Alz-50 immunoreactivity in the thalamic reticular nucleus in Alzheimer's disease

Early and selective localization of tau filaments to glutamatergic subcellular domains within the human anterodorsal thalamus

Widespread cortical accumulation of misfolded pathological tau proteins (ptau) in the form of paired helical filaments is a major hallmark of Alzheimer's disease. Subcellular localization of ptau at various stages of disease progression is likely to be informative of the cellular mechanisms involving its spread. Here, we found that the density of ptau within several distinct rostral thalamic nuclei in post-mortem human tissue (n = 25 cases) increased with the disease stage, with the anterodorsal nucleus (ADn) consistently being the most affected. In the ADn, ptau-positive elements were present already in the pre-cortical (Braak 0) stage. Tau pathology preferentially affected the calretinin-expressing subpopulation of glutamatergic neurons in the ADn. At the subcellular level, we detected ptau immunoreactivity in ADn cell bodies, dendrites, and in a specialized type of presynaptic terminal that expresses vesicular glutamate transporter 2 (vGLUT2) and likely originates from the mammillary body. The ptau-containing terminals displayed signs of degeneration, including endosomal/lysosomal organelles. In contrast, corticothalamic axon terminals lacked ptau. The data demonstrate the involvement of a specific cell population in ADn at the onset of the disease. The presence of ptau in subcortical glutamatergic presynaptic terminals supports hypotheses about the transsynaptic spread of tau selectively affecting specialized axonal pathways.

Hierarchical Distribution of the Tau Cytoskeletal Pathology in the Thalamus of Alzheimer's Disease Patients

In spite of considerable progress in neuropathological research on Alzheimer's disease (AD), knowledge regarding the exact pathoanatomical distribution of the tau cytoskeletal pathology in the thalamus of AD patients in the advanced Braak and Braak AD stages V or VI of the cortical cytoskeletal pathology is still fragmentary. Investigation of serial 100 μm-thick brain tissue sections through the thalamus of clinically diagnosed AD patients with Braak and Braak AD stage V or VI cytoskeletal pathologies immunostained with the anti-tau AT8 antibody, along with the affection of the extraterritorial reticular nucleus of the thalamus, reveals a consistent and severe tau immunoreactive cytoskeletal pathology in the limbic nuclei of the thalamus (e.g., paraventricular, anterodorsal and laterodorsal nuclei, limitans-suprageniculate complex). The thalamic nuclei integrated into the associative networks of the human brain (e.g., ventral anterior and mediodorsal nuclei) are only mildly affected, while its motor precerebellar (ventral lateral nucleus) and sensory nuclei (e.g., lateral and medial geniculate bodies, ventral posterior medial and lateral nuclei, parvocellular part of the ventral posterior medial nucleus) are more or less spared. The highly stereotypical and characteristic thalamic distribution pattern of the AD-related tau cytoskeletal pathology represents an anatomical mirror of the hierarchical topographic distribution of the cytoskeletal pathology in the interconnected regions of the cerebral cortex of AD patients. These pathoanatomical parallels support the pathophysiological concept of a transneuronal spread of the disease process of AD along anatomical pathways. The AD-related tau cytoskeletal pathology in the thalamus most likely contributes substantially to the neuropsychiatric disease symptoms (e.g., dementia), attention deficits, oculomotor dysfunctions, altered non-discriminative aspects of pain experience of AD patients, and the disruption of their waking and sleeping patterns.

Regional acetylcholinesterase activity and its correlation with behavioral performances in 15-month old transgenic mice expressing the human C99 fragment of APP

In addition to Abeta plaques and neurofibrillary tangles, Alzheimer's disease (AD) is characterized by increased brain levels of APP C-terminal fragments. In the present investigation, the cholinergic innervation in forebrain regions of transgenic mice (Tg13592) expressing the human betaAPP C99 fragment was compared to that of non-transgenic controls by measuring the activity of the non-specific catabolic enzyme, acetylcholinesterase (AChE). The AchE activity of Tg13592 mice was altered in several regions implicated in the functional loop of regulation between septum and hippocampus, vulnerable in Alzheimer pathology and critically involved in cognitive functions. In particular, AChE activity was upregulated in three basal forebrain regions containing cholinergic cell bodies, prelimbic cortex, anterior subiculum, and paraventricular thalamus, but downregulated in lateral septum and reticular thalamus. The increased activity in medial septum and anterior subiculum was linearly correlated with poor performances in a spatial learning task, possibly due to cell stress mechanisms. Because of some similarities in terms of neurochemistry and behavior, this mouse model may be of use for studying prodromal AD.

The limbic lobe and its output channels: implications for emotional functions and adaptive behavior

Current dissatisfaction with the limbic system concept reflects a desire to move beyond the limbic system in efforts to explain key facets of emotional functions and motivational behavior. This review promotes an anatomical viewpoint, which originated as a result of histotechnical advances. These improvements paved the way for anatomical discoveries, which in turn led to the concepts of the ventral striatopallidal system and extended amygdala. These two systems, together with the basal nucleus of Meynert and the septum-diagonal band system, serve as output channels for an expanded version of the classic limbic lobe of Broca, which contains all non-isocortical parts of the cortical mantle together with the large laterobasal-cortical amygdaloid complex. Thus defined, the limbic lobe contains all of the major cortical (e.g. orbitofrontal, cingulate and insular cortices in addition to the hippocampal formation) and cortical-like (laterobasal-cortical amygdala) structures known to be especially important for emotional and motivational functions. In their role as output channels for the limbic lobe, the basal forebrain functional-anatomical systems contribute to the establishment of a number of cortico-subcortical circuits, which provide an important part of the anatomical substrate for the elaboration of emotional functions and adaptive behavior.

Differential distribution of butyrylcholinesterase and acetylcholinesterase in the human thalamus

It has been hypothesized that acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) are coregulators of the duration of action of acetylcholine in cholinergic neurotransmission, suggesting that BuChE may also have an important role in the brain. To compare the expression of cholinesterases in the human thalamus, the distributions of BuChE and AChE activity were studied by using a modified Karnovsky-Roots method. BuChE activity was present mainly in neurons, whereas AChE activity was present in both neurons and axons. There was intense staining for BuChE or AChE throughout the thalamus, with some nuclei primarily expressing one or the other cholinesterase. BuChE staining was most intense and widespread in neurons in the anteroventral, mediodorsal, ventral, lateral, and pulvinar thalamic nuclei. AChE was predominantly expressed in neurons of the anterodorsal, midline, ventral, intralaminar, and reticular nuclei. Many nuclei contained both cholinesterases. Considering the overall patterns of labeling in the thalamus for the two cholinesterases, there were both complementary and overlapping relationships of BuChE and AChE activity. Neuronal staining in the subthalamic nucleus and hypothalamus was predominantly positive for AChE activity. The distinct distribution of BuChE activity in neurons in the human thalamus is consistent with an important role for this enzyme in neurotransmission in the human nervous system. Furthermore, BuChE activity, like AChE activity, is found in certain thalamic nuclei related to cognitive and behavioral functions. Involvement of thalamic nuclei in diseases of the nervous system such as Alzheimer's disease and schizophrenia suggests that BuChE could be a potential target for therapeutic intervention in these disorders.

Nerve Growth Factor Receptor-immunoreactive Fibres Innervate the Reticular Thalamic Nucleus: Modulation by Nerve Growth Factor Treatment in Neonate, Adult and Aged Rats

Terminal arborizations expressing nerve growth factor receptor (NGF-R) have been detected with immunohistochemistry in the reticular thalamic nucleus of neonate, adult and aged rats. Intracerebroventricular administration of nerve growth factor (NGF) resulted in a dramatic increase in NGF-R immunoreactivity throughout the lifespan. This effect was paralleled by a concomitant increase in NGF-R immunopositivity in the neurons of the basal forebrain, which was here demonstrated also in aged animals, thus indicating that the NGF-R immunoreactivity within the reticular thalamic nucleus derives in all likelihood from cholinergic neuronal cell bodies of the basal forebrain. Our results demonstrate a prominent ability of NGF to up-regulate its receptors within fibres innervating the reticular thalamic nucleus, and show that this up-regulation of NGF-R is maintained throughout the lifetime. Altogether this indicates that the reticular thalamic nucleus may represent a new, important site of action of endogenous NGF or NGF-like molecules within the brain. In view of the crucial role played by the reticular thalamic nucleus in gating thalamocortical information, the autoregulation of NGF-R in this structure may have important concomitants in both physiological and pathological conditions.

Selective immunolesioning of cholinergic neurons in nucleus basalis magnocellularis impairs prepulse inhibition of acoustic startle

Information processing and attentional abnormalities are prominent in neuropsychiatric disorders. Since the cholinergic neurons located in the nucleus basalis magnocellularis have been shown to be involved in attentional performance and information processing, recent efforts to analyze the significance of the basal forebrain in the context of schizophrenia have focused on this nucleus and its projections to the cerebral cortex. We report here that bilateral selective immunolesioning of the cholinergic neurons in the nucleus basalis magnocellularis is followed by significant deficits in sensorimotor gating measured by prepulse inhibition of the startle reflex in adult rats. This behavioral approach is used in both humans and rodents and has been proposed as a valuable model contributing to the understanding of the neurobiological substrates of schizophrenia. The disruption of prepulse inhibition persisted over repeated testing. The selective lesions were induced by bilateral intraparenchymal infusions of 192 IgG saporin at a concentration having minimal diffusion into adjacent nuclei of the basal forebrain. The infusions were followed by extensive loss of choline acetyltransferase-immunopositive neurons. Our results show that the cholinergic neurons of the nucleus basalis magnocellularis represent a critical station of the startle gating circuitry and suggest that dysfunction of these neurons may result in impaired sensorimotor gating characteristic of schizophrenia.

Pattern of brain destruction in Parkinson's and Alzheimer's diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common age-related degenerative disorders of the human brain. Both diseases involve multiple neuronal systems and are the consequences of cytoskeletal abnormalities which gradually develop in only a small number of neuronal types. In AD, susceptible neurons produce neurofibrillary tangles (NFTs) and neuropil threads (NTs), while in PD, they develop Lewy bodies (LBs) and Lewy neurites (LNs). The specific lesional pattern of both illnesses accrues slowly over time and remains remarkably consistent across cases. In AD, six developmental stages can be distinguished on account of the predictable manner in which the neurofibrillary changes spread across the cerebral cortex. The pathologic process commences in the transentorhinal region (clinically silent stages I and II), then proceeds into adjoining cortical and subcortical components of the limbic system (stages III and IV - incipient AD), and eventually extends into association areas of the neocortex (stages V and VI - fully developed AD). During the course of PD, important components of the limbic system undergo specific lesions as well. The predilection sites include the entorhinal region, the CA2-sector of the hippocampal formation, the limbic nuclei of the thalamus, anterior cingulate areas, agranular insular cortex (layer VI), and - within the amygdala - the accessory cortical nucleus, the ventromedial divisions both of the basal and accessory basal nuclei, and the central nucleus. The amygdala not only generates important projections to the prefrontal association areas but also exerts influence upon all non-thalamic nuclei which in a non-specific manner project upon the cerebral cortex and upon the nuclei regulating endocrine and autonomic functions. All these amygdala-dependent structures themselves exhibit severe PD-specific lesions. In general, the extranigral destructions are in themselves not sufficient to produce overt intellectual deterioration. Similarly, AD-related pathology up to stage III may be asymptomatic as well. Fully developed PD with concurring incipient AD, however, is likely to cause impaired cognition. Presently available data support the view that the occurrence of additional lesions in the form of AD stage III (or more) destruction is the most common cause of intellectual decline in PD.

Substantia innominata: a notion which impedes clinical-anatomical correlations in neuropsychiatric disorders

Comparative neuroanatomical investigations in primates and non-primates have helped disentangle the anatomy of the basal forebrain region known as the substantia innominata. The most striking aspect of this region is its subdivision into two major parts. This reflects the fundamental organizational scheme for this portion of the forebrain. According to this scheme, two major subcortical telencephalic structures, i.e. the striatopallidal complex and extended amygdala, form large diagonally oriented bands. The rostroventral extension of the pallidum accounts for a large part of the rostral subcommissural substantia innominata, while the sublenticular substantia innominata is primarily occupied by elements of the extended amygdala. Also dispersed across this region is the basal nucleus of Meynert, which is part of a more or less continuous collection of cholinergic and non-cholinergic corticopetal and thalamopetal cells, which stretches from the septum diagonal band rostrally to the caudal globus pallidus. The basal nucleus of Meynert is especially prominent in the primate, where it is sometimes inappropriately applied as a synonym for the substantia innominata, thereby tacitly ignoring the remaining components. In most mammals, the extended amygdala presents itself as a ring of neurons encircling the internal capsule and basal ganglia. The extended amygdala may be further subdivided, i.e. into the central extended amygdala (related to the central amygdaloid nucleus) and the medial extended amygdala (related to the medial amygdaloid nucleus), which generally form separate corridors both in the sublenticular region and along the supracapsular course of the stria terminalis. The extended amygdala is directly continuous with the caudomedial shell of the accumbens, and to some extent appears to merge with it. Together the accumbens shell and extended amygdala form an extensive forebrain continuum, which establishes specific neuronal circuits with the medial prefrontal-orbitofrontal cortex and medial temporal lobe. This continuum is particularly characterized by a prominent system of long intrinsic association fibers, and a variety of highly differentiated downstream projections to the hypothalamus and brainstem. The various components of the extended amygdala, together with the shell of the accumbens, are ideally structured to generate endocrine, autonomic and somatomotor aspects of emotional and motivational states. Behavioral observations support this proposition and demonstrate the relevance of these structures to a variety of functions, ranging from the various elements of the reproductive cycle to drug-seeking behavior. The neurochemical and connectional features common to the accumbens shell and the extended amygdala are especially relevant to understanding the etiology and treatment of neuropsychiatric disorders. This is discussed in general terms, and also in specific relation to the neurodevelopmental theory of schizophrenia and to the neurosurgical treatment of neuropsychiatric disorders.

Identification and distribution of axonal dystrophic neurites in Alzheimer's disease

Dystrophic neurites (DNs) are one of the neuropathological characteristics of Alzheimer's disease (AD). Previously, it has been suggested that tau-immunoreactive DNs are of dendritic origin and that axonal and dendritic dystrophic neurites are morphologically indistinguishable. In the present study, two monoclonal antibodies, tau-1 and PHF-1, were used to examine sections of the hippocampal formation from AD and normal aged brains. Both antibodies stained dendritic DNs as well as axonal DNs. Axonal DNs were clearly seen in axonal fiber tracts, white matter and hippocampal terminal regions. Axonal DNs arising from neurofibrillary tangles were easily detected in CA3 and CA1. The morphological appearance of axonal DNs varied with the neuron type from which it originated. The most distinctive feature of tau-1 or PHF-1 immunostained axonal DNs was their uneven contour, alternating swollen and shrunken segments and short rod or cone shaped fragments. In contrast, dendritic dystrophic neurites are thicker and more tortuous. It appears that while DNs are both dendritic and axonal in origin, axonal DNs are more prevalent and widespread in the AD brain than previously realized and may represent one of the main pathological lesions in AD.

The distribution of Alz-50 immunoreactivity in the normal human brain

Alz-50 is a monoclonal antibody that recognizes normal tau proteins as well as phosphorylated tau proteins that are associated with paired helical filaments in Alzheimer's disease. To establish an accurate baseline for future pathological studies, we examined the distribution of Alz-50 immunoreactivity in normal human brain from infancy to senescence. We found extensive staining patterns of somata and axonal profiles in the striatum, amygdala, hypothalamus, brainstem and spinal cord in all normals at all ages. Similar normal staining patterns were seen in the brains of patients who had suffered trauma, tumors, cerebral infarcts, grade 1 periventricular hemorrhages, and in those who had suffered from amyotrophic lateral sclerosis, Parkinson's disease, multi-systems atrophy and Shy-Drager syndrome. An absence of cell body staining and only minimal axonal staining was noted in the same brains with immunocytochemistry using PHF-1, a monoclonal antibody generated against paired helical filament proteins from Alzheimer brains. The characteristic staining pattern of Alz-50 in normal brains is substantially more extensive than has previously been recognized. This pattern, which presumably describes a specific class of tau proteins, must be distinguished from the pathological staining observed in neurodegenerative diseases.

Cell-specific expression of preproenkephalin intronic heteronuclear RNA in the rat forebrain

Using in situ hybridization with multiple probes to the rat preproenkephalin gene, we have identified a novel population of cells in the reticular thalamic nucleus and basal forebrain which express RNA derived from the preproenkephalin gene. These cells contain nuclear RNA from downstream of an alternate transcription start site in intron A of the preproenkephalin gene (Kilpatrick et al., Mol. Cell Biol., 10 (1990) 3717-3726), while in the same cells preproenkephalin exon 2 RNA is undetectable. The results suggest that in this population of cells, preproenkephalin gene transcription initiates from the intron A initiation site, and is regulated by an additional mechanism which results in the accumulation of nuclear preproenkephalin intron A-derived heteronuclear RNA. The anatomical distribution of these cells indicates that they may be involved in the control of cerebral cortical function.

Selective loss and selective sparing of neurons in the thalamic reticular nucleus following human cardiac arrest

Neurons in the portion of the human thalamic reticular nucleus (RT) associated with the prefrontal cortex and mediodorsal thalamic nuclei were found to be selectively vulnerable to ischemic neuronal damage following relatively short (< or = 5-min) duration cardiac arrest. In contrast, selective sparing of these RT neurons occurred in cases with longer (> 10-min) duration of arrest that was sufficient to produce extensive ischemic neuronal damage throughout the cerebral cortex and thalamic relay nuclei. The selective degeneration of RT neurons appears to require the sustained activity of corticothalamic or thalamocortical projections to the RT following the ischemic insult. Loss of RT neurons associated with the frontal cortex and mediodorsal thalamus may be the biological basis of some types of persisting cognitive deficits in attentional processing experienced by patients following cardiac arrest, open heart surgery, or other forms of brief global cerebral ischemia.

Nerve growth factor receptor-containing cholinergic neurons of the basal forebrain project to the thalamic reticular nucleus in the rat

The origin of nerve growth factor receptor-immunoreactive (NGFr-ir) fibers innervating the thalamic reticular nucleus (Rt) was here investigated in the rat using retrograde tracers in combination with immunocytochemistry. Neurons retrogradely labeled from Rt were scattered ipsilaterally throughout the medial septal nucleus and the other cell groups of the basal forebrain, which contained NGFr-ir cells; 10-20% of these retrogradely labeled neurons were also NGFr-ir. Furthermore, a few retrogradely labeled NGFr-ir cells were detected in the basal forebrain on the contralateral side. Retrograde tracing combined with a double immunocytochemical procedure revealed that all the NGFr-ir neurons labeled from Rt also displayed immunoreactivity for choline acetyltransferase. The present results demonstrate that the NGFr-ir neurons of the basal forebrain which project to Rt are cholinergic. The possible functional implications of these findings are discussed.

Selective loss of neurons from the thalamic reticular nucleus following severe human head injury

The GABAergic neurons of the thalamic reticular nucleus, or nucleus reticularis thalami (RT), have been implicated as important components in attentional processing systems. Neurons in the RT are exquisitely sensitive to degeneration following kainic and domoic acid toxicity, experimental global ischemia, human cardiac arrest, and experimental closed head injury in nonhuman primates. The present study was performed to establish whether the selective loss of human RT neurons occurred following severe head injury. Brains from 37 human nonsurvivors of head injury were examined for evidence of RT neuronal loss. RT lesions in were found in 36 of 37 cases, representing 65 of 73 (89%) of the reticular nuclei examined. The incidence of RT lesions was similar in all age groups: 13 of 14 cases (92.9%) in the pediatric (< or = 16 years) group, 33 of 37 (89.2%) in the young adult (18-45 years) group, and 19 of 22 (86.4%) in the older adult (> 45 years) group. RT lesions were characterized by loss of one fourth to three fourths of the neurons from the region of the nucleus associated with the frontal cortex and thalamic mediodorsal (MD) and ventrolateral (VL) nuclei. Sparing of RT neurons correlated highly with the presence of extensive frontal cortical lesions, suggesting that an intact corticothalamic projection was necessary for RT degeneration following head injury. A pathologic cascade with a prominent excitotoxic component is proposed. The loss of these inhibitory thalamic reticular neurons and the resultant thalamic and neocortical neuronal dysfunctions may underlie some forms of attentional deficits that persist following head injury.

Immunohistochemical localization of the D1 dopamine receptor in rat brain reveals its axonal transport, pre- and postsynaptic localization, and prevalence in the basal ganglia, limbic system, and thalamic reticular nucleus

D1 dopamine receptor localization was examined by immunohistochemistry using a polyclonal anti-peptide antibody which (i) immunoprecipitated a protein fragment encoded by a D1 receptor cDNA and (ii) on Western blots of solubilized striatal and hippocampal membranes recognized two proteins of approximately 50 kDa and 75 kDa, corresponding to reported sizes of D1 receptor proteins. Immunoreactivity overlapped with dopamine-containing pathways, patterns of D1 receptor binding, and mRNA expression. Staining was concentrated in prefrontal, cingulate, parietal, piriform, entorhinal, and hippocampal cortical areas and subcortically in the basal ganglia, amygdala, septal area, substantia inominata, thalamus, hypothalamus, and neurohypophysis. Prominent labeling was seen in the thalamic reticular nucleus, a region known to integrate ascending basal forebrain inputs with thalamocortical and corticothalamic pathways and in fiber bundles interconnecting limbic areas. In striatal neuropil, staining appeared in spines (heads and necks), at postsynaptic sites in dendrites, and in axon terminals; in the pars reticulata of the substantia nigra, labeling was prevalent in myelinated and unmyelinated axons and dendrites. These data provide direct evidence for the regional and subcellular distribution of D1 receptor protein in the brain and for its pre- and postsynaptic localization in the basal ganglia. The prominent immunoreactivity seen in the limbic system and thalamic reticular nucleus supports an important role for this receptor subtype in mediating integrative processes involved with learning, memory, and cognition.

Decreased alpha bandwidth responsiveness to photic driving in Alzheimer disease

The power spectra of the photically activated occipital EEGs of 9 mildly to moderately demented probable Alzheimer disease (AD) patients (according to NINCDS-ADRDA criteria), 9 normal age-matched control and 27 normal subjects of different ages were compared. In normal subjects, photic stimulation with rhythmic flashes ranging between 2 and 20 Hz elicited a characteristic response in each EEG bandwidth (delta, theta, alpha, beta1 and beta2). The magnitude of each bandwidth response was a function of the frequency of the photic stimulus. In AD patients the alpha bandwidth response curve was significantly smaller than that of age-matched controls (MANOVA main effect of group, P = 0.018); all the other bandwidth response curves were normal. Therefore, in AD there is a selective abnormality in the alpha bandwidth responsiveness to photic stimulation, probably due to AD pathology in the neuronal generator of the alpha rhythm.

Neuropathological stageing of Alzheimer-related changes

Eighty-three brains obtained at autopsy from nondemented and demented individuals were examined for extracellular amyloid deposits and intraneuronal neurofibrillary changes. The distribution pattern and packing density of amyloid deposits turned out to be of limited significance for differentiation of neuropathological stages. Neurofibrillary changes occurred in the form of neuritic plaques, neurofibrillary tangles and neuropil threads. The distribution of neuritic plaques varied widely not only within architectonic units but also from one individual to another. Neurofibrillary tangles and neuropil threads, in contrast, exhibited a characteristic distribution pattern permitting the differentiation of six stages. The first two stages were characterized by an either mild or severe alteration of the transentorhinal layer Pre-alpha (transentorhinal stages I-II). The two forms of limbic stages (stages III-IV) were marked by a conspicuous affection of layer Pre-alpha in both transentorhinal region and proper entorhinal cortex. In addition, there was mild involvement of the first Ammon's horn sector. The hallmark of the two isocortical stages (stages V-VI) was the destruction of virtually all isocortical association areas. The investigation showed that recognition of the six stages required qualitative evaluation of only a few key preparations.

The axonal origin of a subpopulation of dystrophic neurites in Alzheimer's disease

Dystrophic neurites are observed characteristically in Alzheimer's disease (AD). They are thought to arise from sprouting dendrites and contribute to dementia because of their abnormal growth and close association with neurofibrillary tangles and neuritic plaques. In the present study, dystrophic neurites are demonstrated in the thalamic reticular nucleus in AD in the context of a normal neural and glial architecture. They do not collocalize with somata and dendrites identified by simultaneous labeling with the microtubule-associated protein MAP2, suggesting that they are derived from axons. Throughout the brain, dystrophic neurites may well be comprised of a heterogeneous population of both dendrites and axon terminals and preterminals. While many recent studies have focused upon the dendritic origin of dystrophic neurites, these results emphasize that the interconnectivity of certain brain regions may be compromised by cytoskeletal changes occurring in neurons and their axons in AD.

Alzheimer's disease affects limbic nuclei of the thalamus

Sensitive silver techniques for amyloid and neurofibrillary changes were applied to examine the pathological changes revealed by limbic nuclei of the thalamus in Alzheimer's disease. Large numbers of extracellular amyloid deposits occurred in almost all thalamic nuclei. The antero-ventral nucleus harbored numerous large globular patches, other areas contained more densely packed and smaller deposits, while narrow zones of gray matter subjacent to the ependymal lining of the third ventricle remained virtually devoid of amyloid. Intraneuronal neurofibrillary changes were encountered in the form of distended argyrophilic processes covering the medial convexity of the antero-ventral nucleus. Similar structures, although in considerably lesser density, occurred in the laterally adjoining reticular nucleus. The anterior nuclear complex, the latero-dorsal nucleus, portions of the intralaminar complex, the paraventricular and reuniens nucleus contained numerous neurofibrillary tangles and neuropil threads. The antero-dorsal nucleus showed the most severe involvement. At first glance, the thalamus appeared to be only mildly affected by Alzheimer's disease. Closer inspection revealed that severe changes were confined to only a few limbic nuclei. These changes were virtually identical in amount, type and location in all cases of severe Alzheimer's disease studied. It is assumed that these changes considerably hamper the transport of information through limbic circuits.

Light and electron microscopic evidence for a GABAergic projection from the caudal basal forebrain to the thalamic reticular nucleus in rats

Neurons in the magnocellular nucleus of the caudal basal forebrain extend an axonal projection which arborizes within the reticular nucleus of the thalamus. The present study addresses the ultrastructure and neurochemistry of this projection in rats. Many labeled terminals are apparent within the thalamic reticular nucleus following Phaseolus vulgaris leucoagglutinin injections into the caudal basal nucleus; anterogradely labeled axon terminals most commonly contact both somata and dendrites of reticular nucleus neurons with symmetric membrane specializations. Thus, the majority of the labeled terminals examined contrast with choline acetyltransferase positive terminals which have been previously identified as contacting dendrites and forming asymmetric synapses within this nucleus. Many of the neurons within the caudal basal nucleus which are retrogradely labeled following tracer injections into the thalamic reticular nucleus are gamma-aminobutyric acid (GABA) immunoreactive. In addition, following injections of Phaseolus vulgaris leucoagglutinin or fluoro-ruby into the caudal basal forebrain, some of the labeled axonal swellings and boutons within the thalamic reticular nucleus also contain glutamic acid decarboxylase. These results indicate that a significant component of the projection is GABAergic. These anatomical observations suggest that the projection from the caudal basal nucleus onto the thalamic reticular nucleus could facilitate the relay of information through the dorsal thalamus by inhibiting reticular nucleus neurons, and thus, in turn, disinhibiting thalamic relay neurons.