Inhibition of NMDA-receptor mediated response in the rat medial prefrontal cortical pyramidal cells by the 5-HT3 receptor agonist SR 57227A and 5-HT: intracellular studies

In vivo blockade of 5HT3 receptors in the infralimbic medial prefrontal cortex enhances fear extinction in a rat model of PTSD

Objectives

Treatments that reverse deficits in fear extinction are promising for the management of post-traumatic stress disorder (PTSD). 5-Hydroxytryptamine type 3 (5-HT3) receptor is involved involved in the extinction of fear memories. The present work aims to investigate the role of 5HT3 receptors in the infralimbic part of the medial prefrontal cortex (IL-mPFC) in extinction of conditioned fear in the single prolonged stress (SPS) model of PTSD in rats.

Materials and Methods

The effect of SPS administration was evaluated on the freezing behavior in contextual and cued fear conditioning models. After the behavioral tests, levels of 5HT3 transcription in IL-mPFC were also measured in the same animals using the real-time RT-PCR method. To evaluate the possible role of local 5HT3 receptors on fear extinction, conditioned freezing was evaluated in another cohort of animals that received local microinjections of ondansetron (a 5HT3 antagonist) and ondansetron plus a 5HT3 agonist (SR 57227A) after extinction sessions.

Results

Our findings showed that exposure to SPS increased the freezing response in both contextual and cued fear models. We also found that SPS is associated with increased expression of 5HT3 receptors in the IL-mPFC region. Ondansetron enhanced the fear of extinction in these animals and the enhancement was blocked by the 5HT3 agonist, SR 57227A.

Conclusion

It seems that up-regulation of 5HT3 receptors in IL-mPFC is an important factor in the neurobiology of PTSD and blockade of these receptors could be considered a potential treatment for this condition.

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.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

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.

Are 5-HT3 antagonists effective in obsessive-compulsive disorder? A systematic review of literature

OBJECTIVE

The purpose of this literature database search-based review was to critically consider and evaluate the findings of literature focusing on efficacy and safety of 5-HT3 antagonists in the treatment of obsessive-compulsive disorder (OCD), so as to test whether preclinical data match clinical therapeutic trials.

DESIGN

The PubMed database has been searched for papers on 5-HT3 antagonists and OCD in humans and for animal models of OCD and 5-HT3 receptors.

RESULTS

Of the clinically tested 5-HT3 receptor antagonists, ondansetron has been used to treat OCD in five therapeutic studies, whereas granisetron only in one recent trial. Both showed some efficacy in open studies and superiority to placebo in double-blind studies, along with fair safety. No animal OCD model directly implicated 5-HT3 receptors.

CONCLUSIONS

Overall, results indicate some utility, but the available literature is too scanty to allow for valid conclusions to be drawn. The mismatch between animal models of obsessive-compulsive disorder and clinical data with 5-HT3 antagonists needs more clinical data to ensure that it is not an artefact.

The ascending median raphe projections are mainly glutamatergic in the mouse forebrain

The median raphe region (MRR) is thought to be serotonergic and plays an important role in the regulation of many cognitive functions. In the hippocampus (HIPP), the MRR exerts a fast excitatory control, partially through glutamatergic transmission, on a subpopulation of GABAergic interneurons that are key regulators of local network activity. However, not all receptors of this connection in the HIPP and in synapses established by MRR in other brain areas are known. Using combined anterograde tracing and immunogold methods, we show that the GluN2A subunit of the NMDA receptor is present in the synapses established by MRR not only in the HIPP, but also in the medial septum (MS) and in the medial prefrontal cortex (mPFC) of the mouse. We estimated similar amounts of NMDA receptors in these synapses established by the MRR and in local adjacent excitatory synapses. Using retrograde tracing and confocal laser scanning microscopy, we found that the majority of the projecting cells of the mouse MRR contain the vesicular glutamate transporter type 3 (vGluT3). Furthermore, using double retrograde tracing, we found that single cells of the MRR can innervate the HIPP and mPFC or the MS and mPFC simultaneously, and these double-projecting cells are also predominantly vGluT3-positive. Our results indicate that the majority of the output of the MRR is glutamatergic and acts through NMDA receptor-containing synapses. This suggests that key forebrain areas receive precisely targeted excitatory input from the MRR, which is able to synchronously modify activity in those regions via individual MRR cells with dual projections.

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

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

A placebo-controlled study of tropisetron added to risperidone for the treatment of negative symptoms in chronic and stable schizophrenia

RATIONAL

A growing body of evidence illustrates that 5-HT3 receptor antagonist drugs may be of benefit in the treatment of negative symptoms in schizophrenia.

OBJECTIVE

The objective of this study was to assess the efficacy and tolerability of tropisetron add-on to risperidone on negative symptoms in patients with chronic stable schizophrenia.

METHODS

In a double-blind, placebo-controlled 8-week trial, 40 patients with chronic schizophrenia who were stabilized on risperidone were randomized into tropisetron or placebo add-on groups. Psychotic symptoms were measured using the Positive and Negative Syndrome Scale (PANSS) every 2 weeks. Furthermore, extrapyramidal and depressive symptoms as well as side effects were assessed. The primary outcome measure was the difference in change from baseline of negative subscale scores between the two groups at week 8.

RESULTS

Tropisetron resulted in greater improvement of the total PANSS scores [F(1.860,70.699) = 37.366, p < 0.001] as well as negative scores [F(2.439,92.675) = 16.623, p < 0.001] and general psychopathology [F(1.767,67.158) = 4.602, p = 0.017], but not positive subscale scores [F(1.348, 51.218) = 0.048, p = 0.893] compared to placebo. In a multiple regression analysis controlling for positive, extrapyramidal, and depressive symptoms, treatment group (standardized β = -0.640) significantly predicted changes in primary negative symptoms. The side effect profile did not differ significantly between the two groups.

CONCLUSION

Tropisetron add-on to risperidone improves the primary negative symptoms of patients with chronic stable schizophrenia.

Acute 5-HT7 receptor activation increases NMDA-evoked currents and differentially alters NMDA receptor subunit phosphorylation and trafficking in hippocampal neurons

Background

N-methyl-D-aspartate (NMDA) receptors are regulated by several G protein-coupled receptors (GPCRs) as well as receptor tyrosine kinases. Serotonin (5-HT) type 7 receptors are expressed throughout the brain including the thalamus and hippocampus. Long-term (2–24 h) activation of 5-HT7 receptors promotes the expression of neuroprotective growth factor receptors, including the platelet-derived growth factor (PDGF) β receptors which can protect neurons against NMDA-induced neurotoxicity.

Results

In contrast to long-term activation of 5-HT7 receptors, acute (5 min) treatment of isolated hippocampal neurons with the 5-HT7 receptor agonist 5-carboxamidotryptamine (5-CT) enhances NMDA-evoked peak currents and this increase in peak currents is blocked by the 5-HT7 receptor antagonist, SB 269970. In hippocampal slices, acute 5-HT7 receptor activation increases NR1 NMDA receptor subunit phosphorylation and differentially alters the phosphorylation state of the NR2B and NR2A subunits. NMDA receptor subunit cell surface expression is also differentially altered by 5-HT7 receptor agonists: NR2B cell surface expression is decreased whereas NR1 and NR2A surface expression are not significantly altered.

Conclusions

In contrast to the negative regulatory effects of long-term activation of 5-HT7 receptors on NMDA receptor signaling, acute activation of 5-HT7 receptors promotes NMDA receptor activity. These findings highlight the potential for temporally differential regulation of NMDA receptors by the 5-HT7 receptor.

Granisetron as an add-on to risperidone for treatment of negative symptoms in patients with stable schizophrenia: randomized double-blind placebo-controlled study

Some 5-HT3 antagonists such as ondansetron have shown beneficial effects on negative symptoms of patients with schizophrenia. We aimed to evaluate the efficacy of granisetron (another 5-HT3 antagonist) add-on therapy in the treatment of negative symptoms of patients with stable schizophrenia. In a randomized, double-blind, and placebo-controlled study, forty stable patients with schizophrenia (DSM-IV-TR), were randomized to either granisetron (1 mg twice daily) or placebo (twice daily) in addition to risperidone up to 6 mg/day for eight weeks. The patients were assessed using positive and negative syndrome scale (PANSS) and extrapyramidal symptom rating scale (ESRS) at baseline, week 4 and 8. Hamilton depression rating scale (HDRS) was used to assess depression at baseline and week 8. Thirty-eight patients completed the trial. Granisetron group showed a significantly greater improvement on negative subscale than the placebo group at endpoint [t(38) = 6.046, mean difference (±95% CI) = 3.2(1.8-3.7), P < 0.001]. The same effect was observed for total score [t(38) = 4.168, mean difference (95% CI) = 3.2(1.6-4.7), P < 0.001]. However the placebo and granisetron groups did not differ in their reduction of positive and general psychopathology symptoms scores. HDRS scores and its changes did not differ between the two groups. The ESRS score at week 4 was significantly lower in the granisetron than the placebo group while the two groups showed similar ESRS score at week 8. Frequency of other side effects was similar between the two groups. In summary, granisetron add-on can safely and effectively reduce the primary negative symptoms of patients with schizophrenia.

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

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

Granisetron adjunct to fluvoxamine for moderate to severe obsessive-compulsive disorder: a randomized, double-blind, placebo-controlled trial

BACKGROUND

Several small studies have shown beneficial effects of ondansetron, a serotonin 5-HT(3) receptor antagonist, in the treatment of obsessive-compulsive disorder (OCD). The efficacy of other 5-HT(3) receptor antagonists in patients with OCD is still unclear. Granisetron does not alter cytochrome P450 activity and might have a lower risk of drug interactions, a longer duration of action and a better tolerability profile than other 5-HT(3) receptor antagonists.

OBJECTIVE

The objective of this study was to assess the efficacy and tolerability of granisetron augmentation of fluvoxamine in patients with OCD.

STUDY DESIGN

This was a two-centre, randomized, double-blind, placebo-controlled, parallel-group study conducted from November 2011 to March 2012.

STUDY SETTING

The study setting was outpatient clinics of two large referral centres.

PATIENTS

Study participants were men and women, aged 18-60 years, who met the diagnostic criteria of OCD based on the DSM-IV-TR and who had a Yale-Brown Obsessive Compulsive Scale (Y-BOCS) score of at least 21.

INTERVENTIONS

Participants were randomly assigned to granisetron (Kytril(®); SmithKline Beecham, Philadelphia, PA, USA) 1 mg every 12 hours or placebo every 12 hours in addition to fluvoxamine for 8 weeks.

MAIN OUTCOME MEASURE

Patients were assessed using the Y-BOCS at baseline, second, fourth, sixth and eighth weeks. The primary outcome measure was the difference in the score change of Y-BOCS total score from baseline to week 8 between the two groups. We also compared changes in the obsession and compulsion subscales of the Y-BOCS, and frequencies of partial response (≥25% reduction in Y-BOCS score), complete response (≥35% reduction in Y-BOCS score) and remission (Y-BOCS score ≤16) between the two groups.

RESULTS

Of the 42 included patients, 39 (20 in the placebo group, 19 in the granisetron group) completed the study. Significant time X treatment interaction was observed for total Y-BOCS (F [2.097, 79.678] = 4.941, p = 0.009), obsession (F [2.337, 88.799] = 4.938, p = 0.006) and compulsion (F [2.050, 77.899] = 4.674, p = 0.012) subscales. By week 8, complete response and remission were achieved by 20 (100%) and 18 (90%) patients in the granisetron group and by 7 (35%) patients in the placebo group (p-value of Fisher's exact test <0.001, risk ratio (RR) [95% CI] = 3.857 [2.039, 7.297]). There was no significant difference in the tolerability between the two regimens.

CONCLUSION

Granisetron is an efficacious adjunct for the short-term treatment of patients with moderate to severe OCD and is well tolerated.

CLINICAL TRIAL REGISTRATION NUMBER

IRCT201202041556N32.

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.

Drug therapies for attentional disorders alter the signal-to-noise ratio in the superior colliculus

Despite high levels of use, the mechanism of action of effective pharmacotherapies in attention deficit hyperactivity disorder (ADHD) is unknown. It has recently been hypothesized that one site of therapeutic action is the midbrain superior colliculus, a structure traditionally associated with visual processing, but also strongly implicated in distractibility, a core symptom of ADHD. We used male juvenile Wistar rats to examine the effects of therapeutically relevant doses of methylphenidate and d-amphetamine on collicular activity in vitro. Here we report a novel shared mechanism of the two drugs whereby they enhance the signal-to-noise ratio in the superior colliculus. The effects on the signal-to-noise ratio were mediated by serotonin (5-HT) via a pre-synaptic mechanism. This modulatory action would bias the system towards salient events and lead to an overall decrease in distractibility.

Serotonergic modulation of plasticity of the auditory cortex elicited by fear conditioning

In the awake big brown bat, 30 min auditory fear conditioning elicits conditioned heart rate decrease and long-term best frequency (BF) shifts of cortical auditory neurons toward the frequency of the conditioned tone; 15 min conditioning elicits subthreshold cortical BF shifts that can be augmented by acetylcholine. The fear conditioning causes stress and an increase in the cortical serotonin (5-HT) level. Serotonergic neurons in the raphe nuclei associated with stress and fear project to the cerebral cortex and cholinergic basal forebrain. Recently, it has been shown that 5-HT(2A) receptors are mostly expressed on pyramidal neurons and their activation improves learning and memory. We applied 5-HT, an agonist (alpha-methyl-5-HT), or an antagonist (ritanserin) of 5-HT(2A) receptors to the primary auditory cortex and discovered the following drug effects: (1) 5-HT had no effect on the conditioned heart rate change, although it reduced the auditory responses; (2) 4 mm 5-HT augmented the subthreshold BF shifts, whereas 20 mm 5-HT did not; (3) 20 mm 5-HT reduced the long-term BF shifts and changed them into short-term; (4) alpha-methyl-5-HT increased the auditory responses and augmented the subthreshold BF shifts as well as the long-term BF shifts; (5) in contrast, ritanserin reduced the auditory responses and reversed the direction of the BF shifts. Our data indicate that the BF shift can be modulated by serotonergic neurons that augment or reduce the BF shift or even reverse the direction of the BF shift. Therefore, not only the cholinergic system, but also the serotonergic system, plays an important role in cortical plasticity according to behavioral demands.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

5-HT system and cognition

The study of 5-hydroxytryptamine (5-HT) system has benefited from the identification, classification and cloning of multiple 5-HT receptors (5-HT1 to 5-HT7). Growing evidence suggests that 5-HT is important in learning and memory and all its receptors might be implicated in this. Actually, 5-HT pathways, 5-HT reuptake site/transporter complex and 5-HT receptors show regional distribution in brain areas implicated in learning and memory. Likewise, the stimulation or blockade of presynaptic 5-HT1A, 5-HT1B, 5-HT(2A/2C) and 5-HT3 receptors, postsynaptic 5-HT(2B/2C) and 5-HT4 receptors and 5-HT uptake/transporter sites modulate these processes. Available evidence strongly suggests that the 5-HT system may be important in normal function, the treatment and/or pathogenesis of cognitive disorders. Further investigation will help to specify the 5-HT system nature involvement in cognitive processes, pharmacotherapies, their mechanisms and action sites and to determine under which conditions they could operate. In this regard, it is probable that selective drugs with agonists, neutral antagonist, agonists or inverse agonist properties for 5-HT1A, 5-HT(1B/1D), 5-HT(2A/2B/2C), 5-HT4 and 5-HT7 receptors could constitute a new therapeutic opportunity for learning and memory alterations.

LSD and DOB: interaction with 5-HT2A receptors to inhibit NMDA receptor-mediated transmission in the rat prefrontal cortex

Both the phenethylamine hallucinogen (-)-1-2, 5-dimethoxy-4-bromophenyl-2-aminopropane (DOB), a selective serotonin 5-HT2A,2C receptor agonist, and the indoleamine hallucinogen D-lysergic acid diethylamide (LSD, which binds to 5-HT1A, 1B, 1D, 1E, 1F, 2A, 2C, 5, 6, 7, dopamine D1 and D2, and alpha1 and alpha2 adrenergic receptors), but not their non-hallucinogenic congeners, inhibited N-methyl-D-aspartate (NMDA)-induced inward current and NMDA receptor-mediated synaptic responses evoked by electrical stimulation of the forceps minor in pyramidal cells of the prefrontal cortical slices. The inhibitory effect of hallucinogens was mimicked by 5-HT in the presence of selective 5-HT1A and 5-HT3 receptor antagonists. The inhibitory action of DOB, LSD and 5-HT on the NMDA transmission was blocked by the 5-HT2A receptor antagonists R-(+)-alpha-(2, 3-dimethoxyphenil)-1-[4-fluorophenylethyl]-4-piperidineme thanol (M100907) and ketanserin. However, at low concentrations, when both LSD and DOB by themselves only partially depressed the NMDA response, they blocked the inhibitory effect of 5-HT, suggesting a partial agonist action. Whereas N-(4-aminobutyl)-5-chloro-2-naphthalenesulphonamide (W-7, a calmodulin antagonist) and N-[2-[[[3-(4'-chlorophenyl)- 2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4'-methoxy-b enzenesulphonamide phosphate (KN-93, a Ca2+/CaM-KII inhibitor), but not the negative control 2-[N-4'methoxybenzenesulphonyl]amino-N-(4'-chlorophenyl)-2-propeny l-N -methylbenzylamine phosphate (KN-92), blocked the inhibitory action of LSD and DOB, the selective protein kinase C inhibitor chelerythrine was without any effect. We conclude that phenethylamine and indoleamine hallucinogens may exert their hallucinogenic effect by interacting with 5-HT2A receptors via a Ca2+/CaM-KII-dependent signal transduction pathway as partial agonists and modulating the NMDA receptors-mediated sensory, perceptual, affective and cognitive processes.

A pre- and postsynaptic modulatory action of 5-HT and the 5-HT2A, 2C receptor agonist DOB on NMDA-evoked responses in the rat medial prefrontal cortex

Intracellular recordings were made from pyramidal neurons in layers V and VI of the rat medial prefrontal cortex in slice preparations to investigate the effect of the serotonin 5-HT2A,2C receptor agonist (-)-1-2,5-dimethoxy-4-bromophenol-2-aminopropane (DOB) and 5-hydroxytryptamine (5-HT) on N-methyl-D-aspartate (NMDA)-induced responses. Bath application of either DOB or 5-HT [in the presence of antagonists to 5-HT1A, 5-HT3 and gamma-aminobutytric acid (GABA) receptors] produced a concentration-dependent biphasic modulation of the NMDA responses. They facilitated and inhibited NMDA responses at low (</= 1 microM DOB and </= 50 microM 5-HT) and higher concentrations, respectively. Both the facilitating and inhibitory action were blocked by the highly selective 5-HT2A receptor antagonist R-(+)-alpha-(2, 3-dimethoxyphenil)-1-[4-fluorophenylethyl]-4-piperidineme thanol (M100907) and the 5-HT2 receptor antagonist ketanserin, thus indicating that both facilitation and inhibition were mediated by the activation of the 5-HT2A receptor subtype. However, the facilitating, but not inhibitory, action of DOB showed a marked desensitization, suggesting that the facilitation and inhibition of NMDA responses resulted from activation of different 5-HT2A receptor subtypes and/or signal-transduction pathways. Indeed, the selective PKC inhibitor chelerythrine and the Ca2+/CaM-KII inhibitor KN-93 prevented the facilitating and inhibitory action of DOB, respectively. We have generated several lines of evidence to indicate the following scenario. Low concentrations of DOB, at presynaptic nerve terminals, markedly enhance NMDA-induced release of excitatory amino acids (EAAs), which then act upon both NMDA and non-NMDA receptors to elicit inward current. The massive inward current masks the postsynaptic inhibitory action of DOB. At higher concentrations, DOB inhibits the release of EAAs and discloses the postsynaptic inhibitory action.