Mecanismos neurales de adaptación a la luz y oscuridad. Estudio comparativo en ratones controles y modelos de degeneración retiniana

Abstract

The retina is a portion of the Central Nervous System (CNS) which contains a set of specialized neurons that are organized in an intricate neural network. Such network organization allows discriminating and encoding the many different light signals from the visual world. The light that reaches the retina is transduced in a series of electrical and chemical responses that activate different parallel pathways that transmit the coded signals towards the different areas of the CNS. The classical “imageforming” pathway starts with the transduction of light energy into electric energy by rods and cones photoreceptors. Nevertheless, there exists a second pathway in which these classics photoreceptors do not seem to be involved. This route is termed as the “non-image forming” pathway and the cells responsible for the phototransduction machinery are the intrinsically photosensitive retinal ganglion cells (ipRGCs), which express melanopsin. The central projections of the “no-image forming” pathway are directed to the CNS areas related to no visual functions such us circadian rhythms and pupilay reflex, among others. By now, it remains unclear whether these new photosensitive cells play any modulatory role in the classical “image-forming” vision. In the present Doctoral Thesis we raise the modulatory role of the ipRGCs on retinal molecules involved in light adaptation processes, due to the cell contacts that ipRGC make with other retinal cellular types. All experiments were carried out in mice; wild type animals and mouse models of photoreceptor dysfunction have been used. We initially studied the influence of ipRGCs on the circadian expression of certain cell specific proteins such as the protein kinase C alpha (PKCα), the sodium – potassium – chloride co-transporter (NKCC) and tyrosine hydroxilase (TH) by the use of immunohistochemical technique and western blotting. Secondly, we tried to test the putative modulatory effect of ipRGC on the retinal functional response, analyzed by means of electroretinographic recording (ERG). Finally, we focused on the role played by the ipRGC on the resynchronization of circadian rhythms after changes in light/dark phases of the 24 hours cycle. In summary, we analyzed a series of molecular, functional and behavioural aspect of the light adaptation process that may be modulated by the ipRGCs in the mouse retina. The molecular studies allowed us to demonstrate the existence of a circadian rhythm of the PKCα expression, which may be modulated by the melanopsin containing cells. Likewise, the study of NKCC1and TH showed us that the circadian expression of these proteins needs the presence of, at least, one type of photosensitive cell in the retina. Across the ERG studies we have verified the modulatory role of melanopsin containing cells on the retinal sensitivity. In this sense, retinal light sensitivity is shown to be related to the active presence of the ipRGCs; it depends on the light received by the retina, but is independent of the circadian rhythm. Behavioural experiments allowed us to demonstrate that the resynchronization of the circadian activity after changes in light/dark phases of the 24 hours cycle is dependent solely on the ipRGC activity. The hourly disruption produced by a phase shortening is resynchronized slower than those produced by phase stretching. These experiments, carried out in wild type and dystrophic animals, allowed us to conclude that the ipRGCs are necessary and sufficient for the processes of resynchronization of the circadian activity with the light/dark cycle.