Mechanism of action of acamprosate. Part II. Ethanol dependence modifies effects of acamprosate on NMDA receptor binding in membranes from rat cerebral cortex

Acamprosate is a putative anticraving drug used to maintain abstinence in alcohol-dependent patients. Its mechanism of action is uncertain, but the drug is thought to interact with neuronal NMDA receptors and calcium channels, and these proteins are implicated in the induction of alcohol dependence. In these experiments, the effects of acamprosate were studied on the binding of the NMDA receptor ligand [³H]dizocilpine to rat brain membranes under nonequilibrium conditions; 10 μM glutamate and 1 μM glycine were present in the binding assays to partially activate the receptor. At clinically relevant concentrations (in the micromolar range), acamprosate significantly enhanced [³H]dizocilpine binding to cortical membranes from control animals (suggesting that acamprosate may increase the rate of association of the radioligand), whereas at higher concentrations binding was inhibited. This effect is consistent with a partial agonist effect of acamprosate on the NMDA receptor protein. However, when rats were made dependent on ethanol (exposure to the drug for 10 days by inhalation) and cortical membranes were prepared from these animals, acamprosate in vitro no longer produced any enhancement of [³H]dizocilpine binding. Similar results were obtained when membranes were used from rats that had received 400 mg/kg/day of acamprosate in their drinking water with or without concurrent ethanol inhalation for 10 days. Thus, in brain membranes from all these treatment groups, acamprosate in vitro caused inhibition of [³H]dizocilpine binding only. The results suggest that acamprosate may have excitatory or inhibitory effects on NMDA receptors, depending on the experimental conditions. The effects of the drug on this system appear to be shifted toward inhibition in alcohol dependence, and this finding may be important to its clinical mechanism.