The effects of selective serotonin reuptake inhibitors on extracellular 5-HT levels in the hippocampus of 5-HT(1B) receptor knockout mice

SERT and OCT mediate 5-HT1B receptor regulation of immobility behavior and uptake of 5-HT and HIS

5-HT clearance, commonly mediated by transporters in the uptake-1 and uptake-2 families, has been linked to 5-HT1B receptor's action on behaviors. Since no specific transporters identified yet, effects of serotonin transporter (SERT) and organic cation transporter (OCTs) on 5-HT1B-elicited immobility phenotype, and 5-HT and HIS uptake were then investigated. Intraperitoneal injections of SERT inhibitor fluoxetine (FLX) and/or OCTs inhibitor decynium (D22) were used prior to local perfusion of 5-HT1B agonist CP93129 into the ventral hippocampus to measure immobility times in the FST and TST, to measure 5-HT uptake efficiencies and HIS uptake efficiencies derived from linear regressions using the transient no-net-flux quantitative microdialysis in C57BL/6 mice. Exogenous 5-HT and HIS uptake were measured following incubation of FLX and/or D22 with CP93129 in the RBL-2H3 cells. Moreover, surface membrane levels of SERT and OCT were detected in response to CP93129. Local CP93129 prolonged immobility times, which were attenuated following pretreatment of either inhibitor. Local CP93129 lowered the slopes obtained from the lineal regressions for 5-HT and HIS (slope is reciprocal to uptake efficiency), which were then weakened following pretreatment of either inhibitor. Similar findings were obtained following CP93129 incubation, and co-incubation of CP93129 with either inhibitor in the RBL-2H3. Moreover, CP93129 dose-dependently moved SERT and OCT3 in the cytosol to the surface membrane. Both SERT and OCT are the target effectors mediating 5-HT1B regulation of immobility time and 5-HT uptake, OCT mediates 5-HT1B regulation of HIS uptake. Their underlying signal transductions need to be further explored.

Central Serous Chorioretinopathy and Blood Serotonin Concentrations

BACKGROUND

To investigate blood serotonin (5-hydroxytryptamine (5-HT)) concentrations and their relationships with selected characteristics in patients with central serous chorioretinopathy (CSC).

METHODS

This was a prospective study including 93 patients with active CSC. Blood concentrations of 5-HT, adrenocorticotropic hormone, and cortisol were measured in patients with CSC. Selected patient characteristics, including disease history (acute or chronic), medication use, smoking history, mood status, best-corrected visual acuity (BCVA), subfoveal choroidal thickness (SCT), findings on fluorescein and indocyanine green angiography, and anatomical changes were evaluated during follow-up.

RESULTS

Eleven of the 93 patients had low 5-HT concentrations (<57 ng/mL) (12%, eight men and three women; mean age 55 years); we identified no significant relationship with acute/chronic disease status. The patients with low 5-HT were significantly more likely to have five or more fluorescein leakage sites (p = 0.0275), recurrence of subretinal fluids (p < 0.0001), and failure to achieve significant improvement in BCVA during follow-up (p = 0.862) than patients with 5-HT within the normal range.

CONCLUSIONS

Blood serotonin concentrations may influence the pathophysiology and prognosis of CSC.

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

[Consequences of the monoaminergic systems cross-talk in the antidepressant activity]

Selective serotonin reuptake inhibitors (SSRIs) are the most prescribed antidepressant treatment for treat major depressive disorders. Despite their effectiveness, only 30% of SSRI-treated patients reach remission of depressive symptoms. SSRIs by inhibiting the serotonin transporter present some limits with residual symptoms. Increasing not only serotonin but also norepinephrine and dopamine levels in limbic areas seems to improve remission. Anatomical relationships across serotoninergic, dopaminergic and noradrenergic systems suggest tight reciprocal regulations among them. This review attempts to present, from acute to chronic administration the consequences of SSRI administration on monoaminergic neurotransmission. The serotonin neurons located in the raphe nucleus (RN) are connected to the locus coeruleus (locus coeruleus), the key structure of norepinephrine synthesis, through GABAergic-inhibiting interneurons. Activation of the 5-HT_2A receptors expressed on GABAergic interneurons following SERT-inhibition induces an increase in serotonin leading to inhibitory effect on NE release. Similarly, the serotonin neurons exert negative regulation on dopaminergic neurons from the ventral tegmental area (VTA) through a GABAergic interneuron. These interneurons express the 5-HT_2C and 5-HT₃ receptors inducing an inhibitory effect of 5-HT on DA release. Positive reciprocal connections are also observed through direct projections from the locus coeruleus to the RN and from the VTA to the RN through α₁ and D₂ receptors respectively, both stimulating the serotoninergic activity. Acute SSRI treatment induces only a slight increase in 5-HT levels in limbic areas due to the activation of presynaptic 5-HT_1A and 5-HT_1B autoreceptors counteracting the effects of the transporter blockade. No change in NE levels and a small decrease in the dopaminergic neurotransmission is also observed. These weak changes in monoamine in the limbic areas after acute SSRI treatment seems to be one of key point involved in the onset of action. Following desensitization of the 5-HT_1A and 5-HT_1B autoreceptors, chronic SSRI treatment induces a large increase in the 5-HT neurotransmission. Changes in 5-HT levels at the limbic areas results in a decrease in NE transmission and an increase in DA transmission through an increase in the post-synaptic D₂ receptors sensitivity and not from a change in DA levels, which is mainly due to a desensitization of the 5-HT_2A receptor. The observed decrease of NE neurotransmission could explain some limits of the SSRI therapy and the interest to activate NE system for producing more robust effects. On the other hand, the D₂ sensitization, especially in the nucleus accumbens, stimulates the motivation behavior as well as remission of anhedonia considering the major role of DA release in this structure. Finally, we need to take into account the key role of each monoaminergic neurotransmission to reach remission. Targeting only one system will limit the therapeutic effectiveness. Clinical evidences, including the STAR*D studies, confirmed this by an increase of the remission rate following the mobilization of several monoaminergic transmissions. However, these combinations cannot constitute first line of treatment considering the observed increase of side effects. Such an approach should be adapted to each patient in regard to its particular symptoms as well as clinical history. The next generation of antidepressant therapy will need to take into consideration the interconnections and the interrelation between the monoaminergic systems.

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.

The 5-HT deficiency theory of depression: perspectives from a naturalistic 5-HT deficiency model, the tryptophan hydroxylase 2Arg439His knockin mouse

A decreased level of brain 5-hydroxytryptamine (5-HT) has been theorized to be a core pathogenic factor in depression for half a century. The theory arose from clinical observations that drugs enhancing extracellular levels of 5-HT (5-HT(Ext)) have antidepressant effects in many patients. However, whether such drugs indeed correct a primary deficit remains unresolved. Still, a number of anomalies in putative biomarkers of central 5-HT function have been repeatedly reported in depression patients over the past 40 years, collectively indicating that 5-HT deficiency could be present in depression, particularly in severely ill and/or suicidal patients. This body of literature on putative 5-HT biomarker anomalies and depression has recently been corroborated by data demonstrating that such anomalies indeed occur consequent to severely reduced 5-HT(Ext) levels in a mouse model of naturalistic 5-HT deficiency, the tryptophan hydroxylase 2 His(439) knockin (Tph2KI) mouse. In this review, we will critically assess the evidence for 5-HT deficiency in depression and the possible role of polymorphisms in the Tph2 gene as a causal factor in 5-HT deficiency, the latter investigated from a clinical as well as preclinical angle.

Acute treatment with fluvoxamine elevates rat brain serotonin synthesis in some terminal regions: an autoradiographic study

INTRODUCTION

A considerable body of evidence indicates the involvement of the neurotransmitter serotonin (5-HT) in the pathogenesis and treatment of depression.

METHODS

The acute effect of fluvoxamine, on 5-HT synthesis rates was investigated in rat brain regions, using α-(14)C-methyl-L-tryptophan as a tracer. Fluvoxamine (25 mg/kg) and saline (control) were injected intraperitoneally, one hour before the injection of the tracer (30 μCi).

RESULTS

There was no significant effect of fluvoxamine on plasma free tryptophan. After Benjamini-Hochberg False Discovery Rate correction, a significant decrease in the 5-HT synthesis rate in the fluvoxamine treated rats, was found in the raphe magnus (-32%), but not in the median (-14%) and dorsal (-3%) raphe nuclei. In the regions with serotonergic axon terminals, significant increases in synthesis rates were observed in the dorsal (+41%) and ventral (+43%) hippocampus, visual (+38%), auditory (+65%) and parietal (+37%) cortex, and the substantia nigra pars compacta (+56%). There were no significant changes in the 5-HT synthesis rates in the median (+11%) and lateral (+24%) part of the caudate-putamen, nucleus accumbens (+5%), VTA (+16%) or frontal cortex (+ 6%).

CONCLUSIONS

The data show that the acute administration of fluvoxamine affects 5-HT synthesis rates in a regionally specific pattern, with a general elevation of the synthesis in the terminal regions and a reduction in some cell body structures. The reasons for the regional specific effect of fluvoxamine on 5-HT synthesis are unclear, but may be mediated by the presynaptic serotonergic autoreceptors.

Down-regulation of the serotonin transporter in hyperreactive platelets counteracts the pro-thrombotic effect of serotonin

An elevated plasma concentration of serotonin ([5-HT]) is a common feature of cardiovascular disease often associated with enhanced platelet activation and thrombosis. Whether elevated in vivo plasma 5-HT per se represents an independent risk factor for platelet hyperreactivity or only is an epiphenomenon of cardiovascular disease is poorly understood. We examined in vitro and in vivo platelet function following a 24h elevation of plasma [5-HT] in mice. In vivo administration of 5-HT using osmotic minipumps increased plasma [5-HT] in treated mice compared to control mice instrumented with saline loaded pumps. 5-HT infusion did not increase systolic blood pressure, but markers of platelet activation including P-selectin and (PE)Jon/A staining were increased and these findings coincided with the enhanced aggregation of isolated platelets in response to type I fibrillar collagen. Tail bleeding times and the time to occlusion following chemical damage to the carotid artery were shortened in 5-HT-infused mice. 5-HT-infused mice were treated with paroxetine (Prx) to block 5-HT uptake via the serotonin transporter (SERT). Prx lowered platelet [5-HT] and attenuated platelet activation and aggregation. These results and our biochemical indices of enhanced 5-HT intracellular signaling in the platelets of 5-HT-infused mice reveal a mechanistic link between elevated plasma [5-HT], abnormal intracellular 5-HT signaling and accentuated platelet aggregation. Although a down-regulation of the serotonin transporter (SERT) on the platelet surface may counteract the pro-thrombotic influence of elevated plasma [5HT], this compensatory mechanism may fail to prevent the increased thrombotic risk caused by elevated plasma [5-HT].

5-HT(1A) receptor sensitivity in 5-HT(1B) receptor KO mice is unaffected by chronic fluvoxamine treatment

The 5-HT(1B) receptor has been implicated in disorders such as depression, anxiety and obsessive-compulsive disorder. In mice lacking the 5-HT(1B) receptor (5-HT(1B) knockout mice), important changes in physiology and behavior exist. In the absence of presynaptic 5-HT(1B) receptor inhibition, chronic SSRI treatment may differentially affect 5-HT(1A) receptor functionality. The present studies tested the hypothesis that chronically reducing 5-HT transporter (5-HTT) function with selective serotonin reuptake inhibitor (SSRI) treatment would accelerate 5-HT(1A) receptor desensitization in 5-HT(1B) knockout mice. Moreover, as 5-HT(1B) knockout mice have been found to display exaggerated autonomic and locomotor responses to environmental stressors, the effects of chronic SSRI treatment on the hyperreactive phenotype of 5-HT(1B) knockout mice were investigated. The stress-reducing effect of the 5-HT(1A) receptor agonist flesinoxan on increases in body temperature, heart rate and locomotor activity was similar in wild type and 5-HT(1B) knockout mice before and after chronic 21-day treatment with the SSRI fluvoxamine, indicating no apparent alteration of 5-HT(1A) receptor sensitivity in 5-HT(1B) knockout mice. Also, chronic SSRI treatment did not alter the increased stress reactivity to mild environmental stressors in 5-HT(1B) knockout mice. We demonstrate that no apparent differences in 5-HT(1A) receptor sensitivity occur between 5-HT(1B) knockout and wild type mice after chronic fluvoxamine treatment. Also, the hyperreactive phenotype of 5-HT(1B) knockout mice is unresponsive to chronic SSRI treatment. Taken together, these results indicate that constitutive absence of 5-HT(1B) receptors does not result in adaptive changes in 5-HT(1A) receptor functionality and that chronic SSRI treatment does not modify stress reactivity in 5-HT(1B) knockout mice.

Genetic variation in cortico-amygdala serotonin function and risk for stress-related disease

The serotonin system is strongly implicated in the pathophysiology and therapeutic alleviation of stress-related disorders such as anxiety and depression. Serotonergic modulation of the acute response to stress and the adaptation to chronic stress is mediated by a myriad of molecules controlling serotonin neuron development (Pet-1), synthesis (tryptophan hydroxylase 1 and 2 isozymes), packaging (vesicular monoamine transporter 2), actions at presynaptic and postsynaptic receptors (5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6, 5-HT7), reuptake (serotonin transporter), and degradation (monoamine oxidase A). A growing body of evidence from preclinical rodents models, and especially genetically modified mice and inbred mouse strains, has provided significant insight into how genetic variation in these molecules can affect the development and function of a key neural circuit between the dorsal raphe nucleus, medial prefrontal cortex and amygdala. By extension, such variation is hypothesized to have a major influence on individual differences in the stress response and risk for stress-related disease in humans. The current article provides an update on this rapidly evolving field of research.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Development of 5-HT1A receptor radioligands to determine receptor density and changes in endogenous 5-HT

[(18)F]FCWAY and [(18)F]FPWAY, analogues of the high affinity 5-HT(1A) receptor (5-HT(1A)R) antagonist WAY100635, were evaluated in rodents as potential radiopharmaceuticals for determining 5-HT(1A)R density and changes in receptor occupancy due to changes in endogenous serotonin (5-HT) levels. The in vivo hippocampus specific binding ratio [(hippocampus(uptake)/cerebellum(uptake))-1] of [(18)F]FPWAY was decreased to 32% of the ratio of [(18)F]FCWAY, indicating that [(18)F]FPWAY has lower affinity than [(18)F]FCWAY. The 5-HT(1A)R selectivity of [(18)F]FPWAY was confirmed using ex vivo autoradiography studies with 5-HT(1A)R knockout, heterozygous, and wildtype mice.Pre- or post-treatment of awake rodents in tissue dissection studies with paroxetine had no effect on hippocampal binding of [(18)F]FCWAY or [(18)F]FPWAY compared to controls, indicating neither tracer was sensitive to changes in endogenous 5-HT. In mouse ex vivo autoradiography studies in which awake mice were treated with fenfluramine following the [(18)F]FPWAY, a significant decrease was not observed in the hippocampus specific binding ratios. In rat dissection studies with fenfluramine administered following [(18)F]FPWAY or [(18)F]FBWAY ([(18)F]-MPPF) in awake or urethane-anesthetized rats, no significant differences in the specific binding ratios of the hippocampus were observed compared to their respective controls. [(18)F]FPWAY and [(18)F]FBWAY uptakes in all brain regions were increased variably in the anesthetized group (with the greatest increase in the hippocampus) vs. the awake group, but were decreased in the fenfluramine-treated anesthetized group vs. the anesthetized group. These data are best explained by changes in blood flow caused by urethane and fenfluramine, which varies from region to region in the brain.

Mutant mouse models of depression: Candidate genes and current mouse lines

Depression is a multifactorial and multigenetic disease. At present, three main theories try to conceptualize its molecular and biochemical mechanisms, namely the monoamine-, the hypothalamus-pituitary-adrenal- (HPA-) system- and the neurotrophin-hypotheses. One way to explore, validate or falsify these hypotheses is to alter the expression of genes that are involved in these systems and study their respective role in animal behavior and neuroendocrinological parameters. Following an introduction in which we briefly describe each hypothesis, we review here the different mouse lines generated to study the respective molecular pathways. Among the many mutant lines generated, only a few can be regarded as genetic depression models or as models of predisposition for a depressive syndrome after stress exposure. However, this is likely to reflect the human situation where depressive syndromes are complex, can vary to a great extent with respect to their symptomatology, and may be influenced by a variety of environmental factors. Mice with mutations of candidate genes showing depression-like features on behavioral or neurochemical levels may help to define a complex molecular framework underlying depressive syndromes. Because it is conceivable that manipulation of one single genetic function may be necessary but not sufficient to cause complex behavioral alterations, strategies for improving genetic modeling of depression-like syndromes in animals possibly require a simultaneous targeted dysregulation of several genes involved in the pathogenesis of depression. This approach would correspond to the new concept of 'endophenotypes' in human depression research trying to identify behavioral traits which are thought to be encoded by a limited set of genes.

In search of a depressed mouse: utility of models for studying depression-related behavior in genetically modified mice

The ability to modify mice genetically has been one of the major breakthroughs in modern medical science affecting every discipline including psychiatry. It is hoped that the application of such technologies will result in the identification of novel targets for the treatment of diseases such as depression and to gain a better understanding of the molecular pathophysiological mechanisms that are regulated by current clinically effective antidepressant medications. The advent of these tools has resulted in the need to adopt, refine and develop mouse-specific models for analyses of depression-like behavior or behavioral patterns modulated by antidepressants. In this review, we will focus on the utility of current models (eg forced swim test, tail suspension test, olfactory bulbectomy, learned helplessness, chronic mild stress, drug-withdrawal-induced anhedonia) and research strategies aimed at investigating novel targets relevant to depression in the mouse. We will focus on key questions that are considered relevant for examining the utility of such models. Further, we describe other avenues of research that may give clues as to whether indeed a genetically modified animal has alterations relevant to clinical depression. We suggest that it is prudent and most appropriate to use convergent tests that draw on different antidepressant-related endophenotypes, and complimentary physiological analyses in order to provide a program of information concerning whether a given phenotype is functionally relevant to depression-related pathology.

From monoamines to genomic targets: a paradigm shift for drug discovery in depression

Depression, a complex psychiatric disorder that affects approximately 15% of the population, has an enormous social cost. Although the disorder is thought to be the outcome of gene-environmental interactions, the causative genes and environmental factors underlying depression remain to be identified. All the antidepressant drugs now in use--the forerunner of which was discovered serendipitously 50 years ago--modulate monoamine neurotransmission, and take six to eight weeks to exert their effects, but each drug is efficacious in only 60-70% of patients. A conceptually novel antidepressant that acted rapidly and safely in a high proportion of patients would almost certainly become the world's bestselling drug. Yet such a drug is not on the horizon. Here, we cover the different phases of antidepressant drug discovery in the past, present and future, and comment on the challenges and opportunities for antidepressant research.

Role of 5-HT1B receptors in the regulation of extracellular serotonin and dopamine in the dorsal striatum of mice

To test the hypothesis that 5-HT1B receptors modulate serotonin (5-hydroxytryptamine, 5-HT) and dopamine release in the striatum, we used in vivo microdialysis in mice lacking 5-HT1B receptors. Local administration by reversed microdialysis of the selective 5-HT reuptake inhibitor, fluvoxamine (0.1-10 microM), concentration dependently increased 5-HT to the same extent in wildtype and in 5-HT1B knockout (KO) mice. Fluvoxamine (10 microM) increased dopamine levels similarly in both genotypes. The 5-HT releaser, fenfluramine (50 microM), increased both 5-HT and dopamine levels, but no difference was found between the genotypes. The 5-HT1B receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP-93,129), reduced 5-HT levels in the wildtype, but not in 5-HT1B KO mice. CP-93,129 at a concentration of 0.5 microM did not affect striatal dopamine outflow in either genotype, whereas dopamine outflow was increased 5-fold by 50 microM CP-93,129 in both genotypes. The CP-93,129-induced dopamine increase was not attenuated by ritanserin, a 5-HT2A/2C receptor antagonist, but was completely blocked by tetrodotoxin, demonstrating that the dopamine release was of neuronal origin. In conclusion, 5-HT1B autoreceptors are functionally present in the mouse striatum, but do not appear to play a significant role in the effects of a selective 5-HT reuptake inhibitor on extracellular 5-HT. In addition, the results in 5-HT1B knockout mice do not support a role of 5-HT1B heteroreceptors in the striatum on dopamine outflow in this brain area of mice.

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.

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.