Abstracts

Previously we have shown that transient application of competitive NMDA antagonists induce long-term depression (LTD) at recurrent collateral synapses and thereby decrease CA3 burst probability. Molecular studies suggest that the persistence of this effect is due to metaplasticity induced by endocytosis of membrane-bound NMDA receptors. Here we further tested this hypothesis by isolating NMDA receptor activity from AMPA receptor activity. We used the CA3 region of the hippocampus as a model of network synchronization to test the effects of NMDA and AMPA antagonists on burst propagation. Slices were superfused with a modified ACSF (in mM: 3.3 KCl, 1.3 CaCl2, 0.9 MgCl2) and extracellular recordings of spontaneous burst intervals and durations were analyzed. Long-term depression of the recurrent synapses was induced by transient blockade of the NMDA receptors through application of 2.5-10 [mu]M D-APV, a competitive NMDA antagonist. In experiments where the spontaneous bursts were solely NMDA driven (ACSF with zero Mg2+), AMPA and kainate receptors were blocked with NBQX (50 [mu]M) or GYKI 52466 (40 [mu]M), potassium channels were blocked with 4-AP (10 [mu]M), and GABAA and GABAB receptors were blocked with picrotoxin (100 [mu]M) and CGP 52432 (1 [mu]M), respectively. In control recordings, sequentially decreasing applications of D-APV significantly increased the interburst interval by 101% after washout and decreased burst duration by 17% compared to control (n=15, P[lt]0.05). However when spontaneous bursts are driven solely by NMDA receptor activation (i.e., AMPA and kainate receptors are blocked by NBQX or GYKI), interburst intervals did not increase compared to control (n=4 and n=2, respectively). Burst duration decreased by 21-38% in the presence of either NBQX or GYKI. These electrophysiological data suggest that NMDA receptor internalization is not prominent when AMPA receptors are blocked. Further studies are ongoing to determine more precisely the conditions under which NMDA receptors undergo endocytosis. (Supported by NIH)