Growth hormone signaling and hippocampal neurogenesis: insights from genetic models

The impact of voluntary wheel-running exercise on hippocampal neurogenesis and behaviours in response to nicotine cessation in rats

RATIONALE

The literature indicates that nicotine exposure or its discontinuation impair adult hippocampal neurogenesis in rats, though the impact of exercise on this process remains unclear. We have previously shown that disturbances in the number of doublecortin (DCX, a marker of immature neurons)-positive (DCX+) cells in the dentate gyrus (DG) of the hippocampus during nicotine deprivation may contribute to a depression-like state in rats.

OBJECTIVES

This study aimed to investigate the effect of running on hippocampal neurogenesis, depression-like symptoms, and drug-seeking behaviour during nicotine deprivation.

METHODS

The rats were subjected to nicotine (0.03 mg/kg/inf) self-administration via an increasing schedule of reinforcement. After 21 sessions, the animals entered a 14-day abstinence phase during which they were housed in either standard home cages without wheels, cages equipped with running wheels, or cages with locked wheels.

RESULTS

Wheel running increased the number of Ki-67+ and DCX+ cells in the DG of both nicotine-deprived and nicotine-naive rats. Wheel-running exercise evoked an antidepressant effect on abstinence Day 14 but had no effect on nicotine-seeking behaviour on abstinence Day 15 compared to rats with locked-wheel access.

CONCLUSIONS

In summary, long-term wheel running positively affected the number of immature neurons in the hippocampus, which corresponded with an antidepressant response in nicotine-weaned rats. One possible mechanism underlying the positive effect of running on the affective state during nicotine cessation may be the reduction in deficits in DCX+ cells in the hippocampus.

Identification of Differentially Expressed MicroRNAs in the Rat Hippocampus during Adolescence through an Epigenome-Wide Analysis

INTRODUCTION

Epigenetic mechanisms involving microRNAs (miRNAs) play a fundamental role in many biological processes, particularly during prenatal and early postnatal development. Their role in adolescent brain development, however, has been poorly described. The present study aimed to explore miRNA expression in the hippocampus during adolescence compared to adulthood in rats.

METHOD

The brains of female and male Wistar rats were extracted, and the hippocampus was freshly dissected at postnatal day 41 (adolescence) and postnatal day 98 (adulthood). An epigenome-wide analysis was conducted to identify the miRNAs significantly expressed in adolescence compared to adulthood. Additionally, target genes of such miRNAs were considered to perform an exploratory Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis.

RESULTS

We identified 16 differentially expressed miRNAs in adolescent male rats compared with adult male rats and 4 differentially expressed miRNAs in adolescent females compared with adult females. Enrichment analysis reinforced that the target genes found are related to neurodevelopmental processes such as cell proliferation, cell migration, and nervous system development.

CONCLUSION

Our findings suggest a complex pattern of miRNA expression during adolescence, which differs from that in adulthood. The differential expression of miRNA in the hippocampus during adolescence may be associated with the late developmental changes occurring in this brain region. Furthermore, the observed sex differences in miRNA expression patterns indicate potential sexual differentiation in hippocampal development. Further comprehensive investigations are needed to elucidate the roles of miRNA in normal brain development.

Acromegalic Rat Model Presented Cognitive Impairments and Tau Hyperphosphorylation in the Hippocampus

INTRODUCTION

Acromegaly patients, in addition to the most prominent physical and endocrine changes, also exhibit a higher risk of cognitive dysfunction. However, the reasons and mechanisms underlying cognitive impairments in acromegaly patients remain unknown.

METHODS

Acromegalic rats were induced by subcutaneous injection of tumor cells, with continuous monitoring of the body weight and hormones to confirm the occurrence of acromegaly. Behavioral assessments, including open field test, novel object recognition test, and Barnes maze test, were conducted to evaluate the animals' cognitive function. Western blotting, immunohistochemistry, and immunofluorescence techniques were employed to examine changes in the hippocampal tau protein, Aβ, and associated signaling pathways.

RESULTS

The tumor cells secreting growth hormone increased the secretion of growth hormone, resulting in changes in body size and endocrine functions, thus causing acromegaly. The acromegaly model showed deficiencies in working memory and spatial memory. Hyperphosphorylation of tau protein was observed in the hippocampus of the acromegaly model, but no Aβ deposition was observed. The acromegaly model exhibits hippocampal growth hormone (GH) resistance, decreased expression of GH receptors, and subsequently reduced expression activity of the PI3K-Akt-GSK3β signaling pathway, which is responsible for the hyperphosphorylation of tau protein.

CONCLUSION

The prolonged elevation of GH and insulin-like growth factor 1 caused by acromegaly may lead to abnormalities in the SD rat's PI3K-Akt-GSK3β signaling pathway, subsequently resulting in hyperphosphorylation of the hippocampal tau protein and cognitive impairment.

Suppressor of cytokine signaling 2 is induced in Huntington's disease and involved in autophagy

Suppressor of cytokine signaling (SOCS) proteins are primarily feedback inhibitors of cytokine signaling. The two conserved domains of SOCS proteins have distinct functions. Src homology 2 (SH2) domain inhibits cytokine receptor, while SOCS box acts as an E3 ubiquitin ligase. SOCS2, a cytokine signaling suppressor, has been primarily implicated in regulating inflammatory conditions in neuronal diseases. However, SOCS proteins have been suggested to play diverse roles in healthy and diseased nervous system including neurodegenerative disorders. In this study, SOCS2 was found to be upregulated in Huntington's disease and was substantially induced in extended polyglutamine (polyQ)-expressing striatal cells. The induced level was augmented under aging conditions. In extended polyQ-expressing cells, downregulated SOCS2 improved autophagic dysfunction rather than altered inflammatory conditions. Overall, we suggest that SOCS2 involves in regulating autophagy by functioning as an E3 ligase in extended polyQ conditions, and consequently regulates cell damage and cell death type.

Early Treatment with Growth Hormone (GH) and Rehabilitation Recovers Hearing in a Child with Cerebral Palsy

Neonatal hearing loss is one of the most common anomalies and is frequently associated with delivery problems. The effects of growth hormone (GH) on brain regeneration after an injury are well known. This paper looks at a male child diagnosed with cerebral palsy, psychomotor affectation, left spastic hemiparesis, and bilateral sensorineural hearing loss after fetal distress due to ruptured membranes before the delivery of more than 30 hours of evolution and several episodes of severe hypoglycemia. From 3.5 months of age, we treated him with GH (0.04 mg/kg/day), Melatonin (5 mg/day and 6 months later 10 mg/day) and rehabilitation, for a period of 14 months; at discharge, the child fully recovered all the disabilities produced by his cerebral palsy, including normal hearing; GMFM-88 increased from 7.84% to 48.23%; Battelle scores increased from 2 to 9 after 7 months of treatment, and to 30, 1 year after discharge. Most likely hearing loss was recovered due to the effect of GH on the production of hair cells from stem cells (only present in very young children) in the cochlear sensory epithelium. This is the first case of recovery of hearing loss in humans after GH administration. Moreover, GH administration is useful and safe for early treatment of cerebral palsy.

Tropomyosin-Related Kinase B (TrkB) Regulates Neurite Outgrowth via a Novel Interaction with Suppressor of Cytokine Signalling 2 (SOCS2)

Brain-derived neurotrophic factor (BDNF) is highly expressed in the hippocampus, where it can initiate signalling pathways leading to neurite outgrowth, neuron survival, spine maturation and increased synapse strength. Although suppressor of cytokine signalling 2 (SOCS2) is primarily known to negatively regulate cytokine signalling, it is also highly expressed in the hippocampus and exerts neuron-specific functions in the brain, effecting the length and architecture of neurons. However, little is known about the role of SOCS2 in the hippocampus. In this study, we hypothesised that SOCS2 may have a regulatory role in BDNF-dependent neurite growth and hippocampal neuronal function. Here our data demonstrate that SOCS2 interacts with the kinase domain of the BDNF receptor TrkB. Germline overexpression of SOCS2 results in a BDNF-dependent increase in hippocampal neurite outgrowth, whereas deletion of SOCS2 results in shorter neurite outgrowth. Expression of SOCS2 also results in increased ubiquitination of the juxtamembrane region of TrkB, and alters the trafficking of TrkB into recycling endosomes. Collectively, our data suggest a novel role for SOCS2 in interacting with and regulating the trafficking of TrkB, leading to increased neurite outgrowth in hippocampus neurons.

Suppressor of Cytokine Signalling 2 (SOCS2) Regulates Numbers of Mature Newborn Adult Hippocampal Neurons and Their Dendritic Spine Maturation

Overexpression of suppressor of cytokine signalling 2 (SOCS2) has been shown to promote hippocampal neurogenesis in vivo and promote neurite outgrowth of neurons in vitro. In the adult mouse brain, SOCS2 is most highly expressed in the hippocampal CA3 region and at lower levels in the dentate gyrus, an expression pattern that suggests a role in adult neurogenesis. Herein we examine generation of neuroblasts and their maturation into more mature neurons in SOCS2 null (SOCS2KO) mice. EdU was administered for 7 days to label proliferative neural precursor cells. The number of EdU-labelled doublecortin+ neuroblasts and NeuN+ mature neurons they generated was examined at day 8 and day 35, respectively. While no effect of SOCS2 deletion was observed in neuroblast generation, it reduced the numbers of EdU-labelled mature newborn neurons at 35 days. As SOCS2 regulates neurite outgrowth and dentate granule neurons project to the CA3 region, alterations in dendritic arborisation or spine formation may have correlated with the decreased numbers of EdU-labelled newborn neurons. SOCS2KO mice were crossed with Nes-CreERT2/mTmG mice, in which membrane eGFP is inducibly expressed in neural precursor cells and their progeny, and the dendrite and dendritic spine morphology of newborn neurons were examined at 35 days. SOCS2 deletion had no effect on total dendrite length, number of dendritic segments, number of branch points or total dendritic spine density but increased the number of mature "mushroom" spines. Our results suggest that endogenous SOCS2 regulates numbers of EdU-labelled mature newborn adult hippocampal neurons, possibly by mediating their survival and that this may be via a mechanism regulating dendritic spine maturation.

Altered learning, memory, and social behavior in type 1 taste receptor subunit 3 knock-out mice are associated with neuronal dysfunction

The type 1 taste receptor member 3 (T1R3) is a G protein-coupled receptor involved in sweet-taste perception. Besides the tongue, the T1R3 receptor is highly expressed in brain areas implicated in cognition, including the hippocampus and cortex. As cognitive decline is often preceded by significant metabolic or endocrinological dysfunctions regulated by the sweet-taste perception system, we hypothesized that a disruption of the sweet-taste perception in the brain could have a key role in the development of cognitive dysfunction. To assess the importance of the sweet-taste receptors in the brain, we conducted transcriptomic and proteomic analyses of cortical and hippocampal tissues isolated from T1R3 knock-out (T1R3KO) mice. The effect of an impaired sweet-taste perception system on cognition functions were examined by analyzing synaptic integrity and performing animal behavior on T1R3KO mice. Although T1R3KO mice did not present a metabolically disrupted phenotype, bioinformatic interpretation of the high-dimensionality data indicated a strong neurodegenerative signature associated with significant alterations in pathways involved in neuritogenesis, dendritic growth, and synaptogenesis. Furthermore, a significantly reduced dendritic spine density was observed in T1R3KO mice together with alterations in learning and memory functions as well as sociability deficits. Taken together our data suggest that the sweet-taste receptor system plays an important neurotrophic role in the extralingual central nervous tissue that underpins synaptic function, memory acquisition, and social behavior.

Suppressor of Cytokine Signaling-2 (SOCS2) Regulates the Microglial Response and Improves Functional Outcome after Traumatic Brain Injury in Mice

Traumatic brain injury (TBI) is frequently characterized by neuronal, axonal and myelin loss, reactive gliosis and neuroinflammation, often associated with functional deficits. Endogenous repair mechanisms include production of new neurons from precursor cells, but usually the new neurons fail to integrate and survive more than a few weeks. This is in part mediated by the toxic and inflammatory environment present in the injured brain which activates precursor cells to proliferate and differentiate but limits survival of the newborn progeny. Therefore, an understanding of mechanisms that regulate production and survival of newborn neurons and the neuroinflammatory response after brain injury may lead to therapeutic options to improve outcomes. Suppressor of Cytokine Signaling 2 (SOCS2) promotes hippocampal neurogenesis and survival of newborn neurons in the adult brain and regulates anti-inflammatory responses in the periphery, suggesting it may be a useful candidate to improve outcomes of TBI. In this study the functional and cellular responses of SOCS2 over-expressing transgenic (SOCS2Tg) mice were compared to wildtype littermates following mild or moderately severe TBI. Unlike wildtype controls, SOCS2Tg mice showed functional improvement on a ladder test, with a smaller lesion volume at 7d post injury and increased numbers of proliferative CD11b⁺ microglia/macrophages at 35d post-injury in the mild injury paradigm. At 7d post-moderately severe injury there was an increase in the area covered by cells expressing an anti-inflammatory M2 phenotype marker (CD206⁺) but no difference in cells with a pro-inflammatory M1 phenotype marker (CD16/32⁺). No effect of SOCS2 overexpression was observed in production or survival of newborn neurons, even in the presence of the neuroprotective agent erythropoietin (EPO). Therefore, SOCS2 may improve outcome of TBI in mice by regulating aspects of the neuroinflammatory response, promoting a more anti-inflammatory environment, although this was not sufficient to enhance survival of newborn cortical neurons.

Neurobiological effects of exercise on major depressive disorder: A systematic review

Exercise displays promise as an efficacious treatment for people with depression. However, no systematic review has evaluated the neurobiological effects of exercise among people with major depressive disorder (MDD). The aim of this article was to systematically review the acute and chronic biological responses to exercise in people with MDD. Two authors conducted searches using Medline (PubMed), EMBASE and PsycINFO. From the searches, twenty studies were included within the review, representing 1353 people with MDD. The results demonstrate that a single bout of exercise increases atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), copepetin and growth hormone among people with MDD. Exercise also potentially promotes long-term adaptations of copeptin, thiobarbituric acid reactive species (TBARS) and total mean frequency (TMF). However, there is limited evidence that exercise promotes adaptations on neurogenesis, inflammation biomarkers and brain structure. Associations between depressive symptoms improvement and hippocampus volume and IL-1β were found. Nevertheless, the paucity of studies and limitations presented within, precludes a more definitive conclusion of the underlying neurobiological explanation for the antidepressant effect of exercise in people with MDD. Further trials should utilize appropriate assessments of neurobiological markers in order to build upon the results of our review and further clarify the potential mechanisms associated with the antidepressant effects of exercise.

Control of Cell Survival in Adult Mammalian Neurogenesis

The fact that continuous proliferation of stem cells and progenitors, as well as the production of new neurons, occurs in the adult mammalian central nervous system (CNS) raises several basic questions concerning the number of neurons required in a particular system. Can we observe continued growth of brain regions that sustain neurogenesis? Or does an elimination mechanism exist to maintain a constant number of cells? If so, are old neurons replaced, or are the new neurons competing for limited network access among each other? What signals support their survival and integration and what factors are responsible for their elimination? This review will address these and other questions regarding regulatory mechanisms that control cell-death and cell-survival mechanisms during neurogenesis in the intact adult mammalian brain.

Dopaminergic activity and behaviour in SOCS2 transgenic mice: Revealing a potential drug target for schizophrenia

Alterations in immune function have been implicated in the aetiopathogenesis of schizophrenia. Specifically, the induction of inflammatory cytokines, which are important immunological factors in infection or inflammation, may be critical factors altering the normal course of brain development and increasing schizophrenia risk. Suppressor of cytokine signalling 2 (SOCS2) can negatively regulate the signalling of cytokines. The present study aimed to determine the behavioural phenotype of transgenic mice over-expressing SOCS2 (SOCS2 Tg) in paradigms of relevance to schizophrenia. Both male and female SOCS2 Tg mice displayed reduced locomotor hyperactivity after the administration of the dopamine releaser, amphetamine, compared to wildtype controls (WT). However, only male SOCS2 Tg mice showed enhanced prepulse inhibition compared to WT. Dopamine D2 receptors mRNA expression was reduced and dopamine transporter mRNA expression was increased in the nucleus accumbens of female, but not male, SOCS2 Tg mice, compared to WT. The role of hyperdopaminergia has long been implicated in the aetiology of schizophrenia. This study shows that over-expression of SOCS2 reduces the psychostimulant effects of amphetamine, enhances PPI, and alters mesolimbic dopaminergic activity. SOCS2 may provide a novel target in the development of treatments for schizophrenia.

Could androgens maintain specific domains of mental health in aging men by preserving hippocampal neurogenesis?

Interest surrounds the role of sex-hormones in regulating brain function outside of reproductive behaviour. Declining androgen production in aging males has been associated with cognitive impairment, depression and increased risk of developing Alzheimer's disease. Indication for testosterone replacement therapy is based on biochemically determined low circulating testosterone combined with manifest symptoms. However, which aspects of age-related cognitive decline are attributable to low circulating testosterone remain ambiguous. Studies examining cognition in aging men receiving testosterone replacement therapy have yielded equivocal results. The exact role of testosterone in maintaining cognitive function and the underlying neural mechanisms are largely unknown, though it would appear to be domain specific. Clarity in this area will provide clinical direction toward addressing an increasing healthcare burden of mental health decline coincident with increasing longevity. The premise that androgens contribute to maintaining aspects of mental health in aging men by preserving hippocampal neurogenesis will be used as a forum in this review to discuss current knowledge and the need for further studies to better define testosterone replacement strategies for aging male health.

Is integration and survival of newborn neurons the bottleneck for effective neural repair by endogenous neural precursor cells?

After two decades of research the existence of adult neural precursor cells and the phenomenon of adult neurogenesis is well established. However, there has been little or no effective harnessing of these endogenous cells to promote functional neuronal replacement following neural injury or disease. Neural precursor cells can respond to neural damage by proliferating, migrating to the site of injury, and differentiating into neuronal or glial lineages. However, after a month or so, very few or no newborn neurons can be detected, suggesting that even though neuroblasts are generated, they generally fail to survive as mature neurons and contribute to the local circuitry. Is this lack of survival and integration one of the major bottlenecks that inhibits effective neuronal replacement and subsequent repair of the nervous system following injury or disease? In this perspective article the possibility that this bottleneck can be targeted to enhance the integration and subsequent survival of newborn neurons will be explored and will suggest some possible mechanisms that may need to be modulated for this to occur.

Restoration of hippocampal growth hormone reverses stress-induced hippocampal impairment

Though growth hormone (GH) is synthesized by hippocampal neurons, where its expression is influenced by stress exposure, its function is poorly characterized. Here, we show that a regimen of chronic stress that impairs hippocampal function in rats also leads to a profound decrease in hippocampal GH levels. Restoration of hippocampal GH in the dorsal hippocampus via viral-mediated gene transfer completely reversed stress-related impairment of two hippocampus-dependent behavioral tasks, auditory trace fear conditioning, and contextual fear conditioning, without affecting hippocampal function in unstressed control rats. GH overexpression reversed stress-induced decrements in both fear acquisition and long-term fear memory. These results suggest that loss of hippocampal GH contributes to hippocampal dysfunction following prolonged stress and demonstrate that restoring hippocampal GH levels following stress can promote stress resilience.

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.

Impaired basal and running-induced hippocampal neurogenesis coincides with reduced Akt signaling in adult R6/1 HD mice

Huntington's disease (HD) is a fatal neurodegenerative disorder affecting a range of cellular and molecular functions in the brain. Deficits in adult hippocampal neurogenesis (AHN) have been documented in the R6/1 mouse model of HD. Here we examined basal and running-induced neuronal precursor proliferation in adult female and male R6/1 HD mice. We further tested whether sequential delivery of voluntary running followed by environmental enrichment could synergistically enhance functional AHN in female R6/1 HD mice. R6/1 HD mice engaged in significantly reduced levels of voluntary running, with males showing a more severe deficit. Basal neural precursor proliferation in the hippocampal sub-granular zone remained unchanged between female and male R6/1 HD mice and neither sex significantly responded to running-induced proliferation. While discrete provision of running wheels and enriched environments doubled AHN in adult female R6/1 HD mice it did not reflect the significant 3-fold increase in female wildtypes. Nevertheless, triple-label c-Fos/BrdU/NeuN immunofluorescence and confocal microscopy provided evidence that the doubling of AHN in female R6/1 HD mice was functional. Intrinsic cellular dysfunction mediated by protein aggregates containing mutant huntingtin (mHtt) did not appear to coincide with AHN deficits. In the hippocampus of female R6/1 HD mice, proliferating precursors and 6 week old adult-generated neurons were devoid of mHtt immuno-reactive aggregates, as were endothelial, microglial and astroglial cells populating the neurogenic niche. Serum transforming growth factor-β concentrations remained unaltered in female R6/1 HD mice as did the hippocampal levels of proliferating microglia and glial fibrillarly acidic protein expression. Examining the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis showed no change in base-line serum GH between genotypes. However, despite a reduced distance, acute running increases serum GH in both female wildtype and R6/1 HD mice. Serum IGF-1 levels were increased in female R6/1 HD mice compared to wildtypes during daytime inactive period, while hippocampal levels of the IGF-1 receptor remained unchanged. Running induced Akt phosphorylation in the hippocampus of female wildtype mice, which was not reflected in R6/1 HD mice. Total Akt levels were decreased in the hippocampus of both control and running R6/1 HD mice. Our results show adult-generated hippocampal neurons in female R6/1 HD mice express c-Fos and that running and Akt signaling deficits may mediate reduced basal and running-induced AHN levels.

Regulation of endogenous neural stem/progenitor cells for neural repair-factors that promote neurogenesis and gliogenesis in the normal and damaged brain

Neural stem/precursor cells in the adult brain reside in the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus. These cells primarily generate neuroblasts that normally migrate to the olfactory bulb (OB) and the dentate granule cell layer respectively. Following brain damage, such as traumatic brain injury, ischemic stroke or in degenerative disease models, neural precursor cells from the SVZ in particular, can migrate from their normal route along the rostral migratory stream (RMS) to the site of neural damage. This neural precursor cell response to neural damage is mediated by release of endogenous factors, including cytokines and chemokines produced by the inflammatory response at the injury site, and by the production of growth and neurotrophic factors. Endogenous hippocampal neurogenesis is frequently also directly or indirectly affected by neural damage. Administration of a variety of factors that regulate different aspects of neural stem/precursor biology often leads to improved functional motor and/or behavioral outcomes. Such factors can target neural stem/precursor proliferation, survival, migration and differentiation into appropriate neuronal or glial lineages. Newborn cells also need to subsequently survive and functionally integrate into extant neural circuitry, which may be the major bottleneck to the current therapeutic potential of neural stem/precursor cells. This review will cover the effects of a range of intrinsic and extrinsic factors that regulate neural stem/precursor cell functions. In particular it focuses on factors that may be harnessed to enhance the endogenous neural stem/precursor cell response to neural damage, highlighting those that have already shown evidence of preclinical effectiveness and discussing others that warrant further preclinical investigation.

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.

Deficits in spermatogenesis but not neurogenesis are alleviated by chronic testosterone therapy in R6/1 Huntington's disease mice

Despite the well established central pathophysiology of Huntington's disease (HD), less is known about systemic impairments that are emerging as significant contributors to the morbidity of this neurodegenerative condition. Given the evidence of neuroendocrine dysfunction in HD patients and the pro-neural properties of sex-hormones, we explored the therapeutic potential of hormone therapy in the HD R6/1 mouse model (HD mice). HD mice over-express exon-1 of the defective human HD gene and replicate many of the clinical behavioural, biochemical and physiological impairments. Seven-week-old HD and wild-type littermate mice had either saline (control) or testosterone (treatment; 160μg/day over 90days) pellets implanted s.c. and were subsequently subjected to behavioural, molecular and cellular analysis. Separate mice were used to establish a decrease in serum testosterone concentrations in HD mice at 12weeks of age. Baseline serum testosterone was significantly reduced in control 19-week-old HD mice, whereas treatment significantly raised serum testosterone in both wild-type and HD mice. Testosterone treatment had a limited effect on the development of rotarod deficiencies in HD mice and no effect on progressive body weight loss or the development of central mutant huntingtin-containing aggregates. Testosterone treatment induced hypo-locomotion in both genotypes. Deficits in hippocampal-dependent cognition and neurogenesis were not rescued in testosterone-treated HD mice. By contrast, wild-type-treatment mice experienced significantly increased neuronal survival and differentiation. Testosterone treatment in HD mice did rescue androgen receptor levels in the hippocampus and testes, significantly improved severe testicular atrophy and restored spermatogenesis. We conclude that chronic testosterone provides systemic efficacy in treating spermatogenesis deficits and testicular atrophy but not central cellular and behavioural pathologies in R6/1 HD mice.

Genetic models of sensorimotor gating: relevance to neuropsychiatric disorders

Sensorimotor gating, or the ability of a sensory event to suppress a motor response, can be measured operationally via prepulse inhibition (PPI) of the startle response. PPI is deficient in schizophrenia patients as well as other neuropsychiatric disorders, can be measured across species, and has been used widely as a translational tool in preclinical neuropharmacological and genetic research. First developed to assess drug effects in pharmacological and developmental models, PPI has become one of the standard behavioral measures in genetic models of schizophrenia and other neuropsychiatric disorders that exhibit PPI deficits. In this chapter we review the literature on genetic models of sensorimotor gating and discuss the utility of PPI as a tool in phenotyping mutant mouse models. We highlight the approaches to genetic mouse models of neuropsychiatric disease, discuss some of the important caveats to these approaches, and provide a comprehensive table covering the more recent genetic models that have evaluated PPI.

Messenger RNA and microRNA profiling during early mouse EB formation

Embryonic stem (ES) cells can be induced to differentiate into embryoid bodies (EBs) in a synchronised manner when plated at a fixed density in hanging drops. This differentiation procedure mimics post-implantation development in mouse embryos and also serves as the starting point of protocols used in differentiation of stem cells into various lineages. Currently, little is known about the potential influence of microRNAs (miRNAs) on mRNA expression patterns during EB formation. We have measured mRNA and miRNA expression in developing EBs plated in hanging drops until day 3, when discrete structural changes occur involving their differentiation into three germ layers. We observe significant alterations in mRNA and miRNA expression profiles during this early developmental time frame, in particular of genes involved in germ layer formation, stem cell pluripotency and nervous system development. Computational target prediction using Pictar, TargetScan and miRBase Targets reveals an enrichment of binding sites corresponding to differentially and highly expressed miRNAs in stem cell pluripotency genes and a neuroectodermal marker, Nes. We also find that members of let-7 family are significantly down-regulated at day 3 and the corresponding up-regulated genes are enriched in let-7 seed sequences. These results depict how miRNA expression changes may affect the expression of mRNAs involved in EB formation on a genome-wide scale. Understanding the regulatory effects of miRNAs during EB formation may enable more efficient derivation of different cell types in culture.

Increased adult hippocampal neurogenesis and abnormal migration of adult-born granule neurons is associated with hippocampal-specific cognitive deficits in phospholipase C-β1 knockout mice

Schizophrenia is a devastating psychiatric illness with a complex pathophysiology. We have recently documented schizophrenia-like endophenotypes in phospholipase C-β1 knockout (PLC-β1(-/-)) mice, including deficits in prepulse inhibition, hyperlocomotion, and cognitive impairments. PLC-β1 signals via multiple G-protein coupled receptor pathways implicated in neural cellular plasticity; however, adult neurogenesis has yet to be explored in this knockout model. In this study, we employed PLC-β1(-/-) mice to elucidate possible correlates between aberrant adult hippocampal neurogenesis (AHN) and schizophrenia-like behaviors. Using stereology and bromodeoxyuridine (BrdU) immunohistochemistry we demonstrated a significant increase in the density of adult-generated cells in the granule cell layer (GCL) of adult PLC-β1(-/-) mice compared with wild-type littermates. Cellular phenotype analysis using confocal microscopy revealed these cells to be mature granule neurons expressing NeuN and calbindin. Increased neuronal survival occurred concomitant with reduced caspase-3(+) cells in the GCL of PLC-β1(-/-) mice. Stereological analysis of Ki67(+) cells in the subgranular zone suggested that neural precursor proliferation is unchanged in PLC-β1(-/-) mice. We further showed aberrant migration of mature granule neurons within the GCL of adult PLC-β1(-/-) mice with excessive adult-generated mature neurons residing in the middle and outer GCL. PLC-β1(-/-) mice exhibited specific behavioral deficits in location recognition, a measure of hippocampal-dependent memory, but not novel object recognition. Overall, we have shown that PLC-β1(-/-) mice have a threefold increase in net AHN, and have provided further evidence to suggest a specific deficit in hippocampal-dependent cognition. We propose that abnormal cellular plasticity in these mice may contribute to their schizophrenia-like behavioral endophenotypes.

Enhanced expression of SOCS-2 in the rat hippocampus after transient forebrain ischemia

Suppressor of cytokine signaling-2 (SOCS-2) has recently been identified as an important regulator involved in neuronal differentiation and maturation. However, the role of SOCS-2 in ischemia-induced hippocampal neurogenesis remains to be clarified. Here we investigated the spatiotemporal expression of SOCS-2 in the rat hippocampus following transient forebrain ischemia, and particular attention was paid to changes in the dentate gyrus. SOCS-2 mRNA was constitutively expressed in hippocampal neurons and astrocytes in control animals. However, its upregulation occurred specifically in reactive astrocytes in the hippocampus proper, in particular the CA1 and dentate hilar regions, at day 3 after reperfusion, and was sustained for more than 2 weeks. In addition to the CA1 and hilar regions, SOCS-2 was transiently increased in the subgranular zone (SGZ) of the dentate gyrus on days 3-7 after reperfusion. This correlated with the post-ischemic upregulation of SOCS-2 in the CA1 or dentate gyrus subfield, including the SGZ detected by semiquantitative reverse transcriptase-polymerase chain reaction analysis. The majority of the SOCS-2-expressing cells in the SGZ were co-labeled with glial fibrillary acidic protein (GFAP), and a subpopulation of GFAP/SOCS-2 double-labeled cells in the SGZ co-expressed the neural progenitor marker nestin, or the proliferation marker proliferating cellular nuclear antigen. In addition, a subset of SOCS-2-labeled cells in the SGZ expressed the immature neuronal marker polysialic acid-neural cell adhesion molecule. These data suggest that SOCS-2 may be involved in glial reactions, and possibly adult hippocampal neurogenesis during ischemic insults.