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Serotonin sets the day state in the neurons that control coupling between the optic lobe circadian pacemakers in the cricket Gryllus bimaculatus

A. S. M. Saifullah and Kenji Tomioka^*

Department of Physics, Biology and Informatics, Faculty of Science and Research Institute for Time Studies, Yamaguchi University, Yamaguchi 753-8512, Japan

View larger version (23K): [in a new window]   Fig. 1. The arrangement used to record neural activity from the optic stalk brain efferents. Neural activity was recorded from the separated optic stalk using a suction electrode. Light stimulation and injection of chemicals were performed on the contralateral side. AN, antennal nerve; CE, compound eye; CL, cerebral lobe; LM, lamina medulla complex; LO, lobulla; OS, optic stalk.

View larger version (55K): [in a new window]   Fig. 2. Spontaneous and light-evoked activity recorded from the separated proximal optic stalk of a dark-adapted eye before and after injection of 10 pmol serotonin at daytime. Responsiveness increased with increments in light intensity. Injection of serotonin into the optic lobe reduced the spontaneous activity as well as the light-induced responses. Duration of light pulses to the contralateral eye was 1000 ms, as shown at the bottom of the waveform. The highest intensity tested was 0.4mW cm-2 designated as logI=0.

View larger version (16K): [in a new window]   Fig. 3. Average intensity—response curves for day (open circles, N=19) and night (filled circles, N=14). The photo-responsiveness of medulla bilateral neruons (MBNs) was greater during the night than the day. During the day the response was approximately 60% of that during the night. Vertical bars indicate ±1 S.E.M. Significant differences between day and night are indicated by asterisks (P<0.01, t-test).

View larger version (15K): [in a new window]   Fig. 4. Average intensity—response curves showing the effect of injection of Ringer's solution into the contralateral optic lobe on light-induced responses of medulla bilateral neurons (MBNs) during day (A) and night (B). Filled circles indicate activity before injection and open circles after injection. Vertical bars indicate ±1 S.E.M. Administration of Ringer's solution had little effect on MBN photo-responsiveness with the suppression index SI=1.4±2.3 (mean ± S.E.M., N=6) and 5.2±1.3 (N=8) for day and night, respectively.

View larger version (29K): [in a new window]   Fig. 5. Effects of serotonin on the spontaneous activity recorded from the separated proximal optic stalk. (A) Application of serotonin into the optic lobe suppressed the spontaneous activity in a dose-dependent manner during both day (open squares) and night (filled squares). a and b indicate a significant difference (P<0.05, t-test) compared with the Ringer-injected control for day and night, respectively. Values are means ± S.E.M. of 5-8 preparations. (B) Serotonin (10 pmol) injected into the optic lobe significantly suppressed the spontaneous firing rate, which was further reduced when the contralateral optic stalk was severed. When the contralateral optic lobe was removed, the spontaneous activity was significantly reduced, and this value was increased only slightly when serotonin was injected into the protocerebral lobe after the removal of the optic lobe. Values are means ± S.E.M. of 5 preparations, given as percentage of the value before treatments (untreated). *P<0.01, t-test.

View larger version (23K): [in a new window]   Fig. 6. Effects of serotonin on light-induced responses of medulla bilateral neurons (MBNs) recorded from the separated proximal optic stalk. (A,B) Average intensity—response curves showing the effect of serotonin (10 pmol) on MBN photo-responsiveness during the day (A) and the night (B). The number of spikes induced by a light pulse is plotted against the corresponding light intensities. Filled and open circles indicate values either before (filled circles) or after injection (open circles). The effect of serotonin was greater during the night than during the day. (C) Dose—response curve showing the ability of serotonin to suppress light-evoked MBN activity during the day (open squares) and night (filled squares). Note that the sensitivity of MBN to serotonin was always significantly greater during the night than the day (c, P<0.01, t-test). Values are means ± S.E.M. of 5-8 preparations. a and b indicate a significant difference (P<0.01, t-test) compared with Ringer-injected control values for day and night, respectively.

View larger version (14K): [in a new window]   Fig. 7. An example of recovery of medulla bilateral neuron (MBN) photo-responsiveness from serotonergic suppression. Full (100 %) recovery took only 30 min during the night (filled squares), while during the day complete recovery was not achieved even after 90 min (open squares). Similar results were obtained in other preparations (day, N=3; night, N=3).

View larger version (18K): [in a new window]   Fig. 8. (A) Examples of electroretinogram (ERG) waveform recorded from the compound eye of the cricket before (a1) and after (a2) the optic nerve was severed. The response was elicited with a 500 ms light pulse (shown beneath ERG recordings). (B) ERG waveforms before (b1) and after (b2) 10nl of serotonin solution (10 mmoll-1) was injected into the optic lobe. (C,D) Representative intensity—response curves for the on-component of the ERG obtained before (filled circles) and after (open circles) serotonin injection into the optic lobe during the day (C) and the night (D). Similar results were obtained in other preparations (day, N=3; night, N=3). Note that injection of serotonin into the optic lobe had no significant (P>0.05, N=4) effect on both waveform and amplitude of the ERG.

View larger version (24K): [in a new window]   Fig. 9. (A,B) Average intensity—response curves showing that quipazine (10 pmol) suppressed medulla bilateral neuron (MBN) photo-responsiveness during both day and night, but more effectively during the night. Filled and open circles indicate results before and after quipazine injection, respectively. Vertical bars indicate ± 1 S.E.M. (B) Dose dependency of the effect of quipazine on MBN photo-responsiveness. Open and filled squares indicate results for the day and night, respectively. Values are means ± S.E.M. of 4-8 preparations. a and b indicate a significant difference (P<0.01, t-test) compared with Ringer-injected control value for day and night, respectively. c indicates a significant difference between day and night values (P<0.01, t-test).

View larger version (32K): [in a new window]   Fig. 10. Effects of metergoline (100pmol) on medulla bilateral neuron (MBN) photo-responsiveness. (A,B) Average intensity—response curve before (filled circles) and after injection (open circles) of metergoline. Vertical bars indicate ±1 S.E.M. Metergoline increased the photo-responsiveness during both day (A) and night (B), but more effectively during the day. (C—E) Integrated rate histograms illustrating the activity per 100ms. Serotonin suppressed the light-induced responses (D), but metergoline attenuated the suppressing effect of serotonin when administered together with serotonin (E). The white and black bars at the bottom indicate light (white) and dark (black) periods.

View larger version (14K): [in a new window]   Fig. 11. Possible involvement of serotonin in the coupling mechanism between the optic lobe pacemakers. The circadian pacemakers in the optic lobe synchronize not only to the environmental light/dark cycle through photic information from the compound eye (1) but also to their contralateral partner. The medulla bilateral neurons (MBNs) are the major component of the coupling system and receive photic information from the photoreceptor (2) and circadian information from the pacemaker (3) on their own side. Serotonin (5-HT) is released from the serotonergic neurons under regulation by the circadian pacemaker (4) and the contralateral MBNs (5). The released serotonin shifts the phase of the pacemaker (6) in a phase-dependent manner, and at the same time, it reduces the coupling signals (7) by suppressing the activity of MBNs.