Ultradian rhythm of growth hormone secretion and sleep in the adult male rat

Abstract

The correlation between growth hormone (GH) secretion and the sleep-wakefulness cycle in the adult male rat was studied by serial blood sampling at 10 min intervals. Electroencephalograms (e.e.g.s) recorded continuously during the blood sampling were scored into wakefulness and sleep, and the amount of sleep for every 10 min was plotted against the GH value as assessed by radioimmunoassay. 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 (+/- S.E. of the mean) periodicity was 2.93 +/- 0.10 h for the period from 00.00 to 12.00 h in five rats and 2.85 +/- 0.06 h for the period from 12.00 to 24.00 h in eight rats. For the animals in which the e.e.g. was recorded simultaneously with blood samplings from 12.00 to 24.00 h, the cross-correlation analysis performed between the time series of the amount of sleep and the GH value revealed a significant positive correlation at time lags of 0-10 min in six and at time lags of 20-50 min in four out of fourteen rats. The number of animals having a positive correlation at short time lags seemed relatively less, but in most animals, there was observed a definite relationship that each GH peak occurred with a consistent time lag ranging from 40 to 70 min following the onset of sleep cycle. Sleep deprivation performed from 13.00 to 16.00 h during the sampling period from 11.00 to 19.00 h was effective in preventing high-level GH pulses which otherwise should appear during this time of the day. Concurrent measurements of corticosterone concentrations in the sleep-deprivation experiment revealed that peak values of corticosterone secretory episodes were not influenced by sleep deprivation. These findings indicate that the GH secretion in the adult rat is also correlated with the sleep-wakefulness cycle, although somewhat differently from the immature rat.