5-HT1B Autoreceptors limit the effects of selective serotonin re-uptake inhibitors in mouse hippocampus and frontal cortex

Altered Serotonin 1B Receptor Binding After Escitalopram for Depression Is Correlated With Treatment Effect

BACKGROUND

Major depressive disorder (MDD) is commonly treated with selective serotonin reuptake inhibitors (SSRIs). SSRIs inhibit the serotonin transporter (5-HTT), but the downstream antidepressant mechanism of action of these drugs is poorly understood. The serotonin 1B (5-HT1B) receptor is functionally linked to 5-HTT and 5-HT1B receptor binding and 5-HT1B receptor mRNA is reduced in the raphe nuclei after SSRI administration in primates and rodents, respectively. The effect of SSRI treatment on 5-HT1B receptor binding in patients with MDD has not been examined previously. This positron emission tomography (PET) study aimed to quantify brain 5-HT1B receptor binding changes in vivo after SSRI treatment for MDD in relation to treatment effect.

METHODS

Eight unmedicated patients with moderate to severe MDD underwent PET with the 5-HT1B receptor radioligand [11C]AZ10419369 before and after 3 to 4 weeks of treatment with the SSRI escitalopram 10 mg daily. Depression severity was assessed at time of PET and after 6 to 7 weeks of treatment with the Montgomery-Åsberg Depression Rating Scale.

RESULTS

We observed a significant reduction in [11C]AZ10419369 binding in a dorsal brainstem (DBS) region containing the median and dorsal raphe nuclei after escitalopram treatment (P = .036). Change in DBS [11C]AZ10419369 binding correlated with Montgomery-Åsberg Depression Rating Scale reduction after 3-4 (r = 0.78, P = .021) and 6-7 (r = 0.94, P < .001) weeks' treatment.

CONCLUSIONS

Our findings align with the previously reported reduction of 5-HT1B receptor binding in the raphe nuclei after SSRI administration and support future studies testing change in DBS 5-HT1B receptor binding as an SSRI treatment response marker.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Serotonergic neurons in the treatment of mood disorders: The dialogue with astrocytes

Astrocytes were traditionally regarded as cells important to neuronal activity, providing both metabolic and structural supports. Recent evidence suggests that they may also play a crucial role in the control of higher brain functions. In keeping with this hypothesis, it is now well accepted that astrocytes contribute to stress but also react to antidepressant drugs as they express serotonergic transporters and receptors. However, the downstream mechanisms leading to the fine-tuned regulation of mood are still unknown. This chapter pays attention to the role of astrocytes in the regulation of emotional behavior and related serotonergic neurotransmission. In particular, it gives a current state of the clinical and preclinical evidence showing that astrocytes respond to environmental conditions and antidepressant drugs through the release of gliotransmitters and neurotrophic factors which in turn, influence serotonergic tone in discrete brain areas. This state-of-the-art review aims at demonstrating the remarkable potential for novel therapeutic antidepressant strategies targeting these glial cells.

The 5-HT1B receptor - a potential target for antidepressant treatment

Major depressive disorder (MDD) is the leading cause of disability worldwide. The serotonin hypothesis may be the model of MDD pathophysiology with the most support. The majority of antidepressants enhance synaptic serotonin levels quickly, while it usually takes weeks to discern MDD treatment effect. It has been hypothesized that the time lag between serotonin increase and reduction of MDD symptoms is due to downregulation of inhibitory receptors such as the serotonin 1B receptor (5-HT1BR). The research on 5-HT1BR has previously been hampered by a lack of selective ligands for the receptor. The last extensive review of 5-HT1BR in the pathophysiology of depression was published 2009, and based mainly on findings from animal studies. Since then, selective radioligands for in vivo quantification of brain 5-HT1BR binding with positron emission tomography has been developed, providing new knowledge on the role of 5-HT1BR in MDD and its treatment. The main focus of this review is the role of 5-HT1BR in relation to MDD and its treatment, although studies of 5-HT1BR in obsessive-compulsive disorder, alcohol dependence, and cocaine dependence are also reviewed. The evidence outlined range from animal models of disease, effects of 5-HT1B receptor agonists and antagonists, case-control studies of 5-HT1B receptor binding postmortem and in vivo, with positron emission tomography, to clinical studies of 5-HT1B receptor effects of established treatments for MDD. Low 5-HT1BR binding in limbic regions has been found in MDD patients. When 5-HT1BR ligands are administered to animals, 5-HT1BR agonists most consistently display antidepressant-like properties, though it is not yet clear how 5-HT1BR is best approached for optimal MDD treatment.

Regulation of Hippocampal 5-HT Release by P2X7 Receptors in Response to Optogenetic Stimulation of Median Raphe Terminals of Mice

Serotonergic and glutamatergic neurons of median raphe region (MRR) play a pivotal role in the modulation of affective and cognitive functions. These neurons synapse both onto themselves and remote cortical areas. P2X7 receptors (P2rx7) are ligand gated ion channels expressed by central presynaptic excitatory nerve terminals and involved in the regulation of neurotransmitter release. P2rx7s are implicated in various neuropsychiatric conditions such as schizophrenia and depression. Here we investigated whether 5-HT release released from the hippocampal terminals of MRR is subject to modulation by P2rx7s. To achieve this goal, an optogenetic approach was used to selectively activate subpopulation of serotonergic terminals derived from the MRR locally, and one of its target area, the hippocampus. Optogenetic activation of neurons in the MRR with 20 Hz was correlated with freezing and enhanced locomotor activity of freely moving mice and elevated extracellular levels of 5-HT, glutamate but not GABA in vivo. Similar optical stimulation (OS) significantly increased [³H]5-HT and [³H]glutamate release in acute MRR and hippocampal slices. We examined spatial and temporal patterns of [³H]5-HT release and the interaction between the serotonin and glutamate systems. Whilst [³H]5-HT release from MRR neurons was [Ca²⁺]_o-dependent and sensitive to TTX, CNQX and DL-AP-5, release from hippocampal terminals was not affected by the latter drugs. Hippocampal [³H]5-HT released by electrical but not OS was subject to modulation by 5- HT1_B/D receptors agonist sumatriptan (1 μM), whereas the selective 5-HT_1A agonist buspirone (0.1 μM) was without effect. [³H]5-HT released by electrical and optical stimulation was decreased in mice genetically deficient in P2rx7s, and after perfusion with selective P2rx7 antagonists, JNJ-47965567 (0.1 μM), and AZ-10606120 (0.1 μM). Optical and electrical stimulation elevated the extracellular level of ATP. Our results demonstrate for the first time the modulation of 5-HT release from hippocampal MRR terminals by the endogenous activation of P2rx7s. P2rx7 mediated modulation of 5-HT release could contribute to various physiological and pathophysiological phenomena, related to hippocampal serotonergic transmission.

Rational Design of a Stimuli-Responsive Polymer Electrode Interface Coupled with in Vivo Microdialysis for Measurement of Sialic Acid in Live Mouse Brain in Alzheimer's Disease

Sensitive and selective monitoring of sialic acid (SA) in cerebral nervous system is of great importance for studying the role that SA plays in the pathological process of Alzheimer's disease (AD). In this work, we first reported an electrochemical biosensor based on a novel stimuli-responsive copolymer for selective and sensitive detection of SA in mouse brain. Notably, through synergetic hydrogen-bonding interactions, the copolymer could translate the recognition of SA into their conformational transition and wettability switch, which facilitated the access and enrichment of redox labels and targets to the electrode surface, thus significantly improving the detection sensitivity with the detection limit down to 0.4 pM. Besides amplified sensing signals, the proposed method exhibited good selectivity toward SA in comparison to potential interference molecules coexisting in the complex brain system due to the combination of high affinity between phenylboronic acid (PBA) and SA and the directional hydrogen-bonding interactions in the copolymer. The electrochemical biosensor with remarkable analytical performance was successfully applied to evaluate the dynamic change of SA level in live mouse brain with AD combined with in vivo midrodialysis. The accurate concentration of SA in different brain regions of live mouse with AD has been reported for the first time, which is beneficial for progressing our understanding of the role that SA plays in physiological and pathological events in the brain.

Serotonin receptors in depression: from A to B

The role of serotonin in major depressive disorder (MDD) is the focus of accumulating clinical and preclinical research. The results of these studies reflect the complexity of serotonin signaling through many receptors, in a large number of brain regions, and throughout the lifespan. The role of the serotonin transporter in MDD has been highlighted in gene by environment association studies as well as its role as a critical player in the mechanism of the most effective antidepressant treatments – selective serotonin reuptake inhibitors. While the majority of the 15 known receptors for serotonin have been implicated in depression or depressive-like behavior, the serotonin 1A (5-HT _1A) and 1B (5-HT _1B) receptors are among the most studied. Human brain imaging and genetic studies point to the involvement of 5-HT _1A and 5-HT _1B receptors in MDD and the response to antidepressant treatment. In rodents, the availability of tissue-specific and inducible knockout mouse lines has made possible the identification of the involvement of 5-HT _1A and 5-HT _1B receptors throughout development and in a cell-type specific manner. This, and other preclinical pharmacology work, shows that autoreceptor and heteroreceptor populations of these receptors have divergent roles in modulating depression-related behavior as well as responses to antidepressants and also have different functions during early postnatal development compared to during adulthood.

A Lack of Serotonin 1B Autoreceptors Results in Decreased Anxiety and Depression-Related Behaviors

The effects of serotonin (5-HT) on anxiety and depression are mediated by a number of 5-HT receptors, including autoreceptors that act to inhibit 5-HT release. While the majority of anxiety and depression-related research has focused on the 5-HT_1A receptor, the 5-HT_1B receptor has a lesser known role in modulating emotional behavior. 5-HT_1B receptors are inhibitory GPCRs located on the presynaptic terminal of both serotonin and non-serotonin neurons, where they act to inhibit neurotransmitter release. The autoreceptor population located on the axon terminals of 5-HT neurons is a difficult population to study due to their diffuse localization throughout the brain that overlaps with 5-HT_1B heteroreceptors (receptors located on non-serotonergic neurons). In order to study the contribution of 5-HT_1B autoreceptors to anxiety and depression-related behaviors, we developed a genetic mouse model that allows for selective ablation of 5-HT_1B autoreceptors. Mice lacking 5-HT_1B autoreceptors displayed the expected increases in extracellular serotonin levels in the ventral hippocampus following administration of a selective serotonin reuptake inhibitor. In behavioral studies, they displayed decreased anxiety-like behavior in the open field and antidepressant-like effects in the forced swim and sucrose preference tests. These results suggest that strategies aimed at blocking 5-HT_1B autoreceptors may be useful for the treatment of anxiety and depression.

Antinociceptive activity of the new triple reuptake inhibitor NS18283 in a mouse model of chemotherapy-induced neuropathic pain

BACKGROUND

Chronic neuropathic pain can lead to anxiety and depression. Drugs that block reuptake of serotonin, norepinephrine and/or dopamine are widely used to treat depression, and have emerged as useful drugs in the treatment of neuropathic pain. This study compared the acute antinociceptive effects of NS18283, a novel triple monoamine reuptake inhibitor (MRI) with indatraline, venlafaxine and escitalopram in a mouse model of neuropathic pain.

METHOD

Neuropathic pain-like behaviours were induced in mice by repeated injections of oxaliplatin (OXA), and assessed using the von Frey hair test, the cold plate test and the thermal preference plate test. Anxio/depressive phenotype and antidepressant-like properties of compounds were assessed by the novelty suppressed feeding test and the tail suspension test, respectively.

RESULTS

In vivo microdialysis experiments showed that each MRI increased extracellular serotonin, norepinephrine and/or dopamine levels in the cingulate cortex, in agreement with their in vitro reuptake inhibitory properties. Indatraline (3 mg/kg) reversed the full repertoire of OXA-induced neuropathic hypersensitivity. NS18283 (10 mg/kg) reversed OXA-induced mechano-hypersensitivity and cold allodynia. Venlafaxine (16 mg/kg) and escitalopram (4 mg/kg) only reversed cold allodynia and mechano-hypersensitivity, respectively. All MRIs produced antidepressant-like activity in anxio/depressive phenotype of OXA mice.

CONCLUSIONS

Acute administration of drugs that enhance the activity of serotonin, norepinephrine and dopamine neurotransmission within nociceptive pathways may provide a broader spectrum of antinociception than dual or selective reuptake inhibitors in animal models of neuropathic pain. Whether similar observations would occur after repeated administration of such compounds in an attempt to simulate dosing in humans, or be compromised by dopaminergic-mediated adverse effects warrants further investigation.

Distinct Circuits Underlie the Effects of 5-HT1B Receptors on Aggression and Impulsivity

Impulsive and aggressive behaviors are both modulated by serotonergic signaling, specifically through the serotonin 1B receptor (5-HT1BR). 5-HT1BR knockout mice show increased aggression and impulsivity, and 5-HT1BR polymorphisms are associated with aggression and drug addiction in humans. To dissect the mechanisms by which the 5-HT1BR affects these phenotypes, we developed a mouse model to spatially and temporally regulate 5-HT1BR expression. Our results demonstrate that forebrain 5-HT1B heteroreceptors expressed during an early postnatal period contribute to the development of the neural systems underlying adult aggression. However, distinct heteroreceptors acting during adulthood are involved in mediating impulsivity. Correlating with the impulsivity, dopamine in the nucleus accumbens is elevated in the absence of 5-HT1BRs and normalized following adult rescue of the receptor. Overall, these data show that while adolescent expression of 5-HT1BRs influences aggressive behavior, a distinct set of 5-HT1B receptors modulates impulsive behavior during adulthood.

5-Hydroxytryptamine1A receptor-activation hyperpolarizes pyramidal cells and suppresses hippocampal gamma oscillations via Kir3 channel activation

Rhythmic cortical neuronal oscillations in the gamma frequency band (30-80 Hz, gamma oscillations) have been associated with cognitive processes such as sensory perception and integration, attention, learning, and memory. Gamma oscillations are disrupted in disorders for which cognitive deficits are hallmark symptoms such as schizophrenia and Alzheimer's disease.In vitro, various neurotransmitters have been found to modulate gamma oscillations. Serotonin(5-HT) has long been known to be important for both behavioural and cognitive functions such as learning and memory. Multiple 5-HT receptor subtypes are expressed in the CA3 region of the hippocampus and high doses of 5-HT reduce the power of induced gamma oscillations.Hypothesizing that 5-HT may have cell- and receptor subtype-specific modulatory effects, we investigated the receptor subtypes, cell types and cellular mechanisms engaged by 5-HT in the modulation of gamma oscillations in mice and rats. We found that 5-HT decreases the power of kainate-induced hippocampal gamma oscillations in both species via the 5-HT1A receptor subtype. Whole-cell patch clamp recordings demonstrated that this decrease was caused by a hyperpolarization of CA3 pyramidal cells and a reduction of their firing frequency, but not by alteration of inhibitory neurotransmission. Finally, our results show that the effect on pyramidal cells is mediated via the G protein-coupled receptor inwardly rectifying potassium channel Kir3.Our findings suggest this novel cellular mechanism as a potential target for therapies that are aimed at alleviating cognitive decline by helping the brain to maintain or re-establish normal gamma oscillation levels in neuropsychiatric and neurodegenerative disorders.

Serotonergic pharmacology in animal models: from behavioral disorders to dyskinesia

Serotonin (5-HT) dysfunction has been involved in both movement and behavioral disorders. Serotonin pharmacology improves dyskinetic movements as well as depressive, anxious, aggressive and anorexic symptoms. Animal models have been useful to investigate more precisely to what extent 5-HT is involved and whether drugs targeting the 5-HT system can counteract the symptoms exhibited. We review existing rodent and non-human primate (NHP) animal models in which selective 5-HT or dual 5-HT-norepinephrine (NE) transporter inhibitors, as well as specific 5-HT receptors agonists and antagonists, monoamine oxidase A inhibitors (IMAO-A) and MDMA (Ecstasy) have been used. We review overlaps between the various drug classes involved. We confront behavioral paradigms and treatment regimen. Some but not all animal models and associated pharmacological treatments have been extensively studied in the litterature. In particular, the impact of selective serotonin reuptake inhibitors (SSRI) has been extensively investigated using a variety of pharmacological or genetic rodent models of depression, anxiety, aggressiveness. But the validity of these rodent models is questioned. On the contrary, few studies did address the potential impact of targeting the 5-HT system on NHP models of behavioral disorders, despite the fact that those models may match more closely to human pathologies. Further investigations with carefull behavioral analysis will improve our understanding of neural bases underlying the pathophysiology of movement and behavioral disorders.

Lack of BDNF expression through promoter IV disturbs expression of monoamine genes in the frontal cortex and hippocampus

Brain-derived neurotrophic factor (BDNF) is implicated in the pathophysiology of psychiatric conditions including major depression and schizophrenia. Mice lacking activity-driven BDNF expression through promoter IV (knock-in promoter IV: KIV) exhibit depression-like behavior, inflexible learning, and impaired response inhibition. Monoamine systems (serotonin, dopamine, and noradrenaline) are suggested to be involved in depression and schizophrenia since many of the current antidepressants and antipsychotics increase the brain levels of monoamines and/or act on monoamine receptors. To elucidate the impact of activity-driven BDNF on the monoamine systems, we examined mRNA levels for 30 monoamine-related genes, including receptors, transporters, and synthesizing enzymes, in KIV and control wild-type mice by using quantitative reverse-transcription polymerase chain reaction (qRT-PCR). mRNA levels were measured in the frontal cortex and hippocampus, which are regions related to depression and schizophrenia and where promoter IV is active. The frontal cortex of KIV mice showed reduced levels of mRNA expression for serotonin receptors 1b, 2a, and 5b (5HTR1b, 5HTR2a, 5HTR5b), dopamine D2 receptors (DRD2), and adrenergic receptors alpha 1a and 1d (AdRα1a and AdRα1b), but increased levels for serotonin synthesizing enzyme, tryptophan hydroxylase (TPH), and dopamine D4 receptor (DRD4) when compared to control wild-type mice. The hippocampus of KIV mice showed decreased levels of 5HTR5b. Our results provide causal evidence that lack of promoter IV-driven BDNF disturbs expression of monoaminergic genes in the frontal cortex and hippocampus. These disturbed expression changes in the monoamine systems may mediate the depression- and schizophrenia-like behavior of KIV mice. Our results also suggest that antidepressant and antipsychotic treatments may actually interfere with and normalize the disturbed monoamine systems caused by reduced activity-dependent BDNF, while the treatment responses to these drugs may differ in the subject with reduced BDNF levels caused by stress and lack of neuronal activity.

5-HT1B receptor modulation of the serotonin transporter in vivo: studies using KO mice

The serotonin transporter (SERT) controls the strength and duration of serotonergic neurotransmission by the high-affinity uptake of serotonin (5-HT) from extracellular fluid. SERT is a key target for many psychotherapeutic and abused drugs, therefore understanding how SERT activity and expression are regulated is of fundamental importance. A growing literature suggests that SERT activity is under regulatory control of the 5-HT1B autoreceptor. The present studies made use of mice with a constitutive reduction (5-HT1B+/-) or knockout of 5-HT1B receptors (5-HT1B-/-), as well as mice with a constitutive knockout of SERT (SERT-/-) to further explore the relationship between SERT activity and 5-HT1B receptor expression. High-speed chronoamperometry was used to measure clearance of 5-HT from CA3 region of hippocampus in vivo. Serotonin clearance rate, over a range of 5-HT concentrations, did not differ among 5-HT1B receptor genotypes, nor did [(3)H]cyanoimipramine binding to SERT in this brain region, suggesting that SERT activity is not affected by constitutive reduction or loss of 5-HT1B receptors; alternatively, it might be that other transport mechanisms for 5-HT compensate for loss of 5-HT1B receptors. Consistent with previous reports, we found that the 5-HT1B receptor antagonist, cyanopindolol, inhibited 5-HT clearance in wild-type mice. However, this effect of cyanopindolol was lost in 5-HT1B-/- mice and diminished in 5-HT1B+/- mice, indicating that the 5-HT1B receptor is necessary for cyanopindolol to inhibit 5-HT clearance. Likewise, cyanopindolol was without effect on 5-HT clearance in SERT-/- mice, demonstrating a requirement for the presence of both SERT and 5-HT1B receptors in order for cyanopindolol to inhibit 5-HT clearance in CA3 region of hippocampus. Our findings are consistent with SERT being under the regulatory control of 5-HT1B autoreceptors. Future studies to identify signaling pathways involved may help elucidate novel therapeutic targets for the treatment of psychiatric disorders, particularly those linked to gene variants of the 5-HT1B receptor.

Antidepressant activity: contribution of brain microdialysis in knock-out mice to the understanding of BDNF/5-HT transporter/5-HT autoreceptor interactions

Why antidepressants vary in terms of efficacy is currently unclear. Despite the leadership of selective serotonin reuptake inhibitors (SSRIs) in the treatment of depression, the precise neurobiological mechanisms involved in their therapeutic action are poorly understood. A better knowledge of molecular interactions between monoaminergic system, pre- and post-synaptic partners, brain neuronal circuits and regions involved may help to overcome limitations of current treatments and identify new therapeutic targets. Intracerebral in vivo microdialysis (ICM) already provided important information about the brain mechanism of action of antidepressants first in anesthetized rats in the early 1990s, and since then in conscious wild-type or knock-out mice. The principle of ICM is based on the balance between release of neurotransmitters (e.g., monoamines) and reuptake by selective transporters [e.g., serotonin transporter for serotonin 5-hydroxytryptamine (5-HT)]. Complementary to electrophysiology, this technique reflects pre-synaptic monoamines release and intrasynaptic events corresponding to ≈80% of whole brain tissue content. The inhibitory role of serotonergic autoreceptors infers that they limit somatodendritic and nerve terminal 5-HT release. It has been proposed that activation of 5-HT_1A and 5-HT_1B receptor sub-types limits the antidepressant-like activity of SSRIs. This hypothesis is based partially on results obtained in ICM experiments performed in naïve, non-stressed rodents. The present review will first remind the principle and methodology of ICM performed in mice. The crucial need of developing animal models that display anxiety and depression-like behaviors, neurochemical and brain morphological phenotypes reminiscent of these mood disorders in humans, will be underlined. Recently developed genetic mouse models have been generated to independently manipulate 5-HT_1A auto and heteroreceptors and ICM helped to clarify the role of the pre-synaptic component, i.e., by measuring extracellular levels of neurotransmitters in serotonergic nerve terminal regions and raphe nuclei. Finally, we will summarize main advantages of using ICM in mice through recent examples obtained in knock-outs (drug infusion through the ICM probe allows the search of a correlation between changes in extracellular neurotransmitter levels and antidepressant-like activity) or alternatives (infusion of a small-interfering RNA suppressing receptor functions in the mouse brain). We will also focus this review on post-synaptic components such as brain-derived neurotrophic factor in adult hippocampus that plays a crucial role in the neurogenic and anxiolytic/antidepressant-like activity of chronic SSRI treatment. Limitations of ICM will also be considered.

Selective serotonin reuptake inhibitor antidepressant treatment discontinuation syndrome: a review of the clinical evidence and the possible mechanisms involved

Besides demonstrated efficacy, selective serotonin reuptake inhibitors (SSRIs) hold other advantages over earlier antidepressants such as greater tolerability and a wider range of clinical applications. However, there is a growing body of clinical evidence which suggests that SSRIs could, in some cases, be associated with a withdrawal reaction upon cessation of regular use. In addition to sensory and gastrointestinal-related symptoms, the somatic symptoms of the SSRI discontinuation syndrome include dizziness, lethargy, and sleep disturbances. Psychological symptoms have also been documented, usually developing within 1–7 days following SSRI discontinuation. The characteristics of the discontinuation syndrome have been linked to the half-life of a given SSRI, with a greater number of reports emerging from paroxetine compared to other SSRIs. However, many aspects of the neurobiology of the SSRI discontinuation syndrome (or SSRI withdrawal syndrome) remain unresolved. Following a comprehensive overview of the clinical evidence, we will discuss the underlying pathophysiology of the SSRI discontinuation syndrome and comment on the use of animal models to better understand this condition.

Lu AA21004, a novel multimodal antidepressant, produces regionally selective increases of multiple neurotransmitters--a rat microdialysis and electrophysiology study

The monoaminergic network, including serotonin (5-HT), norepinephrine (NE), and dopamine (DA) pathways, is highly interconnected and has a well-established role in mood disorders. Preclinical research suggests that 5-HT receptor subtypes, including 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors as well as the 5-HT transporter (SERT), may have important roles in treating depression. This study evaluated the neuropharmacological profile of Lu AA21004, a novel multimodal antidepressant combining 5-HT3 and 5-HT7 receptor antagonism, 5-HT1B receptor partial agonism, 5-HT1A receptor agonism, and SERT inhibition in recombinant cell lines. Extracellular 5-HT, NE and DA levels were evaluated in the ventral hippocampus (vHC), medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) after acute and subchronic treatment with Lu AA21004 or escitalopram. The acute effects of LuAA21004 on NE and DA neuronal firing were also evaluated in the locus coeruleus (LC) and ventral tegmental area (VTA), respectively. Acute Lu AA21004 dose-dependently increased 5-HT in the vHC, mPFC and NAc. Maximal 5-HT levels in the vHC were higher than those in the mPFC. Furthermore, mPFC 5-HT levels were increased at low SERT occupancy levels. In the vHC and mPFC, but not the NAc, high Lu AA21004 doses increased NE and DA levels. Lu AA21004 slightly decreased LC NE neuronal firing and had no effect on VTA DA firing. Results are discussed in context of occupancy at 5-HT3, 5-HT1B and 5-HT1A receptors and SERT. In conclusion, Lu AA21004, acting via two pharmacological modalities, 5-HT receptor modulation and SERT inhibition, results in a brain region-dependent increase of multiple neurotransmitter concentrations.

Brain monoamines and antidepressant-like responses in MRL/MpJ versus C57BL/6J mice

The MRL/MpJ mouse demonstrates enhanced wound healing and tissue regeneration and increased neurotrophic mobilization to chronic antidepressant drug treatments. This study compared brain monoamine systems between MRL/MpJ and C57BL/6J mice as a potential basis for strain differences after chronic antidepressant treatment. MRL/MpJ mice had significantly higher tissue levels of serotonin and dopamine in multiple brain regions. Microdialysis studies demonstrated that baseline levels of extracellular serotonin did not differ between strains. However, acute administration of the selective serotonin reuptake inhibitor citalopram produced an increase in extracellular serotonin in the ventral hippocampus of MRL/MpJ mice that was twice as large as achieved in C57BL/6J mice. The greater effects in MRL/MpJ mice on 5-HT levels were not maintained after local perfusion of citalopram, suggesting that mechanisms outside of the hippocampus were responsible for the greater effect of citalopram after systemic injection. The density of serotonin and norepinephrine transporters in the hippocampus was significantly higher in MRL/MpJ mice. In addition, the expression of 5-HT(1A) mRNA was lower in the hippocampus, 5-HT(1B) mRNA was higher in the hippocampus and brainstem and SERT mRNA was higher in the brain stem of MRL/MpJ mice. The exaggerated neurotransmitter release in MRL/MpJ mice was accompanied by reduced baseline immobility in the tail suspension test and a greater reduction of immobility produced by citalopram or the tricyclic antidepressant desipramine. These data suggest that differences in the response to acute and chronic antidepressant treatments between the two strains could be attributed to differences in serotonin or catecholamine transmission.

Metabotropic glutamate 2/3 receptor antagonists improve behavioral and prefrontal dopaminergic alterations in the chronic corticosterone-induced depression model in mice

Metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists have an antidepressant-like effect, but the exact mechanism still remains unclear. This study examined the effects of mGlu2/3 receptor antagonists in chronic corticosterone-treated mice which could be used as an animal model of depression. In the forced swim test, the mGlu2/3 receptor antagonists MGS0039 (1.0 mg/kg, i.p.) and LY341495 (0.3 mg/kg, i.p) significantly reduced the increased immobility time of mice pretreated with corticosterone (20 mg/kg, s.c.) for 21 days, while desipramine (30 mg/kg, i.p.) and fluoxetine (30 mg/kg, i.p.) did not. The antidepressant-like effect of LY341495 was not blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist NBQX (10 mg/kg, i.p.). Systemic administration of LY341495 did not affect basal release of glutamate, dopamine or serotonin in the prefrontal cortex of the control or chronic corticosterone-treated mice. Chronic corticosterone markedly enhanced high K(+)-induced release of dopamine, but not serotonin or glutamate, in the prefrontal cortex. This neurochemical change was blocked by systemic administration of MGS0039 and LY341495, but not desipramine or fluoxetine. These results suggest that chronic corticosterone-treated mice could be used as an animal model of treatment-resistant depression. This study also suggests that the prefrontal dopaminergic system is involved in the antidepressant-like effect of mGlu2/3 receptor antagonists in the chronic corticosterone-induced depression model.

Altered response to the selective serotonin reuptake inhibitor escitalopram in mice heterozygous for the serotonin transporter: an electrophysiological and neurochemical study

A serotonin (5-HT) transporter (5-HTT; SERT) polymorphism has been associated with depressive states and poor responses to selective serotonin reuptake inhibitors (SSRIs). Given the similar attenuation of SERT activity in SERT+/- mice and in humans with short allele(s) of SERT in its promoter region, it is conceivable that SERT+/- mice offer an adequate model to mimic the human subpopulation with respect to their altered response to SSRIs. This study investigated the effects of the most selective SSRI escitalopram, in heterozygous SERT+/- mice using a combined electrophysiological and neurochemical approach. Results indicated that administration of escitalopram for 2 d resulted in a 72% and 63% decrease in dorsal raphe 5-HT neuronal firing rate in SERT+/+ and SERT+/- mice, respectively. In contrast, administration of escitalopram for 21 d produced a gradual recovery of 5-HT neuronal firing rate to basal level in SERT+/+, but not in SERT+/- mice. In the hippocampus, microdialysis revealed that sustained administration of escitalopram produced a greater increase in extracellular 5-HT ([5-HT]ext) outflow in SERT+/- than in the wild-types with or without a washout of the SSRI. Nevertheless, the ability of microiontophoretically applied 5-HT to inhibit the firing rate of CA3 pyramidal neurons was not different between SERT+/+ and SERT+/- mice given escitalopram for 21 d. The data indicate that the poor response to SSRIs of depressive patients with short allele(s) of SERT is not attributable to a lesser increase in 5-HT transmission in the hippocampus.

Alpha7-nicotinic receptors modulate nicotine-induced reinforcement and extracellular dopamine outflow in the mesolimbic system in mice

RATIONALE

Nicotine is the main addictive component of tobacco and modifies brain function via its action on neuronal acetylcholine nicotinic receptors (nAChRs). The mesolimbic dopamine (DA) system, where neurons of the ventral tegmental area (VTA) project to the nucleus accumbens (ACb), is considered a core site for the processing of nicotine's reinforcing properties. However, the precise subtypes of nAChRs that mediate the rewarding properties of nicotine and that contribute to the development of addiction remain to be identified.

OBJECTIVES

We investigated the role of the nAChRs containing the α7 nicotinic subunit (α7 nAChRs) in the reinforcing properties of nicotine within the VTA and in the nicotine-induced changes in ACb DA outflow in vivo.

METHODS

We performed intra-VTA self-administration and microdialysis experiments in genetically modified mice lacking the α7 nicotinic subunit or after pharmacological blockade of α7 nAChRs in wild-type mice.

RESULTS

We show that the reinforcing properties of nicotine within the VTA are lower in the absence or after pharmacological blockade of α7 nAChRs. We also report that nicotine-induced increases in ACb DA extracellular levels last longer in the absence of these receptors, suggesting that α7 nAChRs regulate the action of nicotine on DA levels over time.

CONCLUSIONS

The present results reveal new insights for the role of α7 nAChRs in modulating the action of nicotine within the mesolimbic circuit. These receptors appear to potentiate the reinforcing action of nicotine administered into the VTA while regulating its action over time on DA outflow in the ACb.

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

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

Serotonin synthesis, release and reuptake in terminals: a mathematical model

BACKGROUND

Serotonin is a neurotransmitter that has been linked to a wide variety of behaviors including feeding and body-weight regulation, social hierarchies, aggression and suicidality, obsessive compulsive disorder, alcoholism, anxiety, and affective disorders. Full understanding of serotonergic systems in the central nervous system involves genomics, neurochemistry, electrophysiology, and behavior. Though associations have been found between functions at these different levels, in most cases the causal mechanisms are unknown. The scientific issues are daunting but important for human health because of the use of selective serotonin reuptake inhibitors and other pharmacological agents to treat disorders in the serotonergic signaling system.

METHODS

We construct a mathematical model of serotonin synthesis, release, and reuptake in a single serotonergic neuron terminal. The model includes the effects of autoreceptors, the transport of tryptophan into the terminal, and the metabolism of serotonin, as well as the dependence of release on the firing rate. The model is based on real physiology determined experimentally and is compared to experimental data.

RESULTS

We compare the variations in serotonin and dopamine synthesis due to meals and find that dopamine synthesis is insensitive to the availability of tyrosine but serotonin synthesis is sensitive to the availability of tryptophan. We conduct in silico experiments on the clearance of extracellular serotonin, normally and in the presence of fluoxetine, and compare to experimental data. We study the effects of various polymorphisms in the genes for the serotonin transporter and for tryptophan hydroxylase on synthesis, release, and reuptake. We find that, because of the homeostatic feedback mechanisms of the autoreceptors, the polymorphisms have smaller effects than one expects. We compute the expected steady concentrations of serotonin transporter knockout mice and compare to experimental data. Finally, we study how the properties of the the serotonin transporter and the autoreceptors give rise to the time courses of extracellular serotonin in various projection regions after a dose of fluoxetine.

CONCLUSIONS

Serotonergic systems must respond robustly to important biological signals, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of the serotonin transporters and the serotonin autoreceptors. Many difficult questions remain in order to fully understand how serotonin biochemistry affects serotonin electrophysiology and vice versa, and how both are changed in the presence of selective serotonin reuptake inhibitors. Mathematical models are useful tools for investigating some of these questions.

Assessing the neuronal serotonergic target-based antidepressant stratagem: impact of in vivo interaction studies and knockout models

Depression remains a challenge in the field of affective neuroscience, despite a steady research progress. Six out of nine basic antidepressant mechanisms rely on serotonin neurotransmitter system. Preclinical studies have demonstrated the significance of serotonin receptors (5-HT_1-3,6,7), its signal transduction pathways and classical down stream targets (including neurotrophins, neurokinins, other peptides and their receptors) in antidepressant drug action. Serotonergic control of depression embraces the recent molecular requirements such as influence on proliferation, neurogenesis, plasticity, synaptic (re)modeling and transmission in the central nervous system. The present progress report analyses the credibility of each protein as therapeutically relevant target of depression. In vivo interaction studies and knockout models which identified these targets are foreseen to unearth new ligands and help them transform to drug candidates. The importance of the antidepressant assay selection at the preclinical level using salient animal models/assay systems is discussed. Such test batteries would definitely provide antidepressants with faster onset, efficacy in resistant (and co-morbid) types and with least adverse effects. Apart from the selective ligands, only those molecules which bring an overall harmony, by virtue of their affinities to various receptor subtypes, could qualify as effective antidepressants. Synchronised modulation of various serotonergic sub-pathways is the basis for a unique and balanced antidepressant profile, as that of fluoxetine (most exploited antidepressant) and such a profile may be considered as a template for the upcoming antidepressants. In conclusion, 5-HT based multi-targeted antidepressant drug discovery supported by in vivo interaction studies and knockout models is advocated as a strategy to provide classic molecules for clinical trials.

Effects of acute and chronic administration of atomoxetine and methylphenidate on extracellular levels of noradrenaline, dopamine and serotonin in the prefrontal cortex and striatum of mice

Acute administration of atomoxetine and methylphenidate, attention-deficit/hyperactivity disorder (ADHD) drugs, activates catecholaminergic systems in rat brain, but the effects of their chronic administration are not known. This study examined the effects of acute and chronic administration of ADHD drugs on the extracellular levels of noradrenaline (NA), dopamine (DA) and serotonin (5-HT), and the expression of the neuronal activity marker c-Fos in the prefrontal cortex and striatum of mice. Acute ADHD drugs increased NA and DA, but not 5-HT, levels in the prefrontal cortex of mice. Maximal effects of atomoxetine and methylphenidate were observed at 1 mg/kg and 3 mg/kg, respectively. At these doses, both drugs did not affect the spontaneous locomotor activity of mice. Chronic administration of atomoxetine 1 mg/kg and methylphenidate 3 mg/kg for 21 days also increased NA and DA, but not 5-HT, levels in the prefrontal cortex. The increases in NA levels induced by atomoxetine, but not methylphenidate, were reduced by chronic treatment. In contrast, acute and chronic administration of atomoxetine 1 mg/kg and methylphenidate 3 mg/kg did not affect the monoamine levels in the striatum. Acute and chronic atomoxetine 1 mg/kg and methylphenidate 3 mg/kg increased the expression of c-Fos in the prefrontal cortex, but not in the striatum, to a similar extent. These results suggest that acute and chronic administration of the ADHD drugs selectively activate the prefrontal catecholamine systems in mice.

A longitudinal study of 5-HT outflow during chronic fluoxetine treatment using a new technique of chronic microdialysis in a highly emotional mouse strain

The onset of a therapeutic response to antidepressant treatment exhibits a delay of several weeks. The present study was designed to know whether extracellular serotonin (5-HT) levels need to be increased in territories of 5-HT innervation in order to obtain beneficial effects from a chronic treatment with a serotonin-selective reuptake inhibitor (SSRI). Thus, we performed a longitudinal study of a chronic fluoxetine treatment in a model of highly emotional mice (BALB/cJ). The function of the 5-HT system in the raphe nuclei and hippocampus, was assessed by using repeated in vivo microdialysis sessions in awake freely moving mice, then studying its relation with behavior, analyzed mainly with open field paradigm. One of the neural mechanisms underlying such delay has been proposed to be the functional status of 5-HT1A autoreceptors in raphe nuclei. Thus, we also assessed the degree of 5-HT1A autoreceptor desensitization by using a local infusion in the raphe of the antagonist, WAY 100635 via reverse microdialysis. We report that the anxiolytic-like effects of fluoxetine correlate in time and amplitude with 5-HT1A autoreceptor desensitization, but neither with the extracellular levels of 5-HT in the raphe nuclei, nor in the hippocampus. Our study suggests that the beneficial anxiolytic/antidepressant-like effects of chronic SSRI treatment indeed depend on 5-HT1A autoreceptor internalization, but do not require a sustained increase in extracellular 5-HT levels in a territory of 5-HT projection such as hippocampus.

Interest of using genetically manipulated mice as models of depression to evaluate antidepressant drugs activity: a review

Among the multiple possibilities to study human depressive disorders, animal models remain important preclinical tools. They allow the understanding of the mechanisms of action of antidepressant drugs. Primarily developed in rat, animal models of depression have been adapted to the mouse, an easy-to-use mammal with better genetic possibilities than rats. As an example, genetic manipulation of the serotoninergic 5-hydroxytryptamine-HT; (5-HT) system provided important opportunities to investigate the role of this monoamine in mood disorders. The contribution of either constitutive knockout (KO), tissue specific, or inducible KO mice and animal models in the current knowledge of the pathophysiology and treatment of depression is unanimously recognized. The phenotype of genetically manipulated animals is strongly influenced by both the genetic background of the animal as well as environmental factors. For these reasons, it is necessary to underline that KO mice have been generated on various genetic backgrounds, which strongly influence the behavioral and neurochemical responses to the tests. The present review will thus focus on KO mice lacking G protein-coupled monoaminergic receptors (e.g; 5-HT1B, 5-HT1A, and 5-HT4 receptors) and the 5-HT serotonin transporter, which is the main target of antidepressant drugs (or strategies). The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its main receptor displaying a tyrosine kinase activity, will also be addressed to illustrate the fact that in preclinical studies, combination of genetic manipulations with pharmacological ones should allow further progress in the field of neuropsychopharmacology.

Mutant mouse models and antidepressant drug research: focus on serotonin and brain-derived neurotrophic factor

Several lines of knockout (KO) mice have been evaluated as models of depression-related behavioral and neurobiological changes, and used to investigate molecular and cellular mechanisms underlying the activity of antidepressant drugs. Adult neurogenesis and brain 5-hydroxytryptamine (5-HT)/neurotrophic factor interactions have recently attracted great interest in relation to the mechanism of action of antidepressant drugs. The present review focuses primarily on genetic manipulation of the serotoninergic (5-HT) system. Basal neurochemical and behavioral changes occurring in mice lacking the 5-HT transporter (SERT), which is the main target of antidepressant drugs, as well as in those lacking G protein-coupled serotonin receptors (e.g. 5-HT1B, 5-HT1A, and 5-HT4 receptors) are described and evaluated. The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its high-affinity receptor (R-TrkB), is also addressed. Constitutive KO, tissue specific, or inducible KO mice targeting both 5-HT and brain-derived neurotrophic factor systems may potentially make an important contribution to knowledge of the pathophysiology and treatment of depression.

Genetic variation in cortico-amygdala serotonin function and risk for stress-related disease

The serotonin system is strongly implicated in the pathophysiology and therapeutic alleviation of stress-related disorders such as anxiety and depression. Serotonergic modulation of the acute response to stress and the adaptation to chronic stress is mediated by a myriad of molecules controlling serotonin neuron development (Pet-1), synthesis (tryptophan hydroxylase 1 and 2 isozymes), packaging (vesicular monoamine transporter 2), actions at presynaptic and postsynaptic receptors (5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6, 5-HT7), reuptake (serotonin transporter), and degradation (monoamine oxidase A). A growing body of evidence from preclinical rodents models, and especially genetically modified mice and inbred mouse strains, has provided significant insight into how genetic variation in these molecules can affect the development and function of a key neural circuit between the dorsal raphe nucleus, medial prefrontal cortex and amygdala. By extension, such variation is hypothesized to have a major influence on individual differences in the stress response and risk for stress-related disease in humans. The current article provides an update on this rapidly evolving field of research.

Brain-derived neurotrophic factor-deficient mice exhibit a hippocampal hyperserotonergic phenotype

Growing evidence supports the involvement of brain-derived neurotrophic factor (BDNF) in mood disorders and the mechanism of action of antidepressant drugs. However, the relationship between BDNF and serotonergic signalling is poorly understood. Heterozygous mutants BDNF +/- mice were utilized to investigate the influence of BDNF on the serotonin (5-HT) system and the activity of the serotonin transporter (SERT) in the hippocampus. The zero net flux method of quantitative microdialysis revealed that BDNF +/- heterozygous mice have increased basal extracellular 5-HT levels in the hippocampus and decreased 5-HT reuptake capacity. In keeping with these results, the selective serotonin reuptake inhibitor paroxetine failed to increase hippocampal extracellular 5-HT levels in BDNF +/- mice while it produced robust effects in wild-type littermates. Using in-vitro autoradiography and synaptosome techniques, we investigated the causes of attenuated 5-HT reuptake in BDNF +/- mice. A significant decrease in [3H]citalopram-binding-site density in the CA3 subregion of the ventral hippocampus and a significant reduction in [3H]5-HT uptake in hippocampal synaptosomes, revealed mainly a decrease in SERT function. However, 5-HT1A autoreceptors were not desensitized in BDNF +/- mice. These results provide evidence that constitutive reductions in BDNF modulate SERT function reuptake in the hippocampus.

Interaction between BDNF and serotonin: role in mood disorders

Brain-derived neurotrophic factor (BDNF) and serotonin (5-hydroxytryptamine, 5-HT) are two seemingly distinct signaling systems that play regulatory roles in many neuronal functions including survival, neurogenesis, and synaptic plasticity. A common feature of the two systems is their ability to regulate the development and plasticity of neural circuits involved in mood disorders such as depression and anxiety. BDNF promotes the survival and differentiation of 5-HT neurons. Conversely, administration of antidepressant selective serotonin reuptake inhibitors (SSRIs) enhances BDNF gene expression. There is also evidence for synergism between the two systems in affective behaviors and genetic epitasis between BDNF and the serotonin transporter genes.

Consequences of changes in BDNF levels on serotonin neurotransmission, 5-HT transporter expression and function: studies in adult mice hippocampus

In vivo intracerebral microdialysis is an important neurochemical technique that has been applied extensively in genetic and pharmacological studies aimed at investigating the relationship between neurotransmitters. Among the main interests of microdialysis application is the infusion of drugs through the microdialysis probe (reverse dialysis) in awake, freely moving animals. As an example of the relevance of intracerebral microdialysis, this review will focus on our recent neurochemical results showing the impact of Brain-Derived Neurotrophic Factor (BDNF) on serotonergic neurotransmission in basal and stimulated conditions. Indeed, although the elevation of 5-HT outflow induced by chronic administration of selective serotonin reuptake inhibitors (SSRIs) causes an increase in BDNF protein levels and expression (mRNA) in the hippocampus of rodents, the reciprocal interaction has not been demonstrated yet. Thus, the neurochemical sight of this question will be addressed here by examining the consequences of either a constitutive decrease or increase in brain BDNF protein levels on hippocampal extracellular levels of 5-HT in conscious mice.

Naphthyl piperazines with dual activity as 5-HT1D antagonists and 5-HT reuptake inhibitors

SAR around a known molecule with dual 5-HT(1D) antagonist and 5-HT(transporter) inhibitory activity has led to the discovery of molecules with improved dual activity and reduced cross-reactivity toward other aminergic receptors (5-HT(1B), alpha(1), and D(2)).

Blockade of 5-HT1A receptors by (+/-)-pindolol potentiates cortical 5-HT outflow, but not antidepressant-like activity of paroxetine: microdialysis and behavioral approaches in 5-HT1A receptor knockout mice

Selective serotonin reuptake inhibitors like paroxetine (Prx) often requires 4-6 weeks to achieve clinical benefits in depressed patients. Pindolol shortens this delay and it has been suggested that this effect is mediated by somatodendritic 5-hydroxytryptamine (5-HT) 1A autoreceptors. However clinical data on the beneficial effects of pindolol are conflicting. To study the effects of (+/-)-pindolol-paroxetine administration, we used genetical and pharmacological approaches in 5-HT1A knockout mice (5-HT1A-/-). Two assays, in vivo intracerebral microdialysis in awake mice and the forced swimming test (FST), were used to assess the antidepressant-like effects of this drug combination. Basal levels of extracellular serotonin, 5-HT ([5-HT]ext) in the frontal cortex (FCX) and the dorsal raphe nucleus (DRN) did not differ between the two strains of mice, suggesting a lack of tonic control of 5-HT1A autoreceptors on nerve terminal 5-HT release. Prx (1 and 4 mg/kg) dose-dependently increased cortical [5-HT]ext in both genotypes, but the effects were greater in mutants. The selective 5-HT1A receptor antagonist, WAY-100635 (0.5 mg/kg), or (+/-)-pindolol (5 and 10 mg/kg) potentiated the effects of Prx (4 mg/kg) on cortical [5-HT]ext in 5-HT1A+/+, but not in 5-HT1A-/- mice. Similar responses were obtained following local intra-raphe perfusion by reverse microdialysis of either WAY-100635 or (+/-)-pindolol (100 microM each). In the FST, Prx administration dose-dependently decreased the immobility time in both strains of mice, but the response was much greater in 5HT1A-/- mice. In contrast, (+/-)-pindolol blocked Prx-induced decreases in the immobility time while WAY-100635 had no effect in both genotypes. These findings using 5-HT1A-/- mice confirm that (+/-)-pindolol behaves as an antagonist of 5-HT1A autoreceptor in mice, but its blockade of paroxetine-induced antidepressant-like effects in the FST may be due to its binding to other neurotransmitter receptors.

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

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

Effects of continuous administration of paroxetine on ligand binding site and expression of serotonin transporter protein in mouse brain

Selective serotonin reuptake inhibitors (SSRIs), such as paroxetine, are utilized in the treatment of depression and anxiety disorders. Although SSRIs potently interfere with the activity of brain serotonin transporter (SERT) after acute treatment, clinical improvement of psychiatric diseases is observed only after the repeated administration for several weeks (2-6 weeks). The present study was undertaken to investigate the effects of continuous administration of paroxetine on specific [3H]paroxetine binding sites and expression of SERT protein in mouse brain. Mice continuously and subcutaneously received paroxetine at doses of 2.67 or 13.3 mumol/kg/day for 21 days by using osmotic minipumps, and the steady-state plasma drug levels were within the range of reported concentrations in the clinical therapy. Continuous administration of paroxetine at theses doses produced significant (25-46%) reduction of [3H]paroxetine binding in each brain region (cerebral cortex, striatum, hippocampus, thalamus, midbrain) of mice. In Western blot analysis, expression levels of SERT protein in the thalamus and midbrain of mice were significantly (51% and 61%, respectively) decreased on day 21 after the implantation of minipumps at the higher dose. In conclusion, this study has firstly shown that continuous administration of paroxetine induces significant reduction of not only ligand binding sites of SERT but the protein expression level in mouse brain. Such down-regulation of SERT may partly underlie the therapeutic effect of long-term treatment with SSRIs in human.

Compréhension du mécanisme d’action des antidépresseurs anciens ou nouveaux : apport des modèles de souris génétiquement modifiées en pharmacologie in vivo

The main hypothesis regarding the mechanism of action of antidepressant drugs is monoaminergic and mainly involves two neurotransmitters, serotonin and noradrenaline. Despite the well-recognized therapeutic efficacy of selective serotonin reuptake inhibitors (SSRIs), some disadvantages still occur. For example, they often require 4-6 weeks to achieve clinical benefits in depressed patients. In the past, some molecules that could shorten this long delay of action have been identified. The role of presynaptic autoreceptors - the activation of which leads to an inhibitory feedback control on neurotransmitter synthesis and release - has been extensively studied for antidepressant effects. In our laboratory, we studied the combined effects of an SSRI and a serotonin autoreceptor antagonist of the 5-HT1B subtype using intracerebral in vivo microdialysis in awake, freely moving mice. Important information on SSRIs has been obtained by applying this technique to genetically modified animals, such as constitutive knockout (KO) mice lacking 5-HT1B receptors (5-HT1B KO) generated by homologous recombination: we compared the effects of a combined treatment on extracellular/intrasynaptic levels of serotonin in various nerve terminals area in wild-type control and KO mice. Thus, we found that indirect activation of 5-HT1B autoreceptors limits the effects of SSRIs on dialysate 5-HT levels at serotonergic nerve terminals such as the ventral hippocampus. The study of substance P (neurokinin 1 receptor [R-NK1]) offers another example of the use of KO mice in the development of a new class of antidepressant drugs. NK1 receptor antagonists may display anxiolytic/antidepressant-like properties. The lack of selective compounds for each tachykinin receptor subtype (R-NK 1, R-NK2 or R-NK3) and differences in their affinity between animal species have made R-NK1 KO mice a very useful experimental tool. In collaborative work we found that genetic (R-NK1 KO mice) or pharmacological (GR205171) blockade of R-NK1 is associated with several changes: the increase in cortical 5-HT outflow caused by systemic injection of paroxetine was 4- to 6-fold higher in freely moving R-NK1 KO mice than in wild-type controls. The constitutive lack of NK1 receptors is associated with a functional desensitization of somatodendritic 5-HT1A autoreceptors, resembling that induced by chronic treatment with SSRI antidepressants. These results highlight the link between a neurotransmitter (serotonin) and a neuropeptide (substance P). This genetic strategy allowed us to point out that multiple targets participate to the effects of classical antidepressant drugs within the brain. We hope that, soon, some mice lines (constitutive or tissue specific, conditional rescue mice having alterations of sleep/wakefulness and/or food intake, altered central serotonin and/or noradrenaline neurotransmission, deficit in neurotrophic factors, but increases in intrasynaptic concentrations of substance P) could be a relevant model of the physiopathology of depressive disorders, and could help us understand the appearance of some symptoms. These recent findings suggest that instead of being rejected, the monoaminergic hypothesis of depression should be improved, corrected and completed by studying the role of other neurotransmitter, neuromodulatory compounds (substance P, BDNF [brain-derived neurotrophic factor]). By doing so, it thus could be possible to improve antidepressant drug treatment, i.e. shorten their long delay of action and/or to decrease treatment resistance or improve its tolerance.

Sex differences in the regulation of serotonergic transmission and behavior in 5-HT receptor knockout mice

Few studies have examined the relationship between genetics, stress, and sex-linked differences in neurotransmitter systems. Examining serotonin (5-HT) receptor knockout mice on stress-induced behavioral depression, female 5-HT1B receptor knockout mice demonstrated significantly reduced immobility than either male 5-HT1B receptor knockout mice or male and female wild-type mice on the tail suspension test (TST) and forced swimming test. The behavioral phenotype was identified as likely due to a disinhibition of 5-HT release, because depletion of 5-HT with parachlorophenylalanine selectively reduced immobility of female 5-HT1B receptor knockout mice in the TST. In contrast, male and female 5-HT1A receptor knockout mice demonstrated reduced immobility compared with control mice, but the depletion of 5-HT with PCPA did not reverse the antidepressant-like phenotype. Microdialysis studies confirmed significantly higher baseline levels of hippocampal 5-HT in female, but not male, 5-HT1B receptor knockout mice. Both male and female 5-HT1B receptor knockout mice demonstrated augmented dialysate responses to fluoxetine. Also, both male and female 5-HT1B receptor knockout mice demonstrated reductions of immobility in the TST after treatment with fluoxetine. Therefore, female 5-HT1B receptor knockout mice demonstrate a sex-linked disinhibition of 5-HT release that sustained higher baseline levels of hippocampal 5-HT and behavioral vulnerability to 5-HT depletion.

Rapid and precise method to locate microdialysis probe implantation in the rodent brain

An important concern about microdialysis methodology is the histological validation of the dialysis probe implantation site in brain tissue of rodents (rat, mouse). Several methods have been described on standard histological staining (i.e., cresyl violet, formalin fixation, fast green perfusion, etc.). However, this methodology is time consuming. These requirements are not compatible with a histological validation prior to analysis of microdialysis samples. Here, we developed a new method to locate the track of the dialysis probe in the rodent brain. This method is based on a digital photomicrograph of a coronal section of the rodent frozen brain. The fitting of an appropriate coronal diagram of the rats' and mice' brain atlas with this photomicrograph, allowed us to locate precisely and quickly the track of the dialysis probe.

Altering the course of neurodevelopment: a framework for understanding the enduring effects of psychotropic drugs

Childhood is a time filled with wondrous changes, as brain plasticity permits experiences to shape the immature brain to meet the demands of the environment. Change occurs at various levels--from neuroanatomy, including within a given region and its connectivity to other regions, to the function of neurotransmitter systems and their reactivity to pharmacological agents in the short- and long-term. The nature and degree to which drug exposure influences the final adult topography is influenced greatly by the maturational phase of these critical factors. Moreover, evidence is slowly emerging that suggests that the long-term effects of drug exposure are delayed and expressed once the vulnerable system reaches maturation (i.e., typically during adulthood). This phenomenon is known as neuronal imprinting and occurs when the effects of drug exposure outlast the drug itself. Thus, understanding the persistent effects critically depends on the window of observation. Embracing this concept should influence how we conduct preclinical assessments of developmental drug exposure, and ultimately how we conduct clinical assessments of drug efficacy, effectiveness, and safety for the treatment of childhood psychiatric disorders. In this article, we present a model to provide a heuristic framework for making predictions about imprinted effects of childhood drug exposure. We then review epidemiological data on attention deficit hyperactivity disorder (ADHD) and childhood depression, prescription practices, and what is known regarding the long-term consequences of drug exposure in these populations. We conclude with a discussion of the current status of preclinical studies on juvenile stimulant exposure.

Exploring the relationship between serotonin and brain-derived neurotrophic factor: analysis of BDNF protein and extraneuronal 5-HT in mice with reduced serotonin transporter or BDNF expression

Serotonin (5-HT) has been proposed to promote neuronal plasticity during the treatment of mood and anxiety disorders and following neurodegenerative insult by altering the expression of critical genes including brain-derived neurotrophic factor (BDNF). In this study, mice with constitutive reductions in the serotonin transporter (SERT) or BDNF were investigated to further assess the functional relationship between serotonin neurotransmission and BDNF expression. Using a modified extraction procedure and a commercial enzyme-linked immunosorbant assay, 50% decreases in BDNF protein in hippocampus, frontal cortex and brain stem were confirmed in 4-month-old mice lacking one copy of the BDNF gene (BDNF(+/-)). By contrast, 4-month-old male and female mice with partial (SERT(+/-)) or complete (SERT(-/-)) reductions in SERT expression showed no differences in BDNF protein levels compared to SERT(+/+) mice, although male SERT knockout mice of all genotypes had higher BDNF levels in hippocampus, frontal cortex, and brain stem than female animals. Microdialysis also was performed in BDNF(+/-) mice. In addition to other phenotypic aspects suggestive of altered serotonin neurotransmission, BDNF(+/-) mice show accelerated age-related degeneration of 5-HT forebrain innervation. Nevertheless, extracellular 5-HT levels determined by zero net flux microdialysis were similar between BDNF(+/+) and BDNF(+/-) mice in striatum and frontal cortex at 8-12 months of age. These data illustrate that a 50% decrease in BDNF does not appear to be sufficient to cause measurable changes in basal extracellular 5-HT concentrations and, furthermore, that constitutive reductions in SERT expression are not associated with altered BDNF protein levels at the ages and in the brain regions examined in this study.

Serotonin transporter in substance P (neurokinin 1) receptor knock-out mice

We recently demonstrated that mice lacking the gene for substance P (neurokinin 1) receptors (NK1-/-) show improved cortical dialysate serotonin (5-HT) responses to paroxetine [J. Neurosci. 21 (2001) 8188]. To test for changes that may involve the 5-HT transporter (5-HTT) in these mutant mice, in vivo/in vitro studies were performed. Autoradiographic quantification of 5-HTT was performed: [3H]citalopram binding did not reveal any modification of 5-HT binding sites in the dorsal raphe nucleus (DRN) of wild-type NK1+/+ control and mutant NK1-/- mice. These results were further confirmed by 5-HTT mRNA quantification using competitive reverse transcription and polymerase chain reaction (RT-PCR) assay, which showed similar messenger levels in the DRN of both mice genotypes. The functional status of 5-HTT in vivo was tested by using the zero net flux method of quantitative microdialysis in two serotonergic nerve terminal regions, the frontal cortex and ventral hippocampus, of wild-type NK1+/+ and NK1-/- mice. Neither basal extracellular 5-HT levels nor the 5-HT extraction fraction of the probe (Ed an index of 5-HT uptake in vivo) differed between wild-type and mutant mice in the two brain regions studied. These results suggest that no compensatory response to the constitutive deletion of the tachykinin NK1 receptor involving changes in the activity of the selective 5-HT transporter occurred in the DRN, frontal cortex and ventral hippocampus in mice.