Diurnal variations and sleep deprivation-induced changes in rat hypothalamic GHRH and somatostatin contents

Previous reports indicate that hypothalamic growth hormone-releasing hormon e (GHRH) promotes sleep and is involved in sleep regulation. The aim of our experiments was to determine whether the GHRH and somatostatin contents of the rat hypothalamus have diurnal variations and whether they are altered by sleep deprivation (SD). Hypothalamic samples were collected at 10 time p oints during the 24-h light-dark cycle. SD started at light onset. Hypothal amic samples were obtained after 4 and 8 h of SD and after 1 and 2 h of rec overy following 8 h of SD. The peptides were determined by means of radioim munoassay. GHRH displayed significant diurnal variations with low levels in the morning (a transient rise occurred at 1h after light onset), gradual i ncreases in the afternoon (peak at the end of the light period and beginnin g of the dark period), and decreases at night. SD induced significant GHRH depletion, which persisted during recovery. The afternoon rise was delayed, and the nocturnal decline of somatostatin was more rapid than the changes in GHRH. Although the patterns of the diurnal variations in GHRH and somato statin were similar, there was no significant correlation between them. SD did not alter somatostatin significantly. Comparisons of the present result s with previously reported changes in hypothalamic GHRH mRNA suggest that p eriods of deep nonrapid eye movement sleep (first portion of the light peri od and recovery sleep after SD) are associated with intense hypothalamic GH RH release.