City Research Online

Abstract

Retinomotor movements are the movements of cones, rods, and retinal pigment epithelium (RPE) melanosomes associated with the transitions between light and darkness. They are most prevalent and strongest in fish and amphibia, less in reptiles, and birds, and least in mammals. During the light phase, cones, rods, and RPE melanosomes occupy positions different to those found during the dark phase. Not only do retinomotor movements occur as a result of changes in the ambient light levels, but they are also under endogenous control as shown by their rhythmic continuation in constant darkness where cones, rods, and RPE melanosomes move to light adapted positions during ‘expected' day and to dark adapted positions during ‘expected’ night.

This work commences by outlining the basic rhythms of both cone and RPE melanosome movements during a normal light/dark cycle and in constant darkness, in an equatorial teleost fish, the glowlight tetra (Hemigrammus erythrozonus). Both the cone and RPE melanosomes movement rhythms persist far longer in constant darkness than in any other species examined to date which is interpreted as being due to strong circadian drive of retinomotor movements in the glowlight tetra. Further, after reviewing retinomotor literature, it is suggested that the glowlight tetra and other equatorial fish may have relatively more robust circadian drive than species from more temperate regions.

The relatively long duration of retinomotor movements in constant darkness afforded a good opportunity to study circadian organisation of these movements in more detail. For example, the light control of cone and RPE melanosome movements were examined. The light control of cone movement was found to be poor in response to light exposure and dark placement in the middle of the dark and light phases respectively. In comparison, strong light control of RPE melanosome movement was found during light in the middle of the dark phase, but not to a dark pulse in the middle of the light phase. However, more light control of cone movement was recorded at two other times during a full cycle of cone movements suggesting that there is a circadian rhythm in the light control of cone movements. Although these intervals of light control of cone movements were relatively short in comparison to the period of the cone rhythm, the rhythm of cone movements in glowlight tetras entrained to a 180° phase shifted light/dark cycle in a similar time found in other animals.

As the glowlight tetra is an equatorial species, there would be little need for oscillators governing retinomotor movements to be sensitive to photoperiod. However, the rhythm of cone movement was found to entrain well to all 24 hour light/dark cycles with photoperiods varying from 1-14 hours. Models using a one or two-component oscillator were able to explain the mechanism by which the rhythm of cone movements in the glowlight tetra could entrain to the different light cycles. Further, following entrainment to most light/dark cycles, the free- running period remained at 24 hours.

Another ocular rhythm, the formation and degradation of horizontal cell spinules, was examined in the glowlight tetra. In constant darkness, the rhythm of spinule dynamics showed signs of damping within the first 24 hours. This was interpreted as representing less strong circadian drive than that for both cone and RPE melanosome movement. In comparison to both the cone and melanosome movements, spinule dynamics was affected by both light exposure and dark placement in the middle of the dark and light phases respectively. Following exposure to light in the middle of the dark phase, spinule formation followed a time course similar to all other species examined to date.

Retinomotor movements, in particular the movements of both cones and RPE melanosomes, were also examined to determine whether they played a possible role in regulating visual sensitivity as determined by the dorsal light reaction (DLR). Retinomotor responses in the glowlight tetra and the zebrafish (Danio rerio) were examined in this series of experiments. During light exposure in the middle of the dark phase, RPE melanosomes but not cones, undergo retinomotor movements in the glowlight tetra whereas both cones and RPE melanosomes show retinomotor movements in the zebrafish. When fish are exposed to light in the middle of the light phase, no retinomotor movements are seen in both species of fish. However, under both conditions of light exposure in the middle of the light and dark phases, there was a similar change in sensitivity as measured by the DLR in both species. Thus, it was concluded that it was unlikely that retinomotor movements have a role to play in sensitivity control.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RZ Other systems of medicine
Departments:	School of Health & Psychological Sciences > Psychology School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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