Living organisms have evolved circadian clocks that anticipate daily changes in their environment. Their clockwork is fully endogenous, but can be reset by external cues. (Light is the most efficient cue.) The circadian neuronal network of the fruit fly (Drosophila) brain perceives light through the visual system and a dedicated photoreceptor molecule, cryptochrome. Flies exhibit a bimodal locomotor activity pattern that peaks at dawn and dusk in light-dark conditions. These morning and evening activity bouts are controlled by two distinct neuronal clocks in the fly brain. By using flies with a deficient cryptochrome pathway, we have uncovered an unexpected role for light in the circadian system. In addition to synchronizing the two oscillators to solar time, light also controls their behavioral output. The morning oscillator can periodically rouse the fly when in constant darkness, but not in constant light, whereas the evening oscillator can do the same in constant light, but not in constant darkness. This suggests the existence of a light-dependent switch between oscillators that appears to require the visual system. Such a mechanism likely contributes to better separate the active periods of the fly at dawn and dusk, and may help the animal to adapt to seasonal changes in day length.
A Policy Forum published in this month’s PLoS Medicine highlights an issue that the PLoS Medicine editors have often debated: when research takes place within the context of clinical care, how can we distinguish which activities constitute care, and which research? The World Medical Association Code of Medical Ethics declares that a physician must always “act in the patient’s best interest when providing medical care” . Yet increasingly physicians also undertake research, which involves possibly unknown risks and benefits. These risks, and the uncertainty of benefit, might therefore conflict with a patient’s best interests.