5-HT systems: emergent targets for memory formation and memory alterations

Revisiting serotonin's role in spatial memory: A call for sensitive analytical approaches

The serotonergic system is involved in various psychiatric and neurological conditions, with serotonergic drugs often used in treatment. These conditions frequently affect spatial memory, which can serve as a model of declarative memory due to well-known cellular components and advanced methods that track neural activity and behavior with high temporal resolution. However, most findings on serotonin's effects on spatial learning and memory come from studies lacking refined analytical techniques and modern approaches needed to uncover the underlying neuronal mechanisms. This In Focus review critically investigates available studies to identify areas for further exploration. It finds that well-established behavioral models could yield more insights with modern tracking and data analysis approaches, while the cellular aspects of spatial memory remain underexplored. The review highlights the complex role of serotonin in spatial memory, which holds the potential for better understanding and treating memory-related disorders.

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Electroacupuncture Alleviates Memory Deficits in APP/PS1 Mice by Targeting Serotonergic Neurons in Dorsal Raphe Nucleus

OBJECTIVE

Alzheimer's disease (AD) has become a significant global concern, but effective drugs able to slow down AD progression is still lacked. Electroacupuncture (EA) has been demonstrated to ameliorate cognitive impairment in individuals with AD. However, the underlying mechanisms remains poorly understood. This study aimed at examining the neuroprotective properties of EA and its potential mechanism of action against AD.

METHODS

APP/PS1 transgenic mice were employed to evaluate the protective effects of EA on Shenshu (BL 23) and Baihui (GV 20). Chemogenetic manipulation was used to activate or inhibit serotonergic neurons within the dorsal raphe nucleus (DRN). Learning and memory abilities were assessed by the novel object recognition and Morris water maze tests. Golgi staining, western blot, and immunostaining were utilized to determine EA-induced neuroprotection.

RESULTS

EA at Shenshu (BL 23) and Baihui (GV 20) effectively ameliorated learning and memory impairments in APP/PS1 mice. EA attenuated dendritic spine loss, increased the expression levels of PSD95, synaptophysin, and brain-derived neurotrophic factor in hippocampus. Activation of serotonergic neurons within the DRN can ameliorate cognitive deficits in AD by activating glutamatergic neurons mediated by 5-HT1B. Chemogenetic inhibition of serotonergic neurons in the DRN reversed the effects of EA on synaptic plasticity and memory.

CONCLUSION

EA can alleviate cognitive dysfunction in APP/PS1 mice by activating serotonergic neurons in the DRN. Further study is necessary to better understand how the serotonergic neurons-related neural circuits involves in EA-induced memory improvement in AD.

Structural determinants for activation of the Tau kinase CDK5 by the serotonin receptor 5-HT7R

BACKGROUND

Multiple neurodegenerative diseases are induced by the formation and deposition of protein aggregates. In particular, the microtubule-associated protein Tau leads to the development of so-called tauopathies characterized by the aggregation of hyperphosphorylated Tau within neurons. We recently showed that the constitutive activity of the serotonin receptor 7 (5-HT7R) is required for Tau hyperphosphorylation and aggregation through activation of the cyclin-dependent kinase 5 (CDK5). We also demonstrated physical interaction between 5-HT7R and CDK5 at the plasma membrane suggesting that the 5-HT7R/CDK5 complex is an integral part of the signaling network involved in Tau-mediated pathology.

METHODS

Using biochemical, microscopic, molecular biological, computational and AI-based approaches, we investigated structural requirements for the formation of 5-HT7R/CDK5 complex.

RESULTS

We demonstrated that 5-HT7R domains responsible for coupling to Gs proteins are not involved in receptor interaction with CDK5. We also created a structural model of the 5-HT7R/CDK5 complex and refined the interaction interface. The model predicted two conserved phenylalanine residues, F278 and F281, within the third intracellular loop of 5-HT7R to be potentially important for complex formation. While site-directed mutagenesis of these residues did not influence Gs protein-mediated receptor signaling, replacement of both phenylalanines by alanine residues significantly reduced 5-HT7R/CDK5 interaction and receptor-mediated CDK5 activation, leading to reduced Tau hyperphosphorylation and aggregation. Molecular dynamics simulations of 5-HT7R/CDK5 complex for wild-type and receptor mutants confirmed binding interface stability of the initial model.

CONCLUSIONS

Our results provide a structural basis for the development of novel drugs targeting the 5-HT7R/CDK5 interaction interface for the selective treatment of Tau-related disorders, including frontotemporal dementia and Alzheimer's disease.

Interleukin-15 alters hippocampal synaptic transmission and impairs episodic memory formation in mice

Cytokines are potent immunomodulators exerting pleiotropic effects in the central nervous system (CNS). They influence neuronal functions and circuit activities with effects on memory processes and behaviors. Here, we unravel a neuromodulatory activity of interleukin-15 (IL-15) in mouse brain. Acute exposure of hippocampal slices to IL-15 enhances gamma-aminobutyricacid (GABA) release and reduces glutamatergic currents, while chronic treatment with IL-15 increases the frequency of hippocampal miniature inhibitory synaptic transmission and impairs memory formation in the novel object recognition (NOR) test. Moreover, we describe that serotonin is involved in mediating the hippocampal effects of IL-15, because a selective 5-HT3A receptor antagonist prevents the effects on inhibitory neurotransmission and ameliorates mice performance in the NOR test. These findings provide new insights into the modulatory activities of cytokines in the CNS, with implications on behavior.

Learning Deficits Induced by High-Calorie Feeding in the Rat are Associated With Impaired Brain Kynurenine Pathway Metabolism

In addition to be a primary risk factor for type 2 diabetes and cardiovascular disease, obesity is associated with learning disabilities. Here we examined whether a dysregulation of the kynurenine pathway (KP) of tryptophan (Trp) metabolism might underlie the learning deficits exhibited by obese individuals. The KP is initiated by the enzymatic conversion of Trp into kynurenine (KYN) by indoleamine 2,3-dioxygenase (IDO). KYN is further converted to several signaling molecules including quinolinic acid (QA) which has a negative impact on learning. Wistar rats were fed either standard chow or made obese by exposure to a free choice high-fat high-sugar (fcHFHS) diet. Their learning capacities were evaluated using a combination of the novel object recognition and the novel object location tasks, and the concentrations of Trp and KYN-derived metabolites in several brain regions determined by ultra-performance liquid chromatography-tandem mass spectrometry. Male, but not female, obese rats exhibited reduced learning capacity characterized by impaired encoding along with increased hippocampal concentrations of QA, Xanthurenic acid (XA), Nicotinamide (Nam), and oxidized Nicotinamide Adenine Dinucleotide (NAD+). In contrast, no differences were detected in the serum levels of Trp or KP metabolites. Moreover, obesity enhanced the expression in the hippocampus and frontal cortex of kynurenine monooxygenase (KMO), an enzyme involved in the production of QA from kynurenine. QA stimulates the glutamatergic system and its increased production leads to cognitive impairment. These results suggest that the deleterious effects of obesity on cognition are sex dependent and that altered KP metabolism might contribute to obesity-associated learning disabilities.

The role of serotonin in declarative memory: A systematic review of animal and human research

The serotonergic system is involved in diverse cognitive functions including memory. Of particular importance to daily life are declarative memories that contain information about personal experiences, general facts, and events. Several psychiatric or neurological diseases, such as depression, attention-deficit-hyperactivity disorder (ADHD), and dementia, show alterations in serotonergic signalling and attendant memory disorders. Nevertheless, understanding serotonergic neurotransmission and its influence on memory remained a challenge until today. In this systematic review, we summarize recent psychopharmacological studies in animals and humans from a psychological memory perspective, in consideration of task-specific requirements. This approach has the advantage that comparisons between serotonin (5-HT)-related neurochemical mechanisms and manipulations are each addressing specific mnemonic circuits. We conclude that applications of the same 5-HT-related treatments can differentially affect unrelated tasks of declarative memories. Moreover, the analysis of specific mnemonic phases (e.g., encoding vs. consolidation) reveals opposing impacts of increased or decreased 5-HT tones, with low 5-HT supporting spatial encoding but impairing the consolidation of objects and verbal memories. Promising targets for protein synthesis-dependent consolidation enhancements include 5-HT₄ receptor agonists and 5-HT₆ receptor antagonists, with the latter being of special interest for the treatment of age-related decline. Further implications are pointed out as base for the development of novel therapeutic targets for memory impairment of neuropsychiatric disorders.

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

Association Between Season of Birth and Cognitive Aging in Older Adults: Pan-European Population-Based Study on 70,000 Individuals

BACKGROUND

Several early-life factors have been associated with higher risk of developing dementia. It is unclear whether season of birth (SOB) can affect cognitive aging in older adults or not.

OBJECTIVE

We aimed to study the association of SOB with the level of cognitive performance as well as with the rate of cognitive decline.

METHODS

We studied 70,203 individuals who participated in the Survey of Health, Aging and Retirement in Europe. Cognition was measured with tests on verbal fluency and immediate and delayed recall. We assessed the association of SOB with the level of cognitive performance using multiple linear regression and with the rate of cognitive decline using linear mixed-effects models.

RESULTS

When compared to individuals born in winter and adjusted for sociodemographic and health-related characteristics, being born in summer was associated with a higher level of delayed recall (B 0.05; 95%CI 0.01 to 0.09) and verbal fluency (B 0.15; 95%CI 0.00 to 0.29) and being born in fall with a higher level of immediate recall (B 0.04; 95%CI 0.01 to 0.08) and verbal fluency (B 0.15; 95%CI 0.01 to 0.29). Individuals born in summer had a higher yearly decline in delayed recall (B -0.005; 95%CI -0.009 to 0.000), while the scores in delayed recall in participants born in spring showed an inverse trend (B 0.005; 95%CI 0.000 to 0.010).

CONCLUSION

Individuals born in winter seem to carry a life-long disadvantage in a lower level of cognitive performance; however, being born in winter does not seem to affect the rate of cognitive decline.

Long-term high-intensity interval training increases serum neurotrophic factors in elderly overweight and obese Chinese adults

PURPOSE

To compare the effects of 12-week high-intensity interval training (HIIT) and vigorous-intensity continuous training (VICT) on cognitive function, physical fitness, VO₂max, serum neurotransmitters and neurotrophic factors in overweight and obese elderly individuals.

METHODS

Twenty-nine physically inactive older adults (18 males and 11 females) with a mean age of 64.8 ± 3.9 years were randomly divided into a control group (CON, n = 9), an HIIT group (4 × 3 min at 90% VO₂max interspersed with 3 min at 60% VO₂max, n = 10) and a VICT group (25 min at 70% VO₂max, n = 10) and submitted to 12 weeks of training. Cognitive function questionnaires, physical fitness, VO₂max, serum neurotransmitters and neurotrophic factors were determined at baseline and post training.

RESULTS

Twelve weeks of HIIT and VICT improved the VO₂max (4.19 ± 2.21 and 1.84 ± 1.63 mL/kg/min, respectively, p = 0.005), sit-and-reach distance (8.7 ± 3.0 and 7.8 ± 3.8 cm, p = 0.033), choice reaction time (- 0.115 ± 0.15 and - 0.09 ± 0.15 s, p = 0.004) and one-leg stand time (4.4 ± 3.4 and 4.2 ± 4.0 s, p < 0.001) of the elderly participants. The serum concentrations of brain-derived neurotrophic factor (375.5 ± 247.9 and 227.0 ± 137.1 pg/ml, p = 0.006), nerve growth factor (33.9 ± 16.7 and 23.3 ± 14.5 pg/ml, p = 0.037), neurotrophin-3 (24.2 ± 9.33 and 16.3 ± 5.91 pg/ml, p = 0.006) and neurotrophin-4 (10.4 ± 3.8 and 7.8 ± 5.0 pg/ml, p = 0.029) increased significantly in the HIIT and VICT groups after training. In addition, compared to VICT, HIIT significantly increased VO₂max and the serum neurotrophin-3 concentration. Serum concentrations of the neurotransmitters acetylcholine, dopamine and serotonin trended upward with training. No significant change was observed in the cognitive function questionnaire scores (p > 0.05).

CONCLUSION

HIIT is suitable for elderly adults and is more effective than VICT for improving VO₂max and serum neurotrophin-3 concentrations.

CHINESE CLINICAL TRIAL REGISTRY NUMBER

No. ChiCTR1900022315, date of registration: 4 April 2019.

miR-96 Inhibits SV2C to Promote Depression-Like Behavior and Memory Disorders in Mice

Accumulating evidence continues to emphasize the role of microRNAs as significant contributors to depression-like behavior and memory disorders. The current study aimed to investigate the mechanism by which miR-96 influences depression-like behavior and memory deficit in mice. A depression-like behavior and memory disorder mouse model was initially established by means of intraperitoneal injection with lipopolysaccharide. Memory deficits in the mice were evaluated using the Novel Object Recognition Test and Morris water maze experiments, whereas the Sucrose Preference Experiment and forced swimming experiments were performed to identify depression-like behavior in mice. The levels of tumor necrosis factor-α, malondialdehyde, superoxide dismutase, glutathione, and the monoamine transmitters 5-hydroxytryptamine and dopamine were subsequently detected in the serum. Reverse transcription-quantitative polymerase chain reaction and Western blot analysis evaluated the expression of miR-96 and SV2C expression in the CA1 hippocampal region of the mice. Finally, the relationship of miR-96 and SV2C was verified by dual-luciferase reporter gene assay. Our data indicated that the expression of miR-96 was increased, whereas that of SV2C was decreased in the CA1 region of mice exhibiting depression-like behavior and memory impairment. When miR-96 was downregulated or SV2C was overexpressed via intra-cerebroventricular injection with a miR-96 antagonist (miR-96 antagomir) or overexpression of SV2C vector, the Novel Object Recognition Test and sucrose preference index were increased, whereas the escape latency, the number of water maze platform crossings, and the immobility time of the mice were decreased. The serum levels of tumor necrosis factor-α, interleukin-1β, and malondialdehyde in the mouse CA1 region of mice were reduced, whereas the levels of superoxide dismutase and glutathione were elevated after the downregulation of miR-96 or overexpression of SV2C. Collectively, our study demonstrates that miR-96 negatively regulates the expression of SV2C, which consequently leads to depression-like behavior and memory impairment in mice. Our findings highlight the potential of miR-96-targeted therapeutics.

Tropisetron enhances recognition memory in ovariectomized female rats

The present study evaluated the acute effects of the 5-HT3 receptor antagonist, tropisetron, on recognition memory in ovariectomized adult female rats. The non-spatial novel object recognition task was used to assess recognition memory. In this task, ovariectomized rats explored two identical objects during Trial 1. Immediately after Trial 1, rats were primed either with oil, 250 µg progesterone, 20 µg of estrogen, or 20 µg of estrogen + 250 µg progesterone. Four hours later, the test trial (Trial 2) was initiated. Thirty minutes before Trial 2, rats were injected intraperitoneally with either saline, 1.5 or 2.5 mg/Kg tropisetron. During Trial 2, one arm of the T maze contained an object from Trial 1 (familiar or previously encountered), and a new object (novel) was introduced into the other arm. Exploration times with the novel and familiar objects were recorded and data were converted to percent time spent with the novel object. In oil-primed ovariectomized female rats, treatment with 2.5 mg/Kg tropisetron significantly increased percent time with the novel object. Hormonal-priming with estrogen, progesterone, or estrogen + progesterone did not further accentuate the effects of tropisetron. These results suggest that although tropisetron, estrogen, and progesterone all act as antagonists at the 5-HT3 receptors and blocking 5-HT3 receptors enhances cognition, there appears to be no interaction between tropisetron and these hormones on object recognition.

Serotonin signaling by maternal neurons upon stress ensures progeny survival

Germ cells are vulnerable to stress. Therefore, how organisms protect their future progeny from damage in a fluctuating environment is a fundamental question in biology. We show that in Caenorhabditis elegans, serotonin released by maternal neurons during stress ensures the viability and stress resilience of future offspring. Serotonin acts through a signal transduction pathway conserved between C. elegans and mammalian cells to enable the transcription factor HSF1 to alter chromatin in soon-to-be fertilized germ cells by recruiting the histone chaperone FACT, displacing histones, and initiating protective gene expression. Without serotonin release by maternal neurons, FACT is not recruited by HSF1 in germ cells, transcription occurs but is delayed, and progeny of stressed C. elegans mothers fail to complete development. These studies uncover a novel mechanism by which stress sensing by neurons is coupled to transcription response times of germ cells to protect future offspring.

Global Transcriptome Changes That Accompany Alterations in Serotonin Levels in Caenorhabditis elegans

Serotonin (5-hydroxytryptamine, 5-HT), is a phylogenetically ancient molecule best characterized as a neurotransmitter that modulates multiple aspects of mood and social cognition. The roles that 5-HT plays in normal and abnormal behavior are not fully understood but have been posited to be due to its common function as a 'defense signal'. However, 5-HT levels also systemically impact cell physiology, modulating cell division, migration, apoptosis, mitochondrial biogenesis, cellular metabolism and differentiation. Whether these diverse cellular effects of 5-HT also share a common basis is unclear. C. elegans provides an ideal system to interrogate the systemic effects of 5-HT, since lacking a blood-brain barrier, 5-HT synthesized and released by neurons permeates the organism to modulate neuronal as well as non-neuronal cells throughout the body. Here we used RNA-Seq to characterize the systemic changes in gene expression that occur in C. elegans upon altering 5-HT levels, and compared the transcriptomes to published datasets. We find that an acute increase in 5-HT is accompanied by a global decrease in gene expression levels, upregulation of genes involved in stress pathways, changes that significantly correlate with the published transcriptomes of animals that have activated defense and immune responses, and an increase in levels of phosphorylated eukaryotic initiation factor, eIF2α. In 5-HT deficient animals lacking tryptophan hydroxylase (tph-1(mg280)II) there is a net increase in gene expression, with an overrepresentation of genes related to development and chromatin. Surprisingly, the transcriptomes of animals with acute increases in 5-HT levels, and 5-HT deficiency do not overlap with transcriptomes of mutants with whom they share striking physiological resemblance. These studies are the first to catalog systemic transcriptome changes that occur upon alterations in 5-HT levels. They further show that in C. elegans changes in gene expression upon altering 5-HT levels, and changes in physiology, are not directly correlated.

Acute and Chronic Nicotine Exposures Differentially Affect Central Serotonin 2A Receptor Function: Focus on the Lateral Habenula

Nicotine addiction is a serious public health problem causing millions of deaths worldwide. Serotonin (5-hydroxytryptamine; 5-HT) is involved in central nervous system (CNS) nicotine effects, and it has been suggested as a promising pharmacological target for smoking cessation. In this regard, what is particularly interesting are the 5-HT_2A receptors (5-HT_2ARs) and the lateral habenula (LHb), a central area in nicotine addiction that we showed to be under a strong 5-HT_2AR-modulation. Single-cell extracellular recording of LHb neurons was used to study the 5-HT_2AR function by intravenously administrating the potent agonist TCB-2. Acute nicotine (2 mg/kg, intraperitoneal, i.p.) and chronic nicotine (6 mg/kg/day for 14 days) differently affected both the 5-HT_2AR-immuno reactive (IR) neuron number and the 5-HT_2AR immunostaining area in the different brain areas studied. After acute nicotine, TCB-2 cumulative doses (5–640 µg/kg, intravenous, i.v.) bidirectionally affected the activity of 74% of LHb recorded neurons. After chronic nicotine treatment, TCB-2 was only capable of decreasing the LHb firing rate. The expression of 5-HT_2AR under acute and chronic nicotine exposure was studied in the LHb and in other brain areas involved in nicotine effects in rats by using immunohistochemistry. These data reveal that acute and chronic nicotine differentially affect the 5-HT_2AR function in different brain areas and this might be relevant in nicotine addiction and its treatment.

Spaced Training Enhances Contextual Fear Memory via Activating Hippocampal 5-HT2A Receptors

Spaced training is robustly superior to massed training, which is a well-documented phenomenon in humans and animals. However, the mechanisms underlying the spacing effect still remain unclear. We have reported previously that spacing training exerts memory-enhancing effects by inhibiting forgetting via decreasing hippocampal Rac1 activity. Here, using contextual fear conditioning in rat, we found that spaced but not massed training increased hippocampal 5-HT2A receptors' expression. Furthermore, hippocampal administration of 5-HT2A receptor antagonist MDL11939 before spaced training blocked the enhanced memory, while hippocampal administration of 5-HT2A receptor agonist TCB-2 before massed training promoted the memory. Moreover, MDL11939 activated hippocampal Rac1, while TCB-2 decreased hippocampal Rac1 activity in naïve rats. These results indicated the possibility of interaction between 5-HT2A receptors and Rac1, which was demonstrated by co-immunoprecipitation experiments. Our study first demonstrates that activation of hippocampal 5-HT2A is a mechanism underlying the spacing effect, and forgetting related molecular Rac1 is engaged in this process through interacting with 5-HT2A receptors, which suggest a promising strategy to modulate abnormal learning in cognitive disorders.

Serotonin and serotonin transporter levels in autistic children

OBJECTIVES

To assess the possible correlation between serotonin and serotonin transporter (SERT) with the autism severity and investigate the association between these parameters in autistic children to assess their possible role for diagnosis of autism severity.

METHODS

A comparative cross-sectional study was carried out in the Chemistry and Biochemistry Department, College of Medicine, Al-Nahrain University, Baghdad, Iraq while the samples were taken from 60 male autistic children recruited to the Department of Pediatrics at Al-Sader Hospital, Baghdad, Iraq between November 2014 amd April 2015. Levels of serotonin and serotonin transporters (SERT) were determined in 60 male autistic Iraqi patients classified into mild, moderate and severe (20 for each). These levels were compared with those of 26 healthy control children. Results: Levels of serotonin and SERT were significantly increased in autistic children than that of gender and age-matched controls. Serotonin levels were 80.63± 21.83 ng/ml in mild, 100.39±23.07 ng/ml moderate, and 188.7±31.72 ng/ml severe autistic patients. Serotonin transporter levels were 10.13±4.51 ng/ml in mild, 13.15±4.71 ng/ml moderate, and 16.32±6.7 ng/ml in  severe autistic patients. The increase of both serotonin and SERT levels were associated with severity of autism. Receiver operating characteristic (ROC) analysis  can be used for diagnostic and prognostic purposes.

CONCLUSIONS

High serotonin and SERT levels may indicate that these biomarkers have a role in the autism pathogenesis and support the possibility of using serotonin and SERT to diagnose autism severity.

Aerobic exercise upregulates the BDNF-Serotonin systems and improves the cognitive function in rats

Aerobic exercise (AE) benefits brain health and behavior. Serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) are known to mediate and shape cognitive processes. Both systems share some actions: BDNF is involved in the maturation and function of 5-HT neurons. In turn, 5-HT is involved in neuroplasticity phenomena mediated by BDNF and stimulated by exercise. The aim of this work was to study the long-term effects of AE on BDNF- 5-HT systems and cognitive function in rats at different ages. A lifelong moderate-intensity aerobic training program was designed, in which aerobically exercised (E) and sedentary control (C) rats were studied at middle (8 months) and old age (18 months) by means of biochemical, immunohistochemical and behavioral assays. The levels and expression of BDNF, 5-HT, serotonin transporter (SERT) and 5-HT_1A receptor were determined in selected brain areas involved in memory and learning. Immunopositive cells to neuronal nuclear protein (NeuN) in the hippocampus CA1 area were also quantified. The cognitive function was evaluated by the object recognition test (ORT). Results indicate that AE enhanced spatial and non-spatial memory systems, modulated by age. This outcome temporarily correlated with a significant upregulation of cortical, hippocampal and striatal BDNF levels in parallel with an increase in the number of hippocampal CA1-mature neurons. AE also increased brain and raphe 5-HT levels, as well as the expression of SERT and 5-HT_1A receptor in the cortex and hippocampus. Old AE rats showed a highly conserved response, indicating a remarkable protective effect of exercise on both systems. In summary, lifelong AE positively affects BDNF-5-HT systems, improves cognitive function and protects the brain against the deleterious effects of sedentary life and aging.

Hippocampal 5-HT Input Regulates Memory Formation and Schaffer Collateral Excitation

The efficacy and duration of memory storage is regulated by neuromodulatory transmitter actions. While the modulatory transmitter serotonin (5-HT) plays an important role in implicit forms of memory in the invertebrate Aplysia, its function in explicit memory mediated by the mammalian hippocampus is less clear. Specifically, the consequences elicited by the spatio-temporal gradient of endogenous 5-HT release are not known. Here we applied optogenetic techniques in mice to gain insight into this fundamental biological process. We find that activation of serotonergic terminals in the hippocampal CA1 region both potentiates excitatory transmission at CA3-to-CA1 synapses and enhances spatial memory. Conversely, optogenetic silencing of CA1 5-HT terminals inhibits spatial memory. We furthermore find that synaptic potentiation is mediated by 5-HT4 receptors and that systemic modulation of 5-HT4 receptor function can bidirectionally impact memory formation. Collectively, these data reveal powerful modulatory influence of serotonergic synaptic input on hippocampal function and memory formation.

Interaction effect between 5-HTTLPR and HTR1A rs6295 polymorphisms on the frontoparietal network

Previous studies have shown a close relationship between the serotonin system and working memory (WM), but the neural mechanism for the role of the serotonin system on the WM is unclear. The frontoparietal network is involved in WM and is associated with the serotonin system. Therefore, this study investigated the interaction effect of the serotonin transporter-linked polymorphic region (5-HTTLPR) and the polymorphism in the serotonin 1A receptor gene (rs6295) on the frontoparietal network obtained from the independent component analysis in a large, young Chinese sample population. The current study found a significant interaction effect of 5-HTTLPR and rs6295 on the connectivity within the right frontoparietal network, specifically in the middle frontal gyrus and inferior parietal lobule. Moreover, the mean connectivity in the right inferior parietal lobule was positively correlated with WM performance. These brain network analysis findings could provide a new perspective on the neural mechanisms of gene-gene interactions and on individual differences in cognitive functions.

Lesions of the lateral habenula improve working memory performance in hemiparkinsonian rats

The lateral habenula (LHb) is an important structure involved in various brain functions, because it controls the activity of dopaminergic and serotonergic systems in the midbrain. The impairment of working memory commonly occurs in Parkinson's disease; however, it is not clear whether the LHb involves in the regulation of working memory in rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle (MFB). In this study, we found that the MFB lesions in rats decreased choice accuracy as measured by the T-maze rewarded alternation test compared to control rats, indicating the induction of working memory impairment, and decreased dopamine (DA) levels in the medial prefrontal cortex (mPFC), hippocampus and amygdala. Further, rats in the MFB and LHb lesion group showed increased choice accuracy compared to rats in the MFB lesion group, indicating the enhancement of working memory after lesioning the LHb. Neurochemical results found that lesions of the LHb increased DA levels in the mPFC, hippocampus and amygdala in the MFB and LHb lesion group, as well as serotonin (5-HT) level in the mPFC. These findings suggest that DA depletion plays a key role in working memory impairment, and lesions of the LHb improve working memory in the MFB-lesioned rats, which involves in increases in the levels of DA and 5-HT in the mPFC, hippocampus and amygdala. Additionally, the present results may have implications for improving our understanding of the neuropathology and/or treatment of PD.

Molecular imaging of serotonin degeneration in mild cognitive impairment

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

Contribution of Hippocampal 5-HT3 Receptors in Hippocampal Autophagy and Extinction of Conditioned Fear Responses after a Single Prolonged Stress Exposure in Rats

One of the hypotheses about the pathogenesis of posttraumatic stress disorder (PTSD) is the dysfunction of serotonin (5-HT) neurotransmission. While certain 5-HT receptor subtypes are likely critical for the symptoms of PTSD, few studies have examined the role of 5-HT3 receptor in the development of PTSD, even though 5-HT3 receptor is critical for contextual fear extinction and anxiety-like behavior. Therefore, we hypothesized that stimulation of 5-HT3 receptor in the dorsal hippocampus (DH) could prevent hippocampal autophagy and the development of PTSD-like behavior in animals. To this end, we infused SR57227, selective 5-HT3 agonist, into the DH after a single prolonged stress (SPS) treatment in rats. Three weeks later, we evaluated the effects of this pharmacological treatment on anxiety-related behaviors and extinction of contextual fear memory. We also accessed hippocampal autophagy and the expression of 5-HT3A subunit, Beclin-1, LC3-I, and LC3-II in the DH. We found that SPS treatment did not alter anxiety-related behaviors but prolonged the extinction of contextual fear memory, and such a behavioral phenomenon was correlated with increased hippocampal autophagy, decreased 5-HT3A expression, and increased expression of Beclin-1 and LC3-II/LC3-I ratio in the DH. Furthermore, intraDH infusions of SR57227 dose-dependently promoted the extinction of contextual fear memory, prevented hippocampal autophagy, and decreased expression of Beclin-1 and LC3-II/LC3-I ratio in the DH. These results indicated that 5-HT3 receptor in the hippocampus may play a critical role in the pathogenesis of hippocampal autophagy, and is likely involved in the pathophysiology of PTSD.

Brain serotonin 4 receptor binding is inversely associated with verbal memory recall

BACKGROUND

We have previously identified an inverse relationship between cerebral serotonin 4 receptor (5-HT ₄R) binding and nonaffective episodic memory in healthy individuals. Here, we investigate in a novel sample if the association is related to affective components of memory, by examining the association between cerebral 5-HT ₄R binding and affective verbal memory recall.

METHODS

Twenty-four healthy volunteers were scanned with the 5-HT ₄R radioligand [¹¹C]SB207145 and positron emission tomography, and were tested with the Verbal Affective Memory Test-24. The association between 5-HT ₄R binding and affective verbal memory was evaluated using a linear latent variable structural equation model.

RESULTS

We observed a significant inverse association across all regions between 5-HT ₄R binding and affective verbal memory performances for positive (p = 5.5 × 10^-4) and neutral (p = .004) word recall, and an inverse but nonsignificant association for negative (p = .07) word recall. Differences in the associations with 5-HT ₄R binding between word categories (i.e., positive, negative, and neutral) did not reach statistical significance.

CONCLUSION

Our findings replicate our previous observation of a negative association between 5-HT ₄R binding and memory performance in an independent cohort and provide novel evidence linking 5-HT ₄R binding, as a biomarker for synaptic 5-HT levels, to the mnestic processing of positive and neutral word stimuli in healthy humans.

Neurocognitive performance following acute mephedrone administration, with and without alcohol

Recreational use of mephedrone, alone and in combination with alcohol, has increased over the past years. Pharmacological properties of mephedrone share similarities with methylenedioxymethamphetamine (MDMA), but its effect on neurocognitive function has not been well established in humans. The present study assessed the effect of mephedrone alone and after co-administration with alcohol on neurocognitive function. It was hypothesised that mephedrone would improve psychomotor performance but impair memory performance, when administered alone. Neurocognitive performance was expected to be impaired following mephedrone when combined with alcohol. Eleven participants received single doses of 200 mg mephedrone or placebo combined with 0.8 g/kg alcohol or placebo. Neurocognitive performance was assessed at baseline (T₀), at one hour (T₁) and four hours after (T₂) mephedrone administration, by means of the Divided Attention Task (DAT), Critical Tracking Task (CTT), and the Spatial Memory Test (SMT). Mephedrone intoxication impaired short-term spatial memory at T₁ and improved critical tracking performance at T₂ Mephedrone alone did not affect divided attention, but did show an interaction with alcohol on reaction time at T₂ Reaction time decreased when mephedrone was combined with alcohol as compared to alcohol alone. Alcohol intoxication impaired both short- and long-term spatial memory at T₁ and divided attention at T₁ and T₂ Critical tracking performance was not affected by alcohol intoxication. The current findings support the hypothesis that mephedrone improves psychomotor performance, impairs spatial memory and does not affect divided attention performance. Stimulatory effects of mephedrone were not sufficient to compensate for the impairing effects of alcohol on most performance parameters.

Monoaminergic and Histaminergic Strategies and Treatments in Brain Diseases

The monoaminergic systems are the target of several drugs for the treatment of mood, motor and cognitive disorders as well as neurological conditions. In most cases, advances have occurred through serendipity, except for Parkinson's disease where the pathophysiology led almost immediately to the introduction of dopamine restoring agents. Extensive neuropharmacological studies first showed that the primary target of antipsychotics, antidepressants, and anxiolytic drugs were specific components of the monoaminergic systems. Later, some dramatic side effects associated with older medicines were shown to disappear with new chemical compounds targeting the origin of the therapeutic benefit more specifically. The increased knowledge regarding the function and interaction of the monoaminergic systems in the brain resulting from in vivo neurochemical and neurophysiological studies indicated new monoaminergic targets that could achieve the efficacy of the older medicines with fewer side-effects. Yet, this accumulated knowledge regarding monoamines did not produce valuable strategies for diseases where no monoaminergic drug has been shown to be effective. Here, we emphasize the new therapeutic and monoaminergic-based strategies for the treatment of psychiatric diseases. We will consider three main groups of diseases, based on the evidence of monoamines involvement (schizophrenia, depression, obesity), the identification of monoamines in the diseases processes (Parkinson's disease, addiction) and the prospect of the involvement of monoaminergic mechanisms (epilepsy, Alzheimer's disease, stroke). In most cases, the clinically available monoaminergic drugs induce widespread modifications of amine tone or excitability through neurobiological networks and exemplify the overlap between therapeutic approaches to psychiatric and neurological conditions. More recent developments that have resulted in improved drug specificity and responses will be discussed in this review.

Neurobiology of Anorexia Nervosa: Serotonin Dysfunctions Link Self-Starvation with Body Image Disturbances through an Impaired Body Memory

The etiology of anorexia nervosa (AN) is still unclear, despite that it is a critical and potentially mortal illness. A recent neurobiological model considers AN as the outcome of dysfunctions in the neuronal processes related to appetite and emotionality (Kaye et al., 2009, 2013). However, this model still is not able to answer a critical question: What is behind body image disturbances (BIDs) in AN? The article starts its analysis from reviewing some of the studies exploring the effects of the serotonin systems in memory (episodic, working, and spatial) and its dysfunctions. The review suggests that serotonin disturbances may: (a) facilitate the encoding of third person (allocentric) episodic memories; (b) facilitate the consolidation of emotional episodic memories (e.g., teasing), if preceded by repeated stress; (c) reduce voluntary inhibition of mnestic contents; (d) impair allocentric spatial memory. If we discuss these results within the interpretative frame suggested by the “Allocentric Lock Hypothesis” (Riva, 2012, 2014), we can hypothesize that altered serotoninergic activity in AN patients: (i) improves their ability to store and consolidate negative autobiographical memories, including those of their body, in allocentric perspective; (ii) impairs their ability to trigger voluntary inhibition of the previously stored negative memory of the body; (iii) impairs their capacity to retrieve/update allocentric information. Taken together, these points suggest a possible link between serotonin dysfunctions, memory impairments and BIDs: the impossibility of updating a disturbed body memory using real time experiential data—I'm locked to a wrong body stored in long term memory—pushes AN patients to control body weight and shape even when underweight.

Effects of 5-HT-7 receptor ligands on memory and cognition

The 5-HT7R is the most recently cloned serotonin receptor and thus one the least studied. Many drugs, experimental and in clinical use bind to 5-HT7 with high affinity, though their effects have yet to be clearly elucidated. Its physiological function, though not completely clear, is mostly associated with learning and memory, with both agonists and antagonists possessing subtle procognitive and promnesic properties. We consider it a promising area of research, though still in its infancy, which may one day lead to clinical benefits for patients with various afflictions characterised by cognitive dysfunction, particularily autism spectrum disorder, fragile X syndrome and Alzheimer's disease.

Cognitive Function before and during Treatment with Selective Serotonin Reuptake Inhibitors in Patients with Depression or Obsessive-Compulsive Disorder

Objectives. Identification of adverse effects of selective serotonin reuptake inhibitors (SSRIs) is of great importance due to their extensive use in medicine. Some studies have reported the effects of SSRIs on cognitive functions, but the results are conflicting. This study was designed to assess the effect of these drugs on cognition of patients with depression or obsessive-compulsive disorder (OCD). Methods. Patients with depression or OCD, naïve to therapy, and candidates of receiving one drug from SSRI class, voluntarily, entered this study. Mini-Mental State Examination (MMSE) test was the tool to assess their cognitive functions. MMSE scores of each patient were recorded prior to taking SSRIs and at weeks 3, 5, and 8 of drug therapy. Results. 50 patients met our inclusion criteria, with a baseline mean MMSE score of 23.94. At 3, 5, and 8 weeks of treatment, the mean scores were 22.1, 21.4, and 20.66, respectively. With a p value of <0.0001, the gradual decline was statistically significant. Conclusion. The MMSE scores of our patients showed a gradual decline over the consecutive weeks after taking SSRI drugs. It seems that the use of SSRIs in patients with depression or OCD, can cause cognitive dysfunction in the acute phase of treatment.

The serotonergic system and cognitive function

Symptoms of cognitive dysfunction like memory loss, poor concentration, impaired learning and executive functions are characteristic features of both schizophrenia and Alzheimer’s disease (AD). The neurobiological mechanisms underlying cognition in healthy subjects and neuropsychiatric patients are not completely understood. Studies have focused on serotonin (5-hydroxytryptamine, 5-HT) as one of the possible cognitionrelated biomarkers. The aim of this review is to provide a summary of the current literature on the role of the serotonergic (5-HTergic) system in cognitive function, particularly in AD and schizophrenia.

The role of the 5-HTergic system in cognition is modulated by the activity and function of 5-HT receptors (5-HTR) classified into seven groups, which differ in structure, action, and localization. Many 5-HTR are located in the regions linked to various cognitive processes. Preclinical studies using animal models of learning and memory, as well as clinical in vivo (neuroimaging) and in vitro (post-mortem) studies in humans have shown that alterations in 5-HTR activity influence cognitive performance.

The current evidence implies that reduced 5-HT neurotransmission negatively influences cognitive functions and that normalization of 5-HT activity may have beneficial effects, suggesting that 5-HT and 5-HTR represent important pharmacological targets for cognition enhancement and restoration of impaired cognitive performance in neuropsychiatric disorders.

Effect of agomelatine on adult hippocampus apoptosis and neurogenesis using the stress model of rats

Agomelatine (AG) is an agonist of melatonin receptors and an antagonist of the 5-HT2C-receptor subtype. The chronobiotic properties of AG are of significant interest due to the disorganization of internal rhythms, which might play a role in the pathophysiology of depression. The present study was designed to assess the effects of the antidepressant-like activity of AG, a new antidepressant drug, on adult neurogenesis and apoptosis using stress-exposed rat brains. Over the period of 1 week, the rats were exposed to light stress twice a day for 1h. After a period of 1 week, the rats were given AG treatment at a dose of either 10mg/kg or 40mg/kg for 15 days. The animals were then scarified, and the obtained tissue sections were stained with immuno-histochemical anti-BrdU, Caspase-3, and Bcl-2 antibodies. Serum brain-derived neurotrophic factor (BDNF) concentrations were measured biochemically using a BDNF Elisa kit. Biochemical BDNF analysis revealed a high concentration of BDNF in the serum of the stress-exposed group, but the concentrations of BDNF were much lower those of the AG-treated groups. Immuno-histochemical analysis revealed that AG treatment decreased the BrdU-positive and Bcl-2-positive cell densities and increased the Caspase-3-positive cell density in the hippocampus of stress-induced rats as compared to those of the stress group. The results of the study demonstrated that AG treatment ameliorated the hippocampal apoptotic cells and increased hippocampal neurogenesis. These results also strengthen the possible relationship between depression and adult neurogenesis, which must be studied further.

Serotonergic drugs for the treatment of neuropsychiatric symptoms in dementia

Behavioral and psychological symptoms of dementia (known also as neuropsychiatric symptoms) are essential features of Alzheimer's disease and related dementias. The near universal presence of neuropsychiatric symptoms in dementia (up to 90% of cases) has brought significant attention of clinicians and experts to the field. Non-pharmacological and pharmacological interventions are recommended for various types of neuropsychiatric symptoms. However, most pharmacological interventions for the treatment of behavioral and psychological symptoms of dementia are used off-label in many countries. Cognitive decline and neuropsychiatric symptoms can be linked to alterations in multiple neurotransmitter systems, so modification of abnormalities in specific systems may improve clinical status of patients with neuropsychiatric symptoms. Use of serotonergic compounds (novel particles acting on specific receptors and widely acting drugs) in the treatment of neuropsychiatric symptoms is reviewed.

The effects of a 5-HT5A receptor antagonist in a ketamine-based rat model of cognitive dysfunction and the negative symptoms of schizophrenia

Serotonin (5-HT) receptors still represent promising targets for the development of novel multireceptor or stand-alone antipsychotic drugs with a potential to ameliorate cognitive impairments and negative symptoms in schizophrenia. The 5-HT5A receptor, one of the least known members of the serotonin receptor family, has also drawn attention in this regard. Although the antipsychotic efficacy of 5-HT5A antagonists is still equivocal, recent experimental data suggest the cognitive-enhancing activity of this strategy. The aim of the present study was to evaluate pro-cognitive and pro-social efficacies of the 5-HT5A receptor antagonist in a rat pharmacological model of schizophrenia employing the administration of the NMDA receptor antagonist, ketamine. The ability of SB-699551 to reverse ketamine-induced cognitive deficits in the attentional set-shifting task (ASST) and novel object recognition task (NORT) was examined. The compound's efficacy against ketamine-induced social withdrawal was assessed in the social interaction test (SIT) and in the social choice test (SCT). The results demonstrated the efficacy of SB-699551 in ameliorating ketamine-induced impairments on the ASST and NORT. Moreover, the tested compound also enhanced set-shifting performance in cognitively unimpaired control rats and improved object recognition memory in conditions of delay-induced natural forgetting. The pro-social activity of SB-699551 was demonstrated on both employed paradigms, the SIT and SCT. The present study suggests the preclinical efficacy of a strategy based on the blockade of 5-HT5A receptors against schizophrenia-like cognitive deficits and negative symptoms. The utility of this receptor as a target for improvement of cognitive and social dysfunctions warrants further studies.

SiRNA-mediated serotonin transporter knockdown in the dorsal raphe nucleus rescues single prolonged stress-induced hippocampal autophagy in rats

The neurobiological mechanisms underlying the development of post-traumatic stress disorder (PTSD) remain elusive. One of the hypotheses is the dysfunction of serotonin (5-HT) neurotransmission, which is critically regulated by serotonin transporter (SERT). Therefore, we hypothesized that attenuation of SERT gene expression in the hippocampus could prevent hippocampal autophagy and the development of PTSD-like behavior. To this end, we infused SLC6A4 siRNAs into the dorsal raphe nucleus (DRN) to knockdown SERT gene expression after a single prolonged stress (SPS) treatment in rats. Then, we evaluated the effects of SERT gene knockdown on anxiety-related behaviors and extinction of contextual fear memory. We also examined the histological changes and the expression of Beclin-1, LC3-I, and LC3-II in the hippocampus. We found that SPS treatment did not alter anxiety-related behaviors but prolonged the extinction of contextual fear memory, and such a behavioral phenomenon was correlated with increased hippocampal autophagy, decreased 5-HT level, and increased expression of Beclin-1 and LC3-II/LC3-I ratio in the hippocampus. Furthermore, intra-DRN infusion of SLC6A4 siRNAs promoted the extinction of contextual fear memory, prevented hippocampal autophagy, increased 5-HT level, and decreased expression of Beclin-1 and LC3-II/LC3-I ratio. These results indicated that SERT may play a critical role in the pathogenesis of hippocampal autophagy, and is likely involved in the development of PTSD.

Stress psychobiology in the context of addiction medicine: from drugs of abuse to behavioral addictions

In this chapter, we briefly review the basic biology of psychological stress and the stress response. We propose that psychological stress and the neurobiology of the stress response play in substance use initiation, maintenance, and relapse. The proposed mechanisms for this include, on the one hand, the complex interactions between biological mediators of the stress response and the dopaminergic reward system and, on the other hand, mediators of the stress response and other systems crucial in moderating key addiction-related behaviors such as endogenous opioids, the sympathetic-adrenal-medullary system, and endocannabinoids. Exciting new avenues of study including genomics, sex as a moderator of the stress response, and behavioral addictions (gambling, hypersexuality, dysfunctional internet use, and food as an addictive substance) are also briefly presented within the context of stress as a moderator of the addictive process.