Relationship between the three-hour-period sleep-wakefulness cycle and growth hormone secretion in the immature rat

The temporal pattern of growth hormone (GH) secretion throughout a day and its correlation with the sleep-wakefulness cycle in the 29-31-day-old female rat were studied by serial blood sampling through an intracardiac cannula at 10-min intervals. Electroencephalograms (e.e.g.s) recorded continuously during blood sampling periods were scored into wakefulness and sleep, and the amount of sleep for every 10 min was plotted against the GH value as assessed by radio-immunoassay. B analysis of the power spectrum using the least-squares method, the time series of GH concentration was found to have a periodicity of approximately 3.0 h in any of the 6-h periods sampled, beginning at 12.00, 18.00, 00.00 and 06.00 h, and in either sex. The peak time which was estimated for the time series of GH concentrations was found to occur consistently around 02.00-03.00, 05.00-06.00, 08.00-09.00, 11.00-12.00, 14.00-15.00, 17.00-18.00, 20.00-21.00 and 23.00-24.00 h in either the male or the female rat. The cross-correlation analysis performed between the time series of the amount of sleep and the GH concentration, which were obtained for the 6-h period from 12.00 to 18.00 h, revealed a significant positive correlation between the GH level and the amount of sleep during the preceding 10 min. The time series of the amount of sleep over a 24-h period from the data obtained in the continuous recording of e.e.g.s from 27 to 31 days of age revealed an ultradian rhythm with a periodicity of approximately 3.0 h. These findings indicate that the rat GH secretory rhythm with a 3.0-h periodicity develops by 30 days of age in both sexes, relating to the 3.0-h-period ultradian rhythm of the sleep-wakefulness cycle entrained by the light cycle.