FUNCTIONAL-DEVELOPMENT OF INTRINSIC-PROPERTIES IN GANGLION-CELLS OF THE MAMMALIAN RETINA

Sensory neurons manifest pronounced changes in excitability during mat uration, but the factors contributing to this ubiquitous developmental phenomenon are not well understood. To assess the contribution of int rinsic membrane properties to such changes in excitability, in the pre sent study whole cell patch-clamp recordings were made from developing ganglion cells in the intact retina of postnatal rats. During a relat ively brief developmental period (postnatal days P7-P27) ganglion cell s exhibited pronounced changes in the discharge patterns generated by depolarizing current injections. The youngest cells (P7-P17) typically responded to maintained depolarizations with only a single spike or a rapidly adapting discharge pattern. In contrast, the predominant resp onse mode of more mature cells (P21-P27) was a series of repetitive di scharges that lasted for the duration of the depolarization period, an d by P25 all cells responded in this manner. These functional changes characterized all three morphologically defined cell classes identifie d by intracellular labeling with Lucifer yellow. To determine if expre ssion of the potassium current (I-a) and the kinetics of the Na-channe l related to the increased excitability of developing ganglion cells d escribed above, current-and voltage-clamp recordings were made from in dividual neurons. The different firing patterns manifested by developi ng retinal ganglion cells did not reflect the presence or absence of t he I-a conductance, although cells expressing I-a tended to generate s pikes of shorter duration. With maturation the speed of recovery from inactivation of the Na current increased markedly and this related to the increased excitability of developing ganglion cells. Neurons yield ing only a single spike to maintained depolarization were characterize d by the slowest speed of recovery; cells with rapidly adapting discha rges showed a faster recovery and those capable of repetitive firing r ecovered fastest from Na-channel inactivation. It is suggested that th ese changes in intrinsic membrane properties may relate to the differe nt functional roles subserved by ganglion cells during development.