Differential expression of immediate early genes in distinct layers of rat cerebral cortex after selective immunolesion of the forebrain cholinergic system

d-ribose improves cardiac contractility and hemodynamics, and reduces expression of c-fos in the hippocampus during sustained slow ventricular tachycardia in rats

BACKGROUND

Moderate hypotension during hemodynamically stable ventricular tachycardia (VT), leads to cerebral ischemia. Supplementation of d-ribose has been shown to improve cardiac metabolism. We hypothesized that cerebral ischemia during slow VT may lead to the expression of immediate early genes related to neurodegeneration. This expression may be prevented by d-ribose substitution.

METHODS

Slow VT was induced over 20 min by external left ventricular pacing after infusion of physiologic saline or d-ribose (450 mg/kg) in 44 rats. Different coloured microspheres were used for tissue blood flow measurements. Histochemistry of c-fos in cerebral tissue sections was performed.

RESULTS

With the onset of VT, the mean arterial pressure (MAP) significantly dropped in both groups. However, the MAP in the d-ribose group was significantly higher (p<0.05) than in the control group (111+/-21 mm Hg vs. 80+/-40 mm Hg). The rate pressure product (RPP) during VT was significantly higher in the d-ribose group than in the control group (75,000 vs. 59,000, p<0.05). The occurrence of lethal VT was significantly higher in the control group and could be prevented by d-ribose. A stable activation of c-fos was observed in the control group. This ischemic stress response of the brain could not be seen after d-ribose infusion.

CONCLUSION

d-ribose improves hemodynamic parameters, cardiac contractility and prevents the activation of pro-apoptotic c-fos, demonstrating a neuroprotective effect of d-ribose during slow VT.

Behavioral and immunohistological effects of cholinergic damage in immunolesioned rats: Alteration of c-Fos and polysialylated neural cell adhesion molecule expression

The aim of this study was to determine the brain structures as well as the plasticity events associated with the behavioral effects of cholinergic damage. Rats were submitted to injection of 192 IgG-saporin in the medial septum/diagonal band of Broca complex and the nucleus basalis magnocellularis. The immunohistochemical expression of c-Fos protein and PSA-NCAM (polysialylated neural cell adhesion molecule) and the behavioral performances in the nonmatching-to-position task were assessed at various post-lesion times. Thus, 3 days after injection of the immunotoxin, increased c-Fos labeling was observed in the areas of infusion, indicating these cells were undergoing some plastic changes and/or apoptotic processes. A drastic increase was observed in the number of PSA-NCAM positive cells and in their dendritic arborization in the dentate gyrus. At 7 days post-lesion, no behavioral deficit was observed in immunolesioned rats despite the drastic loss of cholinergic neurons. These neurons showed decreased c-Fos protein expression in the piriform and entorhinal cortex and in the dentate gyrus. In the latter, PSA-NCAM induction was high, suggesting that remodeling occurred, which in turn might contribute to sustaining some mnemonic function in immunolesioned rats. At 1 month, cholinergic neurons totally disappeared and behavioral deficits were drastic. c-Fos expression showed no change. In contrast, the increased PSA-NCAM-labeling observed at short post-lesion times was maintained but the plastic changes due to this molecule could not compensate the behavioral deficit caused by the immunotoxin. Thus, as the post-lesion time increases, a gradual degeneration process should occur that may contribute to mnemonic impairments. This neuronal loss leads to molecular and cellular alterations, which in turn may aggravate cognitive deficits.

Changes in cortical acetyl-CoA metabolism after selective basal forebrain cholinergic degeneration by 192IgG-saporin

The aim of the present study was to reveal whether reduced cortical cholinergic input affects the acetyl-CoA metabolism in cholinoceptive cortical target regions which may play a causative role for the deficits in cerebral glucose metabolism observed in Alzheimer's disease. The effect of cortical cholinergic denervation produced by a single intracerebroventricular application of the cholinergic immunotoxin 192IgG-saporin, on activities of pyruvate dehydrogenase and adenosine triphosphate (ATP)-citrate lyase as well as on the level of synaptoplasmic and mitochondrial acetyl-CoA and acetylcholine release in cortical target regions was studied. Cholinergic lesion produced 83%, 72% and 32% decreases in the activities of choline acetyltransferase, acetylcholinesterase and ATP-citrate lyase in nerve terminals isolated from rat brain cortex, respectively, but no change in pyruvate dehydrogenase activity. Spontaneous and Ca2+-evoked acetylcholine release from synaptosomes was inhibited by 76% and 73%, respectively, following immunolesion. The lesion-induced 39% decrease of acetyl-CoA level in synaptosomal mitochondria was accompanied by 74% increase in synaptoplasmic fraction. Levels of acetyl-CoA and CoASH assayed in fraction of whole brain mitochondria from lesioned cortex were 61% and 48%, respectively, higher as compared to controls. The data suggest a preferential localization of ATP-citrate lyase in cholinergic nerve terminals, where it may contribute to the transport of acetyl-CoA from the mitochondrial to the cytoplasmic compartment. They provide evidence on differential distribution of acetyl-CoA in subcellular compartments of cholinergic and non-cholinergic nerve terminals. There are also indications that cholinergic activity affects acetyl-CoA level and its intracellular distribution in glial and other non-cholinergic cortical cells.

Changes in activity and expression of phosphofructokinase in different rat brain regions after basal forebrain cholinergic lesion

Selective lesion of rat basal forebrain by the cholinergic immunotoxin 192IgG-saporin was used as an animal model to address the question of whether the changes in cortical glucose metabolism observed in patients with Alzheimer's disease may be related to impaired cholinergic transmission. At different times after creating the immunolesion, the isoenzyme pattern and steady-state mRNA levels of the key glycolytic enzyme phosphofructokinase were determined in cortex, hippocampus, basal forebrain and nucleus caudatus. The loss of cholinergic input was accompanied by a persistent decrease in choline acetytransferase and acetylcholine esterase activities in the cortical target areas similar to the cholinergic malfunction seen in Alzheimer's dementia. The basal forebrain lesion induced by the immunotoxin resulted in a transient increase in phosphofructokinase activity peaking on day 7 after inducing the lesion in cortical areas. In parallel, an increased steady-state level of phosphofructokinase mRNA was determined by RT/real-time PCR and in situ hybridization. In contrast, analysis by western blotting and quantitative PCR revealed no changes in the phosphofructokinase isoenzyme pattern after immunolesion. It is concluded that common metabolic mechanisms may underlie the degenerative and repair processes in denervated rat brain and in the diseased Alzheimer's brain.

Rat basal forebrain cholinergic lesion affects neuronal nitric oxide synthase activity in hippocampal and neocortical target regions

Nitric oxide (NO)-mediated mechanisms have been assigned a role in cortical perfusion, learning and memory as well as in neuronal plasticity. Dysfunction of cortical cholinergic transmission has also been associated with reduced cortical cerebral blood flow and impaired performance in learning and memory tasks suggesting a link between the basal forebrain cholinergic system and cortical NO-mediated mechanisms. The aim of this study was therefore to study the influence of cholinergic input on neuronal NO-synthase (nNOS) activity in cortical cholinoceptive target neurons. A nearly complete loss of rat basal forebrain cholinergic cells was induced by a single intracerebroventricular application of the cholinergic immunotoxin 192IgG-saporin. Basal forebrain cholinergic hypofunction resulted in reduced catalytic and substrate binding activity of nNOS in a number of hippocampal and neocortical subregions 7 days after lesion as revealed by NADPH-diaphorase enzyme histochemistry and quantitative autoradiography of [3H]L-N(G)-nitro-arginine binding, respectively. The total amount of nNOS protein assayed by Western analysis, was not affected in the cortical and hippocampal regions examined. The data indicate that cortical cholinergic deafferentation results in reduced nNOS activity in select cholinoceptive neocortical and hippocampal neurons. As the total amount of cortical nNOS protein was not affected by basal forebrain cholinergic lesion, the results suggest that the ratio of catalytically active and inactive cortical nNOS is driven by basal forebrain cholinergic input presumably via M1-muscarinic cholinergic receptors.

Repeated immunolesions display diminished stress response signal

Cholinergic basal forebrain neurons (CBFNs) retrogradely transport neurotrophins released in the hippocampus and cortex as part of a general response to injury in a process that is impaired in the aged rodent and can be spared by the exogenous addition of pharmacological doses of nerve growth factor (NGF). This observation suggests that components of stress response signal transduction pathways in the aged CNS can be exogenously activated. The extent and mechanism of the endogenous stimulation of NGF in response to injury can be mimicked via treatment with 192 IgG-saporin of rat CNS, an immunolesion model. Here we report on the use of a conditioning lesion paradigm to determine if repeated partial immunolesions have a conditioning effect on the immunolesion-induced increases in NGF protein or decreases in choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity. We report that chronic repeated immunolesions, as used here, were not as effective as a one time equivalent immunolesion in terms of induced NGF protein increases or decreasing ChAT and AChE activity in the hippocampus and cortex. Thus, chronic lesions resulting in cholinergic impairment typical of the aged CNS may differ from acute toxic models as a result of desensitization due to a conditioning effect of chronic subthreshold lesioning events in the CNS.

Intracerebroventricular infusion of CHO5, a rat monoclonal antibody directed against mouse low-affinity nerve growth factor receptor (p75NTR), specifically labels basal forebrain cholinergic neurons in mouse brain

The finding that basal forebrain cholinergic cells are specifically endowed with the low-affinity nerve growth factor receptor p75NTR has been employed to develop a cholinergic immunotoxin in rats by covalently linking the monoclonal antibody 192IgG against the rat p75NTR with the cytotoxic protein saporin (192IgG-saporin). Following intracebroventricular application of 192IgG-saporin, the antibody conjugate is taken up into cholinergic cells via the p75NTR, retrogradely transported to the cell body, where saporin exerts cytotoxic action. The lack of an appropriate antibody directed against mouse p75NTR has been hampered the development of a mouse-specific cholinergic immunotoxin, which should be a useful tool to study effects of cortical cholinergic deficits on processing of amyloid precursor protein in transgenic mice with Alzheimer pathology. To develop an appropriate mouse-specific immunotoxin, a variety of antibodies directed against mouse p75NTR were tested. Using double labeling immunocytochemistry, the rat monoclonal antibody CHO5 against mouse p75NTR was found to label mouse basal forebrain neurons, which also demonstrated immunoreactivity for choline acetyltransferase and the high-affinity nerve growth factor receptor, TrkA. Intracerebroventricular infusion of CHO5 in mice resulted in an accumulation of the antibody in cholinergic cells within the basal forebrain, suggesting that CHO5 is a suitable candidate to develop a mouse-specific cholinergic immunotoxin.

NGF treatment potentiates c-fos expression in the rat nucleus basalis upon excitotoxic lesion with quisqualic acid

The induction of the c-fos gene in the rat brain by NGF was studied in a model of acute cholinergic hypofunction, i.e., the lesion of the nucleus basalis magnocellularis (NBM) with quisqualic acid. Choline acetyltransferase and Fos immunoreactivity (IR) in the NBM were analyzed at different times after the excitotoxic lesion. NGF treatment induced a potentiation of Fos expression 4 and 24 h after lesion. The possibility is discussed that c-fos induction is one of the early mechanisms of the neuroprotective action of NGF.

In vivo [125I]-iodobenzovesamicol binding reflects cortical cholinergic deficiency induced by specific immunolesion of rat basal forebrain cholinergic system

In this study, radiolabeled iodobenzovesamicol (IBVM), which is known to bind with high affinity to the vesicular acetylcholine transporter, was tested for its usefulness in imaging cortical cholinergic deficits in vivo. To induce reductions in cortical cholinergic input, the cholinergic immunotoxin 192IgG-saporin was employed. This has been shown to selectively and efficiently destroy basal forebrain cholinergic neurons in rats. The efficiency of the immunolesion was verified by histochemical acetylcholinesterase staining. [125I]-IBVM binding before and after lesioning was measured using autoradiography. Basal forebrain cholinergic cell loss resulted in a considerable reduction in [125I]-IBVM binding in the cholinoceptive target regions, but not in the striatum and cerebellum, brain regions that do not receive a cholinergic input by the basal forebrain cholinergic nuclei, suggesting that [123I]-IBVM has potential in imaging cortical cholinergic deficits in vivo, at least in animals.

Iodo-QNB cortical binding and brain perfusion: effects of a cholinergic basal forebrain lesion in the rat

This study deals with the question of whether in vivo application of [125I]iodo-quinuclidinyl-benzilate (QNB) is able to demonstrate changes in cortical muscarinic receptor density induced by a cholinergic immunolesion of the rat basal forebrain cholinergic system, and whether the potential effects on IQNB distribution in vivo are also associated with effects on regional cerebral perfusion. Immunolesioned and control animals were injected with (R,S) [125]iodo-QNB and with [99mTc]-d,l-hexamethylpropyleneamine oxime (HMPAO). The cerebral distribution of both tracers was imaged using double tracer autoradiography. Impaired cholinergic transmission was paralleled by a 10-15% increase of [125I]iodo-QNB binding in the regions of cortex and hippocampus. The local cerebral blood flow remained unchanged after cholinergic lesion.

Simulation of cortical cholinergic deficits--a novel experimental approach to study pathogenetic aspects of Alzheimer's disease

Cholinergic lesion paradigms have been used to study the role of the cholinergic system in cognitive function, and its implication in cognitive deficits that occur in Alzheimer's disease. In the last few years an increasing number of studies have applied neurotoxins including excitotoxins or cholinotoxins to produce reductions in cortical cholinergic activity. One of the most serious limitations of these lesion paradigms is the fact that the cytotoxins used are far from being selective to cholinergic cells. Recently, a monoclonal antibody to the low-affinity nerve growth factor (NGF) receptor, 192IgG, coupled to a cytotoxin, saporin, has been described as an efficient and selective immunotoxin for the NGF-receptor bearing cholinergic neurons in rat basal forebrain. Here we demonstrate the usefulness of 192IgG-saporin as a powerful tool for producing an animal model with selective and specific basal forebrain cholinergic lesions in rats which can be applied to simulate some neurochemical sequelae of Alzheimer's disease including cholinergic mechanisms in processing of the amyloid precursor protein, and could be of particular value to elaborate and to test therapeutical strategies compensating for the reduced cortical cholinergic input.

Induction of apoptosis in the CNS during development by the combination of hyperoxia and inhibition of glutathione synthesis

Apoptosis in the central nervous system (in contrast to necrosis) is an endogenous cell suicide mechanism triggered in response to biological factors and genotoxic stimuli often resulting from oxidative stress. Excessive neural apoptosis may result in longterm brain dysfunction. A significant proportion of prematurely born infants are exposed to high oxygen and nutritional regimens deficient in antioxidant precursors. Such infants frequently display cognitive deficits when studied in later childhood. Studies in cell culture have characterized a close relationship between oxidative stress, glutathione availability and cell death. Here, we assessed this relationship in rat brain, as a model approximation of the situation that occurs in human infants. Two day old rats were exposed to an atmosphere of 95% oxygen and treated with buthionine sulfoximine (BSO), a glutathione synthesis inhibitor. Control groups consisted of rat-pups kept in air, air plus BSO, or oxygen alone. At the end of 5 days of treatment, brains were harvested, dissected and nerve growth factor protein (NGF), glutathione, and extent of apoptosis were measured. Hyperoxia induced a decrease in NGF protein while BSO induced a decrease in glutathione concentrations. Animals treated with both hyperoxia and BSO had a dramatic increase in the extent of brain apoptosis detected. We conclude from these studies that the brains of animals exposed to both oxidative stress and limited antioxidant protection are liable to pro-apoptotic changes. Increased cell death via apoptosis reflecting changes in neurotrophin and glutathione homeostasis may represent the mechanism responsible for the induction of the longterm cognitive deficits observed in some preterm infants.

Immunolesion of the cholinergic basal forebrain: effects on functional properties of hippocampal and septal neurons

Deficits in cholinergic function have been documented in a variety of brain disorders including Alzheimer's Disease and, to a lesser extent, in normal ageing. In the present article, we have reviewed our recent findings on the effects of the loss of basal forebrain cholinergic neurons on the functional properties of the septohippocampal pathway. In vivo and ex vivo investigations were performed in rats following basal forebrain cholinergic lesion with the specific immunotoxin 192 IgG-saporin. Our results suggest a significant contribution of cholinergic neurons in the rhythmically bursting activity recorded within the medial septum. In addition, they give evidence that acetylcholine may tonically decrease the glutamatergic synaptic responses in the hippocampus whereas the GABAergic mediated inhibitory potentials are not affected. The possible contribution of these cholinergic mechanisms in the age-related functional alterations of the septohippocampal activity is discussed.

Cerebrospinal fluid cholinesterases--markers for loss of cholinergic basal forebrain neurons?

The present study was conducted to test the hypothesis that cholinergic basal forebrain neurons are a major source of cerebrospinal fluid (CSF) cholinesterases. To address this question enzyme activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in both CSF and parietal cortex were assayed following selective lesion of basal forebrain cholinergic neurons by a single intracerebroventricular application of the cholinergic immunotoxin 192IgG-saporin. Cholinergic immunolesions led to a dramatic decrease in total AChE activity in parietal cortex, which was due to the specific loss of the G4 molecular form while the activity of the G1 form was increased as compared to nonlesioned animals. In contrast, the total enzyme activity of BChE and its molecular forms were not affected by cholinergic lesion in both parietal cortex and CSF. The data suggest, that cholinergic basal forebrain neurons are seemingly not a major source of cholinesterases in the CSF, and do not provide any evidence for using CSF cholinesterases as a diagnostic marker of basal forebrain cholinergic cell loss in humans.

Responses in the aged rat brain after total immunolesion

In the present study, we compare the effects of cholinergic deafferentation of the hippocampus, cortex, and olfactory bulb of young and aged rats on nerve growth factor (NGF) protein levels in these areas. We also describe glial responses to intraventricular injections of the immunotoxin, 192 IgG-saporin in the aged. Choline acetyltransferase (ChAT) activity was dramatically decreased in the basal forebrain and target areas of the cholinergic basal forebrain neurons (CBFNs) in the young immunolesioned rats and to a lesser extent in their aged counterparts. After total immunolesion, NGF protein levels significantly increased in the hippocampus, cortex, and olfactory bulb of the young rats but not of the aged rats, except for small increases in the olfactory bulb after two weeks. After immunolesion NGF protein levels in the basal forebrain increased in young rats and less so in the aged rats. The total immunolesions had no effects on NGF and BDNF mRNA levels in the hippocampus and cortex. Two weeks after injection of the immunotoxin, the profiles of AChE- and p75NTR-positive cells significantly decreased in medial septum, vertical and horizontal limbs of diagonal band and nucleus basalis of Meynert. There was also an increase in microglia while but not astrocytes in the subnuclei of basal forebrain. In conclusion, 192 IgG-saporin was effective in producing cholinergic lesions in both young and aged rat brains, the lesion-induced NGF response was partially extinguished in the aged rat brains and immunolesions induced a microglial response in aged brain.

Long term changes in brain cholinergic markers and nerve growth factor levels after partial immunolesion

There are deficits in cholinergic basal forebrain neurons (CBFNs) in the aged brain and patients suffering Alzheimer's disease associated with a partial loss of the CBFNs. To mimic this partial loss and assess its long term effects on residual cholinergic activity and resultant target-derived nerve growth factor (NGF) levels, we produced a partial immunolesion to CBFNs with 192 IgG-saporin, an immunotoxin selectively taken up by p75NTR-bearing neurons. We measured two cholinergic markers, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and NGF protein levels at 10 days, 1, 6 and 12 months postlesion. There were no significant changes in the cholinergic markers and the NGF protein levels in the sham-treated animal controls during the one year experiment. Ten days after 192 IgG-saporin treatment, ChAT activity decreased to 35-50% of controls in the olfactory bulb, hippocampus, and cortex. There was a minor but significant recovery of ChAT activity one year after the immunolesion in the hippocampus. Changes in AChE activity mirrored the ChAT changes but were less robust. There were transient increases in NGF protein levels in the hippocampus and cortex that returned to basal levels at 6 months and 12 months postlesion, respectively. In summary, partial immunolesions resulted in partial region-specific and time-dependent recoveries of cholinergic activity in the target areas of the basal forebrain after a partial elimination of CBFNs and a return to basal levels of NGF protein consistent with the hypothesis that the remaining CBFNs compensated for losses of ChAT and NGF due to changes in cholinergic innervation of basal forebrain target areas.

Muscarinic acetylcholine receptors activate the acetylcholinesterase gene promoter

The acetylcholinesterase (AChE) gene promoter contains several overlapping binding sites for Sp1 and Egr-1 transcription factors. Cotransfection experiments and promoter assays showed that Egr-1 can potently activate transcription from the human AChE promoter. Muscarinic acetylcholine receptors (mAChR) rapidly activate, via protein kinase C-mediated signaling, expression of the Egr-1 gene, leading to dramatically increased nuclear concentrations of Egr-1 protein, and to increased binding of Egr-1 to specific DNA recognition sequences. These mAChR-induced increases are followed by increased transcription from the human AChE promoter. In vivo studies with intraventricular infusions of the cholinergic immunotoxin 192 IgG saporin showed more than 80% decrease of AChE activity in cholinergic target areas of the hippocampus and brain cortex. The results are compatible with a combination of decreased AChE activity in degenerating subcortical cholinergic projections, and additional decreases in postsynaptic AChE gene expression. Together our data show that mAChR can activate transcription from the AChE promoter via increased synthesis of Egr-1. The results suggest a feedback mechanism by which the AChE gene is activated by cholinergic neurotransmission, possibly leading to increased formation of AChE protein and accelerated degradation of acetylcholine at cholinergic synapses. This possibility suggests testing of cholinomimetic compounds currently in development for the treatment of Alzheimer's disease for their potential ability to increase AChE gene expression.

Interleukin-6 is not expressed in activated microglia and in reactive astrocytes in response to lesion of rat basal forebrain cholinergic system as demonstrated by combined in situ hybridization and immunocytochemistry

Interleukin-6 may play an essential role in early inflammatory processes as response to degenerating cholinergic cells in the nucleus basalis of Meynert in patients suffering Alzheimer's disease. The cholinergic immunotoxin, 192IgG-saporin, was applied to produce selective and specific degenerations of basal forebrain cholinergic cells. To disclose the lesion-induced temporal cascade of the expression pattern of IL-6, and to reveal the cellular source for production and secretion of IL-6 in vivo after endogeneously induced basal forebrain cholinergic cell loss, both in situ hybridization and immunocytochemistry for IL-6 were performed. To identify the cell types expressing IL-6 mRNA, double labeling techniques were applied combining in situ hybridization technique with immunocytochemistry and lectin histochemistry for both micro- and astroglia and a number of neuronal markers including choline acetyltransferase, parvalbumin, and neurofilaments. In the intact brain, IL-6 is mainly localized in neurons, in particular in both cholinergic and GABAergic neurons of the basal forebrain. Although basal forebrain cholinergic lesion resulted in a dramatic increase in the number of micro- and astroglial cells at the lesion site, IL-6 expression could not be detected in any of the lesion-induced activated glial cell types. Moreover, cholinergic lesion led to a reduced number of IL-6-expressing cells in the basal forebrain, which is assumed to be due to the loss of cholinergic cells. The predominantly neuronal localization in rat brain suggests a role for IL-6 in activating micro- and astroglial cells in response to degenerating cholinergic neurons.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.

In vivo regulation of amyloid precursor protein secretion in rat neocortex by cholinergic activity

The proteolytic cleavage of the amyloid precursor protein (APP) has been shown to be modulated through specific muscarinic receptor activation in vitro in both transfected cell lines and native brain slices, whereas a demonstration of receptor-mediated control of APP processing under in vivo conditions is still lacking. To simulate alterations in muscarinic receptor stimulation in vivo, we have (i) specifically reduced the cortical cholinergic innervation in rats using partial immunolesions with 192IgG-saporin, and (ii) restored cholinergic function in lesioned rats by transplantation of nerve growth factor producing fibroblasts. While total APP levels in cortical homogenates were unaffected by cholinergic deafferentation, we observed a significant reduction in the abundance of secreted APP and a concomitant increase in membrane-bound APP. These changes were reversed in immunolesioned rats with nerve growth factor-producing fibroblasts. There was a strong positive correlation between the ratio of secreted APP to membrane-bound APP and the activity of choline acetyltransferase and M1 muscarinic acetylcholine receptor density (measured by [3H]pirenzepine binding) in experimental groups. Additionally, we observed a transient decrease in the ratio of cortical APP transcripts containing the Kunitz protease inhibitor domain (APP 770 and APP 751) versus APP 695 in rats with cholinergic hypoactivity. The data presented suggest that cortical APP processing is under basal forebrain cholinergic control, presumably mediated through M1 muscarinic acetylcholine receptors on cholinoceptive cortical target cells.

Basal forebrain cholinergic immunolesion by 192IgG-saporin: evidence for a presynaptic location of subpopulations of alpha 2- and beta-adrenergic as well as 5-HT2A receptors on cortical cholinergic terminals

To study whether the changes in cortical noradrenergic and serotonergic mechanisms observed in patients with Alzheimer's disease are the consequence of reduced cortical cholinergic activity, a novel colinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the lower affinity nerve growth factor receptor with the cytotoxic protein saporin, 192IgG-saporin) was used to produce a specific and selective loss of cholinergic cells in rat basal forebrain nuclei. To correlate the responses to cholinergic immunolesion in cholinoceptive cortical target regions with cholinergic hypoactivity, quantitative receptor autoradiography to measure adrenoceptors and 5-hydroxytryptamine (5-HT) receptor subtypes, and histochemistry to estimate acetylcholinesterase activity, were performed in adjacent brain sections. alpha 1-adrenoceptor and 5-HT1A receptor binding were not affected by cholinergic immunolesion in any of the cortical and hippocampal regions studied. However, cholinergic immunolesion resulted in significantly reduced alpha 2- and beta-adrenoceptor as well as 5-HT2A receptor binding in a number of cortical and hippocampal regions displaying a reduced activity of acetylcholinesterase, already detectable seven days after a single injection of 192IgG-saporin and persisting up to three months post lesion without any significant recovery. The data suggest that at least a subpopulation of alpha 2- and beta-adrenoceptor as well 5-HT2A receptor subtype is present on cortical and hippocampal cholinergic terminals originating in the basal forebrain. The lesion-induced receptor changes suggest that the alterations in cortical 5-HT2 receptor binding observed in patients with Alzheimer's disease might be secondary to cholinergic deficits.

Electron microscopic evidence for microglial phagocytic activity and cholinergic cell death after administration of the immunotoxin 192IgG-saporin in rat

192IgG-saporin represents a novel cholinergic immunotoxin which selectively and specifically destroys cholinergic cells in rat basal forebrain. Activated microglial cells are known to play an important role in phagocytosis in regions of neuronal loss. To study the immunotoxin-induced phagocytic events in the basal forebrain activated microglial cells were visualized by lectin cytochemistry using Griffonia simplicifolia agglutinin and analyzed by electron microscopy. Three and 7 days following an intracerebro-ventricular injection of 4 microg 192IgG-saporin, increased numbers of activated microglial cells were observed at both survival times, but the number was strikingly increased at day 7 postlesion. Three days after immunotoxin application microglial cells displayed features similar to those of resting microglia. Only translucent vacuole-like hollows were found intracellularly beneath the plasma membrane of microglial cells and in the adjoining extracellular space. Most neurons in the vicinity of microglial cells did not show any signs of degeneration. However, 7 days after injection of the immunotoxin microglial cells revealed different stages of phagocytosis. The majority of microglial cells were localized in perineuronal positions attached by processes to large areas of neuronal soma or dendrites, which in general showed signs of severe degeneration. The present study provides electron microscopic evidence for phagocytic microglial reactions in the rat basal forebrain after cholinergic lesion by 192IgG-saporin.

Expression of amyloid precursor protein mRNA isoforms in rat brain is differentially regulated during postnatal maturation and by cholinergic activity

Pathological processing of the amyloid precursor protein (APP) is assumed to be responsible for the amyloid deposits in Alzheimer-diseased brain tissue, but the physiological function of this protein in the brain is still unclear. The aim of this study is to reveal whether the expression of different splicing variants of APP transcripts in distinct brain regions is driven by postnatal maturation and/or regulated by cortical cholinergic transmission, applying quantitative in situ hybridization histochemistry using 35S-labeled oligonucleotides as specific probes to differentiate between APP isoforms. In cortical brain regions, the expression of both APP695 and APP751 is high at birth and exhibits nearly adult levels. The developmental expression pattern of cortical APP695 displays a peak value around postnatal day 10, while the age-related expression of APP751 demonstrates peak values on postnatal days 10 and 25, with the highest steady state levels of APP751 mRNA on day 25. During early development, the cortical laminar distribution of the APP695, but not APP751, mRNA transiently changes from a more homogeneous distribution at birth to a pronounced laminar pattern with higher mRNA levels in cortical layer III/IV detectable at the age of 4 days and persisting until postnatal day 10. The distinct age-related changes in cortical APP695 and APP751 mRNA levels reflect the functional alterations during early brain maturation and suggest that APP695 might play a role in establishing the mature connectional pattern between neurons, whereas APP751 could play a role in controlling cellular growth and synaptogenesis. Lesion of basal forebrain cholinergic system by the selective cholinergic immunotoxin 192IgG-saporin resulted in decreased levels of APP695 but not APP751 and APP770 transcripts by about 15-20% in some cortical (cingulate, frontal, parietal, piriform cortex), hippocampal regions (CA1, dentate gyrus), and basal forebrain nuclei (medial septum, vertical limb of diagonal band), detectable not earlier than 30 days after lesion and persisting until 90 days postlesion, suggesting that the nearly complete loss of cortical cholinergic input does not have any significant impact on the expression of APP mRNA isoforms in cholinoceptive cortical target regions.

Trans-synaptic stimulation of cortical acetylcholine release after partial 192 IgG-saporin-induced loss of cortical cholinergic afferents

Environmental and pharmacological stimulation of cortical acetylcholine (ACh) efflux was determined in rats sustaining partial deafferentation of cortical cholinergic inputs. Rats were bilaterally infused with the selective cholinotoxin 192 IgG-saporin (0.005 microgram/0.5 microliter/site) into the frontoparietal cortex. In the first experiment, animals were pretrained to associate the onset of darkness with presentation of a palatable fruit cereal reward. The ability of this stimulus to enhance frontoparietal ACh efflux alone, and with the benzodiazepine receptor (BZR) weak inverse agonist ZK 93,426 (1.0 or 5.0 mg/kg, i.p.), was determined in lesioned and sham-lesioned rats. Intracortical infusions of 192 IgG-saporin reduced basal cortical ACh efflux by 47% of sham-lesioned values, consistent with reductions in the density of AChE-positive fibers. In spite of this deafferentation, ZK 93,426 produced a transient potentiation of the cortical ACh efflux induced by the darkness/cereal stimulus similar to that observed in control animals. In the second experiment, the ability of the more efficacious BZR partial inverse agonist FG 7142 (8.0 mg/kg, i.p.) to enhance basal cortical ACh efflux was compared in lesioned and sham-lesioned rats. Again, lesioned rats exhibited an increase comparable to control animals after FG 7142. This drug-induced stimulation of cortical ACh efflux was comparably and completely blocked in both groups by co-perfusion with tetrodotoxin (1.0 microM). These results suggest similarities in the modulation of cortical ACh efflux in intact and partially deafferented rats and indicate the potential of BZR inverse agonists for restoring transmission in animals with partial loss of cortical cholinergic inputs.

Effects of intraventricular transplantation of NGF-secreting cells on cholinergic basal forebrain neurons after partial immunolesion

The aim of the present study was to examine the effects of nerve growth factor on brain cholinergic function after a partial immunolesion to the rat cholinergic basal forebrain neurons (CBFNs) by 192 IgG-saporin. Two weeks after intraventricular injections of 1.3 micrograms of 192 IgG-saporin, about 50% of CBFNs were lost which was associated with 40-60% reductions of choline acetyltransferase (ChAT) and high-affinity choline uptake (HACU) activities throughout the basal forebrain cholinergic system. Two groups of lesioned animals received intraventricular transplantations of mouse 3T3 fibroblasts retrovirally transfected with either the rat NGF gene (3T3NGF+) or the retrovirus alone (3T3NGF-) and were sacrificed eight weeks later. In vivo production of NGF by 3T3NGF+ cells was confirmed by NGF immunohistochemistry on the grafts and NGF immunoassay on cerebrospinal fluid (CSF) samples. Both ChAT and HACU activities returned to normal control levels in the basal forebrain and cortex after 3T3NGF+ transplants, whereas no recovery was observed in 3T3NGF- transplanted animals. There was a 25% increase in the size of remaining CBFNs and an increased staining intensity for NGF immunoreactivity in these cells after NGF treatments. Acetylcholinesterase (AChE) histochemistry revealed that the optical density of AChE-positive fibers in the cerebral cortex and hippocampus were reduced by about 60% in immunolesioned rats which were completely restored by 3T3NGF+ grafts. In addition, decreases in growth-associated protein (GAP)-43 immunoreactivity after immunolesion and increases in synaptophysin immunoreactivity after 3T3NGF+ grafts were observed in the hippocampus. Our results further confirm the notion that transfected NGF-secreting cells are useful in long-term in vivo NGF treatment and NGF can upregulate CBFN function. They also highly suggest that NGF induces terminal sprouting from remaining CBFNs.

192IgG-saporin-induced immunotoxic lesions of cholinergic basal forebrain system differentially affect glutamatergic and GABAergic markers in cortical rat brain regions

To study the effect of reduced cortical cholinergic activity on GABAergic and glutamatergic mechanisms in cholinoceptive cortical target regions a novel cholinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor with the cytotoxic protein saporin) was applied, which specifically and selectively destroys cholinergic cells in rat basal forebrain nuclei. To correlate the responses to cholinergic immunolesion in cholinoceptive cortical target regions with cholinergic hypoactivity, quantitative receptor autoradiography to measure NMDA, AMPA and kainate glutamate receptor subtypes, GABAA and benzodiazepine receptors as well as choline uptake sites, and histochemistry to estimate acetylcholinesterase activity were performed in adjacent brain sections. One week after a single intraventricular injection of 4 micrograms of 192IgG-saporin, NMDA receptor binding was markedly reduced in cortical regions displaying a reduced activity of acetylcholinesterase and high-affinity choline uptake sites as a consequence of cholinergic lesion, whereas AMPA and kainate binding sites were significantly increased in these regions. Muscimol binding to GABAA receptors was increased in the caudal portions of frontal and parietal cortices as well as occipital and temporal cortex as compared to the corresponding brain regions from vehicle-injected control rats. Binding levels of benzodiazepine receptors were not affected by the lesion in any of the cortical regions studied. The differential changes in glutamate and GABA receptor subtypes following cholinergic immunolesion might be regarded as the consequence of a cortical reorganization compensating for the reduced cholinergic presynaptic input. The data further suggest that presynaptic cortical cholinergic deficits might affect both glutamatergic and GABAergic functions with different intensity and different directions.

192IgG-saporin immunotoxin-induced loss of cholinergic cells differentially activates microglia in rat basal forebrain nuclei

To characterize the specificity of a novel cholinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor with the cytotoxic protein saporin), coronal sections through the basal forebrain of adult rats, that received a single intracerebro-ventricular injection of 4 micrograms of 192IgG-saporin conjugate, were subjected to histochemical and immunocytochemical procedures to evaluate cholinergic (choline acetyltransferase (ChAT)-immunoreactive, acetylcholinesterase-positive, NADPH-diaphorase-positive) and GABAergic structures (parvalbumin-immunoreactive, labeling of perineuronal nets with Wisteria floribunda agglutinin) as well as microglia (visualized with Griffonia simplicifolia agglutinin) and astrocytes (immunostaining for glial fibrillary acidic protein). Seven days following injection of the immunotoxin, ChAT-immunoreactive cells nearly completely disappeared throughout the magnocellular basal forebrain complex, including globus pallidus, as compared to vehicle-injected controls. However, there was no significant difference in the number of ChAT-positive cells in the adjacent ventral pallidum and in the caudate-putamen of immunolesioned and control animals. NADPH-diaphorase-containing cells, including a significant subpopulation of cholinergic cells, also strikingly decreased in number by more than 90% in the magnocellular basal forebrain complex following immunolesion, and only a few noncholinergic diaphorase-positive cells survived in the medial septum, vertical and horizontal diagonal band, and nucleus basalis of Meynert. In contrast, the number of parvalbumin-containing GABAergic projection neurons in the septum-diagonal band of Broca complex and nucleus basalis of Meynert from immunolesioned rats was not different from that of vehicle-injected control animals. Immunolesioning also did not result in any change in either number or shape of cells surrounded by perineuronal nets, which are frequently associated with parvalbumin-containing GABAergic neurons. Seven days following injection of the immunotoxin, a very strong activation of microglia with an identical distribution pattern was observed in all experimental animals. Large numbers of activated microglia were found in all magnocellular basal forebrain nuclei, corresponding to the distribution of degenerating cholinergic cells. Additionally, immunolesioning also resulted in a dramatic activation of microglia in the lateral septal nuclei, which are known to be almost free of cholinergic cells, but not of penetrating cholinergic dendrites in adjacent zones, and in the ventral pallidum, where there was no observed loss of cholinergic cells. There was no significant increase in microglia activation in striatum and cortical areas, and no astrocytic response in any of the basal forebrain nuclei at this particular time point of survival. These results suggest that 192IgG-saporin specifically destroys basal forebrain cholinergic neurons and does not suppress their neuronal activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential changes in cholinergic markers from selected brain regions after specific immunolesion of the rat cholinergic basal forebrain system

The aim of this study was to characterize the effects of cortical cholinergic denervation on cholinergic parameters in the cerebral cortex and basal forebrain using a novel immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor armed with cytotoxin saporin) to efficiently and selectively lesion cholinergic neurons in rat basal forebrain. Seven days following an intracerebroventricular injection of the cholinergic immunotoxin 192IgG-saporin the binding levels of nicotinic and M1- and M2-muscarinic acetylcholine receptors (mAChR), high-affinity choline uptake sites, as well as the m1-m4 mAChR mRNA were determined in coronal brain sections by both receptor autoradiography and in situ hybridization, and quantified by image analysis. Hemicholinium-3 binding to high-affinity choline uptake sites was decreased by up to 45% in all cortical regions and in the hippocampus after a single injection of the immunotoxin compared to controls. In contrast, M1-mAChR sites were increased over the corresponding control value in the anterior parts of cingulate, frontal, and piriform cortex by about 20%, in the hindlimb/forelimb areas (18%), in the parietal cortex (35%), in the occipital cortex area 2 (17%), as well as in the temporal cortex (25%) following immunolesion. M2-mAChR levels were found to be significantly increased in the posterior part of the parietal cortex area 1 (by about 22%) and in the occipital cortex area 2 (20%) only. With respect to laminar cortical localization, M2-mAChRs and choline uptake sites were altered in all cortical layers, whereas M1-mAChRs were preferentially affected in the upper cortical layers by the immunolesion. The increase in M1-mAChR binding in the temporal and occipital cortex as a consequence of the immunolesion was complemented by an increase in the amount of m1 and m3 mAChR mRNA of about 20% in these regions. The elevated levels of M2-mAChR sites in the occipital and temporal cortex following immunolesion were accompanied by an increase in the m4 (by 25%) but not m2 mAChR mRNA. There was no effect of the immunolesion on the m1-m4 mAChR mRNA in frontal cortical regions. in the basal forebrain, however, immunolesioning caused about a 40% decrease in the level of m2 mAChR mRNA in the medial and lateral septum as well as in the vertical and horizontal limb of the diagonal band, whereas M1- and M2-mAChR binding and the levels of m1, m3, and m4 mAChR mRNA were not affected by the immunolesion in any of the basal forebrain nuclei studied.(ABSTRACT TRUNCATED AT 400 WORDS)

192IGG-saporin-induced selective lesion of cholinergic basal forebrain system: neurochemical effects on cholinergic neurotransmission in rat cerebral cortex and hippocampus

A novel cholinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor with the cytotoxin saporin) producing selective lesions of cholinergic neurons in rat basal forebrain was applied to study its effect on hippocampal and cerebral cortical cholinergic neurotransmission. Intracerebroventricular injection of 4 micrograms 192IgG-saporin conjugate resulted in a selective loss of cholinergic cells in the basal forebrain nuclei 1 week after application, which was accompanied by decreased activities of choline acetyltransferase and by reduced high-affinity uptake of [3H]choline into cholinergic nerve terminals in the cerebral cortex and hippocampus, as well as by a significant activation of micro- and to a lesser extent of astroglial cells in the hippocampus, but hardly in the cerebral cortex.. The K(+)-stimulated release of [3H]acetylcholine from cortical and hippocampal slices of immunolesioned rats was found to be markedly decreased 1 week after injection. Cholinergic immunolesion led to enhanced cortical M1-muscarinic acetylcholine receptor numbers, but did not alter muscarinic receptor sensitivity as measured by carbachol-stimulated inositol phosphate production or phorbol ester binding to membrane-bound protein kinase C. In the hippocampal formation differential enhancements in binding levels of M1-muscarinic cholinergic receptor sites in the CA1 region and in the dentate gyrus were observed, whereas the nicotinic and M2-muscarinic receptor subtype are seemingly not affected by the immunotoxin in either of the subfields studied. Cholinergic immunolesioning did not result in any alterations in the hybridization signals for m1 through m4 muscarinic acetylcholine receptor mRNA in any region or layer of the hippocampus. The data suggest that (i) the novel cholinergic immunotoxin 192IgG-saporin is an appropriate tool to mimic cholinergic hypofunction in the hippocampal formation and cerebral cortex, and (ii) selective and specific immunolesion of cholinergic cells in medial septal nuclei differentially affects cholinergic receptors in particular hippocampal subfields.