S 14506: novel receptor coupling at 5-HT1A receptors
Introduction
The presence of diverse functional consequences of the specific interaction of drugs at single receptors may be underestimated. Agonists have been hypothesised to allow the possibility of “differential trafficking” by specifically activating certain subsets of G proteins after induction of particular conformational states of a receptor (Kenakin, 1995) and some evidence has been obtained that agonists at the 5-HT1A receptor may induce differential coupling to different G proteins (Gettys et al., 1994), which might lead to different pharmacological profiles for the different agonists concerned. The rich diversity of the different coupling pathways activated by 5-HT1A receptors has been reviewed by Raymond et al. (1999). Nevertheless, drugs that are very close structural congeners [e.g., the partial agonists at 5-HT1A receptors: buspirone, ipsapirone, tandospirone, gepirone, Fig. 1(A)] would be expected to interact with the receptor in a very similar way (Jacoby et al., 1999), which may explain why the pharmacology is essentially the same for these drugs and why there has been no major improvement over the first drug, buspirone. However, contrasting pharmacological profiles have been shown in binding experiments from agonists, antagonists or inverse agonists acting at 5-HT1A receptors (McLoughlin and Strange, 2000).
5-HT1A receptors are important targets for drug action in that presynaptic 5-HT1A receptors in the raphe nuclei control 5-HT release throughout the brain, and postsynaptic 5-HT1A receptors in, for example, the hippocampus are important for memory and stress modulation. However, with the possible exception of the weak anxiolytic, buspirone, the therapeutic potential of this target has not been realised. This may be because both anxiogenic and anxiolytic effects follow from stimulating different sites within the 5-HT1A receptor (Spedding et al., 1998).
Spiperone is a potent inverse agonist at 5-HT1A receptors (Barr and Manning, 1997; Newman-Tancredi et al., 1997a, Newman-Tancredi et al., 1997b; Kellett et al., 1999; McLoughlin and Strange, 2000). S 14506, although an analogue of spiperone [Fig. 1(B)], has been reported to be a highly potent agonist at 5-HT1A receptors in behavioural tests, and to be a potent anxiolytic (Colpaert et al., 1992). S 14506 fully substitutes for the 5-HT1A receptor cue of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in drug discrimination paradigms in the rat, with an EC50 of 9 μg/kg subcutaneous. (Schreiber et al., 1995). However, the binding of [ 3H]-S 14506 to 5-HT1A receptors in brain membranes is atypical (Lima et al., 1997). Furthermore, a distictive feature of the drug was the steep Hill slope (nH=1.4) in displacing [ 3H]-8-OH-DPAT binding (Newman-Tancredi et al., 1998; Assie et al., 1999), which may be associated with positive cooperativity (Forsén and Linse, 1995; Assie et al., 1999). We have assessed the binding characteristics and agonist properties of this drug in an attempt to explain its exceptional potency and potential positive cooperativity. In order to assess whether S 14506 showed any selective activation of 5-HT1A receptors coupled to Gi or Go, 5-HT1A receptor–G protein (Gi1α or Go1α) fusions were used (Milligan, 2000). It is known that 5-HT1A receptors couple to Gi and Go proteins, and it has recently been shown that the receptors couple to Gz proteins in the hypothalamus, in both in vitro and in vivo (Serres et al., 2000). Differential coupling is therefore possible at this receptor (Raymond et al., 1999).
Because of the unique effects observed with S 14506 we have also modelled the compound in the 5-HT1A receptor using the three-site model developed to explain the unique pharmacology of RS-30199, an anxiolytic agent acting at 5-HT1A receptors (Spedding et al., 1998; Jacoby et al., 1999). The three-site model allows the interaction of drugs with different sites within the same receptor, and permits the explanation of several distinct structure/activity relationships between chemical series. This principle has been established for many years in ion channnel pharmacology, where drugs may act at different sites, which interact allosterically, at the alpha subunit of calcium channels. Binding at different sites confers distinct structure/activity, allosteric interactions between the sites, and distinct pharmacological and clinical profiles (Spedding, 1985; Spedding and Paoletti, 1992). Binding at three (or more) sites in G-protein-coupled receptors (GPCRs) provides an analagous rationale for distinct pharmacology, and is an extension of Kenakin's (1995) proposition of differential coupling. The transduction mechanisms of GPCRs are becoming clear at a molecular level. An Asp–Arg–Tyr (DRY in standard one-letter code) domain located at the bottom of transmembrane helix 3 (TM3) is a common feature of rhodopsin-like GPCRs and is considered crucial for G protein activation; activation is dependent on an arginine shift out of a lipophilic pocket at the intracellular side of the receptor (Oliveira et al., 1994). We report that S 14506 is the first drug which may span from the agonist binding site to activate the DRY domain, producing a unique and powerful activation mechanism. Preliminary reports of some of this work have been communicated (Dacquet et al., 1994, Dacquet et al., 1996).
Section snippets
Recombinant h 5-HT1A receptors
Membranes were prepared from recombinant Chinese hamster ovary (CHO) cells stably expressing the human 5-HT1A receptor (Newman-Tancredi et al., 1992, Newman-Tancredi et al., 1997a, Newman-Tancredi et al., 1997b). Cells grown in suspension culture were harvested by centrifugation and homogenised in buffer [20 mM N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES), pH 7.5 and 5 mM MgSO4] using a Kinematica Polytron. The homogenate was centrifuged at 50,000g for 30 min and the membrane
Hippocampal membranes
[ 3H]-S 14506 binding to 5-HT1A receptors from rat hippocampal membranes was saturable and its displacement by S 14506 was of high affinity. The Kd for [ 3H]-S 14506 binding was 0.79±0.2 nM (Bmax=400±32 fmol/mg protein). S 14506 displaced 100% of specific [ 3H]-S 14506 binding with a pKi of 9.1±0.2, nH=1.03±0.07. [ 3H]-8-OH-DPAT specific binding was characterised as having a Kd of 1.5±0.5 nM and a Bmax of 303±20 fmol/mg protein. However, the specific binding of [ 3H]-S 14506 was increased in a
Discussion
S 14506 is one of the most potent agonists at 5-HT1A receptors thus far reported. The compound is highly potent in both in vitro and in vivo conditions (Colpaert et al., 1992; Goudie and Leathley, 1994; Schreiber et al., 1994, Schreiber et al., 1995; Lima et al., 1997; Newman-Tancredi et al., 1998; Assie et al., 1999). [ 3H]-S 14506 is a highly potent radioligand and S 14506 was the most potent agent in displacing [ 3H]-S 14506 binding. Spiperone, a close structural analogue and an inverse
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