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ABSTRACT: Mirtazapine, an antidepressant, antagonizes α(2)-adrenergic autoreceptors and heteroreceptors, which leads to enhanced noradrenergic and serotonergic transmission without inhibiting monoamine transporters. Using a microdialysis technique, we investigated whether co-administration of mirtazapine and a serotonin noradrenaline reuptake inhibitor (SNRI), milnacipran, augments the effects of each drug on the extracellular levels of monoamines by pharmacological synergy. Mirtazapine increased the extracellular levels of noradrenaline and serotonin in the dorsal hippocampus. In contrast, it increased the levels of noradrenaline and dopamine without changing serotonin levels in the prefrontal cortex. Milnacipran increased the levels of all monoamines evaluated in both areas, and the combined treatment with mirtazapine augmented these changes. The combined treatment with idazoxan, an α(2) adrenoceptor antagonist, and milnacipran also increased all monoamine levels in the prefrontal cortex. Ketanserin, a serotonin 5-HT(2A) receptor antagonist, showed no effect in combination with milnacipran, while SB242084, a 5-HT(2C) receptor antagonist, augmented the effects of milnacipran on the levels of serotonin and dopamine in the prefrontal cortex. These results suggest that combined treatment with mirtazapine and milnacipran augments the extracellular levels of noradrenaline, serotonin and dopamine through the blockade of α(2) adrenoceptors without regional specificity, whereas mirtazapine enhances serotonergic transmission in a region-specific manner. 5-HT(2C) receptor antagonism may also partly contribute to the amplification effects of mirtazapine on serotonin and dopamine levels. These neurochemical changes could play a role in reported advantageous clinical effects in patients treated with an SNRI and mirtazapine.
Neuropharmacology 02/2012; 62(7):2278-87. · 4.81 Impact Factor
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ABSTRACT: As a part of our search for novel histamine H3 receptor agonists, we designed and synthesized hybrid compounds in which the lipophilic (4'-alkylphenylthio)ethyl moiety of a novel H3 receptor agonist, 4-(2-(4'-tert-butylphenylthio)ethyl)-1H-imidazole (1), was incorporated into N(alpha)-methylhistamine, immepip, and immethridine derivatives. These hybrid compounds were expected to interact concurrently with the histamine-binding site and a putative hydrophobic region in the H3 receptor. Among them, piperidine- and pyridine-type derivatives displayed partial agonist activity, and (S)-4-(1-(1H-imidazol-4-yl)-2-(4-(trifluoromethyl)phenylthio)ethyl)piperidine (36) was identified as a potent H3 agonist. We performed computational docking studies to examine the binding mode of the agonists. The results indicated that immepip interacts with the key residues, Asp114 and Glu206, in a different manner from histamine. The binding mode of 36 to these residues is similar to that of immepip, and the lipophilic tail of 36 has an additional interaction with a hydrophobic region in transmembrane helix 6 of the receptor. These results indicated that 36 served as a useful tool for studies on receptor-agonist interactions and drug design.
Journal of Medicinal Chemistry 09/2010; 53(17):6445-56. · 4.80 Impact Factor
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ABSTRACT: 4-((1H-imidazol-4-yl)methyl)-1-aryl-piperazine and piperidine derivatives were designed and synthesized as candidate human histamine type 3 agonists. The piperazine derivatives were found to have low (or no) affinity for human histamine H3 receptor, whereas the piperidine derivatives showed moderate to high affinity, and their agonistic activity was greatly influenced by substituents on the aromatic ring. Among the piperidine-containing compounds, 17d and 17h were potent human histamine H3 receptor agonists with high selectivity over the closely related human H4 receptor. Our results indicate that appropriate conformational restriction, that is, by the piperidine spacer moiety, favors specific binding to the human histamine H3 receptor.
Bioorganic & medicinal chemistry 07/2010; 18(14):5441-8. · 2.82 Impact Factor
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ABSTRACT: The terminal nitrogen atom of histamine was modified with lipophilic substituents to investigate the structure-activity relationship of histamine type 3 receptor (H3R) agonists. The introduction of an alkylated benzene rings maintained or increased the H3R binding affinity. The most potent compound, 4-(2-(4-tert-butylphenylthio)ethyl)-1H-imidazole, possessed in vivo agonistic activity, decreasing brain N(tau)-methylhistamine levels in mice after oral administration. It also exhibited antistress activity in the mouse resident-intruder test.
Journal of Medicinal Chemistry 04/2010; 53(9):3840-4. · 4.80 Impact Factor
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ABSTRACT: Histamine H3 receptor functions as a presynaptic auto- and hetero-receptor on histaminergic and non-histaminergic neurons in the brain regulating the synaptic release of numerous neurotransmitters. Therefore, the ligands for this receptor have been proposed to be of therapeutic interest for the treatment of various neuropsychiatric disorders. At present, however, the psychopharmacological profiles of H3 ligands, particularly H3 agonists, have not been extensively studied.
The present study investigated the anxiolytic-like profiles of H3-selective agonists in a variety of classical (benzodiazepine-sensitive) and atypical (antidepressant-effective) animal models of anxiety. Comparator drugs used were diazepam and both fluvoxamine and desipramine in the former and latter models, respectively.
H3 agonist R-alpha-methylhistamine and immepip were inactive in rat elevated plus maze test and Vogel type conflict test where diazepam (5 mg/kg) produced significant anxiolytic-like effects. Meanwhile, these H3 agonists (10-30 mg/kg) significantly reduced isolation-induced vocalizations in guinea pig pups and isolation-induced aggressive behavior in mouse resident-intruder test. Moreover, in rat conditioned fear stress test, R-alpha-methylhistamine (30 mg/kg) and immepip (10 mg/kg) significantly decreased freezing time, which were completely reversed by concomitant treatment with H3 antagonist, thioperamide (10 mg/kg). In contrast to the limited efficacy obtained with desipramine (30 mg/kg), fluvoxamine (20-60 mg/kg) exhibited anxiolytic-like effects in all the latter three atypical models.
These data suggest that the H3 agonists may have anxiolytic-like effects similar to those of selective serotonin reuptake inhibitors but not benzodiazepine anxiolytics and represent a novel strategy for the treatment of some anxiety disorders in which selective serotonin reuptake inhibitors are prescribed.
Psychopharmacologia 05/2009; 205(2):177-87. · 4.08 Impact Factor
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ABSTRACT: Mirtazapine is an antidepressant with a unique mechanism of action and has been categorized as a Noradrenergic and Specific Serotonergic Antidepressant (NaSSA). Although numerous clinical trials suggested the usefulness of mirtazapine for not only major depressive disorders but also a variety of anxiety disorders, efficacy studies in animal anxiety models have been rarely reported. The present study investigated a potential anxiolytic-like profile of mirtazapine in rat conditioned fear stress model. A 5-hydroxytryptamine (5-HT) 1A receptor partial agonist, buspirone (1-5 mg/kg) exhibited a significant reduction in freezing time, and its maximal effect was reversed by a selective 5-HT(1A) antagonist, WAY-100635 (1 mg/kg). Mirtazapine (1-10 mg/kg) also reduced the freezing time in a dose-related fashion, a substantial proportion (approx. 50%) of which was likewise antagonized by WAY-100635 (1 mg/kg). Mianserin (1-30 mg/kg), a structural analogue for mirtazapine, was ineffective. Furthermore, co-administration of alpha1 adrenoceptor antagonist, prazosin (0.03 mg/kg) completely reversed mirtazapine (10 mg/kg)-induced reduction of freezing time. These findings represent the first demonstration that the anxiolytic-like action of mirtazapine involves activation of 5-HT(1A) receptor and alpha1 adrenoceptor to different extents, and are compatible with one aspect of mirtazapine's pharmacological profile as NaSSA.
Pharmacology Biochemistry and Behavior 02/2009; 92(3):393-8. · 2.53 Impact Factor
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ABSTRACT: Some selective serotonin reuptake inhibitors (SSRIs) have recently been approved for the treatment of anxiety disorders such as obsessive-compulsive disorder, panic disorder and social anxiety disorder, and they are considered first-line treatment for anxiety disorders in Japan as well as in other countries. Previous clinical studies have suggested that the 5-HT2C receptors in subjects with anxiety disorders are hypersensitive. We recently reported that chronic treatment with fluvoxamine or paroxetine desensitized 5-HT2C receptor function. The desensitization of 5-H T2C receptor function has also been reported with other SSRIs and is considered to be a common mechanism of action of SSRIs in the treatment of anxiety disorders. In addition, some studies have suggested that 5-HT2A receptors and 5-HTIA receptors participate in anxiety disorders and the therapeutic mechanism. Both clinical studies and animal studies have indicated that the amygdala plays an essential role in anxiety and fear response. Thus, it may be important to elucidate functional changes in these 5-HT receptor subtypes in brain regions including the amygdala under the chronic administration of SSRIs to understand the anxiolytic mechanism of SSRIs.
Nihon shinkei seishin yakurigaku zasshi = Japanese journal of psychopharmacology 12/2006; 26(5-6):193-8.
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ABSTRACT: We have previously shown that chronic treatment with selective serotonin reuptake inhibitors (SSRIs), fluvoxamine and paroxetine, attenuated m-chlorophenylpiperazine (mCPP)-induced hypolocomotion in rats. The effect of these SSRIs on the response to mCPP is thought to be caused by the desensitization of 5-HT2C receptor function. In the present study, we investigated whether chronic administration of SSRI could reduce another pharmacological response to mCPP in rats, i.e., the induction of the secretion of corticosterone. The mCPP-induced increase in the serum concentration of corticosterone was not blocked by the 5-HT2C antagonist SB242084, but was blocked by the 5-HT2A antagonist ketanserin. Chronic treatment with fluvoxamine and paroxetine attenuated the response to mCPP, while these SSRIs had no effects in control rats. These results suggest that the desensitization of 5-HT2A receptor function occurs in the same way as that of 5-HT2C receptor function through chronic treatment with either fluvoxamine or paroxetine as a consequence of prolonged exposure to elevated levels of serotonin. The hypersensitivity of 5-HT2A receptors is observed in depressed patients, and chronic treatment with many antidepressants such as tricyclic antidepressants have been reported to reduce 5-HT2A receptor density and/or efficacy. The desensitization of 5-HT2A receptor function might contribute to the therapeutic mechanism of action of these SSRIs, as seen with other classes of antidepressants.
Brain Research 02/2006; 1067(1):164-9. · 2.73 Impact Factor
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ABSTRACT: The effectiveness of fluvoxamine, a selective serotonin re-uptake inhibitor (SSRI), in the treatment of anxiety disorders, such as obsessive-compulsive, panic and social anxiety disorders, has been confirmed in clinical studies. The hypersensitivity of 5-HT2C receptors has been reported in subjects with these disorders, and SSRIs have been suggested to have therapeutic effects in such cases through the desensitization of the 5-HT2C receptor function. In the present study, we investigated whether chronic administration of fluvoxamine desensitizes 5-HT2C receptors using a putative in vivo rat model of 5-HT2C receptor function. Acute treatment with fluvoxamine or another SSRI, paroxetine, reduced spontaneous locomotion, as observed with the administration of m-chlorophenylpiperazine (mCPP). This effect of fluvoxamine was reversed by treatment with a selective 5-HT2C receptor antagonist, SB 242084. On the other hand, chronic treatment with fluvoxamine or paroxetine inhibited mCPP-induced hypolocomotion, while they had no effects in control rats. In addition, chronic treatment with these drugs had no effects on the mCPP concentration in the rat brain. These results suggest that 5-HT2C receptors are desensitized by chronic treatment with fluvoxamine, as well as paroxetine. Thus, the clinical efficacy of fluvoxamine on anxiety disorders might involve the normalization of the 5-HT2C receptor function.
Pharmacology Biochemistry and Behavior 09/2004; 78(4):683-9. · 2.53 Impact Factor
