Effects of Acamprosate on Neuronal Receptors and Ion Channels Expressed in Xenopus Oocytes

Relapse into drug taking after a period of sobriety is a major problem in the treatment of addiction and in the treatment of alcoholism especially (Stalcup et al., 2006). Efforts have focused on the development of pharmacological agents that have the potential to curb the drug craving which precedes and usually precipitates a relapse (Heilig and Egli, 2006). Acamprosate (calcium acetylhomotaurine) was approved first in France in 1989 for the treatment of alcohol dependence and later in the United States in 2002. Over 1.4 million alcohol-dependent patients world-wide have been treated with acamprosate, which shows moderate efficacy at reducing relapse (Mason, 2001). However, attempts at determining the site of action of acamprosate in the central nervous system have produced equivocal results (De Witte et al., 2005; Littleton and Zieglgansberger, 2003). Mouse models of alcoholism have shown clear effects of acamprosate on behavioral responses to alcohol. Mutant mice that have increased brain glutamatergic tone drink large quantities of alcohol, and acamprosate administration reduces both the excessive alcohol consumption and high glutamate tone (Spanagel et al., 2005). In addition to reducing alcohol consumption, acamprosate inhibits the acute activating effects of alcohol and locomotor sensitization in wild-type mice (Kotlinska et al., 2006). Alcohol locomotor sensitization is also reduced by acamprosate to a greater degree in a high alcohol-preferring mouse line compared to a low alcohol-preferring mouse line (Chester et al., 2001). Acamprosate appears to alter conditioned responses to alcohol. For example, acamprosate inhibits the development of conditioned place preference to alcohol (McGeehan and Olive, 2003) and conditioned abstinence behavior from repeated alcohol exposure, a measure of alcohol negative reinforcement (Cole et al., 2000). Most studies aimed at identifying a mechanism of action of acamprosate have focused on the glutamatergic system (De Witte et al., 2005). However, the majority of these studies are difficult to interpret because the concentrations of acamprosate tested were at least 10 times greater than that seen in humans during treatment (Johnson et al., 2003; Mason et al., 2002). For example, in 2 independent clinical trials, healthy patients were administered 1 to 3 g of acamprosate orally once a day for 6 days, and the peak plasma concentration of acamprosate achieved was approximately 1 μM (Johnson et al., 2003; Mason et al., 2002). Early studies suggested that acamprosate at high concentrations (1 mM) inhibits glutamate receptor activated responses in vitro and in vivo (Zeise et al., 1993), and 300 μM acamprosate enhanced N-methyl-D-aspartate (NMDA) receptor function in hippocampal and accumbens brain slices (Berton et al., 1998; Madamba et al., 1996). Furthermore, additional studies on NMDA receptors concluded that acamprosate may interact with the polyamine site as a partial agonist (Naassila et al., 1998; Popp and Lovinger, 2000; al Qatari et al., 2001), while another study suggested that acamprosate may have weak antagonistic properties at NMDA receptors (Rammes et al., 2001). Finally, one study suggested that acamprosate may inhibit metabotropic glutamate receptors (mGluR), in particular mGluR5 (Harris et al., 2002). None of these studies detected direct actions of clinically relevant (e.g., 1 to 3 μM) concentrations of acamprosate on receptor or channel function. Other neurotransmitter systems have been less studied with respect to the site of action of acamprosate. It is plausible that acamprosate may interact with inhibitory neurotransmitter systems, such as GABA and glycine because of the similarity in structure between acamprosate and these neurotransmitters (Olive, 2002). However, there is no direct evidence that acamprosate modulates the function of GABAA or glycine receptors. Finally, voltage-gated channels could also represent a target for the action of acamprosate in the central nervous system because of their important role in cellular excitability. In this study, we tested the effects of acamprosate on several metabotropic receptors including, mGluR1, mGluR5 and M1 muscarinic receptors. We also determined whether acamprosate has any effects on ligand-gated and voltage-gated channels including: GABAA, glycine, vanilloid-1 receptors (VR1) and voltage-gated Na+ channels (Nav). We chose these candidate receptors and channels in part because with the exception of mGluR1, all of them are affected by ethanol in vitro. In addition, we tested the ability of acamprosate to alter ethanol responses on some of these receptors. We found no effects of acamprosate on these receptors or ion channels when recombinantly expressed in Xenopus oocytes.