Phototransduction in retinal rods and cones: effects of temperature and background light, and an application for testing drug delivery

The photoreceptor cells of the vertebrate retina share a common morphological design and molecular scheme for phototransduction. Within this framework, there are great functional differences with respect to response amplification, kinetics, and adaptability to different mean light levels, first, between the two main classes of photoreceptors, rods and cones, and second, between different taxonomic groups. The present thesis analyses functional differences and similarities between i) mammalian and amphibian photoreceptors, and ii) rods and cones by studying effects of temperature on electrophysiological response properties. The research is based on characterisation of sensitivity and photoresponse kinetics in rods and cones of two mammals (rat and mouse), and two amphibians (frog and toad). Photoresponses to light pulses of incremental strength were recorded by the electroretinogram (ERG) technique across isolated aspartate-treated retinas at different temperatures in the range 2 - 37 °C, and at different levels of mean illumination. One objective was to investigate how the major functional differences between different vertebrate photoreceptors can be explained without assuming large differences in the properties of the phototransduction molecules. A general conclusion is that at the same temperature photoreceptors of mammals and amphibians exhibit similar functional properties. In rods, the remaining differences in the electrophysiological properties can largely be explained by differences in outer-segment size and morphology. In cones the picture is more complex due to the highly folded structure of the outer segment as well as the presence of thermal isomerizations of visual pigment, which may occur at a rate possibly high enough to 'light-adapt' cones in darkness. Another objective was to relate the capacity for temporal integration of dark-adapted rod photoreceptors to the integration time of vision and absolute visual sensitivity. A strong correlation was found between temporal integration in rods and in a visually guided behaviour of toads at different temperatures. The results allow the conclusion that temporal integration is mainly set by the rods and explains a considerable part of differences in absolute visual sensitivity between amphibians and mammals. The thesis also includes a project in which the aim was to develop a method where the vertebrate retina could be used as a biosensor for monitoring controlled drug release from temperature-sensitive polymeric carriers. The developed method enabled accurate concentration determinations of the model drug 3-isobutyl-1-methylxanthine (IBMX) based on the square root dependence of photoresponse kinetics on [IBMX] discovered in the work. Moreover, the biocompatibility of drug carriers can be assessed by the degree to which rods retain stable function in the presence of the carrier molecule, or its monomers