Differentiation between partial agonists and neutral 5-HT1B antagonists by chemical modulation of 3-[3-(N,N-dimethylamino)propyl]-4-hydroxy- N-[4-(pyridin-4-yl)phenyl]benzamide (GR-55562)

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats

This study assessed the possible antinociceptive role of peripheral 5-HT(1) receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 microl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT(1) receptor agonists R(+)-UH-301 (5-HT(1A); 0.1-3 microg/paw), CGS-12066A (5-HT(1B); 0.01-0.3 microg/paw), GR46611 (5-HT(1B/1D); 0.3-10 microg/paw), BRL54443 (5-HT(1E/1F); 3-300 microg/paw) or LY344864 (5-HT(1F); 3-300 microg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 microg/paw) was significantly reduced by WAY-100635 (30-100 microg/paw; a 5-HT(1A) receptor antagonist). Moreover, the antagonists GR55562 (30-100 microg/paw; 5-HT(1B/D)) or SB224289 (30-100 microg/paw; 5-HT(1B)) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 microg/paw) whereas GR55562 (30-100 microg/paw) or BRL15572 (30-100 microg/paw, 5-HT(1D)) reduced the antinociceptive effect of GR46611 (0.3 microg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 microg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT(1A,) 5-HT(1B), 5-HT(1D), 5-HT(1F) and, probably, 5-HT(1E) receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT(1) receptor agonists could be a therapeutic strategy to reduce inflammatory pain.

Sterically-directed consecutive and size-selective self-assembly of palladium diphosphane complexes with an Ar-BIAN ligand: unexpected formation of pentameric and hexameric aggregates

The coordination properties of N,N'-bis[4-(4-pyridyl)phenyl]acenaphthenequinonediimine (L(1)) and N,N'-bis[4-(2-pyridyl)phenyl]acenaphthenequinonediimine (L(2)) were investigated in self-assembly with palladium diphosphane complexes [Pd(P;P)(H(2)O)(2)](OTf)(2) (OTf = triflate) by using various analytical techniques, including multinuclear ((1)H, (15)N, and (31)P) NMR spectroscopy and mass spectrometry (P;P = dppp, dppf, dppe; dppp = bis(diphenylphosphanyl)propane, dppf = bis(diphenylphosphanyl)ferrocene, and dppe = bis(diphenylphosphanyl)ethane). Beside the expected trimeric and tetrameric species, the interaction of an equimolar mixture of [Pd(dppp)](2+) ions and L(1) also generates pentameric aggregates. Due to the E/Z isomerism of L(1), a dimeric product was also observed. In all of these species, which correspond to the general formula [Pd(dppp)L(1)](n)(OTf)(2n) (n = 2-5), the L(1) ligand is coordinated to the Pd center only through the terminal pyridyl groups. Introduction of a second equivalent of the [Pd(dppp)](2+) tecton results in coordination to the internal, sterically more encumbered chelating site and induces enhancement of the higher nuclearity components. The presence of higher-order aggregates (n = 5, 6), which were unexpected for the interaction of cis-protected palladium corners with linear ditopic bridging ligands, has been demonstrated both by mass-spectrometric and DOSY NMR spectroscopic analysis. The sequential coordination of the [Pd(dppp)](2+) ion is attributed to the dissimilar steric properties of the two coordination sites. In the self-assembled species formed in a 1:1:1 mixture of [Pd(dppp)](2+)/[Pd(dppe)](2+)/L(1), the sterically more demanding [Pd(dppp)](2+) tectons are attached selectively to the pyridyl groups, whereas the more hindered imino nitrogen atoms coordinate the less bulky dppe complexes, thus resulting in a sterically directed, size-selective sorting of the metal tectons. The propensity of the new ligands to incorporate hydrogen-bonded solvent molecules at the chelating site was confirmed by X-ray diffraction studies.

Novel orally active NPY Y5 receptor antagonists: Synthesis and structure-activity relationship of spiroindoline class compounds

Spiroindoline urea derivatives, designed to act as NPY Y5 receptor antagonists, were synthesized and their structure-activity relationships were investigated. Of these derivatives, compound 3a showed good Y5 binding affinity with favorable pharmacokinetic properties. Compound 3a significantly inhibited bPP Y5 agonist-induced food intake in rats, and suppressed body weight gain in DIO mice.

Biphasic alterations in Serotonin-1B (5-HT1B) receptor function during abstinence from extended cocaine self-administration

Alterations in 5-HT1B receptor function during cocaine abstinence were evaluated in rats given either limited- or extended access (LA and EA, respectively) to cocaine self-administration. The locomotor response to the 5-HT1B/1A agonist RU24969 was significantly reduced in cocaine-experienced animals relative to cocaine-naïve controls following 6 h of abstinence but became sensitized over the subsequent 14 days of abstinence. Both the early phase subsensitivity and later phase supersensivity to RU 24969-induced activity were greater in EA versus LA animals. Intra-nucleus accumbens administration of the 5-HT1B agonist CP 93, 129 produced significantly greater increases in dialysate dopamine levels in EA versus control animals following 14 days of abstinence. However, there was no difference between EA and cocaine-naïve control animals in the augmentation of cocaine-induced increases in nucleus accumbens DA produced by intra-VTA CP 93, 129. Collectively these findings demonstrate that 5-HT1B receptor function is persistently altered by cocaine self-administration.

5-HT1B receptors play a prominent role in the proliferation of T-lymphocytes

Serotonin plays a role in T cell activation, but there is no clear consensus of which of the 14 serotonergic receptors control this activations pathway. We have used a broad range of serotonergic receptor antagonists to define the functional involvement of these receptors governing the proliferation of primary T cells as well as in T cell lines. Our data shows that antagonism of the 5-HT(1B) receptor inhibits the proliferation of both human and murine primary helper T cells and of human helper T cell lines. As a whole, our data suggest that other serotonergic receptors may contribute to the proliferative signals, but the 5-HT(1B) receptor plays the most dominant role.

Inverse agonism or neutral antagonism at G-protein coupled receptors: A medicinal chemistry challenge worth pursuing?

The identification of constitutive, or intrinsic, activity of G-protein coupled receptors has had major impact on receptor theory, the identification of agents that inhibit this ligand-independent receptor activity has led, in turn, to the concept of inverse agonism. It has subsequently emerged that the majority, around 85%, of all known G-protein coupled receptor antagonists are, in fact, inverse agonists. Agents that affect only ligand-dependent receptor activation, i.e. have no effect on constitutive receptor signalling, are termed neutral antagonists and turn out to be relatively rare in pharmacology. Is this relevant for medicinal chemistry? That question is difficult to answer with certainty because there has been little or no effort to understand the structure activity relationships of neutral antagonist vs. inverse agonist molecules. In this review, we suggest that these pharmacological differences may well be translated to differential effects in the whole animal and in medicine. We argue that having either option to inhibit a particular receptor may reveal differences in efficacy and tolerability thus increasing the potential value of a G-protein coupled receptor inhibitor programme. However, since inverse agonists appear to constitute a default inhibitor mode, a systematic survey of the structure activity relationships around what makes a neutral antagonist will be an essential first step towards this goal.

Serotonin1B receptors in the ventral tegmental area modulate cocaine-induced increases in nucleus accumbens dopamine levels

Previous work has demonstrated that peripheral serotonin(1B) (5-HT(1B)) receptor agonist administration facilitates the behavioral and neurochemical effects of cocaine. This study used dual probe microdialysis to investigate whether activation of serotonin(1B) (5-HT(1B)) receptors in the ventral tegmental area (VTA) alters the ability of peripherally administered cocaine to elevate dopamine (DA) levels in the ipsilateral nucleus accumbens (NAcc) of drug-naive Wistar rats. Intra-VTA administration of the selective 5-HT(1B) agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo [3,2-b]pyridin-5-one dihydrochloride (CP 93,129) by reverse dialysis produced a dose-dependent (30 and 100 microM) potentiation of cocaine-induced (10 mg/kg i.p.) increases in NAcc DA efflux and concurrent cocaine-induced decreases in VTA GABA efflux. There was no effect of either local CP 93,129 or peripheral cocaine on VTA glutamate efflux. Intra-VTA administration of the 5-HT(1A/7) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT; 100 microM) did not alter cocaine-induced alterations in NAcc DA or VTA GABA, suggesting that the effects of CP 93,129 were not mediated through 5-HT(1A) receptors. Moreover, the effects of intra-VTA CP 93,129 (100 microM) on both cocaine-induced increases in NAcc DA levels and cocaine-induced decreases in VTA GABA levels were reversed by coadministration of the selective 5-HT(1B) receptor antagonist 3-[3-(dimethylamine)propyl]-4-hydroxy-N-[4-(4-pyridinyl] phenyl] benzamide dihydrochloride (GR 55562; 300 microM). In the absence of cocaine, intra-VTA CP 93,139 produced an increase in NAcc DA and decrease in VTA GABA levels. However, intra-VTA GR 55562 alone had no effect on any of our neurochemical measures. These findings indicate that activation of VTA 5-HT(1B) receptors potentiates cocaine-induced increases in NAcc DA levels by enhancing the ability of cocaine to decrease VTA GABA efflux.

Effects of 5-HT1B receptor ligands microinjected into the ventral tegmental area on cocaine discrimination in rats

Some recent data indicate a significant interaction between serotonin (5-hydroxytryptamine; 5-HT) and dopamine in mesolimbic brain structures (e.g. the ventral tegmental area) which modulate the behavioral effects of cocaine in rats. The present study investigated the role of 5-HT(1B) receptors in the ventral tegmental area in the discriminative stimulus effects of cocaine in rats. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, intraperitoneally (i.p.)) from saline (i.p.) in a two-choice, water-reinforced fixed-ratio 20 procedure. After reaching the cocaine-saline discrimination criterion, the rats were stereotaxically implanted with bilateral cannulae in the ventral tegmental area and were then microinjected with selective 5-HT(1B) receptor ligands. In substitution studies, microinjections of the 5-HT(1B) receptor antagonist, 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; 0.1-1 microg/side), did not evoke cocaine-lever responding, whereas the 5-HT(1B) receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP 93129; 0.3-1 microg/side), induced partial substitution for cocaine. Intra-tegmental microinjections with the 5-HT(1B) receptor antagonist, GR 55562 (0.1-1 microg/side), before cocaine (5 mg/kg), which alone produced 98% cocaine-lever responses, decreased in a dose-dependent manner the discriminative stimulus effects of the psychostimulant. On the other hand, combination tests using a fixed dose of CP 93129 (0.3 or 1 microg/side), given into the ventral tegmental area prior to low systemic doses of cocaine (1.25-2.5 mg/kg), increased cocaine discrimination. These results seem to indicate that tegmental 5-HT(1B) receptors are necessary to express the discriminative stimulus effects of cocaine.

Pharmacological characterization of 5-HT(1B) receptor-mediated inhibition of local excitatory synaptic transmission in the CA1 region of rat hippocampus

1 In the hippocampus, axon collaterals of CA1 pyramidal cells project locally onto neighbouring CA1 pyramidal cells and interneurones, forming a local excitatory network which, in disinhibited conditions, feeds polysynaptic epscs (poly-epscs). 5-hydroxytryptamine (5-HT) has been shown to inhibit poly-epscs through activation of a presynaptic receptor. The aim of the present work was the pharmacological characterization of the 5-HT receptor involved in this 5-HT action. 2 Poly-epscs, evoked by electrical stimulation of the stratum radiatum and recorded in whole-cell voltage-clamp from CA1 pyramidal neurones, were studied in mini-slices of the CA1 region under pharmacological block of GABA(A), GABA(B), and 5-HT(1A) receptors. 3 The 5-HT(1B) receptor selective agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one dihydrochloride (CP 93129) inhibited poly-epscs (EC(50)=55 nM), an effect mimicked by the 5-HT(1B) ligands 5-carboxamidotryptamine (5-CT; EC(50)=14 nM) and methylergometrine (EC(50)=78 nM), but not by 1-(3-chlorophenyl)piperazine dihydrochloride (mCPP; 10 micro M) or 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline dimaleate (CGS 12066B; 10 micro M). 4 The effects of CP 93129 and 5-CT were blocked by the selective 5-HT(1B) receptor antagonist 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; K(B) approximately 100 nM) and by cyanopindolol (K(B)=6 nM); methiothepin (10 micro M) and dihydroergotamine (1 micro M). For both GR 55562 and methiothepin, application times of at least two hours were required in order to achieve their full antagonistic effects. 5 Our results demonstrate that 5-HT(1B) receptors are responsible for the presynaptic inhibition of neurotransmission at CA1/CA1 local excitatory synapses exerted by 5-HT.

Effect of different 5-hydroxytryptamine receptor subtype antagonists on the micturition reflex in rats

OBJECTIVE

To evaluate the effects of antagonists of different subfamilies of 5-hydroxytryptamine (5-HT) receptors on bladder function in anaesthetized and conscious rats. MATEERIALS AND METHODS: The urinary bladder of female anaesthetized rats was catheterized urethrally and filled with physiological saline until spontaneous bladder contractions occurred. Infravesical pressure was measured by a pressure transducer and displayed continuously on a chart recorder. The time of bladder quiescence (to the disappearance of rhythmic contractions) after injection with different compounds tested was recorded. Conscious rats underwent cystometry with chronically (infravesical) implanted catheters to continuously record bladder capacity (evaluated as amount of saline infused between voiding cycles) and maximal voiding pressure. The affinity for the human recombinant serotoninergic 5-HT1A subtype (inhibition of specific binding of [3H]8-hydroxy-2-(di-n-propylamino) tetralin) and the effects on the [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding in HeLa cells was also evaluated.

RESULTS

Among the compounds tested, only 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethyl-piperazine (p-MPPI) and methiothepin showed nanomolar affinity for the 5-HT1A receptors, the former being a neutral antagonist and the latter an inverse agonist in the [35S]GTPgammaS binding model. Intravenous injection of low doses of p-MPPI and methiothepin induced a dose-dependent disappearance of isovolumic bladder contractions in anaesthetized rats (> 10 min). At the highest doses, the dose-response curves were bell-shaped. The amplitude of bladder contractions was not markedly altered. The tested antagonists of 5-HT2, 5-HT3, 5-HT4, and 5-HT6 serotoninergic subtypes were poorly active or inactive in the model. Similarly, these compounds were inactive on cystometry in conscious rats, whereas p-MPPI and methiothepin induced a consistent increase in bladder capacity. Methiothepin also decreased the voiding pressure, whereas p-MPPI did not affect this variable.

CONCLUSIONS

These findings confirm that only selective 5-HT1A receptor antagonists have favourable effects on the bladder, inducing an increase in bladder capacity with no derangement of bladder contractility.

Identification of an amino acid residue important for binding of methiothepin and sumatriptan to the human 5-HT(1B) receptor

Site-directed mutagenesis of the human 5-HT1B receptor was performed to investigate the role of the amino acid residues cysteine 326 and tryptophan 327 in transmembrane region VI and aspartic acid 352 in transmembrane region VII in ligand binding. Binding studies were performed with the antagonist radioligand [3H]GR125743 on mutant and wild-type receptors stably expressed in Chinese hamster ovary cells (CHO)-K1 cells. Substitution of tryptophan 327 by alanine resulted in decreased affinities of all ligands tested. The most prominent changes in affinity were observed for the antagonist methiothepin and the antimigraine drug sumatriptan, which were reduced approximately 300- and 60-fold, respectively. Nevertheless, the affinity of 5-HT remained the same. Replacement of the aspartic acid 352 by alanine reduced high-affinity binding of 5-HT. Substitution of cysteine 326 by alanine had minor effects on ligand binding. Some of these results agree with the results from mutagenesis studies of the corresponding amino acids in other receptors. However, some notable differences also emerge showing that functional roles of individual amino acid residues must be tested experimentally in each receptor subtype.

Partial agonism at serotonin 5-HT1B and dopamine D2L receptors using a luciferase reporter gene assay

We have used a luciferase reporter gene assay to study the functional responses of two G-protein-coupled receptors in Chinese hamster ovary (CHO) cells. The rank order of potency of drugs for the endogenous 5-HT1B receptor was 5-Hydroxytryptamine (5-HT) > zolmitriptan > dihydroergocristine > (-)lisuride (with no response to bromocriptine). However, only 5-HT and (-)lisuride produced a full functional response, with zolmitriptan and dihydroergocristine achieving 69+/-2% and 50+/-1% of the maximal response. In the same cells stably transfected with the rat dopamine D2L receptor, dopamine and bromocriptine produced a full agonist functional response, whilst (-)lisuride produced a biphasic response curve, indicating activity at both the endogenous 5-HT1B and exogenous dopamine D2L receptors. Using the receptor specific antagonists, pindolol and (+)butaclamol, (-)lisuride was shown to produce 52% of the maximal response at the dopamine D2 receptor relative to dopamine. In comparison to a cAMP accumulation assay, the rank orders of potency and intrinsic activity were the same for all compounds used. These results demonstrate that this reporter gene assay is capable of discriminating both potency and efficacy of drugs and can be used to characterise partial agonists at endogenously and heterologously expressed receptors in CHO cells.