On the duplex nature of the skate retina

The skate retina contains only one type of photoreceptor which has rod-like properties in the dark-adapted state. In the presence of background illumination, the receptors take on cone-like properties, i.e., their photoresponses become much faster, are less sensitive to light, and can be elicited in the presence of very bright backgrounds. Although the transformation is an extremely slow process, the skate retina performs like that of animals with mixed, rod-cone retinae. In this report we examine some postreceptoral features which may relate to this remarkable behavior. We show, for example, that the skate photoreceptor terminals make two kinds of junctions (ribbon synapses and basal junctions) that appear analogous to the two kinds of synaptic contacts made by cone terminals in other species. Furthermore, two types of horizontal cells are seen in the skate retina. Although the light-evoked responses of these horizontal cells are similar, there are differences in their current profiles recorded under voltage clamp, and in the nature of their dendritic processes. We have also observed several unusual postreceptoral structural features that may have some bearing on the response properties of the skate retina. In addition, a comparison of the adaptive properties of the receptor potential with those of intraretinal responses (b-wave, PNR) reveals differences that suggest strongly the presence of a network adaptive mechanism originating within the proximal retina. The network mechanism appears to be controlled to a large extent by the extracellular concentration of potassium [K+]0; i.e., changes in [K+]0 affect significantly the sensitivities of the b-wave and PBR, but have little effect on responses arising in the distal retina.