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Abstract
This study sought to define the generality of a previous finding that the spontaneously
hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rat strains differ in free-running
circadian period when maintained in running-wheel cages under constant light. Circadian
drinking rhythms were monitored in SHRs and WKYs housed without access to running
wheels under an increasing series of light intensities beginning with constant darkness.
Strain differences in circadian period were seen only at relatively high light intensities,
indicating that SHRs and WKYs differ in circadian light sensitivity. Since SHRs and
WKYs differ in circadian period with or without access to running wheels, this strain
difference is not likely to depend on differential locomotor activity levels. SHRs
and WKYs also differed in spectral profile and circadian waveform, but only under
low light intensities. At higher intensities, dissociation of rhythmicity was seen
in both strains.