Effect of 5-HT1A receptor antagonists on citalopram-induced increase in extracellular serotonin in the frontal cortex, striatum and dorsal hippocampus

Molecular imaging of the association between serotonin degeneration and beta-amyloid deposition in mild cognitive impairment

BACKGROUND

Degeneration of the serotonin system has been observed in Alzheimer's disease (AD) and in mild cognitive impairment (MCI). In transgenic amyloid mouse models, serotonin degeneration is detected prior to widespread cortical beta-amyloid (Aβ) deposition, also suggesting that serotonin degeneration may be observed in preclinical AD.

METHODS

The differences in the distribution of serotonin degeneration (reflected by the loss of the serotonin transporter, 5-HTT) relative to Aβ deposition was measured with positron emission tomography in a group of individuals with MCI and a group of healthy older adults. A multi-modal partial least squares (mmPLS) algorithm was applied to identify the spatial covariance pattern between 5-HTT availability and Aβ deposition.

RESULTS

Forty-five individuals with MCI and 35 healthy older adults were studied, 22 and 27 of whom were included in the analyses who were "amyloid positive" and "amyloid negative", respectively. A pattern of lower cortical, subcortical and limbic 5-HTT availability and higher cortical Aβ deposition distinguished the MCI from the healthy older control participants. Greater expression of this pattern was correlated with greater deficits in memory and executive function in the MCI group, not in the control group.

CONCLUSION

A spatial covariance pattern of lower 5-HTT availability and Aβ deposition was observed to a greater extent in an MCI group relative to a control group and was associated with cognitive impairment in the MCI group. The results support the application of mmPLS to understand the neurochemical changes associated with Aβ deposition in the course of preclinical AD.

Decreased Brain Ventricular Volume in Psychiatric Inpatients with Serotonin Reuptake Inhibitor Treatment

Background

Brain ventricles have been reported to be enlarged in several neuropsychiatric disorders and in aging. Whether human cerebral ventricular volume can decrease over time with psychiatric treatment is not well-studied. The aim of this study was to examine whether inpatients taking serotonin reuptake inhibitors (SRI) exhibited reductions in cerebral ventricular volume.

Methods

Psychiatric inpatients, diagnosed mainly with depression, substance use, anxiety, and personality disorders, underwent two imaging sessions (Time 1 and Time 2, approximately 4 weeks apart). FreeSurfer was used to quantify volumetric features of the brain, and ANOVA was used to analyze ventricular volume differences between Time 1 and Time 2. Inpatients' brain ventricle volumes were normalized by dividing by estimated total intracranial volume (eTIV). Clinical features such as depression and anxiety levels were collected at Time 1, Time 1.5 (approximately 2 weeks apart), and Time 2.

Results

Inpatients consistently taking SRIs (SRI + , n = 44) showed statistically significant reductions of brain ventricular volumes particularly for their left and right lateral ventricular volumes. Reductions in their third ventricular volume were close to significance (p = .068). The inpatients that did not take SRIs (SRI-, n = 25) showed no statistically significant changes in brain ventricular volumes. The SRI + group also exhibited similar brain structural features to the healthy control group based on the 90% confidence interval comparsions on brain ventricular volume parameters, whereas the SRI- group still exhibited relatively enlarged brain ventricular volumes after treatment.

Conclusions

SRI treatment was associated with decreased brain ventricle volume over treatment.

Depression in multiple sclerosis: Is one approach for its management enough?

BACKGROUND

Major depression disorder (MDD) and severe depression symptoms are highly prevalent in multiple sclerosis (MS). Depression can worsen symptoms of MS and is associated with significantly reduced quality of life and increased risk of suicide. Currently, there is no gold-standard, single treatment available for depression in MS. Pharmacotherapy, cognitive behavior therapy (CBT), and exercise training individually are moderately, yet incompletely, efficacious for managing depression in the general population and MS.

PURPOSE

This review provides an overview of evidence from meta-analyses and systematic reviews for current treatments of depression in persons with MS. This review further develops the rationale for using a combinatory treatment approach in persons with MS.

METHODS

We performed a narrative review of meta-analyses and systematic reviews regarding the current state of evidence for the three most common treatments of depression in persons with MS (i.e., antidepressant medication, cognitive-behavior therapy, and exercise training). We provide a concise assessment of the overall effect of these treatments on depression in the general population and then persons with MS. We further note short-comings of research on these treatments for depression.

CONCLUSION

There is no single, gold-standard treatment for depression in MS, and we proposed that combinatory treatments should be considered for the management of depression in MS. However, there is a paucity of evidence for the use of combinatory therapy on depression and its outcomes in persons with MS, and this supports direct examination of the feasibility and efficacy of such combinatory approaches for MDD in MS.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

Effects of transcranial electrical stimulation on episodic memory in physiological and pathological ageing

Memory for personally-relevant past events (episodic memory) is critical for activities of daily living. Decline in this type of declarative long-term memory is a common characteristic of healthy ageing, a process accelerated in patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD). Transcranial electrical stimulation (tES) has been used as a strategy to ameliorate episodic memory. Here, we critically review studies investigating whether tES may improve episodic memory in physiological and pathological ageing. Most of the studies suggest that tES over the prefrontal or temporoparietal cortices can have a positive effect on episodic memory, but the transfer to improvement of execution of daily living activities is still unknown. Further work is needed to better understand the mechanisms underlying the effects of stimulation, combine tES with neuroimaging and optimizing the dosing of stimulation. Future studies should also investigate the optimal timing of stimulation and the combination with medications to induce long-lasting beneficial effects in pathological ageing. More open science efforts should be done to improve rigor and reliability of tES in ageing research.

Effects of combined escitalopram and aripiprazole in rats: role of the 5-HT1a receptor

RATIONALE

Pre-clinical and clinical studies have suggested that the antidepressant efficacy of escitalopram (ESC) can be augmented by co-administration of aripiprazole (ARI).

OBJECTIVE

To establish if the effects of ESC + ARI can be altered by modulating the 5-HT_1a receptor.

METHODS

Sprague-Dawley male rats received ESC + ARI (10 and 2 mg/kg/day, respectively, via osmotic or by cumulative injections), as well as the 5-HT_1a antagonist WAY-100635 (WAY; 0.01-1 mg/kg) and the 5-HT_1a agonist 8-OH-DPAT (DPAT; 0.3-1 mg/kg) prior to testing in locomotion chambers and in the forced swim test (FST). Expression of the 5-HT_1a receptor mRNA in the dorsal raphe nucleus, hippocampus, septum, and entorhinal cortex was also assessed.

RESULTS

WAY generally synergized, while DPAT antagonized, the effect of ESC + ARI on motor activity. All groups showed significantly lower 5-HT_1a mRNA in the dorsal raphe nucleus. In the hippocampus, ESC + ARI and WAY + ESC + ARI groups displayed equivalent elevations of 5-HT_1a mRNA, but this was not observed in groups that received DPAT + ESC + ARI. Finally, the addition of ARI to ESC augmented the effect that ESC alone had on reducing immobility in the FST. Importantly, WAY antagonized this effect, while DPAT had no consequences.

CONCLUSIONS

Taken together, these results in rats indicate that the 5-HT_1a receptor is involved in the behavioral and brain region-specific mRNA effects of ESC + ARI.

Somato-Dendritic Regulation of Raphe Serotonin Neurons; A Key to Antidepressant Action

Several lines of evidence implicate serotonin (5-hydroxytryptamine, 5-HT)in regulating personality traits and mood control. Serotonergic neurons are classically thought to be tonic regular-firing, “clock-like” neurons. Neurotransmission by serotonin is tightly regulated by the serotonin transporter (SERT) and by autoreceptors (serotonin receptors expressed by serotonin neurons) through negative feedback inhibition at the cell bodies and dendrites (5-HT_1A receptors) of the dorsal raphe nuclei or at the axon terminals (5-HT_1B receptors). In dorsal raphe neurons, the release of serotonin from vesicles in the soma, dendrites, and/or axonal varicosities is independent of classical synapses and can be induced by neuron depolarization, by the stimulation of L-type calcium channels, by activation of glutamatergic receptors, and/or by activation of 5-HT₂ receptors. The resulting serotonin release displays a slow kinetic and a large diffusion. This process called volume transmission may ultimately affect the rate of discharge of serotonergic neurons, and their tonic activity. The therapeutic effects induced by serotonin-selective reuptake inhibitor (SSRI) antidepressants are initially triggered by blocking SERT but rely on consequences of chronic exposure, i.e., a selective desensitization of somatodendritic 5-HT_1A autoreceptors. Agonist stimulation of 5-HT_2B receptors mimicked behavioral and neurogenic SSRI actions, and increased extracellular serotonin in dorsal raphe. By contrast, a lack of effects of SSRIs was observed in the absence of 5-HT_2B receptors (knockout-KO), even restricted to serotonergic neurons (Htr2b^5-HTKO mice). The absence of 5-HT_2B receptors in serotonergic neurons is associated with a higher 5-HT_1A-autoreceptor reactivity and thus a lower firing activity of these neurons. In agreement, mice with overexpression of 5-HT_1A autoreceptor show decreased neuronal activity and increased depression-like behavior that is resistant to SSRI treatment. We propose thus that the serotonergic tone results from the opposite control exerted by somatodendritic (Gi-coupled) 5-HT_1A and (Gq-coupled) 5-HT_2B receptors on dorsal raphe neurons. Therefore, 5-HT_2B receptors may contribute to SSRI therapeutic effects by their positive regulation of adult raphe serotonergic neurons. Deciphering the molecular mechanism controlling extrasynaptic release of serotonin, and how autoreceptors interact in regulating the tonic activity of serotonergic neurons, is critical to fully understand the therapeutic effect of SSRIs.

Molecular imaging of serotonin degeneration in mild cognitive impairment

Neuropathological and neuroimaging studies have consistently demonstrated degeneration of monoamine systems, especially the serotonin system, in normal aging and Alzheimer's disease. The evidence for degeneration of the serotonin system in mild cognitive impairment is limited. Thus, the goal of the present study was to measure the serotonin transporter in vivo in mild cognitive impairment and healthy controls. The serotonin transporter is a selective marker of serotonin terminals and of the integrity of serotonin projections to cortical, subcortical and limbic regions and is found in high concentrations in the serotonergic cell bodies of origin of these projections (raphe nuclei). Twenty-eight participants with mild cognitive impairment (age 66.6±6.9, 16 males) and 28 healthy, cognitively normal, demographically matched controls (age 66.2±7.1, 15 males) underwent magnetic resonance imaging for measurement of grey matter volumes and high-resolution positron emission tomography with well-established radiotracers for the serotonin transporter and regional cerebral blood flow. Beta-amyloid imaging was performed to evaluate, in combination with the neuropsychological testing, the likelihood of subsequent cognitive decline in the participants with mild cognitive impairment. The following hypotheses were tested: 1) the serotonin transporter would be lower in mild cognitive impairment compared to controls in cortical and limbic regions, 2) in mild cognitive impairment relative to controls, the serotonin transporter would be lower to a greater extent and observed in a more widespread pattern than lower grey matter volumes or lower regional cerebral blood flow and 3) lower cortical and limbic serotonin transporters would be correlated with greater deficits in auditory-verbal and visual-spatial memory in mild cognitive impairment, not in controls. Reduced serotonin transporter availability was observed in mild cognitive impairment compared to controls in cortical and limbic areas typically affected by Alzheimer's disease pathology, as well as in sensory and motor areas, striatum and thalamus that are relatively spared in Alzheimer's disease. The reduction of the serotonin transporter in mild cognitive impairment was greater than grey matter atrophy or reductions in regional cerebral blood flow compared to controls. Lower cortical serotonin transporters were associated with worse performance on tests of auditory-verbal and visual-spatial memory in mild cognitive impairment, not in controls. The serotonin system may represent an important target for prevention and treatment of MCI, particularly the post-synaptic receptors (5-HT4 and 5-HT6), which may not be as severely affected as presynaptic aspects of the serotonin system, as indicated by the observation of lower serotonin transporters in MCI relative to healthy controls.

Effects of Anodal Transcranial Direct Current Stimulation and Serotonergic Enhancement on Memory Performance in Young and Older Adults

In the absence of effective therapies for dementia and its precursors, enhancing neuroplasticity by means of non-invasive brain stimulation such as anodal transcranial direct current stimulation (atDCS) might be a promising approach to counteract or delay the onset of cognitive decline, but effect sizes have been moderate so far. Previous reports indicate that increasing serotonin levels may enhance atDCS-induced neuroplasticity. However, evidence for serotonergic modulation of atDCS effects on memory is still lacking. Here, we conducted a double-blind, randomized, sham-/placebo-controlled trial to investigate the impact of a selective serotonin reuptake inhibitor (SSRI; single dose of 20 mg citalopram) and atDCS over the right temporoparietal cortex (1 mA, 20 min) on memory formation. Twenty young and 20 older subjects completed an object-location learning task in each of the four conditions: sham+placebo, sham+SSRI, atDCS+placebo, and atDCS+SSRI. Outcome measures were performance in immediate (primary outcome) and delayed cued recall. While we found an SSRI effect, but no statistically significant effect of atDCS on immediate recall scores, young and older adults benefited most from the combined application (comparisons: atDCS+SSRI>atDCS+placebo and atDCS+SSRI>sham+placebo). Thus, our data provide evidence that atDCS improves memory formation if serotonergic neurotransmission is enhanced simultaneously. Further studies are needed to assess whether these findings extend to clinical populations with memory impairment and translate into clinically relevant improvements after long-term serotonergic enhancement and repeated stimulation.

L-DOPA elicits non-vesicular releases of serotonin and dopamine in hemiparkinsonian rats in vivo

The control of the secretory activity of serotonergic neurons has been pointed out to reduce motor and non-motor side effects of the antiparkinsonian drug L-DOPA. This strategy deserves further investigation because it is presently unclear whether L-DOPA promotes a non-vesicular release of dopamine and serotonin from serotonergic neurons. To get a full neurochemical picture compatible with the existence of such a mechanism, we combined multisite intracerebral microdialysis, post mortem tissue measurement and single unit extracellular recordings in the dorsal raphe nucleus from hemiparkinsonian rats. L-DOPA (3-100mg/kg, ip.) non-homogeneously decreased extracellular serotonin levels in the striatum, substantia nigra pars reticulata, hippocampus and prefrontal cortex and homogenously serotonin tissue content in the striatum, cortex and cerebellum. L-DOPA (12mg/kg) did not modify the firing rate or pattern of serotonergic-like neurons recorded in the dorsal raphe nucleus. When focusing on serotonin release in the prefrontal cortex and the hippocampus, we found that L-DOPA (12 or 100mg/kg) enhanced serotonin extracellular levels in both regions upon Ca(2+) removal. Concomitantly, L-DOPA-stimulated dopamine release partly persisted in the absence of Ca(2+) in a region-dependent manner. Local application of the serotonin reuptake inhibitor citalopram (1µM) blunted the responses to L-DOPA (3-12mg/kg), measured as extracellular dopamine levels, most prominently in the hippocampus. These data stress that L-DOPA, already at low to moderate doses, promotes non-vesicular releases of serotonin and dopamine in a region-dependent manner.

Serotonergic modulation of intrinsic functional connectivity

Serotonin functions as an essential neuromodulator that serves a multitude of roles, most prominently balancing mood. Serotonergic challenge has been observed to reduce intrinsic functional connectivity in brain regions implicated in mood regulation. However, the full scope of serotonergic action on functional connectivity in the human brain has not been explored. Here, we show evidence that a single dose of a serotonin reuptake inhibitor dramatically alters functional connectivity throughout the whole brain in healthy subjects (n = 22). Our network-centrality analysis reveals a widespread decrease in connectivity in most cortical and subcortical areas. In the cerebellum and thalamus, however, we find localized increases. These rapid and brain-encompassing connectivity changes linked to acute serotonin transporter blockade suggest a key role for the serotonin transporter in the modulation of the functional macroscale connectome.

Chronic effects of antidepressants on serotonin release in rat raphe slice cultures: high potency of milnacipran in the augmentation of serotonin release

Most clinically-used antidepressants acutely increase monoamine levels in synaptic clefts, while their therapeutic effects often require several weeks of administration. Slow neuroadaptive changes in serotonergic neurons are considered to underlie this delayed onset of beneficial actions. Recently, we reported that sustained exposure of rat organotypic raphe slice cultures containing abundant serotonergic neurons to selective serotonin (5-HT) reuptake inhibitors (citalopram, fluoxetine and paroxetine) caused the augmentation of exocytotic serotonin release. However, the ability of other classes of antidepressants to evoke a similar outcome has not been clarified. In this study, we investigated the sustained actions of two tricyclic antidepressants (imipramine and desipramine), one tetracyclic antidepressant (mianserin), three 5-HT and noradrenaline reuptake inhibitors (milnacipran, duloxetine and venlafaxine) and one noradrenergic and specific serotonergic antidepressant (mirtazapine) on serotonin release in the slice cultures. For seven of nine antidepressants, sustained exposure to the agents at concentrations of 0.1-100 μ m augmented the level of increase in extracellular serotonin. The rank order of their potency was as follows: milnacipran>duloxetine>citalopram>venlafaxine>imipramine>fluoxetine>desipramine. Neither mirtazapine nor mianserin caused any augmentation. The highest augmentation by sustained exposure to milnacipran was partially attenuated by an α 1-adrenoceptor antagonist, benoxathian, while the duloxetine-, venlafaxine- and citalopram-mediated increases were not affected. These results suggest that inhibition of the 5-HT transporter is required for the enhancement of serotonin release. Furthermore, the potent augmentation by milnacipran is apparently due to the accompanied activation of the α 1-adrenoceptor.

Multisite intracerebral microdialysis to study the mechanism of L-DOPA induced dopamine and serotonin release in the parkinsonian brain

L-DOPA is currently one of the best medications for Parkinson's disease. It was assumed for several years that its benefits and side effects were related to the enhancement of dopamine release in the dopamine-depleted striatum. The use of intracerebral microdialysis combined with a pharmacological approach has led to the discovery that serotonergic neurons are responsible for dopamine release induced by L-DOPA. The subsequent use of multisite microdialysis has further revealed that L-DOPA-stimulated dopamine release is widespread and related to the serotonergic innervation. The present Review emphasizes the functional impact of extrastriatal release of dopamine induced by L-DOPA in both the therapeutic and side effects of L-DOPA.

Acute pharmacologically induced shifts in serotonin availability abolish emotion-selective responses to negative face emotions in distinct brain networks

Pharmacological manipulation of serotonin availability can alter the processing of facial expressions of emotion. Using a within-subject design, we measured the effect of serotonin on the brain's response to aversive face emotions with functional MRI while 20 participants judged the gender of neutral, fearful and angry faces. In three separate and counterbalanced sessions, participants received citalopram (CIT) to raise serotonin levels, underwent acute tryptophan depletion (ATD) to lower serotonin, or were studied without pharmacological challenge (Control). An analysis designed to identify distributed brain responses identified two brain networks with modulations of activity related to face emotion and serotonin level. The first network included the left amygdala, bilateral striatum, and fusiform gyri. During the Control session this network responded only to fearful faces; increasing serotonin decreased this response to fear, whereas reducing serotonin enhanced the response of this network to angry faces. The second network involved bilateral amygdala and ventrolateral prefrontal cortex, and these regions also showed increased activity to fear during the Control session. Both drug challenges enhanced the neural response of this set of regions to angry faces, relative to Control, and CIT also enhanced activity for neutral faces. The net effect of these changes in both networks was to abolish the selective response to fearful expressions. These results suggest that a normal level of serotonin is critical for maintaining a differentiated brain response to threatening face emotions. Lower serotonin leads to a broadening of a normally fear-specific response to anger, and higher levels reduce the differentiated brain response to aversive face emotions.

Serotonin modulation of cerebral glucose metabolism: sex and age effects

The serotonin system is implicated in a variety of psychiatric disorders whose clinical presentation and response to treatment differ between males and females, as well as with aging. However, human neurobiological studies are limited. Sex differences in the cerebral metabolic response to an increase in serotonin concentrations were measured, as well as the effect of aging, in men compared to women. Thirty-three normal healthy individuals (14 men/19 women, age range 20-79 years) underwent two resting positron emission tomography studies with the radiotracer [18F]-2-deoxy-2-fluoro-D-glucose ([(18)F]-FDG) after placebo and selective serotonin reuptake inhibitor (SSRI, citalopram) infusions on two separate days. Results indicated that women demonstrated widespread areas of increased cortical glucose metabolism with fewer areas of decrease in metabolism in response to citalopram. Men, in contrast, demonstrated several regions of decreased cortical metabolism, but no regions of increased metabolism. Age was associated with greater increases in women and greater decreases in men in most brain regions. These results support prior studies indicating that serotonin function differs in men and women across the lifespan. Future studies aimed at characterizing the influences of age and sex on the serotonin system in patients with psychiatric disorders are needed to elucidate the relationship between sex and age differences in brain chemistry and associated differences in symptom presentation and treatment response.

Effects of acute tryptophan depletion on brain serotonin function and concentrations of dopamine and norepinephrine in C57BL/6J and BALB/cJ mice

Acute tryptophan depletion (ATD) is a method of lowering brain serotonin (5-HT). Administration of large neutral amino acids (LNAA) limits the transport of endogenous tryptophan (TRP) across the blood brain barrier by competition with other LNAAs and subsequently decreases serotonergic neurotransmission. A recent discussion on the specificity and efficacy of the ATD paradigm for inhibition of central nervous 5-HT has arisen. Moreover, side effects such as vomiting and nausea after intake of amino acids (AA) still limit its use. ATD Moja-De is a revised mixture of AAs which is less nauseating than conventional protocols. It has been used in preliminary clinical studies but its effects on central 5-HT mechanisms and other neurotransmitter systems have not been validated in an animal model. We tested ATD Moja-De (TRP−) in two strains of mice: C57BL/6J, and BALB/cJ, which are reported to have impaired 5-HT synthesis and a more anxious phenotype relative to other strains of mice. ATD Moja-De lowered brain TRP, significantly decreased 5-HT synthesis as indexed by 5-HTP levels after decarboxlyase inhibition, and lowered 5-HT and 5-HIAA in both strains of mice, however more so in C57BL/6J than in BALB/cJ. Dopamine and its metabolites as well as norepinephrine were not affected. A balanced (TRP+) control mixture did not raise 5-HT or 5-HIAA. The present findings suggest that ATD Moja-De effectively and specifically suppresses central serotonergic function. These results also demonstrate a strain- specific effect of ATD Moja-De on anxiety-like behavior.

Imbalanced Dopaminergic Transmission Mediated by Serotonergic Neurons in L-DOPA-Induced Dyskinesia

L-DOPA-induced dyskinesias (LIDs) are one of the main motor side effects of L-DOPA therapy in Parkinson's disease. The review will consider the biochemical evidence indicating that the serotonergic neurons are involved in the dopaminergic effects of L-DOPA in the brain. The consequences are an ectopic and aberrant release of dopamine that follows the serotonergic innervation of the brain. After mid- to long-term treatment with L-DOPA, the pattern of L-DOPA-induced dopamine release is modified. In several brain regions, its effect is dramatically reduced while, in the striatum, its effect is quite preserved. LIDs could appear when the dopaminergic effects of L-DOPA fall in brain areas such as the cortex, enhancing the subcortical impact of dopamine and promoting aberrant motor responses. The consideration of the serotonergic system in the core mechanism of action of L-DOPA opens an important reserve of possible strategies to limit LIDs.

The acute and chronic effects of combined antipsychotic-mood stabilizing treatment on the expression of cortical and striatal postsynaptic density genes

The detection of changes in postsynaptic gene expression after the administration of mood stabilizers, alone or in combination with antipsychotics, and antidepressants in animal models of drug treatment, may represent a valuable strategy to explore the molecular targets of the mainstay treatments for bipolar disorder. In this study we investigated, in both acute and chronic paradigms, the expression of specific postsynaptic density genes (Homer1a, Homer1b/c, and PSD95) and genes putatively implicated in mood stabilizers mechanism of action (GSK3b, ERK) after administration of first (haloperidol) or second generation antipsychotics (quetiapine 30 mg/kg), alone or in combination with valproate. Moreover, we compared the effects of an antidepressant agent widely used in bipolar depression (citalopram) with a low dose of quetiapine (15 mg/kg), which has been demonstrated to display antidepressant action in bipolar depression. In striatal regions, Homer1a expression was strongly induced by haloperidol compared to all the other treatments. Haloperidol plus valproate also markedly induced Homer1a, but to a significant lesser extent than haloperidol alone. Also in the chronic paradigm haloperidol, but not haloperidol plus valproate, induced Homer1a expression in all the subregions of the caudate-putamen and in the nucleus accumbens core. The high dose of quetiapine significantly induced Homer1a in anterior cingulated, premotor and motor subregions of the cortex, and the extent of induction was significantly higher as compared to the lower dose. Oppositely, Homer1a expression was decreased in the cortex by citalopram acute administration. ERK gene was upregulated in cortex and striatum by the acute treatment with valproate and with the combination of haloperidol or quetiapine plus valproate, whereas no significant differences were noticed in GSK3b expression among treatments. PSD95 showed a significant upregulation by acute citalopram and by haloperidol plus valproate in both cortical and subcortical regions. Haloperidol and quetiapine 30 mg/kg, oppositely, significantly reduced the expression of the gene in the cortex. In conclusion, these results suggest that the combined treatment with a typical or an atypical antipsychotic plus valproate may induce differential modulation of postsynaptic genes expression when compared to the effects of these drugs individually administered.

Chronic L-DOPA therapy alters central serotonergic function and L-DOPA-induced dopamine release in a region-dependent manner in a rat model of Parkinson's disease

The therapeutic benefit of L-DOPA is commonly attributed to restoration of dopamine (DA) extracellular levels in the striatum of Parkinsonian patients. However, the loss of efficacy of L-DOPA after chronic use is paradoxically associated with a similar or enhanced striatal DA response. Release of L-DOPA-derived DA depends on the widespread serotonergic (5-HT) innervation in the brain. Chronic exposure of 5-HT neurons to L-DOPA could lead to aberrant neurochemical responses beyond the striatum. Using multi-site intracerebral microdialysis in a rat model of Parkinson's disease, we showed that chronic L-DOPA treatment at a therapeutic dose (12 mg/kg/day for 10 days) homogeneously reduced basal 5-HT release and metabolism. These effects were paralleled by a decrease in tissue content of 5-HT and its metabolite. Chronic L-DOPA treatment severely altered the brain pattern of 5-HT and DA release responses to L-DOPA (3-12 mg/kg) with an overall loss of efficacy of L-DOPA to increase DA release. Our data demonstrate for the first time in vivo that the impairment of 5-HT neuronal function induced by chronic L-DOPA alters in a region-dependent manner L-DOPA-induced DA release. Changes in neurochemical pattern of L-DOPA in the brain may favour the occurrence of both motor and non-motor side effects.

High-frequency stimulation of the subthalamic nucleus and L-3,4-dihydroxyphenylalanine inhibit in vivo serotonin release in the prefrontal cortex and hippocampus in a rat model of Parkinson's disease

High-frequency stimulation of the subthalamic nucleus (STN-HFS) and l-3,4-dihydroxyphenylalanine (l-DOPA) medication are the most used therapeutic approaches in Parkinson's disease (PD), but their beneficial motor effects are burdened by the emergence of cognitive and depressive disorders. Although a reduced serotonergic function has been linked to the psychiatric effects of antiparkinsonian treatments, biochemical evidence supporting this hypothesis is still lacking. By using a microdialysis approach in anesthetized rats, we investigated the ability of STN-HFS (130 Hz, 30 muA, 20 min) and l-DOPA (6-12 mg/kg) to change extracellular levels of serotonin (5-HT) monitored simultaneously in the prefrontal cortex (PFC) and hippocampus (HIPP), two brain regions involved in the regulation of mood and cognition that receive a distinct 5-HT innervation. The results show that STN-HFS inhibited 5-HT levels in the PFC and HIPP of sham-lesioned and 6-hydroxydopamine (6-OHDA)-lesioned rats. The effect elicited by STN-HFS was blocked by the administration of the 5-HT(1A) agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin. l-DOPA (6 and 12 mg/kg) reduced 5-HT levels in the PFC and HIPP of 6-OHDA rats. STN-HFS did not further decrease 5-HT levels induced by l-DOPA, but attenuated l-DOPA-induced dopamine release in the PFC and HIPP. These neurochemical data show that STN-HFS inhibits 5-HT release by modulating serotonergic neuron activity, while the decrease in 5-HT levels induced by l-DOPA may include its direct action inside serotonergic neurons. These results support the premise that antiparkinsonian treatments reduce central serotonergic transmission, which may favor the development of nonmotor side effects in PD.

Serotonin modulation of cerebral glucose metabolism in depressed older adults

BACKGROUND

Monoamine dysfunction, particularly of the serotonin system, has been the dominant hypothesis guiding research and treatment development in affective disorders. The majority of research has been performed in midlife depressed adults. The importance of understanding the neurobiology of depression in older adults is underscored by increased rates of mortality and completed suicide and an increased risk of Alzheimer's dementia. To evaluate the dynamic response of the serotonin system, the acute effects of citalopram infusion on cerebral glucose metabolism was measured in depressed older adults and control subjects. The hypothesis was tested that smaller decreases in metabolism would be observed in cortical and limbic regions in depressed older adults relative to control subjects.

METHODS

Sixteen depressed older adults and 13 control subjects underwent two resting positron emission tomography (PET) studies with the radiotracer [18F]-2-deoxy-2-fluoro-D-glucose after placebo and citalopram infusions.

RESULTS

In control subjects compared with depressed older adults, greater citalopram-induced decreases in cerebral metabolism were observed in the right anterior cingulate, middle temporal (bilaterally), left precuneus, and left parahippocampal gyri. Greater decreases in the depressed older adults than control subjects were observed in left superior and left middle frontal gyri and increases in left inferior parietal lobule, left cuneus, left thalamus, and right putamen.

CONCLUSIONS

In depressed older adults relative to control subjects, the cerebral metabolic response to citalopram is blunted in cortico-cortical and cortico-limbic pathways and increased in the left hemisphere (greater decrease interiorly and increases posteriorly). These findings suggest both blunted and compensatory cerebral metabolic responses to citalopram in depressed older adults.

Enhancement of cortical extracellular 5-HT by 5-HT1A and 5-HT2C receptor blockade restores the antidepressant-like effect of citalopram in non-responder mice

We recently found that the response of DBA/2 mice to SSRIs in the forced swim test (FST) was impaired and they also had a smaller basal and citalopram-stimulated increase in brain extracellular serotonin (5-HT) than 'responder' strains. We employed intracerebral microdialysis, FST and selective antagonists of 5-HT1A and 5-HT2C receptors to investigate whether enhancing the increase in extracellular 5-HT reinstated the anti-immobility effect of citalopram in the FST. WAY 100635 (0.3 mg/kg s.c.) or SB 242084 (1 mg/kg s.c.), respectively a selective 5-HT1A and 5-HT2C receptor antagonist, raised the effect of citalopram (5 mg/kg) on extracellular 5-HT in the medial prefrontal cortex of DBA/2N mice (citalopram alone 5.2+/-0.3 fmol/20 microl, WAY 100635+citalopram 9.9+/-2.1 fmol/20 microl, SB 242084+ citalopram 7.6+/-1.0 fmol/20 microl) to the level reached in 'responder' mice given citalopram alone. The 5-HT receptor antagonists had no effect on the citalopram-induced increase in extracellular 5-HT in the dorsal hippocampus. The combination of citalopram with WAY 100635 or SB 242084 significantly reduced immobility time in DBA/2N mice that otherwise did not respond to either drug singly. Brain levels of citalopram in mice given citalopram alone or with 5-HT antagonists did not significantly differ. The results confirm that impaired 5-HT transmission accounts for the lack of effect of citalopram in the FST and suggest that enhancing the effect of SSRIs on extracellular 5-HT, through selective blockade of 5-HT1A and 5-HT2C receptors, could be a useful strategy to restore the response in treatment-resistant depression.

Evidence for modulation of facial emotional processing bias during emotional expression decoding by serotonergic and noradrenergic antidepressants: an event-related potential (ERP) study

RATIONALE

Serotonergic (SSRI) and noradrenergic (NRI) antidepressants modulate biases in emotional processing such that perceptual bias is shifted away from negative and towards positive emotional material. However, the effects of serotonergic and noradrenergic modulation on the temporal course (occurring in milliseconds) of emotional processing, and in particular, the rapid physiological changes associated with the different stages of emotional processing, are unknown.

OBJECTIVE

The current study assessed the effects of acute serotonergic (i.e. with citalopram) and noradrenergic (i.e. with reboxetine) augmentation on event-related potential (ERP) measures associated with 'structural encoding' (N170) and emotion expression decoding (N250 and late positive potential [LPP]) for positive (happy) and negative (sad) facial stimuli relative to neutral facial stimuli.

MATERIALS AND METHODS

This study employed a double-blind, placebo-controlled, cross-over design in which 12 healthy male participants completed a facial expression recognition task tested under three acute conditions: (a) placebo, (b) citalopram (20 mg) and (c) reboxetine (4 mg).

RESULTS

Both citalopram and reboxetine had no effect on the N170 ERP component associated with structural encoding, but potentiated the N250 associated with happy (relative to neutral) emotional facial expression decoding. Both drugs had no valence effects on later ERP measures of emotion expression decoding (LPP).

CONCLUSIONS

Citalopram and reboxetine have selective effects on the temporal course of emotional processing with evidence to suggest specific effects on emotion expression decoding of positive (happy) emotional facial stimuli as evidenced by changes in the attention-modulated N250 but not structural encoding. These findings provide physiological evidence that antidepressants may shift perceptual biases in emotional processing away from negative and towards positive stimuli.

Antipsychotic and antidepressant co-treatment: effects on transcripts of inducible postsynaptic density genes possibly implicated in behavioural disorders

Selective serotonin reuptake inhibitors (SSRIs) and antipsychotics co-administration is a widely used strategy to treat both psychotic depression and depressive symptoms in schizophrenia. Nonetheless, the molecular mechanisms involved in the therapeutic benefits of antidepressant-antipsychotic combination are still elusive. It has been suggested that co-administration of SSRIs and antipsychotics may result in molecular changes different from their individual effects. In the present study, we evaluated the acute effects of two SSRIs, citalopram and escitalopram, alone or in combination with haloperidol, on the expression of Homer1a together with its splice variant ania-3, and p11, two genes linked respectively to dopaminergic and serotonergic neurotransmission and involved in synaptic plasticity. Homer1a and ania-3 were induced in the striatum by haloperidol, alone and in combination with SSRIs, but not by SSRIs only. Haloperidol+citalopram co-administration induced a stronger Homer1a expression than haloperidol alone in the ventrolateral caudate-putamen. No signal was detected for p11 in striatum, while there were no significant differences among treatments in cortical subregions. Homer1a was significantly down-regulated in the parietal cortex by all treatments. These results demonstrated that haloperidol+citalopram combination exerts synergistic effects on Homer expression, suggesting that citalopram may influence the impact by haloperidol on the dopaminergic neurotransmission. Moreover, present findings confirm that Homer1a and ania-3 are strongly induced in striatum by haloperidol, while they are not influenced by citalopram or escitalopram in this region. Oppositely, in the cortex the two transcripts are modulated by both haloperidol and SSRIs, suggesting a possible role of both dopamine and serotonin in their cortical regulation.

Acute effects of combining citalopram and pindolol on regional brain serotonin synthesis in sham operated and olfactory bulbectomized rats

The olfactory bulbectomized (OBX) rat is considered to be a good model of the pathology of human depression and also of the functional actions of antidepressant drug therapy. It has been proposed that antidepressant effects of selective serotonin reuptake inhibitors (SSRIs) can be accelerated by blocking 5-HT(1A/B) autoreceptors with pindolol. The underlying mechanism is thought to involve acute unrestricting of 5-HT release and, consequently, relatively enhanced 5-HT turnover throughout the forebrain serotonergic networks. The effect of this combination on 5-HT turnover in sham operated or OBX rats can be assessed at the level of 5-HT synthesis, a very important presynaptic step in serotonergic neurotransmission, using the alpha-[(14)C]methyl-l-tryptophan autoradiography method. In sham rats, acute citalopram (20mg/kg) treatment increased synthesis at almost all serotonergic terminal regions but slightly decreased synthesis at serotonergic cell body regions (i.e. dorsal and median (not significant) raphe; approximately 16%). Combining pindolol (10mg/kg) with citalopram further increased synthesis at many regions in sham rats (relative to treatment with only citalopram). In OBX rats, citalopram decreased synthesis at a few terminal regions and greatly decreased synthesis at the dorsal and median raphe ( approximately 45%; relative to OBX rats treated with saline). Combining pindolol with citalopram greatly increased synthesis at almost all regions in OBX rats (relative to treatment with only citalopram). These results suggest that acute citalopram effects result in elevated terminal 5-HT synthesis, but these effects are restrained by 5-HT(1A/B) autoreceptor feedback to different degrees in sham and OBX rats. Moreover, 5-HT(1A/B) autoreceptor feedback is stronger in OBX rats and may underlie the delay of SSRI effects in OBX rats and, correspondingly, in human depression. Pindolol acceleration and augmentation of SSRI antidepressant therapy for human depression may be mediated by attenuation of 5-HT(1A/B) autoreceptor feedback, permitting unhindered SSRI effects on serotonergic terminals.

Delta(9)-tetrahydrocannabinol prolongs the immobility time in the mouse forced swim test: involvement of cannabinoid CB(1) receptor and serotonergic system

In the present study, we investigated the effect of Delta(9)-tetrahydrocannabinol (THC), the principal psychoactive component of marijuana, on immobility time during the forced swim test. THC (2 and 6 mg/kg, i.p.) significantly prolonged the immobility time. In addition, THC at the same doses did not significantly affect locomotor activity in the open-field test. The selective cannabinoid CB(1) receptor antagonist rimonabant (3 mg/kg, i.p.) significantly reduced the enhancement of immobility by THC (6 mg/kg). Similarly, the selective serotonin (5-HT) reuptake inhibitor (SSRI) citalopram (10 mg/kg, i.p.) and 5-HT(1A/7) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.3 mg/kg, i.p.) significantly reduced this THC-induced effect. Moreover, the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide dihydrochloride (WAY100635, 1 mg/kg, i.p.) and the postsynaptic 5-HT(1A) receptor antagonist MM-77 (0.1 mg/kg, i.p.) reversed this reduction effect of 8-OH-DPAT (0.3 mg/kg). In contrast, the selective 5-HT(7) receptor antagonist (R)-3-[2-[2-(4-methylpiperidin-1-yl)ethyl]pyrrolidine-1-sulfonyl]phenol hydrochloride (SB269970) had no effect on this reduction effect of 8-OH-DPAT. WAY100635 (1 mg/kg) also reversed the reduction effect of citalopram (10 mg/kg). These findings suggest that the 5-HT(1A) receptors are involved in THC-induced enhancement of immobility.

Brain metabolic changes following 4-week citalopram infusion: increased 18FDG uptake and gamma-amino butyric acid levels

We used 2-week and 4-week citalopram infusion (10 mg/kg/day) to determine how this selective serotonin reuptake inhibitor (SSRI) would alter 2-deoxy-2-[18F]-fluoro-D-glucose (18FDG) uptake and neurotransmitter tissue levels in male Sprague-Dawley rodents. A weekly time course of 18FDG uptake altered by chronic citalopram treatment was determined in vivo with small animal positron emission tomography (microPET). Additionally, end of study monoamine levels were measured ex vivo using high pressure liquid chromatography (HPLC) and amino acid levels were determined ex vivo with proton nuclear magnetic resonance spectroscopy (1H-NMRS). We found increased striatal 18FDG uptake, reduced tissue levels of noradrenaline and serotonin in the striatum and prefrontal cortex, and increased striatal gamma-amino-butyric acid following 4-week citalopram infusion.

Strain differences in basal and post-citalopram extracellular 5-HT in the mouse medial prefrontal cortex and dorsal hippocampus: relation with tryptophan hydroxylase-2 activity

We used the microdialysis technique to compare basal extracellular serotonin (5-HT) and the response to citalopram in different strains of mice with functionally different allelic forms of tryptophan hydroxylase-2 (TPH-2), the rate-limiting enzyme in brain 5-HT synthesis. DBA/2J, DBA/2N and BALB/c mice carrying the 1473G allele of TPH-2 had less dialysate 5-HT in the medial prefrontal cortex and dorsal hippocampus (DH) (20-40% reduction) than C57BL/6J and C57BL/6N mice carrying the 1473C allele. Extracellular 5-HT estimated by the zero-net flux method confirmed the result of conventional microdialysis. Citalopram, 1.25, 5 and 20 mg/kg, dose-dependently raised extracellular 5-HT in the medial prefrontal cortex of C57BL/6J mice, with maximum effect at 5 mg/kg, but had significantly less effect in DBA/2J and BALB/c mice and in the DH of DBA/2J mice. A tryptophan (TRP) load enhanced basal extracellular 5-HT in the medial prefrontal cortex of DBA/2J mice but did not affect citalopram's ability to raise cortical and hippocampal extracellular 5-HT. The impairment of 5-HT synthesis quite likely accounts for the reduction of basal 5-HT and the citalopram-induced rise in mice carrying the mutated enzyme. These findings might explain why DBA/2 and BALB/c mice do not respond to citalopram in the forced swimming test. Although TRP could be a useful strategy to improve the antidepressant effect of citalopram (Cervo et al. 2005), particularly in subjects with low 5-HT synthesis, the contribution of serotonergic and non-serotonergic mechanisms to TRP's effect remains to be elucidated.

Oxytocin Involvement in SSRI-Induced Delayed Ejaculation: A Review of Animal Studies

INTRODUCTION

Selective serotonin reuptake inhibitors (SSRIs) differ in the severity of induced ejaculation delay. Various studies indicate that oxytocin is involved in sexual behavior.

AIM

To review and evaluate the involvement of oxytocin in SSRI-induced ejaculation delay.

MAIN OUTCOME MEASURES

Oxytocine release, 5-hydroxytryptamine (5-HT) neurotransmission, and desensitization of 5-HT(1A) receptors.

METHODS

A review and critical analysis of animal studies investigating the interaction of serotonergic and oxytocinergic neurotransmission in relation to the ejaculation process.

RESULTS

Although acute treatment with the SSRIs fluoxetine and paroxetine immediately causes increased serotonin levels, delayed ejaculation does not occur. The increased serotonin levels induce oxytocin release via activation of 5-HT(1A) receptors, and this might compensate for the inhibitory actions of serotonin on sexual behavior. Chronic treatment with fluoxetine and paroxetine desensitizes 5-HT(1A) receptors on oxytocin neurons, and that might in part determine the onset of delayed ejaculation. Desensitization of 5-HT(1A) receptors is less strong following chronic treatment with the SSRIs fluvoxamine or citalopram, which may attenuate the degree of delayed ejaculation.

CONCLUSIONS

Preliminary data suggest that the severity of chronic SSRI treatment-induced delayed ejaculation and the differences between the various SSRIs in inducing ejaculation delay is related to gradual desensitization of 5-HT(1A) receptors on oxytocin neurons.

Dorsal raphe vs. median raphe serotonergic antagonism. Anatomical, physiological, behavioral, neuroendocrinological, neuropharmacological and clinical evidences: relevance for neuropharmacological therapy

Monoaminergic neurons located in the central nervous system (CNS) are organized into complex circuits which include noradrenergic (NA), adrenergic (Ad), dopaminergic (DA), serotonergic (5-HT), histaminergic (H), GABA-ergic and glutamatergic systems. Most of these circuits are composed of more than one and often several types of the above neurons. Such physiologically flexible circuits respond appropriately to both external and internal stimuli which, if not modulated adequately, can trigger pathophysiologic responses. A great deal of research has been devoted to mapping the multiple functions of the CNS circuitry, thereby forming the basis for effective neuropharmacological therapeutic approaches. Such lineal strategies that seek to normalize complex and mixed physiological disorders, however, meet only partial therapeutic success and are often followed by undesirable side effects and/or total failure. In light of these, we have worked to develop possible models of CNS circuitry that are less affected by physiological interaction using the models to design more effective therapeutic approaches. In the present review, we cite and present evidence supporting the dorsal raphe versus median raphe serotonergic circuitry as one model of a reliable paradigm, necessary to the clear understanding and therapy of many psychiatric and even non-psychiatric disturbances.

Investigation of the SSRI augmentation properties of 5-HT2 receptor antagonists using in vivo microdialysis

Recent evidence that 5-HT(2) receptors exert a negative influence on central 5-hydroxytryptamine (5-HT) neurones suggests that 5-HT(2) receptor antagonists may augment the effects of serotonin selective reuptake inhibitors (SSRIs). The present study investigated whether pre-treatment with 5-HT(2) receptor antagonists enhances the effect of SSRI administration on hippocampal extracellular 5-HT of freely moving rats. Administration of the SSRI citalopram at a low (2mg kg(-1)) and higher (4 mg kg(-1)) dose, increased dialysate 5-HT by 5- and 8-fold, respectively. Pre-treatment with the 5-HT(2) receptor antagonist ketanserin (4 mg kg(-1)) augmented the effect of 4 mg kg(-1) but not 2mg kg(-1) citalopram. The effect of 4 mg kg(-1) citalopram was also augmented by pre-treatment with either the 5-HT(2C) receptor antagonist SB 242084 (0.5mg kg(-1)) or the 5-HT(2A) receptor antagonist MDL 100907 (0.5mg kg(-1)). As with citalopram, fluoxetine elevated dialysate 5-HT at both a low (5mg kg(-1)) and higher (20mg kg(-1)) dose. However, neither dose of fluoxetine was augmented by ketanserin (4 mg kg(-1)). These results confirm recent findings that 5-HT(2) receptor antagonists augment the effect of citalopram on extracellular 5-HT, and indicate the involvement of 5-HT(2C) and possibly 5-HT(2A) receptors. The lack of augmentation of fluoxetine might reflect the intrinsic 5-HT(2) receptor antagonist properties of this drug.

Augmentation by citalopram of risperidone-induced monoamine release in rat prefrontal cortex

RATIONALE

A typical antipsychotics (APDs), e.g. olanzapine and risperidone, have been reported to be effective adjunctive treatment for depression if selective serotonin (5-HT) reuptake inhibitors (SSRIs) alone are ineffective.

OBJECTIVES AND METHODS

We utilized microdialysis in awake, freely moving rats to study the effect of risperidone in combination with citalopram, an SSRI, on extracellular 5-HT, dopamine (DA), and norepinephrine (NE) efflux in rat medial prefrontal cortex (mPFC).

RESULTS

Risperidone (1.0 mg/kg, s.c.), given alone, significantly increased 5-HT, DA, and NE concentrations in the mPFC. Citalopram (10 mg/kg, s.c.), by itself, produced a significant increase in 5-HT levels only. The combination of risperidone and citalopram produced significantly greater increases in efflux of both DA and NE than risperidone alone. However, the effect of this combination on extracellular 5-HT concentrations was not significantly different than that of citalopram alone. The augmentation of DA and NE efflux induced by risperidone plus citalopram could be partially blocked by the selective 5-HT1A antagonist, WAY 100635 (0.2 mg/kg, s.c.).

CONCLUSIONS

The results suggest that the ability of atypical APDs to augment the therapeutic efficacy of SSRIs in major depression and treatment-resistant depression may be due, at least in part, to potentiation of SSRI-induced increases in cortical DA and NE. The contributions of 5-HT1A receptor stimulation and 5-HT2A and alpha2 adrenergic receptor antagonism to this augmentation are discussed.

The effect of chronic selective serotonin reuptake inhibitor treatment on serotonin 1B receptor sensitivity and HPA axis activity

The authors have investigated 5-HT 1B receptor function in prefrontal cortex and dorsal hippocampus as well as the HPA axis response after subchronic (24 h) and chronic (15 days) treatment with the SSRI citalopram. All experiments were carried out in presence of citalopram to prevent rapid resensitization of the 5-HT(1B) receptors. Moreover, this more closely resembles the clinical situation. The concentration of citalopram was measured in both brain areas to ensure comparable levels in the different treatment groups. Using microdialysis, the authors found that under those conditions the effect of the 5-HT 1B receptor antagonists SB 224289 and the mixed 5-HT 1B/1D receptor antagonist GR 127935 on extracellular levels of 5-HT was unaltered by duration of treatment. Basal levels of 5-HT, however, were increased in the dorsal hippocampus following chronic treatment. In addition, plasma levels of the catecholamines adrenaline and noradrenaline and the HPA axis hormones ACTH and corticosterone were all decreased after chronic treatment.

The 5-HT1A receptor modulates the effects of cocaine on extracellular serotonin and dopamine levels in the nucleus accumbens

The regulation of extracellular levels of serotonin (5-HT) and dopamine in response to cocaine by 5-HT1A receptors was examined using in vivo microdialysis and the 5-HT1A receptor antagonist 4-(2'-methoxy-)-phenyl-1-[2'-(N-2''-pyridinyl)-p-fluorobenzamido-]ethyl-piperazine (p-MPPF). Pretreatment with p-MPPF significantly augmented the increase in extracellular levels of both 5-HT and dopamine in the nucleus accumbens produced by systemic administration of cocaine. Levels of 5-HT or dopamine were unaffected by p-MPPF given alone. Extracellular levels of 5-HT and dopamine were increased dramatically by cocaine infused locally into the nucleus accumbens. Systemic injection of cocaine given during the cocaine infusion reduced 5-HT and dopamine levels, presumably by activating inhibitory 5-HT and dopamine autoreceptors outside of the locus of infusion. The reduction of 5-HT and dopamine levels by systemic cocaine during accumbal infusion was blocked by pretreatment with the 5-HT1A receptor antagonist p-MPPF. Taken together, these findings suggest that the 5-HT1A autoreceptor acts to modulate the effects of cocaine on both 5-HT and dopamine levels in the nucleus accumbens.

Gene dose-dependent alterations in extraneuronal serotonin but not dopamine in mice with reduced serotonin transporter expression

Serotonin (5-HT) plays an integral regulatory role in mood, anxiety, cognition, appetite and aggressive behavior. Many therapeutic and illicit drugs that modulate these functions act at the serotonin transporter (SERT), thus a mouse model with reduced transporter expression was created to further investigate the effects of differential serotonin reuptake. In the present study, in vivo microdialysis was used to determine homeostatic alterations in extracellular 5-HT levels in unanesthetized SERT knockout mice. SERT(-/-) mice had significantly higher levels of basal dialysate 5-HT than SERT(+/+) mice in striatum and frontal cortex. In addition, although gene-specific increases in 5-HT were evident, neuroadaptive alterations in dialysate dopamine levels were not detected in striatum. Zero net flux microdialysis was utilized to further investigate alterations in extracellular 5-HT. Using this method, a gene dose-dependent increase in extraneuronal 5-HT was observed in striatum (2.8 +/- 1, 9.4 +/- 1 and 18 +/- 3 nM) and frontal cortex (1.4 +/- 0.4, 3.5 +/- 0.9 and 14 +/- 1 nM) in SERT(+/+), SERT(+/-) and SERT(-/-) mice, respectively. Potassium stimulation revealed greater depolarization-induced increases in striatal 5-HT but not dopamine in SERT(-/-) mice. Furthermore, dialysate 5-hydroxyindoleacetic acid (5-HIAA) levels were reduced in striatum in a gene dose-dependent manner, while DOPAC was unchanged in SERT knockout mice. Finally, determination of monoamine oxidase (MAO) activity revealed no significant differences in KM or Vmax of type-A or type-B isozymes indicating that alterations in SERT expression do not cause adaptive changes in the activities of these key catabolic enzymes. Overall, these results demonstrate that constitutive reductions in SERT are associated with increases in 5-HT in the extracellular signaling space in the absence of changes in dopamine neurochemistry. Furthermore, use of zero net flux microdialysis appears warranted in investigations of serotonergic synaptic function where modest changes in extracellular 5-HT are thought to occur in response to altered uptake.

Effects of serotonin transporter promoter polymorphisms on serotonin function

The serotonin transporter promoter polymorphism (5-HTTLPR) has been associated with vulnerability to stress-induced depressive symptoms and with the speed and rate of response to antidepressant treatment. The goal of the present study was to evaluate the association between the 5-HTTLPR and the functional response of the serotonin system as measured by the neuroendocrine and cerebral metabolic response to intravenous administration of the selective serotonin reuptake inhibitor citalopram in normal control subjects. Genotyping was performed for 5-HTTLPR insertion/deletion polymorphism long (l) and short (s) variant alleles. The ll genotype was compared with the combined sl+ss and with the ss genotype alone. Citalopram plasma concentrations did not differ significantly between groups. The s allele was associated with a less of an increase in prolactin and cortisol than the ll genotype. The s allele was associated with greater decreases in left frontal, precentral and middle temporal gyri compared to the ll genotype. The ll genotype was associated with greater decreases in right frontal, insula and superior temporal gyrus compared to the ss genotype. These findings suggest that 5-HTTLPR is associated with an altered functional response of the serotonin system, which may represent a neurobiologic substrate for the differential response to antidepressant treatment in late life and the emergence of neuropsychiatric symptoms in neurodegenerative disorders.

Electrophysiological evidence for the tonic activation of 5-HT(1A) autoreceptors in the rat dorsal raphe nucleus

Serotonin (5-hydroxytryptamine, 5-HT) and norepinephine (NE) neurons have reciprocal connections. These may thus interfere with anticipated effects of selective pharmacological agents targeting these neurons. The main goal of the present study was to assess whether the somatodendritic 5-HT(1A) autoreceptor is tonically activated by endogenous 5-HT in anesthetised rats, using in vivo extracellular unitary recordings. In rats with their NE neurons lesioned using 6-hydroxydopamine (6-OHDA) and in controls administered the NE reuptake inhibitor desipramine to suppress NE neuronal firing, the alpha2-adrenoceptor agonist clonidine no longer inhibited 5-HT neuron firing, therefore indicating the important modulation of the firing activity of 5-HT neurons by NE neurons. In control rats, the administration of the potent and selective 5-HT(1A) receptor antagonist WAY 100,635 ((N-[2-[4(2-methoxyphenyl)-1-piperazinyl]ethy]-N-(2-pyridinyl)cyclohexanecarboxamide trihydroxychloride) (100 microg/kg, i.v.) did not modify the spontaneous firing activity of 5-HT neurons, but in NE-lesioned rats using either 6-OHDA or DSP-4, WAY 100,635 produced a mean firing increase of 80 and 69%, respectively. When desipramine and D-amphetamine were used in control rats to prevent alterations in the availability of NE in the dorsal raphe, again WAY 100,635 produced a significant disinhibition of the firing of 5-HT neurons (83 and 53%, respectively). These data support the notion that the NE system tonically activates the firing activity of 5-HT neurons. When the fluctuations of the function of NE neurons normally produced by WAY 100,635 were prevented, a tonic activation of 5-HT(1A) autoreceptors by endogenous 5-HT was unmasked.

Role of presynaptic alpha2-adrenoceptors in antidepressant action: recent findings from microdialysis studies

The therapeutic effect of an antidepressant drug takes at least 2 to 3 weeks to develop and a significant proportion of patients have no or only partial benefit regardless of the class of antidepressant used. Research into the neurobiological basis of antidepressant action has suggested new strategies to improve the antidepressant effect. Recent microdialysis studies show that hypofunction of the presynaptic autoreceptors enhances the increase of extracellular serotonin (5-HT) induced by selective serotonin reuptake inhibitors (SSRIs) so it has been suggested that the antidepressant effect may be speeded up by blockade of the autoreceptors. The similarity between the synaptic mechanisms controlling serotonergic and noradrenergic transmission has stimulated preclinical research into the role of presynaptic alpha(2)-adrenoceptors in the effect of noradrenaline (NA) reuptake inhibitors (NRIs) on NA availability at central synapses. The microdialysis studies reviewed here indicate that NRIs including desipramine, reboxetine and atomoxetine, the mixed 5-HT/NA reuptake inhibitors sibutramine, duloxetine, venlafaxine or the NA/DA reuptake inhibitor amineptine, increased extracellular NA in various regions of the rat brain. The effect was enhanced by chronic treatment and even more by the co-administration of alpha(2)-adrenoceptor antagonists. The results support the theory that desensitization of the alpha(2)-adrenoceptor contributes to enhancing the effect of NRIs seen after chronic administration and may account for the slow onset of the antidepressant effect. Finally, they suggest that co-administration of an alpha(2)-adrenoceptor antagonist may improve the therapeutic effect of NRI.

Effects of chronic treatment with escitalopram or citalopram on extracellular 5-HT in the prefrontal cortex of rats: role of 5-HT1A receptors

1 Microdialysis was used to study the acute and chronic effects of escitalopram (S-citalopram; ESCIT) and chronic citalopram (CIT), together with the 5-HT1A receptor antagonist WAY100,635 (N-[2-[methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride) and the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on extracellular 5-hydroxytryptamine (5-HT) levels in the rat prefrontal cortex. 2 Extracellular 5-HT rose to 234 and 298% of basal values after subcutaneous (s.c.) acute doses of 0.15 and 0.63 mg kg(-1) ESCIT. No further increase was observed at 2.5 mg kg(-1) ESCIT (290%). 3 The effect of 13-day s.c. infusion of 10 mg kg(-1) day(-1) ESCIT on extracellular 5-HT (422% of baseline) was greater than after 2 days (257% of baseline), whereas exposure to ESCIT was similar. In contrast, the increase in extracellular 5-HT induced by the infusion of CIT for 2 (306%) and 13 days (302%) was similar. However, brain and plasma levels of S-citalopram in rats infused with CIT for 13 days were lower than after 2 days. 4 Acute treatment with 2.5 mg kg(-1) ESCIT or 5 mg kg(-1) CIT raised extracellular 5-HT by 243 and 276%, respectively, in rats given chronic vehicle but had no effect in rats given ESCIT (10 mg kg(-1) day(-1)) or CIT (20 mg kg(-1) day(-1)) for 2 or 13 days, suggesting that the infused doses had maximally increased extracellular 5-HT. WAY100,635 (0.1 mg kg(-1) s.c.) increased extracellular 5-HT levels by 168, 174 and 169% of prechallenge values in rats infused with vehicle or ESCIT for 2 or 13 days, respectively. WAY100,635 enhanced extracellular 5-HT levels to 226, 153 and 164% of prechallenge values in rats infused with vehicle or CIT for 2 and 13 days, respectively. 5 8-OH-DPAT (0.025 mg kg(-1)) reduced extracellular 5-HT by 54% in control rats, but had no effect in those given ESCIT and CIT for 13 days. 6 This series of experiments led to the conclusion that chronic treatment with ESCIT desensitizes the 5-HT1A receptors, regulating the release of 5-HT in the prefrontal cortex and enhances the effect of the drug on extracellular 5-HT. They also indicate that chronic treatment with ESCIT and CIT did not prevent WAY100,635 from raising extracellular 5-HT.

Differential neurochemical properties of central serotonergic transmission in Roman high- and low-avoidance rats

The selective breeding of Roman high- (RHA/Verh) and low-avoidance (RLA/Verh) rats for rapid versus poor acquisition of active avoidant behaviour has produced two behavioural phenotypes with different performances in a variety of animal models of anxiety, in which RLA/Verh rats are consistently more fearful than RHA/Verh rats. In addition, these two lines display different functional properties of brain neurotransmitters like serotonin (5-HT), known to be involved in the expression of anxiety- and depression-related behaviours. Therefore, we used brain microdialysis and [3H]-citalopram binding autoradiography to characterize further the neurochemical properties of 5-HTergic transmission in the two lines. No significant line-related differences were detected in the basal 5-HT output in the frontoparietal cortex (FPCx). In contrast, the increase in the cortical 5-HT output elicited by the systemic administration or the local application, via reverse dialysis, of chlorimipramine and fluoxetine was more robust in RHA/Verh than in RLA/Verh rats. Moreover, the binding signal of [3H]-citalopram to 5-HT re-uptake sites was more intense in the FPCx of RHA/Verh rats than in their RLA/Verh counterparts. These findings suggest that the functional tone of the 5-HTergic projection to the FPCx is stronger in the RHA/Verh line relative to the RLA/Verh line. It is proposed that RLA/Verh rats may be used as a model with heuristic value for studying the role of 5-HTergic transmission in anxiety and in the anxiolytic effects of monoamine re-uptake inhibitors.

Use of Arc expression as a molecular marker of increased postsynaptic 5-HT function after SSRI/5-HT1A receptor antagonist co-administration

An increase in central postsynaptic 5-hydroxytryptamine (5-HT) function activates expression of activity-related cytoskeletal protein (Arc). Here, Arc expression was used to test whether, in rats, co-administration of a 5-HT re-uptake inhibitor (paroxetine) and a 5-HT1A receptor antagonist (WAY 100635) increases postsynaptic 5-HT function. After pre-treatment with WAY 100635 (0.3 mg/kg s.c.), paroxetine (5 mg/kg s.c.) caused a threefold increase in 5-HT in prefrontal cortex microdialysates. In situ hybridization studies found that neither paroxetine (5 mg/kg s.c.) nor WAY 1000635 (0.3 mg/kg s.c.) altered Arc mRNA abundance in any region examined. In contrast, paroxetine (5 mg/kg s.c.) increased Arc mRNA after pre-treatment with WAY 100635 (0.3 mg/kg s.c.). This increase was apparent in cortical regions (frontal, parietal and cingulate) and caudate nucleus but was absent in hippocampus (CA1). Increases in Arc mRNA were accompanied by an increase in c-fos mRNA. The increase in Arc expression induced by paroxetine/WAY 100635 was abolished by the 5-HT synthesis inhibitor, p-chlorophenylalanine (300 mg/kg i.p., daily for two days). In conclusion, paroxetine and WAY 100635 injected in combination (but not alone) caused a region-specific, 5-HT-mediated increase in Arc expression. These data provide molecular evidence that co-administration of a 5-HT re-uptake inhibitor and 5-HT1A receptor antagonist increases 5-HT function at the postsynaptic level.

Noradrenergic and serotonergic blockade inhibits BDNF mRNA activation following exercise and antidepressant

Antidepressants and physical exercise have been shown to increase the transcription of hippocampal brain-derived neurotrophic factor (BDNF). Much evidence regarding the initial actions of antidepressant medications as well as exercise leads to the hypothesis that noradrenergic (NE) and/or serotonergic (5-HT) activation is a key element in the BDNF transcriptional elevation common to both interventions. Currently, we used short-term beta-adrenergic, 5-HT(1A), or 5-HT(2A/C) receptor blockade to characterize the influence of NE and 5-HT systems on BDNF transcription during physical exercise and antidepressant treatment. In situ hybridization revealed that beta-adrenergic blockade significantly blunted the BDNF mRNA elevations due to exercise, and also inhibited the modest elevations in the CA3 and dentate gyrus following short-term treatment with tranylcypromine. In contrast, 5-HT(2A/C) blockade only minimally altered exercise-induced BDNF mRNA levels, but inhibited up-regulation of BDNF transcription via tranylcypromine. Finally, 5-HT(1A) blockade did not inhibit exercise-induced BDNF mRNA elevations, but significantly enhanced levels above those achieved with exercise alone in the CA4. These results suggest that NE activation via beta-adrenergic receptors may be essential for both exercise and antidepressant-induced BDNF regulation. 5-HT(1A) and 5-HT(2A/C) activation, on the other hand, appear to be most important for antidepressant-induced BDNF regulation, but may also participate significantly in exercise-induced regulation in the CA4.