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First published online November 17, 2006
Journal of Experimental Biology 209, 4747-4750 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02573
Magnetic compass in the cornea: local anaesthesia impairs orientation in a mammal
Department of General Zoology, Institute for Biology, University of Duisburg-Essen, Campus Essen, Universitätsstraße 5, 45141 Essen, Germany
* Author for correspondence (e-mail: regina.wegner{at}uni-due.de)
Accepted 2 October 2006
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Summary |
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Key words: magnetic compass orientation, sensory transduction, mole-rat, cornea, magnetite, Bathyergidae
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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; Gudmunsson and Sandberg,
2000; Wiltschko and Wiltschko,
1995). Both magnetite-based receptors and retinal biochemical
processes have been shown to play roles in the biophysical magnetic signal
transduction in birds (Wiltschko and
Wiltschko, 2005). In mammalian compass orientation, Zambian
Ansell's mole-rats of the genus Fukomys (Bathyergidae) have been the
model species of choice. These subterranean, microphthalmic rodents orientate
themselves in their constantly dark ecotope with the help of the Earth's
magnetic field (Burda et al.,
1990). However, the primary biophysical signal transduction for
their light-independent polarity compass
(Marhold et al., 1997a) has
remained unclear.
Two hypotheses are currently considered to explain the sensory mechanism of
magnetic compass orientation. The first is based on retinal chemo-physical
radical-pair reactions (Ritz et al.,
2000), a system suggested as the primary signal transduction
process for migratory birds (Ritz et al.,
2004). This obviously light-dependent mechanism seems unlikely,
however, for the rodent genus Fukomys, whose members spend the
majority of their life underground in sealed burrow systems. A recent study
has ruled out the radical-pair mechanism for this rodent
(Thalau et al., 2006). The
second principal hypothesis, based on a primary involvement of magnetite,
seems much more reasonable for mole-rats, particularly as this mechanism is
light-independent. Magnetite (Fe3O4) has been considered
a possible basis for magnetic compass orientation in diverse species
(Fleissner et al., 2003;
Kirschvink and Gould, 1981;
Kirschvink et al., 2001;
Presti and Pettigrew, 1980;
Winklhofer et al., 2001). In
trouts (Walker et al., 1997)
and some bird species (Fleissner et al.,
2003; Hanzlik et al.,
2000; Williams and Wild,
2001; Winklhofer et al.,
2001), clusters of tiny magnetite crystals (diameter 
1-3 m)
were found in regions innervated by the ophthalmic branch of the trigeminal
nerve. Physiological studies indicated that this nerve may carry magnetic
field information to the brain (Beason and
Semm, 1996; Mora et al.,
2004). The immediate and long-term impairment of mole-rat nesting
orientation induced by a magnetic pulse designed to change the magnetisation
of magnetite (Marhold et al.,
1997b), implicates magnetite in the signal transduction mechanism
in these rodents. The highly mechano-sensitive structure of the cornea makes
it a prime candidate for the location of receptors translating magnetic field
information into mechanical signals.
In addition to these considerations, our preliminary findings of ferrous
inclusions in the corneal epithelium (Fig.
1) motivated behavioural tests on a possible corneal involvement
in magnetic compass orientation in mole-rats, i.e. on a prominent source of
ophthalmic nerve-mediated mechanosensibility. We made use of the spontaneous
nest-building drive of mole-rats to examine whether mechano-sensitive
desensitisation due to local anaesthesia of the corneal region affected their
magnetic compass orientation. In our established experimental design,
mole-rats place their nests predominantly in the southern sector of a circular
arena under control conditions (Burda et
al., 1990). We expected that any direct impairment of primary
magneto-receptors would result in random nest placement rather than the usual
directional behaviour.
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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), and hybrids between them [Fukomys were formerly
known as Cryptomys (Kock et al.,
2006)]. Importantly, these species do not differ in their
directional preferences. Animals were tested in pairs, since testing these
highly social animals in nesting experiments has proved more efficient when
done with at least two individuals (personal observations). Six pairs of
mole-rats were tested repeatedly, and other pairs were tested once in each
condition: control (N=40) and treatment (N=42) with
n being the number of pairs. This difference in sample size resulted
from two pairs that were split by natural death of one partner during the
study and could not be used for the respective control experiment. Most pairs
used in this study consisted of a male and a female breeder; a few were
sibling pairs. The animals derived from our breeding stock at the University
of Duisburg-Essen and were either born in the laboratory or captured in the
field, but had been living under laboratory conditions for at least 18 months
prior to experiments. Each animal carried a tissue compatible, subcutaneous
transponder (bio-capsule, 12 2.1 mm, ISO-standard 11784) with a unique number
code (ALVIC-transponder, ALVETRA GmbH, Neumünster, Germany), to ensure
individual identification. Mole-rats were housed at ambient room temperature
under natural daylight in glass cages filled with a layer of horticultural
peat and were fed ad libitum with carrots, potatoes, salad and
apples.
Experimental set-up
We initially tested orientation in six adult pairs of mole-rats under the
local geomagnetic field of Essen, Germany (45 T; 66° inclination) with
four replications in each condition. Then, we tested all available pairs of
mole-rats from our breeding stock once per condition (N=40 in
controls; N=42 in treatments) thus obtaining a large data set without
replicates (Batschelet, 1981).
In addition to the treatment conditions with local corneal anaesthesia, we
performed control experiments in which the cornea was treated with sodium
chloride solution.
Testing followed a previously used procedure
(Burda et al., 1990;
Marhold et al., 1997a):
mole-rat pairs were tested on warm days in an opaque plastic arena (80 cm
diameter, 30 cm height) in silent, outside premises of the University campus
in Essen, in the undisturbed local geomagnetic field. The arena was covered
with a thin layer of peat; tissue paper stripes and carrot pieces were spread
radially across the surface. A bucket was placed in the middle of the arena to
prevent undirected, central nesting. Animals were introduced into the arena at
random directional segments. During testing, the arena was covered with a
light impervious lid to exclude possible visual orientation. Mole-rat pairs
collected the tissue paper and built a nest; the exact nest position was then
recorded with reference to geographic north.
To exclude order effects, half of the subjects were tested first in the sodium chloride condition, and the others were tested first in the corneal anaesthesia condition. In the repeated tests group, the mole-rat pairs were tested alternately in the sodium chloride and corneal treatment conditions with at least a day between tests. Tests lasted from 30 min to 1 h. In tests lasting more than 30 min, the anaesthetic was re-applied to ensure that the cornea remained anaesthetised.
Although the magnetic compass of these rodents has been described as
light-independent (Marhold et al.,
1997a), the possibility that corneal anaesthesia disrupted a
photoreceptorbased magneto-sensory system in the mole-rats' eyes had to be
ruled out. Therefore in this study, we also tested the effect of the same
anaesthetic treatment on the animals' ability to discriminate light from dark
and to nest preferentially in darkness with a two-armed maze preference test
(see Wegner et al., 2006).
Mole-rat pairs had to make a choice between a dark and an illuminated chamber
for nesting; their choice was recorded. Controls and anaesthetic treatment
were also performed in the same alternating manner detailed above.
Study treatment
In the controls, mole-rat eyes were treated with sterile sodium chloride
solution used for medical and physiological purposes. For application, animals
were motivated to crawl into a paper roll (4.5 cm diameter; 9.5 cm length).
When inside this roll, the mole-rats were immobilized by gently gripping a
skin fold above the tail. Sodium chloride solution was then gently dropped
into the animals' eyes. In the same way, the animals' eyes were anaesthetised
with 2% Xylocain® solution (active substance: Lidocain hydrochloride;
Astra GmbH, Wedel, Germany), a surface anaesthetic used routinely in medical
practice for mucous membrane anaesthesia. The viscous liquid was applied
generously to the opened eyes with a soft brush. During this procedure the
animals did not show any adverse behaviour such as teeth chattering, distress
or aggression vocalisations. No efforts to clean their eyes were observed.
Experiments and Xylocain®-treatment conformed to the relevant regulatory
standards and were approved by the authorities of the University of
Duisburg-Essen and the District Government, Düsseldorf
(50.05-230-37/06).
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p) and length
(rp). The mean directions (p) of the six
repeatedly tested pairs were pooled in grand mean vectors for each testing
condition, with direction (M) and length
(rM). From the nesting data of the `once per condition'
group, we calculated the overall mean vector with direction
(A) and length (rA). The group mean
vectors (p) as well as the mean vector of the once per
condition group (A) were examined for significant
directional preferences with the Rayleigh-test of uniformity
(Batschelet, 1981; ORIANA 2.02,
Kovach Computing Services, Anglesey, UK); grand mean vectors
M and the two mean vectors of the once per condition group
were tested for differences in distribution between the study conditions with
the Watson's U2 test
(Batschelet, 1981; ORIANA
2.02).
The data from the two-armed maze preference tests which examined the
possible influence of the anaesthetic on the animals' retinal performance was
analyzed for a preferential choice using 
2 tests (SPSS®
12.0 for Windows).
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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In the experiment examining a possible retinal disturbance from the
Xylocain®, the mole-rats' behavioural response clearly showed that corneal
anaesthesia did not affect photoreceptor performance; their ability to
perceive light and prefer darkness for nesting was not disturbed
(N=11, 2=7.4, P=0.007).
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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)
reported findings of retinal crystalloid bodies in the inner photoreceptor
segment of the Ansell's mole-rat. The authors interpreted these structures as
potential magnetite grains, and suggested the retinal photoreceptors as the
magnetite-based structure. However, the ability of the mole-rats to
discriminate between light and dark
(Wegner et al., 2006) during
corneal anaesthesia suggests that magnetic compass orientation in
Fukomys is not photoreceptorbased because the application of
anaesthetics did not influence retinal performance or differential orientation
behaviour per se in our study. Our results point instead to a peripheral
ophthalmic locus as the stimulus mediator in these mammals. Given the
behavioural evidence presented here, previous neuroanatomical findings of
magnetoresponsive neurons in F. anselli identified within the inner
sublayer of the intermediate grey layer of the superior colliculus
(Nemec et al., 2001), i.e. in
a layer dominated by trigeminal input in other mammals
(Huerta and Harting, 1984),
could be interpreted as supporting the hypothesis that the putative primary
magnetoreceptors are located in the cornea. Systematic impairment experiments
involving specific bilateral section of the ophthalmic branch of the
trigeminal nerve and enucleation will provide definitive evidence for or
against the corneal magneto-receptor location.
We further hypothesized that in Zambian mole-rats, the mechano-sensors
mediating signals during magnetic orientation are magnetite-based. Along with
the previous findings that ruled out retinal chemo-physical radical-pair
reactions as the underlying signal mediating mechanism
(Marhold 1997b;
Thalau et al., 2006), our
results support innervated magnetite as the responsible sensory structure.
This is because desensitisation of the cornea significantly affected
mechano-sensibility and therefore magnetic stimulus transmission. Further
histological, histochemical, and ultramicroscopic examination of the mole-rat
cornea is clearly warranted. Given the accessibility of the cornea, our
finding may open new vistas for detailed studies of the primary transduction
mechanisms of magnetite-based magnetoreception in mammals.
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Acknowledgments |
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