Animal Models to Produce Cortical Cholinergic Dysfunction

Behavioural effects in mice orally exposed to domoic acid or ibotenic acid are influenced by developmental stages and sex differences

The structure of the brain is dramatically altered during the critical period. Physiological substances (neurotransmitters, hormones, etc.) in the body fluctuate significantly before and after sexual maturation. Therefore, the effect of chemical exposure on the central nervous system often differs depending on the developmental stage and sex. We aimed to compare the behavioural effects that emerged from the administration of chemicals to mice of different life stages (immature or mature) and different sex (male or female). We administered mice with domoic acid (DA), a marine poison, and ibotenic acid (IA), found in poisonous mushrooms. These excitatory amino acids act as agonists for glutamate and are potent neurotoxins. Interestingly, the behavioural effects of these chemicals were completely different. Following DA administration, we observed memory deficits only in groups of male mice treated at maturity. Following IA administration, we observed deviations in emotional behaviour in groups of male mice treated at both immaturity and maturity. In contrast, few characteristic changes were detected in all groups of females. Our results support the theory that the behavioural effects of chemical administration vary considerably with developmental stages and sex. In conclusion, our findings promote better understanding of individual differences in excitatory chemical-induced neurotoxicity and provide evidence for future risk strategies and treatments.

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.

Deafferentation of the septo-hippocampal pathway in rats as a model of the metabolic events in Alzheimer's disease

Changes in the metabolic activity within the brain of patients suffering from Alzheimer's disease (AD) were investigated and compared with biochemical alterations in the hippocampus induced by fimbria/fornix transection in the rat. The deafferentation of the hippocampus results in a degeneration of cholinergic septo-hippocampal terminals accompanied by a persistent decrease of choline acetyltransferase (ChAT) and acetylcholine esterase (AChE) activities similar to the cholinergic malfunction in AD. In the animal model the [3H]-cytochalasin B binding to the glucose transporters was elevated up to the day 7 after surgery as was the activity of the phosphofructokinase (PFK) on day 3. A reactive astrogliosis could be evidenced by the upregulation of glial fibrillary acidic protein (GFAP). An increase of the PFK activity was also found in AD being accompanied by enhanced level of GFAP as well. A higher concentration of mRNA for all three isoenzymes of PFK was shown by reverse transcription (RT)-real time polymerase chain reaction (PCR) amplification. However, the pattern of PFK isoenzyme proteins and mRNAs did neither change in diseased human nor in the lesioned rat brain. The activities of the mitochondrial enzymes pyruvate dehydrogenase complex (PDHC) and cytochrome c oxidase (CO) were diminished in the lesioned rat hippocampus on day 7 as well as in AD brain. Subcellular fractionation showed that the activity of these enzymes was affected in the synaptosomal as well as in the extrasynaptosomal mitochondria indicating a loss of neuronal input and also a vulnerability of intrinsic hippocampal neurons and/or non-neuronal cells. The recovery of the mitochondrial enzyme activity in the animal model at later post lesion intervals may be the result of compensatory responses of surviving cells or of sprouting of other non-affected inputs. It is concluded that common metabolic mechanisms may underlie the concurrent degenerative and repair processes in the denervated hippocampus and the diseased Alzheimer brain.

Glucose metabolism in cholinoceptive cortical rat brain regions after basal forebrain cholinergic lesion

To address the question whether the changes in cortical glucose metabolism observed in patients with Alzheimer's disease are interrelated with, or consequences of, basal forebrain cholinergic cell loss, an experimental approach was employed to produce cortical cholinergic dysfunction in rat brain by administration of the cholinergic immunotoxin 192IgG-saporin. [14C]D-glucose utilization in brain homogenates, D-glucose-displaceable [3H]cytochalasin B binding to glucose transporters (GLUT). Northern and Western analyses, as well as in vivo [14C]2-deoxyglucose autoradiography were used to quantify the regional glucose metabolism. Basal forebrain cholinergic lesion resulted in transient increases in glucose transporter binding in cortical regions displaying reduced acetylcholinesterase activity, already detectable seven days after lesion with peak values around 30 days post lesion. Western analysis revealed that the changes in total glucose transporter binding are mainly due to changes in the GLUT3 subtype only, while the levels of GLUT1 and GLUT3 mRNA (Northern analysis) were not affected by cholinergic lesion. Both immunocytochemistry and in situ hybridization demonstrated preferential localizations of GLUT1 on brain capillaries and GLUT3 on neurons, respectively. A lesion-induced transient decrease in [14C]D-glucose utilization seven days post lesion was detected in the lesion site, whereas cholinoceptive cortical regions were not affected. In vivo [14C]deoxyglucose uptake was transiently increased in cholinoceptive cortical regions and in the lesion site being highest between three to seven days after lesion. The cholinergic lesion-induced transient up-regulation of cortical glucose transporters and deoxyglucose uptake reflects an increased glucose demand in regions depleted by acetylcholine suggesting functional links between cortical cholinergic activity and glucose metabolism in cholinoceptive target regions.

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.

Electron microscopic evidence for a cholinergic innervation of GABAergic parvalbumin-immunoreactive neurons in the rat medial septum

The presence of interconnections between cholinergic and parvalbumin (PARV)-containing gamma aminobutyric acid (GABA)ergic septohippocampal projection neurons is still a matter of debate. To search for contacts of cholinergic collateral axon terminals in the septal-diagonal band region the immunotoxin 192IgG-saporin was applied, which was proved to selectively destroy cholinergic basal forebrain neurons. Seven and 10 days after administration of the immunotoxin, choline acetyltransferase immunoreactivity had disappeared, and numerous neuronal somata and dendrites as well as axonal terminals revealed characteristics of electron-lucent degeneration. Electron-dense degeneration was never observed in dendrites and synaptic boutons. Degenerating terminals were found in contact with PARV-immunopositive and PARV-negative neurons. Because only cholinergic cells were degenerating, the terminals should be collaterals from cholinergic neurons. In addition to such contacts, PARV-immunoreactive boutons were seen in contact with PARV-positive and PARV-negative cells, but were not identified at degenerating postsynaptic profiles. As suggested in other studies, cholinergic boutons contacting GABAergic PARV-containing septal projection cells may influence hippocampal theta activity. Furthermore, multiple synaptic connections of both neuronal populations forming the septohippocampal pathway may contribute to their high rate of survival after fimbria-fornix transection.

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.

Selective induction of c-Jun and NGF in reactive astrocytes after cholinergic degenerations in rat basal forebrain

Cholinergic basal forebrain neurons are the major source of cortical cholinergic innervation. The number of these neurons is regulated by the availability of nerve growth factor (NGF) during development while in adulthood their cholinergic activity is modulated by NGF. In previous studies we have shown that cholinergic immunolesions of basal forebrain neurons increase local immediate early gene expression and NGF synthesis in the regions of degeneration. In this study we identify the cellular source of c-Jun and NGF expression using dual immunolabeling of c-Jun and NGF in combination with neuronal and glial markers. We demonstrate that both c-Jun and NGF are exclusively expressed in reactive astrocytes but not in microglia or in GABAergic basal forebrain neurons. These observations support the hypothesis that reactive astrocytes synthesize neurotrophic substances in vivo in response to neuronal degeneration in the basal forebrain.

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.

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.