Medium supplementation with zinc enables detection of imipramine-induced adaptation in glycine/NMDA receptors labeled with [3H]L-689,560

Antidepressant drugs after chronic administration induce adaptive changes in the NMDA receptor complex. Radioligand-receptorbinding studies using 3H]5,7-dichlorokynurenic acid demonstrated a "down-regulation" of the glycine site/NMDA receptor following chronic treatment with antidepressants and electroconvulsive shock. However, binding procedure using this radioligand is time consuming because it requires the use of centrifugation method in the separation process. The introduction of a new radioligand of glycine/NMDA receptor, 3H]L-689,560 enables the application of a rapid filtration method. In the present study we demonstrate that 2-week treatment with imipramine (15 mg/kg ip) did not evoke alterations in specific 3H]L-689,560 binding and in IC50 value of glycine in displacing 3H]L-689,560 binding in the mouse or rat cortex. However, longer, a 4-week treatment with imipramine induced a significant 71% increase in IC50 value in displacing 3H]L-689.560 binding in the mouse cortex. Moreover, the presence of zinc in the incubation media, dose-dependently enhances detection of imipramine-induced increase in IC50 value of glycine in displacing 3H]L-689,560 binding in the rat cortex. The present data indicate that: (1) 3H]L-689,560 may be a suitable ligand for assessing adaptation of the glycine/NMDA sites and (2) the presence of zinc enhances detection of imipramine-induced reduction of glycine affinity for glycine/NMDA receptors labeled with 3H]L-689,560 which further indicates a significance of zinc in the mechanism of antidepressant treatment.