Differential modulation of ethanol-induced sedation and hypnosis by metabotropic glutamate receptor antagonists in C57BL/6J mice.

The amino acid glutamate is the primary excitatory neurotransmitter in the mammalian central nervous system. Glutamate receptors are divided into 2 categories: iono-tropic receptors (iGluRs), glutamate-gated cation channels including NMDA, α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate responsive receptors, and metabotropic receptors (mGluRs), a heterogeneous family of 7-transmembrane G-protein coupled receptors. Eight mGluR subtypes have been cloned. These receptors are divided into 3 broad groups based on amino acid sequence similarity, agonist pharmacology, and second messenger coupling. Group I mGlu receptors (mGluR1 and mGluR5) stimulate inositol phosphate metabolism and mobilization of intracellular Ca2+, whereas group II (mGluR2 and mGluR3) and group III (mGluR4, -R6, -R7, and -R8) inhibit adenylyl cyclase and reduce synaptic transmission (Gereau and Conn, 1995; Kew and Kemp, 2005; Pin and Duvoisin, 1995). In contrast to the iGluRs, which are responsible for the fast, excitatory responses to glutamate, mGluRs mediate the slower, modulatory responses to glutamate. In this capacity, mGluRs can modulate neurotransmission at both glutamatergic and nonglutamatergic synapses (Benquet et al., 2002; Diaz-Cabiale et al., 2002). The availability of selective pharmacological agents has begun to reveal basic functional roles for the group I and II mGluR subtypes (Kew and Kemp, 2005). Group I selective agonists have been shown to increase glutamate and GABA levels in vivo (de Novellis et al., 2003) and enhance glutamate-evoked depolarization (Doherty et al., 1997; Pisani et al., 2001) and GABA-gated Cl− currents (Hoffpauir and Gleason, 2002). Group II receptors are predominantly presynaptic autoreceptors (Cartmell and Schoepp, 2000) and selective activation of these receptors has been shown to reduce glutamate-evoked excitatory postsynaptic currents through inhibition of glutamate release (Shen and Johnson, 2003). mGluR selective ligands have also demonstrated behavioral activity in rodent models of anxiety and epilepsy. Group I antagonists and group II agonists have been shown to have anxiolytic (Klodzinska et al., 1999; Spooren et al., 2000) and anticonvulsant properties (Chapman et al., 2000; Moldrich et al., 2001). Emerging evidence implicates mGluR function in ethanol’s neurobiological and behavioral effects. Ethanol alters neuronal firing rates (Netzeband et al., 1997) and Ca2+ levels (Gruol et al. 1997) mediated by mGluRs in vitro. Chronic exposure to ethanol reduces mGluR1 mRNA levels in cerebellar Purkinje neurons of mice (Simonyi et al., 1996), and early withdrawal from ethanol leads to alterations in mGluR-evoked Ca2+ signaling in cerebellar neurons (Netzeband et al., 2002). In rats, chronic exposure to an ethanol containing liquid diet decreased mRNA levels for mGluR3 and mGluR5 in the dentate gyrus, whereas mGluR1, mGluR5, and mGluR7 mRNA was decreased in the CA3 regions of the hippocampus (Simonyi et al., 2004). Neurobehaviorally, mGluR5 selective antagonists reduce ethanol self-administration in mice and rats (Cowen et al., 2005, 2007; Hodge et al., 2006; Lominac et al., 2006), decrease relapse to ethanol self-administration in rats (Backstrom et al., 2004; Schroeder et al., 2005), and block the discriminative stimulus effects of ethanol (Besheer and Hodge, 2005; Besheer et al., 2006). The purpose of this study was to investigate the role of specific mGluRs in the acute sedative hypnotic effects of ethanol in mice. The 3 selective, systemically active mGluR antagonists 2-methyl-6-(phenylethynyl)pyridine (MPEP; mGluR5), 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt; mGluR1), and LY341495 (mGluR2/ 3) were examined for their ability to alter ethanol-induced inhibition of spontaneous locomotor activity (sedation) and ethanol-induced loss of righting reflex (LORR) (hypnosis). To explore potential mechanisms by which mGluRs might modulate responses to ethanol, these antagonists were also tested with other sedative-hypnotics: the GABAA positive modulators pentobarbital and midazolam, and the NMDA antagonist ketamine. Blood-ethanol clearance studies were conducted to ensure that the observed effects were not due to alterations in ethanol clearance.