Commitment and differentiation of ganglion cells in the developing chick retina

Abstract

This chapter reviews the electrical and molecular properties of ion channels and the specific function they subserve during the light response. The cyclic GMP-gated channels of vertebrate rod and cone cells play a central role in phototransduction by controlling the flow of Na+ and Ca 2+ ions into the photoreceptor outer segment. Photoexcitation of the rod cell is initiated when a photon is captured by rhodopsin resulting in the conversion of the 11-cis retinal chromophore to its all-trans isomer. The resulting formation of Meta II rhodopsin triggers the activation of the visual enzyme cascade system and an amplification of the signal. Meta II rhodopsin catalyzes the exchange of guanosine diphosphate (GDP) for Guanosine-5'-triphosphate (GTP) on the -subunit of transducin. Interaction of this subunit with inhibitory subunit of phosphodiesterase (PDE) activates PDE leading to the hydrolysis of cGMP. As the intracellular concentration of cGMP decreases, the cGMP-gated channels close causing a hyperpolarization of the rod cell. This in turn results in an inhibition of glutamate transmitter release at the synapse and the transmission of the signal to other retinal neurons. Since the Na/Ca-K exchanger continues to extrude Ca 2+ from the outer segment, a marked decrease in intracellular Ca 2+ concentration (below 100 nM) also occurs. This reduction in Ca 2+ plays a central role in photorecovery and light adaptation.