Not only do circadian
rhythms modulate virtually all sensory systems that interpret
environmental changes for their owners, but they also provide
a temporal framework which supports much of the physiological
adaptation to such changes. Work over the past 40 years
has established firmly that circadian rhythmicity is pervasive:
it is found (and displays the same general characteristics)
in prokaryotic blue green "algae" (bacteria), in human beings
and, with rare exceptions, everywhere in between; within
individual organisms multiple circadian rhythms have been
described at many levels of organization. Work over the
past 20 years has made it clear that the circadian system
is tractable: pacemaking oscillators have been identified
in many organisms, mutations affecting the timing process
have been found and, in some cases, the genes involved have
been cloned, promising work has begun on the biochemistry
and molecular biology that underlies the generation of rhythmicity,
and in complex organisms great progress has been made in
understanding the ways in which the central nervous system
(and its endocrine partners) generates and controls the
many behavioral and physiological rhythms that are essential
to life in the real world.
In the vertebrates
we can already see the outlines of a complete first level
explanation of circadian organization which will include
answers to such questions as: where are circadian oscillators
located? How do they interact with each other? How are they
influenced by the environment? How do they control the downstream
processes whose rhythmicity depends on their influence?
Furthermore,
in the vertebrates, we can begin to define the evolutionary
relationships between the circadian systems of non- mammalian
and mammalian vertebrates. Investigation of the comparative
physiology underlying these relationships has led to the
hypothesis that all vertebrates share a common "circadian
axis", the main components of which are the pineal gland,
the retina and the suprachiasmatic nucleus. This hypothetical
axis (like the adrenal axis) contains both neural and humoral
components and functions as a unit. Individual components
of the axis can be studied in vitro and (with greater difficulty)
the workings of the axis can be studied in intact, behaving
organisms. Surprisingly, the circadian axis is simpler in
mammals than in other vertebrates. I will briefly describe
what is known about the circadian axes of non-mammalian
vertebrates, contrast this with what we have learned about
the mammalian axis, speculate about the selection pressures
that have shaped the latter, and indicate the areas that
I consider most promising for future work.
