Nocturnal wakefulness inhibits growth hormone (GH)-releasing hormone-induced GH secretion.

Recent studies have suggested that spontaneous release of GH as well as GH secretion stimulated by exogenous GHRH are influenced by central nervous mechanisms that regulate sleep and wakefulness. Here, the effect of nocturnal wakefulness on GH secretion stimulated by i.v. administration of GHRH was examined in two experiments in healthy men. On all nights, GHRH (1 microgram/kg BW) was injected after the subjects had slept for about 2.5 h to minimize interference of endogenous release of GH during early sleep with the response to exogenous GHRH. Both experiments included a control condition to assess GH secretory responses to GHRH during undisturbed sleep and an experimental condition to assess the effect of wakefulness. In the control conditions, subjects slept throughout the night, and GHRH was administered 170 min after sleep onset. In the experimental condition of Exp I (n = 10), subjects were awakened 150 min after sleep onset and stayed awake. GHRH was given 20 min after awakening. In the experimental condition of Exp II (n = 8), subjects were awakened 30 min after GHRH treatment, which was administered 170 min after sleep onset. GHRH administrations during sleep fell into epochs of stage 2 sleep or rapid eye movement sleep. GH secretion and sleep characteristics before GHRH administrations were comparable for experimental and control conditions of both experiments. GH secretory responses were inhibited when the subject was awake at the time of GHRH administration compared to GH responses during undisturbed sleep. Awakening the subject 30 min after GHRH administration abruptly interrupted the initiated GH secretory response. The results demonstrate a profound inhibitory effect of nocturnal awakenings on GHRH-induced GH secretion. They indicate that the GH secretory response to GHRH is strongly determined by central nervous system sleep-wake activity.