PROPERTIES OF K-CELLS DURING THE PERIOD OF ACTIVITY-MEDIATED REFINEMENTS IN RETINOFUGAL PATHWAYS( CONDUCTANCES IN CAT RETINAL GANGLION)

During ontogeny retinal ganglion cells manifest pronounced changes in excitable membrane properties. To further our understanding of the ion ic conductances underlying such functional changes, the whole-cell vol tage-clamp Variation of the patch-clamp technique was used to record p otassium currents in 220 ganglion cells dissociated from cat retinas r anging in age from embryonic day 31 to postnatal day 10. Potassium cur rents were isolated by blocking voltage-gated Na+ and Ca2+ currents wi th tetrodoxin (TTX) and CoCl2 respectively and were characterized by t heir pharmacology, kinetics and voltage-dependence of activation and i nactivation. In all cases, a combination of three currents accounted f or the total outward calcium-independent K+ current: (i) a steady line ar conductance; (ii) a voltage-gated transient current, I-A, and (iii) a voltage-gated sustained current, I-K. Both voltage-gated currents w ere affected by the application of 4-aminopyridine and tetraethylammon ia (TEA): I-A showed a greater sensitivity to 4-aminopyridine, while I -K was more sensitive to TEA. Both voltage-gated currents were present throughout the developmental period examined; however, the percentage of retinal ganglion cells (RGCs) expressing I-A showed a marked decli ne from 82% at E31 to 45% at postnatal ages. During this developmental period there was an increase in the density of the two voltage-gated and the linear conductance. Additionally, with maturation, significant ly slower inactivation kinetics were observed for I-K. These findings, and our previous results dealing with maturational changes in the TTX -sensitive voltage-gated Na current, are related to the generation of excitability in developing retinal ganglion cells. Furthermore, the pr esence of cells with and without transient K+ conductance throughout d evelopment suggests that the different spiking patterns observed in RG C classes may be partially due to differences in their membrane proper ties.