Strain differences in basal and post-citalopram extracellular 5-HT in the mouse medial prefrontal cortex and dorsal hippocampus: relation with tryptophan hydroxylase-2 activity

We used the microdialysis technique to compare basal extracellular serotonin (5-HT) and the response to citalopram in different strains of mice with functionally different allelic forms of tryptophan hydroxylase-2 (TPH-2), the rate-limiting enzyme in brain 5-HT synthesis. DBA/2J, DBA/2N and BALB/c mice carrying the 1473G allele of TPH-2 had less dialysate 5-HT in the medial prefrontal cortex and dorsal hippocampus (DH) (20-40% reduction) than C57BL/6J and C57BL/6N mice carrying the 1473C allele. Extracellular 5-HT estimated by the zero-net flux method confirmed the result of conventional microdialysis. Citalopram, 1.25, 5 and 20 mg/kg, dose-dependently raised extracellular 5-HT in the medial prefrontal cortex of C57BL/6J mice, with maximum effect at 5 mg/kg, but had significantly less effect in DBA/2J and BALB/c mice and in the DH of DBA/2J mice. A tryptophan (TRP) load enhanced basal extracellular 5-HT in the medial prefrontal cortex of DBA/2J mice but did not affect citalopram's ability to raise cortical and hippocampal extracellular 5-HT. The impairment of 5-HT synthesis quite likely accounts for the reduction of basal 5-HT and the citalopram-induced rise in mice carrying the mutated enzyme. These findings might explain why DBA/2 and BALB/c mice do not respond to citalopram in the forced swimming test. Although TRP could be a useful strategy to improve the antidepressant effect of citalopram (Cervo et al. 2005), particularly in subjects with low 5-HT synthesis, the contribution of serotonergic and non-serotonergic mechanisms to TRP's effect remains to be elucidated.     

Metabolism and Pharmacokinetics of Selective Serotonin Reuptake Inhibitors

1. Five drugs with the predominant pharmacologic effect of inhibiting the neuronal reuptake of serotonin are available worldwide for clinical use. This class of psychoactive drugs, known as selective serotonin reuptake inhibitors (SSRIs), is comprised of fluoxetine, sertraline, paroxetine, fluvoxamine, and citalopram.2. The SSRIs appear to share similar pharmacodynamic properties which translate to efficacy in the treatment of depression and anxiety syndromes. The drugs are differentiated by their pharmacokinetic properties with regard to stereochemistry, metabolism, inhibition of cytochrome enzymes, and participation in drug–drug interactions. Studies focusing on the relationship of plasma drug concentration to therapeutic and adverse effects have not confirmed the value of plasma concentration monitoring.3. This review summarizes the metabolism and relevant pharmacokinetic properties of the SSRIs.     

Species-scanning mutagenesis of the serotonin transporter reveals residues essential in selective, high-affinity recognition of antidepressants

The serotonin transporter (SERT) is a high-affinity sodium/chloride-dependent neurotransmitter transporter responsible for reuptake of serotonin from the extracellular space. SERT is a selective target of several clinically important antidepressants. In a cross-species analysis comparing human and bovine SERTs, the kinetic parameters for serotonin uptake were found to be similar, however, the pharmacological profiles of the two transporters differ. Following transient expression in COS-1 cells, IC(50) values were determined for several antidepressants and psychostimulants. The potencies of the antidepressants citalopram, fluoxetine, paroxetine and imipramine were several-fold higher at hSERT compared with bSERT. No species selectivity was observed for the antidepressants fluvoxamine, and sertraline or for the psychostimulants cocaine, the cocaine analogue beta-carbomethoxy-3beta-(4-iodophenyl)tropane, or for 3,4-methylenedioxymethamphetamine (MDMA). Analysis of six hSERT/bSERT chimeras and subsequent species-scanning mutagenesis of each isoform revealed methionine-180, tyrosine-495, and phenylalanine-513 to be responsible for the increase in citalopram and paroxetine potencies at hSERT and methionine-180 and phenylalanine-513 to confer species selectivity at hSERT for fluoxetine and imipramine. Results were obtained by doing the forward, bovine to human, mutations and confirmed by doing the reverse mutations. Citalopram analogues were used to define the roles of methionine-180, tyrosine-495, and phenylalanine-513 and to reveal molecular interactions with individual functional groups of citalopram. We suggest that methionine-180 interacts with the heterocyclic nucleus of citalopram or stabilizes the binding pocket and phenylalanine-513 to be a steric blocker of antidepressant recognition.     

Selective serotonin reuptake blockade increases extracellular dopamine in noradrenaline-rich isocortical but not prefrontal areas: dependence on serotonin-1A receptors and independence from noradrenergic innervation

The present study investigated the effects of two serotonin (5-HT) uptake inhibitors, citalopram and paroxetine, and of a non-selective noradrenaline (NA) and 5-HT uptake blocker, imipramine, on extracellular NA and dopamine (DA) in the prefrontal cortex (PfCX), parietal cortex (ParCX) and occipital cortex (OccCX). Citalopram, the most selective 5-HT uptake blocker, increased dialysate DA in the OccCX and ParCX but not in the PfCX and this effect was prevented in the OccCX by WAY-100635, an antagonist of serotonin-1A (5-HT(1A)) receptors, but not by dorsal noradrenergic bundle (DNAB) lesions that reduced to unmeasurable levels basal dialysate NA but did not affect dialysate DA. Paroxetine, a less selective 5-HT uptake inhibitor than citalopram, at the dose of 5 mg/kg, increased DA in the OccCX but not in the PfCX; however, at doses of 10 mg/kg, which increase PfCX NA, paroxetine increased DA also in this area. Imipramine increased dialysate DA and NA both in the PfCX and in the OccCX and this effect was abolished by DNAB lesions and was reduced but not abolished by WAY-100635. Administration of doses of reboxetine and citalopram that do not increase DA release in the OccCX if given separately, markedly increased DA when combined. These results indicate that endogenous 5-HT, raised by selective blockade of the 5-HT carrier, can increase extracellular DA in the OccCX and in the ParCX by stimulating 5-HT(1A) receptors independently from the presence of NA terminals, although blockade of 5-HT and NA carrier can strongly interact to raise extracellular DA in this area. These observations are consistent with the existence of DA neurons separate from the NA ones contributing to extracellular DA even in NA-rich/DA poor isocortical areas.     

西酞普兰对应激大鼠额叶皮质神经细胞PCNA、C-fos表达及凋亡的影响

目的：研究西酞普兰对慢性应激大鼠额叶皮质神经细胞增殖细胞核抗原（PCNA）、原癌基因蛋白（C-fos）表达及凋亡的影响。方法：将24只健康雄性SD大鼠随机分为3组（n=8）：对照组（不做任何处理）、应激组（应激＋生理盐水灌胃）、实验组（应激＋西酞普兰灌胃）,采用强迫游泳建立慢性应激模型,用免疫组化法检测PCNA、C-fos蛋白表达水平;TUNEL法检测细胞凋亡情况;尼康图像分析软件测量各指标阳性细胞数量。结果：应激组与对照组比较,可见少量PCNA表达阳性细胞、大量c-fos表达阳性细胞,阳性细胞的体积明显缩小。实验组与应激组比较,可见PCNA表达阳性细胞增多、C-fos表达阳性细胞明显减少,TUNEL阳性细胞数量减少,核浓缩现象较应激组明显减轻,染色也明显变淡,上述差异均有统计学意义（P〈0．05）。结论：慢性应激可影响大鼠额叶皮质神经细胞PCNA、C-fos蛋白表达水平,促进细胞凋亡,西酞普兰可调控额叶皮质神经细胞PCNA、C-fos蛋白表达水平,拮抗细胞凋亡,这可能是西酞普兰预防和治疗慢性应激引起的精神心理疾病的机制之一。     

Antagonism of 5-HT1A receptors uncovers an excitatory effect of SSRIs on 5-HT neuronal activity, an action probably mediated by 5-HT7 receptors

Both microdialysis and electrophysiology were used to investigate whether another serotonin (5‐HT) receptor subtype next to the 5‐HT_1A autoreceptor is involved in the acute effects of a selective serotonin reuptake inhibitor on 5‐HT neuronal activity. On the basis of a previous study, we decided to investigate the involvement of the 5‐HT₇ receptors. Experiments were performed with the specific 5‐HT₇ antagonist SB 258741 and the putative 5‐HT₇ agonist AS19. In this study WAY 100.635 was used to block 5‐HT_1A receptors. Systemic administration of SB 258741 significantly reduced the effect of combined selective serotonin reuptake inhibitor and WAY 100.635 administration on extracellular 5‐HT in the ventral hippocampus as well as 5‐HT neuronal firing in the dorsal raphe nucleus. In the microdialysis study, co‐administration of AS19 and WAY 100.635 showed a biphasic effect on extracellular 5‐HT in ventral hippocampus, hinting at opposed 5‐HT₇ receptor mediated effects. In the electrophysiological experiments, systemic administration of AS19 alone displayed a bell‐shaped dose–effect curve: moderately increasing 5‐HT neuronal firing at lower doses while decreasing it at higher doses. SB 258741 was capable of blocking the effect of AS19 at a low dose. This is consistent with the pharmacological profile of AS19, displaying high affinity for 5‐HT₇ receptors and moderate affinity for 5‐HT_1A receptors. The data are in support of an excitatory effect of selective serotonin reuptake inhibitors on 5‐HT neuronal activity mediated by 5‐HT₇ receptors. It can be speculated, that the restoration of 5‐HT neuronal firing upon chronic antidepressant treatment, which is generally attributed to desensitization of 5‐HT_1A receptors alone, in fact results from a shift in balance between 5‐HT_1A and 5‐HT₇ receptor function.     

Simultaneous determination of citalopram and tadalafil by the second derivative synchronous fluorescence method in biological fluids; application of Box–Behnken optimization design

This is the first study focusing solely on that determination of tadalafil in the presence of citalopram as an antidepressant drug. The determination in biological fluids of a co‐administered antidepressant drug and a sexual stimulation drug is a very critical and important step for psychotic and ischaemic heart disease patients, especially in cases of emergency and this requires therapeutic drug monitoring. A sensitive, efficient and rapid assay was selected satisfactorily and applied for simultaneous determination of citalopram and tadalafil either in their pure forms, in tablet dosage forms or in spiked human plasma. There was a large overlap for both drugs, forming the broad band found in conventional fluorescence spectra and their related synchronous fluorescence intensity. Therefore, the development of a highly sensitive second derivative synchronous fluorescence method was demonstrated that removed this overlap. The proposed method depended on measuring the amplitudes of the second derivative of synchronous fluorescence intensity at suitable wavelengths of 301 nm and 367 nm for citalopram and tadalafil at Δλ = 60 nm, respectively. Box–Behnken design as a response surface methodology was used to fit models and create an optimization process encompassing a set of factors and resulting in an optimum response value specifically designed for this method. Under optimum conditions, the linear dynamic ranges for citalopram and tadalafil estimation were 20–900 and 5–400 ng ml^?1 with detection limits of 5.40 and 1.43 ng ml^?1, respectively.