Vision begins with rod and cone photoreceptors that absorb light from reflected objects and signal the brain. They can do this because they "express" (build) light-absorbing pigment molecules and a signaling pathway linked to light absorption.
Rod
photoreceptors are so sensitive to light that they are useful only at
night. Cone photoreceptors need bright light to work, so they are useful
only in the day. Because cones come in 3 types, the brain can compare
their signals to perceive color.
If photoreceptors degenerate due to injury or disease, there’s
nothing to start the signal to the brain and blindness results. These
pictures simulate a normal vision scene (left) and the same scene when
cone disease is present (right). Cone-degenerating diseases damage our
very sharpest "central" vision, leaving us unable to see faces,
read, or drive. Therefore, it's important to try to develop treatments.
There are 2 treatment strategies to consider:
1. Stop the disease from spreading; and
2. Regenerate the cones back to full function.
We cannot do research on humans, so research is done with animal models.
Ground
squirrels are a valuable model because (like humans) they depend on
daytime vision, and because they hibernate (November to March). In the
cold dark burrow, hibernating ground squirrels do not eat, drink, or
eliminate wastes. Body temperature falls to near freezing, their hearts
beat slowly, and they hardly breathe. Yet they remain pink and alive,
because their cold bodies are able to live off their fat stores. When
those run out, they "cannibalize" body parts that are not
being used, such as their sex organs and intestines. These unused tissues
shrink, but do not die.
From microscope studies in the 1970s, we believe that hibernating cones
shrink, too, in a process that resembles human cone-degenerating diseases.
In contrast, hibernating rods don't seem to shrink at all. In spring,
ground squirrels arouse to feed and mate. Within a week, their cones
regenerate fully and daytime vision is restored. This "degeneration-regeneration
cycle" has lots in common with the "disease-treatment cycle"
that medical scientists pursue hoping for a cure.
The project in my lab will be to trace the fate of rods and cones in winter and summer animals by tracking marker molecules that identify rods and cones. We will test the hypothesis that light absorbing molecules specific to cones will be absent from hibernating retinas, but that light absorbing molecules specific to rods will be present. This is a first step in understanding how cones regenerate successfully in the spring.
For more information about Dana Vaughan, go to her web page.
