McNair Research Journal - University of St. Thomas

Humanities & Social Sciences
How Early Exposure to Light Changes Retinal Input
to the Subcortical Visual System in the Albino Rat
Clemon J. Dabney III, Dr. J. Roxanne Prichard
University of St. Thomas - Department of Psychology
Abstract
We observed the variation in retinofugal projections and c-FOS responses to
light in retinorecipient nuclei of light-dark-reared (LD) and light-reared (LL)
rats for 3 months; we also tested the affects of going from the LL group to the
light-dark-reared (LLLD) group and from the LD group to a light-reared
environment (LDLL) for 3 months; and compared them with the rats that
had been in the LL or LD group for the whole experiment. The LD, LL,
LDLL, LLLD groups were exposed to an hour of light then 18 hours of darkness.
The rats were injected with anterograde tracer cholera-toxin (CT ) into
the rats and quantified the density as well as the distribution of CT –stain in
select regions of the subcortical visual system. Our research observed that LL
had a significant increase in CT -stain in the ventral lateral geniculate
nucleus (VLGN) and an increase in CT -stain that was approaching statistical
significance in ventral lateral hypothalamus (VLH) and intergeniculate
leaflet (IGL). These differences were decreased but not eliminated in the
distribution of stain in the IGL and density of VLH when a switch to the
LD happened at 3 months of age. The differences were increased by a switch to
LD at 3 months in the VLGN group. The implications of this research study
could play a critical role in understanding the development of the sensory
nervous system.
For most animals, their capacity to respond to differences in light is
essential for their continued survival. Light is the external stimulus that
cues the twenty-four hour circadian rhythm to the light-dark cycle; it
activates seasonal and regulatory changes in both physiology and behavior.
Light also acts to stimulate instantaneous changes in behavior; light can
affect changes in vigilance. For example, darkness can promote sleep in
diurnal creatures, whereas the opposite is true for nocturnal creatures.
When the rat wakes up, when it eats and its motor activity are all
inhibited by light (Borbely, Huston, & Waser, 1975). Changes in light
stimuli as well as temperature can prompt seasonal adaptations in both
behavior and physiology (Palchykova, Deboer, & Tobler, 2003). Early in
an animal’s development photic stimuli has a significant influence on the
timing of rapid eye movement (REM) sleep (Prichard, Fahy, Obermeyer,
Behan & Benca, 2004).
Sleep is essential for survival; it is a state that places an animal at an
increased risk for predation, thus establishes significance to the timing of
sleep. The amount of non-rapid eye movement (NREM) sleep, and REM
sleep in each twenty-four hour day, as well as the amount of time spent
awake is governed by homeostatic mechanisms. Franken’s research team
found that sleep deprivation causes an enhanced EEG slow-wave activity
and a reduced number of awakenings as well as an overall increase in the
period of time spent asleep (Franken, Dijk, Tobler, & Borbely, 1991).
The actual amount of time which an animal spends awake or asleep is
determined by exchanges between the circadian rhythms, the sleep
homeostat, and stimuli from the animal’s environment, which includes
acute changes in light (Mrosovsky, Foster, & Salmon, 1999).
There are separate anatomical systems which process visual stimuli. The
primary visual system receives neuronal stimuli from the geniculocortical
pathway. The geniculocortical pathway mediates the conscious visual
perception of the primary visual system. The other the subcortical visual
system mediates the unconscious responses to photic stimuli (Morin &
Blanchard, 1999). There are two means discovered to date by which the
subcortical visual system receives retinal input. One involves the rod-cone
ospin system. This rod-cone ospin system includes specialized rod and
University of St. Thomas McNair Research Journal
Clemon J. Dabney III
Early Light Exposure
cone photoreceptor cells; these photoreceptor cells absorb light through
rhodopsin and cone-opsins, light sensitive membrane proteins which
deduce light energy into an electrochemical signal that is ultimately transmitted
by neuronal cells to the visual cortex (Garriga & Manyosa, 2002).
The other way in which the subcortical visual system receives retinal input
is from a specific type of photosensitive melanopsin-containing retinal
ganglion cell (Berson, 2003). The hypothalamic suprachiasmatic nucleus
(SCN) and the intergeniculate leaflet (IGL) are within the subcortical
visual system and entrain the circadian responses to light. (Harrington,
1997; Moore & Eichler, 1976).
The subcortical visual system entrains the circadian and acute responses to
light as well as acute changes in behavioral state; the superior collicullus
(SC) mediates compensatory and reflexive movements of the eyes, as well
as the neck and head, and the pretectum (PT) mediates pupil dilation and
constriction. The PT experiences an acute change in sleep patterns in
response to light and dark stimuli in albino rats and is mediated by the
subcortical visual system, and is regulated independently from cortical
vision and circadian rhythms. The PT may regulate REM sleep in
response to photic stimulus (Miller, Miller, Obermeyer, Behan, & Benca,
1999). The neurons of the olivary pretectal nuclei (OPN) receive photic
stimuli and mediate the pupillary light reflex (Gamlin, Zhang & Clark,
1995). The IGL and ventral lateral geniculate nucleus (VLGN) seem to be
very important in both photic and nonphotic phase shifting. The IGL
and VLGN provide a mechanism for entrainment of circadian rhythms to
a light-dark cycle. When the IGL and VLGN are ablated, a change in the
phase shift responses to light pulses as well as responses of circadian
rhythms to continuous light is observed (Harrington, 1997).
The primary visual system needs early photic stimulation in order to
function properly. This can be observed in the 1963 Wiesel and Hubel
experiment, when kittens were deprived of sensory input to the eyes for
the first three months of life there is a lack of photic stimuli to the
primary visual system and thus a change in physiology, behavior and
morphology. The lack of stimuli in the kittens led to extremely defective
vision, in which visual placing and following reactions were not present,
and it appeared that the kittens lacked the ability to perceive form (Wiesel
& Hubel, 1963). This research as well as another research study led by the
pair led to the findings that there is a critical period in the development of
the primary visual cortex where, without proper stimuli, the process of
synaptic refinement in the brain will not occur properly and the cat will
have a significant reduction in visual acuity due to the visual stimuli
deprivation. This observation is further seen in Hubel and Wiesel’s 1969
experiment that found that if an eye is closed for six days in the fourth
and fifth week since birth the percentage of cells that the visually deprived
eye can manipulate drops from 85 to 7% (Hubel & Wiesel, 1969).
The early light environment of an animal has an affect on its later development.
Canal-Corretger research study showed that differences in early
lighting conditions have an effect on the circadian rhythm in rats; the
lighting condition in which a rat was born and reared had a crucial impact
on the future acute responses to light stimuli. Rats reared under constant
bright light became arrhythmic and showed longer phases shifts; rats
reared in constant darkness had shorter circadian rhythm phase shifts and
were more responsive to photic stimuli. This may mean that the development
of the circadian rhythm depends on the environmental stimuli as
well as that the circadian system’s need to be developed before it can be
influenced and manipulated by external stimuli like that of light
(Canal-Corretger, Vilaplana, Cambras, & Noguera, 2001).
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