First published online September 15, 2004
Journal of Experimental Biology 207, 3765-3774 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01205
Regulation of serotonin levels by multiple light-entrainable endogenous rhythms
M. Wildt1,2,
E. M. Goergen1,
J. L. Benton1,
D. C. Sandeman1 and
B. S. Beltz1,*
1 Department of Biological Sciences, Wellesley College, Wellesley, MA 02481,
USA
2 Universität Ulm, Neurobiologie, D-89097, Ulm, Germany
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Fig. 1. Diagram of the lobster brain, reconstructed from serial sections, showing
the location of the neuropils and cell somata relevant to this study.
Accessory lobe (AL); anterior median protocerebral neuropil (AMPN); antenna II
neuropil (AN); central body (CB); deutocerebral commissure (DC); dorsal giant
neuron (DGN); lateral antennular neuropil (LAN); oesophageal connectives
(OES); olfactory globular tract (OGT); olfactory lobe (OL); protocerebral
bridge (PCB); protocerebral tract (PCT); posterior median protocerebral
neuropil (PMPN); tegumentary nerves (TEG); soma cluster 9 (9); soma cluster 10
(10); soma cluster 11 (11). Terminology of Sandeman et al.
(1992 ).
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Fig. 2. HPLC chromatograms (A/D Instruments Inc.; PowerChrom version 2.2.4) showing
the peaks contained in a brain sample (A) compared with a serotonin (5-HT)
external standard (B).
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Fig. 3. Chronograms in which the serotonin (5-HT) levels in lobster brains measured
by HPLC are shown over a 24-h period, plotted as means ±
S.E.M. All three graphs show the measurements
taken after the animals had been in darkness (D:D) for 3 days without food.
The timing of light-on and light-off experienced by the animals during their
entraining L:D periods is marked by the black triangles. Note that in C
light-off has been shifted by 4 h in time and occurred at 14:00 h. (A) Pooled
data from B and C. (B) Serotonin levels in the brains of juvenile lobsters
that had been maintained for 2 weeks on an entraining 12 h:12 h L:D cycle. (C)
Serotonin levels in brains of animals that had been subjected to a
phase-shifted light cycle. In pooling the data, we matched the subjective
light-on and light-off transitions of the two groups. The pooled data (A) show
a pre-dusk peak and pre-dawn trough [N=8 for each time point; the
peak prior to subjective dusk is significantly different (P=0.039)
from the trough prior to subjective dawn]. (B,C) Plotting the data for the two
groups separately illustrates the phase shift of the peak and trough in the
animals that had been subjected to the shifted light regime (N=4 for
each time point).
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Fig. 4. An `anticipatory' rise or fall in the level of serotonin in the brains of
lobsters is revealed when the means of serotonin (5-HT) content in whole
brains (data from Fig. 3A) are
pooled and plotted against the hours since light-on or light-off in a
three-way ANOVA. Little change in serotonin levels follows light-on or
light-off either in the 0–4 or 4–8 h time periods after they have
occurred. In the 8–12 h period, however, the levels in the animals
approaching dusk increase and those approaching dawn decrease. These changes
occur well before the light-on or light-off transition.
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Fig. 5. Separate analyses of serotonin (5-HT) levels in individual regions of the
brains of juvenile lobsters that were entrained to a 12 h:12 h L:D light
regime followed by 3 days in D:D. Chronograms on the left show the raw data
collected from four individuals at each time point (values for some
measurements were the same and so appear as a single point). Chronograms on
the right show the arithmetic means (±
S.E.M.) of the data. All measurements were
made from animals that had been in constant darkness for 3 days. The most
distinctive aspect of the serotonin levels of the olfactory lobe (OL) and the
brain remainder (consisting of the protocerebral and tritocerebral areas, the
medial deutocerebral regions, the lateral antennular neuropil, the medial
antennular neuropil, the olfactory globular tract and the antenna II neuropil)
is the abrupt rise to a peak that occurs prior to subjective dusk followed by
an exponential-like decay (A,B,D,E). By contrast, serotonin levels in the
accessory lobes (ALs) rise during the subjective day to a high around dusk,
are sustained throughout the night hours and fall at subjective dawn (C,F)
(N=4 for each time point). t-tests between the peaks and
troughs in serotonin levels show significant differences in each of these
brain regions (AL, P=0.016; OL, P=0.029; brain remainder,
P=0.00034).
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Fig. 6. Combined serotonin levels in various areas in the brains of juvenile
lobsters. The levels of serotonin in all the accessory lobes (ALs; black),
olfactory lobes (OLs; dark gray) and brain remainders (light gray) are added
together for each of the six time points (data from
Fig. 5A–C). As in the
whole-brain studies (Fig. 3),
the pre-dusk peak is the most pronounced feature. Comparisons of the changes
in serotonin content in individual brain areas at the pre-dusk peak (11:00 h
compared with 15:00 h) show that serotonin levels in the ALs rise by 18%, the
OLs by 123% and the brain remainder by 197% during this period.
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© The Company of Biologists Ltd 2004
