Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality?

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.

Alzheimer Disease-Link With Major Depressive Disorder and Efficacy of Antidepressants in Modifying its Trajectory

Alzheimer disease (AD) is a devastating neurodegenerative disorder that affects millions of individuals worldwide, with no effective cure. The main symptoms include learning and memory loss, and the inability to carry out the simplest tasks, significantly affecting patients' quality of life. Over the past few years, tremendous progress has been made in research demonstrating a link between AD and major depressive disorder (MDD). Evidence suggests that MDD is commonly associated with AD and that it can serve as a precipitating factor for this disease. Antidepressants such as selective serotonin reuptake inhibitors, which are the first line of treatment for MDD, have shown great promise in the treatment of depression in AD, although their effectiveness remains controversial. The goal of this review is to summarize current knowledge regarding the association between AD, MDD, and antidepressant treatment. It first provides an overview of the interaction between AD and MDD at the level of genes, brain regions, neurotransmitter systems, and neuroinflammatory markers. The review then presents current evidence regarding the effectiveness of various antidepressants for AD-related pathophysiology and then finally discusses current limitations, challenges, and future directions.

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.

Design of fast-onset antidepressant by dissociating SERT from nNOS in the DRN

Major depressive disorder (MDD) is one of the most common mental disorders. We designed a fast-onset antidepressant that works by disrupting the interaction between the serotonin transporter (SERT) and neuronal nitric oxide synthase (nNOS) in the dorsal raphe nucleus (DRN). Chronic unpredictable mild stress (CMS) selectively increased the SERT-nNOS complex in the DRN in mice. Augmentation of SERT-nNOS interactions in the DRN caused a depression-like phenotype and accounted for the CMS-induced depressive behaviors. Disrupting the SERT-nNOS interaction produced a fast-onset antidepressant effect by enhancing serotonin signaling in forebrain circuits. We discovered a small-molecule compound, ZZL-7, that elicited an antidepressant effect 2 hours after treatment without undesirable side effects. This compound, or analogous reagents, may serve as a new, rapidly acting treatment for MDD.

Delineating a serotonin 1B receptor circuit for appetite suppression in mice

Triptans are a class of commonly prescribed antimigraine drugs. Here, we report a previously unrecognized role for them to suppress appetite in mice. In particular, frovatriptan treatment reduces food intake and body weight in diet-induced obese mice. Moreover, the anorectic effect depends on the serotonin (5-HT) 1B receptor (Htr1b). By ablating Htr1b in four different brain regions, we demonstrate that Htr1b engages in spatiotemporally segregated neural pathways to regulate postnatal growth and food intake. Moreover, Htr1b in AgRP neurons in the arcuate nucleus of the hypothalamus (ARH) contributes to the hypophagic effects of HTR1B agonists. To further study the anorexigenic Htr1b circuit, we generated Htr1b-Cre mice. We find that ARH Htr1b neurons bidirectionally regulate food intake in vivo. Furthermore, single-nucleus RNA sequencing analyses revealed that Htr1b marks a subset of AgRP neurons. Finally, we used an intersectional approach to specifically target these neurons (Htr1bAgRP neurons). We show that they regulate food intake, in part, through a Htr1bAgRP→PVH circuit.

The impact of HTR1A and HTR1B methylation combined with stress/genotype on early antidepressant efficacy

AIMS

Antidepressants are effective in the treatment of major depressive disorder (MDD), while many patients fail to respond to antidepressants. Both 5-HT1A (HTR1A) and 5-HT1B (HTR1B) receptors play an important role in antidepressant activity. Meanwhile, DNA methylation is associated with MDD and antidepressant efficacy. In this study we investigate the influence of HTR1A and HTR1B methylation combined with stress/genotype on antidepressant efficacy.

METHODS

A total of 291 MDD patients and 100 healthy controls received the Life Events Scale (LES) and the Childhood Trauma Questionnaire (CTQ) as stress assessment. Eight single nucleotide polymorphisms (SNPs) of HTR1A and HTR1B involved in antidepressant mechanisms were tested. Methylation status in 181 cytosine-phosphate-guanine (CpG) sites of HTR1A and HTR1B were assessed. All MDD patients were divided into response (RES) and non-response (NRES) after 2 weeks of antidepressant treatment. Logistic regression was conducted for interactions between methylation, NLES/CTQ score and genotype.

RESULTS

Low HTR1A-2-143 methylation is connected with better antidepressant efficacy in subgroup. Low HTR1A-2-143 methylation combined with low CTQ score is related to better antidepressant efficacy. The interaction between high HTR1B methylation with the rs6298 AA/AG genotype affects better antidepressant efficacy.

CONCLUSIONS

HTR1A and HTR1B methylation combined with stress/genotype is associated with antidepressant efficacy.

Depression and Pain: use of antidepressant

Background:

Emotional disorders are common comorbid affectations that exacerbate the severity and persistence of chronic pain. Specifically, depressive symptoms can lead to an excessive duration and intensity of pain. Clinical and preclinical studies have been focused on the underlying mechanisms of chronic pain and depression comorbidity and the use of antidepressants to reduce pain.

Aim:

This review provides an overview of the comorbid relationship of chronic pain and depression, the clinical and pre-clinical studies performed on the neurobiological aspects of pain and depression, and the use of antidepressants as analgesics.

Methods:

A systematic search of literature databases was conducted according to pre-defined criteria. The authors independently conducted a focused analysis of the full-text articles.

Results:

Studies suggest that pain and depression are highly intertwined and may co-exacerbate physical and psychological symptoms. One important biochemical basis for pain and depression focuses on the serotonergic and norepinephrine system, which have been shown to play an important role in this comorbidity. Brain structures that codify pain are also involved in mood. It is evident that using serotonergic and norepinephrine antidepressants are strategies commonly employed to mitigate pain

Conclusion:

Literature indicates that pain and depression impact each other and play a prominent role in the development and maintenance of other chronic symptoms. Antidepressants continue to be a major therapeutic tool for managing chronic pain. Tricyclic antidepressants (TCAs) are more effective in reducing pain than Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin-Noradrenaline Reuptake Inhibitors (SNRIs).

The behavioral and neurochemical effects of an inescapable stressor are time of day dependent

Circadian rhythms are ∼24 h fluctuations in physiology and behavior that are synchronized with the light-dark cycle. The circadian system ensures homeostatic balance by regulating multiple systems that respond to environmental stimuli including stress systems. In rats, acute exposure to a series of uncontrollable tailshocks (inescapable stress, IS) produces an anxiety and depression-like phenotype. Anxiety- and fear-related behavioral changes produced by IS are driven by sensitization of serotonergic (5-hydroxytryptamine, 5-HT) neurons in the dorsal raphe nucleus (DRN). Because the circadian and serotonergic systems are closely linked, here we tested whether the DRN-dependent behavioral and neurochemical effects of IS are time of day dependent. Exposure to IS during the light (inactive) phase elicited the expected changes in mood related behaviors. In contrast, rats that underwent IS during the dark (active) phase were buffered against stress-induced changes in juvenile social exploration and shock-elicited freezing, both DRN-dependent outcomes. Interestingly, behavioral anhedonia, which is not a DRN-dependent behavior, was comparably reduced by stress at both times of day. Neurochemical changes complimented the behavioral results: IS-induced activation of DRN 5-HT neurons was greater during the light phase compared to the dark phase. Additionally, 5-HT1AR and 5-HTT, two genes that regulate 5-HT activity were up-regulated during the middle of the light cycle. These data suggest that DRN-dependent behavioral outcomes of IS are time of day dependent and may be mediated by circadian gating of the DRN response to stress.Lay summaryHere we show that the time of day at which a stressor occurs impacts the behavioral and neurochemical outcomes of the stressor. In particular, animals appear more vulnerable to a stressor that occurs during their rest phase. This work may have important implications for shift-workers and other populations that are more likely to encounter stressors during their rest phase.

Enhancing the efficacy of 5-HT uptake inhibitors in the treatment of attention deficit hyperactivity disorder

Attention Deficit Hyperactivity Disorder (ADHD) is one of the most common childhood behavioural disorders, the frontline treatments for which are drugs with abuse potential. As a consequence, there is an urgent need to develop non addictive drug treatments with equivalent efficacy. Preclinical evidence suggests that selective serotonin uptake inhibitors (SSRIs) are likely to be effective in ADHD, however clinical reports suggest that SSRIs are of limited therapeutic value for the treatment of ADHD. We propose that this disconnect can be explained by the pattern of drug administration in existing clinical trials (administration for short periods of time, or intermittently) leading to inadequate control of the autoregulatory processes which control 5-HT release, most notably at the level of inhibitory 5-HT_1A somatodendritic autoreceptors. These autoreceptors reduce the firing rate of 5-HT neurons (limiting release) unless they are desensitised by a long term, frequent pattern of drug administration. As such, we argue that the participants in earlier trials were not administered SSRIs in a manner which realises any potential benefits of targeting 5-HT in the pharmacotherapy of ADHD. In light of this, we hypothesise that there may be under-researched potential to exploit 5-HT transmission therapeutically in ADHD, either through changing the administration regime, or by pharmacological means. Recent pharmacological research has successfully potentiated the effects of SSRIs in acute animal preparations by antagonising inhibitory 5-HT_1A autoreceptors prior to the administration of the SSRI fluoxetine. We suggest that combination therapies linking SSRIs and 5-HT_1A antagonists are a potential way forward in the development of efficacious non-addictive pharmacotherapies for ADHD.

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.

Serotonin and Migraine: Biology and Clinical Implications

Migraine is the most frequent neurological disorder in the adult population worldwide, affecting up to 12% of the general population and more frequent in women (~25%). It has a high impact on our society due to its disabling nature and, therein, reduced quality of life and increased absenteeism from work. Headache is the primary clinical manifestation and it has been associated with ‘a hereditary or predisposed sensitivity of neurovascular reactions to certain stimuli or to cyclic changes in the central nervous system’ (1). Amongst the many neurotransmitters in the brain, the serotonergic (serotonin, 5-HT) system from the brainstem raphe nucleus has been most convincingly implicated in migraine pathophysiology. The documented changes in 5-HT metabolism and in the processing of central 5-HT-mediated responses during and in between migraine attacks have led to the suggestion that migraine is a consequence of a central neurochemical imbalance that involves a low serotonergic disposition. Although the exact cascade of events that link abnormal serotonergic neurotransmission to the manifestation of head pain and the accompanying symptoms has yet to be fully understood, recent evidence suggests that a low 5-HT state facilitates activation of the trigeminovascular nociceptive pathway, as induced by cortical spreading depression. In this short review, we present and discuss the original and most recent findings that support a role for altered serotonergic neurotransmission in the manifestation of migraine headache.

Postnatal Treadmill Exercise Alleviates Prenatal Stress-Induced Anxiety in Offspring Rats by Enhancing Cell Proliferation Through 5-Hydroxytryptamine 1A Receptor Activation

Purpose:

Stress during pregnancy is a risk factor for the development of anxiety-related disorders in offspring later in life. The effects of treadmill exercise on anxiety-like behaviors and hippocampal cell proliferation were investigated using rats exposed to prenatal stress.

Methods:

Exposure of pregnant rats to a hunting dog in an enclosed room was used to induce stress. Anxiety-like behaviors of offspring were evaluated using the elevated plus maze test. Immunohistochemistry for the detection of 5-bromo-2ʹ- deoxyuridine and doublecortin (DCX) in the hippocampal dentate gyrus and 5-hydroxytryptamine 1A receptors (5-HT_1A) in the dorsal raphe was conducted. Brain-derived neurotrophic factor (BDNF) and tyrosine kinase B (TrkB) levels in the hippocampus were evaluated by western blot analysis.

Results:

Offspring of maternal rats exposed to stress during pregnancy showed anxiety-like behaviors. Offspring also showed reduced expression of BDNF, TrkB, and DCX in the dentate gyrus, decreased cell proliferation in the hippocampus, and reduced 5-HT_1A expression in the dorsal raphe. Postnatal treadmill exercise by offspring, but not maternal exercise during pregnancy, enhanced cell proliferation and expression of these proteins.

Conclusions:

Postnatal treadmill exercise ameliorated anxiety-like behaviors in offspring of stressed pregnant rats, and the alleviating effect of exercise on these behaviors is hypothesized to result from enhancement of cell proliferation through 5-HT_1A activation in offspring rats.

Elucidation of The Behavioral Program and Neuronal Network Encoded by Dorsal Raphe Serotonergic Neurons

Elucidating how the brain's serotonergic network mediates diverse behavioral actions over both relatively short (minutes-hours) and long period of time (days-weeks) remains a major challenge for neuroscience. Our relative ignorance is largely due to the lack of technologies with robustness, reversibility, and spatio-temporal control. Recently, we have demonstrated that our chemogenetic approach (eg, Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)) provides a reliable and robust tool for controlling genetically defined neural populations. Here we show how short- and long-term activation of dorsal raphe nucleus (DRN) serotonergic neurons induces robust behavioral responses. We found that both short- and long-term activation of DRN serotonergic neurons induce antidepressant-like behavioral responses. However, only short-term activation induces anxiogenic-like behaviors. In parallel, these behavioral phenotypes were associated with a metabolic map of whole brain network activity via a recently developed non-invasive imaging technology DREAMM (DREADD Associated Metabolic Mapping). Our findings reveal a previously unappreciated brain network elicited by selective activation of DRN serotonin neurons and illuminate potential therapeutic and adverse effects of drugs targeting DRN neurons.

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.

Postnatal treadmill exercise attenuates prenatal stress-induced apoptosis through enhancing serotonin expression in aged-offspring rats

Maternal stress during pregnancy affects negative impact on health of offspring. In the present study, we compared the effects of maternal treadmill exercise and offspring treadmill exercise on prenatal stress-induced apoptosis and serotonin expression in offspring. Stress to the pregnant rats was induced by exposure of maternal rats to the hunting dog in an enclosed room. Exposure time was 10 min, three times per day, with a 1-h interval between exposures. This regimen was maintained from the seventh day of gestation until delivery. The pregnant rats in the exercise group were forced to run on a motorized tread-mill for 30 min once a day, started 7 days after pregnancy until delivery. The offspring in the exercise group were forced to run on a motorized treadmill for 30 min once a day, started 4 weeks after birth for 4 weeks. In the present results, offspring exposed to prenatal stress exhibited lower Bcl-2 level and higher Bax level in the hippocampus, lower 5-hydroxytryptamine (5-HT) and tryptophan hydroxylase (TPH) expression in the dorsal raphe, and higher c-Fos expression in the locus coeruleus compared to age-matched control rats. Treadmill exercise of offspring suppressed Bax expression and enhanced Bcl-2 expression in the hippocampus, increased 5-HT and TPH expression in the dorsal raphe, and enhanced c-Fos expression in the locus coeruleus of offspring. Tread-mill exercise of offspring suppressed prenatal stress-induced apoptosis and normalized prenatal stress-induced alterations in serotonin synthesis and neuronal activation. However maternal treadmill exercise during pregnancy exerted no significant effect on offspring.

Interaction between 5-HTTLPR and 5-HT1B genotype status enhances cerebral 5-HT1A receptor binding

Serotonergic neurotransmission is thought to underlie a dynamic interrelation between different key structures of the serotonin system. The serotonin transporter (SERT), which is responsible for the reuptake of serotonin from the synaptic cleft into the neuron, as well as the serotonin-1A (5-HT1A) and -1B (5-HT1B) receptors, inhibitory auto-receptors in the raphe region and projection areas, respectively, are likely to determine serotonin release. Thereby, they are involved in the regulation of extracellular serotonin concentrations and the extent of serotonergic effects in respective projection areas. Complex receptor interactions can be assessed in vivo with positron emission tomography (PET) and single-nucleotide-polymorphisms, which are thought to alter protein expression levels. Due to the complexity of the serotonergic system, gene × gene interactions are likely to regulate transporter and receptor expression and therefore subsequently serotonergic transmission. In this context, we measured 51 healthy subjects (mean age 45.5 ± 12.9, 38 female) with PET using [carbonyl-(11)C]WAY-100635 to determine 5-HT1A receptor binding potential (5-HT1A BPND). Genotyping for rs6296 (HTR1B) and 5-HTTLPR (SERT gene promoter polymorphism) was performed using DNA isolated from whole blood. Voxel-wise whole-brain ANOVA revealed a positive interaction effect of genotype groups (5-HTTLPR: LL, LS+SS and HTR1B: rs6296: CC, GC+GG) on 5-HT1A BPND with peak t-values in the bilateral parahippocampal gyrus. More specifically, highest 5-HT1A BPND was identified for individuals homozygous for both the L-allele of 5-HTTLPR and the C-allele of rs6296. This finding suggests that the interaction between two major serotonergic structures involved in serotonin release, specifically the SERT and 5-HT1B receptor, results in a modification of the inhibitory serotonergic tone mediated via 5-HT1A receptors.

Computational modeling of spike generation in serotonergic neurons of the dorsal raphe nucleus

Serotonergic neurons of the dorsal raphe nucleus, with their extensive innervation of limbic and higher brain regions and interactions with the endocrine system have important modulatory or regulatory effects on many cognitive, emotional and physiological processes. They have been strongly implicated in responses to stress and in the occurrence of major depressive disorder and other psychiatric disorders. In order to quantify some of these effects, detailed mathematical models of the activity of such cells are required which describe their complex neurochemistry and neurophysiology. We consider here a single-compartment model of these neurons which is capable of describing many of the known features of spike generation, particularly the slow rhythmic pacemaking activity often observed in these cells in a variety of species. Included in the model are 11 kinds of ion channels: a fast sodium current INa, a delayed rectifier potassium current IKDR, a transient potassium current IA, a slow non-inactivating potassium current IM, a low-threshold calcium current IT, two high threshold calcium currents IL and IN, small and large conductance potassium currents ISK and IBK, a hyperpolarization-activated cation current IH and a leak current ILeak. In Sections 3-8, each current type is considered in detail and parameters estimated from voltage clamp data where possible. Three kinds of model are considered for the BK current and two for the leak current. Intracellular calcium ion concentration Cai is an additional component and calcium dynamics along with buffering and pumping is discussed in Section 9. The remainder of the article contains descriptions of computed solutions which reveal both spontaneous and driven spiking with several parameter sets. Attention is focused on the properties usually associated with these neurons, particularly long duration of action potential, steep upslope on the leading edge of spikes, pacemaker-like spiking, long-lasting afterhyperpolarization and the ramp-like return to threshold after a spike. In some cases the membrane potential trajectories display doublets or have humps or notches as have been reported in some experimental studies. The computed time courses of IA and IT during the interspike interval support the generally held view of a competition between them in influencing the frequency of spiking. Spontaneous activity was facilitated by the presence of IH which has been found in these neurons by some investigators. For reasonable sets of parameters spike frequencies between about 0.6Hz and 1.2Hz are obtained, but frequencies as high as 6Hz could be obtained with special parameter choices. Topics investigated and compared with experiment include shoulders, notches, anodal break phenomena, the effects of noradrenergic input, frequency versus current curves, depolarization block, effects of cell size and the effects of IM. The inhibitory effects of activating 5-HT1A autoreceptors are also investigated. There is a considerable discussion of in vitro versus in vivo firing behavior, with focus on the roles of noradrenergic input, corticotropin-releasing factor and orexinergic inputs. Location of cells within the nucleus is probably a major factor, along with the state of the animal.

Evaluation of heritable determinants of blood and brain serotonin homeostasis using recombinant inbred mice

The biogenic amine serotonin (5-HT, 5-hydroxytryptamine) exerts powerful, modulatory control over multiple physiological functions in the brain and periphery, ranging from mood and appetite to vasoconstriction and gastrointestinal motility. In order to gain insight into shared and distinct molecular and phenotypic networks linked to variations in 5-HT homeostasis, we capitalized on the stable genetic variation present in recombinant inbred mouse strains. This family of strains, all derived from crosses between C57BL/6J and DBA/2J (BXD) parents, represents a unique, community resource with approximately 40 years of assembled phenotype data that can be exploited to explore and test causal relationships in silico. We determined levels of 5-HT and 5-hydroxyindoleacetic acid from whole blood, midbrain and thalamus/hypothalamus (diencephalon) of 38 BXD lines and both sexes. All 5-HT measures proved highly heritable in each region, although both gender and region significantly impacted between-strain correlations. Our studies identified both expected and novel biochemical, anatomical and behavioral phenotypes linked to 5-HT traits, as well as distinct quantitative trait loci. Analyses of these loci nominate a group of genes likely to contribute to gender- and region-specific capacities for 5-HT signaling. Analysis of midbrain mRNA variations across strains revealed overlapping gene expression networks linked to 5-HT synthesis and metabolism. Altogether, our studies provide a rich profile of genomic, molecular and phenotypic networks that can be queried for novel relationships contributing risk for disorders linked to perturbed 5-HT signaling.

Monitoring serotonin signaling on a subsecond time scale

Serotonin modulates a variety of processes throughout the brain, but it is perhaps best known for its involvement in the etiology and treatment of depressive disorders. Microdialysis studies have provided a clear picture of how ambient serotonin levels fluctuate with regard to behavioral states and pharmacological manipulation, and anatomical and electrophysiological studies describe the location and activity of serotonin and its targets. However, few techniques combine the temporal resolution, spatial precision, and chemical selectivity to directly evaluate serotonin release and uptake. Fast-scan cyclic voltammetry (FSCV) is an electrochemical method that can detect minute changes in neurotransmitter concentration on the same temporal and spatial dimensions as extrasynaptic neurotransmission. Subsecond measurements both in vivo and in brain slice preparations enable us to tease apart the processes of release and uptake. These studies have particularly highlighted the significance of regulatory mechanisms to proper functioning of the serotonin system. This article will review the findings of FSCV investigations of serotonergic neurotransmission and discuss this technique's potential in future studies of the serotonin system.

Modulation of firing and synaptic transmission of serotonergic neurons by intrinsic G protein-coupled receptors and ion channels

Serotonergic neurons project to virtually all regions of the central nervous system and are consequently involved in many critical physiological functions such as mood, sexual behavior, feeding, sleep/wake cycle, memory, cognition, blood pressure regulation, breathing, and reproductive success. Therefore, serotonin release and serotonergic neuronal activity have to be precisely controlled and modulated by interacting brain circuits to adapt to specific emotional and environmental states. We will review the current knowledge about G protein-coupled receptors and ion channels involved in the regulation of serotonergic system, how their regulation is modulating the intrinsic activity of serotonergic neurons and its transmitter release and will discuss the latest methods for controlling the modulation of serotonin release and intracellular signaling in serotonergic neurons in vitro and in vivo.

Oscillatory serotonin function in depression

Oscillations in brain activities with periods of minutes to hours may be critical for normal mood behaviors. Ultradian (faster than circadian) rhythms of mood behaviors and associated central nervous system activities are altered in depression. Recent data suggest that ultradian rhythms in serotonin (5HT) function also change in depression. In two separate studies, 5HT metabolites in cerebrospinal fluid (CSF) were measured every 10 min for 24 h before and after chronic antidepressant treatment. Antidepressant treatments were associated with enhanced ultradian amplitudes of CSF metabolite levels. Another study used resting-state functional magnetic resonance imaging (fMRI) to measure amplitudes of dorsal raphé activation cycles following sham or active dietary depletions of the 5HT precursor (tryptophan). During depletion, amplitudes of dorsal raphé activation cycles increased with rapid 6 s periods (about 0.18 Hz) while functional connectivity weakened between dorsal raphé and thalamus at slower periods of 20 s (0.05 Hz). A third approach studied MDMA (ecstasy, 3,4-methylenedioxy-N-methylamphetamine) users because of their chronically diminished 5HT function compared with non-MDMA polysubstance users (Karageorgiou et al., 2009). Compared with a non-MDMA using cohort, MDMA users showed diminished fMRI intra-regional coherence in motor regions along with altered functional connectivity, again suggesting effects of altered 5HT oscillatory function. These data support a hypothesis that qualities of ultradian oscillations in 5HT function may critically influence moods and behaviors. Dysfunctional 5HT rhythms in depression may be a common endpoint and biomarker for depression, linking dysfunction of slow brain network oscillators to 5HT mechanisms affected by commonly available treatments. 5HT oscillatory dysfunction may define illness subtypes and predict responses to serotonergic agents. Further studies of 5HT oscillations in depression are indicated.

5-HT(1B) autoreceptor regulation of serotonin transporter activity in synaptosomes

Serotonin-1B (5-HT(1B) ) autoreceptors are located in serotonin (5-HT) terminals, along with serotonin transporters (SERT), and play a critical role in autoregulation of serotonergic neurotransmission and are implicated in disorders of serotonergic function, particularly emotional regulation. SERT modulates serotonergic neurotransmission by high-affinity reuptake of 5-HT. Alterations in SERT activity are associated with increased risk for depression and anxiety. Several neurotransmitter receptors are known to regulate SERT K(m) and V(max) , and previous work suggests that 5-HT(1B) autoreceptors may regulate 5-HT reuptake, in addition to modulating 5-HT release and synthesis. We used rotating disk electrode voltammetry to investigate 5-HT(1B) autoreceptor regulation of SERT-mediated 5-HT uptake into synaptosomes. The selective 5-HT(1B) antagonist SB224289 decreased SERT activity in synaptosomes prepared from wild-type but not 5-HT(1B) knockout mice, whereas SERT uptake was enhanced after pretreatment with the selective 5-HT(1B) agonist CP94253. Furthermore, SERT activity varies as a function of 5-HT(1B) receptor expression-specifically, genetic deletion of 5-HT(1B) decreased SERT function, while viral-mediated overexpression of 5-HT(1B) autoreceptors in rat raphe neurons increased SERT activity in rat hippocampal synaptosomes. Considered collectively, these results provide evidence that 5-HT(1B) autoreceptors regulate SERT activity. Because SERT clearance rate varies as a function of 5-HT(1B) autoreceptor expression levels and is modulated by both activation and inhibition of 5-HT(1B) autoreceptors, this dynamic interaction may be an important mechanism of serotonin autoregulation with therapeutic implications.

Egr-1 antisense oligodeoxynucleotide administration into the olfactory bulb impairs olfactory learning in the greater short-nosed fruit bat Cynopterus sphinx

Postsynaptic densities (PSDs) contain proteins that regulate synaptic transmission. We examined two important examples of these, calcium/calmodulin-dependent protein kinase II (CaMKII) and PSD-95, in regard to the functional role of early growth response gene-1 (egr-1) in regulation of olfactory learning in the greater short-nosed fruit bat Cynopterus sphinx (family Pteropodidae). To test whether activation of egr-1 in the olfactory bulb (OB) is required for olfactory memory of these bats, bilaterally canulated individuals were infused with antisense (AS) or non-sense (NS)-oligodeoxynucleotides (ODN) of egr-1, or with phosphate buffer saline (PBS), 2h before the olfactory training. Our results showed that behavioral training significantly up-regulates immediate early gene (IEG) EGR-1 and key synaptic proteins Synaptotagmin-1(SYT-1), CaMKII and PSD-95, and phosphorylation of CaMKII in the OB at the protein level per se. Subsequently, we observed that egr-1 antisense-ODN infusion in the OB impaired olfactory memory and down regulates the expression of CaMKII and PSD-95, and the phosphorylation of CaMKII but not SYT-1. In contrast, NS-ODN or PBS had no effect on the expression of the PSDs CaMKII or PSD-95, or on the phosphorylation of CaMKII. When the egr-1 NS-ODN was infused in the OB after training for the novel odor there was no effect on olfactory memory. These findings suggest that egr-1 control the activation of CaMKII and PSD-95 during the process of olfactory memory formation.

Pharmacology for the treatment of premature ejaculation

Male sexual response comprises four phases: excitement, including erection; plateau; ejaculation, usually accompanied by orgasm; and resolution. Ejaculation is a complex sexual response involving a sequential process consisting of two phases: emission and expulsion. Ejaculation, which is basically a spinal reflex, requires a tight coordination between sympathetic, parasympathetic, and somatic efferent pathways originating from different segments and area in the spinal cord and innervating pelvi-perineal anatomical structures. A major relaying and synchronizing role is played by a group of lumbar neurons described as the spinal generator of ejaculation. Excitatory and inhibitory influences from sensory genital and cerebral stimuli are integrated and processed in the spinal cord. Premature ejaculation (PE) can be defined by ≤1-min ejaculatory latency, an inability to delay ejaculation, and negative personal consequences. Because there is no physiological impairment in PE, any pharmacological agent with central or peripheral mechanism of action that is delaying the ejaculation is a drug candidate for the treatment of PE. Ejaculation is centrally mediated by a variety of neurotransmitter systems, involving especially serotonin and serotonergic pathways but also dopaminergic and oxytocinergic systems. Pharmacological delay of ejaculation can be achieved either by inhibiting excitatory or reinforcing inhibitory pathways from the brain or the periphery to the spinal cord. PE can be treated with long-term use of selective serotonin-reuptake inhibitors (SSRIs) or tricyclic antidepressants. Dapoxetine, a short-acting SSRI, is the first treatment registered for the on-demand treatment of PE. Anesthetics applied on the glans penis have the ability to lengthen the time to ejaculation. Targeting oxytocinergic, neurokinin-1, dopaminergic, and opioid receptors represent future avenues to delaying ejaculation.

Chronic inhibition of mammalian target of rapamycin by rapamycin modulates cognitive and non-cognitive components of behavior throughout lifespan in mice

Aging is, by far, the greatest risk factor for most neurodegenerative diseases. In non-diseased conditions, normal aging can also be associated with declines in cognitive function that significantly affect quality of life in the elderly. It was recently shown that inhibition of Mammalian TOR (mTOR) activity in mice by chronic rapamycin treatment extends lifespan, possibly by delaying aging {Harrison, 2009 #4}{Miller, 2011 #168}. To explore the effect of chronic rapamycin treatment on normal brain aging we determined cognitive and non-cognitive components of behavior throughout lifespan in male and female C57BL/6 mice that were fed control- or rapamycin-supplemented chow. Our studies show that rapamycin enhances cognitive function in young adult mice and blocks age-associated cognitive decline in older animals. In addition, mice fed with rapamycin-supplemented chow showed decreased anxiety and depressive-like behavior at all ages tested. Levels of three major monoamines (norepinephrine, dopamine and 5-hydroxytryptamine) and their metabolites (3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindolacetic acid) were significantly augmented in midbrain of rapamycin-treated mice compared to controls. Our results suggest that chronic, partial inhibition of mTOR by oral rapamycin enhances learning and memory in young adults, maintains memory in old C57BL/6J mice, and has concomitant anxiolytic and antidepressant-like effects, possibly by stimulating major monoamine pathways in brain.

Sensorimotor function is modulated by the serotonin receptor 1d, a novel marker for gamma motor neurons

Gamma motor neurons (MNs), the efferent component of the fusimotor system, regulate muscle spindle sensitivity. Muscle spindle sensory feedback is required for proprioception that includes sensing the relative position of neighboring body parts and appropriately adjust the employed strength in a movement. The lack of a single and specific genetic marker has long hampered functional and developmental studies of gamma MNs. Here we show that the serotonin receptor 1d (5-ht1d) is specifically expressed by gamma MNs and proprioceptive sensory neurons. Using mice expressing GFP driven by the 5-ht1d promotor, we performed whole-cell patch-clamp recordings of 5-ht1d::GFP⁺ and 5-ht1d::GFP⁻ motor neurons from young mice. Hierarchal clustering analysis revealed that gamma MNs have distinct electrophysiological properties intermediate to fast-like and slow-like alpha MNs. Moreover, mice lacking 5-ht1d displayed lower monosynaptic reflex amplitudes suggesting a reduced response to sensory stimulation in motor neurons. Interestingly, adult 5-ht1d knockout mice also displayed improved coordination skills on a beam-walking task, implying that reduced activation of MNs by Ia afferents during provoked movement tasks could reduce undesired exaggerated muscle output. In summary, we show that 5-ht1d is a novel marker for gamma MNs and that the 5-ht1d receptor is important for the ability of proprioceptive circuits to receive and relay accurate sensory information in developing and mature spinal cord motor circuits.

Sequential and opposing alterations of 5-HT(1A) receptor function during withdrawal from chronic morphine

Addiction is a brain chronic relapsing disorder associated with emotional distress. The serotonergic system and especially the 5-HT(1A) receptor crucially regulate emotional behaviors both in humans and rodents. Using [(35)S]GTPγS autoradiography in mice, we show that 5-HT(1A) receptor function is enhanced by chronic morphine treatment in the medial prefrontal cortex, and decreased in dorsal raphe nucleus one week later, two regions involved in emotional processing. These molecular adaptations could contribute to the development of emotional disorders experienced by former opiate addicts.

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.

Neurologic bases for comorbidity of balance disorders, anxiety disorders and migraine: neurotherapeutic implications

The comorbidity among balance disorders, anxiety disorders and migraine has been studied extensively from clinical and basic research perspectives. From a neurological perspective, the comorbid symptoms are viewed as the product of sensorimotor, interoceptive and cognitive adaptations that are produced by afferent interoceptive information processing, a vestibulo-parabrachial nucleus network, a cerebral cortical network (including the insula, orbitofrontal cortex, prefrontal cortex and anterior cingulate cortex), a raphe nuclear-vestibular network, a coeruleo-vestibular network and a raphe-locus coeruleus loop. As these pathways overlap extensively with pathways implicated in the generation, perception and regulation of emotions and affective states, the comorbid disorders and effective treatment modalities can be viewed within the contexts of neurological and psychopharmacological sites of action of current therapies.

Immunocytochemical localization of TASK-3 (K(2P)9.1) channels in monoaminergic and cholinergic neurons

Monoaminergic and cholinergic systems are important regulators of cortical and subcortical systems, and a variety of vegetative functions are controlled by the respective neurotransmitters. Neuronal excitability and transmitter release of these neurons are strongly regulated by their potassium conductances carried by Kir and K(2P) channels. Here we describe the generation and characterization of a polyclonal monospecific antibody against rat TASK-3, a major brain K(2P) channel. After removal of cross-reactivities and affinity purification the antibody was characterized by ELISA, immunocytochemistry of TASK-3 transfected cells, and Western blots indicating that the antibody only detects TASK-3 protein, but not its paralogs TASK-1 and TASK-5. Western blot analysis of brain membrane fractions showed a single band around 45 kD, close to the predicted molecular weight of the TASK-3 protein. In addition, specific immunolabeling using the anti-TASK-3 antibody in Western blot analysis and immunocytochemistry was blocked in a concentration dependent manner by its cognate antigen only. Immunocytochemical analysis of rat brain revealed strong expression of TASK-3 channels in serotoninergic neurons of the dorsal and median raphe, noradrenergic neurons of the locus coeruleus, histaminergic neurons of the tuberomammillary nucleus and in the cholinergic neurons of the basal nucleus of Meynert. Immunofluorescence double-labeling experiments with appropriate marker enzymes confirmed the expression of TASK-3 in cholinergic, serotoninergic, and noradrenergic neurons. In the dopaminergic system strong TASK-3 expression was found in the ventral tegmental area, whereas TASK-3 immunoreactivity in the substantia nigra compacta was only weak. All immunocytochemical results were supported by in situ hybridization using TASK-3 specific riboprobes.

Substitution of 5-HT1A receptor signaling by a light-activated G protein-coupled receptor

Understanding serotonergic (5-HT) signaling is critical for understanding human physiology, behavior, and neuropsychiatric disease. 5-HT mediates its actions via ionotropic and metabotropic 5-HT receptors. The 5-HT(1A) receptor is a metabotropic G protein-coupled receptor linked to the G(i/o) signaling pathway and has been specifically implicated in the pathogenesis of depression and anxiety. To understand and precisely control 5-HT(1A) signaling, we created a light-activated G protein-coupled receptor that targets into 5-HT(1A) receptor domains and substitutes for endogenous 5-HT(1A) receptors. To induce 5-HT(1A)-like targeting, vertebrate rhodopsin was tagged with the C-terminal domain (CT) of 5-HT(1A) (Rh-CT(5-HT1A)). Rh-CT(5-HT1A) activates G protein-coupled inward rectifying K(+) channels in response to light and causes membrane hyperpolarization in hippocampal neurons, similar to the agonist-induced responses of the 5-HT(1A) receptor. The intracellular distribution of Rh-CT(5-HT1A) resembles that of the 5-HT(1A) receptor; Rh-CT(5-HT1A) localizes to somatodendritic sites and is efficiently trafficked to distal dendritic processes. Additionally, neuronal expression of Rh-CT(5-HT1A), but not Rh, decreases 5-HT(1A) agonist sensitivity, suggesting that Rh-CT(5-HT1A) and 5-HT(1A) receptors compete to interact with the same trafficking machinery. Finally, Rh-CT(5-HT1A) is able to rescue 5-HT(1A) signaling of 5-HT(1A) KO mice in cultured neurons and in slices of the dorsal raphe showing that Rh-CT(5-HT1A) is able to functionally compensate for native 5-HT(1A). Thus, as an optogenetic tool, Rh-CT(5-HT1A) has the potential to directly correlate in vivo 5-HT(1A) signaling with 5-HT neuron activity and behavior in both normal animals and animal models of neuropsychiatric disease.

Role of olfactory bulb serotonin in olfactory learning in the greater short-nosed fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae)

The role of olfactory bulb (OB) serotonin [5-hydroxytryptamine (5-HT)] in olfactory learning and memory was tested in the greater short-nosed fruit bat, Cynopterus sphinx (family Pteropodidae). Graded concentrations (25, 40, and 60microg) of 5,7-dihydroxytryptamine (5,7-DHT) or saline were injected into the OB of bats one day before training to the novel odor. In a behavioral test, 5,7-DHT (60microg) injected bats made significantly fewer feeding attempts and bouts when compared to saline-injected bats during learning and in the memory test. Subsequent biochemical analysis showed that 5-HT level was effectively depleted in the OB of 5,7-DHT injected bats. To test odor-induced 5-HT mediated changes in 5-HT receptors and second messenger cascade in the OB, we examined the expression of 5-HT receptors and mitogen-activated protein kinase (MAPK)/Erk cascade after training to the novel odor. We found that odor stimulation up-regulated the expression of 5-HT(1A) receptor, Erk1 and Creb1 mRNA, and phosphorylation of ERK1 and CREB1. Odor stimulation failed to induce expression in 5-HT-depleted bats, which is similar to control bats and significantly low compared to saline-treated bats. Together these data revealed that the level of 5-HT in the OB may regulate olfactory learning and memory in C. sphinx through Erk and CREB.

The pharmacology of the neurochemical transmission in the midbrain raphe nuclei of the rat

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain, loaded with [(3)H]serotonin ([(3)H]5-HT), superfused and the release of [(3)H]5-HT was determined at rest and in response to electrical stimulation. Compartmental analysis of [(3)H]5-HT taken up by raphe tissue indicated various pools where the neurotransmitter release may originate from these stores differed both in size and rate constant. 5-HT release originates not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process establishing synaptic and non-synaptic neurochemical transmission in the serotonergic somatodendritic area. Manipulation of 5-HT transporter function modulates extracellular 5-HT concentrations in the raphe nuclei: of the SSRIs, fluoxetine was found 5-HT releaser, whereas citalopram did not exhibit this effect. Serotonergic projection neurons in the raphe nuclei possess inhibitory 5-HT(1A) and 5-HT(1B/1D) receptors and facilitatory 5-HT(3) receptors, which regulate 5-HT release in an opposing fashion. This observation indicates that somatodendritic 5-HT release in the raphe nuclei is under the control of several 5-HT homoreceptors. 5-HT(7) receptors located on glutamatergic axon terminals indirectly inhibit 5-HT release by reducing glutamatergic facilitation of serotonergic projection neurons. An opposite regulation of glutamatergic axon terminals was also found by involvement of the inhibitory 5-HT(7) and the stimulatory 5-HT(2) receptors as these receptors inhibit and stimulate glutamate release in raphe slice preparation, respectively, Furthermore, postsynaptic 5-HT(1B/1D) heteroreceptors interact with release of GABA in inhibitory fashion in raphe GABAergic interneurons. Serotonergic projection neurons also possess glutamate and GABA heteroreceptors; NMDA and AMPA receptors release 5-HT, whereas both GABAA and GABAB receptors inhibit somatodendritic 5-HT release. Evidence was found for reciprocal interactions between serotonergic and glutamatergic as well as serotonergic and GABAergic innervations in the raphe nuclei. Serotonergic neurons in the raphe nuclei also receive noradrenergic innervation arising from the locus coeruleus and alpha-1 and alpha-2 adrenoceptors inhibited [(3)H]5-HT release in our experimental conditions. The close relation between 5-HT transporter and release-mediating 5-HT autoreceptors was also shown by addition of L-deprenyl, a drug possessing inhibition of type B monoamine oxidase and 5-HT reuptake. L-Deprenyl selectively desensitizes 5-HT(1B) but not 5-HT(1A) receptors and these effects are not related to inhibition of 5-HT metabolism but rather to inhibition of 5-HT transporter.

Distribution of 5-HT1B and 5-HT1D receptors in the inner ear

Migraine and anxiety disorders are frequently co-morbid with balance disorders. This study examined the relative distribution of subtypes of serotonin (5-HT) receptor in the inner ear of monkeys and rats. Most vestibular ganglion cells were immunoreactive for 5-HT(1B) and 5-HT(1D) receptors in macaques and rats. In the inner ear, 5-HT(1B) and 5-HT(1D) receptor immunopositivity was associated with endothelial cells of the vestibular ganglion, spiral ganglion, vestibulocochlear nerve, spiral ligament and stria vascularis. It was noteworthy that 5-HT(1B) and 5-HT(1D) receptors are expressed in parallel sites in peripheral vestibular and trigeminal systems, which may be a factor underlying the efficacy of triptans in treating migraine and migrainous vertigo. Because the vestibular ganglion and trigeminal ganglion are both within the subarachnoid space, an interaction between 5-HT(1B) and TRPV1 receptors on blood vessel and ganglion cells may also contribute to the vasospasm and the comorbid headache, dizziness, nausea and vomiting that accompany subarachnoid hemorrhage.

5-HT(1B) receptor regulation of serotonin (5-HT) release by endogenous 5-HT in the substantia nigra

Axonal release of serotonin (5-hydroxytryptamine, 5-HT) in the CNS is typically regulated by presynaptic 5-HT autoreceptors. Release of 5-HT in substantia nigra pars reticulata (SNr), a principal output from the basal ganglia, has seemed an interesting exception to this rule. The SNr receives one of the highest densities of 5-HT innervation in mammalian brain and yet negative feedback regulation of axonal 5-HT release by endogenous 5-HT has not been identified here. We explored whether we could identify autoregulation of 5-HT release by 5-HT(1B) receptors in rat SNr slices using fast-scan cyclic voltammetry at carbon-fiber microelectrodes to detect 5-HT release evoked by discrete stimuli (50 Hz, 20 pulses) paired over short intervals (1-10 s) within which any autoreceptor control should occur. Evoked 5-HT release exhibited short-term depression after an initial stimulus that recovered by 10 s. Antagonists for 5-HT(1B) receptors, isamoltane (1 microM) or SB 224-289 (1 microM), did not modify release during a stimulus train, but rather, they modestly relieved depression of subsequent release evoked after a short delay (< or =2 s). Release was not modified by antagonists for GABA (picrotoxin, 100 microM, saclofen, 50 microM) or histamine-H(3) (thioperamide, 10 microM) receptors. These data indicate that 5-HT release can activate a 5-HT(1B)-receptor autoinhibition of subsequent release, which is mediated directly via 5-HT axons and not via GABAergic or histaminergic inputs. These data reveal that 5-HT release in SNr is not devoid of autoreceptor regulation by endogenous 5-HT, but rather is under modest control which only weakly limits 5-HT signaling.

Real-time measurements of synaptic autoinhibition produced by serotonin release in cultured leech neurons

We studied autoinhibition produced immediately after synaptic serotonin (5-HT) release in identified leech Retzius neurons, cultured singly or forming synapses onto pressure-sensitive neurons. Cultured Retzius neurons are isopotential, thus allowing accurate recordings of synaptic events using intracellular microelectrodes. The effects of autoinhibition on distant neuropilar presynaptic endings were predicted from model simulations. Following action potentials (APs), cultured neurons produced a slow hyperpolarization with a rise time of 85.4 +/- 5.2 ms and a half-decay time of 252 +/- 17.4 ms. These inhibitory postpotentials were reproduced by the iontophoretic application of 5-HT and became depolarizing after inverting the transmembranal chloride gradient by using microelectrodes filled with potassium chloride. The inhibitory postpotentials were reversibly abolished in the absence of extracellular calcium and absent in reserpine-treated neurons, suggesting an autoinhibition due to 5-HT acting on autoreceptors coupled to chloride channels. The autoinhibitory responses increased the membrane conductance and decreased subsequent excitability. Increasing 5-HT release by stimulating with trains of ten pulses at 10 or 30 Hz produced 23 +/- 6 and 47 +/- 2% of AP failures, respectively. These failures were reversibly abolished by the serotonergic antagonist methysergide (140 muM). Moreover, reserpine-treated neurons had only 5 +/- 4% of failures during trains at 10 Hz. This percentage was increased to 35 +/- 4% by iontophoretic application of 5-HT. Increases in AP failures correlated with smaller postsynaptic currents. Model simulations predicted that the autoinhibitory chloride conductance reduces the amplitude of APs arriving at neuropilar presynaptic endings. Altogether, our results suggest that 5-HT autoinhibits its subsequent release by decreasing the excitability of presynaptic endings within the same neuron.

Role of serotonin 5-HT1A receptors in the antidepressant-like effect and the antinociceptive effect of venlafaxine in mice

The present study was undertaken to evaluate the potential role of 5-HT1A receptors in the antidepressant-like effect and antinociceptive effect of venlafaxine. With this aim, the effect of either a selective 5-HT1A receptor antagonist (WAY-100635; N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT1A receptor agonist (8-OH-DPAT; 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) was investigated in mice in combination with venlafaxine by means of the forced swimming test, a paradigm aimed at screening potential antidepressants, and the hot-plate test, a phasic pain model. Surprisingly, the results showed that WAY-100635 produced a large decrease in the antidepressant-like effect of venlafaxine, while 8-OH-DPAT rendered effective a non-effective dose of this antidepressant. However, in the hot-plate test WAY-100635 significantly enhanced the antinociceptive effect of venlafaxine, whereas 8-OH-DPAT counteracted its antinociceptive effect. These findings show that 5-HT1A receptors play differing roles in modulating the antidepressant-like and antinociceptive effects of venlafaxine in the models investigated. The results imply that blockade of the 5-HT1A receptors in the forebrain will counteract the favourable (antidepressant-like) effect at raphe nuclei level, and consequently, the overall effect evidenced is an antagonism. This suggests a predominant role of 5-HT1A receptors located in the forebrain area for the antidepressant-like effect. In contrast, the antinociceptive effect of venlafaxine is probably potentiated due to the blockade of somatodendritic 5-HT1A receptors in the same raphe nuclei, facilitating the descending monoaminergic pain control system.

Demonstration of decreased activity of dorsal raphe nucleus neurons in depressed suicidal patients by the AgNOR staining method

BACKGROUND

Suicide and depression are closely related yet distinct phenomena. In both these phenomena, research has focused on central serotonergic system disturbances. The dorsal raphe nucleus (DRN) is the main source of serotonergic innervation of limbic structures crucial for the regulation of emotionally influenced behaviour.

METHODS

The study was carried out on paraffin-embedded brains from 23 depressed patients (12 suicides and 11 non-suicides) and 26 matched controls without mental disorders. The karyometric parameters of DRN neurons were evaluated by the AgNOR silver staining method.

RESULTS

The significant effect of suicide on the nuclear area was found in the cumulative analysis of all DRN subnuclei (ANOVA, P=0.032). A decreased mean value of this parameter was observed in the suicides group versus controls (t-test, P=0.032). This effect was especially pronounced in the violent suicide victims (t-test, P=0.001), who also demonstrated a decreased AgNOR area versus controls (t-test, P=0.007). No significant effect of depression or polarity on AgNOR parameters was found.

LIMITATIONS

A major limitation of this study is relatively small case number. A further limitation is given by the lack of data on drug exposure across the whole life span.

CONCLUSION

Our findings suggest that hypoactivity of DRN neurons is a distinct phenomenon in depression, specific only for suicidal subgroup of depressed patients.

Tyrosine hydroxylase immunoreactivity in the locus coeruleus is elevated in violent suicidal depressive patients

Our postmortem study aimed to determine the impact of suicide on the number of noradrenergic neurons of the locus coeruleus (LC) in suicidal depressive patients. Noradrenergic neurons were shown by immunostaining tyrosine hydroxylase in the LC of 22 non-elderly patients with mood disorders compared to 21 age- and sex-matched normal controls. Eleven patients were suicide victims and the other eleven died of natural causes. Seven violent suicide victims revealed an increased number of tyrosine hydroxylase immunoreactive (TH-ir) neurons compared with non-violent suicide victims and controls. No difference was found between the number of TH-ir neurons in all suicidal patients and controls and between non-suicidal patients and controls. The differences of TH-immunoreactivity could neither be attributed to medication nor to the polarity of depressive disorder (unipolar/bipolar). The numbers of TH-ir neurons in suicidal patients correlated negatively with the mean doses of antidepressants. The study suggested a presynaptic noradrenergic dysregulation in the LC related to the level of self-aggression. Traditional antidepressants may, therefore, regulate noradrenergic activity of the LC in suicide patients, however, without demonstrating the suicide-preventing effect.

Effects of Rizatriptan on the Contingent Negative Variation in Healthy Women

The effect of the antimigraine drug rizatriptan on the amplitude and habituation of the contingent negative variation (CNV) in healthy women was examined in a randomized, double-blind, placebo-controlled trial. The test persons were assigned either to a drug ( n = 20) or a placebo group ( n = 20). The CNV was recorded three times: before, directly after, and 24 h after drug or placebo intake. The CNV paradigm was presented in a standard, a cued and a choice version. Rizatriptan led to an increase of CNV amplitude that depended on the level of difficulty of the task. Whereas there was no drug effect in the standard version, an amplitude increase was obtained mainly in the choice task. The results are in line with the ceiling theory of migraine, which assumes a rise of CNV amplitude if the serotonin level is lowered.

5-HT1A gene variants and psychiatric disorders: a review of current literature and selection of SNPs for future studies

5-HT1A receptors are key components of the serotonin system, acting both pre- and post- synaptically in different brain areas. There is a growing amount of evidence showing the importance of 5-HT1A in different psychiatric disorders, from mood to anxiety disorders, moving through suicidal behaviour and psychotic disorders. Findings in the literature are not consistent with any definite 5-HT1A influence in psychiatric disorders. 5-HT1A gene variants have been reported to play some role in mood disorders, anxiety disorders and psychotic disorders. Again, the literature findings are not unequivocal. Concerning response to treatment, the C(-1019)G variant seems to be of primary interest in antidepressant response: C allele carriers generally show a better response to treatment, especially in Caucasian samples. Together with the C(-1019)G (rs6295) variant, the Ile28Val (rs1799921), Arg219Leu (rs1800044) and Gly22Ser (rs1799920) variants have been investigated in possible associations with psychiatric disorders, also with no definitive results. This lack of consistency can be also due to an incomplete gene investigation. To make progress on this point, a list of validated single nucleotide polymorphisms (SNPs) covering the whole gene is proposed for further investigations.