[The relationship between growth hormone secretion and the sleep-wakefulness cycle in the immature rat]

In order to study the relationship between growth hormone (GH) secretion and the sleep-wakefulness cycle in the rat, 29- to 31-day-old male and female rats were bled serially through an intracardiac cannula at 10-min intervals from 1200 to 1800 h. EEGs recorded continuously during the blood sampling period were scored into wakefulness and sleep, and the amount of sleep for every 10 min was plotted against the GH values as assessed by RIA. The power spectrum analysis and the least squares method applied to the time series of GH concentrations in the control rat revealed that the mean (+/- SE) periodicity was 3.03 +/- 0.17 (n = 11) h for the male and 2.90 +/- 0.13 (n = 11) h for the female. The cross-correlation analysis performed between the time series of the amount of sleep and the GH value revealed a significant positive correlation between GH level and the amount of sleep during the preceding 10 min. Further, the time series of sleep amount had periodicities of 3.22 +/- 0.20 h for the male and 2.96 +/- 0.12 h for the female. Forced-wakefulness imposed on 8 male rats between 1300 and 1500 h was effective in preventing GH secretory bursts, which otherwise should appear during this time of the day. Corticosterone measurement by protein-binding assay revealed that the procedure of forced-wakefulness did not act as the "stress" which has been known to inhibit GH secretion in rats. These findings indicate that GH secretion in the rat is sleep-related as has been proved in humans.