Differential distribution of excitatory amino acid receptors on embryonic rat spinal cord neurons in culture

Abstract

1. Excitatory amino acid (EAA) receptors mediate synaptic transmission in many areas of the vertebrate CNS. To map the distribution of the EAA receptors, three agonists selective for EAA receptor subtypes [kainate, quisqualate, and N-methyl-D-aspartate (NMDA)] were applied by pressure to the cell bodies and neurites of voltage-clamped, embryonic rat spinal cord neurons in culture. 2. Current loss along the neurite between the site of activation and the recording electrode at the soma was estimated independently of variations in receptor density by focal application of high [K+] solution. This estimate was used to compensate agonist-evoked responses for current loss due to leak. K(+)-evoked current amplitudes progressively decreased as applications were made at more distal positions along the neurite. 3. Response amplitudes to EAAs showed a monotonic decay, similar to that with high [K+] solution, in only a few of the cells tested with each agonist. The majority of neurons had areas of high agonist sensitivity along the neurites, implying a nonuniform density of receptors. Such regions sometimes occurred at the most proximal segment of the neurite. Most neurites had distal regions of agonist sensitivity that ranged from 40 to 300% higher than at the soma. 4. The relative distributions of the three EAA receptors were compared by applying two agonists to the same sites along neurites and observing variations in normalized response amplitudes. When comparing NMDA versus non-NMDA receptor distributions on the same neurite, approximately 40% of the cells had similar patterns to two agonists, and the remaining 60% had different patterns. When sensitivity to the two non-NMDA agonists kainate and quisqualate were compared, about one-half of the cells tested had different patterns of agonist sensitivity. 5. Areas of high sensitivity persisted after block of calcium channels by addition of La3+ to the bath solution and after prevention of evoked transmitter release by a low [Ca2+]/high [Mg2+] solution. 6. These results show that spinal cord neurons can have regions of high agonist sensitivity to NMDA, kainate, and quisqualate along their neurites and that the sensitivity to any one of the EAA receptor subtypes can be elevated independently of the others.