The emerging role of circular RNAs in transcriptome regulation.

Circular RNAs (circRNAs) are a recently discovered form of RNA that has been found to regulate mammalian transcription. CircRNAs are covalently closed, single-stranded transcripts produced from precursor mRNA. While initially circRNAs were considered to be splicing artefacts, next-generation RNA sequencing of non-polyadenylated transcriptomes has recently shown that the expression of circRNAs is widespread and over 20% of expressed genes in examined cells and tissues can produce these transcripts. Until now thousands of circRNAs have been discovered in organisms ranging from Drosophila melanogaster to Homo sapiens. Functional studies indicate that these transcripts regulate expression of protein-coding linear transcripts and thus comprise an important component of gene expression regulation. Here we provide a comprehensive overview on the biology of circRNAs, including the expression patterns and function. Moreover, we discuss current methodologies for the discovery and validation of circular transcripts. Finally, perspectives on the utilization of circRNA as molecular markers of complex diseases are presented.

Reorganization of Synaptic Connections and Perineuronal Nets in the Deep Cerebellar Nuclei of Purkinje Cell Degeneration Mutant Mice.

The perineuronal net (PN) is a subtype of extracellular matrix appearing as a net-like structure around distinct neurons throughout the whole CNS. PNs surround the soma, proximal dendrites, and the axonal initial segment embedding synaptic terminals on the neuronal surface. Different functions of the PNs are suggested which include support of synaptic stabilization, inhibition of axonal sprouting, and control of neuronal plasticity. A number of studies provide evidence that removing PNs or PN-components results in renewed neurite growth and synaptogenesis. In a mouse model for Purkinje cell degeneration, we examined the effect of deafferentation on synaptic remodeling and modulation of PNs in the deep cerebellar nuclei. We found reduced GABAergic, enhanced glutamatergic innervations at PN-associated neurons, and altered expression of the PN-components brevican and hapln4. These data refer to a direct interaction between ECM and synapses. The altered brevican expression induced by activated astrocytes could be required for an adequate regeneration by promoting neurite growth and synaptogenesis.

Perisynaptic aggrecan-based extracellular matrix coats in the human lateral geniculate body devoid of perineuronal nets.

The extracellular matrix surrounds different neuronal compartments in the mature nervous system. In a variety of vertebrates, most brain regions are loaded with a distinct type of extracellular matrix around the somatodendritic part of neurons, termed perineuronal nets. The present study reports that chondrotin sulfate proteoglycan-based matrix is structured differently in the human lateral geniculate body. Using various chondrotin sulfate proteoglycan-based extracellular matrix antibodies, we show that perisomatic matrix labeling is rather weak or absent, whereas dendrites are contacted by axonal coats appearing as small, oval structures. Confocal laser scanning microscopy and electron microscopy demonstrated that these typical structures are associated with synaptic loci on dendrites. Using multiple labelings, we show that different chondrotin sulfate proteoglycan components of the extracellular matrix do not associate exclusively with neuronal structures but possibly associate with glial structures as well. Finally, we confirm and extend previous findings in primates that intensity differences of various extracellular matrix markers between magno- and parvocellular layers reflect functional segregation between these layers in the human lateral geniculate body.

Effect of leukotriene inhibitors on evolution of experimental brain contusions.

AIMS: Leukotriene levels increase in cerebrospinal fluid (CSF) following controlled cortical impact (CCI) injury in rats. We investigated the impact of two different leukotriene inhibitors in the CCI model on CSF leukotriene levels, brain water content (BWC), brain swelling (BS) contusion size and cellular response. METHODS: 134 male Sprague Dawley rats were investigated at 4, 24 and 72 h after CCI for CSF leukotriene levels and BWC/BS, lesion size in T2-weighted magnetic resonance imaging and immunohistochemistry. Animals received vehicle, MK-886, an inhibitor of 5-lipoxygenase activating protein, or Boscari(®) , a mixture of boswellic acids, acting as competitive nonredox 5-lipoxygenase inhibitors before trauma and then every 8 h until sacrifice. RESULTS: The intracranial pressure (ICP) was unaffected by treatment. Boscari treatment reduced CSF leukotriene C4 increase by -45% at 4 h (P < 0.03) and increase of BWC and BS by 49% (P < 0.05) and -58% at 24 h. Treatment with both substances showed a reduction of lesion volume at 72 h by -21% (P < 0.01) in T(2) -weighted magnetic resonance imaging, which was reflected in a smaller lesion area determined from a NeuN labelled section (-17% to -20%, P < 0.05). Triple immunofluorescence and Fluoro-Jade B staining showed rarefaction of neurones, glia and vasculature in the contusion core, whereas in the pericontusional zone astro- and microglia were upregulated in the presence of dying neurones. Treatment resulted in an improved survival of NeuN labelled neurones in the pericontusional cortex (+15% to +20%, P < 0.05). CONCLUSIONS: Leukotriene inhibition should be further investigated as therapeutic option to counteract secondary growth of traumatic brain contusions and to possibly improve pericontusional neuronal survival.

Cell specific quantitative iron mapping on brain slices by immuno-µPIXE in healthy elderly and Parkinson's disease.

Iron is essential for neurons and glial cells, playing key roles in neurotransmitter synthesis, energy production and myelination. In contrast, high concentrations of free iron can be detrimental and contribute to neurodegeneration, through promotion of oxidative stress. Particularly in Parkinson's disease (PD) changes in iron concentrations in the substantia nigra (SN) was suggested to play a key role in degeneration of dopaminergic neurons in nigrosome 1. However, the cellular iron pathways and the mechanisms of the pathogenic role of iron in PD are not well understood, mainly due to the lack of quantitative analytical techniques for iron quantification with subcellular resolution. Here, we quantified cellular iron concentrations and subcellular iron distributions in dopaminergic neurons and different types of glial cells in the SN both in brains of PD patients and in non-neurodegenerative control brains (Co). To this end, we combined spatially resolved quantitative element mapping using micro particle induced X-ray emission (µPIXE) with nickel-enhanced immunocytochemical detection of cell type-specific antigens allowing to allocate element-related signals to specific cell types. Distinct patterns of iron accumulation were observed across different cell populations. In the control (Co) SNc, oligodendroglial and astroglial cells hold the highest cellular iron concentration whereas in PD, the iron concentration was increased in most cell types in the substantia nigra except for astroglial cells and ferritin-positive oligodendroglial cells. While iron levels in astroglial cells remain unchanged, ferritin in oligodendroglial cells seems to be depleted by almost half in PD. The highest cellular iron levels in neurons were located in the cytoplasm, which might increase the source of non-chelated Fe3+, implicating a critical increase in the labile iron pool. Indeed, neuromelanin is characterised by a significantly higher loading of iron including most probable the occupancy of low-affinity iron binding sites. Quantitative trace element analysis is essential to characterise iron in oxidative processes in PD. The quantification of iron provides deeper insights into changes of cellular iron levels in PD and may contribute to the research in iron-chelating disease-modifying drugs.

Transgenic activation of Ras in neurons promotes hypertrophy and protects from lesion-induced degeneration.

Ras is a universal eukaryotic intracellular protein integrating extracellular signals from multiple receptor types. To investigate its role in the adult central nervous system, constitutively activated V12-Ha-Ras was expressed selectively in neurons of transgenic mice via a synapsin promoter. Ras-transgene protein expression increased postnatally, reaching a four- to fivefold elevation at day 40 and persisting at this level, thereafter. Neuronal Ras was constitutively active and a corresponding activating phosphorylation of mitogen-activated kinase was observed, but there were no changes in the activity of phosphoinositide 3-kinase, the phosphorylation of its target kinase Akt/PKB, or expression of the anti-apoptotic proteins Bcl-2 or Bcl-X(L). Neuronal Ras activation did not alter the total number of neurons, but induced cell soma hypertrophy, which resulted in a 14.5% increase of total brain volume. Choline acetyltransferase and tyrosine hydroxylase activities were increased, as well as neuropeptide Y expression. Degeneration of motorneurons was completely prevented after facial nerve lesion in Ras-transgenic mice. Furthermore, neurotoxin-induced degeneration of dopaminergic substantia nigra neurons and their striatal projections was greatly attenuated. Thus, the Ras signaling pathway mimics neurotrophic effects and triggers neuroprotective mechanisms in adult mice. Neuronal Ras activation might become a tool to stabilize donor neurons for neural transplantation and to protect neuronal populations in neurodegenerative diseases.

Enhanced Ras activity preserves dendritic size and extension as well as synaptic contacts of neurons after functional deprivation in synRas mice.

The monomeric GTP-binding protein p21Ras has been repeatedly implicated in neuronal stability and plastic changes of the adult nervous system. Recently, we have shown that expression of constitutively active Ras protein in transgenic synRas mice results in a significant increase in the dendritic size and complexity of differentiated pyramidal neurons as well as in increased synaptic connectivity. In the present study, we examined the organization of the vibrissae-barrel cortex in synRas mice and the effects of enhanced Ras activity on deprivation-induced dendritic reorganization after vibrissectomy. The results demonstrate a significant increase in vibrissae-barrel sizes and proportional spacing between barrels in synRas mice, suggesting that the neuronal target specificity of thalamocortical terminals is preserved. Accordingly, the arrangement of double bouquet cells at the borders of barrel columns ensuring functional distinctness is unchanged. Partial vibrissectomy is followed by significant dendritic regression of corresponding pyramidal neurons in the barrel cortex of wild-type mice, which, however, could not be observed in synRas mice. The results provide the first evidence for a role of Ras in preserving neuronal structure after functional deprivation in vivo.

Aggrecan-based extracellular matrix is an integral part of the human basal ganglia circuit.

The extracellular matrix is known to be involved in neuronal communication and the regulation of plastic changes, and also considered to protect neurons and synapses against damage. The goal of this study was to investigate how major extracellular matrix components (aggrecan, link protein, hyaluronan) constitute the pathways of the nigral system in the human basal ganglia circuit affected by neurodegeneration in Parkinson's disease. Here we show that aggrecan- and link protein-related components form clear regional distribution patterns, whereas hyaluronan is widely distributed in gray and white matter. Two predominant phenotypes of the aggrecan-based matrix can be discriminated: (1) perineuronal nets (PNs) and (2) axonal coats (ACs) encapsulating preterminal fibers and synaptic boutons. Clearly contoured PNs are associated with GABAergic projection neurons in the external and internal division of the globus pallidus, the lateral and reticular part of the substantia nigra, as well as subpopulations of striatal and thalamic inhibitory interneurons. Dopaminergic nigral neurons are devoid of PNs but are contacted to a different extent by matrix-coated boutons forming subnucleus-specific patterns. A very dense network of ACs is characteristic especially of the posterior lateral cell groups of the compact substantia nigra (nigrosome 1). In the subthalamic nucleus and the lateral thalamic nuclei numerous AC-associated axons were attached to principal neurons devoid of PNs. We conclude from the region-specific patterns that the aggrecan-based extracellular matrix is adapted to the fast processing of sensorimotor activities which are the therapeutic target of surgery and deep brain stimulation in the treatment of advanced stages of Parkinson's disease.

The neuronal extracellular matrix restricts distribution and internalization of aggregated Tau-protein.

Alzheimer's disease (AD) is a chronic degenerative disorder characterized by fibrillary aggregates of Aß and Tau-protein. Formation and progression of these pathological hallmarks throughout the brain follow a specific spatio-temporal pattern which provides the basis for neuropathological staging. Previously, we could demonstrate that cortical and subcortical neurons are less frequently affected by neurofibrillary degeneration if they are enwrapped by a specialized form of the hyaluronan-based extracellular matrix (ECM), the so called 'perineuronal net' (PN). PNs are composed of large aggregating chondroitin sulfate proteoglycans connected to a hyaluronan backbone, stabilized by link proteins and cross-linked via tenascin-R. Recently, PN-associated neurons were shown to be better protected against iron-induced neurodegeneration compared to neurons without PN, indicating a neuroprotective function. Here, we investigated the role of PNs in distribution and internalization of exogenous Tau-protein by using organotypic slice cultures of wildtype mice as well as mice lacking the ECM-components aggrecan, HAPLN1 or tenascin-R. We could demonstrate that PNs restrict both distribution and internalization of Tau. Accordingly, PN-ensheathed neurons were less frequently affected by Tau-internalization, than neurons without PN. Finally, the PNs as well as their three investigated components were shown to modulate the processes of distribution as well as internalization of Tau.

Phosphorylation of the tau protein sequence 199-205 in the hippocampal CA3 region of Syrian hamsters in adulthood and during aging.

Paired helical filaments formed by the abnormally phosphorylated microtubule-associated tau are a main sign of Alzheimer's disease and other neurodegenerative disorders. The hippocampal CA3 region, a brain region with a high degree of synaptic plasticity, is known to be strongly involved in tau hyperphosphorylation in several neurodegenerative diseases. In addition, reversible tau phosphorylation was observed during hibernation in European ground squirrels. The present study provides data on the tau phosphorylation status in the hippocampus of euthermic Syrian hamsters (Mesocricetus auratus), laboratory animals potentially prone to hibernation. Mossy fibers in the CA3 region of all investigated hamsters were immunostained using an antiserum detecting phospho-serine 199 of tau. A similar staining pattern was obtained with CP-13 detecting phospho-serine 202. In contrast, the monoclonal antibody AT8, recognizing both phosphorylated serine 202 and threonine 205, stained the CA3 region only in old hamsters. These findings implicate an additional link between aging, tau phosphorylation and synaptic plasticity. Furthermore, the presented data allow analyses whether tau phosphorylation is reversible in these facultative hibernators and versatile laboratory animal as it was recently shown for the hibernation cycle of European ground squirrels.

Alzheimer's disease as a loss of differentiation control in a subset of neurons that retain immature features in the adult brain.

Degeneration in AD primarily occurs in a subset of neurons that in the adult brain retain a high degree of structural plasticity and in these neurons is associated with the activation of mitogenic pathways and a cell cycle re-entry. Brain areas affected by AD pathology are those structures involved in the regulation of "higher brain functions" that become increasingly predominant as the evolutionary process of encephalization progresses, such as hippocampus, neocortical association areas and the cholinergic basal forebrain neurons. The functions these areas subserve such as learning, memory, perception, self-awareness, and consciousness require a life-long re-fitting of synaptic contacts that allows for the acquisition of new epigenetic information. This adaptive reorganization of neuronal connectivity in the mature brain is based upon the strengthening of existing synapses, the formation of new synapses and the destabilization of previously established synaptic contacts. With the increasing need during evolution to organize brain structures of increasing complexity, these processes of dynamic stabilization and de-stabilization become more and more important but might also provide the basis for an increasing rate of failure. A hypothesis is proposed that it is the 'labile state of differentiation' (G0-arrest) of a subset of neurons in the adult brain that allows for ongoing morphoregulatory processes after development is completed but at the same time renders these neurons particularly vulnerable. The delicate balance between G0-arrest and G1-entry might be prone to a variety of potential disturbances during the lifetime of an individual. Morphodysregulation in AD, accompanied by an activation of intracellular mitogenic signaling might, thus, be a slowly progressing dysfunction that eventually overrides the differentiation control and results in dedifferentiation, a condition in conflict with the otherwise 'mature' background of the nervous system. Cell-cycle and differentiation control might thus provide the link between structural brain self-organization and neurodegeneration that both are unique to human.

Alzheimer's disease as a presumptive threshold phenomenon.

With the example of the basal forebrain cholinergic system as a site of primary importance in AD, the existence of a critical neuron loss as a threshold between normal aging and AD is suggested. If the extent of degeneration exceeds this threshold the system decompensates and the clinical picture becomes apparent. The fully developed stage of AD might, therefore, represent a condition beyond the capacity of compensation where plastic adaptive changes are still present but functionally insufficient.

Postmortem changes in the phosphorylation state of tau-protein in the rat brain.

The phosphorylation state of tau-protein is crucial for the regulation of neuronal microtubule organization. Functional conclusions on tau-protein require an accurate assessment of phosphorylated sites. Therefore, the in vivo distribution and postmortem preservation of some phospho-epitopes on tau-protein were examined in the rat brain under different fixation and preparation conditions. Detection of tau-protein with a phosphorylation-independent antiserum revealed both axonal and somatodendritic localizations, which were not influenced by a postmortem interval of 30 min. The phospho-epitopes recognized by 12E8, AT8, and PHF-1 were mainly localized in the somatodendritic compartment. The binding sites of AT8 and PHF-1 were rapidly dephosphorylated postmortem, whereas the Tau-1 epitope was unmasked in the somatodendritic region. The axonally located phospho-epitope of AT270 and the nuclear epitope of AT100 were still detectable after a postmortem interval of 30 min. Postmortem dephosphorylation and inhibition of this process by PP1 and/or PP2A was further demonstrated on Western blot. In conclusion, rapid processing of tau-protein is essential for the correct assessment of investigations on phospho-isoforms.

Phosphorylation of tau, Abeta-formation, and apoptosis after in vivo inhibition of PP-1 and PP-2A.

Chronic inhibition of protein phosphatases 1 and 2A in vivo was induced by infusion of okadaic acid into lateral ventricles of rat brain for up to 4 months. Cytoskeletal pathology, alterations of the amyloid precursor protein, and apoptotic cell death induced by this treatment followed a certain sequence and spatial distribution. Changes in the expression, phosphorylation, and subcellular distribution of neurofilament proteins and tau, as well as first signs of apoptotic cell death, occurred already after about 2 weeks. The distribution of apoptotic cells, however, was different from those revealing a high accumulation of hyperphosphorylated tau, indicating that those cytoskeletal pathology had no obvious sequelae for the viability of these neurones. A continuation of treatment for longer than 2 weeks induced diffuse deposits of both hyperphosphorylated tau and A beta-amyloid-immunoreactive material in white matter areas that increased in size and number over time. Because tau-phosphorylation is a regulator of the dynamic stability of microtubules, the pathology observed in the present experimental paradigm in the white matter might be viewed as an indication of a disturbed axonal transport. It is hypothesized that perturbations of the axonal transport might also be critically involved in the formation of paired helical filaments and amyloid deposits in Alzheimer's disease.

Progression of neurofibrillary changes and PHF-tau in end-stage Alzheimer's disease is different from plaque and cortical microglial pathology.

In terminal Alzheimer's disease (AD) the frequency of plaques was found to be reduced in single cases. To test this finding in a larger sample, and in order to determine whether the number of plaques labeled with different markers and the distribution of neurofibrillary tangles are correlated positively to each other and to the degree of dementia, a sample of 134 autopsy brains with and 15 without AD-related pathology has been examined. All of the cases were staged according to Braak and Braak. Both the frequency of plaques immunopositive for beta-amyloid, amyloid precursor protein, and apolipoprotein E and that of microglial cells in the cortex and in the white matter were determined semiquantitatively. The content and distribution of PHF-tau was ascertained by ELISA and immunohistochemistry. Both the clinical dementia rating and the global deterioration scale were used as clinical parameters retrospectively. Correlation coefficients were calculated for all parameters and differences were evaluated statistically. With progressive distribution of neurofibrillary tangles and increasing content of PHF-tau the plaque stages and the degree of cortical microglia reaction increased up to the Braak-stages IV and V, thereafter showing a slightly decreasing tendency in the investigated regions. In end-stage AD resorption of beta-amyloid seems to surpass its deposition. The microglial reaction in the white matter correlated neither with the Braak-stage nor with the accumulation of amyloid. With regard to the degree of dementia, both scales correlated well with the pathological changes. Our data show that neuronal cytoskeletal alterations progressively increase with progressive dementia until the end stage of AD in contrast to the frequencies of plaques and cortical microglial cells, and are therefore preferable for staging purposes.

Hyperphosphorylated protein tau is restricted to neurons devoid of perineuronal nets in the cortex of aged bison.

Hyperphosphorylated tau in the cortex and hippocampal formation of two aged bisons was characterized by its immunoreactivity to the phospho-epitope-recognizing monoclonal antibodies AT8, AT100, PHF-1 and TG-3. Gallyas silver staining revealed sparsely scattered cortical tangles and neuropil threads. In dual-peroxidase staining experiments, the immunocytochemical detection of vulnerable neurons was combined with the demonstration of chondroitin sulphate proteoglycan-rich perineuronal nets of the extracellular matrix. Such polyanionic lattice-like neuronal coatings were revealed lectin- and immunocytochemically. Hyperphosphorylated tau was exclusively observed in neurons devoid of perineuronal nets. The present findings in the aged bison parallel previously obtained results from a quantitative study of human brains affected by Alzheimer's disease. In conclusion, the low susceptibility of different types of neurons to the abnormal phosphorylation of tau corresponds to high proportions of certain chondroitin sulphate proteoglycans in their microenvironment.

Hippocampal volume discriminates between normal cognition; questionable and mild dementia in the elderly.

The sensitivity of MRI volumetric measures to detect cognitive dysfunction is examined in 39 participants of an epidemiological field study (age 75-85, MMSE 19-30). According to Clinical dementia rating (CDR), 17 subjects had normal cognition (CDR 0), 12 had questionable (CDR 0.5) and 10 mild dementia (CDR 1). Discriminant analysis based on four hippocampal measures resulted in a correct classification of 76.9% of all subjects. Left-sided and posterior hippocampal measures were more responsible for group discrimination than right-sided and anterior measures. In CDR 0.5, a significant hippocampal volume reduction of 14.3% vs.11.3% (left vs. right) relative to normal was found. The right hippocampus was significantly greater than the left in CDR 0 and CDR 0.5, but not in CDR 1. The magnitude of non-directional hippocampal asymmetry increased with decreasing cognitive state. We conclude that hippocampal atrophy is sensitive to detect cognitive dysfunction and subjects at risk for Alzheimer's disease in the elderly population.

Distribution of isoforms of the microtubule-associated protein tau in grey and white matter areas of human brain: a two-dimensional gelelectrophoretic analysis.

The microtubule-associated protein tau in human brain consists of six molecular isoforms derived from a single gene by alternative mRNA-splicing and further modified by posttranslational processing. In the present study, the distribution of tau isoforms in grey and white matter of human temporal cortex was investigated by two-dimensional gelelectrophoresis. More than 80 isoforms were detected. The pattern of isoforms obtained after treatment with alkaline phosphatase was still more complex than those of recombinant tau, indicating that posttranslational modifications other than phosphorylation contribute to the molecular heterogeneity of tau. The tau isoform D according to Goedert containing four tubulin-binding regions shown to promote tubulin polymerisation most efficiently was present in higher amounts in white as compared to grey matter. The pattern of isoform distribution was not significantly altered in Alzheimer's disease. It is concluded that molecular isoforms that differ in their tubulin-binding characteristics are differentially distributed in subcellular neuronal compartments and/or neuronal types.

Dendritic reorganisation in the basal forebrain under degenerative conditions and its defects in Alzheimer's disease. I. Dendritic organisation of the normal human basal forebrain.

In the present study, the dendritic organisation of neurones in the normal human basal forebrain was analysed as a prerequisite for the evaluation of pathological changes occurring in Alzheimer's disease and related conditions (see other Arendt et al. papers in this issue). Neurones in the basal nucleus of Meynert (NbM), the nucleus of the vertical limb of the diagonal band, and the medial septal nucleus were examined after Golgi impregnation. Cells were classified according to the dendritic branching pattern and soma shape as either reticular neurones or multipolar giant neurones. The reticular type of neurones constitutes more than 90% of neurones in the BnM. Cholinergic neurones also belong to this cell type. Reticular neurones were further subdivided into four subtypes. Morphological features and arrangement of reticular basal forebrain neurones were identical to those described for "reticular formation cells" or "isodendritic" neurones. Dendritic trees of reticular neurones show a spatial orientation perpendicular to passing fibres as well as a high degree of overlap, both of which are hallmarks of "open nuclei." The qualitative classification of Golgi-impregnated basal forebrain neurones was substantiated by a computer-based three-dimensional analysis. Topologic and metric parameters of the dendritic tree were calculated for each type of neurone to characterise the degree of dendritic branching, the shape and orientation of the dendritic arborisation, the spatial extension of the dendritic tree, and soma size. The classification criteria were evaluated according to their power of discrimination between different cell types by means of a discriminant analysis. The quantitative approach applied in the present study not only provides an objective measure for the description and comparison of the structure of various types of neurones but also makes it possible to elucidate fine structural changes that might occur under pathologic conditions and that are not evident during qualitative studies alone.

Dendritic reorganisation in the basal forebrain under degenerative conditions and its defects in Alzheimer's disease. III. The basal forebrain compared with other subcortical areas.

The distribution of the reticular neuronal type in the human brain and its involvement in both degeneration and dendritic reorganisation under the conditions of ageing, Korsakoff's disease (KD), Alzheimer's disease (AD), and Parkinson's disease (PD) was comparatively investigated after Golgi impregnation. Reticular neurones are distributed throughout different areas along the brain axis. The cholinergic basal forebrain nuclei, i.e., the basal nucleus of Meynert, the nucleus of the diagonal band, and the medial septal nucleus form the most rostral part of this network of "open nuclei," which is collectively referred to as the "reticular core." Reticular neurones of the following areas were quantitatively investigated by a computer-based three-dimensional analysis: caudate nucleus, globus pallidus, medial septal nucleus, nucleus of the vertical limb of the diagonal band, basal nucleus, medial amygdaloid nucleus, reticular thalamic nucleus, lateral hypothalamic area, subthalamic nucleus, substantia nigra, locus coeruleus, pedunculopontine tegmental nucleus, and raphe magnus nucleus. There are three major findings. First, neurones that were found to be susceptible to degeneration in AD were largely part of the same neuronal populations prone to degeneration during ageing, in KD and PD. Thus, areas could be classified according to their overall degree of vulnerability under the present degenerative conditions as being highly vulnerable (basal forebrain nuclei, caudate nucleus, locus coeruleus), moderately vulnerable (medial amygdaloid nucleus, raphe magnus nucleus, lateral hypothalamic area, substantia nigra, pedunculopontine tegmental nucleus), or marginally vulnerable (globus pallidus, subthalamic nucleus, reticular thalamic nucleus). Second, neuronal populations that are particularly vulnerable to degenerative changes show a high degree of structural plasticity. Third, the degree of this dendritic plasticity is inversely related to the complexity of dendritic arborisation of the neurone. It is concluded that the sparsely ramified reticular type of neurone forms a pool of pluripotent neurones that have retained their plastic capacity throughout life, which makes them vulnerable to a variety of perturbations.