The manifestation of the motor activity circadian rhythm of blinded rats depends on the lighting conditions during lactation

Programming of mice circadian photic responses by postnatal light environment

Early life programming has important consequences for future health and wellbeing. A key new aspect is the impact of perinatal light on the circadian system. Postnatal light environment will program circadian behavior, together with cell morphology and clock gene function within the suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian clock in mammals. Nevertheless, it is still not clear whether the observed changes reflect a processing of an altered photic input from the retina, rather than an imprinting of the intrinsic molecular clock mechanisms. Here, we addressed the issue by systematically probing the mouse circadian system at various levels. Firstly, we used electroretinography, pupillometry and histology protocols to show that gross retinal function and morphology in the adult are largely independent of postnatal light experiences that modulate circadian photosensitivity. Secondly, we used circadian activity protocols to show that only the animal's behavioral responses to chronic light exposure, but not to constant darkness or the acute responses to a light stimulus depend on postnatal light experience. Thirdly, we used real-time PER2::LUC rhythm recording to show long-term changes in clock gene expression in the SCN, but also heart, lung and spleen. The data showed that perinatal light mainly targets the long-term adaptive responses of the circadian clock to environmental light, rather than the retina or intrinsic clock mechanisms. Finally, we found long-term effects on circadian peripheral clocks, suggesting far-reaching consequences for the animal's overall physiology.

Development of circadian rhythms: role of postnatal light environment

Mammals are born with an immature circadian system, which completes its development postnatally. Evidence suggests that the environment experienced by a newborn will impact and shape its development, which will have future consequences at the levels of circadian system function, circadian behaviour and physiology, and potentially, the animal's long-term health and welfare. Here we review the various stages in postnatal development of the circadian system, and discuss the data available on the long-term effects of early environment, in particular light environment, on the animal's brain, physiology and behaviour.

Circadian desynchronization

The suprachiasmatic nucleus (SCN) coordinates via multiple outputs physiological and behavioural circadian rhythms. The SCN is composed of a heterogeneous network of coupled oscillators that entrain to the daily light-dark cycles. Outside the physiological entrainment range, rich locomotor patterns of desynchronized rhythms are observed. Previous studies interpreted these results as the output of different SCN neural subpopulations. We find, however, that even a single periodically driven oscillator can induce such complex desynchronized locomotor patterns. Using signal analysis, we show how the observed patterns can be consistently clustered into two generic oscillatory interaction groups: modulation and superposition. In seven of 17 rats undergoing forced desynchronization, we find a theoretically predicted third spectral component. Combining signal analysis with the theory of coupled oscillators, we provide a framework for the study of circadian desynchronization.

Expression of circadian neuropeptides in the hypothalamus of adult mice is affected by postnatal light experience

The suprachiasmatic nuclei (SCN) of the hypothalamus are the principal pacemaker in mammals, controlling daily, circadian rhythms in physiology and behaviour. Environmental light during development has long-term effects on circadian behaviour, but it is still unclear what the relevant adaptations within the brain are. In the present study, we examined the manifestation of the circadian rhythm of locomotor activity, and the expression of arginine-vasopressin (AVP) and vasointestinal polypeptide (VIP) in the SCN of adult mice reared under different light environments during the suckling period, and synchronised to light/dark cycles after weaning. We found that animals reared under constant light had higher amplitude and more stable activity rhythms, together with lower levels of VIP- and AVP-immunostaining in the SCN, compared to mice reared under light/dark cycles or constant darkness. Differences in AVP expression were also found in the paraventricular nucleus and the supraoptic nucleus, two brain areas which receive SCN projections. These results indicate that the postnatal light experience may affect clock function and clock output, and suggest implications for the control of hormonal homeostasis and circadian behaviour.