Fluoxetine, but not other selective serotonin uptake inhibitors, increases norepinephrine and dopamine extracellular levels in prefrontal cortex

RATIONALE: The selective serotonin uptake inhibitor (SSRI) fluoxetine has been shown to not only increase the extracellular concentrations of serotonin, but also dopamine and norepinephrine extracellular concentrations in rat prefrontal cortex. The effect of other SSRIs on monoamine concentrations in prefrontal cortex has not been thoroughly studied. OBJECTIVE: The aim of this study was to compare the ability of five systemically administered selective serotonin uptake inhibitors to increase acutely the extracellular concentrations of serotonin, norepinephrine and dopamine in rat prefrontal cortex. METHODS: The extracellular concentrations of monoamines were determined in the prefrontal cortex of conscious rats using the microdialysis technique. RESULTS: Fluoxetine, citalopram, fluvoxamine, paroxetine and sertraline similarly increased the extracellular concentrations of serotonin from 2- to 4-fold above baseline. However, only fluoxetine produced robust and sustained increases in extracellular concentrations of norepinephrine and dopamine after acute systemic administration. Fluoxetine at the same dose blocked ex vivo binding to the serotonin transporter, but not the norepinephrine transporter, suggesting that the increase of catecholamines was not due to non-selective blockade of norepinephrine uptake. Prefrontal cortex extracellular concentrations of fluoxetine at the dose that increased extracellular monoamines were 242 nM, a concentration sufficient to block 5-HT(2C) receptors which is a potential mechanism for the fluoxetine-induced increase in catecholamines. CONCLUSION: Amongst the SSRIs examined, only fluoxetine acutely increases extracellular concentrations of norepinephrine and dopamine as well as serotonin in prefrontal cortex, suggesting that fluoxetine is an atypical SSRI.     

Modification of norepinephrine and serotonin, but not dopamine, neuron firing by sustained bupropion treatment

RATIONALE: Bupropion is widely used in the treatment of depression and as an anti-craving medication for the cessation of tobacco smoking. Because it is a very weak inhibitor of norepinephrine (NE) and dopamine (DA) reuptake, its mechanisms of action remain to be elucidated. METHODS: Bupropion was administered subcutaneously via osmotic minipumps over 2 days to determine its effects on the spontaneous firing activity of NE, serotonin (5-HT), and DA neurons in the brain of anaesthetised male Sprague-Dawley rats. This treatment was used in order to obtain levels of the parent compound and its putatively active metabolites that would more adequately reflect the clinical condition than utilizing acute injections. RESULTS: When given by minipump for 2 days, bupropion produced a dose-dependent attenuation of the mean spontaneous firing NE neurons (7.5 mg/kg per day: 15%; 15 mg/kg per day: 61%; 30 mg/kg per day: 80%) which was reversed by the alpha 2-adrenoceptor antagonist idazoxan. At the highest regimen, the mean firing rate of 5-HT neurons was 100% higher than in control rats, but unaffected in NE-lesioned rats. In contrast, DA neurons in the ventral tegmental area displayed a normal firing rate during the latter bupropion treatment. CONCLUSIONS: Sustained bupropion administration decreased the firing rate of NE neurons due to an increased activation of their inhibitory somatodendritic alpha 2-adrenoceptors. This effect of the bupropion treatment would be attributable mainly to an enhancement of NE release and not to reuptake inhibition. This contention is based essentially on the observation that NE reuptake blockers leave unaltered the firing rate of 5-HT neurons, whereas bupropion enhanced it via a NE-dependent mechanism. The present study did not put into evidence any DA activity of bupropion at the level of the cell body of mesolimbic/cortical DA neurons at a regimen exerting profound alterations of the firing activity of NE and 5-HT neurons.     

MDMA (Ecstasy) and human dopamine,norepinephrine, and serotonin transporters: implications for MDMA-induced neurotoxicity and treatment

Rationale 3,4-Methylenedioxymethamphetamine (MDMA, designated as “Ecstasy” if illicitly marketed in tablet form) induces significant decrements in neuronal serotonin (5-HT) markers in humans, nonhuman primates, and rats as a function of dosing and dosing regimen. In rats, MDMA-mediated effects are attributed, in part, to selective high-affinity transport of MDMA into 5-HT neurons by the 5-HT transporter (SERT), followed by extensive 5-HT release.Objectives To clarify whether SERT-selective effects of MDMA at human monoamine transporters can account for the reported MDMA-induced selective toxicity of serotonin neurons in primate brain.Methods We investigated the interaction of [³H](±, RS)- (+, S)- and (−, R)-MDMA with the human SERT, dopamine (DA) transporter (DAT), and norepinephrine (NE) transporter (NET) in stably transfected human embryo kidney (HEK)-293 cells.Results The human DAT, NET, and SERT actively transported [³H]RS(±)-MDMA saturably, stereoselectively, and in a temperature-, concentration-, and transporter-dependent manner. MDMA exhibited the highest affinity for the NET≫SERT≥DAT, the same rank order for MDMA inhibition of [³H]DA, [³H]NE, and [³H]5-HT transport and stimulated release of the [³H]monoamines, which differed from reports derived from rodent monoamine transporters. The extent of MDMA-induced release of 5-HT was higher compared with release of DA or NE.Conclusions The affinity of MDMA for the human SERT in transfected cells does not clarify the apparent selective toxicity of MDMA for serotonin neurons, although conceivably, its higher efficacy for stimulating 5-HT release may be a distinguishing factor. The findings highlight the need to investigate MDMA effects in DAT-, SERT-, and NET-expressing neurons in the primate brain and the therapeutic potential of NET or DAT inhibitors, in addition to SERT-selective inhibitors, for alleviating the pharmacological effects of MDMA.Klaus A. Miczek as Principal Editor—the special issue “A Contemporary View of MDMA.”This research was presented by C. Verrico et al. in abstract forms at the annual meeting of the Society for Neuroscience 2003 and 2004.     

Extracts of Fructus Akebiae:A novel serotonin,norepinephrine and dopamine reuptake inhibitor

OBJECTIVE Fructus Akebiae(FAE) is a traditional Chinese herbal that has been used for the treatment of depressive disorders in China.To determine the content of hederagenin and identify the substance exerting antidepressant activity from the extracts of FAE which was extracted by systemic solvent segregation.To assess pharmacological acute and sub-chronic antidepressant activity of FAE by using behavioural despair animal models.In currently study,we used a series of competitive radio ligand binding and transporter inhibition bioassays to determined FAE affinities at rat and cloned human(h) SERTs,NETs,and DA transporters(DAT).In contrast,we also evaluated the affinity of FAE to numerous other targets(D1,D3,D2,D4,D5,5-HT1A,5-HT1B,5-HT1D,5-HT2a,5-HT2C,5-HT3,5-HT5A,5-HT6,5-HT7,α1A,α1B,α2A,β1,M1,M2,M3,M4,M5,μ,κ,δ,A1,A2A,H1,H2,H3,H4) to determine its selectivity.METHODS As monoaminergic system is one of the most important targets in the pathophysiology and therapy of depression,our current studies were designed to explore the effect of FAE on monoaminergic systems.Here,we used a series of competitive radio ligand binding and transporter inhibition bioassays to determined FAE affinities to the rat and cloned human(h) SERTs,NETs,and DATs.We also evaluated the affinity of FAE to numerous other targets to determine its selectivity.In addition,we tested the effect of FAE on the transport activities of three monoamines,5-HT,NE and DA,using transporter-transfected HEK cell line as well as rat brain synaptosomal preparations.RESULTS We found that FAE showed a high affinity for rat monoamine transporters including SERT,NET and DAT with Ki value 3.89 nmol·L-1,0.22 nmol·L-1 and 2.87 nmol·L-1.FAE showed high affinity to human monoamine transporters with IC50 value 1.65 nmol·L-1,0.14 nmol·L-1 and 1.03 nmol·L-1,consistent with results from rats.Our results showed that FAE inhibited all three transporters(SERT,DAT and NET) in a dose-dependent manner with IC50 value 3.04 nmol·L-1,0.86 nmol·L-1 and 2.08 nmol·L-1.We found FAE also significantly inhibited the activities of NET,SERT and DAT in these transfected cell lines.The Ki values for FAE to inhibit the uptake of -5-HT,-5-NE,and -DA into cells expressing the corresponding human recombinant transporters were 1.34±0.04,0.15±0.02 nmol·L-1 and(1.09±0.12)nmol·L-1,respectively.CONCLUSION In our present study,we found that FAE bound to NET,SERT and DAT with high affinity,and potently inhibited 5-HT,NE and DA reuptake in both rat synaptosomal preparations and intact cell line expressing human transporters,suggesting that FAE is a novel triple reuptake inhibitor.We conducted our assays using both rat brain synaptosomes and human transporter-transfected cell line to evaluate effect of FAE for clarifying whether there would be an issue with species difference.FAE is a novel potent triple reuptake inhibitor that can bind to all three-monoamine transporters(NET,SERT and DAT) with high affinities and selectivity.Our results suggest that FAE may represent a new class of antidepressants with fewer side effects.     

Orexin A promotes histamine, but not norepinephrine or serotonin, release in frontal cortex of mice

Aim: To investigate the effects of orexin A on release of histamine, norepinephrine, and serotonin in the frontal cortex of mice. Methods: Samples for measuring histamine, norepinephrine, and serotonin contents were collected by in vivo microdialysis of the frontal cortex of anesthetized mice. The histamine, noradrenaline, and serotonin content in dialysates were measured by HPLC techniques. Results: Intracrebroventricular injection of orexin A at doses of 12.5, 50, and 200 pmol per mouse promoted histamine release from the frontal cortex in a dose-dependent manner. At the highest dose given, 200 pmol, orexin A significantly induced histamine release, with the maximal magnitude being 230% over the mean basal release. The enhanced histamine release was sustained for 140 min, and then gradually returned to the basal level. However, no change in norepinephrine or serotonin release was observed under application of the same dose of orexin A. Conclusion: These results suggest that the arousal effect of orexin A is mainly mediated by histamine, not by norepinephrine or serotonin.     

Pharmacological antagonism of facilitatory but not inhibitory effects of serotonin and norepinephrine on excitability of spinal motoneurons

Serotonin (5HT) and norepinephrine (NE) produced long-lasting facilitation of glutamate-evoked activity of spinal motoneurons when applied iontophoretically with small ejection currents into the ventral horn. The facilitation was usually preceded by a brief period of inhibition at the onset of current application to the monoamine-containing barrels. This inhibition did not outlast the current application. Ejection of hydrogen ions produced only inhibition of glutamate-evoked activity with no subsequent facilitation at current offset. The 5HT antagonists, methysergide and metergoline, blocked the facilitation, but not the inhibition of motoneuron excitability caused by 5HT. Similarly, the alpha-adrenergic antagonists, piperoxane and phentolamine, blocked the facilitatory, but not the inhibitory, effects of NE on excitability of motoneurons. Since the inhibitory effects of 5HT and NE could not be blocked with the antagonists used, and since ejection of hydrogen ions also produced inhibition, non-specific causes for the inhibitory effects of 5HT and NE could not be rejected. However, the facilitatory effects of 5HT and NE on excitability of motoneurons were readily blocked by antagonists and were, therefore, attributed to actions on separate 5HT and NE receptors in the ventral horn.     

The effect of hepatic or renal impairment on the pharmacokinetics of edivoxetine, a selective norepinephrine transporter reuptake inhibitor

Purpose

To assess the impact of hepatic or renal impairment on the pharmacokinetics (PK) of edivoxetine.

Methods

Two separate multi-center, open-label studies with males and females were conducted. Subjects were categorized according to their hepatic function, determined by the Child–Pugh classification, or renal function, determined by creatinine clearance using the Cockcroft–Gault equation. Subjects received a single dose of 18 mg in the hepatic impairment study or 6 mg in the renal impairment study. Noncompartmental PK parameters were computed from the edivoxetine plasma concentration–time data.

Results

In the hepatic study, the geometric least squares mean (GLSM) and 90 % confidence interval (CI) of the ratio [impaired : normal] of area under the concentration versus time curve from time zero to infinity (AUC0-∞; h?×?ng/mL) was 1.24 (0.93, 1.64) in the mild, 1.60 (1.21, 2.12) in the moderate, and 1.70 (1.28, 2.24) in the severe group. In the renal impairment study, the GLSM (90 % CI) of the ratio [impaired : normal] of AUC0-∞ was 1.13 (0.73, 1.73) in mild, 1.90 (1.28, 2.82) in moderate, 1.55 (0.94, 2.55) in severe, and 1.03 (0.66, 1.59) in ESRD groups. Overall, the GLSM of the ratio [impaired : normal] of C_max was slightly less than or approximately 1 across the hepatic and renal impairment groups. Across both studies, there were no clinically significant changes in vital signs and laboratory values, the adverse events were mild in severity and mostly related to nervous system and gastrointestinal disorder-related events.

Conclusions

PK changes in subjects with hepatic or renal impairment were of small magnitude and did not appear to impact overall subject tolerability. Daily dosing of edivoxetine in a larger population of impaired subjects, including those with dual impairment, would aid in establishing edivoxetine tolerability and PK in a clinical practice scenario.     

Dopamine but not norepinephrine or serotonin uptake inhibitors protect mice against neurotoxicity of MPTP

Combined administration of nomifensine, a DA reuptake inhibitor, and MPTP completely prevented the long-term (30 days post-treatment) striatal DA depletions induced by MPTP in mice. Cotreatment with desipramine and clomipramine or fluoxetine, inhibitors of NE and 5-HT, respectively had no effect on DA neurotoxicity of MPTP. The findings indicate that MPTP (or MPP+) is a substrate for the specific DA reuptake system and may explain, in part, its selective toxic effects on DA neurons.     

Neonatal intraspinal 6-hydroxydopamine: Depletion of norepinephrine but not dopamine

Neonatal intraspinal 6-hydroxydopamine (6-OHDA), administered in two 10 μg doses at 24 h intervals, severely depleted spinal norepinephrine (NE) for at least 200 days. This neurochemical lesion was accompanied by a functional loss of spinal NE neurotransmission, as measured by the absence of the post decapitation reflex. While these data contraindicated regeneration of spinal NE terminals, enhanced brainstem NE levels in these rats provided evidence for terminal arborization, possibly representing aborted regrowth of NE fibres into the cord. Neither 6-OHDA alone nor in combination with desmethylimipramine pretreatment depleted spinal cord dopamine of serotonin.     

Sustained administration of bupropion alters the neuronal activity of serotonin, norepinephrine but not dopamine neurons in the rat brain

Bupropion is widely used in the treatment of depression. There are, however, limited data on its long-term effects on monoaminergic neurons and therefore the mechanism of its delayed onset of action is at present not well understood. The present study was conducted to examine the effects of prolonged bupropion administration on the firing activity of dorsal raphe nucleus (DRN), locus coeruleus (LC), and ventral tegmental area (VTA) neurons. Spontaneously firing neurons were recorded extracellularly in rats anesthetized with chloral hydrate. Bupropion (30 mg/kg/day) was administered using subcutaneously implanted minipumps. In the DRN, the firing rate of serotonin (5-HT) neurons was significantly increased after 2, 7 and 14 days of administration. The suppressant effect of LSD was significantly diminished after the two-day regimen, indicating a desensitization of 5-HT_1A autoreceptors. In the LC, the firing rate of norepinephrine (NE) neurons was significantly attenuated after a 2-day regimen, but recovered progressively over 14 days of administration. The suppressant effect of clonidine on NE neuronal firing was significantly attenuated in rats treated with bupropion for 14 days, indicating a desensitization of α₂-adrenoceptors. In the VTA, neither 2 nor 14 days of bupropion administration altered the firing and burst activity of dopamine neurons. These results indicate that bupropion, unlike 5-HT reuptake inhibitors, promptly increased 5-HT neuronal activity, due to early desensitization of the 5-HT_1A autoreceptor. The gradual recovery of neuronal firing of NE neurons, due to the desensitization of α₂-adrenoceptors, in the presence of the sustained increase in 5-HT neuronal firing, may explain in part the delayed onset of action of bupropion in major depression.     

Chronic administration of the selective dopamine uptake inhibitor GBR 12909, but not cocaine, produces marked decreases in dopamine transporter density

The chronic continuous infusion of cocaine produces partial behavioral tolerance to cocaine and tolerance to the inhibition of dopamine uptake by cocaine, without changing dopamine transporter binding. In order to examine more closely the dopaminergic contribution to this effect, the selective dopamine uptake inhibitor GBR 12909 (30mg/kg/day), cocaine (50mg/kg/day), or vehicle, were continuously infused via osmotic minipump, and their effects on the dopamine transporter examined. Drug and vehicle pumps were implanted into male Sprague-Dawley rats and removed after seven days. [³H]WIN 35,428 binding and [³H]dopamine uptake were measured in caudate putamen and nucleus accumbens at varying intervals after pump removal. The B _max for [³H]WIN 35,428 binding was decreased by approximately 75% in the caudate putamen and by 40% in the nucleus accumbens of GBR 12909-treated rats both 1 and 4 days after pump removal, and was still significantly decreased after 10 days, but had returned to normal by 20 days post-treatment. In contrast, cocaine did not significantly alter [³H]WIN 35,428 binding. GBR 12909 produced both tolerance to the inhibition of [³H]dopamine uptake by cocaine, and a decrease in total uptake of dopamine, in the caudate putamen, with no change in the nucleus accumbens. The persistent reduction of [³H]WIN 35,428 binding following continuous GBR 12909 does not appear to result from residual drug binding. These findings suggest that GBR 12909 and cocaine may bind to and regulate the dopamine transporter in different ways.     

Resistin, but not adiponectin, inhibits dopamine and norepinephrine release in the hypothalamus

Adiponectin (Adipocyte Complement-Related Protein of 30 kDa, ACRP30) and resistin are adipocyte-derived polypeptide hormones playing a role in metabolic homeostasis. Their plasma levels are inversely (adiponectin) or directly (resistin) correlated to obesity and they have opposite effects on insulin sensitivity. Adipose tissue hormones such as leptin have been shown to modulate neurotransmitters which control feeding in the hypothalamus. We have studied the effects of adiponectin and resistin on dopamine, norepinephrine and serotonin release from hypothalamic neuronal endings (synaptosomes) in vitro. We have found that adiponectin does not modify either basal or depolarization-induced amine release, while resistin inhibits the stimulated release of dopamine and norepinephrine, leaving unaffected serotonin release. We can conclude that, similarly to leptin, but differently from adiponectin, the adipose tissue hormone resistin could affect the central mechanisms of feeding by inhibiting catecholamine release in the hypothalamus.     

From the selective serotonin transporter inhibitor citalopram to the selective norepinephrine transporter inhibitor talopram: synthesis and structure-activity relationship studies

Citalopram and talopram are structurally closely related, but they have very distinct pharmacological profiles as selective inhibitors of the serotonin and norepinephrine transporters, respectively. A systematic structure-activity relationship study was performed, in which each of the four positions distinguishing the two compounds were varied. The inhibitory potencies of the resulting 16 compounds were tested at both serotonin and norepinephrine transporters. This showed that particularly two of the four positions are determinants for the biological activity.     

Reduced cocaine-induced serotonin,but not dopamine and noradrenaline,release in rats with a genetic deletion of serotonin transporters

It has recently been proposed that the increased reinforcing properties of cocaine and ecstasy observed in rats with a genetic deletion of serotonin transporters are the result of a reduction in the psychostimulant-induced release of serotonin. Here we provide the neurochemical evidence in favor of this hypothesis and show that changes in synaptic levels of dopamine or noradrenaline are not very likely to play an important role in the previously reported enhanced psychostimulant intake of these serotonin transporter knockout rats. The results may very well explain why human subjects displaying a reduced expression of serotonin transporters have an increased risk to develop addiction.     

Effects of the selective norepinephrine reuptake inhibitor reboxetine on norepinephrine and serotonin transmission in the rat hippocampus.

Given that norepinephrine (NE) and serotonin (5-HT) neurons are implicated in the mechanisms of action of antidepressant drugs and both project to the hippocampus, the impact of acute and long-term administration of the selective NE inhibitor reboxetine was assessed on CA(3) pyramidal neuron firing in this postsynaptic structure. Cumulative injections of reboxetine (1-4 mg/kg, i.v.) dose-dependently increased the recovery time of the firing of these neurons following iontophoretic applications of NE, but not 5-HT. In rats treated with reboxetine for 2.5 mg/kg/day for 21 days, a robust increase in the recovery time following NE applications was observed, and a small but significant prolongation occurred following 5-HT applications. In controls and reboxetine-treated rats, 1 and 5 Hz stimulations of the afferent 5-HT bundle to the hippocampus, which allows determination of terminal 5-HT(1B) autoreceptor sensitivity, produced similar frequency-dependent decreases in pyramidal neuron firing in both groups. However, after low and high doses of clonidine (10 and 400 microg/kg, i.v.), which assesses alpha(2)-adrenergic auto- and heteroreceptor sensitivity, respectively, only the effect of the high dose of clonidine was attenuated. Interestingly, administration of the selective 5-HT(1A) receptor antagonist WAY 100,635 induced a 140% increase in basal pyramidal neuron firing in reboxetine as compared to saline-treated rats. This increase in tonic activation of postsynaptic 5-HT(1A) receptors might be attributable in part to a desensitization of alpha(2)-adrenergic heteroreceptors, presumably resulting from sustained NE reuptake inhibition. These results indicate that even a selective NE reuptake inhibitor can modulate 5-HT transmission.     

Fluvoxamine,a selective serotonin reuptake inhibitor,suppresses tetrahydrobiopterin levels and dopamine as well as serotonin turnover in the mesoprefrontal system of mice

Rationale Tetrahydrobiopterin (BH₄) is a coenzyme of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), rate-limiting enzymes of monoamine biosynthesis. According to the monoamine hypothesis of depression, antidepressants will restore the function of the brain monoaminergic system, and BH₄ concentration.Objective To investigate the effects of fluvoxamine on BH₄ levels and dopamine (DA) and serotonin (5-HT) turnover in the mesoprefrontal system, incorporating two risk factors of depression, social isolation and acute environmental change.Methods Male ddY mice (6W) were divided into two housing groups, i.e. group-housing (eight animals per cage; 35 days), and isolation-housing (one per cage; 35 days), SC injected with fluvoxamine (20 or 40 mg/kg; days 29–35), and exposed to 20-min novelty stress (day 35). The levels of BH₄, DA, homovanilic acid (HVA), 5-HT, and 5-hydroxyindoleacetic acid (5-HIAA) were measured in the prefrontal cortex and midbrain.Results Under the group-housing condition, novelty stress significantly increased BH₄ levels in both regions, and the HVA/DA ratio in the midbrain, whereas it did not change any parameters in either region under the isolation-housing condition. In the prefrontal cortex, fluvoxamine significantly decreased the 5-HIAA/5-HT ratio under the group-housing condition, and BH₄ levels and the HVA/DA ratio under the isolation-housing condition. In the midbrain, fluvoxamine significantly decreased all parameters, except for an increasing in the 5-HIAA/5-HT ratio under the isolation-housing condition.Conclusion Isolation-housing suppressed the increase of BH₄ levels and DA turnover elicited by novelty stress. Fluvoxamine suppressed BH₄ levels, and DA and 5-HT turnover. Fluvoxamine may have altered DA turnover by suppressing BH₄ levels.