Diurnal variation of autonomic nervous activity in the rat - Investigationby power spectral analysis of heart rate variability

We studied the diurnal variations of autonomic nervous function in rats. Fo r this purpose, a long-term electrocardiogram (ECG) was recorded from consc ious and unrestrained rats using a telemetry system, and the autonomic nerv ous function was investigated by the power spectral analysis of heart rate variability. No arrhythmias were observed in the ECG of any of the rats. No cturnal patterns, in which the values of heart rate in the dark phase (2000 -0600) were higher than those in the light phase (0600-2000), were observed . All normal rats shared a characteristic pattern in their power spectrum a nalysis. Both low-frequency and high-frequency power in the light phase wer e higher than those in the dark phase. However, these differences were not statistically significant (P > .05). The low frequency to high frequency ra tio also showed a nocturnal pattern. The value in the dark phase was signif icantly higher (P < .05) than that in the light phase. These results sugges t that the sympathetic nervous activity is predominant in the dark phase in rats. Therefore, we believe that this information may be useful for future biobehavioral studies.