Many organisms have evolved an internal clock that regulates the daily control of complex behaviours, known as circadian rhythms. Circadian literally means `about a day'. This intrinsic clock synchronizes behaviours, such as sleeping and waking, within a 24 h cycle. The molecular and cellular components of circadian rhythms have been studied extensively in the fruit fly, Drosophila melanogaster. Scientists have studied how Drosophila neural networks synchronize environmental cues with gene expression and behavioural responses and have identified specific genes and neurons involved in the coding of circadian patterns. Environmental cues that may impact circadian rhythms are those which themselves fluctuate predictably throughout the day; for example, light and temperature. A wealthy body of evidence indicates that light is a powerful regulator of circadian rhythms but the impact of temperature is less well characterized. The study by Ania Busza and colleagues from the University of Massachusetts Medical School in the 1st October edition of the Journal of Neuroscience investigates how circadian neurons interact to form a network that synchronizes Drosophilabehaviour with temperature cues.
The team addressed this issue using the well-defined cyclic locomotor behaviour of Drosophila, which exhibits surges of activity in the morning and evening. The team performed the experiments in constant darkness to remove any circadian influence from light, monitoring the flies' activity levels as the temperature in the experimental chamber was slowly cycled between 20°C and 29°C. In this way, the authors could examine the cyclic pattern of locomotor activity in the presence of thermal cues alone. Using these techniques, together with genetic manipulations, the authors unveiled novel interactions between neurons in the circadian circuit. This circuit mediates the response to temperature cues by raising the insect's activity levels in the morning and evening when temperatures are generally cooler.
In Drosophila, light/dark cycles stimulate two populations of neurons in the circadian circuit, which interact to generate the morning and evening peaks in activity. These are known as `M cells' and `E cells',controlling the morning and evening peaks, respectively. Using a genetic manipulation that allowed the investigators to selectively inhibit the activity of either M or E cells, they determined that these neuronal populations are also involved in the synchronization of behaviour with temperature cues. They found that M cells respond slowly to temperature changes compared with the more rapidly responsive E cells. The authors suggested that M cells set the pace of behavioural synchronization and help to prevent over-compensation to erratic temperature variations that occur naturally in response to the weather. E cells may help to fine tune the response of the M cells and it is the functional coupling between the E and M cells that allows for the proper timing of behavioural activity in response to temperature cycles. A final interesting point was the discovery of a distinct population of cells, termed temperature-sensitive cells, that contribute to an increase in evening activity and responded exclusively to thermal cues when both M and E cells were genetically inhibited.
It seems that temperature, like light, utilizes specialized neuronal mechanisms to influence activity at different times during the day. So the next time you walk into a warm, dark lecture hall and feel the overwhelming urge to sleep, perhaps you can be forgiven.