Altered glucocorticoid rhythm attenuates the ability of a chronic SSRI to elevate forebrain 5-HT: implications for the treatment of depression

Early life adversity shapes neural circuit function during sensitive postnatal developmental periods

Early life adversity (ELA) is a major risk factor for mental illness, but the neurobiological mechanisms by which ELA increases the risk for future psychopathology are still poorly understood. Brain development is particularly malleable during prenatal and early postnatal life, when complex neural circuits are being formed and refined through an interplay of excitatory and inhibitory neural input, synaptogenesis, synaptic pruning, myelination, and neurogenesis. Adversity that influences these processes during sensitive periods of development can thus have long-lasting and pervasive effects on neural circuit maturation. In this review, we will discuss clinical and preclinical evidence for the impact of ELA on neural circuit formation with a focus on the early postnatal period, and how long-lasting impairments in these circuits can affect future behavior. We provide converging evidence from human and animal studies on how ELA alters the functional development of brain regions, neural circuits, and neurotransmitter systems that are crucial for cognition and affective behavior, including the hippocampus, the hypothalamus-pituitary-adrenal (HPA) axis, neural networks of fear responses and cognition, and the serotonin (5-HT) system. We also discuss how gene-by-environment (GxE) interactions can determine individual differences in susceptibility and resilience to ELA, as well as molecular pathways by which ELA regulates neural circuit development, for which we emphasize epigenetic mechanisms. Understanding the molecular and neurobiological mechanisms underlying ELA effects on brain function and psychopathology during early postnatal sensitive periods may have great potential to advance strategies to better treat or prevent psychiatric disorders that have their origin early in life.

Male long-Evans rats: An outbred model of marked hypothalamic-pituitary-adrenal hyperactivity

Rat and mouse strains differ in behavioral and physiological characteristics, and such differences can contribute to explain discrepant results between laboratories and better select the most appropriate strain for a particular purpose. Differences in the activity of the hypothalamic-pituitary-adrenal (HPA) axis are particularly important given the pivotal role of this system in determining consequences of exposure to stressors. In this regard, Long-Evans (LE) rats are widely used in stress research, but there is no specific study aiming at thoroughly characterizing HPA activity in LE versus other extensively used strains. In a first experiment, LE showed higher resting ACTH and corticosterone levels only at certain points of the circadian rhythm, but much greater ACTH responsiveness to stressors (novel environment and forced swim) than Sprague-Dawley (SD) rats. Accordingly, enhanced corticotropin-releasing hormone (CRH) expression in the paraventricular nucleus of the hypothalamus and reduced expression of glucocorticoid receptors were observed in the hippocampal formation. Additionally, they are hyperactive in novel environments, and prone to adopt passive-like behavior when compared to SD rats. Supporting that altered HPA function has a marked physiological impact, we observed in another set of animals much lower thymus weight in LE than SD rats. Finally, to demonstrate that LE rats are likely to have higher HPA responsiveness to stressors than most strains, we studied resting and stress levels of HPA hormones in LE versus Wistar and Fischer rats, the latter considered an example of high HPA responsiveness. Again, LE showed higher resting and stress levels of ACTH than both Wistar and Fischer rats. As ACTH responsiveness to stressors in LE rats is stronger than that previously reported when comparing other rat strains and they are commercially available, they could be an appropriate model for studying the behavioral and physiological implications of a hyper-active HPA axis under normal and pathological conditions.

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Strain differences in hypothalamic-pituitary-adrenal (HPA) function were studied.

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Long-Evans (LE) rats show greater HPA response to stressors than other strains.

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CRH expression in critical brain areas is greater in LE than Sprague-Dawley (SD) rats.

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Glucocorticoid receptor expression was lower in the hippocampal formation of LE rats.

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LE rats are more active in novel environments but showed more passive coping.

New Pharmacological Interventions in Bipolar Disorder

The biological bases of bipolar disorder include aspects related, among others, to neurohormonal pathways, neurotransmission, signal transduction, regulation of gene expression, oxidative stress, neuroplasticity, and changes in the immune system. There is still a gap in understanding its complex neurobiology and, consequently, developing new treatments. Multiple factors probably interact in this complex equation of pathophysiology of bipolar disorder, such as genetic, biochemical, psychosocial, and environmental stress events, correlating with the development and severity of the bipolar disorder. These mechanisms can interact to exacerbate inflammation, impair neurogenesis, and increase oxidative stress damage, cellular mitochondrial dysfunction, changes in neurotrophins and in epigenetic mechanisms, neuroendocrine dysfunction, activation of neuronal death pathways, and dysfunction in neurotransmission systems. In this review, we explore the up-to-date knowledge of the neurobiological underpinnings of bipolar disorders. The difficulty in developing new drugs for bipolar disorder is very much associated with the lack of knowledge about the precise pathophysiology of this disorder. Pharmacological treatment for bipolar patients is vital; to progress to effective medications, it is essential to understand the neurobiology in bipolar patients better and identify novel therapeutic targets.

Chronic fluoxetine reverses the effects of chronic corticosterone treatment on α2-adrenoceptors in the rat frontal cortex but not locus coeruleus

Disruption of the hypothalamic-pituitary-adrenal axis is an established finding in patients with anxiety and/or depression. Chronic corticosterone administration in animals has been proposed as a model for the study of these stress-related disorders and the antidepressant action. Alterations of the central noradrenergic system and specifically of inhibitory α₂-adrenoceptors seem to be part of the pathophysiology of depression and contribute to the antidepressant activity. The present study evaluates in male rats the effect of chronic corticosterone treatment during 35 days (16-20 mg kg^-1 day^-1) on the sensitivity of α₂-adrenoceptors expressed in the somatodendritic and terminal noradrenergic areas locus coeruleus (LC) and prefrontal cortex (PFC), respectively. Further, the effect of chronic fluoxetine treatment (5 mg kg^-1, i.p., since the 15th day) on the sensitivity of α₂-adrenoceptors was examined under control conditions and in corticosterone-treated rats. The α₂-adrenoceptor functionality was analysed in vitro by agonist-mediated [³⁵S]GTPγS binding stimulation and in vivo through the modulation of noradrenaline (NA) release evaluated by dual-probe microdialysis. The concentration-effect curves of the [³⁵S]GTPγS binding stimulation by the agonist UK14304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) demonstrated a desensitization of cortical α₂-adrenoceptors induced by corticosterone (-logEC₅₀ = 6.7 ± 0.2 vs 8.2 ± 0.3 in controls) that was reverted by fluoxetine treatment (-logEC₅₀ = 7.5 ± 0.3). Local administration of the α₂-adrenoceptor antagonist RS79948 ((8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulfonyl)-6H-isoquino[2,1-g][1,6]naphthyridine) (0.1-100 μmol L^-1) into the LC induced a concentration-dependent NA increase in the PFC of the control group (E_max = 191 ± 30%) but non-significant effect was observed in corticosterone-treated rats (E_max = 133 ± 46%), reflecting a desensitization of α₂-adrenoceptors that control the firing of noradrenergic neurons. Fluoxetine treatment did not alter the corticosterone-induced desensitization in this area (E_max = 136 ± 19%). No effect of fluoxetine on α₂-adrenoceptor functionality was observed in control animals (E_max = 223 ± 30%). In PFC, the local administration of RS79948 increased NA in controls (E_max = 226 ± 27%) without effect in the corticosterone group (E_max = 115 ± 26%), suggesting a corticosterone-induced desensitization of terminal α₂-adrenoceptors. Fluoxetine administration prevented the desensitization induced by corticosterone in the PFC (E_max = 233 ± 33%) whereas desensitized α₂-adrenoceptors in control animals (E_max = -24 ± 10%). These data indicate that chronic corticosterone increases noradrenergic activity by acting at different α₂-adrenoceptor subpopulations. Treatment with the antidepressant fluoxetine seems to counteract these changes by acting mainly on presynaptic α₂-adrenoceptors expressed in terminal areas.

Adaptive dynamics of the 5-HT systems following chronic administration of selective serotonin reuptake inhibitors: a meta-analysis

Selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed antidepressants. However, a major concern is their delayed onset of action, which is hypothesized to be associated with the time required for serotonin (5-HT) autoreceptors to desensitize, which should be reflected by actual neurochemical changes. Numerous in vivo microdialysis studies have been published that report on 5-HT levels in different brain sites following SSRI administration. Here, we performed a meta-analysis on dynamic changes of 5-HT neurotransmission during the course of chronic SSRI treatment. We conducted a meta-analysis on research articles of 5-HT neurotransmission measured by in vivo microdialysis in rat brain after subchronic and chronic SSRI administrations. In total, data from 42 microdialysis studies (798 rats) were analyzed. Within the first week of SSRI treatment, extracellular 5-HT concentrations drop in frontal cortex. Over the next 2 weeks of treatment, a linear increase in extracellular 5-HT levels up to 350% of prior treatment baseline is evident (n = 269). However, in hippocampus, prefrontal cortex, nucleus accumbens, and ventral tegmental area we found increased 5-HT levels within the first 3 days of SSRI administration. The time course of 5-HT dynamics in frontal cortex is in line with the hypothesis that 5-HT autoreceptors desensitize over 2-3 weeks of SSRI treatment and thereby enhanced extracellular 5-HT levels ensue. Yet, in other regions we did not find evidence supporting the traditional autoreceptor-mediated feedback loops hypothesis and thus other neurobiological adaptation mechanisms may also play a role in the delayed onset of SSRI action.

Effect of subchronic corticosterone administration on α2-adrenoceptor functionality in rat brain: an in vivo and in vitro study

RATIONALE

Noradrenergic system plays a critical role in the hypothalamic-pituitary-adrenal (HPA) axis regulation and the stress response. A dysregulated HPA axis may be indicative of an increased biological vulnerability for depression. In addition, a variety of studies have focused on specific alterations of α₂-adrenoceptors as a mechanism involved in the pathogenesis of mood disorders and antidepressant response.

OBJECTIVES

This study aimed to evaluate the effect of subchronic corticosterone administration on rat brain α₂-adrenoceptor functionality by in vitro [³⁵S]GTPγS binding stimulation assays and in vivo dual-probe microdialysis determination of extracellular noradrenaline concentrations.

RESULTS

Implantation of a time release corticosterone pellet during 14 days induced sustained changes in endocrine function. However, there were no differences in α₂-adrenoceptor agonist UK14304-induced stimulation of [³⁵S]GTPγS binding in prefrontal cortex (PFC) between corticosterone-treated and control rats. In the same way, the in vivo evaluation of α₂-adrenoceptor-mediated noradrenaline release responses to the α₂-adrenoceptor agonist clonidine local administration into the locus coeruleus (LC), and the PFC did not show differences between the groups.

CONCLUSIONS

The present results show that subchronic corticosterone administration does not induce changes on functionality of α₂-adrenoceptors neither in the LC nor in noradrenergic cortical terminal areas.

Managing inadequate antidepressant response in depressive illness

INTRODUCTION OR BACKGROUND

Depression frequently fails to respond to initial treatment.

SOURCES OF DATA

Predominantly meta-analyses and RCTs but supplemented where necessary by additional data and the authors' clinical experience.

AREAS OF AGREEMENT

A systematic assessment to identify remedial causes of poor response should be followed by planned sequential treatment trials. Joint decision making by the patient and clinician is essential. Strategies with the strongest support are antidepressant augmentation with lithium or second generation antipsychotics and adding cognitive behavioural treatment. Electroconvulsive therapy is highly effective in resistant depression but there is a high relapse rate when treatment ends.

AREAS OF CONTROVERSY

Some pharmacological strategies have inconsistent data (e.g. antidepressant combinations, T3 augmentation) or limited preliminary data (e.g. ketamine, antidepressant augmentation with pramipexole). The efficacy of vagus nerve stimulation, deep brain stimulation and repetitive transcranial magnetic stimulation is unclear.

GROWING POINTS

A greater understanding of the causes of depression may assist the development of more effective treatments.

AREAS TIMELY FOR DEVELOPING RESEARCH

Role of glutamate antagonists and psychological treatments, other than cognitive behavioural therapy, as adjunctive treatments.

Antiglucocorticoid therapy for older adults with anxiety and co-occurring cognitive dysfunction: results from a pilot study with mifepristone

OBJECTIVES

In older adults with anxiety disorders, chronically elevated cortisol may contribute to cognitive impairment and elevated anxiety. We conducted a pilot study with mifepristone, a glucocorticoid receptor antagonist, as a potential treatment for late-life anxiety disorders and co-occurring cognitive dysfunction.

METHODS

Fifteen individuals 60 years and older with an anxiety disorder plus cognitive dysfunction participated in the 12-week study. In the first week, participants were randomly assigned to mifepristone 300 mg daily or placebo. In the subsequent 3 weeks, all participants received mifepristone 300 mg. Mifepristone was then discontinued, and the participants were reassessed 8 weeks later. We examined the following: (1) cognitive changes; (2) worry symptom severity; (3) safety and tolerability; and (4) salivary cortisol before, during, and after mifepristone exposure.

RESULTS

Overall safety, tolerability, and high retention supported the feasibility of this research. Participants with higher baseline cortisol levels (peak cortisol >6.0 ng/ml, n = 5) showed improvements in memory, executive function, and worry severity after 3-4 weeks of mifepristone with persistent memory and worry improvements 8 weeks after mifepristone discontinuation. Individuals with low-to-normal baseline cortisol (n = 8) showed little to no improvement. As expected, cortisol levels rose during mifepristone exposure and returned to pretreatment levels 8 weeks after mifepristone discontinuation. In the first week of treatment, there were no differences between placebo-treated and mifepristone-treated participants.

CONCLUSION

The results of this pilot study warrant further testing of antiglucocorticoid agents in late-life anxiety disorders with co-occurring cognitive dysfunction. Mifepristone is hypothesized to have benefits in patients with evidence of glucocorticoid excess. Directions for further study are discussed.

Altered neuronal activity and differential sensitivity to acute antidepressants of locus coeruleus and dorsal raphe nucleus in Wistar Kyoto rats: a comparative study with Sprague Dawley and Wistar rats

The Wistar Kyoto rat (WKY) has been proposed as an animal model of depression. The noradrenergic nucleus, locus coeruleus (LC) and the serotonergic nucleus, dorsal raphe (DRN) have been widely implicated in the ethiopathology of this disease. Thus, the goal of the present study was to investigate in vivo the electrophysiological properties of LC and DRN neurons from WKY rats, using single-unit extracellular techniques. Wistar (Wis) and Sprague Dawley (SD) rats were used as control strains. In the LC from WKY rats the basal firing rate was higher than that obtained in the Wis and SD strain, and burst firing activity also was greater compared to that in Wis strain but not in SD. The sensitivity of LC neurons to the inhibitory effect of the α2-adrenoceptor agonist, clonidine and the antidepressant reboxetine was lower in WKY rats compared to Wis, but not SD. Regarding DRN neurons, in WKY rats burst activity was lower than that obtained in Wis and SD rats, although no differences were observed in other firing parameters. Interestingly, while the sensitivity of DRN neurons to the inhibitory effect of the 5-HT1A receptor agonist, 8-OH-DPAT was lower in the WKY strain, the antidepressant fluoxetine had a greater inhibitory potency in this rat strain compared to that recorded in the Wis group. Overall, these results point out important electrophysiological differences regarding noradrenergic and serotonergic systems between Wis and WKY rats, supporting the utility of the WKY rat as an important tool in the research of cellular basis of depression.

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

Chronic stressful life events are risk factors for developing major depression, the pathophysiology of which is strongly linked to impairments in serotonin (5-HT) neurotransmission. Exposure to chronic unpredictable stress (CUS) has been found to induce depressive-like behaviours, including passive behavioural coping and anhedonia in animal models, along with many other affective, cognitive, and behavioural symptoms. The heterogeneity of these symptoms represents the plurality of corticolimbic structures involved in mood regulation that are adversely affected in the disorder. Chronic stress has also been shown to negatively regulate adult hippocampal neurogenesis, a phenomenon that is involved in antidepressant effects and regulates subsequent stress responses. Although there exists an enormous body of data on stress-induced alterations of 5-HT activity, there has not been extensive exploration of 5-HT adaptations occurring presynaptically or at the level of the raphe nuclei after exposure to CUS. Similarly, although hippocampal neurogenesis is known to be negatively regulated by stress and positively regulated by antidepressant treatment, the role of neurogenesis in mediating affective behaviour in the context of stress remains an active area of investigation. The goal of this review is to link the serotonergic and neurogenic hypotheses of depression and antidepressant effects in the context of stress. Specifically, chronic stress significantly attenuates 5-HT neurotransmission and 5-HT1A autoreceptor sensitivity, and this effect could represent an endophenotypic hallmark for mood disorders. In addition, by decreasing neurogenesis, CUS decreases hippocampal inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, exacerbating stress axis overactivity. Similarly, we discuss the possibility that adult hippocampal neurogenesis mediates antidepressant effects via the ventral (in rodents; anterior in humans) hippocampus' influence on the HPA axis, and mechanisms by which antidepressants may reverse chronic stress-induced 5-HT and neurogenic changes. Although data are as yet equivocal, antidepressant modulation of 5-HT neurotransmission may well serve as one of the factors that could drive neurogenesis-dependent antidepressant effects through these stress regulation-related mechanisms.

A cholinergic hypothesis of the unconscious in affective disorders

The interactions between distinct pharmacological systems are proposed as a key dynamic in the formation of unconscious memories underlying rumination and mood disorder, but also reflect the plastic capacity of neural networks that can aid recovery. An inverse and reciprocal relationship is postulated between cholinergic and monoaminergic receptor subtypes. M1-type muscarinic receptor transduction facilitates encoding of unconscious, prepotent behavioral repertoires at the core of affective disorders and ADHD. Behavioral adaptation to new contingencies is mediated by the classic prototype receptor: 5-HT1A (Gi/o) and its modulation of M1-plasticity. Reversal of learning is dependent on increased phasic activation of midbrain monoaminergic nuclei and is a function of hippocampal theta. Acquired hippocampal dysfunction due to abnormal activation of the hypothalamic-pituitary-adrenal (HPA) axis predicts deficits in hippocampal-dependent memory and executive function and further impairments to cognitive inhibition. Encoding of explicit memories is mediated by Gq/11 and Gs signaling of monoamines only. A role is proposed for the phasic activation of the basal forebrain cholinergic nucleus by cortical projections from the complex consisting of the insula and claustrum. Although controversial, recent studies suggest a common ontogenetic origin of the two structures and a functional coupling. Lesions of the region result in loss of motivational behavior and familiarity based judgements. A major hypothesis of the paper is that these lost faculties result indirectly, from reduced cholinergic tone.

Study protocol for the randomised controlled trial: antiglucocorticoid augmentation of anti-Depressants in Depression (The ADD Study)

BACKGROUND

Some patients with depression do not respond to first and second line conventional antidepressants and are therefore characterised as suffering from treatment refractory depression (TRD). On-going psychosocial stress and dysfunction of the hypothalamic-pituitary-adrenal axis are both associated with an attenuated clinical response to antidepressants. Preclinical data shows that co-administration of corticosteroids leads to a reduction in the ability of selective serotonin reuptake inhibitors to increase forebrain 5-hydroxytryptamine, while co-administration of antiglucocorticoids has the opposite effect. A Cochrane review suggests that antiglucocorticoid augmentation of antidepressants may be effective in treating TRD and includes a pilot study of the cortisol synthesis inhibitor, metyrapone. The Antiglucocorticoid augmentation of anti-Depressants in Depression (The ADD Study) is a multicentre randomised placebo controlled trial of metyrapone augmentation of serotonergic antidepressants in a large population of patients with TRD in the UK National Health Service.

METHODS/DESIGN

Patients with moderate to severe treatment refractory Major Depression aged 18 to 65 will be randomised to metyrapone 500 mg twice daily or placebo for three weeks, in addition to on-going conventional serotonergic antidepressants. The primary outcome will be improvement in Montgomery-Åsberg Depression Rating Scale score five weeks after randomisation (i.e. two weeks after trial medication discontinuation). Secondary outcomes will include the degree of persistence of treatment effect for up to 6 months, improvements in quality of life and also safety and tolerability of metyrapone. The ADD Study will also include a range of sub-studies investigating the potential mechanism of action of metyrapone.

DISCUSSION

Strengths of the ADD study include broad inclusion criteria meaning that the sample will be representative of patients with TRD treated within the UK National Health Service, longer follow up, which to our knowledge is longer than any previous study of antiglucocorticoid treatments in depression, and the range of mechanistic investigations being carried out. The data set acquired will be a rich resource for a range of research questions relating to both refractory depression and the use of antiglucocorticoid treatments.

TRIAL REGISTRATION

Current Controlled Trials: ISRCTN45338259; EudraCT Number: 2009-015165-31.

Are BDNF and glucocorticoid activities calibrated?

One hypothesis to account for the onset and severity of neurological disorders is the loss of trophic support. Indeed, changes in the levels and activities of brain-derived neurotrophic factor (BDNF) occur in numerous neurodegenerative and neuropsychiatric diseases. A deficit promotes vulnerability whereas a gain of function facilitates recovery by enhancing survival, synapse formation and synaptic plasticity. Implementation of 'BDNF therapies', however, faces numerous methodological and pharmacokinetic issues. Identifying BDNF mimetics that activate the BDNF receptor or downstream targets of BDNF signaling represent an alternative approach. One mechanism that shows great promise is to study the interplay of BDNF and glucocorticoid hormones, a major class of natural steroid secreted during stress reactions and in synchrony with circadian rhythms. While small amounts of glucocorticoids support normal brain function, excess stimulation by these steroid hormones precipitates stress-related affective disorders. To date, however, because of the paucity of knowledge of underlying cellular mechanisms, deleterious effects of glucocorticoids are not prevented following extreme stress. In the present review, we will discuss the complementary roles shared by BDNF and glucocorticoids in synaptic plasticity, and delineate possible signaling mechanisms mediating these effects.

Chronic effects of corticosterone on GIRK1-3 subunits and 5-HT1A receptor expression in rat brain and their reversal by concurrent fluoxetine treatment

Dysregulation of the serotonergic system and abnormalities of the hypothalamic-pituitary-adrenal axis have been demonstrated in major depression. Animal studies indicate that 5-HT1A receptor expression may be reduced by long-term administration of corticosterone. However, similar studies on the regulation of GIRK channels, one of the most important effectors of the neuronal 5-HT1A receptor, are limited. In order to address these issues, slow-release corticosterone pellets were implanted subcutaneously to adrenal intact male rats (200mg pellets, 35 days release). Starting on day 15, animals were treated for 21 days with fluoxetine (5mg/kg/day, i.p.), or vehicle. Using in situ hybridization histochemistry and receptor autoradiography, we found that chronic corticosterone treatment was accompanied by a significant decrease on the mRNAs coding for mineralocorticoid receptors in hippocampal areas. Under these conditions, 5-HT1A receptor mRNA expression decreased in dorsal raphe nucleus and dentate gyrus. However, 5-HT1A receptor levels, as measured by [(3)H]-8-OH-DPAT binding, diminished significantly only in dentate gyrus. It is noteworthy that chronic treatment with fluoxetine reversed the alterations on 5-HT1A receptor mRNA levels only in dorsal raphe. Finally, chronic corticosterone treatment produced an increase on the mRNA coding for the GIRK2 subunit in several hypothalamic and thalamic areas, which was reversed by fluoxetine. Measurements of cell density and volume of the granular layer of the dentate gyrus did not reveal significant changes after corticosterone or corticosterone plus fluoxetine treatments. These data are relevant for a better understanding of the differential regulation of pre- and postsynaptic 5-HT1A receptors by corticosterone flattened rhythm.

A randomized trial to examine the effect of mifepristone on neuropsychological performance and mood in patients with bipolar depression

BACKGROUND

Deficits in neuropsychological performance are found in patients with bipolar disorder and represent a potential treatment target for novel therapeutic strategies. We have previously demonstrated a beneficial effect on spatial working memory (SWM) of treatment for 1 week with the progesterone and glucocorticoid receptor antagonist mifepristone, evident 2 weeks after the cessation of treatment.

METHODS

We examined the longer-term efficacy of 600 mg/day of mifepristone as an adjunctive treatment, for 1 week, in a placebo-controlled, randomized, double-blind trial in 60 patients with bipolar depression, with SWM as the primary outcome measure. A comparator group of healthy control subjects was also recruited.

RESULTS

At baseline, neuropsychological performance of patients was impaired, but hypothalamic-pituitary-adrenal axis function did not differ from that of control subjects. Mifepristone treatment was associated with a time-limited increase in cortisol awakening response and with a sustained improvement in SWM performance, which was evident 7 weeks after the cessation of treatment. The magnitude of this neuropsychological response was predicted by the magnitude of the cortisol response to mifepristone. The response occurred in the absence of a significant improvement in depressed mood.

CONCLUSIONS

These data accord with the findings of animal studies and demonstrate that brief treatment with mifepristone is associated with a sustained improvement in SWM, an effect that might be mediated by a persistent enhancement in hippocampal mineralocorticoid receptor function.

Metyrapone in treatment-resistant depression

Depression affects a significant proportion of the population, with 1-year and lifetime prevalence of 3-5% and 10-30% respectively. Full remission is achieved in only a third of patients following treatment with first-line antidepressant. There is a need for novel treatments for treatment-resistant depression (TRD). Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis has been described in patients with depression. There is persistent rise in the levels of cortisol (end product of the HPA axis) and impairment of the negative feedback inhibition mechanism of the HPA axis. Dysregulation of the HPA axis has been found to be linked to nonresponse to antidepressants and relapse following successful treatment. The efficacy of pharmacological agents that intervene with the mechanisms involved in dysregulation of cortisol synthesis and release are being explored in depression, particularly in TRD. Studies have been carried out with these drugs as augmenting agents for antidepressants or as monotherapy. The strongest evidence has come from studies using metyrapone, a cortisol synthesis inhibitor, and this has been described in detail in this review. The most robust evidence for its antidepressant efficacy in depression comes from a double-blind, randomized, placebo-controlled study of augmentation of serotonergic antidepressants with metyrapone. A 3-week augmentation of serotonergic antidepressants with 1 g metyrapone daily was shown to be superior to placebo in reducing the Montgomery-Asberg Depression Rating Scale by 50%, 5 weeks following initiation of treatment. The mechanism of the antidepressant action of metyrapone is not clear but the evidence for various potential mechanisms is discussed.

Functional status of somatodendritic serotonin 1A autoreceptor after long-term treatment with fluoxetine in a mouse model of anxiety/depression based on repeated corticosterone administration

Most preclinical studies investigating the effects and the mechanism of action of antidepressants have been performed in naive rodents. This is inappropriate because antidepressants act on specific symptoms of the pathological condition, such as distress and anxiety. We have developed a mouse model of anxiety/depression based on addition of corticosterone to drinking water. This model is highly reproducible and easy to set up compared with unpredictable chronic mild stress. The serotonin 1A (5-HT(1A)) autoreceptor is known to play a role in mood disorders and their treatments. An increase in somatodendritic 5-HT(1A) autoreceptor density in the dorsal raphe (DR) attenuates the therapeutic activity of selective serotonin-reuptake inhibitors (SSRIs), whereas their functional desensitization promotes activation of brain serotonergic transmission, thereby representing an adaptive change relevant to their therapeutic effect. Here we assessed the effects of sustained administration of the SSRI fluoxetine on 5-HT(1A) autoreceptor sensitivity in mice administered with corticosterone. Fluoxetine attenuated hypothermia induced by the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin, decreased DR 5-HT neuronal activity, and decreased 5-HT release in both vehicle- and corticosterone-pretreated mice. However, such desensitization was more pronounced in corticosterone-pretreated mice. This change had an overall effect on serotonergic tone because we found a greater firing rate of 5-HT neurons associated with an enhancement of 5-HT outflow in the DR of corticosterone-pretreated mice in response to fluoxetine compared with the corresponding group of vehicle-pretreated mice. These results provide cellular explanations for the antidepressant effects produced by SSRIs in subjects with pathological conditions but not in naive animals or healthy volunteers.

Neuropsychological functioning in health and mood disorder: Modulation by glucocorticoids and their receptors

Numerous studies have shown that disturbances in hypothalamic-pituitary-adrenal (HPA) axis function and consequent hypercortisolaemia occur in a significant proportion of patients with mood disorders. This dysfunction has been proposed to be an exacerbating factor of depressive symptoms and may predict symptomatic relapse. Glucocorticoids are also known to have a specific role in learning and memory processes. In this review we present a brief overview of the relationship between HPA axis dysfunction and neuropsychological impairment in mood disorders and the specific links between glucocorticoids and cognition in health and illness states. Finally we examine the neuropsychological effects of drugs that specifically target glucocorticoid receptor function.

5-Hydroxytryptophan rescues serotonin response to stress in prefrontal cortex of hyperphenylalaninaemic mice

Adult early treated hyperphenylalaninaemic patients can show specific deficits of prefrontal cortical functions. The development of additional therapeutic strategies for these patients requires the understanding of the mechanisms involved in phenylalanine-dependent impairment of fronto-cortical functions. We tested the hypothesis of phenylalanine interference with aminergic neurotransmission in the prefrontal cortex by evaluating, in vivo, amine release in adult Pah(enu2) mice, the genetic model of phenylketonuria. Mice of healthy background responded to a psychogenic stressor with the classic time-dependent increase of norepinephrine, dopamine and serotonin release from prefrontal cortical terminals. Neither the dopaminergic nor the serotoninergic responses were observable in the Pah(enu2) mice. Temporary reduction of circulating phenylalanine, by phenylalanine-free diet without amino- acid supplement, promoted recovery of the serotonin response only, demonstrating direct interference with serotonin synthesis in the mature brain. Evaluation of different steps of serotonin synthesis in the prefrontal cortex of hyperphenylalaninaemic mice demonstrated inhibition of cortical tryptophan hydroxylase activity. Finally, systemic administration of 5-hydroxytryptophan, the product of tryptophan hydroxylase activity, allowed frontal cortical serotonin response to stress in hyperphenylalaninaemic mice. Collectively, these results demonstrate that hyperphenylalaninaemia interferes with the ability of the mature prefrontal cortex to respond to psychological challenges, point to serotonin synthesis as the target of phenylalanine interference, and support the use of 5-hydroxytryptophan in lifelong treatment of hyperphenylalaninaemic subjects.

Effect of aspirin on hypothalamic-pituitary-adrenal function and on neuropsychological performance in healthy adults: a pilot study

RATIONALE

Hypothalamic-pituitary-adrenal axis dysregulation predicts poor clinical and biochemical response to antidepressants. Antiglucocorticoids have therapeutic benefits but most have a troublesome adverse event profile. Aspects of neuropsychological performance, notably working memory, are susceptible to corticosteroid modulation and are impaired in depression. Aspirin has been shown to attenuate the adrenocorticotropic hormone (ACTH) and cortisol response to physiological challenge suggesting its potential to act as an augmenting agent in depression.

OBJECTIVES

To examine the effect of sub-acute (300 mg daily for 7 days) aspirin pre-treatment on the cortisol awakening response and the effect of acute (600 mg) and sub-acute aspirin on the neuroendocrine and neuropsychological response to the arginine vasopressin analogue, desmopressin.

RESULTS

We demonstrated that aspirin pre-treatment did not attenuate the cortisol or ACTH response to desmopressin but, as hypothesised, significantly reduced the cortisol awakening response and improved working memory.

CONCLUSIONS

Further studies to examine the impact of aspirin on neuropsychological performance and HPA axis function are warranted.

Prediction of relapse in melancholic depressive patients in a 2-year follow-up study with corticotropin releasing factor test

PURPOSE

To study the power of CRF stimulation test to predict relapse in a sample of melancholic depressive patients in depressed phase, followed-up over a two-year period from the moment they achieved complete remission of depressive symptoms.

METHODS

Fifty-one outpatients diagnosed with unipolar depressive disorder with melancholic features according to DSM-IV were assessed with the CRF test. The Structured Clinical Interview for DSM-IV (SCID-IV) was used for diagnosis. Monthly follow-up visits were held over a two-year period after remission; relapse was established using HDRS according to Frank's criteria [Frank E, Prien RF, Jarret RB, Keller MB, Kupfer DJ, Lavori PW, et al. Conceptualization and rationale for consensus definitions of terms in major depressive disorder: remission, recovery, relapse, and recurrence. Arch Gen Psychiatry 1991;48:851-5]. Forty-three patients completed the study. Non-controlled antidepressant treatment protocols were used. Predictive statistical analysis was performed through logistic regression.

FINDINGS

The final predictive model included three variables: net area under cortisol curve (NAUCC), previous suicide attempt, and stress during follow-up. Sensitivity was of 89%, and specificity was of 92%. NAUCC has shown a predictive power of 80%, with an optimal cut-off point of 251.24 microg/ml/min.

CONCLUSIONS

Cortisol is the hormone of the HPA axis which shows the highest power to predict relapse. NAUCC is the most relevant variable. The complete predictive model is a complex combination of biological, clinical and psychoenvironmental variables (NAUCC, previous suicide attempts, and stress during follow-up). Further studies with better control of the psychoenvironmental variables are required to obtain more precise neuroendocrine findings.

Glucocorticoid receptor antagonism augments fluoxetine-induced downregulation of the 5-HT transporter

The effects of combined treatment with a glucocorticoid receptor (GR) antagonist, Org 34850, and a selective serotonin reuptake inhibitor (SSRI), fluoxetine, were investigated on pre- and postsynaptic aspects of 5-HT neurotransmission. Rats were treated for 14 days with Org 34850 (15 mg per kg per day subcutaneously), fluoxetine (10 mg per kg per day intraperitoneally), or a combination of both drugs. [(3)H]-citalopram binding (an index of 5-HT transporter (5-HTT) expression) was only slightly affected by Org 34850 alone: decreased in cortex and midbrain and increased in hippocampus. In contrast, chronic fluoxetine markedly decreased 5-HTT levels in all regions. Importantly, this decrease was significantly enhanced by combined Org 34850/fluoxetine treatment. There were no changes in the expression of 5-HTT mRNA, suggesting these effects were not due to changes in gene transcription. Expression of tryptophan hydroxylase mRNA and both 5-HT(1A) autoreceptor mRNA and protein (assessed using [(3)H]-8-OH-DPAT binding) were unchanged by any treatment. The expression of postsynaptic 5-HT(1A) receptor protein in the forebrain was unaltered by fluoxetine, Org 34850 or the combined Org 34850/fluoxetine treatment. This downregulation of 5-HTT by fluoxetine and its enhancement by Org 34850 can explain our recent observation that GR antagonists augment the SSRI-induced increase in extracellular 5-HT. In addition, these data suggest that the augmentation of forebrain 5-HT does not result in downregulation of forebrain 5-HT(1A) receptor expression. Given the importance of 5-HT(1A) receptor-mediated transmission in the forebrain to the antidepressant response, these data indicate that co-administration of GR antagonists may be effective in augmenting the antidepressant response to SSRI treatment.

Antidepressants: will new mechanisms of action improve poor outcomes?

Depression is one of the most common mental illnesses and has a high impact on individuals and society. Despite the numerous treatment approaches, poor response combined with the burden of residual symptoms negatively affects the overall outcome. Novel pharmacological approaches could help to address this issue.

Effects of chronic citalopram treatment on 5-HT1A and 5-HT2A receptors in group- and isolation-housed mice

Selective serotonin reuptake inhibitors (SSRI) are characterized by high clinical effectiveness and good tolerability. A 2-3 week delay in the onset of effects is caused by adaptive mechanisms, probably at the serotonergic (5-HT) receptor level. To analyze this in detail, we measured 5-HT(1A) and 5-HT(2A) receptor bindings in vitro after 3 weeks of citalopram treatment (20 mg/kg i.p. daily) in group-housed as well as isolation-housed mice, reflecting neurobiological aspects seen in psychiatric patients. Isolation housing increased somatodendritic (+52%) and postsynaptic (+30-95%) 5-HT(1A) as well as postsynaptic 5-HT(2A) receptor binding (+25-34%), which confirms previous findings. Chronic citalopram treatment did not induce alterations in raphe 5-HT(1A) autoreceptor binding, independent of housing conditions. Housing-dependent citalopram effects on postsynaptic 5-HT(1A) receptor binding were found with increases in group- (+11-42%) but decreases in isolation-housed (-11 to 35%) mice. Forebrain 5-HT(2A) receptor binding decreased between 11 and 38% after chronic citalopram administration, independent of housing conditions. Citalopram's long-term action comprises alterations at the postsynaptic 5-HT(1A) and 5-HT(2A) receptor binding levels. Housing conditions interact with citalopram effects, especially on 5-HT(1A) receptor binding, and should be more strongly considered in pharmacological studies. In general, SSRI-induced alterations were more pronounced and affected more brain regions in isolates, supporting the concept of a higher responsiveness in "stressed" animals. Isolation-induced receptor binding changes were partly normalized by chronic citalopram treatment, suggesting the isolation housing model for further analyses of SSRI effects, especially at the behavioral level.

Glucocorticoid receptor antagonists hasten and augment neurochemical responses to a selective serotonin reuptake inhibitor antidepressant

BACKGROUND

Selective serotonin reuptake inhibitor (SSRI) antidepressant drugs have a delayed onset and commonly produce an incomplete therapeutic response. The therapeutic actions of SSRIs are thought to depend on increased forebrain extracellular serotonin (5-HT), after desensitization of somatodendritic 5-HT(1A) autoreceptors. Here we determined whether concurrent glucocorticoid receptor (GR) blockade enhances these neurochemical responses to the SSRI fluoxetine.

METHODS

Male rats were treated (3, 7, or 14 days) with either fluoxetine (10 mg/kg IP) or vehicle once daily, in combination with either a GR antagonist (Org 34850 15 mg/kg SC or Org 34517 25 mg/kg SC) or vehicle twice daily. After treatment, 5-HT in the medial prefrontal cortex was measured by microdialysis.

RESULTS

Chronic fluoxetine treatment (14 days) raised basal 5-HT and also attenuated the fall in 5-HT after acute systemic administration of fluoxetine (10 mg/kg IP), indicating desensitization of 5-HT(1A) autoreceptors. Concurrent chronic administration (14 days) of Org 34850 or Org 34517 enhanced the fluoxetine-induced increase in basal 5-HT. Org 34850 also hastened the 5-HT(1A) autoreceptor desensitization induced by chronic fluoxetine treatment. Org 34850 alone (14 days) failed to alter basal 5-HT or 5-HT(1A) autoreceptor desensitization.

CONCLUSIONS

Antidepressant response is proposed to depend on 5-HT(1A) autoreceptor desensitization and elevation of forebrain 5-HT. These data suggest adjunctive GR antagonists might both hasten and enhance antidepressant responses to SSRIs.

Research into putative biological mechanisms of mental disorders has been of no value to clinical psychiatry

In 1845 Griesinger declared that mental disorders were physical in origin (Kendell, 2001). The discovery of the bacterial cause of general paresis and the anatomical basis for Alzheimer's disease seemed to confirm this belief. However, is it still reasonable, a century later, to continue to devote increasing amounts of financial and expert human resource to pursuing further possible physical causes for mental disorders? The belief that there remain undiscovered and important biological causes for mental disorders continues to exert a major influence on the direction of research, practice and public education. But has it helped us to understand aetiology, improve management or destigmatise mental disorders?

Differential regulation of neurotrophins and serotonergic function in mice with genetically reduced glucocorticoid receptor expression

The neurotrophin and serotonin (5-HT) hypotheses of depression were studied in a mouse model of reduced glucocorticoid receptor (GR) function (GR(+/-) mice), which recently has been proven as a murine model of predisposition for depressive behaviour under stressful conditions. In this model we studied diurnal changes in neurotrophins and serotonergic function in candidate brain regions mediating depressive behaviour. Morning and evening levels of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were analyzed in representative brain regions of GR(+/-) and wildtype mice. The diurnal variation of hippocampal BDNF in wildtypes with higher levels in the morning was absent in GR(+/-) mice. Hypothalamus and parietal cortex displayed enhanced BDNF levels in GR(+/-) mice. In the frontal cortex, striatum and hypothalamus NGF increased from morning to evening in both genotypes, with an exaggeration in GR(+/-) mice. The diurnal variation of 5-HT levels and turnover did not differ significantly between genotypes. It was only in the hypothalamus that the evening level of 5-HIAA was lower in GR(+/-) mice than in wildtype mice. In conclusion, the present data indicate a contribution of altered BDNF and NGF protein levels to the predisposition for depressive behaviour in the GR(+/-) mouse model of depression, but argue against an eminent role of the serotonergic system.

Stress-resistant mice overexpressing glucocorticoid receptors display enhanced BDNF in the amygdala and hippocampus with unchanged NGF and serotonergic function

Dysfunctional glucocorticoid receptor (GR) signaling has been shown to be involved in the pathogenesis of depressive behavior in mice and humans. In accordance with this hypothesis GR overexpressing mice are less susceptible to develop depressive-like behavior when subjected to stressful events. Here, we analyzed GR overexpressing mice for morning and evening content of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and the tissue levels of serotonin and its metabolite 5-hydroxyindoleacetic acid) in brain areas suspected to be involved in stress adaptation. BDNF concentrations in the hippocampus and amygdala/piriform cortex were significantly enhanced in GR overexpressing mice (by maximally +103%) compared to wildtype animals. Diurnal variations, as detected for NGF in the hypothalamus, for BDNF in the frontal cortex and striatum and for serotonergic function in the frontal cortex and hypothalamus, were not affected by the genotype. In conclusion, GR overexpression-dependent increases of hippocampal and amygdala BDNF content presumably represent a dynamic correlate of enhanced stress resistance.

Abnormalities of the HPA axis in affective disorders: clinical subtypes and potential treatments

BACKGROUND

New evidence is emerging regarding abnormalities of hypothalamic-pituitary-adrenal (HPA) axis function in subtypes of affective disorders. Adverse effects of HPA axis dysregulation may include dysfunction of monoaminergic transmitter systems, cognitive impairment and peripheral effects. Newer treatments specifically targeting the HPA axis are being developed.

OBJECTIVE

To review these developments focusing particularly on the glucocorticoid receptor (GR) antagonist mifepristone.

METHOD

A selective review of the literature.

RESULTS

The function of GRs is increasingly being defined. The role of corticotrophin-releasing hormone (CRH) and dehydroepiandrosterone (DHEA) in the brain is also increasingly understood. HPA axis function is particularly likely to be abnormal in psychotic depression and bipolar disorder, and it is in these conditions that trials of the GR antagonist mifepristone are being focused. CRH antagonists and DHEA are also being investigated as potential treatments.

CONCLUSION

Initial studies of mifepristone and other HPA-axis-targeting agents in psychotic depression and bipolar disorder are encouraging and confirmatory studies are awaited.

Chronic corticosterone decreases brain-derived neurotrophic factor (BDNF) mRNA and protein in the hippocampus, but not in the frontal cortex, of the rat

This study examined the effects of chronic corticosterone (32 mg/kg/day, s.c., 21 days) on brain-derived neurotrophic factor (BDNF) mRNA and protein in the frontal cortex and hippocampus of the rat. Because evidence suggests that BDNF is an important determinant of the function of the 5-hydroxytryptamine (5-HT) system, we also quantified tissue levels of 5-HT and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), to investigate if changes in BDNF mRNA and protein paralleled changes in the 5-HT system. Corticosterone modestly decreased BDNF protein (-16.6%) in whole hippocampus and BDNF mRNA (-19%) in the CA3 area. In contrast, BDNF mRNA and protein in the frontal cortex were unchanged. In both the frontal cortex and hippocampus, tissue levels of 5-HT and 5-HIAA were increased and decreased, respectively. Combined, these data suggests that the effects of corticosterone on the BDNF system are not linked to the effects on the 5-HT systems. However, our findings do suggest that chronic corticosterone impairs hippocampal BDNF function, a finding with potential relevance for the hippocampal atrophy reported in major depression. Additionally, as inferred from the alterations in tissue levels of 5-HT and 5-HIAA, chronic corticosterone may influence the function of the 5-HT system.

Stimulation of neurogenesis in the hippocampus of the adult rat by fluoxetine requires rhythmic change in corticosterone

BACKGROUND

Fluoxetine stimulates proliferation of progenitor cells in the dentate gyrus of the adult hippocampus. There are suggestions that this action may underlie the therapeutic effects of such drugs in depression. Glucocorticoids also regulate neurogenesis, and there are multiple interactions between serotonin and corticoids. Diurnal cortisol rhythms are dysregulated in depression. We explored the role of diurnal variations in corticosterone on the ability of fluoxetine to alter neurogenesis in the dentate gyrus.

METHODS

We manipulated plasma corticosterone by implanting corticosterone pellets or giving daily corticosterone injections to corticosterone-clamped adrenalectomized or intact rats that received fluoxetine or vehicle treatment. Proliferation of progenitor cells in the dentate gyrus was measured using BrdU or Ki-67.

RESULTS

Our results strongly suggest that a diurnal rhythm in corticosterone is necessary for fluoxetine to stimulate neurogenesis in the adult dentate gyrus in the male rat. Preliminary data suggest this may be related to the 5-HT1A receptor.

CONCLUSIONS

If altered neurogenesis in the dentate gyrus is part of the therapeutic response to antidepressants such as fluoxetine, the results we report suggest that concurrent manipulation of the HPA axis might improve sensitivity to selective serotonin reuptake inhibitors in some treatment-resistant patients.

Effects of acute olanzapine after sustained fluoxetine on extracellular monoamine levels in the rat medial prefrontal cortex

The combination of selective serotonin reuptake inhibitors with atypical antipsychotic drugs exhibits beneficial effects in treatment-resistant depression. We investigated the effects of a 2-week treatment with a low fluoxetine dose (3 mg/kg per day) plus a single injection of olanzapine (3 mg/kg) on the dialysate concentration of noradrenaline, dopamine and serotonin (5-HT) in the medial prefrontal cortex of the rat. Chronic fluoxetine increased only 5-HT levels whereas single olanzapine administration increased the concentration of catecholamines and decreased that of 5-HT to a comparable extent in vehicle- and fluoxetine-treated rats. Therefore, it is possible that the therapeutic benefit of this pharmacological combination may not be associated to changes in the cortical concentration of monoamines, but to postsynaptic blockade of monoaminergic receptors.

Serotonin modulates the suppressive effects of corticosterone on proliferating progenitor cells in the dentate gyrus of the hippocampus in the adult rat

This series of experiments explores the interaction between corticosterone and serotonin (5-HT) in the regulation of cell proliferation in the dentate gyrus of the adult rat. Intracerebroventricular 5,7-DHT (5,7-dihydroxytryptamine) (either 200 or 300 microg) resulted in highly significant depletion of 5-HT as measured by high performance liquid chromatography in the frontal cortex but had no effect on the number of proliferating cells in the dentate gyrus by measuring 5-bromo-2'-deoxyuridine (BrdU) and Ki-67 cytochemistry. Treatment with PCPA (p-chlorophenylalanine: a tryptophan hydroxylase inhibitor: 300 mg/kg initially followed by 100 mg/kg/day) resulted in reduced proliferation as measured by Ki-67 after 3 days treatment, but not by BrdU uptake, and not after 14 days treatment by either method. In addition, injection of corticosterone (10-40 mg/kg/day) for 8 days significantly reduced proliferation in the dentate gyrus, as expected, measured by both BrdU uptake and Ki-67 immunostaining. Adrenalectomized (ADX) rats with a replacement subcutaneous pellet of corticosterone showed reduced proliferation when given additional corticosterone (10 mg/kg/day for 8 days), but this was prevented by 5-HT depletion (i.c.v. 5,7-DHT). Finally, a dose-response study showed that progressive doses of corticosterone (0-40 mg/kg/day) in ADX rats resulted in diminished suppression of proliferation in 5-HT-depleted compared with 5-HT-intact rats. These results strongly suggest that 5-HT regulates the sensitivity of proliferating cells in the dentate gyrus to corticosterone.

Chronic fluoxetine microdialysis into the medullary raphe nuclei of the rat, but not systemic administration, increases the ventilatory response to CO2

In conscious rats, focal CO2 stimulation of the medullary raphe increases ventilation, whereas interference with serotonergic function here decreases the ventilatory response to systemic hypercapnia. We sought to determine whether repeated administration of a selective serotonin reuptake inhibitor in this region would increase the ventilatory response to hypercapnia in unanesthetized rats. In rats instrumented with electroencephalogram-electromyogram electrodes, 250 or 500 μM fluoxetine or artificial cerebrospinal fluid (aCSF) was microdialyzed into the medullary raphe for 30 min daily over 15 days. To compare focal and systemic treatment, two additional groups of rats received 10 mg·kg−1·day−1 fluoxetine or vehicle systemically. Ventilation was measured in normocapnia and in 7% CO2 before treatment ( day 0), acutely ( days 1 or 3), on day 7, and on day 15. There was no change in normocapnic ventilation in any treatment group. Rats that received 250 μM fluoxetine microdialysis showed a significant 13% increase in ventilation in wakefulness during hypercapnia on day 7, due to an increase in tidal volume. In rats microdialyzed with 500 μM fluoxetine, there were 16 and 32% increases in minute ventilation during hypercapnia in wakefulness and sleep on day 7, and 20 and 28% increases on day 15, respectively, again due to increased tidal volume. There was no change in the ventilatory response to CO2 in rats microdialyzed with aCSF or in systemically treated rats. Chronic fluoxetine treatment in the medullary raphe increases the ventilatory response to hypercapnia in an unanesthetized rat model, an effect that may be due to facilitation of chemosensitive serotonergic neurons.