When reflected from a surface, light can provide a representation of the spatial environment,
whilst gross changes in environment light can signal the time of day. The differing
sensory demands of using light to detect environmental space and time appear to have
provided the selection pressures for the evolution of different photoreceptor systems
in the vertebrates, and probably all animals. This point has been well recognised
in the non-mammals, which possess multiple opsin/vitamin A-based photoreceptor populations
in a variety of sites distributed both within and outside the CNS. By contrast, eye
loss in mammals abolishes all responses to light, and as a result, all photoreception
was attributed to the rods and cones of the retina. However, studies over the past
decade have provided overwhelming evidence that the mammalian eye contains a novel
photoreceptor system that does not depend upon the input from the rods and cones.
Mice with eyes but lacking rod and cone photoreceptors can still detect light to regulate
their circadian rhythms, suppress pineal melatonin, modify locomotor activity, and
modulate pupil size. Furthermore, action spectra for some of these responses in rodents
and humans have characterised at least one novel opsin/vitamin A-based photopigment,
and molecular studies have identified a number of candidate genes for this photopigment.
Parallel studies in fish showing that VA opsin photopigment is expressed within sub-sets
of inner retina neurones, demonstrates that mammals are not alone in having inner
retinal photoreceptors. It therefore seems likely that inner retinal photoreception
will be a feature of all vertebrates. Current studies are directed towards an understanding
of their mechanisms, determining the extent to which they contribute to physiology
and behaviour in general, and establishing how they may interact with other photoreceptors,
including the rods and cones. Progress on each of these topics is moving very rapidly.
As a result, we hope this review will serve as an introduction to the cascade of papers
that will emerge on these topics in the next few years. We also hope to convince the
more casual reader that there is much more to vertebrate photoreceptors than the study
of retinal rods and cones.