Responses of the Diurnal Locomotor and Adrenocortical Rhythms in Rats exposed to a Weekly 8-Hour Phase-Shift of the Zeitgeber

Abstract

Two groups of female rats were kept in a regimen of 8 h light (08.00–16.00) and 16 h darkness. The first group was killed after 1 month to serve as controls for the diurnal corticosterone rhythm (Fig. 1a). Corticosterone was measured by a thin-layer chromatography-fluorimetric method for six rats at each sampling time. The second group remained thereafter at 8L 16D, but was then exposed for 9 weeks to repetitive 8-h phase-shifts of the photophase, by lengthening once a week a single photophase by 8 h (+ 120°). The locomotor rhythm was continuously recorded from four actographic cages housing three rats each. The daily position of the estimated acrophase (Halberg’s least square method) compared with the Zeitgeber is plotted in Figure 1b. The graph shows that after each phase-shift of the synchronizer, the progressive re-synchronization of the locomotor rhythm required almost one full week. In order to check the response of the adrenocortical rhythm that is usually closely coupled with the locomotor rhythm, the levels of plasma corticosterone were measured over a 48-h span (Fig. 1c, 6 rats per point) beginning on the second day of the fourth week of the experiment (indicated at T₁ on Fig. 1b).