The interaction of R(+)- and S(-)-zacopride with PCPA to modify rodent aversive behaviour

A randomised, double-blind, placebo-controlled, duloxetine-referenced study of the efficacy and tolerability of vortioxetine in the acute treatment of adults with generalised anxiety disorder

AIMS

This study aims to evaluate the efficacy and tolerability of vortioxetine 2.5-, 5- and 10-mg once-daily doses vs. placebo in the treatment of generalised anxiety disorder (GAD).

METHODS

In this 8-week, multicentre, double-blind, placebo-controlled, parallel-group, phase 3 study, patients with a primary GAD diagnosis were randomised to receive placebo (n = 157), vortioxetine 2.5 mg, vortioxetine 5 mg, vortioxetine 10 mg or duloxetine 60 mg once daily (n = 156 each). The primary end-point, mean change from baseline in Hamilton Anxiety Scale (HAM-A) total score and key secondary end-points for the 5- and 10-mg vortioxetine doses were analysed in a prespecified sequential testing procedure (all at week 8). Sexual dysfunction was evaluated using the Arizona Sexual Experiences Scale.

RESULTS

Differences from placebo in the primary efficacy end-point were not statistically significant for the vortioxetine groups. The mean difference from placebo was significant in the duloxetine arm. For all secondary efficacy end-points, results were similar among the vortioxetine groups and did not reach statistical significance. The vortioxetine 10-mg group showed separation from placebo on the Hospital Anxiety and Depression anxiety subscore (nominal p = 0.036). Duloxetine 60 mg significantly improved the primary end-point (p < 0.05 vs. placebo), validating the study. Nausea, dry mouth, diarrhoea, nasopharyngitis, headache, dizziness, somnolence, vomiting, dyspepsia, constipation and fatigue were reported in ≥ 5% of patients receiving vortioxetine. Rates of treatment-emergent sexual dysfunction (TESD) in the vortioxetine dosing groups were similar to placebo.

CONCLUSION

In this study, vortioxetine 2.5-, 5- and 10-mg once-daily doses showed no significant improvement in HAM-A total scores vs. placebo. Vortioxetine was well tolerated at all doses and was not associated with TESD.

The 5-HT3 receptor as a therapeutic target

The 5-HT3 receptor is a neurotransmitter-gated ion channel. It is a member of the Cys-loop family of receptors, which also includes nicotinic acetylcholine, glycine and GABAA receptors. Each member of the family consists of an arrangement of five subunits surrounding a central ion-conducting pore. The 5-HT3 receptor binding site is composed of six loops from two adjacent subunits, and the critical ligand binding residues within these loops are well documented. There are a range of 5-HT3 receptor agonists and competitive antagonists, but it is the antagonists that dominate their clinical use. Studies have proposed a range of disease symptoms that might be amenable to 5-HT3 receptor selective compounds; however, so far only the treatment of emesis and irritable bowel syndrome have been fully realised. In this review, the authors look at the structure, function and distribution of 5-HT3 receptors and how this may influence their role in disease. The authors also describe the existing clinical applications of 5-HT3 antagonists and the future potential of these drugs.

Reduced 5-HT3 receptor binding and lower baseline plus maze anxiety in the alcohol-preferring inbred fawn-hooded rat

The present investigation sought to explore the relationship between the 5-HT(3) receptor and anxiety-like behavior in fawn-hooded (FH/Wjd) rats, an inbred strain that exhibits a high intake and preference for ethanol, and the alcohol-nonpreferring ACI/N strain. Using quantitative autoradiography, we examined whether there were differences in central 5-HT(3) receptor binding in FH/Wjd versus ACI/N rats. Ten to 14 days prior to being used in the autoradiographic studies, rats were first confirmed to be representative of their strains by subjecting them to a two-bottle choice procedure for 2 weeks. The binding of [3H]LY 278584 to 5-HT(3) receptors was significantly reduced in frontal cortex, CA1 region of hippocampus, and in the medial and lateral nuclei of the amygdala of FH/Wjd versus ACI/N rats. In the anterior cingulate cortex and in the dentate gyrus region of the hippocampus the reduction in [3H]LY 278548 binding in the FH/Wjd versus ACI/N strain (40% and 41%, respectively) did not reach statistical significance. In a separate group of animals, the effects of the 5-HT(3) receptor antagonist MDL 72222 (3 mg/kg ip) on anxiety-related behaviors were assessed in the elevated plus maze. In vehicle-treated rats, the FH/Wjd strain exhibited significantly greater percent of time spent on the open arms and percent open arm entries, an indication of less anxiety. Pretreatment with MDL 72222 did not alter these behaviors in the FH/Wjd rats, but had an anxiolytic-like effect in the ACI/N strain, significantly increasing the percent of time spent on the open arms and percent open arm entries. Further research into 5-HT(3) receptor function in the alcohol-preferring FH/Wjd rats is needed to elucidate the relationship among 5-HT(3) receptors, alcohol drinking, and anxiety.

A review of 25 years of the social interaction test

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

The mouse light/dark box test

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

The mouse light-dark paradigm: a review

1. The light/dark paradigm is based on the innate aversion of rodents to brightly illuminated areas and on the spontaneous exploratory behaviour of the animals, applying mild stressors i.e. novel environment and light. The test apparatus consists of a small dark secure compartment (one third) and a large illuminated aversive compartment (two thirds). 2. The test was developed with male mice. The strain, weight and age may be crucial factors. 3. The extent to which an anxiolytic compound can facilitate the exploratory activity depends on the baseline level in the control group. Differences between the type and severity of external stressors might account for variable results reported by different laboratories. 4. In conclusion, the black and white test may be useful to predict anxiolytic-like or anxiogenic-like activity in mice. Transitions have been reported to be an index of activity-exploration because of habituation over time and the time spent in each compartment to be a reflection of aversion. Classic anxiolytics (benzodiazepines) as well as the newer anxiolytic-like compounds (e.g. serotonergic drugs) can be detected using this paradigm. It has the advantages of being quick and easy to use, without requiring the prior training of animals.

Distribution of S(-)-zacopride-insensitive [125I]R(+)-zacopride binding sites in the rat brain and peripheral tissues

Increasing evidence indicates that the 5-HT3 receptor antagonist R(+)-zacopride labels an additional site in brain tissue that is not sensitive to 5-HT (non-5-HT R(+)-zacopride site, R(+)-site). Since the levels of R(+)-sites in the brain are relatively low, the present studies explored the use of [125I]R(+)-zacopride to label the R(+)-site; the incorporation of an [125I] atom considerably increasing the specific activity of the radioligand relative to [3H]R(+)-zacopride that has been utilised previously. Competition experiments with [125I]R(+)-zacopride (1.0 nM) binding to rat whole brain homogenates, in the presence of the 5-HT3 receptor antagonist granisetron (1.0 microM), identified that R(+)-zacopride and prazosin bound to two sites (pIC50: 7.59 and 5.28, respectively, for R(+)-zacopride; 6.75 and 4.42, respectively, for prazosin) whereas S(-)-zacopride and mianserin possessed relatively low affinity (pIC50: 4.37 and 3.80, respectively) while (-)sulpiride and 5-HT failed to compete for [125I]R(+)-zacopride binding at concentrations up to 10 microM. Autoradiographic radioligand binding studies using [125I]R(+)-zacopride (0.5 nM) identified a heterogeneous distribution of specific binding sites (defined by unlabelled R(+)-zacopride, 1.0 microM) throughout the rat brain. In the presence of a saturating concentration of granisetron (1.0 microM), highest levels of specific [125I]R(+)-zacopride, binding sites (defined by R(+)-zacopride, 1.0 microM; R(+)-site), were detected in the olfactory tubercle, thalamus, corpus callosum, colliculus, dorsal and median raphe nucleus, spinal cord and the pons (8.0-13.0 fmol/mg). Moderate densities of R(+)-sites were located in the striatum, nucleus accumbens, substantia nigra, ventral tegmental area, globus pallidus, septal nuclei, frontal cortex and cerebellum (2.0-7.9 fmol/mg). In the hippocampus, amygdala and cortical areas. R(+)-site levels were low but detectable (0.1-1.9 fmol/mg). [125I]R(+)-zacopride labelled R(+)-sites were also detected in some rat peripheral tissues, for instance kidney cortex, adrenal gland and liver (2.4-6.8 fmol/mg). The present results indicate that specific non-5-HT [125I]R(+)-zacopride sites are heterogeneously distributed throughout the rat brain and are expressed in various peripheral tissues.

The enantiomers of zacopride: an intra-species comparison of their potencies in functional and anxiolytic models

1. The 5-HT3 receptor antagonist, zacopride, and its enantiomers, R(+)-zacopride and S(-)-zacopride, were examined in three pharmacological models: (i) 5-HT-induced depolarization of the mouse isolated vagus nerve preparation, (ii) the 5-HT-evoked von Bezold-Jarisch reflex in the mouse, and (iii) the mouse light:dark box model of anxiety. Other standard 5-HT3 receptor antagonists were also included for comparison in these studies. 2. Racemic zacopride, and both of the enantiomers, displayed potent 5-HT3 receptor antagonist activity in the isolated vagus nerve and in the von Bezold-Jarisch model. No 5-HT3 receptor agonist or partial agonist effects of these compounds were detected. 3. In the isolated vagus nerve, R(+)-zacopride and ondansetron were surmountable 5-HT3 receptor antagonists (pA2 values of 9.3 and 8.3, respectively), whereas racemic zacopride, S(-)-zacopride and tropisetron were insurmountable antagonists, markedly suppressing the maximum response to 5-HT. 4. In vivo, racemic zacopride, R(+)-zacopride, S(-)-zacopride and WAY100289 were potent antagonists of the 5-HT-evoked von Bezold-Jarisch reflex, with minimum effective doses (lowest dose required to reduce the reflex by > or = 85%; MED85) of 1.0, 3.0, 0.3 and 3.0 micrograms kg-1, s.c., respectively. 5. Racemic zacopride, R(+)-zacopride and S(-)-zacopride were active in the mouse light:dark box model of anxiety, with similar potencies (minimum effective dose 1 microgram kg-1, s.c.) and similar active dose-ranges (1-1000 micrograms kg-1, s.c.). 6. The doses of racemic zacopride, R( + )-zacopride and S(-)-zacopride required to block 5-HT3receptors in vivo correlated reasonably well with their potencies in an anxiety model within the same species. In these studies, there was no evidence of a marked difference between the anxiolytic potencies ofR( + )-zacopride and S(-)-zacopride.

5-Hydroxytryptamine-interacting drugs in animal models of anxiety disorders: more than 30 years of research

An overview of the behavioral data arising from the vast literature concerning the involvement of 5-hydroxytryptamine (5-HT) neurotransmission in the regulation of anxiety is presented. More than 1300 experiments were carried out in this area and they provide evidence that: (1) results obtained in ethologically based animal models of anxiety with drugs stimulating 5-HT transmission are most consistent with the classic 5-HT hypothesis of anxiety in that they show an increase in animals' emotional reactivity; (2) no category of anti-anxiety models are selectively sensitive to the anxiolytic-like effects of drugs targetting 5-HT1A, 5-HT2A or 5-HT2C receptor subtypes; (3) anxiolytic-like effects of 5-HT3 receptor antagonists, in the great part, are revealed by models based on spontaneous behaviors. Taken together, these observations lead to the conclusion that different 5-HT mechanisms, mediated by different receptor subtypes, are involved in the genesis of anxiety.

Evidence that mCPP-induced anxiety in the plus-maze is mediated by postsynaptic 5-HT2C receptors but not by sympathomimetic effects

1-(3-Chlorophenyl)piperazine (mCPP) (0.125-1.0 mg/kg i.p.), previously shown to inhibit social interaction, dose-dependently reduced exploration of the open arms of an elevated plus-maze. These findings suggest anxiogenic properties. The effect of mCPP was more potently inhibited by 1-(1-naphthyl)piperazine than by ketanserin, indicative of its mediation via activation of 5-HT2C rather than 5-HT2A receptors. The 5-HT1B receptor agonist CGS 12066B did not antagonise the anxiety-like response to mCPP, and further reduced exploration at the highest dose tested (10 mg/kg i.p.). Depletion of serotonin (5-HT) by p-chlorophenylalanine (PCPA, 150 mg/kg/day x 3) did not prevent the response, although PCPA itself increased open arm exploration. The 5-HT1A/B and beta-adrenoceptor antagonist 1-propanolol (5 mg/kg i.p.) and the peripheral beta 1-receptor antagonist atenolol (20 mg/kg i.p.) showed no significant activity on the plus-maze either alone or against the anxiogenic effect of mCPP. These results indicate that mCPP induces anxiety in the rat in the elevated plus-maze primarily by stimulation of postsynaptic 5-HT2C receptors, and suggest that sympathomimetic effects of mCPP are not involved.

The profiles of interaction of yohimbine with anxiolytic and putative anxiolytic agents to modify 5-HT release in the frontal cortex of freely-moving rats

1. The interaction of yohimbine with anxiolytic and putative anxiolytic agents to modify 5-hydroxytryptamine (5-HT) release in the frontal cortex of the freely-moving rat was assessed using the microdialysis technique. 2. The alpha 2-adrenoceptor antagonist, yohimbine (5.0 mg kg-1, i.p.) increased maximally the extracellular levels of 5-HT in the rat frontal cortex by approximately 230% of the basal levels. 3. The alpha 2-adrenoceptor agonist, clonidine (30-100 micrograms kg-1, i.p.) decreased dose-dependently the extracellular levels of 5-HT in the rat frontal cortex by approximately 0-60% of the basal levels. A 5 min pretreatment with clonidine (50 micrograms kg-1, i.p.) prevented the yohimbine-induced increase in the extracellular 5-HT levels. 4. The benzodiazepine receptor agonist, diazepam (2.5 mg kg-1, i.p.) and the 5-HT3 receptor antagonist, ondansetron (100 micrograms kg-1, i.p.) (5 min pretreatment) completely prevented the yohimbine (5.0 mg kg-1, i.p.)-induced increases in the extracellular levels of 5-HT. The 5-HT1A receptor agonist, 8-OH-DPAT (0.32 mg kg-1, s.c.) partially antagonized the yohimbine response. 5. A 5 min pretreatment with the 5-HT3/5-HT4 receptor ligand R(+)-zacopride (10 micrograms kg-1, i.p.) reversed the yohimbine (5.0 mg kg-1, i.p.)-induced increase in the extracellular levels of 5-HT to approximately 30% below the basal levels. A 5 min pretreatment with S(-)-zacopride (100 micrograms kg-1, i.p.) failed to modify the response to yohimbine. 6. The present study provides evidence of the ability of the anxiogenic agent, yohimbine, to increase the activity of the central 5-hydroxytryptaminergic system and the ability of clonidine and various anxiolytic and putative anxiolytic agents to prevent the yohimbine response.

Differential modulation of extracellular levels of 5-hydroxytryptamine in the rat frontal cortex by (R)- and (S)-zacopride

1. The ability of various anxiolytic and potential anxiolytic agents to modify 5-hydroxytryptamine (5-HT) release in the frontal cortex of the rat was assessed by the microdialysis technique. 2. The benzodiazepine receptor agonist, diazepam (2.5 mg kg-1, i.p.), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.32 mg kg-1, s.c.) and the 5-HT1A receptor partial agonist buspirone (4.0 mg kg-1, i.p.) maximally reduced extracellular levels of 5-HT in the rat frontal cortex by approximately 50-60%, 70-80% and 30-40%, respectively. 3. (R)-zacopride (1.0-100 micrograms kg-1, i.p.) dose-dependently reduced extracellular levels of 5-HT in the rat frontal cortex (approximately 80% maximal reduction) whereas the other 5-HT3 receptor antagonists ondansetron (10 micrograms kg-1, i.p.) and (S)-zacopride (10-100 micrograms kg-1, i.p.) were ineffective. 4. In contrast to (S)-zacopride (100 nM; administered via the microdialysis probe), (R)-zacopride (1.0-100 nM; administered via the microdialysis probe) induced a concentration-dependent reduction in extracellular levels of 5-HT in the rat frontal cortex (approximately 70% maximal reduction). 5. In contrast to ondansetron (100 micrograms kg-1, i.p.), (S)-zacopride (10-100 micrograms kg-1, i.p.) dose-dependently reversed the (R)-zacopride (10 micrograms kg-1, i.p.) induced reduction in extracellular levels of 5-HT in the rat frontal cortex. The highest dose of (S)-zacopride (100 micrograms kg-1, i.p.) completely prevented the (R)-zacopride response.In addition, (S)-zacopride (100 nM; administered via the microdialysis probe) attenuated the inhibitory action of (R)-zacopride (10 nM; administered via the microdialysis probe) on extracellular levels of 5-HT in the rat frontal cortex.6. In conclusion, the present study provides further evidence of the ability of diazepam, 8-OH-DPAT and buspirone to reduce the activity of the central 5-hydroxytryptaminergic system in vivo. Furthermore,the results indicate that the ability of (R)-zacopride to reduce the in vivo release of 5-HT in the rat frontal cortex does not correlate with its 5-HT3 receptor antagonism. However, the differential affinity of (R)- and (S)-zacopride for a (S)-zacopride-insensitive (R)-zacopride site in rat cerebral cortex mirrors the relative activity of the two zacopride stereoisomers to modify the in vivo release of 5-HT in the frontal cortex of the rat and their ability to release suppressed behaviour in animal models of anxiety.