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Department of Physiology, Juntendo University, School of Medicine, Tokyo, Japan.

1. The distribution of excitatory amino acid receptors on ventral horn neurones was investigated using slices of newborn rat spinal cord. 2. The neurone and the tip of the pipette used to inject amino acids were visualized using Lucifer Yellow under a fluorescent microscope. The pipette was precisely located on the soma and dendrite of the neurone under visual control, and L-glutamate (Glu), L-aspartate (Asp), N-methyl-D-aspartate (NMDA), kainate (KA) and quisqualate (Quis) were ionophoretically applied with a short pulse. The potential changes were intracellularly recorded from the soma. 3. Sensitivity to Glu as tested with short pulses (1-2 ms) was almost the same at the soma and along dendrites. 4. The amplitude of the responses to NMDA produced at the soma and the proximal part of the dendrite was about the same as that of Glu, but smaller than that of Glu at the distal part of the dendrite. Suppression of the Glu potential by an NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), was greater at the soma than at the dendrite, suggesting that the contribution of NMDA receptors to the Glu potential was greater at the soma. 5. Sensitivity to Asp was about one-half that to Glu sensitivity on the soma and even less on the dendrite. Sensitivity to KA was high at the soma and low at the dendrite. However, Quis responses were produced throughout the neurone. 6. The Quis response induced by the application of a short pulse showed two phases: a fast response followed by a very slow depolarization that lasted more than 10 s. 7. The fast Quis response was easily desensitized and insensitive to APV. The time course of the fast Quis potential was shorter than that of Glu. 8. The slow Quis response was more pronounced at the dendrites than at the soma and was reduced by the intracellular injection of EGTA, suggesting the contribution of Ca2+ in the cell, possibly mediated by a second messenger system. 9. Experimental results suggest that the distribution of excitatory amino acid receptors differs between the soma and the dendrites of spinal neurones.