Figure 5. Activation of GluN2B-containing NMDARs by ambient glutamate regulates mEPSC frequency and depression-like behavior.
Whole-cell recordings of cortical pyramidal neurons (Vhold −65 mV) under control conditions (ACSF) and in 0Mg2+ ACSF were used to test the subunit contribution of the NMDA receptor pool activated by ambient glutamate. (A) Perfusion of 0Mg2+ ACSF uncovered a tonic current in cultured cortical neurons (14 DIV) evidenced by an increase in baseline current noise that could be reversed by the NMDAR antagonist APV. This tonic NMDAR-mediated current was completely absent in GluN2B null neurons, including those in which GluN2B had been genetically replaced by GluN2A (GluN2B→2A). (B) Ambient glutamate also evoked a tonic current in acute brains slices that was suppressed by ketamine and was absent in 2BΔCtx slices (P21). (C–D) Manipulation of tonic activation of GluN2B-containing NMDARs alters mEPSC frequency in cortical cultures and pyramidal neurons in vivo, as well as expression of depression-like behavior. (C) Acute treatment of cultures with the glutamate transporter antagonist dl-TBOA enhanced the evoked current, while NDGA, which enhances glutamate transporter function, decreased this current. (D) Elevating ambient glutamate by blocking transporters with dl-TBOA, or decreasing it by enhancing glutamate transporter function with NDGA, or upregulating glutamate transporter expression using ceftriaxone bidirectionally regulated mEPSC frequency in cultured cortical neurons. (E) Enhancing glutamate transporter expression by i.p. injection of ceftriaxone resulted in a decrease in mEPSC IEI, and significant antidepressant-like action in control mice. (F) Injections of NDGA resulted in a significant decrease in mEPSC IEI in pyramidal neurons and decreased immobility in the TST. The effect of NDGA was mimicked and occluded on both measures in 2BΔCtx mice. Data points are mean ± sem. p-values are *<0.05, **<0.01, ***<0.001, and n values are shown for each experiment. n.c. = no significant change.