Variable distributions of Ca²⁺-permeable and Ca²⁺-impermeable AMPA receptors on embryonic rat dorsal horn neurons

1. By measuring the apparent reversal potential (aE(rev)) of kainate- and α-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA)-evoked currents associated with changes in extracellular Ca²⁺ concentration ([Ca²⁺](e)), we have been able to identify embryonic dorsal horn neurons grown in tissue culture that express Ca²⁺-permeable non-N-methyl-D-aspartic acid (NMDA) receptors. 2. The relative expression of Ca²⁺-permeable and Ca²⁺-impermeable non-NMDA receptors varies from cell to cell. This was evident from the range of aE(rev)s observed for kainate-evoked currents in a 0 mM [Na⁺](e), 10 mM [Ca²⁺](e) bath. Under these conditions, aE(rev) ranged from -96 to -21 mV, suggesting that the percentage of the non-NMDA receptors on each neuron that are Ca²⁺-permeable is variable. 3. To determine the extent to which the variability in aE(rev) is due to variable receptor expression rather than experimental variability, we compared the effects of changes in [Ca²⁺](e) on kainate-evoked currents and NMDA-evoked currents on the same cells. Assuming that all of the NMDA receptors on each neuron have a similar Ca²⁺ permeability, this approach provides an index of the sensitivity of our assay system. The reversal potential of NMDA-evoked currents in 10 mM [Ca²⁺](e) ranged from -30 to -7 mV, whereas on the same population of neurons, the aE(rev) of kainate-evoked currents ranged from - 92 to -40 mV. 4. The rectification properties of the non-NMDA currents were generally linear or outwardly rectifying in normal bath solution. When the P(Ca)/P(Cs) ratio in 0 mM [Na⁺](e), 10 mM [Ca²⁺](e) bath solution was assessed as a function of the rectification index in standard bath, a poor correlation was found between Ca²⁺ permeability and the rectification index. 5. The aE(rev) of kainate-evoked currents was similar to that of cyclothiazide enhanced AMPA-evoked currents observed on the same cells (- 66.5 ± 18.4 and -64.0 ± 13.9 mV, mean ± SD, respectively). This suggests that kainate is primarily activating the AMPA receptor and that the majority of non-NMDA receptors on embryonic dorsal horn neurons in culture are high- affinity AMPA receptors. 6. Immunocytochemical evidence suggests that the AMPA receptor subunits GluR1-4 are expressed to a variable degree from cell to cell in our cultures. We found evidence for low levels of expression of the kainate receptor subunits GluR5-7. The immunocytochemical observations support the physiological data indicating that much of the kainate-evoked current recorded in our experiments can be accounted for by kainate activation of AMPA receptors. 7. Expression levels of functional Ca²⁺- permeable AMPA receptors are different from cell to cell in these maturing dorsal horn neurons, suggesting a possible role in development. Ca²⁺ entry through AMPA receptors has already been shown to have the important physiological consequence of desensitizing colocalized NMDA receptors. This may mediate short-term or long-term changes in synaptic function.