A discriminative stimulus produced by 1-(3-chlorophenyl)-piperazine (mCPP) as a putative animal model of anxiety

Effects of anxiogenic drugs on the emission of 22- and 50-kHz ultrasonic vocalizations in adult rats

RATIONALE

Adult rat 22-kHz vocalizations are often associated with alarm or distress, whereas a subset of 50-kHz calls is preferentially emitted in response to amphetamine and other rewarding stimuli. Whether any 50-kHz calls reflect anxiety is unknown.

OBJECTIVE

To determine the effects of anxiogenic drugs on 50-kHz call rate and call subtype profile, in comparison with D-amphetamine.

METHODS

Adult male rats received systemic amphetamine (1 mg/kg) three times several days before testing. Ultrasonic vocalizations were then recorded after acute intraperitoneal injection of amphetamine or one of five anxiogenic drugs: yohimbine (2.5 mg/kg), N-methyl-β-carboline-3-carboxamide (FG 7142, 5 mg/kg), pentylenetetrazol (PTZ, 20 mg/kg), m-chlorophenylpiperazine (mCPP, 1 mg/kg), caffeine (25 mg/kg), or vehicle.

RESULTS

The duration of immobility was increased by FG 7142, PTZ, and mCPP; this measure was unchanged by yohimbine and reduced by the locomotor stimulant drugs amphetamine and caffeine. Conversely, the 50-kHz call rate was reduced by FG 7142, PTZ and mCPP, and increased by caffeine and amphetamine. Overall, the most common 50-kHz call subtypes were flat, trill, step-up, and complex. Consistent with previous reports, amphetamine increased the relative prevalence of trill calls while reducing the relative prevalence of flat calls. Yohimbine and caffeine reduced flat call prevalence, whereas mCPP reduced trill call prevalence. No other shifts in the call profile were observed, and no anxiogenic drug induced 22-kHz calls.

CONCLUSION

Anxiogenic drugs, as a class, did not uniformly alter the 50-kHz call rate or subtype profile. Amphetamine-induced effects on 50-kHz call rate and profile do not reflect anxiety.

Interactions of a standardized flavonoid fraction from Tilia americana with Serotoninergic drugs in elevated plus maze

ETHNOPHARMACOLOGICAL RELEVANCE

Tilia americana var. mexicana (Schltdl) Hardin (Tiliaceae) aerial parts (bracts and flowers) are used in the traditional Mexican medicine to treat nervous disorders, as sedative and to treat insomnia. A fraction of this species called FC1 (organic fraction from this plant) was proposed, described as anxiolytic and characterized by the presence of flavonoids. In the present work, this fraction was standardized, and its interaction with different serotonergic drugs was tested. We used the elevated plus maze model as anxiety test and the open field test so as to observe a possible effect on mice׳s motor behavior.

MATERIAL AND METHODOLOGY

HPLC technique was used to quantify the flavonoids contained in a fraction called F1C. Different doses of F1C were administered to ICR mice (12.5, 25, 37.5 and 50mg/kg, oral pathway) then they were exposed to elevated plus maze or open field test. After, each dose of F1C fraction was co-administered with different drugs, in order to evaluate the animal׳s behavior: DOI agonist (2.0mg/kg) and KET antagonist (0.03mg/kg) of 5-HT2A receptors; 8-OH-DPAT (0.1mg/kg) selective agonist and WAY100635 (0.5mg/kg) antagonist of 5HT1 receptors.

RESULTS

The HPLC quantitative analysis revealed the F1C composition (mg/g of extract): tiliroside (28.56), glucoside of quercetin (16.25), quercitrin (7.96), rutin (3.93), Kaempferol (2.83). The Emax for F1C curve was 80.6% for time to open arms with an ED50 of 15.09 mg/kg. The combination of F1C with DOI gives a significant increase of the F1C anxiolytic effect (Emax=111% and ED50=13.51 mg/kg), while KET blocks it completely (Emax=12.25% and ED50=2.4 mg/kg). The administration of F1C with 8-OH-DPAT does not generate significant changes on the time to open arms, although it does induce a decrement in F1C potency (Emax=83.3% and ED50=33.3mg/kg). When F1C and WAY-100365 are combined, the anxiolytic activity of the fraction decreases (Emax=33.3% and ED50=102.10mg/kg).

CONCLUSIONS

The medicinal use attributed to Tilia americana for their effect on central nervous system, could be in part in the flavonoid fraction (F1C) with anxiolytic activity which is dose dependent, and has the ability to interact with the serotonergic system. It is necessary to advance in the study of the mechanism of action, using other techniques such in vitro analysis.

Interaction of the natural anxiolytic Galphimine-B with serotonergic drugs on dorsal hippocampus in rats

AIM OF THE STUDY

Galphimine-B (G-B) is a nor-seco triterpene with an anxiolytic-like effect obtained from the plant species Galphimia glauca Cav. By means of a double blind clinical trial, it has been demonstrated that the extract from this plant, standardized in G-B content, possesses therapeutic effectiveness in patients with generalized anxiety. The mechanism of action of this compound remains unknown to date, but it has already demonstrated a non interaction with the γ-aminobutyric acid (GABA)ergic system. For this reason, the objective of this work was to evaluate the pharmacological interaction between G-B with the 5-hydroxytryptamine 1A (5HT(1A)) and 5-hydroxytryptamine 2A (5HT(2A)) serotonergic receptors on CA1 neurons of hippocampus.

MATERIALS AND METHODS

Electrophysiological records were performed as the frequency of discharge of in vivo CA1 cells from dorsal hippocampus in rats.

RESULTS

G-B was able to increase the frequency of discharge of neurons of the CA1 cells with some characteristics that support an interaction with the serotonergic system in this zone. It was demonstrated that this triterpene modulates the induced response of 5HT(1A) receptors, in an allosteric manner.

CONCLUSION

This effect demonstrated an interaction between G-B and the serotonergic system in dorsal hippocampus and evidenced that the mechanism of action of this compound could involve a complex series of actions on different neurotransmitter systems related with the anxiety disorder.

Multiple anxiogenic drugs recruit a parvalbumin-containing subpopulation of GABAergic interneurons in the basolateral amygdala

The basolateral amygdala is a nodal structure within a distributed and interconnected network that regulates anxiety states and anxiety-related behavior. Administration of multiple anxiogenic drugs increases cellular responses (i.e., increases c-Fos expression) in a subregion of the basolateral amygdala, but the neurochemical phenotypes of these cells are not known. The basolateral amygdala contains glutamatergic projection neurons and several populations of γ-aminobutyric acid-synthesizing (GABAergic) interneurons, including a population of parvalbumin (PV)-expressing GABAergic interneurons that co-express the excitatory 5-HT(2A) receptor. The role for these PV-expressing GABAergic interneurons in anxiety-states is unclear. In this experiment we examined the effects of multiple anxiogenic drugs including the 5-HT(2C/2A) receptor agonist m-chlorophenyl piperazine (mCPP), the adenosine receptor antagonist caffeine, the α(2)-adrenoreceptor antagonist yohimbine and the partial inverse agonist at the benzodiazepine allosteric site on the GABA(A) receptor, N-methyl-beta-carboline-3-carboxamide (FG-7142), on c-Fos expression in PV-immunoreactive (PV-ir) interneurons in subdivisions of the basolateral amygdala. All drugs with the exception of mCPP increased c-Fos expression in PV-ir neurons in the basolateral amygdaloid nucleus, anterior part (BLA). The numbers of c-Fos-immunoreactive (c-Fos-ir)/PV-ir GABAergic interneurons in the BLA were positively correlated with the numbers of c-Fos-ir serotonergic neurons in the mid-rostrocaudal dorsal raphe nucleus (DR) and with a measure of anxiety-related behavior. All four drugs increased c-Fos expression in non-PV-ir cells in most of the subdivisions of the basolateral amygdala that were sampled, compared with vehicle-injected controls. Together, these data suggest that the PV/5-HT(2A) receptor expressing GABAergic interneurons in the basolateral amygdala are part of a DR-basolateral amygdala neuronal circuit modulating anxiety-states and anxiety-related behavior.

Effects of pretest manipulation on elevated plus-maze behavior in adolescent and adult male and female Sprague-Dawley rats

The elevated plus-maze (EPM) is vulnerable to variations in pretest circumstances when testing adult rodents. Because of an increasing interest in adolescence, the present experiments examined the impact of pretest manipulations on anxiety levels in the EPM among adolescent and adult Sprague-Dawley rats of both sexes. In Exp. 1, animals removed from their home cage and immediately placed on the EPM were compared to rats tested following 30 min of social isolation, or following 30-min exposure to a novel context. These pretest manipulations only modestly decreased anxiety levels at both ages. In Exp. 2, more varied pretest conditions were examined: testing directly from the home cage; testing following 30 min of social isolation in a novel environment; or a large saline injection and rehousing 18 h prior to a 30-min period of social isolation in a novelty situation before testing. In adults, anxiety levels decreased linearly as pretest perturbation increased, whereas adolescents showed comparable levels of anxiety with both the moderate and large perturbations. As a result, observed age differences in anxiety differed as a function of pretest circumstances. Therefore, caution is urged when using the EPM for across-age comparisons of anxiolytic and anxiogenic effects of pharmacological or other manipulations.

gamma-Aminobutyric acid-serotonin interactions in healthy men: implications for network models of psychosis and dissociation

BACKGROUND

This study tested the hypothesis that deficits in gamma-aminobutyric acid type A (GABA(A)) receptor function might create a vulnerability to the psychotogenic and perceptual altering effects of serotonergic (5-HT(2A/2C)) receptor stimulation. The interactive effects of iomazenil, an antagonist and partial inverse agonist of the benzodiazepine site of the GABA(A) receptor complex, and m-chlorophenylpiperazine (m-CPP), a partial agonist of 5-HT(2A/2C) receptors, were studied in 23 healthy male subjects.

METHODS

Subjects underwent 4 days of testing, during which they received intravenous infusions of iomazenil/placebo followed by m-CPP/placebo in a double-blind, randomized crossover design. Behavioral, cognitive, and hormonal data were collected before drug infusions and periodically for 200 min after.

RESULTS

Iomazenil and m-CPP interacted in a synergistic manner to produce mild psychotic symptoms and perceptual disturbances without impairing cognition. Iomazenil and m-CPP increased anxiety in an additive fashion. Iomazenil and m-CPP interacted in a synergistic manner to increase serum cortisol.

CONCLUSIONS

Gamma-aminobutyric acid-ergic deficits might increase the vulnerability to the psychotomimetic and perceptual altering effects of serotonergic agents. These data suggest that interactions between GABA(A) and 5-HT systems might contribute to the pathophysiology of psychosis and dissociative-like perceptual states.

Behavioral effects of systemically administered MK-212 are prevented by ritanserin microinfusion into the basolateral amygdala of rats exposed to the elevated plus-maze

RATIONALE

Although 5-HT2 receptors seem to play an important role in anxiety, results from numerous studies are still highly variable. Moreover, little is known about the behavioral effects of centrally administered 5-HT2 compounds in animal models of anxiety.

OBJECTIVE

The current study was performed to: (1) further investigate the effects of 5-HT2 receptor activation in rats exposed to the elevated plus-maze (EPM) and the open-field arena, two widely used animal models for studying anxiety and locomotor activity; and (2) evaluate the involvement of the 5-HT2 receptors within the basolateral nucleus of the amygdala (BLA) in the modulation of such effects.

METHODS

In the first experiment, male Wistar rats were exposed for 5 min to the EPM 27 min following intraperitoneal (i.p.) (1.0 ml/kg) injections of the preferential 5-HT2C receptor agonist 6-chloro-2[1-piperazinyl]pyrazine (MK-212) at doses of 1.0, 2.0, or 4.0 mg/kg. Control animals were injected with saline. The percentage of open-arm entries and the percentage of time spent in these arms were employed as anxiety indexes, whereas the number of closed-arm entries was calculated as indicative of locomotor activity. In the second experiment, rats were exposed for 10 min in an open-field arena to further assess the interference of the same MK-212 doses upon locomotor activity. In Experiment 3, rats were microinjected (0.2 microl) either with the mixed 5-HT 2A/2C receptor antagonist ritanserin (0.5, 1.25, 2.5, and 5.0 microg) or its vehicle into the BLA 12 min following i.p. injections of saline or the intermediate dose of MK-212 (2.0 mg/kg). Fifteen minutes later, each animal was exposed to the EPM as before.

RESULTS

Whereas the highest dose of MK-212 (4.0 mg/kg) induced motor-suppressant effects in both EPM and open-field arena, the intermediate dose of the drug (2.0 mg/kg) reduced open-arm exploration without significantly affecting the number of closed-arm entries. This behavioral profile, consistent with selective anxiogenic effect in the EPM, was dose-dependently prevented by ritanserin microinfusion into the BLA. In saline-pretreated animals, however, ritanserin (all doses) was ineffective.

CONCLUSIONS

MK-212 increases anxiety and decreases locomotor activity. The anxiogenic-like profile of 5-HT2 receptor activation is prevented by the blockade of 5-HT2 receptors within the BLA, which does not have an effect by itself upon basal anxiety levels triggered by the EPM.

Ethanol substitutes for the discriminative stimulus effects of m-chlorophenylpiperazine

Rats were trained to discriminate the 5-HT agonist m-chlorophenylpiperazine (mCPP, 1 mg/kg, i.p.) from saline. Ethanol (0.1 to 1 g/kg, i.p.) partially substituted for the discriminative stimulus effects of 1 mg/kg mCPP. Methysergide (10 mg/kg, i.p.), a 5-HT(1/2) receptor antagonist, blocked the ability of ethanol (1 g/kg) to substitute for mCPP. The largest dose of ethanol markedly reduced response rate. These findings suggest an important role of serotonin receptors in mediating the discriminative stimulus effects of ethanol.

Immediate-early gene expression in the central nucleus of the amygdala is not specific for anxiolytic or anxiogenic drugs

The lateral, basal, and central nuclei of the amygdala are part of a circuitry that instantiates many fear and anxious behaviors. One line of support indicates that immediate-early gene (IEG) expression (e.g., c-fos and egr-1 (zif268)) is increased in these nuclei following fear conditioning. Other research finds that anxiogenic drugs working through various mechanisms induce IEG expression in the central nucleus of the amygdala (CeA) suggesting that expression is a neural marker for fear and anxiety. However, several studies have also found that anxiolytic drugs induce IEG expression in the CeA. Expression of egr-1 in the CeA and lateral nucleus of the amygdala following administration of anxiolytic and anxiogenic benzodiazepine and serotonin agonists and antagonists was investigated. The first experiment determined behaviorally active anxiolytic and anxiogenic doses for two anxiogenic drugs (FG 7142 and mCPP) and two anxiolytic drugs (diazepam and buspirone). The effects of anxiogenic and anxiolytic doses of these drugs on egr-1 expression in the amygdala were then tested in a second experiment. All four drugs increased egr-1 in the CeA indicating that increased egr-1 mRNA expression in the CeA is not specific to anxiolytic or anxiogenic effects of the drugs. We suggest that IEG expression in the CeA may be due to activation of circuits that are associated with systemic physiological homeostasis perturbed by a number of drugs including anxiogenic and anxiolytic compounds.

Serotonin (1A) receptor involvement in acute 3,4-methylenedioxymethamphetamine (MDMA) facilitation of social interaction in the rat

The current study assessed whether various co-administered serotonin (5-HT) receptor antagonists could prevent some of the acute behavioral effects of 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") in rats. In the social interaction test, MDMA (5 mg/kg) significantly increased the duration of total social interaction between two conspecifics meeting for the first time. Microanalysis showed that MDMA increased adjacent lying and approach behaviours while reducing anogenital sniffing. MDMA (5 mg/kg) also caused elements of the serotonin syndrome including low body posture and piloerection. In the emergence test, MDMA significantly increased hide time and emergence latency indicating increased anxiety-like behavior. Pretreatment with the 5HT 1A receptor antagonist, WAY 100635 (1 mg/kg), prevented MDMA-induced increases in social interaction and markers of the serotonin syndrome while the 5-HT 1B receptor antagonist GR 55562 (1 mg/kg) and 5-HT 2A receptor antagonist ketanserin (1 mg/kg) were ineffective. The 5-HT 2B/2C receptor antagonist, SB 206553 (2 mg/kg), prevented MDMA-induced prosocial effects but caused pronounced thigmotaxis (hyperactivity at the periphery of the testing chamber). The anxiogenic effect of MDMA on the emergence test was not prevented by pretreatment with any of the 5-HT receptor antagonists tested. These results indicate that prosocial effect of MDMA may involve 5-HT 1A and possibly 5-HT 2B/2C receptors. In contrast, MDMA-induced generalised anxiety, as measured by the emergence test, seems unlikely to involve the 5-HT 1A, 5-HT 1B or 5-HT 2A, 5-HT 2B or 5-HT 2C receptors.

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.

Decreased social behaviour following 3,4-methylenedioxymethamphetamine (MDMA) is accompanied by changes in 5-HT2A receptor responsivity

This study examined the involvement of the 5-HT(2A) receptor in the long-term anxiogenic effect of a brief exposure of young rats to 3,4-methylenedioxymethamphetamine (MDMA) using the social interaction and elevated plus-maze paradigms. Wistar rats (post-natal day (PND) 28) received either MDMA (5 mg/kg i.p.) or saline (1 ml/kg i.p.) hourly for 4 h on 2 consecutive days. Locomotor activity was measured for 60 min after the first injection and core body temperature was recorded at regular intervals over 4 h. On PND 84, without further drug administration, social interaction was assessed between treatment-matched rat pairs derived from separate litters. On PND 86, rats received either the 5-HT(2A/2C) receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, 1 mg/kg i.p.) or saline and locomotor activity, wet-dog shakes and back muscle contractions were monitored. The change in elevated plus-maze behaviour was assessed following the same injection on PND 87. Acutely, MDMA produced a significant hyperlocomotion and hyperthermia (p<0.01). Following 55 days of abstinence, social interaction was reduced by 27% in MDMA pre-treated rats compared with that in controls (p<0.01). On the elevated plus-maze, pre-treatment with MDMA prevented the anxiogenic effect of DOI. On PND 92, hippocampal, frontal cortical and striatal 5-hydroxytryptamine (5-HT) was significantly reduced in MDMA pre-treated rats by between 16% and 22%, without any accompanying change in [(3)H]paroxetine binding in cortical homogenates. In conclusion, exposure of young rats to repeated MDMA caused serotonin depletion and induced 'anxiety-like' behaviour in the social interaction test accompanied by a long-lasting reduction in specific 5-HT(2A) receptor mediated behaviour.

Different roles of 5-HT2A and 5-HT2C receptors in regulation of female rat paced mating behaviour

Serotonin (5-HT) receptor 2A (5-HT2A) and 2C (5-HT2C) agonists have been reported to facilitate female rat lordosis behaviour. This study investigated the acute effects of the 5-HT2A receptor agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and the 5-HT2C receptor agonist MK-212 (2-chloro-6-(1-piperazinyl) pyrazine) on paced mating behaviour in a population of sexually receptive female rats in order to explore the role of 5-HT2A and 5-HT2C receptors in the mediation of female rat sexual motivation. DOI (0.5 and 1.0 mg/kg) increased female rat sexual motivation during the first of two consecutive copulatory series seen as a tendency towards a decrease in return latencies following ejaculation and decreased inter-intromission intervals. MK-212 generally increased approach latencies. 0.5 mg/kg MK-212 increased post-ejaculatory exit latency, while 1.0 and 2.0 mg/kg MK-212 increased both post-ejaculatory exit latencies and post-ejaculatory return latencies. The possibility that an increased level of anxiety confounded the effects of MK-212 on sexual motivation behaviour is discussed. The results may support the hypothesis that both 5-HT2A and 5-HT2C receptors are important in regulation of female sexual behaviour.

Cytochrome P450 dependent metabolism of the new designer drug 1-(3-trifluoromethylphenyl)piperazine (TFMPP)

1-(3-Trifluoromethylphenyl)piperazine (TFMPP) is a designer drug with serotonergic properties. Previous studies with male Wistar rats (WI) had shown, that TFMPP was metabolized mainly by aromatic hydroxylation. In the current study, it was examined whether this reaction may be catalyzed by cytochrome P450 (CYP)2D6 by comparing TFMPP vs. hydroxy TFMPP ratios in urine from female Dark Agouti rats, a model of the human CYP2D6 poor metabolizer phenotype (PM), male Dark Agouti rats, an intermediate model, and WI, a model of the human CYP2D6 extensive metabolizer phenotype. Furthermore, the human hepatic CYPs involved in TFMPP hydroxylation were identified using cDNA-expressed CYPs and human liver microsomes. Finally, TFMPP plasma levels in the above mentioned rats were compared. The urine studies suggested that TFMPP hydroxylation might be catalyzed by CYP2D6 in humans. Studies using human CYPs showed that CYP1A2, CYP2D6 and CYP3A4 catalyzed TFMPP hydroxylation, with CYP2D6 being the most important enzyme accounting for about 81% of the net intrinsic clearance, calculated using the relative activity factor approach. The hydroxylation was significantly inhibited by quinidine (77%) and metabolite formation in poor metabolizer genotype human liver microsomes was significantly lower (63%) compared to pooled human liver microsomes. Analysis of the plasma samples showed that female Dark Agouti rats exhibited significantly higher TFMPP plasma levels compared to those of male Dark Agouti rats and WI. Furthermore, pretreatment of WI with the CYP2D inhibitor quinine resulted in significantly higher TFMPP plasma levels. In conclusion, the presented data give hints for possible differences in pharmacokinetics in human PM and human CYP2D6 extensive metabolizer phenotype subjects relevant for risk assessment.

New designer drug 1-(3-trifluoromethylphenyl) piperazine (TFMPP): gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry studies on its phase I and II metabolism and on its toxicological detection in rat urine

Studies are described on the phase I and II metabolism and the toxicological analysis of the piperazine-derived designer drug 1-(3-trifluoromethylphenyl)piperazine (TFMPP) in rat urine using gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS). The identified metabolites indicated that TFMPP was extensively metabolized, mainly by hydroxylation of the aromatic ring and by degradation of the piperazine moiety to N-(3-trifluoromethylphenyl)ethylenediamine, N-(hydroxy-3-trifluoromethylphenyl)ethylenediamine, 3-trifluoromethylaniline, and hydroxy-3-trifluoromethylaniline. Phase II reactions included glucuronidation, sulfatation and acetylation of phase I metabolites. The authors' systematic toxicological analysis (STA) procedure using full-scan GC/MS after acid hydrolysis, liquid-liquid extraction and microwave-assisted acetylation allowed the detection of TFMPP and its above-mentioned metabolites in rat urine after single administration of a dose calculated from the doses commonly taken by drug users. Assuming similar metabolism, the described STA procedure should be suitable for proof of an intake of TFMPP in human urine.

Discriminative stimulus effects of m-chlorophenylpiperazine as a model of the role of serotonin receptors in anxiety

Serotonin is known to play a role in anxiety. The roles of serotonin reuptake and 5-HT1A receptors have been well characterized, but the contribution of other serotonin receptor subtypes is not as clear. 1-(3-Chlorophenyl)-piperazine (mCPP), which binds non-selectively to a wide range of serotonin receptors, has often been used to produce anxiety in humans and in animal models. Because functional assays indicate that mCPP is significantly more potent at 5-HT2C receptors, it may serve as a tool to investigate the contribution of 5-HT2C receptors to anxiety. This paper reviews the results of behavioral tests using mCPP, including the drug discrimination assay, to model anxiety. Although the discriminative stimulus effects of mCPP do not seem to be a useful screen for general anxiolytics, they do seem to be useful for characterization of the contribution of 5-HT1B and 5-HT2C receptors to the mediation of anxiety-like behaviors.

The neurobiology and control of anxious states

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

Induction of c-Fos expression in specific areas of the fear circuitry in rat forebrain by anxiogenic drugs

BACKGROUND

The fact that induction of anxiety- and panic-related symptoms is a property common to a range of drugs suggests that common neural substrates underlie their behavioral effects.

METHODS

We used Fos immunocytochemistry to test the effects of four anxiogenic drugs (FG-7142, yohimbine, m-chlorophenylpiperazine [mCPP], and caffeine) on anxiety-related circuitry in rat forebrain.

RESULTS

All four drugs commonly increased Fos-like immunoreactivity in 7 of 41 brain areas investigated, namely, central nucleus of the amygdala, bed nucleus of the stria terminalis, lateral septum, paraventricular nucleus of the hypothalamus, lateral hypothalamus, infralimbic and prelimbic cortex. All drugs but one (mCPP) also increased Fos expression in the basolateral and medial amygdala, the dorsomedial hypothalamus, cingulate cortex, and parts of the motor cortex.

CONCLUSIONS

The results suggest that the anxiogenic drugs selected activate a restricted set of forebrain areas. Most of these areas have previously been shown to be activated by environmentally evoked anxiety and to have anatomic connections with hindbrain regions that are activated by the same drugs and by environmentally evoked anxiety. Together, these data are consistent with the theory of an integrated forebrain and hindbrain neuronal system that is important for anxiety states evoked by both drug and environmental manipulations.

Effects of GABA(A) compounds on mCPP drug discrimination in rats

Male Long-Evans rats were trained to discriminate mCPP (1.4 mg/kg, i.p.) from saline, using a two-lever, food-reinforced operant task. The GABA(A) antagonist, bicuculline (0.16-0.64 mg/kg), partially substituted for mCPP, whereas the benzodiazepine antagonist, flumazenil (1-10 mg/kg), and the benzodiazepine inverse agonist, Ro 15-4513 (0.25-2.5 mg/kg), failed to substitute for mCPP. Bicuculline produced no change in response rate, whereas Ro 15-4513 dose-dependently decreased responding. Flumazenil produced a small increase in response rates. Flumazenil (10 mg/kg), Ro 15-4513 (1.25 mg/kg), and the benzodiazepine agonists alprazolam (0.64 mg/kg) and diazepam (5 mg/kg) full agonist all failed to block the mCPP discriminative stimulus. When given in combination with mCPP, Ro15-4513 and alprazolam both produced lower response rates than did mCPP alone, whereas flumazenil and diazepam did not significantly alter response rates. These findings provide evidence that GABA(A) antagonists modulate the discriminative stimulus effects of mCPP, but that these effects are not mediated by activity at the benzodiazepine site.

The discriminative stimulus effects of pentylenetetrazol as a model of anxiety: recent developments

Pentylenetetrazol (PTZ), a GABA(A) receptor antagonist and prototypical anxiogenic drug, has been extensively utilized in animal models of anxiety. PTZ produces a reliable discriminative stimulus which is largely mediated by the GABA(A) receptor. Several classes of compounds can modulate the PTZ discriminative stimulus including 5-HT(1A), 5-HT(3), NMDA, glycine, and L-type calcium channel ligands. Spontaneous PTZ-lever responding is seen in trained rats during withdrawal from GABA(A) receptor compounds such as chlordiazepoxide and diazepam, and also ethanol, morphine, nicotine, cocaine, haloperidol, and phencyclidine. This effect is largely mediated by the GABA(A) receptor, which suggests that anxiety may be part of a generalized withdrawal syndrome mediated by the GABA(A) receptor. There are also important hormonal influences on PTZ. Corticosterone plays some role in mediation of its anxiogenic effects. There is a marked sex difference in response to the discriminative stimulus effects of PTZ, and estrogens appear to protect against its anxiogenic effects. Further work with the PTZ drug discrimination is warranted for characterization of anxiety during withdrawal, and the hormonal mechanisms of anxiety.

Serotonergic regulation of inhibitory avoidance and one-way escape in the rat elevated T-maze

It has been proposed that distinct 5-HT pathways modulate different types of anxiety. Activation of the ascending dorsal raphe (DR)-5-HT pathway, innervating the amygdala and frontal cortex, would facilitate learned defensive behaviors. On the other hand, activation of the DR-periventricular 5-HT pathway, which innervates the dorsal periaqueductal gray matter (DPAG), would inhibit innate flight or fight reactions. Dysfunction of these pathways has been suggested to relate to generalized anxiety disorder (GAD) and panic disorder (PD) in humans, respectively. The elevated T-maze has been developed to separate conditioned (inhibitory avoidance) from unconditioned (escape) defensive responses in the same rat. Pharmacological validation of this model has shown that the GAD-effective serotonergic anxiolytic buspirone or the putative anxiolytic ritanserin selectively impaired inhibitory avoidance while leaving one-way escape unchanged. Chronic injection of the 5-HT/noradrenaline reuptake inhibitor imipramine impaired inhibitory avoidance and prolonged escape, an effect that may be related to the therapeutic action of this drug on both GAD and PD. Like imipramine, intra-DPAG injection of the 5-HT(1A) agonist 8-OH-DPAT impaired both inhibitory avoidance and one-way escape. Intra-DPAG administration of the 5-HT(2A/2C) agonist DOI prolonged escape, without affecting inhibitory avoidance. The reversible inactivation of the DRN by muscimol impaired inhibitory avoidance, while facilitating escape from the open arm. Taken together, these results suggest that 5-HT exerts differential control on inhibitory avoidance and escape response in the elevated T-maze, mobilizing different types of 5-HT receptors in key structures implicated in fear/anxiety.

Neuroanatomical targets of anxiogenic drugs in the hindbrain as revealed by Fos immunocytochemistry

It is speculated that specific hindbrain transmitter pathways centred on the periaqueductal gray and locus coeruleus are an important integrative neural substrate for the expression of anxiety and the somatic symptoms and cardiovascular changes that accompany severe anxiety states, such as in panic disorder. Here we investigated the effects of various drugs, known to induce panic in humans and to be anxiogenic in animals, on Fos expression in the periaqueductal gray, locus coeruleus and other parts of the rat hindbrain. The drugs tested were the benozodiazepine inverse agonist FG-7142, the alpha(2)-adrenoceptor antagonist yohimbine, the non-selective 5-hydroxytryptamine(2C) receptor agonist m-chlorophenyl piperazine, the adenosine antagonist caffeine and the cholecystokinin analogue BOC-CCK(4). A clear-cut finding was that administration of each anxiogenic drug caused a striking region-specific pattern of Fos expression within the hindbrain. In particular, the drugs commonly increased Fos-like immunoreactivity in the periaqueductal gray and locus coeruleus. Increased Fos expression in the periaqueductal gray was specific to the rostral dorsolateral and caudal ventrolateral regions. All the anxiogenic drugs also increased Fos-like immunoreactivity in the lateral parabrachial nucleus and nucleus of the solitary tract and all but one (BOC-CCK(4)) increased Fos in the dorsal raphe nucleus. Rats habituated to the test environment and injected with saline vehicle displayed little or no Fos-like immunoreactivity in the hindbrain areas investigated. In summary, each of the anxiogenic drugs tested (FG-7142, yohimbine, m-chlorophenyl piperazine, caffeine and BOC-CCK(4)) increased Fos expression in a restricted number of hindbrain regions, including the periaqueductal gray and locus coeruleus. Previous Fos studies have found that these same regions are activated by various fearful environmental stimuli. Therefore, a specific set of hindbrain circuits may be commonly involved in the processing of anxiety-related information evoked by pharmacological and environmental manipulation. The present findings also raise the possibility that measurement of the effect of anxiogenic drugs on Fos expression might be a useful way to model hindbrain pathways activated by anxiety and possibly panic.

Effects of ritanserin on ethanol withdrawal-induced anxiety in rats

This study investigated the ability of ritanserin, a 5-HT2 antagonist, to modify ethanol withdrawal (EW) symptoms in two animal models of anxiety: the elevated plus-maze (EPM) and the pentylenetetrazol (PTZ) discrimination assay. Long-Evans hooded rats were given a nutritionally balanced liquid diet containing 4.5% ethanol for 10 days. Twelve hours after removal of the ethanol diet, rats were tested in the EPM. A significant reduction in the open-arm activity and the number of total arm entries was observed, which is indicative of EW. Acute ritanserin (0.16-0.64 mg/kg, i.p., 60 min) had no effect on EW-induced anxiety-like behavior on the EPM. Ritanserin (0.08-0.64 mg/kg, i.p., b.i.d. 12 h) administered concurrently with the last 5 days of ethanol diet produced an increase in the time spent on the open arms of the EPM and reversed the EW-induced reduction in total arm entries. Rats trained to discriminate between saline and PTZ (an anxiogenic drug), selected the PTZ lever during EW. Chronic ritanserin (0.32 mg/kg, i.p., b.i.d. ) did not block PTZ lever responding during EW. On the rotorod, ritanserin (0.32 mg/kg, i.p.) increased the motor incoordination induced by ethanol. In conclusion, coadministration of ritanserin with ethanol prevented the development of EW-induced anxiety as measured by the EPM, but not in the PTZ drug discrimination.

Psychological stress-induced enhancement of brain lipid peroxidation via nitric oxide systems and its modulation by anxiolytic and anxiogenic drugs in mice

We investigated the effect of psychological stress on lipid peroxidation activity in the mouse brain, the mechanism underlying the psychological stress-induced change in the activity, and the effects of anxiolytic and anxiogenic drugs on the activity in psychologically-stressed animals. Psychological stress exposure using a communication box paradigm for 2-16 h significantly increased the content of thiobarbituric acid reactive substance (TBARS), an index of lipid peroxidation activity, in the brain, and the effect was maximal after peaked by a 4-h stress exposure. In the animals stressed for over 4 h, the increased brain TBARS content lasted for 30 min after the stress exposure, while no significant increase of the TBARS content was observed in the liver or serum. Trolox (67.6 mg/kg, i.p.), an antioxidant drug, but not monoamine oxidase inhibitors, clorgyline (2.5-5 mg/kg, i.p.) or 5-(4-benzylphenyl)-3-(2-cyanoethyl)-(3H)-1,3,4-oxadiazol-2-o ne (1-5 mg/kg, i.p.), significantly suppressed the effect of psychological stress. The non-selective nitric oxide (NO) synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 10-100 mg/kg, i.p.) and the selective neuronal NOS inhibitor 7-nitroindazole (25 and 50 mg/kg, i.p.), but not the inducible NOS inhibitor aminoguanidine (1-100 mg/kg, i.p.), dose dependently suppressed the psychological stress-induced enhancement of lipid peroxidation in the brain. L-Arginine (300 mg/kg, i.p.), a substrate of NOS, antagonized the effect of L-NAME. Measurements of NO metabolites revealed a significant increase of NO production in the brains of stressed mice. The benzodiazepine (BZD) receptor agonist diazepam (0.05-0.5 mg/kg, i.p.), the 5-HT(1A) receptor agonists (+/-)-8-hydroxy-di-propylaminotetralin and buspirone (0.1-1 mg/kg, i. p.), but not the 5-HT(3) receptor agonist MDL72222, dose-dependently suppressed the psychological stress-induced enhancement of brain lipid peroxidation. In contrast, the administration of anxiogenic drugs, FG7142 (an inverse BZD agonist: 1-10 mg/kg, i.p.) and 1-(3-chlorophenyl)piperazine (a mixed 5-HT(2A/2B/2C) agonist: 0.1-1 mg/kg, i.p.), potentiated it. The effects of diazepam and FG7142 were abolished by the BZD receptor antagonist flumazenil (10 mg/kg, i.p.). These results indicate that psychological stress causes oxidative damage to the brain lipid via enhancing constitutive NOS-mediated production of NO, and that drugs with a BZD or 5-HT(1A) receptor agonist profile have a protective effect on oxidative brain membrane damage induced by psychological stress.