LOCALIZATION AND INTERACTION OF N-METHYL-D-ASPARTATE AND NON-N-METHYL-D-ASPARTATE RECEPTORS OF LAMPREY SPINAL NEURONS

Small volumes of N-Methyl-D-Aspartate (NMDA) and non-NMDA excitatory a mino acid receptor agonists were applied to localized regions of the d endritic trees of lamprey spinal neurons along their medial-lateral ax is to obtain a spatial map of glutamate receptor distribution. Voltage clamp and frequency domain methods were used to obtain quantitative k inetic data of the voltage dependent ionic channels located both on th e soma and on highly branched dendritic membranes. Pressure pulses of NMDA applied to the most peripheral regions of the dendritic tree elic ited large somatic impedance increases, indicating that the most perip heral dendrites are well supplied with NMDA receptors, Experiments don e with kainate did not elicit somatic responses to agonist application s on peripheral dendrites. The data obtained are consistent with the h ypothesis that the activation of NMDA receptors by exogenous glutamate is significantly modified by the simultaneous activation of non-NMDA receptors, which shunts the NMDA response. The non-NMDA shunting hypot hesis was tested by a combined application of kainate and NMDA to mimi c the action of glutamate showing that the shunting effect of non-NMDA receptor activation virtually abolished the marked voltage dependency typical of NMDA receptor activation. These data were interpreted with a compartmental neuronal model having both NMDA and non-NMDA receptor s.