Beta-amyloid-induced neurotoxicity of a hybrid septal cell line associated with increased tau phosphorylation and expression of beta-amyloid precursor protein.

Recent evidence suggests that beta-amyloid peptide (beta-AP) may induce tau protein phosphorylation, resulting in loss of microtubule binding capacity and formation of paired helical filaments. The mechanism by which beta-AP increases tau phosphorylation, however, is unclear. Using a hybrid septal cell line, SN56, we demonstrate that aggregated beta-AP(1-40) treatment caused cell injury. Accompanying the cell injury, the levels of phosphorylated tau as well as total tau were enhanced as detected immunochemically by AT8, PHF-1, Tau-1, and Tau-5 antibodies. Alkaline phosphatase treatment abolished AT8 and PHF-1 immunoreactivity, confirming that the tau phosphorylation sites were at least at Ser(199/202) and Ser396. In association with the increase in tau phosphorylation, the immunoreactivity of cell-associated and secreted beta-amyloid precursor protein (beta-APP) was markedly elevated. Application of antisense oligonucleotide to beta-APP reduced expression of beta-APP and immunoreactivity of phosphorylated tau. Control peptide beta-AP(1-28) did not produce significant effects on tau phosphorylation, although it slightly increased cell-associated beta-APP. These results suggest that betaAP(1-40)-induced tau phosphorylation may be associated with increased beta-APP expression in degenerated neurons.

Glutamate uptake impairment and neuronal damage in young and aged rats in vivo.

The extracellular concentration of glutamate increases during hypoxia/ischemia probably due to deficient uptake. Glutamate might contribute to neuronal damage associated with this disorder and to neurodegeneration during aging. In the present study, we have tested the effect of two inhibitors of glutamate transport, L-trans-pyrrolidine-2,4-dicarboxylate and dihydrokainate, on the extracellular levels of glutamate and on neuronal damage, which was quantitatively studied by image analysis of histological brain sections. Drugs were administered by microdialysis and glutamate concentration was determined by HPLC in the striatum and the hippocampus of 3-month-old and 22-24-month-old rats. In both regions studied, the basal concentration of extracellular glutamate was higher in aged than in young rats. Pyrrolidine dicarboxylate induced a substantial elevation of extracellular glutamate in both regions, and although this increase was almost twofold higher in old than in young animals, no neuronal damage was observed. In contrast, dihydrokainate had a poor effect on glutamate levels, but induced clear neuronal damage in the striatum and the hippocampus in both groups of rats. The present results suggest that age appears not to be a significant factor in the sensitivity of neurons to the toxic effect of extracellular glutamate increase via blockade of its transport system.

Degeneration of cocultures of spinal muscular atrophy muscle cells and rat spinal cord explants is not due to secreted factors and cannot be prevented by neurotrophins.

We have shown recently that cocultures of muscle cells from infantile spinal muscular atrophy (SMA) patients innervated by motoneurons of normal rat spinal cord explants undergo a degeneration process, suggesting that muscle may play a role in this atrophy, which previously has been considered to be a pure motoneuron disease. Conditional media of SMA cocultures did not affect control healthy nerve muscle cocultures. Conversely, conditioned media of control cocultures were unable to prevent degeneration of SMA cocultures. Moreover, neurotrophic factors, thought to be of help in motoneuron disease treatment, did not protect SMA cocultures from premature death. Our results suggest that the abnormal phenotype observed in nerve-muscle coculture (1) is not due to the release of a toxic factor nor to the lack of a secreted survival factor, and (2) does not respond to neurotrophin treatment.

Prostaglandin E1 prevents apoptotic cell death in superficial dorsal horn of rat spinal cord.

Neuronal degeneration, distinguished morphologically and biochemically from necrosis, was induced in the dorsal horn of the lumbar spinal cord by chronic constriction of the sciatic nerve in rats. Neuronal apoptosis in the territory of the spinal cord which receives afferent excitatory inputs from the sciatic nerve was confirmed by TUNEL-staining and electrophoresis of genomic DNA. The morphological changes including the appearance of dark neurones, as identified by toluidine-blue staining, were almost completely blocked by 10 microg/kg of the prostaglandin E (EP) receptor agonist lipo-PGE1, incorporating PGE1 in lipid microspheres for chemical stability and targeted delivery, but not by 10 microg/kg of carbacyclin a prostacyclin (IP) receptor agonist. Lipo-PGE1 also blocked the "ladder type" fragmentation of genomic DNA extracted from tissue in the affected area of the spinal cord. Since the regional blood flow in the subfield of the spinal cord was neither influenced by the chronic injury nor by application of the vasodilative prostaglandin, the effect of lipo-PGE1 must have been achieved via another mechanism. These results demonstrate that the neuronal apoptosis in spinal cord induced by sciatic constriction injury is more effectively inhibited by the EP receptor agonist PGE1 than by the IP receptor agonist carbacyclin.

Systemic, but not intraparenchymal, administration of 3-nitropropionic acid mimics the neuropathology of Huntington's disease: a speculative explanation.

The mitochondrial toxin, 3-nitropropionic acid, has been introduced in recent years as a neurotoxin that can be administered systemically to model the many neurobehavioral correlates of Huntington's disease. In this update article, we discuss some of the many findings from experiments using the systemic 3-nitropropionic model, and provide some speculative explanations supporting this model over those utilizing conventional excitotoxins or direct intrastriatal application of 3-nitropropionic acid. We infer from our own studies and those of others that the slow process of neurodegeneration, probably through apoptotic mechanism, and the progressive locomotor dysfunctions (from dyskinesia to akinesia) inherent in Huntington's disease can be accomplished by chronic, low dose systemic administration of 3-nitropropionic acid in rodents as well as in non-human primates. This 3-nitropropionic acid model offers a new venue for investigating experimental treatment strategies for Huntington's disease.

Behaviors and neurodegeneration induced by two blockers of K+ channels, the mast cell degranulating peptide and Dendrotoxin I.

Both the Mast Cell Degranulating (MCD) peptide and Dendrotoxin I (DTX(I)), two blockers of fast activation and slowly inactivating K+ channels, induced epileptiform seizures and brain damage after intracerebroventricular injection (200 pmol) in Sprague-Dawley rats. A considerable variation in the response of the rats was observed for each toxin. The neurodegeneration included the hippocampal formation, subiculum, septum, amygdala, and the cerebellum for both toxins, and the neocortex and anterior thalamic nuclei exclusively for DTX(I) treatment.

Activation of the type 2 adrenal steroid receptor can rescue granule cells from death during development.

To determine whether activation of the type 2 adrenal steroid receptor affects granule cell death in the developing dentate gyrus, we treated rat pups with the type 2 receptor agonist RU28362 and examined degenerating cells using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) and Nissl staining. RU28362 administration decreased the numbers of degenerating granule cells suggesting that type 2 receptor activation can rescue granule cells from degeneration.

Calretinin-containing neurons in trimethyltin-induced neurodegeneration in the rat hippocampus: an immunocytochemical study.

The present study uses immunocytochemistry to investigate the behavior of the calretinin (CR)-containing neuronal subpopulation (interneurons) of the rat hippocampus in neurodegenerative processes induced by the neurotoxicant trimethyltin. Cell counts of CR-immunolabeled interneurons indicated that these cells are spared by the neurotoxicant-induced degeneration, characterized by a generalized neuronal loss, as shown by quantitative analysis after cresyl violet staining.

Riluzole reduces incidence of abnormal movements but not striatal cell death in a primate model of progressive striatal degeneration.

Riluzole has been shown recently to increase life expectancy in patients with amyotrophic lateral sclerosis. A number of experimental studies also suggest that this compound may be a neuroprotectant. We have investigated in baboons whether riluzole would protect striatal neurons from a prolonged 3-nitropropionic acid (3NP) treatment and ameliorate the associated motor symptoms. In animals receiving 3NP and the solvent of riluzole, 12 weeks of high-dose 3NP treatment resulted in the appearance of persistent leg dystonia and significant increases in the incidence of three categories of abnormal movements and in the dyskinesia index in the apomorphine test (0.5 mg/kg i.m.). Quantitative assessment of these behavioral deficits using a video movement analysis system demonstrated a significant decrease in locomotor activity and peak tangential velocity in 3NP-treated animals compared to controls. Histological analysis showed the presence of severe, bilateral, striatal lesions, localized in both caudate and putamen. Cotreatment with riluzole (4 mg/kg i.p., twice daily) significantly reduced the dyskinesia index (-35%, P < 0.02) in the apomorphine test. In the quantitative behavioral analysis, riluzole significantly ameliorated the decrease in peak tangential velocity (P < 0.02) but not the decrease in locomotor activity observed after 3NP. Comparative histological analysis of the two groups of treated animals did not demonstrate a clear neuroprotective effect of riluzole. The present study suggests that one potential therapeutic interest for riluzole in neurodegenerative disorders may reside in the reduction of motor symptoms associated with striatal lesions.

In vivo evidence for free radical involvement in the degeneration of rat brain 5-HT following administration of MDMA ('ecstasy') and p-chloroamphetamine but not the degeneration following fenfluramine.

1. Administration of 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') to several species results in a long lasting neurotoxic degeneration of 5-hydroxytryptaminergic neurones in several regions of the brain. We have now investigated whether this degeneration is likely to be the result of free radical-induced damage. 2. Free radical formation can be assessed by measuring the formation of 2,3- and 2,5-dihydroxybenzoic acid (2,3-DHBA and 2,5-DHBA) from salicylic acid. An existing method involving implantation of a probe into the hippocampus and in vivo microdialysis was modified and validated. 3. Administration of MDMA (15 mg kg-1, i.p.) to Dark Agouti (DA) rats increased the formation of 2,3-DHBA (but not 2,5-DHBA) for at least 6 h. Seven days after this dose of MDMA, the concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was reduced by over 50% in hippocampus, cortex and striatum, reflecting neurotoxic damage. There was no change in the concentration of dopamine or 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. 4. p-Chloroamphetamine (PCA), another compound which produces a neurotoxic loss of cerebral 5-HT content, when given at a dose of 5 mg kg-1 also significantly increased the formation of 2.3-DHBA (but not 2,5-DHBA) in the dialysate for over 4.5 h. post-injection starting 2 h after treatment. 5. In contrast, fenfluramine administration (15 mg kg-1, i.p.) failed to increase the 2,3-DHBA or 2,5-DHBA concentration in the dialysate. A single fenfluramine injection nevertheless also markedly decreased the concentration of 5-HT and 5-HIAA in the hippocampus, cortex and striatum seven days later. 6. When rats pretreated with fenfluramine (15 mg kg-1, i.p.) seven days earlier were given MDMA (15 mg kg-1, i.p.) no increase in 2,3-DHBA was seen in the dialysate from the hippocampal probe. This indicates that the increase in free radical formation following MDMA is occurring in 5-HT neurones which have been damaged by the prior fenfluramine injection. 7. Administration of the free radical scavenging agent alpha-phenyl-N-tert-butyl nitrone (PBN; 120 mg kg-1, i.p.) 10 min before and 120 min after an MDMA (15 mg kg-1, i.p.) injection prevented the acute rise in the 2,3-DHBA concentration in the dialysate and attenuated by 30% the long term damage to hippocampal 5-HT neurones (as indicated by a smaller MDMA-induced decrease in both the concentration of 5-HT and 5-HIAA and also the binding of [3H]-paroxetine). 8. These data indicate that a major mechanism by which MDMA and PCA induce damage to 5-hydroxytryptaminergic neurones in rat brain is by increasing the formation of free radicals. These probably result from the degradation of catechol and quinone metabolites of these substituted amphetamines. In contrast, fenfluramine induces damage by another mechanism not involving free radicals; a proposal supported by some of our earlier indirect studies. 9. We suggest that these different modes of action render untenable the recent suggestion that MDMA will not be neurotoxic in humans because fenfluramine appears safe at clinical doses.

The effect of 3-acetylpyridine on inferior olivary neuron degeneration in Lurcher mutant and wild-type mice.

Lurcher mutant and wild-type mice were given intraperitoneal injections of 3-acetylpyridine to look at the toxic effects of this drug on the inferior olivary neurons. Intraperitoneal administration of 3-acetylpyridine is characterized by the different sensitivity of inferior olivary neurons in Lurcher mutant and wild-type mice. Lurcher mutants suffered a destruction of these neurons while wild-type mice were unaffected. The results show that there is a different effect of 3-acetylpyridine between genetic mutations and wild-type mice on the same inbred strain of mice. The different affinity of 3-acetylpyridine for the inferior olivary neurons of this mutant is briefly discussed.

Suppression of cathepsins B and L causes a proliferation of lysosomes and the formation of meganeurites in hippocampus.

Cultured hippocampal slices exhibited prominent ultrastructural features of brain aging after exposure to an inhibitor of cathepsins B and L. Six days of treatment with N-CBZ-L-phenylalanyl-L-alanine-diazomethylketone (ZPAD) resulted in a dramatic increase in the number of lysosomes in the perikarya of neurons and glial cells throughout the slices. Furthermore, lysosomes in CA1 and CA3 pyramidal cells were not restricted to the soma but instead were located throughout dendritic processes. Clusters of lysosomes were commonly found within bulging segments of proximal dendrites that were notable for an absence of microtubules and neurofilaments. Although pyknotic nuclei were sometimes encountered, most of the cells in slices exposed to ZPAD for 6 d appeared relatively normal. Slices given 7 d of recovery contained several unique features, compared with those processed immediately after incubation with the inhibitor. Cell bodies of CA1 neurons were largely cleared of the excess lysosomes but had gained fusiform, somatic extensions that were filled with fused lysosomes and related complex, dense bodies. These appendages, similar in form and content to structures previously referred to as "meganeurites," were not observed in CA3 neurons or granule cells. Because meganeurites were often interposed between cell body and axon, they have the potential to interfere with processes requiring axonal transport. It is suggested that inactivation of cathepsins B and L results in a proliferation of lysosomes and that meganeurite generation provides a means of storing residual catabolic organelles. The accumulated material could be eliminated by pinching off the meganeurite but, at least in some cases, this action would result in axotomy. Reduced cathepsin L activity, increased numbers of lysosomes, and the formation of meganeurites are all reported to occur during brain aging; thus, it is possible that the infusion of ZPAD into cultured slices sets in motion a greatly accelerated gerontological sequence.

Acute corticosterone replacement reinstates performance on spatial and nonspatial memory tasks 3 months after adrenalectomy despite degeneration in the dentate gyrus.

Long-term adrenalectomy (ADX) causes loss of spatial memory and of dentate gyrus cells. These effects are prevented by chronic replacement of corticosterone (CORT). The effects of acute replacement 3 months after ADX in rats classified as ADX or incomplete ADX (INC) on the basis of saline intake, weight, and plasma CORT levels were investigated. ADX rats swam longer and farther to find a platform in a spatial water-maze task (Exp. 1) and were impaired on a nonspatial object-recognition task (Exp. 2) compared with INC and SHAM rats. In both experiments, ADX decreased the size of the dentate gyrus, and replacement with CORT either 5 or 10 days prior to and during testing restored the performance of ADX rats without affecting the size of the dentate. CORT did not affect INC and SHAM rats. Thus, the adverse effects of ADX on memory may not be due to damage in the dentate, and the effects of CORT replacement may operate outside the hippocampus.

Neurotrophic 3,9-bis[(alkylthio)methyl]-and-bis(alkoxymethyl)-K-252a derivatives.

A series of 3,9 disubstituted [(alkylthio)methyl]- and (alkoxymethyl)-K-252a derivatives was synthesized with the aim of enhancing and separating the neurotrophic properties from the undesirable NGF (trk A kinase) and PKC inhibitory activities of K-252a. Data from this series reveal that substitution in the 3- and 9-positions of K-252a with these groups reduces trk A kinase inhibitory properties approximately 100- to > 500-fold while maintaining or in certain cases enhancing the neurotrophic activity. From this research, 3,9-bis[(ethylthio)methyl]-K-252a (8) was identified as a potent and selective neurotrophic agent in vitro as measured by enhancement of choline acetyltransferase activity in embryonic rat spinal cord and basal forebrain cultures. Compound 8 was found to have weak kinase inhibitory activity for trk A, protein kinase C1 protein kinase A, and myosin light chain kinase. On the basis of the in vitro profile, 8 was evaluated in in vivo models suggestive of neurological diseases. Compound 8 was active in preventing degeneration of cholinergic neurons of the nucleus basalis magnocellularis (NBM) and reduced developmentally programmed cell death (PCD) of female rat spinal nucleus of the bulbocavernosus motoneurons and embryonic chick lumbar motoneurons.

The effect of palytoxin on neuromuscular junctions in the anococcygeus muscle of the rat.

Palytoxin, a highly toxic natural product isolated from zoanthids of the genus Palythoa, is accumulated by a wide range of fishes and marine invertebrates used as food in the Indo-Pacific. It is responsible for many incidents of human morbidity and mortality. The toxin is a potent smooth muscle spasmogen. The cause of the contraction of smooth muscle is unclear, but recent work strongly suggests that it is primarily initiated by the release of neurotransmitters from the motor innervation of the smooth muscle. We show here that palytoxin caused the swelling of the muscle cells and some internal organelles of the anococcygeus muscle of the rat, but no substantial structural damage to the tissue. Axons and Schwann cells were also swollen but the most dramatic feature was the depletion of synaptic vesicles from putative release sites in the axons. Some axons were physically damaged following exposure to the toxin, but this was relatively uncommon (< 10% of all axons studied). In the majority of axons there was no damage to nerve terminal membranes, but there was damage to mitochondria. The depletion of vesicles involved all types-clear, dense-cored, large and small. Our observations and pharmacological data gathered elsewhere, provide a neuropathological basis for the spasmogenic activity of palytoxin.

Alcohol-induced Purkinje cell loss with a single binge exposure in neonatal rats: a stereological study of temporal windows of vulnerability.

Previous research has shown that the early neonatal period of rats is one of enhanced vulnerability to cerebellar Purkinje cell loss associated with binge-like alcohol exposure, with a prominent sensitive period during the first neonatal week. In this study, an unbiased count of the total number of Purkinje cells was obtained using the stereological optical fractionator, in groups of rats given a single binge-like alcohol exposure either during the most vulnerable neonatal period [postnatal day (PD) 4] or during a later, less vulnerable period (PD 9). Using artificial rearing methods, rats were given 6.6 g/kg of alcohol either on PD 4 or on PD 9, delivered as a 15% (v/v) solution in milk formula on two consecutive feedings of the designated day. Control groups included an artificially reared gastrostomy control and a normally reared suckle control. The mean peak blood alcohol concentrations were not different between the PD 4 and PD 9 alcohol groups, averaging 374 and 347 mg/dl, respectively. The rats were perfused on PD 27. A uniform random sample of sections was obtained from serial frozen sections through the cerebellum, stained with thionin, and Purkinje cells were counted from a uniform random sample of locations on each section with the three-dimensional optical fractionator. The number of Purkinje cells in the suckle control and gastrostomy control groups did not differ from each other, averaging 3.94 (+/- 0.19) and 3.58 (+/- 0.22) x 10(5) cells, respectively. Binge exposure on PD 4 induced significant cell loss (mean of 2.05 +/- 0.20 x (10(5) Purkinje cells), whereas binge exposure on PD 9 did not induce significant Purkinje cell loss (3.70 +/- 0.39 x 10(5) Purkinje cells). These findings confirm that a single neonatal binge alcohol exposure produces pathological Purkinje cell loss, provided that it occurs during the period of enhanced vulnerability coinciding with the early stages of dendritic outgrowth.

Mechanisms and effects of intracellular calcium buffering on neuronal survival in organotypic hippocampal cultures exposed to anoxia/aglycemia or to excitotoxins.

Neuronal calcium loading attributable to hypoxic/ischemic injury is believed to trigger neurotoxicity. We examined in organotypic hippocampal slice cultures whether artificially and reversibly enhancing the Ca2+ buffering capacity of neurons reduces the neurotoxic sequelae of oxygen-glucose deprivation (OGD), whether such manipulation has neurotoxic potential, and whether the mechanism underlying these effects is pre- or postsynaptic. Neurodegeneration caused over 24 hr by 60 min of OGD was triggered largely by NMDA receptor activation and was attenuated temporarily by pretreating the slices with cell-permeant Ca2+ buffers such as 1, 2 bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM). This pretreatment produced a transient, reversible increase in intracellular buffer content as demonstrated autoradiographically using slices loaded with 14C-BAPTA-AM and by confocal imaging of slices loaded with the BAPTA-AM analog calcium green-acetoxymethyl ester (AM). The time courses of 14C-BAPTA retention and of neuronal survival after OGD were identical, indicating that increased buffer content is necessary for the observed protective effect. Protection by Ca2+ buffering originated presynaptically because BAPTA-AM was ineffective when endogenous transmitter release was bypassed by directly applying NMDA to the cultures, and because pretreatment with the low Ca2+ affinity buffer 2-aminophenol-N,N,O-triacetic acid acetoxymethyl ester, which attenuates excitatory transmitter release, attenuated neurodegeneration. Thus, in cultured hippocampal slices, enhancing neuronal Ca2+ buffering unequivocally attenuates or delays the onset of anoxic neurodegeneration, likely by attenuating the synaptic release of endogenous excitatory neurotransmitters (excitotoxicity).

Protective effects of neurotrophin-4/5 and transforming growth factor-alpha on striatal neuronal phenotypic degeneration after excitotoxic lesioning with quinolinic acid.

Lesioning of the mammalian striatum with the excitotoxin quinolinic acid results in a pattern of neuropathology that resembles that of post mortem Huntington's disease brain. Certain neurotrophic factors can rescue degenerating cells in a variety of lesion types, including those produced by neurotoxins. Several neurotrophic factors promote the survival of striatal neurons and/or are localized within the striatum. Of these factors, neurotrophin-4/5 and transforming growth factor-alpha were chosen for administration to rats lesioned with quinolinic acid. Adult rats received a single unilateral intrastriatal injection of quinolinic acid (120 nmol) and either trophic factors or the control protein cytochrome c for seven days thereafter. The pattern of phenotypic degeneration was assessed by immunocytochemical labeling of various striatal neuronal populations at five rostrocaudal levels. Quinolinic acid produced a preferential loss in the number of cells immunoreactive for glutamate decarboxylase, with a relative sparing of the number of choline acetyltransferase-immunoreactive cells and, to a lesser degree, calretinin-immunoreactive cells. None of these phenotypic populations was protected by either neurotrophin-4/5 or transforming growth factor-alpha. In contrast, when glutamate decarboxylase cells were alternatively identified by calbindin immunolabeling, both factors were found to have partially reversed the loss in the number of calbindin-positive cells induced by excitolesioning. In addition, the loss in the number of parvalbumin-immunopositive cells due to quinolinic acid was partially reversed by neurotrophin-4/5, while the loss in the number of NADPH-diaphorase-stained cells was partially reversed by transforming growth factor-alpha. These findings reveal a new population of striatal cells, calretinin neurons, that are relatively resistant to quinolinic acid toxicity and that neurotrophin-4/5 and transforming growth factor-alpha partially protect against the phenotypic degeneration of striatal cell populations in an in vivo animal model of Huntington's disease.

Retrograde degeneration and colchicine protection of basal forebrain cholinergic neurons following hippocampal injections of an immunotoxin against the P75 nerve growth factor receptor.

Intracerebroventricular injection of 192 IgG antibody against the p75LNGFR rat low affinity nerve growth factor receptor conjugated with saporin, a ribosome inactivating protein, has been shown to destroy the p75LNGFR-expressing cholinergic neurons of the basal forebrain. We injected this immunotoxin into the hippocampus and studied its retrograde effect upon the cholinergic neurons of the medial septum and the vertical limb of the diagonal band of Broca. Seven days after injection, there was a nearly total depletion of cholinergic axons within the hippocampus. This depletion was associated with a marked and significant decrease in the number of cholinergic neurons of the ipsilateral medial septum and the vertical limb of the diagonal band of Broca. At longer survival times, these changes were more pronounced. Parvalbumin-positive, GABAergic neurons within the same areas of the basal forebrain were not affected by immunotoxin injections. Injections of saporin alone had no effect upon cholinergic neurons. Simultaneous injection of colchicine with the immunotoxin resulted in a significant reduction of retrograde degeneration of cholinergic neurons and relative preservation of hippocampal cholinergic axons. These observations suggest that 192 IgG-saporin is transported retrogradely from the hippocampus to the cholinergic neurons in the medial septum and the vertical limb of the diagonal band of Broca and provide a model for retrograde degeneration of basal forebrain cholinergic neurons following cortically based toxic-pathologic processes.