Bioassayable growth hormone release in rats in response to a single bout of treadmill exercise

The multiple roles of GH in neural ageing and injury

Advanced age is typically associated with a decrease in cognitive function including impairment in the formation and retention of new memories. The hippocampus is critical for learning and memory, especially spatial learning, and is particularly affected by ageing. With advanced age, multiple neural components can be detrimentally affected including a reduction in the number of neural stem and precursor cells, a decrease in the formation of adult born neurons (neurogenesis), and deficits in neural circuitry, all of which ultimately contribute to impaired cognitive function. Importantly, physical exercise has been shown to ameliorate many of these impairments and is able to improve learning and memory. Relevantly, growth hormone (GH) is an important protein hormone that decreases with ageing and increases following physical exercise. Originally described due to its role in longitudinal growth, GH has now been identified to play several additional key roles, especially in relation to the brain. Indeed, the regular decrease in GH levels following puberty is one of the most well documented components of neuroendocrine ageing. Growth hormone deficiency (GHD) has been described to have adverse effects on brain function, which can be ameliorated via GH replacement therapy. Physical exercise has been shown to increase circulating GH levels. Furthermore, we recently demonstrated the increase in exercise-mediated GH is critical for improved cognitive function in the aged mouse. Here we examine the multiple roles that GH plays, particularly in the aged brain and following trauma, irradiation and stroke, and how increasing GH levels can ameliorate deficits in cognition.

Bioactive growth hormone in humans: Controversies, complexities and concepts

OBJECTIVE

To revisit a finding, first described in 1978, which documented existence of a pituitary growth factor that escaped detection by immunoassay, but which was active in the established rat tibia GH bioassay.

METHODS

We present a narrative review of the evolution of growth hormone complexity, and its bio-detectability, from a historical perspective.

RESULTS

In humans under the age of 60, physical training (i.e. aerobic endurance and resistance training) are stressors which preferentially stimulate release of bioactive GH (bGH) into the blood. Neuroanatomical studies indicate a) that nerve fibers directly innervate the human anterior pituitary and b) that hind limb muscle afferents, in both humans and rats, also modulate plasma bGH. In the pituitary gland itself, molecular variants of GH, somatotroph heterogeneity and cell plasticity all appear to play a role in regulation of this growth factor.

CONCLUSION

This review considers more recent findings on this often forgotten/neglected subject. Comparison testing of a) human plasma samples, b) sub-populations of separated rat pituitary somatotrophs or c) purified human pituitary peptides by GH bioassay vs immunoassay consistently yield conflicting results.

The exercise-glucocorticoid paradox: How exercise is beneficial to cognition, mood, and the brain while increasing glucocorticoid levels

Exercise is known to have beneficial effects on cognition, mood, and the brain. However, exercise also activates the hypothalamic-pituitary-adrenal axis and increases levels of the glucocorticoid cortisol (CORT). CORT, also known as the "stress hormone," is considered a mediator between chronic stress and depression and to link various cognitive deficits. Here, we review the evidence that shows that while both chronic stress and exercise elevate basal CORT levels leading to increased secretion of CORT, the former is detrimental to cognition/memory, mood/stress coping, and brain plasticity, while the latter is beneficial. We propose three preliminary answers to the exercise-CORT paradox. Importantly, the elevated CORT, through glucocorticoid receptors, functions to elevate dopamine in the medial prefrontal cortex under chronic exercise but not chronic stress, and the medial prefrontal dopamine is essential for active coping. Future inquiries may provide further insights to promote our understanding of this paradox.

Diagnosis and treatment of osteopenia

Osteopenia is a term to define bone density that is not normal but also not as low as osteoporosis. By definition from the World Health Organization osteopenia is defined by bone densitometry as a T score -1 to -2.5. There are many causes for osteopenia including calcium and vitamin D deficiency and inactivity. Genetics plays an important role in a person's bone mineral density and often Caucasian women with a thin body habitus who are premenopausal are found to have osteopenia. Correction of calcium and vitamin D deficiency and walking 3 to 5 miles a week can often improve bone density in the hip and spine. There are a variety of pharmaceutical agents that have been recommended for the treatment of osteopenia and osteoporosis including hormone replacement therapy, selective estrogen receptor modulator therapy, anti-resorptive therapy. In addition patients with osteoporosis who have failed anti-resorptive therapy can have a significant improvement in their bone density with anabolic therapy.

Exercise increases neural stem cell number in a growth hormone-dependent manner, augmenting the regenerative response in aged mice

The exercise-induced enhancement of learning and memory, and its ability to slow age-related cognitive decline in humans led us to investigate whether running stimulates periventricular (PVR) neural stem cells (NSCs) in aging mice, thereby augmenting the regenerative capacity of the brain. To establish a benchmark of normal aging on endogenous NSCs, we harvested the PVR from serial vibratome sections through the lateral ventricles of juvenile (6-8 weeks), 6-, 12-, 18-, and 24-month-old mice, culturing the cells in the neural colony-forming cell assay. A significant decline in NSC frequency was apparent by 6 months ( approximately 40%), ultimately resulting in a approximately 90% reduction by 24 months. Concurrent with this decline was a progressive loss in regenerative capacity, as reflected by an incomplete repopulation of neurosphere-forming cells following gamma cell irradiation-induced depletion of the PVR. However, voluntary exercise (i.e., 21 days of running) significantly increased NSC frequency in mice > or = 18 months of age, augmenting the regeneration of irradiation-ablated periventricular cells and restoring NSC numbers to youthful levels. Importantly, and consistent with the demonstrated ability of growth hormone (GH) to increase NSC proliferation, and the elevated secretion of GH during exercise, exercise failed to stimulate NSCs in GH receptor-null mice. These findings now provide a novel basis for understanding the ability of exercise to delay the onset and rate of decline in neurodegenerative conditions not typically associated with the hippocampus and suggest that the GH-dependent activation of endogenous NSCs may be effective in reversing or preventing age-related neurodegeneration in humans.

Basal and evoked levels of bioassayable growth hormone are altered by hindlimb unloading

Bioassayable growth hormone (BGH) in rats is released in large quantities from the pituitary in response to the activation of large, proprioceptive afferent fibers from fast and mixed fiber-type hindlimb musculature. We hypothesized that hindlimb unloading (HU) of adult male rats would 1) reduce the basal levels of plasma BGH, and 2) abolish stimulus-induced BGH release. Rats were exposed to HU for 1, 4, or 8 wk. Plasma and pituitaries were collected under isoflurane anesthesia for hormone analyses. Additionally, at 4 and 8 wk, a subset of rats underwent an in situ electrical stimulation (Stim) of tibial nerve proprioceptive afferents. Basal plasma BGH levels were significantly reduced (-51 and -23%) after 1 and 8 wk of HU compared with ambulatory controls (Amb). Although Amb-Stim rats exhibited increased plasma BGH levels (88 and 143%) and decreased pituitary BGH levels (-27 and -22%) at 4 and 8 wk, respectively, stimulation in HU rats had the opposite effect, reducing plasma BGH (-25 and -33%) and increasing pituitary BGH levels (47 and 10%) relative to HU alone at 4 and 8 wk. The 22-kDa form of GH measured by immunoassay and the plasma corticosterone, T3, T4, and testosterone levels were unchanged by HU or Stim at all time points. These data suggest that BGH synthesis and release from the pituitary are sensitive both to chronically reduced neuromuscular loading and to acute changes in neuromuscular activation, independent of changes in other circulating hormones. Thus BGH may play a role in muscle, bone, and metabolic adaptations that occur in response to chronically unloaded states.

Vibration-induced activation of muscle afferents modulates bioassayable growth hormone release

The effects of tendon vibration on bioassayable growth hormone (BGH) secretion from the pituitary gland were investigated in anesthetized adult male rats. The tendons from predominantly fast-twitch ankle extensor muscles (gastrocnemius and plantaris) or a predominantly slow-twitch ankle extensor (soleus) were vibrated by using a paradigm that selectively activates group Ia afferent fibers from muscle spindles. The lower hindlimb was secured with the muscles near physiological length, and the tendons were vibrated for 15 min at 150 Hz and a displacement of 1 mm. Control rats were prepared similarly, but the tendons were not vibrated. Compared with control, vibration of the tendons of the fast ankle extensors markedly increased (160%), whereas vibration of the slow soleus decreased (68%), BGH secretion. Complete denervation of the hindlimb had no independent effects on the normal resting levels of BGH, but it prevented the effects of tendon vibration on BGH secretion. The results are consistent with previous findings showing modulation of BGH release in response to in vivo activation or in situ electrical stimulation of muscle afferents (Bigbee AJ, Gosselink KL, Grindeland RE, Roy RR, Zhong H, and Edgerton VR. J Appl Physiol 89: 2174–2178, 2000; Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, and Edgerton VR. J Appl Physiol 88: 142–148, 2000; Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, Grossman EJ, and Edgerton VR. J Appl Physiol 84: 1425–1430, 1998). These data provide evidence that this previously described muscle afferent-pituitary axis is neurally mediated via group Ia afferents from peripheral skeletal muscle. Furthermore, these data show that activation of this group Ia afferent pathway from fast muscles enhances, whereas the same sensory afferent input from a slow muscle depresses, BGH release.

Effects of pharmacologic androgen treatment duration on glucocorticoid receptor alpha immunoreactivity of lumbosacral motor neurons in the male rat

It has previously been suggested that anabolic-androgenic steroids may affect neuromuscular function through their potential action as glucocorticoid receptor antagonists. Alternatively, androgens may regulate the sensitivity of neuromuscular systems to glucocorticoids by modulating GR levels. The purpose of this study was to determine the effects of chronic pharmacologic testosterone treatment of gonadally intact male rats on glucocorticoid receptor alpha immunoreactivity (GRalpha-IR) of motor neurons in the lumbosacral spinal cord. Circulating testosterone levels were chronically increased by subcutaneous Silastic capsules containing crystalline testosterone propionate (TP) for 7, 14, and 28 days. Age-matched sham-operated gonadally intact males served as controls. Relative cytoplasmic and nuclear GRalpha-IR of motor neurons located in the lateral motor column of spinal cord segments L(3) and L(4) (L(Lat); innervating rat hindlimb muscles) and the spinal nucleus of the bulbocavernosus (SNB; innervating the external anal sphincter, bulbocavernosus and levator ani muscles) was measured densitometrically. TP treatment duration had a significant impact on the mean GRalpha levels of both cellular compartments regardless of motor column (two-way ANOVA, P<0.001). The mean nuclear GRalpha-IR of lumbar motor neurons was significantly reduced after 7 days (OD: 0.239+/-0.013 S.E.M.; P<0.016) and 14 days (OD: 0.196+/-0.013; P<0.001) from the GRalpha-IR levels observed among the control group (OD: 0.296+/-0.012) by 20 and 40%, respectively. Interestingly, nuclear GRalpha-IR levels were similar to control levels after 28 days of TP treatment (OD: 0.307+/-0.010). Treatment-dependent changes in cytoplasmic GRalpha-IR paralleled the observed changes in nuclear GRalpha-IR. These data suggest that pharmacologic testosterone treatment effects on motor neuron gene expression may be mediated by testosterone-induced temporal fluctuations of GRalpha-dependent gene regulation.

Muscle afferent-pituitary axis: a novel pathway for modulating the secretion of a pituitary growth factor

Evidence is presented for a novel muscle-neuro-endocrine pathway for modulating the secretion of an unidentified pituitary growth factor by proprioceptive input from skeletal muscle afferents. Exercise stimulates the release of this growth factor, whereas chronic unloading associated with bed rest or spaceflight abrogates the normal exercise stimulus.