Fig. 5. Reservoir pool size of NMDARs and recovery rate from ketamine blockade also contribute to brain region specificity.

(A) NMDAR-eEPSCs (normalized by baseline) during incubation and washout of 10 µM or 1 mM ketamine in LHb or CA1 PYR neurons. Right: bar graphs showing NMDAR-eEPSCs at the end of the 10-min perfusion period and at 50 to 60 min after perfusion (LHb group: P = 0.25, paired t test; CA1 10 µM group: P = 0.01, paired t test; CA1 10 µM group: P = 0.02, Wilcoxon matched-pairs test). n = 9. (B and D) Schematics illustrating how synaptic blockade [(B) for conditions in (A)] and agonist-induced blockade [(D) for conditions in (C)] of NMDARs by ketamine are affected by lateral movement of NMDARs in and out of synapse. Black circles represent synaptic sites. Blue circles represent the area where NMDARs can be opened by corresponding treatment [synaptic stimulation in (A) or agonist perfusion in (C)]. Red dots represent ketamine. (C) NMDAR-eEPSCs (normalized by baseline) during incubation and washout of ketamine (1 mM) and NMDA (20 µM) in CA1 PYR neurons (n = 5). Right: bar graphs showing NMDAR-eEPSCs at the end of the 5-min perfusion period and at 50 to 60 min after perfusion (P > 0.99, Wilcoxon matched-pairs test). n = 5. (E) Experimental paradigm for slice recording to estimate the proportion of synaptic NMDAR-eEPSCs in total NMDAR currents. (F and G) Bar graphs illustrating the total NMDAR currents [P < 0.0001, Mann-Whitney test (F)] and estimated proportion of synaptic NMDAR [P = 0.001, Mann-Whitney test (G)] of LHb and CA1 PYR neurons. Estimated proportion of synaptic NMDAR is calculated as maximal NMDAR-eEPSCs divided by the total NMDAR currents. n = 20 in the LHb group and n = 14 cells in the CA1 group. (H) Schematic model illustrating why systemic ketamine has a stronger and more sustained blockade of NMDARs in the LHb, but not hippocampal CA1 PYR neurons, under a depressive state. The high basal activity allows LHb neurons for ketamine’s open-channel blockade, and the small reservoir pool and the trapping effect are responsible for a slow recovery of NMDAR transmission. By contrast, in hippocampal CA1 neurons, which are not as active under a depressive-like state, the available pool of open NMDARs for ketamine blockade is small to start with. After this small pool is inhibited, the large, extrasynaptic reservoir pool of NMDARs quickly exchanges with the blocked ones through lateral movement, resulting in a rapid recovery of NMDAR transmission. Circles represent synaptic areas where NMDARs can bind to synaptically released glutamate. *P < 0.05; ***P < 0.001; ****P < 0.0001; NS, not significant. Error bars indicate SEM.