J Neurophysiol 93: 2849 –2855, 2005.
First published December 22, 2004; doi:10.1152/jn.01113.2004.
Brain Responses to Ambient Temperature Fluctuations in Fish: Reduction of
Blood Volume and Initiation of a Whole-Body Stress Response
Erwin H. van den Burg,1,* Ronald R. Peeters,2,* Marleen Verhoye,2 Johannes Meek,1 Gert Flik,1 and
Annemie Van der Linden2
1
Department of Animal Physiology, Faculty of Science, University of Nijmegen, Nijmegen, The Netherlands; and 2Bio-Imaging Lab,
Campus Middelheim, University of Antwerp, Antwerp, Belgium
Submitted 26 October 2004; accepted in final form 17 December 2004
INTRODUCTION
A constant body temperature facilitates the proper functioning of essentially all organisms. In ectothermic animals, spatial
and temporal ambient temperature variations directly influence
cellular biochemistry and thus the physiology of the organism.
Many acclimation strategies have evolved throughout the animal kingdom to deal with temperature fluctuations (Crawshaw
et al. 1985). In the vertebrate brain, the preoptic area is
probably the most important thermoregulator (Boulant 2000;
Crawshaw et al. 1985). It contains both warm- and coldsensitive neurons, integrates temperature information from
different parts of the body, and, in endotherms, determines a
temperature set-point, a reference to which body temperature is
adjusted (Boulant 2000). Many studies have been directed to
responses of temperature-sensitive neurons (Boulant and Dean
1986; Griffin et al. 1996, 2001), but how the thermoregulatory
* E. H. van den Burg and R. R. Peeters contributed equally to this work.
Address for reprint requests and other correspondence: A. Van der Linden,
Bio-Imaging Lab, Campus Middelheim, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium (E-mail: Annemie.VanderLinden@
ua.ac.be).
www.jn.org
system is integrated with whole brain functioning remains
largely unknown. However, especially in ectotherms, proper
whole brain responses to temperature changes are fundamental
to body functioning, as the CNS initiates behavioral, physiological, and acclimational responses (Crawshaw et al. 1985).
Many aquatic ectotherms are subjected to temperature changes
up to as much as 15°C over just a few minutes and still manage
to maintain coordinated sensorimotor function (Montgomery
and MacDonald 1990). This led to the hypothesis that compensation mechanisms may exist within the neural system
(Montgomery and MacDonald 1990), but the nature of such
mechanisms was never elucidated. A recent study on cold
acclimation in fish showed that brain gene expression patterns
altered from 2 h after the onset of a 12°C temperature shift (Ju
et al. 2002). Although expression responses within a time span
of 2 h are considered rapid (Ju et al. 2002), they do not reflect
the immediate brain responses of the first acclimation phase to
an ambient temperature drop. This study was designed to
investigate rapid changes in brain physiology, i.e., within
minutes, after exposure of an ectothermic animal (the common
carp, Cyprinus carpio) to a 10°C temperature drop. Wholebrain responses (changes in cerebral blood volume and oxygenation of hemoglobin) and responses in specific brain areas
were investigated by means of high-resolution functional magnetic resonance imaging (fMRI). The high resolution made it
possible to study pituitary responses as well. The pituitary
gland is a primary target of the brain to orchestrate endocrine
whole-body physiological responses on environmental challenges.
METHODS
Animal handling
Carp (40 – 69 g body wt), acclimated to 25°C for ⱖ4 wk and kept
in 40-l tanks under approved laboratory conditions (density, feeding,
water quality, light regime), were lightly anesthetized with 0.011 wt%
ethyl meta aminobenzoate metanesulfonic acid salt (MS-222, Sigma)
buffered to pH 7.4 with NaHCO3. Next the fish were either transferred
to the fMRI set-up (Antwerp experiments) or an fMRI-like set-up
(Nijmegen experiments) to assess blood cortisol responses. In both
cases, a mouth piece was fitted that secured proper irrigation and
aeration of the gills; in doing this, anesthesia also was controlled. The
experimental procedures were approved by the Ethical Committees of
the Universities of Antwerp and Nijmegen, following federal laws.
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
0022-3077/05 $8.00 Copyright © 2005 The American Physiological Society
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van den Burg, Erwin H., Ronald R. Peeters, Marleen Verhoye,
Johannes Meek, Gert Flik, and Annemie Van der Linden. Brain
responses to ambient temperature fluctuations in fish: reduction of
blood volume and initiation of a whole-body stress response. J
Neurophysiol 93: 2849 –2855, 2005. First published December 22,
2004; doi:10.1152/jn.01113.2004. Spatial and temporal ambient temperature variations directly influence cellular biochemistry and thus
the physiology of ectotherms. However, many aquatic ectothermic
species maintain coordinated sensorimotor function during large acute
body-temperature changes, which points to a compensatory mechanism within the neural system. Here we used high-resolution functional magnetic resonance imaging to study brain responses to a drop
of 10°C of ambient water temperature in common carp. We observed
a strong drainage of blood out of the brain as of 90 s after the onset
of the temperature drop, which would be expected to reduce entry of
cold blood arriving from the gills so that the change in brain temperature would be slower. Although oxygen content in the brain thus
decreased, we still found specific activation in the preoptic area
(involved in temperature detection and stress responses), the pituitary
pars distalis (stress response), and inactivation of the anterior part of
the midbrain tegmentum and the pituitary pars intermedia. We propose that the blood drainage from the brain slows down the cooling of
the brain during an acute temperature drop. This could help to
maintain proper brain functioning including sensorimotor activity,
initiation of the stress response, and the subsequent behavioral
responses.
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VAN DEN BURG ET AL.
fMRI
Cortisol measurements
To determine the effect of the temperature drop on the endocrine
stress response (as measured by blood plasma cortisol levels), carp
were divided into seven groups of six animals each. Samples were
taken from carp from the first (control) group immediately after
anesthesia to determine basal blood plasma cortisol levels. Fish from
the second and third groups were anesthetized and placed individually
for 30 or 75 min in the MRI-like setup (which included scan noise).
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RESULTS
Global brain responses
The temperature drop strongly reduced CBV (increased
signal intensity due to decreased concentration of the applied
paramagnetic blood pool agent) throughout the brain, from 1.5
min after the onset of the drop onwards (Fig. 1, A and B).
Blood dissipated into two vessel systems, located rostroventrally to the brain, and caudally to the cerebellum and the vagal
lobes (Fig. 1, A and C). Angiography showed that the blood
vessels systems around the brain are sinuses, as judged by their
broad appearance (Fig. 2). Further increase of blood volume
was observed around the pituitary gland (Fig. 1A), where
sinus-like vessels appeared absent (Fig. 2); blood redistribution
in this region must therefore have occurred in a capillary
network.
CBV redistribution was accompanied by an increased
BOLD-signal in the brain (decreased deoxyhemoglobin content) and a decreased BOLD-signal in the blood vessels surrounding the brain (increased deoxyhemoglobin content; Fig.
1C). The close temporal relationship between blood volume
and deoxyhemoglobin content strongly suggests that the
BOLD signal in the brain is primarily determined by the
amount of cerebral blood. The BOLD signal in the sinus-like
vessels reaches a minimum after 3 min and then, although
blood is still entering, increases to ⬃20%. This decrease of
deoxyhemoglobin content is due to increased affinity of hemoglobin for oxygen at lower temperatures and is especially
evident in sinus-like blood vessels, as the blood concentration
per pixel is here extremely high when compared with brain
tissue. Thus during a rapid temperature change, carp brain cells
not only receive less oxygen because of the dramatic reduction
of blood volume but also because of increased affinity of
hemoglobin for oxygen. As less oxygen is available, it is
possible that metabolic activity of brain cells is suppressed, but
our BOLD-fMRI data do not discriminate among blood volume reduction, increased oxygen affinity of hemoglobin, and
altered metabolic activity. However, by the use of newly
developed data postprocessing protocols (Peeters and Van der
Linden 2002), it is possible to correct for the CBV and BOLD
changes observed throughout the brain, so that specific (metabolic) activities are revealed.
Specific brain responses
Specific responses in the brain were found in the preoptic
area of the hypothalamus, the anterior part of the midbrain
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Anesthetized carp were inserted into a custom-built stereotactic
apparatus combined with a customized headphone RF transmission
antenna (53 mm) and a circular surface receive antenna (22 mm)
positioned on the head of the carp but excluding the gills. The latter
is required to avoid motion interference on the images due to water
flow. The entire setup was mounted in the bore of the magnet while
anesthesia and sufficient aeration of the gills was ensured by a
flow-through system providing the fish, through a mouth piece, 500
ml water/min (containing 0.011% MS222). The temperature of the
water supplied to the carp was switched rapidly from 25 to 15°C,
which is within the natural preferred temperature range of this eurythermic species (Elliot 1981), through a three-way valve connected to
a second aquarium kept at 15°C; the 10°C-temperature drop took
precisely 5 min (Fig. 5).
All MRI experiments [n ⫽ 4 for both blood oxygenation leveldependent (BOLD) fMRI and cerebral blood volume (CBV) weighted
fMRI] were performed on a 7T horizontal SMIS MR microscope
(SMIS, Guildford, UK) with a horizontal accessible bore of 80 mm
diam and shielded gradients with a strength of 1 Gauss/mm. Sagittal
and horizontal T2-weighted SE high-resolution images [TE/TR ⫽
40/2000 ms, acquisition matrix: 256 ⫻ 128, field of view (FOV) ⫽ 40
mm, slice thickness ⫽ 1 mm, 2 averages] were acquired to accurately
identify different brain structures. BOLD-fMRI experiments, which
reveal changes in deoxyhemoglobin content and are used to estimate
cellular activity (Ogawa et al. 1990), involved collection of 12
consecutive horizontal slices through the carp brain (TE/TR ⫽ 10/450
ms, acquisition matrix: 128 ⫻ 64, FOV ⫽ 40 mm); the acquisition
time was 28.8 s for each set. CBV-weighted MRI was accomplished
in a similar way but after injection of 100 ␮l Clariscan (Amersham
Biosciences Europe GmbH, Roosendaal, The Netherlands) into the
caudal vessels.
Image acquisition was continuous and repeated every 30 s during
the entire experiment (100 acquisitions). After acquisition 20 (10 min
of imaging), the temperature was dropped and the low temperature
maintained during the following 80 acquisitions. The threshold for
plotting changes in BOLD- and CBV-weighted signal intensities was
set at a 2 or 10% change relative to the signal intensity before the
temperature drop.
In the pituitary gland, signal intensity changes were analyzed per
pixel (pixel size is 234 ␮m) as a function of time, both in CBV- and
BOLD-weighted MRI. From computing the slope (which is the
change in time per unit of percentual signal change relative to the
signal intensity before the temperature drop) within the time-interval
from 1 to 3 min after the onset of the temperature drop, an accurate
functional map was obtained that allowed detection of regional slope
changes within the small pituitary gland.
For angiography, gradient echo images were obtained in three
different directions (sagittal, horizontal, and coronal) with 40 slices
with the following parameters: TE 7 ms; TR 20 ms; FOV 30 mm; slice
thickness 0.5 mm; and acquisition matrix 256 ⫻ 256. Each set of 40
subsequent angiographic images was submitted to a maximum intensity projection algorithm (programmed in Interactive Data Language).
Brain structures in the images were identified by light microscopy
on the basis of horizontal sections; nomenclature follows Meek and
Nieuwenhuys (Meek and Nieuwenhuys 1998).
The fish of the remaining four groups were treated similarly to the
second group but were then subjected to a 10°C temperature drop by
switching the water inlet irrigating the gills to 15°C. Blood of animals
belonging to groups 3– 6 were collected 1, 5, 15, and 45 min,
respectively, after the onset of the temperature drop. From each
animal, 300 ␮l of blood was collected by puncture of the caudal
vessels using a 21-Gauge syringe with Na2EDTA to prevent clotting.
After centrifugation (500 g, 10 min, 4°C), plasma was collected and
stored at –20°C. Cortisol was measured by radioimmunoassay
(Arends et al. 1998). The Mann-Whitney U test was used to assess the
effects of handling prior to and during the temperature drop and to
determine the effects of the temperature drop by testing the blood
plasma cortisol values of fish from the shocked groups against these
values of fish from the group that stayed in the MRI-setting for 30 min
prior to the temperature drop.
BRAIN RESPONSES TO TEMPERATURE CHANGES IN FISH
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tegmentum, and the brain stem (Fig. 3). CBV decreased in the
preoptic area but slightly slower than in the rest of the brain
(1.5% decreased signal intensity in the corrected trace; Fig. 3B,
left). Apparently, a mechanism is initiated to deliver more
blood to this region to compensate for the temperature-induced
drainage of blood out of the brain. A compensatory reaction is
generally observed in mammalian brains to supply oxygenated
blood to accommodate increased oxygen consumption by activated cells (Ogawa et al. 1992). Indeed, neurons in the
preoptic area increased their metabolic activity in response to
the temperature drop as evidenced by the immediate (within
30 s) and significant (up to 6% after 1.5 min) decrease of the
BOLD-signal intensity (Fig. 3B, right). Activity of the preoptic
area ceased over time during the experiment: blood volume
decreased to values similar to those observed in the rest of the
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brain and the BOLD-signal intensity remained only 1.5%
lower during the entire exposure period to 15°C. Similar
negative BOLD-fMRI data were reported for hypothalamic
areas in mammals (Becerra et al. 1999; Matsuda et al. 1999;
Yokawa et al. 1995).
Metabolic activity in the anterior part of the midbrain tegmentum, a region relatively poor in cells, was transiently
reduced, keeping in step with activation of the neighboring
preoptic area (Fig. 3C). On the basis of timing of activity
changes, it seems possible that reduced activation in the midbrain tegmentum serves to compensate for increased oxygen
demand in the preoptic area in the early phase of the response
to the temperature drop. This suggests that there must be
connections between the midbrain tegmentum and the preoptic
area, but those have not been investigated to date. Another or
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FIG. 1. Blood redistribution in the carp brain after a 10°C temperature drop. A: horizontal high-resolution (1 and 2) and cerebral blood volume
(CBV)-weighted magnetic resonance images (MRIs, 3 and 4) through the carp brain showing a dramatic CBV-reduction throughout the brain (4, red).
Blood is collected (blue) caudally to the vagal lobes (vl) and the cerebellum, and rostroventrally to the brain (4). fl, facial lobe; eg, granular eminence;
vcm, medial cerebellar valvula; vcl, lateral cerebellar valvula; to, optic tectum; t, telencephalon; and no, olfactory nerve. The color bars reflect the signal
intensity changes relative to the signal intensity before the temperature drop. Scale bar ⫽ 1 cm. B: changes of signal intensity in CBV- and blood
oxygenation level-dependent (BOLD)-weighted MRI as measured in an entire brain slice. C: blood volume and deoxyhemoglobin changes in a sinus-like
blood vessel caudal to the cerebellum.
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alternative explanation for the reduced activity in the midbrain
tegmentum is that this region is involved in an escape reaction
as it is associated with locomotion (Meek and Nieuwenhuys
1998).
Part of the brain stem showed minor activation as judged by
a small (2.5% 2 min after the onset of the temperature drop)
decrease of BOLD-signal intensity (Fig. 3D). In many vertebrates, the brain stem is involved in thermoregulation (Crawshaw et al. 1985), but the small response observed here
suggests that the preoptic area is by far the most important of
the two thermo-sensitive systems in carp.
Pituitary responses
FIG.
The small pituitary gland (diameter: 1.8 ⫾ 0.15 mm; n ⫽ 8)
could easily be identified in anatomical and BOLD images, and
its two lobes, the pars intermedia and the pars distalis (Wendelaar Bonga 1997), could be distinguished separately (Fig.
4A). This is of particular interest because the pituitary gland
receives neuroendocrine input from the preoptic area and
converts this input into an endocrine signal that directs many
physiological processes throughout the body. Therefore we
FIG. 3. CBV and BOLD changes in the
preoptic area (poa), the anterior part of the
midbrain tegmentum (mt), and the brain
stem (bs) of a carp brain after a 10°C temperature drop. A: BOLD-fMRI shows decreased (blue) BOLD-contrast in the poa and
the bs and increased contrast in the mt (red).
A1 was obtained before, and A2 and A3, 5
and 35 min after the onset of temperature
drop, respectively. The color bar reflects the
change of signal intensity relative to the
signal intensity before the temperature drop.
Scale bar ⫽ 1 cm. B, left: CBV-signal intensity changes in the poa. B, right: BOLDsignal intensity changes in the poa. C, left:
CBV-signal intensity changes in the mt. C,
right: BOLD-signal intensity changes in the
mt. D, left: CBV-signal intensity changes in
the bs. D, right: BOLD-signal intensity
changes in the bs. All traces were obtained
after correction for global changes (Peeters
and Van der Linden 2002). Data are presented as means ⫾ SD; n ⫽ 4 in all figures.
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2. Angiogram’s of the brain and surrounding regions in the carp.
Large vessels are shown in white and are found in the gills, anteroventral to the
pituitary gland (pit), caudal to the cerebellum and the vagal lobes, and caudal
to the eyes (B, 4). Blood dissipates to the latter 2 vessel systems after the 10°C
temperature drop. The blood vessels close to the pituitary gland branch off the
arteries arriving from the gills (A, 1). No other large blood vessels are found
in the pituitary region, indicating that the redistribution of blood must have
occurred in capillaries.
BRAIN RESPONSES TO TEMPERATURE CHANGES IN FISH
studied responses in the pituitary pars distalis and pars intermedia, without correction for the over-all effects observed in
the brain, as the pituitary gland is endocrine rather than neural
tissue and situated outside the brain. It appeared that the pars
intermedia and the pars distalis responded differently to the
temperature drop, both on CBV- and BOLD-weighted MRI
(Fig. 4A). The contrast-enhanced signal intensity revealed a
decrease in blood volume (up to 29% signal intensity increase)
in the pars intermedia (Fig. 4B). In contrast, the blood volume
in the pars distalis increased almost immediately (within 90 –
120 s) and reached plateau values (36% lowered signal intensity) from 10 min onward (Fig. 4B). BOLD-fMRI also revealed, consistently, an opposite response in the pars intermedia as compared with the pars distalis (Fig. 4C). The BOLDsignal in the pars intermedia showed an initial transient
increase of 4.9% within 1– 4 min, followed by a second
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increase to a constant elevation of 14% during the exposure to
the low temperature. The BOLD signal in the pars distalis
decreased within 90 –120 s after the onset of the temperature
drop and stayed low (⫺26%) during the remainder of the
experiment. Thus the pars distalis received more blood (Fig.
4B) and consumed more oxygen (Fig. 4C) after the rapid
temperature drop and can therefore be considered as highly
active from 2 min onward. Apparently, the increased blood
delivery does not completely compensate for the increased
oxygen consumption in this region. On the other hand, blood
volume in the pars intermedia decreased simultaneously with
an increase of the oxygenated hemoglobin versus deoxygenated hemoglobin ratio, characterizing an inactivation of this
pituitary lobe.
One of the cell types located in the pars distalis are the
corticotrope cells, which produce and release adrenocorticotropic hormone (ACTH). ACTH stimulates the release of
cortisol by the interrenal cells in the head kidney (equivalent of
the adrenal gland in mammals) (Wendelaar Bonga 1997).
Corticotrope cells are under positive control of corticotropinreleasing hormone (CRH), which is released by neuroendocrine cells in the preoptic area during stress (Wendelaar Bonga
1997). The sequential activation of the preoptic area (within
30 s) and the pars distalis (within 90 –120 s) may thus indicate
the initiation of an endocrine stress response. To investigate
whether a stress response occurred, we analyzed blood plasma
samples for cortisol from carp experiencing fMRI analysis
(including mild anesthesia, intubation, and scan noise). Plasma
cortisol levels increased within 5 min after the onset of the
temperature drop and remained elevated for the next 45 min
(Fig. 5, stressed group), similar to cortisol responses seen in
free-swimming carp (Tanck et al. 2000). The cortisol response
to the temperature drop by far exceeded the mild elevation of
cortisol levels after prolonged (75 min) residence in the MRIlike setup. We conclude that the sequential activation of the
preoptic area and the pars distalis indeed reflects an endocrine
stress response.
DISCUSSION
This paper describes, for the first time, an fMRI study in a
lower vertebrate, and reveals new important insights into how
an ambient temperature change affects the activity of the brain
FIG. 5. Blood plasma cortisol levels following the procedures inherent to
fMRI and the effect of a 10°C ambient temperature drop in anesthetized carp.
Cortisol levels in immediately anesthetized carp (t ⫽ ⫺30 min) and carp that
were anesthetized and placed in a MRI-like setup for 30 min (t ⫽ 0 min) are
not significantly different, whereas they are slightly higher in carp that were
kept for 75 min in this setup (t ⫽ 45 min, #, P ⬍ 0.05). A 10°C temperature
drop, initiated at t ⫽ 0 min, increases plasma cortisol levels 4.5-fold in
anesthetized carp 45 min after the onset of the temperature drop (**, P ⬍ 0.01).
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FIG. 4. CBV and BOLD changes in the pituitary gland of carp after a 10°C
temperature drop without correction for global responses. A: a functional map
obtained from a pixel-to-pixel analysis of BOLD data reveals opposite responses in the pars intermedia (pi; pink) and pars distalis (pd; green/blue). The
color bar reflects the slope value calculated from the change of signal intensity
in time, relative to the signal intensity before the temperature drop. Scale bar ⫽
1 cm. B: CBV changes in pi and pd. Blood volume decreases (increased signal
intensity) in the pi and increases in the pd. C: BOLD-signal intensity changes
in the pi and pd. Oxygen consumption increases in the pd (decreased signal
intensity) and decreases in the pi. Data are presented as means ⫾ SD; n ⫽ 3
in all figures.
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J Neurophysiol • VOL
of the change activates cells in the preoptic area rather than the
absolute temperature change. Our hypothesis is supported by
the observation that the level of activity does not change during
the final phase of the temperature drop and decreases again
when the temperature drop is complete. Activation of the
pituitary pars distalis likely results from a strong activation by
the preoptic area during the initial phase of the response to the
temperature drop and persists even though activity of the
preoptic area ceases. It thus appears that brain initiates a stress
response in the preoptic area, which persists while the preoptic
area inactivates similar to all other brain regions.
The stress response is the first step in the process of acclimation, which leads to a number of biochemical adjustments
of, among others, enzyme expression levels (Hazel 1993; Metz
et al. 2003) and membrane lipid composition (Hazel 1993;
Buda et al. 1994). However, temperature acclimation is energetically costly (Claireaux et al. 1995), and, if possible, fish (as
well as other vertebrates) migrate to an environment of a
temperature at or close to the initial temperature (Claireaux et
al. 1995; Tanaka et al. 2000), before the temperature of the
CNS is changed (Crawshaw 1980). Such a behavioral thermoregulatory response has been attributed to cellular activity
changes in the anterior brain stem (reviewed by Crawshaw et
al. 1985), but our fMRI data do not reveal any major activity
changes in this region. This either means that the brain stem is
not involved in temperature selection in carp, that the thermoregulatory nuclei are too small or diffuse to be separately
detected, or that the behavioral response was suppressed by the
mild anesthesia.
Neuronal activity during a rapid temperature drop not only
depends on the supply of oxygen-rich blood but is also influenced by temperature itself. For instance, temperature directly
influences conduction velocity (Harper et al. 1990), reliability
of synaptic transmission (Hardingham and Larkman 1998), and
probability of transmitter release (Volgushev et al. 2004).
Recently it was shown in goldfish (Carassius auratus) that a
10°C temperature drop in 50 min changes properties of the
Mauthner cell as well as of the feed-forward inhibition to this
cell (Preuss and Faber 2003). As a result, startle-escape behavior to an acoustic stimulus changes such that the probability of
an escape reaction is increased, but more often toward rather
than away from the stimulus. From the preceding text it may be
clear that ambient water temperature has a major impact on
cellular and network responses in the brain to certain stimuli,
and therefore it is important to limit temperature fluctuations as
much as possible. The reduced CBV during the temperature
drop we observed in carp may thus be one of the compensatory
mechanisms present in the CNS of aquatic ectothermic vertebrates to maintain behavioral performance and sensorimotor
function.
ACKNOWLEDGMENTS
We thank F.A.T. Spanings for technical assistance and animal care.
GRANTS
This work was financially supported by the Belgian National Fund for
Scientific Research—Flanders (FWO) and the University of Antwerp Research
Fund of the University of Antwerp—RUCA (RAFO) and Special Research
Fund (BOF).
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and pituitary gland of an ectothermic animal, the common carp.
The possible effects of the light anesthesia on the brain and
pituitary responses are likely small, as indicated by the strong
cortisol response to the temperature drop, which was similar to
that observed in free-swimming carp (Tanck et al. 2000). Also,
the light anesthesia strongly reduced the cortisol response to
handling and restraint that normally induce high blood plasma
cortisol levels (Van den Burg et al. 2004). Thus the use of
anesthesia makes it possible to study the effects of a temperature drop on cerebral blood volume and oxygenation in
isolation from other stressful events.
The drainage of blood from the brain may serve to temporarily isolate the brain from the circulation and protect it
against cooling down too rapidly. A similar thermoregulatory
role of blood volume reduction during cold exposure is brought
about by vasoconstriction in the mammalian skin, where most
of the temperature between the body and the environment takes
place (Charkoudian 2003; Johnson and Proppe 1996). In fish,
body temperature is mainly determined by heat exchange
between ambient water and blood in the gills, as the gills are
the structures where blood gets into close contact with ambient
water over the largest body surface area (in fact, only the
buccal and opercular cavities are in close contact with water in
our setup). Ambient water contacting the head hardly influences brain temperature, as the brain is insulated by a layer of
fat (personal observation). Thus a reduced blood flow through
the brain could provide a means to dampen a temperature drop,
and keep brain temperature initially more constant. In contrast,
cerebral blood flow in mammals (rodents) exposed to cold
increases to accommodate increased metabolic activity of neuroglia in the brain (Szelenyi and Donhoffer 1978). It has been
hypothesized that neuroglia, especially astrocytes, play a key
role in thermogenesis within the brain (Szelenyi 1998).
It is well known that a constant brain temperature permits
fish to expand their niche into colder environments: endothermy arose three times during evolution of large marine fish,
and its development is always associated with a movement into
colder water (Block et al. 1993). Some species, such as swordfish (Xiphias gladius) use cranial endothermy only; a constant
brain temperature permits them to dive to colder waters,
spanning a temperature difference of up to 19°C (Carey 1982).
These observations suggest that the brain cannot withstand low
temperatures without further adaptations. Thus isolation of the
brain from incoming blood during a temperature drop, as we
found in carp, could be a mechanism employed by aquatic
ectotherms to dampen temperature fluctuations in the brain. It
is likely, however, that it is not as efficient as (cranial)
endothermy, and it will not keep brain temperature higher than
ambient water temperature during the total period of acclimation. Also, this mechanism limits oxygenated blood entry into
the brain and may thus compromise neuronal activity. It is
therefore interesting that some brain regions were highly active
during the temperature drop.
Activation of the preoptic area occurred within 30 s after the
onset of the temperature drop. This brain region integrates
peripheral and central temperature information (Boulant 2000)
and contains thermo-sensitive neurons (Nelson and Prosser
1981a,b). These neurons could very well stimulate the neighboring neuroendocrine neurons that release CRH to elicit a
stress response. The ambient temperature change during the
first 30 s is considerable (2°C), and it is likely that the rapidity
BRAIN RESPONSES TO TEMPERATURE CHANGES IN FISH
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