The interaction between cannabinoids and long-term synaptic plasticity: A survey on memory formation and underlying mechanisms

Synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), is an essential phenomenon in memory formation as well as maintenance along with many other cognitive functions, such as those needed for coping with external stimuli. Synaptic plasticity consists of gradual changes in the biochemistry and morphology of pre- and postsynaptic neurons, particularly in the hippocampus. Consuming marijuana as a primary source of exocannabinoids immediately impairs attention and working memory-related tasks. Evidence regarding the effects of cannabinoids on LTP and memory is contradictory. While cannabinoids can affect a variety of specific cannabinoid receptors (CBRs) and nonspecific receptors throughout the body and brain, they exert miscellaneous systemic and local cerebral effects. Given the increasing use of cannabis, mainly among the young population, plus its potential adverse long-term effects on learning and memory processes, it could be a future global health challenge. Indeed, the impact of cannabinoids on memory is multifactorial and depends on the dosage, timing, formula, and route of consumption, plus the background complex interaction of the endocannabinoids system with other cerebral networks. Herein, we review how exogenously administrated organic cannabinoids, CBRs agonists or antagonists, and endocannabinoids can affect LTP and synaptic plasticity through various receptors in interaction with other cerebral pathways and primary neurotransmitters.

The functional connectome of 3,4-methyldioxymethamphetamine-related declarative memory impairments

The chronic intake of 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") bears a strong risk for sustained declarative memory impairments. Although such memory deficits have been repeatedly reported, their neurofunctional origin remains elusive. Therefore, we here investigate the neuronal basis of altered declarative memory in recurrent MDMA users at the level of brain connectivity. We examined a group of 44 chronic MDMA users and 41 demographically matched controls. Declarative memory performance was assessed by the Rey Auditory Verbal Learning Test and a visual associative learning test. To uncover alterations in the whole brain connectome between groups, we employed a data-driven multi-voxel pattern analysis (MVPA) approach on participants' resting-state functional magnetic resonance imaging data. Recent MDMA use was confirmed by hair analyses. MDMA users showed lower performance in delayed recall across tasks compared to well-matched controls with moderate-to-strong effect sizes. MVPA revealed a large cluster located in the left postcentral gyrus of global connectivity differences between groups. Post hoc seed-based connectivity analyses with this cluster unraveled hypoconnectivity to temporal areas belonging to the auditory network and hyperconnectivity to dorsal parietal regions belonging to the dorsal attention network in MDMA users. Seed-based connectivity strength was associated with verbal memory performance in the whole sample as well as with MDMA intake patterns in the user group. Our findings suggest that functional underpinnings of MDMA-related memory impairments encompass altered patterns of multimodal sensory integration within auditory processing regions to a functional heteromodal connector hub, the left postcentral gyrus. In addition, hyperconnectivity in regions of a cognitive control network might indicate compensation for degraded sensory processing.

Molecular imaging of the association between serotonin degeneration and beta-amyloid deposition in mild cognitive impairment

BACKGROUND

Degeneration of the serotonin system has been observed in Alzheimer's disease (AD) and in mild cognitive impairment (MCI). In transgenic amyloid mouse models, serotonin degeneration is detected prior to widespread cortical beta-amyloid (Aβ) deposition, also suggesting that serotonin degeneration may be observed in preclinical AD.

METHODS

The differences in the distribution of serotonin degeneration (reflected by the loss of the serotonin transporter, 5-HTT) relative to Aβ deposition was measured with positron emission tomography in a group of individuals with MCI and a group of healthy older adults. A multi-modal partial least squares (mmPLS) algorithm was applied to identify the spatial covariance pattern between 5-HTT availability and Aβ deposition.

RESULTS

Forty-five individuals with MCI and 35 healthy older adults were studied, 22 and 27 of whom were included in the analyses who were "amyloid positive" and "amyloid negative", respectively. A pattern of lower cortical, subcortical and limbic 5-HTT availability and higher cortical Aβ deposition distinguished the MCI from the healthy older control participants. Greater expression of this pattern was correlated with greater deficits in memory and executive function in the MCI group, not in the control group.

CONCLUSION

A spatial covariance pattern of lower 5-HTT availability and Aβ deposition was observed to a greater extent in an MCI group relative to a control group and was associated with cognitive impairment in the MCI group. The results support the application of mmPLS to understand the neurochemical changes associated with Aβ deposition in the course of preclinical AD.

Intricate role of sleep deprivation in modulating depression: focusing on BDNF, VEGF, serotonin, cortisol, and TNF-α

In this review article, we aimed to discuss intricate roles of SD in modulating depression in preclinical and clinical studies. Decades of research have shown the inconsistent effects of SD on depression, focusing on SD duration. However, inconsistent role of SD seems to be more complicated, and SD duration cannot be the only one factor. Regarding this issue, we chose some important factors involved in the effects of SD on cognitive functions and mood including brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), serotonin, cortisol, and tumor necrosis factor-alpha (TNF-α). It was concluded that SD has a wide-range of inconsistent effects on BDNF, VEGF, serotonin, and cortisol levels. It was noted that BDNF diurnal rhythm is significantly involved in the modulatory role of SD in depression. Furthermore, the important role of VEGF in blood-brain barrier permeability which is involved in modulating depression was discussed. It was also noted that there is a negative correlation between cortisol and BDNF that modulates depression. Eventually, it was concluded that TNF-α regulates sleep/wake cycle and is involved in the vulnerability to cognitive and behavioral impairments following SD. TNF-α also increases the permeability of the blood-brain barrier which is accompanied by depressive behavior. In sum, it was suggested that future studies should focus on these mechanisms/factors to better investigate the reasons behind intricate roles of SD in modulating depression.

Participation of Hippocampal 5-HT5A, 5-HT6 and 5-HT7 Serotonin Receptors on the Consolidation of Social Recognition Memory

Social recognition is the ability of animals to identify and recognize a conspecific. The consolidation of social stimuli in long-term memory is crucial for the establishment and maintenance of social groups, reproduction and species survival. Despite its importance, little is known about the circuitry and molecular mechanisms involved in the social recognition memory (SRM). Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator, which plays a key role in learning and memory. Focusing on the more recently described 5-HT receptors, we investigated in the CA1 region of the dorsal hippocampus the participation of 5-HT_5A, 5-HT₆ and 5-HT₇ receptors in the consolidation of SRM. Male Wistar rats cannulated in CA1 were subjected to a social discrimination task. In the sample phase the animals were exposed to a juvenile conspecific for 1 h. Immediately after, they received different pharmacological treatments. Twenty-four hours later, they were submitted to a 5 min retention test in the presence of the previously presented juvenile (familiar) and a novel juvenile. The animals that received infusions of 5-HT_5A receptor antagonist SB-699551 (10 µg/µL), 5-HT₆ receptor agonist WAY-208466 (0.63 µg/µL) or 5-HT₇ receptor agonist AS-19 (5 µg/µL) intra-CA1 were unable to recognize the familiar juvenile. This effect was blocked by the coinfusion of WAY-208466 plus 5-HT₆ receptor antagonist SB-271046 (10 µg/µL) or AS-19 plus 5-HT₇ receptor antagonist SB-269970 (5 µg/µL). The present study helps to clarify the neurobiological functions of the 5-HT receptors more recently described and extends our knowledge about mechanisms underlying the SRM.

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.

Forgetting Unwanted Memories: Active Forgetting and Implications for the Development of Psychological Disorders

Intrusive memories are a common feature of many psychopathologies, and suppression-induced forgetting of unwanted memories appears as a critical ability to preserve mental health. In recent years, biological and cognitive studies converged in revealing that forgetting is due to active processes. Recent neurobiological studies provide evidence on the active role of main neurotransmitter systems in forgetting, suggesting that the brain actively works to suppress retrieval of unwanted memories. On the cognitive side, there is evidence that voluntary and involuntary processes (here termed "intentional" and "incidental" forgetting, respectively) contribute to active forgetting. In intentional forgetting, an inhibitory control mechanism suppresses awareness of unwanted memories at encoding or retrieval. In incidental forgetting, retrieval practice of some memories involuntarily suppresses the retrieval of other related memories. In this review we describe recent findings on deficits in active forgetting observed in psychopathologies, like post-traumatic stress disorder, depression, schizophrenia, and obsessive-compulsive disorder. Moreover, we report studies in which the role of neurotransmitter systems, known to be involved in the pathogenesis of mental disorders, has been investigated in active forgetting paradigms. The possibility that biological and cognitive mechanisms of active forgetting could be considered as hallmarks of the early onset of psychopathologies is also discussed.

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.

Cell-to-Cell Communication in Learning and Memory: From Neuro- and Glio-Transmission to Information Exchange Mediated by Extracellular Vesicles

Most aspects of nervous system development and function rely on the continuous crosstalk between neurons and the variegated universe of non-neuronal cells surrounding them. The most extraordinary property of this cellular community is its ability to undergo adaptive modifications in response to environmental cues originating from inside or outside the body. Such ability, known as neuronal plasticity, allows long-lasting modifications of the strength, composition and efficacy of the connections between neurons, which constitutes the biochemical base for learning and memory. Nerve cells communicate with each other through both wiring (synaptic) and volume transmission of signals. It is by now clear that glial cells, and in particular astrocytes, also play critical roles in both modes by releasing different kinds of molecules (e.g., D-serine secreted by astrocytes). On the other hand, neurons produce factors that can regulate the activity of glial cells, including their ability to release regulatory molecules. In the last fifteen years it has been demonstrated that both neurons and glial cells release extracellular vesicles (EVs) of different kinds, both in physiologic and pathological conditions. Here we discuss the possible involvement of EVs in the events underlying learning and memory, in both physiologic and pathological conditions.

Anatomical and behavioral impact of a lentiviral tool tapping onto hippocampal serotonin reuptake in rats

We aimed at the further characterization of rats in which SERT gene silencing was achieved by hippocampal injection of a lentiviral vector, carrying three si-RNA to block SERT mRNA at 66% of normal levels. Improved self-control and reduced restlessness were already demonstrated in these rats. Present further studies consisted of male adult rats, bilaterally inoculated within the hippocampus; control rats received lentivirus particles inactivated with heat. Both groups were maintained in isolation for 5 months, starting from inoculation. Neurochemical changes were studied by proton magnetic resonance spectroscopy (1H-MRS): we found increased hippocampal viability and bioenergetic potential; however, rats showed a behaviorally depressive pattern, also characterized by enhanced affiliation. Based on the extent of such effects, the whole lenti-SERT group was divided into two subgroups, termed intermediate- and extreme- phenotype profiles. While all rats had a widespread modification within dorsal/ventral striatum, amygdala, and hypothalamus, only the former subgroup showed an involvement of Raphé medialis, while, for the latter subgroup, an increase of SERT within hippocampus was unexpectedly caused. Within the less-affected "intermediate" rats, hippocampal 5-HT7 receptors were down-modulated, and also similarly within substantia nigra, septum, and neocortex. This picture demonstrates that additional rather than fewer neurobiological changes accompany a lower phenotypic expression. Overall, tapping hippocampal SERT affected the balance between habits versus strategies of coping by promoting morphogenetic processes indicative of a serotonergic fiber plasticity. Supplementary studies about serotonergic dynamics and neurogenesis within fronto-striatal circuits are needed.

No interaction between rivastigmine and citalopram on memory and novelty processing in healthy human volunteers

BACKGROUND

Animal literature suggests an interaction between acetylcholine and serotonin on cognitive functions.

AIMS

The aim of the current study was to assess whether both neurotransmitters interact during memory and novelty processing in humans.

METHODS

We tested the interaction between acetylcholine and serotonin on cognitive functions in healthy volunteers by means of treatment with rivastigmine and citalopram, respectively.

RESULTS

The main result of the study showed that during the verbal learning task participants significantly recalled fewer words after citalopram treatment than after rivastigmine or placebo during both the immediate and delayed recall tasks. Rivastigmine was not able to reverse the impairing effect of citalopram.

CONCLUSIONS

This finding is in line with previous studies in which we manipulated acetylcholine and serotonin in different manners. Taken together, these studies in humans do not support the notion from animal studies that these two neurotransmitters interact on cognitive functions.

Activity of Serotonin 5-HT1A Receptor Biased Agonists in Rat: Anxiolytic and Antidepressant-like properties

Although serotonin 5-HT_1A receptors constitute attractive therapeutic targets, there is a lack of potential clinical candidates that have a high degree of selectivity and full agonist efficacy. Recently, novel 5-HT_1A receptor "biased agonists" F15599 (also known as NLX-101) and F13714 have been reported that exhibit distinctive properties for in vitro signaling, neurochemical, electrophysiological effects, and in brain imaging. The present study characterized their effects in rat models of anxiety (elevated plus-maze, EPM, and Vogel tests), in depressive-like behavior (forced swim test), and on the induction of the three serotonergic behaviors (forepaw treading, flat body posture, and lower lip retraction). The prototypical 5-HT_1A receptor ligands (±)8-OH-DPAT and buspirone were tested as comparators. In the elevated plus-maze, F15599, F13714, and (±)8-OH-DPAT dose-dependently increased the amount and percentage of time spent in the open arms with minimal effective doses (MED) of 5 mg/kg p.o., 2.5 mg/kg p.o. and 1.25 mg/kg s.c., respectively. The effects of the three agonists were abolished by pretreatment with the selective 5-HT_1A receptor antagonist, WAY100635 (0.63 mg/kg s.c.). Buspirone did not show significant activity in the EPM. In contrast, in the Vogel test only buspirone was active, significantly increasing the number of licks and shocks accepted (active dose: 1.25 mg/kg s.c.). However, WAY100635 failed to reverse the effects of buspirone in this test, suggesting that they were not 5-HT_1A receptor-mediated. In the forced swim test, F15599, F13714, and (±)8-OH-DPAT were potently active, abolishing immobility (MED: 0.63 mg/kg p.o., 0.63 mg/kg p.o. and 0.16 mg/kg s.c., respectively). Buspirone was not active. In measures of serotonergic behavior, F13714 and (±)8-OH-DPAT robustly elicited all three signs of serotonergic behaviors, whereas F15599 and buspirone elicited only lower-lip retraction. Taken together, these observations highlight the distinct profiles of activity of 5-HT_1A agonists and suggest that the novel biased agonist F15599 combines pronounced activity in a test of anxiety (elevated plus-maze) with potent antidepressant-like effects and low propensity to induce serotonergic behaviors. These data suggest that selective biased agonists could constitute promising pharmacotherapeutics for mood disorders.

Copolymer-1 enhances cognitive performance in young adult rats

Cognitive impairment is a dysfunction observed as a sequel of various neurodegenerative diseases, as well as a concomitant element in the elderly stages of life. In clinical settings, this malfunction is identified as mild cognitive impairment. Previous studies have suggested that cognitive impairment could be the result of a reduction in the expression of brain-derived neurotrophic factor (BDNF) and/or immune dysfunction. Copolymer-1 (Cop-1) is an FDA-approved synthetic peptide capable of inducing the activation of Th2/3 cells, which are able to release BDNF, as well as to migrate and accumulate in the brain. In this study, we evaluated the effect of Cop-1 immunization on improvement of cognition in adult rats. For this purpose, we performed four experiments. We evaluated the effect of Cop-1 immunization on learning/memory using the Morris water maze for spatial memory and autoshaping for associative memory in 3- or 6-month-old rats. BDNF concentrations at the hippocampus were determined by ELISA. Cop-1 immunization induced a significant improvement of spatial memory and associative memory in 6-month-old rats. Likewise, Cop-1 improved spatial memory and associative memory when animals were immunized at 3 months and evaluated at 6 months old. Additionally, Cop-1 induced a significant increase in BDNF levels at the hippocampus. To our knowledge, the present investigation reports the first instance of Cop-1 treatment enhancing cognitive function in normal young adult rats, suggesting that Cop-1 may be a practical therapeutic strategy potentially useful for age- or disease-related cognitive impairment.

Effects of 5-HT5A receptor blockade on amnesia or forgetting

Previously the effects (0.01-3.0 mg/kg) of post-training SB-699551 (a 5-HT_5A receptor antagonist) were reported in the associative learning task of autoshaping, showing that SB-699551 (0.1 mg/kg) decreased lever-press conditioned responses (CR) during short-term (STM; 1.5-h) or (3.0 mg/kg) long-term memory (LTM; 24-h); relative to the vehicle animals. Moreover, as pro-cognitive efficacy of SB-699551 was reported in the ketamine-model of schizophrenia. Hence, firstly aiming improving performance (conditioned response, CR), in this work autoshaping lever-press vs. nose-poke response was compared; secondly, new set of animals were randomly assigned to SB-699551 plus forgetting or amnesia protocols. Results show that the nose-poke operandum reduced inter-individual variance, increased CR and produced a progressive CR until 48-h. After one week of no training/testing sessions (i.e., interruption of 216 h), the forgetting was observed; i.e., the CR% of control-saline group significantly decreased. In contrast, SB-699551 at 0.3 and 3.0 mg/kg prevents forgetting. Additionally, as previously reported the non-competitive NMDA receptor antagonist dizocilpine (0.2 mg/kg) or the non-selective cholinergic antagonist scopolamine (0.3 mg/kg) decreased CR in STM. SB-699551 (0.3 mg/kg) alone also produced amnesia-like effect. Co-administration of SB-699551-dizocilpine or SB-699551-scopolamine reversed the SB-699551 induced-amnesic effects in LTM (24-h). Nose-poke seems to be a reliable operandum. The anti-amnesic and anti-forgetting mechanisms of amnesic SB-699551-dose remain unclear. The present findings are consistent with the notion that low doses of 5-HT_5A receptor antagonists might be useful for reversing memory deficits associated to forgetting and amnesia. Of course, further experiments are necessary.