SELECTIVE REDUCTION OF N-METHYL-D-ASPARTATE-EVOKED RESPONSES BY 1,3-DI(2-TOLYL)GUANIDINE IN MOUSE AND RAT CULTURED HIPPOCAMPAL PYRAMIDAL NEURONS

1 The effects of 1,3-di(2-tolyl)guanidine (DTG) were examined on the responses of cultured hippocampal neurones to the excitatory amino acid analogues N-methyl-D-aspartate (NMDA), kainate, quisqualate and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). 2 In rat hippocampal neurones loaded with the Ca2+-sensitive dye Fura-2, DTG (10-100 muM) produced a concentration-dependent depression of the NMDA-evoked rises in intracellular free calcium ([Ca2+]i), an effect that was not modified by changes in the extracellular glycine concentration. DTG (at 50 and 100 muM) also attenuated, although to a lesser extent, the rises in [Ca2+]i evoked by naturally-derived quisqualate. In contrast, 50 and 100 muM DTG did not depress responses evoked by kainate, AMPA and synthetic, glutamate-free (+)-quisqualate although on occasions DTG enhanced kainate-and AMPA-evoked rises in [Ca2+]i. 3 DTG attenuated NMDA-evoked currents recorded from mouse hippocampal neurones under whole-cell voltage-clamp with an IC50 (mean +/- s.e.mean) of 37 +/- 5 muM at a holding potential of - 60 mV. The DTG block of NMDA-evoked responses was not competitive in nature and was not dependent on the extracellular glycine or spermine concentration. The block did, however, exhibit both voltage-, and use-, dependency. The steady-state current evoked by naturally-derived quisqualate was also attenuated by DTG whereas those evoked by kainate and AMPA were not. 4 We conclude that DTG, applied at micromolar concentrations, is a selective NMDA antagonist in cultured hippocampal neurones, the block exhibiting both Mg2+- and phencyclidine-like characteristics. Given the nanomolar affinity of DTG for sigma binding sites it is unlikely that the antagonism observed here is mediated by sigma-receptors, but the data emphasize the potential danger of ascribing the functional consequences of DTG administration solely to sigma receptor-mediated events.