Characterization of growth hormone binding sites in rat brain

New findings on brain actions of growth hormone and potential clinical implications

Growth hormone (GH) is secreted by somatotropic cells of the anterior pituitary gland. The classical effects of GH comprise the stimulation of cell proliferation, tissue and body growth, lipolysis, and insulin resistance. The GH receptor (GHR) is expressed in numerous brain regions. Notably, a growing body of evidence indicates that GH-induced GHR signaling in specific neuronal populations regulates multiple physiological functions, including energy balance, glucose homeostasis, stress response, behavior, and several neurological/cognitive aspects. The importance of central GHR signaling is particularly evident when the organism is under metabolic stress, such as pregnancy, chronic food deprivation, hypoglycemia, and prolonged exercise. These particular situations are associated with elevated GH secretion. Thus, central GH action represents an internal signal that coordinates metabolic, neurological, neuroendocrine, and behavioral adaptations that are evolutionarily advantageous to increase the chances of survival. This review summarizes and discusses recent findings indicating that the brain is an important target of GH, and GHR signaling in different neuronal populations regulates essential physiological functions.

The Pattern of GH Action in the Mouse Brain

GH acts in numerous organs expressing the GH receptor (GHR), including the brain. However, the mechanisms behind the brain's permeability to GH and how this hormone accesses different brain regions remain unclear. It is well-known that an acute GH administration induces phosphorylation of the signal transducer and activator of transcription 5 (pSTAT5) in the mouse brain. Thus, the pattern of pSTAT5 immunoreactive cells was analyzed at different time points after IP or intracerebroventricular GH injections. After a systemic GH injection, the first cells expressing pSTAT5 were those near circumventricular organs, such as arcuate nucleus neurons adjacent to the median eminence. Both systemic and central GH injections induced a medial-to-lateral pattern of pSTAT5 immunoreactivity over time because GH-responsive cells were initially observed in periventricular areas and were progressively detected in lateral brain structures. Very few choroid plexus cells exhibited GH-induced pSTAT5. Additionally, Ghr mRNA was poorly expressed in the mouse choroid plexus. In contrast, some tanycytes lining the floor of the third ventricle expressed Ghr mRNA and exhibited GH-induced pSTAT5. The transport of radiolabeled GH into the hypothalamus did not differ between wild-type and dwarf Ghr knockout mice, indicating that GH transport into the mouse brain is GHR independent. Also, single-photon emission computed tomography confirmed that radiolabeled GH rapidly reaches the ventral part of the tuberal hypothalamus. In conclusion, our study provides novel and valuable information about the pattern and mechanisms behind GH transport into the mouse brain.

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.

The role of growth hormone in hippocampal function

Growth hormone is a multifunctional molecule with broad cellular targets. This pituitary hormone is currently used as a therapeutic agent against several brain injuries due to its neurotrophic activity. The hippocampus is one of the brain regions where the growth hormone plays a role in normal and pathologic conditions. This brain structure is associated with several cognitive functions such as learning, memory, and mood, which are frequently affected by brain traumatism. The present chapter describes the experimental and clinical evidence that supports a central role of growth hormone in the hippocampus functionality.

Traumatic Brain Injury as Frequent Cause of Hypopituitarism and Growth Hormone Deficiency: Epidemiology, Diagnosis, and Treatment

Traumatic brain injury (TBI)-related hypopituitarism has been recognized as a clinical entity for more than a century, with the first case being reported in 1918. However, during the 20^th century hypopituitarism was considered only a rare sequela of TBI. Since 2000 several studies strongly suggest that TBI-mediated pituitary hormones deficiency may be more frequent than previously thought. Growth hormone deficiency (GHD) is the most common abnormality, followed by hypogonadism, hypothyroidism, hypocortisolism, and diabetes insipidus. The pathophysiological mechanisms underlying pituitary damage in TBI patients include a primary injury that may lead to the direct trauma of the hypothalamus or pituitary gland; on the other hand, secondary injuries are mainly related to an interplay of a complex and ongoing cascade of specific molecular/biochemical events. The available data describe the importance of GHD after TBI and its influence in promoting neurocognitive and behavioral deficits. The poor outcomes that are seen with long standing GHD in post TBI patients could be improved by GH treatment, but to date literature data on the possible beneficial effects of GH replacement therapy in post-TBI GHD patients are currently scarce and fragmented. More studies are needed to further characterize this clinical syndrome with the purpose of establishing appropriate standards of care. The purpose of this review is to summarize the current state of knowledge about post-traumatic GH deficiency.

Blood-brain barrier transport kinetics of NOTA-modified proteins: the somatropin case

BACKGROUND

Chemical modifications such as PEG, polyamine and radiolabeling on proteins can alter their pharmacokinetic behavior and their blood-brain barrier (BBB) transport characteristics. NOTA, i.e. 1,4,7-triazacyclononane-1,4,7-triacetic acid, is a bifunctional chelating agent that has attracted the interest of the scientific community for its high complexation constant with metals like gallium. Until now, the comparative BBB transport characteristics of NOTA-modified proteins versus unmodified proteins are not yet described.

METHODS

Somatropin (i.e. recombinant human growth hormone), NOTA-conjugated somatropin and gallium-labelled NOTA-conjugated somatropin were investigated for their brain penetration characteristics (multiple time regression and capillary depletion [CD]) in an in vivo mice model to determine the blood-brain transfer properties.

RESULTS

The three compounds showed comparable initial brain influx, with Kin=0.38±0.14 µL/(g×min), 0.36±0.16 µL/(g×min) and 0.28±0.18 µL/(g×min), respectively. CD indicated that more than 80% of the influxed compounds reached the brain parenchyma. All three compounds were in vivo stable in serum and brain during the time frame of the experiments.

CONCLUSIONS

Our results show that modification of NOTA as well as gallium chelation onto proteins, in casu somatropin, does not lead to a significantly changed pharmacokinetic profile at the blood-brain barrier.

Therapeutic approaches of trophic factors in animal models and in patients with spinal cord injury

Trophic factors are naturally produced by different tissues that participate in several functions such as the intercellular communication, in the development, stability, differentiation and regeneration at the cellular level. Specifically, in the case of spinal injuries, these factors can stimulate neuronal recovery. They are applied both in experimental models and in clinical trials in patients. The trophic factors analysed in this review include gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), growth hormone (GH), melatonin, oestrogens, the family of fibroblast growth factors (FGFs), the family of neurotrophins and the glial cell-derived neurotrophic factor (GDNF). There are some trophic (neurotrophic) factors that already been tested in patients with spinal cord injury (SCI), but only shown partial recovery effect. It is possible that, the administration of these trophic factors together with physical rehabilitation, act synergistically and, therefore, significantly improve the quality of life of patients with SCI.

Central growth hormone action regulates metabolism during pregnancy

The maternal organism undergoes numerous metabolic adaptations to become prepared for the demands associated with the coming offspring. These metabolic adaptations involve changes induced by several hormones that act at multiple levels, ultimately influencing energy and glucose homeostasis during pregnancy and lactation. Previous studies have shown that central growth hormone (GH) action modulates glucose and energy homeostasis. However, whether central GH action regulates metabolism during pregnancy and lactation is still unknown. In the present study, we generated mice carrying ablation of GH receptor (GHR) in agouti-related protein (AgRP)-expressing neurons, in leptin receptor (LepR)-expressing cells or in the entire brain to investigate the role played by central GH action during pregnancy and lactation. AgRP-specific GHR ablation led to minor metabolic changes during pregnancy and lactation. However, while brain-specific GHR ablation reduced food intake and body adiposity during gestation, LepR GHR knockout (KO) mice exhibited increased leptin responsiveness in the ventromedial nucleus of the hypothalamus during late pregnancy, although their offspring showed reduced growth rate. Additionally, both Brain GHR KO and LepR GHR KO mice had lower glucose tolerance and glucose-stimulated insulin secretion during pregnancy, despite presenting increased insulin sensitivity, compared with control pregnant animals. Our findings revealed that during pregnancy central GH action regulates food intake, fat retention, as well as the sensitivity to insulin and leptin in a cell-specific manner. Together, the results suggest that GH acts in concert with other "gestational hormones" to prepare the maternal organism for the metabolic demands of the offspring.

Growth Hormone Deficiency Following Traumatic Brain Injury

Traumatic brain injury (TBI) is fairly common and annually affects millions of people worldwide. Post traumatic hypopituitarism (PTHP) has been increasingly recognized as an important and prevalent clinical entity. Growth hormone deficiency (GHD) is the most common pituitary hormone deficit in long-term survivors of TBI. The pathophysiology of GHD post TBI is thought to be multifactorial including primary and secondary mechanisms. An interplay of ischemia, cytotoxicity, and inflammation post TBI have been suggested, resulting in pituitary hormone deficits. Signs and symptoms of GHD can overlap with those of TBI and may delay rehabilitation/recovery if not recognized and treated. Screening for GHD is recommended in the chronic phase, at least six months to a year after TBI as GH may recover in those with GHD in the acute phase; conversely, it may manifest in those with a previously intact GH axis. Dynamic testing is the standard method to diagnose GHD in this population. GHD is associated with long-term poor medical outcomes. Treatment with recombinant human growth hormone (rhGH) seems to ameliorate some of these features. This review will discuss the frequency and pathophysiology of GHD post TBI, its clinical consequences, and the outcomes of treatment with GH replacement.

Growth Hormone (GH) and Gonadotropin-Releasing Hormone (GnRH) in the Central Nervous System: A Potential Neurological Combinatory Therapy?

This brief review of the neurological effects of growth hormone (GH) and gonadotropin-releasing hormone (GnRH) in the brain, particularly in the cerebral cortex, hypothalamus, hippocampus, cerebellum, spinal cord, neural retina, and brain tumors, summarizes recent information about their therapeutic potential as treatments for different neuropathologies and neurodegenerative processes. The effect of GH and GnRH (by independent administration) has been associated with beneficial impacts in patients with brain trauma and spinal cord injuries. Both GH and GnRH have demonstrated potent neurotrophic, neuroprotective, and neuroregenerative action. Positive behavioral and cognitive effects are also associated with GH and GnRH administration. Increasing evidence suggests the possibility of a multifactorial therapy that includes both GH and GnRH.

Possible effects of an early diagnosis and treatment in patients with growth hormone deficiency: the state of art

Growth hormone deficiency (GHD) is a relatively uncommon and heterogeneous endocrine disorder presenting in childhood with short stature. However, during the neonatal period, the metabolic effects of GHD may to require prompt replacement therapy to avoid possible life-threatening complications. An increasing amount of data suggests the importance of an early diagnosis and treatment of GHD because of its auxological, metabolic, and neurodevelopmental features with respect to the patients diagnosed and treated later in life.

The available results show favourable auxological outcomes for patients with GHD diagnosed and treated with r-hGH early in life compared with those from patients with GHD who do not receive this early diagnosis and treatment. Because delayed referral for GHD diagnosis and treatment is still frequent, these results highlight the need for more attention in the diagnosis and treatment of GHD.

Despite these very encouraging data regarding metabolic and neurodevelopmental features, further studies are needed to better characterize these findings. Overall, the importance of early diagnosis and treatment of GHD needs to be addressed.

The mRNA expression of insulin-like growth factor-1 (Igf1) is decreased in the rat frontal cortex following gamma-hydroxybutyrate (GHB) administration

In recent years, growth hormone (GH), together with its secondary mediators insulin-like growth factor-1 (IGF-1) and insulin-like growth factor-2 (IGF-2), have been highlighted for their beneficial effects in the central nervous system (CNS), in particular as cognitive enhancers. Cognitive processes, such as learning and memory, are known to be impaired in individuals suffering from substance abuse. In the present study, we investigated the effect of gamma-hydroxybuturate (GHB), an illicit drug used for its sedating and euphoric properties, on genes associated with the somatotrophic axis in regions of the brain important for cognitive function. Sprague Dawley rats (n=36) were divided into three groups and administered either saline, GHB 50mg/kg or GHB 300mg/kg orally for seven days. The levels of Ghr, Igf1 and Igf2 gene transcripts were analyzed using qPCR in brain regions involved in cognition and dependence. The levels of IGF-1 in blood plasma were also determined using ELISA. The results demonstrated a significant down-regulation of Igf1 mRNA expression in the frontal cortex in high-dose treated rats. Moreover, a significant correlation between Igf1 and Ghr mRNA expression was found in the hippocampus, the frontal cortex, and the caudate putamen, indicating local regulation of the GH/IGF-1 axis. To summarize, the current study concludes that chronic GHB treatment influences gene expression of Ghr and Igf1 in brain regions involved in cognitive function.

Growth hormone and insulin-like growth factor-I alter hippocampal excitatory synaptic transmission in young and old rats

In rats, as in humans, normal aging is characterized by a decline in hippocampal-dependent learning and memory, as well as in glutamatergic function. Both growth hormone (GH) and insulin-like growth factor-I (IGF-I) levels have been reported to decrease with age, and treatment with either GH or IGF-I can ameliorate age-related cognitive decline. Interestingly, acute GH and IGF-I treatments enhance glutamatergic synaptic transmission in the rat hippocampus of juvenile animals. However, whether this enhancement also occurs in old rats, when cognitive impairment is ameliorated by GH and IGF-I (des-IGF-I), remains to be determined. To address this issue, we used an in vitro CA1 hippocampal slice preparation and extracellular recording techniques to study the effects of acute application of GH and IGF-I on compound field excitatory postsynaptic potentials (fEPSPs), as well as AMPA- and NMDA-dependent fEPSPs, in young adult (10 months) and old (28 months) rats. The results indicated that both GH and IGF-I increased compound-, AMPA-, and NMDA-dependent fEPSPs to a similar extent in slices from both age groups and that this augmentation was likely mediated via a postsynaptic mechanism. Initial characterization of the signaling cascades underlying these effects revealed that the GH-induced enhancement was not mediated by the JAK2 signaling element in either young adult or old rats but that the IGF-I-induced enhancement involved a PI3K-mediated mechanism in old, but not young adults. The present findings are consistent with a role for a GH- or IGF-I-induced enhancement of glutamatergic transmission in mitigating age-related cognitive impairment in old rats.

Growth hormone and cognitive function

Emerging data indicate that growth hormone (GH) therapy could have a role in improving cognitive function. GH replacement therapy in experimental animals and human patients counteracts the dysfunction of many behaviours related to the central nervous system (CNS). Various behaviours, such as cognitive behaviours related to learning and memory, are known to be induced by GH; the hormone might interact with specific receptors located in areas of the CNS that are associated with the functional anatomy of these behaviours. GH is believed to affect excitatory circuits involved in synaptic plasticity, which alters cognitive capacity. GH also has a protective effect on the CNS, as indicated by its beneficial effects in patients with spinal cord injury. Data collected from animal models indicates that GH might also stimulate neurogenesis. This Review discusses the mechanisms underlying the interactions between GH and the CNS, and the data emerging from animal and human studies on the relationship between GH and cognitive function. In this article, particular emphasis is given to the role of GH as a treatment for patients with cognitive impairment resulting from deficiency of the hormone.

Recombinant human growth hormone affects the density and functionality of GABAB receptors in the male rat brain

The beneficial effects of growth hormone (GH) on memory and learning have previously been confirmed in both humans and in animal models. An important role of GABAB receptors for multiple forms of learning and memory has also been reported. In this study, we examined the effect of GH on the density and functionality of the metabotropic GABAB receptors in the rat brain. Male Sprague-Dawley rats (n = 24) divided into 3 groups were injected twice daily with recombinant human GH (0.07 or 0.7 IU/kg) for 7 days. The effects of the hormone were determined by quantitative autoradiography and by GABAB stimulated [(35)S]-GTPγS binding using the selective GABAB receptor agonist baclofen. The results demonstrate moderate but significant alterations in both receptor density and functionality in a number of brain regions. For example, a dose-dependent upregulation of GABAB receptors was found in the cingulate cortex, primary motor cortex and caudate putamen, whereas attenuation in the receptor density was encountered in, for example, the medial geniculate nucleus. Although the GH-induced effects on the GABAB receptor in brain areas associated with cognition were fairly pronounced, they were significant and we propose that the physiological responses observed after GH administration at least partly can be mediated through a mechanism involving GABAB receptors.

Growth hormone reverses streptozotocin-induced cognitive impairments in male mice

In recent decades, growth hormone (GH) replacement therapy in human subjects deficient in the hormone has resulted in a number of beneficial effects on cognitive performance. Studies in hypophysectomised rats report similar effects of GH treatment on learning and memory tasks. The purpose of this study was to investigate the ability of GH to reverse learning impairments in mice with streptozotocin (STZ)-induced diabetes. Diabetic and control mice were given recombinant human GH (rhGH) 0.1 IU/kg/day for ten consecutive days. In the latter phase of the treatment the cognitive abilities of the mice were tested using the Barnes maze (BM). A profound hormonal effect was seen when analysing the search patterns used by the animals in the maze. rhGH treatment significantly counteracted the cognitive disabilities expressed as lack of direct search strategies on the last day in the BM. In addition, the number of primary errors made by diabetic mice during the acquisition phase was reduced by rhGH treatment, although the primary escape latency was unchanged in these animals when compared to saline-treated diabetic animals. These results suggest that specific cognitive impairments induced by STZ, i.e. the disabilities seen in strategic behaviour, could be reversed by exogenous hormone treatment. Our findings highlight the influence of GH on brain function and in particular on cognitive behaviour related to learning and memory.

Application of in vitro [³⁵S]GTPγ-S autoradiography in studies of growth hormone effects on opioid receptors in the male rat brain

Chronic treatment with opiates may inhibit cell growth and trigger apoptosis. On the contrary, growth hormone (GH) has been demonstrated to stimulate neurogenesis and counteract apoptosis. We recently demonstrated that recombinant human GH (rhGH) may reverse opiate-induced apoptosis in cells derived from prenatal mouse hippocampus. Thus, GH might be able to prevent the impaired cognitive capabilities that may occur in both humans and other mammals in connection to chronic opiate treatment. In order to explore the mechanism by which GH exerts its beneficial effects we here examined the impact of GH treatment on the levels of delta and mu opioid peptide (DOP and MOP, respectively) receptors in the male rat brain. The rats were treated with rhGH (Genotropin®) at two different doses (0.07 and 0.7 IU/kg), twice daily, during 7 days. Following decapitation, the levels of DOP and MOP receptor functionality were determined using [³⁵S]GTPγS autoradiography. The results demonstrate that rhGH affects the levels of the MOP receptor functionality in certain areas of the brain. These alterations were seen in e.g. amygdala and thalamus, i.e. regions that recently have been implicated in learning and memory. The activity level of DOP receptors was not affected. Thus, the data support that the beneficial effect of GH on counteracting apoptosis might involve a direct or indirect effect on the MOP but not the DOP receptor.

Growth hormone responsive neural precursor cells reside within the adult mammalian brain

The detection of growth hormone (GH) and its receptor in germinal regions of the mammalian brain prompted our investigation of GH and its role in the regulation of endogenous neural precursor cell activity. Here we report that the addition of exogenous GH significantly increased the expansion rate in long-term neurosphere cultures derived from wild-type mice, while neurospheres derived from GH null mice exhibited a reduced expansion rate. We also detected a doubling in the frequency of large (i.e. stem cell-derived) colonies for up to 120 days following a 7-day intracerebroventricular infusion of GH suggesting the activation of endogenous stem cells. Moreover, gamma irradiation induced the ablation of normally quiescent stem cells in GH-infused mice, resulting in a decline in olfactory bulb neurogenesis. These results suggest that GH activates populations of resident stem and progenitor cells, and therefore may represent a novel therapeutic target for age-related neurodegeneration and associated cognitive decline.

Growth hormone modulates hippocampal excitatory synaptic transmission and plasticity in old rats

Alterations in the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPA-R) and N-methyl-D-aspartate receptor (NMDA-R) have been documented in aged animals and may contribute to changes in hippocampal-dependent memory. Growth hormone (GH) regulates AMPA-R and NMDA-R-dependent excitatory transmission and decreases with age. Chronic GH treatment mitigates age-related cognitive decline. An in vitro CA1 hippocampal slice preparation was used to compare hippocampal excitatory transmission and plasticity in old animals treated for 6-8 months with either saline or GH. Our findings indicate that GH treatment restores NMDA-R-dependent basal synaptic transmission in old rats to young adult levels and enhances both AMPA-R-dependent basal synaptic transmission and long-term potentiation. These alterations in synaptic function occurred in the absence of changes in presynaptic function, as measured by paired-pulse ratios, the total protein levels of AMPA-R and NMDA-R subunits or in plasma or hippocampal levels of insulin-like growth factor-I. These data suggest a direct role for GH in altering age-related changes in excitatory transmission and provide a possible cellular mechanism through which GH changes the course of cognitive decline.

Regional distribution and ontogeny of the first exon variants of the rat growth hormone receptor mRNA in the brain and the pituitary gland

Expression of the first exon variants of the rat growth hormone receptor mRNA was studied in the brain and the pituitary gland. Four of the five different variant mRNA previously identified in the liver were detected in the cerebral cortex by a conventional reverse-transcription polymerase chain reaction, and, unlike the data reported previously, a quantitative analysis revealed that approximately 90% of the total growth hormone receptor mRNA in the cerebral cortex is V1 form. The present results suggest that the V1 was also a dominant transcript in other brain areas and the pituitary gland, not only in adult but also in fetal and postnatal period. The growth hormone receptor expression in the brain was lower during fetal period than in adults, while in the pituitary gland, the expression is markedly higher in fetuses, suggesting a yet unknown role of growth hormone in the development of this organ.

The role of the somatotrophic axis in neuroprotection and neuroregeneration of the addictive brain

Early studies have shown that the abuse of alcohol, central stimulants, and opiates such as heroin destroys brain cells, reducing attention span and memory. However, new research has suggested that there may be a way to regain some of the lost attention and recall. It has recently been shown that brain cells targeted for early death by continued opiate use can be salvaged by injections of synthetic human growth hormone (GH). GH is a polypeptide hormone, normally secreted by the anterior pituitary gland, which stimulates cell growth and controls body metabolism. Recombinant human GH is currently used in replacement therapy to alleviate the symptoms of adults and children with GH deficiency syndrome. The recent observation that GH can reverse morphine-induced cell damage could open the door to new ways of treating and preventing damage from the abuse of opiates in addicts and also of treating cell damage induced by alcohol and central stimulants. This article reviews current knowledge of the somatotrophic axis, including GH and insulin-like growth factor-1 (IGF-1), in the brain and also discusses the potential use of GH/IGF-1 as agents for treatment of brain pathology in addictive diseases.

Adult-onset deficiency in growth hormone and insulin-like growth factor-I alters oligodendrocyte turnover in the corpus callosum

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) provide trophic support during development and also appear to influence cell structure, function and replacement in the adult brain. Recent studies demonstrated effects of the GH/IGF-I axis on adult neurogenesis, but it is unclear whether the GH/IGF-I axis influences glial turnover in the normal adult brain. In the current study, we used a selective model of adult-onset GH and IGF-I deficiency to evaluate the role of GH and IGF-I in regulating glial proliferation and survival in the adult corpus callosum. GH/IGF-I-deficient dwarf rats of the Lewis strain were made GH/IGF-I replete via twice daily injections of GH starting at postnatal day 28 (P28), approximately the age at which GH pulse amplitude increases in developing rodents. GH/IGF-I deficiency was initiated in adulthood by removing animals from GH treatment. Quantitative analyses revealed that adult-onset GH/IGF-I deficiency decreased cell proliferation in the white matter and decreased the survival of newborn oligodendrocytes. These findings are consistent with the hypothesis that aging-related changes in the GH/IGF-I axis produce deficits in ongoing turnover of oligodendrocytes, which may contribute to aging-related cognitive changes and deficits in remyelination after injury.

Growth hormone signaling and hippocampal neurogenesis: insights from genetic models

Adult hippocampal neurogenesis (AHN) is modulated by a variety of factors through effects on the proliferation-differentiation-survival regulatory axis. We have employed growth hormone receptor knockout (GH-R-/-) and suppressor of cytokine signaling-2 transgenic (SOCS-2 Tg) mice as models of altered GH-signaling to assess their affects on basal and exercised-induced hippocampal neurogenesis. Assessment of proliferation 24-h after 7-days of bromodeoxyuridine (BrdU) labeling with or without voluntary running showed that the density of BrdU(+) cells in the subgranular zone remained unchanged between genotypes in control housing, while running induced significant increases in BrdU-labeled cells in WT, GH-R-/-, and SOCS-2 Tg mice. The proportion of BrdU/doublecortin and BrdU/S100beta cells did not vary between genotype or running conditions at this time-point. Assessment of cell survival 28-days after BrdU labeling showed that SOCS-2 Tg animals had significantly higher BrdU(+) cell densities in the granule cell layer compared to WT and GH-R-/- animals in control housing and after voluntary running. There were no differences in cell survival between WT and GH-R-/- mice with or without running. Mature phenotype analysis showed similar proportions of BrdU/NeuN and BrdU/S100beta in all groups. While SOCS-2 Tg mice had similar social interaction behaviors and sensorimotor gating, they appeared to be less anxious with heightened basal locomotor activity and showed enhanced performance in the Morris watermaze test. Overall, our data indicated that mice over-expressing SOCS-2 showed increased survival of neurons generated during AHN, which correlated with improved performance in a hippocampal-dependent cognitive task. Furthermore, voluntary running increased AHN in WT, SOCS-2 Tg, and serum-IGF-1-deficient GH-R-/- mice.

Adult-onset deficiency in growth hormone and insulin-like growth factor-I decreases survival of dentate granule neurons: Insights into the regulation of adult hippocampal neurogenesis

Insulin-like growth factor-I (IGF-I), long thought to provide critical trophic support during development, also has emerged as a candidate for regulating ongoing neuronal production in adulthood. Whether and how IGF-I influences each phase of neurogenesis, however, remains unclear. In the current study, we used a selective model of growth hormone (GH) and plasma IGF-I deficiency to evaluate the role of GH and IGF-I in regulating cell proliferation, survival, and neuronal differentiation in the adult dentate gyrus. GH/IGF-I-deficient dwarf rats of the Lewis strain were made GH/IGF-I replete throughout development via twice daily injections of GH, and then GH/IGF-I deficiency was initiated in adulthood by removing animals from GH treatment. Bromodeoxyuridine (BrdU) labeling revealed no effect of GH/IGF-I deficiency on cell proliferation, but adult-onset depletion of GH and plasma IGF-I significantly reduced the survival of newly generated cells in the dentate gyrus. Colabeling for BrdU and markers of immature and mature neurons revealed a selective effect of GH/IGF-I deficiency on the survival of more mature new neurons. The number of BrdU-labeled cells expressing the immature neuronal marker TUC-4 did not differ between GH/IGF-I-deficient and -replete animals, but the number expressing only the marker of maturity NeuN was lower in depleted animals. Taken together, results from the present study suggest that, under conditions of short-term GH/IGF-I deficiency during adulthood, dentate granule cells continue to be produced, to commit to a neuronal fate, and to begin the process of neuronal maturation, whereas survival of the new neurons is impaired.

Growth hormone and insulin-like growth factor-1 (IGF-1) and their influence on cognitive aging

The concept that growth hormone and IGF-1 are required for normal development of the mammalian body and, more recently the brain, is supported by a vast experimental literature. IGF-1 crosses the blood-brain barrier and in recent years, much attention has focused on age-related decreases in serum growth hormone and IGF-1 as potential mechanisms that may influence cognitive function in the elderly. However, interventional studies are needed to establish a definite link between these hormones and function of the aging brain. In rodents, long-term growth hormone/IGF-1 replacement improves learning and memory in aged rats. While the exact mechanism underlying these cognitive improvements is unknown, growth hormone and IGF-1 replacement to aged animals increases neurogenesis, vascular density, and glucose utilization, and alters NMDA receptor subunit composition in brain areas that are implicated in learning and memory. While these observations offer valuable insight into the influence of growth hormone and IGF-1 on neuronal events in the aged mammal, additional functional studies are required to link these changes to cognitive improvements.

Distribution of growth hormone receptor mRNA in the brain stem and spinal cord of the rat

By using in situ hybridization histochemistry the distribution of growth hormone (GH) receptor mRNA was examined in the rat brain stem and spinal cord. Dense labeling was seen in the arcuate nucleus of the hypothalamus, as reported previously, but also in several other areas, including the locus coeruleus, the area postrema, and the commissural part of the nucleus of the solitary tract. Other labeled structures included the superior lateral parabrachial nucleus, the facial, hypoglossal and trigeminal motor nuclei, the nucleus incertus, the dorsal tegmental nucleus, the dorsal raphe nucleus, the nucleus of the trapezoid body, and the superficial layers of the dorsal horn of the spinal cord. These findings provide support for a direct action of GH on brain regions involved in various aspects of homeostatic control. Thus, the distribution of GH receptor mRNA to visceral sensory and motor structures is consonant with a role of GH in the regulation of food intake and energy homeostasis. Its presence in the superficial dorsal horn of the spinal cord indicates a role for GH in the initial processing of fine afferent input, and may help explain the beneficial effects of GH replacement in certain unclear pain conditions.

Insulin-like growth factor-I, cognition and brain aging

Aging is associated with a decline in the activity of the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. As aging also coincides with a decline in specific cognitive functions and as some of these dysfunctions are also observed in subjects with GH deficiency, it has been hypothesised that a causal relationship exists between the reduction in circulating GH and/or IGF-I and the observed cognitive deficits in the elderly. The present review summarises the available data concerning the possible relation between GH, IGF-I and cognitive performance, and regarding possible underlying pathophysiological mechanisms.

Differential inhibition of postnatal brain, spinal cord and body growth by a growth hormone antagonist

Background

Growth hormone (GH) plays an incompletely understood role in the development of the central nervous system (CNS). In this study, we use transgenic mice expressing a growth hormone antagonist (GHA) to explore the role of GH in regulating postnatal brain, spinal cord and body growth into adulthood. The GHA transgene encodes a protein that inhibits the binding of GH to its receptor, specifically antagonizing the trophic effects of endogenous GH.

Results

Before 50 days of postnatal age, GHA reduces spinal cord weight more than brain weight, but less than body weight. Thereafter, GHA ceases to inhibit the increase in body weight, which approaches control levels by day 150. In contrast, GHA continues to act on the CNS after day 50, reducing spinal cord growth to a greater extent and for a longer duration than brain growth.

Conclusions

Judging from its inhibition by GHA, GH differentially affects the magnitude, velocity and duration of postnatal growth of the brain, spinal cord and body. GH promotes body enlargement more than CNS growth early in postnatal life. Later, its CNS effects are most obvious in the spinal cord, which continues to exhibit GH dependence well into adulthood. As normal CNS growth slows, so does its inhibition by GHA, suggesting that reduced trophic effects of GH contribute to the postnatal slowing of CNS growth. GHA is a highly useful tool for studying the role of endogenous GH on organ-specific growth during aging.

Growth hormone in the nervous system: autocrine or paracrine roles in retinal function?

Growth hormone (GH) is primarily produced in the pituitary gland, although GH gene expression also occurs in the central and autonomic nervous systems. GH-immunoreactive proteins are abundant in the brain, spinal cord, and peripheral nerves. The appearance of GH in these tissues occurs prior to the ontogenic differentiation of the pituitary gland and prior to the presence of GH in systemic circulation. Neural GH is also present in neonates, juveniles, and adults and is independent of changes in pituitary GH secretion. Neural GH is therefore likely to have local roles in neural development or neural function, especially as GH receptors (GHRs) are widespread in the nervous system. In recent studies, GH mRNA and GH immunoreactive proteins have been identified in the neural retina of embryonic chicks. GH immunoreactivity is present in the optic cup of chick embryos at embryonic day (ED) 3 of the 21-d incubation period. It is widespread in the neural retina by ED 7 but also present in the nonpigmented retina, choroid, sclera, and cornea. This immunoreactivity is associated with proteins in the neural retina comparable in size with those in the adult pituitary gland, although it is primarily associated with 15-16 kDa moieties rather than with the full-length molecule of approximately 22 kDa. These small GH moieties may reflect proteolytic fragments of "monomer" GH and (or) the presence of different GH gene transcripts, since full-length and truncated GH cDNAs are present in retinal tissue extracts. The GH immunoreactivity in the retina persists throughout embryonic development but is not present in juvenile birds (after 6 weeks of age). This immunoreactivity is also associated with the presence of GH receptor (GHR) immunoreactivity and GHR mRNA in ocular tissues of chick embryos. The retina is thus an extrapituitary site of GH gene expression during early development and is probably an autocrine or paracrine site of GH action. The marked ontogenic pattern of GH immunoreactivity in the retina suggests hitherto unsuspected roles for GH in neurogenesis or ocular development.

Central nervous system actions of growth hormone on brain monoamine levels and behavior of juvenile rainbow trout

Growth hormone (GH) has been demonstrated to alter the behavior of juvenile salmonids. However, the mechanisms behind this action are not yet understood. In mammals and birds, peripheral GH treatment has been shown to affect monoaminergic activity in the central nervous system, which may be a mechanism whereby GH alters behavior. To investigate if GH may influence behavior directly at the central nervous system, juvenile rainbow trout were injected with GH into the third ventricle of the brain, whereupon physical activity and food intake were observed during 2 h. Thereafter, brains were sampled and the content of serotonin, dopamine, and noradrenaline and their metabolites were measured in hypothalamus, telencephalon, optic tectum, and brainstem. The GH-treated fish increased their swimming activity relative to sham-injected controls, while appetite remained unchanged, compared with sham-injected controls. Analysis of brain content of monoamines revealed that the GH treatment caused a decrease in the dopamine metabolite homovanillic acid in the hypothalamus, indicating a lowered dopaminergic activity. It is concluded that GH may alter behavior by acting directly on the central nervous system in juvenile rainbow trout. Furthermore, GH seems to alter the dopaminergic activity in the hypothalamus. Whether this is a mechanism whereby GH affects swimming activity remains to be clarified.

Neural growth hormone: an update

It is now well established that growth hormone (GH) gene expression is not restricted to the pituitary gland and occurs in many extrapituitary tissues, including the central and peripheral nervous systems. Indeed, GH gene expression occurs in the brain prior to its ontogenic appearance in the pituitary gland, and GH may have evolved phylogenetically as a neuropeptide, rather than as an endocrine. Recent studies on the regulation and roles of neural GH in health and disease are the focus of this brief review.

Growth hormone induces age-dependent alteration in the expression of hippocampal growth hormone receptor and N-methyl-D-aspartate receptor subunits gene transcripts in male rats

Studies were conducted to evaluate the effects of s.c. injected recombinant human growth hormone (GH) on the expression of the gene transcript of N-methyl-D-aspartate receptor subunits type 1 (NR1), type 2A (NR2A), and type 2B (NR2B) in the male rat hippocampus. The GH-induced effects on the expression of hippocampal gene transcripts of GH receptor (GHR) and GH-binding protein were also examined. Male Sprague-Dawley rats, kept in four groups of two different ages, was treated with the hormone or saline during 10 days before decapitation and tissue dissection. Brain tissues collected were analyzed for mRNA content by using the Northern blot technique. The results indicated that in adult young rats (11 weeks of age) the hormone elicited a decrease in the mRNA expression of NR1 but an increase in that of the NR2B subunit. In elderly adult rats (57-67 weeks of age) GH induced an increase in the expression of the hippocampal message for NR1 and NR2A. Meanwhile, the hormone induced a significant up-regulation of the GHR transcript in hippocampus of adult young rats but not in elderly adult rats. It was further found that a significant positive correlation exists between the level of GHR mRNA and the expression of the NR2B subunit transcript in adult young rats. The GH-induced increase in the expression of hippocampal mRNA for the NR2B subunit is compatible with a previously observed memory promoting effect seen for the hormone, because overexpression of this N-methyl-D-aspartate receptor subunit is shown to enhance cognitive capabilities.

Transcriptional profiling reveals regulated genes in the hippocampus during memory formation

Transcriptional profiling (TP) offers a powerful approach to identify genes activated during memory formation and, by inference, the molecular pathways involved. Trace eyeblink conditioning is well suited for the study of regional gene expression because it requires the hippocampus, whereas the highly parallel task, delay conditioning, does not. First, we determined when gene expression was most regulated during trace conditioning. Rats were exposed to 200 trials per day of paired and unpaired stimuli each day for 4 days. Changes in gene expression were most apparent 24 h after exposure to 200 trials. Therefore, we profiled gene expression in the hippocampus 24 h after 200 trials of trace eyeblink conditioning, on multiple arrays using additional animals. Of 1,186 genes on the filter array, seven genes met the statistical criteria and were also validated by real-time polymerase chain reaction. These genes were growth hormone (GH), c-kit receptor tyrosine kinase (c-kit), glutamate receptor, metabotropic 5 (mGluR5), nerve growth factor-beta (NGF-beta), Jun oncogene (c-Jun), transmembrane receptor Unc5H1 (UNC5H1), and transmembrane receptor Unc5H2 (UNC5H2). All these genes, except for GH, were downregulated in response to trace conditioning. GH was upregulated; therefore, we also validated the downregulation of the GH inhibitor, somatostatin (SST), even though it just failed to meet criteria on the arrays. By during situ hybridization, GH was expressed throughout the cell layers of the hippocampus in response to trace conditioning. None of the genes regulated in trace eyeblink conditioning were similarly affected by delay conditioning, a task that does not require the hippocampus. These findings demonstrate that transcriptional profiling can exhibit a repertoire of genes sensitive to the formation of hippocampal-dependent associative memories.

Morphine decreases the levels of the gene transcripts of growth hormone receptor and growth hormone binding protein in the male rat hippocampus and spinal cord

In this study we have characterized the nucleotide sequence of the cDNA for the growth hormone receptor (GHR) and examined the effects of morphine on the gene transcripts for GHR as well as GH binding protein (GHBP) in the male rat hippocampus and spinal cord. Using reverse transcription-polymerase chain reaction followed by cloning and sequencing, we found that the entire coding region of the GHR mRNA in the spinal cord is identical to that previously described in liver. A similar observation was made for the partially sequenced GHR cDNA from hippocampus. Northern blot analysis showed that in both tissues the levels of the transcripts for both GHR and GHBP were significantly decreased 4 h after a single dose of morphine. After 24 h the level of both transcripts did not significantly differ from that of control animals. This result indicates that the opiate does not only affect the receptor protein as shown earlier by binding studies, but also reduces the expression or turnover of the GHR as well as GHBP at the transcription level.

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.

Neuron addition and enlargement in juvenile and adult animals

Locomotion requires bilateral symmetry of neural circuitry in the spinal cord. Although not well understood, the mechanisms responsible for establishing and maintaining this symmetry must balance the numbers, sizes, and connectivity of the neurons on both sides of the spinal cord. Those mechanisms do not cease to function after embryogenesis, since there is substantial evidence that these properties continue to change as juvenile animals grow to adult size. We review the evidence that spinal neuron number and size increase in growing juvenile frogs and mammals. We postulate that these increases are regulated by both local and systemic factors. In addition, we discuss evidence that axotomy of spinal sensory and motor neurons also enlists local and systemic regulatory factors, some of which may also be operative in normal growth and development.

Growth hormone in the brain: characteristics of specific brain targets for the hormone and their functional significance

During the past decade studies have shown that growth hormone (GH) may exert profound effects on the central nervous system (CNS). For instance, GH replacement therapy was found to improve the psychological capabilities in adult GH deficient (GHD) patients. Furthermore, beneficial effects of the hormone on certain functions, including memory, mental alertness, motivation, and working capacity, have been reported. Likewise, GH treatment of GHD children has been observed to produce significant improvement in many behavioral problems seen in these individuals. Studies also indicated that GH therapy affects the cerebrospinal fluid levels of various hormones and neurotransmitters. Further support that the CNS is a target for GH emerges from observations indicating that the hormone may cross the blood-brain barrier (BBB) and from studies confirming the presence of GH receptors in the brain. It was previously shown that specific binding sites for GH are present in discrete areas in the CNS of both humans and rats. Among these regions are the choroid plexus, hippocampus, hypothalamus, and spinal cord. The density of GH binding in the various brain regions was found to decline with increasing age. More recently, we were able to clone and determine the structure of several GH receptors in the rat and human brain. Although the brain receptor proteins for the hormone were shown to differ in molecular size compared to those present in peripheral tissues the corresponding transcripts did not seem to differ from their peripheral congeners. GH receptors in the hypothalamus are likely to be involved in the regulatory mechanism for hormone secretion and those located in the choroid plexus have been suggested to have a role in the receptor-mediated transport of GH across the BBB. The functions mediated by the GH receptors identified in the hippocampus are not yet known but recently it was speculated that they may be involved in the hormone's action on memory and cognitive functions.

Effect of growth hormone on brain oedema caused by a cryogenic brain injury model in rats

Although growth hormone (GH) is a potential agent to counteract the catabolic response to general and head injuries, the complications of GH treatment have been little studied. To evaluate the safety of GH in head trauma patients, it was investigated whether GH affects brain oedema caused by brain injury, using a rat freeze-injury model. Male Wistar rats (n = 28) were divided into four groups according to the substance injected (GH vs. saline) and whether brain was injured or not. GH (0.8 IU/kg) or saline was injected subcutaneously every 24 hours. In the injury groups, immediately before the first injection of GH, an aluminum rod cooled to -50 degrees C was placed on the right parietal region for 4 minutes, under anaesthesia. At 96 hours after the insult, brain and skeletal muscle were excised and their water content was measured, by drying. Freeze injury of the brain caused an increase in water content in skeletal muscle. GH injection augmented this oedema in skeletal muscle. Freeze injury of the brain also caused an increase in water content in the injured and non-injured hemispheres of brain and cerebellum. GH administration did not exacerbate brain oedema caused by brain injury in this model.

Growth hormone, insulin-like growth factor I and cognitive function in adults

This review focuses on the possible contribution of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis to cognitive function. Binding sites for GH and IGF-I are found in various areas of the brain. Their distribution suggests that GH and IGF-I contribute to the function of the hippocampus, a brain structure important for the maintenance of cognitive functions such as learning and memory. Evidence for cognitive deficits in GH-deficient individuals has been found in various studies, some of which have shown that these deficits can be reversed by GH substitution therapy. In addition to examining conditions of GH deficiency, this article reviews studies evaluating the correlation between the cognitive deficits associated with ageing and age-related decreases in GH or IGF-I secretion. Based on the available data, one might hypothesize that relative GH or IGF-I deficiency could contribute to the deterioration of cognitive functions observed in the elderly.

Expression of growth hormone receptor in the human brain

This study was designed to investigate the presence of growth hormone receptor (GHR) expression in the human brain tissue, both normal and tumoral, as well as in the human glioblastoma cell line U87MG. Reverse transcription-polymerase chain reaction revealed the presence of GHR mRNA in all brain samples investigated and in U87MG cells. GHR immunoreactivity was also detected in this cell line using both immunocytochemistry and western blotting. All together, our data demonstrate the existence of GHR expression within the central nervous system (CNS), thus supporting a possible role for GH in the CNS physiology.

Blood-brain barrier for human growth hormone and insulin-like growth factor-I

There is a blood-brain barrier (BBB) for GH. A certain, unknown amount of GH passes the BBB, acts on the neuronal GH receptors and directly influences the brain mechanisms serving the feedback and ultradian secretion of GH. The high density of GH receptors in the choroid plexus suggests a possible receptor-mediated transcytosis transport. The effects of GH on brain development, neuronal plasticity and neuroprotection seem to be mediated by IGFs. GH and IGFs are also synthesized in the brain. The relative contributions to brain functions of GHs produced inside and outside the BBB are unknown. The cerebrospinal fluid (CSF) space is the compartment inside the barrier accessible to clinicians. High GH levels in CSF were reported in acromegaly and also a small increase was reported after chronic administration of hGH in GH-deficiency syndromes. For the practitioner it is necessary to determine the normal range of hGH levels in CSF.

Does growth hormone augment brain edema caused by brain injury? A study with a freeze brain injury model in the rat

OBJECTIVE

Because of the known sodium and water retention associated with growth hormone (GH) therapy, it is crucial to evaluate the safety of GH after brain injury. To clarify this issue, we investigated whether GH affects brain edema in a rat brain freeze-injury model.

METHODS

Male Wistar rats (n = 29) were divided into four groups according to the substance injected (GH vs. normal saline) and whether the brain was injured or not. The subcutaneous injections of GH (0.8 IU/kg) or saline were given 24 hours apart. In the injury groups, after the second injection, an aluminum rod (4-mm diameter) cooled to -50 degrees C was placed on the exposed dura mater in the right parietal region for 4 minutes, under anesthesia. At 4 hours after the insult, brain and skeletal muscle were excised and their water content was measured by drying.

RESULTS

Freeze injury of the brain caused an increase in water and sodium content in skeletal muscle. GH injection augmented this edema in skeletal muscle. Freeze injury of the brain also caused an increase in water and sodium content in the injured hemisphere of the brain. GH injection did not exacerbate this edema in injured brain tissue. Neither freeze injury nor GH injection caused brain edema in the noninjured hemisphere or in the cerebellum.

CONCLUSION

GH administration did not augment brain edema caused by brain injury in our model.

Growth hormone. A paracrine growth factor?

A number of tissues, including the brain, pituitary, immune system, placenta, mammary gland, and testis, may be self-contained units of GH regulation, production, and action. The production of GH and GH-releasing factors outside the hypothalamo-pituitary axis complements, rather than replaces, the traditional endocrine interactions between GH-releasing factors, GH, and its target tissues.

Aging effects on growth hormone receptor binding in the brain

Recent years have seen an increasing interest in research focused on the role that growth hormone (GH) may have in the central nervous system. The psychological improvements seen in adults following GH therapy combined with the observation that the hormone may affect the cerebrospinal fluid levels of several brain transmitters have received a great deal of attention. Studies have also revealed the presence of specific GH receptors in distinct areas of the brain of many mammals. This article will review our recent studies on the aging effects on GH binding in these regions. It also includes some data on the age-related effects on the expression of the GH-receptor messenger ribonucleic acid (mRNA) in certain brain areas.

Decreased binding of growth hormone in the rat hypothalamus and choroid plexus following morphine treatment

Male Sprague-Dawley rats were continuously infused with morphine through subcutaneously implanted mini-osmotic pumps over a period of 5 days. The binding of rat growth hormone (rGH) to specific sites in choroid plexus, cortex, hypothalamus, hippocampus and striatum was determined. It was found that in the acute phase of morphine administration the density of growth hormone-binding sites was significantly decreased in choroid plexus and in hypothalamus, but not in any other of the tissues studied. When tolerance to morphine was developed, the level of growth hormone-binding was restored to control level. In the acute phase, the plasma levels of GH, as measured by radioimmunoassay, correlated negatively with the density of the binding sites in choroid plexus and hypothalamus. The decrease in growth hormone-binding in these regions of the rat brain was also confirmed by SDS-polyacrylamide gel electrophoresis of cross-linked complexes of the binding entities to 125I-labelled rGH as visualized by autoradiography. In experiments, where morphine was administrated by intermittent injections, a similar decrease in rGH-binding was observed. However, the time-course of this decrease seemed to be dependent upon the route of administration. Following intracerebroventricular (i.c.v.) injections, the binding of the hormone was already affected after 30 min, whereas the binding of rGH in brain areas after subcutaneous (s.c.) injections was affected at a later stage.