The positive allosteric GABAB receptor modulator rac-BHFF enhances baclofen-mediated analgesia in neuropathic mice.

Neuropathic pain is associated with impaired inhibitory control of spinal dorsal horn neurons, which are involved in processing pain signals. The metabotropic GABAB receptor is an important component of the inhibitory system and is highly expressed in primary nociceptors and intrinsic dorsal horn neurons to control their excitability. Activation of GABAB receptors with the orthosteric agonist baclofen effectively reliefs neuropathic pain but is associated with severe side effects that prevent its widespread application. The recently developed positive allosteric GABAB receptor modulators lack most of these side effects and are therefore promising drugs for the treatment of pain. Here we tested the high affinity positive allosteric modulator rac-BHFF for its ability to relief neuropathic pain induced by chronic constriction of the sciatic nerve in mice. rac-BHFF significantly increased the paw withdrawal threshold to mechanical stimulation in healthy mice, indicating an endogenous GABABergic tone regulating the sensitivity to mechanical stimuli. Surprisingly, rac-BHFF displayed no analgesic activity in neuropathic mice although GABAB receptor expression was not affected in the dorsal horn as shown by quantitative receptor autoradiography. However, activation of spinal GABAB receptors by intrathecal injection of baclofen reduced hyperalgesia and its analgesic effect was considerably potentiated by co-application of rac-BHFF. These results indicate that under conditions of neuropathic pain the GABAergic tone is too low to provide a basis for allosteric modulation of GABAB receptors. However, allosteric modulators would be well suited as an add-on to reduce the dose of baclofen required to achieve analgesia.

Inhibition of alcohol self-administration by positive allosteric modulators of the GABAB receptor in rats: lack of tolerance and potentiation of baclofen.

RATIONALE: Treatment with positive allosteric modulators (PAMs) of the GABAB receptor (GABAB PAMs) inhibits several alcohol-motivated behaviors in rodents, including operant, oral alcohol self-administration. OBJECTIVES: The present study assessed the effects of (a) repeated administration of the GABAB PAMs, GS39783, and rac-BHFF and (b) a combination of an ineffective dose of either GS39783, or rac-BHFF, and an ineffective dose of the prototypic GABAB receptor agonist, baclofen, on operant, oral alcohol self-administration. METHODS: Studies were conducted using selectively bred Sardinian alcohol-preferring (sP) rats exposed to a standard procedure of fixed ratio (FR) 4 (FR4) schedule of reinforcement for 15 % (v/v) alcohol. RESULTS: Repeated treatment with GS39783 (50 mg/kg, i.g.) or rac-BHFF (50 mg/kg, i.g.) produced an initial 40 % reduction in number of lever responses for alcohol and amount of self-administered alcohol that was maintained unaltered throughout the 10-day period of the GS39783 treatment and increased throughout the 5-day period of the rac-BHFF treatment. Combination of per se ineffective doses of GS39783 (5 mg/kg, i.g.), or rac-BHFF (5 mg/kg, i.g.), and baclofen (1 mg/kg, i.p.) reduced, by 35-45 %, both number of lever responses for alcohol and amount of self-administered alcohol. CONCLUSIONS: GS39783 and rac-BHFF (a) reduced alcohol reinforcing properties when given repeatedly, with no development of tolerance, and (b) potentiated baclofen effect. Both sets of data possess translational interest, as they suggest potential effectiveness of GABAB PAMs under chronic treatment and selective potentiation of baclofen effect.

Mapping the binding pocket of a novel, high-affinity, slow dissociating tachykinin NK3 receptor antagonist: biochemical and electrophysiological characterization.

The NK3 receptor is a GPCR that is prominently expressed in limbic areas of the brain, many of which have been implicated in schizophrenia. Phase II clinical trials in schizophrenia with two selective NK3 antagonists (osanetant and talnetant) have demonstrated significant improvement in positive symptoms. The objective of this study was to characterize the properties of a novel dual NK2/NK3 antagonist, RO5328673. [(3)H]RO5328673 bound to a single saturable site on hNK2, hNK3 and gpNK3 with high-affinity. RO5328673 acted as an insurmountable antagonist at both human and guinea-pig NK3 receptors in the [(3)H]IP accumulation assay. In binding kinetic analyses, [(3)H]RO5328673 had fast association and dissociation rates at hNK2 while it had a fast association rate and a remarkably slow dissociation rate at gp and hNK3. In electrophysiological recordings of gp SNpc, RO5328673 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurons with an insurmountable mechanism of action. RO5328673 exhibited in-vivo activity in gerbils, robustly reversing the senktide-induced locomotor activity. The TM2 residue gpNK3-A114(2.58) (threonine in all other species) was identified as the critical residue for the RO5328673's slower dissociation kinetics and stronger insurmountable mode of antagonism in the guinea-pig as compared to hNK3-T139(2.58). Using site-directed mutagenesis, [(3)H]RO5328673 binding and rhodopsin-based modeling, the important molecular determinants of the RO5328673-binding pocket of hNK3 were determined. A comparison of the RO5328673-binding pocket with that of osanetant showed that two antagonists have similar contact sides on hNK3 binding crevice except for three mutations V95L(1.42), Y247W(5.38), V255I(5.46), which behaved differently between interacting modes of two antagonists in hNK3.

Positive allosteric modulation of GABAB receptors ameliorates sensorimotor gating in rodent models.

BACKGROUND: Converging evidence points to the involvement of γ-amino-butyric acid B receptors (GABABRs) in the regulation of information processing. We previously showed that GABABR agonists exhibit antipsychotic-like properties in rodent models of sensorimotor gating deficits, as measured by the prepulse inhibition (PPI) of the acoustic startle reflex. The therapeutic potential of these agents, however, is limited by their neuromuscular side effects; thus, in this study, we analyzed whether rac-BHFF, a potent GABABR-positive allosteric modulator (PAM), could counter spontaneous and pharmacologically induced PPI deficits across various rodent models. METHODS: We tested the antipsychotic effects of rac-BHFF on the PPI deficits caused by the N-methyl-D-aspartate glutamate receptor antagonist dizocilpine, in Sprague-Dawley rats and C57BL/6 mice. Furthermore, we verified whether rac-BHFF ameliorated the spontaneous PPI impairments in DBA/2J mice. RESULTS: rac-BHFF dose-dependently countered the PPI deficits across all three models, in a fashion akin to the GABABR agonist baclofen and the atypical antipsychotic clozapine; in contrast with these compounds, however, rac-BHFF did not affect startle magnitude. CONCLUSIONS: The present data further support the implication of GABABRs in the modulation of sensorimotor gating and point to their PAMs as a novel promising tool for antipsychotic treatment, with fewer side effects than GABABR agonists.

Reduction of alcohol intake by the positive allosteric modulator of the GABA(B) receptor, rac-BHFF, in alcohol-preferring rats.

Previous research has demonstrated that treatment with the positive allosteric modulator (PAM) of the GABA(B) receptor (GABA(B) PAM), rac-BHFF, suppressed lever-responding for alcohol and amount of self-administered alcohol in Sardinian alcohol-preferring (sP) rats. The present study was designed to extend the investigation on the anti-alcohol effects of rac-BHFF to alcohol drinking behavior. To this end, sP rats were exposed to the homecage, 2-bottle "alcohol (10%, v/v) vs water" choice regimen, with unlimited access for 24 h/day. rac-BHFF was administered once daily and for 7 consecutive days at the doses of 0, 50, 100, and 200 mg/kg (i.g.). Treatment with rac-BHFF resulted in an immediate, stable, and dose-related reduction in daily alcohol intake; the overall magnitude of reduction in alcohol intake averaged approximately 25%, 40%, and 65% in 50, 100, and 200 mg/kg rac-BHFF-treated rat groups, respectively. An increase in daily water intake fully compensated the reduction in alcohol intake, so that daily total fluid intake was unaffected by treatment with rac-BHFF. Daily food intake tended to be reduced only by the highest dose of rac-BHFF. These results complement closely with previous data indicating that (a) rac-BHFF suppressed operant, oral alcohol self-administration in sP rats and (b) the prototypic GABA(B) PAMs, CGP7930 and GS39783, reduced alcohol drinking in sP rats. However, while the reducing effect of CGP7930 and GS39783 on the daily alcohol intake tended to vanish after the first 2-3 days of treatment, the reducing effect of rac-BHFF on daily alcohol intake remained unchanged over the entire 7-day treatment period. These data strengthen the hypothesis that GABA(B) PAMs may represent a step forward in the search for GABA(B) receptor ligands with therapeutic potential for alcoholism.

Dual hypocretin receptor antagonism is more effective for sleep promotion than antagonism of either receptor alone.

The hypocretin (orexin) system is involved in sleep/wake regulation, and antagonists of both hypocretin receptor type 1 (HCRTR1) and/or HCRTR2 are considered to be potential hypnotic medications. It is currently unclear whether blockade of either or both receptors is more effective for promoting sleep with minimal side effects. Accordingly, we compared the properties of selective HCRTR1 (SB-408124 and SB-334867) and HCRTR2 (EMPA) antagonists with that of the dual HCRTR1/R2 antagonist almorexant in the rat. All 4 antagonists bound to their respective receptors with high affinity and selectivity in vitro. Since in vivo pharmacokinetic experiments revealed poor brain penetration for SB-408124, SB-334867 was selected for subsequent in vivo studies. When injected in the mid-active phase, SB-334867 produced small increases in rapid-eye-movement (REM) and non-REM (NR) sleep. EMPA produced a significant increase in NR only at the highest dose studied. In contrast, almorexant decreased NR latency and increased both NR and REM proportionally throughout the subsequent 6 h without rebound wakefulness. The increased NR was due to a greater number of NR bouts; NR bout duration was unchanged. At the highest dose tested (100 mg/kg), almorexant fragmented sleep architecture by increasing the number of waking and REM bouts. No evidence of cataplexy was observed. HCRTR1 occupancy by almorexant declined 4-6 h post-administration while HCRTR2 occupancy was still elevated after 12 h, revealing a complex relationship between occupancy of HCRT receptors and sleep promotion. We conclude that dual HCRTR1/R2 blockade is more effective in promoting sleep than blockade of either HCRTR alone. In contrast to GABA receptor agonists which induce sleep by generalized inhibition, HCRTR antagonists seem to facilitate sleep by reducing waking "drive".

Identification of a crucial amino acid in the helix position 6.51 of human tachykinin neurokinin 1 and 3 receptors contributing to the insurmountable mode of antagonism by dual NK1/NK3 antagonists.

The neurokinins are neuropeptides that elicit their effect through three GPCRs called NK(1), NK(2), and NK(3). Compounds 5 and 6 are dual hNK(1) (K(i) of 0.7 and 0.3 nM) and hNK(3) (K(i) of 2.9 and 1.7 nM) antagonists. Both compounds exhibit an insurmountable mode of antagonism at hNK(1), whereas at hNK(3), they differ in that 5 is an insurmountable but 6 a surmountable antagonist. Using homology modeling and site-directed mutagenesis, hNK(1)-Phe264 and hNK(3)-Tyr315 were found to be the molecular determinants of hNK(1) and hNK(3) antagonism by 5 and 6. In [(3)H]IP studies, the mutation hNK(1)-F264Y converted the mode of action of 5 from insurmountable to partial insurmountable antagonism while it had no effect on that of 6. Conversely, the mutation hNK(3)-Y315F enhanced the insurmountable behavior of 5 and converted 6's surmountable to an insurmountable antagonism. This finding was further confirmed by characterizing additional derivatives of 5 and 6, most notably with a hybrid structure.

G protein-coupled receptor transmembrane binding pockets and their applications in GPCR research and drug discovery: a survey.

G protein-coupled receptors (GPCRs) share a common architecture consisting of seven transmembrane (TM) domains. Various lines of evidence suggest that this fold provides a generic binding pocket within the TM region for hosting agonists, antagonists, and allosteric modulators. Hence, an automated method was developed that allows a fast analysis and comparison of these generic ligand binding pockets across the entire GPCR family by providing the relevant information for all GPCRs in the same format. This methodology compiles amino acids lining the TM binding pocket including parts of the ECL2 loop in a so-called 1D ligand binding pocket vector and translates these 1D vectors in a second step into 3D receptor pharmacophore models. It aims to support various aspects of GPCR drug discovery in the pharmaceutical industry. Applications of pharmacophore similarity analysis of these 1D LPVs include definition of receptor subfamilies, prediction of species differences within subfamilies in regard to in vitro pharmacology and identification of nearest neighbors for GPCRs of interest to generate starting points for GPCR lead identification programs. These aspects of GPCR research are exemplified in the field of melanopsins, trace amine-associated receptors and somatostatin receptor subtype 5. In addition, it is demonstrated how 3D pharmacophore models of the LPVs can support the prediction of amino acids involved in ligand recognition, the understanding of mutational data in a 3D context and the elucidation of binding modes for GPCR ligands and their evaluation. Furthermore, guidance through 3D receptor pharmacophore modeling for the synthesis of subtype-specific GPCR ligands will be reported. Illustrative examples are taken from the GPCR family class C, metabotropic glutamate receptors 1 and 5 and sweet taste receptors, and from the GPCR class A, e.g. nicotinic acid and 5-hydroxytryptamine 5A receptor.

Tachykinin neurokinin 3 receptor antagonists: a patent review (2005 - 2010).

INTRODUCTION: The neurokinin 3 (NK(3)) receptor is a GPCR that has been shown to modulate monoaminergic systems within regions of the brain implicated in schizophrenia. Preclinical and Phase II clinical results of osanetant and talnetant in schizophrenic patients have indicated that NK(3) antagonists may provide significant improvement of the positive symptoms and cognitive impairment associated with this disorder. Recent findings have also indicated that neurokinin B (NKB)-NK(3) signaling plays a key role in the hypothalamic regulation of reproduction in humans. AREAS COVERED: This review article discusses the latest medicinal chemistry strategies used to derive novel NK(3) receptor antagonists which have been patented during the period 2005 - 2010. EXPERT OPINION: Since the report of a beneficial effect of osanetant in schizophrenic patients, major pharmaceutical companies have been involved in this field, leading to a very large number of patent applications disclosed. Nevertheless, only three NK(3) selective antagonists entered into Phase II, but were then terminated for various reasons. Currently, the main challenge to move forward a selective NK(3) antagonist into the clinic would be to define a safety margin between the desired therapeutic effect and the effect on testosterone levels. The involvement of NKB-NK(3) signaling in reproduction in humans may also lead to new exciting indications, such as treatment for sex steroid-sensitive cancers of breast and prostate.

Mapping the binding pocket of dual antagonist almorexant to human orexin 1 and orexin 2 receptors: comparison with the selective OX1 antagonist SB-674042 and the selective OX2 antagonist N-ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulfonyl)-amino]-N-pyridin-3-ylmethyl-acetamide (EMPA).

The orexins and their receptors are involved in the regulation of arousal and sleep-wake cycle. Clinical investigation with almorexant has indicated that this dual OX antagonist is efficacious in inducing and maintaining sleep. Using site-directed mutagenesis, beta(2)-adrenergic-based OX(1) and OX(2) modeling, we have determined important molecular determinants of the ligand-binding pocket of OX(1) and OX(2). The conserved residues Asp(45.51), Trp(45.54), Tyr(5.38), Phe(5.42), Tyr(5.47), Tyr(6.48), and His(7.39) were found to be contributing to both orexin-A-binding sites at OX(1) and OX(2). Among these critical residues, five (positions 45.51, 45.54, 5.38, 5.42, and 7.39) were located on the C-terminal strand of the second extracellular loop (ECL2b) and in the top of TM domains at the interface to the main binding crevice, thereby suggesting superficial OX receptor interactions of orexin-A. We found that the mutations W214A(45.54), Y223A(5.38), F227A(5.42), Y317A(6.48), and H350A(7.39) resulted in the complete loss of both [(3)H]almorexant and [(3)H]N-ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulfonyl)-amino]-N-pyridin-3-ylmethyl-acetamide (EMPA) binding affinities and also blocked their inhibition of orexin-A-evoked [Ca(2+)](i) response at OX(2). The crucial residues Gln126(3.32), Ala127(3.33), Trp206(45.54), Tyr215(5.38), Phe219(5.42), and His344(7.39) are shared between almorexant and 1-(5-(2-fluoro-phenyl)-2-methyl-thiazol-4-yl)-1-((S)-2-(5-phenyl-(1,3,4)oxadiazol-2-ylmethyl)-pyrrolidin-1-yl)-methanone (SB-674042) binding sites in OX(1). The nonconserved residue at position 3.33 of orexin receptors was identified as occupying a critical position that must be involved in subtype selectivity and also in differentiating two different antagonists for the same receptor. In summary, despite high similarities in the ligand-binding pockets of OX(1) and OX(2) and numerous aromatic/hydrophobic interactions, the local conformation of helix positions 3.32, 3.33, and 3.36 in transmembrane domain 3 and 45.51 in ECL2b provide the structural basis for pharmacologic selectivity between OX(1) and OX(2).

The positive allosteric modulator of the GABA(B) receptor, rac-BHFF, suppresses alcohol self-administration.

The present study was designed to extend to the newly synthesized rac-BHFF [(R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one] the investigation on the capacity of positive allosteric modulators of the GABA(B) receptor to reduce alcohol self-administration in rats. To this end, selectively bred Sardinian alcohol-preferring (sP) rats were initially trained to respond on a lever [on a fixed ratio 4 (FR4) schedule of reinforcement] to orally self-administer alcohol (15%, v/v) or sucrose (0.7%, w/v) in daily 30-min sessions. Once responding reached stable levels, the effect of rac-BHFF (0, 50, 100, and 200mg/kg; i.g.) on responding for alcohol and sucrose was determined. Pretreatment with rac-BHFF produced a dose-dependent suppression in responding for alcohol; reduction in the total number of responses for alcohol, in comparison to vehicle-treated rats, averaged approximately 30%, 65%, and 90% in 50, 100, and 200mg/kg rac-BHFF-treated rats, respectively. Pretreatment with 200mg/kg rac-BHFF markedly increased the latency to the first response on the alcohol lever. The effect of pretreatment with rac-BHFF on alcohol self-administration was highly specific, since (a) responding for sucrose was reduced (to approximately 50%, in comparison to vehicle-treated rats) only by pretreatment with 200mg/kg rac-BHFF, and (b) latency to the first response on the sucrose lever was completely unaltered by any rac-BHFF dose. Treatment with rac-BHFF did not alter spontaneous locomotor activity in an independent group of sP rats. The present data constitute a further piece of evidence on the capacity of positive allosteric modulators of the GABA(B) receptor to reduce alcohol's reinforcing properties in rats.

Identification of a critical residue in the transmembrane domain 2 of tachykinin neurokinin 3 receptor affecting the dissociation kinetics and antagonism mode of osanetant (SR 142801) and piperidine-based structures.

In this study, we show that compound 3 (osanetant) binds with a pseudoirreversible, apparent noncompetitive mode of antagonism at the guinea pig NK(3), while it behaves competitively at the human NK(3). This difference is caused by a slower dissociation rate of compound 3 at the guinea pig NK(3) compared to human NK(3). The only amino acid difference between the human and guinea pig NK(3) in the binding site (Thr139(2.58) in human, corresponding to Ala114(2.58) in guinea pig) has been shown to be responsible for the different behavior. Compound 1 (talnetant), however, behaves competitively at both receptors. Using these data, 3D homology modeling, and site-directed mutagenesis, a model has been developed to predict the mode of antagonism of NK(3) antagonists based on their binding mode. This model was successfully used to predict the mode of antagonism of compounds of another chemical series including piperidine-based structures at human and guinea pig NK(3).

Biochemical and electrophysiological characterization of almorexant, a dual orexin 1 receptor (OX1)/orexin 2 receptor (OX2) antagonist: comparison with selective OX1 and OX2 antagonists.

Recent preclinical and clinical research has shown that almorexant promotes sleep in animals and humans without disrupting the sleep architecture. Here, the pharmacology and kinetics of [(3)H]almorexant binding to human orexin 1 receptor (OX(1))- and human orexin 2 receptor (OX(2))-human embryonic kidney 293 membranes were characterized and compared with those of selective OX(1) and OX(2) antagonists, including 1-(5-(2-fluoro-phenyl)-2-methyl-thiazol-4-yl)-1-((S)-2-(5-phenyl-(1,3,4)oxadiazol-2-ylmethyl)-pyrrolidin-1-yl)-methanone (SB-674042), 1-(6,8-difluoro-2-methyl-quinolin-4-yl)-3-(4-dimethylamino-phenyl)-urea (SB-408124), and N-ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulfonyl)-amino]-N-pyridin-3-ylmethyl-acetamide (EMPA). The effect of these antagonists was also examined in vitro on the spontaneous activity of rat ventral tegmental area (VTA) dopaminergic neurons. [(3)H]Almorexant bound to a single saturable site on hOX(1) and hOX(2) with high affinity (K(d) of 1.3 and 0.17 nM, respectively). In Schild analyses using the [(3)H]inositol phosphates assay, almorexant acted as a competitive antagonist at hOX(1) and as a noncompetitive-like antagonist at hOX(2). In binding kinetic analyses, [(3)H]almorexant had fast association and dissociation rates at hOX(1), whereas it had a fast association rate and a remarkably slow dissociation rate at hOX(2). In the VTA, orexin-A potentiated the basal firing frequency to 175 +/- 17% of control in approximately half of the neurons tested. In the presence of 1 microM SB-674042 or SB-408124, the effect of orexin-A was only partially antagonized. However, in the presence of 1 microM EMPA or 1 microM almorexant, the effect of orexin-A was completely antagonized. In conclusion, almorexant exhibited a noncompetitive and long-lasting pseudo-irreversible mode of antagonism as a result of its very slow rate of dissociation from OX(2). The electrophysiology data suggest that OX(2) might be more important than OX(1) in mediating the effect of orexin-A on slow-firing of VTA dopaminergic neurons.

Me-talnetant and osanetant interact within overlapping but not identical binding pockets in the human tachykinin neurokinin 3 receptor transmembrane domains.

Recent clinical trials have indicated that neurokinin 3 receptor antagonists (S)-(+)-N-{{3-[1-benzoyl-3-(3,4-dichlorophenyl)-piperidin-3-yl]prop-1-yl}-4-phenylpiperidin-4-yl}-N-methylacetamine (SR142801; osanetant) and (S)-(-)-N-(alpha-ethylbenzyl)-3-hydroxy-2-phenylquinoline-4-carboxamide (SB223412; talnetant) may treat symptoms of schizophrenia. Using site-directed mutagenesis, rhodopsin-based modeling, [(3)H](S)-(-)-N-(alpha-ethylbenzyl)-3-methoxy-2-phenylquinoline-4-carboxamide (Me-talnetant) and [(3)H]osanetant binding, and functional Schild analyses, we have demonstrated the important molecular determinants of neurokinin B (NKB), Me-talnetant, and osanetant binding pockets. The residues Asn138(2.57), Asn142(2.61), Leu232(45.49), Tyr315(6.51), Phe342(7.39), and Met346(7.43) were found to be crucial for the NKB binding site. We observed that the M134(2.53)A, V169(3.36)M, F342(7.39)M, and S341(7.38)I/F342(7.39)M mutations resulted in the complete loss of [(3)H]Metalnetant and [(3)H]osanetant binding affinities and also abolished their functional potencies in an NKB-evoked accumulation of [(3)H]inositol phosphates assay, whereas the mutations V95(1.42)A, N142(2.61)A, Y315(6.51)F, and M346(7.43)A behaved differently between the interacting modes of two antagonists. V95(1.42)A and M346(7.43)A significantly decreased the affinity and potency of Me-talnetant. Y315(6.51)F, although not affecting Me-talnetant, led to a significant decrease in affinity and potency of osanetant. The mutation N142(2.61)A, which abolished the potency and affinity of osanetant, led to a significant increase in the affinity and potency of Me-talnetant. The proposed docking mode was further validated using (S)-2-(3,5-bis-trifluoromethyl-phenyl)-N-[4-(4-fluoro-2-methyl-phenyl)-6-((S)-4-methanesulfonyl-3-methyl-piperazin-1-yl)-pyridin-3-yl]-N-methyl-isobutyramide (RO49085940), from another chemical class. It is noteworthy that the mutation F342(7.39)A caused an 80-fold gain of RO4908594 binding affinity, but the same mutation resulted in the complete loss of the affinity of Me-talnetant and partial loss of the affinity of osanetant. These observations show that the binding pocket of Me-talnetant and osanetant are overlapping, but not identical. Taken together, our data are consistent with the proposed docking modes where Me-talnetant reaches deeply into the pocket formed by transmembrane (TM)1, -2, and -7, whereas osanetant fills the pocket TM3, -5, and -6 with its phenyl-piperidine moiety.

Comparison of the binding pockets of two chemically unrelated allosteric antagonists of the mGlu5 receptor and identification of crucial residues involved in the inverse agonism of MPEP.

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea], a clinically validated non-benzodiazepine anxiolytic, has been shown to be a potent and non-competitive metabotropic glutamate (mGlu)-5 receptor antagonist. In the present study, we have used the site-directed mutagenesis coupled with three-dimensional receptor-based pharmacophore modelling to elucidate the interacting mode of fenobam within the seven-transmembrane domain (7TMD) of mGlu5 receptor and its comparison with that of 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototype antagonist. The common residues involved in the recognition of MPEP and fenobam include Pro654(3.36), Tyr658(3.40), Thr780(6.44), Trp784(6.48), Phe787(6.51), Tyr791(6.55) and Ala809(7.47). The differentiating residues between both modulators' interacting modes are Arg647(3.29), Ser657(3.39) and Leu743(5.47). Our data suggest that these chemically unrelated mGlu5 antagonists act similarly, probing a functionally unique region of the 7TMD. Using [3H]inositol phosphates accumulation assay, we have also identified the critical residues involved in the inverse agonist effect of MPEP. The mutation W784(6.48)A completely blocked the inverse agonist activity of MPEP; two mutations F787(6.51)A and Y791(6.55)A, caused a drastic decrease in the MPEP inverse agonism. Furthermore, these three mutations led to an increased efficacy of quisqualate without having any effect on its potency. The fact that the residues Trp784(6.48) and Phe787(6.51) are essential equally in antagonism and inverse agonism effects emphasizes again the key role of these residues and the involvement of a common transmembrane network in receptor inactivation by MPEP.

Determination of key amino acids implicated in the actions of allosteric modulation by 3,3'-difluorobenzaldazine on rat mGlu5 receptors.

Several mutations in the seven-transmembrane region of rat metabotropic glutamate 5 (rmGlu5) receptors were produced by site-directed mutagenesis and expressed in CHO cells. Using functional intracellular calcium ([Ca(2+)](i)) mobilisation, we identified amino acids implicated in the positive allosteric modulation of quisqualate-induced response by 3,3'-difluorobenzaldazine (DFB). Human and rat mGlu5 receptors displayed a higher potency and a higher efficacy in the presence of DFB. Mutant receptors S657(3.39)C, T780(6.44)A and M801(7.39)T disrupted the DFB-mediated increase in functional response. DFB-induced increase in potency was abolished in mutant receptors N733(45.51)A, Y791(6.55)A, A809(7.47)V, P654(3.36)S/S657(3.39)C and P654(3.36)S/S657(3.39)C/L743(5.47)V without affecting the enhancement of efficacy observed in wild type receptors. Mutations at positions Leu-743(5.47) and Trp-784(6.48) resulted in significantly larger DFB-induced potentiation of EC(50) and E(max) values than in wild type receptors. DFB-mediated increase of efficacy was abolished and EC(50) values were right-shifted in mutant receptor F787A, resulting in DFB acting as a weak partial antagonist at this mutant receptor. Based on these findings, we constructed a homology model concluding that six key residues in transmembranes 3, 5, 6 and 7 are necessary for the allosteric modulation of rmGlu5a receptor by DFB. The model confirms an overlapping but distinct binding site to 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and in particular emphasises the key role of W784 in transmembrane (TM) 6 for controlling the receptor's activation state.

An automated system for the analysis of G protein-coupled receptor transmembrane binding pockets: alignment, receptor-based pharmacophores, and their application.

G protein-coupled receptors (GPCRs) share a common architecture consisting of seven transmembrane (TM) domains. Various lines of evidence suggest that this fold provides a generic binding pocket within the TM region for hosting agonists, antagonists, and allosteric modulators. Here, a comprehensive and automated method allowing fast analysis and comparison of these putative binding pockets across the entire GPCR family is presented. The method relies on a robust alignment algorithm based on conservation indices, focusing on pharmacophore-like relationships between amino acids. Analysis of conservation patterns across the GPCR family and alignment to the rhodopsin X-ray structure allows the extraction of the amino acids lining the TM binding pocket in a so-called ligand binding pocket vector (LPV). In a second step, LPVs are translated to simple 3D receptor pharmacophore models, where each amino acid is represented by a single spherical pharmacophore feature and all atomic detail is omitted. Applications of the method include the assessment of selectivity issues, support of mutagenesis studies, and the derivation of rules for focused screening to identify chemical starting points in early drug discovery projects. Because of the coarseness of this 3D receptor pharmacophore model, however, meaningful scoring and ranking procedures of large sets of molecules are not justified. The LPV analysis of the trace amine-associated receptor family and its experimental validation is discussed as an example. The value of the 3D receptor model is demonstrated for a class C GPCR family, the metabotropic glutamate receptors.

Fenobam: a clinically validated nonbenzodiazepine anxiolytic is a potent, selective, and noncompetitive mGlu5 receptor antagonist with inverse agonist activity.

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea] is an atypical anxiolytic agent with unknown molecular target that has previously been demonstrated both in rodents and human to exert anxiolytic activity. Here, we report that fenobam is a selective and potent metabotropic glutamate (mGlu)5 receptor antagonist acting at an allosteric modulatory site shared with 2-methyl-6-phenylethynyl-pyridine (MPEP), the protypical selective mGlu5 receptor antagonist. Fenobam inhibited quisqualate-evoked intracellular calcium response mediated by human mGlu5 receptor with IC(50) = 58 +/- 2 nM. It acted in a noncompetitive manner, similar to MPEP and demonstrated inverse agonist properties, blocking 66% of the mGlu5 receptor basal activity (in an over expressed cell line) with an IC(50) = 84 +/- 13 nM. [(3)H]Fenobam bound to rat and human recombinant receptors with K(d) values of 54 +/- 6 and 31 +/- 4 nM, respectively. MPEP inhibited [(3)H]fenobam binding to human mGlu5 receptors with a K(i) value of 6.7 +/- 0.7 nM, indicating a common binding site shared by both allosteric antagonists. Fenobam exhibits anxiolytic activity in the stress-induced hyperthermia model, Vogel conflict test, Geller-Seifter conflict test, and conditioned emotional response with a minimum effective dose of 10 to 30 mg/kg p.o. Furthermore, fenobam is devoid of GABAergic activity, confirming previous reports that fenobam acts by a mechanism distinct from benzodiazepines. The non-GABAergic activity of fenobam, coupled with its robust anxiolytic activity and reported efficacy in human in a double blind placebo-controlled trial, supports the potential of developing mGlu5 receptor antagonists with an improved therapeutic window over benzodiazepines as novel anxiolytic agents.

Opposite effects of Zn on the in vitro binding of [3H]LY354740 to recombinant and native metabotropic glutamate 2 and 3 receptors.

We investigated the effect of Zn on agonist binding to both recombinant and native mGlu2 and mGlu3 receptors. Zn had a biphasic inhibitory effect on recombinant mGlu2 with IC(50) values for the high- and low-affinity components of 60 +/- 10 microM and 2 +/- 0.7 mM, respectively. Zn induced a complex biphasic effect of inhibition and enhancement of [(3)H]LY354740 binding to mGlu3. Observations with a series of chimeric mGlu2/3 receptors suggest that the Zn effect resides in the N-terminal domain of mGlu2 and mGlu3. We observed that the His56 of mGlu2, which corresponds to Asp63 in mGlu3 was largely accountable for the second phase of the Zn effect. As revealed by quantitative receptor radioautography, the addition of up to 100 microm Zn to brain sections of wild-type mice resulted in significant decreases in binding density in most brain regions. In particular, the mid-molecular layer of the dentate gyrus (DGmol) and the CA1 lacunosum moleculare of hippocampus (CA1-LMol) showed reductions of 62 and 67%, respectively. In contrast, the addition of 300 microM Zn to brain sections of mGlu2(-/-) mice caused large increases in binding density of 289 and 242% in DGmol and CA1-LMol, respectively. Therefore, Zn might play a role as a physiological modulator of group II mGlu receptor function.

Distribution and abundance of metabotropic glutamate receptor subtype 2 in rat brain revealed by [3H]LY354740 binding in vitro and quantitative radioautography: correlation with the sites of synthesis, expression, and agonist stimulation of [35S]GTPgammas binding.

Until recently, there was a lack of selective radioligands for the subtypes of metabotropic glutamate (mGlu) receptors. [(3)H]LY354740 ((+)-2-aminobicyclo[3,1,0]hexane-2,6-dicarboxylic acid), a selective agonist for group II receptors (mGlu2 and -3, which are negatively coupled to cAMP production), has now been used to map their brain distribution and abundance by in vitro binding and quantitative radioautography. The selective cation dependence of its binding allowed the discrimination between mGlu2 and mGlu3 receptor labeling. Thus, in the presence of Ca(2+) and Mg(2+) ions, the agonist bound selectively to mGlu2 receptors as evidenced by: 1) the correlative distribution and abundance of binding sites (highest in the lacunosum moleculare of the hippocampus and lowest in white matter) with mGlu2 receptor mRNA and protein revealed by in situ hybridization histochemistry and immunohistochemistry, respectively; 2) its selective pharmacology; and 3) the distribution of LY354740-stimulated [(35)S]GTPgammaS binding (25-97% above basal, according to the brain region), revealing G protein-coupled receptor coupling to G(i) proteins. Nonspecific binding (in the presence of 10 muM DCG-IV, a group II-selective, mGlu2-preferring, receptor agonist) was <10% of total. In adjacent sections, the distribution of binding sites for [(3)H]DCG-IV was very similar. This extensive study paves the way for investigations of the regional expression and regulation of mGlu2 receptors in human CNS diseases, such as Alzheimer's disease, which may reveal their functional roles and identify potential therapeutic drug targets. Indeed, it has recently been demonstrated (Higgins et al. [2004] Neuropharmacology 46:907-917) that pharmacological manipulation of mGlu2 receptors influences cognitive performance in the rodent.