Loss of enhanced nocturnal growth hormone secretion in aging rhesus males

Mini-review: Aging of the neuroendocrine system: Insights from nonhuman primate models

The neuroendocrine system (NES) plays a crucial role in synchronizing the physiology and behavior of the whole organism in response to environmental constraints. The NES consists of a hypothalamic-pituitary-target organ axis that acts in coordination to regulate growth, reproduction, stress and basal metabolism. The growth (or somatotropic), hypothalamic-pituitary-gonadal (HPG), hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-thyroid (HPT) axes are therefore finely tuned by the hypothalamus through the successive release of hypothalamic and pituitary hormones to control the downstream physiological functions. These functions rely on a complex set of mechanisms requiring tight synchronization between peripheral organs and the hypothalamic-pituitary complex, whose functionality can be altered during aging. Here, we review the results of research on the effects of aging on the NES of nonhuman primate (NHP) species in wild and captive conditions. A focus on the age-related dysregulation of the master circadian pacemaker, which, in turn, alters the synchronization of the NES with the organism environment, is proposed. Finally, practical and ethical considerations of using NHP models to test the effects of nutrition-based or hormonal treatments to combat the deterioration of the NES are discussed.

Evidence against a role for the growth hormone-releasing peptide axis in human slow-wave sleep regulation

A complex interrelationship exists between sleep and somatotropic activity. In humans, intravenous injections of growth hormone-releasing hormone (GHRH) given during sleep consistently stimulate slow-wave (SW) sleep, particularly when given in the latter part of the night. In the present study, the possible somnogenic effects induced under similar conditions by GH-releasing peptide (GHRP) were investigated in seven young healthy men. Bolus intravenous injections of GHRP-2 (1 microgram/kg body wt) or saline, in randomized order, were given after 60 s of the third rapid-eye-movement period. All GHRP injections were immediately followed by transient prolactin elevations and by GH pulses of a magnitude within or around the upper limit of the physiological range. Except for a nonsignificant tendency to increased amounts of wakefulness during the 1st h after the injection, no effects of GHRP-2 administration on sleep were detected. There was in particular no enhancement of SW sleep. Thus, in contrast to GHRH, late-night single injections of GHRP-2 at a dosage resulting in similar GH elevations have no stimulatory effects on SW sleep. The present data provide evidence against the involvement of the GHRP axis in human SW sleep regulation.

Decreases in cerebral microvasculature with age are associated with the decline in growth hormone and insulin-like growth factor 1

Several reports have demonstrated that cerebral blood flow decreases with age and may contribute to neurodegenerative changes found in aging animals and man. Because GH and insulin-like growth factor 1 (IGF-1) decrease with age and have an important role in vascular maintenance and remodeling, we hypothesized that the decrease in cerebral blood flow is associated with a rarefaction of cerebral blood vessels resulting from a decline in GH and IGF-1. Measurements of vascular density (number of vessels/cortical surface area) in both Brown-Norway and Fisher 344/Brown-Norway rats were made at 5, 13, and 29 months of age using chronic cranial window chambers that allowed viewing of the cortical surface and its corresponding vasculature. Correlations were made with plasma levels of IGF-1. In Brown-Norway rats, arteriolar density decreased from 15.53 +/- 1.08 to 9.49 +/- 0.62 endpoints/mm2 in 7- and 29-month-old animals, respectively (P < 0.05). A decline was observed also in arteriolar anastomoses [3.05 +/- 0.21 to 1.42 +/- 0.24 connections/mm2 in 7- and 29-month-old animals (P < 0.05)]. Venular density did not decrease with age. Similar changes were observed in Fisher 344/Brown-Norway rats. The number of cortical surface arterioles was correlated with plasma IGF-1 levels at the time of vascular mapping (r = 0.772, P < 0.05), and injection of bovine GH (0.25 mg/kg, s.c., twice daily for 35 days) to 30-month-old animals increased both plasma IGF-1 and the number of cortical arterioles. These data indicate that: 1) vascular density on the surface of the cortex decreases with age; 2) vascular density is correlated with plasma levels of IGF-1; and 3) injection of GH increases cortical vascular density in older animals. We conclude that GH and IGF-1 have an important role in the decline in vascular density with age and suggest that decreases in vascular density may have important implications for the age-related decline in cerebral blood flow and brain function.

Behavioural rhythmicity in transgenic growth hormone mice: trade-offs, energetics, and sleep–wake cycles

Transgenic mice with extra rat growth hormone (GH) genes (TRrGH mice) are behaviourally lethargic and sleep 3.4 h/d longer than normal on standard diets. We tested the hypothesis that the doubling of the growth rate of TRrGH mice reduced the energy available for behaviour. Provision of sucrose supplements ad libitum normalized the durations of activity and sleep. Our results support a new allocative theory suggesting that sleep serves as an umbrella function for a suite of synergistic anabolic functions (e.g., growth, immunity, repair). Relegating these to the period of sleep in a secure nest allows full dedication of waking resources to niche interfacing (resource acquisition, risk avoidance and environmental stress resistance). Energy stress in TRrGH mice may arise via specific diversion of energy from waking functions via GH-induced insulin resistance. GH is normally secreted during sleep, but any causal relationship remains unresolved. We examined the circadian and ultradian behaviour of TRrGH mice to determine how a chronically elevated GH level impacts sleep. Remarkably, even the major hormonal distortion in TRrGH mice had little impact on the timing of ultradian or circadian rhythms. Increased sleeping of TRrGH mice on normal diets was due to an increased likelihood and duration of sleep at permitted times. GH did, however, appear to increase the depth of sleep.

Heterogeneity of growth hormone (GH)-producing cells in aging male rats: ultrastructure and GH gene expression in somatotrope subpopulations

Mammalian aging is characterized by a decline in the content and release of pituitary growth hormone (GH). However, few studies on the age-related changes in the population of GH-producing cells (somatotropes) have been carried out. We have investigated whether changes in number, ultrastructure and GH gene expression in subpopulations of somatotropes could explain the reduced GH release in aged rats. Three representative ages were studied: adult (5-month-old), old (19-month-old), and senescent (26-month-old) male rats. The total number of immunoreactive-GH cells per pituitary gland remained invariable to age. The separation of dispersed pituitary cells on a density gradient yielded two somatotrope subpopulations, of low density (LD) and high density (HD). Both subpopulations were equally represented in adults, whereas in old and senescent rats a predominance of LD-somatotropes was observed. Morphometric analysis showed that subpopulations exhibited storage and biosynthetic features inversely related. In LD-somatotropes, rough endoplasmic reticulum (RER) was more prominent but secretory granules (SG) were less abundant than in HD somatotropes. Concurrently, in situ hybridization for GH mRNA showed that GH gene expression was higher in LD-cells. Differences between subpopulations were essentially retained through the animals' lifespan, but small-sized SG, reduced RER, and low GH mRNA levels were inherent to aging both in LD- and in HD-somatotropes. The present findings demonstrate that the reduced content of pituitary GH in aged male rats is not due to a diminished number of GH-producing cells, but to the numerical predominance of scarcely granulated LD-somatotropes, combined with the decline in GH biosynthetic capacity observed in both subpopulations. In addition, age-related changes in ultrastructure and GH gene expression suggest a chronic inhibition of GH release and/or a weak stimulation of GH biosynthesis affecting both subpopulations.

Sleep-promoting effects of growth hormone-releasing hormone in normal men

Growth hormone-releasing hormone (GHRH) promotes rapid-eye-movement (REM) and non-REM sleep in animals, but there is little direct evidence for a hypnogenic action of GHRH in humans. In the present study, the possible somnogenic effects of intravenous bolus injections of a dose of GHRH eliciting physiological elevations of GH secretion in healthy young men were investigated. GHRH (0.3 micrograms/kg body wt) was given in early sleep [i.e., 1st slow-wave (SW) period], late sleep (i.e., 3rd REM period), and early sleep after sleep deprivation until 0400 h (i.e., 1st SW period). In the absence of sleep deprivation, injection of GHRH in early sleep did not modify SW sleep but increased REM sleep. GHRH administration in the third REM period was followed by a marked decrease of wake and an almost 10-fold increase in SW sleep. When GHRH was administered during the first SW period after sleep deprivation until 0400 h, the duration of wake decreased. Thus GHRH has sleep-promoting effects in young adults, particularly when given at a time of decreased sleep propensity.

Growth hormone and thyrotropin secretory profiles and provocative testing in aged rats

The impact of aging on both basal and induced GH and TSH secretion in male and female rats was investigated. Analysis of the individual GH secretory profiles in young (3-4 month) and old (19-20 month) rats indicated that sex-dependent patterns of GH secretion was preserved in old animals. However, we observed a reduction of individual GH peak amplitudes of 66% in old males and 53% in old females when compared to their respective young animals. Further, the GH response to an intravenous bolus of GH-releasing factor (GRF), morphine and clonidine was dramatically blunted or absent in old male and female animals. In contrast to GH, basal TSH secretion was elevated, while the TSH response to thyrotropin-releasing hormone (TRH) was not significantly affected in old animals of either sex. The present data provide evidence that reduced pituitary sensitivity to GRF may be a possible cause for reduced GH secretion in old animals. Further, the elevation in plasma TSH observed in old animals is not the result of an increased pituitary sensitivity to TRH.

The relationship between height velocity and growth hormone secretion in short prepubertal children

We have performed 24 h growth hormone (GH) profiles in 50 short prepubertal children aged between 5.2 and 12.9 years, growing with height velocity standard deviation scores (SDS) between 0.4 and -3.9. There was an asymptotic relationship between height velocity and spontaneous GH secretion described by the equation: height velocity SDS = A-B(e-cx), where A, B and C are constants and x is a measure of spontaneous GH secretion. We considered GH pulse amplitude to be the better description of spontaneous GH secretion as duration of the GH pulse (the time component of area under the curve) contributed little to the relationship between height velocity and area under the pulse. The distribution of GH secretion was continuous and there was no dividing point between GH insufficiency and sufficiency. Similar overlap was observed when the results of GH responses to insulin induced hypoglycaemia were considered; 14% of slowly growing children (height velocity SDS less than -0.8), had a response greater than 15 mU/l. Likewise serum IGF-I concentrations could not clearly separate slowly growing children from normal individuals. We conclude that height velocity, which ultimately determines height achieved, is controlled predominantly by GH pulse amplitude. The findings suggest that short normal children growing along or parallel to the third height centile could be made to grow faster by the administration of exogenous GH.