Adaptation mechanisms in the salmonid visual system
Date
2018-07-20
Authors
Beaudet, Luc
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Abstract
Animals in general, but fish in particular, inhabit environments characterized by
dynamic photic conditions that are influenced by cyclical events such as the night-day
cycle, or by spatial heterogeneity in the distribution of light. Effects of these dynamic
properties on the visual system are compounded in salmonid fishes by migrations that
expose individuals to various types of habitats, at various stages of their ontogeny. This
dissertation examines some of the adaptations that enable the retina of salmonid fishes to
cope with their changes of “visual environment” caused by migration and by the night-day
cycle.
In the first part of this dissertation, I used a combination of optic nerve response
(ONR) recordings and conventional histology of the retina to investigate the ontogeny of
sensitivity to ultraviolet (UV) light in salmonid fishes. I found that the UV cone
mechanism contributed mostly to the ON response of retinal ganglion cells in rainbow
trout (Oncorhpichus mykiss). Furthermore, the presence of UV sensitivity in rainbow
trout was associated with the presence of accessory corner cones in the retinal cone
mosaic, as both UV sensitivity and these cones were absent in larger (59.5-83 5g)
juveniles. These results suggest that corner cones in the salmonid retina are sensitive to
UV light, and that their ontogenetic disappearance leads to the loss of UV sensitivity.
The changes in the photic environment that occur when mature salmonid fishes
return to their natal stream to reproduce mirror those undergone during the first
migration. To determine if the accessory comer cones, lost during this first migration,
reappear at the time of the return migration, I studied the structure of the photoreceptor
layer in sexually mature Pacific salmonids from four species: chinook (O. tschawytscha),
chum (O. keta) and coho (O. kisutch) salmon, and rainbow trout. I found accessory
comer cones over a large area of the dorso-temporal retina in all four species examined,
which provides support for the contention that these cones are the product of late cellular
addition.
I investigated possible pathways for visual information to various brain centers in
rainbow trout by labelling retinal projections and torus semicircularis connections in the
same individuals. Double-labelling of neuronal tracts revealed two possible indirect
pathways between the retina and the torus semicircularis, through the accessory optic
center of the diencephalon and the optic tectum respectively.
In the second part of this dissertation, I qualitatively and quantitatively examined
the effects of various levels and spectral types of ambient lighting conditions on the
sensitivity and time course of multi-unit responses recorded from the optic nerve of
juvenile rainbow trout. Change in threshold from the dark-adapted state to progressively
brighter ambient light conditions was examined at four wavelengths (380, 430, 540 and
620 nm) and found to be linear over most of the scotopic range, with a slope around 0.8.
The results also suggested that, under mesopic conditions, rods and the long-wavelength
cone mechanism were active simultaneously, in their respective parts of the spectrum.
Implicit time, or time-to-peak of the scotopic responses decreased with stimulus intensity
following a logarithmic relationship with a slope of -0.10, suggesting that the scotopic
system of trout acts as an 11-stage low-pass filter, a number similar to that inferred in cat
and rat, but different from other non-mammalian vertebrates. Similarly, implicit time at
threshold decreased logarithmically with background intensity for the scotopic system,
with a slope of -0.09
Varying the spectral content of ambient light led to differences in sensitivity and
time course of ONRs across the spectrum, suggesting physiological differences between
cone mechanisms. Possible implications for the coding of visual information are briefly
discussed. In conclusion I provide a qualitative model of light adaptation in the trout
visual system.
Description
Keywords
Salmonidae, Visual pathways, Retina