Monoamine oxidase inhibitors reduce conditioned fear stress-induced freezing behavior in rats

[Psychopharmacotherapy with the MAO-inhibitor Tranylcypromine Key Aspects and Trends in Theory and Practice]

The irreversible monoamine oxidase inhibitor tranylcypromine has been known as an antidepressant drug for more than 60 years. The aim of this review was to make an assessment of the state of the art and therapy of tranylcypromine. The recent medical-scientific literature is analyzed and discussed with respect to key aspects of and general trends in practical psychopharmacotherapy. Meta-analyses of controlled clinical studies have shown that tranylcypromine is an established approach to treatment-resistant depression. Doses (maximum dose, maintenance dose) are increasingly adapted to the requirements of treatment-resistant depression. Monoamine oxidase is not only the primary pharmacological target of tranylcypromine but determines for the first doses also the pharmacokinetics of tranylcypromine because monoamine oxidase is also an enantioselective drug-metabolizing enzyme of the monoamine oxidase inhibitor. An increased diversity of the antidepressant pharmacotherapy suggests the need to rethink the continuing assessment of tranylcypromine as a therapeutic "ultima ratio" in depression. In conclusion, tranylcypromine as a drug of second choice remains a valuable option in antidepressant treatment. Criteria of a switch from other antidepressant drugs to tranylcypromine should be better defined.

Amygdala ensembles encode behavioral states

Internal states, including affective or homeostatic states, are important behavioral motivators. The amygdala regulates motivated behaviors, yet how distinct states are represented in amygdala circuits is unknown. By longitudinally imaging neural calcium dynamics in freely moving mice across different environments, we identified opponent changes in activity levels of two major, nonoverlapping populations of basal amygdala principal neurons. This population signature does not report global anxiety but predicts switches between exploratory and nonexploratory, defensive states. Moreover, the amygdala separately processes external stimuli and internal states and broadcasts state information via several output pathways to larger brain networks. Our findings extend the concept of thalamocortical "brain-state" coding to include affective and exploratory states and provide an entry point into the state dependency of brain function and behavior in defined circuits.

Effect of Scopolamine on Mice Motor Activity, Lick Behavior and Reversal Learning in the IntelliCage

Automated homecage monitoring systems are now widely recognized and used tools in cognitive neuroscience. However, few of these studies cover pharmacological interventions. Scopolamine, an anticholinergic memory disrupting agent is frequently used to study learning behavior. We studied the impact of scopolamine treatment in a relevant dose-range on activity, drinking behavior and reversal learning of C57BL/DJ mice in a homecage-like, social environment, using the IntelliCage. Naïve mice were first habituated to the IntelliCage, where they learned to nosepoke in any of the four corners in order to gain access to the water reward. Visits, nosepokes, lick numbers and durations were recorded. Mice were then trained to distinguish between a rewarded correct corner and punished, incorrect corners. Later, in the reversal learning phase, the assigned correct corner was rotated clockwise every 24 h. Upon s.c. administration of scopolamine general activity represented by visit and nosepoke numbers increased, but their durations were shorter. Surprisingly, general activity and lick behavior were drastically altered. Scopolamine also significantly reduced the ability to perform a reversal learning task. We not only found significant decline in reversal learning due to scopolamine treatment, but studied the method specific underlying behaviors: the general activity and lick behavior as well.

Perseveration in a spatial-discrimination serial reversal learning task is differentially affected by MAO-A and MAO-B inhibition and associated with reduced anxiety and peripheral serotonin levels

RATIONALE

Impairments in behavioral flexibility lie at the core of anxiety and obsessive-compulsive disorders. Few studies, however, have investigated the neural substrates of natural variation in behavioral flexibility and whether inflexible behavior is linked to anxiety and peripheral markers of stress and monoamine function.

OBJECTIVE

The objective of the study was to investigate peripheral and central markers associated with perseverative behavior on a spatial-discrimination serial reversal learning task.

METHODS

Rats were trained on a reversal learning task prior to blood sampling, anxiety assessment, and the behavioral evaluation of selective monoamine oxidase-A (MAO-A) and MAO-B inhibitors, which block the degradation of serotonin (5-HT), dopamine (DA), and noradrenaline (NA).

RESULTS

Perseveration correlated positively with 5-HT levels in blood plasma and inversely with trait anxiety, as measured on the elevated plus maze. No significant relationships were found between perseveration and the stress hormone corticosterone or the 5-HT precursor tryptophan. Reversal learning was significantly improved by systemic administration of the MAO-A inhibitor moclobemide but not by the MAO-B inhibitor lazabemide. Moclobemide also increased latencies to initiate a new trial following an incorrect response suggesting a possible role in modulating behavioral inhibition to negative feedback. MAO-A but not MAO-B inhibition resulted in pronounced increases in 5-HT and NA content in the orbitofrontal cortex and dorsal raphé nuclei and increased 5-HT and DA content in the basolateral amygdala and dorsomedial striatum.

CONCLUSIONS

These findings indicate that central and peripheral monoaminergic mechanisms underlie inter-individual variation in behavioral flexibility, which overlaps with trait anxiety and depends on functional MAO-A activity.

Genetic expression analysis of E2F-associated phosphoprotein in stress responses in the mouse

Dysfunction of the monoaminergic system is critical in stress and anxiety disorders, but the role of each family member in the development of stress-related psychopathologies is not sufficiently understood. Eapp has been reported to be a transcriptional repressor of monoamine oxidase B (Maob) and down-regulates Maob via the Maob core promoter. In the present study, we more specifically examine the role of Eapp in stress responses by testing the hypothesis that Eapp may be involved in the occurrence and development of stress responses. Western blotting, qPCR and immunohistochemistry were used to investigate the expression variation of Eapp in hypothalamus tissue after exposure to stress. The expression of Eapp is controlled by a cis-acting quantitative trait locus (cis-eQTL). Two genes Sphk2 and Nosip, had trans-eQTLs that mapped to the location of Eapp and altered expression of these two genes was shown following siRNA knockdown of Eapp. Additionally, Mmp9, Npy, Npy5r and Maob were shown to have different expression levels in the Eapp knock-down experiments. Our data provide strong evidence that the cis-modulated gene, Eapp, is associated with stress responses, and that validated downstream targets and members of Eapp gene network may also be involved in the development of stress.

Serotonergic pharmacology in animal models: from behavioral disorders to dyskinesia

Serotonin (5-HT) dysfunction has been involved in both movement and behavioral disorders. Serotonin pharmacology improves dyskinetic movements as well as depressive, anxious, aggressive and anorexic symptoms. Animal models have been useful to investigate more precisely to what extent 5-HT is involved and whether drugs targeting the 5-HT system can counteract the symptoms exhibited. We review existing rodent and non-human primate (NHP) animal models in which selective 5-HT or dual 5-HT-norepinephrine (NE) transporter inhibitors, as well as specific 5-HT receptors agonists and antagonists, monoamine oxidase A inhibitors (IMAO-A) and MDMA (Ecstasy) have been used. We review overlaps between the various drug classes involved. We confront behavioral paradigms and treatment regimen. Some but not all animal models and associated pharmacological treatments have been extensively studied in the litterature. In particular, the impact of selective serotonin reuptake inhibitors (SSRI) has been extensively investigated using a variety of pharmacological or genetic rodent models of depression, anxiety, aggressiveness. But the validity of these rodent models is questioned. On the contrary, few studies did address the potential impact of targeting the 5-HT system on NHP models of behavioral disorders, despite the fact that those models may match more closely to human pathologies. Further investigations with carefull behavioral analysis will improve our understanding of neural bases underlying the pathophysiology of movement and behavioral disorders.

Rines E3 ubiquitin ligase regulates MAO-A levels and emotional responses

Monoamine oxidase A (MAO-A), the catabolic enzyme of norepinephrine and serotonin, plays a critical role in emotional and social behavior. However, the control and impact of endogenous MAO-A levels in the brain remains unknown. Here we show that the RING finger-type E3 ubiquitin ligase Rines/RNF180 regulates brain MAO-A subset, monoamine levels, and emotional behavior. Rines interacted with MAO-A and promoted its ubiquitination and degradation. Rines knock-out mice displayed impaired stress responses, enhanced anxiety, and affiliative behavior. Norepinephrine and serotonin levels were altered in the locus ceruleus, prefrontal cortex, and amygdala in either stressed or resting conditions, and MAO-A enzymatic activity was enhanced in the locus ceruleus in Rines knock-out mice. Treatment of Rines knock-out mice with MAO inhibitors showed genotype-specific effects on some of the abnormal affective behaviors. These results indicated that the control of emotional behavior by Rines is partly due to the regulation of MAO-A levels. These findings verify that Rines is a critical regulator of the monoaminergic system and emotional behavior and identify a promising candidate drug target for treating diseases associated with emotion.

Upregulated mRNA levels of SERT, NET, MAOB, and BDNF in various brain regions of ovariectomized rats exposed to chronic aversive stimuli

Estrogen deficiency increases the risk of anxiety and mood disorders, presumably by deranging metabolism of the monoamine neurotransmitters and the expression of their reuptake transporters in the brain. Although estrogen-deficient individuals were also susceptible to stress, little was known regarding the effect of stress on the levels of transcripts related to brain monoamine metabolism. Herein, we used quantitative real-time PCR to quantify the mRNA levels of serotonin reuptake transporter (SERT), norepinephrine transporter (NET), monoamine oxidase-B (MAOB), tryptophan hydroxylase (TPH), and tyrosine hydroxylase (TH) in various brain regions of ovariectomized (OVX) rats which had been exposed for 4 weeks to chronic aversive stimuli (CAS), such as water deprivation, cage tilt, and illumination. We found that CAS induced stress responses in OVX rats as indicated by increases in the adrenal gland weight and sucrose intake. After CAS exposure, mRNA levels of SERT and NET were upregulated in the frontal cortex, hippocampus, amygdala, and periaqueductal gray. In addition, CAS also increased the mRNA levels of MAOB, an enzyme for dopamine degradation, in the same brain regions. However, CAS did not alter the mRNA levels of TPH or TH, both of which are rate-limiting enzymes for the synthesis of serotonin and norepinephrine in the dorsal raphé and locus coeruleus, respectively. Interestingly, mRNA expression of brain-derived neurotrophic factor precursor was upregulated in the hippocampus of CAS-exposed OVX rats, suggesting a compensatory mechanism which might counteract the stress-induced depression. Therefore, the present data have provided evidence to explain how stress affected brain monoamine metabolism in estrogen-deficient stressed patients.

Using the conditioned fear stress (CFS) animal model to understand the neurobiological mechanisms and pharmacological treatment of anxiety

SUMMARY

The mechanisms underlying the etiology and pathophysiology of anxiety disorders - the most prevalent class of mental disorders - remain unclear. Over the last 30 years investigators have used the animal model of conditioned fear stress (CFS) to investigate the brain structures and neurotransmitter systems involved in aversive emotional learning and memory. Recent studies have focused on the neuronal circuitry and cellular mechanisms of fearful emotional experiences. This review describes the CFS paradigm, discusses the neural circuit and neurotransmission underlying CFS, and explains the mechanism of action of pharmacological treatments of CFS. The focus of the review is on the molecular mechanisms of fear extinction, a phenomenon directly implicated in the clinical treatment of anxiety. Based on our assessment of previous work we will conclude by considering potential molecular targets for treating symptoms of anxiety and fear.

Modeling anxiety using adult zebrafish: a conceptual review

Zebrafish (Danio rerio) are rapidly emerging as a useful animal model in neurobehavioral research. Mounting evidence shows the suitability of zebrafish to model various aspects of anxiety-related states. Here, we evaluate established and novel approaches to uncover the molecular substrates, genetic pathways and neural circuits of anxiety using adult zebrafish. Experimental approaches to modeling anxiety in zebrafish include novelty-based paradigms, pharmacological and genetic manipulations, as well as innovative video-tracking, 3D-reconstructions, bioinformatics-based searchable databases and omics-based tools. Complementing traditional rodent models of anxiety, we provide a conceptual framework for the wider application of zebrafish and other aquatic models in anxiety research. This article is part of a Special Issue entitled 'Anxiety and Depression'.

SSRIs and conditioned fear

Among drugs that act on serotonergic neurotransmission, selective serotonin (5-HT) reuptake inhibitors (SSRIs) are now the gold standard for the treatment of anxiety disorders. The precise mechanisms of the anxiolytic actions of SSRIs are unclear. We reviewed the literature related to the effects of SSRIs and the neurochemical changes of 5-HT in conditioned fear. Acute SSRIs and 5-HT(1A) receptor agonists reduced the acquisition and expression of contextual conditioned fear. Chronic SSRI administration enhanced anxiolytic-like effects. Microinjection studies revealed the amygdala as the target brain region of both classes of serotonergic drugs, and the hippocampus as the target of 5-HT(1A) receptor agonists. These findings highlight the contribution of post-synaptic 5-HT receptors, especially 5-HT(1A) receptors, to the anxiolytic-like effects of serotonergic drugs. These results support the new 5-HT hypothesis of fear/anxiety: the facilitation of 5-HT neurotransmission ameliorates fear/anxiety. Furthermore, these behavioral data provide a new explanation of neurochemical adaptations to contextual conditioned fear: increased 5-HT transmission seems to decrease, not increase, fear.

The MAO inhibitor phenelzine improves functional outcomes in mice with experimental autoimmune encephalomyelitis (EAE)

Multiple sclerosis (MS) and the animal model, experimental autoimmune encephalomyelitis (EAE), are both accompanied by motor and non-motor symptoms. Pathological changes in the activities of key neurotransmitters likely underlie many of these symptoms. We have previously described disturbances in the levels of 5-hydroxytryptamine (5-HT/serotonin), noradrenaline (NE) and γ-aminobutyric acid (GABA) in a mouse model of EAE. The potential therapeutic effect of a drug that targets these three neurotransmitters, the antidepressant and anti-panic drug phenelzine (PLZ), was assessed in mice with MOG(35-55) induced EAE. The neurotransmitter content of EAE and control tissue after PLZ administration was first evaluated by HPLC. The ability of PLZ treatment to modulate EAE disease course and clinical signs was then assessed. Daily PLZ treatment, starting seven days after disease induction, delayed EAE onset, reduced disease severity in the chronic phase and was associated with substantial improvements in exploratory behavior and a novel measure of sickness and/or depression. Upon completion of the experiment, PLZ's effects on histopathological markers of the disease were examined. No differences were observed in T cell infiltration, microglia/macrophage reactivity, demyelination or axonal injury in PLZ-treated spinal cords. However, EAE mice treated with PLZ showed a normalization of 5-HT levels in the ventral horn of the spinal cord that might account for the improvements in behavioral outcomes. These results demonstrate the therapeutic potential of MAO inhibitors such as PLZ in MS. Additionally, the behavioral changes observed in EAE mice indicate that alterations in non-motor or 'affective' measures may be valuable to consider in addition to traditional measures of gross locomotor function.

Learning new tricks from an old dog: using experimental autoimmune encephalomyelitis to study comorbid symptoms in multiple sclerosis

SUMMARY Multiple sclerosis (MS) is a chronic disease of the CNS characterized by inflammation, demyelination and axonal injury. In addition to the well-recognized features of the disease such as weakness, fatigue and paralysis, patients with MS may also experience a number of other comorbid disorders. Chronic pain, anxiety and depression affect a large percentage of MS patients. While a number of animal models are available to study the pathophysiology of MS, it is only recently that these models have been used to ask questions about other comorbid conditions associated with the disease. We will now summarize some of the major findings in this area. Although these animal models have been in use for many decades, it is clear that they are still capable of addressing novel and clinically relevant questions about the disease.

Contextual conditioning in rats as an animal model for generalized anxiety disorder

Animal models of psychiatric disorders are important translational tools for exploring new treatment options and gaining more insight into the disease. Thus far, there is no systematically validated animal model for generalized anxiety disorder (GAD), a severely impairing and difficult-to-treat disease. In this review, we propose contextual conditioning (CC) as an animal model for GAD. We argue that this model has sufficient face validity (there are several symptom similarities), predictive validity (it responds to clinically effective treatments), and construct validity (the underlying mechanisms are comparable). Although the refinement and validation of an animal model is a never-ending process, we want to give a concise overview of the currently available evidence. We suggest that the CC model might be a valuable preclinical tool to enhance the development of new treatment strategies and our understanding of GAD.

Pharmacological modulation of anxiety-like phenotypes in adult zebrafish behavioral models

Zebrafish (Danio rerio) are becoming increasingly popular in neurobehavioral research. Here, we summarize recent data on behavioral responses of adult zebrafish to a wide spectrum of putative anxiolytic and anxiogenic agents. Using the novel tank test as a sensitive and efficient behavioral assay, zebrafish anxiety-like behavior can be bi-directionally modulated by drugs affecting the gamma-aminobutyric acid, monoaminergic, cholinergic, glutamatergic and opioidergic systems. Complementing human and rodent data, zebrafish drug-evoked phenotypes obtained in this test support this species as a useful model for neurobehavioral and psychopharmacological research.

Effect of afobazole on mitochondrial monoamine oxidase A activity in vitro

Selective anxiolytic afobazole (1 mM) inhibits monoamine oxidase A activity in mitochondria from rat brain and liver (IC(50) 0.36 and 0.43, respectively). Effect of the compound does not depend on the time of preincubation with mitochondria. Triple washout of mitochondria is followed by complete recovery of initial enzyme activity.

Perinatal treatment

Serotonin (5-hydroxytryptamine, 5HT) regulates the development of 5HT neurons and target tissues during neurogenesis, while later it assumes the function of a neurotransmitter. Alterations in serotonin neurotransmission are indicated as biological substrates in several neuropsychiatric disorders, including autism. The most consistent 5HT-related finding in autistic disorder is hyperserotonemia, but the mechanism of its development and its relation to central 5HT dysfunction are still unclear. In an attempt to pharmacologically induce hyperserotonemia during the period of most intensive development of 5HT neurons, and to later investigate its effects on central 5HT functions, we have treated rats from gestational day 13 until postnatal day 21 with 2 mg/kg of the nonselective irreversible MAO inhibitor tranylcypromine (TCP). The control group received saline in the same manner. TCP treated rats displayed a long-lasting significant increase in platelet 5HT concentrations compared to the control rats. The TCP treated group had smaller litters, significantly lower pup survival rate, and slower weight gain during the post-weaning free-feeding period than the control group. Pups from the TCP group returned to their dams significantly slower than the control pups suggesting lower separation anxiety. Our results indicate that the perinatal treatment of rats with tranylcypromine has induced both, disregulation of the peripheral 5HT homeostasis and disturbances in central 5HT physiology in pups and young rats. The extent of the changes in the central serotonergic compartment in adult rats will be explored in our further studies.

Effects of cytochrome P450 (CYP) 3A4 inhibitors on the anxiolytic action of tandospirone in rat contextual conditioned fear

The azapirone derivatives, including tandospirone and buspirone, are anxiolytics with 5-HT(1A) receptor agonistic action. Previous in vitro studies have suggested these azapirone derivatives are mainly metabolized by the cytochrome P450 (CYP) 3A4 isoform. The purpose of this study was to clarify the effects CYP3A4 inhibitors have on the anxiolytic action of tandospirone in a conditioned fear stress rat model. One day after fear conditioning, the orally administered tandospirone (30-100 mg/kg) significantly inhibited conditioned freezing in a dose-dependent manner. Co-administration of oral tandospirone and CYP3A4 inhibitors [ketoconazole (10 mg/kg, i.p.) and cimetidine (200 mg/kg, p.o.)] markedly inhibited conditioned freezing. Ketoconazole significantly increased the anxiolytic effect of buspirone similar to tandospirone. As with freezing behavior, the plasma concentrations of tandospirone and buspirone were increased by CYP3A4 inhibitors. This suggests the CYP3A4 isoform is involved in the metabolism of tandospirone, in vivo. Therefore, drugs with CYP3A4 inhibitory property may facilitate the anxiolytic effect of tandospirone when treating human anxiety disorders.

L-methamphetamine and selective MAO inhibitors decrease morphine-reinforced and non-reinforced behavior in rats; Insights towards selegiline's mechanism of action

Selegiline is an inhibitor of type B monoamine oxidase (MAO) with psychostimulant effects that can decrease morphine-reinforced and non-reinforced responding. The present study was undertaken to compare the effects of MAO inhibition and treatment with L-methamphetamine, the major psychostimulant metabolite of selegiline, on these behaviors. After rats acquired a stable pattern of morphine self-administration under a progressive ratio schedule, chronic treatment was initiated with vehicle, L-methamphetamine, clorgyline (a selective inhibitor of MAO-A), or rasagiline (a selective inhibitor of MAO-B); with both MAO inhibitors administered at a dose selective for one MAO isoform and a higher dose that inhibited both isoforms. Rats were evaluated for up to four cycles of opiate dependence maintained by morphine self-administration and withdrawal during which extinction responding was recorded. Most behavioral measures (92.4%) did not differ in animals evaluated during an initial and subsequent cycles of dependence and withdrawal. All active treatments attenuated non-reinforced responding during extinction. Morphine reinforcement was also decreased by each of the three active treatments, but greater and more prolonged effects were observed following inhibition of MAO-B with rasagiline. Responding during either cue- or morphine-induced reinstatement was attenuated by either clorgyline or rasagiline administered at nonselective doses, but not by either compound administered at selective dose levels. Treatment with L-methamphetamine did not produce significant effects on cue-induced reinstatement, but decreased non-reinforced responding during morphine-induced reinstatement. These findings indicate that morphine reinforcement and different non-reinforced behaviors differ greatly in their susceptibility to modification by psychostimulant treatment or MAO inhibition.

The pharmacokinetics and pharmacodynamics of tandospirone in rats exposed to conditioned fear stress

The 5-HT1, agonist tandospirone is generally thought to have a weak anxiolytic effect with a slow onset of action. Our recent clinical study suggested that a comparatively high dose of tandospirone has excellent anxiolytic efficacy and is without significant adverse effects. The present study was designed to clarify the relationship between the anxiolytic effect of tandospirone and its plasma and brain concentrations. The anxiolytic effect was estimated by determining the conditioned fear stress-induced freezing behavior in rats after tandospirone administration. Obvious correlations between anxiolytic effect and brain concentration of tandospirone were observed 0.5 and 4 h after tandospirone administration, while the anxiolytic effect was dependent on the plasma concentration of at 0.5 h but not 4 h after tandospirone administration. The plasma concentration was significantly correlated with the brain concentration. These findings suggest that the potency of the anxiolytic effect is dependent on both the plasma and brain concentration.

Acquisition and extinction of a conditioned passive avoidance reflex in mice with genetic knockout of monoamine oxidase A

We report here the results obtained from comparative analysis of learning and the dynamics of extinction of a conditioned passive avoidance response in mice with genetic knockout of monoamine oxidase A (MAO A) and the progenitor line C3H. Mice of both lines acquired the conditioned passive avoidance reaction efficiently. Mice with genetic knockout of MAO A were characterized by prolonged retention of reproduction of the memory trace, as compared with rapid extinction in C3H mice. Smaller numbers of transfers, and vertical rearings on days 7-13 and the numbers of glances into and rom the dark sector on days 11-13 of extinction in MAO A-knockout mice appear to reflect their more marked fear reactions when confronted with the "dangerous" sector, along with increased anxiety, these facilitating longer-lasting retention of the memory trace.

Effect of co-administration of subchronic lithium pretreatment and acute MAO inhibitors on extracellular monoamine levels and the expression of contextual conditioned fear in rats

We investigated the effects of clorgyline [a selective MAO (monoamine oxidase inhibitor)-A inhibitor] and lazabemide (a selective MAO-B inhibitor) on extracellular serotonin, dopamine and noradrenaline concentrations in the medial prefrontal cortex after 1-week treatment with subchronic 0.2% or 0.05% Li2CO3 (p.o.) and the effects on expression of contextual conditioned fear, previously reported to be reduced by facilitation of serotonin neurotransmission. As compared to normal diet controls, the subchronic 0.2% Li2CO3 group showed significantly higher levels of extracellular serotonin, but not noradrenaline. No changes were observed in the 0.05% Li2CO3 group. Acute clorgyline (10 mg/kg) treatments combined with subchronic 0.2% Li2CO3 treatments showed significant increases in extracellular serotonin concentrations, but not in dopamine or noradrenaline, as compared with clorgyline treatment alone. There was an additive effect with combined treatment of subchronic 0.2% Li2CO3 and acute clorgyline on the reduction of conditioned freezing, an index of conditioned fear, and this was not observed with subchronic 0.05% Li2CO3. These behavioral data indicate the functional significance of increased extracellular serotonin concentrations due to combined use of a MAO-A inhibitor with subchronic lithium. Effects of lazabemide (10 mg/kg) on extracellular monoamine concentrations and conditioned fear were slight or negligible, and were not affected by subchronic lithium treatment. The present study suggests that lithium augmentation of the antidepressant effect of MAO inhibitors is mediated by additional increases in the extracellular serotonin concentrations induced by MAO-A inhibition and suggests that the anxiolytic action of MAO inhibitors may be enhanced by lithium.

Natriuretic peptide interaction with [3H]isatin binding sites in rat brain

Isatin is an endogenous indole, which has a distinct and discontinuous distribution in the brain and exhibits a wide range of physiological and pharmacological effects. In the present study, we have demonstrated that atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) inhibited [3H]isatin binding to rat brain sections and isolated membrane fractions. Isatin itself antagonised not only natriuretic peptide receptor type A (NPR-A) (ANP-stimulation of guanylyl cyclase) but also NPR-C (ANP and CNP mediated inhibition of adenylyl cyclase) signalling. These results suggest that some [3H]isatin binding in the brain may be to NPR-A and NPR-C. Competitive interactions between isatin and natriuretic peptides and their receptors give a possible explanation of the known anxiogenic effect of low doses of isatin, interacting at NPR-A, and the sedative effects of higher doses, antagonising respectively the anxiolytic effect of ANP and the anxiogenic effect of CNP.

Context fear conditioning inhibits panic-like behavior elicited by electrical stimulation of dorsal periaqueductal gray

Context fear conditioning has been widely used as an animal model of anxiety whereas electrical stimulation of the dorsal portion of the periaqueductal gray (DPAG) as a model of panic attack. The present study employed these two animal models in order to investigate the influence of anxiety in the occurrence of panic attack. Results indicated that animals exposed to contextual cues that were previously associated with electrical footshocks engaged in robust defensive freezing behavior and were less likely to display flight evoked by electrical stimulation of the DPAG when compared with control animals that were not exposed to the context fear conditioning procedure. These results indicate that activation of the brain mechanisms that underlie anxiety produces an inhibitory effect on panic attack.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Prior exposure to the elevated plus-maze sensitizes mice to the acute behavioral effects of fluoxetine and phenelzine

A single undrugged experience of the elevated plus-maze modifies future drug responses in the test. The present study investigated the effects of maze-experience on the acute behavioral effects of the monoamine oxidase inhibitor phenelzine and the serotonin reuptake inhibitor fluoxetine. Phenelzine (2.5-12.5 mg/kg) had no clear effect on plus-maze behavior in test-naive Swiss Webster mice, but dose-dependently increased anxiety-like behavior in maze-experienced subjects. Similarly, fluoxetine (5-20 mg/kg) produced non-significant trends for increased anxiety-like behavior in maze-naive mice, but significantly and dose-dependently increased anxiety-like behavior and suppressed locomotor activity in maze-experienced mice. The anxiogenic effects of the benzodiazepine receptor inverse agonist N-methyl-beta-carboline-3-carboxamide (FG 7142) (20 mg/kg) was abolished by prior test experience, suggesting an alteration in gamma-aminobutyric acid (GABA)/benzodiazepine receptor function with maze-experience. However, the benzodiazepine receptor antagonist flumazenil (5-20 mg/kg) produced a silent profile regardless of maze-experience. Present findings provide further evidence demonstrating that prior test history is a critical consideration in mouse studies of anxiety-related behavior.

Fluvoxamine suppresses the long-term potentiation in the hippocampal CA1 field of anesthetized rats: an effect mediated via 5-HT1A receptors

A selective 5-HT reuptake inhibitor, fluvoxamine (10 and 30 mg/kg, i.p.) suppressed long-term potentiation (LTP) in the hippocampal CA1 field of anesthetized rats. Fluvoxamine (30 mg/kg, i.p.)-induced suppression of LTP was completely reversed by the 5-HT(1A) receptor antagonist NAN-190 (0.5 mg/kg, i.p), but not by the 5-HT(4) receptor antagonist GR 113808 (20 microg/rat, i.c.v.) and the 5-HT(7) receptor antagonist DR 4004 (10 microg/rat, i.c.v.). These data suggest that the inhibitory effect of fluvoxamine on LTP induction is mediated via 5-HT(1A) receptors.

Effect of chronic treatment with the protein kinase C inhibitor staurosporine on the acquisition and expression of contextual fear conditioning

The present study investigated the effects of acute and chronic administration of the protein kinase C inhibitor, staurosporine, on the acquisition and expression of conditioned freezing behavior, an index of anxiety induced by conditioned fear stress. Results revealed that acute staurosporine (0.01 and 0.1 mg/kg, i.p.) did not affect either acquisition or expression of conditioned freezing. Chronic staurosporine administration (0.01 or 0.1 mg/kg, i.p., for 14 days) significantly reduced the acquisition of conditioned freezing at a dose of 0.1 mg/kg, but failed to affect the expression of conditioned freezing at any dose. These results suggest the involvement of protein kinase C in synaptic and cellular plasticity underlying emotional learning and memory.

The anxiolytic-like effect of nicotine undergoes rapid tolerance in a model of contextual fear conditioning in rats

The effects of repeated administration of nicotine on contextual fear conditioning, locomotor activity, and pain threshold, were examined in rats. It was found that a single injection of nicotine prior to the training session (three 0.7-mA footshocks, each 0.5 s long), decreased the freezing reaction during the retest 24 h later. The locomotor activity was moderately enhanced, and the pain threshold remained unchanged. The baseline freezing measured immediately after administration of a single dose of nicotine was not significantly different from the saline-treated group. The anxiolytic-like effect of nicotine was as potent as that of midazolam, a benzodiazepine derivative. After five day-by-day injections, the anxiolytic-like effect of nicotine (0.6 mg/kg, sc) was no longer present, independently whether the last drug injection was given 24 h or 5 min (i.e., the sixth, additional, nicotine injection), prior to the training session. Thus, it appeared that the expression of tolerance to the nicotine-induced anxiolytic-like action did not require a direct stimulation of nicotinic receptors. Simultaneously, in this group of animals, nicotine caused a potent stimulation of locomotor activity in the open field test. The applied dosage and regimen of nicotine administration did not change rat pain threshold (flinch-jump test). Collectively, the present data showed for the first time, that short-term, intermittent, administration of nicotine was sufficient to induce tolerance to the anxiolytic-like effect of this drug, in the model of fear conditioning to context. Importantly, a clear dissociation between the locomotor and anxiolytic-like effects of nicotine was present. This effect appeared independent also of changes in rat pain threshold. The possible mechanisms of this phenomenon are discussed.

Anxiolytic effects of aniracetam in three different mouse models of anxiety and the underlying mechanism

The anxiolytic effects of aniracetam have not been proven in animals despite its clinical usefulness for post-stroke anxiety. This study, therefore, aimed to characterize the anxiolytic effects of aniracetam in different anxiety models using mice and to examine the mode of action. In a social interaction test in which all classes (serotonergic, cholinergic and dopaminergic) of compounds were effective, aniracetam (10-100 mg/kg) increased total social interaction scores (time and frequency), and the increase in the total social interaction time mainly reflected an increase in trunk sniffing and following. The anxiolytic effects were completely blocked by haloperidol and nearly completely by mecamylamine or ketanserin, suggesting an involvement of nicotinic acetylcholine, 5-HT2A and dopamine D2 receptors in the anxiolytic mechanism. Aniracetam also showed anti-anxiety effects in two other anxiety models (elevated plus-maze and conditioned fear stress tests), whereas diazepam as a positive control was anxiolytic only in the elevated plus-maze and social interaction tests. The anxiolytic effects of aniracetam in each model were mimicked by different metabolites (i.e., p-anisic acid in the elevated plus-maze test) or specific combinations of metabolites. These results indicate that aniracetam possesses a wide range of anxiolytic properties, which may be mediated by an interaction between cholinergic, dopaminergic and serotonergic systems. Thus, our findings suggest the potential usefulness of aniracetam against various types of anxiety-related disorders and social failure/impairments.