Introduction
Spreading depression (SD)-like depolarizations may augment neuronal damage in neurovascular disorders such as stroke, traumatic brain injury, and subarachnoid hemorrhage. Under physiological conditions, SDs are blocked by N-methyl-D-aspartate receptor (NMDAR) antagonists. However, under pathological conditions, SD-like depolarizations occur in presence of an increased extracellular potassium concentration ([K+]o). We here tested whether this increase in baseline [K+]o would reduce the efficacy of NMDAR antagonists to block SD-like depolarizations in human and rat brain slices.
Methods
We prepared acute neocortical slices from rats and from patients undergoing surgery for pharmacoresistant epilepsy. SD was recorded by two K+-sensitive/reference microelectrodes and by measuring intrinsic optical signals. To investigate SD under high [K+]o, the extracellular K+ concentration in the artificial cerebrospinal fluid ([K+]ACSF) was increased in a stepwise manner until SD occurred spontaneously. To study SD under physiological [K+]o, SD was evoked by a local neocortical microinjection of 3 M KCl using a glass capillary. The effects of the competitive NMDAR antagonist D-2-amino-5-phosphonovaleric acid (2-APV, 30 μM) and the non-competitive NMDAR antagonist dizocilpine (MK-801, 20 μM) were investigated.
Results
In slices perfused with 2-APV in which [K+]ACSF was increased in a stepwise manner (n = 6), SD occurred when [K+]o had reached 12.3 ± 1.7 mM. Following post-SD recovery, slices were perfused with 2-APV under continuously elevated [K+]ACSF. No SD occurred under this condition, whereas SDs were detected at the same [K+]ACSF in control slices of the contralateral hemisphere. Subsequently, [K+]ACSF was further raised by 5 mM during continuous perfusion with 2-APV, inducing SD in all slices. The same protocol was followed in another group (n = 6), but MK-801 was applied instead. The first SD occurred when [K+]o had reached 11.6 ± 3.1 mM. No further SD was detected during perfusion with MK-801 at this [K+]ACSF, contrarily to control slices. When [K+]ACSF was further increased by 5 mM, SD occurred in all slices perfused with MK-801. In human slices in which [K+]ACSF was increased in a stepwise manner (n = 5), SD occurred when [K+]o reached 23.9 ± 3.6 mM. Subsequently, MK-801 was perfused at the elevated [K+]ACSF. No further SD occurred during that period, whereas SDs were seen in neighboring slices that served as controls. Subsequently, [K+]ACSF was further raised by 5 mM during continuous perfusion with MK-801, inducing SD in all slices. In slices perfused with physiological [K+]ACSF, in which SD was triggered by a local KCl microinjection, SD was completely blocked by either MK-801 or 2-APV, whereas repetitive SDs were inducible in control slices.
Discussion
The efficacy of NMDAR antagonists to block SD was significantly reduced by high [K+]ACSF in rat and human brain slices. Our data suggest that a pathological rise in baseline [K+]o may critically reduce the efficacy of NMDAR inhibitors on SD-like depolarizations in cortical areas moderately affected by energy depletion, e.g., in the ischemic penumbra.