Ketamine, a general anesthetic, has rapid and sustained antidepressant effects when administered at lower doses. Anesthetic levels of ketamine reduce excitatory transmission by binding deep into the pore of NMDA receptors where it blocks current influx. In contrast, the molecular targets responsible for antidepressant levels of ketamine remain controversial. We used electrophysiology, structure-based mutagenesis, and molecular and kinetic modeling to investigate the effects of ketamine on NMDA receptors across an extended range of concentrations. We report functional and structural evidence that, at nanomolar concentrations, ketamine interacts with membrane-accessible hydrophobic sites on NMDA receptors, which are distinct from the established pore-blocking site. These interactions stabilize receptors in pre-open states and produce an incomplete, voltage- and pH-dependent reduction in receptor gating. Notably, this allosteric inhibitory mechanism spares brief synaptic-like receptor activations and preferentially reduces currents from receptors activated tonically by ambient levels of neurotransmitters. We propose that the hydrophobic sites we describe here account for clinical effects of ketamine not shared by other NMDA receptor open-channel blockers such as memantine and represent promising targets for developing safe and effective neuroactive therapeutics.

氯胺酮是一种全身麻醉剂，在较低剂量下具有快速且持续的抗抑郁作用。麻醉水平的氯胺酮通过深入结合到 NMDA 受体的孔中来阻止电流流入，从而减少兴奋性传递。相比之下，决定氯胺酮抗抑郁水平的分子靶点仍然存在争议。我们使用电生理学、基于结构的诱变以及分子和动力学模型来研究氯胺酮在较大浓度范围内对 NMDA 受体的影响。我们报告的功能和结构证据表明，在纳摩尔浓度下，氯胺酮与 NMDA 受体上的膜可接近疏水位点相互作用，该位点与已建立的孔阻塞位点不同。这些相互作用将受体稳定在开放前状态，并在受体门控中产生不完全的、电压和 pH 依赖性的减少。值得注意的是，这种变构抑制机制避免了短暂的突触样受体激活，并优先减少由环境水平的神经递质激活的受体的电流。我们提出，我们在此描述的疏水位点解释了氯胺酮的临床效应，而其他 NMDA 受体开放通道阻滞剂（例如美金刚）则不具有这种临床效应，并且代表了开发安全有效的神经活性疗法的有希望的目标。