Parameters of photic resetting of the circadian system of a durnal rodent, the nile grass rat

Roberto Refinetti

doi:10.22456/1679-9216.16783

Abstract

Research on circadian rhythms has been hampered by the lack of a suitable rodent model with diurnal habits. The goal of this study was to characterize the parameters of photic entrainment of the circadian system of the Nile grass rat, a small rodent that can potentially serve as a model for the study of circadian rhythms in diurnal animals. The state of the circadian system was accessed through the rhythm of running-wheel activity and manipulated by photic stimulation (white light). The results indicated that the grass rat exhibits a robust rhythm of running-wheel activity with a mean free-running period of 23.9 hours (range: 23.6 to 24.3 hours). The animal is clearly diurnal and starts activity daily approximately 50 minutes before lights-on under an L12:D12 light-dark cycle. Its photic phase-response curve shows a dead zone during most of subjective day, a phase delay region during early subjective night (maximal delay: 1.5 hours), and a phase advance region during late subjective night (maximal advance: 2.2 hours). When exposed to constant illumination, the grass rat exhibits longer circadian periods as the intensity of illumination increases, with a ceiling between 100 and 1,000 lux. Finally, its range of entrainment extends from at least 23.0 to 25.0 hours (from 22.0 to 25.5 in some animals).

Highlights

The temporal niche of a species may be as important for survival as its geographical niche [2, 3]
It is still unclear whether the organization of the circadian system of the two groups is substantially different, as the phase-response curves relating phase shifts to the time of photic stimulation are very similar in the two groups [1, 5]
There is clear entrainment, with activity limited to the light phase of the light-dark cycle, followed by clear free-run with a period shorter than 24 hours

Summary

Introduction

The temporal niche of a species may be as important for survival as its geographical niche [2, 3]. From the perspective of circadian physiology, diurnality and nocturnality are expressions of the “phase angle of entrainment”—that is, they reflect the effects of photic stimulation on the daily resetting of the circadian clock [6, 12]. It is still unclear whether the organization of the circadian system of the two groups is substantially different, as the phase-response curves relating phase shifts to the time of photic stimulation are very similar in the two groups [1, 5]. Diurnal animals in the wild are exposed to much greater intensities of light than nocturnal animals are, which may have significant effects on entrainment [4]

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