N-methyl-D-aspartate receptors contribute to the baseline noise of retinal ganglion cells

Whole-cell recordings of tiger salamander ganglion cells were obtained using a superfused retinal slice preparation. Membrane current fluctuations were recorded under voltage-clamp conditions with cells usually held at -70 mV. Current fluctuations at rest (Mg²⁺ = 1 mM) were reduced by adding D-2-amino-7-phosphonoheptanoate (D-AP7). Resting fluctuations were increased by adding N-methyl-D-aspartate (NMDA) or by removing extracellular Mg ²⁺. These increased fluctuations were blocked by D-AP7. Blocking NMDA receptors under control conditions also reduced a tonic inward current by -1 to -15 pA. Fluctuation analysis of current noise shows that the noise power spectrum measured in the presence of NMDA is similar to that measured under Mg²⁺-free conditions. We conclude that NMDA receptors are active in cells held at -70 mV even in the presence of 1 mM Mg²⁺. We believe this activation is due to the presence of endogenous glutamate in the retina. The observations of this study strongly suggest that NMDA receptors contribute to the resting noise and conductance properties of retinal ganglion cells. Our results suggest NMDA receptors are activated by an ambient level of extracellular glutamate whose source has yet to be determined.