present data on a cellular effect of norepinephrine that could contribute to the influence on signal-to-noise ratio. In brain slice preparations of the rat piriform (olfactory) cortex, perfusion of norepinephrine causes a dose-dependent suppression of excitatory synaptic potentials in the layer containing synapses among pyramidal cells in the cortex (layer Ib), while having a weaker effect on synaptic potentials in the afferent fiber layer (layer Ia). Effects of norepinephrine were similar in dose-response characteristics and laminar selectivity to the effects of the cholinergic agonist carbachol, and combined perfusion of both agonists caused effects similar to an equivalent concentration of a single agonist. In a computational model of the piriform cortex, we have analyzed the effect of noradrenergic suppression of synaptic transmission on signal-to-noise ratio. The selective suppression of excitatory intrinsic connectivity decreases the background activity of modeled neurons relative to the activity of neurons receiving direct afferent input. This can be interpreted as an increase in signal-to-noise ratio, but the term noise does not accurately characterize activity dependent on the intrinsic spread of excitation, which would more accurately be described as interpretation or retrieval. Increases in levels of norepinephrine mediated by locus coeruleus activity appear to enhance the influence of extrinsic input on cortical representations, allowing a pulse of norepinephrine in an arousing context to mediate formation of memories with a strong influence of environmental variables.

Effects of noradrenaline on frequency tuning of rat auditory cortex neurons

The selectivity of rat auditory cortex neurons for pure tone frequency was studied during and after ionophoretic application (5-40 nA) of noradrenaline in urethane-anaesthetized rats. The dominant effect induced by noradrenaline was a significant decrease in spontaneous (93/268 cells) and evoked activity (133/268 cells) which outlasted the application. In the whole population of cells (n = 268) the signal-to-noise ratio, computed using as the signal either the mean evoked response or the response at the best frequency, was unchanged during noradrenaline application. It was significantly increased only for cells showing significantly decreased spontaneous activity, and was significantly decreased for cells showing increased spontaneous activity. Frequency selectivity was significantly increased for the whole population during and after noradrenaline application. It was also significantly increased for cells showing significantly decreased evoked activity, and was significantly decreased for cells showing increased evoked activity. The noradrenaline-induced inhibition was not blocked by propranolol (beta antagonist); it was blocked by prazosin (alpha1 antagonist) and partly mimicked by phenylephrine (alpha1 agonist). GABA, which also inhibited spontaneous and evoked activity, slightly increased the signal-to-noise ratio and significant increased frequency selectivity. However, when noradrenaline was ejected in the presence of bicuculline at doses that were able to block GABAergic inhibition, the inhibitory effects of noradrenaline on spontaneous and evoked activity were still observed. The possible function of noradrenaline-induced inhibitions in sensory cortices is briefly discussed.

Arousal systems and attentional processes

Unitary concepts of arousal have outlived their usefulness and their psychological fractionation corresponds to a similar chemical differentiation of the reticular formation of the brain. Neurobiological characteristics of the monoaminergic and cholinergic systems can be described in terms of their anatomical, electrophysiological and neurochemical properties. Functional studies suggest that the coeruleo-cortical noradrenergic system, under certain circumstances, is implicated in processes of selective attention, that the mesolimbic and mesostriatal dopaminergic systems contribute to different forms of behavioural activation, and that the cortical cholinergic projections have fundamental roles in the cortical processing of signals, affecting attentional and mnemonic processes. The ascending serotoninergic systems contribute to behavioural inhibition and appear to oppose the functions of the other systems in several ways.