Abstract:
After GA (general anaesthesia), patients often report symptoms of circadian misalignment that can last up to five days. Animal studies have now demonstrated that GA may be, at least in part, responsible for these effects, causing timedependent shifts on circadian rhythms of behaviour, physiology, and cognitive parameters. However, due to the lack of consistency in the study design between experiments, the specific effect of GA on circadian rhythms in mammals, and the mechanism by which this occurs, is still unclear. In this thesis, I sought to investigate the time-dependent effect of GA on behavioural rhythms in mice (C57BL/6VJU), with two of the most commonly used halogenated inhalational anaesthetic agents (isoflurane and sevoflurane), administered at different times of the day (circadian times, CTs), over a period of 24 hours. To do so, I collected data to construct a full phase response curve (PRC) for each anaesthetic agent. Animals received either a six-hour GA treatment or a 10-minutes GA treatment (control group) with isoflurane (1.5%) or sevoflurane (2.6%). 80 mice were treated with isoflurane for six hours and 78 mice were treated with isoflurane for 10 minutes. For the sevoflurane PRC, 75 animals were treated with sevoflurane for six hours and 24 mice were treated with sevoflurane for 10 minutes. The two anaesthetic agents produced (weak) type one PRCs. The results showed that after a six-hour treatment with isoflurane mice locomotor activity rhythms were delayed 1.11 hours (on average), when administered between CTs 8-12, with a maximum phase delay of 1.69 hours observed at CT 11.9. Six hours of sevoflurane administration phase delayed mice wheel running rhythms with 0.54 hours (on average) between CTs 6-11. A maximum phase delay after of 1.45 hours was observed at CT 8. No significant phase shifts were observed on mice circadian rhythms of locomotor activity after the control treatment (10 minutes of GA), at any time, with any of the anaesthetic agents studied. These results suggest that the effect of GA on the clock is time-dependent. No evidence of a change in the fundamental period of the mouse clock (tau) was found after anaesthesia, which suggests that the effect of GA on the clock is mediated by non-parametric phase shifting rather than parametric or continuous shifting. also investigated the combined effect of GA and light on mice circadian rhythms of locomotor activity. However, due to the time limitation of my PhD research, only two time points (CTs) were investigated for isoflurane and one for sevoflurane. Animals received either a light pulse (400 lux LED light source) for four hours or the same light while anaesthetised with either isoflurane (1.5%) or sevoflurane (2.6%). Isoflurane administered with light between CTs 14-17 blocked the phase-shifting effect of light. The phase shift evoked by the concomitant treatment (light and isoflurane) between CTs 14-17 was 0.13±0.40 hours (n=5). However, when isoflurane and light were administered between CTs 7-11, the average of the induced phase delay in behavioural rhythms was 2.18±0.21 hours (n=12), showing that isoflurane was not blocking the effect of light on the clock. On the other hand, sevoflurane reduced the phase delay evoked by light when light and sevoflurane were administered together between CTs 14-17 but did not completely block it. The sevoflurane plus light-induced phase delay between CTs 14-17 was 0.61±0.3 hours (n=11). These results suggest that GA does not simply pharmacologically block the effect of light on the clock at all circadian times as was previously thought, but more that the effect of GA and light on the clock depends on the circadian phase of the administration. Further studies need to be done to construct a full light and anaesthesia PRC for both anaesthetic agents in order to determine whether light could be used to reduce the anaesthesia-induced phase shift on circadian rhythms in mammals, as it does in invertebrates.