Antidepressant-induced regulation of 5-HT(1b) mRNA in rat dorsal raphe nucleus reverses rapidly after drug discontinuation

Chronic fluoxetine treatment desensitizes serotoninergic inhibition of GABA inputs and the intrinsic excitability of dorsal raphe serotonin neurons

Dorsal raphe serotonin (5-hydroxytryptamine, 5-HT) neurons are spontaneously active and release 5-HT that is critical to normal brain function such mood and emotion. Serotonin reuptake inhibitors (SSRIs) increase the synaptic and extracellular 5-HT level and are effective in treating depression. Treatment of two weeks or longer is often required for SSRIs to exert clinical benefits. The cellular mechanism underlying this delay was not fully understood. Here we show that the GABAergic inputs inhibit the spike firing of raphe 5-HT neurons; this GABAergic regulation was reduced by 5-HT, which was prevented by G-protein-activated inwardly rectifying potassium (Girk) channel inhibitor tertiapin-Q, indicating a contribution of 5-HT activation of Girk channels in GABAergic presynaptic axon terminals. Equally important, after 14 days of treatment of fluoxetine, a widely used SSRI type antidepressant, this 5-HT inhibition of GABAergic inputs was substantially downregulated. Furthermore, the chronic fluoxetine treatment substantially downregulated the 5-HT activation of the inhibitory Girk current in 5-HT neurons. Taken together, our results suggest that chronic fluoxetine administration, by blocking 5-HT reuptake and hence increasing the extracellular 5-HT level, can downregulate the function of 5-HT1B receptors on the GABAergic afferent axon terminals synapsing onto 5-HT neurons, allowing extrinsic, behaviorally important GABA neurons to more effectively influence 5-HT neurons; simultaneously, chronic fluoxetine treatment also downregulate somatic 5-HT autoreceptor-activated Girk channel-mediated hyperpolarization and decrease in input resistance and intrinsic excitability, rendering 5-HT neurons resistant to autoinhibition and leading to increased 5-HT neuron activity, potentially contributing to the antidepressant effect of SSRIs.

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 Serotonylation and SERT Posttranslational Modifications in Alzheimer's Disease Pathogenesis

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is implicated mainly in Alzheimer's disease (AD) and reported to be responsible for several processes and roles in the human body, such as regulating sleep, food intake, sexual behavior, anxiety, and drug abuse. It is synthesized from the amino acid tryptophan. Serotonin also functions as a signal between neurons to mature, survive, and differentiate. It plays a crucial role in neuronal plasticity, including cell migration and cell contact formation. Various psychiatric disorders, such as depression, schizophrenia, autism, and Alzheimer's disease, have been linked to an increase in serotonin-dependent signaling during the development of the nervous system. Recent studies have found 5-HT and other monoamines embedded in the nuclei of various cells, including immune cells, the peritoneal mast, and the adrenal medulla. Evidence suggests these monoamines to be involved in widespread intracellular regulation by posttranslational modifications (PTMs) of proteins. Serotonylation is the calcium-dependent process in which 5-HT forms a long-lasting covalent bond to small cytoplasmic G-proteins by endogenous transglutaminase 2 (TGM2). Serotonylation plays a role in various biological processes. The purpose of our article is to summarize historical developments and recent advances in serotonin research and serotonylation in depression, aging, AD, and other age-related neurological diseases. We also discussed several of the latest developments with Serotonin, including biological functions, pathophysiological implications and therapeutic strategies to treat patients with depression, dementia, and other age-related conditions.

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

Serotonin 5-HT1B receptors mediate the antidepressant- and anxiolytic-like effects of ventromedial prefrontal cortex deep brain stimulation in a mouse model of social defeat

BACKGROUND

Deep brain stimulation (DBS) delivered to the ventromedial prefrontal cortex (vmPFC) induces antidepressant- and anxiolytic-like responses in various animal models. Electrophysiology and neurochemical studies suggest that these effects may be dependent, at least in part, on the serotonergic system. In rodents, vmPFC DBS reduces raphe cell firing and increases serotonin (5-HT) release and the expression of serotonergic receptors in different brain regions.

METHODS

We examined whether the behavioural responses of chronic vmPFC DBS are mediated by 5-HT_1A or 5-HT_1B receptors through a series of experiments. First, we delivered stimulation to mice undergoing chronic social defeat stress (CSDS), followed by a battery of behavioural tests. Second, we measured the expression of 5-HT_1A and 5-HT_1B receptors in different brain regions with western blot. Finally, we conducted pharmacological experiments to mitigate the behavioural effects of DBS using the 5-HT_1A antagonist, WAY-100635, or the 5-HT_1B antagonist, GR-127935.

RESULTS

We found that chronic DBS delivered to stressed animals reduced the latency to feed in the novelty suppressed feeding test (NSF) and immobility in the forced swim test (FST). Though no significant changes were observed in receptor expression, 5-HT_1B levels in DBS-treated animals were found to be non-significantly increased in the vmPFC, hippocampus, and nucleus accumbens and reduced in the raphe compared to non-stimulated controls. Finally, while animals given vmPFC stimulation along with WAY-100635 still presented significant responses in the NSF and FST, these were mitigated following GR-127935 administration.

CONCLUSIONS

The antidepressant- and anxiolytic-like effects of DBS in rodents may be partially mediated by 5-HT1_B receptors.

Combined HTR1A/1B methylation and human functional connectome to recognize patients with MDD

OBJECTIVES

This study aimed to use a machine-learning method to identify HTR1A/1B methylation and resting-state functional connectivity (rsFC) related to the diagnosis of MDD, then try to build classification models for MDD diagnosis based on the identified features.

METHODS

Peripheral blood samples were collected from all recruited participants, and part of the participants underwent the resting-state fMRI scan. Features including HTR1A/1B methylation and rsFC were calculated. Then, the initial feature sets of epigenetics and neuroimaging were separately input into an all-relevant feature selection to generate significant discriminative power for MDD diagnosis. Random forest classifiers were constructed and evaluated based on identified features. In addition, the SHapley Additive exPlanations (SHAP) method was adapted to interpret the diagnostic model.

RESULTS

A combination of selected HTR1A/1B methylation and rsFC feature sets achieved better performance than using either one alone - a distinction between MDD and healthy control groups was achieved at 81.78% classification accuracy and 0.8948 AUC.

CONCLUSION

A high classification accuracy can be achieved by combining multidimensional information from epigenetics and cerebral radiomic features in MDD. Our approach can be helpful for accurate clinical diagnosis of MDD and further exploring the pathogenesis of MDD.

In vivo correlation of serotonin transporter and 1B receptor availability in the human brain: a PET study

Synaptic serotonin levels in the brain are regulated by active transport into the bouton by the serotonin transporter, and by autoreceptors, such as the inhibitory serotonin (5-HT) 1B receptor which, when activated, decreases serotonin release. Animal studies have shown a regulatory link between the two proteins. Evidence of such coupling could translate to an untapped therapeutic potential in augmenting the effect of selective serotonin reuptake inhibitors through pharmacological modulation of 5-HT_1B receptors. Here we will for the first time in vivo examine the relationship between 5-HT_1B receptors and serotonin transporters in the living human brain. Seventeen healthy individuals were examined with PET twice, using the radioligands [11C]AZ10419369 and [¹¹C]MADAM for quantification of the 5-HT_1B receptor and the 5-HT transporter, respectively. The binding potential was calculated for a set of brain regions, and the correlations between the binding estimates of the two radioligands were studied. [¹¹C]AZ10419369 and [¹¹C]MADAM binding was positively correlated in all examined brain regions. In most cortical regions the correlation was strong, e.g., frontal cortex, r(15) = 0.64, p = 0.01 and parietal cortex, r(15) = 0.8, p = 0.0002 while in most subcortical regions, negligible correlations was observed. Though the correlation estimates in cortex should be interpreted with caution due to poor signal to noise ratio of [¹¹C]MADAM binding in these regions, it suggests a link between two key proteins involved in the regulation of synaptic serotonin levels. Our results indicate a need for further studies to address the functional importance of 5-HT_1B receptors in treatment with drugs that inhibit serotonin reuptake.

Dose-dependent opposite effects of nortriptyline on affective-like behavior in adolescent rats: Comparison with adult rats

Antidepressant drugs elicit different behavioral and neurochemical responses with age. In fact, the use of antidepressants during adolescence is associated with an increased risk of suicidal thinking, being the best pharmacological treatment during this critical period a matter of constant debate in terms of its risk-benefit outcome. In this regard, the present study compared the effects of nortriptyline (3-10 mg/kg, 7 days) on regulating different aspects of affective-like behavior by screening adolescent and adult Sprague-Dawley rats through several consecutive tests (forced-swim, open field, sucrose preference). Brains were later collected to evaluate hippocampal neurogenesis and mBDNF protein content as potential molecular correlates of the observed behavioral responses. The main results in adolescent rats showed that nortriptyline induced dose-dependent opposite effects: while 3 mg/kg decreased immobility and increased mBDNF (indicative of an antidepressant-like response), 10 mg/kg decreased exploratory time in the open field and mBDNF (suggestive of an anxiogenic-like response). These effects were not associated with changes in neurogenesis regulation. In adult rats, nortriptyline failed to modulate affective-like behavior or the neuroplasticity markers evaluated at the doses tested. In conclusion, clear behavioral and neurochemical differences were observed between adolescent and adult rats in response to nortriptyline treatment. Interestingly, while nortriptyline displayed an antidepressant-like potential at the lowest dose examined in adolescence, a higher dose shifted these results towards a negative outcome, thus reinforcing the need to extreme caution when considering this treatment for our younger population.

Electroacupuncture improves synaptic plasticity by regulating the 5-HT1A receptor in hippocampus of rats with chronic unpredictable mild stress

Objectives

To investigate the antidepressant effects of electroacupuncture (EA) on chronic unpredictable mild stress (CUMS) in rats, as well as the effects of EA on hippocampal neurons, synaptic morphology, and 5-hydroxytryptamine (HT) receptor expression.

Methods

Forty adult male Wistar rats were randomly divided into normal control, CUMS, EA, and paroxetine groups. CUMS modeling was performed for 21 days, followed by 14 days of intervention: rats in the EA group underwent stimulation of GV20 and GV29 acupuncture points for 30 minutes daily; rats in the paroxetine group were administered paroxetine daily. Behavioral tests, transmission electron microscopy, western blotting, and real-time quantitative polymerase chain reaction were used to evaluate the effects of the intervention.

Results

EA treatment reversed the behavioral changes observed in rats due to CUMS modeling; it also improved the pathological changes in organelles and synaptic structures of hippocampal neurons, and upregulated the protein and mRNA expression levels of 5-HT1A receptor. There were no significant differences in 5-HT1B receptor protein and mRNA expression levels among the groups.

Conclusions

EA treatment can alleviate depression-like symptoms in CUMS rats. The underlying mechanism may include promoting the expression of 5-HT1A receptor mRNA and protein, thereby improving synaptic plasticity in the hippocampus.

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.

Antidepressant-like effects of a novel curcumin derivative J147: Involvement of 5-HT1A receptor

Depression is a dysthymia disorder characterized by a pervasive or persistent mental disorder that causes mood, cognitive and memory deficits. J147, a curcumin analogue, increases brain derived neurotrophic factor (BDNF) levels and facilitates memory in animals. Because curcumin has the antidepressant-like activity, the present study investigated the potential antidepressant-like effects of J147 in the forced swimming test (FST) and tail suspension tests (TST) and the involvement of 5-HT receptors related to cAMP signaling. The results suggested that acute treatment of J147 at doses of 5 and 10 mg/kg via gavage markedly reduced the duration of immobility in both TST and FST, either 1 h or 3 h after treatment, respectively. It did not alter locomotor activity but influence the immobile response. The molecular biological assays showed that 5-HT_1A receptor expression was significantly increased at 1 h after treatment with J147 at a dose of 10 mg/kg. In addition, pre-treatment of mice with WAY-100635 blocked the J147's effect in the FST. 5-HT_1B receptor expression was not significantly increased with increasing doses of J147. The 5-HT_1B receptors antagonist isamoltan partially prevented J147's effect in the FST. The levels of downstream molecular targets, cAMP, PKA, pCREB and BDNF were significantly increased 1 h after treatment with J147 at doses of 5 and 10 mg/kg. The up-regulated pCREB and BDNF levels lasted for 3 h after 10 mg/kg of J147. These findings demonstrated that J147 has antidepressant-like effects that are mediated, at least in part, by activating the 5-HT_1A/cAMP/PKA/CREB/BDNF-signaling pathway.

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.

Epigenetic and genetic variants in the HTR1B gene and clinical improvement in children and adolescents treated with fluoxetine

The serotonin 1B receptor (5-HT_1B) is important to both the pathogenesis of major depressive disorder and the antidepressant effects of selective serotonin reuptake inhibitors. Although fluoxetine has been shown to be effective and safe in children and adolescents, not all patients experience a proper clinical response, which has led to further study into the main factors involved in this inter-individual variability. Our aim was to study the effect of epigenetic and genetic factors that could affect 5-hydroxytryptamine receptor 1B (HTR1B) gene expression, and thereby response to fluoxetine. A total of 83 children and adolescents were clinically assessed 12weeks after of initiating an antidepressant treatment with fluoxetine for the first time. We evaluated the influence of single nucleotide polymorphisms (SNPs) specifically located in transcription factor binding sites (TFBSs) on their clinical improvement. A combined genetic analysis considering the significant SNPs together with the functional variant rs130058 previously associated in our population was also performed. Moreover, we assessed, for the first time in the literature, whether methylation levels of the HTR1B promoter region could be associated with the pharmacological response. Two, rs9361233 and rs9361235, were significantly associated with clinical improvement after treatment with fluoxetine. The heterozygous genotype combination analysis showed a negative correlation with clinical improvement. The lowest improvement was experienced by patients who were heterozygous for all three SNPs. Moreover, a negative correlation was found between clinical improvement and the average methylation level of the HTR1B promoter. These results give new evidence for the role of epigenetic and genetic factors which could modulate HTR1B expression in the pharmacological response to antidepressants.

Low 5-HT1B receptor binding in the migraine brain: A PET study

Background The pathophysiology of migraine may involve dysfunction of serotonergic signaling. In particular, the 5-HT_1B receptor is considered a key player due to the efficacy of 5-HT_1B receptor agonists for treatment of migraine attacks. Aim To examine the cerebral 5-HT_1B receptor binding in interictal migraine patients without aura compared to controls. Methods Eighteen migraine patients, who had been migraine free for >48 hours, and 16 controls were scanned after injection of the 5-HT_1B receptor specific radioligand [¹¹C]AZ10419369 for quantification of cerebral 5-HT_1B receptor binding. Patients who reported migraine <48 hours after the PET examination were excluded from the final analysis. We defined seven brain regions involved in pain modulation as regions of interest and applied a latent variable model (LVM) to assess the group effect on binding across these regions. Results Our data support a model wherein group status predicts the latent variable ( p = 0.038), with migraine patients having lower 5-HT_1B receptor binding across regions compared to controls. Further, in a whole-brain voxel-based analysis, time since last migraine attack correlated positively with 5-HT_1B receptor binding in the dorsal raphe and in the midbrain. Conclusion We report here for the first time that migraine patients have low 5-HT_1B receptor binding in pain modulating regions, reflecting decreased receptor density. This is either a primary constitutive trait of the migraine brain or secondary to repeated exposure to migraine attacks. We also provide indirect support for the dorsal raphe 5-HT_1B receptors being temporarily downregulated during the migraine attack, presumably in response to higher cerebral serotonin levels in the ictal phase.

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.

Revisiting the Serotonin Hypothesis: Implications for Major Depressive Disorders

Major depressive disorder (MDD) is a heritable neuropsychiatric disease associated with severe changes at cellular and molecular levels. Its diagnosis mainly relies on the characterization of a wide range of symptoms including changes in mood and behavior. Despite the availability of antidepressant drugs, 10 to 30 % of patients fail to respond after a single or multiple treatments, and the recurrence of depression among responsive patients is very high. Evidence from the past decades suggests that the brain neurotransmitter serotonin (5-HT) is incriminated in MDD, and that a dysfunction of 5-HT receptors may play a role in the genesis of this disease. The 5-HT membrane transporter protein (SERT), which helps regulate the serotonergic transmission, is also implicated in MDD and is one of the main targets of antidepressant therapy. Although a number of behavioral tests and animal models have been developed to study depression, little is known about the neurobiological bases of MDD. Understanding the role of the serotonergic pathway will significantly help improve our knowledge of the pathophysiology of depression and may open up avenues for the development of new antidepressant drugs. The overarching goal of this review is to present recent findings from studies examining the serotonergic pathway in MDD, with a focus on SERT and the serotonin 1A (5-HT1A), serotonin 1B (5-HT1B), and serotonin 2A (5-HT2A) receptors. This paper also describes some of the main molecules involved in the internalization of 5-HT receptors and illustrates the changes in 5-HT neurotransmission in knockout mice and animal model of depression.

Chronic SSRI stimulation of astrocytic 5-HT2B receptors change multiple gene expressions/editings and metabolism of glutamate, glucose and glycogen: a potential paradigm shift

It is firmly believed that the mechanism of action of SSRIs in major depression is to inhibit the serotonin transporter, SERT, and increase extracellular concentration of serotonin. However, this undisputed observation does not prove that SERT inhibition is the mechanism, let alone the only mechanism, by which SSRI's exert their therapeutic effects. It has recently been demonstrated that 5-HT2B receptor stimulation is needed for the antidepressant effect of fluoxetine in vivo. The ability of all five currently used SSRIs to stimulate the 5-HT2B receptor equipotentially in cultured astrocytes has been known for several years, and increasing evidence has shown the importance of astrocytes and astrocyte-neuronal interactions for neuroplasticity and complex brain activity. This paper reviews acute and chronic effects of 5-HT2B receptor stimulation in cultured astrocytes and in astrocytes freshly isolated from brains of mice treated with fluoxetine for 14 days together with effects of anti-depressant therapy on turnover of glutamate and GABA and metabolism of glucose and glycogen. It is suggested that these events are causally related to the mechanism of action of SSRIs and of interest for development of newer antidepressant drugs.

Changes in tau phosphorylation levels in the hippocampus and frontal cortex following chronic stress

Studies have indicated that early-life or early-onset depression is associated with a 2- to 4-fold increased risk of developing Alzheimer's disease (AD). In AD, aggregation of an abnormally phosphorylated form of the tau protein may be a key pathological event. Tau is known to play a major role in promoting microtubule assembly and stabilization, and in maintaining the normal morphology of neurons. Several studies have reported that stress may induce tau phosphorylation. The main aim of the present study was to investigate possible alterations in the tau protein in the hippocampus and frontal cortex of 32 male Sprague-Dawley rats exposed to chronic unpredictable mild stress (CUMS) and then re-exposed to CUMS to mimic depression and the recurrence of depression, respectively, in humans. We evaluated the effects of CUMS, fluoxetine, and CUMS re-exposure on tau and phospho-tau. Our results showed that a single exposure to CUMS caused a significant reduction in sucrose preference, indicating a state of anhedonia. The change in behavior was accompanied by specific alterations in phospho-tau protein levels, but fluoxetine treatment reversed the CUMS-induced impairments. Moreover, changes in sucrose preference and phospho-tau were more pronounced in rats re-exposed to CUMS than in those subjected to a single exposure. Our results suggest that changes in tau phosphorylation may contribute to the link between depression and AD.

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.

The genetics of selective serotonin reuptake inhibitors

Selective serotonin reuptake inhibitors (SSRIs) are among the most widely prescribed drugs in psychiatry. Based on the fact that SSRIs increase extracellular monoamine levels in the brain, the monoamine hypothesis of depression was introduced, postulating that depression is associated with too low serotonin, dopamine and noradrenaline levels. However, several lines of evidence indicate that this hypothesis is too simplistic and that depression and the efficacy of SSRIs are dependent on neuroplastic changes mediated by changes in gene expression. Because a coherent view on global gene expression is lacking, we aim to provide an overview of the effects of SSRI treatment on the final targets of 5-HT receptor signal transduction pathways, namely the transcriptional regulation of genes. We address gene polymorphisms in humans that affect SSRI efficacy, as well as in vitro studies employing human-derived cells. We also discuss the molecular targets affected by SSRIs in animal models, both in vivo and in vitro. We conclude that serotonin transporter gene variation in humans affects the efficacy and side-effects of SSRIs, whereas SSRIs generally do not affect serotonin transporter gene expression in animals. Instead, SSRIs alter mRNA levels of genes encoding serotonin receptors, components of non-serotonergic neurotransmitter systems, neurotrophic factors, hypothalamic hormones and inflammatory factors. So far little is known about the epigenetic and age-dependent molecular effects of SSRIs, which might give more insights in the working mechanism(s) of SSRIs.

Understanding the molecular pharmacology of the serotonergic system: using fluoxetine as a model

OBJECTIVES

Serotonin is a monoamine neurotransmitter that is widely distributed in the body and plays an important role in a variety of psychological and other body functions such as mood, sexual desire and function, appetite, sleep, memory and learning, temperature regulation and social behaviour. This review will assess the use of fluoxetine, one of the most commonly used selective serotonin reuptake inhibitors, as a model for understanding the molecular pharmacology of the serotoninergic system.

KEY FINDINGS

Seven serotonin receptor families have been discovered to date. All serotonin receptors, except 5-HT(3), are G-protein coupled, seven transmembrane receptors that activate an intracellular second messenger cascade. The 5-HT(3) receptor is a ligand-gated ion channel. Furthermore, 5-HT(1A) receptors are known as autoreceptors since their stimulation inhibits the release serotonin in nerve terminals. A transporter protein found in the plasma membrane of serotonergic neurones is responsible for the reuptake of this neurotransmitter. Selective serotonin reuptake inhibitors, such as fluoxetine, act primarily at the serotonin transporter protein and have limited, if any, reaction with other neurotransmitter systems. Selective serotonin reuptake inhibitors appear to bind with the serotonin transporter with different rates of occupancy, duration and potency.

SUMMARY

The following review focuses on the interaction of serotonin with this membrane transporter in the body and assesses the use of fluoxetine as a reference drug in the understanding of this interaction.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

Efficacy of exercise in reducing depressive symptoms across 5-HTTLPR genotypes

INTRODUCTION

Exercise is effective in the alleviation of depressive symptoms and may have physiological effects similar to those of selective serotonin reuptake inhibitors (SSRI). Recent research has identified the difference in treatment effects across genetic polymorphisms of the serotonin transporter polymorphic region (5-HTTLPR), in which the l allele has been associated with a better response to SSRI compared with the s allele. The purpose of the current research was to examine the antidepressant effects of exercise across 5-HTTLPR genotypes.

METHODS

Participants, ages 18–23 yr, were randomly assigned to a 5-wk exercise intervention or a no-treatment control group. Participants completed the Beck Depression Inventory before and after the intervention and provided a saliva sample for DNA analysis.

RESULTS

Exercise resulted in a significant reduction in depressive symptoms compared with the control group. In addition, individuals with at least one l allele demonstrated greater reductions in depressive symptoms compared with ss individuals.

CONCLUSIONS

The effects of exercise on depressive symptoms appear to be moderated by 5-HTTLPR genotype, suggesting that the mechanisms responsible for the alleviation of depressive symptoms are similar for exercise and SSRI treatment. Furthermore, these findings suggest that 5-HTTLPR genotype should be a factor in determining the proper line of treatment for depression.

The role of serotonin receptor subtypes in treating depression: a review of animal studies

RATIONALE

Serotonin reuptake inhibitors (SSRIs) are effective in treating depression. Given the existence of different families and subtypes of 5-HT receptors, multiple 5-HT receptors may be involved in the antidepressant-like behavioral effects of SSRIs.

OBJECTIVE

Behavioral pharmacology studies investigating the role of 5-HT receptor subtypes in producing or blocking the effects of SSRIs were reviewed.

RESULTS

Few animal behavior tests were available to support the original development of SSRIs. Since their development, a number of behavioral tests and models of depression have been developed that are sensitive to the effects of SSRIs, as well as to other types of antidepressant treatments. The rationale for the development and use of these tests is reviewed. Behavioral effects similar to those of SSRIs (antidepressant-like) have been produced by agonists at 5-HT(1A), 5-HT(1B), 5-HT(2C), 5-HT(4), and 5-HT(6) receptors. Also, antagonists at 5-HT(2A), 5-HT(2C), 5-HT(3), 5-HT(6), and 5-HT(7) receptors have been reported to produce antidepressant-like responses. Although it seems paradoxical that both agonists and antagonists at particular 5-HT receptors can produce antidepressant-like effects, they probably involve diverse neurochemical mechanisms. The behavioral effects of SSRIs and other antidepressants may also be augmented when 5-HT receptor agonists or antagonists are given in combination.

CONCLUSIONS

The involvement of 5-HT receptors in the antidepressant-like effects of SSRIs is complex and involves the orchestration of stimulation and blockade at different 5-HT receptor subtypes. Individual 5-HT receptors provide opportunities for the development of a newer generation of antidepressants that may be more beneficial and effective than SSRIs.

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.

Effect of triiodothyronine on 5-HT1A and 5-HT1B receptor expression in rat forebrain and on latency to feed in the novelty suppressed feeding test

Thyroid hormones, particularly triiodothyronine (T3), have long been used for the treatment of depression, most frequently to enhance the therapeutic activity of other antidepressants. The purpose of this study was to evaluate possible underlying mechanisms for the antidepressant activity of T3. The effects of T3 20 microg/kg/d S.C. and fluoxetine 5mg/kg/d I.P. given alone or in combination for 7 days on the transcription rates of inhibitory serotonergic receptors (5-HT1A and 5-HT1B) were studied in different brain areas of male Sabra rats using real-time PCR. Significant effects of fluoxetine were found on the expression of 5-HT1B receptors in the frontal cortex and of T3 on the expression of 5-HT1A receptors in the amygdala and hippocampus and 5-HT1B receptors in the frontal and entorhinal cortices, the expression being reduced in all cases. An effect of the combination of T3 plus fluoxetine to reduce transcription was observed for 5-HT1A receptors, in the amygdala and dentate gyrus and for 5-HT1B receptors in the entorhinal cortex and anterior raphe nucleus. In the second experiment, the novelty suppressed feeding test (NFST) was used to examine the effects of fluoxetine 5mg/kg/d I.P. and T3 20 or 50 microg/kg/d, alone or in combination for 12 days, on latency to feed. Only the combinations of T3 (20 or 50 microg/kg/d) and fluoxetine (5mg/kg/d) yielded significant behavioral effects in this test. The results of our studies suggest that the mechanism underlying the antidepressant effect of T3 may involve a reduction in 5-HT1A and 5-HT1B receptor transcription rates.

Transcriptional expression of serotonergic regulators in laser-captured microdissected dorsal raphe neurons of subjects with major depressive disorder: sex-specific differences

The relationship between serotonin (5-HT) and major depressive disorder (MDD) has been extensively studied but certain aspects are still ambiguous. Given the evidence that 5-HT neurotransmission is reduced in depressed subjects, it is possible that one or more of the 5-HT regulators may be altered in the dorsal raphe nucleus (DR) of depressed subjects. Candidates that regulate 5-HT synthesis and neuronal activity of 5-HT neurons include intrinsic regulators such as tryptophan hydroxylase 2, 5-HT autoreceptors, 5-HT transporter and transcription factors, as well as afferent regulators such as estrogen and brain-derived neurotrophic factor. The present study was designed to quantify mRNA concentrations of the above 5-HT regulators in an isolated population of 5-HT-containing DR neurons of MDD subjects and gender-matched psychiatrically normal control subjects. We found that mRNA concentrations of the 5-HT1D receptor and the transcription factors, NUDR and REST, were significantly increased in DR-captured neurons of female MDD subjects compared to female control subjects. No significant differences were found for the transcripts in male MDD subjects compared to male controls. This study reveals sex-specific alterations in gene expression of the pre-synaptic 5-HT1D autoreceptors and 5-HT-related transcription factors, NUDR and REST, in DR neurons of women with MDD.

Chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain

BACKGROUND

Serotonin (5-HT) is a neurotransmitter with important roles in the regulation of neurobehavioral processes, particularly those regulating affect in humans. Drugs that potentiate serotonergic neurotransmission by selectively inhibiting the reuptake of serotonin (SSRIs) are widely used for the treatment of psychiatric disorders. Although the regulation of serotonin synthesis may be an factor in SSRI efficacy, the effect of chronic SSRI administration on 5-HT synthesis is not well understood. Here, we describe effects of chronic administration of the SSRI citalopram (CIT) on 5-HT synthesis and content in the mouse forebrain.

METHODOLOGY/PRINCIPAL FINDINGS

Citalopram was administered continuously to adult male C57BL/6J mice via osmotic minipump for 2 days, 14 days or 28 days. Plasma citalopram levels were found to be within the clinical range. 5-HT synthesis was assessed using the decarboxylase inhibition method. Citalopram administration caused a suppression of 5-HT synthesis at all time points. CIT treatment also caused a reduction in forebrain 5-HIAA content. Following chronic CIT treatment, forebrain 5-HT stores were more sensitive to the depleting effects of acute decarboxylase inhibition.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrate that chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain. Furthermore, our results indicate that chronic 5-HT reuptake inhibition renders 5-HT brain stores more sensitive to alterations in serotonin synthesis. These results suggest that the regulation of 5-HT synthesis warrants consideration in efforts to develop novel antidepressant strategies.

Estrogen decreases 5-HT1B autoreceptor mRNA in selective subregion of rat dorsal raphe nucleus: inverse association between gene expression and anxiety behavior in the open field

We have recently shown that estrogen decreases anxiety and increases expression of tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme for 5-HT synthesis. However, the effects of estrogen on 5-HT release and reuptake may also affect the overall availability of 5-HT in the forebrain. Estrogen has been previously shown to have no effect on the inhibitory 5-HT 1A autoreceptor (5-HT(1A)) in the rat dorsal raphe nuclei (DRN); however the regulation of the inhibitory 5-HT 1B autoreceptor (5-HT(1B)) in the midbrain raphe by estrogen has not yet been investigated. Therefore, we examined the effects of estrogen on 5-HT(1B) mRNA in the rat DRN, focusing on specific subregions, and whether 5-HT(1B) mRNA levels correlated with TPH2 mRNA levels and with anxiety-like behavior. Ovariectomized rats were treated for 2 weeks with estrogen or placebo, exposed to the open field test, and 5-HT(1A) and 5-HT(1B) mRNA was quantified by in situ hybridization histochemistry. Estrogen had no effect on 5HT(1A) mRNA in any of the DRN subregions examined, confirming a previous report. In contrast, estrogen selectively decreased 5-HT(1B) mRNA in the mid-ventromedial subregion of the DRN, where 5-HT(1B) mRNA was associated with higher anxiety-like behavior and inversely correlated with TPH2 mRNA levels. These results suggest that estrogen may reduce 5-HT(1B) autoreceptor and increase TPH2 synthesis in a coordinated fashion, thereby increasing the capacity for 5-HT synthesis and release in distinct forebrain regions that modulate specific components of anxiety behavior.

Estradiol-sertraline synergy in ovariectomized rats

This study investigated estradiol (E(2)) modulation of the antidepressant effects of a selective serotonin (5-HT) reuptake inhibitor (SSRI; sertraline) and a tricyclic antidepressant (imipramine) as measured by the forced swim test (FST) followed by assessment of gene and protein expression for the 5-HT transporter (SERT) and multiple 5-HT receptors. Female Sprague-Dawley rats were ovariectomized (OVX) and two-thirds of the rats received E(2) implants (OVE). 4 weeks later, implants were withdrawn in half of the OVE rats (OVW) to capture a time point when E(2) levels were rapidly declining. Rats in each hormone group were treated with vehicle, sertraline (10 mg/kg) or imipramine (10 mg/kg), 24, 5 and 1h before the FST. Immediately after the FST, midbrain, hippocampus and prefrontal cortex tissue was removed and frozen for analysis of gene expression via quantitative real-time PCR (midbrain tissue) and protein expression via Western blot (prefrontal cortex and hippocampal tissue). In the FST, sertraline decreased immobility and increased swimming in OVE rats, as well as increased swimming in OVW rats. In contrast, no sertraline effect was observed in OVX rats. Rats treated with imipramine showed increased climbing but no changes in immobility or swimming. No changes in protein expression were detected in any treatment group. However, in vehicle-treated rats, E(2) increased midbrain SERT mRNA expression, with no effect on midbrain mRNA for the 5-HT receptors. In sertraline-treated rats, E(2) decreased 5-HT(2A) receptor mRNA, and E(2)-withdrawal increased 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor mRNA. In imipramine-treated rats, E(2) (and E(2)-withdrawal) did not affect mRNA expression for any of the target genes. Thus, E(2) synergized behaviorally and neurochemically with an SSRI but not a tricyclic antidepressant.

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.

Wheel running alters serotonin (5-HT) transporter, 5-HT1A, 5-HT1B, and alpha 1b-adrenergic receptor mRNA in the rat raphe nuclei

BACKGROUND

Altered serotonergic (5-HT) neurotransmission is implicated in the antidepressant and anxiolytic properties of physical activity. In the current study, we investigated whether physical activity alters factors involved in the regulation of central 5-HT neural activity.

METHODS

In situ hybridization was used to quantify levels of 5-HT transporter (5-HTT), 5-HT(1A), 5-HT(1B), and alpha(1b)-adrenergic receptor (alpha(1b) ADR) messenger ribonucleic acids (mRNAs) in the dorsal (DRN) and median raphe (MR) nuclei of male Fischer rats after either sedentary housing or 3 days, 3 weeks, or 6 weeks of wheel running.

RESULTS

Wheel running produced a rapid and lasting reduction of 5-HT(1B) mRNA in the ventral DRN. Three weeks of wheel running decreased 5-HTT mRNA in the DRN and MR and increased alpha(1b) ADR mRNA in the DRN. After 6 weeks of wheel running, 5-HTT mRNA remained reduced, but alpha(1b) ADR mRNA returned to sedentary levels. Serotonin(1A) mRNA was increased in the MR and certain DRN subregions after 6 weeks only.

CONCLUSIONS

Data suggest that the central 5-HT system is sensitive to wheel running in a time-dependent manner. The observed changes in mRNA regulation in a subset of raphe nuclei might contribute to the stress resistance produced by wheel running and the antidepressant and anxiolytic effects of physical activity.

Increased expression of 5-HT1B receptor in dorsal raphe nucleus decreases fear-potentiated startle in a stress dependent manner

5-HT(1B) autoreceptors regulate serotonin release from terminals of dorsal raphe nucleus (DRN) projections. Due to postsynaptic 5-HT(1B) receptors in DRN terminal fields, it has not previously been possible to manipulate 5-HT(1B) autoreceptor activity without also changing 5-HT(1B) heteroreceptor activity. We have developed a viral gene transfer strategy to express epitope-tagged 5-HT(1B) and green fluorescent protein in vivo, allowing us to increase 5-HT(1B) expression in DRN neurons. We have shown that increased 5-HT(1B) autoreceptor expression reduced anxiety in unstressed animals but increased anxiety following inescapable stress. These findings suggest that effects of increased 5-HT(1B) autoreceptor expression are dependent on stress context. To better understand the mechanisms underlying these observations, we have used fear-potentiated startle (FPS). FPS is especially sensitive to the activity of the amygdala, which shares reciprocal connections with DRN. In the absence of an inescapable stressor, increased 5-HT(1B) autoreceptor expression attenuated FPS response compared with animals injected with a virus expressing only green fluorescent protein. Administration of the 5-HT(1B) antagonist SB224289 (5 mg/kg i.p.) before startle testing blocked the effects of increased 5-HT(1B) autoreceptor expression. Since SB224289 had no effect on FPS in the absence of viral gene transfer, these results suggest that the antagonist reversed the behavioral effects of increased 5-HT(1B) autoreceptor expression through blockade of transgenic receptors. When tested 24 h following water-restraint stress, animals with increased 5-HT(1B) autoreceptors demonstrated restoration of robust FPS response. These results extend our previous studies and suggest explanations for the complex relationship between 5-HT(1B) autoreceptor expression, stress, and anxiety behavior.

New insights into the mechanisms of antidepressant therapy

Depressive disorders are among the most frequent psychiatric diseases in the Western world with prevalence numbers between 9% and 18%. They are characterized by depressed mood, a diminished interest in pleasurable activities, feelings of worthlessness or inappropriate guilt, decrease in appetite and libido, insomnia, and recurrent thoughts of death or suicide. Among other findings, reduced activity of monoaminergic neurotransmission has been postulated to play a role in the pathogenesis of depression. Consistent with this hypothesis, most antidepressive drugs exert their action by elevating the concentration of monoamines in the synaptic cleft. However, it is not the enhancement of monoaminergic signaling per se, but rather long-term, adaptive changes that may underlie the therapeutic effect. These include functional and structural changes that are discussed later. In addition, in the last years, evidence has emerged that remissions induced in patients using lithium or electroconvulsive therapy are accompanied by structural changes in neuronal networks thereby affecting synaptic plasticity in various regions of the brain.

Comprehensive expression analysis of a rat depression model

Herein we report on a large-scale analysis of gene expression in the 'learned helplessness' (LH) rat model of human depression, using DNA microarrays. We compared gene expression in the frontal cortex (FC) and hippocampus (HPC) of untreated controls, and LH rats treated with saline (LH-S), imipramine or fluoxetine. A total of 34 and 48 transcripts were differentially expressed in the FC and HPC, respectively, between control and LH-S groups. Unexpectedly, only genes for NADH dehydrogenase and zinc transporter were altered in both the FC and HPC, suggesting limited overlap in the molecular processes from specific areas of the brain. Principal component analysis revealed that sets of upregulated metabolic enzyme genes in the FC and downregulated genes for signal transduction in the HPC can distinguish clearly between depressed and control animals, as well as explain the responsiveness to antidepressants. This comprehensive data could help to unravel the complex genetic predispositions involved in human depression.

Neurobiology of antidepressant withdrawal: implications for the longitudinal outcome of depression

Inappropriate discontinuation of drug treatment and noncompliance are a leading cause of long-term morbidity during treatment of depression. Increasing evidence supports an association between depressive illness and disturbances in brain glutamate activity, nitric oxide synthesis, and gamma-amino butyric acid. Animal models also confirm that suppression of glutamate N-methyl-D-aspartate receptor activity or inhibition of the nitric oxide-cyclic guanosine monophosphate pathway, as well as increasing brain levels of gamma-amino butyric acid, may be key elements in antidepressant action. Imaging studies demonstrate, for the most part, decreased hippocampal volume in patients with depression, which may worsen with recurrent depressive episodes. Preclinical models link this potentially neurodegenerative pathology to continued stress-evoked synaptic remodeling, driven primarily by the release of glucocorticoids, glutamate, and nitric oxide. These stress-induced structural changes can be reversed by antidepressant treatment. In patients with depression, antidepressant withdrawal after chronic administration is associated with a stress response as well as functional and neurochemical changes. Preclinical data also show that antidepressant withdrawal evokes a behavioral stress response that is associated with increased hippocampal N-methyl-D-aspartate receptor density, with both responses dependent on N-methyl-D-aspartate receptor activation. Drawing from both clinical and preclinical studies, this article proposes a preliminary molecular perspective and hypothesis on the neuronal implications of adherence to and discontinuation of antidepressant medication.

5-HT1B receptor mRNA levels in dorsal raphe nucleus: inverse association with anxiety behavior in the elevated plus maze

Serotonergic neurons in the dorsal raphe nucleus, the major source of forebrain serotonin projections, synthesize a terminal autoreceptor that inhibits serotonin release-the 5-HT(1B) autoreceptor. Overexpression of this autoreceptor is hypothesized to contribute to anxiety. Antidepressants decrease (while learned helplessness increases) 5-HT(1B) mRNA in dorsal raphe neurons, and viral-mediated overexpression of 5-HT(1B) here increases anxiety behavior after stress. However, 5-HT(1B) mRNA levels in dorsal raphe are substantially elevated in unstressed rats in two models of stress resistance. Thus, the role of dorsal raphe 5-HT(1B) autoreceptors in anxiety is complex. Therefore, we tested whether different stressors differentially affect dorsal raphe 5-HT(1B) mRNA [via in situ hybridization histochemistry] and anxiety behavior (using the elevated plus maze). Rats were assigned to a stressor (either forced swim, water restraint, dry restraint, or electric tail shock) or a control condition, then were tested and sacrificed 24 h later. Overall, controls exhibited less anxiety than stressed rats as indicated by a higher ratio of open arm to total arm entries (OTR). The stressors did not differentially affect the OTR, nor did any alter dorsal raphe 5-HT(1B) mRNA levels. There was, however, a significant positive correlation between the OTR and 5HT(1B) mRNA intensity in controls (r=.64; P=.006), but not in stressed rats (r=.16, P=.36), providing further evidence that elevated dorsal raphe 5-HT(1B) levels are associated with reduced anxiety in animals that have not been exposed to stress.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Overexpression of 5-HT1B receptor in dorsal raphe nucleus using Herpes Simplex Virus gene transfer increases anxiety behavior after inescapable stress

5-HT(1B) autoreceptors have been implicated in animal models of stress and are regulated selectively by serotonin-selective reuptake inhibitors such as fluoxetine. These terminal autoreceptors regulate serotonin release from dorsal raphe nucleus (DRN) projections throughout rat forebrain. However, it has not been previously possible to manipulate 5-HT(1B) autoreceptor activity selectively without also changing 5-HT(1B) activity in other neurons mediating different behavioral responses. Therefore, we have developed a viral-mediated gene transfer strategy to express hemagglutinin-tagged 5-HT(1B) and manipulate these autoreceptors in DRN. Green fluorescent protein (GFP) was coexpressed from a separate transcriptional unit on the same amplicon to assist in monitoring infection and expression. We confirmed the expression and biological activity of both transgenic proteins in vitro. When injected directly into DRN using stereotaxic procedure, HA-5-HT(1B) receptors were expressed in serotonergic neurons and translocated to the forebrain. The effect of DRN expression of HA-5-HT(1B) on stress-induced behaviors was compared with control rats that received GFP-only amplicons. There was no change in immobility in the forced swim test. However, HA-5-HT(1B) expression significantly reduced entrances into the central region of an open-field arena after water-restraint stress without altering overall locomotor activity, but not in the absence of stress exposure. HA-5-HT(1B) expression also reduced entries into the open arms of the elevated plus maze after water restraint. Because these tests are sensitive to increases in anxiety-like behavior, our results suggest that overactivity of 5-HT(1B) autoreceptors in DRN neurons may be an important mediator of pathological responses to stressful events.

5-HT(1B) mrna regulation in two animal models of altered stress reactivity

BACKGROUND

Acute stress has profound effects on serotonergic activity, but it is not known whether alterations in the serotonin system can predispose individuals to exaggerated stress responses. We examined the regulation of 5-HT(1B) and 5-HT(1A) mRNA in two rodent models of differential sensitivity to stress: congenital learned helplessness (cLH) and handling and maternal separation (HMS).

METHODS

5-HT(1B) and 5-HT(1A) mRNAs in brain tissue sections were quantitated by in situ hybridization from control, stress-sensitive, and stress-resistant male rats in the HMS model and stress-sensitive and stress-resistant rats (both males and females) in the cLH model. Dorsal raphe nucleus, striatum, and hippocampus were examined.

RESULTS

The main result was that dorsal raphe 5-HT(1B) mRNA was substantially elevated (63-73%) in male rats in the stress-resistant group of both models compared with stress-sensitive animals. 5-HT(1B) mRNA in female rats did not differ between groups in the cLH model. There were no differences in 5-HT(1A) mRNA between HMS groups.

CONCLUSIONS

These findings suggest that 5-HT(1B) autoreceptor regulation is altered in animals with diminished stress reactivity. These results suggest that 5-HT(1B) autoreceptors in unstressed and acutely stressed animals differ, indicating the importance of state versus trait changes in serotonin function in animal models of anxiety and depression.

NMDA receptor involvement in imipramine withdrawal-associated effects on swim stress, GABA levels and NMDA receptor binding in rat hippocampus

Abrupt antidepressant withdrawal after chronic treatment is associated with a stress response that may negatively affect the long-term outcome of depression, the neurochemical correlates, of which, remain undetermined. Prolonged depression involves the stress-related release of glucocorticoids and glutamate, while response to antidepressants involves gamma-amino butyric acid (GABA) and the glutamate N-methyl-D-aspartate (NMDA) receptor. Here, imipramine (IMI) was administered to rats for three weeks followed by acute withdrawal for seven days. Levels of GABA in the hippocampus (HC), and effects on swim stress immobility (SSI), were determined. Furthermore, glutamate/NMDA receptor binding properties were determined using [(3)H]-CGP-39653. Finally, the ability of dizocilpine (MK801), a glutamate NMDA antagonist, to reverse IMI withdrawal was determined. Chronic IMI (15 mg/kg ip) significantly reduced SSI together with a slight but insignificant decrease in HC GABA levels. However, IMI significantly reduced specific binding (B(max)) of [(3)H]-CGP-39653. Withdrawal of IMI for 7 days resulted in a loss of efficacy on SSI, a slight increase in GABA and a significant reversal of IMI effects on [(3)H]-CGP-39653 binding. MK801 (0.2 mg/kg ip) alone for seven days caused a significant decrease in SSI, a significant suppression of HC GABA, and significantly decreased [(3)H]-CGP-39653 B(max). MK801 during IMI-withdrawal significantly decreased GABA, prompted recovery on SSI, though not significantly, but significantly reversed withdrawal effects on [(3)H]-CGP-39653 B(max). In conclusion, acute antidepressant discontinuation is associated with subtle changes on HC GABA, a resurgence of NMDA receptor density and a loss of its anti-immobility response. These responses are reversed by a NMDA antagonist suggesting that abrupt antidepressant discontinuation mobilises glutamate activity.