D-cycloserine enhances rapid tolerance to ethanol motor incoordination.

In a recent study, we showed that D-cycloserine, an agonist at the glycine site of the NMDA receptor, enhances the development of rapid tolerance to ethanol. In the present study, we report that the acquisition of rapid tolerance to the motor incoordination effect of ethanol (tilt-plane test) was increased only when D-cycloserine was injected before, but not after, the intoxicated practice under ethanol. The effect of D-cycloserine on tolerance when this agonist was administered in divided doses before and after test was similar to that obtained when D-cycloserine was injected before test. Higher doses of D-cycloserine did not produce a further enhancement of rapid tolerance. Moreover, when the dose of ethanol on day 1 was large enough to induce rapid tolerance per se, D-cycloserine did not further enhance the tolerance. The enhancement of tolerance by D-cycloserine was antagonized by previous administration of ketamine. The enhancement of ethanol tolerance by D-cycloserine and the antagonism of this effect by ketamine cannot be attributed to changes in pharmacokinetics of ethanol. Taken together, these results confirm the participation of the NMDA receptor system in the development of tolerance to ethanol, and reinforce earlier findings about the involvement of learning in tolerance.