Involvement of GABA systems in acetylcholine release induced by 5-HT3 receptor blockade in slices from rat entorhinal cortex

Novel Pitolisant-Derived Sulfonyl Compounds for Alzheimer Disease

Alzheimer's disease (AD) is a complex and multifactorial neurodegenerative disorder characterized by cognitive decline, memory loss, behavioral changes, and other neurological symptoms. Considering the urgent need for new AD therapeutics, in the present study we designed, synthesized, and evaluated multitarget compounds structurally inspired by sulfonylureas and pitolisant with the aim of obtaining multitarget ligands for AD treatment. Due to the diversity of chemical scaffolds, a novel strategy has been adopted by merging into one structure moieties displaying H3R antagonism and acetylcholinesterase inhibition. Eight compounds, selected by their binding activity on H3R, showed a moderate ability to inhibit acetylcholinesterase activity in vitro, and two of the compounds (derivatives 2 and 7) were also capable of increasing acetylcholine release in vitro. Among the tested compounds, derivative 2 was identified and selected for further in vivo studies. Compound 2 was able to reverse scopolamine-induced cognitive deficits with results comparable to those of galantamine, a drug used in clinics for treating AD. In addition to its efficacy, this compound showed moderate BBB permeation in vitro. Altogether, these results point out that the fragment-like character of compound 2 leads to an optimal starting point for a plausible medicinal chemistry approach for this novel strategy.

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.

Granisetron Alleviates Alzheimer's Disease Pathology in TgSwDI Mice Through Calmodulin-Dependent Protein Kinase II/cAMP-Response Element Binding Protein Pathway

Alzheimer's disease (AD) is characterized by a compromised blood-brain barrier (BBB) and disrupted intracellular calcium homeostasis in the brain. Therefore, rectifying the BBB integrity and restoring calcium homeostasis could provide an effective strategy to treat AD. Recently, we developed a high throughput-screening assay to screen for compounds that enhance a cell-based BBB model integrity, which identified multiple hits among which is granisetron, a Food and Drug Administration approved drug. Here, we evaluated the therapeutic potential of granisetron against AD. Granisetron was tested in C57Bl/6J young and aged wild-type mice, and in a transgenic mouse model of AD namely TgSwDI for its effect on BBB intactness and amyloid-β (Aβ)-related pathology. Our study findings showed that granisetron enhanced BBB integrity in both aged and TgSwDI mice. This effect was associated with an overall reduction in Aβ load and neuroinflammation in TgSwDI mice brains. In addition, and supported by proteomics analysis, granisetron significantly reduced Aβ induced calcium influx in vitro, and rectified calcium dyshomeostasis in TgSwDI mice brains by restoring calmodulin-dependent protein kinase II/cAMP-response element binding protein pathway, which was associated with cognitive improvement. These results support granisetron repurposing as a potential drug to hold, slow, and/or treat AD.

5-HT3 Receptors: A Potential Therapeutic Target for Epilepsy

BACKGROUND

Epilepsy is a syndrome of brain dysfunction caused by spontaneous, abnormal discharge. Many anti-epileptic drugs have developed in past decades. 5-HT is an important neurotransmitter in the central and peripheral nervous system of the human body which is involved in a number of physiological activities, such as sensation, movement, and behavior. 5-HT subtype have been divided into seven sub-groups from 5-HT1 to 5HT7. However, the role of 5-HT3 receptor on epilepsy is unclear. Therefore, in this article, the possible role of 5-HT3 receptor on epilepsy was systemically reviewed.

METHODS

Data were collected from Web of Science, Medline, Pubmed, Scopus, through searching of these keywords: "5-HT3" and "epilepsy".

RESULTS

An increasing number of studies have shown that the activation of the 5-HT3 receptor can inhibit epileptic seizures, while inhibition of the 5-HT3 receptor can promote spike waves.

CONCLUSION

In this review, we discuss the relationship between the 5HT3 receptor and epilepsy; this review may provide a new insight for clinical application of epilepsy treatment.

The wonderland of neuronal nicotinic acetylcholine receptors

Nearly 30 years of experimental evidence supports the argument that ligands of nicotinic acetylcholine receptors (nAChRs) have potential as therapeutic agents. However, as in the famous Lewis Carroll novel "Alice in Wonderland", there have been many unexpected adventures along the pathway of development, and few nAChR ligands have been approved for any clinical condition to date with the exception of nicotine dependence. The recent failures of nAChR ligands in AD and schizophrenia clinical trials have reduced enthusiasm for this therapeutic strategy and many pharmaceutical companies have now abandoned this field of research. As with other clinical failures, multiple questions arise as to the basis for the failure. More generic questions focus on a potential translational gap between the animal models used and the human clinical condition they are meant to simulate, or the clinical trial mindset that large Ns have to be achieved for statistical power (often requiring multiple trial sites) as opposed to smaller patient cohorts at limited sites where conditions can be better controlled and replicated. More specific to the nAChR field are questions about subtype selectivity, dose selection, whether an agonist, antagonist, or allosteric modulator strategy is best, etc. The purpose of this review is to discuss each of these questions, but also to provide a brief overview of the remarkable progress that has been made over the last three decades in our understanding of this unique ligand-gated ion channel and how this new knowledge may help us improve drug development successes in the future.

Effect of canagliflozin and metformin on cortical neurotransmitters in a diabetic rat model

BACKGROUND

The rapid economic development in the Arabian Gulf has resulted in lifestyle changes that have increased the prevalence of obesity and type 2 diabetes, with the greatest increases observed in Kuwait. Dyslipidemia and diabetes are risk factors for disruptions in cortical neurotransmitter homeostasis. This study investigated the effect of the antidiabetic medications canagliflozin (CAN) and metformin (MET) on the levels of cortical neurotransmitters in a diabetic rat model.

MATERIALS AND METHODS

The rats were assigned to the control (C) group, the diabetic group that did not receive treatment (D) or the diabetic group treated with either CAN (10 mg/kg) or MET (100 mg/kg) for 2 or 4 weeks. Blood and urine glucose levels and cortical acetylcholinesterase (AChE) activity were assayed, and amino acid and monoamine levels were measured using HPLC.

RESULTS

The diabetic group exhibited a significant increase in AChE activity and a decrease in monoamine and amino acid neurotransmitter levels. In the CAN group, AChE was significantly lower than that in the D and D + MET groups after 2 weeks of treatment. In addition, a significant increase in some cortical monoamines and amino acids was observed in the D + MET and D + CAN groups compared with the D group. Histopathological analysis revealed the presence of severe focal hemorrhage, neuronal degeneration, and cerebral blood vessel congestion, with gliosis in the cerebrum of rats in the D group. The CAN-treated group exhibited severe cerebral blood vessel congestion after 2 weeks of treatment and focal gliosis in the cerebrum after 4 weeks of treatment. Focal gliosis in the cerebrum of rats in the MET-treated group was observed after 2 and 4 weeks of treatment.

CONCLUSIONS

We conclude that the effect of CAN and MET on neurotransmitters is potentially mediated by their antihyperglycemic and antihyperlipidemic effects. In addition, the effects of CAN on neurotransmitters might be associated with its receptor activity, and the effect of MET on neurotransmitters might be associated with cerebral metabolism.

Molecular and Functional Characterization of Bacopa monniera: A Retrospective Review

Over the last 50 years, laboratories around the world analyzed the pharmacological effect of Bacopa monniera extract in different dimensions, especially as a nerve tonic and memory enhancer. Studies in animal model evidenced that Bacopa treatment can attenuate dementia and enhances memory. Further, they demonstrate that Bacopa primarily either acts via antioxidant mechanism (i.e., neuroprotection) or alters different neurotransmitters (serotonin (5-hydroxytryptamine, 5-HT), dopamine (DA), acetylcholine (ACh), γ-aminobutyric acid (GABA)) to execute the pharmacological effect. Among them, 5-HT has been shown to fine tune the neural plasticity, which is a substrate for memory formation. This review focuses on the studies which trace the effect of Bacopa treatment on serotonergic system and 5-HT mediated key molecular changes that are associated with memory formation.

Deterministic functions of cortical acetylcholine

Traditional descriptions of the basal forebrain cholinergic projection system to the cortex have focused on neuromodulatory influences, that is, mechanisms that modulate cortical information processing but are not necessary for mediating discrete behavioral responses and cognitive operations. This review summarises and conceptualises the evidence in support of more deterministic contributions of cholinergic projections to cortical information processing. Through presynaptic receptors expressed on cholinergic terminals, thalamocortical and corticocortical projections can evoke brief cholinergic release events. These acetylcholine (ACh) release events occur on a fast, sub-second to seconds-long time scale ('transients'). In rats performing a task requiring the detection of cues as well as the report of non-cue events cholinergic transients mediate the detection of cues specifically in trials that involve a shift from a state of monitoring for cues to cue-directed responding. Accordingly, ill-timed cholinergic transients, generated using optogenetic methods, force false detections in trials without cues. We propose that the evidence is consistent with the hypothesis that cholinergic transients reduce detection uncertainty in such trials. Furthermore, the evidence on the functions of the neuromodulatory component of cholinergic neurotransmission suggests that higher levels of neuromodulation favor staying-on-task over alternative action. In other terms, higher cholinergic neuromodulation reduces opportunity costs. Evidence indicating a similar integration of other ascending projection systems, including noradrenergic and serotonergic systems, into cortical circuitry remains sparse, largely because of the limited information about local presynaptic regulation and the limitations of current techniques in measuring fast and transient neurotransmitter release events in these systems.

Intermittent versus continuous central administration of clozapine in DBA/2 mice, improvement in sensory inhibition deficits

Deficient sensory inhibition, the failure to inhibit responses to repeated stimuli, is a hallmark of schizophrenia, and is thought to be related to difficulties with attention and working memory. Sensory inhibition is assessed by comparing the auditory-evoked EEG responses to 2 closely-spaced identical stimuli. Normal individuals show suppressed response to the second stimulus while schizophrenia patients have responses of similar magnitude to both stimuli. This deficit has been linked to polymorphisms in the promoter for the α7 nicotinic receptor gene, resulting in reduced numbers of receptors on hippocampal interneurons. This deficit is modeled in DBA/2 mice which also show a polymorphism in the promoter for the α7 nicotinic receptor gene and reduced numbers of hippocampal α7 receptors. Systemic administration of clozapine, the most efficacious antipsychotic medication, improves sensory inhibition deficits in both schizophrenia patients and DBA/2 mice. We have previously shown that acute intracerebroventricular (ICV) injections of clozapine induced similar improvement in sensory inhibition in DBA/2 mice. Here we demonstrate the efficacy of chronic ICV clozapine administration in improving sensory inhibition in DBA2 mice. Mice received ICV vehicle, 3, 7.5, 15 or 30 μg of clozapine, either continuously or as a once-per-day injection. Mice were recorded on the 7th day of drug delivery. Both approaches produced improved sensory inhibition, but the daily bolus injection was effective at a lower dose (3 μg/day) than the continuous delivery (15 μg/day). The bolus injections also showed significant improvement up to 36 h post injection thus suggesting that this approach may be more efficacious.

Pharmacological treatment of schizophrenia: a critical review of the pharmacology and clinical effects of current and future therapeutic agents

Since the introduction of chlorpromazine and throughout the development of the new-generation antipsychotic drugs (APDs) beginning with clozapine, the D(2) receptor has been the target for the development of APDs. Pharmacologic actions to reduce neurotransmission through the D(2) receptor have been the only proven therapeutic mechanism for psychoses. A number of novel non-D(2) mechanisms of action of APDs have been explored over the past 40 years but none has definitively been proven effective. At the same time, the effectiveness of treatments and range of outcomes for patients are far from satisfactory. The relative success of antipsychotics in treating positive symptoms is limited by the fact that a substantial number of patients are refractory to current medications and by their lack of efficacy for negative and cognitive symptoms, which often determine the level of functional impairment. In addition, while the newer antipsychotics produce fewer motor side effects, safety and tolerability concerns about weight gain and endocrinopathies have emerged. Consequently, there is an urgent need for more effective and better-tolerated antipsychotic agents, and to identify new molecular targets and develop mechanistically novel compounds that can address the various symptom dimensions of schizophrenia. In recent years, a variety of new experimental pharmacological approaches have emerged, including compounds acting on targets other than the dopamine D(2) receptor. However, there is still an ongoing debate as to whether drugs selective for singe molecular targets (that is, 'magic bullets') or drugs selectively non-selective for several molecular targets (that is, 'magic shotguns', 'multifunctional drugs' or 'intramolecular polypharmacy') will lead to more effective new medications for schizophrenia. In this context, current and future drug development strategies can be seen to fall into three categories: (1) refinement of precedented mechanisms of action to provide drugs of comparable or superior efficacy and side-effect profiles to existing APDs; (2) development of novel (and presumably non-D(2)) mechanism APDs; (3) development of compounds to be used as adjuncts to APDs to augment efficacy by targeting specific symptom dimensions of schizophrenia and particularly those not responsive to traditional APD treatment. In addition, efforts are being made to determine if the products of susceptibility genes in schizophrenia, identified by genetic linkage and association studies, may be viable targets for drug development. Finally, a focus on early detection and early intervention aimed at halting or reversing progressive pathophysiological processes in schizophrenia has gained great influence. This has encouraged future drug development and therapeutic strategies that are neuroprotective. This article provides an update and critical review of the pharmacology and clinical profiles of current APDs and drugs acting on novel targets with potential to be therapeutic agents in the future.

Effects of acetylcholine on neuronal properties in entorhinal cortex

The entorhinal cortex (EC) receives prominent cholinergic innervation from the medial septum and the vertical limb of the diagonal band of Broca (MSDB). To understand how cholinergic neurotransmission can modulate behavior, research has been directed toward identification of the specific cellular mechanisms in EC that can be modulated through cholinergic activity. This review focuses on intrinsic cellular properties of neurons in EC that may underlie functions such as working memory, spatial processing, and episodic memory. In particular, the study of stellate cells (SCs) in medial entorhinal has resulted in discovery of correlations between physiological properties of these neurons and properties of the unique spatial representation that is demonstrated through unit recordings of neurons in medial entorhinal cortex (mEC) from awake-behaving animals. A separate line of investigation has demonstrated persistent firing behavior among neurons in EC that is enhanced by cholinergic activity and could underlie working memory. There is also evidence that acetylcholine plays a role in modulation of synaptic transmission that could also enhance mnemonic function in EC. Finally, the local circuits of EC demonstrate a variety of interneuron physiology, which is also subject to cholinergic modulation. Together these effects alter the dynamics of EC to underlie the functional role of acetylcholine in memory.

Involvement of nitric oxide in granisetron improving effect on scopolamine-induced memory impairment in mice

Granisetron, a serotonin 5-HT(3) receptor antagonist, widely used as an antiemetic drug following chemotherapy, has been found to improve learning and memory. In this study, effects of granisetron on spatial recognition memory and fear memory and the involvement of nitric oxide (NO) have been determined in a Y-maze and passive avoidance test. Granisetron (3, 10mg/kg, intraperitoneally) was administered to scopolamine-induced memory-impaired mice prior to acquisition, consolidation and retrieval phases, either in the presence or in the absence of a non-specific NO synthase inhibitor, l-NAME (3, 10mg/kg, intraperitoneally); a specific inducible NO synthase (iNOS) inhibitor, aminoguanidine (100mg/kg); and a NO precursor, l-arginine (750 mg/kg). It is demonstrated that granisetron improved memory acquisition in a dose-dependent manner, but it was ineffective on consolidation and retrieval phases of memory. The beneficial effect of granisetron (10mg/kg) on memory acquisition was significantly reversed by l-NAME (10mg/kg) and aminoguanidine (100mg/kg); however, l-arginine (750 mg/kg) did not potentiate the effect of sub-effective dose of granisetron (3mg/kg) in memory acquisition phase. It is concluded that nitric oxide is probably involved in improvement of memory acquisition by granisetron in both spatial recognition memory and fear memory. This article is part of a Special Issue entitled The Cognitive Neuroscience.

Attenuation of 1-(m-chlorophenyl)-biguanide induced hippocampus-dependent memory impairment by a standardised extract of Bacopa monniera (BESEB CDRI-08)

Bacopa monniera is a well-known medhya-rasayana (memory enhancing and rejuvenating) plant in Indian traditional medical system of Ayurveda. The effect of a standardized extract of Bacopa monniera (BESEB CDRI-08) on serotonergic receptors and its influence on other neurotransmitters during hippocampal-dependent learning was evaluated in the present study. Wistar rat pups received a single dose of BESEB CDRI-08 during postnatal days 15-29 showed higher latency during hippocampal-dependent learning accompanied with enhanced 5HT(3A) receptor expression, serotonin and acetylcholine levels in hippocampus. Furthermore, 5HT(3A) receptor agonist 1-(m-chlorophenyl)-biguanide (mCPBG) impaired learning in the passive avoidance task followed by reduction of 5HT(3A) receptor expression, 5HT and ACh levels. Administration of BESEB CDRI-08 along with mCPBG attenuated mCPBG induced behavioral, molecular and neurochemical alterations. Our results suggest that BESEB CDRI-08 possibly acts on serotonergic system, which in turn influences the cholinergic system through 5-HT(3) receptor to improve the hippocampal-dependent task.

Alzheimer's disease and age-related memory decline (preclinical)

An unfortunate result of the rapid rise in geriatric populations worldwide is the increasing prevalence of age-related cognitive disorders such as Alzheimer's disease (AD). AD is a devastating neurodegenerative illness that is characterized by a profound impairment of cognitive function, marked physical disability, and an enormous economic burden on the afflicted individual, caregivers, and society in general. The rise in elderly populations is also resulting in an increase in individuals with related (potentially treatable) conditions such as "Mild Cognitive Impairment" (MCI) which is characterized by a less severe (but abnormal) level of cognitive impairment and a high-risk for developing dementia. Even in the absence of a diagnosable disorder of cognition (e.g., AD and MCI), the perception of increased forgetfulness and declining mental function is a clear source of apprehension in the elderly. This is a valid concern given that even a modest impairment of cognitive function is likely to be associated with significant disability in a rapidly evolving, technology-based society. Unfortunately, the currently available therapies designed to improve cognition (i.e., for AD and other forms of dementia) are limited by modest efficacy and adverse side effects, and their effects on cognitive function are not sustained over time. Accordingly, it is incumbent on the scientific community to develop safer and more effective therapies that improve and/or sustain cognitive function in the elderly allowing them to remain mentally active and productive for as long as possible. As diagnostic criteria for memory disorders evolve, the demand for pro-cognitive therapeutic agents is likely to surpass AD and dementia to include MCI and potentially even less severe forms of memory decline. The purpose of this review is to provide an overview of the contemporary therapeutic targets and preclinical pharmacologic approaches (with representative drug examples) designed to enhance memory function.

How human electrophysiology informs psychopharmacology: from bottom-up driven processing to top-down control

This review surveys human event-related brain potential (ERP) and event-related magnetic field (ERF) approaches to psychopharmacology and psychopathology, and the way in which they complement behavioral studies and other neuroimaging modalities. The major paradigms involving ERP/ERF are P50 suppression, loudness-dependent auditory evoked potential (LDAEP), mismatch negativity (MMN), P300, mental chronometry, inhibitory control, and conflict processing (eg, error-related negativity (ERN)). Together these paradigms cover a range of more bottom-up driven to more top-down controlled processes. A number of relationships between the major neurotransmitter systems and electrocortical mechanisms are highlighted. These include the role of dopamine in conflict processing, and perceptual processing vs motor preparation; the role of serotonin in P50 suppression, LDAEP, and MMN; glutamate/NMDA and MMN; and the role of acetylcholine in P300 generation and memory-related processes. A preliminary taxonomy for these relationships is provided, which should be helpful in attuning possible new treatments or new applications of existing treatments to various disorders.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Effects of add-on mirtazapine on neurocognition in schizophrenia: a double-blind, randomized, placebo-controlled study

Mirtazapine added to antipsychotics appears to improve the clinical picture of schizophrenia, including both negative and positive symptoms. This study explored the effect of adjunctive mirtazapine on neurocognition in patients with schizophrenia who had shown an insufficient response to first-generation antipsychotics (FGAs). Thirty-seven schizophrenia patients, who were at least moderately ill despite their FGA treatment, received add-on mirtazapine (n=19) or placebo (n=18) in a 6-wk double-blind, randomized trial. Widely used neuropsychological tests were performed to explore visual-spatial functions, verbal and visual memory, executive functions, verbal fluency and general mental and psychomotor speed. The data were analysed on the modified intent-to-treat basis with last observation carried forward. False discovery rate was applied to correct for multiple testing. Mirtazapine outperformed placebo in the domains of visual-spatial ability and general mental speed/attentional control as assessed by, correspondingly, Block Design and Stroop dots. The difference in the degree of change (i.e. change while on mirtazapine minus that on placebo) was 18.6% (p=0.044) and 11.1% (p=0.044), respectively. Adjunctive mirtazapine might offer a safe, effective and cost-saving option as a neurocognitive enhancer for FGA-treated schizophrenia patients. Mirtazapine+FGA combinations may become especially useful in light of the currently increasing attention towards FGAs. Larger and longer studies that incorporate functional outcomes, as well as comparisons with second-generation antipsychotics are, however, still needed for more definite conclusions.

The effects of increased serotonergic activity on human sensory gating and its neural generators

RATIONALE

Schizophrenia is a disabling illness with deficits in core mental functions such as sensory gating. The P50 amplitude is an (usually auditory) evoked brain potential that, in a so-called double-click paradigm, can be used to quantify sensory gating. Reports on serotonergic modulation of P50 suppression are sparse.

OBJECTIVE

The objective of this study was to study the effects of increased serotonergic activity on parameters of P50 suppression in healthy volunteers.

MATERIALS AND METHODS

In a double-blind placebo-controlled crossover design, 21 healthy male volunteers received either placebo or a dose of 10 mg of escitalopram (selective serotonin reuptake inhibitor), after which they were tested in a P50 suppression paradigm. Furthermore, an attempt was made to identify the neural generators of the P50 evoked potential.

RESULTS

Escitalopram did not affect P50 suppression but was found to increase P50 amplitude to the first (or conditioning) stimulus. Two bilateral sources located in the temporal cortex, two bilaterally located near the eyes, and one in a fronto-central location were identified, the latter correlating positively with the P50 amplitude.

CONCLUSIONS

In the current study, escitalopram did not affect P50 suppression in healthy male volunteers, which indicates that sensory gating is not affected by a nonspecific increase in serotonergic activity. Furthermore, a generator with a fronto-central location in the brain (possibly the anterior cingulate) was found to be the primary source of the P50 evoked potential.

Effect of selective cholinergic denervation on the serotonergic system: implications for learning and memory

The cholinergic system has been widely implicated in cognitive processes and cholinergic loss is a classical hallmark in Alzheimer disease. Increasing evidence supports a role of the serotonergic system in cognition, possibly through a modulation of cholinergic activity. We compared selective cholinergic denervation by administration of the immunotoxin 192 IgG-saporin in the nucleus basalis of Meynert (NBM) with intracerebroventricular (ICV) lesions of the basal forebrain in male rats 7 days after lesioning. NBM lesions induced significant changes in cholinergic markers in the frontal cortex, whereas ICV lesions produced significant decreases in cholinergic markers both in the frontal cortex and hippocampus. Only ICV lesions lead to memory impairments in passive avoidance and Morris water maze tasks. Both models lead to reductions of serotonin levels in the frontal cortex. Similar changes in 5-hydroxytriptophan levels were observed, suggesting a downregulation of the rate-limiting enzyme for the synthesis of serotonin along with the cholinergic deficit. Neither 5-HT1A nor 5-HT1B receptors seem to mediate this process. These data imply that the serotonergic system in the frontal cortex can compensate for diminished cholinergic function and support the investigation of the serotonergic system as a therapeutic target to treat Alzheimer disease.

Lack of localization of 5-HT6 receptors on cholinergic neurons: implication of multiple neurotransmitter systems in 5-HT6 receptor-mediated acetylcholine release

The involvement of the cholinergic system in learning and memory together with the cognitive enhancing properties of 5-HT6 receptor antagonists led us to study the relationship between 5-HT6 receptors and cholinergic neurotransmission. A selective cholinergic lesion, induced by injection of the immunotoxin 192-IgG-Saporin into the nucleus basalis magnocellularis, failed to alter the density of 5-HT6 receptor mRNA or protein expression in the deafferentated frontal cortex, suggesting that 5-HT6 receptors are not located on cholinergic neurons. The 5-HT6 receptor antagonist SB-357134 (0.001-1 microM) induced a concentration-dependant K+-evoked [3H]acetylcholine (ACh) release in vitro in rat cortical and striatal slices, which was blocked by tetrodotoxin. SB-357134, up to 1 microM, stimulated glutamate release in cortical and striatal slices. In the cortex, riluzole (1 microM) blocked the SB-357134-induced K+-stimulated [3H]ACh release, and simultaneous administration of MK-801 (1 microM) and SB-357134 (0.05 microM) elicited an increase in K+-evoked ACh release. In the striatum, SB-357134, 1 microM, decreased dopamine release, and the increase in K+-evoked [3H]ACh release induced by 5-HT6 receptor blockade was reversed by the D1 receptor antagonist, SCH23390 (1 microM). In both the frontal cortex and striatum, bicuculline, 1 microM, showed no effect on SB-357134-evoked [3H]ACh. These results are discussed in terms of neurochemical mechanisms involved in 5-HT6 receptor functions.

Opposite effects of presynaptic 5-HT3 receptor activation on spontaneous and action potential-evoked GABA release at hippocampal synapses

5-HT(3) (serotonin type 3) receptors are targets of antiemetics, antipsychotics, and antidepressants and are believed to play a role in cognition. Nevertheless, contrasting results have been obtained with respect to their functions in the CNS and in the control of transmitter release. We used rat hippocampal neurons in single-neuron microcultures to identify the roles of presynaptic 5-HT(3) receptors at central synapses. 5-HT (10 microm) caused a transient > 10-fold increase in the frequency of miniature inhibitory postsynaptic currents without affecting amplitudes or kinetics. This effect was abolished by tropisetron (30 nm) and when Ca(2+) channels were blocked by 100 microm Cd(2+) it was mimicked and occluded when neurons were depolarized by 20 mm, but not 10 mm, K(+). Thus, activation of presynaptic 5-HT(3) receptors increased spontaneous GABA release by causing depolarization and opening of voltage-gated Ca(2+) channels. In microculture neurons, 5-HT transiently reduced action potential-evoked inhibitory autaptic currents by > 50%; this effect was blocked by tropisetron and mimicked by 20 mm, but not 10 mm, K(+). Miniature excitatory postsynaptic currents were not altered by 5-HT. Excitatory autaptic currents were tonically reduced, an effect attenuated by 5-HT(1A) antagonists. Thus, presynaptic 5-HT(3) receptors control GABA, but not glutamate, release and mediate opposite effects on spontaneous and action potential-dependent release.

Microinjection of ritanserin into the CA1 region of hippocampus improves scopolamine-induced amnesia in adult male rats

The effect of ritanserin (5-HT2 antagonist) on scopolamine (muscarinic cholinergic antagonist)-induced amnesia in Morris water maze (MWM) was investigated. Rats were divided into eight groups and bilaterally cannulated into CA1 region of the hippocampus. One week later, they received repeatedly vehicles (saline, DMSO, saline+DMSO), scopolamine (2 microg/0.5 microl saline/side; 30 min before training), ritanserin (2, 4 and 8 microg/0.5 microl DMSO/side; 20 min before training) and scopolamine (2 microg/0.5 microl; 30 min before ritanserin injection)+ritanserin (4 microg/0.5 microl DMSO) through cannulae each day. Animals were tested for four consecutive days (4 trial/day) in MWM during which the position of hidden platform was unchanged. In the fifth day, the platform was elevated above the water surface in another position to evaluate the function of motor, motivational and visual systems. The results showed a significant increase in escape latencies and traveled distances to find platform in scopolamine-treated group as compared to saline group. Ritanserin-treated rats (4 microg/0.5 microl/side) showed a significant decrease in the mentioned parameters as compared to DMSO-treated group. However, scopolamine and ritanserin co-administration resulted in a significant decrease in escape latencies and traveled distances as compared to the scopolamine-treated rats. Our findings show that microinjection of ritanserin into the CA1 region of the hippocampus improves the scopolamine-induced amnesia.

Tropisetron improves deficient inhibitory auditory processing in DBA/2 mice: role of alpha 7 nicotinic acetylcholine receptors

RATIONALE

Deficient inhibitory processing of the P50 auditory evoked potential is a pathophysiological feature of schizophrenia. Several lines of evidence suggest that alpha 7 nicotinic receptors play a critical role in this phenomenon. Similar to schizophrenic patients, DBA/2 mice spontaneously exhibit a deficit in inhibitory processing of the P20-N40 auditory evoked potential, which is thought to be a rodent analog of the human P50 auditory evoked potential.

OBJECTIVE

The present study was undertaken to examine whether tropisetron, a partial agonist at alpha 7 nicotinic receptors and an antagonist at 5-hydroxytryptamine-3 receptors, improves this deficit in DBA/2 mice.

RESULTS

Administration of tropisetron (1 mg/kg i.p.) significantly improved the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice. Coadministration of methyllycaconitine (MLA; 3 mg/kg i.p.), a partially selective antagonist at alpha 7 nicotinic receptors, significantly blocked the normalizing effect of tropisetron. Furthermore, MLA alone did not alter the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice.

CONCLUSIONS

The data suggest that tropisetron improves the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice by effects on alpha 7 and perhaps alpha 4 beta 2 nicotinic receptors. Tropisetron may be useful for the treatment of deficient inhibitory processing in schizophrenia.

Facilitation of cholinergic transmission by combined treatment of ondansetron with flumazenil after cortical cholinergic deafferentation

We have studied the effects of concomitant blockade of 5-HT(3) and GABA(A) receptors on acetylcholine (ACh) release in the frontal cortex of rats with a selective cholinergic lesion. Lesions were performed by microinjection of the cholinergic toxin 192 IgG-saporin into the nucleus basalis magnocellularis. Single treatment with either the 5-HT(3) receptor antagonist ondansetron, 0.1 microg/kg, or the GABA(A) receptor benzodiazepine site antagonist flumazenil, 10 mg/kg, did not affect ACh release. However, the combined ondansetron + flumazenil administration significantly increased ACh release to a similar extent as a depolarising stimulus with K(+), 100 mM, at both 7 and 30 days post-lesion. Cortical perfusion with the combined ondansetron + flumazenil treatment also increased [(3)H]ACh efflux "in vitro" 30 days after lesion, suggesting that local events within the frontal cortex may participate in the interaction of ondansetron with GABAergic neurons, modulating ACh release in situations of cholinergic hypoactivity. No differences in the expression of 5-HT(3) and GABA(A) receptors in the frontal cortex were found after the cholinergic lesion. These results suggest that a combined ondansetron + flumazenil treatment would contribute to restoring a diminished cholinergic function and may provide a basis for using this treatment in the therapy of cognitive disorders associated with degeneration of the cholinergic system.

Differential serotonergic inhibition of in vitro striatal [3H]acetylcholine release in prenatally cocaine-exposed male and female rats

Previous research indicates that prenatal cocaine (pCOC)-exposure results in greater 5-HT3 agonist-induced inhibition of electrically evoked [3H]acetylcholine (ACh) overflow in rat striatal slices. The present study examines the effects of fluoxetine (FLU)-induced and exogenous serotonin (5-HT) on electrically evoked ACh release from striatal slices prepared from adult male and female (in periods of diestrus or proestrus) rats exposed to saline or cocaine in utero. Additionally, we assessed the impact of monoaminergic receptor stimulation on evoked ACh release by superfusion with selective 5-HT2, 5-HT3 and D2 receptor antagonists in the presence of FLU-induced and exogenous 5-HT and measuring the capacity of these drugs to reverse inhibitory effects of 5-HT. Given our previous findings of accentuated inhibition of ACh release by 5-HT3 agonism in striata of pCOC-exposed adult rats, we hypothesized that superfusion of endogenous and exogenous 5-HT would lead to greater suppression of evoked ACh release in this group of animals. Our results indicated that ACh release from slices of all prenatal saline (pSAL) rats was inhibited comparably by FLU (10 microM)-elicited increases in endogenous 5-HT or by increases elicited with application of exogenous 5-HT (5 microM). Robust FLU-mediated inhibition of ACh release was evident in slices from pCOC male and pCOC diestrus female rats vs. their respective PSAL control groups. Superfusion of striatal slices with 5-HT (5 microM) produced a pattern of ACh inhibition similar to that produced by FLU; however, the magnitude of ACh inhibition was consistently greater than that observed with FLU. Inhibition of ACh overflow by FLU was blocked by co-superfusion with ketanserin, a 5-HT2 receptor antagonist. ICS-205,930, a 5-HT3 receptor antagonist or sulpiride, a D2 receptor antagonist. Conversely, serotonergic inhibition of ACh overflow was only blocked by a high concentration of ICS-205,930 (5 microM) and was completely reversed by sulpiride (1 microM). Collectively, these findings demonstrate serotonergic modulation of cholinergic neurons varying as a function of prenatal treatment, sex and, for females, phase of estrous. Inhibition of ACh release by 5-HT appears to be mediated by a complex relationship between 5-HT2, 5-HT3 and D2 receptor regulation, as the blockade of any of these receptors reversed the inhibitory effects of FLU on ACh release. Conversely, in the case of exogenous 5-HT-induced inhibition, only blockade of D2 receptors and high concentrations of the 5-HT3 receptor antagonists were capable of reversing monoaminergic inhibition. These data support the hypothesis that the enhanced serotonergic modulation of ACh neurons in pCOC-exposed animals is largely mediated by dopamine (DA) and reflect a major biochemical persistence of neurodevelopmental adaptations elicited by early cocaine exposure.

GABA(A) receptor antagonists enhance cortical acetylcholine release induced by 5-HT(3) receptor blockade in freely moving rats

ACh release from the rat frontal cortex was increased by both local, 0.1-1 microM, and systemic, 0.1-10 microg/kg, administration of the 5-HT(3) receptor antagonist ondansetron, reaching a maximum peak of 143% over basal values. Bicuculline, 1-10 microM, and flumazenil, 5-10 mg/kg, antagonists at different sites of the GABA(A) receptor, also enhanced ACh release, with maximum effects of 85 and 124% above baseline, respectively. GABA(A) receptor antagonists potentiated the effect induced by ondansetron on ACh release, reaching a peak increase of 238% (with bicuculline) and 259% (with flumazenil) over basal levels. These results suggest an interaction of ondansetron with GABAergic neurons modulating ACh release in the rat frontal cortex in vivo.

Differential subcellular localization of the 5-HT3-As receptor subunit in the rat central nervous system

Following the cloning and sequencing of the A subunit of the 5-HT3 receptor, two alternatively spliced isoforms, 5-HT3-AS and 5-HT3-AL, have been identified. In order to analyse the distribution of the receptor, a polyclonal antibody has been produced against the short form which is the most abundant in the central nervous system [Doucet et al. (2000) Neuroscience 95, 881-892]. As expected from the recognition of functional 5-HT3 receptors, immunostaining by this anti-5-HT3-R-AS antibody matched the distribution of the high-affinity 5-HT3 binding sites in the rat brain and spinal cord. 5-HT3-AS-like immunoreactivity was detected at low levels in the limbic system, particularly in the amygdala and the hippocampus, and in the frontal, piriform and entorhinal cortices. High levels of immunoreactivity were found in the brainstem, mainly in the nucleus tractus solitarius and the nucleus of the spinal tract of the trigeminal nerve, and in the dorsal horn of the spinal cord. At the ultrastructural level, immunostaining was generally found associated with axons and nerve terminals (70-80%) except in the hippocampus, where labelled dendrites were more abundant (56%). This preferential localization on nerve endings is consistent with the well-documented physiological role of 5-HT3 receptors in the control of neurotransmitter release. However, the different distribution in the hippocampus raises the question of whether differential addressing mechanisms exist for preferentially targeting 5-HT3 receptors to postsynaptic dendritic sites as compared to presynaptic nerve endings, depending on the nature of the neurons bearing these receptors.

Potent antagonism of 5-HT(3) and 5-HT(6) receptors by olanzapine

The interaction of the psychotropic agent olanzapine with serotonin 5-HT(3) and 5-HT(6) receptors was investigated. Olanzapine did not contract the isolated guinea pig ileum, but blocked contractions induced by the 5-HT(3) receptor agonist 2-methyl serotonin (2-CH(3) 5-HT) with a pK(B) value of 6.38+/-0.03, close to the affinity of the 5-HT(3) receptor antagonist ondansetron. The atypical antipsychotic risperidone (1 microM) did not significantly inhibit 2-CH(3) 5-HT-induced contractions. Olanzapine had high affinity (pK(i)=8.30+/-0.06) for human 5-HT(6) receptors in radioligand binding studies. Olanzapine did not stimulate [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding to the G protein G(s) in cells containing human 5-HT(6) receptors, but inhibited 5-HT-stimulated [35S]GTPgammaS binding (pK(B)=7.38+/-0.16). Among other antipsychotics investigated, clozapine antagonized 5-HT(6) receptors with a pK(B)=7.42+/-0.15, ziprasidone was three-fold less potent, and risperidone, quetiapine and haloperidol were weak antagonists. Thus, olanzapine was not an agonist, but was a potent antagonist at 5-HT(6) receptors and had marked antagonism at 5-HT(3) receptors.

Inhibition of synaptically evoked cortical acetylcholine release by intracortical glutamate: involvement of GABAergic neurons

Cortical acetylcholine (ACh) has been shown to regulate diverse cognitive processes and its release can be regulated by neuromodulators that act presynaptically at cholinergic terminals. The neocortex receives dense glutamatergic input from thalamocortical and other fibres. The present study used in vivo microdialysis to examine, and pharmacologically characterize, the effect of glutamate on cortical ACh release evoked by electrical stimulation of the pedunculopontine tegmental nucleus in urethane-anaesthetized rats. All drugs were administered locally within the cortex by reverse dialysis. Application of glutamate had no detectable effect on spontaneous ACh release but reduced evoked cortical ACh efflux in a concentration-dependent manner. This effect was mimicked by the glutamate transporter blocker L-trans-pyrrolidine-2,4-dicarboxylic acid, as well as by the ionotropic glutamate receptor agonists N-methyl-D-aspartic acid and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and was blocked by the ionotropic glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and (+/-)-3-(2-carboxypiperazin-4yl)-propyl-1-phosphonic acid. Glutamate application also increased extracellular adenosine levels but the simultaneous delivery of the broad-spectrum adenosine receptor antagonist caffeine failed to affect the inhibitory action of glutamate on evoked ACh release. However, the effect of glutamate was fully blocked by simultaneous delivery of the GABAA receptor antagonist bicuculline and partially blocked by the GABAB receptor antagonist phaclofen. These results suggest that ionotropic glutamate receptor activation by glutamate inhibits evoked cortical ACh release via an indirect pathway involving GABAergic neurons in the cortex.

Role of 5-HT(1A) and 5-HT(7) receptors in the facilitatory response induced by 8-OH-DPAT on learning consolidation

The present study further explored the mechanisms involved in the facilitatory effect induced by (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) on learning consolidation. For this purpose, we analyzed in parallel the effects of LY215840 and ritanserin, two 5-HT(2) receptor antagonists with high affinity for the 5-HT(7) receptor, and WAY100635, a selective 5-HT(1A) receptor antagonist, on the facilitatory effect induced by 8-OH-DPAT on learning consolidation. We also determined whether LY215840 and/or ritanserin could be beneficial in restoring a deficient learning condition. Using the model of autoshaping task, post-training injection of LY215840 or WAY100635 had no effect on learning consolidation. However, both drugs abolished the enhancing effect of 8-OH-DPAT, with LY215840 being slightly more effective than WAY100635 in this respect. Ritanserin produced an increase in performance by itself and also abolished the effect of 8-OH-DPAT. Remarkably, selective blockade of 5-HT(2A) and 5-HT(2B/2C) receptors with MDL100907 and SB200646, respectively, failed to alter the 8-OH-DPAT effect. LY215840 and ritanserin, at the doses that inhibited the 8-OH-DPAT-induced response, reversed the learning deficits induced by scopolamine and dizocilpine. The present results suggest that the enhancing effect produced by 8-OH-DPAT on learning consolidation involves activation of 5-HT(1A) receptors and an additional mechanism, probably related to the 5-HT(7) receptor. Blockade of 5-HT(2) receptors, and perhaps of 5-HT(7) receptors as well, may provide some benefit in reversing learning deficits associated with decreased cholinergic and/or glutamatergic neurotransmission.

Local GABAergic modulation of acetylcholine release from the cortex of freely moving rats

Cortical perfusion with GABA agonists and antagonists modulates the spontaneous release of cortical acetylcholine and GABA in freely moving rats. Twenty-four hours after implantation of a dialysis fibre, cerebral cortex spontaneously released acetylcholine (3.8 +/- 0.2 pmol/10 min) and GABA (6.6 +/- 0.4 pmol/10 min) at a stable rate. Local administration of GABA (1 or 5 mM) or the GABAA agonist muscimol (25 or 50 microM) had no effect on the spontaneous release of acetylcholine. However, bicuculline (1-25 microM), a GABAA antagonist, added to the dialysis perfusate, elicited a concentration-dependent increase of acetylcholine release to approximately double that of control. This effect of bicuculline (25 microM) was completely prevented by coperfusion with muscimol (50 microM). Local administration of the GABAB receptor agonist baclofen (10 or 50 microM) elicited a concentration-dependent increase in spontaneous acetylcholine release with a maximal increase of about 60%. Intracortical administration of baclofen also decreased the spontaneous release of GABA. The GABAB receptor antagonist CGP 35348 (1 mM), administered alone for 20 min through the dialysis fibre, was without effect on spontaneous acetylcholine release; however, it completely blocked both the baclofen-induced increase in acetylcholine release and the decrease in GABA release. These results suggest that cortically released GABA exerts a tonic influence on cholinergic activity.

Role of serotonin in memory impairment

As a result of its presence in various structures of the central nervous system serotonin (5-HT) plays a role in a great variety of behaviours such as food intake, activity rythms, sexual behaviour and emotional states. Despite this lack of functional specialization, the serotonergic system plays a significant role in learning and memory, in particular by interacting with the cholinergic, glutamatergic, dopaminergic or GABAergic systems. Its action is mediated via specific receptors located in crucial brain structures involved in these functions, primarily the septo-hippocampal complex and the nucleus basalis magnocellularis (NBM)-frontal cortex. Converging evidence suggests that the administration of 5-HT2A/2C or 5-HT4 receptor agonists or 5-HT1A or 5-HT3 and 5-HT1B receptor antagonists prevents memory impairment and facilitates learning in situations involving a high cognitive demand. In contrast, antagonists for 5-HT2A/2C and 5-HT4, or agonists for 5-HT1A or 5-HT3 and 5-HT1B generally have opposite effects. A better understanding of the role played by these and other serotonin receptor subtypes in learning and memory is likely to result from the recent availability of highly specific ligands, such as 5-HT1A, 5-HT1B, 5-HT2A receptor antagonists, and new molecular tools, such as gene knock-out mice, especially inducible mice in which a specific genetic alteration can be restricted both temporally and anatomically.

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.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Modulation of serotonin 5-HT3 receptor expression in injured adult rat spinal cord motoneurons

The effects of sciatic nerve lesions on the expression of serotonin 5-HT3 receptor (5-HT3R) alpha subunit in motoneurons of the spinal cord was investigated by semi-quantitative immunohistochemistry. Following sciatic nerve crush, a significant reduction in density of staining in motoneurons was observed in longitudinal sections of the ventral horn at 3 and 15 days on the lesioned side when compared to the contralateral side (p<0.01). At 30 days after crush, after completion of sciatic nerve regeneration and reinnervation of peripheral targets, intensity of staining had returned to normal. Conversely, after sciatic nerve cut, a lesion that does not allow for target reinnervation, highly significant reductions were observed at 3, 15, 30 and 45 days. These results suggest a role for functional contacts with muscular targets in the maintenance of 5-HT3R expression in spinal motoneurons.

Differential interaction between 5-HT3 receptors and GABAergic neurons inhibiting acetylcholine release in rat entorhinal cortex slices

The 5-HT3 receptor antagonists, ondansetron, MDL 72222 and granisetron (0.01-1 microM), produced a concentration-dependent increase of K+-evoked [3H]ACh efflux in slices from rat entorhinal cortex preloaded with [3H]choline. Bicuculline and flumazenil, antagonists at different sites of the GABAA receptor, also enhanced [3H]ACh efflux. While the ACh releasing effect of ondansetron was markedly potentiated, in a TTX-sensitive manner, by bicuculline, the effects of MDL 72222 and granisetron were not significantly modified. A qualitatively identical interaction was found by using flumazenil, a GABAA antagonist at the benzodiazepine recognition site, in combination with the 5-HT3 receptor antagonists. The potentiation by the GABAA antagonists of [3H]ACh efflux was also observed in a superfusion medium deficient in Cl-. The nonspecific K+-channel blockers TEA and Ba2+ also increased K+-evoked [3H]ACh efflux in this preparation but the releasing effect was not modified by bicuculline. The results support the functional interaction of ondansetron with GABAergic interneurons in the rat entorhinal cortex, GABA-independent mechanisms may however be involved in the regulation of cortical cholinergic function by other 5-HT3 receptor antagonists.

5-HT2 receptor regulation of acetylcholine release induced by dopaminergic stimulation in rat striatal slices

The role of 5-hydroxytryptamine (5-HT) receptor subtypes in acetylcholine (ACh) release induced by dopamine or neurokinin receptor stimulation was studied in rat striatal slices. The dopamine D1 receptor agonist SKF 38393 potentiated in a tetrodotoxin-sensitive manner the K(+)-evoked [3H]ACh release while SCH 23390, a dopamine D1 receptor antagonist, had no effect. [3H]ACh release was decreased by the dopamine D2 receptor agonist LY 171555 (quinpirole) and slightly potentiated by the dopamine D2 receptor antagonist haloperidol. The selective neurokinin NK1 receptor agonist [Sar9, met(O2)11]SP also potentiated K(+)-evoked release of [3H]ACh. GR 82334, a NK1 receptor antagonist, blocked not only the effect of [Sar9, met(O2)11]SP but also the release of ACh induced by the D1 receptor agonist SKF 38393. Among the 5-HT agents studied, only the 5-HT2A receptor antagonists ketanserin and ritanserin were able to reduce the ACh release induced by dopamine D1 receptor stimulation. Mesulergine, a more selective 5-HT2C antagonist, showed an intrinsic releasing effect but did not affect K(+)-evoked ACh release induced by SKF 38393. Methysergide and methiothepin, mixed 5-HT1/2 antagonists, as well as ondansetron, a 5-HT3 receptor antagonist, showed an intrinsic effect on ACh release, their effects being additive to that of SKF 38393. 5-HT2 receptor agonists were ineffective. However, the 5-HT2 agonist DOI was able to prevent the antagonism by ketanserin of the increased [3H]ACh efflux elicited by SKF 38393, suggesting a permissive role of 5-HT2A receptors. None of the above indicated 5-HT agents was able to reduce the ACh release induced by the selective NK1 agonist. The results suggest that 5-HT2 receptors, probably of the 5-HT2A subtype, modulate the release of ACh observed in slices from the rat striatum after stimulation of dopamine D1 receptors. It seems that this serotonergic control is exerted on the interposed collaterals of substance P-containing neurons which promote ACh efflux through activation of NK1 receptors located on cholinergic interneurons.

Serotonin receptors in cognitive behaviors

The serotonergic system appears to play a role in behaviors that involve a high cognitive demand and in memory improvement or recovery from impaired cognitive performance, as made evident after administration of serotonin 5-HT2A/5-HT2C or 5-HT4 receptor agonists or 5-HT1A or 5-HT3 receptor antagonists. These serotonin receptor subtypes are localized on 'cognitive' pathways, with the hippocampus and frontal cortex as the main target structures. A better understanding of the role played by these and other serotonin receptor subtypes in cognition is likely to result from the recent availability of new specific ligands and new molecular tools, such as gene knock-out and transgenic mice.