The earliest manifestations of aging are changes in metabolism that result in increased fat deposition and reduced muscle mass, which lead to increased likelihood of developing "metabolic disease" (type II diabetes, hyperlipidemia, arteriosclerosis, and hypertension).
Increased fat deposition in young (5 months old), in middle-aged (14 months old), and old (26 months old) male BN rats is illustrated by dual-energy x-ray absorptiometry scans shown in Fig. 1.
These metabolic changes are associated with declining GH, IGF-I, and sex steroid levels in the face of relative increases in glucocorticoid production, as well as insulin resistance and leptin resistance.
Anorexia is commonly associated with aging.
Normal aging is associated with a decrease in appetite and energy intake, which has been termed the anorexia of aging.
Generally, after age 70-75 yr, the reduction in energy intake exceeds energy expenditure, resulting in weight loss where loss of muscle (sarcopenia) predominates and predisposes older subjects to protein energy malnutrition.
The observed malnutrition and sarcopenia correlate with increased morbidity, mortality, and a number of hospitalizations with extended stays.
The causes of the physiological anorexia typified during aging are unknown; they are probably multifactorial and include a reduction in feeding drive with increased activity of satiety signals.
Healthy elderly subjects apparently retain their sensitivity to the satiating effects of cholecystokinin (CCK) and have higher fasting and postprandial CCK concentrations than young adults.
Indeed, it has been reported that CCK concentrations are higher in undernourished elderly subjects compared with the healthy elderly.
Although circulating ghrelin concentrations increase between early adulthood and middle age in humans, there is evidence that old age is associated with decreased ghrelin concentrations in rodents and in humans.
Therefore, enhanced effects of CCK and/or reduced effects of ghrelin may contribute to the development of anorexia and, in some cases, protein malnutrition during aging.
Aging, ghrelin, and energy balance
Ghrelin, which is mainly produced and secreted by the gastric mucosa, stimulates food intake as well as GH secretion.
It is possible that circulating ghrelin levels decline during aging because of impaired function of the gastric mucosa.
Indeed, the thickness of the membrane, the length of the glands, and the number of the endocrine cells in the gastric mucosa decrease in animals between puberty and old age.
If indeed this mechanism is operative in old subjects, we must elucidate how peripheral and central components of ghrelin action are functionally interrelated.
Fig. 1. Representative whole-animal dual-energy x-ray absorptiometry scans of young (5 months old), middle-aged (14 months old), and old (26 months old) male BN rats illustrating increased fat deposition during aging.
The age-related decline of plasma ghrelin concentrations might be related to the anorexia often observed in aged subjects.
However, before we can make definitive conclusions, much larger cohorts of subjects must be evaluated to support the finding that ghrelin decreases during aging.
Chronic treatment of elderly subjects with ghrelin mimetics restores the age-related decline in amplitude of GH pulsatility and circulating IGF-I to levels typical of young adults.
These results suggest that during aging either ghrelin production declines or ghrelin resistance occurs.
The orexigenic property of ghrelin coupled with its anabolic effects via the GH/IGF-I axis and its inhibition of the production of inflammatory cytokines indicate that rescue of reduced GHS-R activity by treatment with exogenous ghrelin or ghrelin mimetics may prove beneficial in the anorexia of aging.
Ghrelin production in CNS orexigenic centers
Ghrelin produced by A cells in the stomach appears to be an important peripheral orexigenic signal to the brain.
By using a selective antibody for ghrelin and using ghrelin knockout mice as controls, the question of whether ghrelin was expressed in areas of the hypothalamus involved in regulating energy balance was addressed.
Ghrelin immunoreactive cells were identified that fill the internuclear space between the lateral arcuate hypothalamus (LAH), ventral medial hypothalamus (VMH), dorsomedial hypothalamus, paraventricular nucleus (PVN), and the ependymal layer of the third ventricle.
This unique distribution does not overlap with known hypothalamic cell populations, such as those that produce NPY, AGRP, POMC, melanin-concentrating hormone, orexin, dopamine, and somatostatin 8–14.
These observations suggest specific roles for locally produced ghrelin in the CNS.
Immunoelectron microscopy showed that ghrelin is located in axons where it is associated with dense-cored vesicles in presynaptic terminals.
These axon terminals innervate the arcuate nucleus, dorsomedial hypothalamus, LAH, PVN, and ghrelin boutons and appear to make synaptic contact with cell bodies, dendrites of NPY/AGRP, POMC neurons in the arcuate nucleus, and NPY and GABA axon terminals in the arcuate nucleus and PVN.
Such interactions suggest a presynaptic mode of action for ghrelin in the hypothalamus.
Some ghrelin axons in the PVN innervate CRH cells, which is consistent with the increase in ACTH and glucocorticoid secretion observed following treatment with ghrelin and its mimetics.
These observations delineate an anatomical basis for pre- and postsynaptic interactions between ghrelin and NPY/AGRP, POMC, and CRH circuits.
Hypothalamic localization of the GHS-R was investigated in coronal slices of rat brain using biotin-labeled ghrelin.
Binding of biotinylated ghrelin was observed in the arcuate nucleus, LAH, and PVN was mainly associated with presynaptic boutons.
Axon terminals that bound ghrelin were frequently found to contain NPY.
Together, the binding data and the localization of expression of ghrelin in axons adjacent to presynaptic nerve terminals support the notion that ghrelin modulates neurotransmission.
In summary, ghrelin is produced in the hypothalamus where it is localized to a previously uncharacterized group of neurons adjacent to the third ventricle between the dorsal, ventral, paraventricular, and arcuate hypothalamic nuclei (Fig. 2).
These neurons send efferents onto key hypothalamic circuits, which include those producing NPY, AGRP, POMC products, and CRH.
In the hypothalamus, ghrelin binds mainly to presynaptic terminals of NPY neurons.
Electrophysiological recordings showed that ghrelin stimulated the activity of arcuate NPY neurons and mimicked the effect of NPY in the PVN.
We propose that at these sites release of ghrelin stimulates the release of orexigenic peptides and neurotransmitters, thus representing a novel regulatory circuit controlling energy homeostasis (Fig. 2).
The involvement of NPY/AGRP neurons was confirmed by Chen and colleagues, who showed that like Ghsr knockout mice, AGRP/NPY double knockout mice were insensitive to the orexigenic effects of ghrelin.
Metabolism and changes in ghrelin activity during aging
One possible explanation for altered metabolism during aging is reduced ghrelin/GHS-R signaling caused by lower production of ghrelin. Rigamonti et al. found that plasma ghrelin values in old subjects (67-91 yr, n=7) of normal weight were similar to those of young (16-36 yr, n=7) morbidly obese, but were markedly lower than in young adults (27-39 yr, n=12) of normal weight.
Therefore, because body mass index was within normal limits, an altered nutritional state was not implicated in the old subjects.
The lower ghrelin levels in the old subjects were accompanied by increased insulin levels and low serum IGF-I.
The former was a predicted compensatory mechanism for age-related insulin resistance, and the latter is consistent with age-dependent hyposomatotropism rather than malnutrition.
Fig. 2. Proposed regulatory circuit involving hypothalamic ghrelin neurons that control energy homeostasis via NPY/AGRP, POMC neurons, and GABA axon terminals.
Had the elderly subjects been malnourished, the low IGF-I level would have been coupled to high rather than low circulating levels of ghrelin as observed in anorexia nervosa.
Sturm et al. evaluated healthy young and older women and undernourished older women.
Plasma ghrelin concentrations (total active ghrelin and inactive desoctanoylghrelin) were higher in undernourished older than in the well-nourished older and young subjects.
Despite the fact that ghrelin stimulates appetite and food intake, the highest circulating ghrelin concentrations were found in underweight, undernourished, older women.
However, this does not preclude the possibility that ghrelin activity is reduced in the undernourished older subjects because of ghrelin resistance and/or increased ratio of desoctanoylated ghrelin/ghrelin.
When ghrelin concentrations were compared in well-nourished young and old women, they were found to be 20% lower in older women.
Although this difference was not statistically significant, another study evaluated a similar number of well-nourished young and old men and women and found that plasma ghrelin concentrations were significantly (~35%) lower in older subjects.
A caveat is that although these studies suggest ghrelin production declines during adult aging, the assays used did not discriminate active ghrelin from desacyl-ghrelin.
Leptin, metabolism, and aging
Leptin, through its action on the hypothalamus, regulates food intake and metabolism.
Mutations identified in the leptin gene of rodents and humans are associated with altered metabolism and obesity.
Secretion of leptin is subject to ultradian pulsatile rhythmicity, although the episodic profile is not as distinct as that illustrated by pituitary hormones.
However, the pulsatile pattern becomes more organized at night, where fluctuations become synchronous with those of LH and estradiol.
In contrast to the reproductive hormones, variations in circadian and ultradian rhythms of leptin are inversely related to ACTH and cortisol rhythms.
In vitro studies have shown that leptin regulates biosynthesis of TSH releasing hormone, and recent studies on the synchrony of circadian/ultradian rhythms of TSH suggest that leptin also regulates TSH oscillations.
Clearly, the compelling data in support of such a relationship do not preclude the possibility that a common pulse generator in the hypothalamus controls both leptin and TSH rhythms.
The collective findings imply a permissive role for leptin in linking nutritional status and pulsatile activity of the hypothalamic-pituitary peripheral axis, they but do not prove causality.
Leptin decreases food intake and increases energy expenditure in rodents by inhibiting neurones in the hypothalamic arcuate nucleus.
Ghrelin stimulates appetite, and its receptor (GHS-R), like the leptin receptor (Ob-Rb), is expressed in the arcuate nucleus.
Ghrelin induces activation of c-fos expression in the arcuate nucleus, and 57% percent of these cells stain positive for Ob-Rb.
Electrophysiology studies on hypothalamic slices show that ghrelin dose-dependently stimulates the electrical activity of these cells.
Leptin is inhibitory, and ghrelin increases the electrical activity in 76% of all cells that are inhibited by leptin.
These results show that ghrelin interacts with the leptin hypothalamic network in the arcuate nucleus and illustrate that ghrelin and leptin serve as mutual functional antagonists.
Hence, ghrelin resistance can potentially be induced by increased activity of leptin and leptin-receptor in hypothalamic neurons.
Leptin resistance and aging
Animal models of aging have been used to investigate changes in leptin sensitivity.
In rats, leptin administration selectively decreases VF by approximately 60% and inhibits hepatic glucose production by approximately 80%.
Surgical removal of VF improves hepatic insulin action and decreases leptin and TNF-α gene expression in sc adipose tissue.
Therefore, the relationship between the age-related increase in VF and increased insulin resistance may involve the failure of centrally acting leptin to regulate fat distribution.
Manipulation of serum leptin levels by fasting causes hypothalamic NPY mRNA to increase in young but not in old rats.
Leptin infusion reduces food consumption and hypothalamic NPY concentrations by 50% in young rats; however, in old rats, food consumption is reduced by only 20% and NPY is unaffected.
A comparison of pair-fed rats with infused with saline or leptin showed that leptin caused a 24% increase in oxygen consumption in young rats but produced no change in oxygen consumption in old rats.
These results support the conclusion that aged rats are less responsive to leptin because of impaired suppression of hypothalamic NPY synthesis.
The age-related altered response to leptin has also been investigated in Zucker diabetic fatty rats, where leptin was delivered by adenovirus-mediated leptin gene transfer.
Leptin caused markedly different responses in old (18 months old) compared with young rats (2 months old).
For example, free fatty acid and triacylglycerol levels fell precipitously in the young rats but were unaffected in the old animals.
Although leptin reduced food intake, body weight, and fat deposition in old rats, the effects were less pronounced than in young animals.
Similarly, important metabolic markers, such as acyl coenzyme A oxidase, carnitine palmitoyl transferase-1, and peroxisome proliferator receptor-α markedly increased in response to leptin in young rats but not in old rats, confirming that the beneficial metabolic effect of leptin is attenuated during aging.
The mechanism of age-dependent leptin resistance is unknown.
However, one possibility is that leptin receptor signaling is attenuated because of an age-dependent increase in the expression of suppressor of cytokine signaling-3 (SOCS-3).
In addition to aging, leptin resistance accompanies obesity and in most cases insulin resistance.
In nonobese animals, both insulin and leptin act on the hypothalamus to inhibit feeding behavior.
If the anorexic action of leptin is dependent on normal insulin signaling, insulin resistance would also present as leptin resistance.
To test this hypothesis, Matsumoto et al. chronically administered the insulin sensitizer troglitazone (a peroxisome proliferator-activated receptor-α agonist) to old BN male rats.
Troglitazone reduced their high insulin, high leptin, and high body fat; furthermore, their body weight gain in response to fasting was corrected.
Interestingly, restoration of this metabolic phenotype did not alter NPY gene expression in the arcuate nucleus.
These results provide an important link between insulin and leptin resistance that apparently contributes to impairments in energy and weight regulation.
Important questions must now be addressed: 1) is the mechanism independent of improved insulin sensitivity; 2) is normalization of leptin action mediated by cross talk between the insulin and leptin receptor signal transduction pathways; or 3) by improving insulin sensitivity, do asynchronous interdependent pathways essential for optimizing the biological responses to leptin become resynchronized?
Clearly, additional studies are necessary to establish the mechanism of the apparent link among insulin, leptin resistance, and aging.