Modification of the anxiolytic action of 5-HT1A compounds by GABA-benzodiazepine agents in rats

Neuromodulation by the immune system: a focus on cytokines

Interactions between the immune system and the nervous system have been described mostly in the context of diseases. More recent studies have begun to reveal how certain immune cell-derived soluble effectors, the cytokines, can influence host behaviour even in the absence of infection. In this Review, we contemplate how the immune system shapes nervous system function and how it controls the manifestation of host behaviour. Interactions between these two highly complex systems are discussed here also in the context of evolution, as both may have evolved to maximize an organism's ability to respond to environmental threats in order to survive. We describe how the immune system relays information to the nervous system and how cytokine signalling occurs in neurons. We also speculate on how the brain may be hardwired to receive and process information from the immune system. Finally, we propose a unified theory depicting a co-evolution of the immune system and host behaviour in response to the evolutionary pressure of pathogens.

GABAA/benzodiazepine receptor complex mediates the anxiolytic-like effect of Montanoa tomentosa

ETHNOPHARMACOLOGICAL RELEVANCE

Montanoa tomentosa also named Cihuapatli is a native plant of Mexico that has been used in traditional medicine for the last five centuries mainly as a remedy for reproductive impairments. However, there are reports indicating that this plant was also consumed by Mexican ancient people for its relaxing properties. In order to corroborate this information, the present study was conducted to evaluate the effect of Montanoa tomentosa lyophilisate (MT) on rat׳s anxiety-like behavior and to analyze its mechanism of action.

MATERIALS AND METHODS

The anxiolytic-like action of MT (1.5, 3.0, 6.0 and 12.0 mg/kg) was investigated in male Wistar rats tested in three animal models of anxiety: the burying behavior, the elevated plus maze and the hole-board tests. As a positive control, the anti-anxiety effects of different doses of the selective GABAA receptor agonist muscimol were also analyzed. In order to evaluate the participation of the GABAA and oxytocin receptors in the anxiolytic-like actions of MT, the GABAA receptors blockers picrotoxin (0.25 and 0.50 mg/kg), bicuculline (2.0 mg/kg) and flumazenil (5.00 and 10.0 mg/kg), the neurosteroid inhibitor finasteride (50.0 and 100 mg/kg) and the oxytocin receptor antagonist atosiban (0.25 µg) were used. Finally, to evaluate general activity, and motor coordination, the open field and rota-rod tests were used.

RESULTS

MT at 3.0 mg/kg showed anxiolytic-like effects in the three anxiety paradigms without affecting reactivity, general motor activity or motor coordination; however, at higher doses sedative effects were observed. Picrotoxin (0.25 and 0.50 mg/kg), flumazenil (10.0 mg/kg) and finasteride (100 mg/kg) antagonized the anxiolytic-like actions of MT in the burying behavior test. In the plus maze and hole-board tests bicuculline (2.0 mg/kg) blocked the effects of the plant as well. Atosiban (0.25 µg) did not antagonize the anxiolytic-like actions of MT.

CONCLUSIONS

The results corroborate the anxiolytic-like actions of Montanoa tomentosa and suggest that this effect is mediated through GABAA receptors but not oxytocin receptors.

A possible mechanism for the anxiolytic-like effect of gallic acid in the rat elevated plus maze

This work was performed to characterize the possible mechanisms involved in the anxiolytic-like activity of gallic acid (GA) in the rat elevated plus maze (EPM) test. Male Wistar rats were acutely treated with a single dose of GA (10-500 mg/kg, i.p.) or diazepam and buspirone, 30 min prior to behavioral assessment in the EPM, open-field and rotarod tests. Treatment with GA markedly produced an increase in the time spent and entries in the open arms of EPM at doses of 30 and 300 mg/kg, respectively. These effects were comparable to those of the diazepam (1 mg/kg, i.p.) and buspirone (1 mg/kg, i.p.). Pretreatment with benzodiazepine antagonist flumazenil (3 mg/kg, i.p.) partially blocked the anxiolytic-like effect of GA. However, an increase in the time spent and entries in the open arms of EPM observed with GA treatment were significantly inhibited by the 5-HT1A receptor antagonist WAY-100635 (0.5 mg/kg, i.p.). In the open-field test, only GA at a dose of 500 mg/kg decreased locomotor activity in rats. Moreover, GA (10-300 mg/kg, i.p.) or diazepam and buspirone did not alter motor coordination in the rotarod test. These results indicate that GA is an effective anxiolytic agent at low doses, while at the highest dose it has sedative effect. Also this study suggests that the anxiolytic-like activity of GA is primarily mediated by the 5-HT1A but not benzodiazepine receptors.

Sex- and endocrine-stage-differences in middle-aged rats in an animal model of OCD

Various clinical studies suggest that many features of OCD are influenced by sex, age and fluctuations in hormonal levels. Animal models have confirmed these differences, and suggest they are mediated by the serotonergic system. We compared the perseveration behavior in a T-maze after the administration of the 5-HT1A agonist, 8-OH-DPAT (2.0 mg/kg) and the preventive action of subchronic fluoxetine (10 mg/kg, 3 times) in middle-aged (11-14 months) males and female rats in two endocrine states: irregular cycles (tested in diestrus) or persistent diestrus. After 8-OH-DPAT, females with persistent diestrus presented higher perseveration scores than males and females with irregular cycles. Fluoxetine produced an anticompulsive-like effect only in females with persistent diestrus. Females in persistent diestrus showed higher estradiol levels than those in irregular cycles or males. In all groups 8-OH-DPAT increased ambulation and fluoxetine did not modify this action. In males, the combined administration of fluoxetine and 8-OH-DPAT impaired motor coordination. Data are discussed on the basis of estradiol levels and sex differences.

The role of the serotonergic and GABA system in translational approaches in drug discovery for anxiety disorders

There is ample evidence that genetic factors play an important role in anxiety disorders. In support, human genome-wide association studies have implicated several novel candidate genes. However, illumination of such genetic factors involved in anxiety disorders has not resulted in novel drugs over the past decades. A complicating factor is the heterogeneous classification of anxiety disorders in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) and diverging operationalization of anxiety used in preclinical and clinical studies. Currently, there is an increasing focus on the gene × environment (G × E) interaction in anxiety as genes do not operate in isolation and environmental factors have been found to significantly contribute to the development of anxiety disorders in at-risk individuals. Nevertheless, extensive research on G × E mechanisms in anxiety has not resulted in major breakthroughs in drug discovery. Modification of individual genes in rodent models has enabled the specific study of anxiety in preclinical studies. In this context, two extensively studied neurotransmitters involved in anxiety are the gamma-aminobutyric acid (GABA) and 5-HT (5-hydroxytryptamine) system. In this review, we illustrate the complex interplay between genes and environment in anxiety processes by reviewing preclinical and clinical studies on the serotonin transporter (5-HTT), 5-HT1A receptor, 5-HT2 receptor, and GABAA receptor. Even though targets from the serotonin and GABA system have yielded drugs with known anxiolytic efficacy, the relation between the genetic background of these targets and anxiety symptoms and development of anxiety disorders is largely unknown. The aim of this review is to show the vast complexity of genetic and environmental factors in anxiety disorders. In light of the difficulty with which common genetic variants are identified in anxiety disorders, animal models with translational validity may aid in elucidating the neurobiological background of these genes and their possible role in anxiety. We argue that, in addition to human genetic studies, translational models are essential to map anxiety-related genes and to enhance our understanding of anxiety disorders in order to develop potentially novel treatment strategies.

Sex differences in the burying behavior test in middle-aged rats: effects of diazepam

The full behavioral profile displayed during the burying behavior test was studied in middle aged (11-14 months) males, females with irregular estrous cycles, and females in persistent diestrus, with and without diazepam (0.5-2.0mg/kg). Ambulation and motor coordination were also tested to discern behavioral changes from general motor alterations. Without diazepam treatment, middle-aged males showed longer burying behavior latencies, more prod explorations and less freezing than both groups of females. Untreated middle aged males also showed less cumulative burying and more immobility compared to females with irregular cycles. None of the parameters showed any difference between the female groups. Diazepam (0.5 and 1.0mg/kg) increased burying behavior latency in females, but had no effect on any parameter in middle aged males. However, a higher dose (2.0mg/kg) of diazepam increased immobility, freezing and the number of prod shocks and decreased prod explorations and groomings, but impaired motor coordination in males. In contrast with young males and females, diazepam at any dose reduced cumulative burying. Data are discussed on the bases of (1) sex and age differences in burying behavior and on (2) the anxiolytic-like action of diazepam and its side effects.

5-HT1A receptor blockade reverses GABA(A) receptor alpha3 subunit-mediated anxiolytic effects on stress-induced hyperthermia

RATIONALE

Stress-related disorders are associated with dysfunction of both serotonergic and GABAergic pathways, and clinically effective anxiolytics act via both neurotransmitter systems. As there is evidence that the GABA(A) and the serotonin receptor system interact, a serotonergic component in the anxiolytic actions of benzodiazepines could be present.

OBJECTIVES

The main aim of the present study was to investigate whether the anxiolytic effects of (non-)selective alpha subunit GABA(A) receptor agonists could be reversed with 5-HT(1A) receptor blockade using the stress-induced hyperthermia (SIH) paradigm.

RESULTS

The 5-HT(1A) receptor antagonist WAY-100635 (0.1-1 mg/kg) reversed the SIH-reducing effects of the non-alpha-subunit selective GABA(A) receptor agonist diazepam (1-4 mg/kg) and the GABA(A) receptor alpha(3)-subunit selective agonist TP003 (1 mg/kg), whereas WAY-100635 alone was without effect on the SIH response or basal body temperature. At the same time, co-administration of WAY-100635 with diazepam or TP003 reduced basal body temperature. WAY-100635 did not affect the SIH response when combined with the preferential alpha(1)-subunit GABA(A) receptor agonist zolpidem (10 mg/kg), although zolpidem markedly reduced basal body temperature.

CONCLUSIONS

The present study suggests an interaction between GABA(A) receptor alpha-subunits and 5-HT(1A) receptor activation in the SIH response. Specifically, our data indicate that benzodiazepines affect serotonergic signaling via GABA(A) receptor alpha(3)-subunits. Further understanding of the interactions between the GABA(A) and serotonin system in reaction to stress may be valuable in the search for novel anxiolytic drugs.

The group III mGlu receptor agonist ACPT-I exerts anxiolytic-like but not antidepressant-like effects, mediated by the serotonergic and GABA-ergic systems

Our earlier studies have demonstrated that (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid ACPT-I, a group III mGlu receptor agonist, produced anxiolytic-like and antidepressant-like actions after central administration. Here we describe the anxiolytic-like effects of ACPT-I after intraperitoneal administration in the stress-induced hyperthermia (SIH), elevated plus-maze (PMT) tests in mice and in the Vogel test in rats. However, the compound did not produce antidepressant-like effects in the tail suspension test (TST) or in the forced swim test (FST) in mice. The potential anxiolytic effect of ACPT-I (20 mg/kg) in the SIH test was inhibited by the benzodiazepine receptor antagonist flumazenil (given i.p., 10 mg/kg), and by a 5-HT(1A) receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl) cyclohexane-carboxamide (WAY100635) (0.1 mg/kg s.c.). At the same time, ritanserin (0.5 mg/kg i.p.), the 5-HT2A/C receptor antagonist, did not change the anxiolytic-like effects of ACPT-I. The results of these studies indicate that the GABA-ergic and serotonergic systems are involved in the potential anxiolytic action of ACPT-I.

Animal models of anxiety and anxiolytic drug action

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

GABA(A) signalling is involved in N/OFQ anxiolytic-like effects but not in nocistatin anxiogenic-like action as evaluated in the mouse elevated plus maze

Nociceptin/orphanin FQ (N/OFQ) and nocistatin are two neuropeptides originated from the same precursor prepronociceptin/orphanin FQ (ppN/OFQ). N/OFQ is the endogenous ligand of the NOP receptor, while the target of action of nocistatin is still unknown. N/OFQ modulates various biological functions, including anxiety. Conversely, nocistatin either behaves as a functional N/OFQ antagonist or evokes per se effects opposite to those of N/OFQ. Here we investigated the interaction between the anxiolytic-like effects of N/OFQ and the anxiogenic-like action of nocistatin with those evoked by GABA(A) receptor ligands in the mouse elevated plus maze. The anxiogenic-like effects of the GABA(A) receptor antagonist pentylenetetrazol (20mg/kg; intraperitoneal, i.p.) were abolished by the co-treatment with N/OFQ (10pmol; intracerebroventricular, i.c.v.) while potentiated by the administration of nocistatin (0.01pmol; i.c.v.). The anxiolytic-like effects of the benzodiazepine receptor agonist diazepam (0.75mg/kg, i.p.) were reversed by nocistatin (0.1pmol; i.c.v.), whereas signs of sedation were observed when mice were co-treated with diazepam and N/OFQ (3pmol). Interesting enough, the i.p. treatment with flumazenil (1mg/kg) blocked the anxiolytic-like effects of N/OFQ (10pmol; i.c.v.), but not the anxiogenic effect elicited by nocistatin. Collectively, our findings suggest that the effects on anxiety elicited by pentylenetetrazol and diazepam can be counteracted or potentiated in the presence of N/OFQ and nocistatin. In addition, the effects on anxiety of N/OFQ, but not nocistatin, appear to be dependent on the benzodiazepine site of the GABA(A) receptor.

Translational aspects of pharmacological research into anxiety disorders: the stress-induced hyperthermia (SIH) paradigm

In anxiety research, the search for models with sufficient clinical predictive validity to support the translation of animal studies on anxiolytic drugs to clinical research is often challenging. This review describes the stress-induced hyperthermia (SIH) paradigm, a model that studies the activation of the autonomic nervous system in response to stress by measuring body temperature. The reproducible and robust SIH response, combined with ease of testing, make the SIH paradigm very suitable for drug screening. We will review the current knowledge on the neurobiology of the SIH response, discuss the role of GABA(A) and serotonin (5-HT) pharmacology, as well as how the SIH response relates to infectious fever. Furthermore, we will present novel data on the SIH response variance across different mice and their sensitivity to anxiolytic drugs. The SIH response is an autonomic stress response that can be successfully studied at the level of its physiology, pharmacology, neurobiology and genetics and possesses excellent animal-to-human translational properties.

Anti-anxiety effects of Apocynum venetum L. in the elevated plus maze test

The purpose of this study was to characterize the putative anxiolytic-like activity of an ethanolic extract prepared from the leaves of Apocynum venetum (AV) using the elevated plus maze (EPM) in mice. Male C75BL/6 mice were either treated orally with the AV extract or the positive controls diazepam and buspirone, respectively, 1h before behavioral evaluation in the EPM. A single treatment of AV extract markedly increased the percentage time spent on and the number of entries into the open arms of the EPM in doses of 30 and 125 mg/kg p.o., respectively. This effect was comparable to that of the benzodiazepine diazepam (1.5 mg/kg p.o.) and the 5-HT(1A) agonist buspirone (10 mg/kg p.o.). The effects of AV in 125 mg/kg were effectively antagonized by the benzodiazepine antagonist flumazenil (3 mg/kg i.p.). However, the effects of AV extract could only partially be blocked by the unspecific 5-HT(1A) receptor antagonist WAY-100635 (0.5 mg/kg i.p.). Neither diazepam and buspirone nor the AV extract produced any overt behavioral change or motor dysfunction in the open field test. These results indicate that AV extract is an effective anxiolytic agent, and suggest that the anxiolytic-like activities of this plant are mainly mediated via the GABAergic system.

Indorenate modifies a1-adrenergic and benzodiazepine receptor binding in the rat brain: an autoradiography study

Indorenate (5-methoxytryptamine-beta-methylcarboxylate) is a 5-HT1A receptor agonist that produces antihypertensive, anxiolytic, antidepressant and anticonvulsant effects. However, there is evidence suggesting that these effects could involve the activation of benzodiazepine (BZD) receptors but not the activation of a1-adrenergic receptors. The goal of this study was to analyse the effect of indorenate on a1-adrenergic and BZD receptor binding in specific rat brain areas by using in-vitro autoradiography. Coronal brain sections from male Wistar rats were used for labelling 5-HT1A (3H-8-OH-DPAT, 2 nM), a1-adrenergic (3H-prazosin, 2 nM) and BZD (3H-flunitrazepam, 2 nM) receptor binding in the presence or absence of indorenate (1 microM). Indorenate totally displaced 3H-8-OH-DPAT binding in all the brain areas evaluated. It decreased 3H-prazosin binding just in the frontal (30%) and sensorimotor (32%) cortices and in the thalamus (21%). Additionally, indorenate diminished 3H-flunitrazepam binding only in the cingulate (16%) and piriform (18%) cortices as well as in the dorsal raphe nucleus (18%). These results confirm that indorenate is a 5-HT1A ligand and suggest the possible participation of a1-adrenergic and BZD receptors in its pharmacological properties.

Anxiolytic profile of HG1, a 5-HT-moduline antagonist, in three mouse models of anxiety

HG1 is a new 5-HT-moduline antagonist which is itself an endogenous tetrapeptide specifically acting as an antagonist of 5-HT(1B) auto- and heteroreceptors. Blockade of endogenous 5-HT-moduline might provoke anxiolysis, so it could be a new therapeutic target in anxiety disorders. The aim of our study was to examine the effects of HG1 in three mouse models of anxiety: the four plates test (FPT), the black and white (B&W) model and the elevated plus maze (EPM). Male Swiss mice were intraperitoneally and acutely administered HG1 at the doses of 8, 16, 32 and 64 mg/kg. In these three tests, HG1 exhibited an anxiolytic profile similar to that of diazepam, the referential benzodiazepine compound, without affecting locomotor activity. In the three models used, HG1 was as efficient as benzodiazepine and may consequently exert its anxiolytic effects via the GABA-ergic system. We cannot exclude that it might also act through 5-HT receptors and rather have the profile of a selective serotonin reuptake inhibitor.

Anxiolytic-like effects of MTEP, a potent and selective mGlu5 receptor agonist does not involve GABA(A) signaling

Several lines of evidence suggest a crucial involvement of glutamate in the mechanism of action of anxiolytic drugs including the involvement of group I metabotropic glutamate (mGlu) receptors. Given the recent discovery of a selective and brain penetrable mGlu5 receptor antagonists, the effect of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP), i.e. the most potent mGlu5 antagonist, was evaluated in established models of anxiety after single or repeated administration. We also studied if the anxiolytic effect of MTEP is mediated by mechanism involving the GABA-benzodiazepine (BZD) receptor complex. Experiments were performed on male Wistar rats or male Albino Swiss mice. The anxiolytic-like effects of MTEP were tested in the conflict drinking test and the elevated plus-maze test in rats as well as in the four-plate test in mice. MTEP (0.3-3.0 mg/kg) induced anxiolytic-like effects in the conflict drinking test (after single and repeated administration) and in the elevated plus-maze test in rats. In the four-plate test in mice, it exerted anxiolytic activity at a dose of 20 mg/kg. MTEP had no effect on the locomotor activity of animals. The anxiolytic-like effect of MTEP was not changed by BZD antagonist flumazenil. Moreover, a synergistic interaction between non-effective doses of MTEP and diazepam was observed in the conflict drinking test. These data suggest that selective mGlu5 receptor antagonists mediated anxiolysis is not dependent on GABA-ergic system and that these agents may play a role in the therapy of anxiety.

In the amygdala anxiolytic action of mGlu5 receptors antagonist MPEP involves neuropeptide Y but not GABAA signaling

Several lines of evidence indicate that inhibition of the metabotropic glutamate (mGlu) receptor 5 produces anxiolytic-like effects in rodents. Peptide neurotransmitter neuropeptide Y (NPY) produces an anxiolytic effect in rats after intraventricular or intra-amygdalar administration. Many classes of anxiolytic drugs exert their effect through the GABA-benzodiazepine (BZD) receptor complex. Therefore, in the present study we have investigated whether the anxiolytic action of MPEP (2-methyl-6-(phenylethynyl)pyridyne), an mGlu5 receptor antagonist, is mediated by a mechanism involving either the GABA-BZD receptor complex or NPY receptor. In the behavioral studies, the anxiolytic activity of MPEP (10 mg/kg, i.p.) was examined using plus-maze test. The BZD antagonist flumazenil (10 mg/kg, i.p.) was given to one group of rats and Y1 receptor antagonist BIBO 3304 (((R)-N-[[4-(aminocarbonylaminomethyl) phenyl] methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate)3304) (200 pmol/site, intra-amygdala) to the other. It was found that anxiolytic effects of MPEP were not changed by flumazenil, but were abolished by BIBO 3304. Immunohistochemical studies showed a high density of mGlu5 receptor immunoreactivity (IR) in the amygdala. The effect of MPEP on NPY expression in the amygdala was studied using immunohistochemistry (IH) and radioimmunoassay (RIA). Both methods showed a diminution of NPY IR expression, to about 43% (IH) or 81% (RIA) of the control level after multiple administrations, but we observed an increase up to 148% of the control after single MPEP administration. These effects may suggest a release of NPY from nerve terminals after MPEP administration. Our results indicate that the anxiolytic action of MPEP is conveyed through NPY neurons with the involvement of Y1 receptors in the amygdala and that BZD receptors do not significantly contribute to these effects.

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.

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

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

The neurobiology and control of anxious states

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

Pharmacological characterization of MP349, a novel 5-HT1A-receptor antagonist with anxiolytic-like activity, in mice and rats

The purpose of this study was to further characterize the pharmacological effects of MP349 (trans-1-(2-methoxyphenyl)-4-(4-succinimidocyclohexyl)piperazine), a new serotonin 5-HT(1A) postsynaptic receptor antagonist, using several biochemical and behavioural assays. The silent 5-HT(1A)-receptor antagonist WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide) was used as a reference compound in in-vivo tests, and diazepam served as standard anxiolytic drug in animal models of anxiety. In this study we showed that MP349 bound with moderate affinity (K(i) = 234 nM) for alpha(1)-adrenoceptors, and with very low affinity (K(i) > 2600 nM) for 5-HT(2A), dopamine D(1), D(2) and benzodiazepine receptors. The effects of MP349 on presynaptic 5-HT(1A) receptors were studied in two models (mice and rats). Like WAY 100635, MP349 antagonized the hypothermia induced by the 5-HT(1A)-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin(8-OH-DPAT) in mice. Neither MP349 nor WAY 100635 administered alone induced hypothermia. In a rat microdialysis study, MP349 (like WAY 100635) did not affect 5-HT dialysate level in the prefrontal cortex; however, when given before 8-OH-DPAT, it inhibited the decrease in 5-HT release induced by the 5-HT(1A )agonist. The data demonstrated that MP349 behaved like a functional antagonist of presynaptic 5-HT(1A) receptors. The potential anxiolytic activity of MP349 and reference drugs was examined in a conflict drinking test in rats, a plus-maze test in rats and a four-plate test in mice. MP349 and WAY 100635 produced anxiolytic-like effects, though somewhat weaker than those induced by diazepam, and only in the case of diazepam the anxiolytic-like effects were dose-dependent. Moreover, MP349 administered in doses inducing anxiolytic-like effects did not disturb the locomotor activity (open field test) or locomotor coordination (rota-rod test) of rats. These and earlier results indicated that MP349 was an antagonist of 5-HT(1A) receptors which exhibited anxiolytic-like activity in an animal model of anxiety.

Serotonin-GABA interactions in the modulation of mu- and kappa-opioid analgesia

In the present study, we studied the interaction between serotonergic (5-HTergic) and gamma-aminobutyric acid (GABA)-ergic systems in the modulation of analgesia from morphine, a mu-opioid agonist, and U50,488, a kappa-opioid agonist. All experiments were performed in mice using the 49 degrees C tail-withdrawal assay. The benzodiazepine receptor agonist, diazepam, the serotonin synthesis inhibitor, para-chlorophenylalanine (p-CPA), and the 5-HT(1A) receptor agonist, 8-OH-DPAT, were all found to attenuate morphine and U50,488 analgesia. In each case, the attenuation was itself blocked by treatment with L-5-HTP, a serotonin precursor, bicuculline, a GABA(A) receptor antagonist or picrotoxin, a GABA(A)-gated chloride channel blocker. Neither L-5-HTP nor the GABA(A) receptor antagonists were found to affect morphine or U50,488 analgesia per se. Thus, these findings indicate that a benzodiazepine-GABAergic agent (diazepam) attenuates opioid analgesia through the serotonergic system, and antiserotonergic agents (8-OH-DPAT, p-CPA) attenuate opioid analgesia through the GABAergic system. The intimate interactions between GABA and serotonin in the present study further suggest that these neurotransmitters work in complex ways together rather than alone in the modulation of opioid analgesia.

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

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

Further evidence that the discriminative stimulus properties of indorenate are mediated by 5-HT 1A/1B/2C receptors

Indorenate (5-methoxytryptamine beta-methylcarboxylate, INDO) is a serotonin (5-hydroxytryptamine, 5-HT) agonist that has affinity for 5-HT(1A/1B/2C) receptors. Unlike other anxiolytics such as 5-HT receptor agonists, INDO may not share tolerance or dependency with the benzodiazepine anxiolytics. It has been reported that the discriminative stimulus properties of 5-HT(1A/1B/2C) agonists, but not those of 5-HT(3/4) agonists, generalize to INDO. Therefore, the aim of the present study was to obtain further evidence on the differential involvement of 5-HT(1A/1B/2C) receptors in the discriminative stimulus properties of INDO by evaluating its interactions with antagonists of the 5-HT(1A), 5-HT(1B), 5-HT(2C), and 5-HT(3/4) receptor subtypes. Rats were trained to discriminate INDO from saline in a conditioned taste aversion paradigm. For Group D(+)S(-), administration of INDO signalled that saccharin flavour was followed by LiCl, while injection of vehicle signalled safe consumption of saccharin solution. Group D(-)S(+) had the contingencies reversed. After this training, rats had generalization tests where INDO administration was preceded by different doses of the following antagonists: WAY100635 (5-HT(1A)), NAN190 (5-HT(1A)), methiothepin (5-HT(1A/1B/2C)), GR127935 (5-HT(1B/1D)), ketanserin (5-HT(2A/2C)), ritanserin (5-HT(2C/2A)), mesulergine (5-HT(2C/2A)), metergoline (5-HT(2C/2A)), SB206553 (5-HT(2B/2C)), and tropisetron (5-HT(3/4)). In Group D(+)S(-), the order of potency to block the discriminative stimulus properties of INDO was WAY100635>ketanserin>ritanserin>GR127935>mesulergine congruent with SB206553>metergoline>methiothepin>NAN190, while in Group D(-)S(+), the order was WAY100635>GR127935>ketanserin>ritanserin>mesulergine congruent with SB206553>metergoline>methiothepin>NAN190. Tropisetron did not produce any alteration of the discriminative control by INDO. These results suggest that the discriminative signal of INDO is mediated by 5-HT(1A/2C/1B) receptors and that blockade of any of its components produces a degradation of its discriminative effects.

Full substitution of the discriminative cue of a 5-HT(1A/1B/2C) agonist with the combined administration of a 5-HT(1B/2C) and a 5-HT(1A) agonist

The present study examined whether animals attend to the individual components of the cue produced by a drug that stimulates different 5-hydroxytryptamine (5-HT) receptor populations, using a drug discrimination task based on the conditioned taste aversion (CTA) procedure. The training drug was indorenate (5-methoxytryptamine beta-methylcarboxylate) (INDO) that has been described as a 5-HT(1A/2C/1B) agonist able to exert discriminative control in both operant and CTA procedures. The principal objective was to examine generalization with the combined administration of agonists for the different receptor sites that may mimic the mechanism of action of the training drug. Male Wistar rats, deprived of water, were trained to discriminate INDO from saline; during the drug trials, the administration of INDO preceded saccharin-LiCl pairings, while, during the saline trials, the administration of saline preceded the saccharin-saline pairings. In generalization tests, INDO, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT(1A) agonist), 1-(3-trifluoromethylphenyl)piperazine (TFMPP, a 5-HT(1B) agonist), alpha-methyl-5-HT (a 5-HT(2C) agonist) or 2-methyl-5-HT (a 5-HT(3) agonist), were administered alone or in combination. The results showed that 8-OH-DPAT, TFMPP and alpha-methyl-5-HT produced dose-dependent generalization, up to 88% in the case of 8-OH-DPAT. The combined administration of the following pairs of drugs, 8-OH-DPAT+TFMPP or 8-OH-DPAT+ alpha-methyl-5-HT, at doses that produced only 15-55% generalization when administered alone, produced greater than 80% generalization to INDO. However, the single administration of 2-methyl-5-HT produced only saline-like responding and its combined administration with 8-OH-DPAT did not modify the generalization produced by the single administration of 8-OH-DPAT. These results suggest that animals attend to the individual components of the drug cue; in the case of INDO, which has three elements, each mediated by a different receptor subpopulation (5-HT(1A), 5-HT(1B) and 5-HT(2C) ), the separate stimulation of at least two receptor subpopulations was 'interpreted' by the subject as the presence of the training drug.

Involvement of benzodiazepine binding sites in an antiaggressive effect by 5-HT(1A) receptor activation in isolated mice

The effect of the benzodiazepine receptor antagonist flumazenil was examined on an antiaggressive effect of (S)-5-[3-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3- benzodioxole HCl (MKC-242), a 5-HT(1A) receptor agonist. MKC-242 (0.1-1.0 mg/kg, p.o.) selectively reduced isolation-induced aggressive behavior in a dose-dependent manner. Flumazenil (10 mg/kg, i.p.) antagonized the antiaggressive effects of MKC-242 and diazepam, although it alone did not affect the behaviors of isolated mice. These findings suggest that a gamma-aminobutyric acid(A) (GABA(A)) receptor system is involved in the antiaggressive effect by 5-HT(1A) receptor activation.

Stress-induced hyperthermia in the 5-HT(1A) receptor knockout mouse is normal

BACKGROUND

Several studies on serotonin 1A (5-HT(1A)) receptor knockout mice in different genetic backgrounds indicate that such mice display a more anxious phenotype than their corresponding wild types. We hypothesized that the 5-HT(1A) receptor knockout mice would show a different phenotype than the wild type mice in the stress-induced hyperthermia (SIH) paradigm, which tests putative anxiolytic effects of drugs. Moreover, on pharmacologic challenges with the 5-HT(1A) receptor agonist flesinoxan we expected an absence of the functional response in knockout mice relative to wild type mice.

METHODS

Effects of the 5-HT(1A) receptor agonist flesinoxan, alone or in combination with the 5-HT(1A) receptor antagonist WAY-100635, and the gamma-aminobutyric acid A (GABA(A))-benzodiazepine receptor agonist diazepam were studied in the SIH paradigm in male 129/Sv 5-HT(1A) receptor knockout and wild type mice. In addition, the effects of flesinoxan on plasma corticosterone concentrations were determined.

RESULTS

Plasma corticosterone concentrations were dose dependently elevated by flesinoxan in wild type mice but not in knockout mice. Flesinoxan dose dependently decreased SIH in wild type mice but not in knockout mice. The flesinoxan effect in wild type mice was blocked by WAY-100635. Furthermore, diazepam decreased SIH in both genotypes. There were no differences in basic SIH responses between wild type and knockout mice.

CONCLUSIONS

5 -HT(1A) receptor knockout mice display a normal SIH response, and results indicate, based on the SIH, that the GABA(A)-benzodiazepine receptor complex functions normally.

Anxiolytic-like actions of toluene in the burying behavior and plus-maze tests: differences in sensitivity between 5-HT(1B) knockout and wild-type mice

This paper compares the anxiolytic-like actions of toluene in two anxiety paradigms, the burying behavior and plus-maze tests, in 5-HT(1B) knockout (KO) and 129/Sv-ter wild-type (WT) mice. Static exposures were conducted in 29-l gas chromatographic jars. Animals were exposed to toluene (0, 1000, 2000 or 4000 ppm; n=8-12, each) for 30 min, and immediately after, tested in one of the anxiety paradigms. Motor coordination was evaluated in the rota-rod test in independent groups of mice. Toluene produced a dose-dependent decrease in anxiety-like levels in both anxiety paradigms and in both the strains. However, toluene exerted its effects at lower concentrations in KO mice than in the WT strain. These results cannot be attributed to a decrease in motor coordination since all the animals behaved similarly in the rota-rod test, regardless of the treatment. To discard any inherent difference in the nociception threshold between strains, mice were tested in the hot plate immediately after being exposed to either air or toluene. Toluene increased nociception in a similar fashion in both the strains. Our results suggest that 5-HT(1B) KO mice are more sensitive to those of toluene's actions related to anxiety, but not to those related with motor coordination or nociception. Data are discussed in terms of toluene's mechanisms of action and on differences between WT and KO animals.

The influence of the benzodiazepine receptor antagonist flumazenil on the anxiolytic-like effects of CGP 37849 and ACPC in rats

In this paper we examined the effect of flumazenil (Ro 15-1788, 10 mg/kg), a benzodiazepine receptor antagonist, on the anticonflict activity of DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, and 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at glycine(B) receptors, in the Vogel conflict drinking test in rats. The effect of flumazenil on the anxiolytic-like (in the plus-maze test) and the anticonvulsant (in the maximal electroshock-induced seizures) activities of CGP 37849 in rats was also studied. Diazepam was used as a reference drug. CGP 37849 (2. 5-5 mg/kg), ACPC (50-200 mg/kg) and diazepam (2.5-5 mg/kg) significantly and dose-dependently increased the number of shocks accepted during experimental sessions in the conflict drinking test. Flumazenil partly but significantly reduced the anticonflict effect of CGP 37849, and it fully blocked the anticonflict effect of ACPC and diazepam. CGP 37849 (2.5-5 mg/kg) and diazepam (2.5-5 mg/kg) were also active in the plus-maze test, as they significantly increased the percentage of the time spent in and entries into the open arms of the plus-maze, both those effects having been antagonized by flumazenil. Flumazenil alone was inactive in both the conflict drinking and the plus-maze tests. In the maximal electroshock-induced seizures, both CGP 37849 (2.5-5 mg/kg) and diazepam (5-10 mg/kg) produced anticonvulsant effects, of which only that of diazepam was antagonized by flumazenil. The results of the present study showing antagonism of flumazenil towards the anxiolytic-like effects of CGP 37849 and ACPC suggest involvement of benzodiazepine receptors in such an activity of the NMDA and glycine(B) receptor ligands, respectively, which may be due to a possible interaction between NMDA and GABA/benzodiazepine systems. The lack of effect of the benzodiazepine antagonist on the anticonvulsant activity of CGP 37849 indicates that involvement of benzodiazepine receptors in the pharmacological action of the NMDA antagonist is not a general phenomenon.

Anxiolytic-Like effect of ejaculation under various sexual behavior conditions in the male rat

The purpose of the present study was to analyze the anxiety-like effect induced by ejaculation in male rats subjected to different sexual behavior conditions. The animal model of anxiety used was the conditioned defensive burying test. Results showed that experimental anxiety was reduced after one or six consecutive ejaculations. Six ejaculations did not induce a larger reduction in burying behavior than that produced by two, suggesting that this effect is not cumulative. This anxiolytic-like effect endured a short period (less than 24 h), and was not accompanied by a reduction in ambulatory behavior. The present results also showed a facilitating action of a previous ejaculation on the reduction in burying behavior induced by a second ejaculatory response. This potentiation occurred with an interval of 24 h between ejaculations. In sexually exhausted rats two populations are distinguished: one sexually unresponsive, and one achieving one ejaculation. Interestingly, in the ejaculatory population no reduction in burying behavior was observed, while in the unresponsive one a diminution in defensive burying was found. Data reveal differences in the anxiolytic-like properties of ejaculation between nonsatiated rats and the two populations of sexually exhausted animals.

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.

Blockade of the anxiolytic-like action of ipsapirone and buspirone, but not that of 8-OH-DPAT, by adrenalectomy in male rats

The effect of the 5-HT1A agonists ipsapirone (5 mg/kg), buspirone (5 mg/kg) and 8-OH-DPAT (0.5 mg/kg) on experimental anxiety was examined in sham-operated, adrenalectomized and adrenally demedullated male rats. The animal model of anxiety used was the defensive burying test. At the doses selected, all 5-HT1A compounds produced an anxiolytic-like action by reducing the burying behavior in both sham-operated and demedullated rats. However, in adrenalectomized subjects, while 8-OH-DPAT still reduced burying behavior, ipsapirone and buspirone lost their action. Data suggest that adrenocortical secretions play a role in the anxiolytic-like actions of buspirone and ipsapirone, but not in those of 8-OH-DPAT. Buspirone and ipsapirone also produced a reduction in burying behavior latency in sham-operated animals that was not observed in adrenalectomized or adrenally demedullated rats. These data suggest that adrenaline may be participating in the action of these compounds on the burying behavior latency. Present findings support possible direct relationships between the stimulation of 5-HT1A receptors and adrenal secretions.