Viloxazine Increases Extracellular Concentrations of Norepinephrine, Dopamine, and Serotonin in the Rat Prefrontal Cortex at Doses Relevant for the Treatment of Attention-Deficit/Hyperactivity Disorder.

Background: Viloxazine ER (viloxazine extended-release capsules; Qelbree®), a nonstimulant attention-deficit/hyperactivity disorder (ADHD) treatment, has known activity as a norepinephrine (NE) transporter (NET) inhibitor. In vitro studies have also shown direct pharmacological effects on specific serotonin (5-HT) receptors, but not on the serotonin transporter (SERT). An in vivo microdialysis study in rats showed viloxazine (50 mg/kg i.p.) increased extracellular 5-HT, NE, and dopamine (DA) in the prefrontal cortex (PFC), a key brain region in ADHD pathology. This study evaluated whether these effects occur at clinically relevant concentrations. Methods: Microdialysis experiments were conducted in freely-moving, Sprague-Dawley rats (males, 8 weeks). Viloxazine (1, 3, 10, 30 mg/kg) was administered intraperitoneally to establish the dose range in rats at which viloxazine plasma concentrations aligned with those of individuals with ADHD administered therapeutic doses of viloxazine ER. Concentrations of unbound viloxazine, NE, 5-HT, DA, and NE and 5-HT metabolites (3,5-dihydroxyphenylglycol [DHPG] and 5-hydroxyindoleacetic acid [5-HIAA]) were measured in PFC interstitial fluid. After identifying a therapeutically relevant dose (30 mg/kg), the experiment was repeated using 30 and 50 mg/kg viloxazine (as 50 mg/kg increased NE, 5-HT, and DA in prior studies). Results: Viloxazine unbound (free drug) plasma concentrations in rats at 30 mg/kg were comparable to free drug concentrations in individuals with ADHD taking clinically effective doses (based on validated population PK models). Viloxazine 30 mg/kg significantly increased extracellular NE, 5-HT, and DA PFC levels compared to vehicle. Concomitant decreases in DHPG, but not 5-HIAA, support the inhibitory effect of viloxazine on NET but not SERT. Conclusion: At clinically relevant concentrations, viloxazine increases PFC NE, DA, and 5-HT. Prefrontal augmentation of 5-HT does not appear to result from 5-HT reuptake inhibition but may be related to activation of 5-HT neurons. The potential therapeutic role of serotonergic effects in ADHD treatment merits further exploration.

Effects of the triple reuptake inhibitor amitifadine on extracellular levels of monoamines in rat brain regions and on locomotor activity.

Major depressive disorder is a prevalent disease, and current pharmacotherapy is considered to be inadequate. It has been hypothesized that a triple reuptake inhibitor (TRI) that activates dopamine (DA) neurotransmission in addition to serotonin and norepinephrine (NE) circuitries may result in enhanced antidepressant effects. Here, we investigated the pharmacological effects of a serotonin-preferring TRI-amitifadine (EB-1010, formerly DOV 21947). The effects of amitifadine (10 mg/kg ip.) on extracellular concentrations of monoamines and their metabolites in rat brain regions were investigated using the in vivo microdialysis technique. The effects of amitifadine on locomotor activity and stereotyped behavior were also evaluated. A major metabolite of amitifadine, the 2-lactam compound, was investigated for inhibition of monoamine uptake processes. Amitifadine markedly and persistently increased extracellular concentrations of serotonin, NE, and DA in prefrontal cortex. The extracellular concentrations of DA were also increased in the DA-rich areas striatum and nucleus accumbens. The extracellular concentrations of the metabolites of serotonin, 5-hydroxyindoleacetic acid, and DA, 3,4-dihydroxyphenylacetic and homovanillic acid, were also markedly decreased in brain regions. Amitifadine did not increase locomotor activity or stereotypical behaviors over a broad dose range. The lactam metabolite of amitifadine weakly inhibited monoamine uptake. Thus, amitifadine increased extracellular concentrations of serotonin, NE, and DA, consistent with TRI. Although amitifadine significantly increased DA in the nucleus accumbens, it did not induce locomotor hyperactivity or stereotypical behaviors. The enhancement of serotonin, NE, and DA in rat brain regions associated with depression suggest that amitifadine may have novel antidepressant activity.

Pharmacological characterization of the norepinephrine and dopamine reuptake inhibitor EB-1020: implications for treatment of attention-deficit hyperactivity disorder.

We report on the pharmacological, behavioral, and neurochemical characterization of a novel dual norepinephrine (NE)/dopamine (DA) transporter inhibitor EB-1020 (1R,5S)-1-(naphthalen-2-yl)-3-azabicyclo[3.1.0]hexane HCl). EB-1020 preferentially inhibited monoamine reuptake in cloned cell lines transfected with human transporters with IC50 values of 6 and 38, respectively, for NE and DA transporters. In microdialysis studies, EB-1020 markedly increased NE, and DA concentrations levels in rat prefrontal cortex in vivo with peak increases of 375 and 300%, respectively with the greatest effects on NE, and also increased DA extracellular concentrations in the striatum to 400% of baseline concentrations. Behavioral studies demonstrated that EB-1020 dose-dependently decreased immobility in the mouse tail suspension test of depression to 13% of control levels, and did not stimulate locomotor activity in adult rats in the optimal dose range. EB-1020 dose-dependently inhibited locomotor hyperactivity in juvenile rats lesioned with the neurotoxin 6-hydroxydopamine (100 µg intracisternally) as neonates; a well-established animal model for attention-deficit hyperactivity disorder (ADHD). These data suggest that EB-1020 mediates its actions by stimulating NE and DA neurotransmission, which are typically impaired in ADHD.

Muscarinic mechanisms in psychotic disorders.

Schizophrenia is a devastating disease with several broad symptom clusters and the current monoamine-based treatments do not adequately treat the disease, especially negative and cognitive symptoms. A proposed alternative approach for treating schizophrenia is through the use of compounds that activate certain muscarinic receptor subtypes, the so-called muscarinic cholinergic hypothesis theory. This theory has been revitalized with a number of recent and provocative findings including postmortem reports in schizophrenia patients showing decreased numbers of muscarinic M(1) and M(4) receptors in brain regions associated with schizophrenia as well as decreased muscarinic receptors in an in vivo imaging study. Studies with M(4) knockout mice have shown that there is a reciprocal relationship between M(4) and dopamine receptor function, and a number of muscarinic agonists have shown antidopaminergic activity in a variety of preclinical assays predictive of antipsychotic efficacy in the clinic. Furthermore, the M(1)/M(4) preferring partial agonist xanomeline has been shown to have antipsychotic-like and pro-cognitive activity in preclinical models and in clinical trials to decrease psychotic-like behaviors in Alzheimer's patients and positive, negative, and cognitive symptoms in patients with schizophrenia. Therefore, we propose that an agonist with M(1) and M(4) interactions would effectively treat core symptom clusters associated with schizophrenia. Currently, research is focused on developing subtype-selective muscarinic agonists and positive allosteric modulators that have reduced propensity for parasympathetic side-effects, but retain the therapeutic benefit observed with their less selective predecessors.

Antipsychotic drugs: comparison in animal models of efficacy, neurotransmitter regulation, and neuroprotection.

Various lines of evidence indicate the presence of progressive pathophysiological processes occurring within the brains of patients with schizophrenia. By modulating chemical neurotransmission, antipsychotic drugs may influence a variety of functions regulating neuronal resilience and viability and have the potential for neuroprotection. This article reviews the current literature describing preclinical and clinical studies that evaluate the efficacy of antipsychotic drugs, their mechanism of action and the potential of first- and second-generation antipsychotic drugs to exert effects on cellular processes that may be neuroprotective in schizophrenia. The evidence to date suggests that although all antipsychotic drugs have the ability to reduce psychotic symptoms via D(2) receptor antagonism, some antipsychotics may differ in other pharmacological properties and their capacities to mitigate and possibly reverse cellular processes that may underlie the pathophysiology of schizophrenia.

Cholinergic dysfunction in a mouse model of Alzheimer disease is reversed by an anti-A beta antibody.

Disruption of cholinergic neurotransmission contributes to the memory impairment that characterizes Alzheimer disease (AD). Since the amyloid cascade hypothesis of AD pathogenesis postulates that amyloid beta (A beta) peptide accumulation in critical brain regions also contributes to memory impairment, we assessed cholinergic function in transgenic mice where the human A beta peptide is overexpressed. We first measured hippocampal acetylcholine (ACh) release in young, freely moving PDAPP mice, a well-characterized transgenic mouse model of AD, and found marked A beta-dependent alterations in both basal and evoked ACh release compared with WT controls. We also found that A beta could directly interact with the high-affinity choline transporter which may impair steady-state and on-demand ACh release. Treatment of PDAPP mice with the anti-A beta antibody m266 rapidly and completely restored hippocampal ACh release and high-affinity choline uptake while greatly reducing impaired habituation learning that is characteristic of these mice. Thus, soluble "cholinotoxic" species of the A beta peptide can directly impair cholinergic neurotransmission in PDAPP mice leading to memory impairment in the absence of overt neurodegeneration. Treatment with certain anti-A beta antibodies may therefore rapidly reverse this cholinergic dysfunction and relieve memory deficits associated with early AD.

Case history: the discovery of fluoxetine hydrochloride (Prozac).

In the early 1970s, evidence of the role of serotonin (5-hydroxytryptamine or 5-HT) in depression began to emerge and the hypothesis that enhancing 5-HT neurotransmission would be a viable mechanism to mediate antidepressant response was put forward. On the basis of this hypothesis, efforts to develop agents that inhibit the uptake of 5-HT from the synaptic cleft were initiated. These studies led to the discovery and development of the selective serotonin-reuptake inhibitor fluoxetine hydrochloride (Prozac; Eli Lilly), which was approved for the treatment of depression by the US FDA in 1987. Here, we summarize this research and discuss the many challenges that we encountered during the development of fluoxetine hydrochloride, which has now been widely acknowledged as a breakthrough drug for depression.

Effect of the attention deficit/hyperactivity disorder drug atomoxetine on extracellular concentrations of norepinephrine and dopamine in several brain regions of the rat.

Atomoxetine is a selective inhibitor of norepinephrine transporters and is currently being used in the pharmacotherapy of attention deficit/hyperactivity disorder (ADHD). We have previously shown that atomoxetine increased extracellular (EX) concentrations of norepinephrine and dopamine in prefrontal cortex, but unlike the psychostimulant methylphenidate, did not alter dopamine(EX) in nucleus accumbens or striatum. Using the in vivo microdialysis technique in rat, we investigated the effects of atomoxetine on norepinephrine(EX) and dopamine(EX) concentrations in several other brain regions and also evaluated the role of inhibitory autoreceptors on atomoxetine-induced increases of norepinephrine(EX) concentrations. Atomoxetine (3mg/kg i.p.) increased norepinephrine(EX) robustly in prefrontal cortex, occipital cortex, lateral hypothalamus, dorsal hippocampus and cerebellum, suggesting that norepinephrine(EX) is increased throughout the brain by atomoxetine. In lateral hypothalamus and occipital cortex where dopamine(EX) was quantifiable, atomoxetine did not increase dopamine(EX) concentrations, in contrast to parallel increases of norepinephrine(EX) and dopamine(EX) in prefrontal cortex, indicating a unique effect in prefrontal cortex. Administration of the alpha(2)-adrenergic antagonist idazoxan 1h after atomoxetine resulted in increases in prefrontal cortical norepinephrine efflux greater than either compound alone, indicating an attenuating effect of the adrenergic autoreceptors on norepinephrine efflux.

Clinical assessment of norepinephrine transporter blockade through biochemical and pharmacological profiles.

BACKGROUND: To assess the sensitivity of biochemical, physiological, and pharmacological markers of peripheral norepinephrine (NE) transporter (NET) function, we chronically antagonized NET by a range of doses of duloxetine [(+)-N-methyl-3-(1-naphthalenyloxy)-2 thiophenepropanamine], which blocks the NE reuptake process. METHODS AND RESULTS: Duloxetine was administered in a randomized, placebo-controlled study in 15 healthy volunteers. Plasma from duloxetine-treated subjects (ex vivo effect) dose-dependently decreased radioligand binding to human NET (maximum inhibition was 60%) (P=0.02). The dose of intravenous tyramine required to raise systolic blood pressure by 30 mm Hg (PD30) increased dose-dependently with duloxetine and was significant at the end of the 120-mg/d dosage (P<0.001). The plasma dihydoxyphenylglycol to NE (DHPG/NE) ratio was reduced significantly at 2 weeks of treatment with 80 mg/d duloxetine (11.3 at baseline, 3.4 at 240 mg/d, P<0.001). Plasma NE was significantly increased starting at 120 mg/d duloxetine. Urine results (corrected for 24-hour creatinine excretion) showed a dose-dependent change from the baseline urinary excretion for NE, DHPG, and the DHPG/NE ratio. The most sensitive measure, the DHPG/NE ratio, was significant at the 80-mg dose. Urinary NE excretion was significantly raised after 2 weeks of treatment with 80 mg/d duloxetine (P<0.001), the lowest dose used in the study. CONCLUSIONS: These findings suggest that the degree of NET blockade can be assessed with the plasma or urine DHPG/NE ratio and the pressor effect of tyramine. Also, the DHPG/NE ratio is more sensitive at the lower end of NET inhibition, whereas tyramine exhibits a linear relation, with NET inhibition commencing at a higher dose.

The dual transporter inhibitor duloxetine: a review of its preclinical pharmacology, pharmacokinetic profile, and clinical results in depression.

Major depressive disorder (MDD) poses a significant health problem and is estimated to be the third most costly and disabling disorder in the United States. Pharmacotherapy of depression has been successful, but improvements in response rates, remission rates, side effects, compliance and faster onset of therapeutic action have become prime objectives in drug development. There is considerable support for the hypothesis that dysfunctional serotonergic or noradrenergic neurotransmission may be etiological in depressed patients. Duloxetine is a balanced and potent reuptake inhibitor of serotonin (5-HT) and norepinephrine (NE) being studied as an antidepressant medication. In this review, we highlight the preclinical pharmacology, pharmacokinetic profile, and effects of duloxetine in the pharmacotherapy of depression. Evidence for 5-HT and NE reuptake inhibition by duloxetine comes from in vitro and in vivo transporter binding and functional uptake studies. Taken together with efficacy data from in vivo microdialysis, electrophysiological and behavioral studies, it is evident that duloxetine is balanced as a dual serotonin norepinephrine uptake inhibitor in vivo. The clinical efficacy and safety of duloxetine in the treatment of MDD has been studied in 6 multicenter, randomized, double-blind, placebo-controlled trials. In these studies, duloxetine was found to be effective in the treatment of emotional/psychological and painful physical symptoms associated with depression. More importantly, duloxetine appears to have better response rates and remission from depressive symptoms, perhaps due to its ability to treat a wider range of symptoms.

M4 muscarinic receptors regulate the dynamics of cholinergic and dopaminergic neurotransmission: relevance to the pathophysiology and treatment of related CNS pathologies.

Dopaminergic dysfunction is an important pathogenetic factor for brain pathologies such as Parkinson's disease, ADHD, schizophrenia, and addiction as well as for metabolic disorders and anorexia. Dopaminergic neurons projecting from the midbrain to forebrain regions, such as the nucleus accumbens and the prefrontal cortex, regulate motor and cognitive functions and coordinate the patterned response of the organism to sensory, affective, and rewarding stimuli. In this study, we showed that dopaminergic neurotransmission is highly dependent on M4 cholinergic muscarinic receptor function. Using in vivo microdialysis, we found elevated dopamine (DA) basal values and enhanced DA response to psychostimulants in the nucleus accumbens of M4 knockout mice. We also demonstrated impaired homeostatic control of cholinergic activity that leads to increased basal acetylcholine efflux in the midbrain of these animals. Thus, loss of M4 muscarinic receptor control of cholinergic function effectuates a state of dopaminergic hyperexcitability. This may be responsible for pathological conditions, in which appetitive motivation as well as affective and cognitive processing is impaired. We propose that M4 receptor agonists could represent an innovative strategy for the treatment of pathologies associated with hyperdopaminergia.

Amantadine for weight gain associated with olanzapine treatment.

Patients with schizophrenia (Sch), schizoaffective, schizophreniform, or bipolar (BP) I disorders [Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV)]; not manic or acutely psychotic [Brief Psychiatric Rating Scale (BPRS) total score < or =45]; treated with olanzapine for 1-24 months; and who had gained > or =5% of their initial body weight were examined to determine whether amantadine could attenuate weight gain or promote weight loss. Olanzapine (Olz; 5-20 mg/day) was co-administered with double-blind treatment of 100-300 mg/day amantadine (Olz+Amt, n=60) or placebo (Olz+Plc, n=65). Visit-wise analysis of weight showed that weight change from baseline [last-observation-carried-forward (LOCF)] in the Olz+Amt group was significantly different from the Olz+Plc group at weeks 8 (P=0.042), 12 (P=0.029), and 16 (primary endpoint, mean+/-S.D.: -0.19+/-4.58 versus 1.28+/-4.26 kg, P=0.045). Mean BPRS total score, positive subscale, and anxiety-depression scores improved comparably in both groups, and Montgomery-Asberg Depression Rating Scale (MADRS) total score improved in the Olz+Amt group. Overall, amantadine was safe, was well tolerated, and attenuated weight gain or promoted weight loss in some patients who had gained weight during olanzapine therapy.

Antipsychotic-like effect of the muscarinic acetylcholine receptor agonist BuTAC in non-human primates.

Cholinergic, muscarinic receptor agonists exhibit functional dopamine antagonism and muscarinic receptors have been suggested as possible future targets for the treatment of schizophrenia and drug abuse. The muscarinic ligand (5R,6R)-6-(3-butylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[3.2.1]octane (BuTAC) exhibits high affinity for muscarinic receptors with no or substantially less affinity for a large number of other receptors and binding sites, including the dopamine receptors and the dopamine transporter. In the present study, we wanted to examine the possible antipsychotic-like effects of BuTAC in primates. To this end, we investigated the effects of BuTAC on d-amphetamine-induced behaviour in antipsychotic-naive Cebus paella monkeys. Possible adverse events of BuTAC, were evaluated in the same monkeys as well as in monkeys sensitized to antipsychotic-induced extrapyramidal side effects. The present data suggests that, the muscarinic receptor ligand BuTAC exhibits antipsychotic-like behaviour in primates. The behavioural data of BuTAC as well as the new biochemical data further substantiate the rationale for the use of muscarinic M1/M2/M4-preferring receptor agonists as novel pharmacological tools in the treatment of schizophrenia.

Amitifadine, a triple monoamine re-uptake inhibitor, reduces nicotine self-administration in female rats.

A wider diversity of drug treatments to aid smoking cessation is needed to help tailor the most efficacious treatment for different types of smokers. This study was conducted to determine whether amitifadine, which inhibits re-uptake of dopamine, norepinephrine and serotonin, would decrease nicotine self-administration at doses that do not cause adverse side effects. Adult female Sprague-Dawley rats were trained to self-administer nicotine intravenous (IV) and were given acute doses of amitifadine in a repeated measures counterbalanced design. Effects of amitifadine on locomotor activity and food motivated responding were also evaluated. Chronic amitifadine effects were also examined. The 30 mg/kg amitifadine dose significantly reduced nicotine self-administration. The 5 and 10 mg/kg doses reduced nicotine self-administration during the first 15 min of the session when the greatest amount of nicotine was self-administered. The 30 mg/kg amitifadine dose, but not the lower doses caused a significant reduction in locomotor activity averaged over the one-hour session and reduced food motivated responding. The 10 mg/kg dose caused hypoactivity at the beginning of the session, but 5 mg/kg did not cause any hypoactivity. The effects of chronic amitifadine treatment (10 mg/kg) over the course of 15 sessions was also determined. Amitifadine caused a significant reduction in nicotine self-administration, which was not seen to diminish over two consecutive weeks of treatment and a week after enforced abstinence. Amitifadine significantly reduced nicotine self-administration. This prompts further research to determine if amitifadine might be an effective treatment for smoking cessation.

Muscarinic receptors as a target for drugs treating schizophrenia.

The family of 5 muscarinic acetylcholine receptors belongs to the superfamily of G protein coupled neurotransmitter receptors that serve in part as regulators of synaptic function. Muscarinic receptors are anatomically positioned in cortical and subcortical areas and modulate dopaminergic and glutamatergic neurotransmission thought to be dysfunctional in schizophrenia. Neurochemical studies have shown that dopamine and muscarinic receptors reciprocally modulate one another. For example, the muscarinic agonist xanomeline increases extracellular levels of dopamine and Fos expression in cortical areas greater than subcortical areas, similar to effects of atypical antipsychotics. In electrophysiological studies, xanomeline with acute and chronic administration decreased firing of the mesocorticolimbic dopamine A10 tract, but not the motoric dopamine A9 tract. Behavioral investigations have shown that muscarinic agonists, like dopamine antagonists, inhibit conditioned-avoidance responding and dopamine-agonist-induced behaviors including hyperactivity, climbing behavior and disruption of prepulse inhibition, models for positive symptoms of schizophrenia. Transgenic knockout mice lacking M(4) receptors are hyperactive and hyper-responsive to dopamine D(1) agonists, suggesting a dynamic balance between the dopamine and M(4) receptors. Muscarinic agonists had activity in animal models of negative symptoms, cognitive dysfunction and affective disorders, symptoms that are prominent in schizophrenic patients. Consistent with effects in animal models, preliminary clinical investigation indicates that muscarinic agonists like xanomeline may be effective in the pharmacotherapy of schizophrenia. Thus, we hypothesize that a combined M(1) agonist to promote cognition and a M(4) agonist for antipsychotic-like effects would treat the symptom domains of schizophrenia without parasympathomimetic side effects.

Duloxetine increases serotonin and norepinephrine availability in healthy subjects: a double-blind, controlled study.

Evidence suggests that compounds that increase the synaptic availability of more than one neurotransmitter have greater efficacy in the treatment of depression than single-acting drugs. Preclinical studies indicate that duloxetine acts to inhibit serotonin (5-HT) and norepinephrine (NE) transporters. The ability of duloxetine to alter 5-HT and NE reuptake was tested in 12 healthy male subjects. Placebo, desipramine 50 mg b.i.d., and duloxetine (80 mg q.d. or 60 mg b.i.d.) were compared in a randomized, double-blind, three-period crossover study in 12 healthy male subjects. Whole-blood 5-HT, urinary excretion of NE and major metabolites, and TYR PD30 (IV tyramine pressor dose needed to increase systolic blood pressure by 30 mmHg) were measured at steady state. Vital signs were measured periodically. Duloxetine affected 5-HT reuptake, with whole-blood 5-HT depletion vs placebo (80 mg q.d.: p=0.07; 60 mg b.i.d.: p=0.02; combined regimens: p=0.01). Cardiovascular changes reflecting increased sympathetic tone were observed with both duloxetine and desipramine, and both treatments significantly decreased whole body NE turnover (p<0.01). Duloxetine and desipramine were associated with similar mean increases in fractional extraneuronal NE concentration, although these changes did not reach statistical significance. TYR PD30 increased significantly with desipramine dosing (p<0.01). In conclusion, whole-blood measurements confirm that duloxetine inhibits platelet 5-HT uptake in vivo. Urinary and cardiovascular measurements suggest that duloxetine has an effect on NE synthesis and turnover, indicative of NE reuptake inhibition. The lack of a detectable impact of duloxetine on TYR PD30 suggests that this may not be the most sensitive indirect measure of NE reuptake when assessing dual reuptake inhibitors.

R-fluoxetine increases extracellular DA, NE, as well as 5-HT in rat prefrontal cortex and hypothalamus: an in vivo microdialysis and receptor binding study.

The selective serotonin reuptake inhibitor fluoxetine consists of equal amounts of R and S stereoisomers. In this study, we investigated the pharmacologic properties of the stereoisomers using transporter and receptor binding assays and in vivo microdialysis in freely moving rats. Binding to the transporter confirmed selectivity of R- and S-fluoxetine for the 5-HT transporter versus the dopamine (DA) and norepinephrine (NE) human transporters. Receptor binding studies demonstrated significant affinity of R-fluoxetine, but not S-fluoxetine, for human 5-HT(2A) and 5-HT(2C) receptor subtypes. Functional GTPgammaS binding studies indicated that R-fluoxetine is an antagonist at 5-HT(2A) and 5-HT(2C) receptors. In microdialysis studies, acute R- and S-fluoxetine increased extracellular levels of 5-HT, DA, and NE in prefrontal cortex (PFC), but R-fluoxetine caused significantly greater increases of catecholamines. R-fluoxetine increased extracellular levels of 5-HT and NE in PFC, nucleus accumbens, and hypothalamus, whereas it increased dopamine in PFC and hypothalamus, but not in DA-rich nucleus accumbens and striatum, thus indicating a regionally selective effect. The unexpected increases of extracellular catecholamines by a selective 5-HT uptake inhibitor like R-fluoxetine may be due to its antagonism of 5-HT(2C) receptors.

Atomoxetine increases extracellular levels of norepinephrine and dopamine in prefrontal cortex of rat: a potential mechanism for efficacy in attention deficit/hyperactivity disorder.

The selective norepinephrine (NE) transporter inhibitor atomoxetine (formerly called tomoxetine or LY139603) has been shown to alleviate symptoms in Attention Deficit/Hyperactivity Disorder (ADHD). We investigated the mechanism of action of atomoxetine in ADHD by evaluating the interaction of atomoxetine with monoamine transporters, the effects on extracellular levels of monoamines, and the expression of the neuronal activity marker Fos in brain regions. Atomoxetine inhibited binding of radioligands to clonal cell lines transfected with human NE, serotonin (5-HT) and dopamine (DA) transporters with dissociation constants (K(i)) values of 5, 77 and 1451 nM, respectively, demonstrating selectivity for NE transporters. In microdialysis studies, atomoxetine increased extracellular (EX) levels of NE in prefrontal cortex (PFC) 3-fold, but did not alter 5-HT(EX) levels. Atomoxetine also increased DA(EX) concentrations in PFC 3-fold, but did not alter DA(EX) in striatum or nucleus accumbens. In contrast, the psychostimulant methylphenidate, which is used in ADHD therapy, increased NE(EX) and DA(EX) equally in PFC, but also increased DA(EX) in the striatum and nucleus accumbens to the same level. The expression of the neuronal activity marker Fos was increased 3.7-fold in PFC by atomoxetine administration, but was not increased in the striatum or nucleus accumbens, consistent with the regional distribution of increased DA(EX). We hypothesize that the atomoxetine-induced increase of catecholamines in PFC, a region involved in attention and memory, mediates the therapeutic effects of atomoxetine in ADHD. In contrast to methylphenidate, atomoxetine did not increase DA in striatum or nucleus accumbens, suggesting it would not have motoric or drug abuse liabilities.

Brain region and dose effects of an olanzapine/fluoxetine combination on extracellular monoamine concentrations in the rat.

Clinical studies of patients with treatment-resistant depression have shown that combined treatment with fluoxetine and olanzapine rapidly and significantly improved depressive symptoms. The present study used in vivo microdialysis to investigate the brain regional and dose effects of these drugs on extracellular monoamine concentrations in the rat prefrontal cortex, hypothalamus, nucleus accumbens and striatum. In the prefrontal cortex, the olanzapine/fluoxetine combination (3/10 mg/kg, respectively) increased catecholamine concentrations to a significantly greater extent than either drug alone (dopamine mean+/-S.E.M. percent of baseline: olanzapine (120 +/- 12.4), fluoxetine (123 +/- 6.2), combination (185 +/- 8.8); norepinephrine: olanzapine (124 +/- 7.2), fluoxetine (126 +/- 5.0), combination (215 +/- 15.8)). The combination also increased serotonin concentrations to 156 +/- 11.0% of baseline, but to a lesser extent than fluoxetine alone (210 +/- 14.5%). Similar synergistic effects of the combination were observed in the hypothalamus, but not in the other regions studied. The dose response effects of the drugs alone and in combination were complex, but larger doses of the combinations produced greater monoamine concentration increases than smaller dose combinations. The effects of the olanzapine/fluoxetine combination are meaningful in prefrontal cortex and hypothalamus due to their hypothesized role in the etiology and pharmacotherapy of depression. The wide-ranging neurochemical effects of this drug combination may make it particularly useful as a treatment for complex, resistant depressions.

Comparison of effects of dual transporter inhibitors on monoamine transporters and extracellular levels in rats.

Compounds that block both serotonin (5-HT) and norepinephrine (NE) transporters have been proposed to have improved antidepressant efficacy. We compared the ability of four dual transporter inhibitors-chlorimipramine, duloxetine, milnacipran and venlafaxine-to block monoamine transporters in vitro and in vivo and increase extracellular monoamines in rat brain. Inhibition of radioligand binding to clonal human monoamine transporters in vitro and in vivo in rats was determined. Extracellular concentrations of 5-HT and NE in rat prefrontal cortex (PFC) were quantified using the microdialysis technique. All compounds blocked binding to human 5-HT and NE transporters, although chlorimipramine and venlafaxine had markedly greater affinity for 5-HT than NE transporters. In vivo, chlorimipramine and duloxetine potently blocked both transporters, milnacipran blocked both with lower potency and venlafaxine only blocked the 5-HT transporter. Chlorimipramine and duloxetine increased robustly and approximately equally monoamine extracellular concentrations. Milnacipran produced only small increases in NE, whereas venlafaxine increased 5-HT markedly at the lower doses and both monoamines at high doses. Thus, the dual transporter inhibitors blocked 5-HT and NE transporters in vitro and in vivo with varying potency. Chlorimipramine, duloxetine, and high dose venlafaxine acted as dual transporter inhibitors in rat PFC and increased extracellular concentrations of the monoamines, indicating functional dual transporter inhibition.