A distinct distribution of functional presynaptic 5-HT receptor subtypes on GABAergic nerve terminals projecting to single hippocampal CA1 pyramidal neurons

Serotonergic Neuromodulation of Synaptic Plasticity

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

Maternal exposure to bisphenol S reduces anxiety and impairs collective antipredator behavior of male zebrafish (Danio rerio) offspring through dysregulation of their serotonergic system

Bisphenol S (BPS) is a common endocrine-disrupting chemical globally used in several consumer and industrial products. Although previous studies suggested that BPS induces multiple effects in exposed organisms, very little is known about its intergenerational effect on offspring behavior and/or the potential underlying mechanisms. To this end, adult female zebrafish Danio rerio were exposed to BPS (0, 10, 30 µg/L) and 1 µg/L of 17-β-estradiol (E2) as a positive control for 60 days. Afterwards, female fish were bred with untreated males, and their offspring were raised to 6 months old in control water. Maternal exposure to BPS decreased male offspring anxiety and antipredator behaviors while boldness remained unaffected. Specifically, maternal exposure to 10 and 30 µg/L BPS and 1 µg/L E2 were found to impact male offspring anxiety levels as they decreased the total time that individuals spent in the dark zone in the light/dark box test and increased the total track length in the center of the open field test. In addition, maternal exposure to all concentrations of BPS and E2 disrupted antipredator responses of male offspring by decreasing shoal cohesion in the presence of chemical alarm cues derived from conspecifics, which communicated high risk. To elucidate the possible molecular mechanism underlying these neuro-behavioral effects of BPS, we assessed the serotonergic system via changes in mRNA expression of serotonin receptors, including the 5-HT1A, 5-HT1B, and 5-HT1D subtypes, the serotonin transporter and monoamine oxidase (MAO). The impaired anxiety and antipredator responses were associated with reduced levels of 5-HT1A subtype and MAO mRNA expression within the brain of adult male offspring. Collectively, the results of this study demonstrate that maternal exposure to environmental concentrations of BPS can interfere with the serotonergic signaling pathway in the developing brain, subsequently leading to the onset of a suite of behavioral deficits in adult offspring.

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

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

The Reversal of Empathy-Induced Hypernociception in Male Mice by Intra-Amygdala Administration of Midazolam and Cannabidiol Depends on 5-HT3 Receptors

Introduction: Empathy is a fundamental prosocial behavior. It has been defined as perception, awareness, and understanding of others' emotional states, including painful processes. Mice living in pairs with conspecific chronic suffering from constriction injury exhibit pain hypersensitivity mediated by the amygdaloid complex. Nevertheless, the underlying mechanisms in the amygdala responsible for this response remain to be determined. This study investigated if the anxiolytic benzodiazepine midazolam (MDZ) and cannabidiol (CBD), a phytocannabinoid with multiple molecular targets, would attenuate this behavioral change. We also investigated if serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the amygdala are involved in this effect. Materials and Methods: Male Swiss mice were housed in pairs for 28 days. The pairs were divided into two groups on the 14th day: cagemate nerve constriction and cagemate sham. On the 24th day, cagemates underwent a stereotaxic surgery and, on the 28th day, were evaluated on the writhing test. Results: The results showed that living with chronic pain leads to hypernociception in the cagemate and increases the expression of 5-HT₃ receptor (5-HT₃R) and glutamic acid decarboxylase 67 within the amygdala. MDZ (3.0 and 30 nmol) and CBD (30 and 60 nmol) attenuated the hypernociceptive behavior. The 5-HT₃R antagonist ondansetron (0.3 nmol) prevented the antinociceptive effects of MDZ and CBD. Conclusion: These findings indicate that 5-HT₃R and GABAergic mechanisms within the amygdala are involved in the pain hypersensitivity induced by the empathy for pain model. They also suggest that MDZ and CBD could be a new potential therapy to alleviate emotional pain disorders.

Effect of ramosetron, a 5-HT3 receptor antagonist on the severity of seizures and memory impairment in electrical amygdala kindled rats

The entorhinal cortex (EC) plays a pivotal role in epileptogenesis and seizures. EC expresses high density of serotonergic receptors, especially 5-HT3 receptors. Cognitive impairment is common among people with epilepsy. The present study investigated the role of 5-HT3 receptor on the severity of seizures and learning and memory impairment by electrical kindling of amygdala in rats. The amygdala kindling was conducted in a chronic kindling manner in male Wistar rats. In fully kindled animals, ramosetron (as a potent and selective 5-HT3 receptor antagonist) was microinjected unilaterally (ad doses of 1, 10 or 100 µg/0.5 µl) into the EC 5 min before the novel object recognition (NOR) and Y-maze tests or kindling stimulations. Applying ramosetron at the concentration of 100 μg/0.5 µl (but not at 1 and 10 µg/0.5 µl) reduced afterdischarge (AD) duration and increased stage 4 latency in the kindled rats. Moreover, the obtained data from the NOR test showed that treatment by ramosetron (10 and 100 µg/0.5 µl) increased the discrimination index in the fully kindled animals. Microinjection of ramosetron (10 and 100 µg/0.5 µl) in fully kindled animals reversed the kindling induced changes in the percentage of spontaneous alternation in Y-maze task. The findings demonstrated an anticonvulsant role for a selective 5-HT3 receptor antagonist microinjected into the EC, therefore, suggesting an excitatory role for the EC 5-HT3 receptors in the amygdala kindling model of epilepsy. This anticonvulsive effect was accompanied with a restoring effect on cognitive behavior in NOR and Y-maze tests.

Co-treatment with low doses of buspirone prevents rewarding effects of methylphenidate and upregulates expression of 5-HT1A receptor mRNA in the nucleus accumbens

Accumulating studies consistently show that methylphenidate (MPD), the first line drug for treating Attention-Deficit Hyperactivity Disorder (ADHD), is abused by patients to whom the drug is prescribed. Like other psychostimulants, only low doses of MPD improve cognitive performance while higher doses impair it. Preventing the use of high doses of MPD is important for retaining its therapeutic efficacy. Previously, it has been shown that performance in Morris water maze test is improved in rats treated, orally, with MPD in doses of 2.5 mg/kg; but higher doses (5 mg/kg) impair it. The present study is designed to monitor rewarding effects of 2.5 mg/kg MPD in conditioned place preference (CPP) paradigm and its potential inhibition in buspirone co-treated animals. Our results show that rewarding effects of MPD in CPP paradigm are prevented in rats co-treated with buspirone in doses of 0.1 and 0.3 mg/kg. Animals treated with MPD exhibit a downregulation of 5-HT1A receptor mRNA in the nucleus accumbens which is also prevented in rats co-treated with 0.1 and 0.3 mg/kg but not 1.0 and 2.0 mg/kg buspirone. Administration of buspirone in these doses is not rewarding in CPP test and upregulates 5-HT1A receptor mRNA in the nucleus accumbens. The findings suggest that co-use of low doses of buspirone can prevent rewarding effects of MPD to help retain its therapeutic efficacy.

Transgenerational effects of selenomethionine on behaviour, social cognition, and the expression of genes in the serotonergic pathway in zebrafish

Elevated levels of contaminants from human activities have become a major threat to animals, particularly within aquatic ecosystems. Selenium (Se) is a naturally occurring element with a narrow range of safe intake, but excessive Se has toxicological effects, as it can bioaccumulate and cause cognitive and behavioural impairments. In this study, we investigated whether exposure to Se would also have transgenerational effects, causing changes in the descendants of exposed individuals. We exposed adult female zebrafish to either a control diet or environmentally relevant concentrations of dietary Se-Met (3.6, 12.8, 34.1 μg Se/g dry weight) for 90 days. Then, females from each treatment group were bred with untreated males, and the offspring (F1-generation) were raised to adulthood (6 months old) without Se exposure. In behavioural tests, offspring that were maternally exposed to 34.1 μg Se/g showed signs of elevated stress, weaker group preferences, and impaired social learning. Maternal exposure to high levels of Se-Met also led to dysregulation of the serotonergic system via changes in mRNA expression of serotonin receptors, including the 5-HT1A, 5-HT1B, and 5-HT1D subtypes, the serotonin transporter, and monoamine oxidase (MAO). Such perturbations in the serotonergic system, thus, appear to underlie the neurobehavioural deficits that we observed. These findings suggest that Se contamination can have important transgenerational consequences on social behaviour and cognition.

Differential Regulation of Human Serotonin Receptor Type 3A by Chanoclavine and Ergonovine

Irritable bowel syndrome (IBS) is a chronic disease that causes abdominal pain and an imbalance of defecation patterns due to gastrointestinal dysfunction. The cause of IBS remains unclear, but intestinal-brain axis problems and neurotransmitters have been suggested as factors. In this study, chanoclavine, which has a ring structure similar to 5-hydroxytryptamine (5-HT), showed an interaction with the 5-HT_3A receptor to regulate IBS. Although its derivatives are known to be involved in neurotransmitter receptors, the molecular physiological mechanism of the interaction between chanoclavine and the 5-HT_3A receptor is unknown. Electrophysiological experiments were conducted using a two-electrode voltage-clamp analysis to observe the inhibitory effects of chanoclavine on Xenopus oocytes in which the h5-HT_3A receptor was expressed. The co-application of chanoclavine and 5-HT resulted in concentration-dependent, reversible, voltage-independent, and competitive inhibition. The 5-HT_3A response induced by 5-HT was blocked by chanoclavine with half-maximal inhibitory response concentration (IC₅₀) values of 107.2 µM. Docking studies suggested that chanoclavine was positioned close F130 and N138 in the 5-HT_3A receptor-binding site. The double mutation of F130A and N138A significantly attenuated the interaction of chanoclavine compared to a single mutation or the wild type. These data suggest that F130 and N138 are important sites for ligand binding and activity. Chanoclavine and ergonovine have different effects. Asparagine, the 130th amino acid sequence of the 5-HT_3A receptor, and phenylalanine, the 138th, are important in the role of binding chanoclavine, but ergonovine has no interaction with any amino acid sequence of the 5-HT_3A receptor. The results of the electrophysiological studies and of in silico simulation showed that chanoclavine has the potential to inhibit the hypergastric stimulation of the gut by inhibiting the stimulation of signal transduction through 5-HT_3A receptor stimulation. These findings suggest chanoclavine as a potential antiemetic agent for excessive gut stimulation and offer insight into the mechanisms of 5-HT_3A receptor inhibition.

Ethanol induces persistent potentiation of 5-HT3 receptor-stimulated GABA release at synapses on rat hippocampal CA1 neurons

Several studies have shown that ethanol (EtOH) can enhance the activity of GABAergic synapses via presynaptic mechanisms, including in hippocampal CA1 neurons. The serotonin type 3 receptor (5-HT3-R) has been implicated in the neural actions of ethanol (EtOH) and in modulation of GABA release from presynaptic terminals. In the present study, we investigated EtOH modulation of GABA release induced by 5-HT3-R activation using the mechanically isolated neuron/bouton preparation from the rat CA1 hippocampal subregion. EtOH application before and during exposure to the selective 5-HT3 receptor agonist, m-chlorophenylbiguanide (mCPBG) potentiated the mCPBG-induced increases in the peak frequency and charge transfer of spontaneous GABAergic inhibitory postsynaptic currents. Interestingly, the potentiation was maintained even after EtOH was removed from the preparation. A protein kinase A inhibitor reduced the magnitude of EtOH potentiation. Fluorescent Ca2+ imaging showed that Ca2+ transients in the presynaptic terminals increased during EtOH exposure. These findings indicate that EtOH produces long-lasting potentiation of 5-HT3-induced GABA release by modulating calcium levels, via a process involving cAMP-mediated signaling in presynaptic terminals.

Glucocorticoids in the Physiological and Transcriptional Regulation of 5-HT1A Receptor and the Pathogenesis of Depression

There is growing increase in the global prevalence of depression, but treatment outcome of this highly disabling disease is not satisfactory. Many patients are not benefitted by the currently prescribed antidepressants-together with this partial remission is very common. Improving treatment strategies and developing better therapeutic agents for treating depression is therefore highly needed. Stress-related epigenetic changes play a critical role in the pathogenesis as well as treatment of depression. Stressful events activate hypothalamic-pituitary-adrenal axis to increase circulating levels of glucocorticoids (GCs), and a greater sensitivity to this fright and flight response increases risk of depression. A role of serotonin (5-hydroxytryptamine; 5-HT) in responses to stress and in the pathogenesis and treatment of depression is well established. Substantial evidence supports a critical role of 5-HT1A receptors in these effects of 5-HT. The present article targets stress-induced higher and sustained increases of GCs and mediated influences on the physiological as well transcriptional regulation of 5-HT1A receptors to evaluate their causal role in the pathogenesis of depression. It is suggested that synthetic compounds with antagonist activity for GC receptors and agonist activity for 5-HT1A receptors may prove better therapeutic agents for treating depression.

5-HT1A receptor-mediated attenuation of synaptic transmission in rat medial vestibular nucleus impacts on vestibular-related motor function

KEY POINTS

Chemogenetic activation of medial vestibular nucleus-projecting 5-HT neurons resulted in deficits in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. The 5-HT_1A receptor mediates the vestibular-related behavioural effects of 5-HT in the vestibular nucleus. 5-HT_1A receptor activation attenuated evoked excitatory postsynaptic currents and evoked inhibitory postsynaptic currents via a presynaptic mechanism in the vestibular nucleus.

ABSTRACT

While the anxiolytic effects of serotonergic neuromodulation are well studied, its role in sensorimotor coordination and postural control is unclear. In this study, we show that an increase of serotonin (5-hydroxytryptamine, 5-HT) at the medial vestibular nucleus (MVN), a brainstem centre for vestibulospinal coordination, by either direct cannula administration or chemogenetic stimulation of MVN-projecting serotonergic neurons, adversely affected performance of rats in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. Application of the 5-HT₁ and 5-HT₇ receptor co-agonist 8-hydroxy-2-(di-n-propylamino) tetralin recapitulated the effect of 5-HT, while co-administration of the specific 5-HT_1A receptor antagonist WAY 100135 effectively abolished all 5-HT-induced behavioural deficits. This indicated that 5-HT_1A receptors mediated the effects of 5-HT in the rat MVN. Using whole-cell patch-clamp recording, we demonstrated that 5-HT_1A receptor activation attenuated both evoked excitatory and evoked inhibitory postsynaptic currents through a presynaptic mechanism in the rat MVN. The results thus highlight the 5-HT_1A receptor as the gain controller of vestibular-related brainstem circuits for posture and balance.

Effects of chronic exposure to selenomethionine on social learning outcomes in zebrafish (Danio rerio): serotonergic dysregulation and oxidative stress in the brain

For many species, social learning is crucial for fitness-related activities, but human-induced environmental changes can impair such learning processes. For instance, mining can release the element, selenium (Se), that is vital for physiological functions but also has toxicological properties at elevated concentrations. In this study, we investigated the effects of chronic exposure to Se on social learning outcomes and potential underlying molecular mechanisms in adult zebrafish. After exposure to different levels of dietary selenomethionine (control, 3.6, 12.8, 34.1 μg Se/g dry weight) for 90 days, we examined the ability of observer fish to follow demonstrators (experienced individuals) in escaping an oncoming trawl. Social learning outcomes were then assessed in the absence of demonstrators. Our results indicated that fish in the highest exposure group (34.1 μg/g) displayed significantly slower escape responses compared to fish in the control and lower exposure groups (3.6 and 12.8 μg Se/g). This impaired behavior was associated with higher oxidative stress and dysregulation in genes that are key in the serotonergic pathway, indicating that oxidative stress and alteration in the serotonergic system lead to impairment of social learning.

5-HT3 Receptor Antagonists in Neurologic and Neuropsychiatric Disorders: The Iceberg Still Lies beneath the Surface

5-HT₃ receptor antagonists, first introduced to the market in the mid-1980s, are proven efficient agents to counteract chemotherapy-induced emesis. Nonetheless, recent investigations have shed light on unappreciated dimensions of this class of compounds in conditions with an immunoinflammatory component as well as in neurologic and psychiatric disorders. The promising findings from multiple studies have unveiled several beneficial effects of these compounds in multiple sclerosis, stroke, Alzheimer disease, and Parkinson disease. Reports continue to uncover important roles for 5-HT₃ receptors in the physiopathology of neuropsychiatric disorders, including depression, anxiety, drug abuse, and schizophrenia. This review addresses the potential of 5-HT₃ receptor antagonists in neurology- and neuropsychiatry-related disorders. The broad therapeutic window and high compliance observed with these agents position them as suitable prototypes for the development of novel pharmacotherapeutics with higher efficacy and fewer adverse effects.

Selective serotonin reuptake inhibitor escitalopram inhibits 5-HT3 receptor currents in NCB-20 cells

Escitalopram is one of selective serotonin reuptake inhibitor antidepressants. As an S-enantiomer of citalopram, it shows better therapeutic outcome in depression and anxiety disorder treatment because it has higher selectivity for serotonin reuptake transporter than citalopram. The objective of this study was to determine the direct inhibitory effect of escitalopram on 5-hydroxytryptamine type 3 (5-HT₃) receptor currents and study its blocking mechanism to explore additional pharmacological effects of escitalopram through 5-HT₃ receptors. Using a whole-cell voltage clamp method, we recorded currents of 5-HT₃ receptors when 5-HT was applied alone or co-applied with escitalopram in cultured NCB-20 neuroblastoma cells known to express 5-HT₃ receptors. 5-HT induced currents were inhibited by escitalopram in a concentration-dependent manner. EC₅₀ of 5-HT on 5-HT₃ receptor currents was increased by escitalopram while the maximal peak amplitude was reduced by escitalopram. The inhibitory effect of escitalopram was voltage independent. Escitalopram worked more effectively when it was co-applied with 5-HT than pre-application of escitalopram. Moreover, escitalopram showed fast association and dissociation to the open state of 5-HT₃ receptor channel with accelerating receptor desensitization. Although escitalopram accelerated 5-HT₃ receptor desensitization, it did not change the time course of desensitization recovery. These results suggest that escitalopram can inhibit 5-HT₃ receptor currents in a non-competitive manner with the mechanism of open channel blocking.

Ginkgo biloba treatments reverse the impairment of conditioned suppression acquisition induced by GluN2B-NMDA and 5-HT1A receptor blockade: Modulatory effects of the circuitry of the dorsal hippocampal formation

To improve our understanding of the effects of standardized extract of Ginkgo biloba (EGb) as a cognitive enhancer, we investigated the conditioned lick suppression-induced expression (mRNA and protein) of the GluN2B-containing N-methyl-D-aspartic acid receptor (GluN2B-NMDAR), serotonin (5-HT) 1A receptor (5-HT_1AR), gamma-aminobutyric acid type A receptor (GABA_AR) and glial fibrillary acidic protein (GFAP) in the dorsal hippocampal formation (dHF) of untreated and EGb-treated (0.25, 0.5 and 1.0 g.kg^-1) groups of rats. To substantiate our data, we analysed the molecular changes in dHF following treatment with vehicle, with agonists or antagonists of GABA_AR, GluN2B-NMDAR and 5-HT_1AR or with one of these antagonists prior to EGb and fear memory acquisition. Additionally, we performed a pharmacological analysis of the drug-receptor-receptor interactions and their supplemental role in fear memory by blocking individual receptors and analysed the possible changes in expression level with each of the other receptors in the study as well as astrocytes. Our data show for the first time that EGb treatment not only upregulated GluN2B, GABA_AR-α5, and GFAP compared with the control but also differentially upregulated GABA_AR-α1 in the dHF and 5HT_1AR in the CA3. We found that the activation of GABA_ARs (diazepam) and the inactivation of GluN2B-NMDARs (Ro25-6981) or 5-HT_1AR ((S)-WAY100135) resulted in memory impairment. Further, higher doses of EGb treatment reversed the effect of blocking GluN2B (P < 0.001) and 5-HT_1AR (P < 0.001). Here, treatment with Ro25-6981 + EGb or (S)-WAY100135 + EGb prevented the impairment of the acquisition of lick suppression in association with the upregulation or prevention of the downregulation of Grin2b expression as well as the expression of GluN2B-NMDA and/or α1 and α5 subunit-containing GABA_AR in the CA1 (P < 0.0001). Our data are in line with previous findings concerning the necessity of GluN2B for fear memory formation and add to the current knowledge of the role of the GABA_AR-α1 and -α5 subunits and of GluN2B as a target of cognitive enhancers. Furthermore, our data show that these receptors play a complementary role in controlling the neural circuitry in the dHF that seems to be essential to conditioned lick suppression and the modulatory effects of EGb.

Chronic exposure to dietary selenomethionine dysregulates the genes involved in serotonergic neurotransmission and alters social and antipredator behaviours in zebrafish (Danio rerio)

Selenium (Se) is a metalloid of potential interest from both a toxicological and nutritional perspective, having a range of safe intake. The adverse neuro-behavioural effects of Se have been investigated in both humans and fishes, but little is known about its effects on social behaviours or the serotonergic signaling pathway in the brain. In the present study, we investigated the effects of chorionic dietary exposure to Se (as selenomethionine) at different concentrations (control, 2.1, 11.6 or 31.5 μg/g dry wt.) on antipredator avoidance, shoaling behaviour, and social group preferences in adult zebrafish (Danio rerio). In addition, we also measured the expression of important genes in the serotonergic pathway that influence social behaviours. After 60 days of exposure, the highest dose (31.5 μg/g dry wt.) caused the highest level of baseline fear behaviour, with fish swimming lower in the water column and in tighter shoals compared to fish in the other treatments. With high levels of baseline fear, these fish did not significantly intensify fear behaviours in response to predation risk in the form of exposure to chemical alarm cues. When individual fish were given an opportunity to shoal with groups of differing sizes (3 vs. 4 individuals), fish exposed to the high dose spent less time with groups in general, and only control fish showed a significant preference for the larger group. In the zebrafish brain, we found significant upregulation in the mRNA expression of serotonin receptors (htr1aa and htr1b), a transporter (slc6a4a), and tryptophan hydroxylase-2 (tph2), whereas there was a downregulation of the monoamine oxidase (mao) gene. The results of this study suggest that disruption of serotonergic neurotransmission might have been responsible for Se-induced impairment of antipredator and social behaviour in zebrafish.

Tricyclic antidepressant amitriptyline inhibits 5-hydroxytryptamine 3 receptor currents in NCB-20 cells

Amitriptyline, a tricyclic antidepressant, is commonly used to treat depression and neuropathic pain, but its mechanism is still unclear. We tested the effect of amitriptyline on 5-hydroxytryptamine 3 (5-HT₃) receptor currents and studied its blocking mechanism because the clinical applications of amitriptyline overlapped with 5-HT₃ receptor therapeutic potentials. Using a whole-cell voltage clamp method, we recorded the currents of the 5-HT₃ receptor when 5-HT was applied alone or co-applied with amitriptyline in cultured NCB-20 neuroblastoma cells known to express 5-HT₃ receptors. To elucidate the mechanism of amitriptyline, we simulated the 5-HT₃ receptor currents using Berkeley Madonna® software and calculated the rate constants of the agonist binding and receptor transition steps. The 5-HT₃ receptor currents were inhibited by amitriptyline in a concentration-dependent, voltage-independent manner, and a competitive mode. Amitriptyline accelerated the desensitization of the 5-HT₃ receptor. When amitriptyline was applied before 5-HT treatment, the currents rose slowly until the end of 5-HT treatment. When amitriptyline was co-applied with 5-HT, currents rose and decayed rapidly. Peak current amplitudes were decreased in both applications. All macroscopic currents recorded in whole cell voltage clamping experiments were reproduced by simulation and the changes of rate constants by amitriptyline were correlated with macroscopic current recording data. These results suggest that amitriptyline blocks the 5-HT₃ receptor by close and open state blocking mechanisms, in a competitive manner. We could expand an understanding of pharmacological mechanisms of amitriptyline related to the modulation of a 5-HT₃ receptor, a potential target of neurologic and psychiatric diseases through this study.

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

One of the hypotheses about the pathogenesis of posttraumatic stress disorder (PTSD) is the dysfunction of serotonin (5-HT) neurotransmission. While certain 5-HT receptor subtypes are likely critical for the symptoms of PTSD, few studies have examined the role of 5-HT₃ receptor in the development of PTSD, even though 5-HT₃ receptor is critical for contextual fear extinction and anxiety-like behavior. Therefore, we hypothesized that stimulation of 5-HT₃ receptor in the dorsal hippocampus (DH) could prevent hippocampal autophagy and the development of PTSD-like behavior in animals. To this end, we infused SR57227, selective 5-HT₃ agonist, into the DH after a single prolonged stress (SPS) treatment in rats. Three weeks later, we evaluated the effects of this pharmacological treatment on anxiety-related behaviors and extinction of contextual fear memory. We also accessed hippocampal autophagy and the expression of 5-HT_3A subunit, Beclin-1, LC3-I, and LC3-II in the DH. We found that SPS treatment did not alter anxiety-related behaviors but prolonged the extinction of contextual fear memory, and such a behavioral phenomenon was correlated with increased hippocampal autophagy, decreased 5-HT_3A expression, and increased expression of Beclin-1 and LC3-II/LC3-I ratio in the DH. Furthermore, intraDH infusions of SR57227 dose-dependently promoted the extinction of contextual fear memory, prevented hippocampal autophagy, and decreased expression of Beclin-1 and LC3-II/LC3-I ratio in the DH. These results indicated that 5-HT₃ receptor in the hippocampus may play a critical role in the pathogenesis of hippocampal autophagy, and is likely involved in the pathophysiology of PTSD.

5HT3 receptors: Target for new antidepressant drugs

5HT3 receptors (5HT3Rs) have long been identified as a potential target for antidepressants. Several studies have reported that antagonism of 5HT3Rs produces antidepressant-like effects. However, the exact role of 5HT3Rs and the mode of antidepressant action of 5HT3R antagonists still remain a mystery. Here, we provide a comprehensive overview of 5HT3Rs: (a) regional and subcellular distribution of 5HT3Rs in discrete brain regions, (b) preclinical and clinical evidence supporting the antidepressant effect of 5HT3R antagonists, and (c) neurochemical, biological and neurocellular signaling pathways associated with the antidepressant action of 5HT3R antagonists. 5HT3Rs located on the serotonergic and other neurotransmitter interneuronal projections control their release and affect mood and emotional behavior; however, new evidence suggests that apart from modulating the neurotransmitter functions, 5HT3R antagonists have protective effects in the pathogenic events including hypothalamic-pituitary-adrenal-axis hyperactivity, brain oxidative stress and impaired neuronal plasticity, pointing to hereby unknown and novel mechanisms of their antidepressant action. Nonetheless, further investigations are warranted to establish the exact role of 5HT3Rs in depression and antidepressant action of 5HT3R antagonists.

The 5-HT3A receptor is essential for fear extinction

The 5-HT3 receptor, the only ionotropic 5-HT receptor, is expressed in limbic regions, including the hippocampus, amygdala, and cortex. However, it is not known whether it has a role in fear memory processes. Analysis of 5-HT3A receptor knockout mice in fear conditioning paradigms revealed that the 5-HT3A receptor is not required for the acquisition or retention of fear memory but is essential for the extinction of contextual and tone-cued fear. Our data suggest that the 5-HT3A receptor could be a key molecule regulating fear memory processes and a potential therapeutic target for fear disorders.

Further evidence for involvement of the dorsal hippocampus serotonergic and γ-aminobutyric acid (GABA)ergic pathways in the expression of contextual fear conditioning in rats

Intra-dorsal hippocampus (DH) injections of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a serotonin-1A (5-hydroxytryptamine (5-HT)-1A) receptor agonist, were previously shown to inhibit the expression of contextual fear when administered six hours after conditioning. However, further understanding of the consolidation and expression of aversive memories requires investigations of these and other mechanisms at distinct time points and the regions of the brain to which they are transferred. Thus, the purpose of the present study was to investigate the role of DH serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the expression of contextual fear 24 h after conditioning, reflected by fear-potentiated startle (FPS) and freezing behavior. The recruitment of the amygdala and medial prefrontal cortex (mPFC) in these processes was also evaluated by measuring Fos protein immunoreactivity. Although intra-DH injections of 8-OH-DPAT did not produce behavioral changes, muscimol reduced both FPS and the freezing response. Fos protein immunoreactivity revealed that contextual fear promoted wide activation of the mPFC, which was significantly reduced after intra-DH infusions of muscimol. The present findings, together with previous data, indicate that in contrast to 5-HT, which appears to play a role during the early phases of contextual aversive memory consolidation, longer-lasting GABA-mediated mechanisms are recruited during the expression of contextual fear memories.

The role of different serotonin receptor subtypes in seizure susceptibility

5-Hydroxytryptamine (5-HT) has the most diverse set of receptors in comparison with any other neurotransmitter or hormone in the body. To date, seven families of 5-HT receptors have been characterized. A great number of studies have been published regarding the role of 5-HT and its receptors in seizures. However, with a few exceptions, the net effect of activating or inhibiting each 5-HT receptor subtype on the development or severity of seizures remains controversial. Additionally, the results of studies, which have used knockout animals to investigate the role of 5-HT receptors in seizures, have sometimes been contradictory to those which have used pharmacological tools. The present study aims to review the available data regarding the influence of each receptor subtype on seizure development and, when possible, reconcile between the apparently different results obtained in these studies.

Serotonergic receptor mechanisms underlying antidepressant-like action in the progesterone withdrawal model of hormonally induced depression in rats

Hormonally induced mood disorders such as premenstrual dysphoric disorder (PMDD) are characterized by a range of physical and affective symptoms including anxiety, irritability, anhedonia, social withdrawal and depression. Studies demonstrated rodent models of progesterone withdrawal (PWD) have a high level of constructive and descriptive validity to model hormonally-induced mood disorders in women. Here we evaluate the effects of several classes of antidepressants in PWD female Long-Evans rats using the forced swim test (FST) as a measure of antidepressant activity. The study included fluoxetine, duloxetine, amitriptyline and an investigational multimodal antidepressant, vortioxetine (5-HT(3), 5-HT(7) and 5-HT(1D) receptor antagonist; 5-HT(1B) receptor partial agonist; 5-HT(1A) receptor agonist; inhibitor of the serotonin transporter (SERT)). After 14 days of administration, amitriptyline and vortioxetine significantly reduced immobility in the FST whereas fluoxetine and duloxetine were ineffective. After 3 injections over 48 h, neither fluoxetine nor duloxetine reduced immobility, whereas amitriptyline and vortioxetine significantly reduced FST immobility during PWD. When administered acutely during PWD, the 5-HT(1A) receptor agonist, flesinoxan, significantly reduced immobility, whereas the 5-HT(1A) receptor antagonist, WAY-100635, increased immobility. The 5-HT(3) receptor antagonist, ondansetron, significantly reduced immobility, whereas the 5-HT(3) receptor agonist, SR-57227, increased immobility. The 5-HT(7) receptor antagonist, SB-269970, was inactive, although the 5-HT(7) receptor agonist, AS-19, significantly increased PWD-induced immobility. None of the compounds investigated (ondansetron, flesinoxan and SB-269970) improved the effect of fluoxetine during PWD. These data indicate that modulation of specific 5-HT receptor subtypes is critical for manipulating FST immobility in this model of hormone-induced depression.

The role of glutamatergic and GABAergic systems on serotonin- induced feeding behavior in chicken

It has been reported that serotonin can modulate glutamate and GABA release in central nervous system (CNS). The present study was designed to examine the role of glutamatergic and GABAergic systems on serotonin- induced feeding behavior in chickens. In Experiment 1 intracerebroventricular (ICV) injection of MK- 801(NMDA receptor antagonist, 15 nmol) performed followed by serotonin (10 μg). In experiments 2, 3, 4, 5, 6 and 7 prior to serotonin injection, chickens received CNQX (AMPA/kainate receptor antagonist, 390 nmol), AIDA (mGluR1 antagonist, 2 nmol), LY341495 (mGluR2 antagonist, 150 nmol), UBP1112 (mGluR3 antagonist, 2 nmol), picrotoxin (GABA A receptor antagonist, 0.5 μg), CGP54626 (GABAB receptor antagonist, 20 ng) respectively. Cumulative food intake was determined at 3 h post injection. The results of this study showed that the hypophagic effect of serotonin was significantly attenuated by pretreatment with MK- 801 and CNQX (p < 0.05) but AIDA, LY341495 and UBP1112 had no effect (p > 0.05). Also, the inhibitory effect of serotonin on food intake was amplified by picrotoxin (p < 0.05) while CGP54626 had no effect (p > 0.05). These results suggest that serotonin as a modulator probably interacts with glutamatergic (via NMDA and AMPA/Kainate receptors) and GABAergic (via GABAA receptor) systems on feeding behavior in chicken.

First and second generation antipsychotics influence hippocampal gamma oscillations by interactions with 5-HT3 and D3 receptors

BACKGROUND AND PURPOSE

Disturbed cortical gamma band oscillations (30-80 Hz) have been observed in schizophrenia: positive symptoms of the disease correlate with an increase in gamma oscillation power, whereas negative symptoms are associated with a decrease.

EXPERIMENTAL APPROACH

Here we investigated the effects of first and second generation antipsychotics (FGAs and SGAs, respectively) on gamma oscillations. The FGAs haloperidol, flupenthixol, chlorpromazine, chlorprothixene and the SGAs clozapine, risperidone, ziprasidone, amisulpride were applied on gamma oscillations induced by acetylcholine and physostigmine in the CA3 region of rat hippocampal slices.

KEY RESULTS

Antipsychotics inhibited the power of gamma oscillations and increased the bandwidth of the gamma band. Haloperidol and clozapine had the highest inhibitory effects. To determine which receptor is responsible for the alterations in gamma oscillations, the effects of the antipsychotics were plotted against their pK(i) values for 19 receptors and analysed for correlation. Our results indicated that 5-HT(3) receptors have an enhancing effect on gamma oscillations whereas dopamine D(3) receptors inhibit them. To test this prediction, m-chlorophenylbiguanide, PD 128907 and CP 809101, selective agonists at 5-HT(3) , D(3) and 5-HT(2C) receptors were applied and revealed that 5-HT(3) receptors indeed enhanced the gamma power whereas D(3) receptors reduced it. As predicted, 5-HT(2C) receptors had no effects on gamma oscillations.

CONCLUSION AND IMPLICATIONS

Our data suggest that antipsychotics alter hippocampal gamma oscillations by interacting with 5-HT(3) and dopamine D(3) receptors. Moreover, a correlation of receptor affinities with the biological effects can be used to predict targets for the pharmacological effects of multi-target drugs.

The effects of volatile anesthetics on synaptic and extrasynaptic GABA-induced neurotransmission

Examination of volatile anesthetic actions at single synapses provides more direct information by reducing interference by surrounding tissue and extrasynaptic modulation. We examined how volatile anesthetics modulate GABA release by measuring spontaneous or miniature GABA-induced inhibitory postsynaptic currents (mIPSCs, sIPSCs) or by measuring action potential-evoked IPSCs (eIPSCs) at individual synapses. Halothane increased both the amplitude and frequency of sIPSCs. Isoflurane and enflurane increased mIPSC frequency while sevoflurane had no effect. These anesthetics did not alter mIPSC amplitudes. Halothane increased the amplitude of eIPSCs, with a decrease in failure rate (Rf) and paired-pulse ratio. In contrast, isoflurane and enflurane decreased the eIPSC amplitude and increased Rf, while sevoflurane decreased the eIPSC amplitude without affecting Rf. Volatile anesthetics did not change kinetics except for sevoflurane, suggesting that presynaptic mechanisms dominate changes in neurotransmission. Each anesthetic showed somewhat different GABA-induced response and these results suggest that GABA-induced synaptic transmission cannot have a uniformly common site of action as suggested for volatile anesthetics. In contrast, all volatile anesthetics concentration-dependently enhanced the GABA-induced extrasynaptic currents. Extrasynaptic receptors containing α4 and α5 subunits are reported to have high sensitivities to volatile anesthetics. Also, inhibition of GABA uptake by volatile anesthetics results in higher extracellular GABA concentration, which may lead to prolonged activation of extrasynaptic GABAA receptors. The extrasynaptic GABA-induced receptors may be major site of volatile anesthetic-induced neurotransmission. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

5-HT(3) receptor mediates the dose-dependent effects of citalopram on pentylenetetrazole-induced clonic seizure in mice: involvement of nitric oxide

Citalopram is a selective serotonin reuptake inhibitor (SSRI), widely used in the treatment of depressive disorders. It has been shown that citalopram affects seizure susceptibility. Although the exact mechanism of these effects are not yet fully understood, recent data suggest that 5HT(3) receptors and nitric oxide (NO) might participate in the central effects of SSRIs. In this study in a mouse model of clonic seizure induced by pentylenetetrazole we investigated whether 5-HT(3) receptors are involved in the effects of citalopram on seizure threshold. In our experiments, citalopram at lower doses (0.5 and 1mg/kg, i.p) significantly increased the seizure threshold and at higher doses (≥25mg/kg) showed proconvulsive effects. Moreover, mCPBG (a 5-HT(3) receptor agonist) at low and non-effective doses augmented while non-effective doses of tropisetron prevented the anticonvulsive properties of citalopram. On the other hand, Low doses of nitric oxide synthase inhibitors l-NAME and 7-NI alone or in combination with lower doses of 5-HT(3) receptor agonist enhanced the anticonvulsive property of citalopram, while l-arginine (NO precursor) alone or in combination with tropisetron blocked the protective effect of citalopram. In summary, our findings demonstrate that 5-HT(3) receptor mediates the anticonvulsant properties of low doses of citalopram, whereas it seems that the proconvulsive effect is mostly mediated through the NO pathway and can be totally blocked by NOS inhibitors. This could propose a new approach toward finding the mechanism of citalopram activity, curtailing the adverse effects of citalopram and perhaps managing the convulsions as a vicious consequence of citalopram overdose.

5-HT(3A) receptor-bearing white matter interstitial GABAergic interneurons are functionally integrated into cortical and subcortical networks

In addition to axons and surrounding glial cells, the corpus callosum also contains interstitial neurons that constitute a heterogeneous cell population. There is growing anatomical evidence that white matter interstitial cells (WMICs) comprise GABAergic interneurons, but so far there is little functional evidence regarding their connectivity. The scarcity of these cells has hampered electrophysiological studies. We overcame this hindrance by taking recourse to transgenic mice in which distinct WMICs expressed enhanced green fluorescence protein (EGFP). The neuronal phenotype of the EGFP-labeled WMICs was confirmed by their NeuN positivity. The GABAergic phenotype could be established based on vasoactive intestinal peptide and calretinin expression and was further supported by a firing pattern typical for interneurons. Axons and dendrites of many EGFP-labeled WMICs extended to the cortex, hippocampus, and striatum. Patch-clamp recordings in acute slices showed that they receive excitatory and inhibitory input from cortical and subcortical structures. Moreover, paired recordings revealed that EGFP-labeled WMICs inhibit principal cells of the adjacent cortex, thus providing unequivocal functional evidence for their GABAergic phenotype and demonstrating that they are functionally integrated into neuronal networks.

Serotonin and prefrontal cortex function: neurons, networks, and circuits

Higher-order executive tasks such as learning, working memory, and behavioral flexibility depend on the prefrontal cortex (PFC), the brain region most elaborated in primates. The prominent innervation by serotonin neurons and the dense expression of serotonergic receptors in the PFC suggest that serotonin is a major modulator of its function. The most abundant serotonin receptors in the PFC, 5-HT1A, 5-HT2A and 5-HT3A receptors, are selectively expressed in distinct populations of pyramidal neurons and inhibitory interneurons, and play a critical role in modulating cortical activity and neural oscillations (brain waves). Serotonergic signaling is altered in many psychiatric disorders such as schizophrenia and depression, where parallel changes in receptor expression and brain waves have been observed. Furthermore, many psychiatric drug treatments target serotonergic receptors in the PFC. Thus, understanding the role of serotonergic neurotransmission in PFC function is of major clinical importance. Here, we review recent findings concerning the powerful influences of serotonin on single neurons, neural networks, and cortical circuits in the PFC of the rat, where the effects of serotonin have been most thoroughly studied.

Amphetamine differentially modifies the expression of monoaminergic and GABAergic synaptic boutons and processes in lateral habenula, dorsal and ventral hippocampal formation

The habenular complex is thought to be associated with cognitive functions and indirectly connected with the hippocampal formation (HF). Thus the responses of the monoaminergic and GABAergic neurons were examined in both structures to the psychostimulant, amphetamine (Amph). Immunocytochemical analysis was performed on brain sections prepared from adult mice treated with a single or multiple (2 doses/day, 7 doses in total) injections of saline or Amph, 5mg/kg. The synaptic boutons were verified by immuno-electron microscopy specific for parvalbumin (PV), glutamic acid decarboxylase(67) (GAD(67)), aromatic amino acid decarboxylase (AADC) or dopamine-β-hydroxylase (DBH). In the lateral part of the lateral habenula (LHb), at 4h post-acute Amph, the densities of PV-positive boutons/processes and DBH-boutons were decreased by approximate 75% and 72% respectively, compared with corresponding saline-controls; however, at 4h post-repeated Amph exposure, PV was increased by 244%, and DBH unaltered. In the dorsal HF (DHF), at 4h post-repeated Amph exposure, GAD(67)-boutons and PV resembled controls in CA1 and CA3 pyramidal cell layers, whereas in the granule cell layer of dentate gyrus (DG), PV was increased by 112%, and GAD(67) unchanged. As shown by biochemical methods, at 4h post-repeated Amph, the decreased level of DHF GABA probably correlates with the immunocytochemical changes. In the ventral HF (VHF), at 4h post-repeated Amph treatment, PV and the enzymes of CA1 and DG were unaltered, while CA3 PV was decreased by 63%, and AADC-boutons increased 55%. Double immuno-electron microscopy revealed synaptic contacts between PV and GAD(67) containing presynaptic or postsynaptic elements, and between PV or GAD(67) and DBH or AADC. This ultrastructural evidence may support the functional significance of the Amph-induced differential changes, which could reflect Amph toxicity and distinct characteristics of the LHb, DHF and VHF.

Role of 5-HT3 Receptors in the Antidepressant Response

Serotonin (5-HT)₃ receptors are the only ligand-gated ion channel of the 5-HT receptors family. They are present both in the peripheral and central nervous system and are localized in several areas involved in mood regulation (e.g., hippocampus or prefrontal cortex). Moreover, they are involved in regulation of neurotransmitter systems implicated in the pathophysiology of major depression (e.g., dopamine or GABA). Clinical and preclinical studies have suggested that 5-HT₃ receptors may be a relevant target in the treatment of affective disorders. 5-HT₃ receptor agonists seem to counteract the effects of antidepressants in non-clinical models, whereas 5-HT₃ receptor antagonists, such as ondansetron, present antidepressant-like activities. In addition, several antidepressants, such as mirtazapine, also target 5-HT₃ receptors. In this review, we will report major advances in the research of 5-HT₃ receptor's roles in neuropsychiatric disorders, with special emphasis on mood and anxiety disorders.

Seizure susceptibility alteration through 5-HT(3) receptor: modulation by nitric oxide

There is some evidence that epileptic seizures could be induced or increased by 5-hydroxytryptamine (5-HT) attenuation, while augmentation of serotonin functions within the brain (e.g. by SSRIs) has been reported to be anticonvulsant. This study was performed to determine the effect of selective 5-HT(3) channel/receptor antagonist granisetron and agonist SR57227 hydrochloride on the pentylenetetrazole (PTZ)-induced seizure threshold in mice. The possible interaction of this effect with nitrergic system was also examined using the nitric oxide (NO) synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) and the NO precursor l-arginine. SR57227 (10mg/kg, i.p.) significantly increased the seizure threshold compared to control group, while high dose granisetron (10mg/kg, i.p.) proved proconvulsant. Co-administration of sub-effective doses of the 5-HT(3) agonist with l-NAME (5 and 60mg/kg, i.p., respectively) exerted a significant anticonvulsive effect, while sub-effective doses of granisetron (3mg/kg) was observed to have a proconvulsive action with the addition of l-arginine (75mg/kg, i.p.). Our data demonstrate that enhancement of 5-HT(3) receptor function results in as anticonvulsant effect in the PTZ-induced seizure model, and that selective antagonism at the 5-HT(3) receptor yields proconvulsive effects. Furthermore, the NO system may play a role in 5-HT(3) receptor function.

Activation of the 5-HT(6) receptor attenuates long-term potentiation and facilitates GABAergic neurotransmission in rat hippocampus

The 5-HT(6) receptor is predominantly expressed in the CNS and has been implicated in the regulation of cognitive function. Antagonists of the 5-HT(6) receptor improve cognitive performance in a number of preclinical models and have recently been found to be effective in Alzheimer's disease patients. Systemic administration of 5-HT(6) antagonists increases the release of acetylcholine and glutamate in the frontal cortex and dorsal hippocampus. In contrast, the selective 5-HT(6) agonist, WAY-181187, can elicit robust increases in extracellular levels of GABA. The reported behavioral and neurochemical effects of 5-HT(6) receptor ligands raise the possibility that the 5-HT(6) receptor may modulate synaptic plasticity in the hippocampus. In the present study, selective pharmacological tools were employed to determine the effect of 5-HT(6) receptor activation on long-term potentiation (LTP) in brain slices containing area CA1 of the hippocampus. While having no effect on baseline synaptic transmission, the results demonstrate that the selective 5-HT(6) agonist, WAY-181187, attenuated LTP over a narrow dose range (100-300 nM). The increase in the slope of the field excitatory post synaptic potential (fEPSP) caused by theta burst stimulation in brain slices treated with the most efficacious dose of WAY-181187 (200 nM) was 80.1+/-4.0% of that observed in controls. This effect was dose-dependently blocked by the selective 5-HT(6) antagonist, SB-399885. WAY-181187 also increased the frequency of spontaneous GABA release in area CA1. As assessed by measuring and evaluating spontaneous inhibitory postsynaptic currents (sIPSCs), 200 nM WAY-181187 increased sIPSC frequency by 3.4+/-0.9 Hz. This increase in GABA sIPSCs was prevented by the selective 5-HT(6) antagonist SB-399885 (300 nM). Taken together, these results suggest that the 5-HT(6) receptor plays a role in the modulation of synaptic plasticity in hippocampal area CA1 and that the regulation of GABAergic interneuron activity may underlie the cognition enhancing effects of 5-HT(6) antagonists.

The 5-HT3 receptor--the relationship between structure and function

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

Fluoxetine potentiates GABAergic IPSCs in rat hippocampal neurons

The GABA system is highly involved in the pathophysiology of mood disorders such as depression. Altered GABAergic function is evident in depressed patients and animal models of depression. Currently, the most widely used antidepressants are selective 5-HT reuptake inhibitors, such as fluoxetine. However, the effects of fluoxetine on GABAergic synaptic neurotransmission remain poorly investigated. Whole-cell patch-clamp recordings from cultured rat hippocampal neurons were therefore conducted to investigate the effects of fluoxetine on GABAergic neurotransmission. The spontaneous inhibitory postsynaptic current (sIPSC) was completely blocked by 10 microM bicuculline and reversibly potentiated by 30 microM fluoxetine. The fluoxetine potentiation on either amplitude or frequency of sIPSCs was dose-dependent, with the EC(50) values of 10.96 and 14.26 microM, respectively. This potentiation was also TTX-insensitive, suggesting independence of presynaptic action potentials. The ritanserin (5 microM), a selective 5-HT(2) receptor antagonist, did not alter the fluoxetine potentiation on miniature inhibitory postsynaptic currents. Taken together, our data suggest that fluoxetine can potentiate GABAergic neurotransmission without depending on presynaptic firing of action potentials and its elevating of 5-HT receptor activities. This potentiation by fluoxetine may normalize the hippocampal GABA deficit during depression and in part exert its antidepressant activity.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

The application of positron-emitting molecular imaging tracers in Alzheimer's disease

The symptomatology and known pathology of Alzheimer's disease are restricted to the central nervous system. This review details studies of PET tracers aimed at interrogating cholinergic, serotonergic, opiate, benzodiazepine, and inflammatory pathways as well as PET tracers that illuminate amyloid plaques and neurofibrillary tangles in AD. Progress has been remarkable. Together with studies of brain structure with MRI and of functional regional brain activity, e.g., through measures of blood flow and glucose metabolic rate, molecular imaging promises to dramatically alter our understanding of the structural and physiological abnormalities underlying AD symptomatology. A more immediate impact on the diagnosis and treatment evaluation of AD patients in clinical trials is predicted while the possibility of personalized treatment or prevention of AD may not be that far away.

Serotoninergic modulation of GABAergic synaptic transmission in developing rat CA3 pyramidal neurons

Serotoninergic modulation of GABAergic mIPSCs was investigated in immature (postnatal 12-16-days old) rat CA3 pyramidal neurons using a conventional whole-cell patch clamp technique. Serotonin or 5-hydroxytryptamine (5-HT) (10 micromol/L) transiently and explosively increased mIPSC frequency with a small increase in the current amplitude. However, 5-HT did not affect the GABA-induced postsynaptic currents, indicating that 5-HT acts presynaptically to facilitate the probability of spontaneous GABA release. The 5-HT action on GABAergic mIPSC frequency was completely blocked by 100 nmol/L MDL72222, a selective 5-HT(3) receptor antagonist, and mimicked by mCPBG, a selective 5-HT(3) receptor agonist. The 5-HT action on GABAergic mIPSC frequency was completely occluded either in the presence of 200 mumol/L Cd2+ or in the Na+-free external solution, suggesting that the 5-HT(3) receptor-mediated facilitation of mIPSC frequency requires a Ca2+ influx passing through voltage-dependent Ca2+ channels from the extracellular space, and that presynaptic 5-HT(3) receptors are less permeable to Ca2+. The 5-HT action on mIPSC frequency in the absence or presence of extracellular Na+ gradually increased with postnatal development. Such a developmental change in the 5-HT(3) receptor-mediated facilitation of GABAergic transmission would play important roles in the regulation of excitability as well as development in CA3 pyramidal neurons.