5-HT7 receptor stimulation and blockade: a therapeutic paradox about memory formation and amnesia

Cognitive Effects of Lurasidone and Cariprazine: A Mini Systematic Review

Cognitive deficits are associated with schizophrenia and show a progressive worsening, often being unresponsive to treatment. New antipsychotic molecules acting as antagonist at the serotoninergic 5-hydroxytryptamine receptor 7 (e.g. lurasidone) or partial agonists at dopamine D3 receptor (e.g. cariprazine) could have an impact on cognition in this patient group. The aim of the systematic review is to explore the efficacy of lurasidone and cariprazine in improving cognition in both animal models and human studies. The following terms: (lurasidone AND cognit*) OR (cariprazine AND cognit*) were searched in Web of Science from inception to December 2021. We included all studies that assessed changes in cognitive function after treatment with cariprazine or lurasidone. Of 201 selected articles, 36 were included. Twenty-four articles used animal models (rats, mice and marmosets), five evaluating the effects of cariprazine and 19 the effects of lurasidone. Twelve articles were clinical studies (cariprazine n = 2; lurasidone n = 10). In both animal and human studies lurasidone showed a greater efficacy on cognitive performance compared to placebo, quetiapine, ziprasidone or treatmentas- usual. Cariprazine was superior to other antipsychotics in improving cognitive functions in both animal and human studies. The cognitive effect of lurasidone could be explained by its potent antagonism at the 5-HT7 receptors combined with partial agonism at 5-HT1A receptors. The pro-cognitive effect of cariprazine is probably explained by its very high affinity for D3 receptors. Head-to-head studies comparing lurasidone and cariprazine are needed to establish the "first-choice" treatment for cognitive dysfunction associated with schizophrenia.

Pathophysiology of Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease leading to dementia worldwide. While neuritic plaques consisting of aggregated amyloid-beta proteins and neurofibrillary tangles of accumulated tau proteins represent the pathophysiologic hallmarks of AD, numerous processes likely interact with risk and protective factors and one's culture to produce the cognitive loss, neuropsychiatric symptoms, and functional impairments that characterize AD dementia. Recent biomarker and neuroimaging research has revealed how the pathophysiology of AD may lead to symptoms, and as the pathophysiology of AD gains clarity, more potential treatments are emerging that aim to modify the disease and relieve its burden.

Apathy, anhedonia and cognitive dysfunction: common symptoms of depression and neurological disorders

Depression is one of the most common mental disorders in neurological practice. Among other symptoms of depression, a symptom complex represented by apathy, anhedonia, and cognitive impairment plays an important role. This review presents the clinical characteristics of the symptoms described above and discusses modern neurochemical and neuroimaging concepts of their pathogenesis. The problem of pathogenetically substantiated therapy of depression with a predominance of apathy, anhedonia and cognitive impairment is discussed. Fundamental and clinical arguments are presented in favor of the high efficacy of vortioxetine in depression with a predominance of apathy, anhedonia, and cognitive impairment. 

The role of 5-HT7R in the memory impairment of mice induced by long-term isoflurane anesthesia

General anesthesia is widely utilized in the clinic for surgical and diagnostic procedures. However, growing evidence suggests that anesthetic exposure may affect cognitive function negatively. Unfortunately, little is known about the underlying mechanisms and efficient prevention and therapeutic strategies for the anesthesia-induced cognitive dysfunction. 5-HT₇R, a serotonin receptor family member, is functionally associated with learning and memory. It has recently become a potential therapeutic target in various neurological diseases as its ligands have a wide range of neuropharmacological effects. However, it remains unknown the role of 5-HT₇R in the long-term isoflurane anesthesia-induced memory impairment and whether prior activation or blockade of 5-HT₇R before anesthesia has modulating effects on this memory impairment. In this study, 5-HT₇R selective agonist LP-211 and 5-HT₇R selective antagonist SB-269970 were pretreated intraperitoneally to mice before anesthesia; their effects on the cognitive performance of mice were assessed using fear conditioning test and novel object recognition test. Furthermore, the transcriptional level of 5-HT₇R in the hippocampus was detected using qRT-PCR, and proteomics was conducted to probe the underlying mechanisms. As a result, long-term exposure to isoflurane anesthesia caused memory impairment and an increase in hippocampal 5-HT₇R mRNA expression, which could be attenuated by SB-269970 pretreatment but not LP-211pretreatment. According to the proteomics results, the antiamnestic effect of SB-269970 pretreatment was probably attributed to its action on the gene expression of Slc6a11, Itpka, Arf3, Srcin1, and Epb41l2, and synapse organization in the hippocampus. In conclusion, 5-HT₇R is involved in the memory impairment induced by long-term isoflurane anesthesia, and the prior blockade of 5-HT₇R with SB-269970 protects the memory impairment. This finding may help to improve the understanding of the long-term isoflurane anesthesia-induced memory impairment and to construct potential preventive and therapeutic strategies for the adverse effects after long-term isoflurane exposure.

5-HT2A, 5-HT1B/D, 5HT3 and 5-HT7 receptors as mediators of serotonin-induced direct contractile response of bovine airway smooth muscle

Background: Serotonin (5-hydroxytryptamine; 5-HT) performs a variety of functions in the body including the modulation of muscle tone in respiratory airways. Several studies indicate a possible role of 5-HT in the pathophysiology of bronchial hyperresponsiveness. However, the receptors and the molecular mechanisms by which 5-HT acts on airway smooth muscle (ASM) continue to be controversial. Most of the evidence suggests the participation of different subtypes of receptors in an indirect response. This study supports the proposal that 5-HT directly contracts ASM and characterizes pharmacologically the subtypes of serotonergic receptors involved. The characterization was carried out by using selective antagonists in an organ bath model allowing study of the smooth muscle of segments of bovine trachea. Results: The results obtained show that 5-HT_2A receptors are the main mediators of the direct contractile response of bovine ASM, with the cooperation of the 5-HT₇, 5-HT₃ and 5-HT_1B/D receptors. Also, it was observed that the muscle response to serotonin is developed more slowly and to a lesser extent in comparison with the response to cholinergic stimulation. Conclusion: Overall, the receptors that mediate the direct serotonergic contraction of the smooth muscle of the bovine trachea are 5-HT_2A, 5-HT₇, 5-HT₃ and 5-HT_1B/D receptors.

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.

5-HT7 receptors in Alzheimer's disease

Even though the involvement of serotonin (5-hydroxytryptamine; 5-HT) and its receptors in Alzheimer's disease (AD) is widely accepted, data on the expression and the role of 5-HT₇ receptors in AD is relatively limited. Therefore, the objective of the present work was to study the expression of serotonergic 5-HT₇ receptors in postmortem samples of AD brains and correlate it with neurotransmitter levels, cognition and behavior. The study population consisted of clinically well-characterized and neuropathologically confirmed AD patients (n = 42) and age-matched control subjects (n = 18). Reverse-transcription quantitative polymerase chain reaction (RT-qPCR) and high-performance liquid chromatography were performed on Brodmann area (BA) 7, BA10, BA22, BA24, hippocampus, amygdala, thalamus and cerebellum to measure mRNA levels of 5-HT₇ receptors (HTR7), as well as the concentrations of various monoamine neurotransmitters and their metabolites. Decreased levels of HTR7 mRNA were observed in BA10. A significant association was observed between HTR7 levels in BA10 and BEHAVE-AD cluster B (hallucinations) (rs(28) = 0.444, P < 0.05). In addition, a negative correlation was observed between HTR7 levels in BA10 and both MHPG concentrations in this brain region (rs(45) = -0.311; P < 0.05), and DOPAC levels in the amygdala (rs(42) = -0.311; P < 0.05). Quite surprisingly, no association was found between HTR7 levels and cognitive status. Altogether, this study supports the notion of the involvement of 5-HT₇ receptors in psychotic symptoms in AD, suggesting the interest of testing antagonist acting at this receptor to specifically treat psychotic symptoms in this illness.

5-HT7 receptor activation rescues impaired synaptic plasticity in an autistic-like rat model induced by prenatal VPA exposure

Autism spectrum disorder (ASD) is a severe life-long neuropsychiatric disorder. Alterations and imbalance of several neurochemical systems may be involved in ASD pathophysiology, of them, serotonergic neurotransmission dysfunction and deficiency may underlie behavioral abnormalities associated with ASD. However, the functional importance of serotonergic receptors, particularly 5HT7 receptors in ASD pathology remains poorly defined. Serotonin receptor subtype 7 (5-HT7R) plays a direct regulatory role in the development and also for the mature function of the brain, therefore, further studies are necessary to elucidate the role of these receptors in the etiology of autism. To address this issue, we combined here behavioral, electrophysiological methods to further characterize the contribution of 5-HT7Rs in the prenatal valproic acid (VPA) exposure-induced impairment in synaptic plasticity and their impact on the associated behavioral changes. This may help to unravel the underlying cellular mechanisms involved in ASD and can lead to new treatment and/or prevention therapies based on the role of the serotonergic system for autism. Findings revealed that compared to control, autistic-like offspring showed increased anxiety-like behavior, reduced social interaction, decreased locomotor activity, and impaired identification of the novel object. However, administration of 5-HT7Rs agonist, LP-211, for 7 consecutive days before testing from postnatal day 21 to 27 reversed all behavioral deficits induced by prenatal exposure to VPA in offspring. Also, both short-term depression and long-term potentiation were impaired in the autistic-like pups, but activation of 5-HT7Rs rescued the LTP impairment in the autistic-like group so that there was no significant difference between the two groups. Blockade of 5-HT7Rs caused LTP impairment following HFS in the autistic-like group. Besides, there was a significant difference in LTD induction following SB-269970 application between the control and the autistic-like groups measured at first 10 min following TPS. Moreover, both the number and the size of retrograde fast blue-labelled neurons in the raphe nuclei were reduced. Overall, these results provide for the first time, as far as we know, functional evidence for the restorative role of 5-HT7Rs activation against prenatal VPA exposure induced behavioral deficits and hippocampal synaptic plasticity impairment. Therefore, these receptors could be a potential and promising pharmacotherapy target for the treatment of autism.

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.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Alternative splicing in serotonergic system: Implications in neuropsychiatric disorders

BACKGROUND

The serotonergic system is a key component of physiological brain function and is essential for proper neurological activity. Numerous neuropsychiatric disorders are associated with deregulation of the serotonergic system. Accordingly, many pharmacological treatments are focused on modulation of this system. While providing a promising line of therapeutic moderation, these approaches may be complicated due to the presence of alternative splicing events for key genes in this pathway. Alternative splicing is a co-transcriptional process by which different mRNA transcripts can be produced from the same gene. These different isoforms may have diverse activities and functions, and their relative balance is often critical for the maintenance of homeostasis. Alternative splicing greatly increases the production of proteins, augmenting cell plasticity, and provides an important control point for regulation of gene expression.

AIM

The objective of this narrative review is to discuss the potential impact of alternative splicing of different components of the serotonergic system and speculate on their involvement in several neuropsychiatric disorders.

CONCLUSIONS

The specific role of each isoform in disease and their relative activities in the signalling pathways involved are yet to be determined. We need to gain a better understanding of the basis of alternative isoforms of the serotonergic system in order to fully understand their impact and be able to develop new effective pharmacological isoform-specific targets.

Cross state-dependency of learning between 5-HT1A and/or 5-HT7 receptor agonists and muscimol in the mouse dorsal hippocampus

BACKGROUND

Dysfunction of the serotonergic and GABAergic systems in cognitive disorders has been revealed. Understanding the neurobiological mechanisms of drug-associated learning and memory formation may help treatment of cognitive disorders.

AIMS

The aim of the present study was to investigate: 1) 8-OH-DPAT (5-HT1A agonist), AS19 (5-HT7 agonist) and muscimol (GABA-A agonist) on memory retrieval and state of memory, 2) cross state-dependent learning between 8-OH-DPAT and/or AS19 and muscimol.

METHODS

The dorsal hippocampal CA1 regions of adult male NMRI mice were bilaterally cannulated, and all drugs were microinjected into the intended sites of injection. A single-trial step-down inhibitory avoidance task was used for the evaluation of memory retrieval and state of memory.

RESULTS

Post-training and/or pre-test 8-OH-DPAT, AS19 and muscimol induced amnesia. Pre-test microinjection of the same doses of 8-OH-DPAT, AS19 and muscimol reversed the post-training 8-OH-DPAT-, AS19- and muscimol-induced amnesia, respectively. This event has been named state-dependent learning (SDL). The amnesia induced by 8-OH-DPAT was reversed by muscimol and induced 8-OH-DPAT SDL. The amnesia induced by muscimol was reversed by 8-OH-DPAT and induced muscimol SDL. The amnesia induced by AS19 was reversed by muscimol and induced AS19 SDL. The amnesia induced by muscimol was reversed by AS19 and induced muscimol SDL. Pre-test administration of a selective GABA-A receptor antagonist, bicuculline, 5 min before muscimol, 8-OH-DPAT and AS19 dose-dependently inhibited muscimol-, 8-OH-DPAT- and AS19-induced SDL, respectively.

CONCLUSIONS

The results strongly revealed a cross SDL among 8-OH-DPAT and/or AS19 and muscimol in the dorsal hippocampal CA1 regions.

Neuropharmacological and cognitive effects of Bacopa monnieri (L.) Wettst - A review on its mechanistic aspects

Bacopa monnieri (L.) - (BM) is a perennial, creeping herb which is widely used in traditional ayurvedic medicine as a neural tonic to improve intelligence and memory. Research into the biological effects of this plant has burgeoned in recent years, promising its neuroprotective and memory boosting ability among others. In this context, an extensive literature survey allows an insight into the participation of numerous signaling pathways and oxidative mechanism involved in the mitigation of oxidative stress, along with other indirect mechanisms modulated by bioactive molecules of BM to improve the cognitive action by their synergistic potential and cellular multiplicity mechanism. This multi-faceted review describes the novel mechanisms that underlie the unfounded but long flaunted promises of BM and thereby direct a way to harness this acquired knowledge to develop innovative approaches to manipulate its intracellular pathways.

Fluorinated indole-imidazole conjugates: Selective orally bioavailable 5-HT7 receptor low-basicity agonists, potential neuropathic painkillers

The 5-HT₇ receptor has recently gained much attention due to its involvement in multiple physiological functions and diseases. The insufficient quality of the available molecular probes prompted design of fluorinated 3-(1-alkyl-1H-imidazol-5-yl)-1H-indoles as a new generation of selective 5-HT₇ receptor agonists. A potent and drug-like agonist, 3-(1-ethyl-1H-imidazol-5-yl)-5-iodo-4-fluoro-1H-indole (AGH-192, 35, K_{i 5-HT7R} = 4 nM), was identified by optimizing the halogen bond formation with Ser5.42 as the supposed partner. The compound was characterized by excellent water solubility, high selectivity over related CNS targets, high metabolic stability, oral bioavailability and low cytotoxicity. Rapid absorption into the blood, medium half-life and a high peak concentration in the brain C_max = 1069 ng/g were found after i.p. (2.5 mg/kg) administration in mice. AGH-192 may thus serve as the long-sought tool compound in the study of 5-HT₇ receptor function, as well as a potential analgesic, indicated by the antinociceptive effect observed in a mouse model of neuropathic pain.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

AVN-101: A Multi-Target Drug Candidate for the Treatment of CNS Disorders

Lack of efficacy of many new highly selective and specific drug candidates in treating diseases with poorly understood or complex etiology, as are many of central nervous system (CNS) diseases, encouraged an idea of developing multi-modal (multi-targeted) drugs. In this manuscript, we describe molecular pharmacology, in vitro ADME, pharmacokinetics in animals and humans (part of the Phase I clinical studies), bio-distribution, bioavailability, in vivo efficacy, and safety profile of the multimodal drug candidate, AVN-101. We have carried out development of a next generation drug candidate with a multi-targeted mechanism of action, to treat CNS disorders. AVN-101 is a very potent 5-HT₇ receptor antagonist (Ki = 153 pM), with slightly lesser potency toward 5-HT₆, 5-HT_2A, and 5HT-_2C receptors (Ki = 1.2–2.0 nM). AVN-101 also exhibits a rather high affinity toward histamine H1 (Ki = 0.58 nM) and adrenergic α2A, α2B, and α2C (Ki = 0.41–3.6 nM) receptors. AVN-101 shows a good oral bioavailability and facilitated brain-blood barrier permeability, low toxicity, and reasonable efficacy in animal models of CNS diseases. The Phase I clinical study indicates the AVN-101 to be well tolerated when taken orally at doses of up to 20 mg daily. It does not dramatically influence plasma and urine biochemistry, nor does it prolong QT ECG interval, thus indicating low safety concerns. The primary therapeutic area for AVN-101 to be tested in clinical trials would be Alzheimer’s disease. However, due to its anxiolytic and anti-depressive activities, there is a strong rational for it to also be studied in such diseases as general anxiety disorders, depression, schizophrenia, and multiple sclerosis.

Low-basicity 5-HT7 Receptor Agonists Synthesized Using the van Leusen Multicomponent Protocol

A series of 5-aryl-1-alkylimidazole derivatives was synthesized using the van Leusen multicomponent reaction. The chemotype is the first example of low-basicity scaffolds exhibiting high affinity for 5-HT₇ receptor together with agonist function. The chosen lead compounds 3-(1-ethyl-1H-imidazol-5-yl)-5-iodo-1H-indole (AGH-107, 1o, K_{i 5-HT7} = 6 nM, EC₅₀ = 19 nM, 176-fold selectivity over 5-HT_1AR) and 1e (5-methoxy analogue, K_{i 5-HT7} = 30 nM, EC₅₀ = 60 nM) exhibited high selectivity over related CNS targets, high metabolic stability and low toxicity in HEK-293 and HepG2 cell cultures. A rapid absorption to the blood, high blood-brain barrier permeation and a very high peak concentration in the brain (C_max = 2723 ng/g) were found for 1o after i.p. (5 mg/kg) administration in mice. The compound was found active in novel object recognition test in mice, at 0.5, 1 and 5 mg/kg. Docking to 5-HT₇R homology models indicated a plausible binding mode which explain the unusually high selectivity over the related CNS targets. Halogen bond formation between the most potent derivatives and the receptor is consistent with both the docking results and SAR. 5-Chlorine, bromine and iodine substitution resulted in a 13, 27 and 89-fold increase in binding affinities, respectively, and in enhanced 5-HT_1AR selectivity.

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

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

Effects of the selective 5-HT7 receptor antagonist SB-269970 on premature responding in the five-choice serial reaction time test in rats

The antagonists of serotonin 5-HT7 receptors have been demonstrated to ameliorate cognitive impairments in pharmacological animal models of schizophrenia that involve blockade of N-methyl-D-aspartate receptors (NMDARs). The administration of NMDAR antagonists evokes a broad range of cognitive deficits, including a loss of impulse control. The involvement of 5-HT7 receptors in the modulation of impulsivity has been recently suggested but has not been studied in great detail. The aim of the present study was to examine the effect of a selective 5-HT7 receptor antagonist SB-269970 on a measure of impulsive action, i.e., premature responding on the five-choice serial reaction time task (5-CSRTT) in rats. The antagonist of 5-HT2A receptor M100,907 was used as a positive control. The efficacies of both compounds were assessed in conditions of increased impulsivity that were produced by the administration of the NMDAR antagonist MK-801 or/and non-drug stimuli, i.e., using variable inter-trial intervals (vITIs). To examine the general ability of SB-269970 to counteract the MK-801-induced impairments, a discrete paired-trial delayed alternation task in a T-maze was employed. MK-801 significantly increased the number of premature responses in 5-CSRTT, and this effect was abolished by the administration of M100,907 (0.5 mg/kg) and SB-269970 (1 mg/kg). In addition, M100,907, but not SB-269970, reduced premature responding in the prolonged ITI trials. Both M100,907 and SB-269970 attenuated MK-801-induced working memory impairment in a T-maze. The present study demonstrated the efficacy of SB-269970 against MK-801-induced premature responding in the 5-CSRTT. This anti-impulsive action may offer additional benefits to the cognitive-enhancing effects of pharmacological blockade of 5-HT7 receptors.

Targeting the Serotonin 5-HT7 Receptor in the Search for Treatments for CNS Disorders: Rationale and Progress to Date

The 5-HT7 (5-hydroxytryptamine 7, serotonin 7) receptor is one of the most recently identified members of the serotonin receptor family. Pharmacological tools, including selective antagonists and, more recently, agonists, along with 5-HT7 receptor (5-HT7R) knock-out mice have revealed the involvement of this receptor in central nervous system processes. Its well-established role in controlling body temperature and regulating sleep and circadian rhythms has implicated this receptor in mood disorders. Thus, the 5-HT7R has gained much attention as a possible target for the treatment of depression. Although preclinical data support the antidepressant-like actions of 5-HT7R antagonists, their clinical efficacy has not been yet established. Other evidence has implicated the 5-HT7R in learning and memory. Preclinical findings suggest that blockade of this receptor may be beneficial against schizophrenia-like cognitive deficits. Other possible indications include nociception, epilepsy, migraine, autism spectrum disorders, and Rett Syndrome. However, the question is whether the beneficial effects may be achieved by activation or blockade of 5-HT7Rs. Hence, this review briefly summarises the recent findings on the role of 5-HT7Rs and their ligands in CNS disorders.

Persistent modification of forebrain networks and metabolism in rats following adolescent exposure to a 5-HT7 receptor agonist

RATIONALE

The serotonin 7 receptor (5-HT7-R) is part of a neuro-transmission system with a proposed role in neural plasticity and in mood, cognitive or sleep regulation.

OBJECTIVES

We investigated long-term consequences of sub-chronic treatment, during adolescence (43-45 to 47-49 days old) in rats, with a novel 5-HT7-R agonist (LP-211, 0 or 0.250 mg/kg/day).

METHODS

We evaluated behavioural changes as well as forebrain structural/functional modifications by in vivo magnetic resonance (MR) in a 4.7 T system, followed by ex vivo histology.

RESULTS

Adult rats pre-treated during adolescence showed reduced anxiety-related behaviour, in terms of reduced avoidance in the light/dark test and a less fragmented pattern of exploration in the novel object recognition test. Diffusion tensor imaging (DTI) revealed decreased mean diffusivity (MD) in the amygdala, increased fractional anisotropy (FA) in the hippocampus (Hip) and reduced axial (D||) together with increased radial (D⊥) diffusivity in the nucleus accumbens (NAcc). An increased neural dendritic arborization was confirmed in the NAcc by ex vivo histology. Seed-based functional MR imaging (fMRI) identified increased strength of connectivity within and between "limbic" and "cortical" loops, with affected cross-correlations between amygdala, NAcc and Hip. The latter displayed enhanced connections through the dorsal striatum (dStr) to dorso-lateral prefrontal cortex (dl-PFC) and cerebellum. Functional connection also increased between amygdala and limbic elements such as NAcc, orbito-frontal cortex (OFC) and hypothalamus. MR spectroscopy (1H-MRS) indicated that adolescent LP-211 exposure increased glutamate and total creatine in the adult Hip.

CONCLUSIONS

Persistent MR-detectable modifications indicate a rearrangement within forebrain networks, accounting for long-lasting behavioural changes as a function of developmental 5-HT7-R stimulation.

The serotonin receptor 7 and the structural plasticity of brain circuits

Serotonin (5-hydroxytryptamine, 5-HT) modulates numerous physiological processes in the nervous system. Together with its function as neurotransmitter, 5-HT regulates neurite outgrowth, dendritic spine shape and density, growth cone motility and synapse formation during development. In the mammalian brain 5-HT innervation is virtually ubiquitous and the diversity and specificity of its signaling and function arise from at least 20 different receptors, grouped in 7 classes. Here we will focus on the role 5-HT7 receptor (5-HT7R) in the correct establishment of neuronal cytoarchitecture during development, as also suggested by its involvement in several neurodevelopmental disorders. The emerging picture shows that this receptor is a key player contributing not only to shape brain networks during development but also to remodel neuronal wiring in the mature brain, thus controlling cognitive and emotional responses. The activation of 5-HT7R might be one of the mechanisms underlying the ability of the CNS to respond to different stimuli by modulation of its circuit configuration.