03OQ.9629/8l/690145-04$02.00/O
Copyright 0 1981 Pergamon Press Ltd
Camp Biochem. Physd
Vol. 69A, pp 145 to 148, 1981
Printed in Great Britam. All rights reserved
ABSENCE OF EXTRAOCULAR
PHOTORECEPTION
DIURNAL AND NOCTURNAL
RODENTS
EXPOSED
TO DIRECT SUNLIGHT
RANDY J. NELSON and IRVING ZUCKER
Department of Psychology, University of California, Berkeley, California
IN
94720, U.S.A.
(Receioed 11July 1980)
Abstract-l.
Entrainment of circadian wheel running activity was monitored over a period of several
months for northern grasshopper
mice (Onychomys [eucogasrer) and golden mantled ground squirrels
(Sperm&i/us
lateralis) housed out-of-doors
and exposed to direct sunlight (average light intensity =
55,000 Ix).
2. Sighted animals entrained their activity rhythms to the natural photoperiod;
the mice were nocturnal and the squirrels diurnal in their locomotor
activity.
3. The activity rhythms of all blind animals from each species failed to entrain to the light-dark
cycle
and free-ran with endogenous
periods > 24 hr for squirrels and < 24 hr for grasshopper
mice.
4. Failure of entrainment
in blind ground squirrels and grasshopper
mice indicates that these species
lack functional extraocular
photoreceptors;
they thus differ from non-mammalian
vertebrates
in which
extraocular
photoreception
is well documented.
5. The sirmificance of exclusive reliance bv mammals
on ocular photoreceptors
for entrainment
of
circadian &ythms is discussed.
man, 1975) on water consumption
(Zucker, 1971) and
on photic synchronization
of reproduction
in ferrets
(Herbert et al., 1978).
The potential for extraocular photoreception
does
exist since measurable quantities of natural and artificial light penetrate deep into the mammalian brain
(Ganong et al., 1963; Hartwig & van Veen, 1979).
Light reaching photocells implanted into the hypothalamus was attenuated 10-6-10-9-fold
by the skull
and soft tissues of the sheep, dog and rabbit. The
degree of attenuation was not determined accurately
in the rat because the light intensities exceeded the
measurement
capability of the photocell. Ganong et
al. (1963) suggested that the amount of light penetrating into the brain was inversely proportional
to the
size of the animal. Other experiments have suggested
an extraretinally mediated effect of light on serotonin
levels in the pineals of neonatal, but not adult rats
(Zweig et al., 1966; Wetterberg et al., 1970).
The limited number of mammalian species tested to
date and the near exclusive reliance on nocturnal animals leaves open the possibility of extraocular photoreception in some adult mammals (Rusak & Zucker,
1975; 1979). Furthermore,
laboratory studies of mammalian extraocular photoreception
have utilized light
intensities
on the order of 5&5001x. The light
intensities encountered by animals exposed to natural
sunlight in the temperate zone are orders of magnitude more intense and fall within the range of
70,00&100,000 lx (Benoit, 1964; Wurtman,
1975).
Since light intensity of the magnitude likely to be
encountered in nature can impinge on the deep structures of the mammalian
brain, it is possible that
extraocular
photoreception
exists in mammals, but
may not be demonstrable
under typical, low-intensity
laboratory lighting.
The present experiment was designed to test this
hypothesis. Grasshopper
mice (Onychomys [eucogas-
INTRODUCTION
Circadian rhythms are endogenous
oscillations that
have approximately
24-hr periods
and can be
entrained to the external environment by synchronizing agents or zeitgebers.
A potent zeitgeber
for
entrainment
of vertebrate circadian rhythms is the
light-dark
cycle (Biinning, 1973). The receptors by
which light is received and transduced into entrainment of circadian rhythms have been the subject of
considerable research and speculation during the past
15 yr (Rusak 8~ Zucker, 1979). It is now fully accepted
that the perception
of light by extraocular
photoreceptors plays a significant role in synchronizing
endogenous rhythms with the environmental
l&#-dark
cycle in non-mammalian
vertebrates (see Menaker &
Underwood,
1976; Benoit, 1970, for review). Extraocular photoreception
may be defined as the perception of electromagnetic
radiation with wavelengths
between 400 and 700nm by receptors
other than
those contained in the lateral eyes.
Many species of fish, reptiles and amphibians monitor the light-dark cycle by way of a third eye (parietal
or parapineal eye) or via the pineal itself (Adler, 1976;
Eakin, 1973). Birds, lacking a third eye, perceive light
through extraocular
receptors located in the brain
(Yokoyama & Farner, 1978). These receptors are capable of affecting physiological and behavioral activities in several avian species (Menaker & Underwood,
1976).
Blinded mammals
exposed to light-dark
cycles
exhibit free-running activity rhythms, as if the animals
were insensitive to the photoperiod
(Richter, 1965;
Halberg, Visscher & Bittner, 1954). Further evidence
that light perception in mammals is mediated exclusively through ocular photoreceptors
comes from demonstrations that blinding eliminates the effects of continuous illumination on the rat estrous cycle (Wurt145
146
RANDY J. NELSON and IRVING ZUCKER
rer) and golden mantled ground squirrels (Spermophilus lateralis) were blinded and then transfered to an
out-of-doors
enclosure where they were exposed to
direct sunlight for several hours daily. Locomotor activity cycles were monitored for evidence of entrainment to the illumination cycle.
MATERIALS AND METHODS
Twenty experimentally naive adult female grasshopper
mice and 12 adult female golden mantled ground squirrels
were introduced
to cages equipped
with activity wheels.
Each revolution of the wheel tripped a micro-switch
which
caused a pen to deflect on an event recorder in continuous
operation
at a chart speed of 45.7 cm/24 hr. Each individual’s running record was cut into 24 hr segments and successive days of activity pasted one beneath the other in the
traditional
fashion. During this adaptation
period all animals were exposed to 14 hr of light- per day from 7.00 to
21.00 hr. Pacific Standard Time (PST). After 24 wk. 13 of
the grasshopper
mice and 8 of the ground squirrels were
blinded by bilateral orbital enucleation
under methoxyflurane (Metofane)
anesthesia.
The remaining
animals
were
anesthetized
and served as controls. Grasshopper
mice and
ground squirrels were transported
from the laboratory
to
the Berkeley Field Station for Animal Behavior (37.53”N.
122.17”W) on November
19, 1979 and collectively
maintained in their individual activity wheel units in one of two
out-of-doors
enclosures. The interval between surgery and
transfer to the out-of-doors
facility was 15 days for ground
squirrels and 40 days for grasshopper
mice.
-Animals
were maintained
out-of-doors
from
midNovember.
1979 until March. 1980. Denendine. on cloud
cover, they received 3-7 hr of direct sunlight daily. Light
intensities, at cage level, measured with a Lamda Instruments Quantum
Radiometer-Photometer
(Model LI-170).
ranged from 30,000 to 80,0001x (mean = 55,0001x) for
measures
obtained
from 11.00-l3.00 hr. Animals
were
Fig. 1. Continuous
record of wheel running of a sighted
ground
squirrel
housed
out-of-doors
and exposed
to
natural photoperiod.
Numbers on the left indicate time in
days since this animal was moved out of doors. Time scale
(abscissa) runs from left to right (Pacific Standard Time in
hours) with successive days activity shown beneath each
other. During days 35-38 the animal was moved indoors
due to inclement weather. The heavy vertical lines running
the length of the record indicate
the times of sunset
(between 16.84 and 18.04 hr) and sunrise (between 06.70
and 07.42 hr). Sunset and sunrise times were obtained for
Oakland, California from U.S. Naval Observatory
Tables.
Fig. 2. Continuous
record
grasshopper
mouse. During
mouse was moved indoors.
in
of wheel running of a sighted
days 33 to 37 and 51 to 56 the
All symbols and conventions
as
Fig. 1.
moved indoors
on several occasions
during
inclement
weather. The times and durations
of interruptions
in outdoor exposure are indicated in the figures. Wheel running
activity was monitored for over 90 days beginning the first
day after the animals were placed outdoors.
RESULTS
AND
DISCUSSION
All sighted squirrels and mice entrained their activity rhythms and maintained
entrainment
for the
length of the experiments (Figs 1 and 2). The squirrels
first became active shortly after sunrise and typically
ran in the wheels throughout the daylight hours. Activity ceased at about the time of sunset (Fig. I). By
contrast,
grasshopper
mice were nocturnal,
with
wheel running activity beginning at sunset and terminating prior to sunrise (Fig. 2).
There was no evidence of entrainment
of activity
patterns in blind animals from either species; their
activity cycles free-ran with characteristic periods and
are illustrated for a representative
ground squirrel
(Fig. 3) and grasshopper
mouse (Fig. 4). The freerunning periods ranged from 23.48-23.67 hr for grasshopper mice and from 24.47724.90 hr for ground
squirrels.
Exposure to environmental
temperature
and humidity cycles, as well as to the noises generated by the
activity wheels of entrained, sighted conspecifics were,
in all instances,
ineffective for entraining
activity
rhythms of blind animals. These findings indicate that
photoperiod
is the principal proximate stimulus for
entraining activity rhythms in sighted animals.
Extraocular
photoreception
plays no functional
role in the entrainment of activity rhythms of the two
species investigated. Preliminary data on 6 other species of rodents exposed to intense artificial lighting
(approx 10,OBOlx) also indicate no entrainment of activity rhythms through extraocular
photoreceptors.
Absence
of extraocular
photoreception
in rodents
147
record of wheel running of a blind ground squirrel. The record has been doubleplotted to facilitate visualization of the continuity of the free-runs with time in hours indicated on the
Fig. 3. Continuous
abscissa. The activity cycle fails to entrain to the illumination
cycle and has a free-running
period of
approximately
24.83 hr calculated for Days 6676. During Days 35-38, 73-74 and 8485 the animal was
moved indoors. Symbols and conventions
as in Fig. 1.
These species include Peromyscus leucopus, Mesocricetus auratus, Mus musculus, Sigmodon hispidus,
Rattus norvegicus and Spermophilus beecheyi.
We cannot exclude the possibility that endpoints
other than locomotor
activity are influenced
by
extraocularly perceived light. It also remains possible,
although we consider it unlikely, that rodent extraocular photoreceptors
are functional only in the presence of the eyes.
Extraocularly perceived light influences neuroendo-
Fig. 4. Continuous
record of wheel running of a blind grasshopper
mouse. The record has been double
plotted with time in hours indicated on the abscissa. The activity cycle fails to entrain to the illumination
cycle and has a period of 23.66 hr calculated for Days 23-32. During Days 4446 and 63-66 the mouse
was moved indoors (see Fig. 1 for explanation
of symbols). This record differs from the others in that
this animal was first moved out-of-doors
on Day 10.
RANDY
148
J. NELSONand IRVINGZUCKER
in neonatal rats (Zweig et al., 1966;
Wetterberg at al., 1970). The functional significance of
laboratory demonstrations of extraocularly mediated
photoreception in neonatal rats is not known. This
species is nocturnal and fossorial and in the field the
young presumably are not exposed to light-dark
cycles during the brief period of sensitivity to extraocularly perceived light.
Mammals are alone among adults of the vertebrate
classes in lacking extraocular photoreception. In nonmammalian vertebrates extraocular photoreceptors
tend to be localized in the nervous system at some
remove from the organs whose activites they entrain.
Positioning of extraocular photoreceptors in the brain
may represent an initial step in the evolution of
photo-neuroendocrine
systems that reduces sensory
processing and allows for efficient integration among
these systems (Scharrer, 1964). In mammals, where
many physiological functions and behaviors are
influenced by light, a single set of photoreceptors is
parsimoniously employed to synchronize cellular
rhythms. These photoreceptors entrain a relatively
localized set of central nervous system circadian oscillators (Rusak & Zucker, 1979). This constitutes a different form of organization than is present in other
vertebrates; there is no reason to suppose that exclusive reliance by mammals on classical lateral eye
photoreceptors is disadvantageous relative to the
more diverse multiple receptor mechanisms employed
by other vertebrates.
crine function
Acknowledgements-We thank Darlene Frost, Michael
Bamat, Laura Beasley, Clarence Turtle, Mary Darragh,
Michael Boshes, John Schutz and Kathy Stout for their
assistance. This research was supported
by Grant
HD-14595 from the National Institute of Child Health and
Human Development, United States Public Health Service.
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