Centrally acting hypotensive agents with affinity for 5-HT1A binding sites inhibit forskolin-stimulated adenylate cyclase activity in calf hippocampus

Role of tandospirone, a 5-HT1A receptor partial agonist, in the treatment of central nervous system disorders and the underlying mechanisms

5-hydroxytryptamine (5-HT, serotonin) is an important neurotransmitter in the modulation of the cognitive, behavioral and psychological functions in animals and humans. Among the fourteen subtypes of 5-HT receptor, 5-HT1A receptor has been extensively studied. Tandospirone, an azapirone derivative with strong and selective agonist effect on 5-HT1A receptor, has been used for the treatment of anxiety disorders especially generalized anxiety disorder for decades. Recently, tandospirone showed the efficacy in relieving the syndromes of social anxiety disorder and post-traumatic stress disorder as well as in potentiating the effect of antidepressants in the treatment of depression in both preclinical and clinical studies. More impressively, the beneficial effect of tandospirone has been revealed on improvement of motor dysfunction of Parkinson's disease and cognitive deficits of schizophrenia either in monotherapy or in combination with other drugs. This review discusses the superiority of tandospirone in the treatment of the disorders and associated mechanisms in central nervous system from the literature.

Comparative analysis of pharmacological properties of xanomeline and N-desmethylclozapine in rat brain membranes

BACKGROUND

3(3-Hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine (xanomeline) and N-desmethylclozapine are of special interest as promising antipsychotics with better efficacy, especially for negative symptoms and/or cognitive/affective impairment.

METHODS

The guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding experiments were performed using (1) conventional filtration technique, (2) antibody-capture scintillation proximity assay, and (3) immunoprecipitation method, in brain membranes prepared from rat cerebral cortex, hippocampus, and striatum.

RESULTS

Xanomeline had agonistic activity at the M1 muscarinic acetylcholine receptor (mAChR) in all brain regions, as well as at the 5-HT1A receptor in the cerebral cortex and hippocampus. On the other hand, N-desmethylclozapine exhibited slight agonistic effects on the M1 mAChR, and agonistic properties at the 5-HT1A receptor in the cerebral cortex and hippocampus. This compound also behaved as an agonist at the δ-opioid receptor in the cerebral cortex and striatum. In addition, the stimulatory effects of N-desmethylclozapine on [(35)S]GTPγS binding to Gαi/o were partially mediated through mAChRs (most likely M4 mAChR subtype), at least in striatum.

CONCLUSIONS

The agonistic effects on the mAChRs (particularly M1 subtype, and also probably M4 subtype), the 5-HT1A receptor and the δ-opioid receptor expressed in native brain tissues, some of which are common to both compounds and others specific to either, likely shape the unique beneficial effectiveness of both compounds in the treatment for schizophrenic patients. These characteristics provide us with a clue to develop newer antipsychotics, beyond the framework of dopamine D2 receptor antagonism, that are effective not only on positive symptoms but also on negative symptoms and/or cognitive/affective impairment.

Discovery of Natural Product-Derived 5-HT1A Receptor Binders by Cheminfomatics Modeling of Known Binders, High Throughput Screening and Experimental Validation

The human 5-hydroxytryptamine receptor subtype 1A (5-HT1A) is highly expressed in the raphe nuclei region and limbic structures; for that reason 5-HT1A has served as a promising target for treating human mood disorders and neurodegenerative diseases. We have developed binary quantitative structure-activity relationship (QSAR) models for 5- HT1A binding using data retrieved from the WOMBAT database and the k-Nearest Neighbor (kNN) machine learning method. A rigorous QSAR modeling and screening workflow had been followed, with extensive internal and external validation processes. The models' classification accuracies to discriminate 5-HT1A binders from the non-binders are as high as 96% for the external validation. These models were employed further to mine two major natural products screening libraries, i.e. TimTec Natural Product Library (NPL) and Natural Derivatives Library (NDL). In the end five screening hits were tested by radioligand binding assays with a success rate of 40%, and two Library compounds were confirmed to be binders at the μM concentration against the human 5-HT1A receptor. The combined application of rigorous QSAR modeling and model-based virtual screening presents a powerful means for profiling natural products compounds with important biomedical activities.

Escitalopram Ameliorates Forskolin-Induced Tau Hyperphosphorylation in HEK239/tau441 Cells

To investigate the effect of escitalopram (a widely used and highly efficacious antidepressant from the SSRI class) on tau hyperphosphorylation, HEK293/tau441 cells were pretreated with 4 μM of forskolin for 2 h. Then we treated the cells with different doses of escitalopram (0, 5, 10, 20, 40, 80 μM) for 22 h. We measured the phosphorylation level of tau by Western blotting. It was shown that escitalopram could protect tau from hyperphosphorylation induced by pharmacological activation of protein kinase A (PKA) at a dose of 20, 40, and 80 μM in vitro. Interestingly, the same dose of escitalopram could also increase the level of serine-9-phosphorylated GSK-3β (inactive form) and the phosphorylation level of Akt at Ser473 (active form) with no significant change in the level of total GSK-3β and Akt. Unexpectedly, 5-hydroxytryptamine 1A receptor (5-HT1A) agonist 8-OH-DPAT did not decrease forskolin-induced tau hyperphosphorylation. Our results suggest that escitalopram can ameliorate forskolin-induced tau hyperphosphorylation, which is not through the typical 5-HT1A pathway, and Akt/GSK-3β signaling pathway is involved. These findings may support an effective role of antidepressants in the prevention of dementia associated with depression in patients.

Luciferase reporter gene assay on human 5-HT receptor: which response element should be chosen?

Serotonin (5-HT) receptors are valuable molecular targets for antipsychotic drug discovery. Current reported methods for detecting 5-HT receptors, such as cAMP accumulation and calcium influx assay, are often demanding specialized instruments and inconvenient. The luciferase reporter gene assay, based on the responsible-element-regulated expression of luciferase, has been widely applied in the high-throughput functional assay for many targets because of its high sensitivity and reliability. However, 5-HT receptors couple to multiple G-proteins regulate respective downstream signalling pathways and are usually detected using different response elements. Hence, finding a suitable response element to fulfil the detection of different 5-HT receptors and make the results of luciferase reporter gene assays generalizable is very useful for active compounds screening. Here, we conducted three luciferase reporter assays using CRE, NFAT, and SRE response elements attached to 5-HT to detect the activation of different 5-HT receptors in CHO-K1 cells. The potencies and efficacies of the reported ligands (agonists and antagonists) were determined and compared. Our results indicate that CRE-luciferase reporter gene is sensitive and reliable to detect the activities of G protein-coupled 5-HT receptors.

Cloperastine rescues impairment of passive avoidance response in mice prenatally exposed to diethylstilbestrol

We previously reported that prenatal exposure to diethylstilbestrol (DES) impaired passive avoidance responses in mice. Apart from the above, we also found that cloperastine, a centrally acting antitussive, ameliorated depression-like and anxiety-like behaviors in rodents at antitussive-effective doses. In this study, we investigated whether or not cloperastine rescues impairment of passive avoidance responses in mice prenatally exposed to DES. Male DES-exposed mice were subcutaneously administered cloperastine at 10 or 30 mg/kg twice a day from 32 to 41 days after birth and subjected to behavioral testing 42 to 46 days after birth. Cloperastine at 10 and 30 mg/kg ameliorated DES-induced impairment of passive avoidance responses. In addition, cloperastine affected the levels of 5-HT1A receptors, GIRK and BDNF in the hippocampus of DES-exposed mice. However, the number of BrdU-positive cells in the hippocampus of DES-exposed mice was not changed by chronic administration of cloperastine. These findings suggest that the action of endocrine disruptors in the brain may not always be irreversible, and that the symptoms caused by endocrine disruptors might be curable with drugs such as cloperastine.

Vortioxetine dose-dependently reverses 5-HT depletion-induced deficits in spatial working and object recognition memory: a potential role for 5-HT1A receptor agonism and 5-HT3 receptor antagonism

We previously reported that the investigational multimodal antidepressant, vortioxetine, reversed 5-HT depletion-induced memory deficits while escitalopram and duloxetine did not. The present report studied the effects of vortioxetine and the potential impact of its 5-HT1A receptor agonist and 5-HT3 receptor antagonist properties on 5-HT depletion-induced memory deficits. Recognition and spatial working memory were assessed in the object recognition (OR) and Y-maze spontaneous alternation (SA) tests, respectively. 5-HT depletion was induced in female Long-Evans rats using 4-cholro-DL-phenylalanine methyl ester HCl (PCPA) and receptor occupancies were determined by ex vivo autoradiography. Rats were acutely dosed with vortioxetine, ondansetron (5-HT3 receptor antagonist) or flesinoxan (5-HT1A receptor agonist). The effects of chronic vortioxetine administration on 5-HT depletion-induced memory deficits were also assessed. 5-HT depletion reliably impaired memory performance in both the tests. Vortioxetine reversed PCPA-induced memory deficits dose-dependently with a minimal effective dose (MED) ≤0.1mg/kg (∼80% 5-HT3 receptor occupancy; OR) and ≤3.0mg/kg (5-HT1A, 5-HT1B, 5-HT3 receptor occupancy: ∼15%, 60%, 95%) in SA. Ondansetron exhibited a MED ≤3.0μg/kg (∼25% 5-HT3 receptor occupancy; OR), but was inactive in the SA test. Flesinoxan had a MED ≤1.0mg/kg (∼25% 5-HT1A receptor occupancy; SA); only 1.0mg/kg ameliorated deficits in the NOR. Chronic p.o. vortioxetine administration significantly improved memory performance in OR and occupied 95%, 66%, and 9.5% of 5-HT3, 5-HT1B, and 5-HT1A receptors, respectively. Vortioxetine's effects on SA performance may involve 5-HT1A receptor agonism, but not 5-HT3 receptor antagonism, whereas the effects on OR performance may involve 5-HT3 receptor antagonism and 5-HT1A receptor agonism.

Context-dependent effects of a single administration of mirtazapine on the expression of methamphetamine-induced conditioned place preference

Re-exposure to cues repeatedly associated with methamphetamine (Meth) can trigger Meth-seeking and relapse in the abstinent abuser. Weakening the conditioned Meth-associated memory during cue re-exposure may provide a means for relapse-reduction pharmacotherapy. Accordingly, we sought to determine if the atypical antidepressant mirtazapine disrupted the persistence of Meth-induced conditioned place preference (CPP) when administered in conjunction with re-exposure to contextual conditioning cues, and if this effect was altered by Meth being present during cue re-exposure. First, we evaluated the effect of mirtazapine on the maintenance of Meth-induced CPP during re-exposure to either the saline- or Meth-paired chamber 12 days after conditioning. Meth-conditioned rats subsequently administered mirtazapine expressed CPP independent of re-exposure to the saline- or Meth-paired chamber; but the magnitude of CPP was significantly less for mirtazapine-treated rats re-exposed to the Meth-paired chamber. Next, we evaluated the effect of mirtazapine on a "reinforced re-exposure" to the Meth-paired context. Administration of mirtazapine vehicle and Meth, prior to re-exposure to the Meth-paired chamber did not disrupt the ability of rats to demonstrate CPP 15 days after conditioning; however, CPP was disrupted when rats were administered mirtazapine and Meth prior to re-exposure to the Meth-paired chamber. These results indicate that the capacity of mirtazapine to diminish Meth-induced CPP is promoted if mirtazapine treatment is coupled with Meth administration in the Meth-associated context and thus appears to be the consequence of disrupting processes necessary to reconsolidate CPP following activation of drug-associated memories.

A role for 5-HT1A receptors in the basolateral amygdala in the development of conditioned defeat in Syrian hamsters

The basolateral nucleus of the amygdala (BLA) is a key brain region regulating behavioral changes following stressful events, including social defeat. Previous research has shown that activation of serotonin (5-HT) 1A receptors in the BLA reduces conditioned fear and anxiety-like behavior. The objective of this study was to test whether 5-HT1A receptors in the BLA contribute to conditioned defeat in male Syrian hamsters (Mesocricetus auratus). We tested whether injection of the selective 5-HT1A receptor agonist flesinoxan (400 ng, 800 ng, or 1200 ng in 200 nl saline) into the BLA prior to social defeat would reduce the acquisition of conditioned defeat, and whether a similar injection prior to testing would reduce the expression of conditioned defeat. We also tested whether injection of the selective 5-HT1A receptor antagonist WAY-100635 (400 ng or 1600 ng in 200 nl saline) into the BLA prior to social defeat would enhance the acquisition of conditioned defeat, and whether a similar injection prior to testing would enhance the expression of conditioned defeat. We found that injection of flesinoxan into the BLA decreased both the acquisition and expression of conditioned defeat. However, injection of WAY-100635 into the BLA did not alter the acquisition or expression of conditioned defeat. These data indicate that pharmacological activation of 5-HT1A receptors in the BLA is sufficient to impair the acquisition and expression of conditioned defeat. Our results suggest that pharmacological treatments that activate 5-HT1A receptors in the BLA are capable of reducing the development of stress-induced changes in behavior.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Role of the serotonergic system in alcohol dependence: from animal models to clinics

Alcohol dependence remains among the most common substance abuse problems worldwide, and compulsive alcohol consumption is a significant public health concern. Alcohol is an addictive drug that alters brain function through interactions with multiple neurotransmitter systems. These neurotransmitter systems mediate the reinforcing effects of alcohol. Specifically, the serotonergic system is important in mediating alcohol reward, preference, dependence, and craving. In this review chapter, we first discuss the serotonin system as it relates to alcoholism, and then outline interactions between this system and other neurotransmitter systems. We emphasize the serotonin transporter and its possible role in alcoholism, then present several serotonergic receptors and discuss their contribution to alcoholism, and finally assess the serotonin system as a target for pharmacotherapy, with an emphasis on current and potential treatments.

Therapeutic role of 5-HT1A receptors in the treatment of schizophrenia and Parkinson's disease

5-HT(1A) receptors have long been implicated in the pathogenesis and treatment of anxiety and depressive disorders. Recently, several lines of studies have revealed new insights into the therapeutic role of 5-HT(1A) receptors in treating schizophrenia and Parkinson's disease. Specifically, 5-HT(1A) receptors seem to be a promising target for alleviating antipsychotic-induced extrapyramidal side effects (EPS) and cognitive/affective disorders in schizophrenia. In the treatment of patients with Parkinson's disease, 5-HT(1A) agonists are expected to improve not only affective symptoms (e.g., anxiety and depression), but also the core parkinsonian symptoms as well as antiparkinsonian agents-induced side effects (e.g., L-DOPA-induced dyskinesia). Here, the therapeutic mechanisms mediated by 5-HT(1A) receptors in schizophrenia and Parkinson's disease are reviewed. This evidence should encourage discovery of new 5-HT(1A) ligands, which can resolve the unmet clinical needs in the current therapy.

5-HT1A receptor-regulated signal transduction pathways in brain

Serotonin is an influential monoamine neurotransmitter that signals through a number of receptors to modulate brain function. Among different serotonin receptors, the serotonin 1A (5-HT1A) receptors have been tied to a variety of physiological and pathological processes, notably in anxiety, mood, and cognition. 5-HT1A receptors couple not only to the classical inhibitory G protein-regulated signaling pathway, but also to signaling pathways traditionally regulated by growth factors. Despite the importance of 5-HT1A receptors in brain function, little is known about how these signaling mechanisms link 5-HT1A receptors to regulation of brain physiology and behavior. Following a brief summary of the known physiological and behavioral effects of 5-HT1A receptors, this article will review the signaling pathways regulated by 5-HT1A receptors, and discuss the potential implication of these signaling pathways in 5-HT1A receptor-regulated physiological processes and behaviors.

Activation of 5-HT1A autoreceptors in the dorsal raphe nucleus reduces the behavioral consequences of social defeat

In animal models, serotonin (5-HT) activity contributes to stress-induced changes in behavior. Syrian hamsters (Mesocricetus auratus) exhibit a stress-induced change in behavior in which social defeat results in increased submissive and defensive behavior and a complete loss of normal territorial aggression directed toward a novel, non-aggressive opponent. We refer to this defeat-induced change in agonistic behavior as conditioned defeat. In this study we tested the hypothesis that 5-HT activity in the dorsal raphe nucleus (DRN) contributes to the acquisition and expression of conditioned defeat. We investigated whether injection of the selective 5-HT1A agonist flesinoxan (200 ng, 400 ng, or 800 ng in 200 nl saline) into the DRN would reduce the acquisition and expression of conditioned defeat. Additionally, we investigated whether injection of the selective 5-HT1A antagonist WAY 100635 (400 ng in 200 nl saline) into the DRN would enhance the acquisition and expression of conditioned defeat following a sub-optimal social defeat experience. We found that injection of flesinoxan into the DRN before exposure to a 15-min social defeat reduced the amount of submissive and defensive behavior shown at testing. We also found that injection of flesinoxan into the DRN before testing similarly reduced submissive and defensive behavior. In addition, we found that WAY 100635 enhanced conditioned defeat when injected either before social defeat or before testing. These data support the hypothesis that the activity of 5-HT cells in the DRN, as regulated by 5-HT1A autoreceptors, contributes to the formation and display of conditioned defeat. Further, our results suggest that 5-HT release in DRN projection regions augments defeat-induced changes in social behavior.

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.

5-HT2 and 5-HT3 receptors in the lateral parabrachial nucleus mediate opposite effects on sodium intake

The present study investigated the role of several 5-HT receptor subtypes in the lateral parabrachial nucleus (LPBN) in the control of sodium appetite (i.e. NaCl consumption). Male Holtzman rats had cannulas implanted bilaterally into the LPBN for the injection of 5-HT receptor agonists and antagonists in conjunction with either acute fluid depletion or 24-h sodium depletion. Following these treatments, access to 0.3 M NaCl was provided and the intakes of saline and water were measured for the next 2 h. Bilateral injections of the 5-HT2A receptor antagonist, ketanserin or the 5-HT2C receptor antagonist, mianserin into the LPBN increased 0.3 M NaCl intake without affecting water intake induced by acute fluid-depletion. Bilateral injections of the 5-HT2B receptor agonist, BW723C86 hydrochloride, had no effect on 0.3 M NaCl or water intake under these conditions. Treatment of the LPBN with the 5-HT2B/2C receptor agonist, 2-(2-methyl-4-clorophenoxy) propanoic acid (mCPP) caused dose-related reductions in 0.3 M NaCl intake after 24 h sodium depletion. The effects of mCPP were prevented by pretreating the LPBN with the 5-HT2B/2C receptor antagonist, SDZSER082. Activation of 5-HT3 receptors by the receptor agonist, 1-phenylbiguanide (PBG) caused dose-related increases in 0.3 M NaCl intake. Pretreatment of the LPBN with the 5-HT3 receptor antagonist, 1-methyl-N-[8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584) abolished the effects of PBG, but LY-278,584 had no effects on sodium or water intake when injected by itself. PBG injected into the LPBN did not alter intake of palatable 0.06 M sucrose in fluid replete rats. The results suggest that activation of the 5-HT2A and 5-HT2C receptor subtypes inhibits sodium ingestion. In contrast, activation of the 5-HT3 receptor subtype increases sodium ingestion. Therefore, multiple serotonergic receptor subtypes in the LPBN are implicated in the control of sodium intake, sometimes by mediating opposite effects of 5-HT. The results provide new information concerning the control of sodium intake by LPBN mechanisms.

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.

Activation of serotonergic 5-HT1A receptors in the lateral parabrachial nucleus increases NaCl intake

Previous studies using non-specific serotonergic agonists and antagonists have shown the importance of serotonergic inhibitory mechanisms in the lateral parabrachial nucleus (LPBN) for controlling sodium and water intake. In the present study, we investigated whether the serotonergic 5-HT(1A) receptor subtype in the LPBN participates in this control. Male Holtzman rats had cannulas implanted bilaterally into the LPBN. Bilateral injections of the 5-HT(1A) receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.1, 1.25, and 2.5 microg/0.2 microl), into the LPBN enhanced 0.3 M NaCl and water intake of rats injected subcutaneously with the diuretic furosemide (10 mg/kg bw) and a low dose of the angiotensin-converting enzyme inhibitor, captopril (5 mg/kg bw). The increase in NaCl intake produced by 8-OH-DPAT injections was reduced in dose-related manner by pre-treating the LPBN with the selective 5-HT(1A) serotonergic antagonist, WAY-100635 (WAY, 1 and 2 microg/0.2 microl). In contrast, WAY did not affect water intake produced by 8-OH-DPAT. WAY-100635 injected alone into the LPBN had no effect on NaCl ingestion. Injections of 8-OH-DAPT (0.1 microg/0.2 microl) into the LPBN also increased 0.3 M NaCl intake induced by 24-h sodium depletion (furosemide, 20 mg/kg bw plus 24 h of sodium-free diet). Serotonin (5-HT, 20 mug/0.2 mul) injected alone or combined with 8-OH-DPAT into the LPBN reduced 24-h sodium depletion-induced 0.3 M NaCl intake. Therefore, the activation of serotonergic 5-HT(1A) receptors in the LPBN increases stimulated hypertonic NaCl and water intake, while 5-HT injections into the LPBN reduce NaCl intake and prevent the effects of serotonergic 5-HT(1A) receptor activation.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

5-HT1A receptor-mediated G protein activation assessed by [35S]GTPgammaS binding in rat cerebral cortex

To date, 5-hydroxytryptamine1A (5-HT1A) receptor-mediated functional assays (adenylyl cyclase inhibition, high-affinity GTPase activity and [35S]guanosine-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding) have been performed mainly in hippocampal membranes. In the current study, 5-HT-stimulated G protein activation assays were carried out in rat cerebral cortical membranes. High-affinity GTPase activity was stimulated by 5-HT, but not by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). By contrast, 5-HT- and 8-OH-DPAT-stimulated [35S]GTPgammaS binding displayed sufficient dynamic range enough to warrant further pharmacological analysis. Under standard conditions, which were determined precisely in terms of the concentrations of GDP, MgCl2 and NaCl, the profile of 5-HT-stimulated [35S]GTPgammaS binding investigated using a series of 5-HT receptor agonists and antagonists clearly indicated the involvement of the 5-HT1A receptor subtype. There appeared to be no evidence supporting the presence of regional heterogeneity in coupling efficiency between 5-HT1A and G proteins in the hippocampus or cortex. This method is a useful tool for investigating functional coupling between postsynaptic 5-HT1A receptors and G proteins in cerebral cortical membranes.

Serotonin 5-HT1A receptor blockade enhances Ca2+/calmodulin-dependent protein kinase II function and membrane expression of AMPA receptor subunits in the rat hippocampus: implications for memory formation

Stimulation of hippocampal 5-HT(1A) receptors impairs memory retention. The highly selective 5-HT(1A) antagonist, WAY-100635, prevents the cognitive deficits induced not only by 5-HT(1A) stimulation but also by cholinergic or NMDA receptor blockade. On this basis, the effects of WAY-100635 on molecular events associated with memory storage were explored. In rat hippocampus, WAY-100635 produced a rapid increase in phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and in Ca(2+)-independent CaMKII and protein kinase A (PKA) enzyme activity. This increase was followed a few hours later by an enhanced membrane expression of AMPA receptor subunits, especially of the GluR1 subunit phosphorylated at the CaMKII site, pGluR1(Ser831). The same qualitative effects were found with the weaker 5-HT(1A) antagonist NAN-190. The effects of both antagonists were no longer apparent in rats with a previous 5-HT depletion induced by the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA), suggesting that 5-HT(1A) receptor blockade removes the tonic inhibition of 5-HT through 5-HT(1A) receptor stimulation on excitatory hippocampal neurons, with the consequent increase in PKA activity. In addition, administration of WAY-100635 potentiated the learning-specific increase in the hippocampus of phospho-CaMKII, Ca(2+)-independent CaMKII activity, as well as the phosphorylation of either the CaMKII or the PKA site on the AMPA receptor GluR1 subunit. This study suggests that blockade of hippocampal 5-HT(1A) receptors favours molecular events critically involved in memory formation, and provides an in vivo molecular basis for the proposed utility of 5-HT(1A) receptor antagonists in the treatment of cognitive disorders.

Trazodone and its active metabolite m-chlorophenylpiperazine as partial agonists at 5-HT1A receptors assessed by [35S]GTPgammaS binding

Trazodone is an effective antidepressant drug with a broad therapeutic spectrum, including anxiolytic efficacy. Although trazodone is usually referred to as a serotonin (5-HT) reuptake inhibitor, this pharmacological effect appears to be too weak to fully account for its clinical effectiveness. The present study aimed to elucidate the agonist properties of trazodone and its active metabolite, m-chlorophenylpiperazine (m-CPP), at 5-HT(1A) receptors by means of the guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding assay. In membranes prepared from Chinese hamster ovary cells expressing human 5-HT(1A) receptors (CHO/h5-HT(1A)), trazodone behaved as an almost full agonist and m-CPP was also a highly efficacious partial agonist at 5-HT(1A) receptors. The intrinsic activities of both compounds were higher than those of tandospirone and buspirone, which are clinically effective anxiolytics with well-known 5-HT(1A) partial agonist properties. These effects were replicated in the 5-HT(1A) receptor-mediated [(35)S]GTPgamma(S) binding assay in native rat brain membranes (at least in hippocampal membranes), although the intrinsic activities of the compounds were low and differently ranked compared to those in CHO/h5-HT(1A) cell membranes. When considering the implications of 5-HT(1A) receptors in anxiety and/or depression, as well as the clinical effectiveness of azapirone anxiolytics with partial 5-HT(1A) receptor agonist properties such as buspirone, it is possible that the agonist effects on 5-HT(1A) receptors of trazodone and its active metabolite m-CPP presented in this study contribute, at least in part, to the clinical efficacy of the atypical antidepressant trazodone.

Role of hippocampal CaMKII in serotonin 5-HT(1A) receptor-mediated learning deficit in rats

The serotonin 5-HT(1A) receptor agonist, 8-OH-DPAT (8-hydroxy-2-di-n-propylamino-tetralin), impairs retention performance in a passive avoidance learning task in rats. In the hippocampus of rats trained on this procedure and killed 1 h after the acquisition trial, an increase in the membrane levels of both Ca2+/calmodulin-dependent protein kinase II (CaMKII) and phosphorylated CaMKII, as well as in total and Ca2+-independent enzyme activity in tissue lysates was found. These effects were learning-specific as no changes in CaMKII levels or activity were found in rats receiving a footshock identical to the trained rats. The effect of training on CaMKII was prevented by a low 8-OH-DPAT dose. The 5-HT(1A) agonist also reduced protein kinase A (PKA) activity and increased the membrane levels of phosphatase 1 (PP1) and PP1 enzyme activity in the hippocampus. All of the changes induced by 8-OH-DPAT were reversed by the selective 5-HT(1A) antagonist WAY-100635, indicating receptor-specific effects. We suggest that 5-HT(1A) receptor-mediated disruption of retention performance is a consequence of the reduced PKA activity and the ensuing enhancement in PP1 activity, possibly through decreased phosphorylation/activation of endogenous PP1 inhibitors, that cause a reduced activity of phosphorylated CaMKII, a key enzyme in early stages of memory formation. This study provides an in vivo molecular basis for the cognitive deficits induced by stimulation of hippocampal 5-HT(1A) receptors.

Modulation of reflexly evoked vagal bradycardias by central 5-HT1A receptors in anaesthetized rabbits

1. The role of central 5-HT(1A) receptors in the control of the bradycardia and changes in central respiratory drive, renal nerve activity and blood pressure evoked by stimulating cardiopulmonary afferents with phenylbiguanide, baroreceptors by electrical stimulation of the aortic nerve and chemoreceptors by injections of sodium cyanide (NaCN) in atenolol-pretreated anaesthetized rabbits were studied. 2. Buspirone (100 micro g kg(-1); i.c.) potentiated the bradycardia (increase in R-R interval) and the changes in blood pressure and renal nerve activity evoked by all three reflexes. These effects could be attenuated by pretreatment with the 5-HT(1A) receptor antagonist WAY-100635 (100 micro g kg(-1)); i.v.), which alone had no effect on these reflex-evoked changes. However, WAY-100635 (100 micro g kg(-1); i.c.) did attenuate these reflex-evoked responses produced by activation of cardiopulmonary and aortic baroreceptors but not that caused by stimulation of chemoreceptors. When given i.v., buspirone was less effective in modulating the responses evoked by these three reflexes. 3. The present data are consistent with the view that central 5-HT(1A) receptors play a role in the reflex activation of cardiac preganglionic vagal motoneurones. However, although antagonists of 5-HT(1A) receptors affected the responses evoked by cardiopulmonary and aortic nerve afferents, they were not effective on chemoreceptor reflex-evoked changes. This suggests that 5-HT(1A) receptors play a different role in chemoreceptor pathways compared to that for the other reflexes. This may relate to the fact that the chemoreceptor afferents travel in the IXth (glossopharyngeal) nerve whilst the other afferents travel in the Xth (vagus) nerve and thus may use different central circuitry and neurotransmitters.

Functional characterization of alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries

alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries were characterized using agonists noradrenaline (non-selective) and A61603 (alpha(1A)-selective), the antagonists prazosin (non-selective), 5-methyl-urapidil (alpha(1A)-selective) and BMY7378 (alpha(1D)-selective) and the alkylating agent chloroethylclonidine (preferential for alpha(1B)). Small arteries were obtained from the non-ischaemic skeletal muscle of limbs amputated for critical limb ischaemia and isometric tension recorded using wire myography. Prazosin antagonized responses to noradrenaline with a pA(2) value of 9.18, consistent with the presence of alpha(1)-adrenoceptors, although the Schild slope (1.32) was significantly different from unity. 5-Methyl-urapidil competitively antagonized responses to noradrenaline with a pK(B) value of 8.48 and a Schild slope of 0.99, consistent with the presence of alpha(1A)-adrenoceptors. In the presence of 300 nM 5-methyl-urapidil, noradrenaline exhibited biphasic concentration response curves, indicating the presence of a minor population of a 5-methyl-urapidil-resistant subtype. Contractile responses to noradrenaline were not affected by 1 microM chloroethylclonidine suggesting the absence of alpha(1B)-adrenoceptors. Maximum responses to noradrenaline and A61603 were reduced to a similar extent by 10 microM chloroethylclonidine, suggesting an effect of chloroethylclonidine at alpha(1A)-adrenoceptors at the higher concentration. BMY7378 (10 and 100 nM) had no effect on responses to noradrenaline. BMY7378 (1 microM) poorly shifted the potency of noradrenaline giving a pA(2) of 6.52. These results rule out the presence of the alpha(1D)-subtype. These results show that contractile responses to noradrenaline in human skeletal muscle resistance arteries are predominantly mediated by the alpha(1A)-adrenoceptor subtype with a minor population of an unknown alpha(1)-adrenoceptor subtype.

The alpha(1A)-adrenoceptor subtype mediates contraction in rat femoral resistance arteries

In this study, alpha(1)-adrenoceptor subtypes were characterised in rat femoral resistance arteries mounted on a small vessel myograph. A-61603 was found to be more potent than noradrenaline and phenylephrine in these arteries. Brimonidine (UK 14304) could not evoke any contractile responses and the sensitivity to noradrenaline and phenylephrine was not affected by (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine (RS 79948), ruling out the presence of alpha(2)-adrenoceptors. Prazosin, 5-methyl-urapidil and 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB 4101) produced rightward shifts in the sensitivity to noradrenaline, giving pA(2) values of 9.6, 9.4 and 10.4, respectively, in agreement with the presence of alpha(1A)-adrenoceptors. (8-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378; 1 microM) produced a small shift in the sensitivity of noradrenaline giving a pK(B) of 7.2. In the presence of 300 nM 5-methyl-urapidil, sensitivity to noradrenaline was not further shifted by 1 microM BMY 7378. Responses to noradrenaline were unaffected by the alpha(1B)-adrenoceptor alkylating agent chloroethylclonidine (1 microM). These results suggest alpha(1A)-adrenoceptors mediate contractile responses to noradrenaline in rat femoral resistance arteries.

Methysergide delays the decompensatory responses to severe hemorrhage by activating 5-HT(1A) receptors

Central administration of the serotonin receptor ligand methysergide delays the decompensatory response to hypotensive hemorrhage. This study was performed to determine the receptor subtype that mediates this effect. Lateral ventricular (LV) injection of methysergide (40 microg) delayed the hypotensive, bradycardic, and sympathoinhibitory responses to blood withdrawal (1.26 ml/min) in conscious rats. The response was quantified, in part, as the blood volume withdrawal that produced a 40-mmHg fall in blood pressure. The delayed hypotensive response produced by methysergide (8.2 +/- 0.2 vs. 5.6 +/- 0.2 ml, P < 0.01) was reversed by the 5-hydroxytryptamine (HT)(1A) antagonist WAY-100635 (30 microg iv: 6.7 +/- 0.4 ml, P < 0. 01; 100 microg iv: 5.6 +/- 0.1 ml, P < 0.01). LV injection of the 5-HT(1A) agonist (+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) also delayed the hypotensive (10 microg: 8.6 +/- 0.3, P < 0.01; 20 microg: 9.2 +/- 0.3 ml, P < 0.01), bradycardic, and sympathoinhibitory responses to hemorrhage. WAY-100635 (10 microg iv) completely reversed the effects of 8-OH-DPAT (20 microg: 5.4 +/- 0.3 ml). Neither selective blockade of 5-HT(2) receptors nor stimulation of 5-HT(1B/1D) receptors had any effect on hemorrhage responses. These data indicate that methysergide stimulates 5-HT(1A) receptors to delay the decompensatory responses to hemorrhage.

Inactivation of 5-HT(1A) receptors in hippocampal and cortical homogenates

5-HT(1A) receptor function can be assessed in rat hippocampal and cortical membrane preparations as agonist-stimulated [35S]GTPgammaS binding. Membranes were preincubated in vitro with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). R(+)-8-hydroxy-2-(di-n-propylamino)tetralin [R(+)-8-OH-DPAT]-stimulated [35S]GTPgammaS binding and [3H]8-OH-DPAT binding assays were used to assess 5-HT(1A) receptor function and density, respectively. EEDQ decreased both R(+)-8-OH-DPAT-stimulated [35S]GTPgammaS and [3H]8-OH-DPAT binding in hippocampal and cortical membranes. The E(max) but not the EC(50) of R(+)-8-OH-DPAT to stimulate [35S]GTPgammaS binding was decreased by EEDQ in both preparations. Additionally, the IC(50) for EEDQ to reduce R(+)-8-OH-DPAT-stimulated [35S]GTPgammaS and [3H]8-OH-DPAT binding was the same for both brain regions in both assays. In contrast to EEDQ alone, agonist-stimulated [35S]GTPgammaS binding was not reduced in hippocampal membranes preincubated with EEDQ and the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl- cyclohexanecarboxamide maleate (WAY 100,635), suggesting that EEDQ acts directly on the receptor. Due to parallel reductions in receptor density and maximal functional response, it is concluded that there is little or no reserve for 5-HT(1A) receptor coupling to G(alpha) in these preparations. In addition, the sensitivity of hippocampal and cortical 5-HT(1A) receptors to inactivation by EEDQ in vitro is the same.

Stimulus properties of the selective 5-HT reuptake inhibitor fluvoxamine in conditioned taste aversion procedures

Previous attempts to train pigeons and rats to discriminate between the antidepressant fluvoxamine and its vehicle as assessed in a drug discrimination paradigm have been without success. The present experiments were, therefore, designed to assess in a conditioned taste aversion procedure (CTA) whether or not fluvoxamine possesses stimulus properties. Rats were exposed to a conditioned taste aversion (CTA) procedure. In Experiment I, subjects were given 15 mg/kg fluvoxamine p.o. or vehicle after drinking a novel tasting saccharin solution. In Experiment II, a comparison was made between the effects of 15 mg/kg fluvoxamine i.p., 30 mg/kg fluvoxamine i.p., NaCl, and lithium chloride (LiCl). In Experiment III, subjects were treated with either 10 mg/kg fluoxetine i.p., 30 mg/kg fluvoxamine i.p., or LiCl. CTA was observed after treatment with LiCl, but never after treatment with fluvoxamine or fluoxetine, suggesting that fluvoxamine does not have clear stimulus properties, which can serve as a discriminative stimulus in operant procedures. In a crossfamiliarization CTA procedure in mice, however, fluvoxamine elicited a reliable CTA, suggesting that under certain conditions (species, dose?) selective serotonin reuptake inhibitors (SSRIs) may lead to certain discriminable effects. It is as yet unclear why SSRIs apparently produce such weak and species or situation-dependent discriminable effects.

5-CT stimulation of adenylyl cyclase activity in guinea-pig hippocampus: evidence for involvement of 5-HT7 and 5-HT1A receptors

1. A number of compounds, including the selective 5-HT7 receptor antagonist SB-258719, were investigated for their effect on [3H]-5-carboxamidotryptamine (5-CT) radioligand binding and 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membranes, in order to confirm the presence of functionally coupled 5-HT7 receptors in this tissue. 2. The [3H]-5-CT radioligand binding profile was consistent with binding predominantly to 5-HT7 receptors. The affinity of SB-258719 (pKi 7.2+/-0.1) was similar to its reported human 5-HT7 receptor affinity. 3. In the adenylyl cyclase functional assay, 5-CT was a potent and full agonist compared to 5-HT, whereas 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) was a partial agonist (intrinsic activity 0.4+/-0.1). The rank order of potency for agonists (5-CT>5-HT approximately 8-OH-DPAT) was consistent with activation of 5-HT7 receptors. SB-258719 (5 microM) and methiothepin (1 microM) surmountably antagonized the response to 5-CT, consistent with competitive antagonism. The pKB for SB-258719 (7.2+/-0.1) was in good agreement with its reported antagonist potency at the human cloned 5-HT7 receptor. 4. In the functional assay, WAY-100635 (100 nM) and cyanopindolol (1 microM) induced a biphasic 5-CT response curve, consistent with selective antagonism of a component of the response to 5-CT. The estimated pKB values for WAY-100635 and cyanopindolol (9.6 and 8.4 respectively) were in good agreement with their reported 5-HT1A receptor affinities. 5. The data are consistent with the presence of 5-HT7 receptors in guinea-pig hippocampus which are positively coupled to adenylyl cyclase. In addition, 5-HT7 receptor-mediated stimulation of adenylyl cyclase activity in this tissue appears to be augmented by a mechanism involving 5-HT1A receptor activation.

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.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

Blockade of the anxiolytic-like action of ipsapirone and buspirone, but not that of 8-OH-DPAT, by adrenalectomy in male rats

The effect of the 5-HT1A agonists ipsapirone (5 mg/kg), buspirone (5 mg/kg) and 8-OH-DPAT (0.5 mg/kg) on experimental anxiety was examined in sham-operated, adrenalectomized and adrenally demedullated male rats. The animal model of anxiety used was the defensive burying test. At the doses selected, all 5-HT1A compounds produced an anxiolytic-like action by reducing the burying behavior in both sham-operated and demedullated rats. However, in adrenalectomized subjects, while 8-OH-DPAT still reduced burying behavior, ipsapirone and buspirone lost their action. Data suggest that adrenocortical secretions play a role in the anxiolytic-like actions of buspirone and ipsapirone, but not in those of 8-OH-DPAT. Buspirone and ipsapirone also produced a reduction in burying behavior latency in sham-operated animals that was not observed in adrenalectomized or adrenally demedullated rats. These data suggest that adrenaline may be participating in the action of these compounds on the burying behavior latency. Present findings support possible direct relationships between the stimulation of 5-HT1A receptors and adrenal secretions.

Modulation of the vagal bradycardia evoked by stimulation of upper airway receptors by central 5-HT1 receptors in anaesthetized rabbits

1. The effects of central application of 5-HT1A and 5-HT1B/1D receptor ligands on the reflex bradycardia, apnoea, renal sympathoexcitation and pressor response evoked by stimulating upper airway receptors with smoke in atenolol-pretreated anaesthetized rabbits were studied. 2. Intracisternal administration of the 5-HT1A receptor antagonists WAY-100635 (100 microg kg(-1)) and (-)pindolol (100 microg kg(-1)) significantly reduced the smoke-induced bradycardia, attenuated the pressor response and in the case of (-)pindolol, sympathetic nerve activity. The same dose of WAY-100635 i.v. was without effect. 3. Buspirone (200 microg kg(-1), i.c.) potentiated the reflex bradycardia. This action was prevented if the animals were pretreated with WAY-100635 (100 Hg kg(-1), i.v.) 4. (+)8-OH-DPAT (25 microg kg(-1), i.c.) attenuated the evoked bradycardia, pressor response, apnoea and renal sympathoexcitation. The attenuation of the apnoea and renal sympathoexcitation, but not the bradycardia or pressor response was prevented in animals pretreated with WAY-100635 (100 microg kg(-1), i.v.). The attenuation of the reflex bradycardia and the reduction in the renal sympathoexcitation were reduced by pretreatment with the 5-HT1B/1D receptor antagonist GR127935 (100 microg kg(-1), i.v.). 5. In WAY-100635 (100 microg kg(-1), i.v.) pretreated animals, sumatriptan (a 5-HT1B/1D receptor agonist) reduced the reflex bradycardia and the pressor response. The 5-HT1B/1D receptor antagonist GR127935 (20 microg kg(-1), i.c. or 100 microg kg(-1), i.v.) had no effect on the reflex responses. 6. In conclusion, the present data are consistent with the hypothesis that activation of central 5-HT1A receptors potentiate whilst activation of 5-HT1B/1D receptors attenuate the reflex activation of cardiac preganglionic vagal motoneurones evoked by stimulation of upper airway receptors with smoke in rabbits.

Agonist and antagonist actions of antipsychotic agents at 5-HT1A receptors: a [35S]GTPgammaS binding study

Recombinant human (h) 5-HT1A receptor-mediated G-protein activation was characterised in membranes of transfected Chinese hamster ovary (CHO) cells by use of guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS binding). The potency and efficacy of 21 5-HT receptor agonists and antagonists was determined. The agonists, 5-CT (carboxamidotryptamine) and flesinoxan displayed high affinity (subnanomolar Ki values) and high efficacy (Emax > 90%, relative to 5-HT = 100%). In contrast, ipsapirone, zalospirone and buspirone displayed partial agonist activity. EC50s for agonist stimulation of [35S]GTPgammaS binding correlated well with Ki values from competition binding (r = +0.99). Among the compounds tested for antagonist activity, methiothepin and (+)butaclamol exhibited 'inverse agonist' behaviour, inhibiting basal [35S]GTPgammaS binding. The actions of 17 antipsychotic agents were investigated. Clozapine and several putatively 'atypical' antipsychotic agents, including ziprasidone, quetiapine and tiospirone, exhibited partial agonist activity and marked affinity at h5-HT1A receptors, similar to their affinity at hD2 dopamine receptors. In contrast, risperidone and sertindole displayed low affinity at h5-HT1A receptors and behaved as 'neutral' antagonists, inhibiting 5-HT-stimulated [35S]GTPgammaS binding. Likewise the 'typical' neuroleptics, haloperidol, pimozide, raclopride and chlorpromazine exhibited relatively low affinity and 'neutral' antagonist activity at h5-HT1A receptors with Ki values which correlated with their respective Kb values. The present data show that (i) [35S]GTPgammaS binding is an effective method to evaluate the efficacy and potency of agonists and antagonists at recombinant human 5-HT1A receptors. (ii) Like clozapine, several putatively 'atypical' antipsychotic drugs display balanced serotonin h5-HT1A/dopamine hD2 receptor affinity and partial agonist activity at h5-HT1A receptors. (iii) Several 'typical' and some putatively 'atypical' antipsychotic agents displayed antagonist properties at h5-HT1A sites with generally much lower affinity than at hD2 dopamine receptors. It is suggested that agonist activity at 5-HT1A receptors may be of utility for certain antipsychotic agents.

The role of central 5-HT receptors in the bronchoconstriction evoked by inhaled capsaicin in anaesthetised guinea-pigs

The effects of intracisternal (i.c) injections of the 5-HT1A receptor agonists, buspirone and 8-OH-DPAT, and the antagonists WAY-100635; and (-)-pindolol, the 5-HT1B/1D receptor agonist sumatriptan and antagonist GR127935, the 5-HT2 receptor agonist DOI and the antagonist cinanserin, the 5-HT3 receptor antagonist granisetron, the alpha-adrenoceptor agonist clonidine and the antagonist idazoxan, the D2 receptor antagonists (-)-sulpiride and the 5-HT uptake inhibitor fluoxetine on capsaicin-evoked increase in tracheal inflation pressure (bronchoconstriction) were investigated in alpha-chloralose anaesthetised, neuromuscularly blocked, artificially ventilated guinea-pigs. Buspirone, 8-OH-DPAT and fluoxetine significantly potentiated while WAY-100635 (-)-pindolol and sumatriptan attenuated the evoked bronchoconstriction when applied i.c. Granisetron attenuated the response when applied i.v. but not when given i.c. The 5-HT2, alpha2-adrenoceptor and D2 dopamine receptor ligands did not have any significant effect on the evoked bronchoconstriction. Pretreatment i.v. with WAY-100635 alone had no effect on the capsaicin-evoked bronchoconstriction but blocked the potentiating action of i.c. buspirone. The effects of sumatriptan could be completely blocked by pretreatment i.v. with GR127935. Only DOI, in the presence (i.v.) of the peripheral acting 5-HT2 receptor antagonist BW501C67, caused a significant increase in baseline tracheal inflation pressure. It is concluded that activation of central 5-HT1A and 5-HT1B/1D receptors have opposing roles, facilitation and inhibition respectively, on the reflex activation of bronchoconstrictor vagal preganglionic neurones.

Characterization of [3H]5-hydroxytryptamine and [3H]spiperone binding sites in clathrin-coated vesicles from bovine brain

Coated vesicles prepared from bovine brain cerebral cortex exhibited [3H]5-hydroxytryptamine (5-HT, serotonin) and [3H]spiperone binding activities. The binding activities were localized in the inner core vesicles. Binding reached an equilibrium level by 30-45 min at 30 degreesC, and was reversed by the addition of 100 microM 5-HT for [3H]5-HT binding or 10 microM ketanserin for [3H]spiperone binding. The saturation binding experiments indicated a single class of binding sites for [3H]5-HT and [3H]spiperone with apparent Kd values of 2.4 and 1.75 nM, respectively. The binding of [3H]5-HT was displaced by 5-HT and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), but not by ketanserin. The binding of [3H]spiperone was displaced by spiperone and ketanserin but not by 5-HT or 8-OH-DPAT even at 1 mM. The coated vesicles were shown by immunoblotting assay to contain alpha-subunits of GTP-binding proteins, Galphas, Galphai2, Galphai3, Galphao and Galphaq/11. Forskolin-stimulated adenylate cyclase activity in the coated vesicles was inhibited to 80% of the control level by 5-HT or 8-OH-DPAT. These results suggested that 5-HT1A and 5-HT2A receptors are present in bovine brain coated vesicles and that the 5-HT1A receptors are coupled to adenylate cyclase activity via GTP binding proteins.

Electrophysiological and neurochemical evidence that pindolol has agonist properties at the 5-HT1A autoreceptor in vivo

1. It has been hypothesized that 5-HT1A autoreceptor antagonists may enhance the therapeutic efficacy of SSRIs and other antidepressants. Although early clinical trials with the beta-adrenoceptor/5-HT1 ligand, pindolol, were promising, the results of recent more extensive trials have been contradictory. Here we investigated the actions of pindolol at the 5-HT1A autoreceptor by measuring its effect on 5-HT neuronal activity and release in the anaesthetized rat. 2. Pindolol inhibited the electrical activity of 5-HT neurones in the dorsal raphe nucleus (DRN). This effect was observed in the majority of neurones tested (10/16), was dose-related (0.2-1.0 mg kg(-1), i.v.), and was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.), in 6/7 cases tested. 3. Pindolol also inhibited 5-HT neuronal activity when applied microiontophoretically into the DRN in 9/10 neurones tested. This effect of pindolol was current-dependent and blocked by co-application of WAY 100635 (3/3 neurones tested). 4. In microdialysis experiments. pindolol caused a dose-related (0.8 and 4 mg kg(-1), i.v.) fall in 5-HT levels in dialysates from the frontal cortex (under conditions where the perfusion medium contained 1 microM citalopram). In rats pretreated with WAY 100635 (0.1 mg kg(-1), i.v.), pindolol (4 mg kg(-1), i.v.) did not decrease, but rather increased 5-HT levels. 5. We conclude that, under the experimental conditions used in this study, pindolol displays agonist effects at the 5-HT1A autoreceptor. These data are relevant to previous and ongoing clinical trials of pindolol in depression which are based on the rationale that the drug is an effective 5-HT1A autoreceptor antagonist.

Antagonism of muscarinic receptors in the rabbit iris-ciliary body by 8-OH-DPAT and other 5-HT1A receptor agonists

Physiological studies have shown that serotonin and 5-HT1A agonists can influence muscarinic function in the rabbit iris-ciliary body (ICB). The purpose of this study was to examine whether a direct interaction exists between muscarinic and 5-HT1A receptors in the ICB. At high concentrations, the 5-HT1A agonist 8-OH-DPAT attenuated the carbachol-induced stimulation of inositol phosphates (InsPs) production, but this was not blocked by the presence of 5-HT1A antagonists. In contrast, serotonin failed to influence carbachol-induced InsPs formation. Moreover, 8-OH-DPAT but not serotonin displayed affinity for [3H]QNB binding sites in the ICB. The combined data suggest that activation of 5-HT1A receptors in the ICB does not cause a modulation of muscarinic receptor-stimulated phosphoinositide turnover. The data instead suggest that, at high concentrations, 8-OH-DPAT acts as an antagonist at muscarinic receptors and in this way influences muscarinic receptor function. The mechanism of 5-HT-induced modulation of muscarinic function in the ICB therefore remains to be elucidated.

Involvement of serotonin in the modulation of cocaine-induced locomotor activity in the rat

The influence of serotonin (5-HT) antagonists and a selective serotonin reuptake inhibitor (SSRI) on cocaine-induced locomotor activity, rears, and head bobs was investigated in female Glaxo Wistar rats. The SSRI, fluoxetine (10 mg/kg), and the nonselective 5-HT agent, methysergide, at the dose range of 5 and 15 mg/kg enhanced the behaviors produced by cocaine (15 mg/kg) to a similar extent. Moreover, the potentiation of cocaine-induced locomotor activity, rears, and head bobs was even greater after the combined administration of methysergide ( 15 mg/kg) and fluoxetine (10 mg/kg). In order to investigate a possible involvement of 5-HT1A receptors in the observed potentiation by methysergide and fluoxetine, the potent and selective 5-HT1A antagonist, WAY 100635, was used. WAY 100635 (0.1 and 1.5 mg/kg) markedly reduced the behaviors induced by cocaine preceded by fluoxetine (10 mg/kg) and methysergide (5 and 15 mg/kg) pretreatment, respectively, suggesting an involvement of 5-HT1A receptors in the action of fluoxetine and methysergide on cocaine-induced behaviors. An attenuation of the fluoxetine-enhanced cocaine-induced behaviors was also observed after pretreatment with the 5-HT2A antagonist ketanserin (0.1 and 1.0 mg/kg). Coadministration of ketanserin (1.0 mg/kg) and WAY 100635 (1.5 mg/kg) resulted in the greatest blockade of the fluoxetine-enhanced cocaine-induced behaviors. The antagonists and the SSRI, fluoxetine, did not alter the behaviors in comparison to that of saline-treated animals. These results provide evidence for an involvement of 5-HT1A receptors in the enhancing effect of fluoxetine and methysergide on cocaine-induced locomotor activity, rears, and head bobs, and suggest a stimulatory action of methysergide at the 5-HT1A receptor. In addition, some of the actions may also be mediated by activation of the 5-HT2A receptor and/or inhibition of the 5-HT2C receptor.

Characterization of 5-HT1A receptor-mediated [35S]GTPgammaS binding in rat hippocampal membranes

Stimulation of [35S]GTPgammaS binding by serotonin (5-hydroxytryptamine, 5-HT) receptor ligands was characterized in rat hippocampal membranes. The optimized assay contained 30-50 microg protein, 300 microM GDP and 0.1 nM [35S]GTPgammaS, incubated at 37 degrees C for 20 min. At 10 microM, the 5-HT1A receptor agonist R(+)-8-hydroxy-2-(di-n-propylamino)tetralin [R(+)-8-OH-DPAT] stimulated GTPgammaS binding from 27.1 +/- 2.5 to 45.7 +/- 4.2 fmol/mg protein. Increasing the protein concentration did not affect the absolute difference between basal and maximal GTPgammaS binding nor the EC50, but decreased the percent stimulation. The non-selective agonists serotonin and 5-carboxamidotryptamine were 30-35% more efficacious, whereas the partial agonists buspirone and S(-)-8-hydroxy-2-(di-n-propylamino)tetralin stimulated GTPgammaS binding by 19 +/- 1 and 43 +/- 3%, respectively, compared to R(+)-8-OH-DPAT. Neither the 5-HT2 receptor agonist [(+/-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl] (DOI) nor the 5-HT1A receptor antagonists WAY 100,635 (n-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-n-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) and spiperone altered basal GTPgammaS binding. WAY 100,635 abolished the effect of R(+)-8-OH-DPAT, but only reduced the effect of serotonin by 88 +/- 3%. Finally, methiothepin antagonized R(+)-8-OH-DPAT-stimulated GTPgammaS binding and reduced basal GTPgammaS binding by itself. The reduction was not affected by WAY 100,635. We have characterized a method to assess functional activity at 5-HT1A receptors in rat hippocampal membranes by measuring agonist-induced [35S]GTPgammaS binding.

Intradermal 5-HT induces Fos expression in rat dorsal horn neurons not via 5-HT3 but via 5-HT2A receptors

We investigated the effects of peripherally administered 5-HT on the secondary neurons in the spinal cord of rats using Fos-like immunoreactivity (FLI) as a marker of neuronal activation. The intradermal administration of 5-HT (30, 60 microg) induced a large number of FLI neurons in the ipsilateral dorsal horn. In animals given 5-HT2A receptor agonists (DOI: 0.28 to 2.8 micromol/kg, alpha-methyl 5-HT: 0.28 to 2.8 micromol/kg) intradermally, immunoreactive neurons were evoked in the same manner as those given 5-HT. Other agonists, including 5-HT3 receptor agonists (m-CPG: 16 to 32 micromol/kg, 2-methyl 5-HT: 0.0028 to 2.8 micromol/kg), did not induce FLI neurons at any dose examined. Furthermore, 5-HT2A receptor antagonist (ketanserin: 1 mg/kg, i.p.) suppressed the expression of FLI in the dorsal horn caused by peripheral 5-HT, but 5-HT3 receptor antagonist (tropisetron: 1 mg/kg, i.p.) did not. These findings suggest that the 5-HT-induced nociceptive response is mediated by 5-HT2A receptors in the periphery.

The putative 5-HT1A receptor antagonist DU125530 blocks the discriminative stimulus of the 5-HT1A receptor agonist flesinoxan in pigeons

Twelve homing pigeons were trained to discriminate the 5-HT1A receptor agonist flesinoxan (0.25 mg/kg p.o.) from its vehicle in a fixed ratio (FR) 30 two-key operant drug discrimination procedure. Tests for generalization and antagonism showed that compounds with agonistic action at the 5-HT1A receptor, such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), buspirone and ipsapirone all substituted for the flesinoxan cue. Compounds with mixed agonistic action at the 5-HT(1A/1B) receptor fully (eltoprazine) or partially (RU24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl-1H-indole)) substituted for flesinoxan. TFMPP (1-(3-trifluoromethylphenyl)piperazine) and mCPP (1-(3-chlorophenyl)piperazine), both acting at the 5-HT(1B/2C) receptor, did not substitute for flesinoxan, neither did the selective 5-HT re-uptake inhibitor fluvoxamine. The results of the antagonism tests showed that the 5-HT1A receptor antagonists NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine), WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-he xane-carboxamide) and the newly developed DU125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide) fully (more than 80%) blocked the flesinoxan cue without having substantial effects when given alone. WAY100135 (N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide ), (+/-)-pindolol and (S)-UH-301 ((S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin) all partially antagonized the flesinoxan cue. However, both WAY100135 as well as (+/-)-pindolol also partially substituted for flesinoxan in generalization tests. NAN190, (S)-UH-301, WAY100635 and DU125530 were without any activity in the generalization test at the doses tested. The putative 5-HT1A receptor antagonist S15535 (4-benzodioxan-5-yl) 1-(indan-2-yl)piperazine) was identified as a full agonist in the present procedure. Taken together these results suggest that the flesinoxan cue in pigeons is mediated by the 5-HT1A receptor and that DU125530 acts as a full antagonist on the 5-HT1A receptor.

The effects of the 5-HT1A agonist flesinoxan, in three paradigms for assessing antidepressant potential in the rat

5-HT1A receptor agonists have been shown to be effective clinically in the treatment of depression and anxiety. Flesinoxan is an example which is highly selective for the 5-HT1A receptor subtype. The objective of this study was to appraise the antidepressant potential of flesinoxan (1 and 3 mg/kg s.c.) in three tests which are indicative of antidepressant activity. These are (1) the forced swim test, following sub-acute administration, (2) 'open field' activity in the olfactory bulbectomised (OB) rat, following chronic administration, and (3) 8-OH-DPAT-induced hypothemia following chronic treatment. Both doses of flesinoxan significantly reduced the immobility time in the sham and OB groups when compared to their respective controls. In the 'open field', there was a significant increase in the ambulation of the OB control group. The higher dose of flesinoxan significantly reduced this deficit. In addition both doses of flesinoxan significantly attenuated the 8-OH-DPAT-induced hypothermic response. These effects of flesinoxan are quantitatively similar to those seen following the chronic administration of antidepressants. These studies illustrate the potential antidepressant properties of flesinoxan, and hence further emphasise the role of the 5-HT1A receptor in the pathogenesis of depression.

Inhibition of cAMP accumulation via recombinant human serotonin 5-HT1A receptors: considerations on receptor effector coupling across systems

Inhibition of forskolin-stimulated cyclic AMP accumulation was measured in two stable HeLa cell lines HA6 and HA7 expressing different levels of recombinant human 5-HT1A receptors. These cells were studied previously to characterize another second messenger system activated by 5-HT1A receptors, i.e. calcium mobilization. The pharmacological characterization of the inhibition of cyclic AMP accumulation was made using agonists (5-HT, 8-OH-DPAT, buspirone, MDL 73005) and putative antagonists (SDZ 216-525, NAN-190, WAY-100135, pindolol, propranolol, WAY 100635). It is shown that 5-HT, 8-OH-DPAT, buspirone, MDL 73005 behaved as full (or nearly full) and potent agonists, whereas SDZ 216-525, NAN-190 and WAY-100135 displayed a limited (and similar) degree of intrinsic activity at human 5-HT1A receptors; on the other hand pindolol, propranolol and WAY 100635 behaved as "silent" antagonists. The effects were quantitatively and qualitatively very similar in both cells lines for all drugs tested, suggesting that the coupling between 5-HT1A receptors and inhibition of cyclic AMP accumulation in HeLa cells is very tight. There were, however, significant variations in both the level of agonism and the potency of a number of compounds when calcium mobilization and the inhibition of cyclic AMP accumulation were compared. Especially in HA7 cells which express lower receptor levels, a number of drugs failed to display agonism (e.g. buspirone or MDL 73005), whereas in HA6 cells they acted as partial agonists. Together, the data show that functional responses mediated by the same receptor can vary rather dramatically depending on receptor density and/or on the effector system involved. Interestingly, 5-HT1A receptor-mediated inhibition of adenylate cyclase activity measured in calf hippocampal membranes shows very similar degrees of potency and intrinsic activity for a number of compounds that have been tested on the inhibition of cyclic AMP accumulation in HeLa cells, suggesting that the very tight coupling observed in the recombinant system may apply to native 5-HT1A receptors.

Age-dependent differences in the rat's conditioned defensive burying behavior: effect of 5-HT1A compounds

The ontogeny of conditioned defensive-burying behavior was studied in rats from 2 to 21 weeks of age. At early ages this parameter shows low values that gradually increase until the 11th week, decreasing steadily thereafter. Reactivity, measured by the burying-behavior latency, appeared increased in 2-week-old rats. The effect of the serotonergic1A compounds: 8-OH-DPAT (0.25 and 0.5 mg/kg), ipsapirone, buspirone, and indorenate (at 2.5 and 5.0 mg/kg) was studied at 3, 7, 11, and 21 weeks of age. All compounds produced a dose-dependent decrease in burying behavior in rats of 7 and 11 weeks, while at the Week 21, only 8-OH-DPAT and indorenate reduced it. At 3 weeks of age, burying-behavior latency was increased by all compounds, but burying behavior was not altered. Motor coordination was affected by buspirone at all ages and by 8-OH-DPAT at the Week 21. Data are discussed on the bases of the development of defensive behaviors.

Involvement of central 5-HT1A receptors in the reflex activation of pulmonary vagal motoneurones by inhaled capsaicin in anaesthetized cats

1. The aim of the present experiments was to determine whether 5-HT1A receptors play a role in the control of the reflex activation of pulmonary vagal motoneurones. This was carried out by investigating the effects of intracisternal injections (i.c.) of the 5-HT1A receptor ligands, 8-OH-DPAT (50 micrograms kg-1), buspirone (200 micrograms kg-1), WAY-100635 (100 micrograms kg-1), methiothepin (200 micrograms kg-1) and (-)-pindolol (100 micrograms kg-1) and the 5-HT2 receptor antagonist, cinanserin (200 micrograms kg-1), on the reflex bronchoconstriction evoked by inhaled capsaicin aerosol in alpha-chloralose anaesthetized, neuromuscularly blocked and artificially ventilated cats. Recordings were made of heart rate, blood pressure and upper tracheal pressure. 2. Central application of all the 5-HT1A receptor antagonists (methiothepin, WAY-100635 and (-)-pindolol) attenuated the reflex bronchoconstriction in the upper trachea. However, the same dose of WAY-100635 given i.v. had no effect on this reflex bronchoconstriction. The 5-HT1A receptor agonist, 8-OH-DPAT (50 micrograms kg-1) given i.c., potentiated the capsaicin-evoked reflex bronchoconstriction, whereas buspirone (200 micrograms kg-1) i.c. had no effect. The 5-HT2 receptor antagonist, cinanserin (200 micrograms kg-1) also had no effect. 3. It is concluded that the reflex excitation of pulmonary vagal motoneurones by inhaled capsaicin in alpha-chloralose anaesthetized cats involves the activation of central 5-HT1A receptors.