Antidepressant drug treatment induces Arc gene expression in the rat brain

Involvement of sigma-1 receptor modulation in the antidepressant action of venlafaxine

Multiple lines of investigation have explored the role of sigma receptors in mental depression. Sigma receptors particularly, sigma-1 subtype is known to modulate the release of various catecholamines in the brain and may play, in some way, a role in the mechanism of action of various antidepressants. The present study investigated the possible involvement of sigma receptors in modulating the antidepressant-like effect of venlafaxine (dual serotonin and norepinephrine reuptake inhibitor) in the mouse forced swim test (FST). Immobility period in the forced swim test was registered for a total period of 6 min. Venlafaxine produced dose-dependent (4-16 mg/kg, i.p.) reduction in immobility period. Pretreatment of mice with (+)-pentazocine (2.5 mg/kg, i.p.), a high-affinity sigma-1 receptor agonist, produced synergism with subeffective dose of venlafaxine (2 mg/kg, i.p.). On the contrary, pretreatment with progesterone (10 mg/kg, s.c.), a sigma-1 receptor antagonist neurosteroid, rimcazole (5 mg/kg, i.p.), another sigma-1 receptor antagonist, or BD 1047 (1 mg/kg, i.p.), a novel sigma-1 receptor antagonist, reversed the anti-immobility effects of venlafaxine (8 mg/kg i.p.). The various modulators used in the study did not produce any changes in locomotor activity per se except venlafaxine which at higher dose (16 mg/kg, i.p.) significantly increased the locomotor activity in mice. The results for the first time demonstrated that the anti-immobility effects of venlafaxine in the FST possibly involve an interaction with sigma-1 receptors.

The α7 nicotinic receptor agonist SSR180711 increases activity regulated cytoskeleton protein (Arc) gene expression in the prefrontal cortex of the rat

Nicotinic alpha7 acetylcholine receptors (alpha7 nAChR) have been shown to enhance attentional function and aspects of memory function in experimental models and in man. The protein Arc encoded by the effector immediate early gene arc or arg3.1 has been shown to be strongly implicated in long-term memory function. We have sought to determine if alpha7 nAChR mediate the stimulation of arc gene expression, and if so, where in the brain such activation may occur using semi-quantitative in situ hybridisation. Administration of the novel and selective alpha7 nAChR agonist, SSR180711 (1, 3 and 10 mg/kg) to adolescent rats, produced a dose- and time-dependent increase in the expression of Arc mRNA in the prefrontal cortex and the ventral orbital cortex. By contrast, no change in mRNA levels was detected in the parietal cortex and the CA1 of the hippocampus. These data show that alpha7 nAChR activates a subset of neurons in the rat prefrontal cortex and this activation likely is important for the attentional effects of this new class of drugs.

Expression of brain derived neurotrophic factor, activity-regulated cytoskeleton protein mRNA, and enhancement of adult hippocampal neurogenesis in rats after sub-chronic and chronic treatment with the triple monoamine re-uptake inhibitor tesofensine

The changes of gene expression resulting from long-term exposure to monoamine antidepressant drugs in experimental animals are key to understanding the mechanisms of action of this class of drugs in man. Many of these genes and their products are either relevant biomarkers or directly involved in structural changes that are perhaps necessary for the antidepressant effect. Tesofensine is a novel triple monoamine reuptake inhibitor that acts to increase noradrenaline, serotonin, and dopamine neurotransmission. This study was undertaken to examine the effect of sub-chronic (5 days) and chronic (14 days) administration of Tesofensine on the expression of brain derived neurotrophic factor (BDNF) and activity-regulated cytoskeleton protein (Arc) in the rat hippocampus. Furthermore, hippocampi from the same animals were used to investigate the effect on cell proliferation by means of Ki-67- and NeuroD-immunoreactivity. We find that chronic, but not sub-chronic treatment with Tesofensine increases BDNF mRNA in the CA3 region of the hippocampus (35%), and Arc mRNA in the CA1 of the hippocampus (65%). Furthermore, the number of Ki-67- and neuroD-positive cells increased after chronic, but not sub-chronic treatment. This study shows that Tesofensine enhances hippocampal gene expression and new cell formation indicative for an antidepressant potential of this novel drug substance.

Methylphenidate regulates activity regulated cytoskeletal associated but not brain-derived neurotrophic factor gene expression in the developing rat striatum

Methylphenidate (MPH) is a psychostimulant drug used to treat attention deficit hyperactivity disorder in children. To explore the central effects of chronic MPH, we investigated the expression of an effector immediate early gene, activity regulated cytoskeletal associated (arc), and the neurotrophin, brain-derived neurotrophic factor (bdnf) in the brain of immature and adult rats following repeated MPH. Prepubertal (postnatal day (PD) 25-38) and adult (PD 53-66) male rats were injected once daily for: a) 14 days with saline or MPH (2 or 10 mg/kg; s.c.) or b) 13 days with saline followed by a single dose of MPH (2 or 10 mg/kg; s.c.). To determine possible long-term effects of MPH, prepubertal rats were allowed a drug-free period of 4 weeks following the 14 days of treatment, and then were given a challenge dose of MPH. We demonstrated, for the first time, that an acute injection of MPH increased levels of activity-regulated cytoskeletal protein (ARC) and arc mRNA in the prepubertal rat striatum and cingulate/frontal cortex. This response was significantly attenuated by chronic MPH. The desensitization in arc expression observed in prepubertal rats persisted in the adult striatum following a later MPH challenge. In contrast to these data we observed little effect of MPH on bdnf expression. We also developed an effective, non-stressful technique to treat freely moving immature rats with oral MPH. Consistent with the results described above, we observed that oral MPH (7.5 and 10 mg/kg) also increased arc expression in the prepubertal rat striatum. However, unlike the effects of injected MPH, repeated oral MPH (7.5 mg/kg) did not alter the normal arc response. This result raises the important possibility that oral doses of MPH that reproduce clinically relevant blood levels of MPH may not down-regulate gene expression, at least in the short term (14 days). We confirmed, using mass spectrometry, that the oral doses of MPH used in our experiments yielded blood levels within the clinical range observed in children. The novel oral administration paradigm that we describe thus provides a clinically relevant animal model to further explore the effects of chronic drug exposure on central gene expression in the developing rat brain.

Increased expression of the 5-HT transporter confers a low-anxiety phenotype linked to decreased 5-HT transmission

A commonly occurring polymorphic variant of the human 5-hydroxytryptamine (5-HT) transporter (5-HTT) gene that increases 5-HTT expression has been associated with reduced anxiety levels in human volunteer and patient populations. However, it is not known whether this linkage between genotype and anxiety relates to variation in 5-HTT expression and consequent changes in 5-HT transmission. Here we test this hypothesis by measuring the neurochemical and behavioral characteristics of a mouse genetically engineered to overexpress the 5-HTT. Transgenic mice overexpressing the human 5-HTT (h5-HTT) were produced from a 500 kb yeast artificial chromosome construct. These transgenic mice showed the presence of h5-HTT mRNA in the midbrain raphe nuclei, as well as a twofold to threefold increase in 5-HTT binding sites in the raphe nuclei and a range of forebrain regions. The transgenic mice had reduced regional brain whole-tissue levels of 5-HT and, in microdialysis experiments, decreased brain extracellular 5-HT, which reversed on administration of the 5-HTT inhibitor paroxetine. Compared with wild-type mice, the transgenic mice exhibited a low-anxiety phenotype in plus maze and hyponeophagia tests. Furthermore, in the plus maze test, the low-anxiety phenotype of the transgenic mice was reversed by acute administration of paroxetine, suggesting a direct link between the behavior, 5-HTT overexpression, and low extracellular 5-HT. In toto, these findings demonstrate that associations between increased 5-HTT expression and anxiety can be modeled in mice and may be specifically mediated by decreases in 5-HT transmission.

Chronic fluoxetine induces region-specific changes in translation factor eIF4E and eEF2 activity in the rat brain

The delayed therapeutic onset observed in response to chronic antidepressant drug treatment is little understood. While current theories emphasize effects on gene transcription, possible effects of antidepressant drugs on translation control pathways have not been explored. We examined the effect of the selective serotonergic reuptake inhibitor fluoxetine on regulation of two major determinants of mRNA translation, eukaryotic initiation factor 4E (eIF4E) and eukaryotic elongation factor 2 (eEF2). Chronic fluoxetine treatment induced hyperphosphorylation of eEF2 (Thr56) in prefrontal cortex, hippocampus and dentate gyrus of rats. By contrast, phosphorylation of eIF4E (Ser209) was observed specifically in the dentate gyrus. Acute fluoxetine treatment had no effect on translational factor activity. These findings suggest that region-specific regulation of translation contributes to the delayed action of antidepressant drugs such as fluoxetine.

Stereoselective and region-specific induction of immediate early gene expression in rat parietal cortex by blockade of neurokinin 1 receptors

Antagonists at neurokinin 1 (NK1) receptors are attracting attention as potential treatments for depressive states in light of their actions in behavioural models predictive of antidepressant properties, their modulation of corticolimbic monoaminergic transmission, and their influence upon neural plasticity. Here, we evaluated the influence of NK1 receptor blockade upon two immediate early genes, Arc and c-fos, implicated in mechanisms of synaptic plasticity. Administration of the selective NK1 receptor antagonist, GR 205,171 (40, but not 1, 5 or 10 mg/kg i.p.), elicited a pronounced elevation in mRNA encoding Arc in both outer and inner layers of the parietal cortex of rat brain. This action was region-specific inasmuch as Arc expression did not change in other cortical territories examined including frontal cortex, nor in CA1, CA3 and the dentate gyrus of the hippocampus. In comparison to GR 205,171, its less active isomer GR 226,206 (1-40 mg/kg) did not significantly modify Arc gene expression in parietal cortex or other cortical areas. GR 205,171 (40 mg/kg) also increased the abundance of c-fos mRNA in outer and inner parietal cortex and caused a corresponding increase in c-fos immunoreactivity in this region. GR 226,206 (40 mg/kg i.p.) had no effect on either c-fos mRNA or protein in parietal cortex. In conclusion, administration of GR 205,171 elicits a stereospecific increase in Arc and c-fos expression in rat parietal cortex but not in other cortical regions. These data suggest that the parietal cortex plays a role in the central actions of NK1 receptor antagonists.

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.

Regulation of activity-regulated cytoskeleton protein (Arc) mRNA after acute and chronic electroconvulsive stimulation in the rat

The temporal profile of Arc gene expression after acute and chronic electroconvulsive stimulations (ECS) was studied using semi-quantitative in situ hybridisation in the rat cortex. A single ECS strongly and temporarily increased Arc mRNA levels in dentate granular cells with maximal induction seen up to 4 h after the stimulus, but returned to baseline at 24 h. A single ECS also increased expression of Arc mRNA in the CA1 and the parietal cortex, but the expression peaked within 1 h and returned to baseline levels within 2 h. Repeated or chronic ECS is a model of electroconvulsive therapy and it would be predicted that gene products involved in antidepressant effects accumulate after repeated ECS. However, repeated ECS reduced Arc gene expression in the CA1 24 h after the last stimulus. These results indicate that Arc is an immediate early gene product regulated by an acute excitatory stimulus, but not accumulated by long term repetitive ECS and therefore not a molecular biomarker for antidepressant properties. More likely, Arc is likely a molecular link to the decline in memory consolidation seen in depressive patients subjected to electroconvulsive therapy.

Influence of inhalation anesthesia assessed by comprehensive gene expression profiling

Although general anesthesia is routinely used as an essential surgical procedure and its harmlessness has been evaluated and endorsed by clinical outcomes, little is known about its comprehensive influence that is not reflected in mortality and morbidity. In this paper, we have shown that inhalation anesthesia affected the expression of <1.5% of >10,000 genes, by analyzing the expression profiles for multiple organs of rats anesthetized with sevoflurane. The small number of transcripts affected by the inhalation anesthesia comprised those specific to single and common in multiple organs. The former included genes mainly associated with drug metabolism in the liver and influenced by agents such as amphetamine in the brain. The latter contained multiple circadian genes. In the brain, we failed to detect the alteration of the clock gene expression with the exception of Per2, assuming that anesthesia perturbs circadian rhythms. Our findings provide the first assessment for the influence of inhalation anesthesia by approaches of experimental biology and genome science.

Evidence for increased expression of the vesicular glutamate transporter, VGLUT1, by a course of antidepressant treatment

The therapeutic effect of a course of antidepressant treatment is believed to involve a cascade of neuroadaptive changes in gene expression leading to increased neural plasticity. Because glutamate is linked to mechanisms of neural plasticity, this transmitter may play a role in these changes. This study investigated the effect of antidepressant treatment on expression of the vesicular glutamate transporters, VGLUT1-3 in brain regions of the rat. Repeated treatment with fluoxetine, paroxetine or desipramine increased VGLUT1 mRNA abundance in frontal, orbital, cingulate and parietal cortices, and regions of the hippocampus. Immunoautoradiography analysis showed that repeated antidepressant drug treatment increased VGLUT1 protein expression. Repeated electroconvulsive shock (ECS) also increased VGLUT1 mRNA abundance in regions of the cortex and hippocampus compared to sham controls. The antidepressant drugs and ECS did not alter VGLUT1 mRNA abundance after acute administration, and no change was detected after repeated treatment with the antipsychotic agents, haloperidol and chlorpromazine. In contrast to VGLUT1, the different antidepressant treatments did not commonly increase the expression of VGLUT2 or VGLUT3 mRNA. These data suggest that a course of antidepressant drug or ECS treatment increases expression of VGLUT1, a key gene involved in the regulation of glutamate secretion.

Chronic stress, depression and antidepressants: effects on gene transcription in the hippocampus

Depressive disorders are among the most frequent forms of mental illness. Both genetic and environmental factors, such as stress, are involved in the etiology of depression. Therefore, chronic stress paradigms in laboratory animals constitute an important tool for research in this field. The molecular bases of chronic stress/depression are largely unknown, although a large amount of information has been accumulated during recent years. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis as well as structural and physiological alterations in the hippocampus and neocortex are known to occur. Modifications in the expression level of some genes, such as brain-derived neurotrophic factor, cAMP-response-element binding protein, serotonin receptors and HPA axis components were consistently associated in a number of experimental models. However, recent results suggest that several synaptic proteins, transcription factors and proteins involved in neuronal growth/differentiation, are also modified in their expression in experimental models of chronic stress. In general, these alterations can be reversed by treatment with antidepressants. Thus, a complex pattern of gene expression leading to stress/depression is starting to emerge. We summarize here recent findings on the alterations of gene expression in the hippocampus of chronically stressed and antidepressant treated animals.

The role of monoamines in the actions of established and "novel" antidepressant agents: a critical review

Monoaminergic pathways are highly responsive to aversive stimuli and play a crucial role in the control of affect, cognition, endocrine secretion, chronobiotic rhythms, appetite, and motor function, all of which are profoundly disrupted in depressive states. Accordingly, a perturbation of monoaminergic transmission is implicated in the aetiology of depressive disorders, and all clinically available antidepressants increase corticolimbic availability of monoamines. However, their limited efficacy, delayed onset of action, and undesirable side effects underlie ongoing efforts to identify improved therapeutic agents. Sequencing the human genome has raised the hope not only of better symptomatic control of depression, but even of the prevention or cure of depressive states. In the pursuit of these goals, there is currently a tendency to focus on selective ligands of "novel" nonmonoaminergic targets. However, certain classes of novel agent (such as neurokinin(1) receptor antagonists) indirectly modulate the activity of monoaminergic networks. Others may act "downstream" of them, converging onto common cellular substrates controlling gene expression, synaptic plasticity, and neurogenesis. Further, by analogy to the broad-based actions of currently employed drugs, multitarget agents may be better adapted than selective agents to the management of depression-a complex disorder with hereditary, developmental, and environmental origins. It is, thus, important to continue the creative exploration of clinically validated and innovative monoaminergic strategies within a multitarget framework. In this light, drugs combining monoaminergic and nonmonoaminergic mechanisms of action may be of particular interest. The present article provides a critical overview of monoaminergic strategies for the treatment of depressive states, both established and under development, and discusses interactions of novel "nonmonoaminergic" antidepressants with monoaminergic mechanisms.

Fluoxetine-induced change in rat brain expression of brain-derived neurotrophic factor varies depending on length of treatment

Recent studies indicate that brain-derived neurotrophic factor (BDNF) may be implicated in the clinical action of antidepressant drugs. Repeated (2-3 weeks) administration of antidepressant drugs increases BDNF gene expression. The onset of this response as well as concomitant effects on the corresponding BDNF protein is however, unclear. The present study investigated the effects of acute and chronic administration of the selective serotonin reuptake inhibitor, fluoxetine (10mg/kg p.o.), upon regional rat brain levels of BDNF mRNA and protein expression. To improve the clinical significance of the study, fluoxetine was administered orally and mRNA and protein levels were determined ex vivo using the techniques of in situ hybridisation histochemistry and immunocytochemistry respectively. Direct measurement of BDNF protein was also carried out using enzyme-linked immunosorbent assay (ELISA). Four days of once daily oral administration of fluoxetine induced decreases in BDNF mRNA (hippocampus, medial habenular and paraventricular thalamic nuclei). Whilst 7 days of treatment showed a non-significant increase in BDNF mRNA, there were marked and region-specific increases following 14 days of treatment. BDNF protein levels remained unaltered until 21 days of fluoxetine treatment, when the numbers of BDNF immunoreactive cells were increased, reaching significance in the pyramidal cell layer of CA1 and CA3 regions of Ammon's horn (CA1 and CA3) but not in the other sub-regions of the hippocampus. Indicative of the highly regional change within the hippocampus, the ELISA method failed to demonstrate significant up-regulation at 21 days, measuring levels of BDNF protein in the whole hippocampus. In contrast to the detected time dependent and biphasic response of the BDNF gene, activity-regulated, cytoskeletal-associated protein (Arc) mRNA showed a gradual increase during the 14-day course of treatment. The results presented here show that BDNF is expressed differentially depending on length of fluoxetine administration, which could contribute in explaining the slow onset of antidepressant activity observed with selective serotonin reuptake inhibitors.

Microarray analysis of differentially expressed genes in rat frontal cortex under chronic risperidone treatment

Long-term administration of antipsychotic drugs can induce differential expression of a variety of genes in the brain, which may underscore the molecular mechanism of the clinical efficacy and/or side effects of antipsychotic drugs. We used cDNA microarray analysis to screen differentially expressed genes in rat frontal cortex under 4 weeks' treatment of risperidone (1 mg/kg). Using real-time quantitative PCR, we were able to verify eight genes, whose expression were significantly upregulated in rat frontal cortex under chronic risperidone treatment when compared with control animals. These genes include receptor for activated protein kinase C, amida, cathepsin D, calpain 2, calcium-independent receptor for alpha-latrotoxin, monoamine oxidase B, polyubiquitin, and kinesin light chain. In view of the physiological function of these genes, the results of our study suggest that chronic risperidone treatment may affect the neurotransmission, synaptic plasticity, and proteolysis of brain cells. This study also demonstrates that cDNA microarray analysis is useful for uncovering genes that are regulated by chronic antipsychotic drugs treatment, which may help bring new insight into the molecular mechanism of antipsychotic drugs.

The circadian rhythm of 5-HT biosynthetic and degradative enzymes in immortalized mouse neuroendocrine pineal cell line--a model for studying circadian rhythm

Serotonin (5-HT) plays an important role in circadian rhythm and its level decreases at night and increases during day time in pineal gland. This study investigates the role of 5-HT biosynthetic and metabolic genes in the circadian rhythm using the mouse neuroendocrine pineal cell line (PT811). We first determined which type of monoamine oxdiase (MAO) was present in PT811 cells. Our results showed that only MAO A, a key which enzyme degrades 5-HT, was present in these cells because both 5-HT (the MAO A preferred substrate) and PEA (the MAO B preferred substrate) oxidation were sensitive to clorgyline but insensitive to deprenyl inhibition. This finding was supported by Northern blot analysis in which MAO A, but not MAO B, transcript was detectable. More sensitive quantitative RT-PCR showed the presence of both MAO A and MAO B, however there was no difference between day and night time. In contrast, the expression of tryptophan hydroxylase (TPH, the 5-HT synthetic enzyme), arylalklamine N-acetyltransferase (AANAT) and Arc (activity regulated, cytoskeleton associated protein) genes showed diurnal cycle. AANAT converts 5-HT to N-acetylserotonin, leading to the synthesis of melatonin. The Arc gene is an effector early immediate gene, regulated by 5-HT. This study suggested that the 5-HT circadian rhythm is regulated by TPH and AANAT but not the MAO A gene in this cell line. This cell line will be a valuable model for studying the molecular mechanisms of circadian rhythm.

Erratum to “The novel monoamine reuptake inhibitor and potential antidepressant, S33005, induces Arc gene expression in cerebral cortex” [Eur. J. Pharmacol. 489 (2004) 179–185]

Recent data show that corticolimbic expression of the effector immediate early gene Arc is up-regulated by standard antidepressant drugs. Here, we tested the effect upon Arc expression of a novel antidepressant and selective 5-hydroxytryptamine/noradrenaline reuptake inhibitor (SNRI), (-)1-(1-dimethylaminomethyl) 5-methoxybenzocyclobutan-1-yl) cyclohexanol (S33005). Arc mRNA abundance in frontal, cingulate, orbital and parietal cortices, hippocampus (CA1 pyramidal layer) and striatum was elevated in rats treated daily for 14 but not 7 days with 10 mg/kg i.p. S33005 compared to saline. Fourteen but not 7 days treatment with 10 mg/kg i.p. venlafaxine, the prototypical SNRI, also elevated Arc mRNA, but its effects were not as pronounced and detected in fewer regions, compared to S33005. Neither S33005 nor venlafaxine altered Arc mRNA after acute injection nor altered brain derived neurotrophic factor mRNA after repeated administration. These data demonstrate that sustained treatment with SNRIs increases Arc expression in corticolimbic regions, and underpin previous neurochemical and behavioural evidence that S33005 is efficacious in models predictive of antidepressant action.

Evaluation of the effect of chronic antidepressant treatment on neurokinin-1 receptor expression in the rat brain

Clinically effective antidepressants are thought to exert their therapeutic effects by facilitating central monoamine neurotransmission. However, recent data showing that neurokinin-1 receptor (NK1R) antagonists have antidepressant properties in both animal and clinical studies raise the possibility that classical antidepressants may also influence NK1R expression in the brain. To test this hypothesis, rats were treated with desipramine, paroxetine, venlafaxine, tranylcypromine or vehicle for 14-42 days. NK1R binding sites and mRNA were determined in a wide variety of brain areas using in situ hybridization and quantitative receptor autoradiography. In all areas examined, the abundance of NK1R binding sites was unchanged after 14 days of treatment. None of the treatments altered the number of NK1R binding sites following 42 days treatment with the exception that an increase was found in the locus coeruleus with tranylcypromine. Taken together, we report that repeated treatment with antidepressants of different classes does not cause significant changes in NK1R expression.

The novel monoamine reuptake inhibitor and potential antidepressant, S33005, induces Arc gene expression in cerebral cortex

Recent data show that corticolimbic expression of the effector immediate early gene Arc is up-regulated by standard antidepressant drugs. Here, we tested the effect upon Arc expression of a novel antidepressant and selective 5-hydroxytryptamine/noradrenaline reuptake inhibitor (SNRI), (-)1-(1-dimethylaminomethyl) 5-methoxybenzocyclobutan-1-yl) cyclohexanol (S33005). Arc mRNA abundance in frontal, cingulate, orbital and parietal cortices, hippocampus (CA1 pyramidal layer) and striatum was elevated in rats treated daily for 14 but not 7 days with 10 mg/kg i.p. S33005 compared to saline. Fourteen but not 7 days treatment with 10 mg/kg i.p. venlafaxine, the prototypical SNRI, also elevated Arc mRNA, but its effects were not as pronounced and detected in fewer regions, compared to S33005. Neither S33005 nor venlafaxine altered Arc mRNA after acute injection nor altered brain derived neurotrophic factor mRNA after repeated administration. These data demonstrate that sustained treatment with SNRIs increases Arc expression in corticolimbic regions, and underpin previous neurochemical and behavioural evidence that S33005 is efficacious in models predictive of antidepressant action.