Implication of 5-HT2A subtype receptors in DOI activity in the four-plates test-retest paradigm in mice

The four-plates test (FPT) is an animal model of anxiety which allows the detection of anxiolytic effect not only of benzodiazepines (BZDs) but also of other non-BZDs anxiolytic compounds such as antidepressants (ADs). Furthermore, DOI, a 5-HT(2A/2C) agonist, has been shown to exert an anxiolytic-like effect in this model. Retesting mice in animal models of anxiety (test-retest paradigm) induces an anxiogenic-like and a loss of anxiolytic-like effects in response to BZDs and ADs. On the contrary, DOI has been reported to oppose the fear potentiation induced by trial 1 in the FPT. Despite DOI is considered as one of the most selective 5-HT(2A) available, it acts as agonist at all three 5-HT(2) receptor subtypes (5-HT(2A), 5-HT(2B) and 5-HT(2C)). The aim of this study was thus to investigate in the FPT test-retest paradigm, which 5-HT(2) receptor subtype(s) was involved in the DOI-induced effect in experienced mice. The effect of DOI (0.25-4 mg/kg) and the agonists, 5-HT(2B), BW 723C86 (1-16 mg/kg) and 5-HT(2C), RO 60-0175 (0.25-4 mg/kg) have also been studied. Then, antagonism studies were conducted combinating the 5-HT(2A) receptor antagonist SR 46349B, the 5-HT(2B/2C) receptor antagonist SB 206553 or the selective 5-HT(2C) receptor antagonist RS 10-2221 (at the doses of 0.1 and 1 mg/kg) with the DOI (1 mg/kg). Our study shows that the BW 723C86 had no effect on retesting mice, whereas it exerted an anxiolytic-like effect in naive mice. By contrast to DOI, the RO 60-0175 had no effect neither in naive nor experienced mice. Furthermore, only the SR 46349B antagonized the DOI-induced anti-punishment effect. Diazepam included as a positive control also increased in each case the number of punished passages in naive mice. Our findings altogether also suggest that DOI exerts its anxiolytic-like effect in the FPT test-retest paradigm through 5-HT(2A) receptors.     

Anxiolytic-like effects of 5-HT2 ligands on three mouse models of anxiety

The behavioural effects of 5-HT₂ receptor agonists, 5-HT_2A and 5-HT_2C receptor antagonists were investigated in the mouse four plates test (FPT), light/dark paradigm (L/D) and the elevated plus maze (EPM), in order to elucidate the role of the 5-HT₂ receptor subtypes in these models and to address the inconclusive results previously reported using rat psychopharmacological models. All compounds were administered intraperitoneally 30 min before each test. DOI, a preferential 5-HT_2A agonist (0.5–8 mg/kg) and BW 723C86, a 5-HT_2B agonist (8 and 16 mg/kg) provoked an anxiolytic-like response in the FPT. In the EPM, an anxiolytic-like effect was observed for DOI (0.5, 1 and 2 mg/kg), BW 723C86 (0.5, 4, 8 and 16 mg/kg), RO 60-0175 a 5-HT_2C agonist (4 mg/kg) and the non-selective 5-HT₂ receptor agonist mCPP (0.25 mg/kg.). Ketanserin, a 5-HT_2A/2C non-selective receptor antagonist (0.015 and 0.03 mg/kg), induced an anxiogenic-like effect in the L/D paradigm. The 5-HT_2C antagonists (RS 10-2221, SDZ SER082 and SB 206553) were without effect in all three tests. These behavioural results are indicative of an anxiolytic-like action of 5-HT₂ receptor agonists, an anxiogenic-like effect of 5-HT_2A receptor antagonism, whereas the blockade of 5-HT_2C receptors are without effect in the mouse models studied.     

Anxiolytic-like effect of 5-HT(2) ligands and benzodiazepines co-administration: comparison of two animal models of anxiety (the four-plate test and the elevated plus maze)

Animal models of anxiety remain a useful tool for evaluating the anxiolytic-like effect of new treatments. Even though many tests are similarly based on exploration tasks, using more than one animal model is all the more recommended since there are qualitative differences between such tests. Furthermore, although many tests are excellent tool for detecting benzodiazepines/GABA compounds, inconsistent results have been reported for 5-HT ligands. Here, two animal models have been chosen, the elevated plus maze (EPM) based on the natural aversion of rodents for open spaces and the four-plates test (FPT) a models involving the animal's conditioned response to stressful events. In a recent study, we have demonstrated that the 5-HT(2A/2C) agonist DOI and the 5-HT(2B) agonist BW 723C86 were shown to produce an anxiolytic-like effect in both tests. This study aimed to evaluate a putative interaction between benzodiazepine and 5-HT(2) ligands in the FPT and the EPM. Indeed, close distribution of GABA(A) and 5-HT(2) receptors was found in brain structures leading to functional interrelation. In the FPT, sub-active doses of alprazolam and diazepam were strongly potentiated by DOI. BW 723C86, also potentiated the anxiolytic-like effect of the two benzodiazepines with a weaker effect. In the same way, DOI and benzodiazepines administration induced an increase in the anxiolytic-like parameters in the EPM with a strongest effect observed with alprazolam. Regardless of anxiety models used in this study, 5-HT(2A) ligands exerted facilitatory influence upon GABAergic system. Therefore, the FPT and the EPM might implicate the same kind of anxiety.     

Anxiolytic-like effects of DOI microinjections into the hippocampus (but not the amygdala nor the PAG) in the mice four plates test

Anxiolytic-like effects of DOI, a 5-HT(2A/2C) agonist have been observed in the four plates test (FPT) after intra-peritoneal administrations. In the present study, DOI (1 microg, 2 microg or 5 microg per mice) was directly injected to three brain structures, the hippocampus, the amygdala and the periaqueductal gray matter (PAG). Tests were carried out immediately after injections. In amygdala and PAG, DOI exerted an anxiogenic-like effect. In the hippocampus, a strong anxiolytic-like effect was found only when injecting 5 microg DOI/mice in the FPT, with a size effect comparable to the anxiolytic-like effect of diazepam 1mg/kg injected intra-peritoneally. DOI or vehicle injections did not affect locomotor activity. These results help us to understand mechanisms of action of DOI in animal models of anxiety, probably through an interaction with other neurotransmitter system, which may take place in the hippocampus.     

In vivo evidence for serotonin 5-HT2C receptor-mediated long-lasting excitability of lumbar spinal reflex and its functional interaction with 5-HT1A receptor in the mammalian spinal cord

The purpose of the present study was to investigate the 5-HT(2C) receptor-mediated effects on the spinal monosynaptic mass reflex activities and also its functional interactions with 5-HT(1A) receptors in anesthetized, acutely spinalized mammalian adult spinal cord in vivo. Intravenous administration of (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) (0.1 mg/kg), an agonist of 5-HT(2A/2C) receptors, significantly increased the excitability of spinal motoneurons as reflected by an increase in the spinal monosynaptic mass reflex amplitude to 150-200% of the control. 5-HT(2A/2C) receptor-induced motoneuron excitability was slow, persistent and long-lasting for more than 2h that was significantly inhibited by 5-HT(2C) receptor specific antagonist SB 242084 administered 10 min prior to DOI. Simultaneous administration of DOI (0.1 mg/kg, i.v.) along with (+/-)-8-hydroxy dipropylaminotetraline hydrobromide (8-OH-DPAT) (0.1 mg/kg, i.v.) completely inhibited DOI-induced spinal monosynaptic mass reflex facilitation. In another separate study, administration of 8-OH-DPAT (0.1 mg/kg, i.v.) at the maximum response of DOI also inhibited the motoneuron's excitability; however, the inhibition lasted only for a period of 40-60 min after administration of 8-OH-DPAT, after which the spinal monosynaptic mass reflex amplitude reached its maximum level. These findings suggest that the 5-HT(2C) receptor is primarily involved in the mediation of the long-lasting excitability of spinal motoneurons and possibly interacts with its functional counterpart, 5-HT(1A) receptors in the mammalian adult spinal cord.     

alpha2-Adrenergic agonists antagonise the anxiolytic-like effect of antidepressants in the four-plate test in mice

Selective serotonin reuptake inhibitors (SSRIs) and serotonin/noradrenaline reuptake inhibitors (SNRIs) has been reported to be efficient in anxiety disorders. Some animal models have demonstrated an anxiolytic-like effect following acute administration, however, it is not yet known how noradrenergic receptors are implicated in the therapeutic effects of antidepressants (ADs) in anxiety. The effects of two alpha(2)-adrenoceptor agonists (clonidine, guanabenz) on anxiolytic-like effect of two SSRIs (paroxetine and citalopram) and two SNRIs (venlafaxine and milnacipran) were evaluated in the four-plate test (FPT) in mice. Paroxetine (4 mg/kg), citalopram (8 mg/kg), venlafaxine (8 mg/kg), and milnacipran (8 mg/kg) administered intraperitoneally (i.p.) increased the number of punishments accepted by mice in the FPT. Clonidine (0.0039-0.5 mg/kg) and guanabenz (0.03-0.5mg/kg) had no effect on the number of punishments accepted by mice. Clonidine (0.03 and 0.06 mg/kg) and guanabenz (0.125 and 0.5 mg/kg) (i.p. -45 min) reversed the anti-punishment effect of paroxetine, citalopram, venlafaxine and milnacipran (i.p. -30 min). But if the antidepressants are administered 45 min before the test and alpha(2)-adrenoceptor agonists 30 min before the test, alpha(2)-adrenoceptor agonists failed to alter the anti-punishment effect of antidepressants. The results of this present study indicate that alpha(2)-adrenoceptor agonists antagonise the anxiolytic-like effect of antidepressants in mice when they are administered 15 min before the administration of antidepressant suggesting a close inter-regulation between noradrenergic and serotoninergic system in the mechanism of SSRIs and SNRIs in anxiety-like behaviour.     

Dose- and time-dependent effects of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a serotonergic 5-HT2 receptor agonist, on local cerebral glucose metabolism in awake rats

The time course and the relation to dose of regional cerebral metabolic rates for glucose (rCMRglc) were measured in awake male Fischer-344 rats after administration of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a selective serotonergic 5-HT2 agonist. rCMRglc was determined, using the quantitative autoradiographic [14C]2-deoxyglucose technique, in 75 brain regions at 5, 15, 30, 60 and 90 min after administration of DOI 10 mg/kg i.p., and at 15 min after DOI 2.5, 25 or 50 mg/kg i.p. In non-hippocampal regions, peak effects were observed at 15-30 min, when rCMRglc in 12% of the regions was significantly different from control. In hippocampal regions rCMRglc effects peaked at 30 min (average rCMRglc reduction 21%) and were sustained for at least 60 min. Higher doses of DOI reduced rCMRglc in most prosencephalic regions (25 mg/kg, 35% of all regions studied; 50 mg/kg, 32%), where 5-HT2 receptors are present in high density. These data suggest that selective 5-HT2 receptor stimulation leads to rCMRglc reduction in areas with high densities of 5-HT2 receptors.     

Involvement of 5-HT1A receptors in the antidepressant-like effect of adenosine in the mouse forced swimming test

This study investigated the involvement of 5-HT1 and 5-HT2 receptors in the antidepressant-like effect of adenosine in the mouse forced swimming test (FST). The pre-treatment of mice with PCPA (100mg/kg, i.p., an inhibitor of serotonin synthesis, for four consecutive days), NAN-190 (0.5mg/kg, i.p., a 5-HT1A receptor antagonist), pindolol (32 mg/kg, i.p., a 5-HT1A/1B receptor/beta-adrenoceptor antagonist) or WAY100635 (0.1 and 0.3mg/kg, s.c., a selective 5-HT1A receptor antagonist), but not with ketanserin (5mg/kg, i.p., a 5-HT2A/2C receptor antagonist), prevented the antidepressant-like effect of adenosine (10mg/kg, i.p.) in the FST. Moreover, the pre-treatment of animals with WAY100635 (0.1mg/kg, s.c.) blocked the decrease in immobility time in the FST elicited by adenosine (5 or 10mg/kg, i.p.), but produced a synergistic effect with a sub-effective dose of adenosine (1mg/kg, i.p.) and did not cause any alteration at the highest dose of adenosine administered (50mg/kg, i.p.). Adenosine (1mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5mg/kg, i.p.), WAY100635 (0.03 mg/kg, s.c.), 8-OH-DPAT (1mg/kg, i.p., a 5-HT1A receptor agonist), but not with DOI (1mg/kg, i.p., a preferential 5-HT2A receptor agonist) or ketanserin. The pre-treatment of mice with DPCPX (2mg/kg, i.p., a selective adenosine A1 receptor antagonist) or ZM241385 (1mg/kg, i.p., a selective adenosine A2A receptor antagonist) did not prevent the effect of fluoxetine (32 mg/kg, i.p., a preferential serotonin reuptake inhibitor) in the FST. Besides that, adenosine (1mg/kg, i.p.) did not produce a synergistic antidepressant-like effect with fluoxetine (10mg/kg, i.p.). Taken together, the results indicate that the antidepressant-like effect of adenosine in the FST appears to be mediated, at least in part, by an interaction with 5-HT1A receptors.     

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.     

Effects of the serotonin2A/2C receptor agonist and antagonist on phencyclidine-induced dopamine release in rat medial prefrontal cortex

1. Systemic administration of PCP (7.5 mg/kg, i.p.) produced a greater increase in extracellular DA levels in the mPFC than in the STR and NAC, as determined by in vivo microdialysis of awake, freely moving rats. Preferential activation by PCP of prefrontal DA neurons may be, at least in part, the basis for the pathophysiology of PCP-induced psychosis as well as schizophrenia. 2. Recent studies suggest a possible involvement of 5-HT2A receptors in the pathophysiology and treatment of schizophrenia. This study was designed to examine whether and how 5-HT2A receptors modulate PCP-induced DA release in the mPFC. 3. The 5-HT2A/2C receptor agonist (+/-)-DOI (2.5 mg/kg, but not 0.75 mg/kg, i.p.), administered 60 min prior to PCP, significantly attenuated the PCP-induced increase in extracellular DA levels. Pretreatment of the 5-HT2A/2C receptor antagonist ritanserin (1.0 and 5.0 mg/kg, i.p.), administered 60 min prior to PCP, did not influence the PCP-induced increase. When administered alone, neither DOI (2.5 mg/kg) nor ritanserin (1.0 mg/kg) affected basal extracellular DA levels in the mPFC. 4. The NMDA receptor antagonist MK-801 (1.0 mg/kg, i.p.) also increased extracellular DA levels in the mPFC, but this effect was unaffected by pretreatment with DOI (2.5 mg/kg). 5. These results suggest that the stimulation of 5-HT2A/2C receptors may inhibit DA release in the mPFC when it is facilitated by PCP. Other than the NMDA receptor-mediated mechanism may also be involved in the neurochemical interaction between 5-HT2A receptors and PCP in the mPFC.     

Activation of 5-HT2 receptors enhances the release of acetylcholine in the prefrontal cortex and hippocampus of the rat

The role of 5-HT2 receptors in the regulation of acetylcholine (ACh) release was examined in the medial prefrontal cortex and dorsal hippocampus using in vivo microdialysis. The 5-HT(2A/2C) agonist +/-1-(2,5-dimethoxy-4-iodophenyl) -2- aminopropane hydrochloride (DOI) (1 and 2 mg/kg, i.p.) significantly increased the extracellular concentration of ACh in both brain regions, and this response was attenuated in rats treated with the 5-HT(2A/2B/2C) antagonist LY-53,857 (3 mg/kg, i.p.). Treatment with LY-53,857 alone did not significantly alter ACh release in either brain region The 5-HT(2C) agonist 6-chloro-2-(1-piperazinyl)-pyrazine) (MK-212) (5 mg/kg, i.p.) significantly enhanced the release of ACh in both the prefrontal cortex and hippocampus, whereas the 5-HT2 agonist mescaline (10 mg/kg, i.p.) produced a 2-fold increase in ACh release only in the prefrontal cortex. Intracortical, but not intrahippocampal, infusion of DOI (100 microM) significantly enhanced the release of ACh, and intracortical infusion of LY-53,857 (100 microM) significantly attenuated this response. These results suggest that the release of ACh in the prefrontal cortex and hippocampus is influenced by 5-HT2 receptor mechanisms. The increase in release of ACh induced by DOI in the prefrontal cortex, but not in the hippocampus, appears to be due to 5-HT2 receptor mechanisms localized within this brain region. Furthermore, it appears that the prefrontal cortex is more sensitive than the dorsal hippocampus to the stimulatory effect of 5-HT2 agonists on ACh release.     

5-HT2C receptor involvement in female rat lordosis behavior

Adult, hormone-primed, ovariectomized rats (CDF-344) with bilateral implants within the ventromedial nucleus of the hypothalamus (VMN), were injected with 0.5 microgram estradiol benzoate followed 48 h later with 500 microgram progesterone. This priming produced rats with 2 different levels of sexual receptivity. Rats with a lordosis to mount ratio (L/M)>/=0.5 were used to examine the potential lordosis-inhibiting effects of the 5-HT2A receptor antagonist, R(+)-a-(2, 3-dimethoxyphenyl)-1-[2(4-fluoro-phenylethyl)]-4-piperidine-methanol (MDL 100,907), and the 5-HT2C receptor antagonist, 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2, 3-f]indole (SB 206553). Rats with low sexual receptivity (L/M<0.5) were bilaterally infused with the 5-HT2A/2C receptor agonist, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), or DOI plus either MDL 100,907 or SB 206553 to determine if either drug would attenuate the lordosis-facilitating effects of DOI. The 5-HT2C receptor antagonist, but not the 5-HT2A receptor antagonist, effectively inhibited lordosis behavior. Similarly, SB 206553 was more effective than MDL 100,907 in reducing the DOI-induced increase in lordosis responding. However, both drugs limited the duration of lordosis responding initiated by DOI. These results are consistent with prior suggestions that 5-HT2A/2C receptors within the VMN are involved in the modulation of lordosis behavior and lead to the suggestion that 5-HT2C, rather than 5-HT2A, receptors are primarily responsible for the effects of 5-HT2 receptor-active drugs on lordosis behavior.     

Serotonergic agents modulate antidepressant-like effect of the neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one in mice

The present study demonstrated the antidepressant-like effect of neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha, 5alpha THP) in mouse forced swim test of depression and its modulation by different serotonergic agents. Pretreatment with the selective serotonin reuptake inhibitor, fluoxetine (5 mg/kg, i.p.), the 5-HT releaser, fenfluramine (10 mg/kg, i.p.), the 5-HT(1A) receptor agonist, 8-OH-DPAT (0.1 mg/kg, s.c.), the 5-HT(1B/1C) receptor agonist, TFMPP (4 mg/kg, s.c.) and the 5-HT(2A/1C) receptor agonist, DOI (2 mg/kg, s.c.) potentiated the antidepressant-like effect of 3alpha, 5alpha THP. At these doses the serotonergic agents per se did not modify the duration of immobility. However, fluoxetine (20 mg/kg, i.p.), fenfluramine (20 mg/kg, i.p.) or imipramine (5 or 20 mg/kg, i.p.) not only reduced immobility but also enhanced the antidepressant-like effect of 3alpha, 5alpha THP. Such a potentiating effect of the 5-HT(1A) or the 5-HT(2A/1C) receptor agonist was not antagonized by the sub-effective dose (0.1 mg/kg, s. c.) of their respective antagonists p-MPPI or ketanserin. Pretreatment with p-CPA (300x3 mg/kg, i.p.), a depleter of 5-HT neuronal store failed to block the influence of fluoxetine and fenfluramine on antidepressant-like effect of 3alpha, 5alpha THP. The accelerated effect of 3alpha, 5alpha THP in presence of serotonergic agents was antagonized by the GABA(A) receptor antagonist, bicuculline (1 mg/kg, i.p.) or the 3alpha-hydroxysteroid oxidoreductase enzyme inhibitor, indomethacin (5 mg/kg, i.p.). These findings for the first time demonstrate that serotonergic agents potentiate the antidepressant-like action of 3alpha, 5alpha THP, by enhancing the GABAergic tone as a likely consequence of increased brain content of this neurosteroid.     

DOI, a 5-HT2A/2C receptor agonist, attenuates clozapine-induced cortical dopamine release.

(+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 1.25, 2.5 and 5 mg/kg), a serotonin (5-HT)2A/2C agonist, produced an inverted U-shaped increase in DA release in rat medial prefrontal cortex (mPFC) with a significant effect only at 2.5 mg/kg. This effect was completely abolished by M100907 (0.1 mg/kg), a 5-HT2A antagonist, and WAY100635 (0.2 mg/kg), a 5-HT1A antagonist, neither of which when given alone affected dopamine release. DOI (2.5 mg/kg), but not the 5-HT2C agonist Ro 60-0175 (3 mg/kg), attenuated clozapine (20 mg/kg)-induced mPFC dopamine release. These results suggest that 5-HT2A receptor stimulation increases basal cortical dopamine release via 5-HT1A receptor stimulation, and inhibits clozapine-induced cortical dopamine release by diminishing 5-HT2A receptor blockade.     

Depletion of systemic macrophages by liposome-encapsulated clodronate attenuates striatal macrophage invasion and neurodegeneration following local endotoxin infusion in gerbils

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) induced catalepsy in mice is modified by dopaminergic, adenosinergic and GABAergic agents. In light of serotonergic agents being implicated in antipsychotic-induced catalepsy and their ability to increase brain neurosteroid content, the present study was undertaken to investigate the effect of various 5-HT agents on catalepsy induced by 3alpha,5alpha-THP in mice. Pretreatment with selective serotonin reuptake inhibitor, fluoxetine (5 mg/kg, i.p.), 5-HT releaser, fenfluramine (10 mg/kg, i.p.), 5-HT(1A) receptor agonist, 8-OH-DPAT (0.3 mg/kg, s.c.), 5-HT1B/1C receptor agonist, TFMPP (3 mg/kg, i.p.), 5-HT2A/1C receptor agonist, DOI (1.5 mg/kg, s.c.) and 5-HT3 agonist, 2-methylserotonin (5 mg/kg, i.p.) potentiated the catalepsy induced by exogenous administration of 3alpha,5alpha-THP. Furthermore, FGIN 1-27, an MDR agonist that increases endogenous content of 3alpha,5alpha-THP although per se failed to exhibit any cataleptic effect but enhanced the cataleptic response in combination with these serotonergic agents. The potentiating action of 5-HT1A, 5-HT2A/1C or 5-HT3 receptor agonist on 3alpha,5alpha-THP induced catalepsy was not blocked by prior administration of sub-effective dose (1 mg/kg, s.c.) of their respective receptor antagonists pindolol, ritanserin or ondansetron or by pretreatment with serotonergic neurotoxin 5,7-DHT (100 microg/mouse, i.c.v.). However this effect of different serotonergic agents was antagonized by the GABA(A) receptor antagonist, bicuculline (1 mg/kg, i.p.) or the 3alpha-hydroxysteroid oxidoreductase enzyme inhibitor, indomethacin (5 mg/kg, i.p.). The 5-HT agents enhance neurosteroid-induced catalepsy by increasing GABAergic tone, likely as a consequence of increased brain content of 3alpha,5alpha-THP.     

Monoaminergic agents modulate antidepressant-like effect caused by diphenyl diselenide in rats

In this study, the antidepressant-like effect caused by diphenyl diselenide on rat forced swimming test (FST) was investigated. The involvement of the monoaminergic system in the antidepressant-like effect was also evaluated. Diphenyl diselenide (0.1-30 mg/kg), given by oral route (p.o.), 30 min earlier, reduced the immobility time in the FST, without accompanying changes in ambulation when assessed in an open field. The anti-immobility effect of diphenyl diselenide (1 mg/kg, p.o.) on the FST was prevented by pretreatment of rats with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, i.p., an inhibitor of serotonin synthesis, given once a day, for 3 consecutive days), WAY100635 (0.1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist), ketanserin (1 mg/kg, i.p., a 5-HT(2A)/(2C) receptor antagonist), ondasentron (1 mg/kg, i.p., a 5-HT(3) receptor antagonist), haloperidol (1 mg/kg, i.p., a D(1), D(2) and D(3) receptor antagonist), SCH233390 (0.05 mg/kg, s.c., a D(1) receptor antagonist), sulpiride (50 mg/kg, i.p., a D(2) receptor antagonist), prazosin (1 mg/kg, i.p., an alpha(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an alpha(2)-adrenoceptor antagonist). However, the anti-immobility effect caused by diphenyl diselenide (1 mg/kg, p.o.) on the FST was not affected by pretreatment with propanolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist). Furthermore, monoamine oxidase (MAO) activity was inhibited (39%) in the animals treated with diphenyl diselenide (30 mg/kg, p.o.) when compared to the control group. Taken together these data demonstrated that the antidepressant-like effect caused by diphenyl diselenide seems to be mediated by involvement of the central monoaminergic system.     

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.     

Serotonin 5-HT2c agonists mimic the effect of light pulses on circadian rhythms

The serotonin agonist quipazine has been shown to cause phase shifts in melatonin and activity rhythms and to induce c-fos in the suprachiasmatic nucleus of rats. In this study, in vivo pharmacological characterisation of the phase shifting properties of serotonin agonists has been performed, with a view to determining the receptor sub-types involved. Agonists for the 5-HT2a/2c receptors, (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI, 0.1 mg/k), 1-(3-chlorophenyl)-piperazine HCl (mCPP, 2 mg/kg) and N-(3-trifluoromethylphenyl)-piperazine HCl (TFMPP, 2 mg/kg) injected at CT18 resulted in acute transient inhibition of melatonin production and delays in the onset of production on the following nights of 1.2±0.2, 1.7±0.3 and 1.4±0.8 h respectively. Drugs specific for 5-HT1a/7 and 5-HT3 receptors failed to affect melatonin production. At a dose of 0.07 μmole/kg, the serotonin antagonist, ritanserin inhibited the DOI induced phase delay whereas ketanserin was ineffective at this dose, providing strong evidence that DOI was acting through 5-HT2c receptors. DOI (0.5 mg/kg) at CT18 provoked a phase delay in the core body temperature rhythm of similar magnitude to that following a light pulse. Administration of DOI but not agonists active at other receptor sites resulted in the appearance of c-Fos in the ventrolateral division of the suprachiasmatic nucleus (SCN) at CT18 but not at CT6. Ritanserin was more potent than ketanserin at inhibiting the DOI induced increase in c-Fos labelled cells in the SCN. When rats were pre-treated with metergoline (15 mg/kg), ritanserin (3 mg/kg) or LY 53,857 (3 mg/kg) prior to a 2 lx/ 1 min light pulse, none of the drugs significantly inhibited the responses to light. The results of these experiments indicate that serotonergic agonists active at the 5-HT2c receptor mimic the effects of light on 2 independent rhythms and activate SCN neurones in the rat.     

Modulation of dopamine release by striatal 5-HT2C receptors

Previous work has demonstrated that dopamine (DA) transmission is regulated by serotonin-2C (5-HT2C) receptors but the site(s) in the brain where these receptors are localized is not known. The present work utilized in vivo microdialysis to investigate the modulation of DA release by 5-HT2C receptors localized in the nerve terminal regions of the mesocortical and nigrostriatal DA pathways. Microdialysis probes implanted in the striatum or the prefrontal cortex (PFC) measured dialysate DA concentrations, while the selective 5-HT2B/2C inverse agonist SB 206553 was given locally by reverse dialysis into these terminal regions. Additionally, the effects of the 5-HT2C agonist mCPP on striatal DA were measured. Local administration of SB 206553 (0.1-100 microM) into the striatum increased DA efflux in a concentration-dependent manner. Systemic administration of mCPP (1.0 mg/kg i.p.) decreased striatal DA and attenuated the SB 206553-induced increase. In contrast, infusion of SB 206553 (0.1-500 microM) by reverse dialysis into the PFC had no significant effect on basal DA efflux in this region. Additionally, high concentrations of SB 206553 had no effect on high potassium (K(+))-stimulated DA release in the PFC. These data contribute to a body of evidence indicating that 5-HT2C receptors inhibit nigrostriatal dopaminergic transmission. In addition, the results suggest that the nigrostriatal system is regulated by 5-HT2C receptors localized in the dorsal striatum. Elucidating the mechanisms by which serotonin (5-HT) modulates striatal and prefrontocortical DA concentrations may lead to improvements in the treatment of diverse syndromes such as schizophrenia, Parkinson's disease, anxiety, drug abuse, and/or depression.     

Antidepressant-like effect of the extract from leaves of Schinus molle L. in mice: evidence for the involvement of the monoaminergic system

Schinus molle L. (Anacardiaceae), among other uses, is popularly employed for the treatment of depression. In this study, the antidepressant-like effect of the hexanic extract from leaves of S. molle was investigated in the mouse tail suspension test (TST), a predictive model of depression. The immobility time in the TST was significantly reduced by the extract (dose range 30-600 mg/kg, p.o.), without accompanying changes in ambulation when assessed in an open-field test. The efficacy of extract was found to be comparable to that of fluoxetine (10 mg/kg, p.o.). The anti-immobility effect of the extract (100 mg/kg, p.o.) was prevented by pretreatment of mice with p-chlorophenylalanine methyl ester (PCPA, 100 mg/kg, i.p., an inhibitor of serotonin synthesis, for four consecutive days), NAN-190 (0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), WAY100635 (0.1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist), ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), MDL72222 (0.1 mg/kg, i.p., a 5-HT(3) receptor antagonist), prazosin (1 mg/kg, i.p., an alpha(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an alpha(2)-adrenoceptor antagonist), SCH23390 (0.05 mg/kg, s.c., a D(1) receptor antagonist) or sulpiride (50 mg/kg, i.p., a D(2) receptor antagonist). It may be concluded that the hexanic extract of S. molle produces an antidepressant-like effect that seems to be dependent on its interaction with the serotonergic, noradrenergic and dopaminergic systems. These results provide evidence that the extract from S. molle shares with established antidepressants some pharmacological effects, at least at a preclinical level.