Effects of sustained administration of the serotonin and norepinephrine reuptake inhibitor venlafaxine: I. in vivo electrophysiological studies in the rat

Enriched environmental exposure reduces the onset of action of the serotonin norepinephrin reuptake inhibitor venlafaxine through its effect on parvalbumin interneurons plasticity in mice

Mood disorders are associated with hypothalamic-pituitary-adrenal axis overactivity resulting from a decreased inhibitory feedback exerted by the hippocampus on this brain structure. Growing evidence suggests that antidepressants would regulate hippocampal excitatory/inhibitory balance to restore an effective inhibition on this stress axis. While these pharmacological compounds produce beneficial clinical effects, they also have limitations including their long delay of action. Interestingly, non-pharmacological strategies such as environmental enrichment improve therapeutic outcome in depressed patients as in animal models of depression. However, whether exposure to enriched environment also reduces the delay of action of antidepressants remains unknown. We investigated this issue using the corticosterone-induced mouse model of depression, submitted to antidepressant treatment by venlafaxine, alone or in combination with enriched housing. We found that the anxio-depressive phenotype of male mice was improved after only two weeks of venlafaxine treatment when combined with enriched housing, which is six weeks earlier than mice treated with venlafaxine but housed in standard conditions. Furthermore, venlafaxine combined with exposure to enriched environment is associated with a reduction in the number of parvalbumin-positive neurons surrounded by perineuronal nets (PNN) in the mouse hippocampus. We then showed that the presence of PNN in depressed mice prevented their behavioral recovery, while pharmacological degradation of hippocampal PNN accelerated the antidepressant action of venlafaxine. Altogether, our data support the idea that non-pharmacological strategies can shorten the onset of action of antidepressants and further identifies PV interneurons as relevant actors of this effect.

Effects of acute and chronic administration of trace amine-associated receptor 1 (TAAR1) ligands on in vivo excitability of central monoamine-secreting neurons in rats

Trace amine-associated receptor 1 (TAAR1) has been recently identified as a target for the future antidepressant, antipsychotic, and anti-addiction drugs. Full (e.g. RO5256390) and partial (e.g. RO5263397) TAAR1 agonists showed antidepressant-, antipsychotic- and anti-addiction-like behavioral effects in rodents and primates. Acute RO5256390 suppressed, and RO5263397 stimulated serotonin (5-HT) neurons of the dorsal raphe nucleus (DRN) and dopamine neurons of the ventral tegmental area (VTA) in brain slices, suggesting that the behavioral effects of TAAR1 ligands involve 5-HT and dopamine. For more comprehensive testing of this hypothesis, we examined acute and chronic effects of RO5256390 and RO5263397 on monoamine neurons in in vivo conditions. Excitability of 5-HT neurons of the DRN, noradrenaline neurons of the locus coeruleus (LC), and dopamine neurons of the VTA was assessed using single-unit electrophysiology in anesthetized rats. For acute experiments, RO5256390 and RO5263397 were administered intravenously; neuronal excitability after RO5256390 and RO5263397 administration was compared to the basal activity of the same neuron. For chronic experiments, RO5256390 was administered orally for fourteen days prior to electrophysiological assessments. The neuronal excitability in RO5256390-treated rats was compared to vehicle-treated controls. We found that acute RO5256390 inhibited 5-HT and dopamine neurons. This effect of RO5256390 was reversed by the subsequent and prevented by the earlier administration of RO5263397. Acute RO5256390 and RO5263397 did not alter the excitability of LC noradrenaline neurons in a statistically significant way. Chronic RO5256390 increased excitability of 5-HT neurons of the DRN and dopamine neurons of the VTA. In conclusion, the putative antidepressant and antipsychotic effects of TAAR1 ligands might be mediated, at least in part, via the modulation of excitability of central 5-HT and dopamine neurons.

Psychopharmacological properties and therapeutic profile of the antidepressant venlafaxine

Major depression (MD) is one of the most common psychiatric disorders worldwide. Currently, the first-line treatment for MD targets the serotonin system but these drugs, notably the selective serotonin reuptake inhibitors, usually need 4 to 6 weeks before the benefit is felt and a significant proportion of patients shows an unsatisfactory response. Numerous treatments have been developed to circumvent these issues as venlafaxine, a mixed serotonin-norepinephrine reuptake inhibitor that binds and blocks both the SERT and NET transporters. Despite this pharmacological profile, it is difficult to have a valuable insight into its ability to produce more robust efficacy than single-acting agents. In this review, we provide an in-depth characterization of the pharmacological properties of venlafaxine from in vitro data to preclinical and clinical efficacy in depressed patients and animal models of depression to propose an indirect comparison with the most common antidepressants. Preclinical studies show that the antidepressant effect of venlafaxine is often associated with an enhancement of serotonergic neurotransmission at low doses. High doses of venlafaxine, which elicit a concomitant increase in 5-HT and NE tone, is associated with changes in different forms of plasticity in discrete brain areas. In particular, the hippocampus appears to play a crucial role in venlafaxine-mediated antidepressant effects notably by regulating processes such as adult hippocampal neurogenesis or the excitatory/inhibitory balance. Overall, depending on the dose used, venlafaxine shows a high efficacy on depressive-like symptoms in relevant animal models but to the same extent as common antidepressants. However, these data are counterbalanced by a lower tolerance. In conclusion, venlafaxine appears to be one of the most effective treatments for treatment of major depression. Still, direct comparative studies are warranted to provide definitive conclusions about its superiority.

Triple reuptake inhibition of serotonin, norepinephrine, and dopamine increases the tonic activation of α2-adrenoceptors in the rat hippocampus and dopamine levels in the nucleus accumbens

Clinical studies have shown the therapeutic efficacy of an increase in dopamine (DA) transmission in treatment of major depressive disorder (MDD). In the present study, we investigated whether blockade of DA transporters in addition to serotonin (5-HT) and norepinephrine (NE) produced additional adaptations of monoaminergic systems. In vivo electrophysiological recordings were carried out in male anesthetized rats. Vehicle, the 5-HT reuptake inhibitor escitalopram, the NE/DA reuptake blocker nomifensine and their combination (triple reuptake inhibition; TRI) were delivered for 2 or 14 days. Firing activity of NE, 5-HT and DA neurons was assessed. Tonic activation of 5-HT_1A receptors and α₁- and α₂-adrenoceptors was determined in the hippocampus and extracellular DA levels in the nucleus accumbens (NAc). Unlike escitalopram, nomifensine and TRI administration increased the tonic activation of α₂-adrenoceptors in the hippocampus despite decreasing NE neuronal firing activity after 2 and 14 days of administration. The firing activity of 5-HT neurons was increased after prolonged nomifensine and TRI regimens, while addition of nomifensine to escitalopram prevented the early 2-day suppression of firing by 5-HT reuptake inhibition. The tonic activation of 5-HT_1A receptors was enhanced only with escitalopram. Whereas escitalopram and nomifensine decreased firing activity of DA neurons after a 2-day administration, their combination normalized it to baseline level after 14 days; this was accompanied by a robust increase in extracellular DA levels in the NAc. In summary, these results indicate that TRI increases NE and DA but not 5-HT transmission, suggesting a differential efficacy profile in MDD patients.

In vivo electrophysiological recordings of the effects of antidepressant drugs

Antidepressant drugs are a standard biological treatment for various neuropsychiatric disorders, yet relatively little is known about their electrophysiologic and synaptic effects on mood systems that set moment-to-moment emotional tone. In vivo electrical recording of local field potentials (LFPs) and single neuron spiking has been crucial for elucidating important details of neural processing and control in many other systems, and yet electrical approaches have not been broadly applied to the actions of antidepressants on mood-related circuits. Here we review the literature encompassing electrophysiologic effects of antidepressants in animals, including studies that examine older drugs, and extending to more recently synthesized novel compounds, as well as rapidly acting antidepressants. The existing studies on neuromodulator-based drugs have focused on recording in the brainstem nuclei, with much less known about their effects on prefrontal or sensory cortex. Studies on neuromodulatory drugs have moreover focused on single unit firing patterns with less emphasis on LFPs, whereas the rapidly acting antidepressant literature shows the opposite trend. In a synthesis of this information, we hypothesize that all classes of antidepressants could have common final effects on limbic circuitry. Whereas NMDA receptor blockade may induce a high powered gamma oscillatory state via direct and fast alteration of glutamatergic systems in mood-related circuits, neuromodulatory antidepressants may induce similar effects over slower timescales, corresponding with the timecourse of response in patients, while resetting synaptic excitatory versus inhibitory signaling to a normal level. Thus, gamma signaling may provide a biomarker (or “neural readout”) of the therapeutic effects of all classes of antidepressants.

Transcriptional evidence for the role of chronic venlafaxine treatment in neurotrophic signaling and neuroplasticity including also Glutamatergic [corrected] - and insulin-mediated neuronal processes

OBJECTIVES

Venlafaxine (VLX), a serotonine-noradrenaline reuptake inhibitor, is one of the most commonly used antidepressant drugs in clinical practice for the treatment of major depressive disorder (MDD). Despite being more potent than its predecessors, similarly to them, the therapeutical effect of VLX is visible only 3-4 weeks after the beginning of treatment. Furthermore, recent papers show that antidepressants, including also VLX, enhance the motor recovery after stroke even in non depressed persons. In the present, transcriptomic-based study we looked for changes in gene expressions after a long-term VLX administration.

METHODS

Osmotic minipumps were implanted subcutaneously into Dark Agouti rats providing a continuous (40 mg/kg/day) VLX delivery for three weeks. Frontal regions of the cerebral cortex were isolated and analyzed using Illumina bead arrays to detect genes showing significant chances in expression. Gene set enrichment analysis was performed to identify specific regulatory networks significantly affected by long term VLX treatment.

RESULTS

Chronic VLX administration may have an effect on neurotransmitter release via the regulation of genes involved in vesicular exocytosis and receptor endocytosis (such as Kif proteins, Myo5a, Sv2b, Syn2 or Synj2). Simultaneously, VLX activated the expression of genes involved in neurotrophic signaling (Ntrk2, Ntrk3), glutamatergic transmission (Gria3, Grin2b and Grin2a), neuroplasticity (Camk2g/b, Cd47), synaptogenesis (Epha5a, Gad2) and cognitive processes (Clstn2). Interestingly, VLX increased the expression of genes involved in mitochondrial antioxidant activity (Bcl2 and Prdx1). Additionally, VLX administration also modulated genes related to insulin signaling pathway (Negr1, Ppp3r1, Slc2a4 and Enpp1), a mechanism that has recently been linked to neuroprotection, learning and memory.

CONCLUSIONS

Our results strongly suggest that chronic VLX treatment improves functional reorganization and brain plasticity by influencing gene expression in regulatory networks of motor cortical areas. These results are consonant with the synaptic (network) hypothesis of depression and antidepressant-induced motor recovery after stroke.

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.

Region-specific regulation of 5-HT1B receptors in the rat brain by chronic venlafaxine treatment

RATIONALE

Venlafaxine is a non-selective serotonin and noradrenaline reuptake inhibitor antidepressant drug for which clinical studies have suggested a high level efficacy and a possible early action onset compared to the classical antidepressants. Its therapeutic effects might be due, at least in part, to adaptive changes in serotonergic neurotransmission, through the activation of the different 5-HT receptor subtypes. 5-HT(1B) receptors are located in the axon terminals of both serotonergic and non-serotonergic neurons, where they act as inhibitory autoreceptors or heteroreceptors, respectively. However, the information about the involvement of this subtype in the mechanism of action of antidepressants is limited and quite controversial.

OBJECTIVES

The aim of this study was to evaluate the effect of venlafaxine (10 mg kg⁻¹ day⁻¹, p.o.) after 21 days of treatment on the density of 5-HT(1B) receptors and their functionality in rat brain.

METHODS

Effects of chronic venlafaxine were evaluated at different levels of 5-HT(1B) receptor by using receptor autoradiography, [³⁵S]GTPγS binding, and the regulation of body temperature induced by selective 5-HT(1B) agonist.

RESULTS

Our results show that venlafaxine induced an increase in sensitivity of 5-HT(1B) receptors in hypothalamus both at G-protein level and the control of core temperature without affecting the receptor density.

CONCLUSIONS

These results demonstrate that adaptive changes on 5-HT(1B) receptors induced by chronic administration of venlafaxine exhibit regional differences suggesting that the hypothalamus might be an important site of drug action.

Organic cation transporter 2 controls brain norepinephrine and serotonin clearance and antidepressant response

High-affinity transporters for norepinephrine (NE) and serotonin (5-HT), which ensure neurotransmitter clearance at the synapse, are the principal targets of widely used antidepressant drugs. Antidepressants targeting these high-affinity transporters, however, do not provide positive treatment outcomes for all patients. Other monoamine transport systems, with lower affinity, have been detected in the brain, but their role is largely unknown. Here we report that OCT2, a member of the polyspecific organic cation transporter (OCT) family, is expressed notably in the limbic system and implicated in anxiety and depression-related behaviors in the mouse. Genetic deletion of OCT2 in mice produced a significant reduction in brain tissue concentrations of NE and 5-HT and in ex vivo uptake of both these neurotransmitters in the presence of the dual 5-HT-NE transport blocker, venlafaxine. In vivo clearance of NE and 5-HT evaluated using microiontophoretic electrophysiology was diminished in the hippocampus of OCT2(-/-) mice in the presence of venlafaxine, thereby affecting postsynaptic neuronal activity. OCT2(-/-) mice displayed an altered sensitivity to acute treatments with NE- and/or 5-HT-selective transport blockers in the forced-swim test. Moreover, the mutant mice were insensitive to long-term venlafaxine treatment in a more realistic, corticosterone-induced, chronic depression model. Our findings identify OCT2 as an important postsynaptic determinant of aminergic tonus and mood-related behaviors and a potential pharmacological target for mood disorders therapy.

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

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

The effects of serotonin/norepinephrine reuptake inhibitors and serotonin receptor agonist on morphine analgesia and tolerance in rats

Several studies have demonstrated that serotonergic and noradrenergic systems have important roles in morphine analgesia and tolerance. However, the exact mechanism underlying the development of morphine tolerance is not fully understood. The aim of this study was to investigate the possible role of serotonin/norepinephrine reuptake inhibitors (amitriptyline, venlafaxine) and serotonin receptor (5-HT(1A) and 5-HT(1B/1D)) agonist (dihydroergotamine) in morphine analgesia and tolerance in rats. To constitute morphine tolerance, animals received morphine (50 mg/kg; s.c.) once daily for 3 days. After the last dose of morphine was injected on day 4, morphine tolerance was evaluated. The analgesic effects of amitriptyline (20 mg/kg; i.p.), venlafaxine (20 mg/kg; s.c.), dihydroergotamine (100 μg/kg; i.v.) and morphine (5 mg/kg) were considered at 15- to 30-min intervals (0, 15, 30, 60, 90, and 120 min) by tail-flick and hot-plate analgesia tests. In this study, the data obtained suggested that amitriptyline and venlafaxine significantly increased the analgesic effect of morphine and attenuated the expression of morphine tolerance. However, dihydroergotamine significantly increased the analgesic effect of morphine but did not reduce the expression of morphine tolerance. In conclusion, we determined that co-administration of morphine with amitriptyline and venlafaxine increased the analgesic effects of morphine and attenuated the morphine analgesic tolerance.

5-HT(1A) receptor sensitivity in 5-HT(1B) receptor KO mice is unaffected by chronic fluvoxamine treatment

The 5-HT(1B) receptor has been implicated in disorders such as depression, anxiety and obsessive-compulsive disorder. In mice lacking the 5-HT(1B) receptor (5-HT(1B) knockout mice), important changes in physiology and behavior exist. In the absence of presynaptic 5-HT(1B) receptor inhibition, chronic SSRI treatment may differentially affect 5-HT(1A) receptor functionality. The present studies tested the hypothesis that chronically reducing 5-HT transporter (5-HTT) function with selective serotonin reuptake inhibitor (SSRI) treatment would accelerate 5-HT(1A) receptor desensitization in 5-HT(1B) knockout mice. Moreover, as 5-HT(1B) knockout mice have been found to display exaggerated autonomic and locomotor responses to environmental stressors, the effects of chronic SSRI treatment on the hyperreactive phenotype of 5-HT(1B) knockout mice were investigated. The stress-reducing effect of the 5-HT(1A) receptor agonist flesinoxan on increases in body temperature, heart rate and locomotor activity was similar in wild type and 5-HT(1B) knockout mice before and after chronic 21-day treatment with the SSRI fluvoxamine, indicating no apparent alteration of 5-HT(1A) receptor sensitivity in 5-HT(1B) knockout mice. Also, chronic SSRI treatment did not alter the increased stress reactivity to mild environmental stressors in 5-HT(1B) knockout mice. We demonstrate that no apparent differences in 5-HT(1A) receptor sensitivity occur between 5-HT(1B) knockout and wild type mice after chronic fluvoxamine treatment. Also, the hyperreactive phenotype of 5-HT(1B) knockout mice is unresponsive to chronic SSRI treatment. Taken together, these results indicate that constitutive absence of 5-HT(1B) receptors does not result in adaptive changes in 5-HT(1A) receptor functionality and that chronic SSRI treatment does not modify stress reactivity in 5-HT(1B) knockout mice.

Modulation of the serotonin system by endocannabinoid signaling

The cannabinoid CB(1) receptors and their endogenous agonists, endocannabinoids (eCBs), are ubiquitously distributed throughout the central nervous system (CNS), where they play a key role in the regulation of neuronal excitability. As such, CB signaling has been implicated in the regulation of a myriad of physiological functions ranging from feeding homoeostasis to emotional and motivational processes. Ample evidence from behavioral studies also suggests that eCBs are important regulators of stress responses and a deficit in eCB signaling contributes to stress-related disorders such as anxiety and depression. The eCB-induced modulation of stress-related behaviors appears to be mediated, at least in part, through the regulation of the serotoninergic system. In this article, we review the role of eCB signaling in the regulation of the serotoninergic system with special emphasis on the cellular mechanisms by which cannabinoid CB(1) receptors modulate the excitability of dorsal raphe serotonin neurons.

Reduced signal transduction by 5-HT4 receptors after long-term venlafaxine treatment in rats

BACKGROUND AND PURPOSE

The 5-HT(4) receptor may be a target for antidepressant drugs. Here we have examined the effects of the dual antidepressant, venlafaxine, on 5-HT(4) receptor-mediated signalling events.

EXPERIMENTAL APPROACH

The effects of 21 days treatment (p.o.) with high (40 mg·kg(-1)) and low (10 mg·kg(-1)) doses of venlafaxine, were evaluated at different levels of 5-HT(4) receptor-mediated neurotransmission by using in situ hybridization, receptor autoradiography, adenylate cyclase assays and electrophysiological recordings in rat brain. The selective noradrenaline reuptake inhibitor, reboxetine (10 mg·kg(-1), 21 days) was also evaluated on 5-HT(4) receptor density.

KEY RESULTS

Treatment with a high dose (40 mg·kg(-1)) of venlafaxine did not alter 5-HT(4) mRNA expression, but decreased the density of 5-HT(4) receptors in caudate-putamen (% reduction = 26 ± 6), hippocampus (% reduction = 39 ± 7 and 39 ± 8 for CA1 and CA3 respectively) and substantia nigra (% reduction = 49 ± 5). Zacopride-stimulated adenylate cyclase activation was unaltered following low-dose treatment (10 mg·kg(-1)) while it was attenuated in rats treated with 40 mg·kg(-1) of venlafaxine (% reduction = 51 ± 2). Furthermore, the amplitude of population spike in pyramidal cells of CA1 of hippocampus induced by zacopride was significantly attenuated in rats receiving either dose of venlafaxine. Chronic reboxetine did not modify 5-HT(4) receptor density.

CONCLUSIONS AND IMPLICATIONS

Our data indicate a functional desensitization of 5-HT(4) receptors after chronic venlafaxine, similar to that observed after treatment with the classical selective inhibitors of 5-HT reuptake.

Antidepressant effects of estrogens: a basic approximation

The use of estrogenic compounds as antidepressants or as coadjuvants to facilitate the effect of antidepressants has reported controversial results, suggesting that many factors could influence their actions. This review analyzes, from a basic research perspective, the possible factors that may underlie the antidepressant action of estrogens alone or in combination. The possible mechanisms of action of estrogens alone and in combination with the selective serotonin reuptake inhibitor, fluoxetine, the selective noradrenaline reuptake inhibitor, desipramine, and the mixed serotonin/noradrenaline reuptake inhibitor, venlafaxine are reviewed, focusing on monoaminergic systems and estrogen receptors as main targets. The antidepressant effect of estrogens depends on the type of estrogen, treatment duration, doses, sex, time after ovariectomy, and age. Estrogens potentiate the antidepressant-like action of fluoxetine, venlafaxine, and desipramine and drastically shorten their latency of action. The antidepressant-like effect of estrogens alone or in combination with antidepressants seems to be mediated by monoaminergic and classic estrogen receptors, as WAY100635, an antagonist to the serotonin 1A receptor, idaxozan, an antagonist to alpha2 adrenergic receptors, and RU 58668, an estrogen receptor antagonist, blocked their antidepressant-like effect. In conclusion, estrogens produce antidepressant-like actions by themselves and importantly facilitate the action of clinically used antidepressants.

Maternal use of venlafaxine near term: correlation between neonatal effects and plasma concentrations

The nature of neonatal adverse effects in the offspring after maternal use of venlafaxine at term is not fully understood. We correlated neonatal clinical signs over time with serum concentrations of venlafaxine and its active metabolite in the neonatal period. Women exposed to venlafaxine near term, and their neonates, were studied. Adverse neonatal signs and serum concentrations of venlafaxine and its active metabolite were assessed. Seven mother-child pairs were studied. Median maternal venlafaxine dose was 75 mg/d (37.5-300 mg/d). Five neonates presented with multiple clinical signs during their hospital stay, all including tachypnea and respiratory distress. Respiratory distress was present within the first hours after birth, with other symptoms appearing subsequently when the drug concentration declined. The elimination half-life, calculated for 3 neonates, ranged between 12 and 15 hours for venlafaxine and between 10 and 37 hours for O-desmethylvenlafaxine. Neonatal clinical signs emerged as drug concentrations declined, corroborating discontinuation as an etiology. Respiratory symptoms tended to occur earlier and, in one case, independently from the typical signs of abrupt cessation of venlafaxine. This suggests that it may be part of the wide range of respiratory problems reported in antidepressant-exposed neonates, including persistent pulmonary hypertension of the newborn reported with selective serotonin reuptake inhibitor. Neonatal clinical signs emerged with decreasing concentrations of venlafaxine, supporting that abrupt cessation of venlafaxine leads to discontinuation syndrome. Respiratory problems occurred earlier than typical discontinuation clinical signs. Larger studies are needed to confirm these findings.

Antidepressant drugs with differing pharmacological actions decrease activity of locus coeruleus neurons

Previous studies suggest that all effective antidepressant (AD) drugs decrease activity of locus coeruleus (LC) neurons. However, little data exist regarding blood levels of drug in these studies, and what data do exist suggest blood levels might have been very high. To assess whether decreased LC activity is produced by drugs that selectively block reuptake for either norepinephrine or serotonin at therapeutically relevant blood levels, effects of chronic administration of desipramine, paroxetine, and escitalopram on LC activity were measured across a range of doses and blood levels of drug. Further, effects of a range of doses of mirtazapine were examined; in that mirtazapine blocks alpha2 adrenergic receptors, it might be anticipated to increase rather than decrease LC activity. Finally, to begin to assess whether the response of LC to ADs was specific to these drugs, effects of four non-AD drugs (single dose) were measured. Drugs were administered via osmotic minipump for 14 d. Electrophysiological recording of LC activity (assessment of both spontaneous firing rate and sensory-evoked 'burst' firing) then took place under isoflurane anaesthesia on the last day of drug treatment. The blood level of drugs present at the end of the recording session was also measured. All AD drugs tested decreased LC spontaneous and sensory-evoked 'burst' firing, and this was observed across a wide range of blood levels for the drugs. Non-AD drugs did not decrease LC activity. The findings of this investigation continue to support the possibility that all effective AD drugs decrease LC activity.

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

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

Reduction in the latency of action of antidepressants by 17 beta-estradiol in the forced swimming test

RATIONALE

Antidepressants (ADs) are slow to produce their therapeutic effect. This long latency promotes the development of new strategies to short their onset of action. Previous reports indicated that 17beta-estradiol (E(2)) promotes the antidepressant-like activity of fluoxetine (FLX) and desipramine (DMI) in the forced swimming test (FST).

OBJECTIVE

The aim of the present work was to analyze if E(2) reduces the antidepressant-like onset of action of venlafaxine (VLX), FLX, and DMI.

MATERIALS AND METHODS

Independent groups of ovariectomized female Wistar rats were tested in the FST and in the open field after chronic (1 to 14 days) treatment with VLX (20 mg/kg/day), FLX (1.25 mg/kg/day), or DMI (1.25 mg/kg/day) alone or in combination with a single injection of E(2) (2.5 microg/rat sc, 8 h before FST).

RESULTS

VLX, FLX, or DMI by themselves at these doses did not induce changes in the FST at short intervals after their injection (from 1 to 7 days). The addition of E(2) promoted the antidepressant-like effect of VLX and DMI as early as day 1. Such action was also evident after 3, for FLX, and 14 days for both FLX and DMI, but not for VLX. The behavioral actions of these ADs combined with E(2) were not accompanied by increases in general activity in the open-field test.

CONCLUSION

E(2) clearly reduced the latency to the onset of action for these ADs in the FST. These results represent an interesting therapeutic strategy for the treatment of depression in perimenopausal women.

WAY100635 prevents the changes induced by fluoxetine upon the 5-HT1A receptor functionality

5-HT1A receptor-mediated signalling in rat brain was evaluated after chronic administration (14 days; s.c.) of the selective serotonin reuptake inhibitor (SRRI) fluoxetine (10 mg/kg/day) alone, or in combination with the 5-HT1A receptor antagonist WAY100635 (0.1 mg/kg/day). The density of 5-HT1A binding sites was unchanged following fluoxetine, WAY100635, or the combination of fluoxetine and WAY100635. However, the net stimulation of [35S]GTPgammaS binding induced by the 5-HT1A agonist 8-OH-DPAT was significantly attenuated in dorsal raphe nucleus (DRN), but not in hippocampus, after chronic fluoxetine. Moreover, depending of the area analysed, the basal binding of [35S]GTPgammaS was differentially affected by this treatment: increased in DRN and decreased in hippocampal dentate gyrus. Interestingly, the changes in [35S]GTPgammaS basal binding and on 5-HT1A receptors functionality were prevented by the concomitant administration of WAY100635. The inhibition of dorsal raphe firing by 8-OH-DPAT was also attenuated in fluoxetine-treated rats (ED50 = 2.12 +/- 0.32 microg/kg and 4.34 +/- 0.09 microg/kg, for vehicle and fluoxetine respectively), an effect which was also prevented by the concomitant administration of WAY100635 (ED50 = 2.10 +/- 0.58 microg/kg). Chronic administration of WAY100635 alone did not affect the 5-HT1A receptor-induced stimulation of [35S]GTPgammaS binding, nor the 8-OH-DPAT-induced inhibition of 5-HT neuron firing. These results demonstrate that the concomitant blockade of 5-HT1A receptors when administering fluoxetine prevents those adaptive changes of 5-HT1A receptor function associated with the chronic administration of this antidepressant. These findings could be relevant from the therapeutic point of view, and further support the potential benefit of treatments with a SSRI/5-HT1A receptor antagonist combination.

Blockade of autoreceptor-mediated inhibition of norepinephrine release by atipamezole is maintained after chronic reuptake inhibition

The role of alpha(2)-adrenergic autoreceptor desensitization in the delayed onset of antidepressant efficacy of selective norepinephrine (NE) reuptake inhibitors is unclear. Using the alpha(2)-antagonist yohimbine, we showed previously that chronic treatment with desipramine (DMI) did not alter autoreceptor-mediated inhibition of NE release in the cortex. However, yohimbine may have non-selective effects that could confound this interpretation. Thus, using microdialysis, we measured acute effects of the highly selective alpha(2)-antagonist atipamezole on NE release in the prefrontal cortex following chronic DMI treatment, after 0-8 d washout. Atipamezole induced a similar elevation of extracellular NE in all treatment groups, indicating no change in autoreceptor function. Further, the effect was most rapid in DMI-treated rats with 0- and 2-d washout, suggesting that autoreceptor-mediated inhibition was most prominent when NE levels were highest. This provides further evidence that autoreceptor-mediated inhibition of NE neurotransmission remains functional after chronic DMI treatment, arguing against the hypothesis that desensitization of alpha(2)-autoreceptors accounts for the delayed onset of action of selective NE reuptake inhibitors.

Measurement of 5-HT1A receptor binding in depressed adults before and after antidepressant drug treatment using positron emission tomography and [11C]WAY-100635

OBJECTIVE

To assess effects of chronic antidepressant drug treatment on serotonin type-1A receptor (5-HT(1A)R) binding potential (BP) in major depressive disorder.

METHODS

Depressed subjects (n = 27) were imaged using PET and [(11)C]WAY-100635 at baseline and following a median of 9.4 weeks of treatment with selective serotonin reuptake inhibitor or dual reuptake inhibitor antidepressant agents. Fifteen subjects had complete pre- and post-treatment scan data. The 5-HT(1A)R BP was derived from the tissue time-radioactivity concentrations from regions-of-interest defined a priori, using a simplified reference tissue model (SRTM), and in a subset of subjects, compartmental modeling (CMOD).

RESULTS

Chronic treatment had no effect on pre- or post-synaptic 5-HT(1A)R BP, as confirmed by both the SRTM and CMOD analyses. These results were unaffected by treatment response status and were consistent across brain regions. Among the 22 subjects for whom the clinical response-to-treatment was established, the treatment nonresponders (n = 7) had higher baseline BP values in the left (P = 0.01) and right orbital cortex (P = 0.02) than the responders (n = 15).

CONCLUSIONS

Chronic antidepressant drug treatment did not significantly change cerebral 5-HT(1A)R binding, consistent with preclinical evidence that the alterations in serotonergic function associated with antidepressant drug administration are not accompanied by changes in 5-HT(1A)R density. Higher baseline 5-HT(1A)R binding was associated with poorer response to treatment.

Chronic treatment with desipramine improves cognitive performance of rats in an attentional set-shifting test

Alterations in central monoaminergic neurotransmission are important in the actions of many antidepressants. This study tested the hypothesis that tonic elevation of noradrenergic (NA) neurotransmission in medial prefrontal cortex (mPFC) by chronic treatment with the selective norepinephrine (NE) reuptake blocker desipramine (DMI) may contribute to the beneficial cognitive effects of this antidepressant drug (AD). Male Sprague-Dawley rats were treated with DMI acutely (15 mg/kg, i.p.) or chronically for 21 days (7.5 mg/kg/day via osmotic minipump) before assessing performance on an attentional set-shifting test. The extradimensional set-shifting component of this test reflects a process of cognitive flexibility that is dependent upon mPFC, and that we have shown previously to be facilitated by NA activity in mPFC. Microdialysis was performed to measure NE release in mPFC concurrently with behavioral testing. Acute DMI treatment produced an increase in extracellular NE levels in mPFC, and a modest improvement in overall performance across all task stages of the attentional set-shifting test, but failed to produce a significant improvement in any of the individual specific tasks comprising the test sequence. Chronic DMI treatment tonically elevated basal extracellular NE levels in mPFC, associated with a significant improvement in performance specifically on the extradimensional set-shifting component of the test. There was also a significant reduction in set loss errors in rats treated chronically with DMI. Hence, tonic elevation of NA transmission in mPFC by chronic DMI treatment was associated with a time-dependent facilitation of cognitive flexibility that may contribute to the mechanism whereby chronic treatment with ADs, specifically NE reuptake blockers, may exert a beneficial therapeutic effect on cognition in depressed patients.

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

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

Treatment of anxiety disorders with venlafaxine XR

When venlafaxine was introduced in 1994, it was the first of the newer generation antidepressants to be classified as a serotonin norepinephrine reuptake inhibitor (SNRI). An extended release (XR) formulation of venlafaxine, introduced in 1997, subsequently received regulatory approval for treatment of three anxiety disorders: generalized anxiety disorder, social anxiety disorder and panic disorder. Although less extensively studied, venlafaxine XR also appears to have efficacy for two other anxiety disorders, post-traumatic stress disorder and obsessive-compulsive disorder. In contrast to the treatment of depression, for which meta-analyses suggest an efficacy advantage relative to selective serotonin reuptake inhibitors (SSRIs), evidence of differential efficacy has not yet been established for any of the anxiety disorders. The overall tolerability profile of venlafaxine XR is generally comparable to that of the SSRIs, although there is greater incidence of noradrenergically mediated side effects (i.e., dry mouth and constipation), as well as a dose-dependent risk of treatment-emergent high blood pressure. Concerns about safety in overdose have also recently emerged. Despite these caveats, venlafaxine XR is an effective and generally well-tolerated option for treatment of anxiety disorders.

A comparison of the chronic treatment effects of venlafaxine and other antidepressants on serotonin and norepinephrine transporters

BACKGROUND

Venlafaxine blocks both serotonin and norepinephrine transporters (SERT and NET), with higher affinity for SERT. Serotonergic effects occur with lower doses, whereas both serotonergic and noradrenergic effects occur with higher doses of venlafaxine. Chronic treatment of rats with selective serotonin reuptake inhibitors decreases SERT binding sites, whereas similar treatment with selective norepinephrine reuptake inhibitors decreases NET binding sites. We hypothesized that venlafaxine would affect monoamine transporters dose-dependently, with low doses causing selective reduction of SERT binding sites and higher doses reducing both SERT and NET binding sites.

METHODS

Rats were treated for 21 days with a low (15 mg/kg/day) or high (70 mg/kg/day) dose of venlafaxine, vehicle, or other antidepressants. The SERT and NET density was determined by quantitative autoradiography.

RESULTS

Neither dose of venlafaxine nor amitriptyline reduced binding to either the SERT or NET. In rats with noradrenergic terminals destroyed by 6-hydroxydopamine, venlafaxine still failed to reduce SERT binding. Also, rats treated simultaneously with sertraline plus desipramine exhibited reductions in both SERT and NET binding.

CONCLUSIONS

Chronic venlafaxine treatment affected SERT and NET binding differently from paroxetine or desipramine. The inability of venlafaxine to reduce SERT or NET binding sites is not due to its dual uptake inhibiting properties.

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.

Antidepressant-like effect of nicotinamide adenine dinucleotide in the forced swim test in rats

Nicotinamide adenine dinucleotide (NADH), a cosubstrate for energy transfer in the oxidative phosphorylation, has supposedly beneficial effects on central nervous system (CNS)-related diseases, e.g., shown in an open study with depressive patients. To our knowledge there are no data concerning the efficacy of NADH in animal tests. Acute effects of NADH and the precursor nicotinamide, compared to controls and the antidepressants desipramine and fluoxetine, were examined in the forced swim test (FST) in Wistar rats. NADH, but not nicotinamide, reduced immobility and increased swimming behaviour in the FST, with a minimum effective dose of 5 mg/kg. NADH-induced behavioural profile was similar to fluoxetine, but different from desipramine. Since NADH did not induce hyperlocomotion but even decreased motor activity in the open field test, the antidepressant-like effect cannot be attributed to an increase in motor activity. These data support an antidepressant potential of NADH.

Effects of chronic antidepressants and electroconvulsive shock on serotonergic neurotransmission in the rat hippocampus

The hippocampus may play a critical role in the pathophysiology and treatment of depression. There are two main lines of evidence for this: firstly, many of its functions correspond to those altered in depression, and secondly, many hippocampal functions are regulated by the serotonergic (5-HT) system, which is a common target of antidepressant treatments. Chronic effects of antidepressants and electroconvulsive shock (ECS) have been studied by various methods using electrophysiology, in vivo microdialysis or ex vivo neurochemical measurements. The aim of the current review is to point out possible correlations between these studies based on different methods and to suggest neurochemical mechanisms that result in the observed changes in hippocampal physiology and neurogenesis. These changes in hippocampal neurochemistry are reviewed and compared with the abnormalities associated with stress, corticosterone or depression.

Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration

Adaptive changes in the serotonergic system are generally believed to underlie the therapeutic effectiveness of the azapirone anxiolytics and a variety of antidepressant drugs. The serotonin-1A (5-HT(1A)) receptor has been implicated in affective disorders. Thus, studies of the regulation of 5-HT(1A) receptor function may have important implications for our understanding the role of this receptor in the mechanism of action of these therapeutic agents. This review focuses on the regulation of central 5-HT(1A) receptor function following administration of 5-HT(1A) receptor agonists or antidepressant drugs expected to increase the synaptic concentration of the neurotransmitter 5-HT. The majority of evidence supports regional differences in the regulation of central 5-HT(1A) receptor function following repeated agonist or antidepressant administration, which may be due to differences in processes involved in desensitization of the receptor at the cellular level. Region-specific differences in the regulation of 5-HT(1A) receptor function may be based on compensatory changes distal to the receptor, such as regulatory changes at the level of effector (e.g. adenylyl cyclase or ion channel), or at the level of the G protein such as changes in G protein expression, or phosphorylation of the G protein. It may be that the increase in serotonin neurotransmission, due to somatodendritic autoreceptor desensitization following agonist or antidepressant treatment, to normo-sensitive 5-HT(1A) receptors in certain brain regions (e.g. hippocampus or cortex) and to sub-sensitive 5-HT(1A) receptors in other brain regions (e.g. amygdala or hypothalamus) underlies the therapeutic efficacy of these drugs.

Inhibition by venlafaxine of the increase in norepinephrine output in rat prefrontal cortex elicited by acute stress or by the anxiogenic drug FG 7142

Venlafaxine is an antidepressant drug that inhibits the reuptake of serotonin and norepinephrine with different efficacies. The effects of repeated administration of this drug on the increase in the extracellular concentration of norepinephrine in the prefrontal cortex, induced by stress or by the anxiogenic drug FG 7142, were studied in freely moving rats. Exposure to foot-shock stress induced a marked increase (+120%) in the extracellular norepinephrine concentration in the prefrontal cortex of control rats. Long-term administration of venlafaxine (10 mg/kg i.p., once a day for 21 days) reduced the effect of stress on norepinephrine output by 75%. This effect of venlafaxine persisted for at least 5 days after discontinuation of drug treatment. Acute administration of FG 7142 (20 mg/kg i.p.), a benzodiazepine receptor inverse agonist, increased norepinephrine output (+90%) in control rats. Chronic treatment with venlafaxine prevented the effect of FG 7142. In contrast, the acute administration of this antidepressant had no effect on the stress- or FG 7142-induced increase in norepinephrine output. These plastic changes in the sensitivity of norepinephrine neurones to foot-shock stress and to an anxiogenic drug may reveal an important neuronal mechanism for the physiological regulation of emotional state. Furthermore, this mechanism might be relevant to the anxiolytic and antidepressant effects of venlafaxine.

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

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

Effect of the selective noradrenergic reuptake inhibitor reboxetine on the firing activity of noradrenaline and serotonin neurons

Reboxetine is a non-tricyclic antidepressant with selective noradrenergic (NA) reuptake-blocking effects. The effects of acute and sustained administration of reboxetine, on the firing activity of locus coeruleus NA neurons and dorsal raphe 5-HT neurons, were assessed using in vivo extracellular unitary recording in rats anaesthetized with chloral hydrate. Reboxetine (0.1-1.25 mg/kg, i.v.) dose-dependently decreased the firing activity of NA neurons (ED50 = 480 +/- 14 microg/kg). A 2-day treatment with reboxetine at 1.25, 2.5, 5, or 10 mg/kg per day (using osmotic minipumps implanted subcutaneously) produced significant decreases of 52%, 68%, 81%, and 83%, respectively, of NA firing activity. When the reboxetine treatment (2.5 mg/kg per day) duration was prolonged to 7 days, a 66% decrease in NA firing activity was observed which further decreased to 80% after 21 days of treatment. In contrast, 5-HT neuron firing rate remained unaltered following short- and long-term reboxetine treatments. The suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of NA neurons was unchanged in long-term reboxetine-treated rats, but its effect on the firing activity of 5-HT neurons was blunted. The enhancement of NA firing activity by the 5-HT1A agonist 8-OH-DPAT was abolished in long-term reboxetine-treated rats, whereas, the inhibitory effect of the 5-HT2 agonist DOI was attenuated by about three-fold. In conclusion, sustained NA reuptake blockade by reboxetine lead to profound alterations in the function of NA neurons and of 5-HT receptors modulating their firing activity.