5-Hydroxytryptamine pathways in anxiety and its treatment

Brain mast cells link the immune system to anxiety-like behavior

Mast cells are resident in the brain and contain numerous mediators, including neurotransmitters, cytokines, and chemokines, that are released in response to a variety of natural and pharmacological triggers. The number of mast cells in the brain fluctuates with stress and various behavioral and endocrine states. These properties suggest that mast cells are poised to influence neural systems underlying behavior. Using genetic and pharmacological loss-of-function models we performed a behavioral screen for arousal responses including emotionality, locomotor, and sensory components. We found that mast cell deficient Kit(W-sh/W-sh) (sash(-/-)) mice had a greater anxiety-like phenotype than WT and heterozygote littermate control animals in the open field arena and elevated plus maze. Second, we show that blockade of brain, but not peripheral, mast cell activation increased anxiety-like behavior. Taken together, the data implicate brain mast cells in the modulation of anxiety-like behavior and provide evidence for the behavioral importance of neuroimmune links.

The tryptophan hydroxylase (TPH) 2 gene unlike TPH-1 exhibits no association with stress-induced depression

BACKGROUND

Serotonin (5-HT) has been implicated in the pathophysiology of several psychiatric disorders including major depression (MD). Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin (5-HT), and might be related to the pathogenesis of MD. Two isoforms are known, TPH-1 and TPH-2. Their association with MD is still debated.

METHODS

A case-control design was used for candidate gene-disease association in 194 patients with stress-induced MD, and 246 healthy controls, all North European Caucasians. Five TPH-2 polymorphisms were analyzed in terms of genotype, allele, and haplotype-based associations.

RESULTS

Neither single marker nor haplotype-based analyses showed significant associations between TPH-2 and MD.

LIMITATIONS

The interpretations are limited by the restricted population size.

CONCLUSIONS

There was no association between TPH-2 gene variants and MD in the same population that had shown a strong association with TPH-1. Hence, the results suggest that in this particular group of stress-induced depression patients TPH-1 appears to be more relevant to MD pathogenesis than TPH-2.

Calcium channel antagonists suppress cross-tolerance to the anxiogenic effects of D-amphetamine and nicotine in the mouse elevated plus maze test

The purpose of the current experiments was to examine the anxiety-related effects of repeated amphetamine and nicotine administration using the mouse elevated plus maze (EPM). d-amphetamine was administered daily for 8 days (2 mg/kg, i.p.). On the 9th day, mice were challenged with amphetamine (2 mg/kg, i.p.) or nicotine (0.1 mg/kg, s.c.), and were tested 30 min after this last injection. Additionally, a distinct group of mice was pretreated with nicotine (0.1 mg/kg, s.c., 6 days). These mice were subjected to nicotine (0.1 mg/kg, s.c.) or amphetamine (2 mg/kg, i.p.) challenge on the seventh day to see if full crossover effects developed after the pretreatment of both psychostimulant drugs. Moreover, the L-type voltage-dependent calcium channel antagonists nimodipine (5 and 10 mg/kg, i.p.), flunarizine (5 and 10 mg/kg, i.p.), verapamil (5 and 10 mg/kg, i.p.) and diltiazem (5 and 10 mg/kg, i.p.) were injected prior to each injection of chronic d-amphetamine or nicotine. We observed cross-tolerance to the anxiogenic effects of d-amphetamine and nicotine that was blunted by a pretreatment with calcium channel blockers. Overall our findings imply that similar neural calcium-dependent mechanisms are involved in the anxiety-related responses to chronic amphetamine and nicotine injections. As anxiety seems to be an important factor for the development of psychostimulant dependence, the L-type VDCC antagonists can offer an interesting approach for the pharmacotherapy of addiction, including amphetamine and/or nicotine dependence.

Perinatal exposure to 5-metoxytryptamine, behavioural-stress reactivity and functional response of 5-HT1A receptors in the adolescent rat

Serotonin is involved in a wide range of physiological and patho-physiological mechanisms. In particular, 5-HT1A receptors are proposed to mediate stress-adaptation. The aim of this research was to investigate in adolescent rats: first, the consequences of perinatal exposure to 5-metoxytryptamine (5MT), a 5-HT1/5-HT2 serotonergic agonist, on behavioural-stress reactivity in elevated plus maze, open field and forced swim tests; secondly, whether the behavioural effects induced by perinatal exposure to 5MT on open field and forced swim tests were affected by the selective 5-HT1A receptor agonist LY 228729, a compound able to elicit a characteristic set of motor behaviours on these experimental models, and by the co-administration of the selective and silent 5-HT1A antagonist WAY 100635. Results indicate that a single daily injection of 5MT to, pregnant dams from gestational days 12 to 21 (1mg/kg s.c.), and to the pups from postnatal days 2 to 18 (0.5mg kg s.c.), induce in the adolescent rat offspring: an increase in the percentage of entries and time spent on the open arms in the elevated plus maze; a reduction in locomotor activity and rearing frequency, and an increase in the time spent on the central areas in the open field test; a decrease in immobility and an increase in swimming in the forced swim test. Acute administration of LY 228729 (1.5mg/kg s.c.) strongly decreases rearing frequency and increases peripheral activity in the open field test, and decreases immobility and increases swimming in the forced swim test both in perinatally vehicle and 5MT-exposed offspring. Co-administration of WAY 100635 (0.25mg/kg s.c.) abolishes the effects exerted by LY 228729. These results suggest that, in the adolescent rat, perinatal exposure to 5MT enhances the stress-related adaptive behavioural responses, presumably through a predominant action on presynaptic 5-HT1A receptors and does not deteriorate the functional response of 5-HT1A receptors to selective agonist and antagonist compounds.

Chronic unpredictable stress induces a cognitive deficit and anxiety-like behavior in rats that is prevented by chronic antidepressant drug treatment

Chronic stress is a risk factor for the development of many psychopathological conditions in humans, including major depression and anxiety disorders. There is a high degree of comorbidity of depression and anxiety. Moreover, cognitive impairments associated with frontal lobe dysfunction, including deficits in cognitive set-shifting and behavioral flexibility, are increasingly recognized as major components of depression, anxiety disorders, and other stress-related psychiatric illnesses. To begin to understand the neurobiological mechanisms underlying the cognitive and emotional consequences of chronic stress, it is necessary to employ an animal model that exhibits similar effects. In the present study, a rat model of chronic unpredictable stress (CUS) consistently induced a cognitive impairment in extradimensional set shifting capability in an attentional set shifting test, suggesting an alteration in function of the medial prefrontal cortex. CUS also increased anxiety-like behavior on the elevated plus-maze. Further, chronic treatment both with the selective norepinephrine reuptake blocker, desipramine (7.5 mg/kg/day), and the selective serotonin reuptake blocker, escitalopram (10 mg/kg/day), beginning 1 week before CUS treatment and continuing through the behavioral testing period, prevented the CUS-induced deficit in extradimensional set-shifting. Chronic desipramine treatment also prevented the CUS-induced increase in anxiety-like behavioral reactivity on the plus-maze, but escitalopram was less effective on this measure. Thus, CUS induced both cognitive and emotional disturbances that are similar to components of major depression and anxiety disorders. These effects were prevented by chronic treatment with antidepressant drugs, consistent also with clinical evidence that relapse of depressive episodes can be prevented by antidepressant drug treatment.

Regulation of extracellular serotonin levels and brain-derived neurotrophic factor in rats with high and low exploratory activity

Serotonin (5-HT) system has a significant role in anxiety- and depression-related states and may be influenced by brain-derived neurotrophic factor (BDNF). This study examined extracellular 5-HT levels and expression of BDNF in rats with persistently low or high levels of exploratory activity (LE and HE, respectively). Baseline extracellular levels of 5-HT as assessed by in vivo microdialysis in conscious animals were similar in both groups in medial prefrontal cortex (PFC) and dentate gyrus (DG). No differences were found in parachloroamphetamine-induced 5-HT release in either region. However, LE animals had significantly higher levels of 5-HT transporter (5-HTT) binding in PFC and a larger increase in extracellular 5-HT levels after administration of citalopram (1 μM) into this area by retrograde dialysis. No difference in 5-HTT levels was found in hippocampus, while perfusion with citalopram was accompanied by a greater increase in extracellular 5-HT in the HE group in this brain region. LE-rats had higher levels of BDNF mRNA in the PFC but not hippocampus. In contrast, levels of nerve growth factor mRNA were similar in these brain regions of LE- and HE-rats. The differential regulation of 5-HT-ergic system in LE- and HE-rats in PFC and hippocampus may form the basis for their distinct anxiety-related behaviours.

The influence of modifications in imide fragment structure on 5-HT1A and 5-HT7 receptor affinity and in vivo pharmacological properties of some new 1-(m-trifluoromethylphenyl)piperazines

New, flexible (7, 9, 11 and 13) and rigid (8, 10, 12 and 14) imides with a 1-(m-trifluorophenyl)piperazine fragment and a tetramethylene or a 1e,4e-cyclohexylene spacer, respectively, showed very high affinity (K(i)=0.3-34 nM) and agonistic in vivo activity for 5-HT(1A) receptors. Flexible new compounds and the previously described 5 also bound to 5-HT(7) receptors (K(i)=21-134 nM). Selected glutarimide derivatives, that is, the most potent postsynaptic 5-HT(1A) receptor agonist rigid compound 8 and its flexible analogue 7, as well as the previously described full agonist-rigid compound 6 and the partial agonist-its flexible counterpart 5 exhibited moderate affinity for alpha(1)-adrenoceptors (K(i)=85 - 268 nM), but were practically devoid of any affinity for dopamine D(2) sites. Those glutarimides demonstrated anxiolytic- (5 and 7) and antidepressant-like (5, 6 and 8) activity in the four-plate and the swim tests in mice, respectively; at the same time, however, they inhibited the locomotor activity of mice. The antidepressant-like effect of 8 was significantly stronger than that induced by imipramine used as a reference antidepressant.

Behavioral response to emotional stress in rabbits: role of serotonin and serotonin2A receptors

Exposure to a novel environment is a stressor which modulates behavior, increases stress hormones and enhances the release of several neurotransmitters including serotonin (5-HT). Exposing rabbits to a novel environment significantly increases head-bob behavior but fails to alter either grooming or wet dog shakes compared with those observed in the home-cage. The goal of this study was to determine the role of 5-HT and its receptors in mediating novelty-elicited head-bob behavior. Reduction of central 5-HT levels after treatment with the serotonergic neurotoxin 5,7-DHT significantly decreased novelty-elicited head bobs by 40% compared with those in sham-lesioned rabbits, indicating that 5-HT mediates, in part, this behavior. Additionally, pretreatment with the 5-HT1A partial agonist and clinically used anxiolytic buspirone also significantly attenuated novelty-elicited head bobs. Pretreatment with the selective 5-HT2A antagonist M 100,907 significantly reduced novel environment-elicited head bobs by 40%. Furthermore, agonist-induced reduction of cortical 5-HT2A receptor density resulted in a significant 40% reduction in the number of head bobs elicited by the novel environment. These data demonstrate that rabbit head-bob behavior, an index of the response to novelty stress, is mediated, in part, by 5-HT activation of 5-HT2A receptors.

Anxiolytic-like and antidepressant-like effects produced by the selective 5-HT6 receptor antagonist SB-258585 after intrahippocampal administration to rats

The purpose of this study was to investigate the effect of SB-258585, a selective 5-HT6 receptor antagonist, administered intrahippocampally to rats, in the conflict drinking and forced swim tests, that is models used for evaluating anxiolytic-like and antidepressant-like activity, respectively. Diazepam and imipramine were used as reference drugs. SB-258585 at a dose of 1 microg (but not 0.3 and 3 microg) showed an anticonflict effect that was weaker than that of diazepam (40 microg). SB-258585 at a dose of 3 microg (but not 1 and 10 microg) produced a marked anti-immobility effect comparable with that of imipramine (0.1 microg). The anxiolytic-like and antidepressant-like activity of SB-258585 seemed to be specific, as that compound--when given by the same route in doses effective in each model--did not affect the shock threshold, nonpunished water consumption, or exploratory activity of rats. The results obtained indicate that the hippocampus is one of the neuroanatomical sites involved in the anxiolytic-like and antidepressant-like activity of the selective 5-HT6 receptor antagonist SB-258585.

Temperamental fearfulness in childhood and the serotonin transporter promoter region polymorphism: a multimethod association study

OBJECTIVES

Early-emerging, temperamental differences in fear-related traits may be a heritable vulnerability factor for anxiety disorders. Previous research indicates that the serotonin transporter promoter region polymorphism is a candidate gene for such traits.

METHODS

Associations between 5-HTTLPR genotype and indices of fearful child temperament, derived from maternal report and standardized laboratory observations, were examined in a community sample of 95 preschool-aged children.

RESULTS

Children with one or more long alleles of the 5-HTTLPR gene were rated as significantly more nervous during standardized laboratory tasks than children who were homozygous for the short alleles. Children homozygous for the short alleles were also rated as significantly shyer, by maternal report, than those with at least one copy of the long allele of the 5-HTTLPR gene.

CONCLUSIONS

This study extends the literature linking the short alleles of the serotonin transporter promoter region polymorphism to fear and anxiety-related traits in early childhood and adulthood, and is one of very few studies to examine the molecular genetics of preschoolers' temperament using multiple measures of traits in a normative sample.

The anxiolytic effect of cinnabar involves changes of serotonin levels

The aim of this study was to explore the neuropharmacological mechanism of cinnabar. The anxiolytic effect of cinnabar on anxiety-like behaviors in mice was investigated using the elevated plus maze test. The changes in the levels of monoamine neurotransmitters and their metabolites and the activity of monoamine oxidase (MAO) in the brain of mice were determined. The results indicate that cinnabar possessed anxiolytic effects after chronic administration (p.o.) at effective doses in association with the declined brain serotonin (5-HT) level. The cinnabar showed no effects on 5-HT metabolism pathway. The results suggested the potential importance of the brain serotonergic system. The 5-HT metabolism pathway may be not involved in the anxiolytic effects of cinnabar.

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.

Differential control of cardiac and sympathetic vasomotor activity from the dorsomedial hypothalamus

1. The dorsomedial hypothalamus (DMH) plays a crucial role in mediating the cardiovascular responses to different stressors, including acute psychological stress and cold stress. Activation of neurons in the DMH evokes increases in arterial pressure and in the activity of sympathetic nerves innervating the heart, blood vessels and brown adipose tissue. The descending pathways from the DMH to the spinal sympathetic outflow include synapses with neurons in medullary nuclei and possibly other brain stem regions. 2. Recent studies from our and other laboratories have indicated that neurons in the rostral ventrolateral medulla (RVLM) and in the region of the raphe pallidus (RP) in the medulla are important components of the descending pathways that mediate the cardiovascular response to activation of the DMH. Neurons in the RP primarily mediate the sympathetic cardiac components of the DMH-evoked response, whereas the RVLM neurons primarily mediate the sympathetic vasomotor component. 3. Activation of DMH neurons not only increases heart rate and sympathetic vasomotor activity, but also resets the baroreceptor reflex such that it remains effective, without any decrease in sensitivity, over a higher operating range of arterial pressure. 4. Activation of 5-hydroxytryptamine 5-HT(1A) receptors in the medulla oblongata leads to a selective suppression of cardiac and sympathetic vasomotor components of the DMH-evoked response, but does not affect sympathetic reflex responses evoked from baroreceptors or chemoreceptors. Thus, central 5-HT(1A) receptors modulate cardiovascular responses evoked from the DMH in a highly potent but selective fashion.

Anxiogenic-like effects of mCPP microinfusions into the amygdala (but not dorsal or ventral hippocampus) in mice exposed to elevated plus-maze

Serotonin (5-HT) can either increase or decrease anxiety-like behaviour in animals, actions that depend upon neuroanatomical site of action and 5-HT receptor subtype. Although systemic studies with 5-HT(2) receptor agonists and antagonists suggest a facilitatory role for this receptor subtype in anxiety, somewhat inconsistent results have been obtained when such compounds have been directly applied to limbic targets such as the hippocampus and amygdala. The present study investigated the effects of the 5-HT(2B/2C) receptor agonist mCPP bilaterally microinjected into the dorsal hippocampus (DH: 0, 0.3, 1.0 or 3.0nmol/0.2microl), the ventral hippocampus (VH: 0, 0.3, 1.0 or 3.0nmol/0.2microl) or the amygdaloid complex (0, 0.15, 0.5, 1.0 or 3.0nmol/0.1microl) in mice exposed to the elevated plus-maze (EPM). Test sessions were videotaped and subsequently scored for conventional indices of anxiety (percentage of open arm entries and percentage of open arm time) and locomotor activity (closed arm entries). Results showed that mCPP microinfusions into the DH or VH failed to affect any behavioural measure in the EPM. However, when injected into the amygdaloid complex, the dose of 1.0nmol of this 5HT(2B/2C) receptor agonist increased behavioural indices of anxiety without significantly altering general activity levels. This anxiogenic-like effect of mCPP was selectively and completely blocked by local injection of a behaviourally-inactive dose of SDZ SER-082 (10nmol/0.1microl), a preferential 5-HT(2C) receptor antagonist. These data suggest that 5HT(2C) receptors located within the amygdaloid complex (but not the dorsal or ventral hippocampus) play a facilitatory role in plus-maze anxiety in mice.

Neurochemical and behavioral consequences of striatal injection of 5,7-dihydroxytryptamine

It is known that central serotonin (5HT) is involved in anxiety, but the behavioral results of many studies have been inconsistent. A prevalent research approach is to destroy 5HT neurotoxically. Such lesions were mostly generated by injecting 5,7-dihydroxytryptamine into ventricles or raphé nuclei, leading to rather global losses of 5HT in the brain. However, there is evidence for differential effects of 5HT in different brain structures regarding anxiety. Therefore, we decided to study the effects of injecting 5,7-dihydroxytryptamine into the forebrain. We chose the ventral striatum as the site of injection, since there is evidence that 5HT may be involved in anxiety there. We administered the neurotoxin bilaterally in adult rats, and analyzed neurochemical and behavioral consequences in three experiments. The first one showed that the toxin dose-dependently (10-50 microg) depleted 5HT in the ventral striatum, neostriatum, frontal cortex, and amygdala. Besides 5HT, dopamine was also partly depleted there. This dopaminergic lesion was prevented in a second experiment, where rats were pre-treated systemically with the dopamine reuptake inhibitor nomifensine. In the final experiment, the functional consequences of such 5HT lesions were tested, which yielded moderate anxiogenic effects in the elevated plus maze and in the open field. Also, there were lesion effects on aversively motivated ultrasonic vocalization during an active avoidance test. In contrast, active avoidance performance itself and general activity in the open field were not affected. Lesion effects became discernible there when challenging rats with MDMA. The psycho-stimulatory effectiveness of this drug, which acts largely via the availability of 5HT in the brain, was reduced to degrees that depended on the size of 5HT lesion. These results are discussed with respect to factors such as severity of lesion, anatomical specificity, and the role of 5HT in anxiety.

5-HT1A receptors are involved in the anxiolytic effect of Delta9-tetrahydrocannabinol and AM 404, the anandamide transport inhibitor, in Sprague-Dawley rats

The mechanism mediating the effects of cannabinoids on anxiety-related responses appear to involve cannabinoid CB1 and non-CB1 receptors. However, other neurotransmitters may play a role in such effect. This study shows evidence of an interaction between endocannabinoid system and serotonin (5-HT), 1A receptor subtype on anxiety-like behavior in Sprague-Dawley rats. The exogenous cannabinoid agonist, Delta9-tetrahydrocannabinol (THC), and N-(4-hydroxyphenyl)-arachidonylamide, the anandamide transporter inhibitor (AM 404) were evaluated in the elevated plus maze test. THC (0.075-0.75 mg/kg i.p.), given 30 min and AM 404 (0.75-1.25 mg/kg i. p.), given 60 min before the test, exhibited a dose-response anxiolytic effect evaluated in terms of increase in the percentage of total entries and time spent in the open and decrease of total entries and time spent in the closed arms. The anxiolytic effect obtained with the maximal active dose of both THC (0.75 mg/kg) and AM 404 (1.25 mg/kg) was blocked by the 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl) piperazin-1-yl]ethyl]-N-pyridin-2-yl-cyclohexanecarboxamide dihydro chloride (WAY-100635 (300 microg/kg, s.c.), given 30 min before THC or 15 min before AM 404. The combination of an ineffective dose of THC (0.015 mg/kg) or AM 404 (0.015 mg/kg) on anxiety-related responses with an ineffective dose of the 5HT(1A) receptor agonist, 8-Hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT) (7.5 microg/kg, i.p.), led to a synergistic effect. No interference with spontaneous motor activity, evaluated in an activity cage for 5 min, in rats given the drugs alone or in combination, was found. These data suggest that the anxiolytic effect produced by endo- and eso-cannabinoids is modulated by 5-HT1A receptors.

Relations of serotonin function to personality: current views and a key methodological issue

Studies of biological underpinnings of personality suggest that serotonergic functioning relates to certain personality traits. However, how to interpret the findings depends partly on assumptions about how personality is organized. These assumptions are reflected in the assessment devices used and also in how the data are examined. Review of evidence to date appears to link serotonin function to impulsivity and, to some extent, to hostility. The relation of serotonin function to anxiety proneness is far more questionable. Indeed, when such a relation occurs, it often takes a form opposite to the direction argued by theory. It is recommended that research use measures that discriminate adequately among personality qualities reflecting incentive sensitivity, threat sensitivity, and impulsiveness. Indeed, it is highly desirable to examine facets of each of these qualities separately.

Increased expression of the 5-HT transporter confers a low-anxiety phenotype linked to decreased 5-HT transmission

A commonly occurring polymorphic variant of the human 5-hydroxytryptamine (5-HT) transporter (5-HTT) gene that increases 5-HTT expression has been associated with reduced anxiety levels in human volunteer and patient populations. However, it is not known whether this linkage between genotype and anxiety relates to variation in 5-HTT expression and consequent changes in 5-HT transmission. Here we test this hypothesis by measuring the neurochemical and behavioral characteristics of a mouse genetically engineered to overexpress the 5-HTT. Transgenic mice overexpressing the human 5-HTT (h5-HTT) were produced from a 500 kb yeast artificial chromosome construct. These transgenic mice showed the presence of h5-HTT mRNA in the midbrain raphe nuclei, as well as a twofold to threefold increase in 5-HTT binding sites in the raphe nuclei and a range of forebrain regions. The transgenic mice had reduced regional brain whole-tissue levels of 5-HT and, in microdialysis experiments, decreased brain extracellular 5-HT, which reversed on administration of the 5-HTT inhibitor paroxetine. Compared with wild-type mice, the transgenic mice exhibited a low-anxiety phenotype in plus maze and hyponeophagia tests. Furthermore, in the plus maze test, the low-anxiety phenotype of the transgenic mice was reversed by acute administration of paroxetine, suggesting a direct link between the behavior, 5-HTT overexpression, and low extracellular 5-HT. In toto, these findings demonstrate that associations between increased 5-HTT expression and anxiety can be modeled in mice and may be specifically mediated by decreases in 5-HT transmission.

A study of the role of serotonin in the anxiolytic effect of nitrous oxide in rodents

RATIONALE

In earlier studies, we have shown that nitrous oxide (N2O)-induced behavioral effects in rats and mice are mediated by benzodiazepine receptors.

OBJECTIVES

This two-part study was conducted in order to investigate the possible role of serotonin (5-HT) in the behavioral effects of N2O by clarifying its effects on regional brain concentrations of 5-HT and assessing the influence of 5-HT antagonist and reuptake inhibiting drugs on the anxiolytic-like behavioral effect of N2O.

METHODS

In experiment A, male, 150-200 g Sprague-Dawley rats were killed following a 15-min exposure to room air or 70% N2O. The frontal cortex, hippocampus, corpus striatum and hypothalamus were dissected out and analyzed by HPLC with electrochemical detection for content of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA); dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were also measured. In experiment B, male 18-22 g NIH Swiss mice were pretreated with the 5-HT2 antagonist cinanserin, the 5-HT3 antagonist LY-278,584, the 5-HT reuptake inhibitor fluoxetine or saline and tested in the light/dark exploration test under 70% N2O 30 min after pretreatment.

RESULTS

In experiment A, N2O produced differential effects on 5-HT neurons in distinct brain areas. There was increased 5-HT turnover in the hypothalamus, decreased turnover in the frontal cortex but no changes in either hippocampus or corpus striatum. By comparison, dopamine turnover in these brain regions was unaltered by N2O exposure. In experiment B, pretreatment with neither cinanserin, LY-278,584 nor fluoxetine had any appreciable effect on the N2O-induced increase in time spent in the light compartment. Only cinanserin significantly reduced the N2O-induced increase in transitions.

CONCLUSIONS

While neurochemical results suggest an effect of N2O on brain 5-HT function, there was no effect of 5-HT2 or 5-HT3 antagonists or 5-HT reuptake inhibitor on N2O-induced anxiolytic-like behavior.

Effects of Milnacipran in Animal Models of Anxiety and Memory

Serotonin (5-HT) and noradrenaline (NA) are involved in both pathogenesis and recovery from depression and anxiety. We examined the effects of acute and chronic treatment with milnacipran, a serotonin/noradrenaline reuptake inhibitors (SNRIs) antidepressant, on anxiety and memory retention in rats. Male Wistar rats received acute or chronic administration of milnacipran (12.5, 25 or 50 mg/kg) or saline (control group). The animals were separately submitted to elevated plus-maze, inhibitory avoidance and open-field tasks 1 h after injection, in the acute group, or 23 h after last injection, in the chronic group. Our results showed an anxiolytic-like effect after chronic administration of milnacipran at doses of 25 and 50 mg/kg. The treatment does not interfere in memory retention and habituation to a novel environment at any doses studied. These findings support that milnacipran, an established SNRIs antidepressant, can also be useful in the treatment of anxiety disorders.

5-HT receptor subtypes involved in the anxiogenic-like action and associated Fos response of acute fluoxetine treatment in rats

RATIONALE

We have recently reported that acute treatment with the selective serotonin reuptake inhibitor fluoxetine exacerbates escape responses to airjet and facilitates airjet-induced activation of locus coeruleus (LC) neurons.

OBJECTIVE

Here we aimed to identify the 5-HT receptor subtype(s) mediating the anxiogenic-like effects of acute fluoxetine in this paradigm and to study whether chronic fluoxetine treatment would alter these responses.

METHODS

The expression of the immediate early gene c-fos was used as a marker of neuronal activation.

RESULTS

Acute fluoxetine increased the airjet-induced escape behaviour and Fos expression in the LC of saline-pretreated rats. Pretreatment with the 5-HT(2C/2B) antagonist SB 206553, but not with the 5-HT1A antagonist WAY 100635, the 5-HT1B antagonist SB 224289 or the 5-HT3 antagonist Y-25130 inhibited the fluoxetine-induced increase in escape behaviour and the associated elevated LC Fos response. The selective 5-HT2C agonist MK-212 mimicked the anxiogenic response of fluoxetine. Chronic treatment with fluoxetine abolished the anxiogenic-like effect and led to a normalization of the enhanced fluoxetine-induced Fos response to airjet.

CONCLUSIONS

Taken together, the results indicate that the anxiogenic-like effect as well as the facilitated neuronal reactivity induced by acute fluoxetine in the airjet model is mediated primarily by activation of 5-HT2C receptors.

Haplotype analysis reveals tryptophan hydroxylase (TPH) 1 gene variants associated with major depression

BACKGROUND

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin (5-HT) and might be related to the pathogenesis of major depression (MD). Two isoforms are known, TPH-1 and TPH-2. Tryptophan hydroxylase-1 association with MD is still debated.

METHODS

A single nucleotide polymorphism (SNP) screening strategy was used to define TPH-1 haplotypes spanning over 23 kilobase (kb) of the 29 kb gene length. Genotyping was performed in 228 MD patients and 253 healthy control subjects.

RESULTS

Six SNPs were found at linkage disequilibrium in both patients and control subjects, suggesting a haplotype block structure. Single marker association analyses showed only one SNP significantly associated with MD. Several haplotypes were associated with MD. When all six locus haplotypes were divided into two groups, above or below a 5% threshold, the compound haplotype group below a 5% frequency resulted as associated with the disease (31.6% vs. 18.0% in control subjects, p < 10(-5)). A "sliding window" analysis attributed the strongest disease association to a haplotype configuration localized between introns 7 and 8 (p < 10(-5)).

CONCLUSIONS

Haplotype analysis indicates that TPH-1 associates with MD. The most common TPH-1 variants appear to carry no risk, while some of the less frequent variants might contribute to genetic predisposition to MD.

Modulation of anxiety circuits by serotonergic systems

Anxiety is a complex emotional state associated with sustained heightened autonomic and behavioral arousal and an increase in avoidance behavior. Anxiety-related behavior is a form of risk assessment behavior that is associated with a level of uncertainty or unpredictability regarding the outcome of emotionally salient events, often when both rewarding and aversive outcomes are possible. In this review, we highlight recent advances in our understanding of the neural circuits regulating anxiety states and anxiety-related behavior with an emphasis on the role of brainstem serotonergic systems in modulating anxiety-related circuits. In particular, we explore the possibility that the regulation of anxiety states and anxiety-related behavior by serotonergic systems is dependent on a specific, topographically organized mesolimbocortical serotonergic system that originates in the mid-rostrocaudal and caudal parts of the dorsal raphe nucleus.

Effect of 5-HT depletion on cardiovascular vagal reflex sensitivity in awake and anesthetized rats

Antagonism of central 5-HT1A and 5-HT7 receptors inhibits reflex-evoked vagal bradycardias indicating that 5-HT is released during these reflexes. The present experiments examined the effect of 5-HT depletion with para-chlorophenylalanine (p-CPA) on the cardiac vagal baroreflex and cardiopulmonary reflex in awake and anesthetized rats. Immunocytochemistry and neurochemical detection showed that p-CPA depleted the brainstem of 5-HT, but not of norepinephrine or dopamine. Depletion of 5-HT was associated with an increase in mean arterial pressure (MAP) in awake rats. This difference was abolished by anesthesia, which reduced MAP in both groups of animals. The baroreflex gain, whether calculated from the rise in pressure induced by phenylephrine or the fall in pressure evoked by sodium nitroprusside, was significantly attenuated in depleted rats compared to controls. This attenuation of the baroreflex gain was unaffected by subsequent anesthesia. 5-HT depletion also attenuated the cardiopulmonary reflex vagal bradycardias but this only reached statistical significance when the rats were anesthetized. The data support the view that 5-HT is released in the reflex activation of the cardiac vagal pathway.

Activation of 5-Hydroxytryptamine 1A Receptors Suppresses the Cardiovascular Response Evoked From the Dorsomedial Hypothalamic Nucleus

The dorsomedial hypothalamic nucleus is a key component of the central pathways subserving the cardiovascular response to psychological stress, which is believed to be an important risk factor for hypertension. Previous studies indicate that 5-hydroxytryptamine 1A receptors can modulate the cardiovascular responses associated with stress. In this study, we determined in anesthetized rats the effects of systemic or intracisternal administration of 8-hydroxy-2-(di-n-propylamino)tetralin, a selective agonist of 5-hydroxytryptamine 1A receptors, and then subsequent administration of the selective antagonist WAY-100635 on the cardiovascular response evoked by activation of the dorsomedial hypothalamic nucleus (by microinjection of bicuculline). The increase in mean arterial pressure, heart rate, and renal sympathetic nerve activity (RSNA) evoked by bicuculline injection into the dorsomedial hypothalamic nucleus was greatly reduced (by 80% to 90%) by administration of 8-hydroxy-2-(di-n-propylamino)tetralin and then completely restored by subsequent administration of WAY-100635, whether administered systemically or intracisternally. In contrast, systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin had no significant effect on the baseline level or reflex changes in RSNA evoked by chemoreceptor or baroreceptor stimulation and resulted in only a modest reduction (12 mm Hg) in baseline mean arterial pressure. The results indicate that activation of 5-hydroxytryptamine 1A receptors in the brain stem causes a potent and selective suppression of the hypertensive and sympathoexcitatory response evoked by stimulation of the dorsomedial hypothalamic nucleus but has little effect on the tonic level or baroreceptor or chemoreceptor reflex control of RSNA.

Diurnal variation in serotonin immunoreactivity in the dorsal raphe nucleus

Serotonin (5-HT) immunostained sections were analyzed using integrated optical density (IOD) measures obtained throughout the dorsal raphe nucleus (DRN) in Mongolian gerbils at selected times during a 12:12 h light:dark cycle. Substantial diurnal variation occurred in 5-HT neuronal staining density, with lowest and highest IOD values occurring at the light/dark and dark/light transitions, respectively. The injection of pargyline and tryptophan increased 5-HT immunostaining comparable to the highest level observed in control animals. Transitions between light and dark periods appear to be major environmental events that influence 5-HT levels in the DRN.

Effects of a selective 5-HT1B receptor agonist and antagonists in animal models of anxiety and depression

The purpose of the present study was to investigate the effects of the selective 5-HT1B receptor agonist CP 94253, the selective 5-HT1B receptor antagonist SB 216641, and the 5-HT1B/1D receptor antagonist GR 127935 in behavioral tests commonly used to predict anxiolytic- and antidepressant-like activity. Diazepam and imipramine were used as reference drugs. In the Vogel conflict drinking test, CP 94253 (1.25-5 mg/kg), SB 216641 (2.5-5 mg/kg) and GR 127935 (5-10 mg/kg) showed anxiolytic-like effects comparable to that of diazepam (2.5-5 mg/kg). In the elevated plus-maze test, antianxiety-like activity of all the compounds tested was also observed: the effects of CP 94253 (2.5 mg/kg) and SB 216641 (5 mg/kg) were similar to that of diazepam (5 mg/kg), while GR 127935 (up to 40 mg/kg) was less active. In the four-plate test, the compounds tested (5-10 mg/kg) produced anxiolytic-like effects which were weaker than that of diazepam (2.5-5 mg/kg). In the forced swimming test, CP 94253 (5-10 mg/kg), like imipramine (30 mg/kg), showed anti-immobility action, whereas SB 216641 (2.5-10 mg/kg) and GR 127935 (20-40 mg/kg) did not affect the immobility time in mice. The results indicate that the selective agonist (CP 94253) and antagonists (SB 216641 and GR 127935) of 5-HT1B receptors produce effects that are characteristic of anxiolytics, in the preclinical models used; however, CP 94253 also behaves like an antidepressant drug.

Effects of 5-HT1Areceptor agonist and antagonist on anxiety in intact and ovariectomized female rats

The purpose of this work was to study the role of 5-HT1A receptors on the level of anxiety in adult intact and ovariectomized (OVX) female rats. The influence of chronic administration of 5-HT1A receptor agonist 8-OH-DPAT (0.05 mg/kg, s.c.) and 5-HT1A receptor antagonist NAN-190 (0.1 mg/kg, i.p.) given for 14 days alone or in combination with 17beta-estradiol (0.5 microg i.m./rat/day) was studied on behavior in the elevated plus maze. In intact females administration of NAN-190 resulted in significant increase in the number of enterings and the time spent on the open arms in every phase of the estrous cycle, however, 8-OH-DPAT failed to modify these parameters. In OVX females 8-OH-DPAT alone or in combination with 17beta-estradiol significantly increased the number of enterings and time spent on the open arms. On the contrary, NAN-190 alone or in combination with 17beta-estradiol in OVX females failed to evoke behavioral changes in the elevated plus maze. Thus, the 5-HT1A receptor antagonist NAN-190 induced anxiolytic effect in intact female rats, while 5-HT1A receptor agonist 8-OH-DPAT produced an anxiolytic profile on OVX rats. Results of this work specify the involvement of 5-HT1A receptors in behavioral mechanisms of anxiety in OVX female rats.

Involvement of 5-HT1?receptors in passive avoidance learning in intact and ovariectomized female rats

The influence of chronic administration of 5-HT1A receptor agonist 8-OH-DPAT (0.05 mg/kg, s.c.) and 5-HT1A receptor antagonist NAN-190 (0.1 mg/kg, i.p.) injected for 14 days alone or in combination with 17beta-estradiol (0.5 microg i.m./rat/day) was studied on passive avoidance performance (PAR) and on behavior in the open field test in adult intact and ovariectomized (OVX) female rats. Administration of 5-HT1A receptor antagonist NAN-190 alone significantly improved PAR (p<0.05) in intact females with proestrus and estrus and in OVX females. Administration of 5-HT1A receptor agonist 8-OH-DPAT alone or in combination with 17beta-estradiol significantly (p<0.05) improved PAR in OVX rats and failed to normalize PAR in intact rats with proestrus and estrus. Results of the work specify the involvement of 5-HT1A receptors in the mechanisms of passive avoidance learning in OVX female rats.

Anxiolytic effect of berberine on exploratory activity of the mouse in two experimental anxiety models: interaction with drugs acting at 5-HT receptors

The aim of this study was to assess the anxiolytic effect of berberine (abbrev. BER) using two experimental anxiety models in the mouse. In the black and white test of anxiety, berberine (100, 500 mg/kg) produced an increase in the first time entry, time spent in the white section, and total changes between two compartments. On the other hand, in the elevated plus-maze test, berberine (100, 500 mg/kg) produced an increase in the time spent and arm entries in the open arms, and a decrease in the time spent and arm entries in the closed arms. Berberine (500 mg/kg) decreased locomotor activity in mice. Furthermore, BER at 100, 500 mg/kg decreased concentrations of NE, DA and 5-HT, and increased the concentrations of VMA, HVA and 5-HIAA in the brain stem. BER also attenuated the anxiogenic effect of WAY-100635, 8-OH DPAT and DOI and enhanced the anxiolytic effect of BUS, p-MPPI and RIT in the elevated plus-maze. These results suggested that berberine at 100 mg/kg had a significant anxiolytic-like effect, which was similar to that observed with 1 mg/kg diazepam and 2 mg/kg buspirone. The anxiolytic mechanism of BER might be related to the increase in turnover rates of monoamines in the brain stem and decreased serotonergic system activity. Moreover, BER decreased serotonergic system activity via activation of somatodendritic 5-HT1A autoreceptors and inhibition of postsynaptic 5-HT1A and 5-HT2 receptors.

Anxiety-related behaviour in C57BL/6 <--> BALB/c chimeric mice

In a previous study on anxiety-related behaviours of the genetically and behaviourally distant inbred mouse strains C57BL/6 and BALB/c using the Elevated plus-maze (EPM) and Open-field (OF) apparatuses, we identified a number of variables, the factorial scores of which were grouped by principal component analysis (PCA) into factors specifically describing each inbred strain. We have now studied the effect of C57BL/6 and BALB/c haploid sets of genes on this behaviour by comparing EPM and OF variables of C57BL/6 and BALB/c versus C57BL/6 x BALB/c F1 hybrids (B6CBF1) and chimeric C57BL/6 x BALB/c (CHIM) mice. CHIM mice were made by embryo aggregation and the chimerism degree of their brain was inferred from coat black/white distribution. Discriminant analysis of EPM and OF factorial scores of C57BL/6, BALB/c and CHIM mice showed that CHIM mice with an exceeding (> or =80%) C57BL/6 or BALB/c coat component had behaviours similar to those of the predominant strain, whereas CHIM mice with intermediate chimerism differed from both inbred strains. Additional MANOVA analysis showed that the anxiety behaviour of CHIM mice with intermediate chimerism was similar to that of B6CBF1 mice as for factors not describing the inbred strains, including a motor activity mostly limited to protected areas, with attempts to approach the anxiogenic areas while processing/storing the external information. We conclude that the balanced presence of both C57BL/6 and BALB/c genetic backgrounds, either when carried by the same cell or by different cells, gives rise to a novel stress coping strategy described by factors different from those of the inbred strains.

The effects of MDMA pretreatment on the behavioural effects of other drugs of abuse in the rat elevated plus-maze test

Few preclinical studies have found long-term behavioural consequences of the serotonergic neurotoxicity produced by 3,4-methylenedioxymethamphetamine (MDMA). This study investigated whether pretreatment with MDMA altered the behavioural effects of other drugs of abuse. Adult male Lister hooded rats (n=10/group) were pretreated with 10 mg/kg MDMA or 1 ml/kg saline vehicle intraperitoneally every 2 h for 6 h. Fourteen days later, the behavioural effects of d-amphetamine (2 mg/kg), cocaine (10 mg/kg), ethanol (2.0 g/kg), heroin (0.5 mg/kg), or MDMA (10 mg/kg) were assessed in the elevated plus-maze test. MDMA pretreatment produced approximately 20-25% decrease in hippocampal 5-HT and 5-HIAA concentrations, and [(3)H]paroxetine binding when analysed 2 weeks later. Despite inducing neurotoxicity, this regimen had no effect upon the plus-maze behaviour induced by ethanol, heroin, and MDMA. Acutely, and independent of neurotoxic pretreatment, MDMA produced a clear anxiogenic-like behavioural profile with a reduction of open arm entries and suppression of explorative behaviours. Despite being acutely anxiogenic, pretreatment with a neurotoxic regimen of MDMA has little effect on the anxiety-related effects of other drugs of abuse. It is possible that extended time points would produce significant changes, although the available evidence suggests that the plus-maze may not be a suitable model for detection of behavioural dysfunction after neurotoxic MDMA.

Comparison of deramciclane to benzodiazepine agonists in behavioural activity of mice and in alcohol drinking of alcohol-preferring rats

Interactions between alcohol and traditional benzodiazepine anxiolytics hamper the treatment of alcoholism-related anxiety disorders. Serotonin 5-HT(2) receptor antagonists, such as deramciclane, are anxiolytic, and considering their pharmacological profile, they might benefit alcoholics with comorbid anxiety. We studied the effects of acute deramciclane (1, 3 and 10 mg/kg i.p.) on alcohol drinking of alcohol-preferring AA rats drinking 10% (vol/vol) ethanol solution in a 4-h limited-access paradigm. Thereafter, a 5-day repeated-treatment experiment was carried out, under corresponding test design, with deramciclane (3 mg/kg i.p.) as a test drug and midazolam (1 mg/kg i.p.) as a benzodiazepine reference compound. Deramciclane had no effect on alcohol consumption in either acute or repeated dosing study. Midazolam increased ethanol drinking, as expected, when administered on successive days. A modified functional observational battery (FOB) procedure was applied to study neurological, behavioural and autonomic effects induced by deramciclane (1-30 mg/kg po) and diazepam (1-30 mg/kg po) in mice at 30 min, 2 h and 4 h after dosing. Deramciclane had a mild dopamine D(2) receptor antagonism-like effect at the highest dose. The effects of diazepam were predictable, myorelaxation-induced motor impairment and anxiolysis-related hyperlocomotion in a novel environment being the characteristic features at the two highest doses. Deramciclane appears to be a safe and well-tolerated drug and we suggest that it might be useful in the treatment of anxiety in alcoholics.

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.

Targeting conditional gene modification into the serotonin neurons of the dorsal raphe nucleus by viral delivery of the Cre recombinase

Delivery of viral vectors encoding the Cre recombinase is showing promise to target gene modification in specific brain regions. Here we describe the targeting of the dorsal raphe nucleus (DRN), which contains the majority of the serotonin (5-HT) neurons projecting to the forebrain. First, we demonstrate successful transgene expression in the mouse DRN by stereotaxic delivery of the AdnlslacZ adenoviral vector. Second, we show that expression of the Cre recombinase can be achieved in the 5-HT neurons by optimized injection of the Adcre vector. Using reporter mice in which Cre activity induces beta-galactosidase (beta-gal) expression, we demonstrate efficient Cre-mediated recombination and persistence of beta-gal positive 5-HT neurons at least 1 month postinjection. Together, these results demonstrate that viral delivery provides a valuable method to target Cre recombination throughout the murine DRN and thus to study 5-HT neurotransmission by conditional gene modification.

Chemical Lesion of the Bed Nucleus of the Stria Terminalis Blocks the Behavioral Consequences of Uncontrollable Stress

Uncontrollable or inescapable shock (IS) produces behavioral changes that are characterized by a sensitized fear system and a deficit in fight-flight responding. These behavioral changes have been argued to represent an anxiety-like state produced by the uncontrollability of the stressor. The bed nucleus of the stria terminalis (BNST) has been implicated in the mediation of long-duration responses to unpredictable stressors, which have also been argued to represent anxiety. In the present study, the effects of BNST chemical lesion on the IS-induced sensitization of freezing to an environment previously paired with shock and the IS-induced impairment of escape responding were investigated. BNST chemical lesion blocked the potentiation of freezing and the increases in escape latency that normally follow IS.

Why do some individuals develop social phobia? A review with emphasis on the neurobiological influences

Social phobia, or social anxiety disorder, is now considered the most common anxiety disorder. Still, the etiology of the disorder is to an essential degree unknown. This paper presents an overview of various pathways to be considered in relation to the development of social phobia. The literature concerning genetics and family aggregation, behavioral inhibition, various forms of the learning account, and neurobiological influences was examined. The reviewed studies suggest that social phobia has a neuroanatomical basis in a highly sensitive fear network centered in the amygdaloid-hippocampal region, i.e. "the alarm system" of the brain, and encompassing the prefrontal cortex. This pattern is congruent with genetic studies proposing that the genetic component comprises a general vulnerability to fearfulness rather than to social phobia itself. Further, both family and twin studies support a hereditary contribution to social phobia resulting from genetic and environmental factors, which most likely operate in an interactive way rather than acting in isolation.

Differential neurochemical properties of central serotonergic transmission in Roman high- and low-avoidance rats

The selective breeding of Roman high- (RHA/Verh) and low-avoidance (RLA/Verh) rats for rapid versus poor acquisition of active avoidant behaviour has produced two behavioural phenotypes with different performances in a variety of animal models of anxiety, in which RLA/Verh rats are consistently more fearful than RHA/Verh rats. In addition, these two lines display different functional properties of brain neurotransmitters like serotonin (5-HT), known to be involved in the expression of anxiety- and depression-related behaviours. Therefore, we used brain microdialysis and [3H]-citalopram binding autoradiography to characterize further the neurochemical properties of 5-HTergic transmission in the two lines. No significant line-related differences were detected in the basal 5-HT output in the frontoparietal cortex (FPCx). In contrast, the increase in the cortical 5-HT output elicited by the systemic administration or the local application, via reverse dialysis, of chlorimipramine and fluoxetine was more robust in RHA/Verh than in RLA/Verh rats. Moreover, the binding signal of [3H]-citalopram to 5-HT re-uptake sites was more intense in the FPCx of RHA/Verh rats than in their RLA/Verh counterparts. These findings suggest that the functional tone of the 5-HTergic projection to the FPCx is stronger in the RHA/Verh line relative to the RLA/Verh line. It is proposed that RLA/Verh rats may be used as a model with heuristic value for studying the role of 5-HTergic transmission in anxiety and in the anxiolytic effects of monoamine re-uptake inhibitors.

Involvement of presynaptic 5-HT(1A) and benzodiazepine receptors in the anticonflict activity of 5-HT(1A) receptor antagonists

In the present paper, we examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg/kg), and the influence of flumazenil (Ro 15-1788, 10 mg/kg), a benzodiazepine receptor antagonist, on the anticonflict activity of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY 100635) and trans-1-(2-methoxy-phenyl)-4-[4-succinimidocyclohexyl]piperazine (MP 349), pre- and postsynaptic 5-HT(1A) receptor antagonists, and 1-(2-methoxyphenyl)-4-(4-succinimidobutyl)piperazine (MM 77), a postsynaptic 5-HT(1A) receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. WAY 100635 (0.5-1 mg/kg), MP 349 (0.25-0.5 mg/kg), MM 77 (0.03-0.25 mg/kg) and diazepam (2.5-5 mg/kg) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test. In p-chloroamphetamine-pretreated rats, neither WAY 100635 (1 mg/kg) nor MP 349 (0.25 mg/kg) induced an anticonflict effect, whereas MM 77 (0.06 mg/kg) did produce it. Flumazenil fully blocked the anticonflict effects of WAY 100635 (1 mg/kg) and diazepam (5 mg/kg), and it partly but significantly reduced the anticonflict effects of MP 349 (0.25 mg/kg) and MM 77 (0.06 mg/kg). p-Chloroamphetamine and flumazenil alone were inactive in the conflict drinking test. The present results suggest that, first, the anticonflict effect of the 5-HT(1A) receptor antagonists, WAY 100635 and MP 349, but not MM 77, is linked to the presynaptically located 5-HT(1A) receptors, and second, benzodiazepine receptors are indirectly involved in such effects of WAY 100635, MP 349 and MM 77, due maybe to a possible interaction between the 5-HT and the gamma-aminobutyric acid (GABA)/benzodiazepine systems.

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.

Operant learning and differential-reinforcement-of-low-rate 36-s responding in 5-HT1A and 5-HT1B receptor knockout mice

Previous studies with mice lacking 5-HT(1A) (1AKO) and 5-HT(1B) (1BKO) receptors in hippocampus-dependent learning and memory paradigms, suggest that these receptors play an important role in learning and memory, although their precise role is unclear. In the present study, 1AKO and 1BKO mice were studied in operant behavioural paradigms of decision making and response inhibition, to further study the putative involvement of these receptors in prefrontal cortex-dependent learning and memory. Moreover, because 1AKO mice have been shown to exhibit an antidepressant-like phenotype and 1BKO mice to be more impulsive in ethological studies, mice were trained in a differential-reinforcement-of-low-rates (DRL) procedure. Overall, results indicate that 1AKO and 1BKO mice display subtle differences in operant paradigms of decision making and response inhibition compared to wild type (WT) mice. In addition, when responding under a DRL 36-s schedule had stabilised, 1BKO mice showed a phenotype indicative of increased impulsivity, whereas 1AKO mice did not differ from WT mice. In conclusion, 5-HT(1B) receptors appear to play an important role in impulsivity and a minor role in prefrontal cortex-dependent learning and memory as shown by the results obtained in serial reversal learning and extinction. In contrast, 5-HT(1A) receptors appear to be involved in facilitation of autoshaping, but their role in impulsivity and prefrontal cortex-dependent learning and memory appears to be limited.

The pre-clinical behavioural pharmacology of 3,4-methylenedioxymethamphetamine (MDMA)

3,4-Methylenedioxymethamphetamine (MDMA) is a relatively novel drug of abuse and as such little is currently known of its behavioural pharmacology. This review aims to examine whether MDMA represents a novel class of abused drug. MDMA is known as a selective serotonergic neurotoxin in a variety of animal species but acutely it is a potent releaser and/or reuptake inhibitor of presynaptic serotonin, dopamine, noradrenaline, and acetylcholine. Interaction of these effects contributes to its behavioural pharmacology, in particular its effects on body temperature. Drug discrimination studies indicate that MDMA and related drugs produce unique interoceptive effects which have led to their classification as entactogens. This is supported by results from other behavioural paradigms although there is evidence for dose dependency of MDMA-specific effects. MDMA also produces conditioned place preference but is not a potent reinforcer in self-administration studies. These unique behavioural effects probably underlie its current popularity. MDMA is found in the street drug ecstasy but it may not be appropriate to equate the two as other drugs are routinely found in ecstasy tablets

The mouse light/dark box test

The light/dark test is based on the innate aversion of rodents to brightly illuminated areas and on the spontaneous exploratory behaviour of rodents in response to mild stressors, that is, novel environment and light. The test apparatus consists of a small dark safe compartment (one third) and a large illuminated aversive compartment (two thirds). The test was developed with male mice. The strain, weight and age may be crucial factors. The extent to which an anxiolytic compound can facilitate exploratory activity depends on the baseline level in the control group. Differences between the type and severity of external stressors might account for the variable results reported by different laboratories. The light/dark test may be useful to predict anxiolytic-like or anxiogenic-like activity in mice. Transitions have been reported to be an index of activity-exploration because of habituation over time, and the time spent in each compartment to be a reflection of aversion. Classic anxiolytics (benzodiazepines) as well as the newer anxiolytic-like compounds (e.g. serotonergic drugs or drugs acting on neuropeptide receptors) can be detected using this paradigm. It has the advantages of being quick and easy to use, without requiring the prior training of animals.

Evidence for disturbed cortical signal processing and altered serotonergic neurotransmission in generalized anxiety disorder

BACKGROUND

Current pathophysiological concepts of generalized anxiety disorder (GAD) assume a disturbed exteroceptive sensory system. Furthermore, central serotonergic neurotransmission has been shown to play an important role in anxiety disorder. Cortical signal processing as measured by auditory evoked potentials (AEPs) may reflect the integrity of the exteroceptive sensory system. Because a special aspect of AEP, the loudness dependence of the N1/P2-component (LD), has been related to central serotonergic activity, the LD may be useful for investigating serotonergic dysfunctions in GAD.

METHODS

The LD was recorded in 31 medication-free patients with GAD without any psychiatric co-morbidity and in 31 matched control subjects. Dipole source analysis was performed to separate the LD of regions including the primary (LD-tangential dipole) and regions including the secondary auditory cortex (LD-radial dipole).

RESULTS

A shallower LD-tangential was observed in patients with GAD as compared to healthy control subjects [F(1,60) = 6.727, p =.012; one-way analysis of variance]. The LD-radial showed no differences between groups. Severity of the anxiety symptoms was not related to the LDs.

CONCLUSIONS

The results indicate an altered exteroceptive sensory system in GAD occurring at the level of the primary but not secondary auditory cortex. Because a shallow LD of the primary auditory cortex was related to a high firing rate of neurons in the dorsal raphe nucleus, the results may support evidence for an enhanced serotonergic activity in GAD.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Modulation of defensive behavior by periaqueductal gray NMDA/glycine-B receptor

Glutamate (GLU) associated with glycine, act as co-transmitter at the N-methyl-D-aspartate/glycine-B (NMDA/GLY(B)) receptor. Dorsal periaqueductal gray (dPAG) neurons express NMDA/GLY(B) receptors suggesting a GLU physiological role in mediating the responses elicited by stimulation of this area. Immunohistochemical data provided evidence of a possible correlation among elevated plus-maze (EPM), fear-like defensive behavior, and dPAG activity. The present data show that whereas the NMDA/GLY(B) receptor agonists increased the open-arm avoidance responses in the EPM, the antagonists had the opposite effects. Microinjection of NMDA/GLY(B) receptor agonists within the dPAG during test sessions in the EPM resulted in an enduring learned fear response detected in the retest. Therefore, in addition to the proposed role for the dPAG in panic attacks (escape), these findings suggest that the dPAG can also participate in more subtle anxiety-like behaviors.

Genetic perspectives on the serotonin transporter

The serotonin transporter (5-HTT) is most well known as the site of action of the serotonin reuptake inhibitors, which were initially developed as antidepressants, but now are the most widely used agents in the treatment of many additional neuropsychiatric and related disorders. The discovery that the gene that expresses the 5-HTT possesses a functional promoter-region polymorphism, which is associated with temperament and personality traits such as anxiety and negative emotionality as well as some behaviors, led to many studies examining this polymorphism in individuals with different neuropsychiatric disorders. The subsequent development of mice with a targeted disruption of the 5-HTT in our laboratory has provided an experimental model to examine the many consequences of diminished (in +/-, heterozygote mice) or absent (in -/-, homozygote knockout mice) function of the 5-HTT. The 5-HTT-deficient mice were also crossed with other knockout mice, allowing the study of multiple neurobiologic dysfunctions. As multiple genes are probably involved in the expression of complex behaviors such as anxiety, as well as neuropsychiatric disorders, these more genetically complex mice may more closely model disorders with complex etiologies. Thus, the combination of these comparative human and mouse studies may extend the opportunities to examine genetic alterations from a novel "bottom-up" approach [gene knockout or partial gene knockout in a combinational gene x gene x (yet unknown) gene approach], which is complementary to the traditional "top-down" genetic approach based upon studies of individuals with diagnosed neuropsychiatric disorders and their family members.

Differential effects of 5-HT1A receptor deletion upon basal and fluoxetine-evoked 5-HT concentrations as revealed by in vivo microdialysis

An involvement of serotonin (5-HT) 1A receptors in the etiology of psychiatric disorders has been suggested. Hypo-responsiveness of the 5-HT1A receptor is linked to anxiety and constitutive deletion of the 5-HT1A receptor produces anxiety-like behaviors in the mouse. Evidence that 5-HT1A receptor inactivation increases the therapeutic effects of antidepressants has also been presented. The present studies used in vivo microdialysis and homologous recombination techniques to examine the contribution of 5-HT1A autoreceptors to these effects. Basal and fluoxetine-evoked extracellular concentrations of 5-HT were quantified in the striatum, a projection area of dorsal raphe neurons (DRN), of wild-type (WT) and 5-HT1A receptor knock out (KO) mice. The density of 5-HT transporters was also determined. Basal 5-HT concentrations did not differ in WT and KO mice. Fluoxetine (10 mg/kg) increased 5-HT concentrations in both genotypes. This increase was, however, 2-fold greater in KO mice. In contrast, no differences in K(+)-evoked 5-HT concentrations were seen. Similarly, neither basal nor stimulation-evoked DA differed across genotype. Autoradiography revealed no differences between genotype in the density of 5-HT transporters or post-synaptic 5-HT2A receptors, an index of 5-HT neuronal activity. These experiments demonstrate that, under basal and KCl stimulated conditions, adaptive mechanisms in the 5-HT system compensate for the lack of 5-HT1A autoreceptor regulation of DRN. Furthermore, they suggest that the absence of release-regulating 5-HT1A autoreceptors in the DRN can not account for the anxiety phenotype of KO mice. The enhanced response to fluoxetine in KO mice is consistent with pharmacological studies and suggests that adaptive mechanisms that occur in response to 5-HT1A receptor deletion are insufficient to oppose increases in 5-HT concentrations produced by acute inhibition of the 5-HT transporter.