Pharmacology of (S)-homoquisqualic acid and (S)-2-amino-5-phosphonopentanoic acid [(S)-AP5] at cloned metabotropic glutamate receptors

Abstract

1. In this study we have determined the pharmacological profile of (S)-quisqualic acid, (S)-2-amino-4-phosphonobutyric acid ((S)-AP4) and their higher homologues (S)-homoquisqualic acid, (S)-2-amino-5-phosphonopentanoic acid ((S)-AP5), respectively, and (R)-AP5 at subtypes of metabotropic (S)-glutamic acid (mGlu) receptors expressed in Chinese hamster ovary cells.

2. (S)-Quisqualic acid was a potent mGlu₁/mGlu₅ agonist (EC₅₀ values of 1.1 μM and 0.055 μM, respectively) showing no activity at mGlu₂ and weak agonism at mGlu₄ (EC₅₀∼1000 μM).

3. (S)-Homoquisqualic acid displayed competitive antagonism at mGlu₁ (K_B=184 μM) and full agonism at mGlu₅ (EC₅₀=36 μM) and mGlu₂ (EC₅₀=23 μM), but was inactive at mGlu₄.

4. (S)-AP4 was a potent and selective mGlu₄ agonist (EC₅₀=0.91 μM) being inactive at mGlu₁, mGlu₂ and mGlu₅ both as agonist and antagonist.

5. (S)-AP5 displayed very weak agonist activity at mGlu₄. At the mGlu₂ receptor subtype (S)-AP5 acted as a competitive antagonist (K_B=205 μM), whereas the compound was inactive at mGlu₁ and mGlu₅. (R)-AP5 was inactive at all mGlu receptor subtypes tested both as agonist and antagonist.

6. These studies demonstrate that incorporation of an additional carbon atom into the backbone of (S)-glutamic acid and its analogues, to give the corresponding homologues, and replacement of the terminal carboxyl groups by isosteric acidic groups have profound effects on the pharmacological profiles at mGlu receptor subtypes. Furthermore, (S)-homoquisqualic acid has been shown to be a potentially useful tool for differentiating mGlu₁ and mGlu₅.

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