Basal forebrain cell loss following fimbria/fornix transection

Impaired recollection and initially preserved familiarity in a patient with bilateral fornix transection following third ventricle colloid cyst removal: A two-year follow-up study

OBJECTIVE

Recognition memory is widely accepted as a dual process-based model, namely familiarity and recollection. However, the location of their specific neurobiological substrates remains unclear. Similar to hippocampal damage, fornix damage has been associated with recollection memory but not familiarity memory deficits. To understand the neural basis of recognition memory, determining the importance of the fornix and its hippocampal connections is essential.

METHODS

Recognition memory was examined in a 45-year-old male who underwent a complete bilateral fornix section following the removal of a third ventricle colloid cyst. The application of familiarity and recollection for recognition memory decisions was investigated via an immediate and delayed associative recognition test and an immediate and delayed forced-choice task in the patient and a control group (N = 15) over a two-year follow-up period. Complete demographic, neuropsychological, neuropsychiatric, and neuroradiological characterizations of this patient were performed.

RESULTS

Persistent immediate and delayed verbal recollection memory deficits were observed in the patient. Moreover, delayed familiarity-based recognition memory declined gradually over the follow-up period, immediate familiarity-based recognition memory was unaffected, and reduced non-verbal memory improved.

CONCLUSION

The present findings support models that the extended hippocampal system, including the fornices, does not appear to play a role in familiarity memory but is particularly important for recollection memory. Moreover, our study suggests that bilateral fornix transection may be associated with relatively functional recovery of non-verbal memory.

Nerve growth factor is primarily produced by GABAergic neurons of the adult rat cortex

Within the cortex, nerve growth factor (NGF) mediates the innervation of cholinergic neurons during development, maintains cholinergic corticopetal projections during adulthood and modulates cholinergic function through phenotypic control of the cholinergic gene locus. Recent studies suggest NGF may also play an important role in cortical plasticity in adulthood. Previously, NGF-producing cells have been shown to colocalize with GABAergic cell markers within the hippocampus, striatum, and basal forebrain. Classification of cells producing NGF in the cortex is lacking, however, and cholinergic corticopetal projections have been shown to innervate both pyramidal and GABAergic neurons in the cortex. In order to clarify potential trophic interactions between cortical neurons and cholinergic projections, we used double-fluorescent immunohistochemistry to classify NGF-expressing cells in several cortical regions, including the prefrontal cortex, primary motor cortex, parietal cortex and temporal cortex. Our results show that NGF colocalizes extensively with GABAergic cell markers in all cortical regions examined, with >91% of NGF-labeled cells coexpressing GAD65/67. Conversely, NGF-labeled cells exhibit very little co-localization with the excitatory cell marker CaMKIIα (<5% of cells expressing NGF). NGF expression was present in 56% of GAD-labeled cells, suggesting that production is confined to a specific subset of GABAergic neurons. These findings demonstrate that GABAergic cells are the primary source of NGF production in the cortex, and likely support the maintenance and function of basal forebrain cholinergic projections in adulthood.

Neurotrophic factors rescue basal forebrain cholinergic neurons and improve performance on a spatial learning test

This study investigated whether animals sustaining experimental damage to the basal forebrain cholinergic system would benefit from treatment with exogenous neurotrophic factors. Specifically, we set out to determine whether neurotrophic factors would rescue damaged cholinergic neurons and improve behavioral performance on a spatial learning and memory task. Adult rats received bilateral injections of either saline (controls) or 192 IgG-saporin to damage basal forebrain cholinergic neurons (BFCNs). Two weeks later, animals received implants of an Alzet mini-pump connected to cannulae implanted bilaterally in the lateral ventricles. Animals received infusions of nerve growth factor (NGF), neurotrophin 3 (NT3), a combination of NGF and NT3, or a saline control over a 4-week period. Compared to saline-treated controls, animals sustaining saporin-induced damage to BFCNs took significantly more trials to learn a delayed match to position task and also performed more poorly on subsequent tests, with increasing delays between test runs. In contrast, animals infused with neurotrophins after saporin treatment performed significantly better than animals receiving saline infusions; no differences were detected for performance scores among animals infused with NGF, NT3, or a combination of NGF and NT3. Studies of ChAT immunnocytochemical labeling of BFCNs revealed a reduction in the numbers of ChAT-positive neurons in septum, nucleus of diagonal band, and nucleus basalis in animals treated with saporin followed by saline infusions, whereas animals treated with infusions of NGF, NT3 or a combination of NGF and NT3 showed only modest reductions in ChAT-positive neurons. Together, these data support the notion that administration of neurotrophic factors can rescue basal forebrain cholinergic neurons and improve learning and memory performance in rats.

Axotomy-induced neurotrophic withdrawal causes the loss of phenotypic differentiation and downregulation of NGF signalling, but not death of septal cholinergic neurons

BACKGROUND

Septal cholinergic neurons account for most of the cholinergic innervations of the hippocampus, playing a key role in the regulation of hippocampal synaptic activity. Disruption of the septo-hippocampal pathway by an experimental transection of the fimbria-fornix drastically reduces the target-derived trophic support received by cholinergic septal neurons, mainly nerve growth factor (NGF) from the hippocampus. Axotomy of cholinergic neurons induces a reduction in the number of neurons positive for cholinergic markers in the medial septum. In several studies, the reduction of cholinergic markers has been interpreted as analogous to the neurodegeneration of cholinergic cells, ruling out the possibility that neurons lose their cholinergic phenotype without dying. Understanding the mechanism of cholinergic neurodegeneration after axotomy is relevant, since this paradigm has been extensively explored as an animal model of the cholinergic impairment observed in neuropathologies such as Alzheimer's disease.The principal aim of this study was to evaluate, using modern quantitative confocal microscopy, neurodegenerative changes in septal cholinergic neurons after axotomy and to assess their response to delayed infusion of NGF in rats.

RESULTS

We found that there is a slow reduction of cholinergic cells labeled by ChAT and p75 after axotomy. However, this phenomenon is not accompanied by neurodegenerative changes or by a decrease in total neuronal number in the medial septum. Although the remaining axotomized-neurons appear healthy, they are unable to respond to delayed NGF infusion.

CONCLUSIONS

Our results demonstrate that at 3 weeks, axotomized cholinergic neurons lose their cholinergic phenotype without dying and down-regulate their NGF-receptors, precluding the possibility of a response to NGF. Therefore, the physiological role of NGF in the adult septal cholinergic system is to support phenotypic differentiation and not survival of neurons. This evidence raises questions about the relationship between transcriptional regulation of the cholinergic phenotype by retrograde-derived trophic signaling and the transcriptional changes experienced when retrograde transport is impaired due to neuropathological conditions.

Impairments in trace EB conditioning by knife-cut lesions to the fornix in rabbits: Reversal by galantamine

Previous work in our laboratory demonstrated that galantamine, a cholinesterase inhibitor and weak cholinergic agonist, facilitated classical trace eyeblink conditioning in healthy, young rabbits [Simon, B. B., Knuckley, B., & Powell, D. A. (2004). Galantamine facilitates acquisition of a trace-conditioned eyeblink response in healthy, young rabbits. Learning & Memory, 11(1), 116-122.]. The current study investigated the effects of galantamine (0.0 or 3.0mg/kg) in rabbits sustaining knife-cut lesions to the fimbria-fornix, a major projection pathway connecting the hippocampus to cortical and subcortical brain structures involved in the formation of long-term memories. Two experiments were conducted. Experiment one assessed the effects of knife-cut lesions to the fornix or sham surgeries on trace eyeblink (EB) conditioning. Results indicate that fornix lesions significantly retarded EB conditioning when trace parameters were employed. Experiment 2 assessed whether treatment with galantamine would reverse the deficits caused by fornix damage. Results indicate that 3.0mg/kg GAL reversed trace EB conditioning deficits in animals with fornix knife-cut lesions. These findings suggest that galantamine may provide benefit in the reversal of cognitive dysfunction following certain types of brain damage, especially damage involving hippocampal structures.

Neuroprotection and neurogenesis: Modulation of cornus ammonis 1 neuronal survival after transient forebrain ischemia by prior fimbria-fornix deafferentation

Severe transient forebrain ischemia causes selective neuronal death in the hippocampal cornus ammonis 1 region. We tested the hypothesis that fimbria-fornix deafferentation can provide long-term protection to cornus ammonis 1 neurons and modulate neurogenesis following ischemia. Fimbria-fornix lesion or sham-fimbria-fornix lesion was performed on Wistar rats 13 days prior to 10 min forebrain ischemia or sham ischemia. Temperature was regulated and rats survived for 7, 14 or 28 days. Immunofluorescent bromodeoxyuridine and neuron specific nuclear protein staining and immunochemistry terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining were performed. At 7 days after ischemia, 73%+/-14% of cornus ammonis 1 neurons were damaged, while deafferentation reduced the injury to 36%+/-17% of cornus ammonis 1 neurons. This protection persisted for at least 28 days. Ischemia significantly increased the number of bromodeoxyuridine-positive cells (85-90 cells/section in stroke group vs. 6 to 11 cells/section in normal or sham stroke group), with very few terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-stained cells adjacent to the hippocampal cornus ammonis 1. Fimbria-fornix lesioning followed by ischemia increased the percentage of new neurons 13-fold over ischemia alone and 6.5-fold over sham lesion plus ischemia. The results indicate that fimbria-fornix deafferentation provides long-term neuroprotection in cornus ammonis 1 following forebrain ischemia and promotes neurogenesis after ischemic insults.

Regulated lentiviral NGF gene transfer controls rescue of medial septal cholinergic neurons

Nerve growth factor (NGF) has been shown to promote survival and function of cholinergic neurons in the basal forebrain in various models of neuronal degeneration in rodents and primates. We examined whether a regulatable in vivo expression system can control the survival of cholinergic neurons after injury, using a tetracycline-regulated promoter ("tet-off" system) to modulate lentiviral NGF gene delivery. Two weeks after lesions to cholinergic neurons, significant cell rescue (65+/-8% neuron survival; P<0.005 compared to controls) was observed when NGF expression was activated. Treatment with the tetracycline analog doxycycline to turn gene expression "off" resulted in a significant loss of cholinergic neurons (only 37+/-5% neurons remained, an amount that did not differ from untreated, lesioned controls). Animals treated with a constitutively active and robust nonregulated NGF expression system showed the same degree of neuronal rescue (73+/-8%) as animals treated with activated tet-regulated vectors. ELISA measurements confirmed that oral treatment of animals with doxycycline reduced NGF protein levels to levels in untreated control subjects. These data demonstrate for the first time that NGF delivery by lentiviral gene transfer using tetracycline-regulated promoters can completely regulate neuronal rescue and protein production in the brain.

Removal of cholinergic input to perirhinal cortex disrupts object recognition but not spatial working memory in the rat

The perirhinal cortex of the temporal lobe has a crucial role in object recognition memory. Cholinergic transmission within perirhinal cortex also seems to be important for this function, as the muscarinic receptor antagonist scopolamine disrupts object recognition performance when administered systemically or directly into perirhinal cortex. In the present study, we directly assessed the contribution of cholinergic basal forebrain input to perirhinal cortex in object recognition. Selective bilateral removal of the cholinergic basal forebrain inputs to perirhinal cortex was accomplished by injecting the immunotoxin 192 IgG-saporin directly into perirhinal cortex in rats. These animals were significantly impaired relative to vehicle-injected controls in a spontaneous object recognition task despite intact spatial alternation performance. These results are consistent with recent reports of object recognition impairment following acute cholinergic receptor blockade and extend these findings by demonstrating that chronic removal of cholinergic basal forebrain input to an otherwise intact perirhinal cortex causes a severe object recognition deficit similar to that associated with more extensive cell body lesions of perirhinal cortex.

Memory impairment in aged primates is associated with focal death of cortical neurons and atrophy of subcortical neurons

Mechanisms of cognitive decline with aging remain primarily unknown. We determined whether localized cell loss occurred in brain regions associated with age-related cognitive decline in primates. On a task requiring the prefrontal cortex, aged monkeys were impaired in maintaining representations in working memory. Stereological quantification in area 8A, a prefrontal region associated with working memory, demonstrated a significant 32 +/- 11% reduction in the number of Nissl-stained neurons compared with young monkeys. Furthermore, the number of immunolabeled cholinergic neurons projecting to this region of cortex from the nucleus basalis was also reduced by 50 +/- 6%. In contrast, neuronal number was strikingly preserved in an adjoining prefrontal cortical region also associated with working memory, area 46, and in the component of the nucleus basalis projecting to this region. These findings demonstrate extensive but highly localized loss of neocortical neurons in aged, cognitively impaired monkeys that likely contributes to cognitive decline. Cell degeneration, when present, extends transneuronally.

Anxiety is functionally segregated within the septo-hippocampal system

Previous lesion studies have suggested that the septal-hippocampal system is involved in fear and anxiety. In this study we examined the effects on anxiety of temporary neuronal inhibition of various aspects of the septo-hippocampal system in rats. Infusions of tetrodotoxin (TTX) were used to induce reversible lesions in the fimbria fornix, medial septum, dorsal hippocampus, and ventral hippocampus. To assess anxiety we used the elevated plus-maze and the shock-probe burying tests. A reduction in anxiety in the elevated plus-maze is indicated by increased open arm exploration, whereas a reduction in anxiety in the shock-probe burying test is indicated by decreased burying behavior or increased contacts with the shock-probe. The results suggested that inhibition of the septal-hippocampal system induced site-specific anxiolytic effects that vary in nature. Tetrodotoxin lesions of the fimbria fornix increased both open arm exploration and the number of shocks taken by the rats, while having no effect on burying behavior. Both septal and ventral hippocampal lesions increased open arm exploration and decreased burying behavior, but had no effect on the number of probe shocks. Finally, TTX lesions of the dorsal hippocampus increased the number of shocks taken by the rats, but did not affect open arm activity or burying behavior. Neuroanatomical studies indicated that the effect on the number of shocks induced by dorsal hippocampal TTX lesions was not likely mediated by the amygdala. Collectively, the data suggest that the control of specific anxiety reactions is functionally segregated within different aspects of the septo-hippocampal system.

Amelioration of cholinergic neurons dysfunction in aged rats depends on the continuous supply of NGF

The present study was designed to examine whether NGF-induced improvement in morphology of senile basal forebrain cholinergic neurons persist after discontinuation of NGF treatment. Trophic effect of continuous intraventricular infusion of NGF was tested in the 4- and 28 months old male Wistar rats immediately after cessation of NGF and 3 or 6 weeks after termination of treatment. Immunohistochemical procedure for ChAT, TrkA, and p75(NTR) receptor has been applied to identify cholinergic cells in the basal forebrain structures. Using the quantitative image analyzer, morphometric and densitometric parameters of cholinergic cells were measured. In untreated 28-month-old rats a reduction in the number, size and intensity of staining of cholinergic neurons was observed in all basal forebrain structures. NGF significantly improved morphological parameters of ChAT- and TrkA-positive cells in aged rats. In 28-month-old rats tested within 3 and 6 weeks after discontinuation of infusion a renewed progressive deterioration of cholinergic phenotype of basal forebrain neurons was observed when compared with the NGF-treated immediately tested group. The parallel staining for p75(NTR) revealed normal morphology of the basal forebrain neurons, despite of the age of rats or the NGF treatment. Analysis of Nissl stained sections also showed that 28-month-old rats did not display significant losses of neurons in the basal forebrain when compared with the young animals. These findings demonstrate that senile impairment of cholinergic neurons is induced by a loss of cholinergic phenotype rather than an acute degeneration of cell bodies. NGF may be beneficial in enhancing cholinergic neurochemical parameters, but the protective effects seem to be dependent on the continuous supply of NGF.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Modulation of neuronal survival and axonal growth in vivo by tetracycline-regulated neurotrophin expression

Vector systems for the regulated and reversible expression of therapeutic genes are likely to improve the safety and efficacy of gene therapy for medical disease. In the present study, we investigated whether the expression of genes transferred into the central nervous system by ex vivo gene therapy can be regulated in vivo leading to controlled neuronal survival and axonal growth. Primary rat fibroblasts were transfected with a retrovirus containing a tetracycline responsive promoter for the expression of the neurotrophin nerve growth factor (NGF) or green fluorescent protein as a control (GFP). After lesions of basal forebrain cholinergic neurons, NGF-mediated neuronal rescue and axonal growth could be completely controlled over a 2-week period by the addition or removal of the tetracycline modulator doxycycline in the animals' drinking water. Further, continued expression of the reporter gene GFP could be reliably and repeatedly turned on and off in the injured CNS for at least 3 months post-grafting, the longest time point investigated. These data constitute the first report of regulated neuronal rescue and axonal growth by controlled neurotrophin gene delivery and long-term, regulated expression using ex vivo CNS gene therapy.

Pituitary adenylate cyclase-activating polypeptide promotes the survival of basal forebrain cholinergic neurons in vitro and in vivo: comparison with effects of nerve growth factor

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the vasointestinal polypeptide gene family for which neurotrophic activity has been postulated. PACAP mRNA is expressed in the developing and adult hippocampus, which is the principal target region of septal cholinergic neurons. We therefore studied the effects of PACAP on septal cholinergic neurons. In primary cultures from septum of embryonic and postnatal rats, PACAP increased the number of neurons immunohistochemically stained for the low-affinity nerve growth factor (NGF) receptor p75 and for the enzyme choline acetyltransferase (ChAT). PACAP also caused a corresponding increase in ChAT activity. In comparison, NGF had a greater effect than PACAP on the number of p75- and ChAT-positive neurons in these cultures. In vivo, following fimbria fornix transection, the number of immunohistochemically stained septal cholinergic neurons fell significantly to 18% in rats given continuous intracerebroventricular infusion of vehicle, whereas in rats given NGF the number of these neurons did not differ significantly from unoperated controls. In PACAP-treated rats the number was 48% of unoperated values, which represented a significant increase compared with vehicle-treated rats and a significant decrease compared with NGF-treated rats or unoperated controls. Double-staining experiments revealed that most ChAT-positive neurons in rat medial septum also express PACAP receptor 1. Together the results show that PACAP promotes the survival of septal cholinergic neurons in vitro, and after injury in vivo, suggesting that PACAP acts as a neurotrophic factor influencing the development and maintenance of these neurons.

Up-regulation of genes involved in cellular stress and apoptosis in a rat model of hippocampal degeneration

Changes in gene expression within the hippocampus induced by denervation after electrolytic fimbria-fornix lesion in rat were compared to morphological and biochemical alterations. Fimbria-fornix lesion results in degeneration of hippocampal cholinergic terminals as evidenced by a sustained (2 days to 1 month) decrease in cholineacetyltransferase (ChAT) activity (50%). These changes were accompanied by a decrease in growth associated protein 43 (GAP-43) immunoreactivity in all hippocampal layers 4 days after lesion followed by a subsequent increase and return to normal levels by 20 days postinjury. This increase in GAP-43 expression in the hippocampus between 7 to 20 days after lesion may reflect heterotypic sprouting. TUNEL-positive cells were revealed by in situ assay within the hippocampus at 10 days, but not at 3 days, after lesion. Two subtracted cDNA libraries from the dorsal hippocampus of control and injured rats (at 3 and 10 days postlesion) were constructed in order to search for new genes potentially implicated in degeneration/regeneration phenomena. We analysed 1,536 clones from each library by differential screening and found a total of 46 up-regulated genes. Among the 15 known genes, 6 coded for proteins involved in signal transduction pathways. The upregulation of growth arrest DNA damage induced gene (GADD153), brain-specific RING finger protein, JNK interacting protein (JIP-1), protein kinase A (PKA), and Na+K+ ATPase was studied by quantitative polymerase chain reaction (PCR). Two of these genes, GADD153 and JIP-1, have been previously shown to participate in cell modifications induced by stress and apoptosis.

Functional blockade of tyrosine kinase A in the rat basal forebrain by a novel antagonistic anti-receptor monoclonal antibody

We have exploited a new monoclonal antibody against the tyrosine kinase A (TrkA) nerve growth factor (NGF) receptor to block the NGF-TrkA interaction in the rat basal forebrain. The monoclonal antibody MNAC13 is a potent antagonist that prevents the binding of NGF to TrkA in a variety of systems. This antibody was used to study the maintenance of the cholinergic phenotype in the rat basal forebrain in vivo, by the implant of antibody-secreting cells. Basal forebrain cholinergic neurons (BFCNs) are greatly affected by the antibody treatment, both in terms of cell number and of cell soma size. When antibody-secreting cells are implanted at postnatal day 2 (P2), the effects observed at P8 are as severe as those obtained with anti-NGF antibodies and, interestingly, are observed also if anti-TrkA cells are implanted at P8, when anti-NGF antibodies, delivered by the same route, are no longer effective (). The effects induced by anti-TrkA, as those induced by anti-NGF, are reversible, but the time required for recovery and the critical period in the sensitivity of BFCNs to the functional inactivation of TrkA is twice as long than that observed when NGF is intercepted. These results demonstrate that BFCNs are more sensitive to the block of TrkA activation than they are to the block of NGF. The cloning of MNAC13 variable regions and their assembly into a functional polypeptide of reduced size (single chain Fv fragment) will allow its use in gene transfer applications.

The effects of anti-nerve growth factor monoclonal antibodies on developing basal forebrain neurons are transient and reversible

In order to reassess the role of nerve growth factor (NGF) on rat basal forebrain cholinergic neurons (BFCNs) survival and/or phenotype maturation during the early postnatal life, we immunoneutralized NGF in vivo. Hybridoma cells producing the neutralizing anti-NGF monoclonal antibody alphaD11 were implanted in the lateral ventricle of the rat at different postnatal ages (P2, P8 and P15) and the effects on the number and the soma size of cholinacetyltransferase (ChAT) positive neurons were analysed 1, 2 or 3 weeks after the injection. A marked decrease in the number and in the soma size of BFCNs was observed implanting hybridoma cells at P2 and performing the analysis 1 week later. These effects are reversed 3 weeks after the implant of hybridoma cells at P2. At this time point, the levels of alphaD11 antibodies in the brain parenchyma are still in a vast molar excess over endogenous NGF. No effects on BFCNs were observed implanting alphaD11 cells at P15 while LGN neurons showed marked shrinkage. Our results demonstrate that the reduction in the number of ChAT-positive neurons during the first two postnatal weeks of anti-NGF treatment is not due to cell death. We conclude that NGF is not a survival factor for BFCNs, and that the influence of NGF on BFCNs cell maturation during the first 2 postnatal weeks is transient and reversible. Our results on tyrosine kinase (Trk) coexpression, suggest that NGF may cooperate with other factors in the cholinergic phenotype differentiation and maintenance after the second postnatal week.

Evidence for normal aging of the septo-hippocampal cholinergic system in apoE (-/-) mice but impaired clearance of axonal degeneration products following injury

The association of the epsilon4 allele of apoE with increased risk for Alzheimer's disease (AD) and with poor clinical outcome after certain acute brain injuries has sparked interest in the neurobiology of apoE. ApoE (-/-) mice provide a tool to investigate the role of apoE in the nervous system in vivo. Since integrity of the basal forebrain cholinergic system is severely compromised in AD, with severity of dysfunction correlating with apoE4 gene dosage, the present study tested the hypothesis that apoE is required to maintain the normal integrity of basal forebrain cholinergic neurons (BFCNs). Histological and biochemical analyses of the septo-hippocampal cholinergic system were performed in apoE (-/-) mice during aging and following injury. Using unbiased quantitative methods, there was little or no evidence for defects in the septo-hippocampal cholinergic system, as assessed by p75(NTR)-immunoreactive neuron number and size in the medial septum, cholinergic fiber density in the hippocampus, and choline acetyltransferase activity in the hippocampus, cortex, and striatum in aged apoE (-/-) mice (up to 24 months of age) as compared to age-matched wild-type mice of the same strain. In addition, cholinergic neuronal survival and size following fimbria-fornix transection in apoE (-/-) mice did not differ from controls. However, following entorhinal cortex lesion, there was persistence of degeneration products in the deafferented hippocampus in apoE (-/-) mice. These data suggest that although apoE is not required for the maintenance of BFCNs in vivo, it may play a role in the clearance of cholesterol-laden neurodegeneration products following brain injury.

Recovery of ChAT immunoreactivity in axotomized rat cholinergic septal neurons despite reduced NGF receptor expression

Previous studies have suggested that target-derived nerve growth factor (NGF) is essential for the survival of cholinergic basal forebrain neurons. Thus, axotomy of septohippocampal neurons in adult rats resulting in the withdrawal of target-derived NGF caused a dramatic loss of choline acetyltransferase (ChAT)-immunoreactive neurons in the medial septum-diagonal band complex. We have recently shown that this loss of immunolabelled neurons does not indicate cell death, since many septohippocampal cholinergic neurons recover their immunoreactivity for ChAT after a long survival time despite disconnection from target-derived neurotrophins. One possibility would be that these surviving ChAT-immunoreactive neurons have gained access to other, probably local, NGF sources. Here we provide evidence that the recovery of ChAT immunoreactivity after axotomy is not accompanied by a similar recovery of NGF receptor expression in these neurons. In situ hybridization for p75NTR mRNA and trkA mRNA 6 months after bilateral fimbria-fornix transection revealed a substantial loss of labelled cells. In addition, there was a persisting loss of p75NTR-immunoreactive and NGF-immunoreactive medial septal neurons. Cholinergic neurons in controls did not express NGF mRNA, but were heavily immunostained for NGF protein due to receptor-mediated uptake. These data suggest that at least some cholinergic septohippocampal neurons re-express ChAT either independently of NGF or with a reduced need for NGF.

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

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

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Medial septal cholinergic neurons express c-Jun but do not undergo DNA fragmentation after fornix-fimbria transections

We investigated the expression of inducible transcription factors (ITFs) and the fate of medial septal (MS) cholinergic neurons following fornix fimbria (FF) transection c-Jun, but not c-Fos or Krox 24 was induced in nerve growth factor receptor-immunoreactive (NGFr-ir), parvalbumin-negative MS neurons by 48 h and still highly expressed 2 months after transection. JunD was expressed only at 48 h after transection. Levels of choline acetyl transferase immunoreactivity (ChAT-ir) and NGFr-ir decreased substantially 7 and 14 days respectively following FF transection and remained depressed for up to 2 months. We also investigated other measures of nerve cell death and found that there was a time-dependent loss of cresyl violet staining, but no evidence of DNA fragmentation, acidophilia or clusterin expression in the MS region. There was however, good evidence of microglial activation and astrocyte hypertrophy in the MS. These results suggest that axotomized c-Jun-positive septohippocampal neurons lose their cholinergic phenotype but do not die for up to 2 months after FF transection. The function of c-Jun in axotomized MS neurons remains a mystery, but c-Jun expression alone is clearly not sufficient to elicit death of these neurons.

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.

Induction of clusterin in the immature brain following a hypoxic-ischemic injury

A unilateral hypoxic-ischemic (HI) insult in the 21 day old rat has been used to assess the role of clusterin in nerve cell death. Both clusterin mRNA and protein levels were measured at various time points after moderate (15 min) and severe (60 min) HI insult using in situ hybridisation and immunocytochemistry respectively. The severe HI insult lead primarily to necrotic neuronal death and showed very little if any clusterin mRNA and protein induction on the ligated side of the brain. However, following the moderate HI insult there was a dramatic time-dependent accumulation of clusterin protein in neurons of the CA1-CA2 pyramidal cell layers in the hippocampus and cortical layers 3-5, regions undergoing delayed neuronal death. Clusterin mRNA expression, in contrast to neuronal protein accumulation, appeared to be glial in origin (probably astrocytes) with increases in mRNA in and around the hippocampal fissure and only a weak signal over the CA1-CA2 pyramidal cell layer. These results support the hypothesis that the clusterin protein is synthesised in the astrocytes, secreted and then taken up by dying neurons. Clusterin immunoreactivity and in situ DNA end-labelling performed on the same sections revealed that clusterin was accumulating in neurons destined to die by programmed cell death. However the relative time-courses of DNA fragmentation and clusterin immunoreactivity suggest that clusterin production was a result of the selective delayed neuronal death rather than being involved in the biochemical cascade of events that cause it.

TrkA activation is sufficient to rescue axotomized cholinergic neurons

To test the molecular nature of the NGF receptor responsible for the ability of NGF to rescue septal cholinergic neurons following axotomy, we infused polyclonal antibodies that act as specific agonists of trkA (RTA) into the lateral ventricle of fimbria-fornix lesioned animals. Rats receiving chronic intraventricular infusions of RTA showed significantly more low affinity NGF receptor immunoreactive (p75NGFR-IR) neurons on the lesioned side than did control animals 2 weeks following unilateral fimbria-fornix lesion. RTA also initiated cholinergic sprouting. Infusions of RTA in combination with an antibody that blocks p75NGFR (REX) did not reduce the cell savings effect observed with RTA alone. However, animals infused with RTA plus REX demonstrated significantly less sprouting. These findings suggest that antibody-induced trkA activation is sufficient to mediate NGF-promoted survival of axotomized cholinergic neurons in vivo.

Mechanical lesions of the fimbria fornix in rat brain studied by 1H-magnetic resonance imaging. Evidence for long-lasting dynamic alterations in the ipsilateral ventricular system

In vivo 1H-NMR imaging was employed to study dynamic changes in the status of tissue water as a function of time after mechanical brain injury induced by partial unilateral transection of the fimbria fornix (FF) in the rat brain and was correlated with histology. Changes in the brain tissue were reproducibly found in distinct regions which were exclusively located in the lesioned hemisphere. The most pronounced changes concerned the lateral ventricle. Ventricular enlargement became evident posterior to the site of transection after a few hours and was maximal after 2-4 days. At later time points the posterior ventricular expansion was reduced. The lateral ventricle anterior to the site of transection was significantly enlarged from day 1 and continued to expand for up to 7 months. Tissue response at the site of transection, mainly involving the hippocampal formation and the thalamus, was first manifested after 24 h, while signs of progressive tissue degeneration were apparent in the long term.

Neurotrophin-4/5 maintains the cholinergic phenotype of axotomized septal neurons

We examined the effect of intraseptal or intracerebroventricular (i.c. v.) infusions of NT-4/5 or intraseptal infusions of NGF on the level of immunohistochemical staining of choline acetyltransferase (ChAT)and the low-affinity nerve growth factor receptor (LNGFR)in the rat medial septum following unilateral transection of the fimbria. The extent of cell loss in the septum ipsilateral to the lesion, determined by cell counts of ChAT-immunopositive neurons and expressed as a ratio comparing the lesioned to the intact sides, was 0.28 in animals that received an infusion of phosphate-buffered saline (PBS). The ratios were 0.97 and 1.07 in animals that received an infusion of NT-4/5 into the ipsilateral ventricle and septum respectively. Septal infusions of NGF produced a ratio of ChAT-immunopositive cells of 1.03. The ratios of LNGFR-immunopositive neurons increased from 0.50 in PBS-infused animals to 0.79 and 0.83 in animals infused with NT-4/5 via the i.c. v. infusion of NT-4/5 or septal infusion of NT-4/5 or NGF. As determined by immunohistochemical staining, NT-4/5 infused into the lateral ventricle was detected in the periventricular portions of the forebrain ipsilateral to the infusion, while NT-4/5 or NGF infused intraseptally was detected in much of the septum, bilaterally. Furthermore, exogenous NT-4/5 or NGF was detected in numerous neuronal perikarya in the medial septal and diagonal band nuclei. These data demonstrate that, as with NGF, i.c.v. as well as septal infusions of NT-4/5 can maintain the phenotype of basal forebrain cholinergic neurons following axotomy.

TGF-beta rescues target-deprived preganglionic sympathetic neurons in the spinal cord

Transforming growth factors beta (TGF-beta), a family of pleiotropic cytokines, are widely distributed in the developing and adult nervous system. In order to further determine the neural functions of TGF-beta, we have localized the TGF-beta isoforms 1, 2 and 3 in the adult rat adrenal medulla and studied the neuroprotective capacity of one representative family member, TGF-beta 2, for those spinal cord neurons which innervate adrenal chromaffin cells and which die after destruction of the adrenal medulla. Unilateral electrothermal destruction of the adrenal medulla led to the disappearance of 25% of sympathetic preganglionic neurons, which are located in the intermediolateral (IML) column of thoracic spinal cord segments 7-10 and can be selectively marked by NADPH-diaphorase. The neurons which disappeared following adrenomedullectomy constitute the full set of neurons that innervate the adrenal medulla. Implantation of gelfoam soaked with 0.5 micrograms TGF-beta 2 into the adrenal wound cavity rescued all spinal cord neurons in the IML ipsilaterally to the lesioned side. Cytochrome c was not effective. Injections of [125I]TGF-beta 2 into the adrenal medulla did not result in retrograde transport and subsequent labelling of spinal cord neurons, suggesting that TGF-beta may exert its neuroprotective actions by indirect mechanisms. TGF-beta applied to cultured adrenocortical cells did not overtly increase the amount of mRNA for fibroblast growth factor-2, an established trophic molecule for sympathetic preganglionic spinal cord neurons. The mechanisms by which TGF-beta exerts its neurotrophic effect are therefore unclear. Even so, our data provide the first evidence that TGF-beta may play an important role in vivo in the control of maintenance of a population of spinal cord neurons.

Axonal regeneration and limited functional recovery following hippocampal deafferentation

Although central neurons do not naturally recover following injury, damaged adult septal neurons can regenerate when nerve growth factor (NGF) is provided along with a suitable cellular substrate. This study investigates the outgrowth of axotomized septal neurons grafted with primary fibroblasts genetically modified to produce NGF. Confocal microscope images of double staining for neuritic markers (neurofilament or low-affinity NGF receptor) and the astrocytic marker glial fibrillary acidic protein (GFAP) demonstrated that regenerating neurites crossed dense buildups of astrocytic processes at the edges of NGF-producing grafts and were in apposition with astrocytic processes within NGF-producing grafts. Immunoreactivity for acetylcholinesterase and low-(p75) and high-affinity (TrkA) NGF receptors was dense in NGF-producing grafts but absent in control grafts. NGF-grafted rats exhibited significantly increased hippocampal density of p75-immunoreactive fibers and significantly decreased ectopic hippocampal sympathetic ingrowth as compared to control-grafted rats. Rats with unilateral fimbria-fornix lesions and NGF-producing grafts exhibited ameliorated performance on a simple memory task. These findings demonstrate that implantation of NGF-producing grafts to the lesion cavity allows axotomized septal cholinergic neurons to reinnervate the hippocampus, and that rats receiving these grafts show a partial recovery of function.

NGF mRNA is expressed by GABAergic but not cholinergic neurons in rat basal forebrain

Nerve growth factor (NGF) supports the survival and biosynthetic activities of basal forebrain cholinergic neurons and is expressed by neurons within lateral aspects of this system including the horizontal limb of the diagonal bands and magnocellular preoptic areas. In the present study, colormetric and isotopic in situ hybridization techniques were combined to identify the neurotransmitter phenotype of the NGF-producing cells in these two areas. Adult rat forebrain tissue was processed for the colocalization of mRNA for NGF with mRNA for either choline acetyltransferase, a cholinergic cell marker, or glutamic acid decarboxylase, a GABAergic cell marker. In both regions, many neurons were single-labeled for choline acetyltransferase mRNA, but cells containing both choline acetyltransferase and NGF mRNA were not detected. In these fields, virtually all NGF mRNA-positive neurons contained glutamic acid decarboxylase mRNA. The double-labeled cells comprised a subpopulation of GABAergic neurons; numerous cells labeled with glutamic acid decarboxylase cRNA alone were codistributed with the double-labeled neurons. These data demonstrate that in basal forebrain GABAergic neurons are the principal source of locally produced NGF.

Nerve growth factor prevents apoptotic cell death in injured central cholinergic neurons

Experimental lesions have been widely used to induce neuronal degeneration and to test the ability to trophic molecules to prevent lesion-induced alterations, but these studies have not demonstrated unequivocally that afflicted neurons die as a result of these manipulations. The documentation of neuronal death in the above-described models and the time when it occurs after injury are crucial for the interpretation of trophic effects. In the present study, we combined multiple approaches to investigate the nature of retrograde neuronal changes in cholinergic neurons of the medial septal nucleus (MSN) after complete, unilateral transection of the fimbria-fornix (F-F). Projections neurons of the MSN were prelabeled with the fluorescent tracer Fluoro-gold (FG) 1 week prior to lesion. By counting both FG-labeled and choline acetyltransferase (ChAT)-immunoreactive neurons in the MSN at multiple time points postaxotomy, we differentiated the phenotypic response to injury from the degenerative process and established a critical time between the third and fourth weeks postaxotomy, during which approximately 50% of fluorescent perikarya disappear. Working in the previous time window, we identified dying cells by electron microscopy (EM) and terminal transferase-mediated (TdT) deoxyuridine triphosphate (d-UTP)-biotin nick end labeling (TUNEL) and showed that MSN neurons die via apoptosis, beginning at 16 days postaxotomy. An additional group of animals was allowed to survive for 1 month (i.e., 10 days after cell death has been completed); during this period, animals were treated with intraventricular nerve growth factor (NGF). Quantitative analysis of surviving cholinergic perikarya showed that NGF prevented degeneration of the majority of neurons. In concert, the results of the present study establish that NGF does not merely protect the phenotype but also prevents cell death in lesioned central cholinergic neurons.

Effects of complete and partial lesions of the dopaminergic mesotelencephalic system on skilled forelimb use in the rat

This study compares certain behavioural consequences of partial and complete unilateral lesions of the dopaminergic mesotelencephalic system. We investigated skilled forelimb use, rotations induced by apomorphine and amphetamine, and dopaminergic metabolism of the nigrostriatal system of rats that had received a unilateral injection of 6-hydroxydopamine into the medial forebrain bundle. The rats classified Apo(+), that rotated after the administration of apomorphine, had a complete lesion of the nigrostriatal system, whereas those classified Apo(-), that did not rotate after the administration of apomorphine, had a partial lesion of the nigrostriatal system. In the Apo(+) rats, 99.8% of the dopamine in the striatum was depleted, as was 85% of that in the substantia nigra. For the Apo(-) rats, 72% of the dopamine in the striatum was depleted as was 56% of that in the substantia nigra. When investigated with the staircase test, the animals with the most severe dopamine depletions were those most impaired in the paw reaching task. Complete and partial unilateral depletions of the dopaminergic mesotelencephalic system impaired the hierarchic phases of paw reaching differently. A complete dopamine depletion, but not a partial one, decreased the number of attempts made with the contralateral paw, and induced a bias towards the ipsilateral paw. A partial dopamine lesion impaired the sensorimotor co-ordination of both paws, whereas the complete dopamine lesion had a greater effect on the contralateral paw than on the ipsilateral paw. The mild paw reaching impairments observed in animals with moderate depletions of dopamine are proposed as a model of the early symptoms of Parkinson's disease that may be useful for the development of protective or restorative therapies.

The role of inducible transcription factors in apoptotic nerve cell death

Recent studies have shown that certain types of nerve cell death in the brain occur by an apoptotic mechanism. Researchers have demonstrated that moderate hypoxic-ischemic (HI) episodes and status epilepticus (SE) can cause DNA fragmentation as well as other morphological features of apoptosis in neurons destined to die, whereas more severe HI episodes lead to neuronal necrosis and infarction. Although somewhat controversial, some studies have demonstrated that protein synthesis inhibition prevents HI-and SE-induced nerve cell death in the brain, suggesting that apoptotic nerve cell death in the adult brain is de novo protein synthesis-dependent (i.e., programmed). The identity of the proteins involved in HI-and SE-induced apoptosis in the adult brain is unclear, although based upon studies in cell culture, a number of potential cell death and anti-apoptosis genes have been identified. In addition, a number of studies have demonstrated that inducible transcription factors (ITFs) are expressed for prolonged periods in neurons undergoing apoptotic death following HI and SE. These results suggest that prolonged expression of ITFs (in particular c-jun) may form part of the biological cascade that induces apoptosis in adult neurons. These various studies are critically discussed and in particular the role of inducible transcription factors in neuronal apoptosis is evaluated.

Ciliary neurotrophic factor supports p75NGFR-immunoreactive non-cholinergic, but not cholinergic, developing septal neurons in vitro

Ciliary neurotrophic factor is known to exert both survival and differentiative actions on a number of neuronal populations of the peripheral and central nervous systems. In this study we have compared the trophic effects of ciliary neurotrophic factor and nerve growth factor on developing septal neurons of the rat in vitro. Fetal septal neurons were grown in vitro under glass coverslips in sandwich culture. Septal cultures grown for 14 days in the continual presence of nerve growth factor contain a population of cholinergic neurons that stain intensely for the low-affinity nerve growth factor receptor (p75NGFR), choline acetyltransferase and acetylcholinesterase. Without added nerve growth factor, few neurons stain for these markers. Ciliary neurotrophic factor addition for 14 days from plating in the absence of exogenous nerve growth factor results in the appearance of a population of neurons that stains for p75NGFR. This population is similar in number to that seen in nerve growth factor-treated cultures but is not immunoreactive for choline acetyltransferase and is significantly smaller in mean cross-sectional area. Delayed addition of nerve growth factor to ciliary neurotrophic factor-supported cultures at 14 days for a further seven days fails to induce choline acetyltransferase immunoreactivity in these p75NGFR-positive septal neurons. In cultures grown in the continual presence of nerve growth factor from plating, removal of nerve growth factor and addition of nerve growth factor antibodies at 14 days results in the death of over 80% of the cholinergic neurons after a further four days. Addition of ciliary neurotrophic factor during the period of nerve growth factor withdrawal appears to preserve a p75NGFR-positive, choline acetyltransferase-negative neuronal population. However, seven day re-addition of nerve growth factor to ciliary neurotrophic factor-treated, nerve growth factor-withdrawn cultures fails to induce choline acetyltransferase immunoreactivity in the ciliary neurotrophic factor-supported p75NGFR-positive septal neurons. Simultaneous treatment of cultures with both ciliary neurotrophic factor and nerve growth factor for 14 days from plating approximately doubles the number of p75NGFR-positive neurons relative to cultures treated with either ciliary neurotrophic factor or nerve growth factor alone, but the number of choline acetyltransferase-positive neurons in these cultures is not significantly greater than that found in cultures treated solely with nerve growth factor. These results suggest that ciliary neurotrophic factor does not support the survival and differentiation of developing septal cholinergic neurons in vitro, but can support the development of a p75NGFR-immunoreactive population of non-cholinergic septal neurons.

Distribution of nerve growth factor following direct delivery to brain interstitium

Several studies suggest the potential of nerve growth factor (NGF) in the treatment of patients with Alzheimer's disease. To characterize NGF transport within the brain interstitium, we implanted controlled release polymers containing NGF and [125I]NGF into the brains of adult male rats and measured spatial distributions of NGF for up to one week. NGF concentration in the brain was quantified using ELISA, radiation counting, and autoradiography. At 2 days post-implantation, quantities of NGF in excess of 50 pg per section were detected within thick (1 mm) coronal slices of the hemisphere ipsilateral to the site of implantation up to 3 mm rostral and caudal to the edge of the polymer. Lower levels of radioactivity (> 5 pg but < 50 pg NGF per section) could be detected throughout the rest of the brain. Levels were highest in the tissue sections containing the polymer, reaching 9.5 ng per section. Autoradiography of thin (20 microns) coronal sections indicated that local NGF concentrations immediately adjacent to the polymer approached 40 micrograms/ml. Analysis of sequential sections on the autoradiograph confirmed that NGF was transported only 2-3 mm from the polymer in any direction. At one week post-implantation, the pattern of NGF distribution was similar to that seen at 2 days, and concentrations remained high near the site of the implant. Comparison of local NGF concentration profiles to simple models of diffusion with first-order elimination suggests that the NGF moved through the tissue by diffusion through the interstitial space with a half-life on the order of 0.5 h. The limited range of NGF transport in brain tissue indicates that: (i) protein drug agents such as NGF will probably need to be delivered almost directly to the site of action for efficacy; and (ii) toxicities associated with delivery of NGF and other protein agents to non-target cells, as often occurs with systemic delivery of drugs, may be reduced by local, interstitial delivery since therapy can be restricted to a small volume of the brain.

Maintaining the neuronal phenotype after injury in the adult CNS. Neurotrophic factors, axonal growth substrates, and gene therapy

Multiple genetic and epigenetic events determine neuronal phenotype during nervous system development. After the mature mammalian neuronal phenotype has been determined it is usually static for the remainder of life, unless an injury or degenerative event occurs. Injured neurons may suffer one of three potential fates: death, persistent atrophy, or recovery. The ability of an injured adult neuron to recover from injury in adulthood may be determined by events that also influence neuronal phenotype during development, including expression of growth-related genes and responsiveness to survival and growth signals in the environment. The latter signals include neurotrophic factors and substrate molecules that promote neurite growth. Several adult CNS regions exhibit neurotrophic-factor responsiveness, including the basal forebrain, entorhinal cortex, hippocampus, thalamus, brainstem, and spinal cord. The specificity of neurotrophic-factor responsiveness in these regions parallels patterns observed during development. In addition, neurons of several CNS regions extend neurites after injury when presented with growth-promoting substrates. When both neurotrophic factors and growth-promoting substrates are provided to adult rats that have undergone bilateral fimbria-fornix lesions, then partial morphological and behavioral recovery can be induced. Gene therapy is one useful tool for providing these substances. Thus, the mature CNS remains robustly responsive to signals that shape nervous system development, and is highly plastic when stimulated by appropriate cues.

Fine structure of rat septohippocampal neurons. III. Recovery of choline acetyltransferase immunoreactivity after fimbria-fornix transection

Most cholinergic projection neurons in the medial septal nucleus (MS) lose their capability to synthesize choline acetyltransferase (ChAT) after axotomy by bilateral fimbria-fornix transection. We have recently shown that identified septohippocampal neurons survive axotomy up to 10 weeks and display fine-structural characteristics of cells in control rats. However, the fate and functional role of these neurons remained unclear. Here we describe observations made in rats which survived axotomy for 6 months. Adult Sprague-Dawley rats were subjected to bilateral transection of the fimbria-fornix system. In some animals septohippocampal projection neurons were labeled by the retrograde fluorescent tracer Fluoro-Gold (FG) prior to axotomy. After varying survival times following fimbria-fornix transection, the animals were fixed and sections of the septal region immunostained for ChAT. Three weeks postlesion, the number of ChAT-positive cells in the MS was reduced to 19% of control, suggesting a severe neuronal loss. However, 10 weeks and 6 months after axotomy this value increased to 28% and 54%, respectively. Fine-structural analysis of ChAT-positive neurons after 6 months survival revealed all characteristics of vital cells including normal input synapses. The majority of these cells could be identified as former septohippocampal projection neurons by the presence of FG. We conclude that many neurons in the MS have the capacity to restore their transmitter synthesis in a long-lasting process following axotomy.

Fluid percussion injury causes loss of forebrain choline acetyltransferase and nerve growth factor receptor immunoreactive cells in the rat

Memory dysfunction is a common sequela of human traumatic brain injury (TBI). Cholinergic forebrain neurons are recognized for their role in memory. We tested the hypothesis that forebrain cholinergic neurons are vulnerable to fluid percussion injury (FPI), a model of human TBI. Rodents were subjected to a moderate parasagittal FPI, sham injury, or fimbria/fornix axotomy and then killed 10 days after the procedure. Additional animals underwent FPI or sham injury and were killed 7, 14, and 28 days after the procedure. Neurons in the medial septal nucleus and vertical limb of the nucleus of the diagonal band of Broca were identified and quantitated using choline acetyltransferase (ChAT) and low affinity nerve growth factor receptor (NGF-R) immunohistochemistry. Our results showed a significant decrease in ChAT (17% +/- 5%) and NGF-R (24% +/- 8%) immunoreactive cells in FPI animals killed after 10 days when compared to sham-injured animals. Animals undergoing fimbria/fornix axotomy showed a greater reduction in ChAT (53% +/- 13%) and NGF-R (55% +/- 5%) immunoreactive cells 10 days postaxotomy. The number of ChAT and NGF-R immunoreactive neurons was reduced at all time points. However, statistical significance was present 10 and 14 days postinjury for ChAT immunoreactive neurons and 10 days only for NGF-R immunoreactive neurons. These studies have shown that FPI produces transient loss of ChAT and NGF-R immunoreactive neurons.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Retrograde degenerative changes in the substantia nigra pars compacta following an excitotoxic lesion of the striatum

The retrograde changes induced by an excitotoxic lesion of the striatum (Str) on the neurons in substantia nigra pars compacta (SNc) projecting to the neuron-depleted region were investigated in adult rats. The retrograde tracer Fluoro-Gold (FG) was injected bilaterally into the Str. 2 weeks later, the excitotoxic amino acid ibotenic acid (IA) was injected unilaterally into the same structure. At four different time points after the lesion (1 week and 1, 2 and 3 months, respectively), the size of the FG-labelled cells and number of tyrosine hydroxylase (TH)-positive cells in the SNc were evaluated on the lesioned and control sides. Parallel groups of animals received suspension grafts of fetal striatal tissue into the lesioned striata. At 1 week and 1 month after lesion, there were no changes in cell size, number of TH-positive cells or number of FG-labelled cells expressing TH at the SNc. At 2 and 3 months, however, there was a significant 30% shrinkage of the FG-labelled SNc cells but no evident decrease in TH-positive cell number, or in the expression of the TH protein, on the lesioned side as compared with the non-lesioned control side. Striatal transplants placed into the lesioned Str did not counteract this effect. This finding that an axon-sparing lesion of target cells results in cell shrinkage but no cell loss of the neurons that project to the lesioned area is in line with what has been shown to occur after similar lesions in the cholinergic septohippocampal and basalo-cortical systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of neurotrophin and trk receptor genes in adult rats with fimbria transections: effect of intraventricular nerve growth factor and brain-derived neurotrophic factor administration

The expression of the specific trk receptors for nerve growth factor and brain-derived neurotrophic factor (trkA and trkB) has been assayed by messenger RNA in situ hybridization in adult rats with partial fimbrial transections along with intraventricular treatment of nerve growth factor or brain-derived neurotrophic factor. In the forebrain, specific hybridization labeling for trkA messenger RNA showed an identical pattern to that of choline acetyltransferase messenger RNA, supporting the view that trkA expression is confined to the cholinergic population in the basal forebrain and the cholinergic interneurons in the striatum. After partial unilateral transections of the fimbria there was a progressive loss of choline acetyltransferase and trkA messenger RNA expression in the septal region ipsilateral to the lesion. Daily intraventricular administration of brain-derived neurotrophic factor or nerve growth factor partially prevented the lesion-induced decrease in the levels of both messengers, the latter being more effective than the former. Grain count analysis of individual cells was used to test whether the two factors upregulated choline acetyltransferase or trkA expression in individual cells surviving the lesion. Brain-derived neurotrophic factor treatment failed to induce any change in the levels of both messengers per neuron in the septal area. In contrast, daily intraventricular administration of nerve growth factor upregulated both choline acetyltransferase and trkA messenger RNA expression in individual neurons. This upregulation was evident on ipsilateral and contralateral sides, suggesting that nerve growth factor is able to upregulate these markers in intact and injured cholinergic cells in the basal forebrain. Similar to the situation in the septum, brain-derived neurotrophic factor did not upregulate choline acetyltransferase or trkA expression in the striatum. However, nerve growth factor administration strongly upregulated choline acetyltransferase messenger RNA expression by individual cholinergic neurons of the striatum. A medial to lateral gradient decrease in this upregulation was detected in the striatum ipsilateral to the side of administration, suggesting a limited diffusion of the nerve growth factor protein from the ventricle into brain parenchyma. In contrast to the strong effect on choline acetyltransferase expression, nerve growth factor treatment was ineffective in altering trkA messenger RNA in the striatum. The contrasting findings between septum and striatum suggest different regulatory mechanisms for trkA messenger RNA expression in the two cholinergic populations. Since nerve growth factor was found to upregulate the expression of its trkA receptor, we tested whether brain-derived neurotrophic factor administration had similar effects on the regulation of its trkB receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Progressive degeneration of nigrostriatal dopamine neurons following intrastriatal terminal lesions with 6-hydroxydopamine: a combined retrograde tracing and immunocytochemical study in the rat

In order to develop a rodent model displaying a progressive degeneration of the dopamine neurons of the substantia nigra, we bilaterally injected the tracer substance FluoroGold into the terminal field of the nigrostriatal projection, i.e. the striatum. One week later, rats received unilateral injections of 20 micrograms 6-hydroxydopamine into one of the two striatal tracer deposits. Groups of animals were killed one, two, four, eight and 16 weeks later. Ipsilateral to the lesion there was a progressive loss of FluoroGold-labelled nigral cells, with cell counts dropping from 96% of the contralateral side at one week to 59% at two weeks, 35% at four weeks, 23% at eight weeks and down to 15% at 16 weeks. Labelled nigral neurons ipsilateral to the lesion showed a moderate to marked atrophy at all investigated time points. The number of tyrosine hydroxylase-immunoreactive cells was decreased to 83% of contralateral at one week, 39% at two weeks, 44% at four weeks, 34% at eight weeks and 52% at 16 weeks postlesion. Rhodamine fluorescence immunocytochemistry showed that the proportion of surviving ipsilateral fluorogold-labelled cells displaying immunoreactivity for tyrosine hydroxylase was 69% at one week postlesion, 51% at two weeks, 63% at four weeks, 69% at eight weeks and 76% at 16 weeks. We conclude that injection of 6-hydroxydopamine into the terminal field of nigral dopaminergic neurons causes a progressive degeneration of these cells, starting between one and two weeks after lesion and continuing over eight to 16 weeks. This degeneration is preceded, and accompanied by, cellular atrophy and a partial loss of marker enzyme expression, thus yielding an animal model which mimics the degenerative processes in Parkinson's disease more closely than the animal models available so far. The present model may be helpful in investigating the in vivo effects of putative neuroprotective agents and neurotrophic factors.

Ciliary neurotrophic factor prevents retrograde neuronal death in the adult central nervous system

The neurocytokine ciliary neurotrophic factor (CNTF) was described originally as an activity that supports the survival of neurons of the chicken ciliary ganglia in vitro. The widespread expression of CNTF and its principal binding protein, CNTF receptor alpha, in the central and peripheral nervous systems suggests a broader trophic role for this peptide. In the present study, we report that CNTF prevents axotomy-induced cell death of neurons in the anteroventral and anterodorsal thalamic nuclei of the adult rat. Using the polymerase chain reaction, we also demonstrate the presence of CNTF and CNTF receptor alpha mRNA in these same thalamic nuclei. The coincidence of CNTF and its receptor in a population of neurons responding to the factor suggests a paracrine function for CNTF. The present findings establish that CNTF has significant effects on neurons of the central nervous system in vivo and demonstrate that neurocytokines can prevent cell death in the adult central nervous system.

Induction of c-FOS immunoreactivity in the hippocampus following potassium stimulation

In microdialysis procedures high potassium ion concentrations are generally used to induce neurotransmitter release. However, the widespread effects, if any, of such a treatment have not been described. In order to establish a possible link between c-fos expression and stimulating conditions for neurotransmitter release in microdialysis procedures we administered KCl (100 mM) into the hippocampus. Proto-oncogene c-FOS-like immunoreactivity is upregulated in granule cells of the dentate gyrus, pyramidal cells of the hippocampus, cingulate, piriform and frontoparietal cortices at 2 h, but not 24 h after K+ administration. Neither implantation of the probes nor perfusion with artificial cerebrospinal fluid resulted in similar patterns of c-FOS immunoreactivity. In addition, we investigated whether the impairment of the cholinergic septohippocampal pathway would modify the K(+)-induced expression of the immediate early gene c-fos in the hippocampus. The expression of c-fos induced by KCl was not altered in the animals with fimbria-fornix lesion despite the marked decrease in acetylcholine release in the hippocampus. Glutamate concentrations measured in the same superfusates showed that a significantly greater glutamate release occurs in denervated hippocampi. Furthermore, abolishment of seizure-like activity (induced by KCl) in anesthetized animals did not alter expression of c-FOS immunoreactivity in the K(+)-stimulated hippocampi. The results from these studies confirm that most of the releasable acetylcholine of the hippocampus is linked to the fimbrial input and may suggest that c-FOS upregulation in this model does not respond to any cholinergic input from the medial septum via the fimbria-fornix.