LY393558, a 5-hydroxytryptamine reuptake inhibitor and 5-HT(1B/1D) receptor antagonist: effects on extracellular levels of 5-hydroxytryptamine in the guinea pig and rat

Reduction of the serotonin 5-HT1B and 5-HT2A receptor-mediated contraction of human pulmonary artery by the combined 5-HT1B receptor antagonist and serotonin transporter inhibitor LY393558

Background

LY393558 is a combined antagonist of serotonin (5-HT) 5-HT_1B receptors and inhibitor of serotonin transporter (SERT). LY393558 reduces 5-HT-induced vasoconstriction and remodelling of rat and/or mouse pulmonary arteries. The aim of our study was to examine the effect of LY393558 on the 5-HT-stimulated vasoconstriction of human pulmonary arteries (hPAs) and to determine the underlying mechanism(s).

Methods

Vascular effects of 5-HT receptor agonists, antagonists and a SERT inhibitor were examined in organ bath studies on intralobar hPAs obtained from patients during resection of lung carcinoma.

Results

Serotonin and agonists of the 5-HT_1B receptor (5-carboxamidotryptamine, 5-CT) and 5-HT_2A receptor (α-methyl-5-HT) contracted endothelium-intact hPAs in a concentration-dependent fashion. The 5-HT_1B antagonists SB224289 and GR55562 reduced responses induced by 5-HT and 5-CT and the 5-HT_2A antagonist ketanserin inhibited the effects of 5-HT and α-methyl-5-HT. Administration of the SERT inhibitor citalopram (at a concentration that failed to modify the 5-HT-induced vasoconstriction) in combination with SB224289 or GR55562 was more effective in inhibiting the response to 5-HT than the 5-HT_1B antagonists alone. LY393558 showed the greatest antagonistic effect against the vasoconstriction elicited by 5-HT, 5-CT and α-methyl-5-HT.

Conclusions

LY393558 reduces the 5-HT-induced contraction antagonizing 5-HT_1B and 5-HT_2A receptors probably due to synergic interaction between SERT inhibition and 5-HT_1B receptor antagonism. Thus, it might represent a valuable future option in the pulmonary arterial hypertension therapy.

Adaptive dynamics of the 5-HT systems following chronic administration of selective serotonin reuptake inhibitors: a meta-analysis

Selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed antidepressants. However, a major concern is their delayed onset of action, which is hypothesized to be associated with the time required for serotonin (5-HT) autoreceptors to desensitize, which should be reflected by actual neurochemical changes. Numerous in vivo microdialysis studies have been published that report on 5-HT levels in different brain sites following SSRI administration. Here, we performed a meta-analysis on dynamic changes of 5-HT neurotransmission during the course of chronic SSRI treatment. We conducted a meta-analysis on research articles of 5-HT neurotransmission measured by in vivo microdialysis in rat brain after subchronic and chronic SSRI administrations. In total, data from 42 microdialysis studies (798 rats) were analyzed. Within the first week of SSRI treatment, extracellular 5-HT concentrations drop in frontal cortex. Over the next 2 weeks of treatment, a linear increase in extracellular 5-HT levels up to 350% of prior treatment baseline is evident (n = 269). However, in hippocampus, prefrontal cortex, nucleus accumbens, and ventral tegmental area we found increased 5-HT levels within the first 3 days of SSRI administration. The time course of 5-HT dynamics in frontal cortex is in line with the hypothesis that 5-HT autoreceptors desensitize over 2-3 weeks of SSRI treatment and thereby enhanced extracellular 5-HT levels ensue. Yet, in other regions we did not find evidence supporting the traditional autoreceptor-mediated feedback loops hypothesis and thus other neurobiological adaptation mechanisms may also play a role in the delayed onset of SSRI action.

Assessing the neuronal serotonergic target-based antidepressant stratagem: impact of in vivo interaction studies and knockout models

Depression remains a challenge in the field of affective neuroscience, despite a steady research progress. Six out of nine basic antidepressant mechanisms rely on serotonin neurotransmitter system. Preclinical studies have demonstrated the significance of serotonin receptors (5-HT_1-3,6,7), its signal transduction pathways and classical down stream targets (including neurotrophins, neurokinins, other peptides and their receptors) in antidepressant drug action. Serotonergic control of depression embraces the recent molecular requirements such as influence on proliferation, neurogenesis, plasticity, synaptic (re)modeling and transmission in the central nervous system. The present progress report analyses the credibility of each protein as therapeutically relevant target of depression. In vivo interaction studies and knockout models which identified these targets are foreseen to unearth new ligands and help them transform to drug candidates. The importance of the antidepressant assay selection at the preclinical level using salient animal models/assay systems is discussed. Such test batteries would definitely provide antidepressants with faster onset, efficacy in resistant (and co-morbid) types and with least adverse effects. Apart from the selective ligands, only those molecules which bring an overall harmony, by virtue of their affinities to various receptor subtypes, could qualify as effective antidepressants. Synchronised modulation of various serotonergic sub-pathways is the basis for a unique and balanced antidepressant profile, as that of fluoxetine (most exploited antidepressant) and such a profile may be considered as a template for the upcoming antidepressants. In conclusion, 5-HT based multi-targeted antidepressant drug discovery supported by in vivo interaction studies and knockout models is advocated as a strategy to provide classic molecules for clinical trials.

Naphthyl piperazines with dual activity as 5-HT1D antagonists and 5-HT reuptake inhibitors

SAR around a known molecule with dual 5-HT(1D) antagonist and 5-HT(transporter) inhibitory activity has led to the discovery of molecules with improved dual activity and reduced cross-reactivity toward other aminergic receptors (5-HT(1B), alpha(1), and D(2)).

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.

Morphological and functional determinants of fluoxetine (Prozac)-induced pulmonary disease in an experimental model

Fluoxetine treatment effects were determined by evaluating respiratory mechanics (elastance/resistance) and exhaled nitric oxide, as well as mononuclear and polymorphonuclear cell recruitment into the lungs, in an experimental guinea pig model. Guinea pigs were divided into four groups: Fl (fluoxetine only, n=7); Fl+Sw (fluoxetine and forced swimming, n=7); Ns+Sw (normal saline and forced swimming, n=8); and Ns (normal saline only, n=8). Treated animals received oral fluoxetine (10 mg/(kg day)) for 30 consecutive days. On day 31, all animals were anesthetized and mechanically ventilated so that respiratory system elastance and resistance, as well exhaled nitric oxide, could be determined. The lungs were then excised en bloc for histological and immunohistochemical evaluation. Forced swimming induced bronchodilation in untreated animals and bronchoconstriction in fluoxetine-treated animals. Fluoxetine treatment was also associated with mononuclear infiltration (predominantly into alveolar walls) and neutrophil recruitment. In addition, levels of exhaled nitric oxide, an inflammatory marker, were higher in fluoxetine-treated animals. Swimming-induced stress also amplified mononuclear cell recruitment to the lungs. These results show that, in this experimental model, fluoxetine treatment reproduces the pathology of chronic interstitial pneumonia in humans.

Synthesis of novel quinolone and quinoline-2-carboxylic acid (4-morpholin-4-yl-phenyl)amides: A late-stage diversification approach to potent 5HT1B antagonists

Multiparallel amenable syntheses of 6-methoxy-8-amino-4-oxo-1,4-dihydroquinoline-2-carboxylic acid-(4-morpholin-4-yl-phenyl)amides (I) and 4-amino-6-methoxy-8-(4-methyl-piperazin-1-yl)-quinoline-2-carboxylic acid (4-morpholin-4-yl-phenyl)amides (II) which facilitate late-stage diversification at the 8-position of (I) and at the 4- and 8-positions of (II) are described. The resulting novel series were determined to contain potent 5HT(1B) antagonists. Preliminary SAR data are presented.

Differentiating antidepressants of the future: Efficacy and safety

There have been significant advances in the treatment of depression since the serendipitous discovery that modulating monoaminergic neurotransmission may be a pathological underpinning of the disease. Despite these advances, particularly over the last 15years with the introduction of selective serotonin and/or norepinephrine reuptake inhibitors (SNRI), there still remain multiple unmet clinical needs that would represent substantial improvements to current treatment regimens. In terms of efficacy there have been improvements in the percentage of patients achieving remission but this can still be dramatically improved and, in fact, issues still remain with relapse. Furthermore, advances are still required in terms of improving the onset of efficacy as well as addressing the large proportion of patients who remain treatment resistant. While this is not well understood, collective research in the area suggests the disease is heterogeneous in terms of the multiple parameters related to etiology, pathology and response to pharmacological agents. In addition to efficacy further therapeutic advances will also need to address such issues as cognitive impairment, pain, sexual dysfunction, nausea and emesis, weight gain and potential cardiovascular effects. With these unmet needs in mind, the next generation of antidepressants will need to differentiate themselves from the current array of therapeutics for depression. There are multiple strategies for addressing unmet needs that are currently being investigated. These range from combination monoaminergic approaches to subtype selective agents to novel targets that include mechanisms to modulate neuropeptides and excitatory amino acids (EAA). This review will discuss the many facets of differentiation and potential strategies for the development of novel antidepressants.

Characterisation of the selective 5-HT1B receptor antagonist SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2′-methyl-4′-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride): In vivo neurochemical and behavioural evidence of anxiolytic/antidepressant activity

The 5-HT1B receptor has attracted significant interest as a potential target for the development of therapeutics for the treatment of affective disorders such as anxiety and depression. Here we present the in vivo characterisation of a novel, selective and orally bioavailable 5-HT1B receptor antagonist, SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride). SB-616234-A reversed the 5-HT1/7 receptor agonist, SKF-99101H-induced hypothermia in guinea pigs in a dose related manner with an ED50 of 2.4 mg/kg p.o. Using in vivo microdialysis in freely moving guinea pigs, SB-616234-A (3-30 mg/kg p.o.) caused a dose-related increase in extracellular 5-HT in the dentate gyrus. Evaluation of antidepressant- and anxiolytic-like effects of this 5-HT1B receptor antagonist was performed in a variety of models and species. SB-616234-A produced a decrease in immobility time in the mouse forced swim test; an effect suggestive of antidepressant activity. Furthermore, SB-616234-A produced dose-related anxiolytic effects in both rat and guinea pig maternal separation-induced vocalisation models with an ED50 of 1.0 and 3.3 mg/kg i.p., respectively (vs fluoxetine treatment ED50 = 2.2 mg/kg i.p. in both species). Also a significant reduction in posturing behaviours was observed in the human threat test in marmosets; an effect indicative of anxiolytic activity. In summary, SB-616234-A is a novel, potent and orally bioavailable 5-HT1B receptor antagonist which exhibits a neurochemical and behavioural profile that is consistent with both anxiolytic- and antidepressant-like activity in a variety of species. Taken together these data suggest that SB-616234-A may have therapeutic efficacy in the treatment of affective disorders.

Mechanisms of action of current and potential pharmacotherapies of obsessive-compulsive disorder

A significant body of evidence documented that the orbitofrontal cortex (OFC) and the head of caudate nucleus are involved in the mediation of obsessive-compulsive disorder (OCD) symptoms. Potent serotonin (5-HT) reuptake inhibitors (SRIs) are the only antidepressant agents thus far shown to be effective in the treatment of OCD. The present review summarizes information on 5-HT release and the adaptive changes in pre- and postsynaptic 5-HT receptors sensitivity induced by SRI treatment in rat and guinea pig structures involved in OCD. It emphasizes that the time course for the occurrence of increased 5-HT release and terminal 5-HT1D desensitization is congruent with the delayed therapeutic response to SRI in OCD. In addition, a greater dose of SRI inducing a greater degree of reuptake inhibition may play an essential role in this phenomenon. This is consistent with the common clinical observation that high doses of SRIs are sometimes necessary to obtain an anti-OCD effect, and with the results of some fixed-dose double blind trials showing a dose-dependent therapeutic effect of SRIs. It is hypothesized that enhanced 5-HT release in the OFC is mediated by the activation of normosensitive postsynaptic 5-HT2-like receptors and underlies the therapeutic action of SRI in OCD. This is supported by the beneficial effect of some hallucinogens with 5-HT2 agonistic properties in obtaining a more rapid therapeutic response. Finally, based on this knowledge, new strategies aimed at producing more rapid, effective and safe anti-OCD drugs, such as a selective action on terminal 5-HT1D receptors, on 5-HT2 receptors as well as on the glutamate system, are discussed.

Investigation of the SSRI augmentation properties of 5-HT2 receptor antagonists using in vivo microdialysis

Recent evidence that 5-HT(2) receptors exert a negative influence on central 5-hydroxytryptamine (5-HT) neurones suggests that 5-HT(2) receptor antagonists may augment the effects of serotonin selective reuptake inhibitors (SSRIs). The present study investigated whether pre-treatment with 5-HT(2) receptor antagonists enhances the effect of SSRI administration on hippocampal extracellular 5-HT of freely moving rats. Administration of the SSRI citalopram at a low (2mg kg(-1)) and higher (4 mg kg(-1)) dose, increased dialysate 5-HT by 5- and 8-fold, respectively. Pre-treatment with the 5-HT(2) receptor antagonist ketanserin (4 mg kg(-1)) augmented the effect of 4 mg kg(-1) but not 2mg kg(-1) citalopram. The effect of 4 mg kg(-1) citalopram was also augmented by pre-treatment with either the 5-HT(2C) receptor antagonist SB 242084 (0.5mg kg(-1)) or the 5-HT(2A) receptor antagonist MDL 100907 (0.5mg kg(-1)). As with citalopram, fluoxetine elevated dialysate 5-HT at both a low (5mg kg(-1)) and higher (20mg kg(-1)) dose. However, neither dose of fluoxetine was augmented by ketanserin (4 mg kg(-1)). These results confirm recent findings that 5-HT(2) receptor antagonists augment the effect of citalopram on extracellular 5-HT, and indicate the involvement of 5-HT(2C) and possibly 5-HT(2A) receptors. The lack of augmentation of fluoxetine might reflect the intrinsic 5-HT(2) receptor antagonist properties of this drug.

Mapping the central effects of ketamine in the rat using pharmacological MRI

RATIONALE

Ketamine induces, in both humans and rodents, behaviours analogous to some of the symptoms of schizophrenia.

OBJECTIVES

To utilise pharmacological magnetic resonance imaging (phMRI) techniques that identify changes in blood-oxygenation-level-dependent (BOLD) contrast to determine the temporal and spatial neuronal activation profile of ketamine in the rat brain.

METHOD

To obtain a pharmacodynamic profile of the drug, we assessed changes in locomotor activity after vehicle and 10 and 25 mg/kg ketamine. Separate animals were then anaesthetised and placed in a 4.7-T magnetic resonance (MR) system before receiving the same doses of ketamine during serial MR image acquisition. Subsequent statistical parametric mapping of the main effect of the drug was then undertaken to identify changes in BOLD contrast. Levels of gamma-aminobutyric acid (GABA) and dopamine (DA) in brain areas showing localised changes in BOLD contrast were then assessed via microdialysis.

RESULTS

Both doses of ketamine produced increases in BOLD image contrast in frontal, hippocampal, cortical and limbic areas. A further investigation of the release of DA and its metabolites in the nucleus accumbens, both in anaesthesised and freely moving rats, corroborated these findings. However, an investigation of GABA and DA levels in the ventral pallidum gave no indication of changes in activity.

CONCLUSIONS

Ketamine produced localised dose-dependent alterations in BOLD MR signal, which correlate with the pharmacodynamic profile of the drug. These results can be, at least, partially substantiated with complementary techniques but consideration must be given to the input function applied to the MR signal and the use of anaesthesia during phMRI experimentation.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

The role of monoamines in the actions of established and "novel" antidepressant agents: a critical review

Monoaminergic pathways are highly responsive to aversive stimuli and play a crucial role in the control of affect, cognition, endocrine secretion, chronobiotic rhythms, appetite, and motor function, all of which are profoundly disrupted in depressive states. Accordingly, a perturbation of monoaminergic transmission is implicated in the aetiology of depressive disorders, and all clinically available antidepressants increase corticolimbic availability of monoamines. However, their limited efficacy, delayed onset of action, and undesirable side effects underlie ongoing efforts to identify improved therapeutic agents. Sequencing the human genome has raised the hope not only of better symptomatic control of depression, but even of the prevention or cure of depressive states. In the pursuit of these goals, there is currently a tendency to focus on selective ligands of "novel" nonmonoaminergic targets. However, certain classes of novel agent (such as neurokinin(1) receptor antagonists) indirectly modulate the activity of monoaminergic networks. Others may act "downstream" of them, converging onto common cellular substrates controlling gene expression, synaptic plasticity, and neurogenesis. Further, by analogy to the broad-based actions of currently employed drugs, multitarget agents may be better adapted than selective agents to the management of depression-a complex disorder with hereditary, developmental, and environmental origins. It is, thus, important to continue the creative exploration of clinically validated and innovative monoaminergic strategies within a multitarget framework. In this light, drugs combining monoaminergic and nonmonoaminergic mechanisms of action may be of particular interest. The present article provides a critical overview of monoaminergic strategies for the treatment of depressive states, both established and under development, and discusses interactions of novel "nonmonoaminergic" antidepressants with monoaminergic mechanisms.

Fluoxetine-induced change in rat brain expression of brain-derived neurotrophic factor varies depending on length of treatment

Recent studies indicate that brain-derived neurotrophic factor (BDNF) may be implicated in the clinical action of antidepressant drugs. Repeated (2-3 weeks) administration of antidepressant drugs increases BDNF gene expression. The onset of this response as well as concomitant effects on the corresponding BDNF protein is however, unclear. The present study investigated the effects of acute and chronic administration of the selective serotonin reuptake inhibitor, fluoxetine (10mg/kg p.o.), upon regional rat brain levels of BDNF mRNA and protein expression. To improve the clinical significance of the study, fluoxetine was administered orally and mRNA and protein levels were determined ex vivo using the techniques of in situ hybridisation histochemistry and immunocytochemistry respectively. Direct measurement of BDNF protein was also carried out using enzyme-linked immunosorbent assay (ELISA). Four days of once daily oral administration of fluoxetine induced decreases in BDNF mRNA (hippocampus, medial habenular and paraventricular thalamic nuclei). Whilst 7 days of treatment showed a non-significant increase in BDNF mRNA, there were marked and region-specific increases following 14 days of treatment. BDNF protein levels remained unaltered until 21 days of fluoxetine treatment, when the numbers of BDNF immunoreactive cells were increased, reaching significance in the pyramidal cell layer of CA1 and CA3 regions of Ammon's horn (CA1 and CA3) but not in the other sub-regions of the hippocampus. Indicative of the highly regional change within the hippocampus, the ELISA method failed to demonstrate significant up-regulation at 21 days, measuring levels of BDNF protein in the whole hippocampus. In contrast to the detected time dependent and biphasic response of the BDNF gene, activity-regulated, cytoskeletal-associated protein (Arc) mRNA showed a gradual increase during the 14-day course of treatment. The results presented here show that BDNF is expressed differentially depending on length of fluoxetine administration, which could contribute in explaining the slow onset of antidepressant activity observed with selective serotonin reuptake inhibitors.

Functional interactions between 5-hydroxytryptamine receptors and the serotonin transporter in pulmonary arteries

Pulmonary arterial 5-hydroxytryptamine (serotonin) (5-HT) transporter (SERT)-, 5-HT receptor expression, and 5-HT-induced vasoconstriction can be increased in pulmonary hypertension. These variables were studied in normoxic and hypoxic Fawn-Hooded (FH) and Sprague-Dawley (SD) rats. Furthermore, we compared the functional effects of SERT inhibitors and 5-HT receptor antagonists against 5-HT-induced vasoconstriction of pulmonary arteries. SERT and 5-HT(1B) expression was greater in FH rat lungs than in SD rats, as was 5-HT-mediated vasoconstriction. The 5-HT(2A) receptor antagonist ketanserin and the 5-HT(1B) receptor antagonist SB224289 (1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro-spiro-[furo] 2, 3-f]indole-3,4'-piperidine]) inhibited responses to 5-HT in all vessels. The combined 5-HT(1B) receptor/SERT antagonist LY393558 (1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-1(2H)-pyridinyl]ethyl]-3-isopropyl-6-(methylsulfonyl)-3,4-dihydro-1H-2,1,3-benzothiadiazine-2,2-dioxide) was the most potent inhibitor of constriction in all vessels. SERT inhibitors citalopram and fluoxetine inhibited responses to 5-HT in SD vessels. However, these inhibitors potentiated responses to 5-HT in FH vessels. After exposure of rats to 2 weeks of hypoxia, there was increased 5-HT-mediated vasoconstriction and a profound decrease in SERT expression in both the FH and SD rat lung. Accordingly, citalopram had no effect on 5-HT-induced constriction in SD rat vessels and markedly less effect in FH rat vessels. Ketanserin, SB224289, and LY393558 inhibited responses to 5-HT in all hypoxic rat vessels. LY393558 was the most potent antagonist, and there was synergy between the effects of fluoxetine and SB224289 when given simultaneously. The results suggest that, in FH rats, SERT inhibitors may increase pulmonary vasoconstriction, but this can be inhibited by simultaneous 5-HT(1B) receptor antagonism. There is synergy between the inhibitory effects of 5-HT(1B) receptor antagonists and SERT inhibitors on 5-HT-induced pulmonary vasoconstriction.

SAR development of a selective 5-HT1D antagonist/serotonin reuptake inhibitor lead using rapid parallel synthesis

Incorporation of an SRI (serotonin reuptake inhibitor) pharmacophore into a selective 5-HT(1D) agonist has led to the discovery of a molecule having both 5-HT(1D) antagonist and SRI activity. RPS methodology was used to develop the SAR and identify potential approaches to reduce unwanted adrenergic alpha 1 and dopamine D(2) cross-reactivities.

Ingestive behaviors and metabolic fuels after central injections of 5-HT1A and 5-HT1D/1B receptors agonists in the pigeon

The effects of intracerebroventricular injections of 8-OH-DPAT (a 5-HT1A agonist; 3, 15 or 30 nmol) or GR46611 (a 5-HT1B/1D agonist; 3, 15 or 30 nmol) on feeding, drinking, preening and sleep-like behaviors were investigated in free-feeding (FF) pigeons. The effects of these 5-HT agonists on blood glucose and free fatty acids levels were also examined. Injections of 8-OH-DPAT evoked intense lipolytic and dipsogenic effects, but failed to affect feeding, non-ingestive behaviors and glycemic levels. On the other hand, GR46611 evoked significant increases in food intake (at the higher dose), as well as lipolytic and hyperglycemic effects, but left drinking and other non-ingestive behaviors unchanged. These effects are opposed to those found in rodents, and may be associated with the diverse, species-specific nature and distribution of these receptors, underscoring the need to examine the functional aspects of the 5-HT1 receptor family in a more extensive range of non-rodent species.

In vitro and in vivo effect of fluoxetine on the permeability of3H-serotonin across rat intestine

The aim of this work was to characterize the mucosal-to-serosal (apical to basolateral; AP-BL) and serosal-to-mucosal (basolateral to apical; BL-AP) transport of serotonin (5-HT) across rat jejunum, ileum, and colon, and to determine the influence of serotonin neuronal transporter inhibitors on this transport. The AP-BL apparent permeability (Papp) of3H-5-HT increased in the order colon = jejunum < ileum, and the BL-AP Pappof3H-5-HT increased in the order colon < jejunum = ileum. In vitro, neither fluoxetine (0.02 or 0.2 µmol/L) nor desipramine (0.4 or 4 µmol/L) had a significant effect upon the AP-BL or BL-AP Pappof3H-5-HT in any of the intestinal regions. However, fluoxetine (0.2 µmol/L) decreased the accumulation of3H-5-HT in the ileum (to 65% of control) in the BL-AP experiments. In vivo, chronic fluoxetine (10 mg/kg daily administered orally for 15 days), as assessed in the ileum, significantly increased (to ±180% of control levels) the BL-AP Pappof3H-5-HT and tended to increase the AP-BL Pappof3H-5-HT. In conclusion, the increase in the Pappof3H-5-HT after chronic administration of fluoxetine suggests that this treatment is able to increase the extracellular concentration of3H-5-HT at the intestinal level.Key words: fluoxetine, serotonin, rat intestine, permeability.

The role of the 5-HT1D receptor as a presynaptic autoreceptor in the guinea pig

The present study investigated the role of the 5-hydroxytryptamine (5-HT, serotonin)1D receptor as a presynaptic autoreceptor in the guinea pig. In keeping with the literature, the 5-HT1B selective antagonist, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydrospiro [furo[2,3-f]indole-3,4'-piperidine]oxalate (SB224289) potentiated [3H]5-HT outflow from pre-labelled slices of guinea pig cerebral cortex confirming its role as a presynaptic autoreceptor in this species. In addition, the 5-HT1D receptor-preferring antagonists, 1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-2H-pyridin-1-yl]-ethyl]-3-pyridin-4-yl-methyl-tetrahydro-pyrimidin-2-one (LY367642), (R)-1-[2-(4-(6-fluoro-1H-indol-3-yl-)-3,6-dihydro-1(2H)-pyridinyl)ethyl]-3,4-dihydro-1H-2-benzopyran-6-carboxamide (LY456219), (S)-1-[2-(4-(6-fluoro-1H-indol-3-yl-)-3,6-dihydro-1(2H)-pyridinyl)ethyl]-3,4-dihydro-1H-2-benzopyran-6-carboxamide (LY456220) and 1-[2-[4-(4-fluoro-benzoyl)-piperidin-1-yl]-ethyl]-3,3-dimethyl-1,2-dihydro-indol-2-one (LY310762), potentiated [3H]5-HT outflow from this preparation with potencies (EC50 values=31-140 nM) in the same range as their affinities for the guinea pig 5-HT1D receptor (Ki values=100-333 nM). The selective 5-HT1D receptor agonist, R-2-(4-fluoro-phenyl)-2-[1-[3-(5-[1,2,4]triazol-4-yl-1H-indol-3-yl)-propyl]-piperidin-4-ylamino]-ethanol dioxylate (L-772,405), inhibited [3H]5-HT outflow. In microdialysis studies, administration of either SB224289 or LY310762 at 10 mg/kg by the intraperitoneal (i.p.) route, potentiated the increase in extracellular 5-HT concentration produced by a maximally effective dose of the selective serotonin re-uptake inhibitor, fluoxetine (at 20 mg/kg i.p.). In addition, the 5-HT1D receptor-preferring antagonist and 5-HT transporter inhibitor, LY367642 (at 10 mg/kg i.p.), elevated extracellular 5-HT concentrations to a greater extent than a maximally effective dose of fluoxetine. It is concluded that the 5-HT1D receptor, like the 5-HT1B receptor, may be a presynaptic autoreceptor in the guinea pig.

Role of extracellular serotonin levels in the effect of 5-HT1B receptor blockade

The release of serotonin (5-HT) at serotonergic nerve terminals is regulated by 5-HT(1B) autoreceptors. Several studies have reported that the effects of selective 5-HT reuptake inhibitors (SSRIs) on extracellular 5-HT are augmented by 5-HT(1B) receptor antagonists, whereas administration of these antagonists alone do not enhance 5-HT levels. It has been suggested that 5-HT(1B) receptors have low basal endogenous activity and therefore elevated endogenous 5-HT levels are needed to elicit an effect of 5-HT(1B) receptor antagonists. To test this hypothesis, different strategies were used to enhance 5-HT levels in the rat frontal cortex to assess the effects of locally applied NAS-181, a new selective 5-HT(1B) receptor antagonist. Blockade of 5-HT(1B) receptors with NAS-181 dose dependently augmented 5-HT levels when 5-HT levels were enhanced by a SSRI. No additional effect of NAS-181 on 5-HT output was found when 5-HT levels were enhanced by KCl depolarization-induced release or by preventing degradation of 5-HT with the monoamine oxidase inhibitor pargyline. In the presence of fluvoxamine, the increased 5-HT release evoked by KCl depolarization was augmented by NAS-181, supporting the idea that blockade of 5-HT transporters is necessary to measure an effect of 5-HT(1B) receptor blockade. In conclusion, the results provide circumstantial evidence that the effect of a 5-HT(1B) receptor antagonist depends on extracellular 5-HT levels, but strongly suggest that additional 5-HT reuptake inhibition is required to detect any effect of 5-HT(1B) receptor antagonist on 5-HT levels by in vivo microdialysis.

New approaches to developing antidepressants by enhancing monoaminergic neurotransmission

Major depressive disorder (MDD) is a serious illness with far reaching societal and economic ramifications. The monoamine-deficiency hypothesis that depressive symptoms are associated with reductions in monoamine neurotransmission, particularly serotonin and noradrenaline, is supported by both neurochemical findings and the successful treatment of MDD with compounds that enhance monoaminergic neurotransmission. This review focuses on novel compounds in different stages of development for the treatment of MDD that enhance monoaminergic neurotransmission via a number of different mechanisms, including re-uptake inhibition of one or more monoamines, monoamine oxidase inhibitors, the combination of monoamine antagonists with re-uptake inhibitors and monoamine receptor subtype agonists. Compounds that enhance individual monoamines have antidepressant properties and compounds that enhance multiple monoamines appear to have a synergistic antidepressant effect and potentially faster onset of action. The differing mechanisms of action possessed by these novel monoamine-enhancing compounds will offer greater treatment flexibility in the therapeutic management of MDD.

In vitro activity of LY393558, an inhibitor of the 5-hydroxytryptamine transporter with 5-HT(1B/1D/2) receptor antagonist properties

1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-1(2H)-pyridinyl]ethyl]-3-isopropyl-6-(methylsulphonyl)-3,4-dihydro-1H-2,1,3-benzothiadiazine-2,2-dioxide (LY393558) is a potent inhibitor of [3H]5-hydroxytryptamine ([3H]5-HT) uptake into rat cortical synaptosomes (pIC(50)=8.48+/-0.12). It produces a dextral shift of the 5-HT dose-response curves for the binding of GTPgamma[35S] to human 5-HT(1B) (pK(b)=9.05+/-0.14) and 5-HT(1D) (pK(b)=8.98+/-0.07) receptors and inhibits the contractile response of the rabbit saphenous vein to the 5-HT(1B/D) receptor agonist, sumatriptan (pK(b)=8.4+/-0.2). In addition, it is an antagonist at the 5-HT(2A) (pK(i)=7.29+/-0.19) and 5-HT(2B) (pK(i)=7.35+/-0.11) receptors. Presynaptic autoreceptor antagonist activity was demonstrated by its ability to potentiate the K(+)-induced outflow of [3H]5-HT from guinea pig cortical slices (pEC(50)=7.74+/-0.05 nM) in which the 5-HT transporter had been inhibited by a maximally effective concentration of paroxetine. It is concluded that LY393558 should be an effective antidepressant with the potential to produce an earlier onset of efficacy than selective serotonin uptake inhibitors.