Effects of 5,7-dihydroxytriptamine (5,7-DHT) on circadian locomotor activity of the blow fly, Calliphora vicina

Abstract

The biogenic amine serotonin (5-HT) is a neuromodulator in both vertebrates and invertebrates. It has been shown that serotonin, apart from its distinct effects on behavior, also plays a morphoregulatory role during the ontogeny of the insect's nervous system. The role of serotonin in modulating circadian locomotor activity of the blow fly, Calliphora vicina was explored. Injection of a specific neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the hemolymph appeared to significantly reduced the level of locomotor activity and lengthened the period (tau) of circadian rhythmicity. After drug injection in constant darkness flies continued with their free-running rhythm of a locomotor activity, depending on the time of 5,7-DHT injection. This compound causes phase delay when administered in the early subjective day, and phase advance in the late subjective day. This effect is the opposite of the phase response curve obtained for 5-HT injections. This suggests that 5-HT might act as an entraining agent via the output pathway by feedback to clock neurons in the brain. Some of the injected insects regained their normal level of activity after a few days. These findings suggest a potential role for serotonin as modulator of circadian rhythms in insect including regulation of the level of locomotor activity.

Figure 1.

Locomotor activity rhythms (actograms) of the blow fly, Calliphora vicina in constant dark at 26 °C (double plotted). It shows unperturbated free-running activity with τ = 20.9 h in the pre-treated fly. On Day 9, 0.5 µg of 5,7-DHT in 1µl of insect saline/ascorbic acid was injected (arrow) in early subjective day. This caused phase delay by a few hours. The level of locomotor activity was greatly reduced and period was lengthened to 21.4h. The reduction in activity level started 2 days after drug injection and after 4 more days normal level of locomotor activity was regained. Periodograms for the appropriate sections of the activity records are shown alongside the actograms. Lines are drawn through activity periods for better visual inspection of the phase shifts.

Figure 2.

Locomotor activity rhythms of Calliphora vicina under constant dark conditions. The drug injection in the late subjective day caused a phase advance and lengthening of τ from 20.3h before treatment to 22.6 after administering the drug. Also the level of activity was very low after treatment. Further explanation see Figure 1.

Figure 3.

Actograms (double plotted) of a rhythms of the blow fly in constant darkness before and after injection of 0.5 µg of 5,7-DHT in 1 µl of insect saline/ascorbic acid on Day 10. After drug injection fly became completely arrhythmic with a substantially reduced level of locomotor activity. For further explanation see Figure 1.

Figure 4.

Mean locomotor activity (α) per 24 h of recording and coefficient (α/ρ) before injection of 5,7-DHT and after treatment with this compound. Both parameters were significantly (p<0.001) reduced after drug injection.

Figure 5.

Mean (± SD) length of the period (τ) in hours before and after treatment with 5,7-DHT drug. The period after treatment significantly (p<0.001) increased.

Figure 6.

Cumulative percentage of all investigated flies having different period τ before (rhomboid symbols) and a after 5.7-DHT treatment (square symbols). The highest percentage (about 45%) of control flies had period τ between 21–22 hours, whereas after treatment in some cases the period exceeded 28–29 hours.