Memory formation, amnesia, improved memory and reversed amnesia: 5-HT role

Modafinil, an atypical CNS stimulant?

Modafinil is a central nervous system stimulant approved for the treatment of narcolepsy and sleep disorders. Due to its wide range of biochemical actions, modafinil has been explored for other potential therapeutic uses. Indeed, it has shown promise as a therapy for cognitive disfunction resulting from neurologic disorders like ADHD, and as a smart drug in non-medical settings. The mechanism(s) of actions underlying the therapeutic efficacy of this agent remains largely elusive. Modafinil is known to inhibit the dopamine transporter, thus decreasing dopamine reuptake following neuronal release, an effect shared by addictive psychostimulants. However, modafinil is unique in that only a few cases of dependence on this drug have been reported, as compared to other psychostimulants. Moreover, modafinil has been tested, with some success, as a potential therapeutic agent to combat psychostimulant and other substance use disorders. Modafinil has additional, but less understood, actions on other neurotransmitter systems (GABA, glutamate, serotonin, norepinephrine, etc.). These interactions, together with its ability to activate selected brain regions, are likely one of the keys to understand its unique pharmacology and therapeutic activity as a CNS stimulant. In this chapter, we outline the pharmacokinetics and pharmacodynamics of modafinil that suggest it has an "atypical" CNS stimulant profile. We also highlight the current approved and off label uses of modafinil, including its beneficial effects as a treatment for sleep disorders, cognitive functions, and substance use disorders.

5-HT6 Receptors Control GABAergic Transmission and CA1 Pyramidal Cell Output of Dorsal Hippocampus

The blockade of 5-HT6 receptors represents an experimental approach that might ameliorate the memory deficits associated with brain disorders, including Alzheimer's disease and schizophrenia. However, the synaptic mechanism by which 5-HT6 receptors control the GABAergic and glutamatergic synaptic transmission is barely understood. In this study, we demonstrate that pharmacological manipulation of 5-HT6 receptors with the specific agonist EMD 386088 (7.4 nM) or the antagonist SB-399885 (300 nM) modulates the field inhibitory postsynaptic potentials of the dorsal hippocampus and controls the strength of the population spike of pyramidal cells. Likewise, pharmacological modulation of 5-HT6 controls the magnitude of paired-pulse inhibition, a phenomenon mediated by GABAergic interneurons acting via GABAA receptors of pyramidal cells. The effects of pharmacological manipulation of the 5-HT6 receptor were limited to GABAergic transmission and did not affect the strength of field excitatory postsynaptic potentials mediated by the Schaffer collaterals axons. Lastly, in a modified version of the Pavlovian autoshaping task that requires the activation of the hippocampal formation, we demonstrated that the anti-amnesic effect induced by the blockade of the 5-HT6 receptor is prevented when the GAT1 transporter is blocked, suggesting that modulation of GABAergic transmission is required for the anti-amnesic properties of 5-HT6 receptor antagonists.

Stable in-vivo electrochemical sensing of tonic serotonin levels using PEDOT/CNT-coated glassy carbon flexible microelectrode arrays

Chronic sampling of tonic serotonin (5-hydroxytryptamine, 5-HT) concentrations in the brain is critical for tracking neurological disease development and the time course of pharmacological treatments. Despite their value, in vivo chronic multi-site measurements of tonic 5-HT have not been reported. To fill this technological gap, we batch-fabricated implantable glassy carbon (GC) microelectrode arrays (MEAs) onto a flexible SU-8 substrate to provide an electrochemically stable and biocompatible device/tissue interface. To achieve detection of tonic 5-HT concentrations, we applied a poly(3,4-ethylenedioxythiophene)/carbon nanotube (PEDOT/CNT) electrode coating and optimized a square wave voltammetry (SWV) waveform for selective 5-HT measurement. In vitro, the PEDOT/CNT-coated GC microelectrodes achieved high sensitivity to 5-HT, good fouling resistance, and excellent selectivity against the most common neurochemical interferents. In vivo, our PEDOT/CNT-coated GC MEAs successfully detected basal 5-HT concentrations at different locations within the CA2 region of the hippocampus of both anesthetized and awake mice. Furthermore, the PEDOT/CNT-coated MEAs were able to detect tonic 5-HT in the mouse hippocampus for one week after implantation. Histology reveals that the flexible GC MEA implants caused less tissue damage and reduced inflammatory response in the hippocampus compared to commercially available stiff silicon probes. To the best of our knowledge, this PEDOT/CNT-coated GC MEA is the first implantable, flexible sensor capable of chronic in vivo multi-site sensing of tonic 5-HT.

Nine-month-long Social Isolation Changes the Levels of Monoamines in the Brain Structures of Rats: A Comparative Study of Neurochemistry and Behavior

Social isolation (SI) is chronic psycho-emotional stress for humans and other socially living species. There are few comparative studies that have measured monoamine levels in brain structures in male and female rats subjected to SI. Existing data is highly controversial. In our recent study, we investigated behavioral effects of SI prolonged up to 9 months on a rather large sample of 69 male and female Wistar rats. In the present study, we measured the levels of monoamines-norepinephrine (NE), dopamine (DA), 5-hydroxytryptamine (5-HT), and DA and 5-HT metabolites-in the brain structures of 40 rats from the same sample. The single-housed rats of both sexes showed hyperactivity and reduced reactivity to novelty in the Open Field test, and impaired passive avoidance learning. Regardless of their sex, by the time of sacrifice, the single-housed rats weighed less and had lower pain sensitivity and decreased anxiety compared with group-housed animals. SI decreased NE levels in the hippocampus and increased them in the striatum. SI induced functional activation of the DA-ergic system in the frontal cortex and hypothalamus, with increased DA and 3-methoxytyramine levels. SI-related changes were found in the 5-HT-ergic system: 5-HT levels increased in the frontal cortex and striatum, while 5-hydroxyindoleacetic acid only increased in the frontal cortex. We believe that SI prolonged for multiple months could be a valuable model for comparative analysis of the behavioral alterations and the underlying molecular processes in dynamics of adaptation to chronic psychosocial stress in male and female rats in relation to age-dependent changes.

Implantable flexible multielectrode arrays for multi-site sensing of serotonin tonic levels

Real-time multi-channel measurements of tonic serotonin (5-hydroxytryptamine, 5-HT) concentrations across different brain regions are of utmost importance to the understanding of 5-HT’s role in anxiety, depression, and impulse control disorders, which will improve the diagnosis and treatment of these neuropsychiatric illnesses. Chronic sampling of 5-HT is critical in tracking disease development as well as the time course of pharmacological treatments. Despite their value, in vivo chronic multi-site measurements of 5-HT have not been reported. To fill this technological gap, we batch fabricated implantable glassy carbon (GC) microelectrode arrays (MEAs) on a flexible SU-8 substrate to provide an electrochemically stable and biocompatible device/tissue interface. Then, to achieve multi-site detection of tonic 5-HT concentrations, we incorporated the poly(3,4-ethylenedioxythiophene)/functionalized carbon nanotube (PEDOT/CNT) coating on the GC microelectrodes in combination with a new square wave voltammetry (SWV) approach, optimized for selective 5-HT measurement. In vitro , the PEDOT/CNT coated GC microelectrodes achieved high sensitivity towards 5-HT, good fouling resistance in the presence of 5-HT, and excellent selectivity towards the most common neurochemical interferents. In vivo , our PEDOT/CNT-coated GC MEAs were able to successfully detect basal 5-HT concentrations at different locations of the CA2 hippocampal region of mice in both anesthetized and awake head-fixed conditions. Furthermore, the implanted PEDOT/CNT-coated MEA achieved stable detection of tonic 5-HT concentrations for one week. Finally, histology data in the hippocampus shows reduced tissue damage and inflammatory responses compared to stiff silicon probes. To the best of our knowledge, this PEDOT/CNT-coated GC MEA is the first implantable flexible multisite sensor capable of chronic in vivo multi-site sensing of tonic 5-HT. This implantable MEA can be custom-designed according to specific brain region of interests and research questions, with the potential to combine electrophysiology recording and multiple analyte sensing to maximize our understanding of neurochemistry.

Highlights

PEDOT/CNT-coated GC microelectrodes enabled sensitive and selective tonic detection of serotonin (5-HT) using a new square wave voltammetry (SWV) approach PEDOT/CNT-coated GC MEAs achieved multi-site in vivo 5-HT tonic detection for one week. Flexible MEAs lead to reduced tissue damage and inflammation compared to stiff silicon probes.

Xanthoceraside exerts anti-Alzheimer's disease effect by remodeling gut microbiota and modulating microbial-derived metabolites level in rats

BACKGROUND

Microbial-derived metabolites play important roles in Alzheimer's disease (AD) pathology, yet how intestinal microbes influence AD progression remains uncertain. Xanthoceraside (XAN), a triterpenoid saponin with anti-AD activity, was extracted from the husks of Xanthoceras sorbifolia Bunge. However, it is still unclear that how XAN modulates the gut microbiota community to regulate AD progression through changing the levels of microbial-derived metabolites.

PURPOSE

In this study, we investigated the mechanism underlying the anti-AD effect of XAN.

METHODS

The current combination studies of multiple-targeted metabolomics, natural product chemistry and pharmacology revealed that oral XAN mediated intestinal microbiota to ameliorate Aβ1-42-induced learning and memory deficits in rats, which were confirmed through antibiotic treatments and fecal microbiota transplantation.

RESULTS

As a poor water solubility and low permeability compound that hardly be absorbed into blood-brain barrier, XAN significantly regulated Aβ1-42-induced metabolism disorders directly or indirectly in gut, including neurotransmitters, amino acids, bile acids and SCFAs metabolism that were detected by UHPLC-MS/MS and GC-MS/MS. In particularly, the in vitro evaluation of XAN on SCFAs production not only found a striking increase in the production of SCFAs after fermentation, but revealed the inner relationship among XAN, gut microbiota and SCFAs in vivo. All results demonstrated that XAN could improve AD rats' learning and memory deficits by modulating the community of gut microbiota which was connected through 16S rRNA sequencing and CCA analyses.

CONCLUSIONS

Our study provided a novel mechanism for developing XAN as a potential anti-AD drug and revealed that the gut microbiota might be a potential target for AD treatment .

Associations Between a Polymorphism in the Rat 5-HT1A Receptor Gene Promoter Region (rs198585630) and Cognitive Alterations Induced by Microwave Exposure

The nervous system is a sensitive target of electromagnetic radiation (EMR). Chronic microwave exposure can induce cognitive deficits, and 5-HT system is involved in this effect. Genetic polymorphisms lead to individual differences. In this study, we evaluated whether the single-nucleotide polymorphism (SNP) rs198585630 of 5-HT_1A receptor is associated with cognitive alterations in rats after microwave exposure with a frequency of 2.856 GHz and an average power density of 30 mW/cm². Rats were exposed to microwaves for 6 min three times a week for up to 6 weeks. PC12 cells and 293T cells were exposed to microwaves for 5 min up to 3 times at 2 intervals of 5 min. Transcriptional activity of 5-HT_1A receptor promoter containing rs198585630 C/T allele was determined in vitro. Electroencephalograms (EEGs), spatial learning and memory, and mRNA and protein expression of 5-HT_1A receptor were evaluated in vivo. We demonstrated that transcriptional activity of 5-HT_1A receptor promoter containing rs198585630 C allele was higher than that of 5-HT_1A receptor promoter containing T allele. The transcriptional activity of 5-HT_1A receptor promoter was stimulated by 30 mW/cm² microwave exposure, and rs198585630 C allele was more sensitive to microwave exposure, as it showed stronger transcriptional activation. Rats carrying rs198585630 C allele exhibited increased mRNA and protein expression of 5-HT_1A receptor and were more susceptible to 30 mW/cm² microwave exposure, showing cognitive deficits and inhibition of brain electrical activity. These findings suggest SNP rs198585630 of the 5-HT_1A receptor is an important target for further research exploring the mechanisms of hypersensitivity to microwave exposure.

Modulating role of serotonergic signaling in sleep and memory

Serotonin is an important neurotransmitter with various receptors and wide-range effects on physiological processes and cognitive functions including sleep, learning, and memory. In this review study, we aimed to discuss the role of serotonergic receptors in modulating sleep-wake cycle, and learning and memory function. Furthermore, we mentioned to sleep deprivation, its effects on memory function, and the potential interaction with serotonin. Although there are thousands of research articles focusing on the relationship between sleep and serotonin; however, the pattern of serotonergic function in sleep deprivation is inconsistent and it seems that serotonin has not a certain role in the effects of sleep deprivation on memory function. Also, we found that the injection type of serotonergic agents (systemic or local), the doses of these drugs (dose-dependent effects), and up- or down-regulation of serotonergic receptors during training with various memory tasks are important issues that can be involved in the effects of serotonergic signaling on sleep-wake cycle, memory function, and sleep deprivation-induced memory impairments. This comprehensive review was conducted in the PubMed, Scopus, and ScienceDirect databases in June and July 2021, by searching keywords sleep, sleep deprivation, memory, and serotonin.

The bidirectional effect of prelimbic 5-hydroxytryptamine type-4 (5-HT4) receptors on ACPA-mediated aversive memory impairment in adult male Sprague-Dawley rats

Objectives

This study aimed at investigating the effect of serotonergic 5-HT4 receptor agonist/antagonist on memory consolidation deficit induced by ACPA (a potent, selective CB₁ cannabinoid receptor agonist) in the pre-limbic (PL) cortex.

Materials and Methods

We used the step-through passive avoidance test to evaluate memory consolidation of male Sprague-Dawley (SD) rats. Bilateral post-training microinjections of the drugs were done in a volume of 0.6 μl/rat into the PL area (0.3 μl per side).

Results

The results showed a significant interaction between RS67333 hydrochloride (5-HT4 receptor agonist) or RS23597-190 hydrochloride (5-HT4 receptor antagonist) and ACPA on consolidation of aversive memory. RS67333 hydrochloride (0.5 μg/rat) enhanced consolidation of memory and its co-administration at the ineffective dose of 0.005 μg/rat with ineffective (0.001 μg/rat) or effective (0.1 μg/rat) doses of ACPA improved and prevented impairment of memory caused by ACPA, respectively. In other words, RS67333 had a bidirectional effect on ACPA-caused amnesia. While RS23597-190 hydrochloride had no effect on memory at the doses used (0.005, 0.01, 0.1, or 0.5 μg/rat); but its concomitant use with an effective dose of ACPA (0.1 μg/rat) potentiated amnesia. None of the drugs had an effect on locomotor activity.

Conclusion

This study revealed that activation or deactivation of the 5-HT4 receptors in the PL may mediate the IA memory impairment induced by ACPA indicating a modulatory role for the 5-HT4 serotonergic receptors.

Protein synthesis inhibitor administration before a reminder caused recovery from amnesia induced by memory reconsolidation impairment with NMDA glutamate receptor antagonist

Memory recovery in amnestic animals is one of the most poorly studied processes. In this paper, we examine the role of protein synthesis and a reminder in the mechanisms of amnesia and memory recovery in grape snails trained to conditioned food aversion. Amnesia was induced by the impairment of memory reconsolidation using NMDA (N-methyl d-aspartate) glutamate receptor antagonists. In an early stage of amnesia (day 3), injections of protein synthesis inhibitors into animals combined with a reminder by a conditioned stimulus (CS) led to the recovery of aversive reactions to its presentation. Two types of changes in reactions to CS were revealed. In most animals, a persistent recovery of memory retrieval was found that lasted for at least 10 days. In other snails, aversive responses to CS persisted for 24 h. Isolated injections of inhibitors, injections of inhibitors and a reminder by the learning environment (without presenting a CS), usage of a differentiating stimulus instead of a CS, or inhibitor injections after the reminder did not affect the development of amnesia. The administration of protein synthesis inhibitors and a reminder in the late period after amnesia induction (10 days) did not affect its development or caused a short-term memory recovery. We suggest that amnesia is an active process that develops over time. The reminder induces the reactivation of the amnesia process dependent on protein synthesis, while the administration of protein synthesis inhibitors leads to the impairment of amnesia reactivation and recovery of the state formed before amnesia induction (i.e., recovery of conditioned food aversion memory).

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.

NMDA or 5-HT receptor antagonists impair memory reconsolidation and induce various types of amnesia

Elucidation of amnesia mechanisms is one of the central problems in neuroscience with immense practical application. Previously, we found that conditioned food presentation combined with injection of a neurotransmitter receptor antagonist or protein synthesis inhibitor led to amnesia induction. In the present study, we investigated the time course and features of two amnesias: induced by impairment of memory reconsolidation using an NMDA glutamate receptor antagonist (MK-801) and a serotonin receptor antagonist (methiothepin, MET) on snails trained with food aversion conditioning. During the early period of amnesia (<10th day), the unpaired presentation of conditioned stimuli (CS) or unconditioned stimuli (US) in the same training context did not have an effect on both types of amnesia. Retraining an on 1st or 3rd day of amnesia induction facilitated memory formation, i.e. the number of CS + US pairings was lower than at initial training. On the 10th or 30th day after the MET/reminder, the number of CS + US pairings did not change between initial training and retraining. Retraining on the 10th or 30th day following the MK-801/reminder in the same or a new context of learning resulted in short, but not long-term, memory, and the number of CS + US pairings was higher than at the initial training. This type of amnesia was specific to the CS we used at initial training, since long-term memory for another kind of CS could be formed in the same snails. The attained results suggest that disruption of memory reconsolidation using antagonists of serotonin or NMDA glutamate receptors induced amnesias with different abilities to form long-term memory during the late period of development.

Novel Vilazodone-Tacrine Hybrids as Potential Multitarget-Directed Ligands for the Treatment of Alzheimer's Disease Accompanied with Depression: Design, Synthesis, and Biological Evaluation

Depression is one of the most frequent psychiatric complications of Alzheimer's disease (AD), affecting up to 50% of the patients. A novel series of hybrid molecules were designed and synthesized by combining the pharmacophoric features of vilazodone and tacrine as potential multitarget-directed ligands for the treatment of AD with depression. In vitro biological assays were conducted to evaluate the compounds; among the 30 hybrids, compound 1e showed relatively balanced profiles between acetylcholinesterase inhibition (IC₅₀ = 3.319 ± 0.708 μM), 5-HT_1A agonist (EC₅₀ = 107 ± 37 nM), and 5-HT reuptake inhibition (IC₅₀ = 76.3 ± 33 nM). Compound 1e displayed tolerable hepatotoxicity and moderate hERG inhibition activity, and could penetrate the blood-brain barrier in vivo. Furthermore, an oral intake of 30 mg/kg 1e·HCl could significantly improve the cognitive function of scopolamine-induced amnesia mice and alleviate the depressive symptom in tail suspension test. The effectivity of 1e validates the rationality of our design strategy.

Frameworking memory and serotonergic markers

The evidence for neural markers and memory is continuously being revised, and as evidence continues to accumulate, herein, we frame earlier and new evidence. Hence, in this work, the aim is to provide an appropriate conceptual framework of serotonergic markers associated with neural activity and memory. Serotonin (5-hydroxytryptamine [5-HT]) has multiple pharmacological tools, well-characterized downstream signaling in mammals’ species, and established 5-HT neural markers showing new insights about memory functions and dysfunctions, including receptors (5-HT1A/1B/1D, 5-HT2A/2B/2C, and 5-HT3-7), transporter (serotonin transporter [SERT]) and volume transmission present in brain areas involved in memory. Bidirectional influence occurs between 5-HT markers and memory/amnesia. A growing number of researchers report that memory, amnesia, or forgetting modifies neural markers. Diverse approaches support the translatability of using neural markers and cerebral functions/dysfunctions, including memory formation and amnesia. At least, 5-HT1A, 5-HT4, 5-HT6, and 5-HT7receptors and SERT seem to be useful neural markers and therapeutic targets. Hence, several mechanisms cooperate to achieve synaptic plasticity or memory, including changes in the expression of neurotransmitter receptors and transporters.

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.

Poly(basic red 9) doped functionalized multi-walled carbon nanotubes as composite films for neurotransmitters biosensors

This paper discusses the electrochemical polymerization of basic dye films, which are composed of basic red 9 (BR9), on various electrodes and the enhancement of the electropolymerization by functionalized multiwall carbon nanotubes (f-MWCNTs) modification of the electrode surface. The presence of f-MWCNTs enhances the surface coverage (Γ) and stability. Poly(BR9) films were electrocatalytically active for epinephrine and serotonin oxidation. The electrocatalytic oxidation current developed from the anodic peak of the redox couple. Electrochemical impedance spectroscopy (EIS) was applied to monitor the whole process of the electrode modification. EIS can provide useful information regarding the impedance changes on the electrode surface between each step. We studied the surface morphology of the composite film using scanning electron microscopy (SEM) and atomic force microscopy (AFM), which revealed that BR9 is doped on f-MWCNTs. Cyclic voltammetry (CV) was used for the measurement of the electroanalytical properties of the analytes. The sensitivity values for the f-MWCNTs/BR9 composite film were higher than the poly(BR9) and f-MWCNTs composite film. Finally, differential pulse voltammetry (DPV) was used for the detection of a mixture of analytes at the f-MWCNTs/BR9 composite film. We simulated a more complex system with both serotonin and epinephrine present simultaneously. This system also exhibited oxidation peaks for serotonin in bovine calf serum (BCS) and epinephrine injection for real samples determination at pH 7.0 at the f-MWCNTs/BR9 composite film.

5-HT4-receptors modulate induction of long-term depression but not potentiation at hippocampal output synapses in acute rat brain slices

The subiculum is the principal target of CA1 pyramidal cells and mediates hippocampal output to various cortical and subcortical regions of the brain. The majority of subicular pyramidal cells are burst-spiking neurons. Previous studies indicated that high frequency stimulation in subicular burst-spiking cells causes presynaptic NMDA-receptor dependent long-term potentiation (LTP) whereas low frequency stimulation induces postsynaptic NMDA-receptor-dependent long-term depression (LTD). In the present study, we investigate the effect of 5-hydroxytryptamine type 4 (5-HT4) receptor activation and blockade on both forms of synaptic plasticity in burst-spiking cells. We demonstrate that neither activation nor block of 5-HT4 receptors modulate the induction or expression of LTP. In contrast, activation of 5-HT4 receptors facilitates expression of LTD, and block of the 5-HT4 receptor prevents induction of short-term depression and LTD. As 5-HT4 receptors are positively coupled to adenylate cyclase 1 (AC1), 5-HT4 receptors might modulate PKA activity through AC1. Since LTD is blocked in the presence of 5-HT4 receptor antagonists, our data are consistent with 5-HT4 receptor activation by ambient serotonin or intrinsically active 5-HT4 receptors. Our findings provide new insight into aminergic modulation of hippocampal output.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Design and synthesis of novel series of 5-HT6 receptor ligands having indole, a central aromatic core and 1-amino-4 methyl piperazine as a positive ionizable group

The exclusive distribution of 5-HT6 receptor in the brain regions and high affinity for antipsychotic and antidepressant drugs makes 5-HT6 receptor a promising target in treatment of CNS diseases. Based on a pharmacophore model reported in the literature, we designed and synthesized a novel series of 5-HT6 receptor ligands having indole as a central aromatic core and 1-amino-4-methyl piperazine as positive ionizable group. Out of 32 compounds we have successfully identified 10 new compounds as 5-HT6 receptor antagonists. The structure-activity relationship (SAR) studies have been carried out by mapping the compounds with the 3D QSAR model.

Activation of 5-HT7 serotonin receptors reverses metabotropic glutamate receptor-mediated synaptic plasticity in wild-type and Fmr1 knockout mice, a model of Fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is a genetic cause of intellectual disability and autism. Fmr1 knockout (Fmr1 KO) mice, an animal model of FXS, exhibit spatial memory impairment and synapse malfunctioning in the hippocampus, with abnormal enhancement of long-term depression mediated by metabotropic glutamate receptors (mGluR-LTD). The neurotransmitter serotonin (5-HT) modulates hippocampal-dependent learning through serotonin 1A (5-HT1A) and serotonin 7 (5-HT7) receptors; the underlying mechanisms are unknown.

METHODS

We used electrophysiology to test the effects of 5-HT on mGluR-LTD in wild-type and Fmr1 KO mice and immunocytochemistry and biotinylation assay to study related changes of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) glutamate receptor surface expression.

RESULTS

Application of 5-HT or 8-OH-DPAT (a mixed 5-HT1A/5-HT7 agonist) reversed mGluR-LTD in hippocampal slices. Reversal of mGluR-LTD by 8-OH-DPAT persisted in the presence of the 5-HT1A receptor antagonist WAY-100635, was abolished by SB-269970 (5-HT7 receptor antagonist), and was mimicked by LP-211, a novel selective 5-HT7 receptor agonist. Consistently, 8-OH-DPAT decreased mGluR-mediated reduction of AMPA glutamate receptor 2 (GluR2) subunit surface expression in hippocampal slices and cultured hippocampal neurons, an effect mimicked by LP-211 and blocked by SB-269970. In Fmr1 KO mice, mGluR-LTD was abnormally enhanced; similarly to wild-type, 8-OH-DPAT reversed mGluR-LTD and decreased mGluR-induced reduction of surface AMPA receptors, an effect antagonized by SB-269970.

CONCLUSIONS

Serotonin 7 receptor activation reverses metabotropic glutamate receptor-induced AMPA receptor internalization and LTD both in wild-type and in Fmr1 KO mice, correcting excessive mGluR-LTD. Therefore, selective activation of 5-HT7 receptors may represent a novel strategy in the therapy of FXS.

The 5-HT4 receptor levels in hippocampus correlates inversely with memory test performance in humans

The cerebral serotonin (5-HT) system is involved in cognitive functions such as memory and learning and animal studies have repeatedly shown that stimulation of the 5-HT type 4 receptor (5-HT4 R) facilitates memory and learning and further that the 5-HT4 R modulates cellular memory processes in hippocampus. However, any associations between memory functions and the expression of the 5-HT4 R in the human hippocampus have not been investigated. Using positron emission tomography with the tracer [(11) C]SB207145 and Reys Auditory Verbal Learning Test we aimed to examine the individual variation of the 5-HT4R binding in hippocampus in relation to memory acquisition and consolidation in healthy young volunteers. We found significant, negative associations between the immediate recall scores and left and right hippocampal BPND , (p = 0.009 and p = 0.010 respectively) and between the right hippocampal BPND and delayed recall (p = 0.014). These findings provide evidence that the 5-HT4 R is associated with memory functions in the human hippocampus and potentially pharmacological stimulation of the receptor may improve episodic memory.

Memory monitoring performance and PFC activity are associated with 5-HTTLPR genotype in older adults

Older adults show extensive variability in cognitive performance, including episodic memory. A portion of this variability could potentially be explained by genetic factors. Recent literature shows that the neurotransmitter serotonin plays an important role in memory processes, as enhancements of brain serotonin have led to memory improvement. Here, we have begun to explore genetic contributions to the performance and underlying brain activity associated with source memory monitoring. Using a source recognition memory task during fMRI scanning, this study offers evidence that older adults who carry a short allele (S-car) of the serotonin transporter linked polymorphic region (5-HTTLPR) in the SLC6A4 gene show specific deficits in source memory monitoring relative to older adults who are homozygous for the long allele (LL). These deficits are accompanied by less neural activity in regions of prefrontal cortex that have been shown to support accurate memory monitoring. Moreover, while the older adult LL group's behavioral performance does not differ from younger adults, their brain activation reveals evidence of compensatory activation that likely supports their higher performance level. These results provide preliminary evidence that the long-allele homozygous profile is cognitively beneficial to older adults, particularly for memory functioning.

Role of serotonin in Alzheimer's disease: a new therapeutic target?

Mounting evidence accumulated over the past few years indicates that the neurotransmitter serotonin plays a significant role in cognition. As a drug target, serotonin receptors have received notable attention due in particular to the role of several serotonin-receptor subclasses in cognition and memory. The intimate anatomical and neurochemical association of the serotonergic system with brain areas that regulate memory and learning has directed current drug discovery programmes to focus on this system as a major therapeutic drug target. Thus far, none of these programmes has yielded unambiguous data that suggest that any of the new drug entities possesses disease-modifying properties, and significantly more research in this promising area of investigation is required. Compounds are currently being investigated for activity against serotonin 5-HT(1), 5-HT(4) and 5-HT(6) receptors. This review concludes that most work done in the development of selective serotonin receptor ligands is in the pre-clinical or early clinical phase. Also, while many of these compounds will likely find application as adjuvant therapy in the symptomatic treatment of Alzheimer's disease, there are currently only a few drug entities with activity against serotonin receptors that may offer the potential to alter the progression of the disease.

Procedural and declarative memory task performance, and the memory consolidation function of sleep, in recent and abstinent ecstasy/MDMA users

Ecstasy/MDMA use has been associated with various memory deficits. This study assessed declarative and procedural memory in ecstasy/MDMA users. Participants were tested in two sessions, 24 h apart, so that the memory consolidation function of sleep on both types of memory could also be assessed. Groups were: drug-naive controls (n = 24); recent ecstasy/MDMA users, who had taken ecstasy/MDMA 2-3 days before the first testing session (n = 25), and abstinent users, who had not taken ecstasy/MDMA for at least 8 days before testing (n = 17). Procedural memory did not differ between groups, but greater lifetime consumption of ecstasy was associated with poorer procedural memory. Recent ecstasy/MDMA users who had taken other drugs (mainly cannabis) 48-24 h before testing exhibited poorer declarative memory than controls, but recent users who had not taken other drugs in this 48-24-h period did not differ from controls. Greater lifetime consumption of ecstasy, and of cocaine, were associated with greater deficits in declarative memory. These results suggest that procedural, as well as declarative, memory deficits are associated with the extent of past ecstasy use. However, ecstasy/MDMA did not affect the memory consolidation function of sleep for either the declarative or the procedural memory task.

Inactivation of the dorsal raphé nucleus reduces the anxiogenic response of rats running an alley for intravenous cocaine

Rats traversing a straight alley once a day for delivery of a single i.v. injection of cocaine develop over trials an ambivalence about entering the goal box. This ambivalence is characterized by the increasing occurrence of "retreat behaviors" where animals leave the start box and run quickly to the goal box, but then stop at the entry point and "retreat" back toward the start box. This unique pattern of retreat behavior has been shown to reflect a form of "approach-avoidance conflict" that stems from the animals' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Cocaine blocks reuptake of the serotonergic (5-HT) transporter and serotonin has been implicated in the modulation of anxiety. It was therefore of interest to determine whether inactivation of the serotonergic cell bodies residing in the dorsal raphé nucleus (DRN) and projecting to brain areas critical for the modulation of anxiety, would alter the anxiogenic state exhibited by rats running an alley for single daily i.v. injections of 1.0mg/kg cocaine. Reversible inactivation of the DRN was accomplished by intracranial application of a mixed solution of the GABA agonists baclofen and muscimol. While DRN inactivation had no impact on the subjects' motivation to initiate responding (i.e., latencies to leave the start box were unaffected) it reliably reduced the frequency of approach-avoidance retreat behaviors (conflict behavior). These data suggest that inactivation of the dorsal raphé reduces the conflict/anxiety otherwise present in experienced cocaine-seeking animals.

Differential involvement of hippocampal serotonin1A receptors and re-uptake sites in non-cognitive behaviors of Alzheimer's disease

RATIONALE

Previous studies have shown extensive serotonergic deficits in the hippocampus of Alzheimer's disease (AD) patients. However, it is unclear whether such deficits play a role in non-cognitive, neuropsychiatric behaviors that occur frequently in AD and cause significant caregiver distress.

OBJECTIVES

In this study, we aimed to correlate serotonergic markers in the AD hippocampus with neuropsychiatric behaviors.

METHODS

Using postmortem hippocampal homogenates from aged controls as well as a cohort of longitudinally assessed AD patients, measurements of 5-HT(1A) receptors, 5-HT(2A) receptors, and serotonin re-uptake (5-HTT) sites were performed by binding with (3)H-labeled 8-OH-DPAT, ketanserin, and citalopram, respectively.

RESULTS

Alterations of 5-HT(1A) receptors and 5-HTT were found to be differentially involved in neuropsychiatric behaviors, with loss of 5-HT(1A) receptors specifically correlated with depressive symptoms, while 5-HTT sites were preserved or up-regulated in patients with aggressive behaviors.

CONCLUSIONS

Our data suggest that neuropsychiatric behaviors in AD share certain neurochemical features with psychiatric disorders like major depression and that serotonergic drugs used in psychiatric disorders may also be efficacious against behavioral symptoms in AD.

Serotonin transporter and memory

The serotonin transporter (SERT) has been associated to diverse functions and diseases, though seldom to memory. Therefore, we made an attempt to summarize and discuss the available publications implicating the involvement of the SERT in memory, amnesia and anti-amnesic effects. Evidence indicates that Alzheimer's disease and drugs of abuse like d-methamphetamine (METH) and (+/-)3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") have been associated to decrements in the SERT expression and memory deficits. Several reports have indicated that memory formation and amnesia affected the SERT expression. The SERT expression seems to be a reliable neural marker related to memory mechanisms, its alterations and potential treatment. The pharmacological, neural and molecular mechanisms associated to these changes are of great importance for investigation.

Effects of d-amphetamine on short- and long-term memory in spontaneously hypertensive, Wistar-Kyoto and Sprague-Dawley rats

Diverse studies indicate that the attention deficit hyperactivity disorder (ADHD) is associated with alterations in encoding processes, including working or short-term memory. Some ADHD dysfunctional domains are reflected in the spontaneously hypertensive rat (SHR). Here SHR-saline group showed significantly poor STM and LTM relative to SD and WKY saline rats. SD and WKY rats treated with d-amphetamine displayed better STM and LTM, compared to SD-vehicle, WKY-vehicle or SHR-d-amphetamine groups.

Correlations among central serotonergic parameters and age-related emotional and cognitive changes assessed through the elevated T-maze and the Morris water maze

Emotion and spatial cognitive aspects were assessed in adult and middle-aged rats using the elevated T-maze (ETM) and the Morris water maze (MWM) tasks. Both adult and middle-aged rats were able to acquire inhibitory avoidance behaviour, though the middle-aged subjects showed larger latencies along the trials, including the baseline, which was significantly longer than that showed by adult rats. Further, compared to adult rats, middle-aged rats had longer escape latency. In spite of the worse performance in the second session of the spatial cognitive task, the middle-aged rats were able to learn the task and remember the information along the whole probe trial test. Both thalamic serotonin (5-HT) concentration and amygdala serotonergic activity (5-HIAA/5-HT) are significantly correlated, respectively, to escape latency and behavioural extinction in the MWM only for middle-aged rats. A significant correlation between the 5-HIAA/5-HT ratio in the amygdala and behavioural extinction for middle-aged, but not for adult, rats was observed. This result suggests that serotonergic activity in the amygdala may regulate behavioural flexibility in aged animals. In addition, a significant negative correlation was found between hippocampal 5-HIAA/5-HT ratio and the path length at the second training session of the MWM task, although only for adult subjects. This was the only session where a significant difference between the performance of middle-aged and adult rats has occurred. Although the involvement of the hippocampus in learning and memory is well established, the present work shows, for the first time, a correlation between a serotonergic hippocampal parameter and performance of a spatial task, which is lost with ageing.

Beyond the cholinergic hypothesis: do current drugs work in Alzheimer's disease?

Alzheimer's disease (AD) is a neurodegenerative disease characterized by memory and cognitive loss, and represents the leading cause of dementia in elderly people. Besides the complex biochemical processes involved in the neuronal degeneration (formation of senile plaques containing Abeta peptides, and development of neurofibrillary tangles), other molecular and neurochemical alterations, like cholinergic deficit due to basal forebrain degeneration, also occur. Because acetylcholine has been demonstrated to be involved in cognitive processes, the idea to increase acetylcholine levels to restore cognitive deficits has gained interest (the so-called cholinergic hypothesis). This has led to the development of drugs able to prevent acetylcholine hydrolysis (acetylcholinesterase inhibitors). However, the analysis of clinical efficacy of these drugs in alleviating symptoms of dementia showed unsatisfactory results. Despite such critical opinions on the efficacy of these drugs, it should be said that acetylcholinesterase inhibitors, and for some aspects memantine also, improve memory and other cognitive functions throughout most of the duration of the disease. The pharmacological activity of these drugs suggests an effect beyond the mere increase of acetylcholine levels. These considerations are in agreement with the idea that cognitive decline is the result of a complex and not fully elucidated interplay among different neurotransmitters. The role of each of the neurotransmitters implicated has to be related to a cognitive process and as a consequence to its decline. The current review aims to highlight the positive role of cholinergic drugs in alleviating cognitive deficits during wake as well as sleep. Moreover, we suggest that future therapeutic approaches have to be developed to restore the complex interplay between acetylcholine and other neurotransmitters systems, such as dopamine, serotonin, noradrenaline, or glutamate, that are likely involved in the progressive deterioration of several cognitive functions such as attention, memory, and learning.

Autoradiographic study of serotonin transporter during memory formation

Serotonin transporter (SERT) has been associated with drugs of abuse like d-methamphetamine (METH). METH is well known to produce effects on the monoamine systems but it is unclear how METH affects SERT and memory. Here the effects of METH and the serotonin reuptake inhibitor fluoxetine (FLX) on autoshaping and novel object recognition (NOR) were investigated. Notably, both memory tasks recruit different behavioral, neural and cognitive demand. In autoshaping task a dose-response curve for METH was determined. METH (1.0mg/kg) impaired short-term memory (STM; lasting less of 90min) in NOR and impaired both STM and long-term memory (LTM; lasting 24 and 48h) in autoshaping, indicating that METH had long-lasting effects in the latter task. A comparative autoradiography study of the relationship between the binding pattern of SERT in autoshaping new untrained vs. trained treated (METH, FLX, or both) animals was made. Considering that hemispheric dominance is important for LTM, hence right vs. left hemisphere of the brain was compared. Results showed that trained animals decreased cortical SERT binding relative to untrained ones. In untrained and trained treated animals with the amnesic dose (1.0mg/kg) of METH SERT binding in several areas including hippocampus and cortex decreased, more remarkably in the trained animals. In contrast, FLX improved memory, increased SERT binding, prevented the METH amnesic effect and re-established the SERT binding. In general, memory and amnesia seemed to make SERT more vulnerable to drugs effects.