Anxiogenic-like effect of infusing 1-(3-chlorophenyl) piperazine (mCPP) into the hippocampus

The Multiplicity of Serotonin Receptors: Uselessly Diverse Molecules or an Embarrassment of Riches?

A large number of 5-HT receptors (>15) have been identified by molecular cloning technology over the past 10 years. This review briefly summarizes available information regarding the functional and therapeutic implications of serotonin receptor diversity for neurology and psychiatry. 5-HT receptors are divided into seven main families: 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6, and 5-HT7. Several families (e.g., 5-HT1 family) have many members (e.g., 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F), each of which is encoded by a distinct gene product. In addition to the genomic diversity of 5-HT receptors, splice variants and editing isoforms exist for many of the 5-HT receptors, making the family even more diverse. Evidence that is summarized in this review suggests that 5-HT receptors represent novel therapeutic targets for a number of neurologic and psychiatric diseases including migraine headaches, chronic pain conditions, schizophrenia, anxiety, depression, eating disorders, obsessive compulsive disorder, pervasive developmental disorders, and obesity-related conditions (Type II diabetes, hypertension, obesity syndromes). It is possible that sub-type-selective serotonergic agents may revolutionize the treatment for a number of medical, psychiatric, and neurological disorders.

5-HT2C receptor desensitization moderates anxiety in 5-HTT deficient mice: from behavioral to cellular evidence

BACKGROUND

Desensitization and blockade of 5-HT2C receptors (5-HT2CR) have long been thought to be central in the therapeutic action of antidepressant drugs. However, besides behavioral pharmacology studies, there is little in vivo data documenting antidepressant-induced 5-HT2CR desensitization in specific brain areas.

METHODS

Mice lacking the 5-HT reuptake carrier (5-HTT(-/-)) were used to model the consequences of chronic 5-HT reuptake inhibition with antidepressant drugs. The effect of this mutation on 5-HT2CR was evaluated at the behavioral (social interaction, novelty-suppressed feeding, and 5-HT2CR-induced hypolocomotion tests), the neurochemical, and the cellular (RT-qPCR, mRNA editing, and c-fos-induced expression) levels.

RESULTS

Although 5-HTT(-/-) mice had an anxiogenic profile in the novelty-suppressed feeding test, they displayed less 5-HT2CR-mediated anxiety in response to the agonist m-chlorophenylpiperazine in the social interaction test. In addition, 5-HT2CR-mediated inhibition of a stress-induced increase in 5-HT turnover, measured in various brain areas, was markedly reduced in 5-HTT(-/-) mutants. These indices of tolerance to 5-HT2CR stimulation were associated neither with altered levels of 5-HT2CR protein and mRNA nor with changes in pre-mRNA editing in the frontal cortex. However, basal c-fos mRNA production in cells expressing 5-HT2CR was higher in 5-HTT(-/-) mutants, suggesting an altered basal activity of these cells following sustained 5-HT reuptake carrier inactivation. Furthermore, the increased c-fos mRNA expression in 5-HT2CR-like immune-positive cortical cells observed in wild-type mice treated acutely with the 5-HT2CR agonist RO-60,0175 was absent in 5-HTT(-/-) mutants.

CONCLUSIONS

Such blunted responsiveness of the 5-HT2CR system, observed at the cell signaling level, probably contributes to the moderation of the anxiety phenotype in 5-HTT(-/-) mice.

Simvastatin treatment highlights a new role for the isoprenoid/cholesterol biosynthetic pathway in the modulation of emotional reactivity and cognitive performance in rats

The aim of the present work was to shed light on the role played by the isoprenoid/cholesterol biosynthetic pathway in the modulation of emotional reactivity and memory consolidation in rodents through the inhibition of the key and rate-limiting enzyme 3-hydroxy 3-methylglutaryl Coenzyme A reductase (HMGR) both in vivo and in vitro with simvastatin. Three-month-old male Wistar rats treated for 21 days with simvastatin or vehicle were tested in the social interaction, elevated plus-maze, and inhibitory avoidance tasks; after behavioral testing, the amygdala, hippocampus, prefrontal cortex, dorsal, and ventral striatum were dissected out for biochemical assays. In order to delve deeper into the molecular mechanisms underlying the observed effects, primary rat hippocampal neurons were used. Our results show that HMGR inhibition by simvastatin induces anxiogenic-like effects in the social interaction but not in the elevated plus-maze test, and improves memory consolidation in the inhibitory avoidance task. These effects are accompanied by imbalances in the activity of specific prenylated proteins, Rab3 and RhoA, involved in neurotransmitter release, and synaptic plasticity, respectively. Taken together, the present findings indicate that the isoprenoid/cholesterol biosynthetic pathway is critically involved in the physiological modulation of both emotional and cognitive processes in rodents.

5-HT2C receptor activation prevents stress-induced enhancement of brain 5-HT turnover and extracellular levels in the mouse brain: modulation by chronic paroxetine treatment

Stress is known to activate the central 5-hydroxytryptamine (5-HT) system, and this is probably part of a coping response involving several 5-HT receptors. Although 5-HT(2C) receptors are well known to be implicated in anxiety, their participation in stress-induced changes had not been investigated in parallel at both behavioral and neurochemical levels. We show here that the preferential 5-HT(2C) receptor agonist, m-chlorophenylpiperazine, as well as restraint stress increased anxiety in the mouse social interaction test. The selective 5-HT(2C) receptor antagonist, SB 242,084, prevented both of these anxiogenic effects. Restraint stress increased 5-HT turnover in various brain areas, and this effect was prevented by the 5-HT(2B/2C) receptor agonist RO 60-0175 (1 mg/kg), but not the preferential 5-HT(2A) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1 mg/kg), and in contrast potentiated by SB 242,084 (1 mg/kg), which also blocked the effect of RO 60-0175. Using microdialysis, RO 60-0175 was shown to inhibit cortical 5-HT overflow in stressed mice when 5-HT reuptake was blocked locally. Chronic paroxetine prevented both the anxiogenic effect of m-chlorophenylpiperazine and the inhibitory effect of RO 60-0175 on locomotion and stress-induced increase in 5-HT turnover. The anxiolytic action of chronic paroxetine might be associated with an enhancement of 5-HT neurotransmission caused by a decreased 5-HT(2C) receptor-mediated inhibition of stress-induced increase in 5-HT release.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Fine-tuning serotonin2c receptor function in the brain: molecular and functional implications

The serotonin(2C) receptor (5-HT(2C)R) is a member of the serotonin(2) family of 7-transmembrane-spanning (7-TMS) receptors, which possesses unique molecular and pharmacological properties such as constitutive activity and RNA editing. The 5-HT(2C)R is widely expressed within the central nervous system, where is thought to play a major role in the regulation of neuronal network excitability. In keeping with its ability to modulate dopamine (DA) neuron function in the brain, the 5-HT(2C)R is currently considered as a major target for improved treatments of neuropsychiatric disorders related to DA neuron dysfunction, such as depression, schizophrenia, Parkinson's disease or drug addiction. The aim of this review is to provide an update of the functional status of the central 5-HT(2C)R, covering molecular, cellular, anatomical, biochemical and behavioral aspects to highlight its distinctive regulatory properties, the emerging functional significance of constitutive activity and RNA editing in vivo, and the therapeutic potential of inverse agonism.

The role of hippocampus in anxiety: intracerebral infusion studies

Although current models of hippocampal function stress its well-known role in cognitive functions, historically it has also been viewed as a neural mediator of emotion. Here, we review recent evidence from intrahippocampal infusion studies in animals that support a distinctive role of the hippocampus in anxiety, independent of its roles in learning and memory. Specifically, gamma-aminobutyric acid type A receptor agonists, both direct and indirect, reliably inhibit a number of animals' untrained anxiety reactions when microinfused into the hippocampus, whereas gamma-aminobutyric acid type A receptor antagonists do not. Intrahippocampal infusions of glutamatergic, serotonergic and cholinergic compounds also produce statistically reliable antianxiety effects, but the results vary as a function of specific anxiety reactions, and to some extent specific intrahippocampal targets. One hypothesis that may accommodate some of this variability is that anxiety is functionally segregated within the hippocampus, with ventral subregions more involved in anxiety-related processes, and dorsal subregions more involved with cognitive processes. Another possibility is that different hippocampal functions (e.g. memory and anxiety) are mediated by different neurotransmitter systems and/or different receptor subtypes within the hippocampus. Although there is some evidence that supports the latter hypothesis, the evidence for the former is not conclusive. Overall, however, the data clearly suggest that the hippocampus is importantly and directly involved in the mediation of untrained anxiety reactions in animals.

Effect of meta-chlorophenylpiperazine and cholecystokinin on food intake of Osborne-Mendel and S5B/P1 rats

OBJECTIVE

To investigate whether there is a difference in sensitivity to a serotonin agonist, meta-chlorophenylpiperazine (mCPP), or cholecystokinin (CCK-8), an intestinal hormone that inhibits food intake, between the Osborne-Mendel (OM) rat, which becomes obese eating a high-fat diet, and the S5B/Pl (S5B) rat, which is resistant to dietary-induced obesity.

RESEARCH METHODS AND PROCEDURES

OM and S5B rats were adapted to either a high-saturated-fat diet (56% energy as fat) or a low-fat diet (10% energy as fat) or to both for 14 days and then treated with several doses of mCPP or CCK-8.

RESULTS

Treatment with mCPP reduced food intake in both strains of rats. The dose-response curve showed that the OM rats had an increased sensitivity to the serotonergic agonist. Animals eating the high-fat diet had less response to mCPP; and in the S5B rats, the response was significantly reduced. After treatment with CCK-8, there was a similar dose-related suppression of food intake in both the OM and S5B rats.

DISCUSSION

These data are consistent with the hypothesis that the serotonin system in the S5B rat has a greater activity that could act to inhibit fat intake. The response to CCK was not significantly affected by strain or diet.

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.

Bombesin receptors as a novel anti-anxiety therapeutic target: BB1 receptor actions on anxiety through alterations of serotonin activity

The effects of PD 176252 [3-(1H-indol-3-yl)-N-[1-(5-methoxy-pyridin-2-yl)-cyclohexylmethyl]-2-methyl-2-[3-(nitro-phenyl)ureido]propionamide], a nonpeptide bombesin (BB) BB1/BB2 receptor antagonist, were assessed in rats using several ethologically relevant tests of anxiety. Consistent with a role for the bombesin family of peptides in subserving anxiety behaviors, the antagonist increased social interaction (3.75 and 7.5 mg/kg, i.p.), dose-dependently attenuated the number of vocalizations emitted by guinea pig pups separated from their mother (1-30 mg/kg, i.p.), reduced latency to approach a palatable snack in an anxiogenic (unfamiliar) environment, and reduced the fear-potentiated startle response (5 and 10 mg/kg, i.p., and 100-200 ng per rat, i.c.v.). When administered directly to the dorsal raphé nucleus (DRN), PD 176252 (20-500 ng) increased social interaction under aversive conditions, as did the 5-HT1A receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (50 ng). Furthermore, intra-DRN microinfusion of the peptide antagonist (PD 176252) suppressed, whereas its agonist [neuromedin B (NMB)-30] promoted, the in vivo release of 5-HT in the ventral hippocampus. In parallel, the suppressed social interaction elicited by intra-DRN administration of NMB was attenuated by a systemically administered 5-HT2C (but not 5-HT1A) receptor antagonist. Together, these findings suggest that endogenous BB-like peptides at the DRN evoke the release of 5-HT from the limbic nerve terminals originating from the raphé, specifically at the ventral hippocampus, resulting in anxiogenesis. The finding that this action was attenuated by BB receptor (BB1 and/or BB2) antagonists suggests that these compounds may represent a novel class of anxiolytic agents.

Functional magnetic resonance imaging and c-Fos mapping in rats following an anorectic dose of m-chlorophenylpiperazine

We have used blood-oxygenation-level-dependent (BOLD) contrast functional magnetic resonance imaging (fMRI) to characterise brain regions responsive to a regulator of appetite. An anorectic dose of the 5-HT(1B/2C) receptor agonist m-chlorophenylpiperazine (mCPP; 3 mg/kg s.c.) was used to compare BOLD contrast fMRI with expression of the c-Fos protein. mCPP was administered to rats, which were then anaesthetised and perfused with fixative 90 min later to allow immunohistochemistry. In a separate experiment, rats were imaged using a T(2)*-weighted gradient echo in a 7 T magnet for 70 min under alpha-chloralose anaesthesia. Both methods detected positive activation in areas of the limbic system: cingulate and orbitofrontal cortices, nucleus accumbens, paraventricular and dorsomedial regions of the hypothalamus. fMRI detected increased signal in the pontine nuclei, the hippocampal formation and olfactory cortex, areas that did not display c-Fos. In addition, BOLD signal was diminished in the ventral tegmental area, preoptic area and the cerebellum-presumably due to decreased neuronal signalling and, therefore, unlikely to display c-Fos. Activity in the limbic system may reflect the appetitive agonist activity of mCPP at the 5-HT(2C) receptor. We conclude that c-Fos provides excellent spatial information but is less useful for detecting inhibited regions, whereas fMRI provides greater temporal resolution. Thus, the two methodologies provide complementary details of brain activity following pharmacological challenge.

Monoamine and motor responses to cocaine are co-deficient in the Fawn-Hooded depressed animal model

The Fawn-Hooded (FH) genetic animal model of depression continues to be of interest because the FH model has limited biochemical and immune function. The FH animal has an inherited trait, platelet storage pool deficiency (PSPD), an hemorrhagic disorder that is also a component of Chediak-Higashi syndrome (CHS). CHS is a pyrogenic infectious childhood disease; few patients live past the age of 20. Our hypothesis was that FH animals may exhibit different monoamine and motor responses to cocaine versus the Sprague-Dawley (SD) "normal" animal strain, which does not have the FH trait. Therefore, selective neuromolecular imaging (NMI) of the monoamines, dopamine (DA) and 5-HT within nucleus accumbens (NAcc) of behaving male FH versus SD rats was performed in vivo with BRODERICK PROBE sensors and a semiderivative voltammetric circuit. Each animal was placed in a faraday chamber and electrochemical signals were detected via a mercury commutator and flexible cable. Baseline values for neurotransmitters and behavior were derived during the last half-hour of habituation behavior. Release of DA and 5-HT was detected selectively, at separate oxidation potentials, within seconds, before and after intraperitoneal administration of the psychostimulant, cocaine (10 mg/kg). At the same time, frequencies of ambulations and central ambulations were separately monitored with infrared photobeams, which surrounded the faraday chamber. Data were compared by ANOVA analysis followed by Tukey's post hoc test. The data showed that (1) DA release in NAcc of behaving FH animals did not respond to cocaine; neither first hour nor second hour values significantly differed from baseline (both hours, p>0.05), whereas SD animals exhibited a significant increase in cocaine-induced DA release in NAcc (both hours, p<0.001). The ability for acute cocaine to increase DA release in NAcc was significantly greater in SD than in FH animals (p<0.001). (2) 5-HT release in NAcc of behaving FH animals was not significantly increased by cocaine (both hours, p>0.05), whereas 5-HT release in NAcc of SD animals was significantly increased after cocaine (both hours, p<0.001). The ability for acute cocaine to increase 5-HT release was significantly greater in SD than in FH animals (p<0.001). (3) Ambulations in the FH strain were modestly, yet significantly, enhanced after cocaine during both hours of study (p<0.05, p<0.001, respectively) as were ambulations in the SD strain. Nonetheless, the ability for acute cocaine to increase ambulations was significantly greater in SD than in FH animals in the first hour (p<0.001). (4) Central ambulations in the FH strain was not affected by cocaine (both hours, p>0.05), whereas SD animals showed a significant increase in central ambulatory activity in both hours of the cocaine study (p<0.001). The ability for acute cocaine to increase central ambulations was significantly greater in SD than in FH animals (p<0.001). Thus, this is the first study to determine in vivo the neurochemical response to acute cocaine in the behaving FH animal. Moreover, this is the first study to determine in vivo and simultaneously the neurochemical and behavioral response to acute cocaine in the FH strain in comparison with SD animals, a "normal" strain. Remarkable deficiencies in the ability for acute cocaine to alter neurochemistry and behavior in animals with the FH trait are shown. These studies emphasize the need to look differentially at cocaine effects in biochemically and immune-compromised subjects versus "normal" subjects.

The anxiogenic drug FG-7142 increases serotonin metabolism in the rat medial prefrontal cortex

The neural mechanisms underlying anxiety states are believed to involve interactions among forebrain limbic circuits and brainstem serotonergic systems. Consistent with this hypothesis, FG-7142, a partial inverse agonist at the benzodiazepine allosteric site of the GABAA receptor, increases c-Fos expression within a subpopulation of brainstem serotonergic neurons. Paradoxically, FG-7142 has no effect on extracellular serotonin concentrations, as measured using in vivo microdialysis, in certain anxiety-related brain structures. This study tested the hypothesis that FG-7142 alters serotonin metabolism within one or more nodes of a defined anxiety-related forebrain circuit. Rats received one of four treatments (vehicle, 1.9, 3.8, or 7.5 mg/kg FG-7142, i.p.) and brains were collected 1 h following treatment. Thirteen forebrain regions were microdissected and analyzed for l-tryptophan, serotonin, and 5-hydroxyindoleacetic acid concentrations using high pressure liquid chromatography with electrochemical detection. FG-7142 (7.5 mg/kg) increased l-tryptophan, serotonin, and 5-hydroxyindoleacetic acid concentrations in the prelimbic cortex but not in several other regions studied including subdivisions of the amygdala and bed nucleus of the stria terminalis. These data demonstrate that FG-7142 alters brain tryptophan concentrations and serotonin metabolism in specific components of an anxiety-related forebrain circuit including the medial prefrontal cortex, an important structure involved in executive function and the regulation of emotional behavior.

Serotonergic systems associated with arousal and vigilance behaviors following administration of anxiogenic drugs

Serotonergic systems play important roles in modulating behavioral arousal, including behavioral arousal and vigilance associated with anxiety states. To further our understanding of the neural systems associated with increases in anxiety states, we investigated the effects of multiple anxiogenic drugs on topographically organized subpopulations of serotonergic neurons using double immunohistochemical staining for c-Fos and tryptophan hydroxylase combined with topographical analysis of the rat dorsal raphe nucleus (DR). Anxiogenic drugs with diverse pharmacological properties including the adenosine receptor antagonist caffeine, the serotonin 5-HT2A/2C receptor agonist m-chlorophenyl piperazine (mCPP), the alpha2-adrenoreceptor antagonist yohimbine, and the benzodiazepine receptor partial inverse agonist N-methyl-beta-carboline-3-carboxamide (FG-7142) induced increases in behavioral arousal and vigilance behaviors consistent with an increase in anxiety state. In addition, these anxiogenic drugs, excluding yohimbine, had convergent actions on an anatomically-defined subset of serotonergic neurons within the middle and caudal, dorsal subdivision of the DR. High resolution topographical analysis revealed that at the mid-rostrocaudal level, caffeine and FG-7142 had convergent effects on c-Fos expression in serotonergic neurons that were restricted to a previously undefined region, which we have named the shell region of the dorsal part of the dorsal raphe nucleus (DRDSh), that overlaps the anatomical border between the dorsal part of the dorsal raphe nucleus, the ventral part of the dorsal raphe nucleus (DRV), and the ventrolateral part of the dorsal raphe nucleus (DRVL). Retrograde tracing methods revealed that DRDSh contains large numbers of neurons projecting to the basolateral amygdaloid nucleus, a forebrain structure important for emotional appraisal and modulation of anxiety-related physiological and behavioral responses. Together these findings support the hypothesis that there is a functional topographical organization in the DR and are consistent with the hypothesis that anxiogenic drugs have selective actions on a subpopulation of serotonergic neurons projecting to a distributed central autonomic and emotional motor control system regulating anxiety states and anxiety-related physiological and behavioral responses.

Brain angiotensin and anxiety-related behavior: The transgenic rat TGR(ASrAOGEN)680

The transgenic rat TGR(ASrAOGEN)680, characterized by a transgene-producing antisense RNA against angiotensinogen in the brain, provides an opportunity to study the behavioral effects of angiotensin. While exposed to the elevated plus-maze (EPM) and the light/dark box, TGR(ASrAOGEN)680 rats showed more signs of anxiety compared to parental Sprague-Dawley (SD) rats. In the EPM, they made fewer entries into the open arms, spent less time there and more time on the closed arms. Head dips were reduced and U-turns were increased. In the light/dark box, the latency to the first re-entry into the light compartment was higher in TGR(ASrAOGEN)680. They displayed more SAP out from the dark and a reduced number of transitions between the two compartments. In the social interaction test, active social contacts were reduced, further suggesting an anxious phenotype. Although there was no transgenic effect on distance traveled in the open field, the more anxious TGR(ASrAOGEN)680 spent less time in the inner zone. Self-grooming was increased in TGR(ASrAOGEN)680 during exposure to the EPM and the open field, but was decreased in the social interaction test. In TGR(ASrAOGEN)680, tissue content of 5-HT and its metabolite 5-HIAA was lower in the hippocampus, frontal, and parietal cortex. HIAA and 5-HIAA/5-HT ratios were reduced in the hypothalamus, striatum, and septum. In the open field, the anxiogenic effect of the 5-HT2C/1B receptor agonist mCPP (0.5-1 mg/kg IP) was more pronounced in TGR(ASrAOGEN)680. The data suggest an anxious phenotype in rats with low brain angiotensinogen, possibly related to secondary dysfunctions of the brain serotonergic system.

Anxiolytic-like profile in Wistar, but not Sprague-Dawley rats in the social interaction test

RATIONALE

The social interaction test is a valuable behavioural model for testing anxiolytic drugs in rodents, quantifying the level of social behaviour between pairs of rats.

OBJECTIVE

The aim of the present study was to assess the appropriateness of the social interaction test for use with a Sprague-Dawley rat line, because of increasing use of this strain in targeted mutagenesis research.

METHODS

Sprague-Dawley and Wistar rats received either diazepam or mCPP or were exposed to different environmental conditions (lighting, social isolation prior testing, habituation, testing-time). General anxiety-related parameters measured were: duration of active social contact, frequency of active social contact, latency to first contact. Different forms of active social contact were recorded: number of crawls, follows and sniffs. Secondly, aversion-induced hippocampal serotonin release and serotonin content in brain regions were measured.

RESULTS

In Wistar rats habituation to the test substantially increased the time of social contact, an effect comparable with treatment with diazepam (1 mg/kg), whereas changes in the lighting level had less impact. Latency to the first contact increased under "anxiety-reducing" conditions, the frequency of contacts did not change consistently. Sprague-Dawley rats behaviour did not change under varying environmental conditions, and treatment with diazepam had only sedating effects at higher doses (5 mg/kg). Anxiogenic doses of mCPP caused reduced social interaction in both strains. Serotonin release and serotonin content were higher in the anxious Wistar rats.

CONCLUSIONS

Different rat strains as well as differing test conditions have a major impact on the outcome of this animal test for anxiety.

Anxiogenic effects in the rat elevated plus-maze of 5-HT(2C) agonists into ventral but not dorsal hippocampus

The effect of the non-selective 5-HT2C receptor agonist trifluoromethyl-phenylpiperazine (TFMPP, 0.75, 1.5 and 3.0 microg) and the preferential 5-HT2C agonist 6-chloro-2(1-piperazinyl)pyrazine (MK-212, 0.1, 0.3 and 1.0 microg) microinjected into the ventral or dorsal hippocampus was investigated in anxiety measures of rats exposed to the elevated plus-maze test. Ventral hippocampal (VH) microinjections of the 0.75 or 1.5 microg doses of TFMPP reduced open-arm exploration without affecting the number of closed-arm entries, indicating a selective anxiogenic profile. The highest dose (3.0 microg) reduced open- and closed-arm entries, suggesting interference in locomotor activity. The 0.1 microg dose of MK-212 also caused a selective anxiogenic effect when microinjected into the ventral hippocampus, without disturbing locomotor activity. Microinjections of the two higher doses of MK-212 (0.3 or 1.0 microg) into the ventral hippocampus led to a decrease of exploration in both arms of the maze. In contrast to the anxiogenic effect observed in the VH, neither TFMPP nor MK-212 significantly changed anxiety measures when microinjected into the dorsal hippocampus. These results suggest that activation of 5-HT2C postsynaptic receptors located in the ventral, but not in the dorsal, hippocampus play an important role in anxiety triggered by the elevated plus-maze test.

The serotonergic system and anxiety

The wide use of serotonin reuptake inhibitors and serotonin receptor agonists in anxiety disorders has suggested a key role for the modulatory neurotransmitter in anxiety. However, serotonin's specific role is still uncertain. This article reviews the literature concerning how and where serotonergic agents modulate anxiety. Varying and sometimes conflicting data from human and animal studies argue for both anxiolytic and anxiogenic roles for serotonin, depending on the specific disorder, structure, or behavioral task studied. However, recent data from molecular genetic studies in the mouse point toward two important roles for the serotonin 1A receptor. In development, serotonin acts through this receptor to promote development of the circuitry necessary for normal anxiety-like behaviors. In adulthood, serotonin reuptake inhibitors act through the same receptor to stimulate neurogenesis and reduce anxiety-like behaviors. These studies highlight that the complex serotonin system likely plays various roles in the regulation of anxiety both during development and in adulthood.

Synthesis of 2-N,N-Dimethylaminomethyl-2,3,3a,12b-tetrahydrodibenzo-[b,f]furo[2,3-d]oxepin Derivatives as Potential Anxiolytic Agents

New synthesis approaches that have led to a series of novel tetrahydrodibenzo[b,f]furo[2,3-d]oxepin derivatives are described. According to preliminary data these novel tetracycles can be useful intermediates for the preparation of potential new therapeutic agents.

m-CPP-induced self-grooming is mediated by 5-HT2C receptors

m-Chlorophenylpiperazine (m-CPP), a potent 5-HT receptor agonist, is known to induce self-grooming in rats and exacerbate symptoms in patients with obsessive-compulsive disorder (OCD). To characterise the possible role, 5-HT(2B) and 5-HT(2C) receptors play in m-CPP-induced self-grooming, subtype-selective receptor antagonists were used. m-CPP significantly increased the amount of self-grooming in male Sprague-Dawley rats. This effect followed a bell-shaped dose-response curve with a peak at 0.6 mg/kg, i.p. Pretreatment with SB-242084, a subtype-selective 5-HT(2C) receptor antagonist (0.1-0.5 mg/kg, i.p.), reversed m-CPP-induced self-grooming. In contrast, pretreatment with the subtype-selective 5-HT(2B) receptor antagonist SB-215505 (1 mg/kg, i.p) did not block the effect of m-CPP. Two days after depletion of brain 5-HT by p-chlorophenylalanine (p-CPA, 2 x 50, 2 x 100 mg/kg, i.p.) m-CPP-induced responses were significantly enhanced compared to controls. Our studies provide evidence that direct activation of 5-HT(2C) receptors mediate m-CPP-induced self-grooming and the depletion of brain 5-HT sensitizes these receptors.

A review of 25 years of the social interaction test

The social interaction test of anxiety was developed 25 years ago to provide an ethologically based test that was sensitive to both anxiolytic and anxiogenic effects. It is sensitive to a number of environmental and physiological factors that can affect anxiety. It has detected anxiogenic effects of peptides such as corticotropin-releasing factor (CRF) and adrenocorticotropic hormone (ACTH), and anxiolytic effects of neuropeptide Y and substance P receptor antagonists. It has successfully identified neuropharmacological sites of action of anxiogenic compounds and drug withdrawal. Effects of compounds acting on the gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT) systems have been extensively investigated after both systemic administration and microinjection into specific brain regions. The use of this test has, thus, played a crucial role in unravelling the neural basis of anxiety. It is hoped that in the next 25 years, the test will play a crucial role in determining the genetic basis of anxiety disorders.

2-(Dimethylaminomethyl)-tetrahydroisoxazolopyridobenzazepine derivatives. Synthesis of a new 5-HT(2C) antagonist with potential anxiolytic properties

Following the program started at Johnson & Johnson Pharmaceutical Research & Development searching for 5-HT(2A/2C) antagonists, we now report on the synthesis of 2-(dimethylaminomethyl)-2,3,3a,8-tetrahydroisoxazolo[3,2-a]pyrido[3,4-c]-[2]benzazepine and 2-(dimethylaminomethyl)-2,3,3a,8-tetrahydroisoxazolo[3,2-a]pyrido[3,2-c]-[2]benzazepine. A new method for the synthesis of pyridobenzazepines is described as well. The affinities for several receptors as well as the mCPP antagonistic activity of the compounds synthesised are described.

Synthesis and structure-activity relationship of 2-(aminoalkyl)-2,3,3a,8-tetrahydrodibenzo[c,f]isoxazolo[2,3-a]azepine derivatives: a novel series of 5-HT(2A/2C) receptor antagonists. Part 1

The synthesis of a series of novel 2-(aminoalkyl)-2,3,3a,8-tetrahydrodibenzo[c,f]isoxazolo[2,3-a]azepine derivatives as well as their 5-HT(2A/2C) and H(1) receptor binding affinities are described. The in vivo activity as potential anxiolytics of the synthesised compounds was measured in a mCPP challenge test. One of the compounds, 2a, proved to be a potent 5-HT(2A/2C) receptor antagonist showing as well oral activity and therefore could be considered as a potential anxiolytic/antidepressant agent.

Strain-dependent effects of diazepam and the 5-HT2B/2C receptor antagonist SB 206553 in spontaneously hypertensive and Lewis rats tested in the elevated plus-maze

The 5-HT2B/2C receptor antagonist SB 206553 exerts anxiolytic effects in rat models of anxiety. However, these effects have been reported for standard rat strains, thus raising the issue of SB 206553 effects in rat strains displaying different levels of anxiety. Herein, the effects of SB 206553 in a 5-min elevated plus-maze test of anxiety were compared to those of the reference anxiolytic, diazepam, in two rat strains respectively displaying high (Lewis rats) and low (spontaneously hypertensive rats, SHR) anxiety. Diazepam (0.37, 0.75, or 1.5 mg/kg; 30 min before testing) increased in a dose-dependent manner the behavioral measures in SHR, but not in Lewis rats. On the other hand, SB 206553 (1.25, 2.5, or 5 mg/kg; 30 min before testing) failed to alter the anxiety parameters in both strains, whereas it increased closed arm entries in Lewis rats, suggesting that it elicited hyperactivity in the latter strain. Accordingly, the hypolocomotor effect of the nonselective 5-HT2B/2C receptor agonist m-chlorophenylpiperazine (1.5 mg/kg ip 20 min before a 15-min exposure to an activity cage) was prevented by the 1.25 and 2.5 mg/kg doses of SB 206553 in Lewis rats and SHR, respectively. Compared with SHR, Lewis rats may display a lower response to benzodiazepine-mediated effects and a more efficient control of locomotor activity by 5-HT2B/2C receptors.

Dissociation of systemic and hippocampal modulation of rat locomotor activity by 5-HT(2C) receptors

In the present study, the ability of 5-hydroxytryptamine(2C) (5-HT(2C)) receptors in the hippocampus to enhance locomotor activity in rats was investigated by local infusion. Intraperitoneal injection of the selective 5-HT(2C) antagonist SB 242084 (1 mg/kg) significantly increased rats motor activity, while the effects of non-selective 5-HT(2C) agonist m-chlorophenylpiperazine (m-CPP, 10 mg/kg; i.p.) on motor activity were lower than those of the control group. In the day hours, the local infusion of non-selective 5-HT(2C) agonist, m-CPP (1.0 mM) into the bilateral hippocampus via microdialysis probes increased locomotor activity in contrast with intraperitoneal injection. This increase was completely reversed by the combined infusion of the selective 5-HT(2C) antagonist SB 242084 (100 mM). In the night hours, the local infusion of SB 242084 (100 mM) into the bilateral hippocampus significantly inhibited the nocturnal hyperactivity, which was reversed by the combined infusion of m-CPP. The present study demonstrates that the 5-HT(2C) receptors in the hippocampus act to increase rat locomotor activity.

High social anxiety and low aggression in Fawn-Hooded rats

The Fawn-Hooded (FH) rat strain, with well-documented changes in their serotonergic and noradrenergic systems, is a putative genetic model for some neuropsychiatric disorders like depression, alcohol abuse, and anxiety. Because social phobia frequently occurs in combination with these disorders and there are no social anxiety-related data in FH rats in the literature, we measured the behavior of FH rats in the social interaction test. In addition, the effects of the anxiogenic Serotonin-2C (5-HT2C) receptor agonist, m-chlorophenylpiperazine (m-CPP), were studied. Male FH, Wistar (W), and Sprague-Dawley (SD) rats were used in two different test conditions of the social interaction test: the high light, unfamiliar arena, associated with high anxiety, and the low light, familiar arena, associated with low anxiety-like behavior. All social behaviors were markedly diminished in FH rats that suggested higher anxiety in these animals. Total social interaction time was reduced by 60-70% in FH rats compared either to W or SD rats under high light, unfamiliar or low light, familiar conditions, respectively. Aggressive behavior was reduced at least by 85% in FH rats. Locomotor activity and exploratory behavior were only minimally, in most comparisons, not significantly affected in FH rats. Total social interaction time, aggression, and locomotor activity were decreased, and self-grooming increased by m-CPP (0.5 mg/kg, ip) in all three strains. m-CPP decreased total social interaction time thus, caused anxiety most efficiently in FH rats (reduced by 69%, 50%, and 55% in FH, W, and SD rats, respectively), but other effects of the drug were similar in the three strains. Our studies provide evidence that the FH rat strain may be a genetic model of social phobia or other anxiety disorders with impaired social behavior.

The role of the dorsal hippocampal serotonergic and cholinergic systems in the modulation of anxiety

A review of the literature suggests that the dorsal hippocampal serotonergic system, and, in particular, the postsynaptic 5-HT(1A) receptor, mediates an anxiogenic response, whereas endogenous dorsal hippocampal cholinergic tone mediates an anxiolytic response. Accordingly, it has been shown that direct dorsal hippocampal administration of the 5-HT(1A) receptor agonist, 8-OH-DPAT, the nicotinic receptor antagonist, mecamylamine, and the M(1) muscarinic receptor antagonist, pirenzepine, all have anxiogenic effects in rats tested in the social interaction test. It is therefore surprising that nicotine also has an anxiogenic effect in this test following dorsal hippocampal administration. However, the anxiogenic effects of mecamylamine and nicotine in the dorsal hippocampus are blocked by coadministration of the 5-HT(1A) receptor antagonist, WAY 100635, suggesting that both of these compounds act by enhancing hippocampal serotonergic transmission, thereby stimulating postsynaptic 5-HT(1A) receptors. This conclusion is supported by the observation that both nicotine and mecamylamine stimulate basal [3H]-5-HT release from dorsal hippocampal slices. A possible mechanism by which nicotinic receptor ligands modulate hippocampal 5-HT release is discussed, and it is proposed that the dorsal hippocampal serotonergic and cholinergic systems are tightly coupled and function antagonistically in the modulation of anxiety, as measured in the social interaction test. These systems are relatively unimportant in controlling behaviour on trial 1 in the plus-maze. On trial 2 in the elevated plus-maze, a model of specific phobia, the endogenous cholinergic system, nicotine, and the M(1) receptor agonist, McN-A-343, all mediate an anxiolytic effect, whereas stimulation of 5-HT(1A) receptors mediates an anxiogenic effect. It is proposed that the hippocampus may predominantly control the avoidance components of phobic anxiety, with other regions, such as the dorsomedial hypothalamus, controlling the escape components.

Evidence for accelerated desensitisation of 5-HT(2C) receptors following combined treatment with fluoxetine and the 5-HT(1A) receptor antagonist, WAY 100,635, in the rat

Both pre-clinical and clinical studies suggest that additional treatment with 5-HT(1A) receptor antagonists may accelerate the antidepressant efficacy/onset of selective serotonin re-uptake inhibitors (SSRIs). Given that chronic SSRI treatment has been shown to desensitise 5-HT(2C) receptor mediated responses, we have used the rat social interaction test to determine if combined treatment with WAY 100,635, a selective 5-HT(1A) receptor antagonist, will accelerate this effect. In pairs of unfamiliar rats, acute administration of the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP) or fluoxetine decreased the time spent in social interaction, responses which were reversed by the 5-HT(2C/2B) receptor antagonists SB 200646A and SB 221284. Similar reductions in social interaction were observed in rats treated with fluoxetine (10 mg/kg, i.p. daily) for 4, 7 and 14 days but was no longer apparent after 28 days of treatment. In contrast, only 7 days of combined treatment with WAY 100,635 (1 mg/kg/s.c./day) and fluoxetine were needed to reverse this response. The decrease in social interaction induced by an acute challenge of mCPP (1 mg/kg, i. p.) was also reduced after 6 days co-treatment with WAY 100,635 and fluoxetine. Thus, WAY 100,635 accelerates SSRI-induced desensitisation of 5-HT(2C) receptors, suggesting that this response might contribute towards the therapeutic effects of SSRIs in man.

Effects of centrally administered anxiolytic compounds in animal models of anxiety

The effect of intra-cerebrally infused compounds in animal models of anxiety were reviewed. A large body of evidence suggested that benzodiazepine agonists in different brain regions--including areas of the raphe, hypothalamus, periaqueductal gray, septum, hippocampus, and amygdala--produce reasonably consistent anxiolytic effects in a variety of animal models. However, evidence regarding the effects on anxiety of 5-HT1A agonists, 5-HT2 compounds, and 5-HT3 antagonists was somewhat less extensive, both anatomically and behaviourally, and more complex. For example, establishing receptor specificity for 5-HT ligand effects was often complicated by the lack of 'silent' and/or selective antagonists. Neuropeptides had significant effects on anxiety, but these were shown in a smaller number of animal models and in a limited number of brain regions. Regardless of the compounds tested, however, there seemed to be a surprising number of double dissociations (brain site by behavioural test). In fact in some instances, different fear reactions appeared to be controlled by distinct receptor subpopulations within particular parts of the limbic system. These results suggest that the neural control of anxiety might be analogous in organization to sensorimotor systems, i.e., anxiety is controlled by complex systems of multiple, distributed, parallel pathways.

Pharmacological evaluation of a modified open-field test sensitive to anxiolytic drugs

In a recent study it has been shown that benzodiazepine receptor agonists attenuate novelty-induced suppression of feeding and increase the percentage of animals feeding in the open field. Food-deprived rats were placed in one corner of the open field containing food in the center. The number of rats beginning to eat in the first 5 min was recorded. In the present study this test was validated pharmacologically using known "anxiolytic" or "nonanxiolytic" drugs. The following substances (effective doses, given IP) increased the number of rats feeding within 5 min in the center of the open field: meprobamate (30.0-300 mg/kg), 8-OH-DPAT (10 and 30 microg/kg), ipsapirone (1.0 and 2.0 mg/kg), ritanserin (0.125-0.5 mg/kg), tropisetron (0.1-10.0 microg/kg), ondansetron (0.3-3.0 microg/kg), lisuride (0.28-0.55 mg/kg), morphine (0.3 and 1.0 mg/kg), propranolol (0.3 and 1.0 mg/kg), clozapine (1.0 mg/kg). Drugs without "anxiolytic" effects in other animal models or in humans, including amphetamine, apomorphine, haloperidol, sulpiride, and mCPP did not increase the incidence of food intake in this test. Ethanol and hexobarbital, in nonsedative doses, had no effect in this paradigm. Drugs and doses effective in the modified open-field test caused no increase in food intake in an independent food consumption test using food-deprived rats staying in the familiar cages. The results suggest that the modified open-field test can detect "anxiolytic" drug properties and is valid for the assessment of "anxiolytic" effects from different classes of drugs.

Role of 5-HT2A and 5-HT2C receptor subtypes in the two types of fear generated by the elevated T-maze

To study the role of 5-HT2A and 5-HT2C receptor subtypes in anxiety, the behavioral effects of drugs that either block or stimulate these receptors were measured in an animal model of anxiety, the elevated T-maze. One arm of the maze is enclosed by walls and stands perpendicular to the two open arms. Inhibitory (passive) avoidance--representing learned fear--was measured by placing a rat at the end of the enclosed arm and recording the time to leave this arm with the four paws during three consecutive trials. After 30 s, the same animal was placed at the end of one of the open arms and the time to leave this arm with the four paws was recorded. This one-way escape response represents unconditioned fear. The I.P. injection of the preferential 5-HT2C receptor agonists mCPP and TFMPP (0.1-0.8 mg/kg), 25 min before the experimental session enhanced inhibitory avoidance. In contrast, the same drugs either tended to impair (mCPP) or significantly inhibited (TFMPP) one-way escape. The preferential 5-HT2A agonist DOI (0.03-0.3 mg/kg) did not change either inhibitory avoidance or one-way escape. Inhibitory avoidance was impaired by the selective 5-HT2C antagonists SB 200646A (3.0-30 mg/kg) and SDZ SER 082 (0.1-1.0 mg/kg), by the 5-HT2A antagonist SR 46349B (1.0-10.0 mg/kg), and by the mixed 5-HT(2A,2C) antagonist ritanserin (0.3-3.0 mg/kg). However, it was not affected by the selective 5-HT2A antagonist RP 62203 (0.25-4.0 mg/kg). All the 5-HT2 antagonists used were ineffective on one-way escape. Therefore, conditioned fear seems to be tonically facilitated through 5-HT2C receptor stimulation, although the 5-HT2A receptor may also participate in its regulation. Unconditioned fear might be phasically inhibited by 5-HT2C receptor activation.

Decreased 5-HT1A and increased 5-HT2A receptor binding after chronic corticosterone associated with a behavioural indication of depression but not anxiety

The effects of chronic corticosterone treatment (100 mg pellet implanted for 1 week) were assessed in animal tests of anxiety, exploration and motor activity, and changes in binding to 5-HT1A and 5-HT2A receptors, and the 5-HT transporter, were measured. At the end of the week's treatment, the corticosterone concentration was significantly elevated and there were significant decreases in adrenal, thymus and body weights. However, there were no changes in the measures of anxiety in the social interaction test or on trials 1 and 2 of the elevated plus-maze. Also supporting a dissociation between anxiety and elevated corticosterone concentrations are previous findings that benzodiazepine withdrawal causes increased anxiety but no change in corticosteroid concentrations. Therefore these two situations provide a double dissociation between anxiety and elevated corticosteroids. Decreased 5-HT1A receptor binding in the dentate gyrus and increased 5-HT2A receptor binding in the parietal cortex was found following chronic corticosterone treatment. This reciprocal relationship between 5-HT1A and 5-HT2A receptors has been proposed to be important in mediating depression. The significant decreases in motor activity observed in all the test situations would be compatible with this proposal. Thus the constellation of behavioural and biochemical changes detected after chronic corticosterone treatment is more pertinent to depression than anxiety. One week after removal of the pellets, the behavioural and neurochemical changes had disappeared and the only differences to remain were decreased adrenal, thymus and body weights in the animals that had been treated chronically with corticosterone.

Anxiolytic-like actions of the selective 5-HT4 receptor antagonists SB 204070A and SB 207266A in rats

The highly selective 5-HT4 receptor antagonists, SB 204070A (0.001-0.1 mg/kg s.c., 30 min pretest) and SB 207266A (0.01, 1 and 10 mg/kg p.o., 1 hr pre-test), increased time spent in social interaction without affecting locomotor activity, in a rat 15 min social interaction test under high light, unfamiliar conditions. At 1 and 10 mg/kg s.c., SB 204070A was no longer active. These results are consistent with the profile expected of anxiolytic treatments in this procedure. In a rat 5 min elevated x-maze test, SB 204070A (0.01 and 1 mg/kg s.c., 30 min pre-test) significantly increased the percentage of time spent on the open arms. SB 204070A (0.01 mg/kg s.c.) and SB 207266A (1 mg/kg p.o., 1 hr pre-test) also increased percentage entries to the open arms. Neither compound affected locomotion at any dose tested in the procedure. The effects of both compounds in this procedure are also consistent with anxiolysis. Neither SB 204070A (0.1 or 1 mg/kg s.c., 30 min pre-test) nor SB 207266A (0.1 or 1 mg/kg p.o., 1 hr pre-test) affected either unpunished or punished responding, in a rat Geller-Seifter conflict model of anxiety. The maximal efficacy of both SB 204070A and SB 207266A in the rat social interaction test was similar to that of the benzodiazepine anxiolytic chlordiazepoxide (5 mg/kg s.c. or p.o.) used as a positive control, but was considerably less in the elevated x-maze procedure. The results suggest that 5-HT4 receptor antagonists may have modest anxiolytic-like actions in rats.

Activation of 5-HT2B receptors in the medial amygdala causes anxiolysis in the social interaction test in the rat

In a recent study, we reported the presence of neurones expressing 5-HT2B receptor protein in the medial amygdaloid nucleus of the adult rat brain. In the present study, bilateral micro-injection of the 5-HT2B receptor agonist 1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride (BW 723C86, 0.09 and 0.93 nmol, 5 min pretest) into the medial amygdaloid nuclei increased the total interaction time of a pair of male rats in the social interaction test, to a comparable extent to chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) without altering locomotor activity; indicative of anxiolytic activity. The increase in social interaction was prevented by pretreatment with the 5-HT2C/2B receptor antagonist N-(1-methyl-5-indoyl)-N'-(3-pyridyl) urea hydrochloride (SB 200646A, at 2 but not 1 mg/kg p.o., 1 hr pretest), which did not alter behaviour when given alone. Intra-amygdala BW 723C86 (0.09, 0.31 and 0.93 nmol, 5 min pretest) did not significantly alter the number of punished responses made when the same rats were examined seven days later in a Vogel punished drinking test, although chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) produced the expected anxiolytic profile. The results are consistent with the proposal that activation of 5-HT2B receptors in the medial amygdala induces anxiolysis in the social interaction model but has little effect on behaviour in a punished conflict model of anxiety. These data suggest that serotonergic neurotransmission in this nucleus may selectively affect specific kinds of anxiety generated by different animal models.

Effects of the 5-HT2B receptor agonist, BW 723C86, on three rat models of anxiety

1. BW 723C86 (3 and 10 mg kg-1, s.c. 30 min pretest), a 5-HT2B receptor agonist, increased total interaction, but not locomotion in a rat social interaction test, a profile consistent with anxiolysis. 2. The effect of BW 723C86 in the social interaction test is likely to be 5-HT2B receptor-mediated as it was prevented by pretreatment with the 5-HT2C/2B receptor antagonist, SB 200646A, (1 and 2 mg kg-1, p.o., 1 h pretest) which did not affect basal levels of social interaction at the doses used. 3. An anxiolytic-like action was also observed in the rat Geller-Seifter conflict test, where BW 723C86 (0.5-50 mg kg-1, s.c. 30 min pretest) modestly, but significantly increased punished, but not unpublished responding. 4. In a rat 5 min elevated x-maze test, BW 723C86 (1-10 mg kg-1, s.c.) had no significant effect. 5. The maximal anxiolytic-like effect of BW 723C86 approached that of the benzodiazepine anxiolytic, chloradiazepoxide (5 mg kg-1, s.c. 30 min pretest) in the social interaction test, but was markedly less in the Geller-Siefter test. The effect of BW 723C86 was also clearly less than chlordiazepoxide in the elevated x-maze procedure where it had no significant effect. 6. In conclusion, BW 723C86 exerted an appreciable anxiolytic-like profile in a rat social interaction test, but had a weaker effect in the Geller-Siefter and was ineffective in the elevated x-maze test used. These effects are likely to be 5-HT2B receptor-mediated.

In vitro and in vivo profile of SB 206553, a potent 5-HT2C/5-HT2B receptor antagonist with anxiolytic-like properties

1. SB 206553 (5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2 ,3-f]indole) displays a high affinity (pK1 7.9) for the cloned human 5-HT2C receptor expressed in HEK 293 cells and the 5-HT2B receptor (pA2 8.9) as measured in the rat stomach fundus preparation. SB 206553 has low affinity for cloned human 5-HT2A receptors expressed in HEK 293 cells (pK1 5.8) and (pK1 < 6) for a wide variety of other neurotransmitter receptors. 2. SB 206553 appears to be a surmountable antagonist of 5-HT-stimulated phosphoinositide hydrolysis in HEK 293 cells expressing the human 5-HT2C receptor (pKB 9.0). 3. The compound potently (ID50 5.5 mg kg-1, p.o., 0.27 mg kg-1, i.v.) inhibited the hypolocomotor response to m-chlorophenylpiperazine (mCPP), a putative model of 5-HT2C/5-HT2B receptor function in vivo. 4. At similar doses (2-20 mg kg-1, p.o.) SB 206553 increased total interaction scores in a rat social interaction test and increased punished responding in a rat Geller-Seifter conflict test. These effects are consistent with the possession of anxiolytic properties. 5. SB 206553 also increased suppressed responding in a marmoset conflict model of anxiety at somewhat higher doses (15 and 20 mg kg-1, p.o.) but also reduced unsuppressed responding. 6. These results suggest that SB 206553 is a potent mixed 5-HT2C/5-HT2B receptor antagonist with selectivity over the 5-HT2A and all other sites studied and possesses anxiolytic-like properties.

Increased 5-HT2C receptor responsiveness occurs on rearing rats in social isolation

To investigate whether isolation rearing alters 5-hydroxytryptamine2C (5-HT2C) receptors, the effect of the serotonin agonist m-chlorophenylpiperazine (mCPP) was examined on elevated plus-maze behaviour, plasma corticosterone and brain 5-HT2C receptor protein levels in rats. There was no distinction between behaviour or corticosterone levels in drug-free isolates or socially housed rats exposed to the elevated plus-maze. The anxiogenic response to mCPP (decrease in open arm entry and time and an increase in stretch attend postures) on the elevated plus-maze was greater in isolation than in socially reared controls without any concomitant difference in the hypolocomotor effect of mCPP in the two groups. mCPP produced a greater elevation in plasma corticosterone in isolates than in group-housed controls. Hippocampal 5-HT2C receptor protein-like immunoreactive levels were significantly lower following mCPP than saline only in rats reared in isolation. These results indicate that increased 5-HT2C receptor responsiveness accompanies isolation-rearing and may contribute to the enhanced response to stress and the increased neophobia seen in this animal model of trait anxiety/depression. In isolation reared rats, rapid down-regulation of supersensitive 5-HT2C receptors may occur in the hippocampus following 5-HT agonist challenge.