Effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline in the rat frontal cortex: augmentation via 5-HT1A receptor antagonism

Precision Medicine in Antidepressants Treatment

Precision medicine uses innovative approaches to improve disease prevention and treatment outcomes by taking into account people's genetic backgrounds, environments, and lifestyles. Treatment of depression is particularly challenging, given that 30-50% of patients do not respond adequately to antidepressants, while those who respond may experience unpleasant adverse drug reactions (ADRs) that decrease their quality of life and compliance. This chapter aims to present the available scientific data that focus on the impact of genetic variants on the efficacy and toxicity of antidepressants. We compiled data from candidate gene and genome-wide association studies that investigated associations between pharmacodynamic and pharmacokinetic genes and response to antidepressants regarding symptom improvement and ADRs. We also summarized the existing pharmacogenetic-based treatment guidelines for antidepressants, used to guide the selection of the right antidepressant and its dose based on the patient's genetic profile, aiming to achieve maximum efficacy and minimum toxicity. Finally, we reviewed the clinical implementation of pharmacogenomics studies focusing on patients on antidepressants. The available data demonstrate that precision medicine can increase the efficacy of antidepressants and reduce the occurrence of ADRs and ultimately improve patients' quality of life.

Deciphering the Differential Effective and Toxic Responses of Bupleuri Radix following the Induction of Chronic Unpredictable Mild Stress and in Healthy Rats Based on Serum Metabolic Profiles

The petroleum ether fraction of Bupleuri Radix which is contained in the traditional Chinese medicine prescription of Xiaoyaosan (XYS) may have a therapeutic effect in depressed subjects based on the results of our previous study. It has been reported that Bupleuri Radix can cause liver toxicity following overdosing or long-term use. Therefore, this study aimed to decipher the differential effective and toxic responses of Bupleuri Radix in chronic unpredictable mild stress (CUMS) (with depression) and healthy rats based on serum metabolic profiles. Serum metabolic profiles were obtained using the UHPLC- Q Exactive Orbitrap-MS technique. Our results demonstrated that the petroleum ether fraction of Bupleuri Radix (PBR) produces an antidepressant effect through regulating glycometabolism, amino acid metabolism, sphingolipid metabolism, glycerophospholipid metabolism, and fatty acid metabolism. It also induces more severe toxic reactions in the liver or kidney in healthy rats than in CUMS rats, which exhibited a comparatively mild drug-induced toxic reaction. The altered lysine degradation, sphingolipid metabolism, glycerophospholipid metabolism, fatty acid metabolism, and bile acid metabolism could be at least partly responsible for the PBR toxic responses in healthy rats. The differential effective and toxic response of PBR in CUMS rats and healthy rats provide a new standard for the more rational and safer application of clinical drugs in the future.

Agomelatine, venlafaxine, and running exercise effectively prevent anxiety- and depression-like behaviors and memory impairment in restraint stressed rats

Several severe stressful situations, e.g., natural disaster, infectious disease out break, and mass casualty, are known to cause anxiety, depression and cognitive impairment, and preventive intervention for these stress complications is worth exploring. We have previously reported that the serotonin-norepinephrine-dopamine reuptake inhibitor, venlafaxine, as well as voluntary wheel running are effective in the treatment of anxiety- and depression-like behaviors in stressed rats. But whether they are able to prevent deleterious consequences of restraint stress in rats, such as anxiety/depression-like behaviors and memory impairment that occur afterward, was not known. Herein, male Wistar rats were pre-treated for 4 weeks with anti-anxiety/anti-depressive drugs, agomelatine and venlafaxine, or voluntary wheel running, followed by 4 weeks of restraint-induced stress. During the stress period, rats received neither drug nor exercise intervention. Our results showed that restraint stress induced mixed anxiety- and depression-like behaviors, and memory impairment as determined by elevated plus-maze, elevated T-maze, open field test (OFT), forced swimming test (FST), and Morris water maze (MWM). Both pharmacological pre-treatments and running successfully prevented the anxiety-like behavior, especially learned fear, in stressed rats. MWM test suggested that agomelatine, venlafaxine, and running could prevent stress-induced memory impairment, but only pharmacological treatments led to better novel object recognition behavior and positive outcome in FST. Moreover, western blot analysis demonstrated that venlafaxine and running exercise upregulated brain-derived neurotrophic factor (BDNF) expression in the hippocampus. In conclusion, agomelatine, venlafaxine as well as voluntary wheel running had beneficial effects, i.e., preventing the restraint stress-induced anxiety/depression-like behaviors and memory impairment.

Activation of D1R/PKA/mTOR signaling cascade in medial prefrontal cortex underlying the antidepressant effects of l-SPD

Major depressive disorder (MDD) is a common neuropsychiatric disorder characterized by diverse symptoms. Although several antidepressants can influence dopamine system in the medial prefrontal cortex (mPFC), but the role of D1R or D2R subtypes of dopamine receptor during anti-depression process is still vague in PFC region. To address this question, we investigate the antidepressant effect of levo-stepholidine (l-SPD), an antipsychotic medication with unique pharmacological profile of D1R agonism and D2R antagonism, and clarified its molecular mechanisms in the mPFC. Our results showed that l-SPD exerted antidepressant-like effects on the Sprague-Dawley rat CMS model of depression. Mechanism studies revealed that l-SPD worked as a specific D1R agonist, rather than D2 antagonist, to activate downstream signaling of PKA/mTOR pathway, which resulted in increasing synaptogenesis-related proteins, such as PSD 95 and synapsin I. In addition, l-SPD triggered long-term synaptic potentiation (LTP) in the mPFC, which was blocked by the inhibition of D1R, PKA, and mTOR, supporting that selective activation of D1R enhanced excitatory synaptic transduction in PFC. Our findings suggest a critical role of D1R/PKA/mTOR signaling cascade in the mPFC during the l-SPD mediated antidepressant process, which may also provide new insights into the role of mesocortical dopaminergic system in antidepressant effects.

Chronic effects of antidepressants on serotonin release in rat raphe slice cultures: high potency of milnacipran in the augmentation of serotonin release

Most clinically-used antidepressants acutely increase monoamine levels in synaptic clefts, while their therapeutic effects often require several weeks of administration. Slow neuroadaptive changes in serotonergic neurons are considered to underlie this delayed onset of beneficial actions. Recently, we reported that sustained exposure of rat organotypic raphe slice cultures containing abundant serotonergic neurons to selective serotonin (5-HT) reuptake inhibitors (citalopram, fluoxetine and paroxetine) caused the augmentation of exocytotic serotonin release. However, the ability of other classes of antidepressants to evoke a similar outcome has not been clarified. In this study, we investigated the sustained actions of two tricyclic antidepressants (imipramine and desipramine), one tetracyclic antidepressant (mianserin), three 5-HT and noradrenaline reuptake inhibitors (milnacipran, duloxetine and venlafaxine) and one noradrenergic and specific serotonergic antidepressant (mirtazapine) on serotonin release in the slice cultures. For seven of nine antidepressants, sustained exposure to the agents at concentrations of 0.1-100 μ m augmented the level of increase in extracellular serotonin. The rank order of their potency was as follows: milnacipran>duloxetine>citalopram>venlafaxine>imipramine>fluoxetine>desipramine. Neither mirtazapine nor mianserin caused any augmentation. The highest augmentation by sustained exposure to milnacipran was partially attenuated by an α 1-adrenoceptor antagonist, benoxathian, while the duloxetine-, venlafaxine- and citalopram-mediated increases were not affected. These results suggest that inhibition of the 5-HT transporter is required for the enhancement of serotonin release. Furthermore, the potent augmentation by milnacipran is apparently due to the accompanied activation of the α 1-adrenoceptor.

Sexual side effects of serotonergic antidepressants: mediated by inhibition of serotonin on central dopamine release?

Antidepressant-induced sexual dysfunction adversely affects the quality of life of antidepressant users and reduces compliance with treatment. Animal models provide an instructive approach for examining potential sexual side effects of novel drugs. This review discusses the stability and reproducibility of our standardized test procedure that assesses the acute, subchronic and chronic effects of psychoactive compounds in a 30 minute mating test. In addition, we present an overview of the effects of several different (putative) antidepressants on male rat sexual behavior, as tested in our standardized test procedure. By comparing the effects of these mechanistically distinct antidepressants (paroxetine, venlafaxine, bupropion, buspirone, DOV 216,303 and S32006), this review discusses the putative mechanism underlying sexual side effects of antidepressants and their normalization. This review shows that sexual behavior is mainly inhibited by antidepressants that increase serotonin neurotransmission via blockade of serotonin transporters, while those that mainly increase the levels of dopamine and noradrenaline are devoid of sexual side effects. Those sexual disturbances cannot be normalized by simultaneously increasing noradrenaline neurotransmission, but are normalized by increasing both noradrenaline and dopamine neurotransmission. Therefore, it is hypothesized that the sexual side effects of selective serotonin reuptake inhibitors may be mediated by their inhibitory effects on dopamine signaling in sex brain circuits. Clinical development of novel antidepressants should therefore focus on compounds that simultaneously increase both serotonin and dopamine signaling.

Investigating the roles of different monoamine transmitters and impulse control using the 5-choice serial reaction time task

Previous studies have shown that drugs which block the reuptake of catecholamine neurotransmitters improve impulse control in diseases such as attention deficit hyperactivity disorder (ADHD). Serotonin-specific reuptake inhibitors (SSRI) lack efficacy in ADHD and have been linked to increased suicide risk. The present study investigated drugs with affinity for one or more of the monoamine reuptake transporters using the 5-choice serial reaction time task, a model of attention and impulsivity in rodents. We also tested the effects of the alpha(2)-adreoceptor antagonist, idazoxan and novel antidepressant, agomelatine, which both increase cortical noradrenaline concentrations through non-reuptake mechanisms. Improvements in impulse control were observed with venlafaxine, a serotonin and noradrenaline re-uptake inhibitor (SNRI) but not bupropion (dopamine and noradrenaline re-uptake inhibitor). Sibutramine (SNRI) reduced premature responses by ~50% at the highest dose tested but this was not significant. All three of the SSRIs tested reduced premature responding in a dose-dependent manner, although also slowed response and collection latencies. Neither idazoxan nor agomelatine significantly reduced premature responding, suggesting a lack of efficacy at the doses tested. None of the drugs tested improved attention in this task but sibutramine (SNRI), fluoxetine (SSRI) and paroxetine (SSRI) all increased omissions at the highest dose tested. These data suggest that the SNRIs and SSRIs reduce premature responding but tend to be less specific than noradrenaline specific reuptake inhibitors in this model. SSRIs did not induce any specific impairment in impulse control in this model.

Venlafaxine and neuropathic pain

The possible mechanisms involved in the antinociceptive effect of venlafaxine (VFX), a selective serotonin and noradrenaline reuptake inhibitor, after a single administration and chronic treatment were investigated in a diabetic neuropathic pain (DNP) model. VFX produced a significant antihyperalgesic effect after a single and repeated administration. This effect was reversed by pretreatment with yohimbine (a relatively selective α(2)-adrenergic antagonist) and p-chloroamphetamine (a neurotoxin which destroys serotonergic neurons). Conversely, naloxone (a nonselective opioid antagonist) did not reverse the effect of VFX in a DNP model. It is concluded that both noradrenergic and serotonergic mechanisms participate in the antinociceptive effect of VFX in the DNP model. However, the noradrenergic mechanism probably plays a more important role.

An investigation of the antidepressant action of xiaoyaosan in rats using ultra performance liquid chromatography-mass spectrometry combined with metabonomics

A rapid, highly sensitive, and selective method was applied in a non-invasive way to investigate the antidepressant action of Xiaoyaosan (XYS) using ultra performance liquid chromatography-mass spectrometry (UPLC-MS) and chemometrics. Many significantly altered metabolites were used to explain the mechanism. Venlafaxine HCl and fluoxetine HCl were used as chemical positive control drugs with a relatively clear mechanism of action to evaluate the efficiency and to predict the mechanism of action of XYS. Urine obtained from rats subjected to chronic unpredictable mild stress (CUMS) was analyzed by UPLC-MS. Distinct changes in the pattern of metabolites in the rat urine after CUMS production and drug intervention were observed using partial least squares-discriminant analysis. The results of behavioral tests and multivariate analysis showed that CUMS was successfully reproduced, and a moderate-dose XYS produced significant therapeutic effects in the rodent model, equivalent to those of the positive control drugs, venlafaxine HCl and fluoxetine HCl. Metabolites with significant changes induced by CUMS were identified, and 17 biomarker candidates for stress and drug intervention were identified. The therapeutic effect of XYS on depression may involve regulation of the dysfunctions of energy metabolism, amino acid metabolism, and gut microflora changes. Metabonomic methods are valuable tools for measuring efficacy and mechanisms of action in the study of traditional Chinese medicines.

Chronic serotonin-norepinephrine reuptake transporter inhibition modifies basal respiratory output in adult mouse in vitro and in vivo

Respiratory disturbances are a common feature of panic disorder and present as breathing irregularity, hyperventilation, and increased sensitivity to carbon dioxide. Common therapeutic interventions, such as tricyclic (TCA) and selective serotonin reuptake inhibitor (SSRI) antidepressants, have been shown to ameliorate not only the psychological components of panic disorder but also the respiratory disturbances. These drugs are also prescribed for generalized anxiety and depressive disorders, neither of which are characterized by respiratory disturbances, and previous studies have demonstrated that TCAs and SSRIs exert effects on basal respiratory activity in animal models without panic disorder symptoms. Whether serotonin-norepinephrine reuptake inhibitors (SNRIs) have similar effects on respiratory activity remains to be determined. Therefore, the current study was designed to investigate the effects of chronic administration of the SNRI antidepressant venlafaxine (VHCL) on basal respiratory output. For these experiments, we recorded phrenic nerve discharge in an in vitro arterially-perfused adult mouse preparation and diaphragm electromyogram (EMG) activity in an in vivo urethane-anesthetized adult mouse preparation. We found that following 28-d VHCL administration, basal respiratory burst frequency was markedly reduced due to an increase in expiratory duration (T(E)), and the inspiratory duty cycle (T(I)/T(tot)) was significantly shortened. In addition, post-inspiratory and spurious expiratory discharges were seen in vitro. Based on our observations, we suggest that drugs capable of simultaneously blocking both 5-HT and NE reuptake transporters have the potential to influence the respiratory control network in patients using SNRI therapy.

Venlafaxine enhances the effect of bupropion on extracellular dopamine in rat frontal cortex

Venlafaxine is recognised as an effective treatment for depression and is known to inhibit the reuptake of serotonin (5-HT) and noradrenaline (NA). Another antidepressant, bupropion, acts to inhibit dopamine (DA) and NA reuptake and is commonly co-administered with other antidepressants to improve the efficacy of the antidepressant effect. The present study was designed to investigate the acute effect of combining the 2 drugs on extracellular levels of 5-HT, DA, and NA in rat frontal cortex using brain microdialysis, with the drugs being administered by intraperitoneal injection (i.p). Bupropion (10 mg/kg body mass, i.p.) alone had no effect on extracellular 5-HT levels, whereas venlafaxine (10 mg/kg, i.p.) alone significantly elevated extracellular 5-HT over basal values. As expected, bupropion alone elevated extracellular dopamine above basal values at 40 min post-drug administration, and this effect lasted for a further 2 h. Venlafaxine alone did not statistically elevate extracellular dopamine. The co-administration of venlafaxine with bupropion resulted in a dramatic increase in extracellular dopamine, and this effect was significantly greater than that seen with bupropion alone. In the frontal cortex, NA was elevated by bupropion alone and venlafaxine alone, relative to the control animals. The combination of bupropion and venlafaxine resulted in a marked elevation of NA.

Effects of general anaesthetics on 5-HT neuronal activity in the dorsal raphe nucleus

The ascending 5-HT system has been and continues to be the subject of much research. The majority of in vivo electrophysiological and neurochemical studies of 5-HT function in rodents have been conducted in animals under anaesthesia - usually chloral hydrate or urethane. However, the effects of anaesthetics, on 5-HT function have not been systematically investigated. Here we used in vitro electrophysiology in dorsal raphe slices, to determine the effects of anaesthetically relevant concentrations of chloral hydrate (100 μM and 1 mM), urethane (10 and 30 mM), pentobarbitone (10 and 100 μM) and ketamine (10, 100 and 300 μM) on regulators of 5-HT firing activity. We examined i) basal firing (driven by α(1) adrenoceptors), ii) the excitatory response to N-methyl-d-aspartate (NMDA), iii) the 5-HT(1A) autoreceptor-mediated inhibitory response to 5-HT and iv) the GABA(A) receptor-mediated inhibitory response to 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridinyl-3-ol (THIP, gaboxadol). Pentobarbitone selectively enhanced the response to THIP. Ketamine decreased basal firing, attenuated the response to NMDA, and enhanced responses to both 5-HT and THIP. Chloral hydrate had marginal effects on basal firing, slightly attenuated the NMDA response, and enhanced both the 5-HT and THIP responses. Urethane increased basal firing, decreased the NMDA response, increased the response to THIP, but had no effect on the 5-HT response. Our data indicate that all anaesthetics tested significantly affect the regulators of 5-HT neuronal function. These findings will aid in the interpretation of previous reports of in vivo studies of the 5-HT system and will allow researchers to make a rational selection of anaesthetic for future studies.

Metabonomic study on chronic unpredictable mild stress and intervention effects of Xiaoyaosan in rats using gas chromatography coupled with mass spectrometry

ETHNOPHARMACOLOGY

Xiaoyaosan (XYS), a famous Chinese prescription, composed of Radix Bupleuri (Bupleurum chinense DC.), Radix Angelicae Sinensis (Angelica sinensis (Oliv.) Diels), Radix Paeoniae Alba (Paeonia lactiflora Pall.), Rhizoma Atractylodis Macrocephalae (Atractylodes macrocephala Koidz.), Poria (Poria cocos (Schw.) Wolf), Radix Glycyrrhizae (Glycyrrhiza uralensis Fisch.), Herba Menthae (Mentha haplocalyx Briq.), and Rhizoma Zingiberis Recens (Zingiber officinale Rosc.), has been widely used in the clinic for treating mental disorders. Behavior and biochemical analyses indicate XYS has obvious anti-depression activity. However, there is no report on the effects of XYS using a metabolomics approach.

AIM OF THE STUDY

Depression is a prevalent complex psychiatric disorder and its pathophysiological mechanism is not yet well understood. This paper was designed to study metabonomic characters of the depression induced by chronic unpredictable mild stress (CUMS) and the therapeutic effects of XYS, classic traditional Chinese medicine (TCM) in treating the depression.

MATERIAL AND METHODS

A plasma metabonomics method based on gas chromatography/mass spectrometry (GC/MS) was developed. Principal component analysis (PCA) was utilized to classify and reveal the differences between the model group and control group. In turns, the concentration of these differences was analyzed with t-test to determine whether XYS was possible to influence the metabolic pattern induced by CUMS.

RESULTS

The significant difference in metabolic profiling was observed from model group compared with drug-dose group by using the PCA, indicating the recovery effect of XYS on CUMS rats. Some significantly changed metabolites like glycine, glucose and hexadecanoic acid have been identified. These biochemical changes are related to the disturbance in amino acid metabolism, energy metabolism and glycometabolism, which are helpful to further understand the CUMS and the therapeutic mechanism of XYS.

CONCLUSIONS

Metabonomic approach is helpful to further understanding the pathophysiology of depression and assisting in clinical diagnosis of depression and is also a valuable tool for studying the essence of Chinese medicine's syndrome theory and therapeutic effect mechanism of TCM.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Preclinical characterization of BRL 44408: antidepressant- and analgesic-like activity through selective alpha2A-adrenoceptor antagonism

Biogenic amines such as norepinephrine, dopamine, and serotonin play a well-described role in the treatment of mood disorders and some types of pain. As alpha2A-adrenoceptors regulate the release of these neurotransmitters, we examined the therapeutic potential of BRL 44408, a potent (Ki=8.5 nM) and selective (>50-fold) alpha2A-adrenoceptor antagonist (K(B)=7.9 nM). In rats, BRL 44408 penetrated the central nervous system resulting in peak brain and plasma concentrations of 586 ng/g and 1124 ng/ml, respectively. In a pharmacodynamic assay, pretreatment with BRL 44408 to rats responding under a fixed-ratio 30 operant response paradigm resulted in a rightward shift of the clonidine dose-response curve, an effect indicative of alpha2-adrenoceptor antagonism in vivo. Consistent with presynaptic autoreceptor antagonism and tonic regulation of neurotransmitter release, acute administration of BRL 44408 elevated extracellular concentrations of norepinephrine and dopamine, but not serotonin, in the medial prefrontal cortex. Additionally, BRL 44408, probably by inhibiting alpha2A heteroceptors, produced a significant increase in cortical levels of acetylcholine. In the forced swim test and schedule-induced polydipsia assay, BRL 44408 produced an antidepressant-like response by dose-dependently decreasing immobility time and adjunctive water intake, respectively, while in a model of visceral pain, BRL 44408 exhibited analgesic activity by decreasing para-phenylquinone (PPQ)-induced abdominal stretching. Finally, BRL 44408 did not produce deficits in overall motor coordination nor alter general locomotor activity. This preclinical characterization of the neurochemical and behavioural profile of BRL 44408 suggests that selective antagonism of alpha2A-adrenoceptors may represent an effective treatment strategy for mood disorders and visceral pain.

Regulation of brain-derived neurotrophic factor (BDNF) in the chronic unpredictable stress rat model and the effects of chronic antidepressant treatment

Chronic unpredictable stress (CUS) is a widely used animal model of depression. The present study was undertaken to investigate behavioral, physiological and molecular effects of CUS and/or chronic antidepressant treatment (venlafaxine or imipramine) in the same set of animals. Anhedonia, a core symptom of depression, was assessed by measuring consumption of a palatable solution. Exposure to CUS reduced intake of a palatable solution and this effect was prevented by chronic antidepressant treatment. Moreover, chronic antidepressant treatment decreased depressive-like behavior in a modified forced swim test in stressed rats. Present evidence suggests a role for brain-derived neurotrophic factor (BDNF) in depression. BDNF mRNA levels in the ventral and dorsal hippocampus were assessed by in situ hybridization. Exposure to CUS was not correlated with a decrease but rather with an increase in BDNF mRNA expression in both the dentate gyrus of the dorsal hippocampus and the CA3 region of the ventral hippocampus indicating that there is no simple link between depression-like behaviors per se and brain BDNF levels in rats. However, a significant increase in BDNF mRNA levels in the dentate gyrus of the dorsal hippocampus correlated with chronic antidepressant treatment emphasizing a role for BDNF in the mechanisms underlying antidepressant activity.

Role of serotonin 5-HT1A receptors in the antidepressant-like effect and the antinociceptive effect of venlafaxine in mice

The present study was undertaken to evaluate the potential role of 5-HT1A receptors in the antidepressant-like effect and antinociceptive effect of venlafaxine. With this aim, the effect of either a selective 5-HT1A receptor antagonist (WAY-100635; N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT1A receptor agonist (8-OH-DPAT; 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) was investigated in mice in combination with venlafaxine by means of the forced swimming test, a paradigm aimed at screening potential antidepressants, and the hot-plate test, a phasic pain model. Surprisingly, the results showed that WAY-100635 produced a large decrease in the antidepressant-like effect of venlafaxine, while 8-OH-DPAT rendered effective a non-effective dose of this antidepressant. However, in the hot-plate test WAY-100635 significantly enhanced the antinociceptive effect of venlafaxine, whereas 8-OH-DPAT counteracted its antinociceptive effect. These findings show that 5-HT1A receptors play differing roles in modulating the antidepressant-like and antinociceptive effects of venlafaxine in the models investigated. The results imply that blockade of the 5-HT1A receptors in the forebrain will counteract the favourable (antidepressant-like) effect at raphe nuclei level, and consequently, the overall effect evidenced is an antagonism. This suggests a predominant role of 5-HT1A receptors located in the forebrain area for the antidepressant-like effect. In contrast, the antinociceptive effect of venlafaxine is probably potentiated due to the blockade of somatodendritic 5-HT1A receptors in the same raphe nuclei, facilitating the descending monoaminergic pain control system.

Reciprocal effects of combined administration of serotonin, noradrenaline and dopamine reuptake inhibitors on serotonin and dopamine levels in the rat prefrontal cortex: the role of 5-HT1A receptors

The purpose of the present study was to examine, by in vivo microdialysis technique, the effects of triple acting monoamine reuptake inhibitors, constructed by combinations of a selective serotonin reuptake inhibitor citalopram with a noradrenaline/dopamine reuptake inhibitor methylphenidate and a serotonin/noradrenaline reuptake inhibitor venlafaxine with a dopamine reuptake inhibitor GBR12909, on extracellular levels of serotonin (5-HT), noradrenaline (NA) and dopamine (DA) in the prefrontal cortex (PFC) of anaesthetized rats. At the highest dose tested, adjunctive methylphenidate (10 mg/kg s.c.) to citalopram markedly attenuated by 63% the extracellular levels of 5-HT as compared to the levels induced by citalopram (5 mg/kg i.p.) alone, whereas the overall DA concentrations significantly increased to about 149% of those induced by methylphenidate alone. Similarly, the combination of venlafaxine with GBR12909 (10 mg/kg s.c.) caused a reduction of 5-HT levels to 66% of the levels induced by venlafaxine (10 mg/kg i.p.) alone, whereas the overall DA levels increased to 151% of the venlafaxine-treated group. The extracellular levels of NA were only marginally affected by the treatments with combined reuptake inhibitors compared to the effects induced by methylphenidate or venlafaxine alone. The modulatory effects of combined administration of the DA/NA reuptake inhibitors with the 5-HT reuptake inhibitors (citalopram and venlafaxine) on potentiation of DA and attenuation of 5-HT efflux were completely reversed by a pre-treatment with the 5-HT(1A) receptor antagonist WAY-100635. These findings suggest a crucial role played by the 5-HT(1A) receptors in balancing the reuptake inhibitory efficacy for the enhancement of 5-HT and DA transmission in the PFC by the drugs combining the reuptake inhibition of all three monoamines.

In Vivo Effect of Venlafaxine on Locus Coeruleus Neurons: Role of Opioid, α2-Adrenergic, and 5-Hydroxytryptamine1A Receptors

The locus coeruleus (LC) is involved in several neural pathways responsible for some somatic and emotional processes, such as pain and depression; its activity is regulated by several receptors, such as opioid, alpha(2)-adrenergic, and 5-hydroxytryptamine (5-HT)(1A) receptors. The present study investigates the in vivo effects of venlafaxine, an antidepressant with analgesic properties, on locus coeruleus neurons, and its modulation by opioid, alpha(2)-adrenergic, and 5-HT(1A) receptors. The results show that acute administration of venlafaxine produced a dose-dependent, complete inhibition of LC activity. This inhibitory effect was not reversed by the opioid receptor antagonist naloxone, but subsequent administration of idazoxan, an alpha(2)-adrenoceptor antagonist, did reverse it. The preadministration of the 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) (1 and 40 microg/kg) significantly enhanced the venlafaxine inhibitory effect, decreasing the ED(50) by 56 and 44%, respectively. A 14-day treatment with venlafaxine (40 mg/kg/day) induced a suppression of the firing activity of LC neurons. In these treated animals, venlafaxine produced an inhibitory effect similar to that in nontreated animals. This inhibitory effect was not reversed by naloxone, but it was reversed by idazoxan. In addition, the preadministration of 8-OH-DPAT (40 microg/kg) significantly enhanced the venlafaxine effect, decreasing the ED(50) by 60%. These results suggest that the effect of venlafaxine on LC neurons is modulated by alpha(2)-adrenergic and 5-HT(1A) receptors, and not by opioid receptors. These data could contribute to the further understanding of the antidepressant and analgesic mechanism of action of venlafaxine.

Alleviation of thermoregulatory dysfunction with the new serotonin and norepinephrine reuptake inhibitor desvenlafaxine succinate in ovariectomized rodent models

Hot flushes and night sweats, referred to as vasomotor symptoms (VMS), are presumed to be a result of declining hormone levels and are the principal menopausal symptoms for which women seek medical treatment. To date, estrogens and/or some progestins are the most effective therapeutics for alleviating VMS; however, these therapies may not be appropriate for all women. Therefore, nonhormonal therapies are being evaluated. The present study investigated a new reuptake inhibitor, desvenlafaxine succinate (DVS), in animal models of temperature dysfunction. Both models used are based on measuring changes in tail-skin temperature (TST) in ovariectomized (OVX) rats. The first relies on naloxone-induced withdrawal in morphine-dependent (MD) OVX rats, resulting in an acute rise in TST. The second depends on an OVX-induced loss of TST decreases during the dark phase as measured by telemetry. An initial evaluation demonstrated abatement of the rise in TST with long-term administration of ethinyl estradiol or with a single oral dose of DVS (130 mg/kg) in the MD model. Further evaluation showed that orally administered DVS acutely and dose dependently (10-100 mg/kg) abated a naloxone-induced rise in TST of MD rats and alleviated OVX-induced temperature dysfunction in the telemetry model. Oral administration of DVS to OVX rats caused significant increases in serotonin and norepinephrine levels in the preoptic area of the hypothalamus, a key region of the brain involved in temperature regulation. These preclinical studies provide evidence that DVS directly impacts thermoregulatory dysfunction in OVX rats and may have utility in alleviating VMS associated with menopause.

Pharmacokinetic and pharmacodynamic profiles of the novel serotonin and norepinephrine reuptake inhibitor desvenlafaxine succinate in ovariectomized Sprague-Dawley rats

Desvenlafaxine succinate (DVS) is a novel serotonin (5-HT) and norepinephrine (NE) reuptake inhibitor (SNRI) that is currently in clinical development for the treatment of major depressive disorder and vasomotor symptoms associated with menopause. Previous studies have documented the pharmacokinetic and pharmacodynamic profiles of DVS in male rats. Similar studies, however, have not been performed in ovariectomized (OVX) rats, a model that mimics the loss of ovarian hormones that occurs at menopause. The goal of the present study, therefore, was to characterize the pharmacokinetic and pharmacodynamic properties of DVS in OVX rats. Desvenlafaxine levels peaked in plasma, brain (total brain minus hypothalamus) and hypothalamus at concentrations of 7.0, 10.8 and 9.5 microM (assuming 1 g = 1 ml), respectively, 30 min post-dosing DVS (30 mg/kg, oral). The apparent terminal half-lives of desvenlafaxine in plasma, brain and hypothalamus were 3.0, 2.1 and 2.5 h, respectively. Based on AUC(0-last), brain to plasma and hypothalamus to plasma ratios were 1.7 and 1.3, respectively. Microdialysis experiments in the medial preoptic area of the hypothalamus showed that DVS (30 mg/kg, s.c.), in the presence of WAY-100635 (5-HT(1A) antagonist), increased 5-HT levels 225% at 1 h post-dosing. Norepinephrine levels increased 44% at 3 h post-dosing while dopamine levels were unchanged. Thus, in OVX rats, DVS has good pharmacokinetic properties, rapid brain penetration, excellent brain penetrability and selectively increases 5-HT and NE levels in the hypothalamus. This work supports the notion that DVS could have utility for treating disorders in menopausal women in which changes in 5-HT and/or NE have been implicated.

Dorsal raphe vs. median raphe serotonergic antagonism. Anatomical, physiological, behavioral, neuroendocrinological, neuropharmacological and clinical evidences: relevance for neuropharmacological therapy

Monoaminergic neurons located in the central nervous system (CNS) are organized into complex circuits which include noradrenergic (NA), adrenergic (Ad), dopaminergic (DA), serotonergic (5-HT), histaminergic (H), GABA-ergic and glutamatergic systems. Most of these circuits are composed of more than one and often several types of the above neurons. Such physiologically flexible circuits respond appropriately to both external and internal stimuli which, if not modulated adequately, can trigger pathophysiologic responses. A great deal of research has been devoted to mapping the multiple functions of the CNS circuitry, thereby forming the basis for effective neuropharmacological therapeutic approaches. Such lineal strategies that seek to normalize complex and mixed physiological disorders, however, meet only partial therapeutic success and are often followed by undesirable side effects and/or total failure. In light of these, we have worked to develop possible models of CNS circuitry that are less affected by physiological interaction using the models to design more effective therapeutic approaches. In the present review, we cite and present evidence supporting the dorsal raphe versus median raphe serotonergic circuitry as one model of a reliable paradigm, necessary to the clear understanding and therapy of many psychiatric and even non-psychiatric disturbances.

Alpha 2A-adrenoceptors enhance the serotonergic effects of fluoxetine

The ability of subtype-preferring alpha(2)-adrenoceptor antagonists to enhance the neurochemical effects of the antidepressant, fluoxetine, was evaluated by in vivo microdialysis. Combining the selective alpha(2A)-adrenoceptor antagonist, BRL-44408 (10 mg/kg, s.c.), with fluoxetine (30 mg/kg, s.c.) elevated the extracellular levels of serotonin (5-HT) and noradrenaline in the rat frontal cortex, an effect not observed following antidepressant treatment alone. In contrast, combining fluoxetine with the alpha(2B)- or alpha(2C)-adrenoceptor antagonists, imiloxan (10 mg/kg, s.c.) or rauwolscine (10 mg/kg, s.c.), respectively, did not similarly alter biogenic amine levels. Collectively, these results reveal a specific role for the alpha(2A)-adrenoceptor subtype in augmenting the neurochemical effects of antidepressants.

SNRIs: their pharmacology, clinical efficacy, and tolerability in comparison with other classes of antidepressants

The class of serotonin and norepinephrine reuptake inhibitors (SNRIs) now comprises three medications: venlafaxine, milnacipran, and duloxetine. These drugs block the reuptake of both serotonin (5-HT) and norepinephrine with differing selectivity. Whereas milnacipran blocks 5-HT and norepinephrine reuptake with equal affinity, duloxetine has a 10-fold selectivity for 5-HT and venlafaxine a 30-fold selectivity for 5-HT. All three SNRIs are efficacious in treating a variety of anxiety disorders. There is no evidence for major differences between SNRIs and SSRIs in their efficacy in treating anxiety disorders. In contrast to SSRIs, which are generally ineffective in treating chronic pain, all three SNRIs seem to be helpful in relieving chronic pain associated with and independent of depression. Tolerability of an SNRI at therapeutic doses varies within the class. Although no direct comparative data are available, venlafaxine seems to be the least well-tolerated, combining serotonergic adverse effects (nausea, sexual dysfunction, withdrawal problems) with a dose-dependent cardiovascular phenomenon, principally hypertension. Duloxetine and milnacipran appear better tolerated and essentially devoid of cardiovascular toxicity.

High-sensitive liquid chromatographic method for determination of neuronal release of serotonin, noradrenaline and dopamine monitored by microdialysis in the rat prefrontal cortex

A high-sensitive liquid chromatographic method based on precolumn derivatization and fluorescence detection allowing simultaneous determination of serotonin (5-HT), noradrenaline (NA) and dopamine (DA) in brain microdialysis samples is described. 5-HT, NA and DA were derivatized with benzylamine and 1,2-diphenylethylenediamine in the presence of potassium hexacyanoferrate(III) and glycine, which yielded to highly fluorescent and stable benzoxazoles. The derivatized samples were separated on a microbore column (150 mm x 1.0mm i.d., packed with C18 silica, 5 microm) within 60 min. The mobile phase consisted of acetonitrile-Briton-Robinson buffer (pH 7.2) (32:68, v/v) containing 5 mM Na2EDTA and 5 mM octanesulfonic acid sodium salt. The detection limits (signal-to-noise ratio of 3) for 5-HT, NA and DA were 76, 42 and 95 amol/10 microl injected on-column, respectively. Microdialysis samples were collected at 10-min intervals from the probes implanted in the prefrontal cortex of awake rats. The basal levels of 5-HT, NA and DA were 7.3 +/- 0.7, 5.3 +/- 0.31 and 8.1 +/- 0.47 fmol/5 microl (mean +/- S.E.M., n = 5). Following 90-min perfusion with tetrodotoxin (1 microM) or calcium-free Ringer solution, the DA and NA levels were reduced to about 15 and 20%, respectively and the 5-HT levels to 45 and 60% of the basal levels, respectively. Reserpine, 12h after a dose of 5mg/kg i.p., reduced the extracellular 5-HT, NA and DA concentrations to about 34, 39 and 32% of the basal levels, respectively. In conclusion, the preset microdialysis/analytical method enables simultaneous monitoring of basal and pharmacologically reduced neuronal release of 5-HT, NA and DA in the rat brain.

Acute effect of milnacipran on the relationship between the locus coeruleus noradrenergic and dorsal raphe serotonergic neuronal transmitters

The present studies sought to investigate the effect of milnacipran called the serotonin (5-HT) and noradrenaline (NA) reuptake inhibitor (SNRI) on the interaction of central locus coeruleus noradrenergic and dorsal raphe nucleus serotonergic functional activity by utilizing in vivo microdialysis. A single administration of milnacipran (60 mg/kg, s.c.) markedly decreased the levels of NA and its metabolite, 4-hydroxy-3-methoxymandelic acid (HMMA), in the locus coeruleus and the levels of, a metabolite of 5-hydroxytryptamine (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA) in the dorsal raphe nucleus. Combined administration of yohimbine (2 mg/kg, s.c.),?alpha(2)-adrenoceptor?antagonist, at 2 h after milnacipran (60 mg/kg, s.c.) led to a significant increase in NA levels in the locus coeruleus, although yohimbine alone had no effect on these levels. Under similar experimental condition, 5-HIAA levels in the dorsal raphe nucleus remained unchanged. NAN-190 (1 mg/kg, s.c.), 5-HT(1A) receptor partial agonist, alone markedly decreased the levels of 5-HIAA in the dorsal raphe nucleus, although this level was not affected by WAY100635, the selective 5-HT(1A) receptor antagonist. WAY100635 recovered the milnacipran-induced decrease of 5-HIAA levels in the dorsal raphe nucleus to control levels. On the other hand, NAN-190 did not affect the milnacipran-induced decrease of 5-HIAA levels. Behavioral signs (locomotion and rearing) were markedly observed following milnacipran alone or combined administration of milnacipran and yohimbine. However, the behavioral signs after coadministration of milnacipran and WAY100635 or NAN-190 were relatively poor. These results may suggest that an increase of NA in the locus coeruleus with the treatment of yohimbine after milnacipran results from negative feedback following the blockade of alpha(2)-adrenoceptors achieved with yohimbine, and that WAY100635 but not NAN-190 recovered milnacipran-induced decrease of 5-HIAA in the dorsal raphe nucleus to control levels by preventing the activation for the presynaptic 5-HT(1A) autoreceptor.

Role of presynaptic alpha2-adrenoceptors in antidepressant action: recent findings from microdialysis studies

The therapeutic effect of an antidepressant drug takes at least 2 to 3 weeks to develop and a significant proportion of patients have no or only partial benefit regardless of the class of antidepressant used. Research into the neurobiological basis of antidepressant action has suggested new strategies to improve the antidepressant effect. Recent microdialysis studies show that hypofunction of the presynaptic autoreceptors enhances the increase of extracellular serotonin (5-HT) induced by selective serotonin reuptake inhibitors (SSRIs) so it has been suggested that the antidepressant effect may be speeded up by blockade of the autoreceptors. The similarity between the synaptic mechanisms controlling serotonergic and noradrenergic transmission has stimulated preclinical research into the role of presynaptic alpha(2)-adrenoceptors in the effect of noradrenaline (NA) reuptake inhibitors (NRIs) on NA availability at central synapses. The microdialysis studies reviewed here indicate that NRIs including desipramine, reboxetine and atomoxetine, the mixed 5-HT/NA reuptake inhibitors sibutramine, duloxetine, venlafaxine or the NA/DA reuptake inhibitor amineptine, increased extracellular NA in various regions of the rat brain. The effect was enhanced by chronic treatment and even more by the co-administration of alpha(2)-adrenoceptor antagonists. The results support the theory that desensitization of the alpha(2)-adrenoceptor contributes to enhancing the effect of NRIs seen after chronic administration and may account for the slow onset of the antidepressant effect. Finally, they suggest that co-administration of an alpha(2)-adrenoceptor antagonist may improve the therapeutic effect of NRI.

Regulation by 5-HT1A receptors of the in vivo release of 5-HT and DA in mouse frontal cortex

This study examines the effects of serotonin (5-HT)1A receptor ligands on the in vivo release of 5-HT and dopamine (DA) in the prefrontal cortex of mice. Oral MKC-242 and 8-OH-DPAT, selective 5-HT1A receptor agonists, decreased cortical 5-HT release at low and high doses, while the receptor agonists increased cortical DA release only at a high dose. Local application of the selective 5-HT1A receptor antagonist, WAY100635, via a dialysis probe, antagonized oral MKC-242-induced increase in cortical DA release, but did not affect the decrease in cortical 5-HT release. Local application of 8-OH-DPAT at 100 and 300 nM via a dialysis probe increased cortical DA release, but did not affect cortical 5-HT release. The effects of oral MKC-242 and 8-OH-DPAT on 5-HT release were blocked by low and high doses of WAY100635, while blocking the agonist-induced increase in DA release required a high dose of WAY100635. These results suggest that 5-HT release and DA release in the frontal cortex of mice are regulated by pre- and postsynaptic 5-HT1A receptors, respectively, and that the presynaptic 5-HT1A receptor-mediated response is more sensitive to inhibition by WAY100635 than the postsynaptic 5-HT1A receptor-mediated response in mice.

Effects of acute treatment with paroxetine, citalopram and venlafaxine in vivo on noradrenaline and serotonin outflow: a microdialysis study in Swiss mice

1. This study investigated whether a single administration of a range of doses (1, 4 and 8 mg kg-1, i.p.) of paroxetine, citalopram or venlafaxine may simultaneously increase extracellular levels of 5-HT ([5-HT]ext) and noradrenaline ([NA]ext) by using in vivo microdialysis in the frontal cortex (FCx) of awake, freely moving Swiss mice. 2. In vivo, paroxetine induced similar increases in cortical [5-HT]ext at the three doses tested, and induced a statistically significant increase in cortical [NA]ext at 4 and 8 mg x kg-1. Citalopram increased neither [5-HT]ext nor [NA]ext at the lowest dose, but increased both neurotransmitter levels at 4 and 8 mg x kg-1. At these doses, citalopram induced greater increases in cortical [5-HT]ext than in [NA]ext. Venlafaxine increased [5-HT]ext and [NA]ext to about 400 and 140% of the respective basal values at 8 mg kg-1. 3. Citalopram and paroxetine have the highest potency to increase cortical [5-HT]ext and [NA]ext, respectively. In addition, the rank of order of efficacy of these antidepressant drugs to increase [5-HT]ext in vivo in the FCx of mice was as follows: venlafaxine>citalopram>paroxetine, while the efficacy to increase cortical [NA]ext in mice of paroxetine and citalopram is similar, and greater than that of venlafaxine. 4. In conclusion, extracellular levels of cortical [NA]ext increase with the highest doses of the very selective SSRI citalopram, as well as with the very potent SSRI paroxetine. Surprisingly, the SNRI venlafaxine increased cortical [5-HT]ext to a greater extent rather than [NA]ext in the range of doses studied in mice.

Effects of venlafaxine on extracellular 5-HT, dopamine and noradrenaline in the hippocampus and on peripheral hormone concentrations in the rat in vivo

The purpose of the present study was to study the effect of an acute dose of the serotonin (5-HT) - noradrenaline (NA) reuptake inhibitor venlafaxine on extracellular concentrations of 5-HT, NA and dopamine (DA) in the hippocampus and on the peripheral hormone concentrations in freely moving rats. Blood obtained from a catheter placed in the vena femoralis was analyzed for adrenocorticotropin (ACTH), beta-endorphins, prolactin (PRL), growth hormone (GH) and cortisol. Collections are referred to pre and post injection of 20 mg/kg of venlafaxine. Extracellular hippocampal NA and 5-HT as determined with in vivo microdialysis increased significantly after drug injection. PRL and ACTH were significantly affected by the drug. At the selected dose venlafaxine is able to increase the release of 5-HT but also of NA in rat hippocampus. Due to the dual reuptake properties of the drug and the functional interconnection of the NA and the 5-HT systems, the observed effects on peripheral hormones are possibly mediated by a combined action of these 2 systems.

Simultaneous determination of norepinephrine, serotonin, and 5-hydroxyindole-3-acetic acid in microdialysis samples from rat brain by microbore column liquid chromatography with fluorescence detection following derivatization with benzylamine

A microbore column liquid chromatographic method for the simultaneous determination of norepinephrine (NE), serotonin (5-HT), and 5-hydroxyindole-3-acetic acid (5HIAA) in microdialysis samples from rat brain is described. The method is based on precolumn derivatization of NE, 5HT, and 5HIAA with benzylamine in the presence of potassium hexacyanoferrate(III) resulting in the corresponding highly fluorescent and stable benzoxazole derivatives. A 15-microl sample was mixed with 15 microl derivatization reagent solution containing 0.3M 3-cyclohexylaminopropanesulfonic acid buffer (pH 12.0), 0.5M benzylamine, 10mM potassium hexacyanoferrate(III), and methanol (1/1/1/12, v/v/v/v). The derivatization was carried out at 50 degrees C for 20 min. The benzylamine derivatives of NE, 5HT, and 5HIAA were separated on a reversed-phase column (100 x 1.0mm i.d., packed with C18 silica, 5 microm) within 30 min. The mobile phase consisted of 15 mM acetate buffer (pH 5.0) and acetonitrile (31%, v/v); the flow rate was 50 microl/min. The detection limits (signal-to-noise ratio of 3) for NE, 5HT, and 5HIAA in the injection volume of 20 microl were 90, 210, and 260 amol, respectively. Microdialysis samples were collected in 7.5-min intervals from the probes implanted in the hippocampus and prefrontal cortex of awake rats. The basal levels of NE, 5HT, and 5HIAA in the dialysates from the hippocampus were 4.2+/-0.5, 4.9+/-0.6, and 934.1 +/- 63.4 fmol/20 microl, and those from the prefrontal cortex were 6.0+/-1.2,5.51.3, and 669.1 +/- 96.0 fmol/20 microl (mean +/- SE, n=25), respectively. The NE and 5HT levels were altered by perfusion of high-potassium or low-calcium solution and following antidepressant drugs imipramine and desipramine. It is concluded that the new fluorescence derivatization method in combination with microbore column liquid chromatography allows the simultaneous determination of NE, 5HT, and 5HIAA in the microdialysis samples at higher sensitivity, providing easier maintenance in routine use than that achieved by high-performance liquid chromatographic methods with electrochemical detection.

The 5-HT1A receptor agonist MKC-242 increases the exploratory activity of mice in the elevated plus-maze

The effect of (S)-5-[3-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242), a 5-HT(1A) receptor agonist, on mouse behavior was examined in the elevated plus-maze. MKC-242 significantly increased the percentage of open-arm entries and the percentage of open-arm time, indices of anxiety reduction, while it did not increase the enclosed-arm entries and time spent in enclosed arms. The effect of MKC-242 was antagonized by a low dose of the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide which alone did not affect the behavior. These findings suggest that MKC-242 increases the exploratory activity of mice in the elevated plus-maze via activation of 5-HT(1A) receptors, probably the presynaptic autoreceptors.

Comparison of the effects of antidepressants on norepinephrine and serotonin concentrations in the rat frontal cortex: an in-vivo microdialysis study

The present study employed in-vivo microdialysis techniques in the freely moving rat to systematically compare the neurochemical effects of various antidepressant agents on extracellular concentrations of norepinephrine (NE) and serotonin (5-HT) in the frontal cortex. We found that acute administration of the tricyclic antidepressant, desipramine (3-30 mg/kg, s.c.) and the dual serotonin/norepinephrine reuptake inhibitor, venlafaxine (3-30 mg/kg, s.c.), produced dose-dependent and robust increases in cortical NE concentrations (498% and 403%, respectively). Conversely, acute injection of the selective serotonin reuptake inhibitors, fluoxetine (30 mg/kg, s.c.) and paroxetine (1-10 mg/kg, s.c.), did not alter forebrain NE concentrations. However, paroxetine did produce a significant increase in cortical NE concentrations (164%) when administered at 30 mg/kg. These changes in NE were not paralleled by 5-HT, which showed no increase following administration of desipramine, venlafaxine, paroxetine or fluoxetine. Combination treatment with the 5-HT1A receptor antagonist, WAY-100635 (0.3 mg/kg, s.c.), significantly enhanced extracellular 5-HT concentrations following venlafaxine (10 and 30 mg/kg), fluoxetine (30 mg/kg) and paroxetine (3-30 mg/kg). Alternatively, WAY-100635 produced no augmentation of the antidepressant-induced changes in extracellular NE. Collectively, these studies show that paroxetine, at low to intermediate doses, and fluoxetine are selective for 5-HT versus NE systems, whereas venlafaxine produces similar effects on both 5-HT and NE levels at the effective doses tested.

Dual serotonin and noradrenaline uptake inhibitor class of antidepressants potential for greater efficacy or just hype?

Preclinical and clinical studies support the rationale that development of single molecules, which would promote serotonergic and noradrenergic neurotransmission by inhibiting simultaneously the uptake of both monoamines, would potentially result in improved antidepressant drugs. Currently, the dual inhibitors of serotonin and noradrenaline uptake are venlafaxine, milnacipran and duloxetine. Based on the preclinical studies, the three drugs do show properties of inhibiting uptake of both monoamines in vitro and in vivo in the following order of decreasing potency: duloxetine, venlafaxine and milnacipran, and all exhibit low affinity at neuronal receptors of neurotransmitters, suggesting low side-effect potential. In double-blind, controlled studies, venlafaxine and milnacipran were repeatedly shown to be as efficacious as tricyclic antidepressant drugs in treating major depressive disorder, while one double-blind, placebo-controlled trial showed the antidepressant efficacy of duloxetine. Specifically designed comparative trials of dual uptake inhibitors against the other agents are needed to establish whether the dual uptake inhibitors show improvement in efficacy, rate of responders, antidepressive effects and/or remission.

Induction of hyperlocomotion in mice exposed to a novel environment by inhibition of serotonin reuptake. A pharmacological characterization of diverse classes of antidepressant agents

This study characterized the influence of acute administration of diverse classes of antidepressant agent upon the spontaneous locomotor activity (LA) of mice in a novel, open-field environment. The selective serotonin (5-HT) reuptake inhibitors (SSRIs), citalopram, fluoxetine, paroxetine, fluvoxamine, litoxetine and zimelidine, dose-dependently enhanced LA. Their actions were mimicked by the mixed 5-HT/noradrenaline (NA) reuptake inhibitors (SNRIs), venlafaxine, duloxetine and S33005. In contrast, clomipramine only slightly elevated LA and two further tricyclics, imipramine and amitriptyline, were inactive. Further, the selective NA vs. 5-HT reuptake inhibitors (NARIs), reboxetine, desipramine, maprotiline, nisoxetine and nortriptyline all failed to increase LA. The "atypical antidepressants," mianserin and mirtazapine, neither of which modify 5-HT reuptake, as well as the mixed SSRI/5-HT(2) antagonists, nefazodone and trazodone, also failed to increase LA. Doses of SSRI and SNRI which increased LA did not modify motor performance in the rotarod test. Further, they did not enhance LA in rats, suggesting that this response is characteristic of mice. Finally, upon prehabituation of mice to the activity chamber, the SSRI, citalopram, and the SNRI, venlafaxine, failed to increase LA. In conclusion, in mice exposed to a novel environment, inhibition of 5-HT reuptake by SSRIs and SNRIs enhances spontaneous LA in the absence of a generalized influence upon motor function. This response provides a simple parameter for characterization of SSRIs and SNRIs, and differentiates them from other classes of antidepressant agent. Although an influence upon arousal and/or anxiety is likely related to the increase in LA, the functional significance of this response requires additional elucidation.

Role of 5-HT1A receptors in the mediation of acute citalopram effects: a 8-OH-DPAT challenge study

(1) The study was aimed to investigate the effects of the minimal effective doses of acute citalopram (5 mg/kg), (+/-)-8-hydroxydipropylaminotetralin HBr (8-OH-DPAT; 0.1 mg/kg), and their combined treatment on the rat open field and forced swimming behaviour and post-mortem monoamine content. (2) The animals were prospectively divided into the vehicle- and para-chlorophenylalanine (p-CPA)-pretreated (350 mg/kg) groups. (3) Acute citalopram (5 mg/kg), 8-OH-DPAT (0.1 mg/kg), or their combined treatment had no major effect on the rat open field and forced swimming behaviour. (4) The post-mortem catecholamine content in four brain regions studied was unchanged in all treatment groups. (5) The combined 8-OH-DPAT (0.1 mg/kg) and citalopram (5 mg/kg) treatment partially reversed the p-CPA-induced decrease of serotonin (5-HT) and 5-hydroxy-indolacetic acid (5-HIAA) content. (6) The present experiments demonstrate that the 5-HT1A receptors mediate some of the selective serotonin reuptake inhibitor (SSRI)-induced biochemical phenomena.

Effects of chronically administered venlafaxine on 5-HT receptor activity in rat hippocampus and hypothalamus

The effects of chronic administration of the mixed serotonin [5-hydroxytryptamine (5-HT)]/norepinephrine re-uptake inhibitor venlafaxine (5 mg/kg daily by osmotic minipump for 28 days) on the sensitivity of somatodendritic 5-HT(1A) autoreceptors on serotonergic neurons innervating the hypothalamus, and on 5-HT(1B) autoreceptors in both hypothalamus and hippocampus, were determined using in vivo microdialysis in freely moving rats. Venlafaxine induced a reduction in sensitivity of 5-HT(1B) autoreceptors in hypothalamus, but did not affect the sensitivity of 5-HT(1A) autoreceptors, or of 5-HT(1B) autoreceptors in hippocampus. The corticosterone and oxytocin responses to the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.05 or 0.2 mg/kg), a measure of postsynaptic 5-HT(1A) receptor activity in the hypothalamus, were reduced in animals administered 5 or 10 mg/kg venlafaxine daily by intraperitoneal injection for 21 days. This desensitization of post-synaptic 5- HT(1A) receptors in the hypothalamus may be a consequence of increased 5-HT levels induced by desensitization of the presynaptic 5-HT(1B) receptors. These results taken together with those of previous studies suggest that the hypothalamus might be an important site of drug action, and that venlafaxine has an overall mechanism similar to that of selective serotonin re-uptake inhibitors.

Determination of serotonin in microdialysis samples from rat brain by microbore column liquid chromatography with post-column derivatization and fluorescence detection

The present paper describes a new method for on-line determination of 5-HT in brain microdialysates from awake rats by microbore column liquid chromatography with post-column derivatization and fluorescence detection. The derivatization reagent contained 1 mM benzylamine and 0.5 mM potassium hexacyanoferrate (III), both dissolved in a mixture of acetonitrile and 25 mM borate buffer (pH 11.0) (1:1, v/v). The limit of detection (S/N=3) for 5-HT was 0.5 fmol/20 microl. The samples were injected every 20 min onto a microbore column packed with C18 silica gel. The method exhibits an excellent stability over the periods of at least 12-24 h. The basal levels of 5-HT from 25 awake rats were 7.10+/-1.06 fmol/20 microl in the dorsal hippocampus and 4.64+/-0.91 fmol/20 microl (mean+/-SD) in the striatum. The 5-HT release increased to about 1500% during the perfusion with 100 mM K(+) containing Ringer solution or it was reduced to 60 or 40% during the perfusion with 1 microM tetrodotoxin or calcium free Ringer, respectively. The new method can be used to monitor extracellular 5-HT following acute systemic drug administration.

Neuropharmacology of venlafaxine

Venlafaxine (Effexor) is an effective antidepressant and has also been approved for the treatment of generalized anxiety disorder. Venlafaxine was initially characterized as an inhibitor of both serotonin (5HT) and norepinephrine (NE) uptake and was therefore termed a "dual uptake inhibitor." This chapter reviews data from both in vitro and in vivo studies regarding its effects on 5HT and NE neurotransmission. In addition, the effects of venlafaxine on other systems that may play a role in its therapeutic efficacy effects are described. The data indicate that venlafaxine is a relatively weak inhibitor of NE transport in vitro. In vivo studies indicate that venlafaxine selectively inhibits 5HT uptake at low therapeutic doses and inhibits both 5HT and NE uptake at higher therapeutic doses. This chapter concludes with a discussion of the effects of venlafaxine on various aspects of physiology.

Potentiation of the antinociceptive effect of clomipramine by a 5-ht(1A) antagonist in neuropathic pain in rats

The benefit of antidepressant treatment in human neuropathic pain is now well documented, but the effect is limited and slow to appear. It has been demonstrated that the association of a 5-HT(1A) antagonist and a serotoninergic antidepressant reduced the delay of action and increases the thymoanaleptic effect of the drug. The purpose of this work was to evaluate the combination of an antidepressant and a 5-HT(1A) antagonist in animal models of chronic neuropathic pain. We studied the antinociceptive effect of the co-administration of clomipramine and a 5-HT(1A) antagonist (WAY 100,635) in a pain test applied in normal rats and in two models of neurogenic sustained pain (mononeuropathic and diabetic rats). The results show an increase in the antinociceptive effect of acutely injected clomipramine due to WAY 100,635 in these models, which is majored when the two drugs are repeatedly injected. The 5-HT(1A) antagonist reduced the delay of onset and increased the maximal antinociceptive effect of clomipramine. These new findings argue for using the combination of an antidepressant and a 5-HT(1A) antagonist in human neuropathic pain therapy.

5-HT1B Autoreceptors limit the effects of selective serotonin re-uptake inhibitors in mouse hippocampus and frontal cortex

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5-hydroxytryptamine, 5-HT) 1B receptor subtype in mediating the effects of selective serotonin re-uptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels [5-HT]ext in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in [5-HT]ext in knockout than in wild-type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild-type mice at the 1 mg/kg, but not at 5 mg/kg. In addition, either the absence of the 5-HT1B receptor or its blockade with the mixed 5-HT1B/1D receptor antagonist, GR 127935, potentiated the effect of a single administration of paroxetine on extracellular 5-HT levels more in the ventral hippocampus than in the frontal cortex. These data suggest that 5-HT1B autoreceptors limit the effects of SSRIs on dialysate 5-HT levels at serotonergic nerve terminals.

Venlafaxine and its interaction with WAY 100635: effects on serotonergic unit activity and behavior in cats

The therapeutic efficacy of antidepressant drugs that inhibit the reuptake of serotonin (5-hydroxytryptamine, 5-HT) may be enhanced by blocking their indirect activation of 5-HT(1A) autoreceptors, which mediate feedback inhibition of serotonergic neuronal activity. In this study, we examined the effects of venlafaxine, a dual 5-HT/noradrenaline reuptake inhibitor, alone and in combination with the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY 100635), on the single-unit activity of serotonergic dorsal raphe neurons and concurrent behavior in freely moving cats. Systemic administration of venlafaxine (0.05-1.0 mg/kg, i.v.) produced a dose-dependent decrease in firing rate (ED(50)=0.19 mg/kg), with virtually complete inhibition of neuronal discharge at the highest dose tested. The subsequent administration of WAY 100635 (0.1 mg/kg, i.v.) rapidly reversed the neuronal suppression produced by venlafaxine and significantly elevated the firing rate above baseline levels. The overshoot in neuronal activity was associated with the onset of an adverse behavioral reaction resembling the 5-HT syndrome resulting from excessive levels of brain 5-HT. The intensity of this reaction paralleled the degree of neuronal restoration induced by WAY 100635, suggesting a causal relationship. Such behavioral responses were either not observed previously, or of a low intensity, when WAY 100635 was combined with selective 5-HT reuptake inhibitors. Overall, these results suggest that the risk of inducing adverse effects, such as the 5-HT syndrome, may be higher with dual 5-HT/noradrenaline reuptake inhibitors than with selective 5-HT reuptake inhibitors, when these agents are combined with a potent 5-HT(1A) autoreceptor antagonist. Possible mechanisms that might account for these differences in drug interaction are discussed.

Effect of acute treatment with YM992 on extracellular norepinephrine levels in the rat frontal cortex

The effects of acute treatment with (S)-2-[[(7-fluoroindan-4-yl)oxy]methyl]morpholine monohydrochloride (YM992), venlafaxine, fluoxetine and citalopram on extracellular norepinephrine levels were examined in the rat frontal cortex by in vivo microdialysis. YM992 (3, 10, 30 mg/kg, i.p.) dose-dependently increased extracellular norepinephrine levels (3-fold at 10 mg/kg, 5. 5-fold at 30 mg/kg). While venlafaxine and 30 mg/kg fluoxetine also produced significant increases in norepinephrine levels, 30 mg/kg citalopram had no effect. The combined administration of MDL100,907 (a selective 5-HT(2A) receptor antagonist) and citalopram did significantly increase norepinephrine levels compared with either saline or citalopram treatment. Therefore, a synergistic effect due to 5-HT reuptake inhibition and 5-HT(2A) receptor antagonism of YM992 may partly contribute to the increase of extracellular norepinephrine levels. YM992 enhances the neurotransmission of not only 5-HT system but also norepinephrine, and as such may have a preclinical profile different from that of a selective serotonin reuptake inhibitor.