Evidence that thyroid hormone induces olfactory cellular proliferation in salmon during a sensitive period for imprinting
Center for Animal Behavior and Section of Neurobiology, Physiology and Behavior, One Shields Avenue, University of California at Davis, Davis, CA 95616, USA
* Author for correspondence (e-mail: sclema{at}ucdavis.edu or ganevitt{at}ucdavis.edu)
Accepted 15 June 2004
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Summary |
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Key words: thyroid hormone, olfactory imprinting, cell proliferation, neural progenitor cell, peripheral olfactory system, parr-smolt transformation, salmon, Oncorhynchus kisutch
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Introduction |
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Hasler and Scholz (1983)
suggested that the sensitive period for olfactory imprinting in salmon is tied
to changes in T4 that occur during smolting. In a now classic
study, they showed that hatchery-reared coho salmon (Oncorhynchus
kisutch) learned, remembered and homed to synthetic odors if they were
exposed to them during the parr-smolt transformation
(Scholz et al., 1976
). In a
similar study, Dittman et al.
(1996
) exposed hatchery-reared
coho salmon to synthetic or natural stream odors at several stages of
development and found that only fish that were exposed to odors during
smolting formed an imprinted memory. In addition, artificially elevating
thyroid hormone induced parr to imprint to artificial odorants, while parr
with unaltered hormone levels did not
(Scholz, 1980
).
We have suggested that the relationship between thyroid hormone and
olfactory imprinting may involve differential growth of the peripheral
olfactory system (e.g. Nevitt and Dittman,
2004). The olfactory nerve and glomerular structures in the bulb
grow dramatically during smolting in both coho and Atlantic salmon (Salmo
salar; Jarrard, 1997
),
and changes in the physiological sensitivity of olfactory receptor neurons to
imprinted odors have been documented in salmon
(Nevitt et al., 1994
;
Dittman et al., 1997
).
Combined, these results suggest that proliferation of olfactory receptor
neurons and growth in the input layer of the bulb may be part of the
neuro-substrate for imprinting. At the level of the olfactory epithelium, the
development and turnover of olfactory receptor neurons occurs by proliferation
of multipotent basal stem cells (Caggiano
et al., 1994
; Farbman,
1994
; Huard et al.,
1998
; Jang et al.,
2003
). Proliferation of these cells has been shown to be
stimulated by thyroid hormones in a variety of animals, including rats
(Mackay-Sim and Beard, 1987
;
Paternostro and Meisami, 1989
,
1994
) and frogs (Burd,
1990
,
1992
). Although it is not known
whether thyroid hormones influence proliferation in the salmon olfactory
periphery, the epithelium of masu salmon (Oncorhynchus masou) becomes
enriched with thyroid hormone receptors during smolting, suggesting that
olfactory tissues may be particularly sensitive to effects of thyroid hormones
at this time (Kudo et al.,
1994
).
We have begun to test these ideas by examining whether thyroid hormones
influence cell proliferation in the olfactory epithelium of juvenile coho
salmon. Because T4 is converted extrathryoidally by
5'-deiodinase to the intracellularly active form
3,5,3'-triiodothyronine (T3)
(DeGroot et al., 1978;
Köhrle, 1999
), we
manipulated thyroid hormone levels by intraperitoneally implanting
T3 or placebo pellets for 16-20 days to mimic smolting. We then
used an established 5-bromo-2'-deoxyuridine (BrdU) immunocytochemical
cell birth-dating technique to compare the density and spatial distribution of
mitotic cells and cell clusters within the olfactory epithelium between the
two treatment groups. Finally, we explored whether natural fluctuations in
plasma levels of T4 are associated with changes in cell
proliferation in the epithelium during smolting.
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Materials and methods |
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Experiment 1: thyroid hormone manipulations
Thyroid hormone levels were artificially elevated by implanting
T3 (or placebo) pellets intraperitoneally (5 mg of
3,5,3'-triiodo-L-thryonine per pellet; Innovative Research of
America, Sarasota, FL, USA). Fish were sacrificed 16-20 days after pellet
implantation to mimic the elevated thyroid hormones during smolting. At nine
months of age, parr were lightly anesthetized (MS-222 immersion, 1:1000;
Crescent Research Chemicals, Phoenix, AZ, USA) and the pellet was inserted
with surgical forceps into a small (5 mm) incision in the peritoneum. The
incision was sealed with all-purpose Krazy® glue (Elmer's Products Inc.,
Columbus, OH, USA). We measured the standard length and recorded the mass of
each fish. Placebo- and T3-implanted fish were housed separately
with equal numbers of non-implanted companion fish in two identical holding
tanks. Due to technical limitations, hormone levels and BrdU
immunocytochemical analysis could not be performed on the same individuals.
Separate fish from each treatment were thus used to measure hormone levels and
to quantify cell proliferation.
Experiment 2: natural fluctuations in thyroid hormones
From 18 December 1997 to 9 April 1998, we sampled unmanipulated, juvenile
salmon every two weeks to measure plasma levels of the thyroid hormones
T4 and T3 and to quantify cell proliferation in the
olfactory epithelium using the established BrdU immunocytochemical cell
birth-dating technique. Separate fish from the same cohort and holding tank
were used to measure hormone levels and to quantify cell proliferation for
each sampling day.
Blood collection for plasma hormone analysis
Blood was collected from the caudal peduncle using a heparinized capillary
tube. Blood was centrifuged, and plasma was stored at -80°C for subsequent
hormone analysis. Plasma was assayed for total T3 using a single
antibody enzyme-linked immunosorbent assay (ELISA) procedure (modified from
Schall et al., 1978). Binding
of the free hormone to other proteins was inhibited by ANS (8-anilonaphthalene
sulfonic acid). The binding of peroxide-labeled antibodies was measured by
reacting the peroxide to 3,3',5,5-tetramethylbenzidine. The reaction was
terminated with HCl and read at 450 nm using a microplate reader (Bio-Tek
EL311s). Each plate was prepared with five standards (0-50 ng
ml-1). Standard concentrations regressed against
loge(OD) yielded an r2 of >0.99.
BrdU immunocytochemistry
Fish were injected intraperitoneally with BrdU (0.05 mg g-1 body
mass; Sigma, St Louis, MO, USA). BrdU is incorporated into replicating DNA,
and the systemic application of BrdU is a well-established technique for
labeling mitotically active cells in a variety of taxa including fish (e.g.
Zupanc and Horschke, 1995;
Ekström et al., 2001
).
After a survival time of 1 h, BrdU-injected fish were deeply anesthetized
(MS-222 immersion, 1:1000) and perfused by intracardial injection of chilled
heparinized phosphate-buffered saline (PBS; 0.1 mol l-1) followed
by Bouin's fixative. Olfactory rosettes and the brain were dissected and
postfixed in Bouin's. At this time, the peritoneum was checked to confirm the
presence of the implanted pellet in the event that it might have been exuded
over the course of the experiment.
After postfixation for 12 h, tissue was dehydrated in a graded ethanol series, cleared in toluene and embedded in Paraffin. After serial sectioning (9 µm) and mounting, tissue sections were then deparaffinized and rehydrated. Chromatin was precipitated with 2 mol l-1 HCl (30 min), followed by quenching endogenous peroxidase activity with 3% H2O2 (15 min). Tissue was rinsed between each step with PBS-D (PBS containing 1% dimethylsulfoxide; Sigma). After a 2 h blocking reaction with normal horse serum (1.125% in PBSD), sections were incubated overnight at 4°C with primary anti-BrdU antibody solution (1:1000 PBS-D dilution; Sigma). Antibody binding was visualized by incubation with a biotinylated horse anti-mouse IgG, avidin-biotin-peroxidase complex (mouse IgG ABC Kit; Vector Laboratories, Burlingame, CA, USA) and diaminobenzidine (DAB) with nickel enhancement. Staining controls that included preincubating the primary antibody in the presence of excess BrdU prevented all immunohistochemical staining.
Quantification of BrdU-labeled cell density
The number of cells immunoreactive to BrdU was determined using
computer-aided analysis (NIH Image 1.60; National Institutes of Health,
Gaithersburg, MD, USA) of images captured via digital camera (Cohu
Inc., San Diego, CA, USA) attached to an Axioskop microscope (Zeiss,
Oberkochen, Germany). For each fish, we randomly selected one lamella from the
rosette and counted all BrdU-immunoreactive (BrdU-ir) cells in sections every
90 µm throughout the entire lamella. The progenitor cells that
differentiate into neurons in the olfactory epithelium occur in a specific
cellular layer at the base of the epithelium (e.g.
Caggiano et al., 1994;
Huard et al., 1998
).
Consequently, we divided the olfactory epithelium into two regions for
analysis: (1) the basal region and (2) the mid-apical region. BrdU-ir cells
were classified as within the basal region when any portion of the cell was
located within 10 µm of the basal edge of the epithelium; all other
BrdU-labeled cells were considered within the mid-apical region. This division
allowed us to distinguish between an increase in proliferation along the
basement membrane of the epithelium (the basal region) and proliferation
within the remainder of the epithelium (the mid-apical region).
We also quantified clusters of BrdU-ir cells. We defined a cluster as a group of at least two BrdU-ir cells that appeared to be in physical contact with each other. Results were analyzed as the number of BrdU-ir cells per length (µm) of lamella to standardize for any differences in lamellar sizes between fish.
Statistical analyses
To compare plasma T3 levels between T3-implant and
placebo treatments, the hormone values were loge transformed.
Comparisons were then made using a two-sample t-test.
We analyzed the number of BrdU-labeled cells using Mann-Whitney
U-tests to compare the number of positively immunoreactive cells
between treatments (SyStat 8.0; SyStat, Inc., Point Richmond, CA, USA).
Separate analyses were conducted for the clustered, single and total number of
BrdU-ir cells in the basal and mid-apical regions of the olfactory epithelium.
All statistical values are reported as means ±
S.E.M. As we specifically predicted that
elevated T3 would induce an increase in cell proliferation (e.g.
Nevitt and Dittman, 2004), all
Mann-Whitney U statistics are one-tailed.
To determine whether cells were randomly distributed among clusters, we
first tallied the occurrences of different sizes of clusters (i.e. number of
BrdU-ir cells per cluster) for each individual fish from both treatment
groups. We then compared the mean distribution of cluster sizes for each
treatment group to one created by Poisson (random) process. Because the
cluster data follow a truncated Poisson distribution
(Zar, 1996), we estimated the
Poisson parameter (
) for each distribution using tables provided in
Cohen (1960
). We also used a
2 test to compare the distribution of BrdU-ir cluster sizes
between treatments (Zar,
1996
). Exact P-values were obtained using StaTable 1.0.1
(Cytel Software Corp., Cambridge, MA, USA).
We used analyses of variance (ANOVAs) to test for changes in body mass (g),
standard length (mm) and condition factor {k; calculated as:
[(105)(body mass)/(standard length)3]; see
LeCren, 1951} over the nine
sample dates from December 1997 to April 1998. To evaluate changes in thyroid
hormone levels, we first used a Bartlett's test to examine homogeneity of
variances over the nine sampling days. When variances were equal among
sampling dates, we used an ANOVA to test for changes in hormone levels among
sampling days; when variances were unequal, a Welch test was used
(Zar, 1996
). All analyses were
conducted using JMP 4.0.2 statistical software (SAS Institute, Inc.).
We used Kruskal-Wallis tests to examine changes in the density of
BrdU-labeled cells and cell clusters over the nine sampling days. Spearman
rank correlations were used to examine the relationship between mean thyroid
hormone levels and mean density of BrdU-labeled cells in the epithelium among
sampling dates (Zar, 1996).
Because we predicted that an increase in thyroid hormone would relate to an
increase in the density of BrdU-labeled cells
(Nevitt and Dittman, 2004
),
tests for the significance of correlations were performed one-tailed.
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Results |
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Morphology and distribution of BrdU-labeled cells
BrdU-labeled cells were easily identified because they displayed a black
precipitate typical of the DAB reaction product with nickel enhancement (Figs
1,
2). BrdU-ir cells were
distributed throughout the basal and mid-apical regions of the epithelium. In
the basal epithelium, BrdU-ir cells tended to be round
(Fig. 1A). These basal cells
were morphologically consistent with globose basal stem cells, which give rise
to both olfactory receptor neurons and support cells
(Schwob et al., 1994;
Huard et al., 1998
). In the
mid-apical region, BrdU-ir cells tended to be more elongated
(Fig. 1B). In addition to
labeling single cells, BrdU labeled cell clusters. Clusters were groups of two
or more cells that appeared to be in contact with each other
(Fig. 1C). Although clusters
were identified throughout both the basal and mid-apical regions of the
epithelium, the basal region contained significantly more clusters than the
mid-apical region (Wilcoxon paired-sample test, T=37,
P<0.001, N=24).
|
|
T3 increases BrdU-ir cells in the basal cell layer
Fig. 2 shows representative
photomicrographs of BrdU labeling in the epithelium of both placebo
(N=11; Fig. 2A-C) and
T3-implanted fish (N=13;
Fig. 2D-F). Overall, the total
number of BrdU-labeled cells in the olfactory epithelium was significantly
greater in fish implanted with T3 pellets (19.07±3.41 cells
per 100 µm lamella length; mean ±
S.E.M.) than in fish with placebos
(10.48±1.65 cells per 100 µm lamella length) (Mann-Whitney
U-test, U=105, P=0.026). Within the basal region,
the total number of BrdU-ir cells was greater in T3-implanted fish
than in placebo fish (Fig.
3A:U=105, P=0.026). However, the number of
BrdU-ir cells distributed in clusters in the basal region was not
significantly different between treatments (U=74, P=0.443).
Thus, the observed difference in total BrdU-labeled cells resulted from a
greater number of single BrdU-ir cells in the experimental group
(U=106, P=0.024). By contrast, the total number of
BrdU-labeled cells in the mid-apical region did not differ between
T3 implant and control treatments
(Fig. 3B; U=94,
P=0.096). Both clustered (U=78.5, P=0.342) and
non-clustered BrdU-ir cells (U=93, P=0.107) were just as
frequent in the mid-apical layer in each treatment group. Finally, we found no
evidence for a significant increase in the total number of clusters
(T3 implant, 0.111±0.024 clusters per 100 µm lamella
length; placebo, 0.068±0.012; U=94, P=0.096).
Likewise, the number of clusters was similar between treatments within both
the basal (T3 implant, 0.075±0.021 clusters per 100 µm
lamella length; placebo, 0.046±0.008; U=83, P=0.253)
and mid-apical regions (T3 implant, 0.036±0.009 clusters per
100 µm lamella length; placebo, 0.022±0.006; U=86.5,
P=0.192). Even though we found significant differences in the density
of BrdU-labeled cells, this change in the rate of cell proliferation did not
translate into a difference between treatments in length of the olfactory
lamella (t-test, d.f.=22, t=-0.151, P=0.8815).
|
Evidence for targeted proliferation of clusters
Distributions of cluster sizes differed from a Poisson (random) process for
both T3 and placebo treatments
(Fig. 4; T3 implant,
2=12.6697, P=0.0004; placebo,
2=7.0978, P=0.008), suggesting that, in each case,
certain clusters were targeted to proliferate. We next compared distribution
patterns between treatment groups to examine potential influences of
T3 on this process. We found that, while large clusters seemed to
occur more frequently in the epithelium of T3-implanted fish
(Table 1), the overall
distribution of cluster sizes did not differ significantly between
T3 and placebo treatments (Fig.
5;
2=0.8289, d.f.=4, P=0.9345), suggesting
that targeted proliferation was not influenced by T3.
|
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|
Experiment 2: natural fluctuations in thyroid hormones
Morphological changes during smolting
Over the course of this study, fish lost their parr marks and turned a
silvery color characteristic of smolting. In addition, fish nearly doubled
their body mass (Fig. 6A;
ANOVA, F8,91=8.1153, P<0.0001) and increased
their body length by approximately 20% (F8,91=10.2666,
P<0.0001). Condition factor (k), an indication of fish
body status or `fatness' (LeCren,
1951), also showed significant changes from December 1997 to April
1998 (Fig. 6B;
F8,91=3.0190, P=0.0047). Pairwise comparisons
(Tukey HSD test, overall
=0.05) indicated a significant difference
between condition factors calculated for 3 January 1998 and 7 April 1998,
suggesting a decreasing trend over the four-month sampling period. Taken
together, the change in body coloration, increases in body mass and length,
and the decrease in condition factor suggest that fish transitioned from the
parr to smolt life stages over the course of the experiment (e.g.
Young et al., 1989
).
|
Plasma levels of thyroid hormones
Fig. 7 shows profiles of the
thyroid hormones T4 and T3 for the nine sample dates
spanning the four months of this study. During this time, T4 levels
fluctuated between approximately 10 and 20 ng ml-1 without an
obvious surge (ANOVA, F8,34=0.9853, P=0.4643).
Plasma levels of T3 were consistently low (range, 0.45-0.90 ng
ml-1) relative to T4 concentrations throughout the
duration of sampling and similarly showed no statistically significant changes
over the sampling period (Welch test, F=0.8308,
P=0.5898).
|
Relationship between T4 and cell proliferation
Fig. 8 shows profiles of
mean plasma T4 levels relative to the number of BrdU-labeled cells
per micron in the basal and mid-apical regions of the olfactory epithelium.
While the mean number of labeled cells fluctuated, there were no significant
differences in BrdU labeling among sampling dates in either the basal
(Kruskal-Wallis test, d.f.=8, H=8.428, P=0.393) or the
mid-apical (H=9.327, P=0.315) regions of the olfactory
epithelium. Linear regression of mean number of BrdU-labeled cells against
body mass showed no significant relationship in either the basal region
(r2=0.048, P=0.572) or the mid-apical region
(r2=0.123, P=0.355) of the epithelium. Given that
fish doubled in body size over this sampling period, cell proliferation in the
epithelium appeared to be unaffected by the rapid growth of the fish.
|
BrdU-ir cell counts were significantly correlated to plasma T4 levels. This positive relationship was restricted to the basal epithelium (Fig. 9A; clustered and single cells considered together; Spearman rank correlation, N=9, rs=0.600, P=0.05) and was not expressed in the mid-apical region (Fig. 9B;rs=0.233, P>0.25). The positive correlation between plasma T4 and overall cell proliferation appears to be due to a strong relationship between plasma T4 and the density of single BrdU-labeled cells in the basal epithelium (Fig. 9C; rs=0.683, P<0.05). No significant relationship was found between T4 and single BrdU-labeled cells in the mid-apical region (Fig. 9D; rs=0.433, P>0.05). Likewise, there was no significant relationship between T4 and the number of BrdU-labeled clusters in either the basal (rs=0.433, P>0.05) or mid-apical (rs=0.333, P>0.10) regions of the olfactory epithelium.
|
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Discussion |
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In addition to labeling single cells, we observed labeling in cell clusters
(see also Moulton et al.,
1970; Graziadei and Monti
Graziadei, 1979
; Suzuki and
Takeda, 1993
; Huard and
Schwob, 1995
; Weiler and
Farbman, 1997
). Weiler and Farbman
(1997
) have hypothesized that
this clustering results from a local stimulus activating the cell cycle in
several neighboring progenitor cells. If this is the case, then some clusters
exposed to the local stimulus should be expanding at a faster rate than others
that are either not exposed or are unresponsive to the stimulus. Our sample
size (overall N=24 fish) was large enough to test this idea
statistically, and our results support this idea. We found that the
distributions of cluster sizes differed significantly from random regardless
of treatment, indicating that there were more large clusters than expected by
chance. However, although we noted large (>5 cells) clusters more
frequently in T3-implanted than in placebo fish, we found no
difference in the distribution of cluster sizes among treatments. This
analysis suggests that T3 promotes a uniform increase in
proliferation rather than a shift in the distribution of cluster sizes toward
either larger or smaller clusters.
Identity of the proliferating cells
Our current understanding of neuronal proliferation in the olfactory system
is that the basal region contains globose basal cells that act as multipotent
progenitors within the epithelium (Caggiano
et al., 1994; Huard et al.,
1998
). These globose basal cells become mitotic and give rise to
both olfactory receptor neurons (Caggiano
et al., 1994
; Schwob et al.,
1994
; Jang et al.,
2003
) and non-neuronal cells such as sustentacular cells
(Huard et al., 1998
). It is
thus likely that part of the increased proliferation of basal cells that we
see in response to elevated T3 represents the enhanced production
of olfactory receptor neurons. This interpretation is consistent with other
studies in salmon showing targeted growth in the olfactory nerve and
glomerular layer coincident with the developmentally regulated surges in
thyroid hormone during the parr-smolt transformation. For example, studies in
Atlantic salmon suggest a quadrupling of olfactory receptor cell number, as
well as specific changes in the relative composition of the olfactory bulb
neuropil during this transition (Bowers,
1988
). More extensive investigation of Chinook salmon (O.
tshawytscha) confirmed these findings suggesting growth in the input
layer of the olfactory bulb coincident with smolting
(Jarrard, 1997
).
As a cautionary note, however, it has also recently been argued that BrdU
labeling may not be specific to cells in the process of mitosis (e.g.
Gould and Gross, 2002;
Rakic, 2002
); BrdU may in fact
label any cell that is undergoing DNA synthesis. It is thus possible that some
of these BrdU-ir cells are not mitotic but are in the process of DNA repair.
This intriguing explanation may account for the BrdU labeling we and others
(e.g. Weiler and Farbman,
1997
) have observed in the mid-apical layer of the epithelium
where mature olfactory receptor cells are much more numerous than stem cells.
Since the survival time following BrdU injection was only 1 h, it is doubtful
that these elongated cells represent developing olfactory receptor neurons
labeled in the basal region during S phase. A more likely possibility is that
these mid-apical cells are non-neuronal sustentacular cells. While beyond the
scope of the current study, double-labeling with a neuron-specific marker may
help to determine their identity in the future (but see
Rakic, 2002
).
Timing of olfactory imprinting
Our results examining naturally smolting fish suggest that even small
fluctuations in plasma T4 are associated with increased rates of
proliferation of the olfactory epithelium. This result suggests that even
subtle changes in the thyroid axis may influence growth in the peripheral
olfactory system. This relationship may in part explain why hatchery-reared
and wild salmon appear to imprint at different life stages (reviewed by
Dittman and Quinn, 1996). In
the wild, juvenile salmon leave the natal stream soon after emergence and
often smolt a considerable distance from where they hatched. Yet these fish
imprint on the natal stream and home to it as adults - not to the location
where they smolted. For example, wild Kokanee salmon (the non-anadromous form
of sockeye salmon, Oncorhynchus nerka) have been shown to imprint to
artificial odorants during the alevin and fry life stages (Tilson et al.,
1994
,
1995
). Thus, the sensitive
period for imprinting appears to be more variable than suggested by the
classic studies of Hasler and Scholz
(1983
).
A partial resolution to this paradox can be found in evidence that the
thyroid axis of fish is functional early in life (for review, see
Power et al., 2001).
T3 concentrations in fertilized eggs have been reported as high as
52 ng g-1 body mass for salmonids
(Mylonas et al., 1994
), and
maternal thyroid receptor transcripts have recently been identified in
developing eggs and alevins (Jones et al.,
2002
). In addition, fluctuations in thyroid hormone have been
linked to both hatching and emergence of fry from the gravel streambed
(Sullivan et al., 1987
;
Dickhoff and Sullivan, 1987
;
Leatherland et al., 1989
;
Tagawa and Hirano, 1987
,
1989
). There is also growing
evidence that novel stimuli encountered by juvenile salmon as they emerge from
the gravel, establish territories, forage and migrate downstream can stimulate
increases in circulating levels of thyroid hormones. For example, the thyroid
axis is sensitive to a variety of environmental cues including changes in
lunar phase (Grau et al.,
1981
), photoperiod (Hoar,
1976
; Iwamoto,
1982
), water temperature
(Iwamoto, 1982
;
Lin et al., 1985
) and changes
in water flow rates (Youngson and Simpson,
1984
; Lin et al.,
1985
). And exposing salmon to novel water sources can alter
thyroid activity (Dickhoff et al.,
1982
; Nishioka et al.,
1985
; Hoffnagle and Fivizzani,
1990
).
If the peripheral olfactory tissue is competent to respond to these
hormonal fluctuations, then variation in environmental conditions should lead
to subtle changes in cell proliferation in the olfactory epithelium. In the
case of hatchery-reared fish, this process is co-opted by smolting, simply
because the over-riding consistency of the hatchery environment fails to
stimulate the thyroid axis at other times of development
(Dittman and Quinn, 1996).
Thus, the parr-smolt transformation becomes the sensitive period for
imprinting, not because fish are specifically adapted to learn odor cues
during this stage of life but because the hatchery environment dampens
olfactory learning during earlier life stages.
Local influences of T4 are also regulated in part by enzymatic
deiodination of thyroid hormone at the target tissue (e.g.
Eales and Brown, 1993;
Köhrle, 1999
). While we
did not address deiodinase activity here, recent work suggests that the
olfactory epithelium of salmonids is less tightly regulated to respond to
circulating thyroid hormones than other sensory targets (e.g. the retina; see
Plate et al., 2002
). The
olfactory epithelium may thus be better able to mirror subtle changes in
plasma T4 triggered by changes in the fish's immediate environment.
Whether this process extends to other parts of the brain potentially involved
in imprinting (e.g. olfactory bulb and telencephalon;
Kihslinger et al., 2003
) will
be an interesting area for further research.
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Acknowledgments |
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References |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bowers, S. M. (1988). Morphological differences in the Atlantic salmon olfactory bulb between the parr and smolt stages of development. M.Sc. Thesis. Yale University.
Burd, G. D. (1990). Role of thyroxine in neural development of the olfactory system. In Proceedings of the Tenth International Symposium on Olfaction and Taste (ed. K. B. Doving), pp. 196-205. Oslo: GCA A.S. Press.
Burd, G. D. (1992). Development of the olfactory nerve in the clawed frog, Xenopus laevis: II. Effects of hypothyroidism. J. Comp. Neurol. 315,255 -263.[Medline]
Caggiano, M., Kauer, J. S. and Hunter, D. D. (1994). Globose basal cells are neuronal progenitors in the olfactory epithelium: a lineage analysis using a replication-incompetent retrovirus. Neuron 13,339 -352.[Medline]
Cohen, A. C. (1960). Estimating the parameter in a conditional Poisson distribution. Biometrics 16,203 -211.
DeGroot, L. J., Refetoff, S., Bernal, J., Rue, P. A. and Coleoni, A. H. (1978). Nuclear receptors for thyroid hormone. J. Endocrinol. Invest. 1, 79-88.[Medline]
Dickhoff, W. W., Darling, D. S. and Gorbman, A. (1982). Thyroid function during smoltification of salmonid fish. Gunma Symp. Endocr. 19,45 -61.
Dickhoff, W. W., Folmar, L. C. and Gorbman, A. (1978). Changes in plasma thyroxine during smoltification of coho salmon, Oncorhynchus kisutch. Gen. Comp. Endocrinol. 36,229 -232.[Medline]
Dickhoff, W. W. and Sullivan, C. (1987). Involvement of the thyroid gland in smoltification, with special reference to metabolic and developmental processes. Am. Fish. Soc. Symp. 1,197 -210.
Dittman, A. H. and Quinn, T. P. (1996). Homing
in Pacific salmon: mechanisms and ecological basis. J. Exp.
Biol. 199,83
-91.
Dittman, A. H., Quinn, T. P. and Nevitt, G. A. (1996). Timing of imprinting to natural and artificial odors by coho salmon, Oncorhynchus kisutch. Can. J. Fish. Aquat. Sci. 53,434 -442.[CrossRef]
Dittman, A. H., Quinn, T. P., Nevitt, G. A., Hacker, B. and Storm, D. R. (1997). Sensitization of olfactory guanylyl cyclase to a specific imprinted odorant in coho salmon. Neuron 19,381 -389.[CrossRef][Medline]
Eales, J. G. and Brown, S. B. (1993). Measurement and regulation of thyroidal status in teleost fish. Rev. Fish Biol. Fish. 3,299 -347.
Ekström, P., Johnsson, C.-M. and Ohlin, L.-M. (2001). Ventricular proliferation zones in the brain of an adult teleost fish and their relation to neuromeres and migration (secondary matrix) zones. J. Comp. Neurol. 436,92 -110.[CrossRef][Medline]
Farbman, A. I. (1994). Developmental biology of olfactory sensory neurons. Semin. Cell Biol. 5, 3-10.[Medline]
Gould, E. and Gross, C. G. (2002). Neurogenesis in adult mammals: some progress and problems. J. Neurosci. 22,619 -623.[CrossRef][Medline]
Grau, E. G., Dickhoff, W. W., Nishioka, R. S., Bern, H. A. and Folmar, L. C. (1981). Lunar phasing of the thyroxine surge preparatory to seawater migration of salmonid fish. Science 211,607 -609.[Medline]
Graziadei, P. P. C. and Monti Graziadei, A. G. (1979). Neurogenesis and neuron regeneration in the olfactory system of mammals: I. Morphological aspects of differentiation and structural organization of the olfactory sensory neurons. J. Neurocytol. 8,1 -18.[Medline]
Hasler, A. D. and Scholz, A. T. (1983). Olfactory Imprinting and Homing in Salmon. New York: Springer-Verlag.
Hoar, W. S. (1976). Smolt transformation: evolution, behavior, and physiology. J. Fish. Res. Board Can. 33,1233 -1252.
Hoffnagle, T. L. and Fivizzani, A. J. (1990). Stimulations of plasma thyroxine levels by novel water chemistry during smoltification in Chinook salmon (Oncorhynchus tshawytscha). Can. J. Fish. Aquat. Sci. 43,1513 -1517.
Huard, J. M. T. and Schwob, J. E. (1995). Cell cycle of globose basal cells in rat olfactory epithelium. Dev. Dyn. 203,17 -26.[Medline]
Huard, J. M. T., Youngentob, S. L., Goldstein, B. J., Luskin, M. B. and Schwob, J. E. (1998). Adult olfactory epithelium contains multipotent progenitors that give rise to neurons and non-neural cells. J. Comp. Neurol. 400,469 -486.[CrossRef][Medline]
Iwamoto, R. N. (1982). Strain-photoperiod-temperature interactions in coho salmon: freshwater growth, smoltification and seawater adaptation. Ph.D. Dissertation. University of Washington, Seattle.
Jang, W., Youngentob, S. L. and Schwob, J. E. (2003). Globose basal cells are required for reconstitution of olfactory epithelium after methyl bromide lesion. J. Comp. Neurol. 460,123 -140.[CrossRef][Medline]
Jarrard, H. E. (1997). Postembryonic changes in the structure of the olfactory bulb of the Chinook salmon Oncorhynchus tshawytscha across its life history. Brain Behav. Evol. 49,249 -260.[Medline]
Jones, I., Rogers, S. A., Kille, P. and Sweeney, G. E. (2002). Molecular cloning and expression of thyroid hormone receptor alpha during salmonid development. Gen. Comp. Endocrinol. 125,226 -235.[CrossRef][Medline]
Kihslinger, R., Alvarado, A., Silverman, I., Lema, S., Marchetti, M., Swanson, P. and Nevitt, G. (2003). Environmental rearing conditions produce differences in relative brain size in wild Chinook salmon Oncorhynchus tshawytscha. Program No. 561.7. 2003 Abstract Viewer/Itinerary Planner. Washington DC: Society for Neuroscience.
Köhrle, J. (1999). Local activation and inactivation of thyroid hormones: the deiodinase family. Mol. Cell. Endocrinol. 151,103 -119.[CrossRef][Medline]
Kudo, H., Tsuneyoshi, Y., Nagae, M., Adachi, S., Yamauchi, K., Ueda, U. and Kawamura, H. (1994). Detection of thyroid hormone receptors in the olfactory system and brain of wild masu salmon, Oncorhynchus masou (Brevoort), during smolting by in vitro autoradiography. Aquacult. Fish. Manag. 25 (Suppl. 2),171 -182.
Leatherland, J. F., Lin, L., Down, N. E. and Donaldson, E. M. (1989). Thyroid hormone content of eggs and early development stages of five Oncorhynchus species. Can. J. Fish. Aquat. Sci. 46,2140 -2145.
LeCren, E. D. (1951). The length-weight relationship and seasonal cycle in gonad weight and condition in the perch (Perca fluviatilis). J. Anim. Ecol. 20,201 -219.
Lin, R. J., Rivas, R. J., Grau, E. G., Nishoka, R. S. and Bern, H. A. (1985). Changes in plasma thyroxine following transfer of young coho salmon (Oncorhynchus kisutch) from fresh water to fresh water. Aquaculture 45,381 -382.[CrossRef]
Mackay-Sim, A. and Beard, M. D. (1987). Hypothyroidism disrupts neural development in the olfactory epithelium of adult mice. Dev. Brain Res. 36,190 -198.
McCormick, S. D., Hansen, L. P., Quinn, T. P. and Saunders, R. L. (1998). Movement, migration, and smolting of Atlantic salmon (Salmo salar). Can. J. Fish. Aquat. Sci. 55 (Suppl. 1),77 -92.[CrossRef]
Moulton, D. G., Çelebi, G. and Fink, R. P. (1970). Olfaction in mammals - two aspects: proliferation of cells in the olfactory epithelium and sensitivity to odours. In Symposium on Taste and Smell in Vertebrates (ed. E. Wolstenholme and J. Knight), pp. 227-238. London: Churchill.
Mylonas, C. C., Sullivan, C. V. and Hinshaw J. M. (1994). Thyroid hormones in brown trout (Salmo trutta) reproduction and early development. Fish Physiol. Biochem. 13,485 -493.
Nevitt, G. A., Dittman, A. H., Quinn, T. P. and Moody, W. J., Jr (1994). Evidence for a peripheral olfactory memory in imprinted salmon. Proc. Natl. Acad. Sci. USA 91,4288 -4292.[Abstract]
Nevitt, G. A. and Dittman, A. H. (2004). Olfactory imprinting in salmon: new models and approaches. In The Senses of Fishes: Adaptations for the Reception of Natural Stimuli (ed. G. von der Emde, J. Mogdans and B. G. Kapoor), pp.109 -127. New Delhi, India: Narosa Publishing House.
Nishioka, R. S., Young, G., Bern, H. A., Jochimsen, W. and Hiser, C. (1985). Attempts to intensify the thyroxine surge in coho and king salmon by chemical stimulation. Aquaculture 45,215 -225.[CrossRef]
Paternostro, M. A. and Meisami, E. (1989). Selective effects of thyroid hormonal deprivation on growth and development of the olfactory receptor sheet during the early postnatal period: a morphometric and cell count study. Int. J. Dev. Neurosci. 7, 243-255.[CrossRef][Medline]
Paternostro, M. A. and Meisami, E. (1994). Quantitative [3H] thymidine autoradiography of neurogenesis in the olfactory epithelium of developing normal, hypothyroid, and hypothyroid-rehabilitated rats. Dev. Brain Res. 83,151 -162.[Medline]
Plate, E. M., Adams, B. A., Allison, W. T., Martens, G., Hawryshyn, C. W. and Eales, J. G. (2002). The effects of thyroxine or a GnRH analogue on thyroid hormone deiodination in the olfactory epithelium and retina of the rainbow trout, Oncorhynchus mykiss, and sockeye salmon, Oncorhynchus nerka. Gen. Comp. Endocrinol. 127,59 -65.[CrossRef][Medline]
Power, D. M., Llewellyn, L., Faustino, M., Nowell, M. A., Björnsson, B. Th., Einarsdottir, I. E., Canario, A. V. M. and Sweeney, G. E. (2001). Thyroid hormones in growth and development of fish. Comp. Biochem. Physiol. C 130,447 -459.
Rakic, P. (2002). Adult neurogenesis in mammals: an identity crisis. J. Neurosci. 22,614 -618.[CrossRef][Medline]
Schall, R. F., Fraser, A. S., Hansen, H. W., Kern, C. W. and
Tenoso, H. J. (1978). A sensitive manual enzyme immunoassay
for thyroxine. Clin. Chem.
24,1801
-1804.
Scholz, A. T. (1980). Hormonal regulation of smolt transformation and olfactory imprinting in coho salmon. Ph.D. Dissertation. University of Wisconsin.
Scholz, A. T., Horrall, R. M., Cooper, J. C. and Hasler, A. D. (1976). Imprinting to chemical cues: the basis for homestream selection in salmon. Science 1972,1247 -1249.
Schwob, J. E., Huard, J. M. T., Luskin, M. B. and Youngentob, S. L. (1994). Retroviral lineage studies of the rat olfactory epithelium. Chem. Senses 19,671 -682.[Abstract]
Specker, J. L., Brown, C. L. and Bern, H. A. (1992). Asynchrony of changes in tissue and plasma thyroid hormones during the parr-smolt transformation of coho salmon. Gen. Comp. Endocrinol. 88,397 -405.[Medline]
Sullivan, C. V., Iwamoto, R. N. and Dickhoff, W. W. (1987). Thyroid hormones in blood plasma of developing salmon embryos. Gen. Comp. Endocrinol. 65,337 -345.[Medline]
Suzuki, Y. and Takeda, M. (1993). Basal cells in the mouse olfactory epithelium during development - immunohistochemical and electron-microscopic studies. Dev. Brain Res. 73,107 -113.[Medline]
Tagawa, M. and Hirano, T. (1987). Presence of thyroxine in eggs and changes in its content during early development of chum salmon, Oncorhynchus keta. Gen. Comp. Endocrinol. 68,129 -135.[Medline]
Tagawa, M. and Hirano, T. (1989). Changes in tissue and blood concentrations of thyroid hormones in developing chum salmon. Gen. Comp. Endocrinol. 76,437 -443.[Medline]
Tilson, M. B., Scholz, A. T., White, R. J. and Galloway, H. (1994). Assessment of smolt transformation tendency and the critical period for olfactory imprinting in kokanee salmon. 1993 Annual Report. U.S. Dept of Energy, Bonneville Power Administration. Project No. 88-63. Contract No. DE-8179-88BP91819.
Tilson, M. B., Scholz, A. T., White, R. J. and Hendrickson, J. L. (1995). Artificial imprinting and smoltification in juvenile kokanee salmon: implications for operating Lake Roosevelt kokanee salmon hatcheries. 1994 Annual Report. U.S. Dept of Energy, Bonneville Power Administration. Project No. 88-63. Contract No. DE-8179-88BP91819.
Weiler, E. and Farbman, A. I. (1997).
Proliferation in the rat olfactory epithelium: age-dependent changes.
J. Neurosci. 17,3610
-3622.
Yamauchi, K., Koide, N., Adachi, S. and Nagahama, Y. (1984). Changes in seawater adaptability and blood thyroxine concentrations during smoltification of the masu salmon, Oncorhynchus masou, and the amago salmon, Oncorhynchus rhodurus. Aquaculture 42,247 -256.[CrossRef]
Young, G., Björnsson, B. T., Prunet, P., Lin, R. J. and Bern, H. A. (1989). Smoltification and seawater adaptation in coho salmon (Oncorhynchus kisutch): plasma prolactin, growth hormone, thyroid hormones, and cortisol. Gen. Comp. Endocrinol. 74,335 -345.[Medline]
Youngson, A. F. and Simpson, T. H. (1984). Changes in serum thyroxine levels during smolting in captive and wild Atlantic salmon, Salmo salar L. J. Fish Biol. 24, 29-39.
Zar, J. H. (1996). Biostatistical Analysis. Third edition. New Jersey: Prentice Hall.
Zupanc, G. K. H. and Horschke, I. (1995). Proliferation zones in the brain of adult gymnotiform fish - a quantitative mapping study. J. Comp. Neurol. 353,213 -233.[Medline]