Buspirone enhances duloxetine- and fluoxetine-induced increases in dialysate levels of dopamine and noradrenaline, but not serotonin, in the frontal cortex of freely moving rats

Combined α2- and D2-receptor blockade activates noradrenergic and dopaminergic neurons, but extracellular dopamine in the prefrontal cortex is determined by uptake and release from noradrenergic terminals

Experimental and clinical evidence indicates a deficit of release and function of dopamine in schizophrenia and suggests that α2-adrenoceptor antagonists rescue dopamine deficit and improve the antipsychotic efficacy of D2-receptor antagonists. In anesthetized male rats, we investigated how the blockade of α2- and D2-receptors by atipamezole and raclopride, respectively, modified the firing of noradrenergic neurons in the locus coeruleus (LC) and dopaminergic neurons in the ventral tegmental area (VTA). In freely moving rats, we studied how atipamezole and raclopride modified extracellular noradrenaline, dopamine, and DOPAC levels in the medial prefrontal cortex (mPFC) through microdialysis. When administered alone, atipamezole activated LC noradrenaline but not VTA dopamine cell firing. Combined with raclopride, atipamezole activated dopamine cell firing above the level produced by raclopride. Atipamezole increased extracellular dopamine to the same level, whether administered alone or combined with raclopride. In the presence of the noradrenaline transporter (NET) inhibitor, atipamezole combined with raclopride increased extracellular dopamine beyond the level produced by either compound administered alone. The results suggest that a) the D2-autoreceptor blockade is required for LC noradrenaline to activate VTA cell firing; b) the level of dopamine released from dopaminergic terminals is determined by NET; c) the elevation of extracellular dopamine levels in the mPFC is the resultant of dopamine uptake and release from noradrenergic terminals, independent of dopaminergic cell firing and release; and d) LC noradrenergic neurons are an important target for treatments to improve the prefrontal deficit of dopamine in neuropsychiatric pathologies.

Therapeutic treatment with fluoxetine using the chronic unpredictable stress model induces changes in neurotransmitters and circulating miRNAs in extracellular vesicles

The most widely prescribed antidepressant, fluoxetine (FLX), is known for its antioxidant and anti-inflammatory effects when administered post-stress. Few studies have evaluated the effects of FLX treatment when chronic stress has induced deleterious effects in patients. Our objective was to evaluate FLX treatment (20 mg/kg/day, i.v.) once these effects are manifested, and the drug's relation to extracellular circulating microRNAs associated with inflammation, a hedonic response (sucrose intake), the forced swim test (FST), and corticosterone levels (CORT) and monoamine concentrations in limbic areas. A group of Wistar rats was divided into groups: Control; FLX; CUMS (for six weeks of exposure to chronic, unpredictable mild stress); and CUMS + FLX, a mixed group. After CUMS, the rats performed the FST, and serum levels of CORT and six microRNAs (miR-16, -21, -144, -155, -146a, -223) were analyzed, as were levels of dopamine, noradrenaline, and serotonin in the prefrontal cortex, hippocampus, and hypothalamus. CUMS reduced body weight, sucrose intake, and hippocampal noradrenaline levels, but increased CORT, immobility behavior on the FST, dopamine concentrations in the prefrontal cortex, and all miRNAs except miR-146a expression. Administering FLX during CUMS reduced CORT levels and immobility behavior on the FST and increased the expression of miR-16, -21, -146a, -223, and dopamine. FLX protects against the deleterious effects of stress by reducing CORT and has an antidepressant effect on the FST, with minimally-modified neurotransmitter levels. FLX increased the expression of miRNAs as part of the antidepressant effect. It also regulates both neuroinflammation and serotoninergic neurotransmission through miRNAs, such as the miR-16.

Apocynin-Tandospirone Derivatives Suppress Methamphetamine-Induced Hyperlocomotion in Rats with Neonatal Exposure to Dizocilpine

Accumulating evidence implicates oxidative stress as a potential pathophysiological mechanism of schizophrenia. Accordingly, we synthesized new chemicals using apocynin and tandospirone as lead compounds (A-2, A-3 and A-4). These novel compounds decreased reactive oxygen species (ROS) concentrations in vitro and reversed decreases in glutathione levels in the medial prefrontal cortex of rats transiently exposed to MK-801, an N-methyl-d-aspartate receptor antagonist, in the neonatal period. To determine whether A-2, A-3 and A-4 show behavioral effects associated with antipsychotic properties, the effects of these compounds on methamphetamine (MAP)-induced locomotor and vertical activity were examined in the model rats. A-2 and A-3, administered for 14 days around the puberty period, ameliorated MAP-induced hyperlocomotion in MK-801-treated rats in the post-puberty period, while A-4 suppressed MAP-induced vertical activity. These findings indicate that apocynin-tandospirone derivatives present anti-dopaminergic effects and may alleviate psychotic symptoms of schizophrenia.

Conserved Serotonergic Background of Experience-Dependent Behavioral Responsiveness in Zebrafish (Danio rerio)

Forming effective responses to threatening stimuli requires the adequate and coordinated emergence of stress-related internal states. Such ability depends on early-life experiences and, in connection, the adequate formation of neuromodulatory systems, particularly serotonergic signaling. Here, we assess the serotonergic background of experience-dependent behavioral responsiveness using male and female zebrafish (Danio rerio). For the first time, we have characterized a period during behavioral metamorphosis in which zebrafish are highly reactive to their environment. Absence of social stimuli during this phase established by isolated rearing fundamentally altered the behavioral phenotype of postmetamorphic zebrafish in a challenge-specific manner, partially due to reduced responsiveness and an inability to develop stress-associated arousal state. In line with this, isolation differentially affected whole-brain serotonergic signaling in resting and stress-induced conditions, an effect that was localized in the dorsal pallium and was negatively associated with responsiveness. Administration of the serotonin receptor 1A partial agonist buspirone prevented the isolation-induced serotonin response to novelty in the level of the whole brain and the forebrain as well, without affecting catecholamine levels, and rescued stress-induced arousal along with challenge-induced behaviors, which together indicates functional connection between these changes. In summary, there is a consistent negative association between behavioral responsiveness and serotonergic signaling in zebrafish, which is well recognizable through the modifying effects of developmental perturbation and pharmacological manipulations as well. Our results imply a conserved serotonergic mechanism that context-dependently modulates environmental reactivity and is highly sensitive to experiences acquired during a specific early-life time window, a phenomenon that was previously only suggested in mammals.SIGNIFICANCE STATEMENT The ability to respond to challenges is a fundamental factor in survival. We show that zebrafish that lack appropriate social stimuli in a sensitive developmental period show exacerbated alertness in nonstressful conditions while failing to react adequately to stressors. This shift is reflected inversely by central serotonergic signaling, a system that is implicated in numerous mental disorders in humans. Serotonergic changes in brain regions modulating responsivity and behavioral impairment were both prevented by the pharmacological blockade of serotonergic function. These results imply a serotonergic mechanism in zebrafish that transmits early-life experiences to the later phenotype by shaping stress-dependent behavioral reactivity, a phenomenon that was previously only suggested in mammals. Zebrafish provide new insights into early-life-dependent neuromodulation of behavioral stress-responses.

Involvement of serotonin 5-HT3 receptors in the modulation of noradrenergic transmission by serotonin reuptake inhibitors: a microdialysis study in rat brain

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs), in addition to being able to enhance serotonergic neurotransmission, are able to modulate other brain systems involved in depression.

OBJECTIVES

This study evaluates the neurochemical effect of the SSRI citalopram on brain noradrenergic activity and the serotonin receptor involved in this effect.

METHODS

Dual-probe microdialysis in the locus coeruleus (LC) and prefrontal cortex (PFC) was performed in freely awake rats.

RESULTS

Systemic citalopram (10 mg/kg, i.p.) increased noradrenaline (NA) in the LC (E max = 141 ± 13%) and simultaneously decreased NA in the PFC (Emax = -46 ± 7%). In the local presence into the LC of the α2-adrenoceptor antagonist RS79948 (1 μM), systemic citalopram increased NA in the LC (Emax = 157 ± 25%) and PFC (Emax = 175 ± 24%). Local citalopram (0.1-100 μM) into the LC induced NA increase in the LC (Emax = 210 ± 25%) and decrease in the PFC (Emax = -38 ± 9%). Local LC citalopram effect was abolished by LC presence of the 5-HT3 receptor antagonist MDL72222 (1 μM) but not the 5-HT1/2 receptor antagonist methiothepin (1 μM). Systemic citalopram in the LC presence of MDL72222 did not modify NA in the LC but increased NA in the PFC (Emax = 158 ± 26%). Local citalopram into the PFC enhanced NA (Emax = 376 ± 18%) in the area, which was prevented by MDL72222.

CONCLUSIONS

The SSRI citalopram modulates central noradrenergic neurotransmission by activation, through endogenous serotonin, of 5-HT3 receptors expressed in the somatodendritic (LC) and terminal (PFC) areas, which subsequently promote an enhancement of local NA. Therefore, 5-HT3 receptors and somatodendritic α2-adrenoceptors in the LC play an important role in the global effect of SSRIs.

Antidepressant-like effect of ursolic acid isolated from Rosmarinus officinalis L. in mice: evidence for the involvement of the dopaminergic system

Ursolic acid, a constituent from Rosmarinus officinalis, is a triterpenoid compound which has been extensively known for its anticancer and antioxidant properties. In the present study, we investigated the antidepressant-like effect of ursolic acid isolated from this plant in two predictive tests of antidepressant property, the tail suspension test (TST) and the forced swimming test (FST) in mice. Furthermore, the involvement of dopaminergic system in its antidepressant-like effect was investigated in the TST. Ursolic acid reduced the immobility time in the TST (0.01 and 0.1mg/kg, p.o.) and in the FST (10mg/kg, p.o.), similar to fluoxetine (10mg/kg, p.o.), imipramine (1mg/kg, p.o.) and bupropion (10mg/kg, p.o.). The effect of ursolic acid (0.1mg/kg, p.o.) in the TST was prevented by the pretreatment of mice with SCH23390 (0.05mg/kg, s.c., a dopamine D(1) receptor antagonist) and sulpiride (50mg/kg, i.p., a dopamine D(2) receptor antagonist). The administration of a sub-effective dose of ursolic acid (0.001mg/kg, p.o.) in combination with sub-effective doses of SKF38393 (0.1mg/kg, s.c., a dopamine D(1) receptor agonist), apomorphine (0.5μg/kg, i.p., a preferential dopamine D(2) receptor agonist) or bupropion (1mg/kg, i.p., a dual dopamine/noradrenaline reuptake inhibitor) reduced the immobility time in the TST as compared with either drug alone. Ursolic acid and dopaminergic agents alone or in combination did not cause significant alterations in the locomotor and exploratory activities. These results indicate that the antidepressant-like effect of ursolic acid in the TST is likely mediated by an interaction with the dopaminergic system, through the activation of dopamine D(1) and D(2) receptors.

Duloxetine in the treatment of stress urinary incontinence

This manuscript reviews the pharmacodynamics and pharmacokinetics of duloxetine and its efficacy and safety in women with stress urinary incontinence. Duloxetine is a selective inhibitor of neuronal serotonin and norepinephrine uptake which increases urethral striated muscle activity and bladder capacity. Duloxetine is readily absorbed and extensively metabolized; cytochrome P450 1A2 (CYP1A2) inhibiting drugs can markedly increase duloxetine exposure. The clinical efficacy of duloxetine has consistently been demonstrated in several randomized, double-blind studies in women with moderate-to-severe stress urinary incontinence, but the additional benefit relative to placebo was moderate. Duloxetine treatment is frequently associated with adverse events such as nausea, dry mouth, fatigue, insomnia and constipation, but serious adverse events are rare. Therefore, duloxetine appears suitable for the treatment of stress urinary incontinence.

A new strategy for antidepressant prescription

From our research and literature search we propose an understanding of the mechanism of action of antidepressants treatments (ADTs) that should lead to increase efficacy and tolerance. We understand that ADTs promote synaptic plasticity and neurogenesis. This promotion is linked with stimulation of dopaminergic receptors. Previous evidence shows that all ADTs (chemical, electroconvulsive therapy, repetitive transcranial magnetic stimulation, sleep deprivation) increase at least one monoamine neurotransmitter serotonin (5-HT), noradrenaline (NA) or dopamine (DA); this article focuses on DA release or turn-over in the frontal cortex. DA increased dopaminergic activation promotes synaptic plasticity with an inverted U shape dose–response curve. Specific interaction between DA and glutamate is mediated by D1 receptor subtypes and Glutamate (NMDA) receptors with neurotrophic factors likely to play a modulatory role. With the understanding that all ADTs have a common, final, DA-ergic stimulation that promotes synaptic plasticity we can predict that (1) AD efficiency is related to the compound strength for inducing DA-ergic stimulation. (2) ADT efficiency presents a therapeutic window that coincides with the inverted U shape DA response curve. (3) ADT delay of action is related to a “synaptogenesis and neurogenesis delay of action.” (4) The minimum efficient dose can be found by starting at a low dosage and increasing up to the patient response. (5) An increased tolerance requires a concomitant prescription of a few ADTs, with different or opposite adverse effects, at a very low dose. (6) ADTs could improve all diseases with cognitive impairments and synaptic depression by increasing synaptic plasticity and neurogenesis.

Role of serotonin in central dopamine dysfunction

The interaction between serotonin (5-HT) and dopamine (DA)-containing neurons in the brain is a research topic that has raised the interest of many scientists working in the field of neuroscience since the first demonstration of the presence of monoamine-containing neurons in the mid 1960. The bulk of neuroanatomical data available clearly indicate that DA-containing neurons in the brain receive a prominent innervation from serotonin (5-hydroxytryptamine, 5-HT) originating in the raphe nuclei of the brainstem. Compelling electrophysiological and neurochemical data show that 5-HT can exert complex effects on the activity of midbrain DA neurons mediated by its various receptor subtypes. The main control seems to be inhibitory, this effect being more marked in the mesocorticolimbic DA system as compared to the DA nigrostriatal system. In spite of a direct effect of 5-HT by its receptors located on DA cells, 5-HT can modulate their activity indirectly, modifying gamma-aminobutyric (GABA)-ergic and glutamatergic input to the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Although 5-HT/DA interaction in the brain has been extensively studied, much work remains to be done to clarify this issue. The recent development of subtype-selective ligands for 5-HT receptors will not only allow a detailed understanding of this interaction but also will lead to the development of new treatment strategies, appropriate for those neuropsychiatric disorders in which an alteration of the 5-HT/DA balance is supposed.

The antidepressant effect of Gastrodia elata Bl. on the forced-swimming test in rats

Depression is a common psychiatric disorder with a high morbidity and mortality rate. The pharmacotherapy used in clinic today is not suitable for all patients and causes certain side-effects. Thus, looking for alternative treatments with antidepressant effect and minimal side-effect is important. Gastrodia elata Bl. is a famous Chinese traditional medicine used for centuries. The aim of this study is to test the antidepressant effect of Gastrodia elata Bl. extract (GE) using forced-swimming test (FST). Sprague-Dawley rats were assigned to control, GE, and fluoxetine groups, treated with 10 ml/kg bw (body weight) water, 1 g/kg bw of GE, and 15 mg/kg bw of fluoxetine, respectively. Those samples were administered by gavage to rats 23.5, 4.5 and 1 hour prior to the test session of FST. After FST, the animals were sacrificed and their brains were collected for monoamines analysis. The results indicated that the duration of immobility was significantly decreased in GE group compared to the control (p < 0.05). The concentration of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and the ratio of 5-HIAA/5-HT in frontal cortex, amygdala, and hippocampus were not significantly different between GE and the control groups. However, administration of GE significantly increased the dopamine (DA) concentration (p < 0.05) and decreased the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) (p < 0.01) and DA turnover (p < 0.05) in striatum compared to the control. The results of this study show that Gastrodia elata Bl. extract modulates the turnover of DA in rats, and thus probably possesses antidepressant effect in rats.

Aripiprazole in the treatment of depressive and anxiety disorders: a review of current evidence

Despite the availability of different classes of drugs for the treatment of depressive and anxiety disorders, there are a number of clinically significant unmet needs, such as a high prevalence of treatment resistance, partial response, subsyndromal symptomatology, recurrence and relapse. With the approval of atypical antipsychotics, which are associated with a lower adverse effect burden than typical antipsychotics, consideration of their off-label use for the treatment of affective disorders and various other psychiatric disorders has become a viable option. However, consideration should be given to the US FDA black box warning indicating that atypical antipsychotics may increase mortality risk, particularly in the elderly population with dementia-related psychosis. There has been much conjecture about the utility of these atypical drugs to facilitate traditional antidepressant therapy, either in combination (from the initiation of therapy) or as adjunctive therapy (in the case of partial/incomplete response). Nevertheless, at present, available evidence from randomized, placebo-controlled trials is sparse, and a formal risk/benefit assessment of the use of these agents in a nonpsychotic patient population is not yet possible. As a representative agent from the atypical antipsychotic class with a novel mechanism of action and a relatively low adverse effect burden, aripiprazole represents an interesting potential treatment for depressive and anxiety disorders. In this review, we focus on the rationale for the use of aripiprazole in these disorders. Preclinical data suggests that aripiprazole has a number of possible mechanisms of action that may be important in the treatment of depressive and anxiety disorders. Such mechanisms include aripiprazole action at serotonin (5-HT) receptors as a 5-HT1A partial receptor agonist, a 5-HT2C partial receptor agonist and a 5-HT2A receptor antagonist. Aripiprazole also acts as a dopamine D2 partial receptor agonist, and has a possible action at adrenergic receptors. Furthermore, aripiprazole may have possible neuroprotective effects. Clinical studies demonstrate that aripiprazole may be useful in the treatment of bipolar depression, major depressive disorder, treatment-resistant depression and possibly anxiety disorders. Clinical data also suggest that aripiprazole may have a lower adverse effect burden than the other atypical drugs. Future research may confirm the potential utility of aripiprazole in the treatment of depressive and anxiety disorders.

Differential alpha2-mediated inhibition of dopamine and noradrenaline release in the parietal and occipital cortex following noradrenaline transporter blockade

Parietal and occipital cortices, while densely innervated by noradrenalin 2 (NA) projections, possess a comparatively sparse dopamine 2 (DA) innervation, even sparser than the prefrontal cortex. We previously reported that reboxetine and desipramine, two selective norepinephrine transporter (NET) blockers, at doses that maximally increase DA in the prefrontal cortex, do not increase DA in the parietal and occipital cortices. In the present study, we performed a full dose-response study of the effect of systemic reboxetine and desipramine on DA and NA in dialysates from the parietal and occipital cortices. Seven doses of reboxetine (0.1, 0.25, 0.5, 1.0, 2.5, 5.0 and 10 mg/kg) and four doses of desipramine (0.25, 1.0, 2.5 and 5.0 mg/kg) were tested. Reboxetine and desipramine differentially affected dialysate DA as compared with NA. Reboxetine increased DA maximally by about 100% after doses of 0.25-0.5 mg/kg and showed a bell-shaped dose-response function in both areas; desipramine did not affect DA in the parietal cortex and increased it in the occipital cortex only at 2.5 mg/kg. NA was maximally increased by 275% by 0.5-2.5 mg/kg reboxetine and by about 300% by 5.0 mg/kg desipramine with a more linear dose-response curve. The mechanism of peculiar dose-response function of dialysate DA after reboxetine and desipramine was further investigated by testing the effect of drugs on dialysate DA and NA under alpha(2) receptor blockade. Under local perfusion of the occipital cortex with idazoxan, an otherwise ineffective dose of reboxetine and desipramine (5 mg/kg) became effective in raising extracellular DA. In contrast, the effect of reboxetine on NA was potentiated, while that of desipramine was not affected. These results suggest that, in the parietal and occipital cortices, extracellular NA, raised by NET blockade, exerts a preferential inhibitory influence on DA release by acting on local alpha(2) receptors, thus accounting for the bell-shaped feature of the dose-response function of drugs on dialysate DA in these areas.

Antidepressant-like activity of selective serotonin reuptake inhibitors combined with a NK1 receptor antagonist in the mouse forced swimming test

Substance P antagonists of the neurokinin-1 receptor type (NK1) have growing interest as new antidepressant therapies. It has been postulated that these drugs exert this putative therapeutic effect without direct interactions with serotonin (5-HT) neurons. In line with this assumption, previous intracerebral in vivo microdialysis experiments provided evidence that the NK1 receptor antagonists did not change basal cortical 5-HT levels. However, we found that increases in cortical 5-HT overflow caused by systemic injection of the selective serotonin reuptake inhibitor (SSRI), paroxetine was higher in freely moving (C57BL/6x129sv) NK1-/- mutants than in wild-type NK1+/+ mice. More recently, a pharmacological study has led to a similar conclusion since GR205171, a NK1 receptor antagonist, potentiated paroxetine-induced increases in cortical 5-HT dialysate following its acute systemic or intra-raphe administration to wild-type mice . In the present study, we tested whether an acute combination of SSRI and NK1 receptor antagonist could display antidepressant-like activity using the forced swimming test in Swiss mice. We found that a single systemic dose of GR205171 (10 and 30 mg/kg, i.p.) had no effect by itself. However, it selectively potentiated the antidepressant-like activity of subactive doses of two serotonergic antidepressant drugs, citalopram and paroxetine (without psychomotor stimulant activity), but not that of noradrenaline reuptake inhibitor, desipramine. In agreement with neurochemical data, the present study confirms that co-administration of a NK1 receptor antagonist with an antidepressant drug such as a SSRI may have a therapeutic potential to improve the treatment of major depressive episodes in human compared to SSRI alone.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Dual serotonin and noradrenaline reuptake inhibitors: Focus on their differences

There are three non-tricyclic dual serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs) currently used in human therapeutics for psychiatric disorders. These medications differ in their in vitro potency to inhibit 5-HT and NA reuptake with differential ratios of activity. Using in vivo studies carried out in laboratory animals, which better reflect human physiology than experiments using lysed tissue in a test tube, venlafaxine is about three times more potent on 5-HT than NA reuptake, duloxetine five times, and milnacipran is about twice more potent on NA than 5-HT reuptake. Sustained administration of SNRIs induces different adaptive effects on presynaptic 5-HT and NA receptors controlling the function of 5-HT and NA neurons, suggesting that they may differentially affect transmission of these two neuronal systems. In the treatment of depression, SNRIs appear to have similar effectiveness and when compared to selective 5-HT reuptake inhibitors, they generally exert a superior antidepressant effect. Taken together, these observations suggest that individual patients not responding to a SNRI may present a favourable response to another agent within that family. SNRIs have different pharmacokinetic properties and exert distinct effects on the activity of liver metabolic enzymes. These features of SNRIs can help clinicians tailor treatment to individual patients.

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.

Neurochemical evaluation of the novel 5-HT1A receptor partial agonist/serotonin reuptake inhibitor, vilazodone

Vilazodone has been reported to be an inhibitor of 5-hydoxytryptamine (5-HT) reuptake and a partial agonist at 5-HT1A receptors. Using [35S]GTPgammaS binding in rat hippocampal tissue, vilazodone was demonstrated to have an intrinsic activity comparable to the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Vilazodone (1-10 mg/kg p.o.) dose-dependently displaced in vivo [3H]DASB (N,N-dimethyl-2-(2-amino-4-cyanophenylthio)benzylamine) binding from rat cortex and hippocampus, indicating that vilazodone occupies 5-HT transporters in vivo. Using in vivo microdialysis, vilazodone (10 mg/kg p.o.) was demonstrated to cause a 2-fold increase in extracellular 5-HT but no change in noradrenaline or dopamine levels in frontal cortex of freely moving rats. In contrast, administration of 8-OH-DPAT (0.3 mg/kg s.c.), either alone or in combination with a serotonin specific reuptake inhibitor (SSRI; paroxetine, 3 mg/kg p.o.), produced no increase in cortical 5-HT whilst increasing noradrenaline and dopamine 2 and 4 fold, respectively. A 2-fold increase in extracellular 5-HT levels (but no change in noradrenaline or dopamine levels) was observed after combination of the 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyridinyl)cyclohexanecarboxamide) (WAY-100635; 0.3 mg/kg s.c.) and paroxetine (3 mg/kg p.o.). In summary, vilazodone behaved as a high efficacy partial agonist at the rat hippocampal 5-HT1A receptors in vitro and occupied 5-HT transporters in vivo. In vivo vilazodone induced a selective increase in extracellular levels of 5-HT in the rat frontal cortex. This profile was similar to that seen with a 5-HT1A receptor antagonist plus an SSRI but in contrast to 8-OH-DPAT either alone or in combination with paroxetine.

Duloxetine: A Dual Serotonin-Norepinephrine Reuptake Inhibitor for Treatment of Major Depressive Disorder

The burden of mental illness has been underestimated worldwide. Depression was the fourth leading cause of disease burden in the world in 1990 and is projected to be the second leading cause of disability by 2020. It is a leading cause of morbidity and mortality in the United States, costing billions of dollars annually in direct and indirect medical costs and losses in productivity. Patients with major depressive disorder (MDD) may experience both psychological and medical complaints, including somatic sensations or pain. Some antidepressants have been shown to treat chronic pain syndromes, but despite the variety of antidepressants available in the United States, only 65-70% of patients respond to initial antidepressant treatment. Treatments are limited by delayed onset of antidepressant effects, side effects, partial response, and treatment resistance. Duloxetine, approved by the U.S. Food and Drug Administration for the treatment of MDD, is a reuptake inhibitor at serotonergic and noradrenergic neurons and appears to have low affinity for other neurotransmitter systems. In clinical trials, duloxetine was effective for the treatment of MDD and was well tolerated. Further study is needed to compare its efficacy with that of other antidepressants, to clarify effects on somatic symptoms, and to assess potential adverse cardiovascular and sexual side effects. Duloxetine is also approved for the management of diabetic peripheral neuropathic pain and is under investigation for the treatment of stress urinary incontinence in women.

Perospirone, a novel atypical antipsychotic drug, potentiates fluoxetine-induced increases in dopamine levels via multireceptor actions in the rat medial prefrontal cortex

Perospirone is a novel atypical antipsychotic with a unique combination of 5-HT1A receptor agonism as well as 5-HT2A and D2 receptor antagonism. We investigated the effect of perospirone in combination with fluoxetine on dopamine release in the rat medial prefrontal cortex using microdialysis. Perospirone and fluoxetine increased dopamine release to 270 and 210% of the baseline value, respectively. A combination of perospirone and fluoxetine markedly increased dopamine release to 800% of the baseline value. Pretreatment with a selective 5-HT1A receptor antagonist, WAY 100635, suppressed the increase in dopamine levels induced by the administration of perospirone and fluoxetine to 330% of the baseline value. These findings suggest that perospirone potentiates fluoxetine-induced dopamine increases in part via the action of the 5-HT1A receptor and may augment the effect of fluoxetine in treatment-resistant depression.

Fluoxetine administration potentiates the effect of olanzapine on locus coeruleus neuronal activity

BACKGROUND

As many as 30% of individuals diagnosed with depression are nonresponsive to traditional antidepressant medication. Augmentation and combination strategies have emerged in an attempt to address this issue. Atypical antipsychotics (e.g., olanzapine), when added to a selective serotonin reuptake inhibitor (e.g., fluoxetine) have shown great promise in the treatment of these treatment-resistant patients. As of yet, the precise neural mechanisms responsible for the beneficial clinical effect of these combinations are not completely understood.

METHODS

Separate groups of rats received either saline or fluoxetine (10 mg/kg/day) for 24 hours or 3 weeks via subcutaneously implanted osmotic pumps. The effects of either intravenous saline or olanzapine (.3, 1.0, or 3.0 mg/kg) on locus coeruleus (LC) neuronal activity were then assessed via extracellular single-unit recordings.

RESULTS

Acute administration of olanzapine produced a significant elevation of the firing rate and burst firing of LC cells, and chronic, but not acute, administration of fluoxetine decreased baseline and burst firing of LC cells; however, when given in combination, an interaction of fluoxetine and olanzapine was observed, with olanzapine causing a significantly greater increase in LC firing rate and burst firing after acute and chronic administration of fluoxetine.

CONCLUSIONS

These results provide a potential neural mechanism for the beneficial clinical effects of the olanzapine/fluoxetine combination. The increase in baseline and burst firing of LC neurons in the groups receiving both fluoxetine and olanzapine would result in enhanced norepinephrine release in projection areas (e.g., prefrontal cortex), which could lead to a reduction in depressive symptoms.

Serotonin agonists and antagonists in obstructive sleep apnea: therapeutic potential

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a prevalent disorder associated with substantial cardiovascular and neurobehavioral morbidity. Yet this is a disorder for which there are no widely effective pharmacotherapies. The pathophysiology of obstructive sleep apnea namely, normal respiration in waking with disordered breathing only in sleep, suggests that this disorder should be readily amenable to drug therapy. Over the past 10 years, we have gained tremendous insight into the neurochemical mechanisms involved in state-dependent control of respiration. It is apparent from this work that there are many potential avenues for pharmacotherapies, including several seemingly conflicting directions for serotonergic therapies. Serotonin delivery is reduced to upper airway dilator motor neurons in sleep, and this contributes, at least in part, to sleep-related reductions in dilator muscle activity and upper airway obstruction. The dilator motor neuron post-synaptic serotonin receptors are 5-HT(2A) and 5-HT(2C) subtypes, and in adults the presynaptic 5-HT receptor in motor nuclei is 5-HT(1B), an inhibitory receptor. Serotonin receptors are also found within central respiratory neuronal groups, and these receptor subtypes include 5-HT(1A) (inhibitory) and 5-HT(2) receptors. Peripherally, stimulation of 5-HT(2A), 5-HT(2C) and 5-HT(3) receptor subtypes have an inhibitory effect on respiration via action at the nodose ganglion. Many of these receptor subtypes and their signal transduction pathways may be affected by oxidative stress in obstructive sleep apnea. These alterations will make finding drug therapies for sleep apnea more challenging, but not insurmountable. Future directions are suggested for elucidating safe, well-tolerated serotonergic drugs for this disorder. Tryptophan was one of the first serotonergic drugs tested for OSAHS. This drug was withdrawn from the market as a result of reports linking tryptophan use with eosinophilic myalgia syndrome and life-threatening pulmonary hypertension. Newer drugs with serotonergic activity tested in persons with sleep-disordered breathing include buspirone, fluoxetine and paroxetine. Trials are presently being conducted to evaluate the effects of 5-HT(2A) and 5-HT(3) antagonists on OSAHS. Many of the drugs tested have not shown significant improvement in sleep apnea. However, with continued effort to elucidate the pharmacology of neurochemical control of state-dependent changes in respiratory control, the availability of pharmacological therapy for this disorder is not too far away.

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.

Tandospirone potentiates the fluoxetine-induced increases in extracellular dopamine via 5-HT(1A) receptors in the rat medial frontal cortex

Recent clinical studies suggest that 5-HT(1A) receptor agonists, including buspirone, may have an antidepressant effect and potentiate the efficacy of selective serotonin reuptake inhibitors (SSRI) in major depressive disorders. In the present study, we investigated the effect of tandospirone, a highly potent and selective 5-HT(1A) receptor agonist, on dopamine release and potentiation of fluoxetine-induced dopamine outflow in the medial frontal cortex using microdialysis in freely moving rats. Intraperitoneal injection of tandospirone (5 mg/kg) increased dopamine release to about 190% of basal levels. Pretreatment with the selective 5-HT(1A) receptor antagonist, WAY 100635 (1mg/kg), blocked the effect of tandospirone. Local application of WAY 100635 (10 microM) via microdialysis probe antagonized the increase in dopamine release in the medial frontal cortex induced by systemic injection of tandospirone. Fluoxetine (10 mg/kg) also increased dopamine release in the medial frontal cortex, to 200% of basal levels, and the simultaneous administration of tandospirone and fluoxetine increased the release to 380%. These results indicate that tandospirone potentiates the fluoxetine-induced increase in dopamine release via 5-HT(1A) receptors in the rat medial frontal cortex, and suggest that tandospirone may have therapeutic potential for the treatment of depression.

Acute locomotor effects of fluoxetine, sertraline, and nomifensine in young versus aged Fischer 344 rats

Spontaneous locomotor activity was measured in young (6-8 months) and aged (24-26 months) Fischer 344 (F344) rats. Following habituation to the activity monitors, aged rats demonstrated significantly diminished motor activity as quantified by total distance traveled and vertical activity. Movement speed did not differ significantly between the two groups. Following habituation, rats were administered acute doses of fluoxetine, sertraline, or nomifensine (1.0, 3.0, and 10.0 mg/kg). Fluoxetine diminished all three behavioral measures in the young rats, while in the old rats, fluoxetine's effects were limited to a robust attenuation of vertical activity. Sertraline decreased movement speed and vertical activity, but not total distance traveled, in the young rats. Unlike fluoxetine, sertraline produced no significant effects on any of the three behavioral variables in the old rats. Nomifensine increased behavioral scores for both age groups. The results are discussed in relation to acute motor side effects of selective serotonin reuptake inhibitors (SSRIs) in motor-impaired aged individuals, as these effects may influence their eventual use in the clinic.

Augmentation of standard depression therapy

OBJECTIVE

This article reviews treatment options (eg, augmentation) for depressed patients with suboptimal clinical responses to an antidepressant.

BACKGROUND

Approximately one third of patients treated with antidepressants exhibit suboptimal or delayed clinical response to these medications. In such cases, alternative options include switching to another antidepressant or adding a second antidepressant. Augmentation strategies include addition of lithium carbonate, atypical antipsychotics, psychostimulants, thyroid hormone (triiodothyronine), pindolol, or buspirone.

CONCLUSIONS

In approximately half of all antidepressant-resistant cases of major depressive disorder, controlled clinical trials have indicated that augmentation with lithium or thyroid hormone is effective. Other reports suggest that central nervous system stimulants may augment antidepressant activity, but their use is constrained by possible abuse potential. Pindolol therapy has been shown to accelerate clinical response in some but not all studies. Finally, the favorable safety and tolerability profile of buspirone, together with its desirable anxiolytic effects, render it a sound therapeutic option in antidepressant augmentation.

An investigation of the mechanisms responsible for acute fluoxetine-induced anxiogenic-like effects in mice

Although selective 5-hydroxytryptamine (5-HT) reuptake inhibitors (SSRIs) are widely used in the chronic treatment of several anxiety disorders, increased anxiety has been observed in some patients at the beginning of treatment with these compounds. Similar increases in anxiety-related behaviors have been observed in animal studies following a single injection with SSRIs. The mechanism underlying this effect is unclear. The aim of the present study was to investigate the effects of a variety of psychoactive compounds on the anxiogenic-like activity of fluoxetine. The drugs used included the benzodiazepine diazepam, the 5-HT1A receptor partial agonist buspirone, the 5-HT1A receptor antagonists pindolol and WAY-100635, the non-selective 5-HT2 receptor antagonists methiothepin, mianserin and ritanserin, the non-selective dopamine (DA) receptor antagonist haloperidol, the D1 antagonist SCH23390, the selective D2 antagonist raclopride, the D2/3 agonist quinelorane, the cholecystokininB (CCK(B)) receptor antagonist LY 288513, and the corticotropin-releasing factor1 (CRF1) receptor antagonist CP-154,526. Experiments were performed in the free-exploration test. This model is based on the strong neophobic reactions exhibited by BALB/c mice when confronted simultaneously with a familiar and a novel environment. When administered alone, diazepam (1 and 2 mg/kg), buspirone (1 mg/kg) and mianserin (0.3 mg/kg) produced anxiolytic-like effects as they significantly increased exploratory activity of the novel compartment. In contrast, fluoxetine (20 mg/kg) almost completely suppressed exploration of the novel area. Diazepam reversed the anxiogenic-like as well as the locomotor impairment induced by fluoxetine, while quinelorane blocked only the anxiogenic action of fluoxetine. None of the other compounds was able to counteract this effect. Taken together, these results suggest that dopaminergic mechanisms may underlie, at least in part, the behavioral effects of fluoxetine in the free-exploration test, whereas 5-HT1A 5-HT2, CCK(B) and CRF1 receptors may not be involved primarily in these effects.

Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents

The frontal cortex (FCX) plays a key role in processes that control mood, cognition and motor behaviour, functions which are compromised in depression, schizophrenia and other psychiatric disorders. In this regard, there is considerable evidence that a perturbation of monoaminergic input to the FCX is involved in the pathogenesis of these states. Correspondingly, the modulation of monoaminergic transmission in the FCX and other corticolimbic structures plays an important role in the actions of antipsychotic and antidepressant agents. In order to further understand the significance of monoaminergic systems in psychiatric disorders and their treatment, it is essential to characterize mechanisms underlying their modulation. Within this framework, the present commentary focuses on our electrophysiological and dialysis analyses of the complex and reciprocal pattern of auto- and heteroreceptor mediated control of dopaminergic, noradrenergic and serotonergic transmission in the FCX. The delineation of such interactions provides a framework for an interpretation of the influence of diverse classes of antidepressant agent upon extracellular levels of dopamine, noradrenaline and serotonin in FCX. Moreover, it also generates important insights into strategies for the potential improvement in the therapeutic profiles of antidepressant agents.

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.

Buspirone modulates basal and fluoxetine-stimulated dialysate levels of dopamine, noradrenaline and serotonin in the frontal cortex of freely moving rats: activation of serotonin1A receptors and blockade of alpha2-adrenergic receptors underlie its actions

The serotonin1A receptor partial agonist, buspirone, also displays antagonist properties at D2 receptors and is metabolized to the alpha2-adrenergic receptor antagonist, 1-(2-pyrimidinyl-piperazine). Herein, we examined mechanisms underlying the influence of buspirone alone, and in association with the serotonin reuptake inhibitor, fluoxetine, upon extracellular levels of serotonin, dopamine and noradrenaline simultaneously quantified in the frontal cortex of freely moving rats. Buspirone (0.01-2.5 mg/kg, s.c.) dose-dependently decreased dialysate levels of serotonin (-50%), and increased those of dopamine (+100%) and noradrenaline (+140%). The reduction by buspirone of serotonin levels was abolished by the serotonin1A receptor antagonist, WAY 100,635 (0.16), which did not, however, modify its influence upon dopamine and noradrenaline. In contrast to buspirone, the serotonin reuptake inhibitor, fluoxetine (10.0), increased frontocortical levels of serotonin (+ 120%), dopamine (+55%) and noradrenaline (+90%). Buspirone dose-dependently (0.01-2.5) decreased the induction by fluoxetine of serotonin levels yet potentiated (three-fold) its elevation of dopamine and noradrenaline levels. The serotonin1A agonist, 8-hydroxy-2-(di-n-propyl-amino)-tetralin (0.16), mimicked the action of buspirone in reducing resting levels of serotonin (-60%) and in enhancing those of dopamine (+135%) and noradrenaline (+165%). Like buspirone, it attenuated the influence of fluoxetine upon serotonin levels, yet facilitated its influence upon dopamine and noradrenaline levels. In contrast, WAY 100,635 selectively potentiated the increase in levels of serotonin (two-fold) versus dopamine and noradrenaline elicited by fluoxetine. Further, WAY 100,635 abolished the inhibitory influence of buspirone upon fluoxetine-induced serotonin release, but only partly interfered with its potentiation of fluoxetine-induced increases in dopamine and noradrenaline levels. The D2/D3 receptor antagonist, raclopride (0.16), increased basal dopamine (+60%) levels but little influenced those of serotonin and noradrenaline, and failed to modify the action of fluoxetine. The alpha2-adrenergic receptor antagonist, 1-(2-pyrimidinyl-piperazine) (2.5), which did not modify resting levels of serotonin, markedly increased those of dopamine (+90%) and noradrenaline (+190%) and potentiated (two-fold) the increases in dialysate levels of dopamine, noradrenaline and serotonin provoked by fluoxetine. Further, the alpha2-adrenergic receptor agonist, S18616, attenuated the enhancement by buspirone of the fluoxetine-induced increase in levels of dopamine and noradrenaline. In conclusion, the inhibitory influence of buspirone upon resting and fluoxetine-stimulated serotonin levels reflects its agonist properties at serotonin1A autoreceptors. The facilitatory influence of buspirone upon resting and fluoxetine-stimulated dopamine and noradrenaline levels may also involve its serotonin1A properties. However, its principal mechanism of action in this respect is probably the alpha2-adrenergic antagonist properties of its metabolite, 1-(2-pyrimidinyl-piperazine). The present observations are of significance to experimental and clinical studies of the influence of buspirone upon depressive states, alone and in association with antidepressant agents.

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

Venlafaxine is a novel serotonin/noradrenaline reuptake inhibitor (SNRI) which has been shown clinically to be an effective antidepressant (AD) with a faster onset of action than serotonin specific reuptake inhibitors (SSRI). Preclinically, venlafaxine has been shown to potently inhibit dorsal raphe neuronal (DRN) firing through a 5-HT1A mediated mechanism, in a similar manner to SSRIs. Here we demonstrate the acute neurochemical effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline (NA) from the rat frontal cortex using in vivo microdialysis. Administration of venlafaxine (3-50 mg/kg s.c.) resulted in a significant dose-dependent increase in extracellular NA, but produced no significant increase in 5-HT concentrations. Combination treatment with the selective 5-HT1A antagonist WAY100635 produced a dose-dependent augmentation of venlafaxine-induced (3-30 mg/kg s.c) extracellular 5-HT concentrations, but had no further effect on NA above that produced by venlafaxine alone. WAY100635, at doses as low as 0.03 mg/kg s.c., maintained this potentiation effect. The beta-adrenergic/5-HT1A receptor antagonist (+/-)pindolol and the selective 5-HT1B/D antagonist GR127935 produced no significant augmentation of venlafaxine-induced changes in either 5-HT or NA. Using the alpha1 and alpha2-adrenoceptor antagonists, prazosin and idazoxane, we also demonstrate the role of the alpha-adrenoceptors in the augmentation of venlafaxine-induced changes. The possible mechanisms underlying venlafaxines improved clinical AD action and the potential for further enhancement of this SNRIs clinical effects are discussed.

Modulation of the firing activity of rat serotonin and noradrenaline neurons by (+/-)pindolol

BACKGROUND

(+/-)Pindolol is a beta-adrenergic/5-HT1A receptor antagonist used in combination with certain antidepressant drugs to accelerate the onset of the antidepressive response.

METHODS

The aim of the present study was to assess, using an in vivo electrophysiologic paradigm, the effect of (+/-)pindolol on the spontaneous firing activity of rat dorsal raphe serotonin (5-HT) and locus coeruleus noradrenaline (NA) neurons.

RESULTS

(+/-)Pindolol did not modify the firing activity of dorsal raphe 5-HT neurons at low doses (10 and 200 micrograms/kg, i.v.), but it prevented the suppressant effect of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD, 10 micrograms/kg, i.v.) but not that of the 5-HT1A receptor 8-hydroxy-N,N-dipropyl-aminotetralin (8-OHDPAT, 5 micrograms/kg, i.v.). At a higher dose (500 micrograms/kg, i.v.), (+/-)pindolol decreased 5-HT neuronal firing and this effect was reversed by the selective 5-HT1A receptor antagonist WAY 100635 (100 micrograms/kg, i.v.), suggesting that it could act as a partial 5-HT1A autoreceptor agonist. In the locus coeruleus, the high dose of (+/-)pindolol decreased the firing activity of NA neurons and this effect was reversed by the 5-HT2A receptor antagonist MDL 100907 (200 micrograms/kg, i.v.). Finally, both a lesion of NA neurons and the administration of MDL 100907 prevented the suppressant effect of (+/-)pindolol on the firing of 5-HT neurons.

CONCLUSIONS

It is suggested that, at low doses, (+/-)pindolol acts as a somatodendritic 5-HT1A autoreceptor antagonist whereas at a higher dose, it decreases the tonic excitatory input from NA neurons to 5-HT neurons.

Chronic clomipramine administration reverses NMDA-evoked decreases in dopamine release in the raphe nuclei

The effect of acute or chronic treatment with the antidepressant clomipramine (CIM) on basal and N-methyl-d-aspartate (NMDA) evoked release of dopamine (DA) in rat raphe has been studied using microdialysis. Acute injection of CIM (10 or 20 mg/kg) caused a decrease in raphe DA release, as did infusion of NMDA (25-100 microM) into this region. When NMDA infusion was preceded by a single acute injection of CIM no differences between NMDA and NMDA plus CIM treated groups was observed. Chronic (15 day) treatment with CIM caused a dose-dependent increase in basal extracellular DA. In addition the effect of infusing NMDA into the raphe on DA release was markedly reduced or abolished. This suggests that adaptive changes occur in NMDA receptor function during treatment with CIM.

Chronic but not acute clomipramine alters the effect of NMDA receptor regulation of dopamine release in rat frontal cortex

The effect of acute or chronic treatment with the antidepressant clomipramine (CIM) on N-methyl-D-aspartate (NMDA) evoked release of dopamine (DA) in the frontal cortex of the rat has been studied using microdialysis. Acute injection of CIM (10 or 20 mg/kg) caused a decrease in dialysate DA in the frontal cortex. Infusion of 25-100 microM NMDA into the frontal cortex decreased DA release in this region. When NMDA infusion was preceded by a single injection of CIM no marked differences between NMDA and NMDA + CIM treated groups were observed. Chronic (15 day) treatment with CIM (10 or 20 mg/kg) caused a dose-dependent increase in basal extracellular DA. In these animals, however, the effects of infusion of NMDA on DA release in the cortex were greatly attenuated or abolished. This suggests that adaptive changes occur in NMDA receptor function during treatment with an antidepressant. The possible significance of this in the aetiology and treatment of depression is discussed.

Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)1A receptors: WAY 100,635-reversible actions of the highly selective ligands, flesinoxan and S 15535

This study examined the influence of the highly selective 5-HT1A receptor ligands, flesinoxan, S 15535, and WAY 100,635, upon the electrical activity of dopaminergic neurons in the ventral tegmental area (VTA), as compared to serotonergic neurons in the dorsal raphe nucleus (DRN) of anesthetized rats. Flesinoxan, a high-efficacy agonist at both pre- and postsynaptic 5-HT1A receptors, dose-dependently (inhibitory dose (ID)50 = 19.5 microg/kg, i.v.) inhibited the firing of DRN serotonergic neurons. This action was abolished by WAY 100,635 (31 microg/kg i.v.) which is an antagonist at pre- and postsynaptic 5-HT1A receptors. S 15535, which behaves as an agonist and partial agonist at pre- and postsynaptic 5-HT1A receptors, respectively, similarly abolished DRN firing in a WAY 100,635-reversible fashion with an ID50 of 6.1 microg/kg, i.v. In contrast to these actions, both flesinoxan (> or = 500 microg/kg, i.v.) and S 15535 (> or = 125 microg/kg, i.v.) dose-dependently and monophasically increased the firing rate of dopaminergic neurons in the VTA, with maximal effects of 70.1 +/- 17.2% and 33.7 +/- 5.3%, respectively. Further, VTA dopaminergic neurons displaying a regular firing pattern were transformed into a bursting mode. This influence of flesinoxan and S 15535 on VTA cells was abolished by WAY 100,635. Administered alone, WAY 100,635 did not significantly modify the activity of either serotonergic or dopaminergic neurons. In conclusion, the present findings show that selective activation of 5-HT1A receptors not only inhibits serotonergic neurones but also elicits a (possibly related) increase in VTA dopaminergic output. A facilitatory influence of flesinoxan, S 15535, and other selective 5-HT1A receptor ligands upon mesocortical dopaminergic pathways may contribute to their putative antidepressant properties.

Synergistic neurochemical and behavioral effects of fluoxetine and 5-HT1A receptor antagonists

We studied the ability of WAY 100635 [N-[4-(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide], 0.5 mg/kg, i.v. and (-)-5-Me-8-OH-DPAT [(-)-5-methyl-8-hydroxy-2-(di-n-propylamino)tetralin], 3 mg/kg, i.v. two selective 5-HT1A receptor antagonists, to potentiate: (1) the enhancement of extracellular 5-HT levels ([5-HT(ext)]) induced by a single administration of 5 mg/kg i.p. fluoxetine using in vivo microdialysis in the ventral hippocampus of conscious rats, (2) the decrease in food intake induced by this antidepressant drug in food-deprived rats. The effects of fluoxetine were significantly potentiated, by 30-40%, by WAY 100635 as well as by (-)-5-Me-8-OH-DPAT in the two sets of experiments. Thus, fluoxetine increased [5-HT(ext)] in serotonergic nerve terminal areas and consequently, induced hypophagia, both effects being limited by indirect activation of somatodendritic 5-HT1A autoreceptors.

Effects of 5-HT1A receptor antagonists on fluoxetine-induced changes in extracellular serotonin concentrations in rat frontal cortex

Clinical studies in which serotonin specific reuptake inhibitors have been co-administered with pindolol have demonstrated a shortened time to onset of antidepressant activity. This effect has been attributed to the antagonist effects of pindolol at the presynaptic 5-HT1A receptor which augments the action of the serotonin specific reuptake inhibitors. In the present study, we demonstrate that acute fluoxetine-induced increases in extracellular serotonin concentrations, as measured by microdialysis in the frontal cortex, can be potentiated by 5-HT1A receptor blockade using N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyridinyl)cyclohexa necarboxamide (WAY100635), the silent and selective 5-HT1A receptor antagonist. WAY100635 at doses as low as 0.03 mg/kg s.c. maintained this potentiation effect across a range of fluoxetine doses. In addition, using antagonists with different intrinsic agonist activities for the 5-HT1A receptor, we have determined that only compounds with very low intrinsic agonist activity can produce a potentiation of the acute fluoxetine-induced increases in extracellular serotonin.