Blockade of 5-HT1B receptors facilitates contextual aversive learning in mice by disinhibition of cholinergic and glutamatergic neurotransmission

Neuronal atg1 Coordinates Autophagy Induction and Physiological Adaptations to Balance mTORC1 Signalling

The mTORC1 nutrient-sensing pathway integrates metabolic and endocrine signals into the brain to evoke physiological responses to food deprivation, such as autophagy. Nevertheless, the impact of neuronal mTORC1 activity on neuronal circuits and organismal metabolism remains obscure. Here, we show that mTORC1 inhibition acutely perturbs serotonergic neurotransmission via proteostatic alterations evoked by the autophagy inducer atg1. Neuronal ATG1 alters the intracellular localization of the serotonin transporter, which increases the extracellular serotonin and stimulates the 5HTR7 postsynaptic receptor. 5HTR7 enhances food-searching behaviour and ecdysone-induced catabolism in Drosophila. Along similar lines, the pharmacological inhibition of mTORC1 in zebrafish also stimulates food-searching behaviour via serotonergic activity. These effects occur in parallel with neuronal autophagy induction, irrespective of the autophagic activity and the protein synthesis reduction. In addition, ectopic neuronal atg1 expression enhances catabolism via insulin pathway downregulation, impedes peptidergic secretion, and activates non-cell autonomous cAMP/PKA. The above exert diverse systemic effects on organismal metabolism, development, melanisation, and longevity. We conclude that neuronal atg1 aligns neuronal autophagy induction with distinct physiological modulations, to orchestrate a coordinated physiological response against reduced mTORC1 activity.

Activation of 5-HT1 receptor in Lateral Habenula impaired contextual fear memory and hippocampal LTP in rat

Serotonin (5-hydroxytraptamine, 5-HT) is a neurotransmitter plays important roles in emotion and motivation. The action of 5-HT varies across nucleus and the receptor sub-types. Lateral habenula (LHb) in a brain area reciprocally connects with raphe nucleus and plays important roles in emotion and depression. In this study, we aimed to study the role of 5-HT₁ receptor in LHb on fear learning. 15 minutes before or immediate after the fear conditioning, 5-Carboxyamidotrypamine maleate salt (5-CT), an agonist of 5-HT₁ receptor, was bilaterally delivered into LHb (1μg/μl, 1μl/side) in rats. We found that activation of 5-HT₁ receptor in LHb impaired the acquisition but not consolidation of fear memory in rats, which was accompanied by impaired long-term potentiation (LTP) and decreased phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit 1 (GluA1) at the Ser845 site in hippocampus. In addition, 5-CT decreased the time spent in center area of the open field and time spent in open arm in elevated plus maze. These results suggest that activation of 5-HT₁ receptor in LHb impaired acquisition of hippocampal dependent fear memory and increased anxiety- like behavior in rats.

Alterations and interactions of subcortical modulatory systems in Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is not fully understood. Here we summarize current knowledge on the involvement of the serotonergic, noradrenergic, dopaminergic, cholinergic, and opioid systems in AD, emphasizing the importance of interactions between the serotonergic and the other subcortical modulatory systems during the progression of AD. In physiological conditions, all neurotransmitter systems function in concert and are interdependent at both the neuroanatomical and molecular levels. Through their early involvement in AD, cognitive and behavioral abilities that rely on their interactions also become disrupted. Considering that serotonin (5HT) regulates the release of noradrenaline (NA), dopamine (DA) and acetylcholine (ACh), any alteration in 5HT levels leads to disturbance of NA, DA, and ACh homeostasis in the brain. One of the earliest pathological changes during the prodromal phase of AD is a decrease of serotonergic transmission throughout the brain, with serotonergic receptors being also affected. Additionally, serotonergic and noradrenergic as well as serotonergic and dopaminergic nuclei are reciprocally interconnected. As the serotonergic dorsal raphe nucleus (DRN) is affected by pathological changes early in AD, and the noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental area (VTA) exhibit AD-related pathological changes, their connectivity also becomes altered in AD. Such disrupted interactions among neurotransmitter systems in AD can be used in the development of multi-target drugs. Some of the potential AD therapeutics (such as ASS234, RS67333, tropisetron) target multiple neurotransmitter systems to achieve the best possible improvement of cognitive and behavioral deficits observed in AD. Here, we review how serotonergic system interacts with other subcortical modulatory systems (noradrenergic, dopaminergic, cholinergic, and opioid systems) during AD.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Effect of 4-Fluoro-N-(4-sulfamoylbenzyl) Benzene Sulfonamide on cognitive deficits and hippocampal plasticity during nicotine withdrawal in rats

Withdrawal from chronic nicotine has damaging effects on a variety of learning and memory tasks. Various Sulfonamides that act as carbonic anhydrase inhibitors have documented role in modulation of various cognitive, learning, and memory processing. We investigated the effects of 4-Fluoro-N-(4-sulfamoylbenzyl) Benzene Sulfonamide (4-FBS) on nicotine withdrawal impairments in rats using Morris water maze (MWM), Novel object recognition, Passive avoidance, and open field tasks. Also, Brain-derived neurotrophic factor (BDNF) profiling and in vivo field potential recording were assessed. Rats were exposed to saline or chronic nicotine 3.8 mg/kg subcutaneously for 14 days in four divided doses, spontaneous nicotine withdrawal was induced by quitting nicotine for 72 h (hrs). Animals received 4-FBS at 20, 40, and 60 mg/kg after 72 h of withdrawal in various behavioral and electrophysiological paradigms. Nicotine withdrawal causes a deficit in learning and long-term memory in the MWM task. No significant difference was found in novel object recognition tasks among all groups while in passive avoidance task nicotine withdrawal resulted in a deficit of hippocampus-dependent fear learning. Anxiety like behavior was observed during nicotine withdrawal. Plasma BDNF level was reduced during nicotine withdrawal as compared to the saline group reflecting mild cognitive impairment, stress, and depression. Withdrawal from chronic nicotine altered hippocampal plasticity, caused suppression of long-term potentiation (LTP) in the CA1 area of the hippocampus. Our results showed that 4-FBS at 40 and 60 mg/kg significantly prevented nicotine withdrawal-induced cognitive deficits in behavioral as well as electrophysiological studies. 4-FBS at 60 mg/kg upsurge nicotine withdrawal-induced decrease in plasma BDNF. We conclude that 4-FBS at 40 and 60 mg /kg effectively prevented chronic nicotine withdrawal-induced impairment in long term potentiation and cognitive performance.

Atypical but not typical antipsychotic drugs ameliorate phencyclidine-induced emotional memory impairments in mice

Schizophrenia is associated with cognitive impairments related to hypofunction in glutamatergic N-methyl-D-aspartate receptor (NMDAR) transmission. Phencyclidine (PCP), a non-competitive NMDAR antagonist, models schizophrenia-like behavioral symptoms including cognitive deficits in rodents. This study examined the effects of PCP on emotional memory function examined in the passive avoidance (PA) task in mice and the ability of typical and atypical antipsychotic drugs (APDs) to rectify the PCP-mediated impairment. Pre-training administration of PCP (0.5, 1, 2 or 3 mg/kg) dose-dependently interfered with memory consolidation in the PA task. In contrast, PCP was ineffective when administered after training, and immediately before the retention test indicating that NMDAR blockade interferes with memory encoding mechanisms. The typical APD haloperidol and the dopamine D_2/3 receptor antagonist raclopride failed to block the PCP-induced PA impairment suggesting a negligible role of D₂ receptors in the PCP impairment. In contrast, the memory impairment was blocked by the atypical APDs clozapine and olanzapine in a dose-dependent manner while risperidone was effective only at the highest dose tested (1 mg/kg). The PCP-induced impairment involves 5-HT_1A receptor mechanisms since the antagonist NAD-299 blocked the memory impairment caused by PCP and the ability of clozapine to attenuate the impairment by PCP. These results indicate that atypical but not typical APDs can ameliorate NMDAR-mediated memory impairments and support the view that atypical APDs such as clozapine can modulate glutamatergic memory dysfunctions through 5-HT_1A receptor mechanisms. These findings suggest that atypical APDs may improve cognitive impairments related to glutamatergic dysfunction relevant for emotional memories in schizophrenia.

Modulation of Kalirin-7 expression by hippocampal CA1 5-HT1B receptors in spatial memory consolidation

Serotonin 5-HT1B receptors (5-HT1BRs) are distributed in hippocampal CA1 and play a pivotal role in cognitive function. Activation of 5-HT1BRs regulates synaptic plasticity at the excitatory synapses in the hippocampus. However, the role and its underlying mechanism of 5-HT1BR activation-mediated glutamatergic synaptic plasticity in spatial memory are not fully understood. In this study, spatial memory of Sprague-Dawley (SD) rats was assessed in a Morris water maze after bilateral dorsal hippocampal CA1 infusion of the 5-HT1BR antagonist GR55562 (25 μg/μL) or agonist CP93129 (25 μg/μL). GR55562 did not affect the spatial memory acquisition but significantly increased the target quadrant preference during the memory consolidation probe performed 14 d after the training session, while CP93129 impaired the memory consolidation process. Moreover, GR55562 significantly increased, while CP93129 significantly decreased, the density of dendritic spines on the distal apical dendrites of CA1 pyramidal neurons. Furthermore, western blot experiments indicated that GR55562 significantly increased, but CP93129 significantly reduced, the expression of Kalirin-7 (Kal-7), PSD95, and GluA2/3 subunits of AMPA receptors. Our results suggest that Kal-7 and Kal-7-mediatedalteration of AMPA receptor subtype expression may play crucial roles in the impact of hippocampal CA1 5-HT1BR activation on spatial memory consolidation.

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.

Effects of gravity changes on gene expression of BDNF and serotonin receptors in the mouse brain

Spaceflight entails various stressful environmental factors including microgravity. The effects of gravity changes have been studied extensively on skeletal, muscular, cardiovascular, immune and vestibular systems, but those on the nervous system are not well studied. The alteration of gravity in ground-based animal experiments is one of the approaches taken to address this issue. Here we investigated the effects of centrifugation-induced gravity changes on gene expression of brain-derived neurotrophic factor (BDNF) and serotonin receptors (5-HTRs) in the mouse brain. Exposure to 2g hypergravity for 14 days showed differential modulation of gene expression depending on regions of the brain. BDNF expression was decreased in the ventral hippocampus and hypothalamus, whereas increased in the cerebellum. 5-HT1BR expression was decreased in the cerebellum, whereas increased in the ventral hippocampus and caudate putamen. In contrast, hypergravity did not affect gene expression of 5-HT1AR, 5-HT2AR, 5-HT2CR, 5-HT4R and 5-HT7R. In addition to hypergravity, decelerating gravity change from 2g hypergravity to 1g normal gravity affected gene expression of BDNF, 5-HT1AR, 5-HT1BR, and 5-HT2AR in various regions of the brain. We also examined involvement of the vestibular organ in the effects of hypergravity. Surgical lesions of the inner ear's vestibular organ removed the effects induced by hypergravity on gene expression, which suggests that the effects of hypergravity are mediated through the vestibular organ. In summary, we showed that gravity changes induced differential modulation of gene expression of BDNF and 5-HTRs (5-HT1AR, 5-HT1BR and 5-HT2AR) in some brain regions. The modulation of gene expression may constitute molecular bases that underlie behavioral alteration induced by gravity changes.

RP5063, an atypical antipsychotic drug with a unique pharmacologic profile, improves declarative memory and psychosis in mouse models of schizophrenia

Various types of atypical antipsychotic drugs (AAPDs) modestly improve the cognitive impairment associated with schizophrenia (CIAS). RP5063 is an AAPD with a diverse and unique pharmacology, including partial agonism at dopamine (DA) D₂, D₃, D₄, serotonin (5-HT)_1A, and 5-HT_2A receptors (Rs), full agonism at α₄β₂ nicotinic acetylcholine (ACh)R (nAChR), and antagonism at 5-HT_2B, 5-HT₆, and 5-HT₇Rs. Most atypical APDs are 5-HT_2A inverse agonists. The efficacy of RP5063 in mouse models of psychosis and episodic memory were studied. RP5063 blocked acute phencyclidine (PCP)-as well as amphetamine-induced hyperactivity, indicating antipsychotic activity. Acute administration of RP5063 significantly reversed subchronic (sc)PCP-induced impairment in novel object recognition (NOR), a measure of episodic memory, but not reversal learning, a measure of executive function. Co-administration of a sub-effective dose (SED) of RP5063 with SEDs of a 5-HT₇R antagonist, a 5-HT_1BR antagonist, a 5-HT_2AR inverse agonist, or an α₄β₂ nAChR agonist, restored the ability of RP5063 to ameliorate the NOR deficit in scPCP mice. Pre-treatment with a 5-HT_1AR, a D₄R, antagonist, but not an α₄β₂ nAChR antagonist, blocked the ameliorating effect of RP5063. Further, co-administration of scRP5063 prior to each dose of PCP prevented the effect of PCP to produce a deficit in NOR for one week. RP5063, given to scPCP-treated mice for one week restored NOR for one week only. Acute administration of RP5063 significantly increased cortical DA efflux, which may be critical to some of its cognitive enhancing properties. These results indicate that RP5063, by itself, or as an adjunctive treatment has a multifaceted basis for improving some cognitive deficits associated with schizophrenia.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

5-HT6 receptor agonism facilitates emotional learning

Serotonin (5-HT) and its receptors play crucial roles in various aspects of mood and cognitive functions. However, the role of specific 5-HT receptors in these processes remains to be better understood. Here, we examined the effects of the selective and potent 5-HT6 agonist (WAY208466) on mood, anxiety and emotional learning in mice. Male C57Bl/6J mice were therefore tested in the forced swim test (FST), elevated plus-maze (EPM), and passive avoidance tests (PA), respectively. In a dose-response experiment, mice were treated intraperitoneally with WAY208466 at 3, 9, or 27 mg/kg and examined in an open field arena open field test (OFT) followed by the FST. 9 mg/kg of WAY208466 reduced immobility in the FST, without impairing the locomotion. Thus, the dose of 9 mg/kg was subsequently used for tests of anxiety and emotional learning. There was no significant effect of WAY208466 in the EPM. In the PA, mice were trained 30 min before the treatment with saline or WAY208466. Two separate sets of animals were used for short term memory (tested 1 h post-training) or long term memory (tested 24 h post-training). WAY208466 improved both short and long term memories, evaluated by the latency to enter the dark compartment, in the PA. The WAY208466-treated animals also showed more grooming and rearing in the light compartment. To better understand the molecular mechanisms and brain regions involved in the facilitation of emotional learning by WAY208466, we studied its effects on signal transduction and immediate early gene expression. WAY208466 increased the levels of phospho-Ser⁸⁴⁵-GluA1 and phospho-Ser^217/221-MEK in the caudate-putamen. Levels of phospho-Thr^202/204-Erk1/2 and the ratio mature BDNF/proBDNF were increased in the hippocampus. Moreover, WAY208466 increased c-fos in the hippocampus and Arc expression in both hippocampus and prefrontal cortex (PFC). The results indicate antidepressant efficacy and facilitation of emotional learning by 5-HT6 receptor agonism via mechanisms that promote neuronal plasticity in caudate putamen, hippocampus, and PFC.

Serotonergic modulation of glutamate neurotransmission as a strategy for treating depression and cognitive dysfunction

Monoamine-based treatments for depression have evolved greatly over the past several years, but shortcomings such as suboptimal efficacy, treatment lag, and residual cognitive dysfunction are still significant. Preclinical and clinical studies using compounds directly targeting glutamatergic neurotransmission present new opportunities for antidepressant treatment, with ketamine having a surprisingly rapid and sustained antidepressant effect that is presumably mediated through glutamate-dependent mechanisms. While direct modulation of glutamate transmission for antidepressant and cognition-enhancing actions may be hampered by nonspecific effects, indirect modulation through the serotonin (5-HT) system may be a viable alternative approach. Based on localization and function, 5-HT can modulate glutamate neurotransmission at least through the 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors, which presents a rational pharmacological opportunity for modulating glutamatergic transmission without the direct use of glutamatergic compounds. Combining one or more of these glutamate-modulating 5-HT targets with 5-HT transporter inhibition may offer new therapeutic opportunities. The multimodal compounds vortioxetine and vilazodone are examples of this approach with diverse mechanisms, and their different clinical effects will provide valuable insights into serotonergic modulation of glutamate transmission for the potential treatment of depression and associated cognitive dysfunction.

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.

Prenatal exposure to carbamazepine reduces hippocampal and cortical neuronal cell population in new-born and young mice without detectable effects on learning and memory

Pregnant women with epilepsy have to balance maternal and fetal risks associated with uncontrolled seizures against the potential teratogenic effects from antiepileptic drugs (AEDs). Carbamazepine (CBZ) is among the four most commonly used AEDs for treatment of pregnant epileptic women. We previously reported that new-born children had a decreased head circumference after in utero CBZ exposure. This study investigates how prenatal exposure of CBZ influences the number of neurons in new-born and young mouse hippocampus, amygdala and cortex cerebri. Clinical studies describe inconclusive results on if prenatal CBZ treatment influences cognition. Here we investigate this issue in mice using two well characterized cognitive tasks, the passive avoidance test and the Morris water maze test. Prenatal exposure of CBZ reduced the number of neurons (NeuN-immunoreactive cells) in the new-born mouse hippocampus with 50% compared to non-exposed mice. A reduction of neurons (20%) in hippocampus was still observed when the animals were 5 weeks old. These mice also displayed a 25% reduction of neurons in cortex cerebri. Prenatal CBZ treatment did not significantly impair learning and memory measured in the passive avoidance test and in the Morris water maze. However, these mice displayed a higher degree of thigmotaxic behaviour than the control mice. The body weight of prenatally CBZ exposed five-week old mice were lower compared to control mice not exposed to CBZ (p = 0.001). In conclusion, prenatal exposure to CBZ reduces the number of neurons dramatically in areas important for cognition such as hippocampus and cortex, without severe impairments on learning and memory. These results are in line with some clinical studies, reporting that CBZ has minor negative effects on cognition. The challenge for future studies are to segment out what possible effects a reduction of neurons could have on different types of cognition, like intellectual ability and social interaction.

Bidirectional regulation of emotional memory by 5-HT1B receptors involves hippocampal p11

Cognitive impairments are common in depression and involve dysfunctional serotonin neurotransmission. The 5-HT1B receptor (5-HT(1B)R) regulates serotonin transmission, via presynaptic receptors, but can also affect transmitter release at heterosynaptic sites. This study aimed at investigating the roles of the 5-HT(1B)R, and its adapter protein p11, in emotional memory and object recognition memory processes by the use of p11 knockout (p11KO) mice, a genetic model for aspects of depression-related states. 5-HT(1B)R agonist treatment induced an impairing effect on emotional memory in wild type (WT) mice. In comparison, p11KO mice displayed reduced long-term emotional memory performance. Unexpectedly, 5-HT(1B)R agonist stimulation enhanced memory in p11KO mice, and this atypical switch was reversed after hippocampal adeno-associated virus mediated gene transfer of p11. Notably, 5-HT(1B)R stimulation increased glutamatergic neurotransmission in the hippocampus in p11KO mice, but not in WT mice, as measured by both pre- and postsynaptic criteria. Magnetic resonance spectroscopy demonstrated global hippocampal reductions of inhibitory GABA, which may contribute to the memory enhancement and potentiation of pre- and post-synaptic measures of glutamate transmission by a 5-HT(1B)R agonist in p11KO mice. It is concluded that the level of hippocampal p11 determines the directionality of 5-HT(1B)R action on emotional memory processing and modulates hippocampal functionality. These results emphasize the importance of using relevant disease models when evaluating the role of serotonin neurotransmission in cognitive deficits related to psychiatric disorders.

Vortioxetine (Lu AA21004), a novel multimodal antidepressant, enhances memory in rats

The serotonergic system plays an important role in cognitive functions via various 5-HT receptors. Vortioxetine (Lu AA21004) in development as a novel multimodal antidepressant is a 5-HT3, 5-HT7 and 5-HT1D receptor antagonist, a 5-HT1B receptor partial agonist, a 5-HT1A receptor agonist and a 5-HT transporter (5-HTT) inhibitor in vitro. Preclinical studies suggest that 5-HT3 and 5-HT7 receptor antagonism as well as 5-HT1A receptor agonism may have a positive impact on cognitive functions including memory. Thus vortioxetine may potentially enhance memory. We investigated preclinical effects of vortioxetine (1-10mg/kg administered subcutaneously [s.c.]) on memory in behavioral tests, and on cortical neurotransmitter levels considered important in rat memory function. Contextual fear conditioning and novel object recognition tests were applied to assess memory in rats. Microdialysis studies were conducted to measure extracellular neurotransmitter levels in the rat medial prefrontal cortex. Vortioxetine administered 1h before or immediately after acquisition of contextual fear conditioning led to an increase in freezing time during the retention test. This mnemonic effect was not related to changes in pain sensitivity as measured in the hotplate test. Rats treated with vortioxetine 1h before training spent more time exploring the novel object in the novel object recognition test. In microdialysis studies of the rat medial prefrontal cortex, vortioxetine increased extracellular levels of acetylcholine and histamine. In conclusion, vortioxetine enhanced contextual and episodic memory in rat behavioral models. Further demonstration of its potential effect on memory functions in clinical settings is warranted.

Serotonin: from top to bottom

Serotonin is a monoamine neurotransmitter, which is phylogenetically conserved in a wide range of species from nematodes to humans. In mammals, age-related changes in serotonin systems are known risk factors of age-related diseases, such as diabetes, faecal incontinence and cardiovascular diseases. A decline in serotonin function with aging would be consistent with observations of age-related changes in behaviours, such as sleep, sexual behaviour and mood all of which are linked to serotonergic function. Despite this little is known about serotonin in relation to aging. This review aims to give a comprehensive analysis of the distribution, function and interactions of serotonin in the brain; gastrointestinal tract; skeletal; vascular and immune systems. It also aims to demonstrate how the function of serotonin is linked to aging and disease pathology in these systems. The regulation of serotonin via microRNAs is also discussed, as are possible applications of serotonergic drugs in aging research and age-related diseases. Furthermore, this review demonstrates that serotonin is potentially involved in whole organism aging through its links with multiple organs, the immune system and microRNA regulation. Methods to investigate these links are discussed.

5-HT6 receptor blockade differentially affects scopolamine-induced deficits of working memory, recognition memory and aversive learning in mice

RATIONALE

Blockade of 5-HT6 receptors (5-HT6R) is known to improve cognitive performances in the rodent. This improvement has been hypothesized to be the result, at least in part, of a modulation of the cholinergic neurotransmission.

OBJECTIVE

We assessed the effects of 5-HT6R blockade on selected types of memory relevant to functional deficits of ageing and neurodegenerative diseases, in mice that present a scopolamine-induced cholinergic disruption of memory.

METHOD

Following the selection of an adequate dose of scopolamine to induce cognitive deficits, we have studied the effects of the selective 5-HT6R antagonist SB-271046, alone or in combination with scopolamine, on working memory (spontaneous alternation task in the T-maze), recognition memory (place recognition) and aversive learning (passive avoidance).

RESULTS

SB-271046 alone failed to affect working memory, recognition memory and aversive learning performances. In contrast, SB-271046 was able to reverse the scopolamine-induced deficits in working memory (only at 30 mg kg⁻¹) and those of acquisition and retrieval of aversive learning (dose-dependent effect); scopolamine-induced deficits in episodic-like memory (acquisition and retrieval) were partially counteracted by 5-HT6R blockade.

CONCLUSION

The modulation between 5-HT6R and the cholinergic system appears to be predominant for working memory and aversive learning, but not for other types of memory (i.e. episodic-like memory). Interactions between 5-HT6R and alternative neurotransmission systems (i.e. glutamatergic system) should be further studied. The respective involvement of these interactions in the memory disorders related to ageing and neurodegenerative diseases is of pivotal importance regarding the possible use of 5-HT6R antagonists in the treatment of memory disorders in humans.

A subpopulation of serotonin 1B receptors colocalize with the AMPA receptor subunit GluR2 in the hippocampal dentate gyrus

The serotonin(1B) receptor (5-HT(1B)R) plays a role in cognitive processes that also involve glutamatergic neurotransmission via amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptors. Accumulating experimental evidence also highlights the involvement of 5-HT(1B)Rs in several neurological disorders. Consequently, the 5-HT(1B)R is increasingly implicated as a potential therapeutic target for intervention in cognitive dysfunction. Within the hippocampus, a brain region critical to cognitive processing, populations of pre- and post-synaptic 5-HT(1B)Rs have been identified. Thus, 5-HT(1B)Rs could have a role in the modulation of hippocampal pre- and post-synaptic conductance. Previously, we demonstrated colocalization of 5-HT(1B)Rs with the N-methyl-D-aspartate (NMDA) receptor subunit NR1 in a subpopulation of granule cell dendrites (Peddie et al. [53]). In this study, we have examined the cellular and subcellular distribution of 5-HT(1B)Rs with the AMPA receptor subunit GluR2. Of 5-HT(1B)R positive profiles, 28% displayed colocalization with GluR2. Of these, 87% were dendrites, corresponding to 41% and 10% of all 5-HT(1B)R labeled or GluR2 labeled dendrites, respectively. Dendritic labeling was both cytoplasmic and membranous but was not usually associated with synaptic sites. Colocalization within dendritic spines and axons was comparatively rare. These findings indicate that within the dentate gyrus molecular layer, dendritic 5-HT(1B)Rs are expressed predominantly on GluR2 negative granule cell processes. However, a subpopulation of 5-HT(1B)Rs is expressed on GluR2 positive dendrites. Here, it is suggested that activation of the 5-HT(1B)R may play a role in the modulation of AMPA receptor mediated conductance, further supporting the notion that the 5-HT(1B)R represents an interesting therapeutic target for modulation of cognitive function.

Methylthioninium chloride reverses cognitive deficits induced by scopolamine: comparison with rivastigmine

RATIONALE

The cholinergic system is involved in cognition as well as in age-related cognitive decline and Alzheimer disease (AD). Cholinergic enhancers ameliorate AD symptoms and represent the main current therapy for AD. MTC (Methylthioninium chloride), an antioxidant with metabolism-enhancing properties may be a novel candidate with pro-cognitive capacities.

OBJECTIVES

This study was performed: (1) to assess the pro-cognitive efficacy of MTC and establish its dose-response; (2) to compare the efficacy of MTC with rivastigmine and (3) to determine the potential for combination therapy by co-administration of MTC and rivastigmine.

METHODS

Spatial cognition of female NMRI mice was tested in a reference memory water maze task. Subjects received intra-peritoneal injections of scopolamine (0.5 mg/kg) followed by vehicle, and/or MTC and/or rivastigmine (0.15-4 mg/kg MTC; 0.1-0.5 mg/kg rivastigmine) in mono or combination treatment.

RESULTS

Scopolamine treatment prevented spatial learning in NMRI female mice and the deficit was reversed by both rivastigmine and MTC in a dose-dependent manner. Mono-therapy with high doses of rivastigmine (>0.5 mg/kg) caused severe side effects but MTC was safe up to 4 mg/kg. Co-administration of sub-effective doses of both drugs acted synergistically in reversing learning deficits and scopolamine-induced memory impairments.

CONCLUSIONS

In our model, MTC reversed the spatial learning impairment. When combined with the ChEI rivastigmine, the effect of MTC appeared to be amplified indicating that combination therapy could potentially improve not only symptoms but also contribute beneficially to neuronal metabolism by minimising side effects at lower doses.