Effect of delayed treatment with nerve growth factor on choline acetyltransferase activity in the cortex of rats with lesions of the nucleus basalis magnocellularis: dose requirements

The neurotrophins and their role in Alzheimer's disease

Besides being essential for correct development of the vertebrate nervous system the neurotrophins also play a vital role in adult neuron survival, maintenance and regeneration. In addition they are implicated in the pathogenesis of certain neurodegenerative diseases, and may even provide a therapeutic solution for some. In particular there have been a number of studies on the involvement of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) in the development of Alzheimer's disease. This disease is of growing concern as longevity increases worldwide, with little treatment available at the moment to alleviate the condition. Memory loss is one of the earliest symptoms associated with Alzheimer's disease. The brain regions first affected by pathology include the hippocampus, and also the entorhinal cortex and basal cholinergic nuclei which project to the hippocampus; importantly, all these areas are required for memory formation. Both NGF and BDNF are affected early in the disease and this is thought to initiate a cascade of events which exacerbates pathology and leads to the symptoms of dementia. This review briefly describes the pathology, symptoms and molecular processes associated with Alzheimer's disease; it discusses the involvement of the neurotrophins, particularly NGF and BDNF, and their receptors, with changes in BDNF considered particularly in the light of its importance in synaptic plasticity. In addition, the possibilities of neurotrophin-based therapeutics are evaluated.

Intraparenchymal nerve growth factor improves behavioral deficits while minimizing the adverse effects of intracerebroventricular delivery

Nerve growth factor (NGF) delivered via intracerebroventricular (ICV) infusion restores behavioral and biochemical deficits in animal models of cholinergic hypofunction. However, ICV infusion of NGF induces an array of adverse events including weight loss, thermal hyperalgesia, and Schwann cell hyperplasia. We compared ICV administration with three different doses of intraparenchymally delivered NGF with cytochrome C infusion serving as a control. The goal of the study was to determine whether direct infusion of NGF would result in a more restricted topographical distribution of NGF leading to a reduction or elimination of the adverse events while still augmenting cholinergic functioning sufficiently to restore spatial mnemonic processing. Subsequent to bilateral ibotenic acid lesions of the nucleus basalis magnocellularis (NBM), NGF was delivered into the lateral ventricle or adjacent to the NBM for 11 weeks. Ibotenic acid lesions resulted in reductions in choline acetyltransferase (ChAT) activity in the cortex. The highest and medium dose of NGF led to significant restoration in ChAT activity on par with ICV infusion. The lowest dose was ineffective in altering ChAT activity in any region assayed. Similarly, the two highest doses did not alter weight gain, but ICV-NGF led to a significant weight loss. Intraparenchymal infusion resulted in a dose-dependent attenuation of the development of thermal hyperalgesia. However, the highest dose of intraparenchymal NGF induced Schwann cell hyperplasia at the level of the medulla and upper cervical spinal cord. ICV-NGF was able to completely restore spatial learning and memory as predicted while only the highest intraparenchymal dose was able to able to restore the mnemonic deficits. These data suggest that intraparenchymal infusion of growth factors may provide a viable delivery method in clinical trials using this mode of drug delivery once an optimal dose has been established.

Discovery of CEP-1347/KT-7515, an inhibitor of the JNK/SAPK pathway for the treatment of neurodegenerative diseases

Apoptosis has been proposed as a mechanism of cell death in Alzheimer's, Huntington's and Parkinson's diseases and the occurrence of apoptosis in these disorders suggests a common mechanism. Events such as oxidative stress, calcium toxicity, mitochondria defects, excitatory toxicity, and deficiency of survival factors are all postulated to play varying roles in the pathogenesis of the diseases. However, the transcription factor c-jun may play a role in the pathology and cell death processes that occur in Alzheimer's disease. Parkinson's disease (PD) is also a progressive disorder involving the specific degeneration and death of dopamine neurons in the nigrostriatal pathway. In Parkinson's disease, dopaminergic neurons in the substantia nigra are hypothesized to undergo cell death by apoptotic processes. The commonality of biochemical events and pathways leading to cell death in these diseases continues to be an area under intense investigation. The current therapy for PD and AD remains targeting replacement of lost transmitter, but the ultimate objective in neurodegenerative therapy is the functional restoration and/or cessation of progression of neuronal loss. This chapter will describe a novel approach for the treatment of neurodegenerative diseases through the development of kinase inhibitors that block the active cell death process at an early transcriptional independent step in the stress activated kinase cascade. In particular, preclinical data will be presented on the c-Jun Amino Kinase pathway inhibitor, CEP-1347/KT-7515, with respect to it's properties that make it a desirable clinical candidate for treatment of various neurodegenerative diseases.

Multiple injections of thyrotropin releasing hormone fail to reverse learning and memory deficits in rats with lesions of the nucleus basalis of meynert

The learning and memory enhancing effects of thyrotropin releasing hormone (TRH) was examined in an animal model of Alzheimer's disease. Adult rats were prepared with either sham surgeries or cholinergic lesions of the nucleus basalis of Meynert (nbM). Subjects were injected (ip) with one of three doses of TRH (0, 5, 10 mg/kg) starting on the day of surgery and continuing once every other day for a total of four injections. Performance (four trials/day for 4 days, 30 m inter-trial interval) in a Morris water maze was assessed one week after the last TRH injection (i.e., 2 weeks postoperatively). Latency to find the hidden platform served as the dependent variable. Results indicated that damage to the nbM impaired task performance in that animals with nbM lesions generally required more time to find the platform and showed less trial-to-trial improvement. Treatment with TRH failed to reverse this lesion-induced deficit. These results suggest that multiple injections of TRH do not provide residual protection against the deleterious effects on learning and memory produced by cholinergic lesions of the basal forebrain. Other doses and administration parameters, however, need to be studied in order to determine the generalizability of these findings.

Modulation of sensory inputs and ectopic presence of Schwann cells depend upon the route and duration of nerve growth factor administration

Nerve growth factor (NGF) ameliorates deficits in models of cholinergic hypofunction. However, notable adverse effects of intracerebroventricular (ICV) infusion of NGF include weight loss, Schwann cell hyperplasia (SCH), and aberrant sensory and sympathetic sprouting. In order to maintain efficacy on the cholinergic basal forebrain (CBF) and minimize these detrimental effects, intraparenchymal NGF infusion was compared with ICV administration to assess morphological and functional measures. NGF was delivered intraparenchymally (Intra-NGF) or intracerebroventricularly (ICV-NGF) for 3 and 6 months. Hypertrophy of cholinergic nucleus basalis neurons at 3 and 6 months was not different between both routes of administration, indicating similar efficacy for the CBF. SCH surrounding the medulla was observed in both Intra- and ICV-NGF animals due to the widespread distribution of NGF from the infusion site. The thickness of SCH reached a plateau at 3 months in ICV-NGF animals, while further proliferation occurred in Intra-NGF animals. More importantly, ectopic Schwann cells and aberrant sensory and sympathetic sprouting within the medulla oblongata were found solely in ICV-NGF animals. Differential changes in sensory processing were evident by an exaggerated response to acoustic stimuli in Intra-NGF animals and a decrease in thermal pain threshold in ICV-NGF-treated animals. Intra-NGF treatment did not produce the reduction in body weight exhibited by ICV-NGF-treated rats. These results indicate that different routes of NGF administration are identically efficacious for CBF neurons, but differentially modulate behaviors and structures leading to distinct profiles of adverse effects. Thus, current trophic factor delivery methods require further refinement to abolish detrimental effects.

Liposome-mediated NGF gene transfection following neuronal injury: potential therapeutic applications

We have systematically investigated the therapeutic potential of cationic liposome-mediated neurotrophic gene transfer for treatment of CNS injury. Following determination of optimal transfection conditions, we examined the effects of dimethylaminoethane-carbamoyl-cholesterol (DC-Chol) liposome-mediated NGF cDNA transfection in injured and uninjured primary septo-hippocampal cell cultures and rat brains. In in vitro studies, we detected an increase of NGF mRNA in cultures 1 day after transfection. Subsequent ELISA and PC12 cell biological assays confirmed that cultured cells secreted soluble active NGF into the media from day 2 after gene transfection. Further experiments showed that such NGF gene transfection reduced the loss of chol- ine acetyltransferase (ChAT) activity in cultures following calcium-dependent depolarization injury. In in vivo studies, following intraventricular injections of NGF cDNA complexed with DC-Chol liposomes, ELISA detected nine- to 12-fold increases of NGF in rat CSF. Further studies showed that liposome/NGF cDNA complexes could attenuate the loss of cholinergic neuronal immunostaining in the rat septum after traumatic brain injury (TBI). Since deficits in cholinergic neurotransmission are a major consequence of TBI, our studies demonstrate for the first time that DC-Chol liposome-mediated NGF gene transfection may have therapeutic potential for treatment of brain injury.

Deficit in selective and divided attention associated with cholinergic basal forebrain immunotoxic lesion produced by 192-saporin; motoric/sensory deficit associated with Purkinje cell immunotoxic lesion produced by OX7-saporin

The immunotoxin 192-saporin, infused intracerebroventricularly into rats, destroys cholinergic neurons in the basal forebrain nuclei. Doses required for complete cholinergic loss also kill some Purkinje cells. The immunotoxin OX7-saporin, when infused intraventricularly into rats, destroys Purkinje cells in a pattern similar to that produced by 192-saporin, without affecting cholinergic neurons. Thus, we used OX7-saporin to distinguish behavioral effects of 192-saporin due to cerebellar damage versus those due to cholinergic cell loss. Three doses of 192-saporin (1.6, 2.6, and 3.3 micrograms/rat) were chosen along with a dose of OX7-saporin (2.0 micrograms/rat) that produced Purkinje loss equivalent to the two highest doses of 192-saporin. Groups of Fischer-344 rats were trained in the multiple choice reaction time task and retested with more complex tasks after lesioning. They were also tested in the water maze, passive avoidance, acoustic startle, and open field. The OX7-saporin group exhibited changes in many tests suggesting hypermotility and sensory deficits. The 192-saporin groups differed from the OX7-saporin group when they displayed deficits in multiple choice reaction time tasks in which novel challenges were introduced, including sessions with a noise distractor, shortened and lengthened intertrial intervals, and use of nine instead of five sources of light stimulus. The 192-saporin groups showed no impairment in the other tasks. The cholinergic basal forebrain lesion may mask some of the effects of cerebellar damage up to a threshold after which effects of Purkinje cell loss predominate when 192-saporin is administered intraventricularly.

Preservation of cholinergic activity and prevention of neuron death by CEP-1347/KT-7515 following excitotoxic injury of the nucleus basalis magnocellularis

We have identified a class of small organic molecules, derived from the indolocarbazole K-252a, that promote the survival of cultured neurons. However, many of these indolocarbazoles inhibit protein kinase C and neurotrophin-activated tyrosine kinase receptors. These kinase inhibitory activities may limit the utility of these compounds for neurological disorders. A bis-ethyl-thiomethyl analogue of K-252a, CEP-1347/KT-7515, has been identified that lacks protein kinase C and tyrosine kinase receptor inhibitory activities, yet retains the ability to promote survival of cultured neurons, including cholinergic neurons derived from the basal forebrain. In the present studies, CEP-1347/KT-7515 was assessed for neurotrophic activity on basal forebrain neurons of in vivo rats following excitotoxic insult. Ibotenate infusion into the nucleus basalis magnocellularis reduced levels of choline acetyltransferase activity in the cortex, as well as reduced numbers of choline acetyltransferase-immunoreactive and retrogradely (FluoroGold)-labelled cortically-projecting neurons in the nucleus basalis. Systemically administered CEP-1347/KT-7515 attenuated the loss of cortical choline acetyltransferase activity and the loss of the number of choline acetyltransferase-immunoreactive and retrogradely-labelled FluoroGold neurons in the nucleus basalis. Moreover, CEP-1347/KT-7515 ameliorated the loss of cortical choline acetyltransferase if administration was initiated one day, but not seven days post-lesion. Together, these results demonstrate that CEP-1347/KT-7515 protects damaged cortically-projecting basal forebrain neurons from degeneration. Thus, CEP-1347/KT-7515 may have therapeutic potential in neurodegenerative diseases, such as Alzheimer's disease, in which basal forebrain cholinergic neurons degenerate.

Short-term and complete reversal of NGF effects in rats with lesions of the nucleus basalis magnocellularis

Rats received bilateral quisqualic acid lesions of the nucleus basalis magnocellularis. Three weeks after lesioning, osmotic minipumps were implanted that released recombinant human nerve growth factor or cytochrome c at a dosage of 5.0 microg rat-1 day-1 through intracerebroventricular cannulas for 7 weeks. One quarter of the rats were sacrificed at the end of the treatment, while the rest of the animals were sacrificed 2, 8, and 12 weeks after termination of NGF/cc treatment. ICV administration of nerve growth factor (NGF) transiently reduced weight gain. NGF maximally increased choline acetyltransferase activity in all cortical regions, the olfactory bulb and the hippocampus between 20% and 56% at the end of the treatment. This increase linearly declined and completely regressed during the 12-week withdrawal period both in regions affected and unaffected by the lesion. Administration of NGF induced a short-lasting hypertrophy of low affinity NGF receptor immunoreactive neurons within the nucleus basalis magnocellularis (NBM), the horizontal limb of the diagonal band of Broca, and the medial septum. In contrast, QUIS-induced NBM lesions permanently reduced ChAT activity most pronounced in the frontal and parietal cortex up to 45%. Furthermore, QUIS induced a permanent loss of p75NGFr-immunoreactive neurons within the NBM and the DB without affecting the MS. These findings suggest that degenerating cholinergic neurons of the NBM and HDB do not spontaneously recover after lesioning and may require continuous neurotrophic support by NGF to ameliorate cholinergic hypofunctioning.

Effects of cholinergic deafferentation and NGF on brain electrical coherence

Rats received unilateral lesions of the nucleus basalis and were infused intracerebroventricularly (i.c.v.) over 3 weeks with nerve growth factor (NGF) or vehicle. Electrocortical coherence was assessed at postoperative days 4, 7, 14, and 21 from all possible pairs of eight epidural electrodes in the delta (1-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta-1 (12-20 Hz), and beta-2 (20-28 Hz) bands. On day 21 choline acetyltransferase (ChAT) activity was measured in cortical tissue underlying each electrode site. Lesions resulted in losses of interhemispheric, as well as bilateral intrahemispheric coherence in the theta band (F1,21 = 28.61, p < 0.0001, F1,21 = 4.30, p < 0.05), with no significant differences seen in other bands. Changes were accentuated during immobility compared with walking and exploratory behavior. Intrahemispheric changes were greatest within the lesioned hemisphere (F1,21 = 6.97, p < 0.01) in long connections between electrode pairings connecting frontal to posterior brain regions. Nerve growth factor (NGF) attenuated losses in ChAT (F1,21 = 21.31, p < 0.0001) and intrahemispheric coherence (F1,21 = 9.66, p < 0.005), whereas interhemispheric coherence showed no significant response. Intact animals receiving NGF showed increases in intrahemispheric coherence, as well as modest increases in ChAT. Increases in coherence in response to NGF occurred within 4-7 days following brain lesions, with no significant change during the 2 weeks thereafter. Our results suggest that coherence is sensitive to cholinergic deafferentation, particularly of long corticocortical connections. NGF differentially restores coherence within hemispheres, as opposed to between hemispheres. Our study suggests that brain function in Alzheimer's disease related to damage of transcallosal fiber tracts may not be responsive to cholinergic treatments. Future studies may wish to evaluate the cognitive relevance of NGF's effects on intact brain.

Neurotrophin receptor immunostaining in the rat ventral cochlear nucleus

By virtue of its known segregated distribution of cell types, their known neurotransmitters and neurophysiologic properties, the cochlear nucleus is an excellent model and provides the opportunity to study the relation between neurotrophins and their receptors along with the functional properties of the adult cochlear nucleus. To investigate the potential role of neurotrophins in the mature cochlear nucleus, we determined the expression of the three major neurotrophin tyrosine kinase receptors (Trk) in the adult rat ventral cochlear nucleus, as revealed by antibodies against the full-Trk proteins. A qualitative and a cartographic analysis showed a widespread distribution of the three Trk receptors throughout the nucleus. Immunostaining was mainly restricted to neurons as shown by the lack of double immunostaining with specific markers for glial cells. However, we observed variability in immunostaining for given receptors. Three classes of cells were distinguished by their specificity for Trk receptors. The first one was a cell population that stained for TrkA or TrkB. This population characterizes the majority of small and small round neurons and fusiform cells. The second group consists of TrkC-immunolabeled cells and comprises the majority of spherical, globular, granule and small multipolar cells. The third group consists of cells that seem to be immunopositive for all three Trk receptors. This group includes in part multipolar, giant and octopus cells. A possible correlation between Trk expression and cell functional properties is suggested: TrkA- and TrkB-immunoreactive cells could include inhibitory neurons while cells stained for TrkC could include excitatory neurons. The abundant and widespread neuronal distribution of signal-transducing forms of TrkA, TrkB and TrkC predicts that their cognate ligands may exert significant effects on a large proportion of neurons within the mature ventral cochlear nucleus.

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

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

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

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

Changes in cortical EEG and cholinergic function in response to NGF in rats with nucleus basalis lesions

We examined whether recovery of cholinergic function in response to nerve growth factor (NGF) results in restoration of electrocortical activity. Rats received unilateral lesions of the nucleus basalis and were infused intracerebroventricularly (i.c.v.) over 3 weeks with NGF or vehicle. Cortical electrical activity was assessed at postoperative days 4, 7, 14, and 21 from 8 epidural electrodes. On day 21, choline acetyl transferase (ChAT) activity was measured in cortical tissue underlying each electrode site. Lesions resulted in increases in slow-wave (delta) power and decreases in high-frequency (beta 2) power in the lesioned, as well as the non-lesioned hemisphere. Changes correlated topographically and in magnitude with losses of ChAT activity and suggested that regional electrocortical function was affected by cholinergic activity originating in the ipsilateral, as well as the contralateral hemisphere. NGF attenuated changes in cholinergic and electrocortical function bilaterally, though in the lesioned hemisphere, function did not return to control levels. Likewise, intact animals receiving NGF showed increases in beta 2-power, as well as modest increases in ChAT activity. Changes in brain electrical activity in response to NGF occurred within 4-7 days without significant changes during the 2 weeks thereafter. Our results suggest that outcomes of future animal and human trials-using unilateral i.c.v. infusions of NGF need to consider the reciprocal influences of hemispheric cholinergic function, as well as possible effects of NGF on intact brain.

Gene therapy for central nervous system injury: the use of cationic liposomes: an invited review

This paper briefly reviews general principles of gene therapy with emphasis on the therapeutic potential of cationic liposome-mediated neurotrophin gene transfer to treat central nervous system (CNS) injury. Current developments in studies of gene therapy for CNS injury are both impressive and promising. Ex vivo gene transfer into the CNS is relatively mature in animal studies following more than a decade of experimental studies. In vivo gene transfer into the CNS has gained more attention recently. Although progress has been made using viral vectors, rapid advances in transfection technologies employing cationic liposomes, together with the relatively low toxicity of these nonviral vector systems, suggest that liposomes may have significant potential for clinical applications. Although many investigators have recognized that gene therapy may be useful for treatment of certain genetic defect diseases or cancer, gene therapy for CNS injury is relatively novel. In contrast to genetic defect disorders, temporary induction of transgenes may have therapeutic applications for CNS injuries such as stroke and trauma. Employing gene transfer techniques to achieve therapeutically useful levels of expression of neurotrophins in the CNS could provide a new strategy for treatment of the traumatically injured CNS.

Nerve growth factor induces Fos-like immunoreactivity within identified cholinergic neurons in the adult rat basal forebrain

Immunocytochemical techniques were used to examine and compare the effects of intracerebroventricular administration of nerve growth factor (NGF) on Fos expression within identified cholinergic and non-cholinergic neurons located in different regions of the adult rat basal forebrain. Animals were killed 1, 3, 6, and 12 h after receiving NGF (0.5 or 5.0 microg) or vehicle into the left lateral ventricle and sections through the medial septum, diagonal band of Broca, nucleus basalis magnocellularis, and striatum were processed for the combined immunocytochemical detection of Fos and choline acetyltransferase (a marker for cholinergic neurons), or Fos and parvalbumin (a marker for gamma aminobutyric acid (GABA)-containing neurons). NGF produced a significant increase in the percentage of cholinergic neurons containing Fos-like immunoreactivity within all four regions examined. The largest increases were detected in the medial septum (47.8%) and the horizontal limb of the diagonal band of Broca (67.7%). In these areas, NGF-mediated induction of Fos-like immunoreactivity was detected as early as 3 h, peaked at 6 h, and was reduced by 12 h, postinfusion. Small but significant increases in the percentage of cholinergic neurons containing Fos-like immunoreactivity were also detected in the striatum (4.2%) and in the nucleus basalis magnocellularis (19.2%) 3-12 h following administration of the higher dose of NGF. No evidence for an NGF-mediated induction of Fos within parvalbumin-containing neurons was detected in any of the four regions at any of the time-points examined; however, evidence for an NGF-mediated induction of Fos within epithelial cells lining the lateral ventricle was observed. These data demonstrate that NGF induces Fos expression within cholinergic, and not parvalbumin-containing (GABAergic), neurons in the basal forebrain, and furthermore that intracerebroventricular administration of NGF influences the different subgroups of basal forebrain cholinergic neurons to different degrees.

Reversible Schwann cell hyperplasia and sprouting of sensory and sympathetic neurites after intraventricular administration of nerve growth factor

Substantial dysfunction and loss of cholinergic neurons occur in Alzheimer's disease (AD). Nerve growth factor (NGF) is a potent neurotrophic factor for cholinergic basal forebrain neurons, and the use of NGF to stimulate residual dysfunctional cells in AD is being considered. To define the effects of NGF on other cell populations in the brain, NGF was continuously infused into the lateral ventricle of rats for 7 weeks. At the end of treatment, Schwann cell hyperplasia and abundant sensory and sympathetic neurite sprouting were observed in the subpial region of the medulla oblongata and the spinal cord. Following withdrawal of NGF, the Schwann cell hyperplasia and sprouting of sensory and sympathetic neurites disappeared completely. These findings suggest that better temporal and spatial delivery systems for NGF must be explored to limit potential undesirable side effects while maintaining the survival and function of diseased basal forebrain cholinergic neurons.

Cationic lipid-mediated NGF gene transfection increases neurofilament phosphorylation

We examined the effect of cationic lipid-mediated gene transfection of nerve growth factor (NGF) in primary septo-hippocampal cell cultures. Rat NGF cDNA was subcloned into a pUC19-based plasmid containing a CMV promoter. Two days after NGF gene transfection in primary cell cultures, ELISA confirmed increases in NGF protein secretion from transfected cells. To study the biological effect of cationic lipid-mediated NGF gene transfection, we analyzed the amount of neurofilament protein from NGF-transfected cell cultures. Western blot and immunohistochemical analyses detected significant increases in the phosphorylated form of neurofilament proteins in the cultures after cationic lipid-mediated NGF cDNA transfection. Cationic lipid-mediated NGF cDNA transfection did rot cause significant changes in the total amount of neurofilament protein. Our studies suggest that cationic lipid-mediated NGF gene transfection can increase neurofilament phosphorylation but not total neurofilament protein.

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.

Nerve growth factor facilitates conditioned taste aversion learning in normal rats

Chronic intracerebroventricular (i.c.v.) infusion of 3.2 micrograms/day of nerve growth factor (NGF) in normal rats elevated choline acetyltransferase (ChAT) activity of the striatum, medial septum, and basal forebrain and improved performance of a conditioned taste aversion (CTA) task. Relative to bovine serum albumin (BSA) or Cytochrome C treatments, NGF treatment facilitated acquisition and prolonged extinction of a lithium chloride (LiCl)-induced saccharin aversion. This facilitation was evident at saccharin/LiCl intervals ranging up to 1 h. Also, NGF treatment did not increase reactivity to LiCl-induced illness and neither shifted detection thresholds nor altered hedonic reactions to taste stimuli, indicating that NGF did not produce simple changes in sensory function. NGF treatments that elevate ChAT also facilitate memory of CTA in normal, adult rats.

Reversal of age-dependent cognitive impairments and cholinergic neuron atrophy by NGF-secreting neural progenitors grafted to the basal forebrain

A highly NGF-secreting cell line was generated by retroviral transduction of a conditionally immortalized CNS-derived neural progenitor cell line. After transplantation to the nucleus basalis magnocellularis (NBM), the cells continue to express the NGF transgene for at least 10 weeks, producing sufficient NGF to reverse cholinergic neuron atrophy in aged rats and induce cellular hypertrophy in young rats. In cognitively impaired aged rats, transplants of the NGF-secreting cells placed either in the NBM and septum or in only the NBM induced a near-complete reversal of the spatial learning impairment. This was accompanied by a normalization of the size of the cholinergic neurons in the grafted areas. The results demonstrate that locally increased supply of NGF to the basal forebrain cholinergic nuclei has a significant impact on cognitive function and support the usefulness of neural progenitor cells for a long-term localized delivery of neurotrophins to the CNS.

Cholinergic differentiation factor/leukemia inhibitory factor enhances functional effects of adrenal medulla grafts after hippocampal lesions in rats

We recently found that adrenal medulla grafts implanted into the hippocampus of rats survived for several months and significantly decreased the deficits produced by hippocampal lesions in the radial maze test [Jousselin-Hossaja et al. (1994) Neuroscience 59, 275-284]. These grafts contained choline acetyltransferase immunopositive chromaffin cells and received cholinergic innervation. In the experiments reported here, adrenal medulla grafts implanted in lesioned hippocampus were treated with cholinergic differentiation factor/leukemia inhibitory factor. In the presence of this factor, the number of chromaffin cells with cholinergic phenotypes increased as well as the beneficial effects of the grafts on the performances of rats in the radial maze. These results suggest that the functional effects of adrenal medulla grafted into the hippocampus set into play cholinergic mechanisms. The cholinergic differentiation factor/leukemia inhibitory factor may also have facilitated the survival and recovery of cholinergic neurons in the host tissue. However, due to the large range of action of this cytokine and the richness of the adrenal medulla contents, non cholinergic factors are also probably involved. Our results may help to elucidate the functions of the cholinergic differentiation factor/leukemia inhibitory factor since they provide the first indication that its intracerebral injection may have behavioral effects. Moreover, our data confirm the possibility of improving the efficiency of adrenal medulla implants in the central nervous system by appropriate treatments, not only by facilitating survival but also by selectively amplifying some potential factors of the graft. This might greatly enlarge the field of this grafting technique for analysing the normal functioning of the brain and for repairing it.

Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging

Estrogen replacement can significantly affect the expression of ChAT and NGF receptors in specific basal forebrain cholinergic neurons. The time-course of the effects is consistent with a direct up-regulation of ChAT followed by either direct or indirect down-regulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests ChAT, p75NGFR, trkA, and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with aging and with neurodegenerative disease. It is possible that estrogen may affect cognitive function via effects on NGF-related systems and basal forebrain cholinergic neurons. Effects of estrogen on cognitive function have been reported, as has some preliminary evidence for beneficial effects of estrogen in decreasing the prevalence of and reducing some cognitive deficits associated with Alzheimer's disease. Whether these effects are related to effects on NGF-related systems or basal forebrain cholinergic neurons is currently unknown. Indirect evidence suggests that estrogen interacts with NGF-related systems and that changes in circulating levels of estrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of estrogen replacement therapy in pre- and post-menopausal women. Further studies examining effects of different regimens of estrogen replacement as well as estrogen combined with progesterone on NGF and basal forebrain cholinergic neurons in young and aged animals are required. Prospective studies correlating aging and estrogen replacement with numbers of basal forebrain cholinergic neurons and hippocampal and cortical levels of NGF also need to be performed to better assess the potential benefits of estrogen replacement in reducing age- and disease-related cognitive decline.

Effect of nerve growth factor and GM1 ganglioside on the recovery of cholinergic neurons after a lesion of the nucleus basalis in aging rats

A unilateral ibotenic acid lesion was placed in the nucleus basalis magnocellularis of 3- and 18-month-old rats. In the lesioned aging rats, the number of choline acetyltransferase-immunoreactive neurons of the nucleus basalis magnocellularis was markedly reduced in the ipsilateral side and to a lesser extent in the contralateral side. Twenty-one days after the lesion, the activity of choline acetyltransferase in the ipsilateral cortex was reduced by 40% in both groups of rats and by 24% in the contralateral frontal cortex of the aging rats. Intracerebroventricular administration of nerve growth factor (10 micrograms twice a week) to aging lesioned rats for 3 weeks after surgery resulted in a complete recovery in the number of choline acetyltransferase-immunoreactive neurons in the nucleus basalis of both sides, and choline acetyltransferase activity in the contralateral cortex, with little effect on the ipsilateral cortex. No potentiation was seen after the concurrent administration of GM1 ganglioside and nerve growth factor. Complete recovery in cortical choline acetyltransferase activity was only observed in the lesioned rats treated with nerve growth factor for 1 week before and 3 weeks after lesioning. Nerve growth factor treatment, both after the lesion, and before and after the lesion, improved the passive avoidance performance disrupted by the lesion. In young lesioned rats daily intraperitoneal administration of GM1 (30 mg/kg) for 21 days after surgery promoted both the recovery of choline acetyltransferase activity and passive avoidance performance. In aging rats GM1, even at a dose twice as large, failed to reverse the biochemical and morphological deficits and behavioral impairment induced by the lesion. Only when GM1 administration was started 3 days before the lesion, were a complete recovery in choline acetyltransferase activity in the contralateral cortex and a partial recovery in the ipsilateral cortex obtained. Our results indicate that nerve growth factor and, to some extent, GM1 facilitate the recovery of the cholinergic neurons after a lesion of the nucleus basalis in aging rats, but their efficacy is reduced. The lower efficacy of GM1 as compared to NGF might be due to the different routes of administration used.

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.

Grafting of nerve growth factor-producing fibroblasts reduces behavioral deficits in rats with lesions of the nucleus basalis magnocellularis

Rats received bilateral lesions of the nucleus basalis magnocellularis by infusion of biotenic acid. Two weeks after the lesion, a suspension of genetically modified primary rat fibroblasts was grafted dorsal to the nucleus basalis magnocellularis (2 x 10(5) cells per side). The fibroblasts were either infected with the gene for human beta-nerve growth factor or Escherichia coli beta-galactosidase. The nerve growth factor-producing fibroblasts released 67 ng nerve growth factor/10(5) cells per day in vitro. Two weeks after implantation of the fibroblasts, spatial learning was tested in the Morris water-maze. Nerve growth factor-producing fibroblasts, but not beta-galactosidase-producing fibroblasts ameliorated the deficit in acquisition of the water-maze task. In addition, spatial acuity was improved to near-normal levels by the nerve growth factor-producing grafts. Choline acetyltransferase activity in cortical areas and hippocampus was not affected by the nerve growth factor-producing grafts. Both grafted groups showed a similar reduction in the level of dopamine, but not homovanillic acid or 3-methoxytyramine, in the frontal cortex. Levels of norepinephrine, epinephrine and serotonin and their metabolites in the neocortex and hippocampus were not affected by the lesion or the grafts. Nerve growth factor-producing grafts increased the size of remaining nerve growth factor-receptor (p75) immunoreactive neurons in the nucleus basalis magnocellularis by 25%. Nucleus basalis magnocellularis lesions reduced the integrated optic density of choline acetyltransferase-positive fiber staining in the ventral neocortex by 46%, but nerve growth factor-producing grafts restored this area to 86% of control. These data suggest that nerve growth factor-producing grafts can cause a marked behavioral improvement, probably through the partial restoration of the lesioned projection from nucleus basalis magnocellularis to neocortex.

In situ hybridization detection of trkA mRNA in brain: distribution, colocalization with p75NGFR and up-regulation by nerve growth factor

In situ hybridization techniques were used to examine the distribution and the nerve growth factor (NGF) regulation of trkA mRNA in the adult rat brain in order to identify neurons in discrete regions of the brain that may be NGF responsive. In agreement with previous studies, trkA mRNA was detected within cells located in the medial septum (MS), diagonal band of Broca (DBB), and caudate. trkA mRNA was also detected in many other regions of the brain, including the nucleus basalis of Meynert, substantia innominata, paraventricular nucleus of the thalamus, interpeduncular nucleus, prepositus hypoglossal nucleus, vestibular nuclei, raphe obscuris, cochlear nucleus, sensory trigeminal nuclei, and gigantocellular as well as perigigantocellular neurons in the medullary reticular formation. By combining in situ hybridization detection of trkA mRNA with immunocytochemical detection of p75NGFR, it was determined that the vast majority (> 90%) of the trkA mRNA-containing cells detected in the MS and DBB also express p75NGFR. Likewise, the vast majority of p75NGFR-IR cells detected in the MS and DBB expressed trkA mRNA. Intracerebroventricular infusions of NGF into the third ventricle adjacent to the preoptic area resulted in a 58% increase in relative cellular levels of trkA mRNA in the horizontal limb of the DBB. These data provide evidence that both p75NGFR and trkA are expressed by NGF-responsive neurons in the MS and DBB. In addition, we note that areas that contained trkA mRNA and that also have been reported to contain p75NGFR are areas where high-affinity NGF binding sites have been observed autoradiographically, whereas areas that contain either trkA or p75NGFR alone are areas where no high-affinity NGF binding has been reported. Together, these findings suggest that both trkA and p75NGFR play an important role in the formation of high-affinity NGF receptors in brain and, furthermore, suggest that NGF may have physiological effects within many regions of the brain outside of the basal forebrain.

Time course of cholinergic and monoaminergic changes in rat brain after immunolesioning with 192 IgG-saporin

192 IgG-saporin, an immunotoxin targeted at the low affinity NGF receptor, was infused into the lateral ventricle of rat brain. Three days and one week post lesion, choline acetyltransferase activity was markedly decreased in cortex, hippocampus, olfactory bulbs, and septum (brain regions innervated by the cholinergic neurons of the basal forebrain) with no change in cerebellum, striatum or pons. Measurement of monoamine levels revealed increases in HVA, DOPAC and dopamine, primarily in the olfactory bulbs at the 28-day time point only, suggesting a compensation for cholinergic inactivity. High levels of basal forebrain cholinergic lesioning can be obtained with this immunotoxin with minimal or no effects on monoaminergic or other cholinergic systems.

Effects of brain-derived neurotrophic factor and nerve growth factor on remaining neurons in the lesioned nucleus basalis magnocellularis

Rats received a unilateral lesion of the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid. Starting 2 weeks after the lesion, the animals were treated with nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) by intraparenchymal infusion of 3 micrograms per day for 4 weeks. Lesioned control animals received a similar amount of cytochrome c. The activity of choline acetyltransferase (ChAT) in the frontal neocortex was significantly reduced by the lesion (-39%). However, the intraparenchymal treatment with NGF or BDNF did not affect cortical ChAT activity. The number of p75 NGF receptor-immunoreactive neurons in the NBM was significantly decreased (-49%) by the lesion and was not affected by NGF or BDNF. The size of the remaining neurons was significantly increased by NGF (+32%), but not by BDNF (+12%). Similarly, in situ hybridization showed enhanced expression of the p75 NGF receptor following treatment with NGF, but not with BDNF. These results suggest that although BDNF occurs in the target area of cholinergic NBM neurons, its effects on these neurons are less pronounced than those of NGF.

Memory impairment and neural dysfunction after continuous infusion of anti-nerve growth factor antibody into the septum in adult rats

Nerve growth factor is required for the survival and maintenance of cholinergic neurons in the central nervous system. The direct infusion into the rat's septum of an anti-nerve growth factor monoclonal antibody, which inhibits nerve growth factor bioactivity seven times more strongly than a polyclonal antibody, caused very severe damage to the hippocampal cholinergic system. Anti-nerve growth factor polyclonal antibody also neutralized endogenously occurring nerve growth factor. The infusion of anti-nerve growth factor polyclonal antibody produced a dysfunction of memory and decreased choline acetyltransferase activity and acetylcholinesterase staining in the hippocampus. The cholinergic dysfunction and impairment of memory recovered to the normal level two weeks after cessation of the infusion of the anti-nerve growth factor polyclonal antibody. These results suggest that a deficit of nerve growth factor in the adult brain causes neuronal dysfunction.

Functional consequences of a single nerve growth factor administration following septal damage in rats

This study examined how possible nerve growth factor (NGF)-induced behaviour changes after septal damage might be modulated by the lesion extent, the dose of NGF administered and the delay between surgery and the onset of testing. In a first experiment, young rats which received electrolytic septal lesions of high or low intensity (inducing respectively large and mild lesions) were treated with 10 or 30 micrograms NGF administered intrahippocampally in a single injection. They were tested 4 months postoperatively for open field ambulation, spontaneous alternation and radial maze performance. It was observed that irrespective of the severity of the lesions rats were impaired in the spontaneous alternation and radial maze tests; however, no obvious changes appeared in the open field test. While an NGF injection did not affect behavioural performances in rats with large lesions, it was capable of ameliorating behavioural deficits in the spontaneous alternation and radial maze tests of rats with mild lesions in both NGF dosage groups. It was also seen that lesions produced a general decrease in hippocampal choline acetyltransferase (ChAT) activity, which was not significantly affected by an NGF administration. There was no significant correlation between ChAT activity and behavioural performance of NGF-treated rats. In a second experiment, young rats received mild septal lesions and were treated with 10 micrograms NGF. These rats were tested 2 weeks postoperatively for radial maze performance. NGF rats exhibited similar behaviour to controls with regard to all of the variables measured. The present results suggest that a single NGF administration spares some abilities to use spatial information efficiently providing lesions are partial.

Cholinergic marker deficits induced by lesions of the nucleus basalis of Meynert are attenuated by nerve growth factor in young, but not in aged, F344 rats

To investigate the efficacy of nerve growth factor (NGF) in promoting recovery from cholinergic damage, young (3-4 month old) and aged (22-23 month old) Fischer 344 rats received NMDA-induced unilateral lesions of the nucleus basalis of Meynert and subcutaneous osmotic pumps (2-week duration) connected to permanently implanted cannulas directed at the lateral ventricle ipsilateral to the lesion. Pumps were filled with either artificial CSF/rat serum albumin (the vehicle) or 5.0 micrograms of angiotensin-free, beta-NGF. Fourteen days after surgery, all subjects were sacrificed and their brains regionally dissected (frontal and occipital cortices, striatum, and dorsal and ventral hippocampi) and assayed for choline acetyltransferase (CAT) and acetylcholinesterase (AChE). Results indicated that the lesion decreased CAT and AChE levels within the frontal cortex of both young (29.8% and 39.4% depletion, respectively) and aged (30.5% and 34.8% depletion, respectively) animals. Only in young animals did NGF reduce these lesion-induced CAT (by 34.2%) and AChE deficits (by 65.5%). In fact, NGF exacerbated frontal cortical CAT depletions in aged animals in that percent depletion was 11.3% more following treatment (30.5% vs. 41.8% depletion in Aged/CSF and Aged/NGF groups, respectively). Lower CAT and AChE levels were found in the striatum of aged animals, an effect not reversed by NGF treatment. In contrast, NGF in young animals enhanced striatal CAT activity on the non-lesioned side by 22.2%.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent effects of cholinergic and serotonergic lesions on acetylcholine and serotonin release in the neocortex of the rat

Rats received a unilateral lesion of the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid. In addition, the dorsal raphe nucleus was lesioned by infusion of 5,7-dihydroxytryptamine (5,7-DHT). The release of acetylcholine (ACh), choline, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was measured in the frontal neocortex by means of microdialysis. Lesions of the NBM, but not the raphe nucleus, reduced the release of ACh significantly (-47%). The release of 5-HT and 5-HIAA was reduced by raphe lesions (-44% and -79%+), but not by NBM lesions. In no case did the combined lesion affect neurotransmitter release more than a single lesion. These results suggest that serotonergic projections from the dorsal raphe nucleus are not involved in tonic inhibition of ACh release in the neocortex.

Nerve growth factor increases cortical choline acetyltransferase-positive fiber staining without affecting cortical cholinergic neurons

Lesions of the nucleus basalis magnocellularis (NBM) increased the number of neurons in the frontal neocortex staining for choline acetyltransferase (ChAT). Intracerebroventricular treatment with nerve growth factor (NGF; 10 micrograms per day for 6 weeks) did not further increase this number. NGF increased the size of NBM neurons [Brain Res., 584 (1992) 55-63], but not those in the neocortex. However, NGF increased the area of ChAT-positive fiber staining in the neocortex. These data suggest that NGF enhances cholinergic innervation to the neocortex by affecting residual NBM neurons, rather than cortical cholinergic neurons.