MULTIPLE MECHANISMS OF KETAMINE BLOCKADE OF N-METHYL-D-ASPARTATE RECEPTORS

Background: The N-methyl-D-aspartate (NMDA) subtype of glutamate recep tor is blocked by ketamine, and this action likely contributes to keta mine's anesthetic and analgesic properties. Previous studies suggest t hat ketamine occludes the open channel by binding to a site located wi thin the channel pore. This hypothesis was examined by investigating t he effects of ketamine on single-channel currents from NMDA receptors. Methods: The cell-attached and outside-out configurations of the patc h clamp technique were used to study NMDA-activated currents recorded from cultured mouse hippocampal neurons. Results: In cell-attached pat ches, NMDA evoked currents that had an apparent mean open time (tau(o) ) of 3.26 ms. The probability of at least one channel being open (Po') was 0.058. The addition of ketamine (0.1 mu M or 1 mu M) to the pipet te solution decreased Po' to 53% and 24% of control values, respective ly. At 1 mu M ketamine, this reduction was due to a decrease in both t he frequency of channel opening and the mean open time (44% and 68% of control values, respectively). Ketamine did not influence channel con ductance and no new components were required to fit the open- or close d-duration distributions. Ketamine (50 mu M), applied outside the reco rding pipette, reduced the opening frequency of channels recorded In t he cell-attached configuration. This observation suggests that ketamin e gained access to a binding site by diffusing across the hydrophobic cell membrane. In outside-out patches, ketamine potency was lower than that observed in cell-attached patches: 1 mu M and 10 mu M ketamine r educed Po' to 63% and 34% of control values, respectively, and this re duction was due primarily to a decrease in the frequency of channel op ening with little change in mean open time. Conclusions: These observa tions are consistent with a model whereby ketamine inhibits the NMDA r eceptor by two distinct mechanisms: (1) Ketamine blocks the open chann el and thereby reduces channel mean open time, and (2) ketamine decrea ses the frequency of channel opening by an allosteric mechanism.