O. von Bohlen und Halbach

Immunohistological markers for staging neurogenesis in adult hippocampus

Neurogenesis in the adult dentate gyrus (DG) of the hippocampus occurs constitutively throughout postnatal life, and the rate of neurogenesis within the DG can be altered under various physiological and pathophysiological conditions. Adult neurogenesis includes the process in which the division of a precursor cell takes place and the multi-step process (proliferation, differentiation, migration, targeting, and synaptic integration) that ends with the formation of a postmitotic functionally integrated new neuron. During specific time-frames of adult neurogenesis, various markers are expressed that correlate with the differentiation steps along the pathway from early progenitor cells to newly generated postmitotic neurons within the DG. Markers that are currently widely used for the investigation of adult hippocampal neurogenesis are: glial fibrillary acidic protein, nestin, Pax6, NeuroD, PSA-NCAM, doublecortin, TUC-4, Tuj-1, and calretinin. The discovery and development of specific markers that allow the time-course and fate of neurons to be followed during adult neurogenesis in a detailed and precise fashion are not only helpful for gaining further insights into the genesis of new neurons in the hippocampus, but also might be applicable to the development of strategies for therapeutic interventions.Neurogenesis in the adult dentate gyrus (DG) of the hippocampus occurs constitutively throughout postnatal life, and the rate of neurogenesis within the DG can be altered under various physiological and pathophysiological conditions. Adult neurogenesis includes the process in which the division of a precursor cell takes place and the multi-step process (proliferation, differentiation, migration, targeting, and synaptic integration) that ends with the formation of a postmitotic functionally integrated new neuron. During specific time-frames of adult neurogenesis, various markers are expressed that correlate with the differentiation steps along the pathway from early progenitor cells to newly generated postmitotic neurons within the DG. Markers that are currently widely used for the investigation of adult hippocampal neurogenesis are: glial fibrillary acidic protein, nestin, Pax6, NeuroD, PSA-NCAM, doublecortin, TUC-4, Tuj-1, and calretinin. The discovery and development of specific markers that allow the time-course and fate of neurons to be followed during adult neurogenesis in a detailed and precise fashion are not only helpful for gaining further insights into the genesis of new neurons in the hippocampus, but also might be applicable to the development of strategies for therapeutic interventions.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1–8) [Ang II], angiotensin-(3–8) [Ang IV], and angiotensin-(1–7) [Ang-(1–7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1–7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1–7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

TrkB Modulates Fear Learning and Amygdalar Synaptic Plasticity by Specific Docking Sites

Understanding the modulation of the neural circuitry of fear is clearly one of the most important aims in neurobiology. Protein phosphorylation in response to external stimuli is considered a major mechanism underlying dynamic changes in neural circuitry. TrkB (Ntrk2) neurotrophin receptor tyrosine kinase potently modulates synaptic plasticity and activates signal transduction pathways mainly through two phosphorylation sites [Y515/Shc site; Y816/PLCγ (phospholipase Cγ) site]. To identify the molecular pathways required for fear learning and amygdalar synaptic plasticity downstream of TrkB, we used highly defined genetic mouse models carrying single point mutations at one of these two sites (Y515F or Y816F) to examine the physiological relevance of pathways activated through these sites for pavlovian fear conditioning (FC), as well as for synaptic plasticity as measured by field recordings obtained from neurons of different amygdala nuclei. We show that a Y816F point mutation impairs acquisition of FC, amygdalar synaptic plasticity, and CaMKII signaling at synapses. In contrast, a Y515F point mutation affects consolidation but not acquisition of FC to tone, and also alters AKT signaling. Thus, TrkB receptors modulate specific phases of fear learning and amygdalar synaptic plasticity through two main phosphorylation docking sites.

Modeling neurodegenerative diseases in vivo review.

Parkinson’s disease (PD) is one of the major neurodegenerative disorders. The etiology of this disease is likely due to combinations of environmental and genetic factors. Symptomatic hallmarks of PD are tremor, bradykinesia, rigidity and postural instability. On the morphological and anatomical level, PD is characterized by massive degeneration of dopaminergic neurons in the substantia nigra pars compacta, leading to a severe loss of striatal dopaminergic fibers and to a massive reduction of dopamine levels in the striatum. In addition, PD is characterized by the appearance of Lewy bodies within the surviving dopaminergic neurons. Animal models of PD allow getting insight into the mechanisms of several symptoms of PD thereby providing indispensable tools for basic and applied research. The biochemical and cellular changes that occur following administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in rodents or monkeys are remarkably similar to those seen in idiopathic PD. In this review, the main characteristics of experimental models of PD induced by the neurotoxic compound MPTP are reviewed.Parkinson’s disease (PD) is one of the major neurodegenerative disorders. The etiology of this disease is likely due to combinations of environmental and genetic factors. Symptomatic hallmarks of PD a

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.

Localization of DJ-1 protein in the murine brain

Mutations in the DJ-1 gene have been identified to cause Parkinsons disease. In humans, nonmutated DJ-1 is expressed in specific brain areas but seems to be expressed by astrocytes rather than by neurons. In contrast, DJ-1 mRNA is mainly found in neurons in the mouse brain. We have investigated the distribution of DJ-1 protein in the mouse brain and found that DJ-1 protein is predominantly expressed by neurons but can also be detected in astrocytes. Consistent with a global role of DJ-1 in the brain, we found immunoreactivity, for example, in cortical areas, hippocampus, basolateral amygdala, the reticular nucleus of the thalamus, zona incerta, and locus coeruleus. Within the substantia nigra, however, DJ-1 is localized in both neuronal and nonneuronal cells, suggesting a distinct role in this area.

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.

FGF‐2 deficiency does not alter vulnerability of the dopaminergic nigrostriatal system towards MPTP intoxication in mice

Fibroblast growth factor 2 (FGF‐2) was the first growth factor discovered that exerted prominent protective and regenerative effects in an animal model of Parkinsons disease, the MPTP‐lesioned dopaminergic nigrostriatal system. To address the putative physiological relevance of endogenous FGF‐2 for midbrain dopaminergic neurons, we have analysed densities of tyrosine hydroxylase (TH)‐positive cells in the substantia nigra (SN) and TH‐positive fibers in the striatum and amygdala of adult FGF‐2‐deficient mice. We found that densities of TH‐immunoreactive (ir) cells in the SN as well as densities of TH‐ir fibers in the striatum and amygdala were unaltered as compared with wild‐type littermates. There is evidence to suggest that growth factor deficits do not become apparent unless a system is challenged in a lesioning paradigm. We therefore tested the ability of the nigrostriatal system with respect to its ability to cope with MPTP (1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine) intoxication. Treatment with 20 mg/kg MPTP on three consecutive days reduced dopamine levels in the striatum by about 80%. Densities of TH‐positive neurons in the SN were reduced by 71%. However, both parameters did not significantly differ between FGF‐2(–/–) mice and wild‐type littermates. Our results therefore suggest that FGF‐2, despite its prominent pharmacological potency as a neurotrophic factor for the dopaminergic nigrostriatal system, is not crucial for maintaining its structural integrity and ability to cope with MPTP intoxication.

Neurotrophin receptor heterozygosity causes deficits in catecholaminergic innervation of amygdala and hippocampus in aged mice

Summary.We have recently shown that aged mice with haploinsufficiencies for the neurotrophin receptors trkB, trkC or both, trkB and trkC, display reduced cell numbers in the substantia nigra and in the dentate gyrus, but not in the amygdala. Moreover, both hippocampus and amygdala contain increased numbers of degenerated axonal fragments. Consistent with this observation and the expression of trkB and trkC by midbrain dopaminergic neurons, we show now that heterozygous deletion of the trkB or/and trkC receptor genes significantly reduces catecholaminergic, tyrosine hydroxylase (TH-) positive fiber densities in the hippocampus and amygdala mainly in aged (21–23 month old) mice. In the amygdala the phenotype was restricted to the lateral and basolateral nucleus of the amygdala. In adult (6 month old) mice, reductions in catecholaminergic fiber densities were only found in the hippocampal area CA3 and the dentate gyrus of heterozygous trkB and trkB/C mice. Our observations suggest that signaling through trkB and trkC neurotrophin receptors is important for the maintenance of the catecholaminergic innervation of two limbic key regions, the hippocampus and amygdala.