Single-unit and physiological analyses of brain norepinephrine function in behaving animals

Brainstem noradrenergic neurons: Identifying a hub at the intersection of cognition, motility, and skeletal muscle regulation

Brainstem noradrenergic neuron clusters form a node integrating efferents projecting to distinct areas such as those regulating cognition and skeletal muscle structure and function, and receive dissimilar afferents through established circuits to coordinate organismal responses to internal and environmental challenges. Genetic lineage tracing shows the remarkable heterogeneity of brainstem noradrenergic neurons, which may explain their varied functions. They project to the locus coeruleus, the primary source of noradrenaline in the brain, which supports learning and cognition. They also project to pre-ganglionic neurons, which lie within the spinal cord and form synapses onto post-ganglionic neurons. The synapse between descending brainstem noradrenergic neurons and pre-ganglionic spinal neurons, and these in turn with post-ganglionic noradrenergic neurons located at the paravertebral sympathetic ganglia, support an anatomical hierarchy that regulates skeletal muscle innervation, neuromuscular transmission, and muscle trophism. Whether any noradrenergic neuron subpopulation is more susceptible to damaged protein deposit and death with ageing and neurodegeneration is a relevant question that answer will help us to detect neurodegeneration at an early stage, establish prognosis, and anticipate disease progression. Loss of muscle mass and strength with ageing, termed sarcopenia, may predict impaired cognition with ageing and neurodegeneration and establish an early time to start interventions aimed at reducing central noradrenergic neurons hyperactivity. Complex multidisciplinary approaches, including genetic tracing, specific circuit labelling, optogenetics and chemogenetics, electrophysiology, and single-cell transcriptomics and proteomics, are required to test this hypothesis pre-clinical.

Modulatory Effects of Monoamines and Perineuronal Nets on Output of Cerebellar Purkinje Cells

Classically, the cerebellum has been thought to play a significant role in motor coordination. However, a growing body of evidence for novel neural connections between the cerebellum and various brain regions indicates that the cerebellum also contributes to other brain functions implicated in reward, language, and social behavior. Cerebellar Purkinje cells (PCs) make inhibitory GABAergic synapses with their target neurons: other PCs and Lugaro/globular cells via PC axon collaterals, and neurons in the deep cerebellar nuclei (DCN) via PC primary axons. PC-Lugaro/globular cell connections form a cerebellar cortical microcircuit, which is driven by serotonin and noradrenaline. PCs' primary outputs control not only firing but also synaptic plasticity of DCN neurons following the integration of excitatory and inhibitory inputs in the cerebellar cortex. Thus, strong PC-mediated inhibition is involved in cerebellar functions as a key regulator of cerebellar neural networks. In this review, we focus on physiological characteristics of GABAergic transmission from PCs. First, we introduce monoaminergic modulation of GABAergic transmission at synapses of PC-Lugaro/globular cell as well as PC-large glutamatergic DCN neuron, and a Lugaro/globular cell-incorporated microcircuit. Second, we review the physiological roles of perineuronal nets (PNNs), which are organized components of the extracellular matrix and enwrap the cell bodies and proximal processes, in GABA release from PCs to large glutamatergic DCN neurons and in cerebellar motor learning. Recent evidence suggests that alterations in PNN density in the DCN can regulate cerebellar functions.

The emerging role of the sympathetic nervous system in skeletal muscle motor innervation and sarcopenia

Examining neural etiologic factors'role in the decline of neuromuscular function with aging is essential to our understanding of the mechanisms underlying sarcopenia, the age-dependent decline in muscle mass, force and power. Innervation of the skeletal muscle by both motor and sympathetic axons has been established, igniting interest in determining how the sympathetic nervous system (SNS) affect skeletal muscle composition and function throughout the lifetime. Selective expression of the heart and neural crest derivative 2 gene in peripheral SNs increases muscle mass and force regulating skeletal muscle sympathetic and motor innervation; improving acetylcholine receptor stability and NMJ transmission; preventing inflammation and myofibrillar protein degradation; increasing autophagy; and probably enhancing protein synthesis. Elucidating the role of central SNs will help to define the coordinated response of the visceral and neuromuscular system to physiological and pathological challenges across ages. This review discusses the following questions: (1) Does the SNS regulate skeletal muscle motor innervation? (2) Does the SNS regulate presynaptic and postsynaptic neuromuscular junction (NMJ) structure and function? (3) Does sympathetic neuron (SN) regulation of NMJ transmission decline with aging? (4) Does maintenance of SNs attenuate aging sarcopenia? and (5) Do central SN group relays influence sympathetic and motor muscle innervation?

Postganglionic sympathetic neurons, but not locus coeruleus optostimulation, activates neuromuscular transmission in the adult mouse in vivo

Recent work demonstrated that sympathetic neurons innervate the skeletal muscle near the neuromuscular junction (NMJ), and muscle sympathectomy and sympathomimetic agents strongly influence motoneuron synaptic vesicle release ex vivo. Moreover, reports attest that the pontine nucleus locus coeruleus (LC) projects to preganglionic sympathetic neurons and regulates human mobility and skeletal muscle physiology. Thus, we hypothesized that peripheral and central sympathetic neurons projecting directly or indirectly to the skeletal muscle regulate NMJ transmission. The aim of this study was to define the specific neuronal groups in the peripheral and central nervous systems that account for such regulation in adult mice in vivo by using optogenetics and NMJ transmission recordings in 3-5-month-old, male and female ChR2(H134R/EYFP)/TH-Cre mice. After detecting ChR2(H134R)/EYFP fluorescence in the paravertebral ganglia and LC neurons, we tested whether optostimulating the plantar nerve near the lumbricalis muscle or LC neurons effectively modulates motor nerve terminal synaptic vesicle release in living mice. Nerve optostimulation increased motor synaptic vesicle release in vitro and in vivo, while the presynaptic adrenoceptor blockers propranolol (β1/β2) and atenolol (β1) prevented this outcome. The effect is primarily presynaptic since miniature end-plate potential (MEPP) kinetics remained statistically unmodified after stimulation. In contrast, optostimulation of LC neurons did not regulate NMJ transmission. In summary, we conclude that postganglionic sympathetic neurons, but not LC neurons, increased NMJ transmission by acting on presynaptic β1-adrenergic receptors in vivo.

Behavioral effects of Bj-PRO-7a, a proline-rich oligopeptide from Bothrops jararaca venom

The heptapeptide Bj-PRO-7a, isolated and identified from Bothrops jararaca (Bj) venom, produces antihypertensive and other cardiovascular effects that are independent on angiotensin converting enzyme inhibition, possibly relying on cholinergic muscarinic receptors subtype 1 (M1R). However, whether Bj-PRO-7a acts upon the central nervous system and modifies behavior is yet to be determined. Therefore, the aims of this study were: i) to assess the effects of acute administration of Bj-PRO-7a upon behavior; ii) to reveal mechanisms involved in the effects of Bj-PRO-7a upon locomotion/exploration, anxiety, and depression-like behaviors. For this purpose, adult male Wistar (WT, wild type) and spontaneous hypertensive rats (SHR) received intraperitoneal injections of vehicle (0.9% NaCl), diazepam (2 mg/kg), imipramine (15 mg/kg), Bj-PRO-7a (71, 213 or 426 nmol/kg), pirenzepine (852 nmol/kg), α-methyl-DL-tyrosine (200 mg/kg), or chlorpromazine (2 mg/kg), and underwent elevated plus maze, open field, and forced swimming tests. The heptapeptide promoted anxiolytic and antidepressant-like effects and increased locomotion/exploration. These effects of Bj-PRO-7a seem to be dependent on M1R activation and dopaminergic receptors and rely on catecholaminergic pathways.

Monoaminergic modulation of GABAergic transmission onto cerebellar globular cells

Cerebellar Purkinje cells (PCs) project their axon collaterals to underneath of the PC layer and make GABAergic synaptic contacts with globular cells, a subgroup of Lugaro cells. GABAergic transmission derived from the PC axon collaterals is so powerful that it could inhibit globular cells and regulate their firing patterns. However, the physiological properties and implications of the GABAergic synapses on globular cells remain unknown. Using whole-cell patch-clamp recordings from globular cells in the mouse cerebellum, we examined the monoaminergic modulation of GABAergic inputs to these cells. Application of either serotonin (5-HT) or noradrenaline (NA) excited globular cells, thereby leading to their firing. The 5-HT- and NA-induced firing was temporally confined and attenuated by GABAergic transmission, although 5-HT and NA exerted an inhibitory effect on the release of GABA from presynaptic terminals of PC axon collaterals. Agonists for 5-HT_1B receptors and α₂-adrenoceptors mimicked the 5-HT- and NA-induced suppression of GABAergic activity. Through their differential modulatory actions on the cerebellar inhibitory neural circuits, 5-HT facilitated PC firing, whereas NA suppressed it. These results indicate that 5-HT and NA regulate the membrane excitability of globular cells and PCs through their differential modulation of not only the membrane potential but also GABAergic synaptic circuits. Monoaminergic modulation of the neural connections between globular cells and PCs could play a role in cerebellar motor coordination.

Prepulse inhibition and facilitation of the postauricular reflex, a vestigial remnant of pinna startle

If the postauricular reflex (PAR) is to be used effectively in studies of emotion and attention, its sensitivity to basic modulatory effects such as prepulse inhibition and facilitation must be determined. Two experiments were carried out with healthy young adults to assess the effects of transient and sustained visual prestimuli on the pinna-flexion response to trains of startle probes. In the first experiment, participants passively viewed a small white square. It was displayed from 1,000 ms prior to onset of a train of noise bursts until the end of that train. Relative to no-prepulse control trials, PAR amplitude was inhibited, possibly due to the withdrawal of attentional resources from the auditory modality. In the second experiment, participants performed a visual oddball task in which irrelevant trains of startle probes followed most briefly displayed task stimuli (checkerboards). Prepulse inhibition was observed when a transient stimulus preceded the first probe at a lead time of 100 ms. Amplitude facilitation was observed at longer lead times. In addition to documenting the existence of prepulse inhibition and facilitation, the data suggest that the PAR is not elicited by visual stimuli, that temporal expectancy does not influence its amplitude or latency, and that this vestigial microreflex is resistant to habituation. Results are interpreted in light of a recent theory that the human PAR is a highly degraded pinna startle, in which the reflex arc no longer includes the startle center (nucleus reticularis pontis caudalis).

The brain on stress: Insight from studies using the Visible Burrow System

The discovery of adrenal steroid receptors outside of the hypothalamus in the hippocampus and other forebrain regions catalyzed research on the effects of stress upon cognitive function, emotions and self-regulatory behaviors as well as the molecular, cellular and neuroanatomical mechanisms underlying acute and chronic stress effects on the brain. Indeed, this work has shown that the brain is a plastic and vulnerable organ in the face of acute and chronic stress. The insight that Bob and Caroline Blanchard had in developing and interpreting findings using the Visible Burrow System model made an enormous contribution to the current view that the human brain is very sensitive to the social environment and to agonistic interactions between individuals. Their collaboration with Sakai and McEwen at The Rockefeller University extended application of the Visible Burrow System model to demonstrate that it also was a unique and highly relevant neuroethological model with which to study stress and adaptation to stressors. Those studies focused on the brain and systemic organ responses to stress and, in turn, described that the brain is also very responsive to changes in systemic physiology.

Insomnia Caused by Serotonin Depletion is Due to Hypothermia

STUDY OBJECTIVE

Serotonin (5-hydroxytryptamine, 5-HT) neurons are now thought to promote wakefulness. Early experiments using the tryptophan hydroxylase inhibitor para-chlorophenylalanine (PCPA) had led to the opposite conclusion, that 5-HT causes sleep, but those studies were subsequently contradicted by electrophysiological and behavioral data. Here we tested the hypothesis that the difference in conclusions was due to failure of early PCPA experiments to control for the recently recognized role of 5-HT in thermoregulation.

DESIGN

Adult male C57BL/6N mice were treated with PCPA (800 mg/kg intraperitoneally for 5 d; n = 15) or saline (n = 15), and housed at 20 °C (normal room temperature) or at 33 °C (thermoneutral for mice) for 24 h. In a separate set of experiments, mice were exposed to 4 °C for 4 h to characterize their ability to thermoregulate.

MEASUREMENTS AND RESULTS

PCPA treatment reduced brain 5-HT to less than 12% of that of controls. PCPA-treated mice housed at 20 °C spent significantly more time awake than controls. However, core body temperature decreased from 36.5 °C to 35.1 °C. When housed at 33 °C, body temperature remained normal, and total sleep duration, sleep architecture, and time in each vigilance state were the same as controls. When challenged with 4 °C, PCPA-treated mice experienced a precipitous drop in body temperature, whereas control mice maintained a normal body temperature.

CONCLUSIONS

These results indicate that early experiments using para-chlorophenylalanine that led to the conclusion that 5-hydroxytryptamine (5-HT) causes sleep were likely confounded by hypothermia. Temperature controls should be considered in experiments using 5-HT depletion.

Effect of parasympathetic stimulation on brain activity during appraisal of fearful expressions

Autonomic nervous system activity is an important component of human emotion. Mental processes influence bodily physiology, which in turn feeds back to influence thoughts and feelings. Afferent cardiovascular signals from arterial baroreceptors in the carotid sinuses are processed within the brain and contribute to this two-way communication with the body. These carotid baroreceptors can be stimulated non-invasively by externally applying focal negative pressure bilaterally to the neck. In an experiment combining functional neuroimaging (fMRI) with carotid stimulation in healthy participants, we tested the hypothesis that manipulating afferent cardiovascular signals alters the central processing of emotional information (fearful and neutral facial expressions). Carotid stimulation, compared with sham stimulation, broadly attenuated activity across cortical and brainstem regions. Modulation of emotional processing was apparent as a significant expression-by-stimulation interaction within left amygdala, where responses during appraisal of fearful faces were selectively reduced by carotid stimulation. Moreover, activity reductions within insula, amygdala, and hippocampus correlated with the degree of stimulation-evoked change in the explicit emotional ratings of fearful faces. Across participants, individual differences in autonomic state (heart rate variability, a proxy measure of autonomic balance toward parasympathetic activity) predicted the extent to which carotid stimulation influenced neural (amygdala) responses during appraisal and subjective rating of fearful faces. Together our results provide mechanistic insight into the visceral component of emotion by identifying the neural substrates mediating cardiovascular influences on the processing of fear signals, potentially implicating central baroreflex mechanisms for anxiolytic treatment targets.

Too much of a good thing: blocking noradrenergic facilitation in medial prefrontal cortex prevents the detrimental effects of chronic stress on cognition

Cognitive impairments associated with dysfunction of the medial prefrontal cortex (mPFC) are prominent in stress-related psychiatric disorders. We have shown that enhancing noradrenergic tone acutely in the rat mPFC facilitated extra-dimensional (ED) set-shifting on the attentional set-shifting test (AST), whereas chronic unpredictable stress (CUS) impaired ED. In this study, we tested the hypothesis that the acute facilitatory effect of norepinephrine (NE) in mPFC becomes detrimental when activated repeatedly during CUS. Using microdialysis, we showed that the release of NE evoked in mPFC by acute stress was unchanged at the end of CUS treatment. Thus, to then determine if repeated elicitation of this NE activity in mPFC during CUS may have contributed to the ED deficit, we infused a cocktail of α(1)-, β(1)-, and β(2)-adrenergic receptor antagonists into the mPFC prior to each CUS session, then tested animals drug free on the AST. Antagonist treatment prevented the CUS-induced ED deficit, suggesting that NE signaling during CUS compromised mPFC function. We confirmed that this was not attributable to sensitization of adrenergic receptor function following chronic antagonist treatment, by administering an additional microinjection into the mPFC immediately prior to ED testing. Acute antagonist treatment did not reverse the beneficial effects of chronic drug treatment during CUS, nor have any effect on baseline ED performance in chronic vehicle controls. Thus, we conclude that blockade of noradrenergic receptors in mPFC protected against the detrimental cognitive effects of CUS, and that repeated elicitation of noradrenergic facilitatory activity is one mechanism by which chronic stress may promote mPFC cognitive dysfunction.

Cortical plasticity, excitatory-inhibitory balance, and sensory perception

Experience shapes the central nervous system throughout life. Structural and functional plasticity confers a remarkable ability on the brain, allowing neural circuits to adequately adapt to dynamic environments. This process can require selective adjustment of many excitatory and inhibitory synapses in an organized manner, in such a way as to enhance representations of behaviorally important sensory stimuli while preserving overall network excitability. The rules and mechanisms that orchestrated these changes across different synapses and throughout neuronal ensembles are beginning to be understood. Here, we review the evidence connecting synaptic plasticity to functional plasticity and perceptual learning, focusing on the roles of various neuromodulatory systems in enabling plasticity of adult neural circuits. However, the challenge remains to appropriately leverage these systems and forms of plasticity to persistently improve perceptual abilities and behavioral performance.

Orienting and reorienting: the locus coeruleus mediates cognition through arousal

Mood, motivation, attention, and arousal are behavioral states having a profound impact on cognition. Behavioral states are mediated though the peripheral nervous system and neuromodulatory systems in the brainstem. The noradrenergic nucleus locus coeruleus is activated in parallel with the autonomic system in response to biological imperatives. These responses can be spontaneous, to unexpected salient or threatening stimuli, or they can be conditioned responses to awaited behaviorally relevant stimuli. Noradrenaline, released in forebrain structures, will facilitate sensory processing, enhance cognitive flexibility and executive function in the frontal cortex, and promote offline memory consolidation in limbic structures. Central activation of neuromodulatory neurons and peripheral arousal, together, prepare the organism for a reorientation or reset of cortical networks and an adaptive behavioral response.

Activation of inactivation process initiates rapid eye movement sleep

Interactions among REM-ON and REM-OFF neurons form the basic scaffold for rapid eye movement sleep (REMS) regulation; however, precise mechanism of their activation and cessation, respectively, was unclear. Locus coeruleus (LC) noradrenalin (NA)-ergic neurons are REM-OFF type and receive GABA-ergic inputs among others. GABA acts postsynaptically on the NA-ergic REM-OFF neurons in the LC and presynaptically on the latter's projection terminals and modulates NA-release on the REM-ON neurons. Normally during wakefulness and non-REMS continuous release of NA from the REM-OFF neurons, which however, is reduced during the latter phase, inhibits the REM-ON neurons and prevents REMS. At this stage GABA from substantia nigra pars reticulate acting presynaptically on NA-ergic terminals on REM-ON neurons withdraws NA-release causing the REM-ON neurons to escape inhibition and being active, may be even momentarily. A working-model showing neurochemical-map explaining activation of inactivation process, showing contribution of GABA-ergic presynaptic inhibition in withdrawing NA-release and dis-inhibition induced activation of REM-ON neurons, which in turn activates other GABA-ergic neurons and shutting-off REM-OFF neurons for the initiation of REMS-generation has been explained. Our model satisfactorily explains yet unexplained puzzles (i) why normally REMS does not appear during waking, rather, appears following non-REMS; (ii) why cessation of LC-NA-ergic-REM-OFF neurons is essential for REMS-generation; (iii) factor(s) which does not allow cessation of REM-OFF neurons causes REMS-loss; (iv) the association of changes in levels of GABA and NA in the brain during REMS and its deprivation and associated symptoms; v) why often dreams are associated with REMS.

Pharmacological stimulation of locus coeruleus reveals a new antipsychotic-responsive pathway for deficient sensorimotor gating

Surprisingly little is known about the modulation of core endophenotypes of psychiatric disease by discrete noradrenergic (NE) circuits. Prepulse inhibition (PPI), the diminution of startle responses when weak prestimuli precede the startling event, is a widely validated translational paradigm for information-processing deficits observed in several mental disorders including schizophrenia, Tourette's syndrome, and post-traumatic stress disorder (PTSD). Despite putative NE disturbances in these illnesses, NE regulation of PPI remains poorly understood. In these studies, regulation of PPI by the locus coeruleus (LC), the primary source of NE to forebrain, was evaluated in rats using well-established protocols to pharmacologically activate/inactivate this nucleus. The ability of drugs that treat deficient PPI in these illnesses to reverse LC-mediated PPI deficits was also tested. Stimulation of LC receptors produced an anatomically and behaviorally specific deficit in PPI that was blocked by clonidine (Cataprese, an α2 receptor agonist that reduces LC neuronal firing after peri-LC delivery), a postsynaptic α1 NE receptor antagonist (prazosin), and second-generation antipsychotics (olanzapine, seroquel), but not by drugs that antagonized dopamine-1 (SCH23390), dopamine-2 (the first-generation antipsychotic Haloperidol), or serotonin-2 receptors (ritanserin). These results indicate a novel substrate in the regulation of PPI and reveal a novel functional role for the LC. Hence, a hyperactive LC-NE system might underlie a deficient sensorimotor gating endophenotype in a subset of patients suffering from psychiatric illnesses including schizophrenia, Tourette's syndrome, and PTSD, and the ability to normalize LC-NE transmission could contribute to the clinical efficacy of certain drugs (Cataprese, prazosin, and second-generation antipsychotics) in these conditions.

Central administration of NPY or an NPY-Y5 selective agonist increase in vivo extracellular monoamine levels in mesocorticolimbic projecting areas

Selective NPY-Y5 antagonists are known to reduce NPY-evoked increase of food intake under free feeding conditions and drug-reinforced operant responding in rodents suggesting that NPY-Y5 receptors can regulate reinforcers, potentially by modulating the hypothalamic-limbic reward system. However, evidence published to date has revealed a limited expression of NPY-Y5 in the limbic areas. Thus, the first aim of the present study was to investigate the distribution of NPY-Y5 receptor binding sites in rat mesocorticolimbic projection areas such as the nucleus accumbens (NAc), medial prefrontal cortex (mPFC), and lateral hypothalamus (LH). Since mesocorticolimbic release of monoamines has been typically associated to the rewarding and motivational significance of reinforcers, we then compared the ability of NPY and an NPY-Y5 selective agonist, [cPP1-7,NPY19-23,Ala31,Aib32,Gln34]hPP, to evoke changes in extracellular monoamines from these brain regions using in vivo microdialysis techniques. Intracerebral doses of each compound were selected on the basis of those previously demonstrated to trigger food intake in a separate set of animals. We found that NPY-Y5 receptors were widely distributed in both the NAc and mPFC but not in the LH nuclei. Central administration of either NPY (4.5 nmol/rat) or the NPY-Y5 agonist (0.6 nmol/rat) induced a significant increase of dopamine (DA) output of up to 150% of basal values in the NAc. In addition, NPY induced a stepped increase of norepinephrine (NE) outflow in the NAc area. Also extracellular levels of NE levels were increased by both treatments in the mPFC (150% vs basal concentration). Hypothalamic monoamine levels were unaffected by both treatments. Extracellular serotonin (5-HT) levels were also unchanged in all regions. Given the NPY-Y5 agonist paralleled the in vivo ability of NPY to increase DA, these data suggest that the release of NPY may modulate behaviours associated to accumbal DA release such reward and reinforcement by, at least in part, acting on mesocorticolimbic NPY-Y5 receptors.

Beneficial effects of desipramine on cognitive function of chronically stressed rats are mediated by alpha1-adrenergic receptors in medial prefrontal cortex

Chronic stress is a risk factor for many psychopathological conditions, including depression and anxiety disorders. Cognitive impairments associated with prefrontal cortical dysfunction are a major component of such illnesses. Using an attentional set-shifting test (AST), we have previously shown that elevating noradrenergic activity in rat medial prefrontal cortex (mPFC) can facilitate cognitive set-shifting, and that chronic unpredictable stress (CUS) caused set-shifting deficits. It is not known, however, if noradrenergic modulatory function is compromised by chronic stress, perhaps contributing to the stress-induced cognitive deficit. Thus, the first study investigated whether acutely elevating noradrenergic activity in mPFC still enhances cognitive function after chronic stress. As previously demonstrated, CUS impaired cognitive set-shifting on the AST. This deficit was abolished by acute systemic administration of the alpha(2)-adrenergic autoreceptor antagonist, atipamezole. Microdialysis revealed no differences in extracellular norepinephrine (NE) levels in mPFC of CUS-exposed and unstressed control rats at baseline or during behavioral testing, and comparable increases after atipamezole. In the second experiment, rats were treated chronically with the selective NE reuptake blocker, desipramine, during the CUS treatment through behavioral testing. Again, CUS impaired cognitive set-shifting in vehicle-treated rats, and chronic desipramine treatment prevented such deficits. Acute blockade of post-synaptic alpha(1)-adrenergic receptors in mPFC prior to testing blocked the beneficial effect of desipramine on cognitive set-shifting. These results suggest that desipramine restores cognitive set-shifting capability that has been compromised by chronic stress by activating alpha(1)-adrenergic receptors in the mPFC. Thus, noradrenergic modulatory capability in mPFC remains intact after CUS, and this represents one possible substrate by which antidepressants may exert their beneficial effects in the treatment of depression.

Anxiogenic modulation of spontaneous and evoked neuronal activity in the basolateral amygdala

The amygdala has a well-established role in stress, anxiety, and aversive learning, and anxiolytic and anxiogenic agents are thought to exert their behavioral actions via the amygdala. However, despite extensive behavioral data, the effects of noradrenergic anxiogenic drugs on neuronal activity within the amygdala have not been examined. The present experiments examined how administration of the anxiogenic drug yohimbine affects spontaneous and evoked neuronal activity in the basolateral amygdala (BLA) of rats. Yohimbine produced both excitatory and inhibitory effects on neurons of the BLA, with an increase in spontaneous activity being the predominant response in the lateral and basomedial nuclei of the BLA. Furthermore, yohimbine tended to facilitate neuronal responses evoked by electrical stimulation of the entorhinal cortex, with this facilitation seen more often in lateral and basomedial nuclei of the BLA. These data are the first to examine the effects of the anxiogenic agent yohimbine on BLA neuronal activity, and suggest that neurons in specific subnuclei of the amygdala exhibit unique responses to administration of such pharmacological agents.

Role of locus coeruleus heme oxygenase-carbon monoxide-cGMP pathway during hypothermic response to restraint

Central heme oxigenase-carbon monoxide (HO-CO) pathway has been shown to play a pyretic role in the thermoregulatory response to restraint. However, the specific site in the central nervous system where CO may act modulating this response remains unclear. LC is rich not only in sGC but also in heme oxygenase (HO; the enzyme that catalyses the metabolism of heme to CO, along with biliverdin and free iron). Therefore, the possible role of the HO-CO-cGMP pathway in the restraint-induced-hypothermia by LC neurons was investigated. Body temperature dropped about 0.7 degrees C during restraint. ZnDPBG (a HO inhibitor; 5 nmol, intra-LC) prevented the hypothermic response during restraint. Conversely, induction of the HO pathway in the LC with heme-lysinate (7.6 nmol, intra-LC) intensified the hypothermic response to restraint, and this effect was prevented by pretreatment with ODQ (a sGC inhibitor; given intracerebroventricularly, 1.3 nmol). Taken together, these data suggest that CO in the LC produced by the HO pathway and acting via cGMP is implicated in thermal responses to restraint.

Noradrenergic facilitation of shock-probe defensive burying in lateral septum of rats, and modulation by chronic treatment with desipramine

We have previously shown that acute stress-induced release of norepinephrine (NE) facilitates anxiety-like behavioral responses to stress, such as reduction in open-arm exploration on the elevated-plus maze and in social behavior on the social interaction test. Since these responses represent inhibition of ongoing behavior, it is important to also address whether NE facilitates a response that represents an activation of behavior. Correspondingly, it is unknown how a chronic elevation in tonic steady-state noradrenergic (NA) neurotransmission induced by NE reuptake blockade might alter this acute modulatory function, a regulatory process that may be pertinent to the anxiolytic effects of NE reuptake blockers such as desipramine (DMI). Therefore, in this study, we investigated noradrenergic modulation of the shock-probe defensive burying response in the lateral septum (LS). In experiment 1, shock-probe exposure induced an acute 3-fold increase in NE levels measured in LS of male Sprague-Dawley rats by microdialysis. Shock-probe exposure also induced a modest rise in plasma ACTH, taken as an indicator of perceived stress, that returned to baseline more rapidly in rats that were allowed to bury the probe compared to rats prevented from burying by providing them with minimal bedding, indicating that the active defensive burying behavior is an effective coping strategy that reduces the impact of acute shock probe-induced stress. In experiment 2, blockade of either alpha(1)- or beta-adrenergic receptors in LS by local antagonist microinjection immediately before testing reduced defensive burying and increased immobility. In the next experiment, chronic DMI treatment increased basal extracellular NE levels in LS, and attenuated the acute shock probe-induced increase in NE release in LS relative to baseline. Chronic DMI treatment decreased shock-probe defensive burying behavior in a time-dependent manner, apparent only after 2 weeks or more of drug treatment. Moreover, rats treated chronically with DMI showed no significant rise of plasma ACTH in response to shock-probe exposure. Thus, acute stress-induced release of NE in LS facilitated defensive burying, an active, adaptive behavioral coping response. Chronic treatment with the NE reuptake blocker and antidepressant drug DMI attenuated acute noradrenergic facilitation of the active burying response, and also attenuated the level of perceived stress driving that response. These results suggest that long-term regulation of the acute modulatory function of NE by chronic treatment with reuptake blockers may contribute to the mechanisms by which such drugs exert their anxiolytic effects in the treatment of stress-related psychiatric conditions, including depression and anxiety.

Respiratory and stress-induced activation of low-threshold motor units in the human trapezius muscle

The study aimed to characterize trapezius motor unit firing pattern in low-amplitude contractions, with emphasis on respiratory modulated activity. Constant-amplitude contractions with shoulder elevation, controlled by feedback of the root mean square detected surface electromyographic (SEMG) signal, typing with arm movement and tasks with mental stress were performed. Single motor unit activity was recorded by a quadrifilar fine-wire electrode. A surface electrode simultaneously recorded SEMG activity. Contraction amplitudes ranged from 1 to 10% of the SEMG signal at maximum voluntary contraction (1-10% EMG(max)). The majority ( approximately 80%) of motor units recorded during constant-amplitude contractions showed firing rate modulation at the respiratory frequency. Respiratory firing rate modulation was clear for low amplitude contractions (< 3% EMG(max)), but was reduced at higher amplitudes (3-5.9% EMG(max)). Most motor units had peak firing rate at the transition from inspiration to expiration, but peak firing rate at the transition from expiration to inspiration or at the first harmonic frequency was also observed. The SEMG signal showed little or no respiratory modulation, possibly because respiratory phase varied between motor units. Respiratory modulation of firing rates was significantly reduced in experiments with mental stress and was rarely observed in typing experiments. Both central respiratory drive and peripheral afferent input may contribute to respiratory modulation of firing rates; however, animal studies indicate a central source of the respiratory modulated input. We speculate that the reduction in respiratory modulation of motor activity with mental stress is due to activation of alternative pathways providing excitatory input to trapezius motoneurons.

Noradrenergic modulation of cognitive function in rat medial prefrontal cortex as measured by attentional set shifting capability

The brain noradrenergic system is thought to facilitate neuronal processes that promote behavioral activation, alertness, and attention. One region in which norepinephrine may exert such effects is the medial prefrontal cortex, which has been implicated in many cognitive functions including arousal, attention, motivation, working memory, response inhibition, and behavioral flexibility. The present study addressed the modulatory influence of noradrenergic neurotransmission in medial prefrontal cortex on cognitive function in rats, as measured by performance in an attentional set shifting task. In experiment 1, we tested effects of increasing and decreasing brain noradrenergic neurotransmission by systemic administration of the alpha2-adrenergic autoreceptor antagonist and agonist drugs, atipamezole and clonidine, respectively. Atipamezole pretreatment significantly improved performance on the stages of the attentional task requiring an extradimensional shift in attention, and those involving stimulus reversals, whereas clonidine had no effect at any stage. In experiment 2, we then tested effects of microinjecting alpha1- or beta-adrenergic receptor antagonists into medial prefrontal cortex on the enhancement of performance on the extradimensional task produced by atipamezole. The atipamezole-induced enhancement of performance on the extradimensional set shifting task was blocked by alpha1-, but not beta-adrenergic receptor antagonists in medial prefrontal cortex. Neither antagonist alone had any effect on extradimensional set shift performance in the absence of atipamezole-induced enhancement. These results indicate that elevating noradrenergic activity at alpha1-receptors in medial prefrontal cortex facilitates cognitive performance of rats in an attentional set-shifting task, which may contribute to the role of norepinephrine in behavioral state changes such as arousal, or to the beneficial cognitive effects of psychotherapeutic drugs that target noradrenergic neurotransmission.

Role of brain norepinephrine in the behavioral response to stress

The brain noradrenergic system is activated by acute stress. The post-synaptic effects of norepinephrine (NE), exerted at a cellular or neural circuit level, have been described as modulatory in nature, as NE facilitates responses evoked in target cells by both excitatory and inhibitory afferent input. Over the past few years, we have undertaken a series of studies to understand how these cellular modulatory effects of NE, elicited by acute stress, might translate into modulation of the behavioral-affective components of the whole-animal response to stress. Using microdialysis, we have demonstrated that acute immobilization stress activates NE release in a number of stress-related limbic forebrain target regions, such as the central and medial amygdala, lateral bed nucleus of the stria terminalis, medial prefrontal cortex, and lateral septum. Using microinjections of adrenergic antagonist drugs directly into these regions, we have shown that this stress-induced release of NE facilitates a number of anxiety-like behavioral responses that are mediated in these regions, including stress-induced reduction of open-arm exploration on the elevated plus-maze, stress-induced reduction of social interaction behavior, and activation of defensive burying behavior by contact with an electrified probe. Dysregulation of the brain noradrenergic system may be a factor in determining vulnerability to stress-related pathology, or in the interaction of genetic vulnerability and environmental sensitization. Compared to outbred Sprague-Dawley rats, we have shown that the modulatory effect of NE is deficient in Wistar-Kyoto rats, which also exhibit attenuated behavioral reactivity to acute stress, as well as increased vulnerability to stress-induced gastric ulcers and exaggerated activation of the hypothalamic-pituitary-adrenal (HPA) stress axis. Further, repeated exposure to mild intermittent cold stress resulted in a much greater sensitization of both the brain noradrenergic system and the HPA axis in Wistar-Kyoto rats compared to Sprague-Dawley rats. The recruitment of a robust noradrenergic facilitatory influence following repeated cold exposure in this previously deficient strain resulted in an aberrant HPA response, which may be illustrative of the kinds of neurobiological changes that may contribute to the development of stress-related neuropsychiatric disorders such as depression, post-traumatic stress disorder, or other anxiety disorders in predisposed or susceptible individuals. On the other side of the same issue, regulatory alterations in noradrenergic neurotransmission, or in the stress-modulatory functions of NE, may be important in the behavioral effects of chronic antidepressant drug treatment. We present recent preliminary results addressing the effects of chronic treatment with the selective NE reuptake inhibitor, desipramine, on acute behavioral reactivity to stress. A better understanding of the role of NE in adaptive responses to acute stress, the pathological consequences of prolonged, repeated or severe stress, and the mechanisms of action of drugs used to treat stress-related diseases, may contribute to the future development of more effective strategies for the treatment or even prevention of such disorders.

What should animal models of depression model?

In this article, we discuss what animal models of depression should be attempting to 'model'. One must first determine if the goal is to model the regulatory mechanisms by which antidepressant treatments alleviate the various symptoms of depression, or to model the dysregulatory mechanisms underlying the etiology of those symptoms. When modeling the mechanisms of antidepressant effects, a key feature that is often overlooked is the time course required for behavioral efficacy. Even in the clinical literature, there is considerable confusion and inconsistency in defining and identifying 'time of onset' of clinical effect. Although the 'therapeutic lag' may not be as long as has been commonly believed, it does occur. Observable improvement in either global symptomatology or specific symptoms becomes evident after 7-14 days of treatment, and more complete recovery takes considerably longer. Thus, any model addressing potential mechanisms of antidepressant action should exhibit a similar time-dependency. Second, whether attempting to address mechanisms underlying behavioral effects of antidepressants, or the neurobiological substrates underlying the development and manifestation of depression, it is essential to recognize that the syndrome of depression is a diagnostic construct that includes a variety of disparate symptoms, some of which may be related mechanistically, and others that may not be specific to depression, but may cut across categorical diagnostic schemes. Further, it is critical to recognize the close relationship of depression and anxiety. Psychological studies have suggested that the myriad symptoms of depression and anxiety may be subsumed within a more limited number of distinct behavioral dimensions, such as negative affect (neuroticism), positive affect, or physiologic hyperarousal. These dimensions may be related to the functioning of specific neurobiological systems. Thus, rather than trying to recreate or mimic the entire spectrum of symptoms comprising the syndrome of depression, it may be more informative to develop animal models for these behavioral dimensions. Such models may then provide access not only to the neural regulatory mechanisms underlying effective antidepressant treatment, but may also provide clues to the processes underlying the development and manifestation of depression.

Effect of number of tailshocks on learned helplessness and activation of serotonergic and noradrenergic neurons in the rat

Adult male albino rats were exposed to varying numbers of tailshocks (0, 10, 50 or 100). The following day, their escape latencies in a shuttlebox were measured in order to estimate the degree of learned helplessness (LH) produced by the varying number of shocks. Only the groups exposed to 50 or 100 shocks displayed evidence of LH. In a parallel experiment, c-fos activation was used to determine the degree of activation of raphe serotonergic neurons (FosIR+5-HT) and locus coeruleus (LC) noradrenergic neurons (FosIR+TH) produced by the same shock conditions. Compared to unhandled cage controls, all shock groups (0 shocks was a restrained group) significantly activated both raphe and LC neurons. The 50 and 100 shock groups had significantly higher degrees of activation of serotonergic neurons in the rostral raphe groups and the LC than the 0 and 10 shock groups. These data are consistent with the hypothesis that activation of rostral raphe serotonergic neurons and LC noradrenergic neurons beyond a certain threshold may be critical for the development of LH. The relevance of these results for elucidating the neural bases of psychopathology is discussed.

Recovery of hippocampal cell proliferation and BDNF levels, both of which are reduced by repeated restraint stress, is accelerated by chronic venlafaxine

The present study investigated the poststress (PS) cellular and molecular changes in the hippocampus of rats subjected to repeated restraint stress (RS) and the effects of chronic administration of an antidepressant drug, venlafaxine, on these changes. It was found that RS suppressed hippocampal cell proliferation, decreased brain-derived neurotrophic factor (BDNF) levels, and increased both the levels of copper/zinc superoxide dismutase (Cu/Zn-SOD) and the number of Cu/Zn-SOD immunostained hippocampal interneurons. In venlafaxine-treated rats, the changes in cell proliferation, BDNF levels, and the number of Cu/Zn-SOD interneurons returned to control levels on PS Days 21, 14, 7, respectively. In vehicle-injected rats, BDNF and the number of Cu/Zn-SOD interneurons returned to control levels on PS Days 21 and 14, respectively, but cell proliferation was still suppressed on PS Day 21. The stress-induced elevation of Cu/Zn-SOD protein remained during the 3-week PS period, and it was further increased by about 20% after 3 weeks of venlafaxine administration.

Onset and early behavioral effects of pharmacologically different antidepressants and placebo in depression

This study was aimed at resolving the time course of clinical action of antidepressants (ADs) and the type of early behavioral changes that precede recovery in treatment-responsive depressed patients. The first goal was to identify, during the first 2 weeks of treatment, the onset of clinical actions of the selective serotonin reuptake inhibitor (SSRI), paroxetine, and the selective noradrenergic reuptake inhibitor, desipramine (DMI). The second aim was to test the hypothesis that the two pharmacologic subtypes would induce different early behavioral changes in treatment-responsive patients. The design was a randomized, parallel group, placebo-controlled, double-blind study for 6 weeks of treatment following a 1-week washout period. The study utilized measures of the major behavioral components of the depressive disorder as well as overall severity. The results indicated that the onset of clinical actions of DMI ranged from 3 to 13 days, averaged 13 days for paroxetine, and was 16-42 days for placebo. Furthermore, as hypothesized, the different types of ADs initially impacted different behavioral aspects of the disorder. After 1 week of treatment, DMI produced greater reductions in motor retardation and depressed mood than did paroxetine and placebo, and this difference persisted at the second week of treatment. Early improvement in depressed mood-motor retardation differentiated patients who responded to DMI after 6 weeks of treatment from those that did not. Paroxetine initially reduced anxiety more in responders than in nonresponders, and by the second week, significantly improved depressed mood and distressed expression in responders to a greater extent. Depressed patients who responded to placebo showed no consistent early pattern of behavior improvement. Early drug-specific behavioral changes were highly predictive of ultimate clinical response to the different ADs, results that could eventually be applied directly to clinical practice.

Chronic cold stress sensitizes brain noradrenergic reactivity and noradrenergic facilitation of the HPA stress response in Wistar Kyoto rats

Many psychiatric disorders, including depression, post-traumatic stress disorder and other anxiety disorders, result from an interaction between genetic factors and exposure to a sufficiently sensitizing environmental stressor. The inbred Wistar Kyoto (WKY) rat strain has been proposed as a model of stress vulnerability, exhibiting an exaggerated hypothalamic-pituitary-adrenal (HPA) response to stress and susceptibility to gastric ulceration. Previously, we showed that stress-activation of the brain noradrenergic system was deficient in WKY rats, and they lacked noradrenergic facilitation of the HPA response in the lateral bed nucleus of the stria terminalis (BSTL), compared to outbred Sprague-Dawley (SD) controls. Deficient modulatory function of the noradrenergic system may contribute to the stress susceptibility of WKY rats. Thus, we investigated the influence of a sensitizing stimulus, chronic intermittent cold exposure, on neuroendocrine and noradrenergic stress reactivity, and on noradrenergic facilitation of the HPA response in these two strains. Chronic cold exposure (7 days, 4 h/day, 4 degrees C) potentiated activation of the HPA axis by acute immobilization stress, assessed by measuring plasma adrenocorticotropic hormone (ACTH), in both strains, although to a greater extent in WKY rats, and enhanced stress-induced norepinephrine (NE) release in BSTL of WKY but not SD rats. We then compared the influence of chronic cold exposure on noradrenergic modulation of the HPA stress response in BSTL, by measuring changes in acute stress-induced elevation of plasma ACTH after microinjecting the alpha(1)-adrenoreceptor antagonist benoxathian into the BSTL. As shown previously, benoxathian attenuated stress-induced ACTH secretion in control SD but not control WKY rats. After chronic cold, the ACTH response to acute stress was attenuated by benoxathian administration into BSTL of both strains, such that the WKY response was not different from that of SD rats. Thus, chronic cold not only sensitized the release of NE in BSTL of WKY rats, but also restored noradrenergic facilitation of their already-elevated HPA response. Such functional sensitization of a previously-deficient facilitatory system may be one mechanism whereby exposure to repeated or severe stress may induce pathologic dysregulation of the stress response in susceptible individuals, resulting in psychiatric illness.

Stress reactivity of the brain noradrenergic system in three rat strains differing in their neuroendocrine and behavioral responses to stress: implications for susceptibility to stress-related neuropsychiatric disorders

The brain noradrenergic system is activated by stress, modulating the activity of forebrain regions involved in behavioral and neuroendocrine responses to stress. In this study, we characterized brain noradrenergic reactivity to acute immobilization stress in three rat strains that differ in their neuroendocrine stress response: the inbred Lewis (Lew) and Wistar-Kyoto (WKY) rats, and outbred Sprague-Dawley (SD) rats. Noradrenergic reactivity was assessed by measuring tyrosine hydroxylase mRNA expression in locus coeruleus, and norepinephrine release in the lateral bed nucleus of the stria terminalis. Behavioral measures of arousal and acute stress responsivity included locomotion in a novel environment, fear-potentiated startle, and stress-induced reductions in social interaction and open-arm exploration on the elevated-plus maze. Neuroendocrine responses were assessed by plasma adrenocorticotropic hormone. Compared to SD, adrenocorticotropic hormone responses of Lew rats were blunted, whereas those of WKY were enhanced. The behavioral effects of stress were similar in Lew and SD rats, despite baseline differences. Lew had similar elevations of tyrosine hydroxylase mRNA, and initially greater norepinephrine release in the lateral bed nucleus of the stria terminalis during stress, although both noradrenergic responses returned toward baseline more rapidly than in SD rats. WKY rats showed depressed baseline startle and lower baseline exploratory and social behavior than SD. However, unlike the Lew or SD rats, WKY exhibited a lack both of fear potentiation of the startle response and of stress-induced reductions in exploratory and social behavior, indicating attenuated stress responsivity. Acute noradrenergic reactivity to stress, measured by either tyrosine hydroxylase mRNA levels or norepinephrine release, was also attenuated in WKY rats. Thus, reduced arousal and behavioral responsivity in WKY rats may be related to deficient brain noradrenergic reactivity. This deficit may alter their ability to cope with stress, resulting in the exaggerated neuroendocrine responses and increased susceptibility to stress-related pathology exhibited by this strain.

The locus coeruleus, Barrington's nucleus, and neural circuits of stress

Much attention has focused on the role of the locus coeruleus (LC) as a component of the central neural circuitry involved in stress. Many, though not all, stressful stimuli produce activation of LC neurons, as reflected by increased Fos expression in these neurons. Stimulation of the LC elicits many stress-like responses, including increased ACTH secretion, though not all responses to LC stimulation are readily interpretable in the context of stress. In particular, stimulation of the LC, at least in anesthetized rats, elicits a decrease in blood pressure and heart rate. Inhibition of the LC has been reported to inhibit certain responses to stress, including inhibition of ACTH release in response to certain stressors. Furthermore, local inhibition of the LC prevents foot shock-evoked Fos expression in certain brain areas. In the studies of the role of the LC in stress, one complicating factor has been the inadequate attention given to Barrington's nucleus (BN), which is located adjacent to the LC. Although BN is best recognized for its role in the control of micturition, the fact that it is activated by a great variety of stressful stimuli, and that it is anatomically connected to multiple output systems involved in stress responses, suggest that it may play a role in the neural circuitry subserving responses to stress.

Modulatory effects of norepinephrine, acting on alpha 1 receptors in the central nucleus of the amygdala, on behavioral and neuroendocrine responses to acute immobilization stress

The central nucleus of the amygdala (CeA) is a component of the limbic fear-anxiety circuit, and has also been implicated in regulation of the hypothalamic-pituitary-adrenal (HPA) stress axis. The CeA receives dense noradrenergic innervation, and is rich in expression of alpha(1)-adrenergic receptors. We hypothesized that norepinephrine (NE), acting on alpha(1) receptors in CeA, may modulate stress-induced anxiety-like behavioral responses and HPA activation. To investigate the role of alpha(1) adrenergic receptors in CeA on stress-induced behavioral reactivity, the alpha(1) antagonist benoxathian was microinjected bilaterally into CeA of male Sprague-Dawley rats, and anxiety-like behavioral responses to acute immobilization stress were measured on the Social Interaction (SI) test and on the Elevated Plus-maze (EPMZ). Benoxathian dose dependently blocked the reduction in SI time induced by immobilization stress, whereas beta-receptor antagonists had no effect, consistent with an absence of beta-receptors in CeA. By contrast, in separate experiments, benoxathian had no effect on stress-induced reduction in open-arm exploratory behavior on the EPMZ, nor on stress-induced plasma ACTH secretion. These results confirm that the SI test and EPMZ measure different aspects of behavioral stress reactivity that can be modulated independently, and likewise, that noradrenergic modulation of behavioral stress reactivity can occur independently of modulation of the HPA axis.

Modulatory effects of norepinephrine in the lateral bed nucleus of the stria terminalis on behavioral and neuroendocrine responses to acute stress

The brain noradrenergic system is activated by stress, and modulates the activity of forebrain regions involved in behavioral and neuroendocrine responses to stress, such as the lateral bed nucleus of the stria terminalis (BSTL). This region of the limbic forebrain receives dense noradrenergic innervation, and has been implicated in both anxiety and regulation of the hypothalamic-pituitary-adrenal axis. We hypothesized that stress-induced release of norepinephrine in the BSTL modulates anxiety-like behavioral responses to stress and activation of the hypothalamic-pituitary-adrenal stress axis. Using microdialysis, we showed that release of norepinephrine was increased in the BSTL of male Sprague-Dawley rats during immobilization stress. In the next experiment, we then microinjected noradrenergic antagonists into the BSTL immediately prior to acute immobilization stress to examine noradrenergic modulation of behavioral stress reactivity. Either the alpha(1)-receptor antagonist benoxathian, or a cocktail of beta(1)- and beta(2)-receptor antagonists (betaxolol+ICI 118,551) blocked the anxiety-like reduction in open-arm exploration on the elevated plus-maze, but not the reduction in social behavior induced in the social interaction test. In a third experiment, benoxathian reduced plasma levels of adrenocorticotropic hormone following stress, but beta-receptor antagonists had no effect. From these results we suggest that stress-induced norepinephrine release acts on both alpha(1)- and beta-receptors in the BSTL to facilitate anxiety-like behavioral responses on the plus-maze but not the social interaction test, and modulates hypothalamic-pituitary-adrenal axis activation via alpha(1)-receptors only. Together with previous results in which adrenergic antagonists in central amygdala attenuated behavioral responses on the social interaction test but not the plus-maze, these observations suggest the two behavioral tests measure different dimensions of stress reactivity, and that norepinephrine facilitates different components of the stress response by region- and receptor-specific mechanisms.

Low-frequency oscillations (<0.3 Hz) in the electromyographic (EMG) activity of the human trapezius muscle during sleep

The surface electromyographic (EMG) signal from right and left trapezius muscles and the heart rate were recorded over 24 h in 27 healthy female subjects. The root-mean-square (RMS) value of the surface EMG signals and the heartbeat interval time series were calculated with a time resolution of 0.2 s. The EMG activity during sleep showed long periods with stable mean amplitude, modulated by rhythmic components in the frequency range 0.05-0.2 Hz. The ratio between the amplitude of the oscillatory components and the mean amplitude of the EMG signal was approximately constant over the range within which the phenomenon was observed, corresponding to a peak-to-peak oscillatory amplitude of approximately 10% of the mean amplitude. The duration of the periods with stable mean amplitude ranged from a few minutes to approximately 1 h, usually interrupted by a sudden change in the activity level or by cessation of the muscle activity. Right and left trapezius muscles presented the same pattern of FM. In supplementary experiments, rhythmic muscle activity pattern was also demonstrated in the upper extremity muscles of deltoid, biceps, and forearm flexor muscles. There was no apparent association between the rhythmic components in the muscle activity pattern and the heart rate variability. To our knowledge, this is the first time that the above-described pattern of EMG activity during sleep is documented. On reanalysis of earlier recorded trapezius motor unit firing pattern in experiments on awake subjects in a situation with mental stress, low-FM of firing with similar frequency content was detected. Possible sources of rhythmic excitation of trapezius motoneurons include slow-wave cortical oscillations represented in descending cortico-spinal pathways, and/or activation by monoaminergic pathways originating in the brain stem reticular formation. The analysis of muscle activity patterns may provide an important new tool to study neural mechanisms in human sleep.

In vivo monitoring of evoked noradrenaline release in the rat anteroventral thalamic nucleus by continuous amperometry

Continuous amperometry coupled with untreated carbon-fibre electrodes was used in anaesthetized rats to measure the noradrenaline release evoked in the anteroventral thalamic nucleus by electrical stimulation of the dorsal noradrenergic bundle. As expected, the variations in the oxidation current detected in the anteroventral thalamic nucleus exhibited the characteristics of the in vivo noradrenaline release. They were closely correlated with stimulation and consistent with the anatomy of the noradrenergic system involved. They were abolished by the ejection of tetrodotoxin in the vicinity of the carbon-fibre electrode, diminished by clonidine, an alpha-2 agonist, and restored by yohimbine, an alpha-2 antagonist. Furthermore, the time course of these variations was dramatically increased by desipramine, a specific noradrenaline reuptake blocker. In contrast, neither dopamine nor serotonin reuptake blockers, nor the monoamine oxidase inhibitor pargyline were able to alter them. The main advantage of the present approach is its excellent time resolution. We show here for the first time that after single pulse stimulation, noradrenaline is released and eliminated in 118 milliseconds, this time lapse corresponding to the maximal period beyond which subsequent noradrenaline releases could not add up. These observations are in good agreement with the physiological relationship previously observed between impulse flow and noradrenaline overflow.

Inhibition by venlafaxine of the increase in norepinephrine output in rat prefrontal cortex elicited by acute stress or by the anxiogenic drug FG 7142

Venlafaxine is an antidepressant drug that inhibits the reuptake of serotonin and norepinephrine with different efficacies. The effects of repeated administration of this drug on the increase in the extracellular concentration of norepinephrine in the prefrontal cortex, induced by stress or by the anxiogenic drug FG 7142, were studied in freely moving rats. Exposure to foot-shock stress induced a marked increase (+120%) in the extracellular norepinephrine concentration in the prefrontal cortex of control rats. Long-term administration of venlafaxine (10 mg/kg i.p., once a day for 21 days) reduced the effect of stress on norepinephrine output by 75%. This effect of venlafaxine persisted for at least 5 days after discontinuation of drug treatment. Acute administration of FG 7142 (20 mg/kg i.p.), a benzodiazepine receptor inverse agonist, increased norepinephrine output (+90%) in control rats. Chronic treatment with venlafaxine prevented the effect of FG 7142. In contrast, the acute administration of this antidepressant had no effect on the stress- or FG 7142-induced increase in norepinephrine output. These plastic changes in the sensitivity of norepinephrine neurones to foot-shock stress and to an anxiogenic drug may reveal an important neuronal mechanism for the physiological regulation of emotional state. Furthermore, this mechanism might be relevant to the anxiolytic and antidepressant effects of venlafaxine.

Chronic treatment with imipramine or mirtazapine antagonizes stress- and FG7142-induced increase in cortical norepinephrine output in freely moving rats

The effect of repeated administration of imipramine or mirtazapine, two antidepressant drugs with different mechanisms of action, was studied on the stress-induced increase in the extracellular concentration of norepinephrine in the prefrontal cortex of freely moving rats. Exposure to footshock in control rats induced a marked increase in extracellular norepinephrine concentrations in the prefrontal cortex (+120%). Long-term administration with imipramine or mirtazapine (10 mg/kg, i.p., twice or once a day, respectively, for 14 days) reduced (+50%) the effect of stress on basal norepinephrine output. Acute administration of FG7142 (30 mg/kg, i.p.), an anxiogenic benzodiazepine receptor inverse agonist, induced a marked increase in norepinephrine output (+90%) in control rats. In rats chronically treated with imipramine or mirtazapine this effect was completely antagonized. On the contrary, acute administration of these antidepressant drugs failed to reduce stress- and FG7142-induced increase in norepinephrine output. The plastic changes in the sensitivity of norepinephrine neurons to footshock stress and drug-induced anxiogenic stimuli may reveal a new important neuronal mechanism involved in the long-term modulation of emotional state. This action might be relevant for the anxiolytic and antidepressant effect of antidepressant drugs.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Effects of acute restraint stress on tyrosine hydroxylase mRNA expression in locus coeruleus of Wistar and Wistar-Kyoto rats

Norepinephrine (NE) is thought to play a role in the stress response, and may be involved in stress-related psychopathological conditions such as depression or anxiety. Heterogeneity in individual responses to the same stressor suggest that a genetic susceptibility to the effects of stress may contribute to such pathology. To address possible mechanisms underlying this genetic aspect of the stress response, we examined acute stress-induced changes in mRNA expression for several components of the NE system in the locus coeruleus (LC) and adrenal medullae of stress-susceptible Wistar-Kyoto (WKY) rats and their parent Wistar (W) strain. Expression of tyrosine hydroxylase (TH), NE transporter (NET) and alpha(2A) receptor mRNA were measured in the LC by in situ hybridization 30 min and 2 h after the onset of 30 min restraint stress. Adrenal TH mRNA was measured by slot blots. No basal differences were observed for any measure, but in the LC, expression of TH mRNA increased by 40% in W rats at 30 min (n=8, p<0.05) and returned toward baseline by 2 h, while WKY rats showed only a non-significant 29% increase at 2 h. In contrast, adrenal TH mRNA expression increased in WKY rats at 2 h (n=3, p<0.05), with no significant change in W rats. NET and alpha(2A) mRNA were unaltered by restraint stress in both strains. Differences in the stress-reactivity of TH gene expression in the central and peripheral noradrenergic systems may be related to differences in behavioral coping strategies and autonomic responsivity to stress in these strains, and suggest that differences in noradrenergic reactivity may contribute to genetic susceptibility to stress-related pathology.

Cerebellar and spinal projections of the coeruleus complex in the duck: a fluorescent retrograde double-labeling study

The double fluorescent retrograde tracing technique was used to identify, within the coeruleus complex (Co complex) of the duck, the nerve cells projecting to the cerebellar cortex and to the spinal cord. This technique was also used to investigate the possibility that the cerebellar and spinal projections of the Co complex are collaterals of the same axons. In the same animal, nuclear Diamidino yellow dihydrochloride (DY) fluorescent tracer was placed into the cerebellar cortex of folia V-VII, and cytoplasmic fluorescent Fast blue (FB) dye was injected into C3-C4 spinal cord segments. FB labeled multipolar somata and DY fluorescent nuclei were intermingled within the dorsal caudal region of the locus coeruleus (LCo) and within the dorsal division of the nucleus subcoeruleus (dSCo). Moreover, in the LCo, a low proportion of double-labeled neurons (about 3-4% of labelings) was evidenced among single-labeled neurons. In the ventral division of the nucleus subcoeruleus (vSCo), occasional DY labeled nuclei were found, whereas FB-labeled cells were frequently present. The present findings reveal the location of the coeruleocerebellar and coeruleospinal projecting neurons within the Co complex of the duck. They are intermingled in the caudal portion of the LCo and along the rostrocaudal extent of the subjacent dSco. The LCo and the dSCo are the major source of the projections to the folia V-VII, whereas the vSCo contributes very slightly to the innervation of the cerebellar injected areas. Moreover, the double-labeling study demonstrates that in the duck a low percentage of neurons within the ventrolateral portion of the caudal region of the LCo projects both to the cerebellar cortex of folia V-VII and to C3-C4 spinal cord segments via collaterals. Therefore, these neurons simultaneously influence the cerebellar cortex and spinal cord. The possibility that the projections studied are noradrenergic and that they play a role in feeding is discussed.

Facilitation of cognitive functions by a specific alpha2-adrenoceptor antagonist, atipamezole

The present experiments investigated the effects of a specific and potent alpha2-adrenoceptor antagonist, atipamezole (as a stimulator of the noradrenergic system) on cognitive performance in rats. Atipamezole enhanced the acquisition of a linear-arm maze test and also improved the choice accuracy of poorly performing rats in a delayed (20 min) three-choice maze test. Furthermore, atipamezole improved the achievement of a one-trial appetite-maze when injected immediately after teaching, thus having an effect on consolidation. Atipamezole clearly impaired the acquisition of the active avoidance test. The present results indicate that stimulation of noradrenergic system by atipamezole improves the performance of animals in tasks assessing relational learning and memory, possibly affecting attention, short-term memory and the speed of information processing. It has also an effect on a consolidation process unrelated to attentional or motivational mechanisms. In a stressful test. stimulation of noradrenaline release leads to impairment of performance.

Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. In the present review we argue that noradrenaline affects aggression on three different levels: the hormonal level, the sympathetic autonomous nervous system, and the central nervous system (CNS), in different, but functionally synergistic ways. Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. Indirect CNS effects include: the shift of attention towards socially relevant stimuli; the enhancement of olfaction (a major source of information in rodents); the decrease in pain sensitivity; and the enhancement of memory (an aggressive encounter is very relevant for the future of the animal). Concerning more aggression-specific effects one may notice that a slight activation of the central noradrenergic system stimulates aggression, while a strong activation decreases fight readiness. This biphasic effect may allow the animal to engage or to avoid the conflict, depending on the strength of social challenge. A hypothesis is presented regarding the relevance of different adrenoceptors in controlling aggression. It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors. The latter phenomenon may be dependent on a noradrenaline-induced corticosterone secretion. It appears that by activating very different mechanisms the systems working with adrenaline and/or noradrenaline prepare the animal in a very complex way to answer the demands imposed by, and to endure the effects caused by, fights. It is a challenge for future research to elucidate how precisely these mechanisms interact to contribute to functionally relevant and adaptive aggressive behaviour.

Distribution of alpha1A adrenergic receptor mRNA in the rat brain visualized by in situ hybridization

Norepinephrine has been implicated in a number of physiological, behavioral, and cellular modulatory processes in the brain, and many of these modulatory effects are attributable to alpha1 adrenergic receptors. At least three alpha1 receptor subtypes have been identified by molecular criteria, designated alpha1A, alpha1B, and beta1D. The distributions of alpha1B and alpha1D receptor mRNA expression in rat brain have been described previously, but the cDNA for the rat alpha1A receptor has only recently been cloned and characterized. In the present study, we used a radiolabelled riboprobe derived from the rat alpha1A receptor cDNA to describe the distribution of alpha1A message expression in the rat brain. The highest levels of alpha1A adrenergic receptor mRNA expression were seen in the olfactory bulb, tenia tectae, horizontal diagonal band/magnocellular preoptic area, zona incerta, ventromedial hypothalamus, lateral mammillary nuclei, ventral dentate gyrus, piriform cortex, medial and cortical amygdala, magnocellular red nuclei, pontine nuclei, superior and lateral vestibular nuclei, brainstem reticular nuclei, and several cranial nerve motor nuclei. Dual in situ hybridization combining a radioactive riboprobe for choline acetyltransferase mRNA with a digoxigenin-labeled alpha1A riboprobe in the fifth and seventh cranial nerve motor nuclei showed that the alpha1A mRNA is expressed in cholinergic motor neurons. Prominent alpha1A hybridization signal was also seen in the neocortex, claustrum, lateral amygdala, ventral cochlear nucleus, raphe magnus, and in the ventral horn of thoracic spinal cord. This overall pattern of expression, considered in comparison with that previously described for the other alpha1 adrenergic receptor subtypes, may shed light on the different roles of the alpha1 receptors in mediating the neuromodulatory effects of norepinephrine in processes such as arousal, neuroendocrine control, sensorimotor regulation, and the stress response.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Role of the amygdala in the coordination of behavioral, neuroendocrine, and prefrontal cortical monoamine responses to psychological stress in the rat

Exposure to mild stress is known to activate dopamine (DA), serotonin (5-HT), and norepinephrine (NE) metabolism in the anteromedial prefrontal cortex (m-PFC). Neuroanatomical site(s) providing afferent control of the stress activation of the m-PFC monoaminergic systems is at present unknown. The present study used a conditioned stress model in which rats were trained to fear a substartle-threshold tone paired previously with footshock and assessed for behavioral, neuroendocrine, and neurochemical stress responses. Bilateral NMDA-induced excitotoxic lesioning of the basolateral and central nuclei of the amygdala was performed before or after training. Pretraining amygdala lesions blocked stress-induced freezing behavior, ultrasonic vocalizations, adrenocortical activation, and dopaminergic metabolic activation in the m-PFC. Post-training amygdala lesions blocked stress-induced m-PFC DA, 5-HT, and NE metabolic activation. Post-training amygdala lesions also blocked stress-induced freezing and defecation, and greatly attenuated adrenocortical activation. These data provide evidence of amygdalar control of stress-induced metabolic activation of the monoaminergic systems in the m-PFC, as well as amygdalar integration of behavioral and neuroendocrine components of the rat stress response. These results are discussed in terms of possible relevance to stress-induced exacerbation of schizophrenic symptoms and the pathophysiology of posttraumatic stress disorder.

Serotonergic axons in monkey prefrontal cerebral cortex synapse predominantly on interneurons as demonstrated by serial section electron microscopy

Anatomical approaches were used to describe the distribution, appearance, and synaptic interactions of serotonin (5-HT)-immunoreactive axons in monkey prefrontal cortex. A plexus of 5-HT axons was found throughout the gray matter, with an especially high density in layer I and a slight increase in layer IV. They were strikingly heterogeneous, with a gradient of morphologies ranging from fine and nonvaricose to highly varicose or thick and nonvaricose. Electron microscopy showed that both varicose and nonvaricose axons were typically filled with clear vesicles and less abundant dense core vesicles. A serial section analysis of 5-HT varicosities in layers I, III, and V showed consistent results across layers. Only about 23% of labeled varicosities formed identifiable synapses. These synapses were consistently asymmetric and were 2-5 serial sections (or 0.08-0.38 mu) in diameter. Targets of identified 5-HT synapses were dendritic shafts with the exception of one cell soma. Followed in serial sections, postsynaptic dendrites typically had morphological features of interneurons, i.e. they lacked spines, had a high density of synaptic inputs, and often had a varicose morphology. Only 8% of postsynaptic shafts were classified as pyramidal dendrites. This is in striking contrast to our previous study in this cortex of dopamine axons, which synapsed predominantly on pyramidal dendrites. These are the first results to indicate that interneurons are the major recipient of identifiable 5-HT synapses in the monkey prefrontal cortex.

Potentiation by propranolol of stress-induced changes in passive avoidance and open-field emergence tests in mice

Noradrenergic and serotonergic systems are known to be stimulated during various forms of stress. The present study examined the effect of the beta-adrenergic serotonin1A receptor blocker propranolol on the ability of stress to elicit behavioral inhibition in mice. Mice were given the drug before immobilization or tube-restraint stress, and then were tested for either passive avoidance performance or time to emerge into an open field. Propranolol markedly potentiated stress-induced increases in latency in both of these tests, suggesting that it exacerbated reactions to stress. These results agree with previous data indicating that under certain conditions, propranolol can potentiate the effects of stress in rodents. The results support the hypothesis that the response of the noradrenergic and/or serotonergic systems to stress may have anxiolytic or antistress effects.

Possible noradrenergic dysfunction in schizophrenia

In spite of extensive studies over the last 2 decades to find direct evidence in support of the dopamine hypothesis of schizophrenia, no undisputed experimental data has been obtained. In contrast, estimation of noradrenalin (another major catecholamine) and its metabolites in postmortem brain and in the cerebrospinal fluid appears to be producing consistent results. To understand the meaning of this change for the pathogenesis of the illness, we have carried out animal experiments in which reproducibility of schizophrenic signs and symptoms by noradrenergic dysfunction, and treatability of the disorder by modulation of noradrenergic activity were studied. First, psychophysiological signs in skin conductance responsiveness (nonhabituating or nonresponding change) and smooth pursuit eye movement (spiky or stepwise pursuit) could be reproduced by enhancing or suppressing central noradrenergic activity. Behavioral abnormalities resembling schizophrenic symptoms are known to be reproducible by over- or underactivity of the system (overarousal or underarousal syndrome). Secondly, the action of various drugs capable of modulating schizophrenic symptoms was analyzed in relation to noradrenergic activity. Haloperidol, in particular, had a potent suppressing effect on skin conductance activity (spontaneous fluctuation rate and habituation rate) when administered chronically, suggesting its inhibitory action on noradrenergic activity.