Responses in the aged rat brain after total immunolesion

Aging with alcohol-related brain damage: Critical brain circuits associated with cognitive dysfunction

Alcoholism is associated with brain damage and impaired cognitive functioning. The relative contributions of different etiological factors, such as alcohol, thiamine deficiency and age vulnerability, to the development of alcohol-related neuropathology and cognitive impairment are still poorly understood. One reason for this quandary is that both alcohol toxicity and thiamine deficiency produce brain damage and cognitive problems that can be modulated by age at exposure, aging following alcohol toxicity or thiamine deficiency, and aging during chronic alcohol exposure. Pre-clinical models of alcohol-related brain damage (ARBD) have elucidated some of the contributions of ethanol toxicity and thiamine deficiency to neuroinflammation, neuronal loss and functional deficits. However, the critical variable of age at the time of exposure or long-term aging with ARBD has been relatively ignored. Acute thiamine deficiency created a massive increase in neuroimmune genes and proteins within the thalamus and significant increases within the hippocampus and frontal cortex. Chronic ethanol treatment throughout adulthood produced very minor fluctuations in neuroimmune genes, regardless of brain region. Intermittent "binge-type" ethanol during the adolescent period established an intermediate neuroinflammatory response in the hippocampus and frontal cortex, that can persist into adulthood. Chronic excessive drinking throughout adulthood, adolescent intermittent ethanol exposure, and thiamine deficiency all led to a loss of the cholinergic neuronal phenotype within the basal forebrain, reduced hippocampal neurogenesis, and alterations in the frontal cortex. Only thiamine deficiency results in gross pathological lesions of the thalamus. The behavioral impairment following these types of treatments is hierarchical: Thiamine deficiency produces the greatest impairment of hippocampal- and prefrontal-dependent behaviors, chronic ethanol drinking ensues mild impairments on both types of tasks and adolescent intermittent ethanol exposure leads to impairments on frontocortical tasks, with sparing on most hippocampal-dependent tasks. However, our preliminary data suggest that as rodents age following adolescent intermittent ethanol exposure, hippocampal functional deficits began to emerge. A necessary requirement for the advancement of understanding the neural consequences of alcoholism is a more comprehensive assessment and understanding of how excessive alcohol drinking at different development periods (adolescence, early adulthood, middle-aged and aged) influences the trajectory of the aging process, including pathological aging and disease.

Revisiting the cholinergic hypothesis in the development of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia affecting the elderly population today; however, there is currently no accurate description of the etiology of this devastating disorder. No single factor has been demonstrated as being causative; however, an alternative co-factors theory suggests that the interaction of multiple risk factors is responsible for AD. We have used this model, in combination with the original cholinergic hypothesis of AD to propose a "new" cholinergic hypothesis that we present in this review. This new version takes into account recent findings from the literature and our reports of removal of medial septum cholinergic projections to the hippocampus reduces both behavioural and anatomical plasticity, resulting in greater cognitive impairment in response to secondary insults (stress, injury, disease, etc.). We will first summarize the experimental results and discuss some potential mechanisms that could explain our results. We will then present our 'new' version of the cholinergic hypothesis and how it relates to the field of AD research today. Finally we will discuss some of the implications for treatment that arise from this model and present directions for future study.

The effects of neonatal forebrain cholinergic lesion on adult hippocampal neurogenesis

Previous work in our laboratory indicated that cholinergic denervation by intraventricular infusion of 192-IgG-saporin on postnatal day 7 (N192S) reduced the number of cells in the dentate gyrus expressing doublecortin, a marker for immature neuroblasts. In addition, there was a suggestion that N192S impaired the neurogenic response to environmental enrichment (EE). The purpose of the present study was to further characterize the impact of N192S on the proliferation, differentiation and survival of newborn cells in the dentate gyrus. After 42 days in EE or standard housing, all rats received injections of 5-bromo-2-deoxyuridine (BrdU) to label dividing cells. They were sacrificed either one day (to assess cell proliferation) or 28 days later (to assess survival and differentiation of BrdU-labelled cells). EE failed to increase neurogenesis, thereby preventing determination of the effects of N192S on EE-induced neurogenesis. However, N192S by itself reduced the number of BrdU(+) cells 1 day after BrdU exposure, but did not alter the number of cells expressing the cell cycle marker Ki-67. The number of BrdU(+) cells 28 days after BrdU exposure was not affected by N192S. Confocal analysis of BrdU(+) cells double-immunofluorescently stained to detect NeuN or S100B indicated that N192S did not alter the proportion of new cells that adopted a neuronal or glial identity. The most plausible explanation for these results is that N192S accelerates the death of newborn cells, but does not change their overall survival rate or phenotypic differentiation.

Effect of voluntary running on adult hippocampal neurogenesis in cholinergic lesioned mice

Background

Cholinergic neuronal dysfunction of the basal forebrain is observed in patients with Alzheimer's disease and dementia, and has been linked to decreased neurogenesis in the hippocampus, a region involved in learning and memory. Running is a robust inducer of adult hippocampal neurogenesis. This study aims to address the effect of running on hippocampal neurogenesis in lesioned mice, where septohippocampal cholinergic neurones have been selectively eliminated in the medial septum and diagonal band of Broca of the basal forebrain by infusion of mu-p75-saporin immunotoxin.

Results

Running increased the number of newborn cells in the dentate gyrus of the hippocampus in cholinergic denervated mice compared to non-lesioned mice 24 hours after injection of bromodeoxyuridine (BrdU). Although similar levels of surviving cells were present in cholinergic depleted animals and their respective controls four weeks after injection of BrdU, the majority of progenitors that proliferate in response to the initial period of running were not able to survive beyond one month without cholinergic input. Despite this, the running-induced increase in the number of surviving neurones was not affected by cholinergic depletion.

Conclusion

The lesion paradigm used here models aspects of the cholinergic deficits associated with Alzheimer's Disease and aging. We showed that running still increased the number of newborn cells in the adult hippocampal dentate gyrus in this model of neurodegenerative disease.

Astrocytic reaction to a lesion, under hormonal deprivation

Gonadal hormones can influence the morphology and function of glial cells, particularly astrocytes. Here we explore the hypothesis that 17beta-estradiol (E2) exerts a positive effect on astrocytes within the region of the cholinergic neurons of the basal forebrain, an area heavily implicated in memory and attentional processes. Female rats were ovariectomized at 3 months of age and lesioned with the immunotoxin 192 IgG-saporin before receiving a subcutaneous pellet containing 0.25mg of estrogen or placebo, released over 60 days. The control, non-ovariectomized group was treated identically. At the end of the treatment, we used image analysis procedures to evaluate changes in the levels of glial fibrillary acidic protein (GFAP) expression in the area of the lesion. Infusion of the immunotoxin induced a slight increase in GFAP expression in some subjects, compared to the contralateral side. However, when differences within animals where factored in, GFAP expression in ovariectomized animals treated with E2 was undistinguishable from intact controls. By contrast, in ovariectomized animals treated with placebo, GFAP expression was significantly higher. These results suggest that E2 deprivation may exacerbate the effects of an immunotoxic lesion, and, more importantly, that E2 administration may contribute to structural recovery of lesioned cholinergic neurons by blocking GFAP expression in the area. These results are particularly relevant in the context of female aging and postmenopausal dementia, and further highlight other potential levels at which to design interventions to preserve an intact cholinergic system, which may be crucial to prevent Alzheimer's disease.

Intracranial therapy of glioblastoma with the fusion protein DTAT in immunodeficient mice

A gene splicing technique was used to create a hybrid fusion protein DTAT encoding the 390 amino acid portion of diphtheria toxin (DT(390)), a linker, and the downstream 135-amino terminal fragment portion of human urokinase plasminogen activator. DTAT was assembled to target human glioblastoma cell lines in a murine intracranial model. Previously published in vitro studies demonstrated that DTAT was highly selective and toxic to human glioblastoma cell lines in a flank tumor model. The purpose of this study was to determine the toxicity, specificity and possible therapeutic efficacy of DTAT in an intracranial model. Convection enhanced delivery of DTAT resulted in about a 16-fold increase in maximum tolerated dose. Intracranial administration of DTAT on an every-other-day basis in nude mice with established U87 MG brain tumors resulted in significant reductions in tumor volume and significantly prolonged survival (p < 0.0001). Magnetic resonance imaging proved to be a powerful tool in mice and rats for demonstrating tumor growth in a xenograft intracranial model, assessing the efficacy of DTAT in tumor volume reduction and detecting DTAT-associated intracranial toxicity and vascular damage. These results suggest that the DTAT recombinant fusion protein is highly effective in an intracranial model and DTAT might be an effective treatment for glioblastoma.

Sympathetic sprouting drives hippocampal cholinergic reinnervation that prevents loss of a muscarinic receptor-dependent long-term depression at CA3-CA1 synapses

Degeneration of septohippocampal cholinergic neurons results in memory deficits attributable to loss of cholinergic modulation of hippocampal synaptic circuits. A remarkable consequence of cholinergic degeneration is the sprouting of noradrenergic sympathetic fibers from the superior cervical ganglia into hippocampus. The functional impact of sympathetic ingrowth on synaptic physiology has never been investigated. Here, we report that, at CA3-CA1 synapses, a Hebbian form of long-term depression (LTD) induced by muscarinic M1 receptor activation (mLTD) is lost after medial septal lesion. Unexpectedly, expression of mLTD is rescued by sympathetic sprouting. These effects are specific because LTP and other forms of LTD are unaffected. The rescue of mLTD expression is coupled temporally with the reappearance of cholinergic fibers in hippocampus, as assessed by the immunostaining of fibers for VAChT (vesicular acetylcholine transporter). Both the cholinergic reinnervation and mLTD rescue are prevented by bilateral superior cervical ganglionectomy, which also prevents the noradrenergic sympathetic sprouting. The new cholinergic fibers likely originate from the superior cervical ganglia because unilateral ganglionectomy, performed when cholinergic reinnervation is well established, removes the reinnervation on the ipsilateral side. Thus, the temporal coupling of the cholinergic reinnervation with mLTD rescue, together with the absence of reinnervation and mLTD expression after ganglionectomy, demonstrate that the autonomic-driven cholinergic reinnervation is essential for maintaining mLTD after central cholinergic cell death. We have discovered a novel phenomenon whereby the autonomic and central nervous systems experience structural rearrangement to replace lost cholinergic innervation in hippocampus, with the consequence of preserving a form of LTD that would otherwise be lost as a result of cholinergic degeneration.

Alzheimer's disease beta-secretase BACE1 is not a neuron-specific enzyme

The brains of Alzheimer's disease (AD) patients are morphologically characterized by neurofibrillar abnormalities and by parenchymal and cerebrovascular deposits of beta-amyloid peptides. The generation of beta-amyloid peptides by proteolytical processing of the amyloid precursor protein (APP) requires the enzymatic activity of the beta-site APP cleaving enzyme 1 (BACE1). The expression of this enzyme has been localized to the brain, in particular to neurons, indicating that neurons are the major source of beta-amyloid peptides in brain. Astrocytes, on the contrary, are known to be important for beta-amyloid clearance and degradation, for providing trophic support to neurons, and for forming a protective barrier between beta-amyloid deposits and neurons. However, under certain conditions related to chronic stress, the role of astrocytes may not be beneficial. Here we present evidence demonstrating that astrocytes are an alternative source of BACE1 and therefore may contribute to beta-amyloid plaque formation. While resting astroyctes in brain do not express BACE1 at detectable levels, cultured astrocytes display BACE1 promoter activity and express BACE1 mRNA and enzymatically active BACE1 protein. Additionally, in animal models of chronic gliosis and in brains of AD patients, there is BACE1 expression in reactive astrocytes. This would suggest that the mechanism for astrocyte activation plays a role in the development of AD and that therapeutic strategies that target astrocyte activation in brain may be beneficial for the treatment of AD. Also, there are differences in responses to chronic versus acute stress, suggesting that one consequence of chronic stress is an incremental shift to different phenotypic cellular states.

Hippocampal neurotrophin levels after injury: Relationship to the age of the hippocampus at the time of injury

Aging impairs the competence of the hippocampus for synaptic reorganization after injury. This potentially is due to the inability of the aging hippocampus to up-regulate the critical neurotrophic factors for prolonged periods after injury to levels at which they can stimulate neurite outgrowth and facilitate synaptic reorganization. We hypothesize that the concentrations of neurotrophins in the hippocampus after injury depend on the age at the time of injury. We quantified the concentrations of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) in the hippocampus of young, middle-aged, and aged Fischer 344 rats at 4 days after kainic acid (KA)-induced injury. In comparison with the age-matched intact hippocampus, the KA-lesioned hippocampus exhibited increased levels of BDNF and NGF in all three age groups. In contrast, the NT-3 concentration was unaltered after KA lesion. Notwithstanding similar percentage increases in BDNF after injury, the lesioned middle-aged and aged hippocampus contained 45-52% less BDNF than the lesioned young hippocampus. NGF and NT-3 levels after injury were comparable across the three age groups, however. Furthermore, lower BDNF concentration in the injured aging hippocampus was associated with normal astrocytic response but significantly diminished microglial reaction. Thus, in comparison with the injured young hippocampus, the injured aging hippocampus contains considerably less BDNF but similar levels of NGF and NT-3. Lower BDNF levels in the injured aging hippocampus might underlie the diminished spontaneous healing response observed in the aging hippocampus after injury, particularly in terms of synaptic reorganization and dentate neurogenesis.

Behavioral Effects of Basal Forebrain Cholinergic Lesions in Young Adult and Aging Rats

The interactive effects of age and cholinergic damage were assessed behaviorally in young and middle-aged rats. Rats were lesioned at either 3 or 17 months of age by injection of 192 IgG-saporin immunotoxin into the medial septum and the nucleus basalis magnocellularis, and they were then tested on a range of behavioral tasks: a nonmatching-to-position task in a T-maze, an object-recognition task, an object-location task, and an open-field activity test. Depending on the task used, only an age or a lesion effect was observed, but there was no Age X Lesion interaction. Middle-aged and young rats responded to the cholinergic lesions in the same manner. These results show that in the middle-aged rats in which cholinergic transmission was affected, additional injury to the system was not always accompanied by major cognitive dysfunctions.

Low sensitivity of retina to AMPA-induced calcification

Glutamate is involved in most CNS neurodegenerative diseases. In particular, retinal diseases such as retinal ischemia, retinitis pigmentosa, and diabetic retinopathy are associated with an excessive synaptic concentration of this neurotransmitter. To gain more insight into retinal excitotoxicity, we carried out a dose-response study in adult rats using alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), a glutamate analogue. AMPA intraocular injections (between 0.27 and 10.8 nmol) caused no morphologic modification, but a 10.8 + 21 nmol double injection in a 10-day interval produced a lesion characterized by discrete neuronal loss, astroglial and microglial reactions, and calcium precipitation. Abundant calcium deposits similar to those present in rat and human brain excitotoxicity or hypoxia-ischemia neurodegeneration were detected by alizarin red staining within the retinal surface and the optic nerve. Glial reactivity, associated normally with astrocytes in the nerve fiber, was assessed in Müller cells. GABA immunoreactivity was detected not only in neuronal elements but also in Müller cells. In contrast to the high vulnerability of the brain to excitotoxin microinjection, AMPA-induced retinal neurodegeneration may provide a useful model of low central nervous system sensitivity to excitotoxicity.

Survival of fetal hippocampal CA3 cell grafts in the middle-aged and aged hippocampus: effect of host age and deafferentation

The potential application of neural transplantation to many neurodegenerative disorders at early stages of disease progression would involve middle-aged and aged persons. Hence, it is important to examine critically the extent of graft cell survival in both intact and partially deafferented middle-aged and aged brain. We investigated the degree of survival of 5'-bromodeoxyuridine (BrdU)-labeled fetal hippocampal CA3 cells after grafting into both intact hippocampus and partially deafferented hippocampus (i.e., hippocampus contralateral to intracerebroventricular administration of kainic acid) of middle-aged and aged Fischer 344 rats. Absolute cell survival within these grafts was rigorously analyzed using BrdU immunostaining of serial sections and the optical fractionator cell counting method. In the intact hippocampus, graft cell survival was 23% of injected cells for middle-aged rats and 18% for aged rats, which is consistent with the survival of fetal hippocampal cells in the intact young adult hippocampus reported earlier (Shetty and Turner [1995] Neuroscience 67:561-582). A partial deafferentation at the time of grafting significantly enhanced the degree of graft cell survival to 35% of injected cells in the middle-aged hippocampus and 27% in the aged hippocampus. However, the overall graft cell survival after deafferentation was significantly (30%) greater in the middle-aged hippocampus compared with the aged hippocampus. These results reveal that 1) the degree of survival of fetal neural cells in the intact mature brain remains constant with aging and 2) a partial deafferentation of the mature host brain at the time of grafting enhances survival of grafted fetal cells, regardless of the host age. However, the overall extent of graft cell survival after deafferentation depends on the age of the mature brain at the time of deafferentation.

Attenuated transcriptional responses to oxidative stress in the aged rat brain

The aged nervous system displays impaired cognitive functions, and these impairments are exacerbated in several neurodegenerative diseases. A role for oxidative stress has been suggested for several of these age-associated dysfunctions. In addition, recovery from more acute traumatic insults that also generate oxidative stress is impaired in the aged. Here we examine the response of aged rat hippocampi to normobaric hyperoxia treatments and demonstrate an attenuation in the DNA binding activity of the AP-1 and nuclear factor-kappa B transcription factors, which are important components of stress response signal transduction pathways and can determine shifts in cellular commitments to necrosis, apoptosis, or functional recovery in the central nervous system.

Hippocampal brain-derived neurotrophic factor gene regulation by exercise and the medial septum

Brain-derived neurotrophic factor (BDNF) enhances synaptic plasticity and neuron function. We have reported that voluntary exercise increases BDNF mRNA levels in the hippocampus; however, mechanisms underlying this regulation have not been defined. We hypothesized that medial septal cholinergic and/or gamma amino butyric acid (GABA)ergic neurons, which provide a major input to the hippocampus, may regulate the baseline gene expression and exercise-dependent gene upregulation of this neurotrophin. Focal lesions were produced by medial septal infusion of the saporin-linked immunotoxins 192-IgG-saporin or OX7-saporin. 192-IgG-saporin produced a selective and complete loss of medial septal cholinergic neurons with no accompanying GABA loss. Baseline BDNF mRNA was reduced in the hippocampus of sedentary animals, but exercise-induced gene upregulation was not impaired, despite complete loss of septo-hippocampal cholinergic afferents. OX7-saporin produced a graded lesion of the medial septum characterized by predominant GABA neuron loss with less reduction in the number of cholinergic cells. OX7-saporin lesion reduced baseline hippocampal BDNF mRNA and attenuated exercise-induced gene upregulation, in a dose-dependent manner. These results suggest that combined loss of septal GABAergic and cholinergic input to the hippocampus may be important for exercise-dependent BDNF gene regulation, while cholinergic activity on its own is not sufficient. These results are discussed in relation to their implications for aging and Alzheimer's disease.

Combined neurotrophic supplementation and caspase inhibition enhances survival of fetal hippocampal CA3 cell grafts in lesioned CA3 region of the aging hippocampus

Fetal hippocampal CA3 cells show excellent survival when homotopically grafted into the kainic acid-lesioned CA3 region of the young adult hippocampus, a model of temporal lobe epilepsy. However, survival of these cells in the kainic acid-lesioned CA3 region of the aging hippocampus is unknown. We hypothesize that fetal CA3 grafts into the lesioned CA3 region of the middle-aged and aged hippocampus exhibit significantly diminished cell survival compared with similar grafts in the lesioned young adult hippocampus unless pre-treated and transplanted with factors that augment graft cell survival. We analyzed cell survival of 5'-bromodeoxyuridine-labeled embryonic day 19 CA3 grafts following their transplantation into the lesioned CA3 region of the middle-aged and aged rat hippocampus. Grafts were placed 4 days after an i.c.v. administration of kainic acid, and absolute cell survival of grafts was quantified 1 month after grafting using 5'-bromodeoxyuridine immunostaining of serial sections and the optical fractionator counting method. Grafts into both middle-aged and aged hippocampus exhibited analogous but significantly diminished cell survival (30% of injected cells) compared with similar grafts into the young adult hippocampus (72% cell survival). However, the extent of cell survival of CA3 grafts pre-treated and transplanted with a combination of neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 and the caspase inhibitor acetyl-tyrosinyl-valyl-alanyl-aspartyl-chloro-methylketone was significantly enhanced in both middle-aged and aged hippocampus (51-63% cell survival). These results underscore that aging impairs the conduciveness of the CA3 region for robust survival of homotopic fetal CA3 grafts after lesion. However, a combined neurotrophic supplementation and caspase inhibition significantly enhances survival of fetal CA3 cells in the lesioned aging hippocampus. Thus, pre-treatment and grafting of donor cells with a combination of factors that support growth of specific donor cells may considerably enhance survival and integration of fetal grafts into the lesioned aging CNS in clinical trials.

NGF-mediated alteration of NF-kappaB binding activity after partial immunolesions to rat cholinergic basal forebrain neurons

There are age-associated cognitive and cholinergic deficits in the neurotrophin-dependent cholinergic basal forebrain neurons (CBFNs). There are also increases in the activity of the transcription factor NF-kappaB in the aged rodent brain that may reflect chronic enhancement of stress response signaling. We used partial immunolesions (PIL) to CBFN to examine the role of endogenous NGF on choline acetyltransferase (ChAT) activity and NGF-mediated NF-kappaB alteration after cholinergic deafferentation. We injected 192 IgG-saporin, an immunotoxin selectively taken up by neurotrophin receptor p75(NTR)-bearing neurons, into lateral ventricles, followed by infusions of anti-NGF to assess NF-kappaB, ChAT and NGF responses to PIL after anti-NGF infusion. Treatment with anti-NGF decreased ChAT activity by 17-34% in the cortex, hippocampus, and olfactory bulb and PIL decreased ChAT activity by 47-73%. Changes in AChE activity levels paralleled those observed for ChAT after PIL. NGF protein levels in the olfactory bulb, but not the cortex or hippocampus, increased significantly after PIL treatment. Infusion of anti-NGF abolished the PIL-induced eight-fold NGF increase in CNS. NF-kappaB binding activity to the IgG-kappaB and ChAT specific NF-kappaB consensus sequences, increased in the cortex but not hippocampus after PIL followed by anti-NGF infusion. It is likely that immunolesion-induced changes in ambient NGF levels may perturb NF-kappaB activity.

Differential alterations of NF-kappaB to oxidative stress in primary basal forebrain cultures

Oxidative stress has been linked to neuronal cell death resulting from either acute insults due to ischemia, trauma, excitotoxicity, or chronic neurodegenerative diseases. Cholinergic basal forebrain neurons (CBFNs) compete for nerve growth factor (NGF) synthesized in the hippocampus and cortex via retrograde transport. NGF affects CBFN survival and cholinergic function via activation of the NF-kappaB transcription factor and this signaling pathway appears to be impaired in aged rats. Here, we demonstrate that activation of NF-kappaB in basal forebrain primary culture via treatment with hydrogen peroxide or TNF-alpha is predominantly restricted to CBFNs, and that NF-kappaB activation appears to mostly affect p65 translocation to the nucleus, but not the p50 subunit. These results are consistent with NF-kappaB activation being a part of recovery processes after acute oxidative stress. Since p50 or p49 (also called p52) binding to promoter sites does not stimulate transcription - both p50 and p49 lack an activating domain - and p65 does contain an activating domain and thus can act as a transcription enhancer, differential translocation of different NF-kappaB dimers can act as repressors of constitutive activity or enhancers. These results are in agreement with the hypothesis that p50/p65 is the active trans-activating species of NF-kappaB, as compared to p50/p50 homodimers which bind to NF-kappaB binding sites but do not trans-activate promoters. Our results also suggest that selective activation of different NF-kappaB dimer species may have regulatory significance in neuronal responses to acute or chronic insults to CNS. Thus, increased p65 translocation could have enhancing effects while increased p50 translocation could have a repressor role. Manipulation of the types of NF-kappaB species being translocated could provide a basis for therapeutic strategies.

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.

Model for aging in the basal forebrain cholinergic system

A key component of the cognitive deficits associated with aging is the loss of function of cholinergic neurons in the basal forebrain due to neuronal losses and decreased cholinergic function of spared neurons. A model to mimic one aspect of this phenomenon is to kill cholinergic neurons selectively in the basal forebrain via administration of the immunotoxin IgG-192-saporin. Here we discuss apoptotic regulators, such as nerve growth factor, in age-associated changes present in the cholinergic system and the role of the NF-kappaB signaling system in cellular commitment to apoptosis. We also examine the age-associated decline in intrinsic response mechanisms, which may account for the age-associated reduction in recovery from both acute and chronic insults to the central nervous system.

Immunolesioning of nerve growth factor p75 receptor-containing neurons in the rat brain by a novel immunotoxin: anti-p75-anti-mouse IgG-trichosanthin conjugates

In the present study, a comparison of potency between a commercially available immunotoxin, 192-immunoglobulin-SAP (192-IgG), and a novel immunotoxin produced in our laboratory, anti-p75-anti-mouse IgG-trichosanthin conjugates (p75-TCS), was conducted. Both of the immunotoxins were specific for nerve growth factor p75 receptor. Cholinergic neurons in the rat basal forebrain and in the neostriatum were depleted after the injection of either 192-IgG or p75-TCS. These indicate that both types of immunotoxins are potent and useful in performing immunolesioning experiments. In addition, there were variations in potency among the two immunotoxins in different routes of administration. The 192-IgG was more potent than the p75-TCS in the case of ventricular injections. In case of striatal injections, 192-IgG caused serious tissue necrosis and considerable tissue damage in the brain region. In contrast, p75-TCS was potent and caused a selective and specific depletion of cholinergic neurons in the neostriatum. These results indicate that indirect immunotoxins may be more useful for performing immunolesioning experiments in case of brain parenchyma administration.

The effects of a novel NSAID on chronic neuroinflammation are age dependent

Chronic inflammation may play an important role in the pathogenesis of Alzheimer's disease (AD). The present study compared the effects of chronic neuroinflammation, produced by infusion of lipopolysaccharide (LPS) into the fourth ventricle, upon memory in young, adult, and old rats. Nonsteroidal anti-inflammatory drug (NSAID) therapy may delay the onset of AD. We show that NO-Flurbiprofen (NFP), a novel NSAID that lacks gastrointestinal side effects, attenuated the neuroinflammatory reaction and reduced the inflammation-induced memory deficit. Chronic LPS infusions impaired performance of young rats but not adult or old rats. Treatment with NFP improved the performance of LPS-infused young rats, but not LPS-infused adult or old rats. LPS infusions increased the number of activated microglia in young and adult rats but not old rats. NFP treatment attenuated the effects of LPS upon microglia activation in young and adult rats, but not old rats. The results suggest that NSAID therapies designed to influence the onset of AD should be initiated in adults before age-associated inflammatory processes within the brain have a chance to develop.