RAT RETINAL GANGLION-CELLS EXPRESS CA2+-PERMEABLE NON-NMDA GLUTAMATE RECEPTORS DURING THE PERIOD OF HISTOGENETIC CELL-DEATH

Local application of glutamate agonists to retinal ganglion cells (RGNs) was performed in retinae isolated from pigmented rats aged between 3 and 8 days postnatally. A vast majority of RGNs displayed current responses to glutamate (Glu), N-methyl-D-aspartate (NMDA), quisqualate (QA), alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoazolepropanoic acid (AMPA), kainate (KA) and domoate (DA). In Na+-free extracellular solution with elevated Ca2+, non-NMDA agonists elicited large (up to 200 pA) inward currents that were completely blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX) and Cd2+. MK-801 also induced a partial block of cationic currents in Na+-free saline. In standard salt solutions, current-voltage relationships of Glu-R-mediated currents were often inwardly rectifying in the presence of D-aminophosphonovalerat (D-APV), as is typical of Ca2+-permeable non-NMDA receptors. The presence of inward rectification in the current voltage relationship was always associated with a high value of the cationic permeability ratio P(Ca2+)/P(Cs+) (>0.8). However, in about half of the investigated RGNs no inward rectification was observed under standard recording conditions. Our results lead to the suggestion that expression of Ca2+-permeable Glu receptor subunits may contribute to regulation of cell numbers in the postnatal retina.