5-HT 1B/D receptor antagonists

The role of hippocampal 5-HT1D and 5-HT1F receptors on learning and memory in rats

Serotonin is a neurotransmitter, which is involved in memory via its receptors. The 5-HT_1D and 5-HT_1F receptors mainly exist in the hippocampus, which plays an important role in memory processing. However, few studies have assessed the effect of these serotonin receptors on memory. We evaluated the effect of a 5-HT_1D receptor agonist, PNU142633, 5-HT_1D receptor antagonist, BRL15572 hydrochloride, and 5-HT1F receptor agonist, LY344864, on the recognition and avoidance memory in the hippocampus area. Fifty adult male Wistar rats weighing 200-250 g were divided into the control, sham-operated, PNU, BRL, and LY groups (n=10 per group). Bilateral guide cannulas were implanted into the dentate gyrus area of the hippocampus. The drugs were administered at the dose of 1 μg/μl before the novel object recognition (NOR) and passive avoidance learning (PAL) tests. The results showed that in the NOR test, the administration of PNU and LY had no significant effect on recognition index; however, the recognition index was increased by BRL. In the PAL test, the administration of PNU had no significant effect on recognition index, but the administration of BRL and LY increased the time spent in the dark compartment of the apparatus and decreased the step-through latency into the dark compartment apparatus. It can be concluded that the inhibition of the hippocampal 5-HT_1D receptor improved cognition memory but impaired avoidance memory. Activation of the hippocampal 5-HT_1F receptor had no effect on cognitive memory but impaired avoidance memory.

HTR1B genotype and psychopathy: Main effect and interaction with paternal maltreatment

Psychopathy is a condition characterized by atypical emotions and socially maladaptive behavioral patterns. Among incarcerated people, psychopathy has been associated with higher rates of crimes, recidivism, and resistance to treatment. Many studies have indicated significant heritability of psychopathic traits, but little is known about the specific contribution of genes and their interaction with adverse experiences in life. Considering the primary role that serotonin plays in cognition and emotion, we investigated TPH2-rs4570625, 5-HTTLPR, MAOA-uVNTR, HTR1B-rs13212041 and HTR2A-rs6314 as risk factors for psychopathy in the largest sample of institutionalized individuals studied so far, consisting of 793 US White male incarcerated adults, and in a replication sample of 168 US White male incarcerated adolescents. In a subgroup of the adult sample, the interaction between genetics and parenting style, assessed by the Measure of Parental Style (MOPS) questionnaire, was also evaluated. The HTR1B-rs13212041-T/T genotype, as compared to HTR1B-rs13212041-C allele, predicted higher psychopathy scores in both the adult and the adolescent samples. The interaction between HTR1B-rs13212041-T/T genotype and paternal MOPS scores, investigated in a subgroup of the adult sample, was an even stronger predictor of higher levels of psychopathy than either the genetics or the environment taken individually. Overall, these data, obtained in two independent samples, shed new light on neurobiological correlates of psychopathy with promising implications both at a clinical and forensic level.

Forensic Value of Genetic Variants Associated with Anti-Social Behavior

Insanity defense is sometimes invoked in criminal cases, and its demonstration is usually based on a multifactorial contribution of behavioural, clinical, and neurological elements. Neuroradiological evidence of structural alterations in cerebral areas that involve decision-making and moral reasoning is often accepted as a useful tool in these evaluations. On the other hand, the genetic predisposition to anti-social behavior is still controversial. In this paper, we describe two cases of violent crimes committed by young carriers of genetic variants associated with personality disorder; both the defendants claimed to be insane at the time of the crime. We discuss these cases and review the scientific literature regarding the relationship between legal incapacity/predisposition to criminal behavior and genetic mutations. In conclusion, despite some genetic variants being able to influence several cognitive processes (like moral judgement and impulse control), there is currently no evidence that carriers of these mutations are, per se, incapable of intentionally committing crimes.

Activation of 5-HT 1b/d receptor restores the cognitive function by reducing glutamate release, deposition of β-amyloid and TLR-4 pathway in the brain of scopolamine-induced dementia in rat

OBJECTIVES

This study evaluates the effect of 5-HT 1b/d agonist on cognitive function in scopolamine (SPN)-induced dementia in the rat.

METHODS

Dementia was induced by administration of SPN 2 mg/kg/day, intraperitoneally, for a duration of 21 days. The effect of zolmitriptan (ZMT) 30 mg/kg, intraperitoneally, was observed on cognitive function, and the parameters of oxidative stress like malondialdehyde (MDA) level, nitric oxide (NO), superoxide dismutase (SOD) and glutathione peroxidase (GPX) were estimated at the end. Histopathology study of brain tissue was performed for the determination of β-amyloid peptide, and qRT-PCR was used to determine the mRNA expression of Toll-like receptor 4 (TLR-4), IL-17 and β-amyloid.

KEY FINDINGS

Data of the study suggested that treatment with ZMT alone and in combination with DMP (dextromethorphan) significantly (P < 0.01) decreases the escape latency in conditioned avoidance response (CAR) and transfer latency in elevated plus maze (EPM) as compared with negative control group. Moreover, the result of Morris water maze (MWM) shows an increase in retention time and a decrease in escape latency in ZMT alone and in combination with DMP-treated group of SPN-induced dementia than in the negative control group. There was a significant decrease in MDA and NO and increase in SOD and GPX in the brain tissues of ZMT and ZMT + DMP-treated group than negative control group. Histopathology study also suggested that the concentration of Aβ peptide decreases in the brain tissues in ZMT and ZMT + DMP-treated group than the negative control group. Moreover, ZMT treatment ameliorates the altered mRNA expression of TLR-4 and IL-17 in the brain tissue of SPN-induced dementia rat.

CONCLUSIONS

In conclusion, ZMT restores the cognitive functions and impaired memory in SPN-induced dementia in the rat by decreasing oxidative stress and Aβ peptide in the brain tissue of rat.

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

Crystal structure of the human 5-HT1B serotonin receptor bound to an inverse agonist

5-hydroxytryptamine (5-HT, also known as serotonin) regulates many physiological processes through the 5-HT receptor family. Here we report the crystal structure of 5-HT1B subtype receptor (5-HT1BR) bound to the psychotropic serotonin receptor inverse agonist methiothepin (MT). Crystallization was facilitated by replacing ICL3 with a novel optimized variant of BRIL (OB1) that enhances the formation of intermolecular polar interactions, making OB1 a potential useful tool for structural studies of membrane proteins. Unlike the agonist ergotamine (ERG), MT occupies only the conserved orthosteric binding pocket, explaining the wide spectrum effect of MT on serotonin receptors. Compared with ERG, MT shifts toward TM6 and sterically pushes residues W3276.48, F3306.50 and F3316.51 from inside the orthosteric binding pocket, leading to an outward movement of the extracellular end and a corresponding inward shift of the intracellular end of TM6, a feature shared by other reported inactive G protein-coupled receptor (GPCR) structures. Together with the previous agonist-bound serotonin receptor structures, the inverse agonist-bound 5-HT1BR structure identifies a basis for the ligand-mediated switch of 5-HT1BR activity and provides a structural understanding of the inactivation mechanism of 5-HT1BR and some other class A GPCRs, characterized by ligand-induced outward movement of the extracellular end of TM6 that is coupled with inward movement of the cytoplasmic end of this helix.

The serotonergic system and cognitive function

Symptoms of cognitive dysfunction like memory loss, poor concentration, impaired learning and executive functions are characteristic features of both schizophrenia and Alzheimer’s disease (AD). The neurobiological mechanisms underlying cognition in healthy subjects and neuropsychiatric patients are not completely understood. Studies have focused on serotonin (5-hydroxytryptamine, 5-HT) as one of the possible cognitionrelated biomarkers. The aim of this review is to provide a summary of the current literature on the role of the serotonergic (5-HTergic) system in cognitive function, particularly in AD and schizophrenia.

The role of the 5-HTergic system in cognition is modulated by the activity and function of 5-HT receptors (5-HTR) classified into seven groups, which differ in structure, action, and localization. Many 5-HTR are located in the regions linked to various cognitive processes. Preclinical studies using animal models of learning and memory, as well as clinical in vivo (neuroimaging) and in vitro (post-mortem) studies in humans have shown that alterations in 5-HTR activity influence cognitive performance.

The current evidence implies that reduced 5-HT neurotransmission negatively influences cognitive functions and that normalization of 5-HT activity may have beneficial effects, suggesting that 5-HT and 5-HTR represent important pharmacological targets for cognition enhancement and restoration of impaired cognitive performance in neuropsychiatric disorders.

Functional Selectivity and Antidepressant Activity of Serotonin 1A Receptor Ligands

Serotonin (5-HT) is a monoamine neurotransmitter that plays an important role in physiological functions. 5-HT has been implicated in sleep, feeding, sexual behavior, temperature regulation, pain, and cognition as well as in pathological states including disorders connected to mood, anxiety, psychosis and pain. 5-HT1A receptors have for a long time been considered as an interesting target for the action of antidepressant drugs. It was postulated that postsynaptic 5-HT1A agonists could form a new class of antidepressant drugs, and mixed 5-HT1A receptor ligands/serotonin transporter (SERT) inhibitors seem to possess an interesting pharmacological profile. It should, however, be noted that 5-HT1A receptors can activate several different biochemical pathways and signal through both G protein-dependent and G protein-independent pathways. The variables that affect the multiplicity of 5-HT1A receptor signaling pathways would thus result from the summation of effects specific to the host cell milieu. Moreover, receptor trafficking appears different at pre- and postsynaptic sites. It should also be noted that the 5-HT1A receptor cooperates with other signal transduction systems (like the 5-HT1B or 5-HT2A/2B/2C receptors, the GABAergic and the glutaminergic systems), which also contribute to its antidepressant and/or anxiolytic activity. Thus identifying brain specific molecular targets for 5-HT1A receptor ligands may result in a better targeting, raising a hope for more effective medicines for various pathologies.

Exercise benefits brain function: the monoamine connection

The beneficial effects of exercise on brain function have been demonstrated in animal models and in a growing number of clinical studies on humans. There are multiple mechanisms that account for the brain-enhancing effects of exercise, including neuroinflammation, vascularization, antioxidation, energy adaptation, and regulations on neurotrophic factors and neurotransmitters. Dopamine (DA), noradrenaline (NE), and serotonin (5-HT) are the three major monoamine neurotransmitters that are known to be modulated by exercise. This review focuses on how these three neurotransmitters contribute to exercise affecting brain function and how it can work against neurological disorders.

Reduced ventral striatal/ventral pallidal serotonin1B receptor binding potential in major depressive disorder

RATIONALE

Although serotonin (5-HT) dysregulation is implicated in the pathophysiology of major depressive disorder (MDD), the role of specific receptor subtypes remains to be elucidated. Emerging preclinical research suggests an important role for the 5-HT(1B) receptor in behavioral regulation and depressive phenotypes. In particular, 5-HT(1B) heteroreceptors located within the striatum have been shown to play an essential role in antidepressant action.

OBJECTIVES

The objective of this study was to determine 5-HT(1B) receptor binding potential (BP (ND)) in the region of the ventral striatum/ventral pallidum (VS/VP) in individuals with MDD and healthy control participants.

METHODS

Ten participants with MDD (30.8 ± 9.5 years, five men/five women) in a current major depressive episode (MDE) and ten healthy control participants (30.7 ± 10.5 years, five men/five women) underwent positron emission tomography (PET) scanning with the selective 5-HT(1B) receptor radioligand [(11)C]P943.

RESULTS

Within the VS/VP region of interest, [(11)C]P943 BP (ND) was significantly reduced in the MDD group compared with the healthy control group (1.37 ± 0.13 and 1.68 ± 0.16, respectively; 18.7% between-group difference; p < 0.001).

CONCLUSIONS

Consistent with preclinical and postmortem data, our findings suggest abnormally reduced function of VS/VP 5-HT(1B) receptors in humans with MDD. Abnormal 5-HT(1B) heteroreceptor function may contribute to dysfunctional reward signaling within the striatum, including the nucleus accumbens, via interaction with dopamine, γ-amino-butyric acid, or glutamate systems. Our findings suggest reduced 5-HT(1B) receptor signaling in the VS/VP in MDD and contribute to the therapeutic rationale for testing 5-HT(1B) agonists as a novel class of antidepressants.

Evidence of fast serotonin transmission in frog slowly adapting type 1 responses

The Merkel cell-neurite (MCN) complex generates slowly adapting type 1 (SA1) response when mechanically stimulated. Both serotonin (5-HT) and glutamate have been implicated in the generation of normal SA1 responses, but previous studies have been inconclusive as to what their roles are or how synaptic transmission occurs. In this study, excised dorsal skin patches from common water frogs (Rana ridibunda) were stimulated by von Frey hairs during perfusion in a tissue bath, and single-unit spike activity was recorded from SA1 fibres. Serotonin had no significant effect on the SA1 response at low (10 µM) concentration, significantly increased activity in a force-independent manner at 100 µM, but decreased activity with reduced responsiveness to force at 1 mM. Glutamate showed no effect on the responsiveness to force at 100 µM. MDL 72222 (100 µM), an ionotropic 5-HT3 receptor antagonist, completely abolished the responsiveness to force, suggesting that serotonin is released from Merkel cells as a result of mechanical stimulation, and activated 5-HT3 receptors on the neurite. The metabotropic 5-HT2 receptor antagonist, ketanserin, greatly reduced the SA1 fibre's responsiveness to force, as did the non-specific glutamate receptor antagonist, kynurenic acid. This supports a role for serotonin and glutamate as neuromodulators in the MCN complex, possibly by activation and/or inhibition of signalling cascades in the Merkel cell associated with vesicle release. Additionally, it was observed that SA1 responses contained a force-independent component, similar to a dynamic response observed during mechanical vibrations.

Feeding behavior after metergoline or GR-46611 injections into the paraventricular nucleus of the hypothalamus in the pigeon

The present study examined changes in spontaneous behavior of free-feeding pigeons in response to local injections of metergoline (MET, an antagonist of 5-HT(1/2) receptors; 5, 10 and 20 nmol), GR-46611 (GR, a 5-HT(1B/1D) agonist; 0.6 and 6 nmol) or vehicle into the paraventricular hypothalamic nucleus (PVN). When infused into the PVN, MET and GR promptly and reliably elicited feeding at their higher doses, without affecting drinking or non-ingestive behaviors (locomotion, exploration, preening, sleep) during the first hour after injection. Both GR- and MET-evoked ingestive responses were associated only with an increase in feeding duration, with no changes in latency to start feeding. In a second series of experiments, the effective doses of MET (20 nmol) and GR (6 nmol) were injected into other diencephalic areas. This exploratory study revealed that intense feeding responses to both MET and GR local injections are also observed in the n. medialis hypothalami posterioris and in the adjacent n. lateralis hypothalami posterioris (PMH/PLH complex, in the caudoventral hypothalamus) and in the n. magnocellularis preopticus (PPM, in the caudal preoptic region). The behavioral profiles associated with these hyperphagic responses were nucleus-specific: in the PMH/PLH, MET-induced feeding was accompanied by an increase in total feeding duration and by a reduction in the latency to start feeding, while ingestive responses evoked by MET in the PPM were associated only with an increase in feeding duration (similar to that observed in the PVN experiments). No ingestive effects were observed after intracerebroventricular (ICV, lateral ventricle) injections of MET (10, 30, 100 or 300 nmol), while ICV injections of GR (3, 15 or 30 nmol) increased feeding only at the higher dose [Da Silva RA, De Oliveira ST, Hackl LPN, Spilere CI, Faria MS, Marino-Neto J, Paschoalini MA. Ingestive behaviors and metabolic fuels after central injections of 5-HT1A and 5-HT1D/1B receptors agonists in the pigeon. Brain Res, 2004;1026:275-283]. These data indicate the presence of a tonic inhibitory influence on feeding behavior exerted by 5-HT afferents on these hypothalamic areas, and suggest that these inputs, possibly mediated by non-rodent-type 5-HT1D/1B receptors, can affect both satiety and satiation mechanisms.

Models of PTSD and traumatic stress: the importance of research "from bedside to bench to bedside"

The epidemiology and psychology of PTSD noted above is not often considered in neurobiological models of PTSD. Neurobiological models tend to focus on symptoms. This is an important perspective but it does not capture the brains total response to traumatic events. Similarly, neurobiologists have rarely used the extensive knowledge of animal behavioral responses to stress as a means to define the human stress phenomenology, looking for the human equivalent (rather than the other way around). The development of animal models for PTSD and other traumatic stress-related brain changes is an important part of advancing our neurobiological understanding of the disease process as well as recovery, resilience, and possible therapeutic targets. Animal models should address symptoms but also other aspects of PTSD that are seen in clinical care including the waxing and waning of symptoms, Understanding "forgetting", toxic exposure, failure to recover and how the neural systems fail rather than function are important perspectives on developing animal models. The cognitive process of identification is another important animal model to develop. Using these perspectives recent work has shown new avenues for understanding the trauma response in animal models and human brain tissue of individuals with PTSD. The 5-HT2A receptor and p11 protein and associated regulators are avenues of new investigation that warrant study and consideration in models of PTSD.

The role of central 5-hydroxytryptamine (5-HT, serotonin) receptors in the control of micturition

At present the most investigated 5-HT receptor that has been shown to play a role in the control of micturition is the 5-HT(1A) receptor followed by 5-HT(7), 5-HT(2) and 5-HT(3) receptors. Most experiments focus on the control these receptors have on the parasympathetic outflow to the bladder and the somatic outflow to the external urethral sphincter (EUS) in the rat. Furthermore, 5-HT(1A) and 5-HT(7) receptors have been identified as having an excitatory physiological role in the control of bladder function. 5-HT(1A) receptors act, at least in the rat, at both a spinal (probably a heteroreceptor) and supraspinal (probably an autoreceptor) level, while 5-HT(7) receptors only act at a supraspinal level. Additionally, in the rat, 5-HT administered at a spinal or supraspinal site has an excitatory action, although earlier experiments have shown that activating 5-HT-containing brain areas causes inhibition of the bladder. Recent experiments have also indicated that blockade of the 5-HT(1A) receptor pathway shows rapid tolerance. However, no data exist for the development of tolerance for the 5-HT(7) receptor pathway. Neither receptor seems to play a role in the control of the urethra. Regarding 5-HT(2) receptors, activation of this receptor subtype inhibits micturition, and this inhibitory action may occur at a spinal, supraspinal or both levels. Although no physiological role for 5-HT(2C) receptors can yet be identified, 5-HT(2C) receptors have been implicated in the proposed supraspinal tonically active 5-HT(1A) autoreceptor (negative feedback) pathway. This proposition reconciles the data that central 5-HT-containing pathways are inhibitory to micturition, while 5-HT(1A) receptors, although inhibitory to adenylyl cyclase, have an excitatory function. This is because activation of 5-HT(1A) autoreceptors reduces the release of 5-HT thus reducing the activation of the 5-HT(2C) receptors, which are inhibitory in the control of micturition (disinhibition). Furthermore, 5-HT(2A) receptors in the rat and 5-HT(2C) receptors in the guinea pig cause activation of the EUS. In this respect, 5-ht(5A) receptors have also been identified in Onuf's nucleus, the site of somatic motoneurones controlling this sphincter. In the cat there is very little evidence to indicate that 5-HT receptors are involved in micturition except under pathological conditions in which activation of 5-HT(1A) receptors causes inhibition of micturition. Interestingly, under such conditions 5-HT(1A) receptors cause excitation of the EUS. Nevertheless, spinal 5HT(3) receptors have been implicated in the physiological control of micturition in the cat, but not yet in the rat. Overall, the data support the view that 5-HT receptors are important in the control of micturition. However, many more studies are required to fully understand these roles and why there are such species differences.

Neuronal 5-HT metabotropic receptors: fine-tuning of their structure, signaling, and roles in synaptic modulation

Serotonin (5-hydroxytryptamine, 5-HT) is, without doubt, the neurotransmitter for which the number of receptors is the highest. Fifteen genes encoding functional 5-HT receptors have been cloned in mammalian brain. 5-HT(3) receptors are ionotropic receptors, whereas all the others are metabotropic G-protein-coupled receptors (GPCRs). 5-HT receptor diversity is further increased by post-genomic modifications, such as alternative splicing (up to 10 splice variants for the 5-HT(4) receptor) or by mRNA editing in the case of 5-HT(2C) receptors. The cellular and behavioral implications of 5-HT(2C) receptor editing are of great physiological importance. Signaling of 5-HT receptors involves a great variety of pathways, but only some of these have been demonstrated in neurons. The classical view of neurotransmitter receptors localized within the synaptic cleft cannot be applied to 5-HT receptors, which are mostly (but not exclusively) localized at extra-synaptic locations either pre- or post-synaptically. 5-HT receptors are engaged in pre- or post-synaptic complexes composed of many GPCR-interacting proteins. The functions of these proteins are starting to be revealed. These proteins have been implicated in targeting, trafficking to or from the membrane, desensitization, and fine-tuning of signaling.

Autoradiographic characterisation of [35S]GTPgammaS binding stimulation mediated by 5-HT1B receptor in postmortem human brain

G-protein activation mediated by 5-HT1B receptors was studied in human brain by [35S]GTPgammaS autoradiographic methods. 5-HT (10 microM) increased [35S]GTPgammaS binding in caudate-putamen nucleus, globus pallidus, dentate gyrus, CA1, entorhinal cortex and substantia nigra. In basal ganglia and midbrain, this effect was blocked by GR 127935 (5-HT(1B/1D) antagonist). In contrast, WAY 100635 (selective 5-HT1A antagonist) reversed the effect of 5-HT in hippocampus and entorhinal cortex. Therefore, a detailed pharmacological study was carried out in basal ganglia and substantia nigra using 5-HT and the 5-HT(1B/1D) agonists GTI and CP 93129. In these areas, these agonists stimulated [35S]GTPgammaS binding in a concentration-dependent manner, with no significant differences in the potency for a given structure. Furthermore, GTI was more potent in the putamen than in globus pallidus. In caudate-putamen, the three agonists showed the same efficacy, while in globus pallidus and substantia nigra the efficacy of 5-HT was higher than GTI and CP 93129. The selective 5-HT1B antagonist SB-224289 inhibited GTI- and CP 93129-stimulated [35S]GTPgammaS binding in basal ganglia and substantia nigra, while coincubation with BRL 15572 (selective 5-HT1D antagonist) did not result in any significant change. Here we report the anatomical pattern of distribution of 5-HT1B-dependent functionality by using specific pharmacological tools in human brain sections.

Hippocampal 5-hydroxytryptamine receptors: abnormalities in postmortem brain from schizophrenic subjects

There is strong evidence that hippocampal 5-hydroxytryptamine (5-HT) systems are affected in schizophrenia and hence we have studied a number of markers of the 5-HT system in hippocampi from subjects with schizophrenia. Using in situ radioligand binding with autoradiography we measured [(3)H]proplyamino-8-hydroxy-1,2,3,4-tetrahydronapthalene, [(3)H]ketanserin and [(3)H]sumatriptan binding in hippocampi from 20 schizophrenic and 20 control subjects. There were significant decreases in the density of [(3)H]ketanserin binding to the 5-HT(2A) receptor (5-HT(2A)R) in the Cornu Ammonis (CA) 3 (p=0.006), CA 1 (stratum radiatum p=0.02; pyramidal layer p=0.0008) and subiculum (pyramidal layer p=0.0004), as well as methiothepin-insensitive [(3)H]sumatriptan binding to the 5-HT(1F)R in the CA 1 (p=0.016), stratum radiatum/lacunosum moleculare (p=0.04) and subiculum (p=0.015) from subjects with schizophrenia. There were no differences in the densities of 5-HT(1A)R, 5-HT(1D)R or 5-HT(4)R in hippocampi from subjects with schizophrenia. These data support the hypothesis that regionally specific reductions in the density of the 5-HT(2A)R and 5-HT(1F)R are a component of the pathological processes underlying schizophrenia.

Effect of combined administration of 5-HT1A or 5-HT1B/1D receptor antagonists and antidepressants in the forced swimming test

In the present study, we examined effects of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor citalopram, the 5-HT/noradrenaline reuptake inhibitor imipramine, the selective noradrenaline reuptake inhibitor desipramine or the monoamine oxidase-A inhibitor moclobemide, administered in combination with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635) or the 5-HT(1B/1D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)1,1'-biphenyl-4-carboxamide (GR 127935) and the 5-HT(1B) receptor antagonist N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB 216641) in the forced swimming test in rats. When given alone, citalopram (20 and 30 mg/kg), imipramine (20 mg/kg), desipramine (20 mg/kg), moclobemide (20 mg/kg), WAY 100635 (0.1 and 1 mg/kg), GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) did not shorten the immobility time of rats. Co-administration of WAY 100635 (0.1 and 1 mg/kg) and citalopram (20 mg/kg), or imipramine (20 mg/kg), or moclobemide (20 mg/kg) did not affect the immobility time of rats, whereas WAY 100635 given jointly with desipramine (20 mg/kg) induced a weak anti-immobility effect. GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) co-administered with imipramine, desipramine or moclobemide, but not citalopram, produced a significant anti-immobility action in the forced swimming test in rats. These results indicate that the blockade of 5-HT(1B) rather than 5-HT(1A) receptors may facilitate the anti-immobility effect of imipramine, desipramine or moclobemide in the forced swimming test. No interaction was observed between 5-HT(1A) or 5-HT(1B/1D) receptor antagonists and citalopram.

Pharmacology of a novel selective 5-hydroxytryptamine1B receptor antagonist, AR-A000002

The terminal 5-HT(1B) autoreceptors have attracted great pharmacological interest since they are potential targets for compounds modifying serotonergic neurotransmission. In the present work the in vivo biochemical properties of AR-A000002 ((R)-N-[5-methyl-8-(4-methylpiperazin-1-yl)-1,2,3,4-tetrahydro-2-naphthyl]-4-morpholinobenzamide), a novel selective 5-HT(1B) receptor antagonist, are reported. The effects of AR-A000002 on: 5-HT metabolism was measured as the ratio between 5-HIAA and 5-HT concentrations in different brain regions; 5-HT synthesis was measured as the accumulation of 5-HTP after inhibition of the aromatic amino acid decarboxylase activity with m-hydroxybenzylhydrazine (NSD1015); 5-HT release was measured using the microdialysis technique. 5-HT, 5-HIAA and 5-HTP concentrations were analyzed using high power liquid chromatography (HPLC) with electrochemical detection. AR-A000002 significantly enhanced 5-HT metabolism (5-HIAA/5-HT ratio) and 5-HT synthesis in guinea pig brain in the dose range 0.9-18 mg/kg s.c. (ED(50)=1 mg/kg s.c. in the four brain regions examined) with maximal effect seen after 2-4 h. AR-A000002 (9 mg/kg s.c.) significantly increased the extracellular concentrations of 5-HT and 5-HIAA by 20% in the guinea pig frontal cortex, measured with the in vivo microdialysis technique in freely moving guinea pigs. AR-A000002 (9 mg/kg s.c.) in combination with the 5-HT uptake inhibitor citalopram (5 mg/kg s.c.) increased the extracellular 5-HT concentration in guinea pig frontal cortex from 250 to 400% of the basal level. Citalopram alone decreased the extracellular 5-HIAA levels to 70% of the basal value. AR-A000002 counteracted the citalopram-induced decrease in 5-HIAA. Since the basal level of extracellular 5-HIAA was 160 times higher than that of 5-HT the 20% increase in 5-HIAA concentrations indicates that only a few percent of the exocytotically released 5-HT from the nerve terminals reached the extracellular space when the re-uptake mechanism was intact. The results also show that the terminal 5-HT(1B) autoreceptors are tonically activated under drug-free as well as citalopram conditions. The increase in plasma level of cortisol after AR-A000002 administration may indicate stimulation of post-synaptic 5-HT receptors. AR-A000002 also blocked 5-HT(1B) agonist-induced (CP-135,807) decrease in 5-HT metabolism and hypothermia (ED(50)=1 mg/kg s.c.), thus indicating competition between these two drugs. It is concluded that AR-A000002 is a 5-HT(1B) receptor antagonist that enhances the serotonergic neurotransmission in guinea pig brain.

Efficacy as a Vector: the Relative Prevalence and Paucity of Inverse Agonism

This article describes the expected phenotypic behavior of all types of ligands in constitutively active receptor systems and, in particular, the molecular mechanisms of inverse agonism. The possible physiological relevance of inverse agonism also is discussed. Competitive antagonists with the molecular property of negative efficacy demonstrate inverse agonism in constitutively active receptor systems. This is a phenotypic behavior that can only be observed in the appropriate assay; a lack of observed inverse agonism is evidence that the ligand does not possess negative efficacy only if it can be shown that constitutive receptor activity is present. In the absence of constitutive activity, inverse agonists behave as simple competitive antagonists. A survey of 105 articles on the activity of 380 antagonists on 73 biological G-protein-coupled receptor targets indicates that, in this sample dataset, 322 are inverse agonists and 58 (15%) are neutral antagonists. The predominance of inverse agonism agrees with theoretical predictions which indicate that neutral antagonists are the minority species in pharmacological space.

Studies on serotonergic markers in the human hippocampus: changes in subjects with bipolar disorder

BACKGROUND

Various studies suggest the hippocampus and serotonergic systems are important in the pathology of bipolar disorder (BD). We therefore measured hippocampal serotonergic markers in post-mortem tissue from BD and control subjects.

METHODS

The density and affinity of [3H]citalopram binding to the serotonin transporter (SERT), as well as the density of the 5HT(2A), 5HT(1A), 5HT(1D) and 5HT(1F) receptors were measured.

RESULTS

The density of SERT and 5HT receptors was no different in BD. There was a significant decrease in the affinity of [3H]citalopram binding to SERT in the stratum lacunosum-moleculare (S(lac)) in BD (K(d) mean+/-S.E.M.=4.3+/-0.8 vs. 1.9+/-0.3 nM).

LIMITATIONS

This study was completed using relatively small cohorts.

CONCLUSIONS

There are no generalised changes in hippocampal serotonergic markers in the hippocampus from subjects with BD. There is a decreased affinity of radioligand binding to S(lac) SERT in subjects with BD.

5-HT 1A/1B receptor-mediated effects of the selective serotonin reuptake inhibitor, citalopram, on sleep: studies in 5-HT 1A and 5-HT 1B knockout mice

Selective serotonin reuptake inhibitors (SSRIs) are extensively used for the treatment of depression. Aside from their antidepressant properties, they provoke a deficit in paradoxical sleep (PS) that is most probably mediated by the transporter blockade-induced increase in serotonin concentration in the extracellular space. Such an effect can be accounted for by the action of serotonin at various types of serotonergic receptors involved in PS regulation, among which the 5-HT(1A) and 5-HT(1B) types are the best candidates. According to this hypothesis, we examined the effects of citalopram, the most selective SSRI available to date, on sleep in the mouse after inactivation of 5-HT(1A) or 5-HT(1B) receptors, either by homologous recombination of their encoding genes, or pharmacological blockade with selective antagonists. For this purpose, sleep parameters of knockout mice that do not express these receptors and their wild-type counterparts were monitored during 8 h after injection of citalopram alone or in association with 5-HT(1A) or 5-HT(1B) receptor antagonists. Citalopram induced mainly a dose-dependent inhibition of PS during 2-6 h after injection, which was observed in wild-type and 5-HT(1B)-/- mice, but not in 5-HT(1A)-/- mutants. This PS inhibition was fully antagonized by pretreatment with the 5-HT(1A) antagonist WAY 100635, but only partially with the 5-HT(1B) antagonist GR 127935. These data indicate that the action of the SSRI citalopram on sleep in the mouse is essentially mediated by 5-HT(1A) receptors. Such a mechanism of action provides further support to the clinical strategy of antidepressant augmentation by 5-HT(1A) antagonists, because the latter would also counteract the direct sleep-inhibitory side-effects of SSRIs.

An evaluation of the effect of NAS-181, a new selective 5-HT(1B) receptor antagonist, on extracellular 5-HT levels in rat frontal cortex

In the mammalian brain 5-HT(1B) receptors are present as autoreceptors regulating the release of serotonin (5-HT) by inhibitory feedback. The antagonistic properties of NAS-181 ((R)-(+)-2-[[[3-(Morpholinomethyl)-2H-chromen-8-yl]oxy]methyl] morpholine methane sulfonate), a new selective antagonist for the rodent 5-HT(1B) receptor, were determined by using an agonist-induced decrease of extracellular 5-HT. The 5-HT(1B) receptor agonist CP93129 (0.030.3 microM) applied by reversed microdialysis, dose-dependently reduced 5-HT levels in rat frontal cortex. The suppressant effect of CP93129 (0.1 microM) was smaller in the presence of fluvoxamine (3-10 microM), a 5-HT reuptake inhibitor. The effects of NAS-181 on CP93129 were compared with GR127935, a mixed 5-HT (1B/1D) receptor antagonist, and SB224289, a 5-HT(1B) receptor antagonist. Both in the presence and absence of fluvoxamine, the suppressant effect of CP93129 on extracellular 5-HT was attenuated by NAS-181 (1 microM) and GR127935 (10 microM), but not by SB224289 (1 microM). In the absence of fluvoxamine, GR127935, SB224289 and NAS-181 all reduced 5-HT levels, suggesting partial agonistic properties of these compounds. In conclusion, the results show that NAS-181 is a potent 5-HT(1B) receptor antagonist.

Serotonin mechanisms in heart valve disease I: serotonin-induced up-regulation of transforming growth factor-beta1 via G-protein signal transduction in aortic valve interstitial cells

Clinical disorders associated with increased serotonin [5-hydroxytryptamine (5-HT)] levels, such as carcinoid syndrome, and the use of serotonin agonists, such as fenfluoramine have been associated with a valvulopathy characterized by hyperplastic valvular and endocardial lesions with increased extracellular matrix. Furthermore, 5-HT has been demonstrated to up-regulate transforming growth factor (TGF)-beta in mesangial cells via G-protein signal transduction. We investigated the hypothesis that increased exposure of heart valve interstitial cells to 5-HT may result in increased TGF-beta1 expression and activity because of serotonin receptor-mediated signal transduction with activation of Galphaq, and subsequently up-regulation of phospholipase C. Thus, in the present study we performed a clinical-pathological investigation of retrieved carcinoid and normal valve cusps using immunohistochemical techniques to detect the presence of TGF-beta1 and other proteins associated with TGF-beta expression, including TGF-beta receptors I and II, latent TGF-beta-associated peptide (LAP), and alpha-smooth muscle actin. Carcinoid valve cusps demonstrated the unusual finding of widespread smooth muscle actin involving the interstitial cells in the periphery of carcinoid nodules; these same cells were also positive for LAP. Normal valve cusps were only focally positive for smooth muscle actin and LAP. In sheep aortic valve interstitial cell cultures 5-HT induced TGF-beta1 mRNA production and increased TGF-beta1 activity. 5-HT also increased collagen biosynthesis at the dosages studied. Furthermore, TGF-beta1 added to SAVIC cultures increased the production of sulfated glycan and hyaluronic acid. In addition, overexpression of Galphaq using an adenoviral expression vector for a constitutively active Galphaq mutant (Q209L-Galphaq) resulted in increased phospholipase C activity as well as up-regulation of TGF-beta expression and activity. These results strongly support the view that G-protein-related signal transduction is involved in 5-HT up-regulation of TGF-beta1. In conclusion, 5-HT-associated valve disease may be, in part, because of TGF-beta1 mechanisms.

Molecular targets in the treatment of anxiety

Although the cathecholamine systems have long been the focus of drug therapy in anxiety and depression, the development of novel drugs specifically aimed at new targets within these traditional neurotransmitter systems and at targets outside of these systems is now propelling the field of drug development in anxiety. A greater understanding of regional brain networks implicated in stress, anxiety, and anxious behaviors has provided localized targets for anxiolytics. Within the serotonin and norepinephrine systems, increased understanding of postsynaptic receptor regulation with chronic treatment and cross-system effects of drug therapy have been critical in furthering our understanding of effective pharmacological interventions. Receptors within the glutamate, gamma-aminobutyric acid, and neuropeptide systems provide a rich diversity of drug targets, both in localization and function. While acknowledging significant clinical and biological differences between the various anxiety disorders, an important aspect of modern neurobiological research is to look for similarities among these disorders, given that they are highly comorbid with each other and often respond to the same spectrum of treatments. Here we review current views on both traditional and new molecular targets in the treatment of anxiety, realizing that the ultimate challenge in effective anxiolytic drug development may be achieving specificity in brain regions important in generating and sustaining anxiety.

Molecular, pharmacological and functional diversity of 5-HT receptors

Serotonin (5-hydroxytryptamine, 5-HT) is probably unique among the monoamines in that its effects are subserved by as many as 13 distinct heptahelical, G-protein-coupled receptors (GPCRs) and one (presumably a family of) ligand-gated ion channel(s). These receptors are divided into seven distinct classes (5-HT(1) to 5-HT(7)) largely on the basis of their structural and operational characteristics. Whilst this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity continues to emerge. The challenge for modern 5-HT research has therefore been to define more precisely the properties of the systems that make this incredible diversity possible. Much progress in this regard has been made during the last decade with the realisation that serotonin is possibly the least conservative monoamine transmitter and the cloning of its many receptors. Coupled with the actions of an extremely avid and efficient reuptake system, this array of receptor subtypes provides almost limitless signalling capabilities to the extent that one might even question the need for other transmitter systems. However, the complexity of the system appears endless, since posttranslational modifications, such as alternate splicing and RNA editing, increase the number of proteins, oligomerisation and heteromerisation increase the number of complexes, and multiple G-protein suggest receptor trafficking, allowing phenotypic switching and crosstalk within and possibly between receptor families. Whether all these possibilities are used in vivo under physiological or pathological conditions remains to be firmly established, but in essence, such variety will keep the 5-HT community busy for quite some time. Those who may have predicted that molecular biology would largely simplify the life of pharmacologists have missed the point for 5-HT research in particular and, most probably, for many other transmitters. This chapter is an attempt to summarise very briefly 5-HT receptor diversity. The reward for unravelling this complex array of serotonin receptor--effector systems may be substantial, the ultimate prize being the development of important new drugs in a range of disease areas.

The role of 5-HT1B and 5-HT1D receptors in the selective inhibitory effect of naratriptan on trigeminovascular neurons

The importance of 5-HT(1B) and 5-HT(1D) receptors in the actions of the anti-migraine drug naratriptan was investigated using the relatively selective 5-HT(1) receptor ligands SB224289 and BRL15572. Electrical stimulation of the superior sagittal sinus (SSS) in cats activated neurones in the trigeminal nucleus caudalis. Facial receptive fields (RF) were also electrically stimulated to activate the same neurones. Responses of these neurones to SSS stimulation were suppressed by iontophoretic application of naratriptan (5-50 nA). There were two distinct populations of neurones in the nucleus--those in deeper laminae in which the responses to SSS and RF stimulation were equally suppressed by naratriptan ('non-selective') and more superficial neurones in which only the SSS responses were suppressed by naratriptan ('selective'). Concurrent micro-iontophoretic application (50 nA) of the 5-HT(1D) antagonist BRL15572 antagonised the suppression by naratriptan of the response of 'selective' cells to SSS stimulation. Iontophoretic application of SB224289 (50 nA), a 5-HT(1B) antagonist, antagonised the suppression by naratriptan of responses of 'non-selective' cells to RF stimulation and, to a lesser extent, also antagonised the suppression of responses to SSS stimulation. Intravenous administration of SB224289 antagonised the suppression only of RF responses of "non-selective" neurons by naratriptan and intravenous administration of BRL15572 antagonised the suppression only of SSS responses of "selective" neurons by naratriptan. These results suggest that the response of nucleus caudalis neurons to stimulation of the sagittal sinus can be modulated by both 5-HT(1B) and 5-HT(1D) receptor activation, with the 5-HT(1D) receptors perhaps playing a greater role. The response to RF stimulation is more influenced by 5-HT(1B) receptor modulation with 5-HT(1D) receptors being less important. Therefore, this suggests that selective 5-HT(1D) agonists may be able to target the neuronal population, which is selectively involved in the transmission of dural inputs. We conclude that the central terminals of trigeminal primary afferent fibres contain 5-HT(1B) and 5-HT(1D) receptors. Primary afferents from the dura mater may predominantly express 5-HT(1D) receptors, while facial afferents may predominantly express 5-HT(1B) receptors. Activation of 5-HT(1D) receptors in particular may be important in the anti-migraine effect of naratriptan.

Contribution of the 5-HT(1B) receptor to hypoxia-induced pulmonary hypertension: converging evidence using 5-HT(1B)-receptor knockout mice and the 5-HT(1B/1D)-receptor antagonist GR127935

5-Hydroxytryptamine (5-HT)(1B) receptors mediate contraction in human pulmonary arteries, and 5-HT(1B) receptor-mediated contraction is enhanced in pulmonary arteries from hypoxic rats. Here we further examine the role of this receptor in the development of pulmonary hypertension (PHT) by examining (1) the effects of a 5-HT(1B/1D)-receptor antagonist (GR127935) on hypoxia-induced PHT (CHPHT) in rats and (2) CHPHT in 5-HT(1B)-receptor knockout mice. In rats, hypoxia increased right ventricular pressure and right ventricular hypertrophy and induced pulmonary vascular remodeling associated with an increase in pulmonary arterial wall thickness. GR127935 (3 mg. kg(-1). d(-1)) reduced all of these indices. 5-HT(1)-mediated contraction was enhanced in pulmonary arteries of the CHPHT rats. The effects of GR127935 on PHT indices were associated with an attenuation of the enhanced contractile responses to 5-HT and the 5-HT(1)-receptor agonist, 5-carboxamidotryptamine (5-CT), in isolated pulmonary arteries. In wild-type mice, hypoxia increased right ventricular hypertrophy, which was absent in 5-HT(1B)-receptor knockout mice. Hypoxia increased pulmonary vascular remodeling in wild-type mice, and this was reduced in the 5-HT(1B)-receptor knockout mice. Hypoxia increased 5-HT(1)-mediated contraction in pulmonary arteries from the wild-type mice and this was attenuated in the 5-HT(1B)-receptor knockout mice. In conclusion, the 5-HT(1B) receptor plays a role in the development of CHPHT. One possible mechanism may be via enhanced 5-HT(1) receptor-mediated contraction of the pulmonary arterial circulation.

Autoradiographic mapping of 5-HT(1B) and 5-HT(1D) receptors in the post mortem human brain using [(3)H]GR 125743

The distribution of 5-HT(1B) and 5-HT(1D) receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [(3)H]GR 125743. [(3)H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [(3)H]GR 125743 binding was observed, reflecting low densities of 5-HT(1B/1D) receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT(1D) receptors and the second using SB 224289 to inhibit 5-HT(1B) receptor binding. The autoradiograms indicated that in the human brain, the 5-HT(1B) receptor is much more abundant than the 5-HT(1D) receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [(3)H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT(1B) receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT(1D) receptor in the human brain.

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

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

Diverse signalling by 5-hydroxytryptamine (5-HT) receptors

Fourteen different receptor subtypes might be regarded as a diversity that is sufficient to accommodate the wide-ranging physiological roles of 5-hydroxytryptamine (5-HT). However, it is becoming clear that, for 5-HT as for other neurotransmitters, the concept of a receptor as a gatekeeper for a specific cellular process or event is too restrictive. Multiple receptor-mediated biochemical cascades can be activated in cells in response to an agonist by a number of mechanisms. Whereas it is well established that different agonists do not necessarily elicit the same magnitude of response, they probably also select between various possible signal transduction pathways. Receptor signalling may be diverse via a single receptor subtype as a consequence of specific agonist-receptor-G protein interactions. 5-HT receptors are even more heterogeneous when one considers that the amino acid sequence of these receptor subtypes may vary from individual to individual, and that there is an increasing number of receptor isoforms due to alternative splicing and RNA editing of 5-HT receptor transcripts. Activation, in particular constitutive, agonist-independent activation, of some of these receptor isoforms has been reported to be altered. This implies that ligands with similar binding affinities may display different pharmacological properties (partial agonist, antagonist, or inverse agonist) versus these receptor isoforms, depending on their activation state. Therefore, intervention with receptor ligands to modify hampered neurotransmission pathways is a difficult task, and one needs to consider the growing evidence of diversity in G protein-coupled receptor signalling.

Regulation of central synaptic transmission by 5-HT(1B) auto- and heteroreceptors

Although 5-HT(1B) receptors are believed to be expressed on nerve terminals, their precise mode of action is not fully understood because of the lack of selective antagonists. The 5-HT(1B) receptor knockout mouse was used in the present investigation to assess the function of 5-HT(1B) receptors in the modulation of synaptic transmission in three areas of the central nervous system: the dorsal raphe, the ventral midbrain, and the nucleus accumbens. N-(3-Trifluoromethylphenyl)piperazine, a 5-HT(1B) receptor agonist, potently inhibited 5-HT(1A) receptor-mediated slow inhibitory postsynaptic potentials (IPSPs) in the dorsal raphe of wild-type but not knockout mice. Both synaptically released 5-HT and exogenous 5-HT caused a presynaptic inhibition that outlasted the postsynaptic hyperpolarization only in wild-type mice. In the ventral midbrain, 5-HT(1B) receptor-dependent inhibition of gamma-aminobutyric acid(B) IPSPs in dopamine neurons was present in wild-type animals and absent in knockout animals. Similar results were obtained in the nucleus accumbens measuring glutamate-mediated excitatory postsynaptic currents in medium spiny neurons. Finally, cocaine, which blocks 5-HT uptake, inhibited IPSPs in the dorsal raphe and the ventral midbrain of wild-type but not knockout mice, whereas cocaine produced comparable inhibition of excitatory postsynaptic currents in the nucleus accumbens of both types of animals. These results indicate that 5-HT(1B) receptors function as autoreceptors and heteroreceptors to exert presynaptic inhibition of transmitter release in the central nervous system. Furthermore, this study underscores the role played by presynaptic 5-HT(1B) receptors in mediating the effects of cocaine on synaptic transmission.

Pharmacological analysis of contractile effects of eletriptan and sumatriptan on human isolated blood vessels

Eletriptan, a second-generation triptan with high affinity for 5-HT(1B/1D) receptors, is highly effective in migraine, with or without aura. We compared the effects of eletriptan and sumatriptan on the human isolated middle meningeal and coronary arteries and saphenous vein, used as models for therapeutic efficacy and potential side effects, and have investigated the role of 5-HT(1B/1D) receptors in contractions induced by these triptans. Concentration-response curves to eletriptan and sumatriptan were constructed in the absence or presence of a selective 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-py rid yl) benzamide (GR125743). All three blood vessels constricted in response to eletriptan and sumatriptan, but the middle meningeal artery relaxed following the highest concentration (100 microM) of eletriptan. In the middle meningeal artery, GR125743 antagonised the contractions induced by both eletriptan (pEC(50): 7.34+/-0.13) and sumatriptan (pEC(50): 6.91+/-0.17) to a similar degree (pA(2): 8. 81+/-0.17 and 8.64+/-0.21, respectively). In the human coronary artery and saphenous vein, sumatriptan-induced contractions (pEC(50): 6.24+/-0.14 and 6.19+/-0.12, respectively) were also potently antagonised by GR125743 (pA(2): 8.18+/-0.27 and 8.34+/-0.12, respectively). The eletriptan-induced contractions of the human saphenous vein (pEC(50): 6.09+/-0.13) were antagonised less effectively by GR125743 (pK(B): 7.73+/-0.18), and those of the human coronary artery (pEC(50): 5.54+/-0.22) remained unaffected by GR125743 up to a concentration of 100 nM. These results suggest that (i) based on the differences in pEC(50) values, the cranioselectivity of eletriptan (63-fold) is higher than that of sumatriptan (5-fold) in coronary artery, (ii) the contractile effects of sumatriptan and eletriptan (lower concentrations) in the three blood vessels are mediated via the 5-HT(1B) receptor, and (iii) additional mechanisms seem to be involved in coronary artery and saphenous vein contractions and middle meningeal artery relaxation following high concentrations of eletriptan.

Antagonist binding at 5-HT(2A) and 5-HT(2C) receptors in the rabbit: high correlation with the profile for the human receptors

This study examined the binding of serotonin receptor antagonists at the 5-HT(2A) and 5-HT(2C) receptors of the rabbit's cerebral cortex. The 5-HT(2A) receptor was characterized by the binding of [3H]MDL 100,907 (R(+)-alpha-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methan ol) to cortical membranes and the 5-HT(2C) receptor by the binding of [3H]mesulergine in the presence of the selective 5-HT(2A) receptor ligand spiperone. Both [3H]MDL 100,907 and [3H]mesulergine demonstrated high affinity binding to single sites in rabbit membranes. Based on Scatchard plots of [3H]MDL 100,907 binding, the mean B(max) was 8.5+/-0.7 fmol/mg tissue and the mean K(d) was 33. 1+/-3.5 pM. For [3H]mesulergine binding the mean B(max) was 3.70+/-0. 58 fmol/mg tissue and the mean K(d) was 0.35+/-0.05 nM. Binding of [3H]MDL 100,907 to the 5-HT(2A) receptor and of [3H]mesulergine to the 5-HT(2C) receptor was confirmed by displacement studies with highly selective 5-HT(2A) and 5-HT(2C) receptor ligands. The pharmacological profile of these ligands in rabbits correlated highly with published values for 5-HT(2A) (r=0.91, P<0.001) and 5-HT(2C) (r=0.94, P<0.001) receptors in humans. There was also a high correlation between the profiles for human and rat 5-HT(2C) receptor (r=0.92, P<0.001), but not for 5-HT(2A) receptors (r=0.53, P>0.10). It was concluded that the rabbit provides an appropriate animal model for studies attempting to predict the pharmacology of human 5-HT(2A) and 5-HT(2C) receptors.

Altered expression and functions of serotonin 5-HT1A and 5-HT1B receptors in knock-out mice lacking the 5-HT transporter

By taking up serotonin (5-hydroxytryptamine, 5-HT) released in the extracellular space, the 5-HT transporter (5-HTT) regulates central 5-HT neurotransmission. Possible adaptive changes in 5-HT neurotransmission in knock-out mice that do not express the 5-HT transporter were investigated with special focus on 5-HT1A and 5-HT1B receptors. Specific labelling with radioligands and antibodies, and competitive RT-PCR, showed that 5-HT1A receptor protein and mRNA levels were significantly decreased in the dorsal raphe nucleus (DRN), increased in the hippocampus and unchanged in other forebrain areas of 5-HTT-/- vs. 5-HTT+/+ mice. Such regional differences also concerned 5-HT1B receptors because a decrease in their density was found in the substantia nigra (-30%) but not the globus pallidus of mutant mice. Intermediate changes were noted in 5-HTT+/- mice compared with 5-HTT+/+ and 5-HTT-/- animals. Quantification of [35S]GTP-gamma-S binding evoked by potent 5-HT1 receptor agonists confirmed such changes as a decrease in this parameter was noted in the DRN (-66%) and the substantia nigra (-30%) but not other brain areas in 5-HTT-/- vs. 5-HTT+/+ mice. As expected from actions mediated by functional 5-HT1A and 5-HT1B autoreceptors, a decrease in brain 5-HT turnover rate after i.p. administration of ipsapirone (a 5-HT1A agonist), and an increased 5-HT outflow in the substantia nigra upon local application of GR 127935 (a 5-HT1B/1D antagonist) were observed in 5-HTT+/+ mice. Such effects were not detected in 5-HTT-/- mice, further confirming the occurrence of marked alterations of 5-HT1A and 5-HT1B autoreceptors in these animals.

Serotonin autoreceptor function and antidepressant drug action

This article briefly summarizes, within the context of a brief review of the relevant literature, the outcome of our recent rat microdialysis studies on (1) the relative importance of serotonin (5-HT)1A versus 5-HT1B autoreceptors in the mechanism of action of 5-HT reuptake blocking agents, including putative regional differences in this regard, and (2) autoreceptor responsiveness following chronic SSRI administration. First, our data are consistent with the primacy of 5-HT1A autoreceptors in restraining the elevation of 5-HT levels induced by SSRIs, whereas nerve terminal 5-HT1B autoreceptors appear to have an accessory role in this regard. Second, there is an important interplay between cell body and nerve terminal 5-HT autoreceptors, and recent findings suggest that this interplay may potentially be exploited to obtain regionally preferential effects on 5-HT neurotransmission in the central nervous system, even upon systemic drug administration. In particular, emerging data suggest that somatodendritic 5-HT1A autoreceptor- and nerve terminal 5-HT1B autoreceptor-mediated feedback may be relatively more important in the control of 5-HT output in dorsal raphe-frontal cortex and median raphe-dorsal hippocampus systems, respectively. Third, 5-HT autoreceptors evidently retain the capability to limit the 5-HT transmission-promoting effect of SSRIs after chronic treatment. Thus, although the responsiveness of these sites is probably somewhat reduced, residual autoreceptor capacity still remains an effective restraint on large increases in extracellular 5-HT, even after prolonged treatment. If a further increase in extracellular 5-HT is crucial to the remission of depression in patients responding only partially to prolonged administration of antidepressants, then sustained adjunctive treatment with autoreceptor-blocking drugs may consequently prove useful in the long term.