Sucrose intake and fasting glucose levels in 5-HT(1A) and 5-HT(1B) receptor mutant mice

Neuroprotective effect of niacin in a rat model of obesity induced by high-fat-rich diet

This study investigates the impact of a high-fat-rich diet (HFRD) on behavioral, biochemical, neurochemical, and histopathological studies using the hypothalamus of rats following niacin (NCN) administration. The rats were divided into HFRD and normal diet (ND)-fed groups and administered selected doses of NCN, i.e., 25 mg/mL/kg (low dose) and 50 mg/mL/kg (high dose), for 8 weeks. The grouping of male rats (n = 8) was as follows: (i) Vehicle (Veh) + ND; (ii) ND + NCN (low dose); (iii) ND + NCN (high dose); (iv) Veh + HFRD; (v) HFRD + NCN (low dose); and (vi) HFRD + NCN (high dose). Behavioral tests assessed depression-like symptoms and spatial memory; after that, the hypothalamus was isolated for various analyses of sacrificed animals. NCN at both doses decreased food intake and growth rate in both diet groups and demonstrated antidepressant and memory-enhancing effects. HFRD-induced oxido-neuroinflammation decreased with both doses of NCN. HFRD-induced decreases in serotonergic neurotransmission, 5-HT1A receptor expression, and morphological alterations in the rat's hypothalamus were normalized by both doses of NCN. In conclusion, NCN, as a potential antioxidant and neuromodulator, can normalize feeding behavior and produce antidepressant and memory-improving effects in a rat model of obesity following HFRD intake.

Establishing an OCD Model in BALB/c Mice Using RU24969: A Molecular and Behavioural Study of Optimal Dose Selection

Obsessive-compulsive disorder (OCD) is a disabling disease characterized by distressing obsessions and repetitive compulsions. The etiology of OCD is poorly known, and mouse modeling allows to clarify the genetic and neurochemical basis of this disorder and to investigate potential treatments. This study evaluates the impact of the 5-HT1B agonist RU24969 on the induction of OCD-like behaviours in female BALB/c mice (n = 30), distributed across five groups receiving varying doses of RU24969. Behavioural assessments, including marble test, tail suspension test, sucrose preference test, forced swim test, and nestlet shredding test, were conducted. Gene expression and protein quantitation of Gabra1 and serotonin transporter in mouse brain were also performed. Marble-burying behaviour increased significantly at high doses of RU24969 (15-20 mg/kg). The forced swimming test consistently showed elevated values at the same high concentrations, compared to the control. Altered reward-seeking behaviour was indicated by the sucrose preference test, notably at 15 and 20 mg/kg doses of RU24969. Nestlet shredding results did not show statistical significance among the tested animal groups. Gene expression analysis revealed reduced Gabra1 expression with increasing doses of RU, while serotonin transporter was not related to varying doses of RU24969. Western blotting corroborated these trends. The results underscore complex interactions between the serotonin system, GABAergic signaling, and OCD-relevant behaviours and suggest the use of intraperitoneal injection of 15 mg/kg of RU24969 to induce OCD-like behaviour in BALB/c mouse models.

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.

Effects of social isolation on 50-kHz ultrasonic vocalizations, affective state, cognition, and neurotransmitter concentrations in the ventral tegmental and locus coeruleus of adult rats

Vocal communication, cognition, and affective state are key features of sustained health and wellness, and because vocalizations are often socially-motivated, social experience likely plays a role in these behaviors. The monoaminergic systems of the ventral tegmental area (VTA) and the locus coeruleus (LC) are associated with social and reward processing, vocalization production, and neurotransmitter changes in response to environmental stressors. The effect of social isolation on these complex behaviors and the underlying neural mechanisms is relatively unknown. To add to this body of literature, we randomized adult male Long-Evans rats to control (housed with a cagemate) or isolated (housed individually) conditions and assayed ultrasonic vocalizations, cognition (novel object recognition test), anxiety (elevated plus maze) and anhedonia (sucrose preference test) at 2, 4, 6, 8, and 10 months of age. At 10 months, VTA and LC samples were assayed for dopamine, norepinephrine, and serotonin using high performance liquid chromatography. We tested the hypotheses that isolation 1) diminishes vocalizations and cognition, 2) increases anxiety and depression, and 3) increases levels of dopamine, norepinephrine, and serotonin in the VTA and LC. Results showed isolation significantly reduced vocalization tonality (signal-to-noise ratio) and increased maximum frequency. There were no significant findings for cognition, anxiety, or anhedonia. Dopamine and serotonin and their respective metabolites were significantly increased in the VTA in isolated rats. These findings suggest chronic changes to social condition such as isolation affects vocalization production and levels of VTA neurotransmitters.

5-HT1B receptor-AC-PKA signal pathway in the lateral habenula is involved in the regulation of depressive-like behaviors in 6-hydroxydopamine-induced Parkinson's rats

OBJECTIVE

The aim of the present study was to investigate whether serotonin_1B (5-HT_1B) receptor-adenylate cyclase (AC)-protein kinase A (PKA) signal pathway in the lateral habenula (LHb) is involved in Parkinson's disease-related depression in sham-lesioned and substantia nigra pars compacta (SNc)-lesioned rats.

METHODS

The sucrose preference and forced swim tests were used to measure depressive-like behaviors. In vivo electrophysiology and microdialysis were performed to observe the firing activity of LHb neurons and GABA and glutamate release in the LHb, respectively. Western blotting was used to analyze protein expression of 5-HT_1B receptors, AC and phosphorylated PKA at threonine 197 site (p-PKA-Thr197) in the LHb.

RESULTS

Unilateral 6-hydroxydopamine lesions of the SNc in rats induced depressive-like behaviors. Intra-LHb injection of 5-HT_1B receptor agonist CP93129 produced antidepressant-like effects and the antagonist SB216641 induced depressive-like behaviors in sham-lesioned and SNc-lesioned rats. Further, pretreatment with AC inhibitor SQ22536 and PKA inhibitor KT5720 blocked the behavioral effects of CP93129 in the two groups of rats, respectively. CP93129 decreased the firing rate of LHb neurons and release of GABA and glutamate, but increased the GABA/glutamate ratio, while SB216641 induced the opposite effects. Compared with sham-lesioned rats, effects of CP93129 and SB216641 on the depressive-like behaviors, electrophysiology, and microdialysis were decreased in SNc-lesioned rats, which were associated with decreased expression of 5-HT_1B receptors, AC and p-PKA-Thr197 in the LHb.

CONCLUSION

5-HT_1B receptor-AC-PKA signal pathway in the LHb is involved in the regulation of depressive-like behaviors, and depletion of DA reduces activity of 5-HT_1B receptor-AC-PKA signal pathway.

S100A10 and its binding partners in depression and antidepressant actions

S100A10 (p11) is an emerging player in the neurobiology of depression and antidepressant actions. p11 was initially thought to be a modulator of serotonin receptor (5-HTR) trafficking and serotonergic transmission, though newly identified binding partners of p11 and neurobiological studies of these proteins have shed light on multifunctional roles for p11 in the regulation of glutamatergic transmission, calcium signaling and nuclear events related to chromatin remodeling, histone modification, and gene transcription. This review article focuses on direct binding partners of p11 in the brain including 5-HTRs, mGluR5, annexin A2, Ahnak, Smarca3, and Supt6h, as well as their roles in neuronal function, particularly in the context of depressive-like behavior as well as behavioral effects of antidepressant drug treatments in mice. In addition, we discuss neurobiological insights from recently uncovered p11 pathways in multiple types of neurons and non-neuronal cells and cast major remaining questions for future studies.

Serotonin, food intake, and obesity

The role of serotonin in food intake has been studied for decades. Food intake is mainly regulated by two brain circuitries: (i) the homeostatic circuitry, which matches energy intake to energy expenditure, and (ii) the hedonic circuitry, which is involved in rewarding and motivational aspects of energy consumption. In the homeostatic circuitry, serotonergic signaling contributes to the integration of metabolic signals that convey the body's energy status and facilitates the ability to suppress food intake when homeostatic needs have been met. In the hedonic circuitry, serotonergic signaling may reduce reward-related, motivational food consumption. In contrast, peripherally acting serotonin promotes energy absorption and storage. Disturbed serotonergic signaling is associated with obesity, emphasizing the importance to understand the role of serotonergic signaling in food intake. However, unraveling the serotonin-mediated regulation of food intake is complex, as the effects of serotonergic signaling in different brain regions depend on the regional expression of serotonin receptor subtypes and downstream effects via connections to other brain regions. We therefore provide an overview of the effects of serotonergic signaling in brain regions of the homeostatic and hedonic regulatory systems on food intake. Furthermore, we discuss the disturbances in serotonergic signaling in obesity and its potential therapeutic implications.

Endocannabinoid-serotonin systems interaction in health and disease

Endocannabinoid (eCB) and serotonin (5-HT) neuromodulatory systems work both independently and together to finely orchestrate neuronal activity throughout the brain to strongly sculpt behavioral functions. Surprising parallelism between the behavioral effects of 5-HT and eCB activity has been widely reported, including the regulation of emotional states, stress homeostasis, cognitive functions, food intake and sleep. The distribution pattern of the 5-HT system and the eCB molecular elements in the brain display a strong overlap and several studies report a functional interplay and even a tight interdependence between eCB/5-HT signaling. In this review, we examine the available evidence of the interaction between the eCB and 5-HT systems. We first introduce the eCB system, then we describe the eCB/5-HT crosstalk at the neuronal and synaptic levels. Finally, we explore the potential eCB/5-HT interaction at the behavioral level with the implication for psychiatric and neurological disorders. The precise elucidation of how this neuromodulatory interaction dynamically regulates biological functions may lead to the development of more targeted therapeutic strategies for the treatment of depressive and anxiety disorders, psychosis and epilepsy.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Manganese-induced neurotoxicity and the potential protective effects of lipoic acid and Spirulina platensis

Manganese (Mn) is essential for many physiological processes; however, its excessive accumulation in the brain causes severe dysfunctions in the nervous system. Oxidative stress is thought to be involved in Mn-induced neurotoxicity. The aim of this study was to evaluate the neurotoxic effects of Mn and the potential protective effects of alpha lipoic acid (ALA) and Spirulina platensis (SP), each alone and in combination. Sixty-four male albino rats were divided into eight equal groups: group 1 was used as control, group 2 received saline, which used as a vehicle, group 3 received ALA (50 mg/kg/day), group 4 received SP (300 mg/kg/day), group 5 received Mn (74 mg/kg, 5 days/week), group 6 received Mn + ALA, group 7 received Mn + SP, group 8 received Mn + ALA + SP. Groups 6, 7 and 8 received the same previously mentioned doses. All treatments were orally gavaged for 8 weeks. Mn administration caused neurobehavioral changes, increases of brain and serum Mn and malondialdehyde (MDA), with decreased glutathione peroxidase (GPx), dopamine and acetylcholine levels. The co-treatment with ALA and SP revealed their ability to protect against oxidative damage, neurobehavioral and biochemical changes induced by Mn.

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.

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.

Expression of serotonin receptor genes in cranial ganglia

Taste cells release neurotransmitters to gustatory neurons to transmit chemical information they received. Sweet, umami, and bitter taste cells use ATP as a neurotransmitter. However, ATP release from sour taste cells has not been observed so far. Instead, they release serotonin when they are activated by sour/acid stimuli. Thus it is still controversial whether sour taste cells use ATP, serotonin, or both. By reverse transcription-polymerase chain reaction and subsequent in situ hybridization (ISH) analyses, we revealed that of 14 serotonin receptor genes only 5-HT3A and 5-HT3B showed significant/clear signals in a subset of neurons of cranial sensory ganglia in which gustatory neurons reside. Double-fluorescent labeling analyses of ISH for serotonin receptor genes with wheat germ agglutinin (WGA) in cranial sensory ganglia of pkd1l3-WGA mice whose sour neural pathway is visualized by the distribution of WGA originating from sour taste cells in the posterior region of the tongue revealed that WGA-positive cranial sensory neurons rarely express either of serotonin receptor gene. These results suggest that serotonin receptors expressed in cranial sensory neurons do not play any role as neurotransmitter receptor from sour taste cells.

Mice lacking the serotonin 5-HT2B receptor as an animal model of resistance to selective serotonin reuptake inhibitors antidepressants

Depressive disorders are among the most prevalent neuropsychiatric dysfunctions worldwide, with high rates of resistance to antidepressant treatment. Genetic factors clearly contribute to the manifestation of depression as well as to the response to antidepressants. Transgenic mouse models appear as seminal tools to disentangle this complex disorder. Here, we analyzed new key aspects of the phenotype of knock-out mice for the gene encoding the serotonin 2B receptor (Htr(2B)(-/-)), including basal phenotype, ability to develop a depressive-like phenotype upon chronic isolation, and effect of chronic exposure to fluoxetine on chronically stressed Htr(2B)(-/-) mice. We find, here, that Htr(2B)(-/-) mice display an antidepressant-like phenotype, which includes reduced latency to feed in the novelty suppressed feeding test, basal increase in hippocampal BDNF levels, no change in TrkB and p75 protein levels, and an increased preference for sucrose consumption compared to wild type (Htr(2B)(+/+)) mice. Nevertheless, we show that these mice can develop depressive-like behaviors when socially isolated during four weeks. Selective serotonin reuptake inhibitors (SSRI) have been previously shown to be ineffective in non-stressed Htr(2B)(-/-) mice. We evaluated, here, the effects of the SSRI fluoxetine in chronically stressed Htr(2B)(-/-) mice and similarly no behavioral or plastic effect was induced by this antidepressant. All together, these results highlight the suitability to study resistance to SSRI antidepressants of this mouse model displaying panoply of conditions among which behavioral, neurotrophic and plastic causative factors can be analyzed.

Opposing roles for serotonin in cholinergic neurons of the ventral and dorsal striatum

Little is known about the molecular similarities and differences between neurons in the ventral (vSt) and dorsal striatum (dSt) and their physiological implications. In the vSt, serotonin [5-Hydroxytryptamine (5-HT)] modulates mood control and pleasure response, whereas in the dSt, 5-HT regulates motor behavior. Here we show that, in mice, 5-HT depolarizes cholinergic interneurons (ChIs) of the dSt whereas hyperpolarizing ChIs from the vSt by acting on different 5-HT receptor isoforms. In the vSt, 5-HT1A (a postsynaptic receptor) and 5-HT1B (a presynaptic receptor) are highly expressed, and synergistically inhibit the excitability of ChIs. The inhibitory modulation by 5-HT1B, but not that by 5-HT1A, is mediated by p11, a protein associated with major depressive disorder. Specific deletion of 5-HT1B from cholinergic neurons results in impaired inhibition of ACh release in the vSt and in anhedonic-like behavior.

Viral vector mediated expression of mutant huntingtin in the dorsal raphe produces disease-related neuropathology but not depressive-like behaviors in wildtype mice

Huntington׳s disease (HD) is a neurodegenerative disorder caused by a mutation in the HTT gene (mHTT) encoding the protein huntingtin. An expansion in the gene׳s CAG repeat length renders a misfolded, dysfunctional protein with an abnormally long glutamine (Q) stretch at the N terminus that often incorporates into inclusion bodies and leads to neurodegeneration in many regions of the brain. HD is characterized by motor and cognitive decline as well as mood disorders, with depression being particularly common. Approximately 40% of the HD population suffers from depressive symptoms. Because these symptoms often manifest a decade or more prior to the knowledge that the person is at risk for the disease, a portion of the early depression in HD appears to be a consequence of the pathology arising from expression of the mutant gene. While the depression in HD patients is often treated with serotonin agonists, there is scant experimental evidence that the depression in HD responds well to these serotonin treatments or in a similar manner to how non-HD depression tends to respond. Additionally, at very early sub-threshold depression levels, abnormal changes in several neuronal populations are already detectable in HD patients, suggesting that a variety of brain structures may be involved. Taken together, the serotonin system is a viable candidate. However, at present there is limited evidence of the precise nuclei or circuits that play a role in HD depression. With this in mind, the current study was designed to control for the widespread brain neuropathology that occurs in HD and in transgenic mouse models of HD and focuses specifically on the influence of the midbrain dorsal raphe nucleus (DRN). The DRN provides the majority of the serotonin to the forebrain and exhibits cell loss in non-HD depression. Therefore, we employed a viral vector delivery system to investigate whether the over-expression of mHTT in the DRN׳s ventral sub-nuclei alone is sufficient to produce depressive-like behaviors. Wildtype mice were injected with an adeno-associated virus (AAV2/1) encoding HTT containing either a pathogenic (N171-82Q) or control (N171-16Q) CAG repeat length into the ventral DRN and depressive-like behaviors and motor behaviors were assessed for 12 weeks post-surgery. Quantitative PCR and immunohistochemistry (IHC) verified positive transduction in the ventral aspects of the DRN, including the ventral sub-nucleus (DRv) and interfascicular sub-nucleus (DRif). IHC demonstrated microgliosis in and around the injection site and mHTT-positive inclusions in serotonin-producing neurons and a small percentage of astrocytes in animals injected with N171-82Q compared to controls. Moreover, N171-82Q injected mice showed a 75% reduction in cells that stained positive for the serotonin synthesis enzyme, tryptophan hydroxylase-2 (TPH2) compared to controls (p<0.05). Despite mHTT-mediated pathology in the DRv and DRif, no significant changes in depressive-like behavior were detected. Consequently, we conclude that 12 weeks of N171-82Q expression in the ventral sub-nuclei of the DRN of wildtype mice causes characteristic disease-related cellular neuropathology but is not sufficient to elicit depressive-like behaviors. Ongoing studies are investigating whether a larger injection volume that transfects a larger percentage of the DRN and/or a longer time course of mHTT expression might elicit depressive-like behaviors. Moreover, mHTT expression in other regions of the brain, such as the hippocampal dentate gyrus and/or the frontal cortex might be necessary to elicit HD depression. Together, these results may prove helpful in addressing which therapeutic and/or pharmacological strategies might be most efficacious when treating depressive symptomology in patients suffering from HD.

The expression of platelet serotonin transporter (SERT) in human obesity

Background

Serotonin (5-HT) is a well-known modulator of eating behavior. However, the molecular mechanisms linking its action to body weight balance have been only partially elucidated. Since platelets are a suitable peripheral model to study 5-HT transport, metabolism and release, we herein evaluated the expression of the platelet 5-HT re-uptake system (SERT) by [³H]-paroxetine binding assay. A cohort of 114 unrelated individuals (34 males, 80 females; age, mean ± SD: 38.57 ± 12.47 years) without major psychiatric disorders, was recruited following a naturalistic design regarding age or gender and classified accordingly to their body mass index (BMI). Subjects were divided into 5 groups: normal-weight (NW), overweight (OW) and grade I-III obese (OB) individuals. For gender analyses, data were transformed into [³H]-paroxetine density (B_max)/BMI ratios to overcome both the disparity of women vs. men number and anthropometric differences between sexes.

Results

[³H]-paroxetine B_max (SERT density, fmol/mg proteins) was reduced in platelet membranes of grade II (p < 0.01) and III (p < 0.001) obese subjects vs. controls and in overweight subjects (p < 0.05) vs. grade III obese individuals. Considering all patients together, a strong negative correlation between B_max and BMI (r = −0.449; P < 0.0001) was demonstrated. Conversely, [³H]-paroxetine K_D (dissociation constant, nM) did not differ among groups. No gender-related variation concerning B_max/BMI ratios was observed in this cohort of subjects.

Conclusions

The down-regulation of SERT in platelet membranes of severe human obesity (BMI > 35 Kg/m²) confirms the involvement of 5-HT system in body weight gain. Moreover, this findings may help to elucidate those monoamine-endocrine networks acting on fat storage, adipocyte signaling and energy balance. Targeting 5-HT/5-HT-related markers will possibly uncover the existence of human obesity subtypes.

Local potentiation of excitatory synapses by serotonin and its alteration in rodent models of depression

The causes of major depression remain unknown. Antidepressants elevate concentrations of monoamines, particularly serotonin, but it remains uncertain which downstream events are critical to their therapeutic effects. We found that endogenous serotonin selectively potentiated excitatory synapses formed by the temporoammonic pathway with CA1 pyramidal cells via activation of serotonin receptors (5-HT(1B)Rs), without affecting nearby Schaffer collateral synapses. This potentiation was expressed postsynaptically by AMPA-type glutamate receptors and required calmodulin-dependent protein kinase-mediated phosphorylation of GluA1 subunits. Because they share common expression mechanisms, long-term potentiation and serotonin-induced potentiation occluded each other. Long-term consolidation of spatial learning, a function of temporoammonic-CA1 synapses, was enhanced by 5-HT(1B)R antagonists. Serotonin-induced potentiation was quantitatively and qualitatively altered in a rat model of depression, restored by chronic antidepressants, and required for the ability of chronic antidepressants to reverse stress-induced anhedonia. Changes in serotonin-mediated potentiation, and its recovery by antidepressants, implicate excitatory synapses as a locus of plasticity in depression.

Inducible nitric oxide synthase is involved in the modulation of depressive behaviors induced by unpredictable chronic mild stress

BACKGROUND

Experiences and inflammatory mediators are fundamental in the provocation of major depressive disorders (MDDs). We investigated the roles and mechanisms of inducible nitric oxide synthase (iNOS) in stress-induced depression.

METHODS

We used a depressive-like state mouse model induced by unpredictable chronic mild stress (UCMS). Depressive-like behaviors were evaluated after 4 weeks of UCMS, in the presence and absence of the iNOS inhibitor N-(3-(aminomethyl)benzyl)acetamidine (1400 W) compared with the control group. Immunohistochemistry was used to check the loss of Nissl bodies in cerebral cortex neurons. The levels of iNOS mRNA expression in the cortex and nitrites in the plasma were measured with real-time reverse transcription PCR (RT-PCR) and Griess reagent respectively.

RESULTS

Results showed that the 4-week UCMS significantly induced depressive-like behaviors, including decreased sucrose preference in a sucrose preference test, increased duration of immobility in a forced swim test, and decreased hole-searching time in a locomotor activity test. Meanwhile, in the locomotor activity test, UCMS had no effect on normal locomotor activities, such as resting time, active time and total travel distance. Furthermore, the levels of iNOS mRNA expression in the cortex and nitrites in the plasma of UCMS-exposed mice were significantly increased compared with that of the control group. Neurons of cerebral cortex in UCMS-exposed mice were shrunken with dark staining, together with loss of Nissl bodies. The above-mentioned stress-related depressive-like behaviors, increase of iNOS mRNA expression in the cortex and nitrites in the plasma, and neuron damage, could be abrogated remarkably by pretreating the mice with an iNOS inhibitor (1400 W). Moreover, neurons with abundant Nissl bodies were significantly increased in the 1400 W + UCMS group.

CONCLUSIONS

These results support the notion that stress-related NO (derived from iNOS) may contribute to depressive-like behaviors in a mouse model, potentially concurrent with neurodegenerative effects within the cerebral cortex.

The neurobiology of anhedonia and other reward-related deficits

Anhedonia, or markedly diminished interest or pleasure, is a hallmark symptom of major depression, schizophrenia and other neuropsychiatric disorders. Over the past three decades, the clinical definition of anhedonia has remained relatively unchanged, although cognitive psychology and behavioral neuroscience have expanded our understanding of other reward-related processes. Here, we review the neural bases of the construct of anhedonia that reflects deficits in hedonic capacity and also closely linked to the constructs of reward valuation, decision-making, anticipation and motivation. The neural circuits subserving these reward-related processes include the ventral striatum, prefrontal cortical regions, and afferent and efferent projections. An understanding of anhedonia and other reward-related constructs will facilitate the diagnosis and treatment of disorders that include reward deficits as key symptoms.

Social isolation in adolescence alters behaviors in the forced swim and sucrose preference tests in female but not in male rats

Social interactions in rodents are rewarding and motivating and social isolation is aversive. Accumulating evidence suggests that disruption of the social environment in adolescence has long-term effects on social interactions, on anxiety-like behavior and on stress reactivity. In previous work we showed that adolescent isolation produced increased reactivity to acute and to repeated stress in female rats, whereas lower corticosterone responses to acute stress and decreased anxiety-related behavior were noted in isolated males. These results indicate a sex specific impact on the effects of social stress in adolescence. However, little is known about whether social isolation impacts behaviors related to affect and whether it does so differently in male and female rats. The present study investigated the impact of adolescent social isolation from day 30-50 of age in male and female Sprague Dawley rats on behavior in the forced swim test at the end of adolescence and in adulthood and on behavior in the sucrose preference test in adulthood. Adult female rats that were isolated in adolescence exhibited increased climbing on the first and second day of the forced swim test and showed an increased preference for sucrose compared to adult females that were group-housed in adolescence. There were no effects in male rats. The results indicate that social isolation in adolescence produces a stable and active behavioral phenotype in adult female rats.

Brain serotonin system in the coordination of food intake and body weight

An inverse relationship between brain serotonin and food intake and body weight has been known for more than 30 years. Specifically, augmentation of brain serotonin inhibits food intake, while depletion of brain serotonin promotes hyperphagia and weight gain. Through the decades, serotonin receptors have been identified and their function in the serotonergic regulation of food intake clarified. Recent refined genetic studies now indicate that a primary mechanism through which serotonin influences appetite and body weight is via serotonin 2C receptor (5-HT(2C)R) and serotonin 1B receptor (5-HT(1B)R) influencing the activity of endogenous melanocortin receptor agonists and antagonists at the melanocortin 4 receptor (MC4R). However, other mechanisms are also possible and the challenge of future research is to delineate them in the complete elucidation of the complex neurocircuitry underlying the serotonergic control of appetite and body weight.

Electrical stimulation of the lateral habenula produces enduring inhibitory effect on cocaine seeking behavior

The lateral habenula (LHb) is critical for modulation of negative reinforcement, punishment and aversive responses. In light of the success of deep-brain-stimulation (DBS) in the treatment of neurological disorders, we explored the use of LHb DBS as a method of intervention in cocaine self-administration, extinction, and reinstatement in rats. An electrode was implanted into the LHb and rats were trained to self-administer cocaine (21 days; 0.25-1 mg/kg) until they achieved at least three days of stable performance (as measured by daily recordings of active lever presses in self-administration cages). Thereafter, rats received DBS in the presence or absence of cocaine. DBS reduced cocaine seeking behavior during both self-administration and extinction training. DBS also attenuated the rats' lever presses following cocaine reinstatement (5-20 mg/kg) in comparison to sham-operated rats. These results were also controlled by the assessment of physical performance as measured by water self-administration and an open field test, and by evaluation of depressive-like manifestations as measured by the swim and two-bottles-choice tests. In contrast, LHb lesioned rats demonstrated increased cocaine seeking behavior as demonstrated by a delayed extinction response. In the ventral tegmental area, cocaine self-administration elevated glutamatergic receptor subunits NR1 and GluR1 and scaffolding protein PSD95, but not GABA(A)β, protein levels. Following DBS treatment, levels of these subunits returned to control values. We postulate that the effect of both LHb modulation and LHb DBS on cocaine reinforcement may be via attenuation of the cocaine-induced increase in glutaminergic input to the VTA.

Assessment of glutamate transporter GLAST (EAAT1)-deficient mice for phenotypes relevant to the negative and executive/cognitive symptoms of schizophrenia

Glutamatergic dysfunction is increasingly implicated in the pathophysiology of schizophrenia. Current models postulate that dysfunction of glutamate and its receptors underlie many of the symptoms in this disease. However, the mechanisms involved are not well understood. Although elucidating the role for glutamate transporters in the disease has been limited by the absence of pharmacological tools that selectively target the transporter, we recently showed that glial glutamate and aspartate transporter (GLAST; excitatory amino-acid transporter 1) mutant mice exhibit abnormalities on behavioral measures thought to model the positive symptoms of schizophrenia, some of which were rescued by treatment with either haloperidol or the mGlu2/3 agonist, LY379268 the mGlu2/3 agonist, LY379268. To further determine the role of GLAST in schizophrenia-related behaviors we tested GLAST mutant mice on a series of behavioral paradigms associated with the negative (social withdrawal, anhedonia), sensorimotor gating (prepulse inhibition of startle), and executive/cognitive (discrimination learning, extinction) symptoms of schizophrenia. GLAST knockout (KO) mice showed poor nesting behavior and abnormal sociability, whereas KO and heterozygous (HET) both demonstrated lesser preference for a novel social stimulus compared to wild-type littermate controls. GLAST KO, but not HET, had a significantly reduced acoustic startle response, but no significant deficit in prepulse inhibition of startle. GLAST KO and HET showed normal sucrose preference. In an instrumental visual discrimination task, KO showed impaired learning. By contrast, acquisition and extinction of a simple instrumental response was normal. The mGlu2/3 agonist, LY379268, failed to rescue the discrimination impairment in KO mice. These findings demonstrate that gene deletion of GLAST produces select phenotypic abnormalities related to the negative and cognitive symptoms of schizophrenia.

Pharmacological targeting of the serotonergic system for the treatment of obesity

The attenuation of food intake as induced by an increase in serotonergic (5-hydroxytryptamine, 5-HT) efficacy has been a target of antiobesity pharmacotherapies. However, the induction of tolerance and/or side-effects limited the clinical utility of the earliest serotonin-related medications. With the global prevalence of obesity rising, there has been renewed interest in the manipulation of the serotonergic system as a point of pharmacological intervention. The serotonin(2C) receptor (5-HT(2C)R), serotonin(1B) (rodent)/serotonin(1Dbeta) (human) receptor (5-HT(1B/1Dbeta)R) and serotonin(6) receptor (5-HT(6)R) represent the most promising serotonin receptor therapeutic targets. Canonical serotonin receptor compounds have given way to a myriad of novel receptor-selective ligands, many of which have observable anorectic effects. Here we review serotonergic compounds reducing ingestive behaviour and discuss their clinical potential for the treatment of obesity.