Caffeine-induced modifications of heart rate, temperature, and motor activity circadian rhythms in rats

Even if the effects of caffeine on some physiological parameters are well known, its influence on circadian rhythmicity had not yet been investigated. This possible influence is of particular importance, introducing a possible bias in chronopharmacological studies. Thus, the aim of this study was to evaluate the effects of repeated caffeine administration on the circadian rhythms of heart rate "H," body temperature "T," and motor activity "A" in unrestrained rats maintained under controlled conditions (LD 12:12, light from 0600 to 1800 h) by using radiotelemetry transmitters. The study was divided into three 7-day observation spans: a 1-week control span "P1," a 1-week treatment span "P2," and a 1-week recovery span "P3." P1 was performed for assessing baseline measurements of H. T, and A. During P2, four rats received caffeine (25 mg/kg) at 0900 h, while four rats received saline in the same conditions every day of the observation span. H, T, and A were continuously monitored and plotted every 10 min. For P1, P2, and P3. a power spectrum analysis (Fourier transform) was applied to determine the dominant period of rhythmicity. If H, T, and A circadian rhythms were detected, the characteristics of these rhythms, i.e., mesors, amplitudes and acrophases, were determined by cosinor analysis, expressed as means +/- SEM and compared by analysis of variance. Our results indicated that caffeine did not suppress the circadian rhythmicity of H, T, and A, but significantly increased mesors and decreased amplitudes of the three rhythms and advanced acrophases of temperature and activity compared to the control group.