Somatostatin-like immunoreactivity within neuritic plaques

Somatostatin Ameliorates β-Amyloid-Induced Cytotoxicity via the Regulation of CRMP2 Phosphorylation and Calcium Homeostasis in SH-SY5Y Cells

Somatostatin is involved in the regulation of multiple signaling pathways and affords neuroprotection in response to neurotoxins. In the present study, we investigated the role of Somatostatin-14 (SST) in cell viability and the regulation of phosphorylation of Collapsin Response Mediator Protein 2 (CRMP2) (Ser522) via the blockade of Ca²⁺ accumulation, along with the inhibition of cyclin-dependent kinase 5 (CDK5) and Calpain activation in differentiated SH-SY5Y cells. Cell Viability and Caspase 3/7 assays suggest that the presence of SST ameliorates mitochondrial stability and cell survival pathways while augmenting pro-apoptotic pathways activated by Aβ. SST inhibits the phosphorylation of CRMP2 at Ser522 site, which is primarily activated by CDK5. Furthermore, SST effectively regulates Ca²⁺ influx in the presence of Aβ, directly affecting the activity of calpain in differentiated SH-SY5Y cells. We also demonstrated that SSTR2 mediates the protective effects of SST. In conclusion, our results highlight the regulatory role of SST in intracellular Ca²⁺ homeostasis. The neuroprotective role of SST via axonal regeneration and synaptic integrity is corroborated by regulating changes in CRMP2; however, SST-mediated changes in the blockade of Ca²⁺ influx, calpain expression, and toxicity did not correlate with CDK5 expression and p35/25 accumulation. To summarize, our findings suggest two independent mechanisms by which SST mediates neuroprotection and confirms the therapeutic implications of SST in AD as well as in other neurodegenerative diseases where the effective regulation of calcium homeostasis is required for a better prognosis.

Histological characterization of interneurons in Alzheimer's disease reveals a loss of somatostatin interneurons in the temporal cortex

Neuronal dysfunction and synaptic loss are major hallmarks of Alzheimer's disease (AD) which correlate with symptom severity. Impairment of the γ-aminobutyric acid (GABA)ergic inhibitory interneurons, which form around 20% of the total neuronal network, may be an early event contributing to neuronal circuit dysfunction in neurodegenerative diseases. This study examined the expression of two of the main classes of inhibitory interneurons, parvalbumin (PV) and somatostatin (SST) interneurons in the temporal cortex and hippocampus of AD and control cases, using immunohistochemistry. We report a significant regional variation in the number of PV and SST interneurons with a higher number identified per mm² in the temporal cortex compared to the hippocampus. Fewer SST interneurons, but not PV interneurons, were identified per mm² in the temporal cortex of AD cases compared to control subjects. Our results support regional neuroanatomical effects on selective interneuron classes in AD, and suggest that impairment of the interneuronal circuit may contribute to neuronal dysfunction and cognitive decline in AD.

Somatostatin in Alzheimer's disease: A new Role for an Old Player

The amyloid beta (Aβ) peptide is central to the pathogenesis of Alzheimer's disease (AD). Insights into Aβ-interacting proteins are critical for understanding the molecular mechanisms underlying Aβ-mediated toxicity. We recently undertook an in-depth in vitro interrogation of the Aβ1–42 interactome using human frontal lobes as the biological source material and taking advantage of advances in mass spectrometry performance characteristics. These analyses uncovered the small cyclic neuropeptide somatostatin (SST) to be the most selectively enriched binder to oligomeric Aβ1–42. Subsequent validation experiments revealed that SST interferes with Aβ fibrillization and promotes the formation of Aβ assemblies characterized by a 50–60 kDa SDS-resistant core. The distributions of SST and Aβ overlap in the brain and SST has been linked to AD by several additional observations. This perspective summarizes this body of literature and draws attention to the fact that SST is one of several neuropeptide hormones that acquire amyloid properties before their synaptic release. The latter places the interaction between SST and Aβ among an increasing number of observations that attest to the ability of amyloidogenic proteins to influence each other. A model is presented which attempts to reconcile existing data on the involvement of SST in the AD etiology.

Functional Amyloids and their Possible Influence on Alzheimer Disease

Amyloids play critical roles in human diseases but have increasingly been recognized to also exist naturally. Shared physicochemical characteristics of amyloids and of their smaller oligomeric building blocks offer the prospect of molecular interactions and crosstalk amongst these assemblies, including the propensity to mutually influence aggregation. A case in point might be the recent discovery of an interaction between the amyloid β peptide (Aβ) and somatostatin (SST). Whereas Aβ is best known for its role in Alzheimer disease (AD) as the main constituent of amyloid plaques, SST is intermittently stored in amyloid-form in dense core granules before its regulated release into the synaptic cleft. This review was written to introduce to readers a large body of literature that surrounds these two peptides. After introducing general concepts and recent progress related to our understanding of amyloids and their aggregation, the review focuses separately on the biogenesis and interactions of Aβ and SST, before attempting to assess the likelihood of encounters of the two peptides in the brain, and summarizing key observations linking SST to the pathobiology of AD. While the review focuses on Aβ and SST, it is to be anticipated that crosstalk amongst functional and disease-associated amyloids will emerge as a general theme with much broader significance in the etiology of dementias and other amyloidosis.

Interneurons in the human olfactory system in Alzheimer's disease

The principal olfactory structures display Alzheimer's disease (AD) related pathology at early stages of the disease. Consequently, olfactory deficits are among the earliest symptoms. Reliable olfactory tests for accurate clinical diagnosis are rarely made. In addition, neuropathological analysis postmortem of olfactory structures is often not made. Therefore, the relationship between the clinical features and the underlying pathology is poorly defined. Traditionally, research into Alzheimer's disease has focused on the degeneration of cortical temporal projection neurons and cholinergic neurons. Recent evidence has demonstrated the neurodegeneration of interneuron populations in AD. This review provides an updated overview of the pathological involvement of interneuron populations in the human olfactory system in Alzheimer's disease.

Anatomical, physiological and molecular properties of Martinotti cells in the somatosensory cortex of the juvenile rat

Whole-cell patch-clamp recordings followed by histochemical staining and single-cell RT-PCR were obtained from 180 Martinotti interneurones located in layers II to VI of the somatosensory cortex of Wistar rats (P13-P16) in order to examine their anatomical, electrophysiological and molecular properties. Martinotti cells (MCs) mostly displayed ovoid-shaped somata, bitufted dendritic morphologies, and axons with characteristic spiny boutons projecting to layer I and spreading horizontally across neighbouring columns more than 1 mm. Electron microscopic examination of MC boutons revealed that all synapses were symmetrical and most synapses (71%) were formed onto dendritic shafts. MCs were found to contact tuft, apical and basal dendrites in multiple neocortical layers: layer II/III MCs targeted mostly layer I and to a lesser degree layer II/III; layer IV MCs targeted mostly layer IV and to a lesser degree layer I; layer V and VI MCs targeted mostly layer IV and layer I and to a lesser degree the layer in which their somata was located. MCs typically displayed spike train accommodation (90%; n = 127) in response to depolarizing somatic current injections, but some displayed non-accommodating (8%) and a few displayed irregular spiking responses (2%). Some accommodating and irregular spiking MCs also responded initially with bursts (17%). Accommodating responses were found in all layers, non-accommodating mostly in upper layers and bursting mostly in layer V. Single-cell multiplex RT-PCR performed on 63 MCs located throughout layers II-VI, revealed that all MCs were somatostatin (SOM) positive, and negative for parvalbumin (PV) as well as vasoactive intestinal peptide (VIP). Calbindin (CB), calretinin (CR), neuropeptide Y (NPY) and cholecystokinin (CCK) were co- expressed with SOM in some MCs. Some layer-specific trends seem to exist. Finally, 24 accommodating MCs were examined for the expression of 26 ion channel genes. The ion channels with the highest expression in these MCs were (from highest to lowest); Cabeta1, Kv3.3, HCN4, Cabeta4, Kv3.2, Kv3.1, Kv2.1, HCN3, Caalpha1G, Kv3.4, Kv4.2, Kv1.1 and HCN2. In summary, this study provides the first detailed analysis of the anatomical, electrophysiological and molecular properties of Martinotti cells located in different neocortical layers. It is proposed that MCs are crucial interneurones for feedback inhibition in and between neocortical layers and columns.

Distribution of glutamate receptor subunit NMDAR1 in the hippocampus of normal elderly and patients with Alzheimer's disease

Immunocytochemical techniques were employed to study the distribution and cytological features of NMDAR1-immunoreactive elements in the human hippocampal formation. Subjects with Alzheimer's disease (AD), presenting with a wide range of neuropathology and classified into six Braak stage (I-VI), and nondemented age-matched controls were examined. In control cases, the most intense NMDAR1 immunoreactivity was observed within the soma and dendrites of granule cells in the dentate gyrus and pyramidal neurons in Ammon's horn. Whereas small variations in the pattern of immunoreactivity were noted in control cases, AD subjects were characterized with intersubject variability which in most instances correlated with neuropathologic severity. For example, AD cases, particularly those with mild/modest pathology (Braak I-III), were indistinguishable from controls in the overall pattern of immunolabeling. In contrast, in those more severe AD cases (Braak IV-VI) the intensity of immunolabeling within the CA fields was greater than observed in controls and those with mild AD pathology. In addition, in pathologically severe cases numerous NMDAR1-positive pyramidal neurons were characterized by unique morphologic features including long and often tortuous apical dendrites. These latter findings were most prevalent in the CA1 region and subiculum. In contrast to the marked increase in immunolabeling in the CA fields, in the dentate gyrus we observed a reduction in NMDAR1 labeling particularly within the outer molecular layer (i.e., termination zone of the perforant pathway). This latter region was also the site of a number of NMDAR1-labeled plaques. Notably, the overall pattern of NMDAR1 immunoreactivity is distinct from that observed with antibodies against AMPA receptor subunits and suggests a differential role of various inotropic glutamate receptors in hippocampal plasticity in AD.

Early association of reactive astrocytes with senile plaques in Alzheimer's disease

The fibrillar beta-amyloid protein (A beta) plaques of Alzheimer's disease (AD) are associated with reactive astrocytes and dystrophic neurites and have been suggested to contribute to neurodegenerative events in the disease. We recently reported parallel in vitro and in situ findings, suggesting that the adoption of a reactive phenotype and the colocalization of astrocytes with plaques in AD may be mediated in large part by aggregated A beta. Thus, A beta-mediated effects on astrocytes may directly affect disease progression by modifying the degenerative plaque environment. Alternatively, plaque-associated reactive astrocytosis may primarily represent a glial response to the neural injury associated with plaques and not significantly contribute to AD pathology. To investigate the validity of these two positions, we examined the differential colocalization of reactive astrocytes and dystrophic neurites with plaques. Hippocampal sections from AD brains--ranging in neuropathology from mild to severe--were triple-labeled with antibodies recognizing A beta protein, reactive astrocytes, and dystrophic neurites. We observed not only plaques containing both or neither cell type, but also plaques containing (1) reactive astrocytes but not dystrophic neurites and (2) dystrophic neurites but not reactive astrocytes. The relative proportion of plaques colocalized with reactive astrocytes in the absence of dystrophic neurites is relatively high in mild AD but significantly decreases over the course of the disease, suggesting that plaque-associated astrocytosis may be an early and perhaps contributory event in AD pathology rather than merely a response to neuronal injury. These data underscore the potentially significant contributions of reactive astrocytosis in modifying the plaque environment in particular and disease progression in general.

Somatostatin messenger RNA-containing neurons in Alzheimer's disease: an in situ hybridization study in hippocampus, parahippocampal cortex and frontal cortex

The level of expression of somatostatin messenger RNA-containing neurons in human brain was visualized and quantified by in situ hybridization with a 35S-labelled oligonucleotide complementary to amino acids 96-111 of the preprosomatostatin complementary DNA sequence. The analysis was carried out in the frontal and parahippocampal cortices and hippocampus of six age- and post mortem delay-matched Alzheimer's disease and control brains. By northern blot analysis, in frontal cortex samples, 18S rRNA degradation was identical in control and Alzheimer brains and somatostatin messenger RNAs migrated as a single band of 1 kb. By in situ hybridization, specificity was demonstrated by abolition of the signal using either an excess of unlabelled antisense probe or using a labelled sense probe. Somatostatin messenger RNA-containing neurons displayed a similar regional and subregional distribution in control subjects and patients with Alzheimer's disease, being more abundant in the frontal cortex, followed by the hippocampus and the parahippocampal cortex. An overall reduction of labelled cell density was observed in patients with Alzheimer's disease (frontal cortex gray matter:--41%; white matter:--66%; hippocampus:--44%; parahippocampal cortex white matter:--40%). Due to a great variation between brains, this decrease only reached significance in the parahippocampal cortex (-59%, P < 0.05). A significantly lower level of expression of somatostatin messenger RNA per somatostatinergic cell was observed in the hippocampus of Alzheimer's disease patients (-47%, P < 0.05), but not in frontal cortex gray (-17%) and white (-36%) matter and parahippocampal cortex gray (-42%) and white (-29%) matter. These data are in accordance with the distribution of somatostatin cells as visualized by immunohistochemistry in human brain. They indicate that the ability of cortical cells to express somatostatin messenger RNA is partially preserved in Alzheimer disease brains and that the decrease in the amount of somatostatin messenger RNA per cell is restricted to the hippocampal formation.

Cortical and subcortical neuropeptides in Alzheimer's disease

Given the clinical features of AD, the severe atrophy of cerebral cortex that accompanies the disease, and the predominant cortical location of plaques and tangles, it is not surprising to find the most consistent changes in neuropeptides in this disease occurring in the cerebral cortex. The neuropeptide changes that have been reproducibly demonstrated in AD are reduced hippocampal and neocortical SS and CRF concentrations and a reduced CSF level of SS. In cerebral cortex, SS and CRF are found in GABAergic local circuit neurons in layers II, III, and VI. The function of these neurons is not well established, although these cells may act to integrate the flow of incoming and outgoing information in cerebral cortex. If this is true, then dysfunction of this integration could produce widespread failure of cerebrocortical function, resulting in the various neurobehavioral deficits seen in AD. The interpretation of neuropeptide changes in subcortical brain regions, either those that project to cortex, or those that are the efferent targets of cortical projections, is also uncertain. The observed neuropeptide abnormalities in these brain regions in AD are less consistent than are those seen in cerebral cortex. Perhaps the most intriguing result in these regions is the increases in galanin-immunoreactive terminals seen in the nucleus basalis of AD brains. Galanin has been shown to inhibit acetylcholine release and to impair memory function in rats (46,113).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical distribution of peptidergic and cholinergic fibers in the human amygdala: their depletion in Alzheimer's disease and morphologic alteration in non-demented elderly with numerous senile plaques

As part of an ongoing investigation devoted to understanding the pathogenesis of senile plaques, we employed histochemical and immunocytochemical techniques to examine the distribution and cytologic features of acetylcholinesterase (AChE), choline acetyltransferase (ChAT), somatostatin (SOM), neurotensin (NT) and substance P (SP) containing fibers and neurons within the amygdala of: (1) patients with Alzheimer's disease (AD); (2) age-matched non-demented controls (NC); and (3) a group of non-demented cases, who upon postmortem neuropathologic examination exhibited sufficient numbers of senile plaques to be classified as AD. This latter group was referred to as high plaque non-demented (HPND). For every case, the distribution of immunolabeled fibers and neurons were determined for each transmitter throughout the various subnuclei of the amygdala. In addition, in the AD and HPND cases the topographic distribution of senile plaques was determined throughout the amygdala using thioflavine-S and Bielschowsky silver methods. In the amygdala, the distribution and density of senile plaques were not bound by conventional cytoarchitectural groupings but rather were most dense in the ventromedial regions of the amygdala with decreasing density in dorsal and lateral directions. Importantly, the density and distribution of senile plaques failed to correlate with the normal topography and/or density of the various peptidergic or cholinergic fibers within the amygdala. The finding that plaques do not correlate with the topographic distribution of any specific transmitter system suggests that plaques likely do not arise from the degeneration of a single neurotransmitter system (i.e., the cholinergic system). However, the finding that in AD a transmitter is most markedly depleted in regions of greatest plaque density, suggests certain constituents of the plaque (e.g. beta-amyloid) may be contributing to the degeneration of local fibers. The extent to which a transmitter was depleted in AD patients varied considerably among those four investigated with the cholinergic and NT systems displaying the most dramatic reductions, followed by SP and SOM. Despite these differential reductions in fiber density, all four neurotransmitters were found localized within dystrophic neurites and in most instances these dystrophic neurites were associated with thioflavine-positive senile plaques. In contrast to the AD cases, the HPND cases were characterized by no significant reductions in immunolabeled fibers, although immunostained dystrophic neurites were very prevalent in the HPND cases. These data suggest that dystrophic neurites occur very early in the disease process and likely precede the actual loss of fibers when or if it occurs.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced cerebral cortical but elevated striatal concentration of somatostatin-like immunoreactivity in dominantly inherited olivopontocerebellar atrophy

Somatostatin-like immunoreactivity (SLI) was measured in the brains of nine patients with dominantly inherited olivopontocerebellar atrophy (OPCA), who all had a marked deficit of the cholinergic marker choline-acetyltransferase (ChAT) in the cerebral cortex and striatum. Mean concentrations of SLI in OPCA were significantly reduced by 42-58% in parietal and occipital cortices and frontal cortical eye fields, but were normal in other cortical areas, including two subdivisions of the temporal cortex which show marked depletions of both SLI and ChAT in Alzheimer's disease. This dissociation of SLI and ChAT indicates that a cortical cholinergic deficit does not invariably lead to reduction of somatostatin. In the caudate nucleus, the region of OPCA brain having the most severe ChAT deficit (-81%), SLI levels were significantly elevated by 46% and were negatively and significantly correlated with ChAT activities (r = -0.66). The SLI alterations could be due to abnormal somatostatin metabolism or release, or an increased number of somatostatin-containing neurons and could contribute to the brain dysfunction of OPCA.

The role of cortical connectivity in Alzheimer's disease pathogenesis: a review and model system

Here we review current evidence in support of the cortical disconnection/cortical connectivity model of Alzheimer disease (AD) pathogenesis, a model which predicts that one of the first events in AD is damage to the entorhinal cortex and/or subiculum resulting in the disconnection of the hippocampal formation and neocortex, and the subsequent progression of the disease in a stepwise fashion along cortico-cortical connections. Much of the evidence for this model has been obtained from studies involving the limbic system where investigators have demonstrated a precise correspondence between established patterns of connectivity and the degenerative changes associated with AD. In addition, some studies of the distribution of neuritic plaques (NP) and neuro-fibrillary tangles (NFT) in the neocortex and subcortical structures have yielded corroborative data. The validity of the cortical disconnection/connectivity model in the neocortex remains to be established or refuted. We propose that testing of this model can be accomplished with systematic studies of the laminar and regional distribution of NP and NFT in a series of sequentially interconnected cytoarchitectural regions that also form part of two functional hierarchies--the paralimbic and occipitotemporal visual systems. To adequately control for variation between brains affected by AD, it is imperative that such studies be conducted in a large but varied population of AD cases exhibiting differences in several variables, including clinical and/or neuropathological severity of the disease, temporal duration of the disease, and clinical/neuropsychological profile. We believe that further understanding of the relationship between characteristic AD pathology and intrinsic anatomico-functional circuits will contribute not only to our comprehension of AD pathogenesis but also to our general knowledge of the human brain.

NADPH-diaphorase-positive cell populations in the human amygdala and temporal cortex: neuroanatomy, peptidergic characteristics and aspects of aging and Alzheimer's disease

Previous studies have shown that nerve cells containing NADPH-diaphorase (NADPH-d) are relatively resistant to various damaging processes. NADPH-d has been found to be colocalized with somatostatin (SOM) and neuropeptide Y (NPY) in neuronal populations of several forebrain regions. We have investigated the anatomical distribution, morphology and cell sizes of NADPH-d neurons in amygdala and temporal cortex in Alzheimer's disease (AD) compared to controls of different age. NADPH-d cells and fibers were present in layers II-VI of the cortex and in the white matter below the cortical mantle. In the amygdaloid complex, NADPH-d cells and processes were observed in almost all subnuclei. In the amygdala of aged controls, only insignificant atrophic alterations of NADPH-d neurons and fibers were seen. In AD, a moderate, but significant shift towards an increased number of medium-to small-sized neurons was measured in amygdala and cortex, indicating cell shrinkage during the course of the disease. However, there were no differences when comparing NADPH-d staining in amygdaloid subregions in AD cases that contained numerous neuritic plaques (i.e., accessory basal nucleus) with areas that were relatively free of lesions (i.e., lateral nucleus). Analysis of cell size of SOM- and NPY-immunoreactive cells revealed only slight atrophic changes during aging. In AD, however, a significant atrophy of somatostatin neurons in temporal cortex was found, whereas no further cell shrinkage was noted for NPY as compared to aged controls. Colocalization tests demonstrated a large overlap between NPY, SOM and NADPH-d in the amygdala, whereas a subpopulation of cortical SOM neurons, predominantly localized in upper layers, showed a lack of NADPH-d. Our findings of a relative stability of a selective subclass of neurons during aging and AD support the hypothesis that cellular pathology may affect only specific neuronal populations while others might be spared.

Parvalbumin immunoreactive neurons in normal human temporal neocortex and in patients with Alzheimer's disease

Parvalbumin-immunoreactive (PARV-ir) neurons were studied in the temporal neocortex of 4 normal subjects and in 7 patients with Alzheimer's disease (AD) whose brains were removed from the skull between 1 and 4 h after death and immediately fixed by perfusion through the carotid arteries to minimize pitfalls related to delayed tissue processing. Freezing microtome sections were immunostained free-floating for PARV using a well characterized monoclonal antibody diluted at 1:5000 and the peroxidase-antiperoxidase method. PARV-ir cells predominated in layers III, IV and V and were classified as bitufted cells and small, medium and large multipolar neurons according to their dendritic arbors. Immunoreactive cell processes surrounding the soma of neighbouring cells and immunoreactive vertical strings of buttons were consistent, respectively, with terminal axons of basket cells and chandelier neurons. The number of PARV-ir cells in the superior (T1), middle (T2) and inferior (T3) temporal gyri was variable from one case to another in both normal and pathological cases. Only 1 of 7 patients with AD had significantly reduced numbers of PARV-ir neurons, thus suggesting that PARV-ir cells in the neocortex are relatively resistant to degeneration in Alzheimer's disease.

Regional distribution of somatostatin receptor binding and modulation of adenylyl cyclase activity in Alzheimer's disease brain

We have previously reported a reduction in the inhibitory effect of somatostatin on adenylyl cyclase activity in the superior temporal cortex of a group of Alzheimer's disease cases, compared to a group of matched controls. In the present study, the levels of high affinity 125I-Tyr11-somatostatin-14 binding, its modulation by guanine nucleotides and the effects of somatostatin on adenylyl cyclase activity have been measured in preparations of frontal cortex, hippocampus, caudate nucleus and cerebellum from the same patient and control groups. A significant reduction in 125I-Tyr11-somatostatin-14 binding was observed in the frontal cortex, but not other regions, of the Alzheimer's disease group, compared with control values. The profiles of inhibition of specific 125I-Tyr11-somatostatin-14 binding by Gpp(NH)p were similar in all regions in both groups. No significant differences in basal, forskolin-stimulated, or somatostatin and neuropeptide Y inhibitions of adenylyl cyclase activity were found between the two groups. The pattern of change of somatostatin binding in the Alzheimer's disease cases observed in the present study differs from the reported pattern of loss of somatostatin neurons and may be secondary to the degeneration of somatostatin receptor-bearing cholinergic afferents arising from the nucleus basalis. The results of this study indicate that impaired somatostatin modulation of adenylyl cyclase is not a global phenomenon in Alzheimer's disease brain and also that there are no major disruptions of somatostatin receptor-G-protein coupling or of adenylyl cyclase catalytic activity in this disorder.

Substance P-like immunoreactive neurons are depleted in Alzheimer's disease cerebral cortex

We studied the morphology and distribution of substance P-like immunoreactive elements in normal and Alzheimer's disease brain with a monoclonal anti-substance P antibody. Bands of prominent terminal-like staining were found in the dentate gyrus of normal brain. Multipolar substance P-immunoreactive neurons were seen in dentate polymorphic layer and CA4 and prominent fiber staining was present in the CA fields of the hippocampus and adjacent allocortex. Reactive perikarya, concentrated in deep cortex and infracortical white matter, were found in all isocortical regions. Greatest density was in frontal and parietal association cortex; lowest in visual cortex. Fiber density was generally greatest in layers I and II. In Alzheimer's disease, staining intensity was reduced in the dentate gyrus. Hilar neurons were unaffected but other CA field neurons were distorted with pruned dendritic trees. Isocortical perikarya and fibers were significantly depleted and distorted in all regions. Globular deposits consisting of distorted neurites or dissolving perikarya were frequently seen. Double staining methods showed that the vast majority of isocortical, but not hippocampal, substance P-like immunoreactive neurons are nicotinamide adenine dinucleotide phosphate diaphorase-positive. Despite the modest quantitative depletion of substance P in Alzheimer's disease cortex as measured by radioimmunoassay compared to somatostatin, there is a significant depletion of substance P-like immunoreactive perikarya. This disparity may be due to persistence of afferent projections which make a major contribution to substance P concentrations in cerebral cortex or to the high substance P content of dystrophic fibers in Alzheimer's disease cortex.

Somatostatin in neurodegenerative illnesses

Somatostatin may play a role in several neurodegenerative diseases. Somatostatin concentrations are depleted in cerebral cortex in both Alzheimer's disease and in the dementia that accompanies Parkinson's disease. Somatostatin neurons in both illnesses are markedly dystrophic and may be reduced in number. In Huntington's disease, somatostatin concentrations are increased in the basal ganglia, as is the density of somatostatin neurons. The precise role of somatostatin changes in the pathophysiology of these illnesses requires further study.

A quantitative assessment of somatostatin-like and neuropeptide Y-like immunostained cells in the frontal and temporal cortex of patients with Alzheimer's disease

Immunocytochemical studies utilizing radioimmunoassay and morphological techniques have provided conflicting evidence for the involvement of somatostatin and neuropeptide Y in Alzheimer's disease (AD). However, previous investigators have not considered the effects of cortical atrophy in AD tissue on their findings. This study reports the numbers of somatostatin-like (SLI) and neuropeptide Y-like immunoreactive (NPYLI) neuronal perikarya and the length of SLI and NPYLI immunoreactive fibres, with appropriate corrections for atrophy in 6 control and 6 AD cases. There were significantly fewer SLI neurones in AD in layers II + III combined from the temporal cortex, and fewer NPYLI neurones in layers V + VI in both frontal and temporal cortices. Using a randomized method to quantify immunostained fibre length in the neuropil, an analysis of variance revealed no significant differences in the mean SLI or NPYLI fibre length per cortical strip between control and AD groups in frontal or temporal cortex. However, using a second measure of fibre length by tracing the fibres attached to consecutive immunostained perikarya, there were significant reductions in the AD brains in the mean fibre length per cell in layers V + VI for SLI in the temporal cortex, and for NPYLI in the frontal cortex. This reduction in fibre length per individual cell was presumably masked by the large variation in the fibre length found between cases using the randomized approach. It was concluded that in order to evaluate the involvement of these neuropeptides in AD from any measurements of concentration, it is essential to include some compensation for the extent of cortical atrophy that occurs with the disease.

Tyrosine hydroxylase-like immunoreactivity in senile plaques is not related to the density of tyrosine hydroxylase-positive fibers in patients with Alzheimer's disease

The numbers of silver-stained senile plaques and plaques containing tyrosine hydroxylase (TH)-like immunoreactivity were counted in the neocortex, amygdala and hippocampus of control subjects and patients with Alzheimer's disease, and compared with the density of TH-positive nerve fibers. The number of silver-stained senile plaques was lowest in the hippocampus and highest in the amygdala, and increased in all three structures in relation to the degree of dementia in the patients. A small proportion of plaques in the hippocampus of the most demented subjects and a large proportion of plaques in the amygdala were TH-positive. No TH-like immunoreactivity was found in plaques in the neocortex, although this structure contained almost as many silver-stained senile plaques and was as densely innervated by TH-positive fibers as the amygdala. The number of plaques containing TH-like immunoreactivity was, therefore, not proportional to the innervation of the structures by TH-positive fibers, nor to the total number of plaques in the structure, suggesting that the dissociation between the proportion of TH-positive plaques in the amygdala and neocortex may be due to differences in the populations of TH-positive fibers innervating the structures.

Neuronal responses to injury and aging: lessons from animal models

Alzheimer's disease (AD), the most common type of adult-onset dementia, is characterized by a variety of brain abnormalities, including degeneration of certain populations of nerve cells, alterations in the neuronal cytoskeleton, and the abnormal deposition of amyloid within brain parenchyma. Pathogenetic processes that lead to these brain abnormalities are difficult to study in humans. Recently, investigators have begun to utilize animal models to examine some of the mechanisms that cause cellular/molecular alterations in transmitter systems, cytoskeletal elements, and APP. These investigations have helped to clarify issues related to the lesions that occur in aged humans and individuals with AD.

Monoaminergic neurotransmitters in Alzheimer's disease. An HPLC study comparing presenile familial and sporadic senile cases

Norepinephrine, epinephrine, dopamine, serotonin and their major metabolites were measured in 20 regions of the left hemisphere in 4 presenile familial cases of Alzheimer-type dementia and 4 sporadic senile cases. Both groups were compared to values in normal brains obtained in our laboratory. Quantitative determination of the monoamines was performed by HPLC with electrochemical detection. The clinical diagnosis of Alzheimer-type dementia was confirmed by histological examination of the right hemisphere and brain stem. The serotonergic system was dramatically affected in the familial cases with very low or undetectable serotonin concentrations in most cortical and subcortical areas studied and an important cell loss in the nucleus raphe dorsalis, origin of the main ascending serotonergic system. In the senile demented patients the serotonergic deficit is less important but still clearly present. The noradrenergic, adrenergic and dopaminergic systems were less affected by the disease process in senile sporadic as well as in the presenile familial type of Alzheimer's disease.

Subpopulations of somatostatin 28-immunoreactive neurons display different vulnerability in senile dementia of the Alzheimer type

We tested whether the vulnerability of somatostatin (SST) neurons in senile dementia of the Alzheimer type (SDAT) depended upon their co-localization with neuropeptide Y (NPY). Density estimates of SST28- and NPY-immunoreactive neurons and percentage of double-labeled SST-NPY neurons were obtained in the cortex (areas 9 and 25) and the bed nucleus of stria terminalis (BST), in 6 SDAT and 5 control cases. Counts of senile plaques (SP) and neurofibrillary tangles (NFT) were done on thioflavin S stains. In both cortical areas, a decrease in the density of SST28-IR neurons was found in SDAT cases (-60% in area 25 and -80% in area 9), whereas density of NPY-IR neurons was unchanged. Accordingly, the proportion of single-labeled SST neurons decreased; this decrease was significantly correlated with SP (r = -0.89, P less than 0.001). We conclude that single SST-IR neurons, in cortical layers II-III, and V, are preferentially lost relative to co-localized SST-NPY neurons. In the BST, no significant reduction of SST-IR, NPY-IR neurons nor of the percentage of single labeled SST neurons was found, despite the presence of SP. Thus one subpopulation of SST neurons, defined by associated neurochemical characters (not co-localized with NPY nor with NADPH diaphorase) and by topography (cortical layers III and V) appears to be particularly vulnerable in SDAT. The potential importance of their position in neural circuitry is emphasized.

Direct synaptic contacts of medial septal efferents with somatostatin immunoreactive neurons in the rat hippocampus

Anterogradely labeled projections from the medial septum to hippocampal somatostatin immunoreactive (SOM-i) neurons were studied with double-label immunocytochemistry under light (LM) and electron microscopic (EM) conditions. Medial septal projections were identified after injecting the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) followed by immunohistochemical visualization of PHA-L presynaptic terminal labeling and concurrent immunocytochemical staining of SOM in hippocampal target cell bodies. This double-label procedure yielded blue-black nickel enhanced DAB stained, PHA-L-immunoreactive terminals on light brown SOM-i neurons that were investigated by correlative LM and EM observations. PHA-L-labeled terminal contacts with often basket-like appearance were localized with highest densities on soma and proximal dendrites of SOM-i neurons in stratum oriens of Ammon's horn and hilus of dentate gyrus, and some minor projections to stratum pyramidale and radiatum. Most double-labeled contacts could be identified as symmetric type synapses equally divided over soma and proximal dendrites of several forms of SOM-i neurons. These data indicate monosynaptic regulation of the hippocampal intrinsic SOM system by septal input, which probably represents a peptidergic subpopulation of the hippocampal GABAergic system.

Substance P and somatostatin coexist within neuritic plaques: implications for the pathogenesis of Alzheimer's disease

In recent years the present authors and others have sought to determine the neurochemical composition of the dilated neuronal processes found within neuritic plaques of patients with Alzheimer's disease. To date a number of neurotransmitter and neuropeptide systems have been observed within different plaques, yet at present it is unclear whether individual human plaques contain more than one transmitter substance. In the present study a highly sensitive dual-immunolabeling procedure was employed and it was demonstrated that substance P and somatostatin-immunoreactive profiles coexist within single senile plaques of patients with Alzheimer's disease. Coexistence of somatostatin and substance P immunoreactivity within plaques was observed in the hippocampus and amygdala but not in the neocortex, although the latter region contained plaques within which somatostatin and substance P existed alone. The frequency with which we observed one or more neuropeptide within plaques was relatively low and in fact most plaques contained neither substance P nor somatostatin immunoreactivity. In addition, a large number of swollen peptidergic processes were observed outside of plaques. The significance of these observations with respect to the pathogenesis of Alzheimer's disease is discussed.

Anatomical distribution of somatostatin immunoreactivity in the infant brainstem

The distribution of somatostatin-immunoreactive structures in the infant brainstem was investigated using the peroxidase-antiperoxidase technique. A wide distribution of somatostatin-immunoreactive cell bodies and fibers was observed throughout the brainstem. Numerous somatostatin-immunoreactive cell bodies and fibers were present in several areas of the brainstem including the substantia grisea centralis and the reticular formation. Some immunoreactive cell bodies were seen in cranial nerve nuclei such as the nucleus praepositus, the nucleus nervi hypoglossi and the vestibular nuclei. Immunoreactive fibers were seen in the nucleus cuneatus, the locus coeruleus, the nucleus tractus solitarius, the nucleus ambiguus, the nucleus tractus spinalis nervi trigemini and the dorsal horn of the spinal cord. These data were in agreement with previous works on the human adult. However, a high density of somatostatin-immunoreactive cell bodies and fibers in the interpeduncular nucleus and in the nucleus centralis superior, and a dense network of somatostatin-immunoreactive fibers in the dorsal part of the nucleus inferior olivarius, were also observed. The role of somatostatin in some brainstem nuclei and its probable implication in some specific neuropathological diseases of the infant brainstem is discussed.

Serotoninergic neurites in senile plaques in cingulate cortex of aged nonhuman primate

In immunocytochemical studies, a polyclonal antiserotonin antibody was used to visualize fibers within the cingulate cortex of young and aged rhesus monkeys. Intricate and distinct patterns of serotoninergic processes were seen in anterior and posterior segments of cingulate cortex (Brodmann areas 24 and 23). In these regions of cortex, many multivaricose serotonin-immunoreactive axonal swellings were identified, and some of these immunostained neurites were associated with deposits of amyloid. These observations suggest that serotoninergic processes are involved in the formation of senile plaques in neocortex of aged macaques.

Neuropeptides and neuropathology in the amygdala in Alzheimer's disease: relationship between somatostatin, neuropeptide Y and subregional distribution of neuritic plaques

This study examined the amygdaloid complex in Alzheimer's disease (AD). We compared the distribution and morphology of somatostatin (SOM-) and neuropeptide Y-immunoreactive (NPY-IR) neurons in the amygdala with the distribution of neuritic plaques (NP) and acetylcholinesterase (AChE) staining patterns in various subnuclei. We found that in AD, there was an increase in the number of small, atrophic neurons for both SOM and NPY, and subregional analysis revealed similar size reductions in all subnuclei. In contrast, the highest density of NP was found in the corticomedial nuclei and densest staining for AChE in the basal nucleus. Although NPY- and SOM-IR fibers were occasionally associated with NP, a dense, morphologically preserved peptidergic fiber-network was found in all areas including subnuclei with high numbers of NP. Our study indicates that atrophic SOM- and NPY-IR neurons are not correlated with the subregional distribution of NP or cholinesterase staining pattern of the amygdala, and suggests that alterations in SOM and NPY neurons are not characteristics of the primary pathogenic process that underlie the formation of NP or cholinergic cell loss in AD.

Decreased levels of protein kinase C in Alzheimer brain

Protein kinase C (PK-C) levels were determined using [3H]phorbol-12,13-dibutyrate (PDB) binding and the in vitro phosphorylation of histone H I (III-S), in autopsied human frontal cortex of age- and postmortem time-matched normal and Alzheimer patients. PK-C levels in Alzheimer particulate fractions determined by both methods were about 50% of those in controls. PK-C levels in Alzheimer cytosol fractions were not significantly different from those in controls. In a parallel study, we measured the phosphorylation of a Mr 86,000 protein (P86), the major protein kinase C substrate in the cytosol fraction prepared from Alzheimer frontal cortex, and found it to be reduced to 43% of that in control brains. This reduction in P86 protein phosphorylation compared to controls was not detected in brain samples prepared from demented patients without Alzheimer's disease. We considered 3 extraneous factors (postmortem delay, age and sex) which may have affected the extent of P86 phosphorylation and concluded that the reduced P86 phosphorylation in the Alzheimer samples is not due to any of them. Reduced PK-C levels and Mr 86,000 protein phosphorylation may reflect a biochemical deficit related specifically to the pathogenesis of Alzheimer's disease.

Morphologic alterations of cholinergic processes in the neocortex of aged rats

In the present study we observed enlarged cholinergic processes in the neocortex of aged Fischer 344 rats. These swollen ChAT-positive profiles appeared either as a single axon enlargement or, in many instances, the bulbous processes coalesced to form grape-like clusters of immunoreactivity. The latter structures looked similar to the immunoreactive profiles observed in the cortex of patients with Alzheimer's disease and in the rat septum following fimbria-fornix transection. Together, these data provide evidence that morphologic changes occur within processes of cholinergic neurons in the aged rat. Moreover, the similarity in appearance between the axonal alterations in the aged rat and in patients with Alzheimer's disease suggests a common pathologic process.

Cortical somatostatin, neuropeptide Y, and NADPH diaphorase neurons: normal anatomy and alterations in Alzheimer's disease

Somatostatin and neuropeptide Y are two neuropeptides that are of particular interest in Alzheimer's disease because they are reported to be depleted in cerebral cortex. In the present study we examined somatostatin, neuropeptide Y, and nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase neurons in nine cortical regions in both normal and Alzheimer's disease brains. These three neurochemical markers show a high degree of co-localization (greater than 90%) in nonpyramidal neurons that are primarily distributed in cortical layers II-III, V-VI, and, most prominently, in infracortical white matter. The highest cell density was in temporal and parietal association cortex. The major morphological abnormality in Alzheimer's disease brains was a marked pruning and distortion of fiber plexuses with an apparent reduction in fiber density. In contrast, perikaryal density was preserved except for a reduction in parietal association cortex. Approximately 10 to 15% of senile plaques in the inferior temporal gyrus contained abnormal neurites. Additional abnormal collections of neurites without plaque cores were frequently found in layers II-III and V-VI. Neuropeptide Y and somatostatin were co-localized in abnormal neurites, suggesting an origin from local intrinsic neurons in which the two peptides are co-localized. Double immunofluorescence staining for both tau protein, a major antigenic component of paired helical filaments, and either somatostatin or neuropeptide Y showed that these neurons do not contain tau-immunoreactive neurofibrillary tangles. The morphological correlate of reduced somatostatin and neuropeptide Y content in Alzheimer's disease brain therefore appears to be a distortion and reduction in fiber plexuses. In addition, it is apparent that these neurons can develop widespread morphological abnormalities in the absence of neurofibrillary tangle formation.

An immunohistochemical study of pro-somatostatin-derived peptides in the human brain

The distribution of pro-somatostatin-derived-peptide-positive profiles was examined by indirect immunohistofluorescence in nine post-mortem human brains (age 58-73 years). Three specific antisera were used for this study which recognize, respectively, somatostatin-28, somatostatin-28 (1-12) and somatostatin (1-14). Pro-somatostatin-derived-peptide-positive immunoreactive profiles were observed throughout the neuraxis. Cell bodies were found within archeo-, paleo- and neocortical areas, the subcortical white matter, in the nucleus accumbens, caudate nucleus and putamen, as well as in the hypothalamus, the reticular thalamic nucleus and the reticular formation of the brainstem. Fibers and terminals were seen in the same areas as well as in various thalamic nuclei, in the brainstem and spinal cord. Pro-somatostatin-derived-peptide-positive fibre tracts include the bed nucleus of the stria terminalis, the diagonal band of Broca, the stria medullaris, the inter-thalamic adhesion, the posterior commissure and the spinothalamic tract. Furthermore, differences between human and animal brains were noted and some somatostatin systems reported which may be implicated in certain human neuropathological states.

Senile plaques in aged squirrel monkeys

Aged squirrel monkeys develop senile plaques in the brain that are similar to those occurring in aged rhesus monkeys and aged humans. These plaques consist of abnormal, swollen neurites around an amyloid core. In whole-hemisphere coronal sections through the level of the rostral temporal lobe, plaques are present in temporal cortex, amygdala, hippocampal formation and, occasionally, in other cortical regions. In more rostral sections through the frontal lobe, plaques are most common in orbitofrontal and frontal opercular cortical regions. In immunocytochemical preparations, some neurites show immunoreactivity with antibodies directed against phosphorylated neurofilaments and neuropeptide Y. Thus, plaques in these New World primates are similar in distribution and composition to those occurring in aged Old World primates.

Somatostatin immunoreactive neurons in the human hippocampus and cortex shown by immunogold/silver intensification on vibratome sections: coexistence with neuropeptide Y neurons, and effects in Alzheimer-type dementia

The distribution of somatostatinlike immunoreactivity was studied in the hippocampal formation, retrohippocampal region, and temporal cortex in the human brain. Tissues from surgical biopsy and postmortem cases were used, and the immunogold/silver method on vibratome sections was introduced for routine applications in conjunction with primary antisera that recognise somatostatin-14 or somatostatin-28. Somatostatin-28 antisera readily stained numerous neurons, dendrites, and extensive axonal networks throughout the hippocampus and neighbouring cortex. Liquid phase absorption provided controls for specificity. The most prominent accumulations of somatostatin immunoreactive neurons and axons occurred in the hilus of the area dentata, in CA1, and in the entorhinal and perirhinal cortices. Axonal plexuses occurred throughout the hippocampal subfields but were particularly dense in those regions rich in somatostatin neurons. The distribution of somatostatin immunoreactive neurons and fibers parallels the distribution of neuropeptide Y (NPY) neurons and fibers in the hippocampus and cerebral cortex to a remarkable extent. Double labelling experiments with antisera against neuropeptide Y and somatostatin indicate a considerable frequency of coexistence of the two peptides in single neurons, particularly in large multipolar cortical neurons and also in the small bipolar white matter neurons. Regional variations exist in the amounts of coexistence found in the hippocampal subfields; somatostatin-NPY coexistence is particularly high in the hilus of the area dentata, the subicular complex, and the deep layers of the entorhinal and perirhinal cortices. In the hippocampi and temporal cortices in cases of Alzheimer-type dementia compared to those of age-matched control brains, there is a significant to severe loss of somatostatin immunoreactive neurons and axons. This loss is most severe in those regions with the highest indices of neurofibrillary tangles and neuritic plaques-the hilus of the area dentata, CA1, and the entorhinal and perirhinal cortices. Surviving somatostatin neurons are distorted with short dendrites and truncated axons. Neuritic plaques identified on double label experiments with thioflavin include somatostatin axons but not neurons.

Brain reactive antibodies and the blood-brain barrier: observations in aging rodents and the effects of peripheral kainic acid

This study was initiated to confirm the existence of brain-reactive autoantibodies and to determine if such antibodies have higher affinity for brain regions especially affected in Alzheimer's disease. Serum collected from 90, 300, and 600 day old mice was incubated against brain tissues from these same mice, followed by incubation with fluorescently tagged rabbit antimouse IgG. No antibodies were present in the youngest serum, but considerable antibodies were present at 300 and, especially, at 600 days. Such antibodies were present in the blood vessels, but not in the brains of older animals. These antibodies, applied exogenously, labeled cells equally in all three ages of brains including most cortical and many other neurons, indicating that they are not neurotransmitter specific. In a further study, kainic acid or saline was administered peripherally to 15-month old rats. Kainic acid damaged the blood brain barrier and allowed the CNS entry of brain-reactive antibodies, especially into the subregions of hippocampus most damaged in Alzheimer's.

Acetylcholinesterase fibers and the development of senile plaques

A new, highly sensitive histochemical technique for actylcholinesterase (AChE) was applied to a study of Alzheimer's disease brain tissue. Many immature senile plaques were seen to be developing along AChE-positive axons in the hippocampus and neocortex. Single fibers often displayed multiple lesions, showing stages of initial swelling, ballooning with the appearance of AChE in the surrounding extra-axonal space, and development of an AChE-amyloid intense core accompanied by a weakly staining AChE-positive halo. Except for the core and halo, AChE-positive material was seldom detected in so-called mature plaques which are large and incorporate many degenerating neuritic elements surrounding an amyloid core. Lesion data in rats established the relationship between AChE-positive neocortical axons and medial basal forebrain cholinergic cells. In Alzheimer's disease tissue, many degenerating neurons in the basal forebrain were detected by the AChE histochemical stain, along with pathological alterations in the proximal axons en route to their cortical terminal fields. These data provide direct evidence of an association between the cholinergic system of the basal forebrain and the early formation of senile plaques in the cortex in Alzheimer's disease.

Reinnervation of the hippocampal perforant pathway zone in Alzheimer's disease

The perforant pathway originates from the entorhinal cortex of the anterior parahippocampal gyrus and terminates on the outer dendritic branches of the granule cells of the dentate gyrus and pyramidal cells of the subiculum and hippocampus. It carries the principal cortical input to the hippocampal formation. Destruction of the perforant pathway in experimental animals leads to a partial deafferentation of its target neurons, followed by a robust sprouting of acetylcholinesterase (AChE) terminals in the deafferented perforant pathway zone. In Alzheimer's disease, the cells of origin of the perforant pathway are laden with neurofibrillary tangles. AChE staining in the terminal zone of the perforant pathway in Alzheimer's disease shows several distinct patterns that are not found in control brains. These changes are consistent with the results of experimental studies demonstrating reinnervation in laboratory mammals, including nonhuman primates. The results suggest that in Alzheimer's disease sprouting of AChE-containing systems occurs in the hippocampal formation in response to disease-related cellular damage in the entorhinal cortex.

Immunohistochemical study of neurons containing corticotropin-releasing factor in Alzheimer's disease

In the brains of controls and individuals with Alzheimer's disease (AD), antisera to corticotropin-releasing factor (CRF) were used to immunostain neurons and their processes. In AD, we identified abnormal CRF-immunoreactive axons as well as neurites associated with deposits of amyloid in brain regions showing senile plaques. The number of immunoreactive fibers was decreased in individuals with AD. In contrast, CRF immunoreactivity was markedly increased in some neurons located within the paraventricular nucleus (PVN) of the hypothalamus. These findings support previous neurochemical studies indicating that certain CRF systems are affected in AD.

Increased in vitro phosphorylation of a Mr 60,000 protein in brain from patients with Alzheimer disease

We have established in vitro conditions under which we can reliably measure kinase activity in normal postmortem human brain. Using these conditions, we detected in the brains of patients with Alzheimer disease a 2-fold increase in the level of Mr 60,000 protein phosphorylation compared to age-matched controls. The Mr 60,000 protein phosphorylation was found exclusively in the cytosol fraction. No differences were detected between phosphoproteins in 100,000 X g pellet fractions from brains of Alzheimer disease patients and from age-matched controls. Postmortem time up to 17 hr does not seem to affect the phosphorylation level of the Mr 60,000 protein. Younger Alzheimer disease patients had more prominent changes in the elevation of the Mr 60,000 protein phosphorylation level than older patients, although in the control patient, age did not affect the phosphorylation level of the Mr 60,000 protein. We conclude that in the brain cytosol of Alzheimer disease there may be an abnormality in either the degree of Mr 60,000 protein phosphorylation or in the Mr 60,000 protein concentration.

The topography of cell loss from locus caeruleus in Alzheimer's disease

A topographical analysis of nerve cell loss from locus caeruleus in Alzheimer's disease has shown that cell loss is confined to the dorsal areas and occurs uniformly throughout the rostrocaudal length of the locus. By contrast there is no significant cell loss from ventral parts of the locus, at any point along its rostrocaudal length. Dorsally located neurones of the locus project to cerebral cortex; ventrally located neurones to non-cortical areas such as basal ganglia, cerebellum and spinal cord. These data suggest that damage to nerve cells of locus caeruleus in Alzheimer's disease relates primarily to pathological events within their terminal fields, with perikaryal loss following as a secondary retrograde change. The senile plaque may represent the actual site of the damage to nerve terminals.

Nucleus basalis neuronal loss, neuritic plaques and choline acetyltransferase activity in advanced Alzheimer's disease

All our advanced, severe cases of Alzheimer's disease have dramatic cholinergic cell losses in the nucleus basalis of Meynert even after correction for cell or nucleoli shrinkage. There is a good correlation between choline acetyltransferase activity and "healthy" cell number in the nucleus basalis of Meynert. Half of the Alzheimer disease cases have markedly reduced cortical choline acetyltransferase activity in spite of preserved nucleus basalis of Meynert choline acetyltransferase activity, suggesting a deficiency of cortical origin and/or of axonal transport in Alzheimer disease. The relationship between cell loss in the various sub-divisions of the nucleus basalis of Meynert and plaque counts in corresponding and non-corresponding projection areas of the cortex has also been examined. Globally, this relation appears more obvious when cell loss in a sub-division of the nucleus basalis of Meynert is compared to plaque counts in its cortical projection area. However, the relation is discontinuous with few or no data to document the intermediary stages of the process, probably reflecting the severity of our Alzheimer disease cases.

Choline acetyltransferase immunoreactivity in neuritic plaques of Alzheimer brain

We have observed dystrophic choline acetyltransferase (ChAT)-positive processes surrounding the amyloid core of neuritic plaques in human neocortex, amygdala and hippocampus, using a polyclonal anti-human ChAT antiserum. These data, and those from studies of the aged monkey by other investigators, provide a morphologic counterpart for the biochemical abnormality of the cholinergic system in Alzheimer's disease and senile dementia of the Alzheimer type.

Neurotransmitter and receptor deficits in senile dementia of the Alzheimer type

Multiple neurotransmitter systems are affected in senile dementia of the Alzheimer's type (SDAT). Among them, acetylcholine has been most studied. It is now well accepted that the activity of the enzyme, choline acetyltransferase (ChAT) is much decreased in various brain regions including the frontal and temporal cortices, hippocampus and nucleus basalis of Meynert (nbm) in SDAT. Cortical M2-muscarinic and nicotinic cholinergic receptors are also decreased but only in a certain proportion (30-40%) of SDAT patients. For other systems, it appears that cortical serotonin (5-HT)-type 2 receptor binding sites are decreased in SDAT. This diminution in 5-HT2 receptors correlates well with the decreased levels of somatostatin-like immunoreactive materials found in the cortex of SDAT patients. Cortical somatostatin receptor binding sites are decreased in about one third of SDAT patients. Finally, neuropeptide Y and neuropeptide Y receptor binding sites are distributed in areas enriched in cholinergic cell bodies and nerve fiber terminals and it would be of interest to determine possible involvement of this peptide in SDAT. Thus, it appears that multi-drug clinical trials should be considered for the treatment of SDAT.

Neuropeptides in neurological disease

Neuropeptides are widely distributed in the central nervous system, where they serve as neuroregulators. Recent interest has focused on their role in degenerative neurological diseases. We describe the normal anatomy of neuropeptides in both the cerebral cortex and basal ganglia as a framework for interpreting neuropeptide alterations in Alzheimer's disease (AD), Huntington's disease, and Parkinson's disease. Concentrations of cortical somatostatin are reduced in AD and in dementia associated with Parkinson's disease. Concentrations of neuropeptide Y and corticotropin-releasing factor are also reduced in AD cerebral cortex. The reduced cortical concentrations of somatostatin and neuropeptide Y in AD cerebral cortex may reflect a loss of neurons or terminals in which these two peptides are co-localized. In Huntington's disease, basal ganglia neurons in which somatostatin and neuropeptide Y are co-localized are selectively preserved. Cerebrospinal fluid concentrations of neuropeptides in AD reflect alterations in cortical concentrations. Improved understanding of neuropeptides in degenerative neurological illnesses will help define which neuronal populations are specifically vulnerable to the pathological processes, and this could lead to improved therapy.

An integrative hypothesis concerning the pathogenesis and progression of Alzheimer's disease

Observations, in Alzheimer's disease, in the pattern of nerve cell damage and loss, the pathology, microchemistry and immunology of senile plaques and neurofibrillary tangles and alterations in blood vessels are drawn together into a hypothesis that attempts to explain the pathogenesis and progression of the disorder. At the heart of this hypothesis lies a defect in blood brain barrier function and/or structure within the cerebral cortex and this defect may be the cause of the cerebral vessel amyloidosis common in many patients with Alzheimer's disease. Age-related alterations in blood brain barrier allow for damage to nerve terminals and limited formation of senile plaques within cerebral cortex; neurofibrillary tangles are formed within cortical and subcortical nerve cells which project to or near damaged vessels/senile plaques. Uptake of "neurotoxin" at affected terminals and retrograde transport to perikarya causes neurofibrillary tangles to be formed; their accumulation leads to perikaryal changes culminating in cell death and loss. Loss of cells in cortically projecting areas of subcortex such as nucleus basalis, locus caeruleus and dorsal raphe, which terminate on cerebral vessels, causes further blood brain barrier dysfunction, new plaque formation and continued cell loss in cortex and subcortex. Once started, such a process could be self-perpetuating and the initial site of damage could lie within the amygdala/hippocampus with putative pathogenic agent accessing the brain via the olfactory pathways.

Widespread reduction of somatostatin-like immunoreactivity in the cerebral cortex in Alzheimer's disease

Although several studies have documented reduced concentrations of somatostatin-like immunoreactivity (SLI) in the cerebral cortex in Alzheimer's disease, there is controversy concerning the extent and importance of these changes. We measured SLI in brains obtained post mortem from 12 patients with pathologically confirmed Alzheimer's disease and from 13 neurologically normal controls. All major cortical and subcortical regions were examined. Widespread reductions of SLI in Alzheimer's disease cerebral cortex were found, with the most profound changes seen in temporal lobe; but there also were major reductions in both the frontal and occipital cortex. There were no significant reductions in subcortical regions. Characterization of SLI by high-pressure liquid chromatography showed no significant difference in profiles between Alzheimer's disease and control frontal cortex. These results suggest that the reduction in somatostatin immunoreactivity in Alzheimer's disease may be caused by degeneration of intrinsic somatostatin cortical neurons.