Effect of noradrenergic system on the anxiolytic-like effect of DOI (5-HT2A/2C agonists) in the four-plate test

Rationale Selective serotonin reuptake inhibitors and serotonin and noradrenaline reuptake inhibitors demonstrated an anxiolytic-like effect in the four-plate test (FPT). (+/−)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI; a 5-HT_2A receptor agonist) also possessed strong anxiolytic-like effect in the same test. A 5-HT_2A mechanism seems to be implicated in the mechanism of action of both antidepressants and DOI in this test. On the other hand, the α-adrenergic ligands have also demonstrated an activity in other models of anxiety. A previous study demonstrated that the α₂-adrenoceptor agonists abolished the anxiolytic-like effect of antidepressants. Objectives The aim of the present study was to evaluate the role of noradrenergic system on the regulation of 5-HT₂ receptors implicated in the DOI anxiolytic-like activity in the FPT. Methods First, the effect of noradrenergic and serotonergic lesions on DOI anxiolytic-like activity was studied in the FPT. Second, the effect of co-administration of α-adrenoceptor ligands and DOI was evaluated in the same test. Results The noradrenergic and serotonergic lesions had no effect on DOI (1 mg/kg) anti-punishment activity in the FPT. Adrafinil 0.25 and 4 mg/kg (an α₁-adrenoceptor agonist), prazosin 0.5 and 2 mg/kg (an α₁-adrenoceptor antagonist) and idazoxan 1 and 4 mg/kg (an α₂-adrenoceptor antagonist) did not modify the activity of DOI. Clonidine 0.06 mg/kg, guanabenz 0.125 and 0.5 mg/kg (two α₂-adrenoceptor agonists) and guanfacine 0.06 and 0.125 mg/kg (a specific α_2A-adrenoceptor agonist) completely abolished DOI-induced increase in punished passages. Conclusion These results indicate that the DOI seems to act on the 5-HT₂ receptors post-synaptically located. The effect of DOI is regulated by the α₂-adrenoceptors.     

Anxiolytic-like effect of milnacipran in the four-plate test in mice: mechanism of action

Milnacipran is a serotonin/noradrenaline reuptake inhibitor (SNRI) which has not yet been systematically studied preclinically or clinically for the treatment of anxiety disorders. In the four-plate test (FPT) which is known to predict anxiolytic-like activity in mice, milnacipran (4, 8, 16 and 32 mg/kg) demonstrated strong anti-punishment effects following acute administration. The anxiolytic-like effect of milnacipran was not reversed by the selective GABA(A) receptor antagonist, flumazenil (2 and 4 mg/kg), the selective alpha1-adrenoceptor antagonist, prazosin (0.5 and 2 mg/kg), the selective alpha2-adrenoceptor antagonist, idazoxan (1 and 4 mg/kg) or the selective 5-HT2B receptor antagonist, SB 206553 (0.1 and 1 mg/kg). In contrast, the selective 5-HT2A receptor antagonist, SR 46349B (0.1 and 1 mg/kg), and the non-selective 5-HT2 receptor antagonist, ketanserin (0.125 and 0.5 mg/kg), completely abolished the anxiolytic-like effect of milnacipran in FPT. Neurochemical depletion of NA or 5-HT completely abolished the activity of milnacipran. These results strongly suggest that activation of 5-HT2A receptors is critically involved in the anxiolytic activity of milnacipran. On the other hand the lack of activity of milnacipran after depletion of NA or 5-HT is consistent with milnacipran acting on the locus coeruleus to induce 5-HT release. The present data suggest a strong connection between 5-HT2A receptors and NA neurotransmission.     

5-HT(1A) receptors play an important role in modulation of behavior of rats in a two-compartment black and white box

The involvement of serotonergic receptor subtypes in modulation of rat behavior in a black and white test box was studied. The high efficacy 5-HT(1A) receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetraline] was inactive, and the partial agonist buspirone showed an anxiolytic-like response that was weaker than the response to diazepam. The non-selective 5-HT(1) receptor agonists TFMPP [1-(trifluoromethylphenyl) piperazine] and eltoprazine were inactive. Low doses of the 5-HT(1A) receptor antagonist WAY 100.635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide] induced an anxiolytic-like response; 10-100 times higher doses induced an anxiogenic-like response. The 5-HT(2A/2C) and 5-HT(3) receptor antagonists ritanserin and ondansetron induced anxiolytic-like responses. The 5-HT(2A/2C) receptor agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] and the 5-HT(2A) receptor antagonist-MDL 100.151 [(=) chi-(2,3-dimethoxyphenyl)-1-[2-(4-fluorphenyl)ethyl]-4-piperidin-methanol] were inactive. 8-OH-DPAT potentiated the anxiolytic-like effect of ritanserin. It is suggested that 5-HT(1A) and 5-HT(2C) receptors are involved in mediation of exploratory behavior in rats, and that the 5-HT receptor subtype mediated effects may modulate one another.     

5-HT(2A) and 5-HT(2C) receptor stimulation are differentially involved in the cortical dopamine efflux-Studied in 5-HT(2A) and 5-HT(2C) genetic mutant mice

Both 5-HT(2A) and 5-HT(2C) receptors modulate cortical dopamine efflux, but in opposite directions. We have now compared the ability of the three 5-HT(2A/2C) receptor agonists, DOI (R(-)-2,5-dimethoxy-4-iodoamphetamine), mCPP (meta-chlorophenylpiperazine) and MK-212 (6-Chloro-2-(piperazinyl) pyrazine), to modulate cortical dopamine efflux in 5-HT(2A) and 5-HT(2C) genetic mutant mice. In the 5-HT(2A) mice, the preferential 5-HT(2A) receptor agonist DOI (2.5mg/kg, s.c.) induced a slight but significant increase in cortical dopamine efflux only in the wild type (WT) mice; MK-212 (2.5mg/kg) reduced dopamine efflux in both WT and receptor knockout (KO) mice; moreover, MCPP, 2.5mg/kg, had no effect in either types. In 5-HT(2C) mice, DOI increased dopamine efflux in both types; while MK-212 decreased dopamine efflux in the WT, but not the receptor KO mice. These results provide new evidence that 5-HT(2A) receptor stimulation enhances and 5-HT(2C) receptor stimulation inhibits cortical dopamine efflux, and suggest the effects of DOI, MK-212 and mCPP on the cortical dopamine efflux are due to their different abilities on 5-HT(2A) and 5-HT(2C) receptors stimulation. Of these three agents, only DOI, the more selective 5-HT(2A) receptor agonist, is hallucinogenic. The absence of hallucinations with mCPP may be due to its relatively more potent 5-HT(2C) receptor agonist effect, inhibiting the ability of mCPP to enhance dopamine efflux in cortical and perhaps limbic regions as well. The present data provide additional evidence that hallucinations are due, in part, to 5-HT(2A) rather than 5-HT(2C) receptor stimulation. These findings suggest that 5-HT(2C) receptor agonists may be useful as antipsychotics, consistent with previous suggestions.     

Implication of 5-HT<Subscript>2</Subscript> receptor subtypes in the mechanism of action of antidepressants in the four plates test

Rationale The selective serotonin reuptake inhibitors (SSRIs) and the serotonin and noradrenaline reuptake inhibitors (SNRIs) increase synaptic levels of serotonin, leading to an increased activation of a multitude of specific postsynaptic 5-HT receptors. However, it is not yet known which 5-HT receptor subtypes mediate the therapeutic effects of antidepressants.Methods The effects of the SSRI, paroxetine and the SNRI, venlafaxine were evaluated in the mouse four plates test (FPT).Results Paroxetine administered intraperitoneally (IP) (0.5, 2–8 mg/kg) potently augmented the number of punished passages accepted by mice in this paradigm. The effects of paroxetine (8 mg/kg) were not reversed by the selective 5-HT_2C receptor antagonist, RS 10-2221 (0.1 mg/kg and 1 mg/kg) or the selective 5-HT_2B/2C receptor antagonist SB 206553 (0.1 mg/kg and 1 mg/kg), at doses which lack an effect when administered alone. In contrast, the selective 5-HT_2A receptor antagonist, SR 46349B (0.1 mg/kg and 1 mg/kg) completely abolished the paroxetine-induced increase in punished passages. The acute administration of venlafaxine induced an anxiolytic-like effect in the FPT at the doses of 2–16 mg/kg. This effect was reversed by the 5-HT_2B/2C receptor antagonist as did SR 46349B, for both doses administered. Our results strongly suggest that activation of 5-HT_2A receptors is critically involved in the anxiolytic activity of paroxetine, whereas the 5-HT_2A and 5-HT_2B receptors are involved in the anti-punishment action of venlafaxine in the FPT. The co-administration of selective 5-HT_{2A, 2B, 2C} receptor agonists (DOI, 0.06 mg/kg and 0.25 mg/kg; BW 723C86, 0.5 mg/kg and 2 mg/kg and RO 60-0175, 0.06 mg/kg and 0.25 mg/kg), respectively, was subsequently investigated. The effects of sub-active doses of paroxetine (0.25 mg/kg and 1 mg/kg) were augmented by BW 723C86 and RO 60-0175 receptor agonist challenge. The anti-punishment effects of venlafaxine (0.25 mg/kg and 1 mg/kg) were potentialised only by DOI co-administration.Conclusion These results indicate that the co-administration of 5-HT₂ receptor agonists with paroxetine and venlafaxine may provide a powerful tool for enhancing the clinical efficacy of these antidepressants.     

Regulation of 5-HT(2A) receptor mRNA levels and binding sites in rat frontal cortex by the agonist DOI and the antagonist mianserin

In the present study we have characterized the time course of effect of administration of the serotonin(2) (5-HT(2)) receptor antagonist mianserin, or the 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), on 5-HT(2A) receptor binding sites and mRNA levels in rat frontal cortex. Radioligand binding and ribonuclease protection assays were performed with separate hemispheres of frontal cortex from each animal to examine concomitant changes in 5-HT(2A) receptor sites and mRNA levels. The decrease in cortical 5-HT(2A) receptor sites in response to chronic DOI administration was not accompanied by changes in 5-HT(2A) receptor mRNA. A single injection of DOI produced a transient decrease in 5-HT(2A) receptor mRNA levels detected 1 h post-injection. The density of 5-HT(2A) receptor sites, however, was not significantly reduced following a single injection of DOI. The down-regulation of cortical 5-HT(2A) receptor sites in response to a single injection of mianserin was accompanied by reductions in 5-HT(2A) receptor mRNA levels. Following 4 days of mianserin administration, however, we did not observe a change in 5-HT(2A) receptor mRNA levels, although 5-HT(2A) receptor density was decreased. Thus, changes in receptor mRNA may initially contribute to the down-regulation of 5-HT(2A) receptors in response to acute mianserin administration. Sustained changes in 5-HT(2A) receptor mRNA, however, appear not to be involved in maintaining the down-regulation of 5-HT(2A) receptor number with chronic mianserin administration. Mechanisms other than the regulation of receptor mRNA levels appear to underlie the down-regulation of 5-HT(2A) receptor sites in response to chronic administration of the agonist DOI.     

Factors triggering abolishment of benzodiazepines effects in the Four-Plate Test--retest in mice.

Abolishment of anxiolytic-like effects of diazepam occurs during re-exposure to some animal tests of anxiety. We investigated the loss of anxiolytic-like effects of diazepam during Trial 2 on previously undrugged mice, namely one-trial tolerance (OTT). Swiss mice were subjected to 1) Four-Plate Test (FPT) without punishments in Trial 1 or 2) FPT without punishments in both Trials or 3) FPT with spatial modifications in Trial 1 or 4) Elevated Plus Maze (EPM), then 24 h later to FPT, with saline, diazepam (1 mg/kg) or DOI (1 mg/kg). Removing punishments in Trial 1 does not counteract the effect reduction of diazepam in Trial 2, but spatial modifications of the aversive environment. Previous exposure to EPM does not trigger a loss of efficacy of diazepam in FPT. Electric punishments do not trigger OTT to benzodiazepines; whilst knowledge of the environment seems to be responsible for this phenomenon. FPT may be useful to study OTT because punishments potentate OTT in this model of anxiety.     

In vivo electrophysiological examination of 5-HT2 responses in 5-HT2C receptor mutant mice

The present study used 5-HT2C receptor mutant mice and their wild-type littermates to characterize the 5-HT2 receptor using the 5-HT2 agonists (+/-)-2-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and 1-(3-chlorophenyl)piperazine (mCPP) applied locally in the orbitofrontal cortex (OFC) and head of the caudate nucleus. Microiontophoretically-applied 5-HT, DOI and mCPP induced current-dependent inhibition of neuronal firing activity in both brain regions. There was no difference between 5-HT2C receptor mutants and wild-type mice in the ability of 5-HT or DOI to inhibit neuronal firing at any current used. In contrast, there was a reduced ability of mCPP to inhibit firing activity in the OFC when ejected at 10 nA. Unexpectedly, there was a small but significant increase in mCPP-induced inhibition in the caudate nucleus of mutant mice. In the OFC, the 5-HT2A antagonist MDL 100907 (2 mg/kg, i.p.) significantly antagonized the effect of both DOI and mCPP. In contrast, the non-selective 5-HT antagonist clozapine (10 mg/kg, i.p.) significantly antagonized only mCPP in the wild-type mice. However, neither MDL 100907 nor clozapine antagonized DOI or mCPP in the caudate nucleus. Finally, it required significantly less quisqualate to activate neurons in the 5-HT2C receptor mutants than in the wild-type mice, suggesting that 5-HT2C receptors serve a tonic inhibitory role in membrane excitability. The present results indicate that the inhibitory action of DOI is predominantly mediated by the 5-HT2A receptor in the OFC. mCPP, when applied locally, inhibits OFC firing activity by acting on both 5-HT2A and 5-HT2C receptors. However, DOI and mCPP might be acting in the caudate nucleus through an atypical 5-HT2 receptor yet to be characterized.     

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

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

Effects of repeated treatment with 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on the autoregulatory control of dorsal raphe 5-HT neuronal firing and cortical 5-HT release.

These experiments were designed to examine the effects of repeated 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) treatment on the autoregulatory control of cortical 5-HT release and dorsal raphe nucleus (DRN) 5-HT neuronal cell firing. Repeated DOI treatment decreased the behavioural responsiveness (wet-dog shakes) of 5-HT2 receptors and attenuated the inhibitory effects of the 5-HT1A receptor agonist, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), on both cortical 5-HT release and DRN 5-HT neuronal firing. In contrast, the inhibitory effect of acute DOI on cortical 5-HT release and DRN 5-HT neuronal firing was unaffected by repeated DOI treatment. The results demonstrate that changes in the responsiveness of 5-HT2 receptor function may influence the responsiveness of presynaptic 5-HT1A receptors regulating 5-HT neuronal function. The results also provide further evidence that the inhibition of cortical 5-HT release and DRN 5-HT neuronal firing produced by DOI is not mediated by 5-HT2 receptor activation.     

A comparison of the behavioural effects of 5-HT2A and 5-HT2C receptor agonists in the pigeon

Activity at the 5-HT2A receptor versus that of the 5-HT2C receptor was studied in three behavioural paradigms. In pigeons trained to discriminate 0.32 mg/kg of 1-(2,5-diemethoxy-4-iodophenyl)-2-aminopropane (DOI) (a mixed 5-HT2A/C receptor agonist) from vehicle, quipazine (0.1-1 mg/kg) and m-chlorophenylpiperazine (mCPP) (1-3 mg/kg) substituted for DOI in a dose-related manner, and this generalization was blocked by MDL100907 (0.0001-0.01 mg/kg), a selective 5-HT2A receptor antagonist. RO60-0175 (a relatively selective 5-HT2C agonist) induced partial substitution at 3 mg/kg that was antagonized by both MDL100907 and by 3 mg/kg of SB242084, a relatively selective 5-HT2C antagonist. MK212 (a mixed 5-HT2C/A agonist) induced partial substitution that was antagonized by SB242084, but not by MDL100907. On a progressive ratio 5 operant schedule (PR5) for food reinforcement, DOI, quipazine, mCPP, MK212 and R060-0175 decreased the break point; mCPP, DOI, MK212 and quipazine also induced vomiting. Although MDL100907 antagonized both the reductions of break point and vomiting, SB242084 only partially attenuated the decrease in break point observed with MK212 and DOI, and was unable to eliminate vomiting. Thus pharmacological activity at the 5-HT2A receptor can be behaviourally distinguished from pharmacological activity at the 5-HT2C receptor in the pigeon. Furthermore, the decrease in the break point of a PR5 schedule induced by 5-HT2C receptor agonists may be related to decreased appetite, whereas that induced by 5-HT2A receptor agonists may be due to unrelated factors, such as emesis.     

Effects of the 5-HT2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on plasma glucose and glucagon levels of rats

Effects of the 5-hydroxytryptamine (5-HT)2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on plasma glucagon levels were studied in rats. Systemic injection of DOI induces significant increases in plasma glucagon levels. Hyperglucagonemia induced by DOI was dose-dependently prevented by the 5-HT2A receptor antagonist ketanserin. Adrenodemedullation abolished hyperglucagonemia elicited by DOI. Previous report demonstrated that the peripheral 5-HT2A receptor agonist induces hyperglycemia in rats but does not increase plasma glucagon levels at doses inducing hyperglycemia. Therefore, our findings suggest that DOI-induced glucagon release was elicited by stimulation of the central 5-HT2A receptor, which in turn increasing adrenaline release.     

(+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane(DOI) and alpha-methyl-5-HT: 5-HT2 receptor agonistic action on phosphatidylinositol metabolism in the rat fronto-cingulate and entorhinal cortex.

In the present study, the effects of 5-HT and two 5-HT1c/5-HT2 receptor agonists, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and alpha-methyl-serotonin (alpha-Me-5-HT) on phosphoinositide hydrolysis were compared, to determine whether DOI and alpha-Me-5-HT were full agonists. Consistent with the results obtained from previous studies, both (+/-)-DOI and alpha-Me-5-HT stimulated turnover of phosphoinositide in a concentration-dependent manner. However, the response obtained with these 5-HT1c/5-HT2 receptor agonists was only 30-40% of that of 5-HT. The stimulation of hydrolysis of phosphoinositide, produced by both 5-HT2 receptor agonists, was potently antagonized by ritanserin (a 5-HT1c/5-HT2 receptor antagonist) and alpha-phenyl-1-(2-phenylethyl)-4-piperine methanol [(+)-MDL 11,939, a 5-HT2 receptor antagonist] but not by granisetron (BRL a 5-HT3 receptor antagonist), suggesting that the action of DOI and alpha-Me-5-HT was primarily mediated by 5-HT2 receptors. When the effect of increasing the concentration of 5-HT on turnover of phosphoinositide was measured in the presence of a 1 microM concentration of the 5-HT3 receptor antagonist granisetron, the response obtained was similar to the response produced by the 5-HT2 receptor agonists, DOI and alpha-Me-5-HT. These results confirm the previous finding that 5-HT stimulates hydrolysis of phosphoinositide by interacting with 5-HT1c/5-HT2 and 5-HT3 receptors. Moreover, they suggest that DOI and alpha-Me-5-HT are full agonists at the 5-HT2 receptor, coupled to hydrolysis of phosphoinositide in the cortex of the rat.     

Inhibition of DOI-induced wet dog shakes in the guinea-pig by 5-HT2 receptor antagonists

The preferential 5-HT( 2)/5-HT(1C) receptor agonist DOI (0.1-4 mg/kg s.c.) caused an increase in locomotor activity, grooming and 'wet-dog' shakes (WDS) in the adult guinea-pig. The DOI-induced WDS behaviour was potently inhibited by several antagonists that have high affinity for the 5-HT(2) binding site. The WDS response is likely to be centrally-mediated since the effects of peripherally administered DOI were poorly antagonized by the peripherally-acting 5-HT(2) receptor antagonist BW501C67. Although these studies do not exclude an effect of DOI at 5-HT(1C) receptors, the high potency of ketanserin and spiperone in attenuating the effects of DOI would suggest an effect at the 5-HT(2) receptor. The present data suggest that antagonism of the directly-acting agonist DOI may be useful for assessing the selectivity and duration of action of centrally-acting 5-HT(2) receptor antagonists in the guinea-pig.     

The selective 5-HT2 receptor antagonist amperozide attenuates 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane-induced inhibition of male rat sexual behavior.

This study was aimed at exploring the role of 5-HT2/5-HT1C neurotransmission in male rat sexual behavior. The administration of the 5-HT2/5-HT1C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (1 mg/kg), suppressed sexual activity in most of the animals. The suppressive effect of DOI was antagonized by treatment with amperozide, a selective 5-HT2 receptor antagonist, in doses which did not by themselves affect sexual activity. In addition, several other serotonin antagonists were tested with varying affinity profiles for 5-HT2/5-HT1C receptors, including ketanserin, ritanserin, and mesulergine. All these compounds antagonized the suppressive action of DOI. In contrast, no antagonizing effect was obtained by treatment with (-)-alprenolol, a 5-HT1A antagonist. The present findings suggest that 5-HT2/5-HT1C receptors might be involved in the neural control of male rat sexual behavior, presumably by exerting an inhibitory influence on the behavior.     

Effects of buspirone, diazepam, and zolpidem on open field behavior, and brain [3H]muscimol binding after buspirone pretreatment

The effects of 5-HT(1A) receptor agonist buspirone, a nonselective (diazepam), and a selective (zolpidem) GABA(A) receptor agonist were compared in the open field test of neophobia. Unhabituated rats were pretreated with the drugs once, prior to a first exposure to the open field, and their behavior was recorded both during this test and during a second trial 24 h later. It has been hypothesized that the decrease in exploratory activity observed during the second test session may be considered an adaptive reaction to the first day aversive experience (neophobia). If so, a selective modulation of 5-HT and GABA systems activity during the test could bring about significant changes in animal behavior on the retest. Buspirone at the lowest dose of 0.3 mg/kg revealed anxiolytic-like properties on the first day, whereas the action of diazepam and zolpidem was modulated by the dose-related sedative effect. At the dose of 2.4 mg/kg buspirone elicited delayed in time anxiolytic-like action, i.e., produced the antithigmotactic effect during the retrial 24 h later. Diazepam and zolpidem failed to exhibit similar profile of action. Autoradiography of [3H]muscimol binding after pretreatment of rats with buspirone showed a significant increase in the selective radioligand binding within the frontal cortex and a similar, near-significant tendency in the dentate gyrus of the hippocampus. The behavioral data validate buspirone as important drug for the treatment of anxiety disorders, devoid of disruptive influence on motor and cognitive processes. The open field test, as modified by us, appeared sensitive in distinguishing the behavioral profiles of action of different anxiolytic compounds, including 5-HT(1A) receptor agonist. The present results support the assumption that reduced turnover of 5-HT due to stimulation of 5-HT(1A) autoreceptors, may bring about changes in GABA(A) receptor system activity, in some brain structures, leading to the anxiolytic effect.     

Effects of serotonin 5-HT(1/2) receptor agonists in a limited-access operant food intake paradigm in the rat.

Hypophagic effects of serotonergic drugs have mostly been investigated in free-feeding paradigms and are generally ascribed to drug-induced acceleration of satiety, or to behavioral disruption. The present study investigated the hypophagic effects of various 5-HT(1/2) receptor agonists in an operant paradigm. Because of its limited duration (10-min session) the procedure was considered to be relatively insensitive to satiety processes. The behavioral specificity of the hypophagic effect was assessed by additional testing of the compounds in a locomotor activity assay. Male Wistar rats, maintained at about 80% of their free-feeding weights, were trained to acquire stable operant responding in daily fixed ratio:10 food-reinforced sessions; after which they were tested once a week with a 5-HT receptor agonist. Each compound dose-dependently suppressed the number of earned pellets after i.p. administration: DOI (5-HT(2A/2C) receptor agonist; ED(50): 0.36 mg/kg), TFMPP (5-HT(1B/2C/2A); 0.37 mg/kg), m-CPP (5-HT(2C/1B/2A); 0.54 mg/kg), ORG 37684 (5-HT(2C/2A); 0.85 mg/kg), CP-94,253 (5-HT(1B); 2.09 mg/kg), BW 723C86 (5-HT(2B); 6.26 mg/kg) and ipsapirone (5-HT(1A); 10.17 mg/kg). When tested at the dose equivalent to the ED(50) value in the operant paradigm, only ORG 37684 and DOI weakly suppressed activity counts in a locomotor activity assay; suggesting that the inhibition of operant food intake obtained with the other compounds at these doses is not a direct consequence of unconditioned motor effects. It is suggested that the hypophagic effect induced by relatively low doses of CP-94,253, TFMPP and m-CPP, and by moderate doses of ipsapirone and BW 723C86, is partly due to a drug-induced suppression of appetite. Although the exact contribution of the diverse 5-HT(1/2) receptor subtypes to appetite control remains to be studied in more detail, it is hypothesized that activation of 5-HT(1B) and/or 5-HT(2C) receptors attenuates appetite.     

Chronic treatment with a serotonin(2) receptor (5-HT(2)R) agonist modulates the behavioral and cellular response to (+)-3,4-methylenedioxymethamphetamine [(+)-MDMA

3,4-Methylenedioxymethamphetamine [MDMA; ecstasy] evokes a multifaceted subjective experience in human users which includes stimulation, feelings of well-being, mood elevation, empathy towards others as well as distortions in time, sensation and perception. Aspects of this unique psychopharmacology of MDMA are thought to be related to its potent actions to release serotonin (5-HT) and indirectly stimulate the 5-HT(2A) receptor (5-HT(2A)R). In the present studies, we examined the interrelationship between down-regulation of 5-HT(2A)R expression and the behaviorally stimulatory effects generated by acute administration of (+)-MDMA, the most potent enantiomer of (+/-)-MDMA. Male Sprague-Dawley rats were chronically treated with the preferential 5-HT(2A)R agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) which has been shown to down-regulate expression of the 5-HT(2A)R, but not the closely related 5-HT(2C)R. While chronic DOI treatment did not alter the functional sensitivity of either the 5-HT(2A)R or 5-HT(2C)R, this regimen enhanced (+)-MDMA-evoked hyperactivity. Subsequent analysis of c-Fos and 5-HT(2A)R immunoreactivity in brain sections demonstrated that DOI treatment decreased the number of (+)-MDMA-induced c-Fos immunopositive nuclei and 5-HT(2A)R immunostaining in select cortical and striatal areas. These results indicate that chronic DOI exposure results in an enhanced behavioral response to (+)-MDMA and in a pattern of neuronal activation which resembles that seen in psychostimulant sensitization. These data also suggest that expression of the 5-HT(2A)R in the NAc and PFC may play a role in the sensitivity to the locomotor-stimulating effects of (+)-MDMA and in the processes of neural regulation upon repeated psychostimulant administration.     

Anxiolytic-like action of MTEP expressed in the conflict drinking Vogel test in rats is serotonin dependent

The purpose of the present study was to investigate whether the anxiolytic-like action of a selective and brain penetrable group I metabotropic glutamate (mGlu5) receptor antagonist 3-[(2-methyl-1,3-tiazol-4-yl)ethynyl]-pyridine (MTEP) is dependent upon the serotonergic system. Experiments were performed on male Wistar rats. The Vogel conflict drinking test was used to detect anxiolytic-like activity. MTEP administered intraperitoneally at doses of 1, 3 and 6 mg/kg induced anxiolytic-like effect. The potential anxiolytic effect of MTEP (1 mg/kg) was inhibited by a nonselective 5-HT receptor antagonist metergoline (2 mg/kg i.p.) and 5-HT2A/2C receptor antagonist ritanserin (0.5 mg/kg i.p.), but not by a 5-HT1A receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl)cyclohexane-carboxamide (WAY 100635) (0.1 mg/kg i.p). The anxiolytic effect of MTEP (6 mg/kg) was attenuated by ritanserin (1 mg/kg i.p.). Moreover, MTEP-induced a dose-dependent release of serotonin in the frontal cortex. The obtained results suggest that the potential anxiolytic effect of the mGlu5 receptor antagonist MTEP is due to the increased serotonin release with subsequent activation of 5-HT2A/2C receptors, most probably located postsynaptically, but not by the 5-HT1A receptors.     

Anxiolytic-like effects of baicalein and baicalin in the Vogel conflict test in mice

A previous receptor binding assay indicated that baicalein, one of the active principles of the Chinese herbal drug, Huangqin (Scutellariae Radix), interacts with the benzodiazepine binding site of GABA(A) receptors in mouse cortex membrane preparations with a K(i) value of 13.1 microM. Therefore, the present study examined whether baicalein and its 7-glucuronide, baicalin, have anxiolytic-like effects in a Vogel conflict test adapted for ICR mice. The results showed that both baicalein (10 mg/kg, i.p.) and baicalin (20 mg/kg, i.p.) significantly increased the number of shocks accepted in the Vogel lick-shock conflict paradigm over 9 min, as did a benzodiazepine receptor agonist, chlordiazepoxide (5.0 mg/kg, i.p.) and a 5-HT(1A) receptor agonist, 8-hydroxy-2(di-n-propylamino)tetralin (0.5 mg/kg, i.p.). Since the total volume of water intake and the shock sensitivity of mice were not significantly changed after drug treatment, the effect of baicalein or baicalin was not due to an enhancement of thirst or shock tolerance. Furthermore, this anxiolytic-like effect of baicalein or baicalin was antagonized by co-administration with a benzodiazepine receptor antagonist, flumazenil (2 mg/kg, i.p.), but not with a 5-HT(1A) receptor antagonist, pindolol (10 mg/kg, i.p.). It is concluded that the anxiolytic-like effect of baicalein or baicalin may be mediated through activation of the benzodiazepine binding site of GABA(A) receptors.