Modulation of long-term memory and extinction responses induced by growth hormone (GH) and growth hormone releasing hormone (GHRH) in rats

Dynamic regulation of excitatory and inhibitory synaptic transmission by growth hormone in the developing mouse brain

Normal sensory and cognitive function of the brain relies on its intricate and complex neural network. Synaptogenesis and synaptic plasticity are critical to neural circuit formation and maintenance, which are regulated by coordinated intracellular and extracellular signaling. Growth hormone (GH) is the most abundant anterior pituitary hormone. Its deficiencies could alter brain development and impair learning and memory, while GH replacement therapy in human patients and animal models has been shown to ameliorate cognitive deficits caused by GH deficiency. However, the underlying mechanism remains largely unknown. In this study, we investigated the neuromodulatory function of GH in young (pre-weaning) mice at two developmental time points and in two different brain regions. Neonatal mice were subcutaneously injected with recombinant human growth hormone (rhGH) on postnatal day (P) 14 or 21. Excitatory and inhibitory synaptic transmission was measured using whole-cell recordings in acute cortical slices 2 h after the injection. We showed that injection of rhGH (2 mg/kg) in P14 mice significantly increased the frequency of mEPSCs, but not that of mIPSCs, in both hippocampal CA1 pyramidal neurons and L2/3 pyramidal neurons of the barrel field of the primary somatosensory cortex (S1BF). Injection of rhGH (2 mg/kg) in P21 mice significantly increased the frequency of mEPSCs and mIPSCs in both brain regions. Perfusion of rhGH (1 μM) onto acute brain slices in P14 mice had similar effects. Consistent with the electrophysiological results, the dendritic spine density of CA1 pyramidal neurons and S1BF L2/3 pyramidal neurons increased following in vivo injection of rhGH. Furthermore, NMDA receptors and postsynaptic calcium-dependent signaling contributed to rhGH-dependent regulation of both excitatory and inhibitory synaptic transmission. Together, these results demonstrate that regulation of excitatory and inhibitory synaptic transmission by rhGH occurs in a developmentally dynamic manner, and have important implication for identifying GH treatment strategies without disturbing excitation/inhibition balance.

Therapeutic effects of growth hormone in a rat model of total sleep deprivation: Evaluating behavioral, hormonal, biochemical and electrophysiological parameters

OBJECTIVE

Total sleep deprivation (TSD) causes several harmful changes in the brain, including memory impairment, increased stress and depression levels, as well as reduced antioxidant activity. Growth hormone (GH) has been shown to boost antioxidant levels while improving memory and depression. The present study was conducted to explain the possible effects of exogenous GH against behavioral and biochemical disorders caused by TSD and the possible mechanisms involved.

MAIN METHODS

To induce TSD, rats were housed in homemade special cages equipped with stainless steel wire conductors to induce general and inconsistent TSD. They received a mild repetitive electric shock to their paws every 10 min for 21 days. GH (1 ml/kg, sc) was administered to rats during induction of TSD for 21 days. Memory retrieval, anxiety, depression-like behaviors, pain behaviors, antioxidant activity, hippocampal level of BDNF, and simultaneously brain electrical activity were measured at scheduled times after TSD.

KEY FINDINGS

The results showed that GH treatment improved memory (p < 0.001) in the PAT test of rats exposed to TSD. These beneficial effects were associated with lowering the level of anxiety and depression-like behavior (p < 0.001), rising the pain threshold (p < 0.01), increasing the activity of antioxidants (p < 0.01), hippocampal BDNF (p < 0.001), and regular brain electrical activity.

SIGNIFICANCE

Our findings show that GH plays a key role in modulating memory, anxiety and depression behaviors, as well as reducing oxidative stress and improve hippocampal single-unit activity in the brain during TSD.

Chronic adult-onset of growth hormone/IGF-I hypersecretion improves cognitive functions and LTP and promotes neuronal differentiation in adult rats

AIM

Besides their metabolic and endocrine functions, the growth hormone (GH) and its mediated factor, the insulin-like growth factor I (IGF-I), have been implicated in different brain functions, including neurogenesis. Long-lasting elevated GH and IGF-I levels result in non-reversible somatic, endocrine and metabolic morbidities. However, the subcutaneous implantation of the GH-secreting (GH-S) GC cell line in rats leads to the controllable over-secretion of GH and elevated IGF-I levels, allowing the experimental study of their short-term effects on brain functions.

METHODS

Adult rats were implanted with GC cells and checked 10 weeks later, when a GH/IGF-I-secreting tumour was already formed.

RESULTS

Tumour-bearing rats acquired different operant conditioning tasks faster and better than controls and tumour-resected groups. They also presented better retentions of long-term memories in the passive avoidance test. Experimentally evoked long-term potentiation (LTP) in the hippocampus was also larger and longer lasting in the tumour bearing than in the other groups. Chronic adult-onset of GH/IGF-I hypersecretion caused an acceleration of early progenitors, facilitating a faster neural differentiation, maturation and integration in the dentate gyrus, and increased the complexity of dendritic arbours and spine density of granule neurons.

CONCLUSION

Thus, adult-onset hypersecretion of GH/IGF-I improves neurocognitive functions, long-term memories, experimental LTP and neural differentiation, migration and maturation.

Effects of temperature on the transcriptomes of pituitary and liver in Golden Pompano Trachinotus blochii

Fish growth can be modulated dynamically through the brain-pituitary-liver regulation axis. In the present study, whole transcriptomes of the pituitary and liver from Golden Pompano Trachinotus blochii were sequenced in seawater at 20 °C (T_low) and 25 °C (T_high). A total of 187,277,583 paired-end reads were assembled to obtain 100,495 transcripts, corresponding to 83,974 genes. These reads were mapped to T. blochii transcripts, and the mapping accuracy ranged from 80.4 to 94.9%. Two lists of differentially expressed genes were obtained by comparisons of pituitary and liver T_low versus T_high groups, comprising 458 and 205 genes, respectively. Of these, 33 differentially expressed genes were common between the two lists. Twelve GO terms were overrepresented for the 458 differentially expressed genes in the pituitary, and it is noteworthy that the GO term galanin receptor activity (GO: 0004966) related to the modulation of appetite and metabolism, whose genes made up half of all assembled genes in the term. For the 205 differentially expressed genes in the liver, 19 overrepresented GO terms were mainly related to immune regulation, digestion, and protein metabolism. Among the common differentially expressed genes, there were 32 genes that had identical changing trends in both pituitary and liver comparisons. Furthermore, two GO terms inorganic diphosphatase activity and MHC protein complex were overrepresented. These results indicate that the brain could regulate pituitary function through galanin signal mechanism and that the metabolism of liver was further optimized to modulate immunity and growth under different temperatures.

Hippocampal growth hormone modulates relational memory and the dendritic spine density in CA1

Growth hormone (GH) deficiency is associated with cognitive decline which occur both in normal aging and in endocrine disorders. Several brain areas express receptors for GH although their functional role is unclear. To determine how GH affects the capacity for learning and memory by specific actions in one of the key areas, the hippocampus, we injected recombinant adeno-associated viruses (rAAVs) in male rats to express green fluorescent protein (GFP) combined with either GH, antagonizing GH (aGH), or no hormone, in the dorsal CA1. We found that aGH disrupted memory in the Morris water maze task, and that aGH treated animals needed more training to relearn a novel goal location. In a one-trial spontaneous location recognition test, the GH treated rats had better memory performance for object locations than the two other groups. Histological examinations revealed that GH increased the dendritic spine density on apical dendrites of CA1, while aGH reduced the spine density. GH increased the relative amount of immature spines, while aGH decreased the same amount. Our results imply that GH is a neuromodulator with strong influence over hippocampal plasticity and relational memory by mechanisms involving modulation of dendritic spines. The findings are significant to the increasing aging population and GH deficiency patients.

Childhood-onset growth hormone deficiency and the transition to adulthood: current perspective

Childhood-onset growth hormone deficiency (CO-GHD) is an endocrine condition associated with a broad range of health issues from childhood through to adulthood, which requires particular attention during the transition period from adolescence to young adulthood. There is uncertainty in the clinical practice of the management of CO-GHD during transition regarding the clinical assessment and management of individual patients during and after transition to obtain optimal follow-up and improved health outcomes. Despite the availability of clinical guidelines providing the framework for transition of young adults with CO-GHD, there remains substantial variation in approaching transitional care among pediatric and adult services. A well-structured and coordinated transitional plan with clear communication and direct collaboration between pediatric and adult health care to ensure optimal management of adolescents with CO-GHD during transition is needed.

Mode of GH administration and gene expression in the female rat brain

The endogenous secretion of growth hormone (GH) is sexually dimorphic in rats with females having a more even and males a more pulsatile secretion and low trough levels. The mode of GH administration, mimicking the sexually dimorphic secretion, has different systemic effects. In the brains of male rats, we have previously found that the mode of GH administration differently affects neuron-haemoglobin beta (Hbb) expression whereas effects on other transcripts were moderate. The different modes of GH administration could have different effects on brain transcripts in female rats. Hypophysectomised female rats were given GH either as injections twice daily or as continuous infusion and GH-responsive transcripts were assessed by quantitative reverse transcription polymerase chain reaction in the hippocampus and parietal cortex (cortex). The different modes of GH-administration markedly increased Hbb and 5'-aminolevulinate synthase 2 (Alas2) in both brain regions. As other effects were relatively moderate, a mixed model analysis (MMA) was used to investigate general effects of the treatments. In the hippocampus, MMA showed that GH-infusion suppressed glia- and neuron-related transcript expression levels, whereas GH-injections increased expression levels. In the cortex, GH-infusion instead increased neuron-related transcripts, whereas GH-injections had no significant effect. Interestingly, this contrasts to previous results obtained from male rat cortex where GH-infusion generally decreased expression levels. In conclusion, the results indicate that there is a small but significant difference in response to mode of GH administration in the hippocampus as compared to the cortex. For both modes of GH administration, there was a robust effect on Hbb and Alas2.

Growth hormone biases amygdala network activation after fear learning

Prolonged stress exposure is a risk factor for developing posttraumatic stress disorder, a disorder characterized by the 'over-encoding' of a traumatic experience. A potential mechanism by which this occurs is through upregulation of growth hormone (GH) in the amygdala. Here we test the hypotheses that GH promotes the over-encoding of fearful memories by increasing the number of neurons activated during memory encoding and biasing the allocation of neuronal activation, one aspect of the process by which neurons compete to encode memories, to favor neurons that have stronger inputs. Viral overexpression of GH in the amygdala increased the number of amygdala cells activated by fear memory formation. GH-overexpressing cells were especially biased to express the immediate early gene c-Fos after fear conditioning, revealing strong autocrine actions of GH in the amygdala. In addition, we observed dramatically enhanced dendritic spine density in GH-overexpressing neurons. These data elucidate a previously unrecognized autocrine role for GH in the regulation of amygdala neuron function and identify specific mechanisms by which chronic stress, by enhancing GH in the amygdala, may predispose an individual to excessive fear memory formation.

Growth hormone (GH) increases cognition and expression of ionotropic glutamate receptors (AMPA and NMDA) in transgenic zebrafish (Danio rerio)

The growth hormone/insulin-like factor I (GH/IGF-I) somatotropic axis is responsible for somatic growth in vertebrates, and has important functions in the nervous system. Among these, learning and memory functions related to the neural expression of ionotropic glutamate receptors, mainly types AMPA (α-amino-3hydroxy-5methylisoxazole-4propionic) and NMDA (N-methyl-d-aspartate) can be highlighted. Studies on these mechanisms have been almost exclusively conducted on mammal models, with little information available on fish. Consequently, this study aimed at evaluating the effects of the somatotropic axis on learning and memory of a GH-transgenic zebrafish (Danio rerio) model (F0104 strain). Long-term memory (LTM) was tested in an inhibitory avoidance apparatus, and brain expression of igf-I and genes that code for the main subunits of the AMPA and NMDA receptors were evaluated. Results showed a significant increase in LTM for transgenic fish. Transgenic animals also showed a generalized pattern of increase in the expression of AMPA and NMDA genes, as well as a three-fold induction in igf-I expression in the brain. When analyzed together, these results indicate that GH, mediated by IGF-I, has important effects on the brain, with improvement in LTM as a result of increased glutamate receptors. The transgenic strain F0104 was shown to be an interesting model for elucidating the intricate mechanisms related to the effect of the somatotropic axis on learning and memory in vertebrates.

Effect of dietary γ-aminobutyric acid on the nerve growth factor and the choline acetyltransferase in the cerebral cortex and hippocampus of ovariectomized female rats

The brain protein synthesis and the plasma concentration of growth hormone (GH) is sensitive to the dietary γ-aminobutyric acid (GABA) in ovariectomized female rats; however, the role of dietary GABA on biomarkers including nerve growth factor (NGF) and choline acetyltransferase for the function of cholinergic neurons remains unknown in ovariectomized female rats. The purpose of this study was to determine whether the dietary GABA affects the concentration and mRNA level of NGF, and the activity of choline acetyltransferase in the brains of ovariectomized female rats. Experiments were done on two groups of 24-wk-old ovariectomized female rats given 0 or 0.5% GABA added to a 20% casein diet. The concentrations of NGF and activities of choline acetyltransferase in the cerebral cortex and hippocampus, and mRNA level of NGF in the hippocampus increased significantly with the 20% casein+0.5% GABA compared with the 20% casein diet alone. In the hippocampus, the mRNA level of NGF significantly correlated with the NGF concentration (r=0.714, p<0.01). These results suggest that the administration of GABA to ovariectomized female rats is likely to control the mRNA level and concentration of NGF and cause an increase in the activity of choline acetyltransferase in the brains.

Enhanced performance of aged rats in contingency degradation and instrumental extinction tasks

Normal aging in rats affects behavioral performance on a variety of associative learning tasks under Pavlovian conditions. There is little information, however, on whether aging also impacts performance of instrumental tasks. Young (9-12 months) and aged (24-27 months) Fisher 344 rats were trained to press distinct levers associated with either maltodextrin or sucrose. The rats in both age groups increased their lever press frequency at a similar rate, suggesting that the initial acquisition of this instrumental task is not affected by aging. Using a contingency degradation procedure, we then addressed whether aged rats could adapt their behavior to changes in action-outcome contingencies. We found that young and aged rats do adapt, but that a different schedule of reinforcement is necessary to optimize performance in each age group. Finally, we also addressed whether aged rats can extinguish a lever press action as well as young rats, using 2 40-min extinction sessions on consecutive days. While extinction profiles were similar in young and aged rats on the first day of training, aged rats were faster to extinguish their lever presses on the second day, in spite of their performance levels being similar at the beginning of the session. Together these data support the finding that acquisition of instrumental lever press behaviors is preserved in aged rats and suggest that they have a different threshold for switching strategies in response to changes in action-outcome associations. This pattern of result implies that age-related changes in the brain are heterogeneous and widespread across structures.

Different modes of GH administration influence gene expression in the male rat brain

The endogenous secretion pattern in males of GH is episodic in rats and in humans, whereas GH administration is usually even. Different types of GH administration have different effects on body mass, longitudinal bone growth, and liver metabolism in rodents, whereas possible effects on brain plasticity have not been investigated. In this study, GH was administered as a continuous infusion or as two daily injections in hypophysectomized male rats. Thirteen transcripts previously known to respond to GH in the hippocampus and parietal cortex (cortex) were assessed by RT-PCR. To investigate the effects of type of GH administration on several transcripts with different variations, and categories of transcripts (neuron-, glia-, and GH-related), a mixed model analysis was applied. Accordingly, GH injections increased overall transcript abundance more than GH infusions (21% in the hippocampus, P<0.001 and 10% in the cortex, P=0.09). Specifically, GH infusions and injections robustly increased neuronal hemoglobin beta (Hbb) expression significantly (1.8- to 3.6-fold), and GH injections were more effective than GH infusions in increasing Hbb in the cortex (41%, P=0.02), whereas a 23% difference in the hippocampus was not significant. Also cortical connexin 43 was higher in the group with GH injections than in those with GH infusions (26%, P<0.007). Also, there were differences between GH injections and infusions in GH-related transcripts of the cortex (23%, P=0.04) and glia-related transcripts of the hippocampus (15%, P=0.02). Thus, with the exception of Hbb there is a moderate difference in responsiveness to different modes of GH administration.

Cognitive improvement by acute growth hormone is mediated by NMDA and AMPA receptors and MEK pathway

It has been reported that Growth hormone (GH) has an immediate effect enhancing excitatory postsynaptic potentials mediated by AMPA and NMDA receptors in hippocampal area CA1. As GH plays a role in adult memory processing, this work aims to study the acute effects of GH on working memory tasks in rodents and the possible involvement of NMDA and AMPA receptors and also the MEK/ERK signalling pathway. To evaluate memory processes, two different tests were used, the spatial working memory 8-arm radial maze, and the novel object recognition as a form of non-spatial working memory test. Acute GH treatment (1mg/kg i.p., 1h) improved spatial learning in the radial maze respect to the control group either in young rats (reduction of 46% in the performance trial time and 61% in the number of errors), old rats (reduction of 38% in trial time and 48% in the number of errors), and adult mice (reduction of 32% in the performance time and 34% in the number of errors). GH treatment also increased the time spent exploring the novel object respect to the familiar object compared to the control group in young rats (from 63% to 79%), old rats (from 53% to 70%), and adult mice (from 61 to 68%). The improving effects of GH on working memory tests were blocked by the NMDA antagonist MK801 dizocilpine (0.025 mg/kg i.p.) injected 10 min before the administration of GH, in both young and old rats. In addition, the AMPA antagonist DNQX (1mg/kg i.p.) injected 10 min before the administration of GH to young rats, blocked the positive effect of GH. Moreover, in mice, the MEK inhibitor SL 327 (20mg/kg i.p.) injected 30 min before the administration of GH, blocked the positive effect of GH on radial maze and the novel object recognition. In conclusion, GH improved working memory processes through both glutamatergic receptors NMDA and AMPA and it required the activation of extracellular MEK/ERK signalling pathway. These effects could be related to the enhancement of excitatory synaptic transmission in the hippocampus reported by GH.

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.

Role of growth hormone (GH) in the treatment on neural diseases: from neuroprotection to neural repair

Growth hormone (GH) is a pleiotropic hormone that exerts important functions in the control of brain development as well as in the regulation neuronal differentiation and function, together with several behavioral and psychological effects that have been linked to its modulatory actions on brain neurotransmitters. In addition, the possibility that GH may play a role on brain repair after injury has been also envisaged, and a number of reports have shown that GH administration following injury confers neuroprotection and accelerates the recovery of some neural functions. In this review we have analyzed the state of the art of GH administration in several neural diseases. Though more studies are still necessary in order to completely understand the importance of GH in these processes, the promising results obtained so far, together with the absence of untoward effects during GH therapy, encourages the development of clinical assays in order to further support the use GH treatment in neural diseases in which neuroprotection and/or neuroregeneration are involved.

Sexual dimorphism of growth hormone in the hypothalamus: regulation by estradiol

GH is best known as an anterior pituitary hormone fundamental in regulating growth, differentiation, and metabolism. GH peptide and mRNA are also present in brain, in which their functions are less well known. Here we describe the distribution of GH neurons and fibers and sex differences in Gh mRNA in adult mouse brain. Cell bodies exhibiting GH immunoreactivity are distributed in many brain regions, particularly in the hypothalamus in which retrograde labeling suggests that some of these cells project to the median eminence. To determine whether Gh mRNA is sexual dimorphic, we carried out quantitative RT-PCR on microdissected brain nuclei. Ovary-intact mice had elevated Gh mRNA in the arcuate nucleus and medial preoptic area (MPOA) compared with gonad-intact males. In males, castration increased Gh mRNA in the MPOA, whereas ovariectomy decreased Gh mRNA in both regions. When gonadectomized adults of both sexes were treated with estradiol Gh mRNA increased in females but had no effect in castrated males. Tamoxifen was able to blunt the rise in Gh mRNA in response to estradiol in females. In addition, we found that estrogen receptor-α is coexpressed in GH neurons in the MPOA and arcuate nucleus. In summary, the findings reveal sexual dimorphisms in Gh gene expression in areas of the brain associated with reproduction and behavior. Interestingly, estradiol enhances Gh mRNA in females only, suggesting that multiple factors orchestrate this sexual dimorphism.

Context processing in aging: older mice are impaired in renewal of extinguished fear

Fear conditioning, extinction, and renewal were evaluated in adult (6-month-old) and aging (17-month-old) male C57Bl/6 mice. Mice were subjected to five tone-shock trials and later exposed to 150 tone-alone trials. Thereafter, all mice showed little fear in the extinction context. Adult mice demonstrated return of fear in a distinct context (renewal) but aging mice did not. Aging mice showed normal shock sensitivity, tone fear learning, and extinction. Aging mice thus exhibited a very selective deficit in the contextual gating of extinguished fear. This contextual gating deficit may reflect age-related pathology in the hippocampus and prefrontal cortex.

Peripheral administration of bovine GH regulates the expression of cerebrocortical beta-globin, GABAB receptor 1, and the Lissencephaly-1 protein (LIS-1) in adult hypophysectomized rats

Growth hormone (GH) therapy substantially improves several cognitive functions in hypopituitary experimental animals and in humans. Although a number of biochemical correlates to these effects have been characterized, there are no comprehensive analysis available examining effects of GH on the brain. Hypophysectomized female rats were given replacement therapy with cortisol and thyroxine (=hx). Subcutaneous infusions of bovine GH (bGH, henceforth designated GH) were supplied in osmotic minipumps for 6 days (=hx+GH). To evaluate whether GH normalized specific transcript expression levels in cerebral cortex, pituitary-intact rats were used as normal controls. DNA microarrays (Affymetrix) of cerebrocortical samples showed that 24 transcripts were changed by more than 1.5-fold by GH treatment in addition to being normalized by GH treatment. The expression of three selected highly regulated transcripts was confirmed by quantitative real-time polymerase chain reaction analysis. These were the GABAB receptor 1, Lissencephaly-1 protein (LIS-1), and hemoglobin b or beta-globin. A similar regulation was found for hemoglobin b also in the hippocampus. Both the GABAB receptor 1 and hemoglobin b may have importance for the previously described neuroprotective and perhaps cognitive potential of GH treatment. Altogether, these results show that short term GH treatment affects a number of transcripts in cerebral cortex with various biological functions. These transcripts represent potential novel mechanisms by which GH can interact with the brain.

Growth hormone improves hippocampal adult cell survival and counteracts the inhibitory effect of prolonged sleep deprivation on cell proliferation

Sleep deprivation (SD) produces numerous deleterious changes in brain cells, including apoptosis. It has been demonstrated that growth hormone (GH) stimulates cell growth and counteracts apoptosis, although this anti-apoptotic effect has not been tested against SD. To determine the protective effect of GH administration on cell proliferation and survival in the dentate gyrus (DG) of the hippocampus after sleep deprivation; we injected Wistar adult rats with a low dose of recombinant human GH (rhGH 5 ng/kg) per seven days and then we gently sleep deprived the animals for 48 consecutive hours. 5-Bromodeoxiuridine (BrdU) was administered to assess cell proliferation after the GH treatment and NeuN was used as marker of cell fate. Our results indicate that GH produced a three fold increase in the number of BrdU positive cells within the DG [Control = 1044 ± 106.38 cells, rhGH = 2952 ± 99.84 cells, P<0.01]. In contrast, 48 h of SD significantly reduced cell proliferation but this effect was antagonized by the GH administration [SD = 540 ± 18.3 cells, rhGH + SD = 1116 ± 84.48 cells, P<0.004]. Paradoxically, SD and GH administration increased cell survival separately but no significantly compared with control animals. However, cell survival was increased in animals treated with rhGH+SD compared to rats injected with saline solution [P<0.04]. Within the survival cells, the percentage of neurons was higher in SD animals [95%] compared with saline group, while this percentage (NeuN positive cells) was increased in animals treated with rhGH+SD [120%] compared with rhGH [25%] alone. Our findings indicate that GH strongly promotes cell proliferation in the adult brain and also protects the hippocampal neuronal precursors against the deleterious effect of prolonged sleep loss.

Effects of growth hormone (GH) replacement and cognitive rehabilitation in patients with cognitive disorders after traumatic brain injury

OBJECTIVE

To assess the effects of growth hormone (GH) treatment combined with cognitive rehabilitation in patients with adult growth hormone deficiency (GHD) and cognitive disorders occurring after traumatic brain injury (TBI).

PARTICIPANTS

Nineteen adult patients with TBI: GHD was found in 11 of them.

INTERVENTION

Patients were treated with GH (GHD; sc; 1 mg/day) or vehicle (controls; sc; 1 mg/day); daily cognitive rehabilitation therapy was performed in both groups for 3 months.

MAIN OUTCOME MEASURES

The GHRH-arginine test established GHD. The neuropsychological test WAIS was performed before commencing the treatment and 3 months after commencing it.

RESULTS

Controls achieved significant improvements in digits and in manipulative intelligence quotient (IQ) (p < 0.05 vs. baseline). GHD achieved significant improvements in more cognitive parameters: understanding, digits, numbers and incomplete figures (p < 0.05 vs. baseline) and similarities, vocabulary, verbal IQ, manipulative IQ and total IQ (p < 0.01). GHD reached significantly greater improvements than controls in similarities (p < 0.01) and in vocabulary, verbal IQ and total IQ (p < 0.05).

CONCLUSION

GH administration significantly improved cognitive rehabilitation in GHD patients. Since at the end of treatment period plasma IGF-I levels were similar in both groups it is likely that exogenous GH administration is responsible for the significant differences found.

Sleep, hormones, and memory

Nocturnal sleep is characterized by a unique pattern of endocrine activity, which comprises reciprocal influences on the hypothalamo-pituitary-adrenal (HPA) and the somatotropic system. During early sleep, when slow wave sleep (SWS) prevails, HPA secretory activity is suppressed whereas growth hormone (GH) release reaches a maximum; this pattern is reversed during late sleep when rapid eye movement (REM) sleep predominates. SWS benefits the consolidation of hippocampus-dependent declarative memories, whereas REM sleep improves amygdala-dependent emotional memories and procedural skill memories involving striato-cortical circuitry. Manipulation of plasma cortisol and GH concentration during sleep revealed a primary role of HPA activity for memory consolidation. Pituitary-adrenal inhibition during SWS sleep represents a prerequisite for efficient consolidation of declarative memory; increased cortisol during late REM sleep seems to protect from an overshooting consolidation of emotional memories.

Comparative study of the effects of different growth hormone doses on growth and spatial performance of hypophysectomized rats

This study was designed to examine the effects of recombinant human growth hormone replacement on somatic growth and cognitive function in hypophysectomized (HYPOX) female Sprague-Dawley rats. Rats (5 per group) were randomized by weight to 3 experimental groups: group 1, administered 200 microg/kg of GH once daily for 9 days; group 2, administered 200 microg/kg of GH twice daily; and group 3, administered saline daily. Somatic growth was evaluated by measurement of body weight daily and of the width of the proximal tibial growth plate of the HYPOX rats. Cognitive function was evaluated using the Morris water maze (MWM) test. The results indicated that GH replacement therapy in HYPOX rats promoted an increase in the body weight and the width of the tibial growth plate in a dose-dependent manner. On the third day of the MWM test, the escape latency in the GH-treated groups 1 and 2 was significantly shorter than that in the control rats (P<0.001 and P=0.032, respectively), suggesting that rhGH improved spatial memory acquisition in the MWM test. Therefore it is concluded that rhGH replacement therapy in HYPOX rats stimulates an increase in somatic growth in a dose-dependent manner and also has beneficial effects on cognitive functions.

EFFECTS OF TESTOSTERONE AND GROWTH HORMONE ON LONG-TERM RETENTION AND EXTINCTION OF A PASSIVE AVOIDANCE RESPONSE IN YOUNG AND AGED RATS

Wistar rats were divided into two blocks: young and aged. Each block was then randomly divided into the following groups: (1) Control (saline solution), (2) Growth Hormone (GH), (3) Testosterone enanthate, and (4) GH plus testosterone enanthate. Animals were trained on a one-trial passive avoidance conditioning and tested for retention 24 h after training and thereafter twice a week until the extinction response occurred. Results showed that GH, testosterone, and GH plus testosterone improved long-term memory in young rats. Both GH and testosterone delayed the extinction process in young and aged rats. The GH-testosterone interaction delayed the extinction only in young rats.

Effect of growth hormone on Cyclooxygenase-2 expression in the hippocampus of rats chronically exposed to ozone

The aim of this study was to determine GH-effects on Cyclooxygenase-2 (COX-2) expression on hippocampus alterations caused by ozone exposure. Seventy male rats were divided into: (1) control; (2) exposed to ozone for 7, 15, and 30 days; (3) exposed to ozone and treated with GH, for 7, 15, and 30 days. Results showed that lipoperoxidation levels and number of COX-2-positive cells increased in all groups exposed to ozone compared to control. In the groups treated with GH, COX-2 immunoreactive cell number decreased with respect to the ozone group. Therefore, GH could provide protection against damage induced by oxidative stress.

Extinction learning: neurological features, therapeutic applications and the effect of aging

Extinction learning consists of the usually gradual inhibition of the retrieval of a previously learned response or behavior. It is widely used for the treatment of syndromes of learned fear, such as phobias and post-traumatic stress disorder. It relies on well-identified molecular processes in the hippocampus, basolateral amygdala, ventromedial prefrontal cortex (vmPFC) and entorhinal cortex. In humans, thickness of the orbital cortex, vmPFC and the anterior cingulate cortex correlates with the capacity to extinguish. The three regions are functionally inter-related (see below). The development of learned fear syndromes in humans is viewed by many as being due to a deficit of extinction, and so of the capacity to deal with fear. Blockade of NMDA receptors, inhibition of protein synthesis in the vmPFC or blockade of protein synthesis or of various molecular signaling cascades in the hippocampus, amygdala or entorhinal cortex impairs extinction. d-cycloserine, a partial agonist at NMDA receptors, enhances extinction in animals and humans and may help extinction to exert its therapeutic effect. Cannabinoids also enhance extinction, acting through CB1 receptors, but their therapeutic use is not warranted.

The role of the entorhinal cortex in extinction: influences of aging

The entorhinal cortex is perhaps the area of the brain in which neurofibrillary tangles and amyloid plaques are first detectable in old age with or without mild cognitive impairment, and very particularly in Alzheimer's disease. It plays a key role in memory formation, retrieval, and extinction, as part of circuits that include the hippocampus, the amygdaloid nucleus, and several regions of the neocortex, in particular of the prefrontal cortex. Lesions or biochemical impairments of the entorhinal cortex hinder extinction. Microinfusion experiments have shown that glutamate NMDA receptors, calcium and calmodulin-dependent protein kinase II, and protein synthesis in the entorhinal cortex are involved in and required for extinction. Aging also hinders extinction; it is possible that its effect may be in part mediated by the entorhinal cortex.

Sleep-dependent surges in growth hormone do not contribute to sleep-dependent memory consolidation

In the search for the mechanisms that mediate the effects of sleep on the consolidation of memories, growth hormone (GH) recently became of interest, because in humans it is released mainly during slow-wave sleep (SWS), a period of enhanced declarative memory consolidation. In addition, recent studies showed that GH is involved in proper memory function in GH deficient and elderly humans and this effect has been linked to regulatory influences of GH on hippocampal NMDA receptors. Here, we blocked GH secretion by intravenous infusion of somatostatin in healthy young subjects during the first 3 h of sleep, which contain mainly SWS. Declarative and procedural memory consolidation was tested across this period, using a word pair association task and a mirror tracing task, respectively. Although GH was effectively suppressed, memory performance as well as sleep were entirely unaffected by this suppression. Whereas GH may in the long run generally support brain systems required for maintaining proper memory function, our data exclude a necessary contribution of the nocturnal surge in pituitary GH secretion to the acute processing and formation of specific memories during sleep.

The aging brain: is function dependent on growth hormone/insulin-like growth factor-1 signaling?

The role of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) in normal brain function is not well understood. Studies looking at cognition in humans with GH deficiency have produced controversial results. Experiments in which GH is administered to rodents have shown an apparent improvement in learning and memory. However, studies in which GH deficient or resistant mice were tested in learning and memory tasks reveal that these animals have normal cognitive performance and that their neural function does not deteriorate with age at the same rate as their normal siblings. Further research into this phenomenon revealed that these animals have elevated GH and IGF-1 expression in the hippocampus compared to normal animals. Additional studies with GH deficient and resistant mice suggested that these mutants experience a delay in age-related decline in locomotor activity and exploratory behavior. Data indicate that GH/IGF-1 deficiency and resistance do not impair neural function and instead may offer some degree of protection that results in delayed cognitive and motor aging.

Growth hormone replacement in hypophysectomized rats affects spatial performance and hippocampal levels of NMDA receptor subunit and PSD-95 gene transcript levels

Clinical studies have demonstrated that growth hormone (GH) promotes learning and memory processes in GH-deficient (GHD) patients. In animal studies, GH also influences the N-methyl-D-aspartate (NMDA) receptor system in the hippocampus, an essential component of long-term potentiation (LTP), which is highly involved in memory acquisition. This study was designed to examine the beneficial effects of recombinant human GH (rhGH) on cognitive function in male rats with multiple hormone deficiencies resulting from hypophysectomy (Hx). The performance of an rhGH-treated group and an untreated control group was appraised in the Morris water maze (MWM). The rhGH-treated group performed significantly better in the spatial memory task than the control animals on the second and third trial days. Further training eliminated this difference between the groups. Hippocampal mRNA expression of the NMDA subunits NR1, NR2A and NR2B, insulin-like growth factor type 1 receptor (IGF-1R), and postsynaptic density protein-95 (PSD-95) was then measured in the animals by Northern blot analysis. The results suggest that there may be a relationship between the NMDA receptor subunit mRNA expression levels and learning ability, and that learning is improved by rhGH in Hx rats. Furthermore, a link between MWM performance and PSD-95 was also suggested by this study.

Growth hormone enhances excitatory synaptic transmission in area CA1 of rat hippocampus

The hippocampus produces growth hormone (GH) and contains GH receptors, suggesting a potential role for GH signaling in the regulation of hippocampal function. In agreement with this possibility, previous investigations have found altered hippocampal function and hippocampal-dependent learning and memory after chronic GH administration or deficiency. In this study we applied GH to in vitro rat hippocampal brain slices, to determine whether GH has short-term effects on hippocampal function in addition to previously documented chronic effects. We found that GH enhanced both AMPA- and NMDA-receptor-mediated excitatory postsynaptic potentials (EPSPs) in hippocampal area CA1, but did not alter GABA(A)-receptor-mediated inhibitory synaptic transmission. GH enhancement of excitatory synaptic transmission was gradual, requiring 60-70 min to reach maximum, and occurred without any change in paired-pulse facilitation, suggesting a possible postsynaptic site of action. In CA1 pyramidal neurons, GH enhancement of EPSPs was correlated with significant hyperpolarization and decreased input resistance. GH enhancement of EPSPs required Janus kinase 2 (JAK2), phosphatidylinositol-3 (PI3) kinase, mitogen-activated protein (MAP) kinase kinase (MEK), and synthesis of new proteins. Although PI3 kinase and MEK were required for initiation of GH effects on excitatory synaptic transmission, they were not required for maintained enhancement of EPSPs. GH treatment and tetanus-induced long-term potentiation were mutually occluding, suggesting a common mechanism or mechanisms in both forms of synaptic enhancement. Our results demonstrate that GH has powerful short-term effects on hippocampal function, and extend the timescale for potential roles of GH in regulating hippocampal function and hippocampal-dependent behaviors.

Testosterone inhibition of growth hormone release stimulated by a growth hormone secretagogue: studies in the rat and dog

Anabolic steroids are frequently taken by athletes and bodybuilders together with recombinant human GH (rhGH), though there is some scientific evidence that the use of anabolic steroids reverses the rhGH-induced effects. Recently, we have shown that treatment with rhGH (0.2 IU/kg s.c., daily x 12 days) in the dog markedly reduced the canine GH (cGH) responses stimulated by EP51216, a GH secretagogue (GHS), evaluated after 3 and 5 daily rhGH injections, and that the inhibition was still present a few days after rhGH discontinuation. The aim of the present study was to evaluate in the dog the GH response to EP51216 (125 mug/kg i.v.) in a condition of enhanced androgenic function (i.e. acute injection or 15-day treatment with testosterone at the dose of 2 mg/kg i.m. on alternate days), and in the hypophysectomized rat the hypothalamic and hippocampal expression of ghrelin, the receptor of GHSs (GHS-R), GH-releasing hormone (GHRH) and somatostatin (SS) after specific hormonal replacement therapies (testosterone, 1 mg/kg/day s.c.; hydrocortisone, 500 mug/kg/day s.c.; rhGH, 400 mug/kg/day s.c.; 0.9% saline 0.1 ml/kg/day s.c.; x11 days). In the dog experiments, under baseline conditions, a single injection of EP51216 elicited an abrupt rise of plasma cGH. Twenty-four hours from the acute bolus injection of testosterone, C(max) and AUC(0-90) of the GHS-stimulated cGH response were significantly lower than baseline cGH response; 5 days later, there was still a significant decrease of either parameter versus the original values. Short-term treatment with testosterone markedly reduced the GHS-stimulated cGH responses evaluated during (5th bolus) and at the end (8th bolus) of testosterone treatment. Four and 8 days after testosterone withdrawal, the EP51216-stimulated cGH response was still significantly reduced when compared with that under baseline conditions. Plasma concentrations of insulin-like growth factor 1 (IGF-1) were stable until the 5th bolus of testosterone and decreased progressively in the remaining time of the testosterone treatment; 4 and 8 days from treatment withdrawal, IGF-1 levels were still suppressed. In rat studies, hypothalamic mRNA levels of GHS-R were significantly reduced by treatments with testosterone and hydrocortisone, whereas hippocampal expressions of ghrelin, GHRH and SS were reduced by rhGH replacement therapy. In conclusion, these studies show that a single administration of testosterone can abrogate the cGH response ensuing acute stimulation by a GHS; the inhibitory effect of testosterone on the cGH response to GHS is present during and even 8 days after termination of a short-lived treatment with testosterone; these events occur via a

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Growth hormone secretagogues: prospects and potential pitfalls

The growth hormone secretagogues (GHSs) are the first well-characterised agents that rejuvenate the growth hormone (GH)/insulin-like growth factor (IGF-1) axis. This property was discovered during investigations of the underlying causative mechanisms of age-related endocrine changes. Chronic administration of the long acting GHS, MK-0677, reverses the age-related decline in pulse-amplitude of GH secretion and restores IGF-1 levels producing profiles typical of young adults. This restoration is accompanied by improvements in body composition in frail elderly subjects. When given acutely, the GHSs also increase appetite. Following cloning and characterisation of the GHS-receptor (GHS-R) an endogenous ligand, ghrelin, was isolated and identified. Ghrelin shares the GH releasing and orexigenic properties of the GHSs. Studies using Ghsr-null mice confirmed that the GHS-R was the ghrelin-receptor; hence, the GHSs should be considered to be 'ghrelin mimetics.' Ghrelin levels are reported to decline during ageing, therefore long-acting GHSs are ideal candidates for ghrelin replacement therapy.

Expression of delta opioid receptor mRNA and protein in the rat cerebral cortex and cerebellum is decreased by growth hormone

Hormones released from the pituitary have been shown to regulate the expression of different proteins in the central nervous system. We wanted to examine whether peripheral administration of bovine growth hormone (bGH) regulates the expression of delta-opioid receptor (DOR) in the cerebral cortex and cerebellum. Expression of the DOR protein was quantified using Western blot densitometry. DOR mRNA was quantified with a solution hybridization RNase protection assay. Hypophysectomized (Hx) and untreated normal female rats were included in the study. All Hx rats were hormonally treated with cortisol (400 microg/kg/day) and L-thyroxine (10 microg/kg/day) for 19 days. Hypophysectomy resulted in a threefold increase in cerebral cortex and a twofold increase in cerebellum of the DOR protein compared with normal rats. One subgroup of Hx rats received bGH (1 mg/kg body weight) as a daily subcutaneous injection for 19 days. This treatment normalized the levels of DOR protein in the cerebral cortex and cerebellum. Immunohistochemical experiments showed that GH decreased DOR expression especially in layers II-VI in cerebral cortex and in stratum moleculare in cerebellum. Quantification of DOR mRNA by solution hybridization RNase protection assay corresponded to the DOR protein measurements. We conclude that the expression of DORs in cerebral cortex and cerebellum is regulated by GH.

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.

Evidence that age-induced decline in memory retention is delayed in growth hormone resistant GH-R-KO (Laron) mice

Although the role of growth hormone (GH) in aging is controversial, the recent production of GH-R-KO mice may provide a means for elucidating its importance. Using the inhibitory avoidance learning task as a measure of cognitive aging, the present study compared learning and retention in young and old GH-R-KO mice and their normal siblings. Results for the old normal animals agreed with the current literature, in that the ability of old animals to retain learned information declined over time. However, retention in the old GH-R-KO mice did not decline between the 24-h, 7-day and 28-day retention tests and did not differ from young animals. To determine whether performance differences seen in the old normal vs. old GH-R-KO groups were due to locomotor behavior or emotionality, both groups were tested in the elevated-plus maze. Results showed that the normal and GH-R-KO mice did not differ in number of open or closed arms entered, time spent in closed or open arms or time taken to first enter an open arm. Thus, it was concluded that inhibitory avoidance performance was not affected by differences of locomotor activity or emotionality, and that the absence of GH signaling may be associated with improved long-term memory in aging mice.

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.

Growth hormone, insulin-like growth factor I and the CNS: localization, function and mechanism of action

The growth hormone (GH) receptor and binding protein are synthesized in the CNS and are regulated differentially to their hepatic counterparts. GH is also synthesized in the CNS and is regulated differentially to its hypophyseal counterpart. Insulin-like growth factor I (IGF-I) is synthesized in the CNS and in the early postnatal period is regulated by peripherally secreted GH. Both GH and IGF-I alter the size and morphology of the CNS during development and affect differentiated cell function in the CNS, with consequent modulation of cognitive function. Differential utilization of the same signal transduction molecules indicates that GH and IGF-I possess distinct overlapping roles in CNS function.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

Effects of ozone exposure in rats on memory and levels of brain and pulmonary superoxide dismutase

Exposure to ozone results in increased production of free radicals, which causes oxidative stress. The objective of this study was to determine the effect of different doses of ozone exposure on memory and to correlate this with pulmonary and brain Cu/Zn superoxide dismutase (SOD) levels. Male Wistar rats were exposed for 4 h to one of the following ozone concentrations: 0, 0.1, 0.2, 0.5, or 1 ppm. Subsequently, they were tested in a passive avoidance conditioning protocol to measure short and long-term memory. Motor activity was determined 1 and 24 h after ozone exposure. Cu/Zn SOD levels in the brain and pulmonary tissue were also measured. Rats exposed for 4 h to 0.2, 0.5, and 1 ppm ozone showed long-term memory deterioration and decreased motor activity, which was reversed 24 h later. Brain and pulmonary Cu/Zn SOD levels were increased in animals exposed to 0.1, 0.2, and 0.5 ppm ozone doses, but decreased in animals exposed to 1 ppm ozone. The results suggest that ozone exposure affects long-term memory possibly in association oxidative stress.

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.