Strain-differentiated circadian and ultradian rhythms in locomotor activity of the laboratory rat

Abstract

Spontaneous locomotor activity (LA) was recorded in five inbred strains of laboratory rats (ACI/Ztm, AS/Ztm, BH/Ztm, BS/Ztm, LEW/Ztm) under light-dark (LD 12∶12), continuous-dark (DD), and continuous-light (LL) conditions. Strain-dependent differences in LA were observed under each light condition. Under LD conditions, 24-h rhythms were found in the LA pattern of each strain studied. In addition, the BH strain showed a clear bimodal, and the LEW strain a trimodal, activity pattern. Spectral analysis of the activity patterns revealed distinct 4- and 4.8-h ultradian rhythms in the LEW strain and 12-, 6-, 4.8-, and 4-h rhythms in the BH strain. These strain-characteristic patterns persisted under continuous-dark conditions, therefore demonstrating their endogenous character. Under continuous-bright-light conditions the circadian rhythms decreased to a non-significant level in the AS and BS strains. This was not the case, however, in all animals of the LEW and BH strains. Despite the rapid decrease in the 24-h rhythm under LL, the LEW and BH strains showed significant ultradian rhythms.