Serotonergic mechanisms involved in the exploratory behaviour of mice in a fully automated two-compartment black and white text box

Approach/Avoidance Behavior to Novel Objects is Correlated with the Serotonergic and Dopaminergic Systems in the Brown Rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is known to show three types of behavioral responses to novel objects. Whereas some rats are indifferent to novel objects, neophobic and neophilic rats show avoidance and approach behavior, respectively. Here, we compared the dopaminergic, serotonergic, and noradrenergic systems immunohistochemically among these rats. Trapped wild rats and laboratory rats were first individually exposed to the novel objects in their home cage. Wild rats were divided into neophobic and indifferent rats depending on their behavioral responses. Similarly, laboratory rats were divided into neophilic and indifferent rats. Consistent with the behavioral differences, in the paraventricular nucleus of the hypothalamus, Fos expression in corticotropin-releasing hormone-containing neurons was higher in the neophobic rats than in the indifferent rats. In the anterior basal amygdala, the neophobic rats showed higher Fos expression than the indifferent rats. In the posterior basal amygdala, the neophobic and neophilic rats showed lower and higher Fos expressions than the indifferent rats, respectively. When we compared the neuromodulatory systems, in the dorsal raphe, the number of serotonergic neurons and Fos expression in serotonergic neurons increased linearly from neophobic to indifferent to neophilic rats. In the ventral tegmental area, Fos expression in dopaminergic neurons was higher in the neophilic rats than in the indifferent rats. These results demonstrate that approach/avoidance behavior to novel objects is correlated with the serotonergic and dopaminergic systems in the brown rat. We propose that the serotonergic system suppresses avoidance behavior while the dopaminergic system enhances approach behavior to novel objects.

HDAC I inhibition in the dorsal and ventral hippocampus differentially modulates predator-odor fear learning and generalization

Although predator odors are ethologically relevant stimuli for rodents, the molecular pathways and contribution of some brain regions involved in predator odor conditioning remain elusive. Inhibition of histone deacetylases (HDACs) in the dorsal hippocampus has been shown to enhance shock-induced contextual fear learning, but it is unknown if HDACs have differential effects along the dorso-ventral hippocampal axis during predator odor fear learning. We injected MS-275, a class I HDAC inhibitor, bilaterally in the dorsal or ventral hippocampus of mice and found that it had no effects on innate anxiety in either region. We then assessed the effects of MS-275 at different stages of fear learning along the longitudinal hippocampal axis. Animals were injected with MS-275 or vehicle after context pre-exposure (pre-conditioning injections), when a representation of the context is first formed, or after exposure to coyote urine (post-conditioning injections), when the context becomes associated with predator odor. When MS-275 was administered after context pre-exposure, dorsally injected animals showed enhanced fear in the training context but were able to discriminate it from a neutral environment. Conversely, ventrally injected animals did not display enhanced learning in the training context but generalized the fear response to a neutral context. However, when MS-275 was administered after conditioning, there were no differences between the MS-275 and vehicle control groups in either the dorsal or ventral hippocampus. Surprisingly, all groups displayed generalization to a neutral context, suggesting that predator odor exposure followed by a mild stressor such as restraint leads to fear generalization. These results may elucidate distinct functions of the dorsal and ventral hippocampus in predator odor-induced fear conditioning as well as some of the molecular mechanisms underlying fear generalization.

Evaluation of anxiolytic potency of essential oil and S-(+)-linalool from Cinnamomum osmophloeum ct. linalool leaves in mice

Cinnamomum osmophloeum ct. linalool ( tǔ ròu guì) is one chemotype of the indigenous cinnamons in Taiwan. This study examined the anxiolytic potency of leaf essential oil (LEO) from C. osmophloeum ct. linalool and its main constituent on 4-week ICR mice using an open field test (OFT), a light-dark test (LDT) and an elevated plus maze test (EPT). After oral administration of corn oil, LEO (250 mg/kg and 500 mg/kg), S-(+)-linalool (500 mg/kg), R-(-)-linalool (500 mg/kg), and trazodone hydrochloride (75 mg/kg) for 14 days, the anxiolytic effects on mice behavior were evaluated. The results showed that LEO from C. osmophloeum ct. linalool leaves and S-(+)-linalool can significantly increase the time mice remained in the center area of the OFT, the illuminated area of the LDT and the open arms of the EPT without any side effects affecting motor activity, indicating excellent anxiolytic responses. Furthermore, results from the measurements of monoamines in mice brain revealed decreases in serotonin, dopamine, and norepinephrine, which are consistent with their anxiolytic effects in animal models. The findings obtained suggest that LEO from C. osmophloeum ct. linalool and its major compound, S-(+)-linalool, possess anxiolytic properties without any side effects and thus support their potential use in treatment of anxiety disorders.

Evidence that oxytocin exerts anxiolytic effects via oxytocin receptor expressed in serotonergic neurons in mice

The oxytocin receptor has been implicated in the regulation of reproductive physiology as well as social and emotional behaviors. The neurochemical mechanisms by which oxytocin receptor modulates social and emotional behavior remains elusive, in part because of a lack of sensitive and selective antibodies for cellular localization. To more precisely characterize oxytocin receptor-expressing neurons within the brain, we generated an oxytocin receptor-reporter mouse in which part of the oxytocin receptor gene was replaced with Venus cDNA (a variant of yellow fluorescent protein). Examination of the Venus expression revealed that, in the raphe nuclei, about one-half of tryptophan hydroxylase-immunoreactive neurons were positive for Venus, suggesting a potential role for oxytocin in the modulation of serotonin release. Oxytocin infusion facilitated serotonin release within the median raphe nucleus and reduced anxiety-related behavior. Infusion of a 5-HT(2A/2C) receptor antagonist blocked the anxiolytic effect of oxytocin, suggesting that oxytocin receptor activation in serotonergic neurons mediates the anxiolytic effects of oxytocin. This is the first demonstration that oxytocin may regulate serotonin release and exert anxiolytic effects via direct activation of oxytocin receptor expressed in serotonergic neurons of the raphe nuclei. These results also have important implications for psychiatric disorders such as autism and depression in which both the oxytocin and serotonin systems have been implicated.

Anxiolytic effect of ting-chih-wan in mouse behavior models of anxiety

The anxiolytic effect of the alcoholic extract of ting-chih-wan (TCWa) was studied using the black and white test (BWT) and the elevated plus-maze (EPM). We further demonstrated the anxiolytic mechanism of TCWa by combining with diazepam (DIZ), serotonin (5-HT) agonists or antagonists, and measuring the levels of monoamines and its metabolites in the brain stem and cortex. In the BWT, TCWa (0.1-1.0 g/kg, p.o.) increased the time spent in the white chamber and total change between the two chambers, and decreased the time spent in the black chamber. TCWa (0.1-0.5 g/kg, p.o.) increased the arm entries and the time spent on the open arms, and decreased the arm entries and the time spent on the closed arms in the EPM. On the other hand, TCWa (1.0 g/kg, p.o.) decreased horizontal activity and prolonged pentobarbital-induced sleeping times. TCWa (0.1, 0.5 g/kg) decreased the levels of norepinephrine (NE), dopamine (DA), 5-HT and 5-hydroxy-indoleacetic acid (5-HIAA) and increased the levels of vanillylmandelic acid (VMA) and homovanillic acid (HVA) in the brain stem. TCWa (0.1 and 0.5 g/kg) decreased the levels of NE, DA and increased the levels of VMA and HVA in the cortex. TCWa also attenuated the anxiogenic effect of 5-hydroxytryptophan (5-HTP) and enhanced the anxiolytic effect of 9p-chlorophenylalanine (PCPA), buspirone (BUS) and ritanserin (RIT) in the EPM. From these results, TCWa at 0.1 and 0.5 g/kg possessed an anxiolytic effect T heanxiolytic mechanisms of TCWa might be due to decreased catecholaminergic activity caused by the increase in the turnover rate of catecholamines in the brain and decreased concentrations of 5-HT in the brain stemvia activating somatodendritic 5-HT1A autoreceptors and inhibiting postsynaptic 5-HT receptors.

The dorsal raphe nucleus exerts opposed control on generalized anxiety and panic-related defensive responses in rats

It has been proposed that the ascending dorsal raphe (DR)-serotonergic (5-HT) pathway facilitates conditioned avoidance responses to potential or distal threat, while the DR-periventricular 5-HT pathway inhibits unconditioned flight reactions to proximal danger. Dysfunction on these pathways would be, respectively, related to generalized anxiety (GAD) and panic disorder (PD). To investigate this hypothesis, we microinjected into the rat DR the benzodiazepine inverse receptor agonist FG 7142, the 5-HT(1A) receptor agonist 8-OH-DPAT or the GABA(A) receptor agonist muscimol. Animals were evaluated in the elevated T-maze (ETM) and light/dark transition test. These models generate defensive responses that have been related to GAD and PD. Experiments were also conducted in the ETM 14 days after the selective lesion of DR serotonergic neurons by 5,7-dihydroxytriptamine (DHT). In all cases, rats were pre-exposed to one of the open arms of the ETM 1 day before testing. The results showed that FG 7142 facilitated inhibitory avoidance, an anxiogenic effect, while impairing one-way escape, an anxiolytic effect. 8-OH-DPAT, muscimol, and 5,7-DHT-induced lesions acted in the opposite direction, impairing inhibitory avoidance while facilitating one-way escape from the open arm. In the light/dark transition, 8-OH-DPAT and muscimol increased the time spent in the lighted compartment, an anxiolytic effect. The data supports the view that distinct DR-5-HT pathways regulate neural mechanisms underlying GAD and PD.

Escitalopram, the S-(+)-enantiomer of citalopram, is a selective serotonin reuptake inhibitor with potent effects in animal models predictive of antidepressant and anxiolytic activities

OBJECTIVE

The pharmacological profile of escitalopram, the S-(+)-enantiomer of citalopram, was studied and compared with citalopram and the R-(-)-enantiomer, R-citalopram.

METHODS

Inhibition of the serotonin transporter (5-HTT) was studied in COS-1 cells expressing the human 5-HTT (h-5-HTT) and in rat brain synaptosomes. In vitro selectivity was studied relative to noradrenaline transporter (NAT) and dopamine transporter (DAT) function in rat brain synaptosomes, and affinities for other binding sites were determined. In vivo 5-HT activity was measured as inhibition of neuronal firing rate in rat dorsal raphe nucleus (DRN) and enhancement of 5-hydroxytryptophan (5-HTP)-induced behaviour (mouse and rat). Furthermore, studies were conducted in models of antidepressant (mouse forced-swim test), anxiolytic [foot-shock-induced ultrasonic vocalization (USV) in adult rats and mouse black and white box] and anti-aggressive activity (socially isolated mice).

RESULTS

Escitalopram inhibited 5-HTT functions approximately 2 times more potently than citalopram and at least 40 times more potently than R-citalopram. Escitalopram showed insignificant activity at other monoamine transporters and 144 other binding sites. Escitalopram inhibited 5-HT neuronal firing in DRN and potentiated 5-HTP-induced behaviours more potently than citalopram; R-citalopram was inactive. Escitalopram and citalopram, but not R-citalopram, reduced forced-swimming-induced immobility and facilitated exploratory behaviour in the black and white box. Escitalopram and citalopram inhibited USV potently; R-citalopram was several times less potent. Escitalopram, citalopram and R-citalopram inhibited aggressive behaviour weakly. Escitalopram and citalopram had very potent anti-aggressive effects when co-administered with l-5-HTP.

CONCLUSION

Escitalopram is a very selective 5-HT reuptake inhibitor. It is more potent than its racemate citalopram and is effective in animal models predictive of antidepressant and anxiolytic activities.

Clitoria ternatea and the CNS

The present investigation was aimed at determining the spectrum of activity of the methanolic extract of Clitoria ternatea (CT) on the CNS. The CT was studied for its effect on cognitive behavior, anxiety, depression, stress and convulsions induced by pentylenetetrazol (PTZ) and maximum electroshock (MES). To explain these effects, the effect of CT was also studied on behavior mediated by dopamine (DA), noradrenaline, serotonin and acetylcholine. The extract decreased time required to occupy the central platform (transfer latency, TL) in the elevated plus maze (EPM) and increased discrimination index in the object recognition test, indicating nootropic activity. The extract was more active in the object recognition test than in the EPM. The extract increased occupancy in the open arm of EPM by 160% and in the lit box of the light/dark exploration test by 157%, indicating its anxiolytic activity. It decreased the duration of immobility in tail suspension test (suggesting its antidepressant activity), reduced stress-induced ulcers and reduced the convulsing action of PTZ and MES. The extract exhibited tendency to reduce the intensity of behavior mediated via serotonin and acetylcholine. The effect on DA- and noradrenaline-mediated behavior was not significant. In conclusion, the extract was found to possess nootropic, anxiolytic, antidepressant, anticonvulsant and antistress activity. Further studies are necessary to isolate the active principle responsible for the activities and to understand its mode of action.

Anxiolytic-like effects of 5-HT2 ligands on three mouse models of anxiety

The behavioural effects of 5-HT(2) receptor agonists, 5-HT(2A) and 5-HT(2C) receptor antagonists were investigated in the mouse four plates test (FPT), light/dark paradigm (L/D) and the elevated plus maze (EPM), in order to elucidate the role of the 5-HT(2) receptor subtypes in these models and to address the inconclusive results previously reported using rat psychopharmacological models. All compounds were administered intraperitoneally 30 min before each test. DOI, a preferential 5-HT(2A) agonist (0.5-8 mg/kg) and BW 723C86, a 5-HT(2B) agonist (8 and 16 mg/kg) provoked an anxiolytic-like response in the FPT. In the EPM, an anxiolytic-like effect was observed for DOI (0.5, 1 and 2 mg/kg), BW 723C86 (0.5, 4, 8 and 16 mg/kg), RO 60-0175 a 5-HT(2C) agonist (4 mg/kg) and the non-selective 5-HT(2) receptor agonist mCPP (0.25 mg/kg.). Ketanserin, a 5-HT(2A/2C) non-selective receptor antagonist (0.015 and 0.03 mg/kg), induced an anxiogenic-like effect in the L/D paradigm. The 5-HT(2C) antagonists (RS 10-2221, SDZ SER082 and SB 206553) were without effect in all three tests. These behavioural results are indicative of an anxiolytic-like action of 5-HT(2) receptor agonists, an anxiogenic-like effect of 5-HT(2A) receptor antagonism, whereas the blockade of 5-HT(2C) receptors are without effect in the mouse models studied.

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.

Targeted gene deletion of the 5-HT3A receptor subunit produces an anxiolytic phenotype in mice

Anxiety disorders are the most common psychiatric disorders. Typical medications used to treat patients are benzodiazepines or antidepressants that target serotonin (5-HT) activity. The ionotropic 5-HT(3) receptor has emerged as a potential therapeutic target because selective antagonist compounds reduce anxiety in rodents, primates, and humans. 5-HT binds to the extracellular N-terminus of the 5-HT(3A) receptor subunit, but receptor activation is also enhanced by distinct allosteric sites. It is not known if specific molecular subunits of the 5-HT(3) receptor modulate anxiety. To address this issue, we characterized anxiety-like behavior of mice with a targeted deletion of the 5-HT(3A) receptor subunit gene in the light/dark box, elevated plus maze, and novelty interaction animal models of anxiety. 5-HT(3A) null mice exhibited an anxiolytic behavioral phenotype that was highly correlated across behavioral measures. This evidence indicates that the 5-HT(3A) molecular subunit influences anxiety-like behavior. Pharmacotherapy that targets specifically the 5-HT(3A) receptor subunit may provide a novel treatment for anxiety disorders.

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.

GIRK2 deficient mice. Evidence for hyperactivity and reduced anxiety

G-protein activated inwardly rectifying potassium channel (GIRK2)-deficient (null mutant) mice were examined in three tests for anxiety: the elevated plus-maze, light/dark box and "canopy" test. In the elevated plus-maze test, GIRK2 null mutant mice spent a higher percentage of time in the open arms and showed a higher number of total entries. A short (6 days) period of social isolation decreased anxiety and also increased the total activity in GIRK2 mutant mice. However, the increase of total activity in GIRK2 null mutant mice was mostly due to an increase in the number of entries into the open arms. The behavior of the wild-type animals was not substantially changed after social isolation. In the light/dark box, GIRK2 homozygous (-/-) mice demonstrated a higher level of locomotion and a higher number of rearings in the light area. In the "canopy" test, GIRK2 mutant mice displayed an increased locomotion in the exposed area and a strong trend to decrease in the number of stretched attend postures (SAP) in the most secure "canopy" area. GIRK2 heterozygous (+/-) animals showed behavioral changes intermediate between wild-type and null mutants only in the elevated plus-maze test after social isolation. In all other tests, GIRK2 heterozygous (+/-) animals did not differ from wild-type mice. Taken together, this data demonstrates that GIRK2 null mutant mice have reduced anxiety with signs of hyperactivity. We suggest that the functional block of dopamine D3 receptors may be a reason for this phenotype.

Prior test experience compromises the anxiolytic efficacy of chlordiazepoxide in the mouse light/dark exploration test

It is now well established that prior test experience can alter behavioural baselines and attenuate/abolish the anxiolytic efficacy of benzodiazepines in the elevated plus-maze paradigm. In view of evidence that different models of anxiety measure qualitatively distinct forms of anxiety-like behaviour, it is important to establish whether the effects of prior experience extend to other widely-used tests. The present study assessed the behavioural and pharmacological sequelae of a single undrugged prior exposure to the light/dark exploration (L/D) test in mice, using ethological scoring methods. One group of adult male Swiss-Webster mice was given a single undrugged exposure to the L/D test 24 h prior to drug testing, while another group was completely naïve to the apparatus. On test day, half the animals in each experiential condition were treated with saline and half with an anxiolytic dose (10 mg/kg) of chlordiazepoxide (CDP). When administered to test-naïve animals, CDP induced a clear reduction in anxiety-like behaviour as evidenced by significant increases in exploration of the light compartment (line crossings, % line crossings, and % time) as well as reductions in stretched attend postures (SAPs) and the proportion of SAPs displayed toward the light compartment. The behavioural specificity of these effects was confirmed by the absence of a drug effect on line crossings in the dark compartment, total rearing and grooming. In complete contrast, with the sole exception of a decrease in total SAPs, CDP was without significant behavioural effect in test-experienced mice. As prior test experience did not significantly alter behavioural baselines in the L/D test, a second experiment was designed to investigate the possibility that handling/intraperitoneal injection may have precluded detection of experientially-induced changes in baseline behaviour. Results showed that handling/injection had no effect upon L/D behavioural profiles in either test-naïve or test-experienced subjects, and confirmed that prior experience itself did not modify the primary indices of anxiety in this test. Present data indicate that prior test experience seriously compromises the anxiolytic efficacy of CDP (10 mg/kg) in the mouse L/D test and, together with recent findings in the four-plate test, appear to confirm that an experientially-induced reduction in sensitivity to the anxiolytic effects of benzodiazepines is by no means unique to the elevated plus-maze.

Behavioral and neurochemical effects of anpirtoline and citalopram in isolated and group housed mice

Acute effects of serotonergic drugs acting via different mechanisms were investigated by a social interaction test and subsequent determination of serotonin and dopamine metabolisms in mice housed in groups or isolated for 6 weeks. A resident/intruder test was performed with anpirtoline (5-HT1B receptor agonist in rodents; 1 mg/kg), citalopram (SSRI; 0.5 mg/kg) and saline treatment before animals were decapitated and different brain regions were frozen for subsequent HPLC-analyses. Behavioral investigations indicated a strong increase of aggressive behavior after 6 weeks of isolation housing. Acute citalopram treatment did not influence behavioral parameters of isolated and group housed mice. In contrast, anpirtoline antagonized isolation induced aggressive behavioral components in a specific manner. Analysis of dopamine and serotonin metabolism revealed that citalopram treatment did not affect dopamine metabolism, but reduced serotonin metabolism in the striatum, hippocampus, cortex and midbrain independent of housing conditions. In contrast, anpirtoline treatment increased dopamine metabolism in cortex, striatum and midbrain as well as influenced serotonin metabolism in a structure- and state-specific manner. Whereas anpirtoline decreased serotonin metabolism in the cortex, the midbrain and the hippocampus independent of housing conditions, in the striatum anpirtoline abolished the isolation induced decrease of serotonin metabolism. These results indicate that anpirtoline might induce antiaggressive effects via postsynaptic receptor- and structure-specific activation of serotonergic but also dopaminergic processes, whereas structure independent increase of synaptic serotonin via citalopram was ineffective to reverse aggressivity in isolated mice.

The influence of buspirone, and its metabolite 1-PP, on the activity of paroxetine in the mouse light/dark paradigm and four plates test

Although numerous animal procedures have been employed in the study of antidepressants (ADs) in anxiety, the results following acute administration remain highly variable. The present study investigated the effect of the SSRI paroxetine (4, 8, and 16 mg/kg, IP) in two tests of anxiety in mice: the light/dark test paradigm, and the four plates test (FPT). In both tests, it was found that paroxetine resulted in an anxiolytic-like effect at doses that did not modify motor performance (at the doses of 4 and 8 mg/kg in the light/dark test and at the doses of 4, 8, and 16 mg/kg in the four plates test). In the light/dark paradigm, both doses of buspirone significantly potentiated paroxetine, while in the four plates only one dose of buspirone (a 5HT(1A) partial agonist) (0.06 mg/kg) increased the anxiolytic-like effect of paroxetine. Prior administration of 1-PP was without effect in the light/dark paradigm but antagonized the effect of paroxetine (at the dose of 0.06 and 0. 5 mg/kg) in the FPT. The results suggested that a balance between pre- and postsynaptic 5-HT(1A) receptor was implicated in the anxiolytic-like effect of paroxetine. Buspirone seemed to emphasize the role of paroxetine in 5-HT(1A) receptor modulation and exerted a biphasic influence in the two tests.

Recovery of emotional behaviour in neural cell adhesion molecule (NCAM) null mutant mice through transgenic expression of NCAM180

In the present study we further investigate functions of the neural cell adhesion molecule (NCAM) in the mature central nervous system and its implications for animal behaviour. To this end we generated transgenic mice expressing the major NCAM isoform with the largest cytoplasmic domain, NCAM180, under control of a promoter for the small form neurofilament gene. Transgenic mice were also bred with mice deficient in endogenous NCAM (Ncam-/- mice) so that effects of NCAM180 could be analysed in the presence and absence of endogenous NCAM. While overexpression of transgenic NCAM180 was without apparent behavioural or morphological effect, its expression in Ncam-/- mice counteracted NCAM ablation-induced aggressive, anxiety-like and antidepressant-like behaviour. It furthermore prevented a hypersensitivity of Ncam-/- mice to the anxiolytic serotonin1A (5-HT1A) receptor agonist buspirone. Such recovery of emotional behaviour and behavioural 5-HT1A response occurred in spite of misdevelopment of the olfactory bulb and hippocampus that is characteristic of Ncam-/- mice, and without an apparent change in the expression of 5-HT1A binding sites in the brain. Hippocampus- and amygdala-dependent learning, though disturbed in Ncam-/- mice, remained unaffected by the transgenic NCAM180. We suggest an involvement of NCAM180-mediated cell recognition processes in the serotonergic modulation of emotional behaviour in adult mice.

Citalopram

Citalopram is a selective serotonin (5-HT) reuptake inhibitor (SSRI) developed by H. Lundbeck A/S in Denmark. It is the most selective serotonin antidepressant with proven efficacy, a favourable pharmacokinetic profile and a low potential for interactions with other concomitant medication. The drug has a low incidence of side effects, even when compared to the other SSRIs and good patient compliance and satisfaction is a feature of this drug. These factors make the drug a good choice for depressed patients who require continuation and long-term treatment, as well as for elderly patients. Copyright 2000 John Wiley & Sons, Ltd.

Anxiety and increased 5-HT1A receptor response in NCAM null mutant mice

Mice deficient in the neural cell adhesion molecule (NCAM) show behavioral abnormalities as adults, including altered exploratory behavior, deficits in spatial learning, and increased intermale aggression. Here, we report increased anxiety-like behavior of homozygous (NCAM-/-) and heterozygous (NCAM/-) mutant mice in a light/dark avoidance test, independent of genetic background and gender. Anxiety-like behavior was reduced in both NCAM+/+ and NCAM-/- mice by systemic administration of the benzodiazepine agonist diazepam and the 5-HT1A receptor agonists buspirone and 8-OH-DPAT. However, NCAM-/- mice showed anxiolytic-like effects at lower doses of buspirone and 8-OH-DPAT than NCAM+/+ mice. Such increased response to 5-HT1A receptor stimulation suggests a functional change in the serotonergic system of NCAM-/- mice, likely involved in the control of anxiety and aggression. However, 5-HT1A receptor binding and tissue content of serotonin and its metabolite 5-hydroxyindolacetic acid were found unaltered in every brain area of NCAM-/- mice investigated, indicating that expression of 5-HT1A receptors as well as synthesis and release of serotonin are largely unchanged in NCAM-/- mice. We hypothesize a critical involvement of endogenous NCAM in serotonergic transmission via 5-HT1A receptors and inwardly rectifying K+ channels as the respective effector systems.

Anxiogenic-like effect of acute and chronic fluoxetine on rats tested on the elevated plus-maze

The selective serotonin reuptake inhibitor fluoxetine (FLX) is widely prescribed for depression and anxiety-related disorders. On the other hand, enhanced serotonergic transmission is known to be classically related to anxiety. In this study, the effects of acute (5.0 mg/kg) and chronic (5.0 mg/kg, 22 days) FLX were investigated in both food-deprived and non-deprived rats tested in the elevated plus-maze. Significant main effects of the three factors (drug, food condition and administration regimen) were observed, but no interaction between them. The administration of either acute or chronic FLX resulted in an anxiogenic effect, as detected by a significant reduction in the percentage of time spent in the open arms and in the percentage of open arm entries. Food deprivation yielded an anxiolytic-like profile, probably related to changes in locomotor activity. The administration regimen resulted in an anxiolytic profile in chronically treated rats, as would be expected after 22 days of regular handling. The anxiogenic action of acute FLX is consistent with both its neurochemical and clinical profile. The discrepancy between the anxiogenic profile of chronic FLX and its therapeutic uses is discussed in terms of possible differences between the type of anxiety that is measured in the plus-maze and the types of human anxiety that are alleviated by fluoxetine.

Identification of a new polymorphism in the 3'-untranslated region of the human serotonin receptor 2C (5-HT2C) gene

We identified a polymorphism (2831T > G) in the 3'-untranslated region of 5-HT2C receptor gene, approximately 100 kb from a previously reported coding sequence polymorphism, 796G > C (C23S). Allele frequencies were 0.90 (T) and 0.10 (G) and cosegregation analysis of the alleles at the two loci demonstrated frequencies of 0.82 (GT), 0.08 (CT), 0.10 (GG), and 0 (CC). The increased informativity gained by analysis of both polymorphisms will prove useful for future studies of this gene in X-linked neuropsychiatric disorders.

Level of punishment determines anticonflict activity of ondansetron in pigeons: comparison with buspirone and diazepam

The anticonflict effect of the selective 5-HT3 receptor antagonist, ondansetron, was investigated employing an operant conflict task in pigeons. Behavior (key pecking) was stimulated by food presentation. A fixed-interval program of alternated punished (electrical shocks) and unpunished responding was employed. The effects of drugs were evaluated at two levels punishment intensity; i.e., baseline responding during the punished interval was 5% (higher punishment) or 10% (lower punishment) of the unpunished responding rate. Ondansetron released responding suppressed by punishment only when pigeons were working at the lower levels of punishment. Under these conditions, ondansetron (100 microg/kg, i.v.), increased key pecking by 119% above control and vehicle values, and doubled the number of shocks received by the pigeons during the punished intervals. Similarly to ondansetron, the anticonflict effects of buspirone (0.3 and 1 mg/kg) and diazepam (1 and 1.5 mg/kg) were strongly dependent on the intensity of the punishing stimulus. When punished responding was suppressed to 5% of unpunished responding by applying shocks of higher intensity, diazepam and buspirone had negligible anticonflict action. However, at lower levels of punishment, diazepam and buspirone produced much greater anticonflict effects than ondansetron (p < 0.001). These results indicate that ondansetron exhibits a modest effect in releasing behaviors suppressed by punishment (anxiolytic-like action), which was highly dependent on the intensity of punishment applied. It is proposed that the anxiogenic response to punishment is less sensitive to 5-HT3 antagonists than the behavior induced by aversive, unpunished situations, where 5-HT3 antagonists have shown comparable efficacy to benzodiazepines.

Increased anxiety of mice lacking the serotonin1A receptor

Brain serotonin (5-HT) has been implicated in a number of physiological processes and pathological conditions. These effects are mediated by at least 14 different 5-HT receptors. We have inactivated the gene encoding the 5-HT1A receptor in mice and found that receptor-deficient animals have an increased tendency to avoid a novel and fearful environment and to escape a stressful situation, behaviors consistent with an increased anxiety and stress response. Based on the role of the 5-HT1A receptor in the feedback regulation of the 5-HT system, we hypothesize that an increased serotonergic neurotransmission is responsible for the anxiety-like behavior of receptor-deficient animals. This view is consistent with earlier studies showing that pharmacological activation of the 5-HT system is anxiogenic in animal models and also in humans.

The effect of serotonin depletion on motor activity habituation, and [3H]muscimol binding in the rat hippocampus

The effect of serotonin depletion (p-chlorophenylalanine pretreatment) on habituation of exploratory motor activity, and on cortical and hippocampal [3H]muscimol binding in vitro, was examined in rats. It appeared that the very strong decrease in serotonin concentration abolished motor habituation in the open field and decreased [3H]muscimol binding to cortical and hippocampal brain slices. The GABA(A) receptor down-regulation was due to a decrease in the apparent affinity of the radioligand for the receptors. p-Chlorophenylalanine-induced biochemical changes were selective and most probably secondary to serotonin depletion, as the serotonin synthesis inhibitor did not displace [3H]muscimol from its binding sites in neural membranes taken from the occipital cortex. It is concluded that there is a functional interaction between brain serotonin and GABA (gamma-aminobutyric acid) systems, both at behavioral and biochemical levels, that is involved in the motor activity habituation process.

Electrophysiological and neurochemical evidence that pindolol has agonist properties at the 5-HT1A autoreceptor in vivo

1. It has been hypothesized that 5-HT1A autoreceptor antagonists may enhance the therapeutic efficacy of SSRIs and other antidepressants. Although early clinical trials with the beta-adrenoceptor/5-HT1 ligand, pindolol, were promising, the results of recent more extensive trials have been contradictory. Here we investigated the actions of pindolol at the 5-HT1A autoreceptor by measuring its effect on 5-HT neuronal activity and release in the anaesthetized rat. 2. Pindolol inhibited the electrical activity of 5-HT neurones in the dorsal raphe nucleus (DRN). This effect was observed in the majority of neurones tested (10/16), was dose-related (0.2-1.0 mg kg(-1), i.v.), and was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.), in 6/7 cases tested. 3. Pindolol also inhibited 5-HT neuronal activity when applied microiontophoretically into the DRN in 9/10 neurones tested. This effect of pindolol was current-dependent and blocked by co-application of WAY 100635 (3/3 neurones tested). 4. In microdialysis experiments. pindolol caused a dose-related (0.8 and 4 mg kg(-1), i.v.) fall in 5-HT levels in dialysates from the frontal cortex (under conditions where the perfusion medium contained 1 microM citalopram). In rats pretreated with WAY 100635 (0.1 mg kg(-1), i.v.), pindolol (4 mg kg(-1), i.v.) did not decrease, but rather increased 5-HT levels. 5. We conclude that, under the experimental conditions used in this study, pindolol displays agonist effects at the 5-HT1A autoreceptor. These data are relevant to previous and ongoing clinical trials of pindolol in depression which are based on the rationale that the drug is an effective 5-HT1A autoreceptor antagonist.

Acute stress enhances anxiolytic-like drug responses of mice tested in a black and white test box

Drug-induced facilitation of exploration of a two-compartment black and white test box was studied in mice applying different stressors before test. Tail suspension suppressed exploration of both compartments and enhanced diazepam- or eltoprazine-induced increase of exploration of the white compartment. (-)-Penbutolol reversed the effects of eltoprazine suggesting involvement of 5-HT(1A/1B) receptors. Buspirone had weak effects both under standard conditions, and after tail suspension. Ritanserin suppressed exploratory behavior, and this effect was potentiated by tail suspension. In conclusion, application of acute tail suspension stress immediately before test in the black and white box enhanced the sensitivity to drugs. In contrast to this, forced swimming suppressed general behavioral activity, but in this case diazepam facilitated the behavior in both compartments. Foot shocks immediately before test reduced activity in the black compartment and did not affect the behavior in the white compartment. Thus different acute stressors modified behavior in the black and white box differently. Chronic stress (crowded housing) produced an anxiolytic-like response.

Influence of 5-HT1A receptor antagonism on plus-maze behaviour in mice. II. WAY 100635, SDZ 216-525 and NAN-190

To understand further the role of 5-hydroxytryptamine receptor subtype 1A (5-HT1A) mechanisms in anxiety, the behavioural effects of 5-HT1A receptor antagonists with different selectivity and intrinsic activity were examined using an ethological version of the murine elevated plus-maze test. WAY 100635 (0.03-9.0 mg/kg) produced a behavioural profile indicative of an anxiolyticlike effect, with an apparent bell-shaped dose-response relationship and increases in nonexploratory behaviours at the largest dose tested. SDZ 216-525 exerted a dose-dependent antianxiety action at doses of 0.05-0.8 mg/kg, with some loss of activity at 3.2 mg/kg. In contrast, smaller doses of NAN-190 had a significant effect, whereas higher doses (2.5-10.0 mg/kg) decreased locomotor activity and other active behaviours, a profile similar to that produced by the alpha1-adrenoceptor antagonist prazosin (2.5 mg/kg), which also inhibited open arm activity. Findings are discussed in relation to 5-HT1A receptor and alpha1-adrenoceptor antagonism and corresponding neurochemical changes. The results of the present series support the view that 5-HT1A receptor antagonists have therapeutic potential in the management of anxiety.

Influence of 5-HT1A receptor antagonism on plus-maze behaviour in mice. I. Pindolol enantiomers and pindobind 5-HT1A

Studies on the behavioural effects of 5-hydroxytryptamine receptor subtype 1A (5-HT1A) antagonists may provide important clues to the precise role of 5-HT1A receptor mechanisms in anxiety. In the first of a series of experiments designed to address this issue, the effects of mixed 5-HT1A and beta-adrenergic receptor antagonists pindolol enantiomers and pindobind 5-HT1A and of metoprolol and ICI 118,551 (selective beta1- and beta2-adrenoceptor antagonists, respectively) were assessed in the mouse elevated plus-maze using ethological techniques. Results showed that, at lower doses, (-)pindolol (0.1-1.6 mg/kg) and pindobind 5-HT1A (0.1-0.5 mg/kg) produced changes in both conventional and ethological measures (increased percentage of open arm time and reduced risk assessment) indicative of anxiety reduction. However, these anxiolyticlike actions were less evident at higher doses. In contrast, (+)pindolol (0.1-6.4 mg/kg), metoprolol (2.0-18.0 mg/kg) and ICI 118,551 (1.0-9.0 mg/kg) were behaviourally inert under present test conditions. These data suggest that antagonist actions at 5-HT1A receptors (but not beta-adrenoceptors) are involved in the anxiolyticlike effects of (-)pindolol and pindobind 5-HT1A in the murine elevated plus-maze test.

Anxiety- and activity-related effects of diazepam and chlordiazepoxide in the rat light/dark and dark/light tests

We have investigated, through factor analysis, anxiety- and activity-related variables in rats placed in the light/dark box. Thus, vehicle-, diazepam (DZ)-, and chlordiazepoxide (CDP)-treated rats were submitted 30 min later to 5-min light/dark or dark/light tests (initial placements in light or dark, respectively). Following this test, the animals were tested for 5 min in an automated activity monitor. Doses of DZ (0.75-3.0 mg/kg) and CDP (2.5-10.0 mg/kg) were based on preliminary evidence for 1.5 mg/kg of DZ and 5 mg/kg of CDP being anxiolytic in the elevated plus-maze. In the light/dark test, DZ increased the number of visits to and duration in the light compartment, and locomotor activity in the dark compartment; moreover, DZ decreased the latency to enter the light compartment. These effects were, however, significant for the highest dose only. Although CDP yielded similar behavioural effects, only the highest dose had a significant effect, namely, on latency to enter the light side. Conversely, none of the other variables were benzodiazepine-sensitive. Locomotion in the activity cages was decreased by DZ and CDP, an effect significant for the highest doses of benzodiazepines (dark/light test condition only). In both tests, factor analyses revealed an anxiety-related factor (to which all variables related to the visits in the light and part of the locomotion in the dark contributed), and an activity-related factor (upon which the latency to enter the dark and part of the locomotion in the dark loaded) in the light/dark test only. It is suggested that although the light/dark and dark/light tests capture an approach/avoidance dimension, DZ and CDP are more effective in the former test. Compared to the light/dark test, however, the plus-maze may be more sensitive to the anxiolytic effects of DZ and CDP.

Behavioral profiles of SSRIs in animal models of depression, anxiety and aggression. Are they all alike?

The behavioral profiles of five clinically used selective serotonin reuptake inhibitors (SSRIs) citalopram, paroxetine, sertraline, fluvoxamine and fluoxetine, have been compared in animal models of antidepressant (mouse forced swim test), anxiolytic (exploration of black and white test box and foot-shock-induced ultrasonic vocalization in the rat) and antiaggressive (isolation-induced aggressive behavior in the mouse) activity. the results are discussed in relation to receptor binding data from the literature. Furthermore, affinities for the sigma 1 and sigma 2 binding sites are presented. Citalopram reversed the immobility induced by forced swimming with a potency similar to that of imipramine. Paroxetine, fluvoxamine and fluoxetine reversed swim-induced immobility less potently and with a maximum of 40-50% reversal. Citalopram produced a mixed anxiogenic-/anxiolytic-like response in rats tested in the two-compartment black and white box. Paroxetine induced an anxiogenic-like response at low doses and the other SSRIs were without major effects. Citalopram and paroxetine inhibited footshock-induced ultrasonic vocalization with high potencies. The dose-response curve was biphasic for citalopram with a maximum of 64% inhibition. Sertraline and fluvoxamine inhibited the vocalization less potently, and fluoxetine induced a weak inhibitory effect corresponding to a maximum of 32%. Sertraline, fluvoxamine and fluoxetine inhibited isolation-induced aggressive behavior, whereas citalopram and paroxetine were inactive. Both 5-HT1 and 5-HT2 receptors are involved, and there was a functional interaction between 5-HT1A and 5-HT2A or 5-HT2C receptors, as ritanserin potentiated the antiaggressive effect of 1,5-HTP as well as that of 8-OH-DPAT.