Orser B A, Pennefather P S, MacDonald J F
Anaesthesia Research Laboratory, University of Toronto, Ontario, Canada.
Anesthesiology. 1997 Apr;86(4):903-17. doi: 10.1097/00000542-199704000-00021.
The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is blocked by ketamine, and this action likely contributes to ketamine's anesthetic and analgesic properties. Previous studies suggest that ketamine occludes the open channel by binding to a site located within the channel pore. This hypothesis was examined by investigating the effects of ketamine on single-channel currents from NMDA receptors.
The cell-attached and outside-out configurations of the patch clamp technique were used to study NMDA-activated currents recorded from cultured mouse hippocampal neurons.
In cell-attached patches, 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 microM or 1 microM) to the pipette solution decreased Po' to 53% and 24% of control values, respectively. At 1 microM ketamine, this reduction was due to a decrease in both the frequency of channel opening and the mean open time (44% and 68% of control values, respectively). Ketamine did not influence channel conductance and no new components were required to fit the open- or closed-duration distributions. Ketamine (50 microM), applied outside the recording pipette, reduced the opening frequency of channels recorded in the cell attached configuration. This observation suggests that ketamine 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 microM and 10 microM ketamine reduced Po' to 63% and 34% of control values, respectively, and this reduction was due primarily to a decrease in the frequency of channel opening with little change in mean open time.
These observations are consistent with a model whereby ketamine inhibits the NMDA receptor by two distinct mechanisms: (1) Ketamine blocks the open channel and thereby reduces channel mean open time, and (2) ketamine decreases the frequency of channel opening by an allosteric mechanism.
氯胺酮可阻断谷氨酸受体的N-甲基-D-天冬氨酸（NMDA）亚型，这一作用可能是氯胺酮具有麻醉和镇痛特性的原因。以往研究表明，氯胺酮通过结合通道孔内的位点来堵塞开放通道。通过研究氯胺酮对NMDA受体单通道电流的影响来检验这一假说。
采用膜片钳技术的细胞贴附式和外向式记录模式，研究培养的小鼠海马神经元中NMDA激活的电流。
在细胞贴附式膜片中，NMDA诱发的电流平均开放时间（tau o）明显为3.26毫秒。至少有一个通道开放的概率（Po'）为0.058。向移液管溶液中加入氯胺酮（0.1微摩尔或1微摩尔）后，Po'分别降至对照值的53%和24%。在1微摩尔氯胺酮作用下，这种降低是由于通道开放频率和平均开放时间均减少（分别为对照值的44%和68%）。氯胺酮不影响通道电导，且拟合开放或关闭持续时间分布不需要新的成分。在记录移液管外部施加50微摩尔氯胺酮，可降低细胞贴附式记录模式下记录的通道开放频率。这一观察结果表明，氯胺酮通过扩散穿过疏水细胞膜进入结合位点。在外向式膜片中，氯胺酮的效力低于细胞贴附式膜片中观察到的效力：1微摩尔和10微摩尔氯胺酮分别将Po'降至对照值的63%和34%，这种降低主要是由于通道开放频率降低，平均开放时间变化不大。
这些观察结果与一种模型一致：氯胺酮通过两种不同机制抑制NMDA受体：（1）氯胺酮阻断开放通道，从而缩短通道平均开放时间；（2）氯胺酮通过变构机制降低通道开放频率。
