Regional- and age-dependent reduction in trkB receptor expression in the hippocampus is associated with altered spine morphologies

Morphological and behavioral analysis of Slc35f1-deficient mice revealed no neurodevelopmental phenotype

SLC35F1 is a member of the sugar-like carrier (SLC) superfamily that is expressed in the mammalian brain. Malfunction of SLC35F1 in humans is associated with neurodevelopmental disorders. To get insight into the possible roles of Slc35f1 in the brain, we generated Slc35f1-deficient mice. The Slc35f1-deficient mice are viable and survive into adulthood, which allowed examining adult Slc35f1-deficient mice on the anatomical as well as behavioral level. In humans, mutation in the SLC35F1 gene can induce a Rett syndrome-like phenotype accompanied by intellectual disability (Fede et al. Am J Med Genet A 185:2238-2240, 2021). The Slc35f1-deficient mice, however, display only a very mild phenotype and no obvious deficits in learning and memory as, e.g., monitored with the novel object recognition test or the Morris water maze test. Moreover, neuroanatomical parameters of neuronal plasticity (as dendritic spines and adult hippocampal neurogenesis) are also unaltered. Thus, Slc35f1-deficient mice display no major alterations that resemble a neurodevelopmental phenotype.

Neurotrophic Factors and Dendritic Spines

Dendritic spines are highly dynamic structures that play important roles in neuronal plasticity. The morphologies and the numbers of dendritic spines are highly variable, and this diversity is correlated with the different morphological and physiological features of this neuronal compartment. Dendritic spines can change their morphology and number rapidly, allowing them to adapt to plastic changes. Neurotrophic factors play important roles in the brain during development. However, these factors are also necessary for a variety of processes in the postnatal brain. Neurotrophic factors, especially members of the neurotrophin family and the ephrin family, are involved in the modulation of long-lasting effects induced by neuronal plasticity by acting on dendritic spines, either directly or indirectly. Thereby, the neurotrophic factors play important roles in processes attributed, for example, to learning and memory.

Introduction: What Are Dendritic Spines?

Dendritic spines are cellular specializations that greatly increase the connectivity of neurons and modulate the "weight" of most postsynaptic excitatory potentials. Spines are found in very diverse animal species providing neural networks with a high integrative and computational possibility and plasticity, enabling the perception of sensorial stimuli and the elaboration of a myriad of behavioral displays, including emotional processing, memory, and learning. Humans have trillions of spines in the cerebral cortex, and these spines in a continuum of shapes and sizes can integrate the features that differ our brain from other species. In this chapter, we describe (1) the discovery of these small neuronal protrusions and the search for the biological meaning of dendritic spines; (2) the heterogeneity of shapes and sizes of spines, whose structure and composition are associated with the fine-tuning of synaptic processing in each nervous area, as well as the findings that support the role of dendritic spines in increasing the wiring of neural circuits and their functions; and (3) within the intraspine microenvironment, the integration and activation of signaling biochemical pathways, the compartmentalization of molecules or their spreading outside the spine, and the biophysical properties that can affect parent dendrites. We also provide (4) examples of plasticity involving dendritic spines and neural circuits relevant to species survival and comment on (5) current research advancements and challenges in this exciting research field.

CaMKIV mediates spine growth deficiency of hippocampal neurons by regulation of EGR3/BDNF signal axis in congenital hypothyroidism

Congenital hypothyroidism (CH) will cause cognitive impairment in the condition of delayed treatment. The hippocampus is one of the most affected tissues by CH, in which the functional structures of hippocampal neurons manifest deficiency due to aberrant expression of effector molecules. The Ca²⁺/Calmodulin-dependent protein kinase, CaMKIV, is downregulated in the hippocampal neurons, influencing the growth of dendritic spines in response to CH. However, the underlying mechanism is not fully elucidated. In the present study, the early growth response factor 3 (EGR3) was regulated by CaMKIV in the hippocampal neurons of CH rat pups, as was analyzed by transcriptome sequencing and in vitro cell experiments. EGR3 localized within hippocampal neurons in CA1, CA3, and dentate gyrus regions. Deficient EGR3 in the primary hippocampal neurons significantly reduced the density of dendritic spines by downregulating the expression of BDNF, and such effects could be rescued by supplementing recombinant BDNF protein. Taken together, CH mediates cognitive impairment of pups through the inactivation of CaMKIV in the hippocampal neurons, which decreases the expression of EGR3 and further reduces the production of BDNF, thereby impairing the growth of dendritic spines. Identifying CaMKIV/EGR3/BDNF pathway in the hippocampal neurons in the context of CH will benefit the drug development of intellectual disability caused by CH.

BDNF as a Mediator of Antidepressant Response: Recent Advances and Lifestyle Interactions

Conventional antidepressants are widely employed in several psychiatric and neurologic disorders, yet the mechanisms underlying their delayed and partial therapeutic effects are only gradually being understood. This narrative review provides an up-to-date overview of the interplay between antidepressant treatment and Brain-Derived Neurotrophic Factor (BDNF) signaling. In addition, the impact of nutritional, environmental and physiological factors on BDNF and the antidepressant response is outlined. This review underlines the necessity to include information on lifestyle choices in testing and developing antidepressant treatments in the future.

Deficiency in FTSJ1 Affects Neuronal Plasticity in the Hippocampal Formation of Mice

Simple Summary

Neuronal plasticity refers to the brain’s ability to adapt in response to activity-dependent changes. This process, among others, allows the brain to acquire memory or to compensate for a neurocognitive deficit. We analyzed adult FTSJ1-deficient mice in order to gain insight into the role of FTSJ1 in neuronal plasticity. These mice displayed alterations in the hippocampus (a brain structure that is involved in memory and learning, among other functions) e.g., in the form of changes in dendritic spines. Changes in dendritic spines are considered to represent a morphological hallmark of altered neuronal plasticity, and thus FTSJ1 deficiency might have a direct effect upon the capacity of the brain to adapt to plastic changes. Long-term potentiation (LTP) is an electrophysiological correlate of neuronal plasticity, and is related to learning and to processes attributed to memory. Here we show that LTP in FTSJ1-deficient mice is reduced, hinting at disturbed neuronal plasticity. These findings suggest that FTSJ1 deficiency has an impact on neuronal plasticity not only morphologically but also on the physiological level.

Abstract

The role of the tRNA methyltransferase FTSJ1 in the brain is largely unknown. We analyzed whether FTSJ1-deficient mice (KO) displayed altered neuronal plasticity. We explored open field behavior (10 KO mice (aged 22–25 weeks)) and 11 age-matched control littermates (WT) and examined mean layer thickness (7 KO; 6 WT) and dendritic spines (5 KO; 5 WT) in the hippocampal area CA1 and the dentate gyrus. Furthermore, long-term potentiation (LTP) within area CA1 was investigated (5 KO; 5 WT), and mass spectrometry (MS) using CA1 tissue (2 each) was performed. Compared to controls, KO mice showed a significant reduction in the mean thickness of apical CA1 layers. Dendritic spine densities were also altered in KO mice. Stable LTP could be induced in the CA1 area of KO mice and remained stable at for at least 1 h, although at a lower level as compared to WTs, while MS data indicated differential abundance of several proteins, which play a role in neuronal plasticity. FTSJ1 has an impact on neuronal plasticity in the murine hippocampal area CA1 at the morphological and physiological levels, which, in conjunction with comparable changes in other cortical areas, might accumulate in disturbed learning and memory functions.

Structural and functional consequences in the amygdala of leptin-deficient mice

On the one hand, the emotional state can influence food intake and on the other hand, hunger can have an impact on the emotional state. Leptin, which is encoded by the ob gene, is involved in the energy homeostasis and plays a role in development of obesity. Mice deficient for leptin (ob/ob) are obese and display several behavioral alterations. It has been shown that ob/ob mice display striking changes in neuronal plasticity within the limbic system, e.g., hippocampal formation. We focus on alterations in ob/ob mice that can be related to alter processing in another part of the limbic system, the amygdala. ob/ob mice have a higher food consumption than age-matched controls, which might have an impact on the emotional state of these mice. Since the amygdala is involved in emotional processing, we analyze whether ob/ob mice display alterations in plasticity at the electrophysiological and structural level. No changes were seen in dendritic spine densities in the basolateral and lateral (LA) nucleus of the amygdala. Interestingly and in contrast to the hippocampus (Porter et al. 2013), long-term potentiation in the LA was increased in ob/ob mice. Our results indicate that amygdalar and hippocampal synaptic plasticity are regulated in different ways by leptin deficiency in accordance with the different functions of these limbic structures in stress and anxiety.

Obesity Impairs Mobility and Adult Hippocampal Neurogenesis

Currently, it is controversially discussed whether a relationship between obesity and cognition exists. We here analyzed a mouse model of obesity (leptin-deficient mice) to study the effects of obesity on the morphology of the hippocampus (a brain structure involved in mechanisms related to learning and memory) and on behavior. Mice aged 4 to 6 months were analyzed. At this age, the obese mice have nearly double the body weight as controls, but display smaller brains (brain volume is about 10% smaller) as control animals of the same age. Adult hippocampal neurogenesis, a process that is linked to learning and memory, might be disturbed in the obese mice and contribute to the smaller brain volume. Adult hippocampal neurogenesis was examined using specific markers for cell proliferation (phosphohistone H3), neuronal differentiation (doublecortin), and apoptosis (caspase 3). The number of phosphohistone H3 and doublecortin-positive cells was markedly reduced in leptin-deficient mice, but not the number of apoptotic cells, indicating that adult hippocampal neurogenesis on the level of cell proliferation was affected. In addition, dendritic spine densities of pyramidal neurons in the hippocampal area CA1 were analyzed using Golgi impregnation. However, no significant change in dendritic spine densities was noted in the obese mice. Moreover, the performance of the mice was analyzed in the open field as well as in the Morris water maze. In the open field test, obese mice showed reduced locomotor activity, but in the Morris water maze they showed similar performance compared with control animals.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

The Neuroprotective Effects of Exercise: Maintaining a Healthy Brain Throughout Aging

Physical activity plays an essential role in maintaining a healthy body, yet it also provides unique benefits for the vascular and cellular systems that sustain a healthy brain. While the benefit of exercise has been observed in humans of all ages, the availability of preclinical models has permitted systematic investigations into the mechanisms by which exercise supports and protects the brain. Over the past twenty-five years, rodent models have shown that increased physical activity elevates neurotrophic factors in the hippocampal and cortical areas, facilitating neurotransmission throughout the brain. Increased physical activity (such as by the voluntary use of a running wheel or regular, timed sessions on a treadmill) also promotes proliferation, maturation and survival of cells in the dentate gyrus, contributing to the process of adult hippocampal neurogenesis. In this way, rodent studies have tremendous value as they demonstrate that an 'active lifestyle' has the capacity to ameliorate a number of age-related changes in the brain, including the decline in adult neurogenesis. Moreover, these studies have shown that greater physical activity may protect the brain health into advanced age through a number of complimentary mechanisms: in addition to upregulating factors in pro-survival neurotrophic pathways and enhancing synaptic plasticity, increased physical activity promotes brain health by supporting the cerebrovasculature, sustaining the integrity of the blood-brain barrier, increasing glymphatic clearance and proteolytic degradation of amyloid beta species, and regulating microglia activation. Collectively, preclinical studies demonstrate that exercise initiates diverse and powerful neuroprotective pathways that may converge to promote continued brain health into old age. This review will draw on both seminal and current literature that highlights mechanisms by which exercise supports the functioning of the brain, and aids in its protection.

Forebrain-specific, conditional silencing of Staufen2 alters synaptic plasticity, learning, and memory in rats

Background

Dendritic messenger RNA (mRNA) localization and subsequent local translation in dendrites critically contributes to synaptic plasticity and learning and memory. Little is known, however, about the contribution of RNA-binding proteins (RBPs) to these processes in vivo.

Results

To delineate the role of the double-stranded RBP Staufen2 (Stau2), we generate a transgenic rat model, in which Stau2 expression is conditionally silenced by Cre-inducible expression of a microRNA (miRNA) targeting Stau2 mRNA in adult forebrain neurons. Known physiological mRNA targets for Stau2, such as RhoA, Complexin 1, and Rgs4 mRNAs, are found to be dysregulated in brains of Stau2-deficient rats. In vivo electrophysiological recordings reveal synaptic strengthening upon stimulation, showing a shift in the frequency-response function of hippocampal synaptic plasticity to favor long-term potentiation and impair long-term depression in Stau2-deficient rats. These observations are accompanied by deficits in hippocampal spatial working memory, spatial novelty detection, and in tasks investigating associative learning and memory.

Conclusions

Together, these experiments reveal a critical contribution of Stau2 to various forms of synaptic plasticity including spatial working memory and cognitive management of new environmental information. These findings might contribute to the development of treatments for conditions associated with learning and memory deficits.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1350-8) contains supplementary material, which is available to authorized users.

Autism-like behavior caused by deletion of vaccinia-related kinase 3 is improved by TrkB stimulation

Kang et al. showed that reduced vaccinia-related kinase 3 (VRK3) expression affects synaptic structure and function and results in cognitive dysfunction and autism-like behaviors in mice. TrkB stimulation reverses the altered synaptic properties and restores autism-like behaviors in VRK3-deficient mice.

Vaccinia-related kinases (VRKs) are multifaceted serine/threonine kinases that play essential roles in various aspects of cell signaling, cell cycle progression, apoptosis, and neuronal development and differentiation. However, the neuronal function of VRK3 is still unknown despite its etiological potential in human autism spectrum disorder (ASD). Here, we report that VRK3-deficient mice exhibit typical symptoms of autism-like behavior, including hyperactivity, stereotyped behaviors, reduced social interaction, and impaired context-dependent spatial memory. A significant decrease in dendritic spine number and arborization were identified in the hippocampus CA1 of VRK3-deficient mice. These mice also exhibited a reduced rectification of AMPA receptor–mediated current and changes in expression of synaptic and signaling proteins, including tyrosine receptor kinase B (TrkB), Arc, and CaMKIIα. Notably, TrkB stimulation with 7,8-dihydroxyflavone reversed the altered synaptic structure and function and successfully restored autism-like behavior in VRK3-deficient mice. These results reveal that VRK3 plays a critical role in neurodevelopmental disorders and suggest a potential therapeutic strategy for ASD.

High frequency stimulation of the infralimbic cortex induces morphological changes in rat hippocampal neurons

BACKGROUND

Although a significant subset of patients with major depressive disorder (MDD) fail to respond to medical or behavioural therapy, deep brain stimulation (DBS) applied to the subgenual cingulate cortex (SCC; sg25) has been shown to reduce depressive symptoms in a subset of patients. This area receives projections from neurons in the CA1 region and subiculum of the hippocampus (HC), a brain region implicated in the pathobiology and treatment of MDD.

OBJECTIVE

To assess whether high frequency stimulation (HFS) of the infralimbic cortex is associated with changes in cellular morphology in the HC.

METHODS

Rats were subjected to either infralimbic HFS or sham-stimulation. Measures of cellular morphology, including dendritic length and complexity, were assessed in pyramidal neurons in the CA1 region of the HC by means of the Golgi-Cox histological stain.

RESULTS

Dendritic length (p = 0.013) and number of branch points (p = 0.004) were significantly increased across the entire dendritic tree in animals subjected to HFS. Subsequent Scholl analysis revealed that for dendritic length these effects were localized to the region between 80 and 160 μm from the soma (p < 0.001 for either 40 μm interval) in the basal dendritic tree, while branch point number was predominantly increased between 120 and 160 μm from the soma (p < 0.001) in the apical dendritic tree.

CONCLUSIONS

High-frequency stimulation of the infralimbic cortex increases the complexity of apical dendrites and the length of basal dendritic trees of pyramidal neurons located in the CA1 hippocampal subfield relative to sham-stimulated animals.

FGF-2 deficiency causes dysregulation of Arhgef6 and downstream targets in the cerebral cortex accompanied by altered neurite outgrowth and dendritic spine morphology

Fibroblast growth factor 2 (FGF-2) is an abundant growth factor in the brain and exerts multiple functions on neural cells ranging from cell division, cell fate determination to differentiation. However, many details of the molecular mechanisms underlying the diverse functions of FGF-2 are poorly understood. In a comparative microarray analysis of motor sensory cortex (MSC) tissue of adult knockout (FGF-2(-/-)) and control (FGF-2(+/+)) mice, we found a substantial number of regulated genes, which are implicated in cytoskeletal machinery dynamics. Specifically, we found a prominent downregulation of Arhgef6. Arhgef6 mRNA was significantly reduced in the FGF-2(-/-) cortex, and Arhgef6 protein virtually absent, while RhoA protein levels were massively increased and Cdc42 protein levels were reduced. Since Arhgef6 is localized to dendritic spines, we next analyzed dendritic spines of adult FGF2(-/-) and control mouse cortices. Spine densities were significantly increased, whereas mean length of spines on dendrites of layer V of MSC neurons in adult FGF-2(-/-) mice was significantly decreased as compared to respective controls. Furthermore, neurite length in dissociated cortical cultures from E18 FGF-2(-/-) mice was significantly reduced at DIV7 as compared to wildtype neurons. Despite the fact that altered neuronal morphology and alterations in dendritic spines were observed, FGF-2(-/-) mice behave relatively unsuspicious in several behavioral tasks. However, FGF-2(-/-) mice exhibited decreased thermal pain sensitivity in the hotplate-test.

Emerging roles of ARHGAP33 in intracellular trafficking of TrkB and pathophysiology of neuropsychiatric disorders

Intracellular trafficking of receptor proteins is essential for neurons to detect various extracellular factors during the formation and refinement of neural circuits. However, the precise mechanisms underlying the trafficking of neurotrophin receptors to synapses remain elusive. Here, we demonstrate that a brain-enriched sorting nexin, ARHGAP33, is a new type of regulator for the intracellular trafficking of TrkB, a high-affinity receptor for brain-derived neurotrophic factor. ARHGAP33 knockout (KO) mice exhibit reduced expression of synaptic TrkB, impaired spine development and neuropsychiatric disorder-related behavioural abnormalities. These deficits are rescued by specific pharmacological enhancement of TrkB signalling in ARHGAP33 KO mice. Mechanistically, ARHGAP33 interacts with SORT1 to cooperatively regulate TrkB trafficking. Human ARHGAP33 is associated with brain phenotypes and reduced SORT1 expression is found in patients with schizophrenia. We propose that ARHGAP33/SORT1-mediated TrkB trafficking is essential for synapse development and that the dysfunction of this mechanism may be a new molecular pathology of neuropsychiatric disorders.

The molecular mechanisms of neurotrophin receptor trafficking are only partially understood. Here the authors show that ARHGAP33 interacts with SORT1 to regulate TrkB trafficking, the dysfunction of which impairs synapse development and leads to schizophrenia-related behavioural abnormalities in mice.

Neonatal vaccination with bacillus Calmette-Guérin and hepatitis B vaccines modulates hippocampal synaptic plasticity in rats

Immune activation can exert multiple effects on synaptic transmission. Our study demonstrates the influence of neonatal vaccination on hippocampal synaptic plasticity in rats under normal physiological conditions. The results revealed that neonatal BCG vaccination enhanced synaptic plasticity. In contrast, HBV hampered it. Furthermore, we found that the cytokine balance shifted in favour of the T helper type 1/T helper type 2 immune response in BCG/HBV-vaccinated rats in the periphery. The peripheral IFN-γ:IL-4 ratio was positively correlated with BDNF and IGF-1 in the hippocampus. BCG raised IFN-γ, IL-4, BDNF and IGF-1 and reduced IL-1β, IL-6, and TNF-α in the hippocampus, whereas, HBV triggered the opposite effects.

Impact of a deletion of the full-length and short isoform of p75NTR on cholinergic innervation and the population of postmitotic doublecortin positive cells in the dentate gyrus

Analyses of mice carrying a deletion of the pan-neurotrophin receptor p75NTR have allowed identifying p75NTR as an important structural regulator of the hippocampus. Most of the previous analyses were done using p75NTR (ExIII) knockout mice which still express the short isoform of p75NTR. To scrutinize the role of p75NTR in the hippocampus, we analyzed adult and aged p75NTR (ExIV) knockout mice, in which both, the short and the full-length isoform are deleted. Deletion of these isoforms induced morphological alterations in the adult dentate gyrus (DG), leading to an increase in the thickness of the molecular and granular layer. Based on these observations, we next determined the morphological substrates that might contribute to this phenotype. The cholinergic innervation of the molecular and granular layer of the DG was found to be significantly increased in the knockout mice. Furthermore, adult neurogenesis in the DG was found to be significantly altered with increased numbers of doublecortin (DCX) positive cells and reduced numbers of apoptotic cells in p75NTR (ExIV) knockout mice. However, cell proliferation as measured by phosphohiston H3 (PH3) positive cell numbers was not affected. These morphological alterations (number of DCX-positive cells and increased cholinergic fiber densities) as well as reduced cell death in the DG are likely to contribute to the observed thickening of the granular layer in p75NTR (ExIV) knockout mice. In addition, Sholl-analysis of DCX-positive neurons revealed a higher dendritic complexity and could thus be a possible morphological correlate for the increased thickness of the molecular layer in p75NTR deficient animals. Our data clearly demonstrate that deletion of both, the short and the full-length isoform of p75NTR affects DG morphology, due to alterations of the cholinergic system and an imbalance between neurogenesis and programmed cell death within the subgranular zone.

Mood disorders in Huntington's disease: from behavior to cellular and molecular mechanisms

Huntington's disease (HD) is a neurodegenerative disorder that is best known for its effect on motor control. Mood disturbances such as depression, anxiety, and irritability also have a high prevalence in patients with HD, and often start before the onset of motor symptoms. Various rodent models of HD recapitulate the anxiety/depressive behavior seen in patients. HD is caused by an expanded polyglutamine stretch in the N-terminal part of a 350 kDa protein called huntingtin (HTT). HTT is ubiquitously expressed and is implicated in several cellular functions including control of transcription, vesicular trafficking, ciliogenesis, and mitosis. This review summarizes progress in efforts to understand the cellular and molecular mechanisms underlying behavioral disorders in patients with HD. Dysfunctional HTT affects cellular pathways that are involved in mood disorders or in the response to antidepressants, including BDNF/TrkB and serotonergic signaling. Moreover, HTT affects adult hippocampal neurogenesis, a physiological phenomenon that is implicated in some of the behavioral effects of antidepressants and is linked to the control of anxiety. These findings are consistent with the emerging role of wild-type HTT as a crucial component of neuronal development and physiology. Thus, the pathogenic polyQ expansion in HTT could lead to mood disorders not only by the gain of a new toxic function but also by the perturbation of its normal function.

Implications of p75NTR for dentate gyrus morphology and hippocampus-related behavior revisited

The pan-neurotrophin receptor p75NTR is expressed in the adult brain in a discrete pattern. Although numerous studies have addressed its implications for hippocampal functions, the generated sets of data are surprisingly conflicting. We have therefore set out to re-investigate the impact of a deletion of the full-length p75NTR receptor on several parameters of the dentate gyrus (DG), including neurogenesis and hippocampus-related behavior by using p75NTR(ExIII) knockout mice. Moreover, we investigated further parameters of the DG (cholinergic innervation, dendritic spines). In addition, we analyzed on the morphological level the impact of aging by comparing adult and aged p75NTR(ExIII) mice and their age-matched littermates. Adult (4-6 months old), but not aged (20 months old), p75NTR(ExIII) knockout mice display an enhanced volume of the DG. However, adult neurogenesis within the adult DG was unaffected in both adult and aged p75NTR(ExIII) knockout mice. We could further demonstrate that the change in the volume of the DG was accompanied by an increased cholinergic innervation and increased spine densities of granule cells in adult, but not aged p75NTR deficient mice. These morphological changes in the adult p75NTR deficient mice were accompanied by specific alterations in their behavior, including altered behavior in the Morris water maze test, indicating impairments in spatial memory retention.

Analysis of morphological changes as a key method in studying psychiatric animal models

A major interest in the analysis of animal models of psychiatric diseases is their underlying cellular pathology and to gain information regarding whether pharmacological treatments, genetic differences or an altered environment exert an impact upon the brain morphology or on the morphology or activity of single neurones. In this review, several key methods will be introduced that allow the analysis of morphological changes that are frequently observed in psychiatric animal models. An overview of the techniques that enable dendritic arborisation, alterations in dendritic spines and changes in fibre densities to be analysed are described. Moreover, methods for the analysis of adult neurogenesis and neurodegeneration and for the analysis of neuronal activity in fixed brain tissue are described. An important step during the analysis of morphological changes is the estimation of the number of stained cells. Since conventional cell counting methods have several limitations, two different approaches that permit an estimate of the number of stained cells within three-dimensional tissue are also discussed.

The Role of Neurotrophins in Major Depressive Disorder

Neurotrophins and other growth factors have been advanced as critical modulators of depressive behavior. Support for this model is based on analyses of knockout and transgenic mouse models, human genetic studies, and screens for gene products that are regulated by depressive behavior and/or antidepressants. Even subtle alteration in the regulated secretion of brain-derived neurotrophic factor (BDNF), for example, due to a single nucleotide polymorphism (SNP)-encoded Val-Met substitution in proBDNF that affects processing and sorting, impacts behavior and cognition. Alterations in growth factor expression result in changes in neurogenesis as well as structural changes in neuronal cytoarchitecture, including effects on dendritic length and spine density, in the hippocampus, nucleus accumbens, and prefrontal cortex. These changes have the potential to impact the plasticity and stability of synapses in the CNS, and the complex brain circuitry that regulates behavior. Here we review the role that neurotrophins play in the modulation of depressive behavior, and the downstream signaling targets they regulate that potentially mediate these behavioral pro-depressant and antidepressant effects.

Differential response of hippocampal subregions to stress and learning

The hippocampus has two functionally distinct subregions-the dorsal portion, primarily associated with spatial navigation, and the ventral portion, primarily associated with anxiety. In a prior study of chronic unpredictable stress (CUS) in rodents, we found that it selectively enhanced cellular plasticity in the dorsal hippocampal subregion while negatively impacting it in the ventral. In the present study, we determined whether this adaptive plasticity in the dorsal subregion would confer CUS rats an advantage in a spatial task-the radial arm water maze (RAWM). RAWM exposure is both stressful and requires spatial navigation, and therefore places demands simultaneously upon both hippocampal subregions. Therefore, we used Western blotting to investigate differential expression of plasticity-associated proteins (brain derived neurotrophic factor [BDNF], proBDNF and postsynaptic density-95 [PSD-95]) in the dorsal and ventral subregions following RAWM exposure. Lastly, we used unbiased stereology to compare the effects of CUS on proliferation, survival and neuronal differentiation of cells in the dorsal and ventral hippocampal subregions. We found that CUS and exposure to the RAWM both increased corticosterone, indicating that both are stressful; nevertheless, CUS animals had significantly better long-term spatial memory. We also observed a subregion-specific pattern of protein expression following RAWM, with proBDNF increased in the dorsal and decreased in the ventral subregion, while PSD-95 was selectively upregulated in the ventral. Finally, consistent with our previous study, we found that CUS most negatively affected neurogenesis in the ventral (compared to the dorsal) subregion. Taken together, our data support a dual role for the hippocampus in stressful experiences, with the more resilient dorsal portion undergoing adaptive plasticity (perhaps to facilitate escape from or neutralization of the stressor), and the ventral portion involved in affective responses.

Remodeling of axo-spinous synapses in the pathophysiology and treatment of depression

Dendritic spines provide a compartment for assembly and functional organization of synaptic machinery that plays a fundamental role in neuronal communication and neuroplasticity. Studies in humans as well as in animal models have demonstrated abnormal spine architecture in several psychiatric disorders, including depression and other stress-related illnesses. The negative impact of stress on the density and organization of spines is thought to contribute to the behavioral deficits caused by stress exposure. Moreover, there is now evidence that medication-induced recovery involves changes in synaptic plasticity and dendrite morphology, including increased expression of pre- and postsynaptic plasticity-related proteins, as well as the density and function of axo-spinous synapses. Here we review the evidence from brain imaging and postmortem studies demonstrating that depression is accompanied by structural and functional alterations of cortical and limbic brain regions, including the prefrontal cortex, hippocampus and amygdala. In addition, we present more direct evidence from basic research studies that exposure to stress alters spine morphology, function and plasticity and that antidepressants, particularly new rapid acting agents, reverse these effects. Elucidation of the signaling pathways and molecular mechanisms that control spine synapse assembly and plasticity will contribute to a better understanding of the pathophysiology of depression and development of novel, more effective therapeutic agents.

7,8-dihydroxyflavone rescues spatial memory and synaptic plasticity in cognitively impaired aged rats

7,8-dihydroxyflavone (7,8-DHF) has recently been identified as a potential TrkB agonist that crosses the blood-brain barrier after i.p. administration. We previously demonstrated that 7,8-DHF in vitro rescues long-term synaptic plasticity in the hippocampus of aged rats. This study assessed the rescue effect of 7,8-DHF in vivo on aging-related cognitive impairment in rats, and further determined whether the effect of 7,8-DHF is age dependent. Aged rats at 22 and 30 months of age were pretested for spatial memory in Morris water maze. The aged-impaired rats were retested twice during 7,8-DHF or vehicle treatment, which started 3 weeks after the completion of the pretest. In the 22-month-old rats, daily i.p. administration of 7,8-DHF for 2 weeks improved spatial memory. The improvement in behavioral tests was associated with increases in synapse formation and facilitation of synaptic plasticity in the hippocampus, as well as the activation of several proteins crucial to synaptic plasticity and memory. A more extended treatment paradigm with 7,8-DHF was required to achieve a significant memory improvement in the severely impaired 30-month-old rats. Moreover, 7,8-DHF moderately facilitated the synaptic plasticity, modified the density but not number of spines in the hippocampus of the oldest rats. Taken together, our results suggest that 7,8-DHF can act in vivo to counteract aging-induced declines in spatial memory and synaptic plasticity and morphological changes of hippocampal neurons. The effect of 7,8-DHF is more pronounced in relatively younger impaired rats than in those of more advanced age. These findings demonstrate the reversal of age-dependent memory impairment by in vivo 7,8-DHF application and support the benefit of early treatment for cognitive aging.

The brain-derived neurotrophic-factor (BDNF) val66met polymorphism is associated with geriatric depression: a meta-analysis

Depression has been associated with reduced expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. Genetic association studies of the BDNF Val66Met polymorphism (rs6265) in geriatric depression have produced inconsistent results. A meta-analysis of studies was conducted to compare the frequency of the BDNF Val66Met variant between cases with geriatric depression and age-matched controls. A total of five studies involving 523 cases with geriatric depression and 1,220 psychiatrically healthy controls was included. Met allele carriers had an increased risk for geriatric depression when compared to Val/Val homozygotes (P = 0.004, OR = 1.48, 95% CI = 1.13-1.93). Our findings suggest the BDNF Met allele may confer increased risk for depression as individual age.

Divergent roles of p75NTR and Trk receptors in BDNF's effects on dendritic spine density and morphology

Activation of TrkB receptors by brain-derived neurotrophic factor (BDNF) followed by MAPK/ERK signaling increases dendritic spine density and the proportion of mature spines in hippocampal CA1 pyramidal neurons. Considering the opposing actions of p75(NTR) and Trk receptors in several BDNF actions on CNS neurons, we tested whether these receptors also have divergent actions on dendritic spine density and morphology. A function-blocking anti-p75(NTR) antibody (REX) did not affect spine density by itself but it prevented BDNF's effect on spine density. Intriguingly, REX by itself increased the proportion of immature spines and prevented BDNF's effect on spine morphology. In contrast, the Trk receptor inhibitor k-252a increased spine density by itself, and prevented BDNF from further increasing spine density. However, most of the spines in k-252a-treated slices were of the immature type. These effects of k-252a on spine density and morphology required neuronal activity because they were prevented by TTX. These divergent BDNF actions on spine density and morphology are reminiscent of opposing functional signaling by p75(NTR) and Trk receptors and reveal an unexpected level of complexity in the consequences of BDNF signaling on dendritic morphology.

Immunohistological markers for proliferative events, gliogenesis, and neurogenesis within the adult hippocampus

Biologists long believed that, once development is completed, no new neurons are produced in the forebrain. However, as is now firmly established, new neurons can be produced at least in two specific forebrain areas: the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampal formation. Neurogenesis within the adult DG occurs constitutively throughout postnatal life, and the rate of neurogenesis within the DG can be altered under various physiological and pathophysiological conditions. The process of adult neurogenesis within the DG is a multi-step process (proliferation, differentiation, migration, targeting, and synaptic integration) that ends with the formation of a post-mitotic functionally integrated new neuron. Various markers are expressed during specific stages of adult neurogenesis. The availability of such markers allows the time-course and fate of newly born cells to be followed within the DG in a detailed and precise fashion. Several of the available markers (e.g., PCNA, Ki-67, PH3, MCM2) are markers for proliferative events, whereas others are more specific for early phases of neurogenesis and gliogenesis within the adult DG (e.g., nestin, GFAP, Sox2, Pax6). In addition, markers are available allowing events to be distinguished that are related to later steps of gliogenesis (e.g., vimentin, BLBP, S100beta) or neurogenesis (e.g., NeuroD, PSA-NCAM, DCX).

Potential of cystamine and cysteamine in the treatment of neurodegenerative diseases

Neurodegenerative disorders are a subset of disabling pathologies characterized, in part, by a progressive and specific loss of certain brain cell populations. Current therapeutic approaches for the treatment of these disorders are mainly designed towards symptom management and do not manifestly block their typified neuronal loss. However, research conducted over the past decade has reflected the increasing interest and need to find disease-modifying molecules. Among the several neuroprotective agents emerging from experimental animal work, cystamine, as well as its reduced form cysteamine, have been identified as potential candidate drugs. Given the significant benefits observed in a Huntington's disease (HD) model, cysteamine has recently leaped to clinical trial. Here, we review the beneficial properties of these compounds as reported in animal studies, their mechanistic underpinnings, and their potential implications for the future treatment of patients suffering from neurodegenerative diseases, and more specifically for HD and Parkinson's disease (PD).

Involvement of BDNF in age-dependent alterations in the hippocampus

It is known since a long time that the hippocampus is sensitive to aging. Thus, there is a reduction in the hippocampal volume during aging. This age-related volume reduction is paralleled by behavioral and functional deficits in hippocampus-dependent learning and memory tasks. This age-related volume reduction of the hippocampus is not a consequence of an age-related loss of hippocampal neurons. The morphological changes associated with aging include reductions in the branching pattern of dendrites, as well as reductions in spine densities, reductions in the densities of fibers projecting into the hippocampus as well as declines in the rate of neurogenesis. It is very unlikely that a single factor or a single class of molecules is responsible for all these age-related morphological changes in the hippocampus. Nevertheless, it would be of advantage to identify possible neuromodulators or neuropeptides that may contribute to these age-related changes. In this context, growth factors may play an important role in the maintenance of the postnatal hippocampal architecture. In this review it is hypothesized that brain-derived neurotrophic factor (BDNF) is a factor critically involved in the regulation of age-related processes in the hippocampus. Moreover, evidences suggest that disturbances in the BDNF-system also affect hippocampal dysfunctions, as e.g. seen in major depression or in Alzheimer disease.

Exercise improves cognition and hippocampal plasticity in APOE epsilon4 mice

BACKGROUND

Human studies on exercise, cognition, and apolipoprotein E (APOE) genotype show that epsilon4 carriers may benefit from regular physical activity.

METHODS

We examined voluntary wheel-running, memory, and hippocampal plasticity in APOE epsilon3 and APOE epsilon4 transgenic mice at 10-12 months of age.

RESULTS

Sedentary epsilon4 mice exhibited deficits in cognition on the radial-arm water maze (RAWM), a task dependent on the hippocampus. Six weeks of wheel-running in epsilon4 mice resulted in improvements on the RAWM to the level of epsilon3 mice. Hippocampal brain-derived neurotrophic factor (BDNF) levels were similar in epsilon3 and epsilon4 mice, and after exercise BDNF was similarly increased in both epsilon3 and epsilon4 mice. In sedentary epsilon4 mice, tyrosine kinase B (Trk B) receptors were reduced by 50%. Exercise restored Trk B in epsilon4 mice to the level of epsilon3 mice, and in epsilon4 mice, exercise dramatically increased synaptophysin, a marker of synaptic function.

CONCLUSIONS

Our results support the hypothesis that exercise can improve cognitive function, particularly in epsilon4 carriers.

Structure and function of dendritic spines within the hippocampus

Most excitatory input in the hippocampus impinges on dendritic spines. Therefore, the dendritic spines are likely to be of major importance for neural processing. The morphology of dendritic spines is very diverse and changes in spine size as well as in their density are thought to reflect changes in the strength of synaptic transmission. Thus, alterations in dendritic spine densities or shape are suspected to be morphological manifestations of psychopathological, pathophysiological, physiological and/or behavioural changes. However, in spite of a long history of research, the specific function of dendritic spines within the hippocampal formation is still not well understood. This review will shed light on the hippocampal dendritic spines, their ultrastructure and morphology, as well as their supposed roles in neuronal plasticity and in certain mental illnesses.

Inflammation regulates functional integration of neurons born in adult brain

Inflammation influences several steps of adult neurogenesis, but whether it regulates the functional integration of the new neurons is unknown. Here, we explored, using confocal microscopy and whole-cell patch-clamp recordings, whether a chronic inflammatory environment affects the morphological and electrophysiological properties of new dentate gyrus granule cells, labeled with a retroviral vector encoding green fluorescent protein. Rats were exposed to intrahippocampal injection of lipopolysaccharide, which gave rise to long-lasting microglia activation. Inflammation caused no changes in intrinsic membrane properties, location, dendritic arborization, or spine density and morphology of the new cells. Excitatory synaptic drive increased to the same extent in new and mature cells in the inflammatory environment, suggesting increased network activity in hippocampal neural circuitries of lipopolysaccharide-treated animals. In contrast, inhibitory synaptic drive was more enhanced by inflammation in the new cells. Also, larger clusters of the postsynaptic GABA(A) receptor scaffolding protein gephyrin were found on dendrites of new cells born in the inflammatory environment. We demonstrate for the first time that inflammation influences the functional integration of adult-born hippocampal neurons. Our data indicate a high degree of synaptic plasticity of the new neurons in the inflammatory environment, which enables them to respond to the increase in excitatory input with a compensatory upregulation of activity and efficacy at their afferent inhibitory synapses.

Altered morphology of hippocampal dentate granule cell presynaptic and postsynaptic terminals following conditional deletion of TrkB

Dentate granule cells play a critical role in the function of the entorhinal-hippocampal circuitry in health and disease. Dentate granule cells are situated to regulate the flow of information into the hippocampus, a structure required for normal learning and memory. Correspondingly, impaired granule cell function leads to memory deficits, and, interestingly, altered granule cell connectivity may contribute to the hyperexcitability of limbic epilepsy. It is important, therefore, to understand the molecular determinants of synaptic connectivity of these neurons. Brain-derived neurotrophic factor and its receptor TrkB are expressed at high levels in the dentate gyrus (DG) of the hippocampus, and are implicated in regulating neuronal development, neuronal plasticity, learning, and the development of epilepsy. Whether and how TrkB regulates granule cell structure, however, is incompletely understood. To begin to elucidate the role of TrkB in regulating granule cell morphology, here we examine conditional TrkB knockout mice crossed to mice expressing green fluorescent protein in subsets of dentate granule cells. In stratum lucidum, where granule cell mossy fiber axons project, the density of giant mossy fiber boutons was unchanged, suggesting similar output to CA3 pyramidal cell targets. However, filopodial extensions of giant boutons, which contact inhibitory interneurons, were increased in number in TrkB knockout mice relative to wildtype controls, predicting enhanced feedforward inhibition of CA3 pyramidal cells. In knockout animals, dentate granule cells possessed fewer primary dendrites and enlarged dendritic spines, indicative of disrupted excitatory synaptic input to the granule cells. Together, these findings demonstrate that TrkB is required for development and/or maintenance of normal synaptic connectivity of the granule cells, thereby implying an important role for TrkB in the function of the granule cells and hippocampal circuitry.

TrkB but not trkC receptors are necessary for postnatal maintenance of hippocampal spines

Dendritic spines are major sites of excitatory synaptic transmission and changes in their densities have been linked to alterations in learning and memory. The neurotrophins brain-derived neurotrophic factor and neurotrophin-3 and their receptors, trkB and trkC, are thought to be involved in learning, memory and long-term potentiation (LTP). LTP is known to induce trkB and trkC gene expression as well as spinogenesis in the hippocampus. In the aging hippocampus, declines in trkB and trkC mRNA levels may underlie, at least in part, impairments in spatial memory and reductions in spine densities. To determine the significance of trkB and trkC for the maintenance of dendritic spines, we have analyzed Golgi-impregnated hippocampi of adult and aged mice heterozygous for trkB, trkC, or both along with respective wildtype littermates. Deletion of one allele of trkB, but not trkC, significantly reduces spine densities of CA1 pyramidal neurons in both adult and aged mice, as compared to age-matched controls. This indicates that trkB, but not trkC, receptors are necessary for the maintenance of hippocampal spines during postnatal life.

Impaired spatial memory and altered dendritic spine morphology in angiotensin II type 2 receptor-deficient mice

Mental retardation is the most frequent cause of serious handicap in children and young adults. Mutations in the human angiotensin II type 2 receptor (AT2) have been implicated in X-linked forms of mental retardation. We here demonstrate that mice lacking the AT2 receptor gene are significantly impaired in their performance in a spatial memory task and in a one-way active avoidance task. As no difference was observed between the genotypes in fear conditioning, the detected deficit in spatial memory may not relate to fear. Notably, receptor knockout mice showed increased motility in an activity meter and elevated plus maze. Importantly, these mice are characterized by abnormal dendritic spine morphology and length, both features also found to be associated with some cases of mental retardation. These findings suggest a crucial role of AT2 in normal brain function and that dysfunction of the receptor has impact on brain development and ultrastructural morphology with distinct consequences on learning and memory.

Expression of trkB and trkC receptors and their ligands brain-derived neurotrophic factor and neurotrophin-3 in the murine amygdala

The neurotrophin brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their cognate receptors, trkB and trkC, have a variety of physiological brain functions, ranging from cell survival to mechanisms involved in learning and memory and long-term potentiation (LTP). LTP can be induced in the cortex and hippocampus, as well as within the amygdala. However, the role of neurotrophins in amygdalar LTP is largely unknown. Expression patterns of BDNF and NT-3 and their cognate receptors in the adult mouse amygdala have not been analyzed in detail. We have therefore examined the expression of trkB, trkC, BDNF, and NT-3 mRNA and protein in different amygdalar nuclei as well as in the hippocampal areas CA1-CA3 and the dentate gyrus. The distribution pattern of trkB, trkC, BDNF, and NT-3 mRNA in the murine hippocampus is comparable to that seen in rats. Within most amygdalar nuclei, a moderate BDNF mRNA expression was found; however, BDNF mRNA was virtually absent from the central nucleus. No expression of NT-3 mRNA was found within the amygdala, but trkC mRNA-expressing cells were widely distributed within this brain region. trkB mRNA was strongly expressed in the amygdala. Because trkB is expressed in a full-length and a truncated form (the latter form is also expressed by nonneuronal cells), we also investigated the distribution of full-length trkB mRNA-expressing cells and could demonstrate that this version of trkB receptors is also widely expressed in the amygdala. These results can serve as a basis for studies elucidating the physiological roles of these receptors in the amygdala.

The role of neurotrophins during early development

The effects of neurotrophins during the middle and late stages of development are well known. It was previously thought that neurotrophins had no role during early development, but this is not the case and is the subject of this review article. The earliest neurotrophin receptor expressed is that for neurotrophin-3 (NT-3). TrkC is detected in the neural plate and is present in the neural tube. Initially, the distribution of TrkC is homogenous, but it becomes localized to specific regions of the neural tube as the neural tube differentiates. The receptor for brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5), TrkB, is detected somewhat later than TrkC in the neural tube where it is also differentially localized. In contrast, the NGF receptor, TrkA, was not detected during early development. Both NT-3 and BDNF have been shown to have effects in vitro during early development. NT-3 caused an increase in neurite outgrowth and apoptosis in neural plate explants, and promoted differentiation of progenitors into motoneurons. BDNF increased the number of motoneurons in neural tube explants. These data suggest that NT-3 and BDNF may play a role during early development in vivo.

The role of neurotrophic factors in adult hippocampal neurogenesis, antidepressant treatments and animal models of depressive-like behavior

Major depressive disorder (MDD) is characterized by structural and neurochemical changes in limbic structures, including the hippocampus, that regulate mood and cognitive functions. Hippocampal atrophy is observed in patients with depression and this effect is blocked or reversed by antidepressant treatments. Brain-derived neurotrophic factor and other neurotrophic/growth factors are decreased in postmortem hippocampal tissue from suicide victims, which suggests that altered trophic support could contribute to the pathophysiology of MDD. Preclinical studies demonstrate that exposure to stress leads to atrophy and cell loss in the hippocampus as well as decreased expression of neurotrophic/growth factors, and that antidepressant administration reverses or blocks the effects of stress. Accumulating evidence suggests that altered neurogenesis in the adult hippocampus mediates the action of antidepressants. Chronic antidepressant administration upregulates neurogenesis in the adult hippocampus and this cellular response is required for the effects of antidepressants in certain animal models of depression. Here, we review cellular (e.g. adult neurogenesis) and behavioral studies that support the neurotrophic/neurogenic hypothesis of depression and antidepressant action. Aberrant regulation of neuronal plasticity, including neurogenesis, in the hippocampus and other limbic nuclei may result in maladaptive changes in neural networks that underlie the pathophysiology of MDD.

Brain-derived neurotrophic factor: linking fear learning to memory consolidation

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, plays an important role in synaptic plasticity. In this issue of Molecular Pharmacology, Ou and Gean (p. 350) thoroughly describe the molecular cascade by which fear learning leads to an increase in BDNF expression in the lateral amygdala (LA). Calcium influx through N-methyl-D-aspartate receptors and L-type voltage-dependent calcium channels, which occurs in the LA during fear conditioning, activates protein kinase A and Ca2+/calmodulin-dependent protein kinase IV. Each induces phosphorylation of cAMP response element-binding protein, which binds to the BDNF promoter, leading to BDNF expression in the LA, and contributes to fear memory consolidation.

Status epilepticus induces region-specific changes in dendritic spines, dendritic length and TrkB protein content of rat brain cortex

Induction of status epilepticus (SE) with kainic acid results in a large reorganization of neuronal brain circuits, a phenomenon that has been studied primarily in the hippocampus. The neurotrophin BDNF, by acting through its receptor TrkB, has been implicated in such reorganization. In the present work we investigated, by Western blot and immunohistochemistry, whether regional changes of TrkB expression within the rat brain cortex are correlated with altered neuronal morphology and/or with apoptotic cell death. We found that the full-length TrkB protein decreased within the cortex when measured 24 h to 1 week after induction of SE. Analysis by immunohistochemistry revealed that TrkB staining diminished within layer V of the retrosplenial granular b (RSGb) and motor cortices, but not within the auditory cortex. In layer II/III, differential changes were also observed: TrkB decreased in the motor cortex, did not change within the RSGb but increased within the auditory cortex. Reduced TrkB was associated with dendritic atrophy and decreased spine density in pyramidal neurons within layer V of the RSGb. No correlation was observed between regional and cellular changes of TrkB protein and apoptosis, measured by the TdT-mediated dUTP nick end labeling (TUNEL) method. The global decrease of TrkB within the neocortex and the associated dendritic atrophy may counteract seizure propagation in the epileptic brain but may also underlie cognitive impairment after seizures.

Transient receptor potential channels as novel effectors of brain-derived neurotrophic factor signaling: potential implications for Rett syndrome

In addition to their prominent role as survival signals for neurons in the developing nervous system, neurotrophins have established their significance in the adult brain as well, where their modulation of synaptic transmission and plasticity may participate in associative learning and memory. These crucial activities are primarily the result of neurotrophin regulation of intracellular Ca(2+) homeostasis and, ultimately, changes in gene expression. Outlined in the following review is a synopsis of neurotrophin signaling with a particular focus upon brain-derived neurotrophic factor (BDNF) and its role in hippocampal synaptic plasticity and neuronal Ca(2+) homeostasis. Neurotrophin signaling through tropomyosin-related kinase (Trk) and pan-neurotrophin receptor 75 kD (p75(NTR)) receptors are also discussed, reviewing recent results that indicate signaling through these two receptor modalities leads to opposing cellular outcomes. We also provide an intriguing look into the transient receptor potential channel (TRPC) family of ion channels as distinctive targets of BDNF signaling; these channels are critical for capacitative Ca(2+) entry, which, in due course, mediates changes in neuronal structure including dendritic spine density. Finally, we expand these topics into an exploration of mental retardation (MR), in particular Rett Syndrome (RTT), where dendritic spine abnormalities may underlie cognitive impairments. We propose that understanding the role of neurotrophins in synapse formation, plasticity, and maintenance will make fundamental contributions to the development of therapeutic strategies to improve cognitive function in developmental disorders associated with MR.