Serotonin1A receptors are expressed by a subpopulation of cholinergic neurons in the rat medial septum and diagonal band of Broca--a double immunocytochemical study

Distinct septo-hippocampal cholinergic projections separately mediate stress-induced emotional and cognitive deficits

Patients suffering from chronic stress develop numerous symptoms, including emotional and cognitive deficits. The precise circuit mechanisms underlying different symptoms remain poorly understood. We identified two distinct basal forebrain cholinergic subpopulations in mice projecting to the dorsal hippocampus (dHPC) or ventral hippocampus (vHPC), which exhibited distinct input organizations, electrophysiological characteristics, transcriptomics, and responses to positive and negative valences of stimuli and were critical for cognitive and emotional modulation, respectively. Moreover, chronic stress induced elevated anxiety levels and cognitive deficits in mice, accompanied by enhanced vHPC but suppressed dHPC cholinergic projections. Chemogenetic activation of dHPC or inhibition of vHPC cholinergic projections alleviated stress-induced aberrant behaviors. Furthermore, we identified that the acetylcholinesterase inhibitor donepezil combined with blockade of muscarinic receptor 1-type muscarinic acetylcholine receptors in the vHPC rescued both stress-induced phenotypes. These data illuminated distinct septo-hippocampal cholinergic circuits mediated specific symptoms independently under stress, which may provide promising strategies for circuit-based treating of stress-related psychiatric disorders.

Transcriptomic mapping of the 5-HT receptor landscape

Serotonin (5-HT) is crucial for regulating brain functions such as mood, sleep, and cognition. This study presents a comprehensive transcriptomic analysis of 5-HT receptors (Htrs) across ≈4 million cells in the adult mouse brain using single-cell RNA sequencing (scRNA-seq) data from the Allen Institute. We observed differential transcription patterns of all 14 Htr subtypes, revealing diverse prevalence and distribution across cell classes. Remarkably, we found that 65.84% of cells transcribe RNA of at least one Htr, with frequent co-transcription of multiple Htrs, underscoring the complexity of the 5-HT system even at the single-cell dimension. Leveraging a multiplexed error-robust fluorescence in situ hybridization (MERFISH) dataset provided by Harvard University of ≈10 million cells, we analyzed the spatial distribution of each Htr, confirming previous findings and uncovering novel transcription patterns. To aid in exploring Htr transcription, we provide an online interactive visualizer.

Septal medial/diagonal band of Broca citalopram infusion reduces place learning efficiency and alters septohippocampal theta learning-related activity in rats

Increases in power and frequency of hippocampal theta activity have been related to efficient place learning and memory acquisition in hippocampal-dependent tests. The complex medial septum-diagonal band of Broca (MS/DBB) is the pacemaker of hippocampal theta activity, influenced by the ascending synchronizing system, and modulated by serotonergic raphe medial afferents, acting on cholinergic and GABAergic septal neurons. The suppression of hippocampal theta expression and the modulation of hippocampal learning and memory are attributed to serotonin. To simultaneously test these hypotheses, a daily local serotonin increase was induced by citalopram (CIT) infusion (100 µM, 0.88 µl, 0.2 µl/m) 15 min before training in the Morris water maze. The theta activity was recorded in the MS/DBB, dentate gyrus (DG) and CA1 of one group infused with artificial cerebrospinal liquid (ACL) and the other with CIT on Days 1-6 of training. After a probe trial (Day 7) and one resting day, the treatments were reversed (Days 8-11). The CIT MS/DBB infusion in the first 6 training days reduced the efficiency of spatial learning in association with reduced power in the DG, reduced MS/DBB-DG coherence, increased DG-CA1 coherence, and a lack of a negative correlation between MS/DBB power and swam distances. No effect of the CIT occurred once the information was acquired under ACL training. These results support a role of serotonin, in acting on the MS/DBB in the fine tuning of hippocampal learning and memory efficiency through the modulation of learning-related theta activity power and septohipocampal synchronization.

Neural Circuits for Sleep-Wake Regulation

The neural mechanisms of sleep, a fundamental biological behavior from invertebrates to humans, have been a long-standing mystery and present an enormous challenge. Gradually, perspectives on the neurobiology of sleep have been more various with the technical innovations over the recent decades, and studies have now identified many specific neural circuits that selectively regulate the initiation and maintenance of wake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep. The cholinergic system in basal forebrain (BF) that fire maximally during waking and REM sleep is one of the key neuromodulation systems related to waking and REM sleep. Here we outline the recent progress of the BF cholinergic system in sleep-wake cycle. The intricate local connectivity and multiple projections to other cortical and subcortical regions of the BF cholinergic system elaborately presented here form a conceptual framework for understanding the coordinating effects with the dissecting regions. This framework also provides evidences regarding the relationships between the general anesthesia and wakefulness/sleep cycle focusing on the neural circuitry of unconsciousness induced by anesthetic drugs.

Activation of somatodendritic 5-HT1A autoreceptors reduces the acquisition and expression of cued fear in the rat fear-potentiated startle test

RATIONALE

Fear conditioning is an important factor in the etiology of anxiety disorders. Previous studies have demonstrated a role for serotonin (5-HT)_1A receptors in fear conditioning. However, the relative contribution of somatodendritic 5-HT_1A autoreceptors and post-synaptic 5-HT_1A heteroreceptors in fear conditioning is still unclear.

OBJECTIVE

To determine the role of pre- and post-synaptic 5-HT_1A receptors in the acquisition and expression of cued and contextual conditioned fear.

METHODS

We studied the acute effects of four 5-HT_1A receptor ligands in the fear-potentiated startle test. Male Wistar rats were injected with the 5-HT_1A receptors biased agonists F13714 (0-0.16 mg/kg, IP), which preferentially activates somatodendritic 5-HT_1A autoreceptors, or F15599 (0-0.16 mg/kg, IP), which preferentially activates cortical post-synaptic 5-HT_1A heteroreceptors, with the prototypical 5-HT_1A receptor agonist R(+)8-OH-DPAT (0-0.3 mg/kg, SC) or the 5-HT_1A receptor antagonist WAY100,635 (0-1.0 mg/kg, SC).

RESULTS

F13714 (0.16 mg/kg) and R(+)-8-OH-DPAT (0.03 mg/kg) injected before training reduced cued fear acquisition. Pre-treatment with F15599 or WAY100,635 had no effect on fear learning. In the fear-potentiated startle test, F13714 (0.04-0.16 mg/kg) and R(+)-8-OH-DPAT (0.1-0.3 mg/kg) reduced the expression of cued and contextual fear, whereas F15599 had no effect. WAY100,635 (0.03-1.0 mg/kg) reduced the overall startle response.

CONCLUSIONS

The current findings indicate that activation of somatodendritic 5-HT_1A autoreceptors reduces cued fear learning, whereas 5-HT_1A receptors seem not involved in contextual fear learning. Moreover, activation of somatodendritic 5-HT_1A autoreceptors may reduce cued and contextual fear expression, whereas we found no evidence for the involvement of cortical 5-HT_1A heteroreceptors in the expression of conditioned fear.

Review : Serotonin Transporter and Psychiatric Disorders: Listening to the Gene

Mood, cognition, and many other physiological functions are modulated by the midbrain raphe serotonin (5- HT) system. By directing the magnitude and duration of postsynaptic receptor-mediated signaling, the 5-HT transporter (5-HTT) plays a crucial role in the integration of 5-HT neurotransmission. Considerable progress has been made in the molecular characterization of the 5-HTT, and research is currently focusing on the organization of 5-HTT gene (SLC6A4, OMIM accession number 182138), on the regulation of 5-HTT ex pression, on alterations in expression because of allelic variation in gene transcription, on structure-activity relationships of the 5-HTT protein, and on mechanisms of 5-HT and ion translocation. In the psychobiological dimension, it is becoming increasingly evident that inadequate adaptive responses to environmental stress ors, in conjunction with predisposing genes like the 5-HTT, contribute to the etiopathogenesis of behavioral and psychiatric disorders. A polymorphism in the regulatory region of the 5-HTT gene is associated with anxiety- and depression-related personality traits, and preliminary studies suggest that it influences the risk to develop affective disorders, alcohol dependence, and late-onset dementias. Finally, transgenic strategies are gaining momentum for the validation of the concept of the 5-HTT gene as a susceptibility locus for emotional instability (neuroticism) and psychiatric disorders. This approach addresses the pertinent question: to what extent does targeted disruption of the 5-HTT gene affect biochemistry, electrophysiology, and phar macology of the 5-HT system and modulate neural development and synaptic plasticity? It may also provide a model system that facilitates the dissection of successive events that lead to disease states as well as to the testing of novel therapeutic concepts. NEUROSCIENTIST 4:25-34, 1998

Is 'sinus' pain really sinusitis?

So-called 'sinus pain' is a common complaint in GP and ear, nose and throat clinics, and patients often receive treatment with antibiotics and decongestants. Recent evidence suggests that facial pain may not be related to the sinuses at all and that doctors may have to rethink their prescribing strategy.

Cellular profile of the dorsal raphe lateral wing sub-region: relationship to the lateral dorsal tegmental nucleus

As one of the main serotonergic (5HT) projections to the forebrain, the dorsal raphe nucleus (DRN) has been implicated in disorders of anxiety and depression. Although the nucleus contains the densest population of 5HT neurons in the brain, at least 50% of cells within this structure are non-serotonergic, including a large population of nitric oxide synthase (NOS) containing neurons. The DRN has a unique topographical efferent organization and can also be divided into sub-regions based on rostro-caudal and medio-lateral dimensions. NOS is co-localized with 5HT in the midline DRN but NOS-positive cells in the lateral wing (LW) of the nucleus do not express 5HT. Interestingly, the NOS LW neuronal population is immediately rostral to and in line with the cholinergic lateral dorsal tegmental nucleus (LDT). We used immunohistochemical methods to investigate the potential serotonergic regulation of NOS LW neurons and also the association of this cell grouping to the LDT. Our results indicate that >75% of NOS LW neurons express the inhibitory 5HT1A receptor and are cholinergic (>90%). The findings suggest this assembly of cells is a rostral extension of the LDT, one that it is subject to regulation by 5HT release. As such the present study suggests a link between 5HT signaling, activation of cholinergic/NOS neurons, and the stress response including the pathophysiology underlying anxiety and depression.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Mast cell expression of the serotonin1A receptor in guinea pig and human intestine

Serotonin [5-hydroxytryptamine (5-HT)] is released from enterochromaffin cells in the mucosa of the small intestine. We tested a hypothesis that elevation of 5-HT in the environment of enteric mast cells might degranulate the mast cells and release mediators that become paracrine signals to the enteric nervous system, spinal afferents, and secretory glands. Western blotting, immunofluorescence, ELISA, and pharmacological analysis were used to study expression of 5-HT receptors by mast cells in the small intestine and action of 5-HT to degranulate the mast cells and release histamine in guinea pig small intestine and segments of human jejunum discarded during Roux-en-Y gastric bypass surgeries. Mast cells in human and guinea pig preparations expressed the 5-HT1A receptor. ELISA detected spontaneous release of histamine in guinea pig and human preparations. The selective 5-HT1A receptor agonist 8-hydroxy-PIPAT evoked release of histamine. A selective 5-HT1A receptor antagonist, WAY-100135, suppressed stimulation of histamine release by 5-HT or 8-hydroxy-PIPAT. Mast cell-stabilizing drugs, doxantrazole and cromolyn sodium, suppressed the release of histamine evoked by 5-HT or 8-hydroxy-PIPAT in guinea pig and human preparations. Our results support the hypothesis that serotonergic degranulation of enteric mast cells and release of preformed mediators, including histamine, are mediated by the 5-HT1A serotonergic receptor. Association of 5-HT with the pathophysiology of functional gastrointestinal disorders (e.g., irritable bowel syndrome) underlies a question of whether selective 5-HT1A receptor antagonists might have therapeutic application in disorders of this nature.

Mast cell expression of the serotonin1A receptor in guinea pig and human intestine

Serotonin [5-hydroxytryptamine (5-HT)] is released from enterochromaffin cells in the mucosa of the small intestine. We tested a hypothesis that elevation of 5-HT in the environment of enteric mast cells might degranulate the mast cells and release mediators that become paracrine signals to the enteric nervous system, spinal afferents, and secretory glands. Western blotting, immunofluorescence, ELISA, and pharmacological analysis were used to study expression of 5-HT receptors by mast cells in the small intestine and action of 5-HT to degranulate the mast cells and release histamine in guinea pig small intestine and segments of human jejunum discarded during Roux-en-Y gastric bypass surgeries. Mast cells in human and guinea pig preparations expressed the 5-HT1A receptor. ELISA detected spontaneous release of histamine in guinea pig and human preparations. The selective 5-HT1A receptor agonist 8-hydroxy-PIPAT evoked release of histamine. A selective 5-HT1A receptor antagonist, WAY-100135, suppressed stimulation of histamine release by 5-HT or 8-hydroxy-PIPAT. Mast cell-stabilizing drugs, doxantrazole and cromolyn sodium, suppressed the release of histamine evoked by 5-HT or 8-hydroxy-PIPAT in guinea pig and human preparations. Our results support the hypothesis that serotonergic degranulation of enteric mast cells and release of preformed mediators, including histamine, are mediated by the 5-HT1A serotonergic receptor. Association of 5-HT with the pathophysiology of functional gastrointestinal disorders (e.g., irritable bowel syndrome) underlies a question of whether selective 5-HT1A receptor antagonists might have therapeutic application in disorders of this nature.

Cholinergic connectivity: it's implications for psychiatric disorders

Acetylcholine has been implicated in both the pathophysiology and treatment of a number of psychiatric disorders, with most of the data related to its role and therapeutic potential focusing on schizophrenia. However, there is little thought given to the consequences of the documented changes in the cholinergic system and how they may affect the functioning of the brain. This review looks at the cholinergic system and its interactions with the intrinsic neurotransmitters glutamate and gamma-amino butyric acid as well as those with the projection neurotransmitters most implicated in the pathophysiologies of psychiatric disorders; dopamine and serotonin. In addition, with the recent focus on the role of factors normally associated with inflammation in the pathophysiologies of psychiatric disorders, links between the cholinergic system and these factors will also be examined. These interfaces are put into context, primarily for schizophrenia, by looking at the changes in each of these systems in the disorder and exploring, theoretically, whether the changes are interconnected with those seen in the cholinergic system. Thus, this review will provide a comprehensive overview of the connectivity between the cholinergic system and some of the major areas of research into the pathophysiologies of psychiatric disorders, resulting in a critical appraisal of the potential outcomes of a dysregulated central cholinergic system.

Evidence for the differential co-localization of neurokinin-1 receptors with 5-HT receptor subtypes in rat forebrain

Studies suggest that like selective 5-hydroxytryptamine (5-HT; serotonin) reuptake inhibitors, antagonists at neurokinin-1 receptors (NK(1)Rs) may have antidepressant and anxiolytic properties. NK(1)Rs are present in 5-HT innervated forebrain regions which may provide a common point of interaction between these two transmitter systems. This study aimed to investigate for cellular co-localization between NK(1)Rs and 5-HT receptor subtypes in mood-related brain regions in the rat forebrain. With experiments using fluorescence immunocytochemistry, double-labelling methods demonstrated a high degree of co-localization between NK(1)Rs and 5-HT(1A) receptors in most regions examined. Co-localization was highest in the medial septum (88% NK(1)R expressing cells were 5-HT(1A) receptor-positive) and hippocampal regions (e.g. dentate gyrus, 65%), followed by the lateral/basolateral amygdala (35%) and medial prefrontal cortex (31%). In contrast, co-localization between NK(1)Rs and 5-HT(2A) receptors was infrequent (< 8%) in most areas examined except for the hippocampus (e.g. CA3, 43%). Overall co-localization between NK(1)Rs and 5-HT(1A) receptors was much greater than that between NK(1)Rs and 5-HT(2A) receptors. Thus, these experiments demonstrate a high degree of co-localization between NK(1)Rs and 5-HT(1A) receptors in cortical and limbic regions of the rat forebrain. These findings suggest a novel site of interaction between NK(1)R antagonists and the 5-HT system.

Spatial memory alterations by activation of septal 5HT 1A receptors: no implication of cholinergic septohippocampal neurons

INTRODUCTION

In rats, activation of medial septum (MS) 5-HT(1A) receptors with the 5-HT(1A)/5-HT(7) receptor agonist 8-OH-DPAT disrupts encoding and consolidation, but not retrieval of a spatial memory in the water maze task. These findings might be explained by an action of 8-OH-DPAT on 5-HT(1A) receptors located on cholinergic neurons which the drug could transiently hyperpolarise. If so, selective damage of these neurons should mimic the effects of 8-OH-DPAT, or, at least, synergistically interfere with them.

METHODS

To test this hypothesis, rats were subjected to intraseptal infusions of 8-OH-DPAT (or phosphate-buffered saline) during acquisition of a water maze task before and/or after 192 IgG-saporin-induced MS cholinergic lesion (vs. sham-operated).

RESULTS

We confirmed that only pre-acquisition intraseptal 8-OH-DPAT infusions prevented learning and subsequent drug-free retrieval of the platform location in intact rats and found that (1) the cholinergic lesion did not prevent recall of the platform location, and (2) the impairing effects of 8-OH-DPAT were similar in sham-operated and lesioned rats, whether naïve or not, to the task before lesion surgery.

CONCLUSIONS

An action of 8-OH-DPAT on only MS cholinergic neurons is not sufficient to account for the drug-induced memory impairments. A concomitant 8-OH-DPAT-induced hyperpolarisation of cholinergic and/or GABAergic and/or glutamatergic neurons (intact rats), or of only GABAergic and/or glutamatergic ones after cholinergic lesion, might be necessary to obliterate task acquisition, confirming that, in the MS, (1) the three neuronal populations could cooperate to process hippocampal-dependent information, and (2) non-cholinergic septohippocampal neurons might be more important than cholinergic ones in serotonin-induced modulation of hippocampus-dependent memory processing.

The selective 5-HT(1A) receptor antagonist NAD-299 increases acetylcholine release but not extracellular glutamate levels in the frontal cortex and hippocampus of awake rat

The effects of the HT(1A) receptor antagonist NAD-299 on extracellular acetylcholine (ACh) and glutamate (Glu) levels in the frontal cortex (FC) and ventral hippocampus (HPC) of the awake rats were investigated by the use of in vivo microdialysis. Systemic administration of NAD-299 (0.3; 1 and 3micromol/kg s.c.) caused a dose-dependent increase in ACh levels in FC and HPC (peak value of 209% and 221%, respectively) and this effect was comparable to that induced by donepezil (2.63micromol/kg s.c.). Moreover, the ACh levels in the FC increased even after repeated (14days) treatment with NAD-299 and when NAD-299 was injected locally into the nucleus basalis magnocellularis or perfused through the microdialysis probe implanted in the cortex. In contrast, NAD-299 failed to alter the extracellular levels of glutamate after systemic (3micromol/kg s.c.) or local (100microM) administration. The present data support the hypothesis that cholinergic transmission in cortico-limbic regions can be enhanced via blockade of postsynaptic 5-HT(1A) receptors, which may underlie the proposed cognitive enhancing properties of NAD-299 in models characterized by cholinergic deficit.

Morphology and distribution of neurons expressing serotonin 5-HT1A receptors in the rat hypothalamus and the surrounding diencephalic and telencephalic areas

Disorders of serotonergic neurotransmission are involved in disturbances of numerous hypothalamic functions including circadian rhythm, mood, neuroendocrine functions, sleep and feeding. Among the serotonin receptors currently recognized, 5-HT(1A) receptors have received considerable attention due to their importance in the etiology of mood disorders. While previous studies have shown the presence of 5-HT(1A) receptors in several regions of the rat brain, there is no detailed map of the cellular distribution of 5-HT(1A) receptors in the rat diencephalon. In order to characterize the distribution and morphology of the neurons containing 5-HT(1A) receptors in the diencephalon and the adjacent telencephalic areas, single label immunohistochemistry was utilized. Large, multipolar, 5-HT(1A)-immunoreactive (IR) neurons were mainly detected in the magnocellular preoptic nucleus and in the nucleus of diagonal band of Broca, while the supraoptic nucleus contained mainly fusiform neurons. Medium-sized 5-HT(1A)-IR neurons with triangular or round-shaped somata were widely distributed in the diencephalon, populating the zona incerta, lateral hypothalamic area, anterior hypothalamic nucleus, substantia innominata, dorsomedial and premamillary nuclei, paraventricular nucleus and bed nucleus of stria terminalis. The present study provides schematic mapping of 5-HT(1A)-IR neurons in the rat diencephalon. In addition, the morphology of the detected 5-HT(1A)-IR neural elements is also described. Since rat is a widely used laboratory animal in pharmacological models of altered serotoninergic neurotransmission, detailed mapping of 5-HT(1A)-IR structures is pivotal for the neurochemical characterization of the neurons containing 5-HT(1A) receptors.

Regional changes in the cholinergic system in mice lacking monoamine oxidase A

Elevated brain monoamine concentrations resulting from monoamine oxidase A genetic ablation (MAOA knock-out mice) lead to changes in other neurotransmitter systems. To investigate the consequences of MAOA deficiency on the cholinergic system, we measured ligand binding to the high-affinity choline transporter (CHT1) and to muscarinic and nicotinic receptors in brain sections of MAOA knock-out (KO) and wild-type mice. A twofold increase in [(3)H]-hemicholinium-3 ([(3)H]-HC-3) binding to CHT1 was observed in the caudate putamen, nucleus accumbens, and motor cortex in MAOA KO mice as compared with wild-type (WT) mice. There was no difference in [(3)H]-HC-3 labeling in the hippocampus (dentate gyrus) between the two genotypes. Binding of [(125)I]-epibatidine ([(125)I]-Epi), [(125)I]-alpha-bungarotoxin ([(125)I]-BGT), [(3)H]-pirenzepine ([(3)H]-PZR), and [(3)H]-AFDX-384 ([(3)H]-AFX), which respectively label high- and low-affinity nicotinic receptors, M1 and M2 muscarinic cholinergic receptors, was not modified in the caudate putamen, nucleus accumbens, and motor cortex. A small but significant decrease of 19% in M1 binding densities was observed in the hippocampus (CA1 field) of KO mice. Next, we tested acetylcholinesterase activity and found that it was decreased by 25% in the striatum of KO mice as compared with WT mice. Our data suggest that genetic deficiency in MAOA enzyme is associated with changes in cholinergic activity, which may account for some of the behavioral alterations observed in mice and humans lacking MAOA.

Activation of septal 5-HT1A receptors alters spatial memory encoding, interferes with consolidation, but does not affect retrieval in rats subjected to a water-maze task

Using Long-Evans rats tested in a water maze, this study assessed the role of 5-HT1A/5-HT7 receptors of the medial septum in encoding, consolidation, and retrieval of spatial information. The testing protocol (acquisition: daily four-trial sessions over three consecutive days; retention: probe trial on day 4) was first validated by showing that intraseptal infusions of lidocaine (LIDO; 40 microg/0.5 microL) disrupted acquisition and retrieval of the task. 8-OH-DPAT (4 microg/0.5 microL) infused before each acquisition session prevented learning/retention of the platform location, an effect attenuated by pretreatment with the 5-HT1A receptor antagonist WAY 100635. With the 5-HT7 antagonist SB 269970, the 8-OH-DPAT-induced acquisition deficit seemed attenuated, but there was no subsequent retention. When infused immediately, 1, 4, or 6 h after each acquisition session, 8-OH-DPAT did not hinder consolidation. When the infusions were performed 2 h postacquisition, however, consolidation was disrupted. Finally, when infused before a probe trial after drug-free acquisition, 8-OH-DPAT had no effect, suggesting no interference with retrieval processes. We also established that 8-OH-DPAT had no effects when the platform was visible, and altered neither home-cage activity nor anxiety-related behavior (elevated plus-maze). Altogether, these results show that 5-HT1A receptors in the septal region contribute both to declarative-like information encoding and subsequently, within a given postacquisition time window, to its consolidation. They do not participate in the retrieval of recently learned declarative-like information. These observations suggest that 5-HT1A receptors of the medial septum contribute to a serotonin-mediated mechanism involved in the encoding and consolidation, not the retrieval of spatial hippocampal-dependent knowledge. These results might have some relevance to approaches aimed at modifying serotonergic functions in the brain for the treatment of disorders such as depression, anxiety, post-traumatic stress, and amnesia.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Cognitive dysfunction in neuropsychiatric disorders: selected serotonin receptor subtypes as therapeutic targets

The indolamine, serotonin (5-hydroxytryptamine-5-HT) was identified and initially characterized around the middle of the twentieth century and it is now known to participate in multiple physiologic processes in mammalians. As a neurotransmitter, 5-HT is well documented to play a significant role in the pathophysiology and treatment of a variety of psychiatric disorders including anxiety, depression, and schizophrenia. In addition, there is also some evidence to suggest that 5-HT function in the brain may be important (particularly in the behavioral disturbances) in various forms of dementia including Alzheimer's disease. While 5-HT is undoubtedly involved in cognitive function, its role in specific domains of cognition (attention, learning, and memory, etc.) is poorly understood. This understanding has been impeded to some extent by the many complex interactions between 5-HT neurons and other neuronal phenotypes, 5-HT receptor heterogeneity, and the conflicting results of some behavioral experiments in animals conducted to date. Through the combined use of modern molecular biology, transgenic animal models, and other more traditional research methods such as medicinal chemistry and classical pharmacology, a clearer picture of the role of serotonin and its receptor subtypes in mnemonic processes is beginning to emerge, however. Considerable data now support the argument that selective ligands at specific 5-HT receptor subtypes can serve as therapeutic agents designed to enhance cognitive function in psychiatric disorders such as schizophrenia as well as age-related neurodegenerative illnesses such as Alzheimer's disease. The purpose of this review is to provide a brief overview of these therapeutic targets within the 5-HT system and the pharmacologic approaches (including the most recently developed compounds) designed to enhance memory function.

5-HT receptor regulation of neurotransmitter release

Serotoninergic neurons in the central nervous system impinge on many other neurons and modulate their neurotransmitter release. This review focuses on 1) the function of presynaptic 5-hydroxytryptamine (5-HT) heteroreceptors on axon terminals of central cholinergic, dopaminergic, noradrenergic, or GABAergic neurons and 2) the role of GABAergic interneurons expressing 5-HT heteroreceptors in the regulation of acetylcholine, dopamine, or noradrenaline release. In vitro studies on slices or synaptosomes and in vivo microdialysis experiments have shown that 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C), 5-HT(3), and/or 5-HT(4) heteroreceptors mediate this modulation. 5-HT(1B) receptors on neocortical cholinergic, striatal dopaminergic, or hippocampal GABAergic axon terminals are examples for release-inhibiting 5-HT heteroreceptors; 5-HT(3) receptors on hippocampal GABAergic or 5-HT(4) receptors on hippocampal cholinergic axon terminals are examples for release-facilitating 5-HT heteroreceptors. GABA released from GABAergic interneurons upon activation of facilitatory 5-HT receptors, e.g., 5-HT(2A) or 5-HT(3) receptors, mediates inhibition of the release of other neurotransmitters such as prefrontal neocortical dopamine or neocortical acetylcholine release, respectively. Conversely, attenuated GABA release in response to activation of inhibitory 5-HT heteroreceptors, e.g., 5-HT(1A) or 5-HT(1B) receptors on GABAergic interneurons is involved in paradoxical facilitation of hippocampal acetylcholine and striatal dopamine release, respectively. Such 5-HT heteroreceptors are considered potential targets for appropriate 5-HT receptor ligands which, by enhancing the release of a relevant neurotransmitter, can compensate for its hypothesized deficiency in distinct brain areas. Examples for such deficiencies are the impaired release of hippocampal or neocortical acetylcholine, striatal dopamine, and hippocampal or neocortical noradrenaline in disorders such as Alzheimer's disease, Parkinson's disease, and major depression, respectively.

Differential effects of adjunctive methylphenidate and citalopram on extracellular levels of serotonin, noradrenaline and dopamine in the rat brain

Several clinical studies have suggested that the combined treatment with methylphenidate and citalopram may accelerate the onset of antidepressant action and induce an improvement even in treatment-refractory patients. In the present study, in vivo microdialysis was used to monitor the extracellular levels of serotonin, noradrenaline and dopamine in the prefrontal cortex, hippocampus, nucleus accumbens and striatum of the rat. Administration of methylphenidate (2.5 mg/kg s.c.) with citalopram (5 mg/kg i.p.) compared to methylphenidate alone caused a marked enhancement of dopamine levels in the prefrontal cortex, n. accumbens and hippocampus, but not in the striatum. Citalopram-induced increase in serotonin levels was strongly enhanced by adjunctive methylphenidate in the hippocampus, but attenuated in the cortex. These findings suggest that the proposed augmentation effects of adjuvant methylphenidate to citalopram are most likely associated with enhanced dopamine transmission in the corticolimbic areas, whereas serotonin and noradrenaline levels show differential and region specific responses.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

5-HT1A receptors and memory

The study of 5-hydroxytryptamine (5-HT) systems has benefited from the identification, classification and cloning of multiple 5-HT receptors (5-HT(1)-5-HT(7)). Increasing evidence suggests that 5-HT pathways, reuptake site/transporter complex and 5-HT receptors represent a strategic distribution for learning and memory. A key question still remaining is whether 5-HT markers (e.g., receptors) are directly or indirectly contributing to the physiological and pharmacological basis of memory and its pathogenesis or, rather, if they represent protective or adaptable mechanisms (at least in initial stages). In the current paper, the major aim is to revise recent advances regarding mammalian 5-HT(1A) receptors in light of their physiological, pathophysiological and therapeutic implications in memory. An attempt is made to identify and discuss sources of discrepancies by employing an analytic approach to examine the nature and degree of difficulty of behavioral tasks used, as well as implicating other factors (for example, brain areas, training time or duration, and drug administration) which might offer new insights into the understanding and interpretation of these data. In this context, 8-OH-DPAT deserves special attention since for many years it has been the more selective 5-HT drug and, hence, more frequently used. As 5-HT(1A) receptors are key components of serotonergic signaling, investigation of their memory mechanisms and action sites and the conditions under which they might operate, could yield valuable insights. Moreover, selective drugs with agonists, neutral antagonists or inverse agonist properties for 5-HT(1A) (and 5-HT(7)) receptors may constitute a new therapeutic opportunity for learning and memory disorders.

Reduced density of functional 5-HT1A receptors in the brain, medulla and spinal cord of monoamine oxidase-A knockout mouse neonates

Abnormally high brain 5-HT levels in monoamine oxidase-A knockout (MAO-A KO) mouse neonates raise the question of whether the distribution and density of the 5-HT1A receptors (5-HT1AR) expressed in the brain by postnatal day P7 are affected and, if so, whether the 5-HT1A autoreceptors in the dorsal raphe are modified in the same way as the postsynaptic 5-HT1AR present in raphe target structures. [3H]8-OH-DPAT binding and quantitative autoradiography were performed to answer these questions. Binding specificity was first confirmed in adult wild-type mice and rat brain sections. 5-HT1AR binding was then analyzed in four MAO-A mutant vs. five wild-type neonatal brains, from olfactory bulb to cervical cord. Among 12 structures expressing postsynaptic 5-HT1AR in wild-type neonates, the highest densities involved the retrosplenial cortex, entorhinal cortex, and septum (52-46 fmol/mg tissue); low densities occurred in the hippocampus and spinal cord (24 fmol/mg tissue); in addition, the raphe autoreceptor density was only 20 fmol/mg tissue. In mutants, the distribution of postsynaptic 5-HT1AR was unchanged, but an overall decrease in density occurred (-32% to -63%); the raphe autoreceptors decreased in mutants by at least -79%. Data are discussed with reference to the ectopic 5-HT uptake and accumulation reported to occur during the first 10 postnatal days in wild-type and MAO-A KO mice. As previously suggested to explain the raphe autoreceptor loss in 2-month-old MAO-A KO mice, the overall 5-HT1AR down-regulation in mutant pups probably results from extracellular 5-HT excess in both raphe and target structures. The greater the 5-HT excess, the more the functional receptor density decreases.

5-Hydroxytryptamine 1A receptor blockade facilitates aversive learning in mice: interactions with cholinergic and glutamatergic mechanisms

The effects of 5-hydroxytryptamine 1A (5-HT(1A)) receptor ligands on aversive learning were examined in the passive avoidance (PA) task in mice. Anxiety and autonomic functions were investigated using the elevated plus-maze and heart rate measurements. The main findings from this study are as follows. 1) Pretraining administration of the 5-HT(1A) receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide] facilitated PA retention at low doses (0.01 and 0.03 mg/kg) but impaired PA retention at higher doses (0.1-1.0 mg/kg), consistent with previous findings in the rat. 2) Similar to the acetylcholinesterase inhibitor physostigmine, pretraining administration of the 5-HT(1A) receptor antagonists [(R)-3-N,N-dicyclobutylamino-8 fluoro-3,4-dihydro-3H-1-benzopyran-5-carboxamide hydrogen(2R,3R)-tartrate monohydrate] NAD-299 (0.1-2 mg/kg) and [N-2-4-(2-methoxyphenyl)-1-piperazinylethyl-N-(2-pyridinyl)cyclohexane carboxamide trihydrochloride] WAY-100635 (0.3-3 mg/kg) enhanced PA retention. 3) The impairment (1 mg/kg) but not the facilitation (0.03 mg/kg) induced by 8-OH-DPAT was fully blocked by NAD-299 (0.3 mg/kg). 4) 5-HT(1A) receptor ligands given immediate post-training failed to alter PA retention. 5) NAD-299 (0.3-1 mg/kg) blocked the impairment of PA retention caused by a) the nonselective muscarinic receptor antagonist scopolamine and b) the non-competitive N-methyl-D-aspartate receptor antagonist MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate]. 6) A subthreshold dose of scopolamine completely blocked the facilitatory effect of NAD-299 on PA retention. 7) Anxiety-related behaviors and autonomic function were unchanged by NAD-299. 8) In situ hybridization showed that septal neurons expressing 5-HT(1A) receptor mRNA were codistributed with markers for cholinergic, GABAergic, and glutamatergic neurons. These results indicate that systemic administration of 5-HT(1A) receptor antagonists can facilitate cognitive performance, most likely by enhancing hippocampal/cortical cholinergic and glutamatergic neurotransmissions. Selective 5-HT(1A) receptor antagonists may be useful in the treatment of cognitive deficits such as Alzheimer's disease.

Analysis of the role of 5-HT1A receptors in spatial and aversive learning in the rat

The role of the brain 5-HT1A receptor in cognition was examined in the water maze (WM) and passive avoidance (PA) tasks in the male rat. Pre-training administration of the 5-HT1A receptor agonist 8-OH-DPAT impaired WM performance and facilitated PA retention at low doses (0.01 and 0.03 mg/kg) and impaired PA retention at higher doses (0.1-1.0 mg/kg). The 5-HT1A receptor antagonist NAD-299 produced a dose-dependent facilitation of PA retention. In contrast, the 5-HT1A receptor antagonists NAD-299 and WAY-100635 failed to alter acquisition and retention in the WM. The impairments in WM and PA (but not facilitation in PA) induced by 8-OH-DPAT were blocked by NAD-299. Furthermore, NAD-299 prevented the PA impairments induced by the muscarinic antagonist scopolamine or the NMDA receptor antagonist MK-801. In contrast, NAD-299 and WAY-100635 failed to attenuate the WM impairment induced by scopolamine, probably due to the failure of 5-HT1A receptor blockade to attenuate the sensorimotor disturbances induced by scopolamine. These results indicate that 5-HT1A receptor stimulation and blockade result in opposite effects in two types of cognitive tasks in the rat, and that 5-HT1A receptor blockade can facilitate some aspects of cognitive function, probably via modulation of cholinergic and glutamatergic transmissions. This suggests that 5-HT1A receptor antagonists may have a potential role in the treatment of human degenerative disorders associated with cognitive deficits.

5-HT1A receptor mRNA and immunoreactivity in the rat medial septum/diagonal band of Broca—relationships to GABAergic and cholinergic neurons

Activation of 5-HT1A receptors results in a variety of physiological responses, depending on their localization on neurons with different phenotypes in the brain. This study investigated the localization of 5-HT1A receptor mRNA and 5-HT1A receptor immunoreactivity in cell bodies of the rat septal complex using in situ hybridization and immunohistochemistry. In adjacent sections of the medial septum/diagonal band of Broca (MSDB), the distribution of cell bodies expressing 5-HT1A receptor mRNA was closely related to cells labeled with oligonucleotide probes to GAD (glutamic acid decarboxylase), VAChT (vesicular acetylcholine transporter) or parvalbumin mRNA. Using antiserum to GAD and antibodies to GABA, 5-HT1A receptor immunoreactivity was demonstrated in a majority of GABAergic cells in the MSDB. 5-HT1A receptor-immunoreactive GABAergic cells in the MSDB were also demonstrated to contain the calcium-binding protein parvalbumin, a marker for septohippocampal projecting GABAergic neurons. In the lateral septum, 5-HT1A receptor immunoreactivity was colocalized with the calcium-binding protein calbindin D-28k, a marker for septal GABAergic somatospiny neurons. 5-HT1A receptor immunoreactivity was also detected in a subpopulation of VAChT-containing cholinergic neurons of the MSDB. In MSDB neurons, colocalization of 5-HT1A and 5-HT2A receptor immunoreactivities was demonstrated. These observations suggest that serotonin via 5-HT1A receptors may represent an important modulator of hippocampal transmission important for cognitive and emotional functions through actions on both GABAergic and cholinergic neurons of the rat septal complex. In addition, 5-HT may exert its effects in the MSDB via cells expressing both 5-HT1A and 5-HT2A receptors.

The serotonin1A receptor partial agonist S15535 [4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine] enhances cholinergic transmission and cognitive function in rodents: a combined neurochemical and behavioral analysis

These studies examined the influence of the selective 5-hydroxytryptamine (serotonin) (5-HT)(1A) receptor partial agonist S15535 [4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine] upon cholinergic transmission and cognitive function in rodents. In the absence of acetylcholinesterase inhibitors, S15535 dose-dependently (0.04-5.0 mg/kg s.c.) elevated dialysis levels of acetylcholine in the frontal cortex and dorsal hippocampus of freely moving rats. In the cortex, the selective 5-HT(1A) receptor antagonist WAY100,635 [(N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide) fumarate] dose-dependently (0.0025-0.63) blocked this action of S15535. By contrast, in dorsal hippocampus, WAY100,635 mimicked the induction of acetylcholine release by S15535. In a social recognition paradigm, S15535 dose-dependently (0.16-10.0) improved retention, an action blocked by WAY100,635 (0.16), which was ineffective alone. Furthermore, S15535 dose-dependently (0.04-2.5) and WAY100,635 reversibly abolished amnesic properties of the muscarinic antagonist scopolamine (0.63) in this procedure. Cognitive deficits provoked by scopolamine in autoshaping and Morris water-maze procedures were likewise blocked by S15535 at doses of 0.63 to 10.0 and 0.16 to 2.5, respectively. In a two-platform spatial discrimination task, in which S15535 similarly abrogates cognitive deficits elicited by scopolamine, injection of S15535 (1.0 and 10.0 microg) into dorsal hippocampus blocked amnesic effects of the 5-HT(1A) agonist 8-hydroxy-2-dipropylaminotetralin (0.5 microg). Finally, S15535 (0.16-0.63) improved performance in a spatial, delayed nonmatching to sample model in mice, and in an operant delayed nonmatching to sample model in old rats, S15535 (1.25-5.0 mg/kg p.o.) increased response accuracy and reduced latency to respond. In conclusion, S15535 reinforces frontocortical and hippocampal release of acetylcholine and displays a broad-based pattern of procognitive properties. Its actions involve both blockade of postsynaptic 5-HT(1A) receptors and engagement of 5-HT(1A) autoreceptors.

Effects of beta-amyloid protein on serotoninergic, noradrenergic, and cholinergic markers in neurons of the pontomesencephalic tegmentum in the rat

The effects on serotoninergic, noradrenergic and cholinergic markers on neurons of the pontomesencephalic tegmentum nuclei were studied in rats following local administration of fibrillar beta-amyloid peptide (Abeta1-40) into the left retrosplenial cortex. Focal deposition of Abeta in the retrosplenial cortex resulted in a loss of serotoninergic neurons in the dorsal and median raphe nuclei. The dorsal raphe nucleus showed a statistically significant reduction of 31.7% in the number of serotoninergic neurons and a decrease (up to 17.38%) in neuronal density in comparison with the same parameters in uninjected controls. A statistically significant reduction of 50.3%, together with a significant decrease of 53.94% in the density of serotoninergic neurons, was also observed in the median raphe nucleus as compared with control animals. Furthermore, a significant reduction of 35.07% in the number of noradrenergic neurons as well as a statistically significant decrease of 56.55% in the density of dopamine-beta-hydroxylase-immunoreactive neurons were also found in the locus coeruleus as compared with the corresponding hemisphere in uninjected controls. By contrast, a reduction of 24.37% in the number of choline acetyltransferase-positive neurons and a slight decrease (up to 22.28%) in the density of cholinergic neurons, which were not statistically significant, was observed in the laterodorsal tegmental nucleus in comparison with the same parameters in control animals. These results show that three different neurochemically defined populations of neurons in the pontomesencephalic tegmentum are affected by the neurotoxicity of Abeta in vivo and that Abeta might indirectly affect serotoninergic, noradrenergic and cholinergic innervation in the retrosplenial cortex.

Reversal of visual attention dysfunction after AMPA lesions of the nucleus basalis magnocellularis (NBM) by the cholinesterase inhibitor donepezil and by a 5-HT1A receptor antagonist WAY 100635

RATIONALE

Degeneration of the cholinergic magnocellular neurons in the basal forebrain and their cortical projections is a major feature of the neuropathology of Alzheimer's disease (AD). In addition to memory dysfunction, attentional functions are also impaired in AD.

OBJECTIVE

We investigated the extent to which the cholinesterase inhibitor donepezil reversed the attentional performance deficit in nucleus basalis magnocellularis (NBM) lesioned rats. We also examined the effects of a selective and potent 5-HT(1A) receptor antagonist, WAY 100635, on the attentional deficit of NBM lesioned rats.

METHODS

We injected alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) into the NBM to selectively destroy cholinergic neurons projecting to the neocortex. Attentional functions were examined using the 5-CSRT task, in which hungry rats were required to locate brief visual targets presented randomly in one of five locations in a specially designed chamber. RESULTS. AMPA lesions of the NBM caused marked reductions in choline acetyltransferase activity (ChAT) ranging from 30 to 46% in medial areas of the cortex (medial-frontal and cingulate) and from 58 to 72% in more lateral areas (anterior-dorso-lateral and parietal). AMPA lesioned rats made fewer correct responses (choice accuracy), longer latency to correct response and an increase in the number of premature and perseverative responses. These impairments showed some recovery over the next 12 weeks. Reducing the duration of the visual stimulus reinstated the impairments in choice accuracy. The anticholinesterase inhibitor donepezil at 1.0 mg/kg but not 0.5 mg/kg reversed the impairments in choice accuracy and correct response latency. The premature and perseverative over-responding of AMPA lesioned rats remained unchanged. A dose of 0.1 mg/kg WAY 100635 to AMPA-lesioned rats improved their choice accuracy but did not shorten correct response latencies. The number of premature responses was reduced by WAY 100635 but perseverative over-responding was not affected.

CONCLUSIONS

The attentional impairments induced due to cortical cholinergic dysfunction may be ameliorated by cholinergic treatments such as cholinesterase inhibitors. In addition, 5-HT(1A) receptors and the cortical cholinergic system exert balanced opposition in regulating attentional performance in the rat. Blockade of 5-HT(1A) receptors may be useful to treat some aspects of attentional dysfunction in AD.

Selective 5-HT1A antagonists WAY 100635 and NAD-299 attenuate the impairment of passive avoidance caused by scopolamine in the rat

Systemic administration of the muscarinic-receptor antagonists atropine and scopolamine produces cognitive deficits in humans, nonhuman primates and rodents. In humans, these deficits resemble symptoms of dementia seen in Alzheimer's disease. The passive avoidance (PA) task has been one of the most frequently used animal models for studying cholinergic mechanisms in learning and memory. The present study examined the ability of two selective 5-HT(1A) receptor antagonists WAY 100635 and NAD-299 (robalzotan) and two acetylcholinesterase (AChE) inhibitors tacrine and donepezil to attenuate the impairment of PA retention caused by the nonselective muscarinic receptor antagonist scopolamine in the rat. Although demonstrating differences in their temporal kinetics, both WAY 100635 and NAD-299 attenuated the impairment of PA caused by scopolamine (0.3 mg/kg s.c.). Donepezil did not block the PA deficit caused by the 0.3 mg/kg dose of scopolamine, but it prevented the inhibitory effects of the 0.2 mg/kg dose of scopolamine. In contrast, tacrine was effective vs both the 0.2 and 0.3 mg/kg doses of scopolamine. These results indicate that (1). a functional 5-HT(1A) receptor antagonism can attenuate the anterograde amnesia produced by muscarinic-receptor blockade, and (2). the AChE inhibitors tacrine and donepezil differ in their ability to modify muscarinic-receptor-mediated function in vivo. These results suggest that 5-HT(1A) receptor antagonists may have a potential in the treatment of cognitive symptoms in psychopathologies characterized by reduced ACh transmission such as Alzheimer's disease.

Decreased G-protein coupling of serotonin 5-HT(1A) receptors in the brain of 5-HT(1B) knockout mouse

The firing of central serotonin (5-hydroxytryptamine, 5-HT) neurons and their capacity to release 5-HT are subjected to a receptor-mediated auto-control via 5-HT(1A) and 5-HT(1B) receptors respectively located on the somata/dendrites (5-HT(1A) autoreceptors) and preterminal axon arborizations (5-HT(1B) autoreceptors) of these neurons. To further characterize mutual adaptations of these two receptor subtypes in the absence of one of them, activation of G-protein coupling by agonist was measured and compared to wild-type (WT) in 5-HT(1A) and 5-HT(1B) homozygous knockout (KO) mice. As expected, in WT, the non-selective 5-HT(1A/1B) receptor agonist 5-carboxyamidotryptamine (5-CT) stimulated guanosine 5'-O-(gamma-[(35)S]thio)triphosphate ([(35)S]GTP(gamma)S) incorporation in many brain regions endowed with one and/or the other receptor. In the respective KOs, no stimulation was measured in regions known to express only or mainly the deleted receptor. In the 5-HT(1A) KOs, the amplitude of G-protein activation in regions endowed with 5-HT(1B) receptors was unchanged by comparison to WT. In the 5-HT(1B) KOs, the magnitude of the 5-CT stimulation was the same as WT in all regions containing 5-HT(1A) receptors, except in the amygdala, where it was significantly lower, even if this region was one of the most strongly activated in the WT. A similar result was obtained in the amygdala of 5-HT(1B) KOs after activation by the selective 5-HT(1A) receptor agonist R-(+)8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). Under these conditions, however, there was in addition a significant lowering of the stimulated (but not basal) [(35)S]GTP(gamma)S incorporation by comparison to WT in all regions endowed with 5-HT(1A) receptors, including the dorsal raphe nucleus. Thus, eventhough agonist radioligand binding to either 5-HT(1A) or 5-HT(1B) receptors is unchanged in the reciprocal KOs, it appears that a compensatory decrease in the efficiency of G-protein coupling to 5-HT(1A) receptors has developed in the 5-HT(1B) mutant. This could represent the first indication of a cross-talk between these two 5-HT receptor subtypes, at least in brain regions where they are co localized in the same neurons.

GABA(B) receptors in the median raphe nucleus: distribution and role in the serotonergic control of hippocampal activity

Previous studies have shown that serotonergic neurons of the median raphe nucleus have a suppressive effect on theta synchronization in the hippocampus. Median raphe lesion, suppression of 5-HT neuronal activity by administration of GABA(A) receptor antagonist or by glutamate blockade or depletion produced long-lasting non-interrupted hippocampal theta in freely behaving rats independent of behavior and in rats anesthetized with urethane. Serotonergic neurons show a characteristic sleep-wake pattern of activity and there is evidence that GABAergic mechanisms play an important role in their regulation. In this study we analyzed the distribution and subcellular localization of GABA(B) receptors in the midbrain raphe complex using combined 5-HT/GABA(B) receptor immunohistochemistry at the light and electron microscopic levels and studied the effects of their pharmacological manipulation on hippocampal electroencephalographic activity in urethane-anesthetized rats. We found that sustained infusion of the GABA(B) receptor agonist baclofen into the median raphe nucleus, using the microdialysis technique, elicited lasting theta activity in the hippocampus. The effect was antagonized by selective GABA(B) receptor antagonists. The predominant localization of GABA(B) receptors in the median, as well as in dorsal raphe was found on serotonergic neurons which strongly indicates that the increase in theta occurrence after baclofen injection resulted from suppression of the serotonergic output originating from the median raphe. On the electron microscopic level, we found GABA(B) receptors located extrasynaptically indicating that these receptors are preferentially activated by strong inputs, i.e. when GABA released from the synaptic terminals is sufficient to spill over from the synaptic cleft. Such conditions might be satisfied during rapid eye movement sleep when GABAergic neurons in the raphe are firing at their highest rate and in rhythmic synchronized bursts. Our data indicate that midbrain raphe GABA(B) mechanisms play an important role in behavioral state control and in hippocampal activity, in particular.

Serotonergic lesion of median raphe nucleus alters nerve growth factor content and vulnerability of cholinergic septohippocampal neurons in rat

About 45% of the serotonergic raphe neurons are reported to express nerve growth factor (NGF) receptors. We therefore investigated whether selective serotonergic lesions of the median or dorsal raphe nuclei are associated with changes in NGF protein levels of the brain and whether the loss of serotonergic function alters the vulnerability of cholinergic septohippocampal neurons. In adult rats the hippocampal NGF content changed in a biphasic way after lesion of the median raphe nucleus by 5,7-dihydroxytryptamine (5,7-DHT), with a significant increase after 2-3 weeks of up to 35%, followed by a significant reduction of 22% below control levels after 7 weeks, and a return to control levels within the following 4 weeks. By contrast, the decrease in hippocampal serotonin and 5-hydroxyindoleacetic acid remained throughout the observation period of 11 weeks, being still reduced to 15 and 30% of the control levels, respectively. In the frontal cortex the partial loss of the serotonergic innervation projecting from the median raphe was associated 5 weeks after 5,7-DHT injection with an increase in NGF protein of 39.7+/-9.6% (P<0.05), which remained elevated up to 11 weeks. At 9 weeks after 5,7-DHT, the lesion of the septohippocampal cholinergic neurons induced by the cholinotoxin ethylcholine aziridinium (AF64A) was exaggerated (P<0.05) as compared to AF64A-treated rats with intact serotonergic innervation. The present data indicate that a serotonergic lesion of the median raphe nucleus results in biphasic changes of NGF protein content and in a delayed increase in the vulnerability of septohippocampal cholinergic neurons.

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.

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.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

Serotonergic systems, their phylogeny and ontogeny have been thoroughly described up to the ultrastructural level, thanks to the multiplicity of methodological approaches. They have often been referred to as a 'Rosetta stone', as several features first described for serotonin neurons or paraneurons have been then extended to other neurotransmitter systems: coexistence with neuropeptides or even a canonical neurotransmitter (GABA), volume transmission, regrowth after lesioning, and characterization of multiple receptor subtypes. This review deals with the contributions of neuroanatomical approaches for studying serotoninergic systems, and focuses on recent advances concerning the topological relationships between serotonergic innervation, receptors and target cells. This aspect is particularly important with regard to the possibility for serotonin to act through classical synaptic transmission and/or non-junctional transmission. Serotonin then can selectively regulate different neuronal systems through the activation of distinct receptor subtypes, which in turn can be linked to different transduction pathways. Neurocytochemical approaches constitute unique tools to analyse both anatomical and functional characteristics of complex neuronal systems.

Intraseptal infusions of 8-OH-DPAT in the rat impairs water-maze performances: effects on memory or anxiety?

In the rat, 5-HT1A receptors are found on medial septal cholinergic neurons. The effects of intraseptal infusions of the 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propyl-amino)-tertralin) were assessed on reference memory performances in a water maze. Compared with vehicle infusions, 0.5 and 4 microg of 8-OH-DPAT significantly impaired (but did not prevent) acquisition of the task and probe-trial performances. The results suggest that activation of 5-TH1A receptors in the (medial) septal area impairs spatial learning, perhaps directly by reducing the hippocampal cholinergic tonus, or indirectly by an effect on anxiety.

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.

Stimulation of 5-HT1A receptors by systemic or medial septum injection induces anxiogenic-like effects and facilitates acquisition of a spatial discrimination task in mice

1. In the present study, the authors addressed the issue of the possible modulation of both emotional and learning processes by the stimulation of 5-HT1A receptors. In this respect, we have carried out two series of experiments: the first series examined the effects of systemic injections of 8-OH-DPAT, a 5-HT1A receptor agonist, successively on a model of anxiety and on a learning task; secondly the effects of selective infusions into the medial septum were studied in the same experimental design. 2. Mice were tested in an elevated plus-maze before being submitted to a spatial discrimination task in an 8-arm radial maze. The 8-OH-DPAT (1 mg/kg) administered intraperitoneally 30 minutes prior to testing, induced anxiogenic-like effects in the plus-maze and improved the acquisition of the spatial discrimination. 3. Moreover, a regression analysis showed that the index of anxiety measured in the elevated plus-maze was positively correlated with the performance level reached at the forth day of training in the spatial discrimination task. The intraseptal infusion of the drug (1 microgram) demonstrated the same pattern of results, although the effects were less pronounced. Again a correlation between the index of anxiety and acquisition performance was obtained. 4. These results suggest that anxiogenic-like effects induced by selective stimulation of 5-HT1A receptors have a positive influence on the acquisition of a memory task. As systemic injections appeared to be more effective than intra-septal infusions, these effects might be mediated by both pre- and postsynaptic 5-HT1A receptors.