Intracellular MK801 does not fully block NMDA receptors

MK801 is an open-channel blocker of the NMDA receptor pore. That is, when the receptor is activated and the channel opens, MK801 enters the pore and blocks it. Usually, MK801 is used extracellularly, but multiple studies have included it in the intracellular pipette solution during whole-cell patch clamp recordings. We show here that MK801 has a much lower effective affinity for NMDA receptors when used intracellularly, suggesting caution is needed when interpreting such studies.