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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

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Regulation of serotonin levels by multiple light-entrainable endogenous rhythms

M. Wildt^1,2, E. M. Goergen¹, J. L. Benton¹, D. C. Sandeman¹ and B. S. Beltz^1,*

¹ Department of Biological Sciences, Wellesley College, Wellesley, MA 02481, USA
² Universität Ulm, Neurobiologie, D-89097, Ulm, Germany

View larger version (55K): [in a new window]   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).

View larger version (35K): [in a new window]   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).

View larger version (13K): [in a new window]   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).

View larger version (15K): [in a new window]   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.

View larger version (20K): [in a new window]   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).

View larger version (29K): [in a new window]   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.