Neurotropism of nerve growth factor for adult rat septal cholinergic axons in vivo

Reorganization of the septohippocampal cholinergic fiber system in experimental epilepsy

The septohippocampal cholinergic neurotransmission has long been implicated in seizures, but little is known about the structural features of this projection system in epileptic brain. We evaluated the effects of experimental epilepsy on the areal density of cholinergic terminals (fiber varicosities) in the dentate gyrus. For this purpose, we used two distinct post-status epilepticus rat models, in which epilepsy was induced with injections of either kainic acid or pilocarpine. To visualize the cholinergic fibers, we used brain sections immunostained for the vesicular acetylcholine transporter. It was found that the density of cholinergic fiber varicosities was higher in epileptic rats versus control rats in the inner and outer zones of the dentate molecular layer, but it was reduced in the dentate hilus. We further evaluated the effects of kainate treatment on the total number, density, and soma volume of septal cholinergic cells, which were visualized in brain sections stained for either vesicular acetylcholine transporter or choline acetyltransferase (ChAT). Both the number of septal cells with cholinergic phenotype and their density were increased in epileptic rats when compared to control rats. The septal cells stained for vesicular acetylcholine transporter, but not for ChAT, have enlarged perikarya in epileptic rats. These results revealed previously unknown details of structural reorganization of the septohippocampal cholinergic system in experimental epilepsy, involving fiber sprouting into the dentate molecular layer and a parallel fiber retraction from the dentate hilus. We hypothesize that epilepsy-related neuroplasticity of septohippocampal cholinergic neurons is capable of increasing neuronal excitability of the dentate gyrus.

Treatment of focal cerebral ischemia with liposomal nerve growth factor

Liposomal nerve growth factor (NGF) was used for the treatment of focal cerebral ischemia in a rat model. Positive charge inducing agents of sphingosine (SP) and stearylamine (S) were formulated in the liposomal NGF. Dose-response of intraventricular injection of liposomal NGF showed significant reduction in infarct volume at the dose of 5 and 10 microg/rat of NGF. The liposomal NGF formulated with SP or S demonstrated similar results in the reduction of total infarct volume in rats. When we increased the molar ratio of SP and S from 0.15 to 0.3, the infarct volume from rats showed a similar value as that of the control treated with NGF solution. Liposomal NGF was given prior to the development of ischemia. We found that NGF was effective in prevention of neuronal death. The NGF concentrations in brain for liposomal NGF were maintained in a level significantly higher than those for NGF solution. This was attributed to the positively charged liposomal NGF bound effectively in brain ventricle and caused longer retention time than free NGF for localization in brain. Therefore, the effect of liposomal NGF on reduction of infarct volume was significant. We assumed that the transportation of NGF might go through the cerebrospinal fluid pathway throughout the ventricular system and subarachnoid system to cerebral cortex to produce a therapeutic effect on ischemia.

Neurogenesis in diseases of the central nervous system

Neurogenesis is altered in ageing, and diseases of the central nervous system (CNS) such as neurodegenerative disorders. We discuss the process of neurogenesis, its relevance for disorders of the CNS, the dynamic nature of neurogenesis, how and why it may be abnormal in ageing, and disease, and possibilities to ameliorate abnormal neurogenesis in disease.

Protective effects of bone marrow stromal cell transplantation in injured rodent brain: synthesis of neurotrophic factors

Several groups have suggested that transplantation of marrow stromal cells (MSCs) promotes functional recovery in animal models of brain trauma. Recent studies indicate that tissue replacement by this method may not be the main source of therapeutic benefit, as transplanted MSCs have only limited ability to replace injured central nervous system (CNS) tissue. To gain insight into the mechanisms responsible for such effects, we systematically investigated the therapeutic potential of MSCs for treatment of brain injury. Using in vitro studies, we detected the synthesis of various growth factors, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and neurotrophin-3 (NT-3). Enzyme-linked immunosorbent assay (ELISA) demonstrated that MSCs cultured in Dulbecco's modified Eagle medium (DMEM) produced substantial amounts of NGF for at least 7 weeks, whereas the levels of BDNF, GDNF and NT-3 remained unchanged. In studies in mice, after intraventricular injection of MSCs, NGF levels were increased significantly in cerebrospinal fluid by ELISA, confirming our cell culture results. Further studies showed that treatment of traumatic brain injury with MSCs could attenuate the loss of cholinergic neuronal immunostaining in the medial septum of mice. These studies demonstrate for the first time that by increasing the brain concentration of NGF, intraventricularly transplanted MSCs might play an important role in the treatment of traumatic brain injury.

The effect of nerve growth factor on functional recovery after injury to the chorda tympani and lingual nerves

Nerve growth factor (NGF) is known to ameliorate central changes and enhance the regeneration of damaged axons in the early stages after peripheral nerve injury. We have assessed the long-term outcome of placing NGF at a nerve repair site by determining the functional characteristics of several groups of sensory afferent and autonomic efferent fibres in the cat lingual nerve. Six months after entubulation repair, with or without the incorporation of NGF, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in spontaneous discharge and persistent reductions in conduction velocity, receptor sensitivity, proportion of gustatory units, and rate of salivary secretion. Comparisons between the outcome of repair with or without NGF revealed few differences. In the NGF group the conduction velocity of afferents in the lingual nerve was lower, and the level of spontaneous activity was higher. However, NGF appeared to preferentially enhance the regeneration of thermosensitive afferents, suggesting that it may play a role in determining the phenotypic profile of the regenerating axonal population. This suggests that future therapeutic enhancement of regeneration after peripheral nerve injury may require a combination of factors to encourage regeneration of specific fibre groups.

Environmental influences on brain neurotrophins in rats

Environmental factors can have profound influences on the brain. Enriching environments with physical, social and sensory stimuli are now established to be beneficial to brain development and ageing. A multitude of responses from cellular and molecular mechanisms to macroscopic changes in neural morphology and neurogenesis have been considered in the context for evidences that environmental inputs can regulate brain plasticity in the rat at all stages of life. Data from our laboratory have revealed that enriched environment increased nerve growth factor (NGF) gene expression and protein levels in the hippocampus, and this may contribute to events underlying environmentally induced neural plasticity. Because neurotrophic factors are essential for neural development and survival, they are likely to be involved in the cerebral consequences modified by enriched experiences.

Nontropic actions of neurotrophins: subcortical nerve growth factor gene delivery reverses age-related degeneration of primate cortical cholinergic innervation

Normal aging is associated with a significant reduction in cognitive function across primate species. However, the structural and molecular basis for this age-related decline in neural function has yet to be defined clearly. Extensive cell loss does not occur as a consequence of normal aging in human and nonhuman primate species. More recent studies have demonstrated significant reductions in functional neuronal markers in subcortical brain regions in primates as a consequence of aging, including dopaminergic and cholinergic systems, although corresponding losses in cortical innervation from these neurons have not been investigated. In the present study, we report that aging is associated with a significant 25% reduction in cortical innervation by cholinergic systems in rhesus monkeys (P < 0.001). Further, these age-related reductions are ameliorated by cellular delivery of human nerve growth factor to cholinergic somata in the basal forebrain, restoring levels of cholinergic innervation in the cortex to those of young monkeys (P = 0.89). Thus, (i) aging is associated with a significant reduction in cortical cholinergic innervation; (ii) this reduction is reversible by growth-factor delivery; and (iii) growth factors can remodel axonal terminal fields at a distance, representing a nontropic action of growth factors in modulating adult neuronal structure and function (i.e., administration of growth factors to cholinergic somata significantly increases axon density in terminal fields). These findings are relevant to potential clinical uses of growth factors to treat neurological disorders.

Long-term microglial and astroglial activation in the hippocampus of trimethyltin-intoxicated rat: stimulation of NGF and TrkA immunoreactivities in astroglia but not in microglia

In the present study we investigated the microglial and astroglial response after trimethyltin (TMT) exposure over a prolonged period of time. Male Wistar rats were given a single dose of TMT (8 mg/kg, i.p.) and survived 4, 7, 21, 60 and 180 days after the administration of the toxin. Histochemistry (Griffonia simplicifolia lectin staining) and immunocytochemistry for GFAP were applied to identify micro- and astroglial cells, respectively. To assess the trophic response of glial cells (NGF and TrkA expression), single or double staining experiments were performed. In addition, the biochemical evaluation of GFAP and NGF were carried out at chosen timepoints using immunoblotting technique and ELISA, respectively. The main findings of our study were as follows. (1) A protracted activation of microglia (at least up to 2 months posttreatment). (2) A long-lasting expression of GFAP immunoreactivity (at least up to 6 months posttreatment) and a steady increase in GFAP content (at least up to 2 months posttreatment). (3) The appearance of enormously enlarged, round-shape astrocytes exclusively localized to CA1 and observed 2 months posttreatment. (4) The stimulation of NGF and TrkA expression in reactive astrocytes. (5) The strongest activation of micro- and astroglia coincided with the most prominent neurodegeneration in the hippocampus, i.e., in CA4/CA3c and CA1. It is tempting to assume that the activation of glial cells in the hippocampal areas particularly vulnerable to TMT may affect neuronal fate after neurotoxic insult.

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.

Schwann cells genetically modified to secrete human BDNF promote enhanced axonal regrowth across transected adult rat spinal cord

The infusion of BDNF and NT-3 into Schwann cell (SC) grafts promotes regeneration of brainstem neurones into the grafts placed in adult rat spinal cord transected at T8 (Xu et al., 1995b). Here, we compared normal SCs with SCs genetically modified to secrete human BDNF, grafted as trails 5 mm long in the cord distal to a transection site and also deposited in the transection site, for their ability to stimulate supraspinal axonal regeneration beyond the injury. SCs were infected with the replication-deficient retroviral vector pL(hBDNF)RNL encoding the human preproBDNF cDNA. The amounts of BDNF secreted (as detected by ELISA) were 23 and 5 ng/24 h per 106 cells for infected and normal SCs, respectively. Biological activity of the secreted BDNF was confirmed by retinal ganglion cell bioassay. The adult rat spinal cord was transected at T8. The use of Hoechst prelabelled SCs demonstrated that trails were maintained for a month. In controls, no SCs were grafted. One month after grafting, axons were present in SC trails. More 5-HT-positive and some DbetaH-positive fibres were observed in the infected vs. normal SC trails. When Fast Blue was injected 5 mm below the transection site (at the end of the trail), as many as 135 retrogradely labelled neurones could be found in the brainstem, mostly in the reticular and raphe nuclei (normal SCs, up to 22, mostly in vestibular nuclei). Numerous neurones were labelled in the ventral hypothalamus (normal SCs, 0). Also, following Fast Blue injection, a mean of 138 labelled cells was present in dorsal root ganglia (normal SCs, 46) and spinal cord (39 vs. 32) rostral to the transection. No labelled spinal neurones rostral to the transection were seen when SCs were not transplanted. Thus, the transplantation of SCs secreting increased amounts of BDNF improved the regenerative response across a transection site in the thoracic cord. Moreover, the enhanced regeneration observed with infected SCs may be specific as the largest response was from neurones known to express trkB.

Absence of p75NTR causes increased basal forebrain cholinergic neuron size, choline acetyltransferase activity, and target innervation

Emerging evidence suggests that the p75 neurotrophin receptor (p75NTR) mediates cell death; however, it is not known whether p75NTR negatively regulates other neuronal phenotypes. We found that mice null for p75NTR displayed highly significant increases in the size of basal forebrain cholinergic neurons, including those that are TrkA-positive. Cholinergic hippocampal target innervation also was increased significantly. Activity of the cholinergic neurotransmitter synthetic enzyme choline acetyltransferase (ChAT) was increased in both the medial septum and hippocampus. Upregulation of these cholinergic features was not associated with increased basal forebrain or hippocampal target NGF levels. In contrast, striatal cholinergic neurons, which do not express p75NTR, showed no difference in neuronal number, size, or ChAT activity between wild-type and p75NTR null mutant mice. These findings indicate that p75NTR negatively regulates cholinergic neuronal phenotype of the basal forebrain cholinergic neurons, including cell size, target innervation, and neurotransmitter synthesis.

Nerve growth factor attenuates cholinergic deficits following traumatic brain injury in rats

Traumatic brain injury (TBI) results in chronic derangements in central cholinergic neurotransmission that may contribute to posttraumatic memory deficits. Intraventricular cannula (IVC) nerve growth factor (NGF) infusion can reduce axotomy-induced spatial memory deficits and morphologic changes observed in medial septal cholinergic neurons immunostained for choline acetyltransferase (ChAT). We examined the efficacy of NGF to (1) ameliorate reduced posttraumatic spatial memory performance, (2) release of hippocampal acetylcholine (ACh), and (3) ChAT immunoreactivity in the rat medial septum. Rats (n = 36) were trained prior to TBI on the functional tasks and retested on Days 1-5 (motor) and on Day 7 (memory retention). Immediately following injury, an IVC and osmotic pump were implanted, and NGF or vehicle was infused for 7 days. While there were no differences in motor performance, the NGF-treated group had significantly better spatial memory retention (P < 0.05) than the vehicle-treated group. The IVC cannula was then removed on Day 7, and a microdialysis probe was placed into the dorsal hippocampus. After a 22-h equilibration period, samples were collected prior to and after administration of scopolamine (1 mg/kg), which evoked ACh release by blocking autoreceptors. The posttraumatic reduction in scopolamine-evoked ACh release was completely reversed with NGF. Injury produced a bilateral reduction in the number and cross-sectional area of ChAT immunopositive medial septal neurons that was reversed by NGF treatment. These data suggest that cognitive but not motor deficits following TBI are, in part, mediated by chronic deficits in cholinergic systems that can be modulated by neurotrophic factors such as NGF.

Developmental profile of NGF immunoreactivity in the rat brain: a possible role of NGF in the establishment of cholinergic terminal fields in the hippocampus and cortex

In the current investigation, we have examined the developmental profile of nerve growth factor immunoreactivity (NGF-ir) in the postnatal rat. During the first 3 weeks after birth, NGF-ir was observed within the hippocampal mossy fiber region, where it persists throughout adulthood and appeared transiently within three additional zones-the dentate gyrus supragranular zone, the tenia tecta/intermediate lateral septum, and the cingulate/retrosplenial cortex. In all cases, the appearance of NGF-ir progressed in a rostrocaudal pattern over time. A strong correlation was seen between the pattern of NGF-ir and cholinergic innervation in the dentate gyrus supragranular zone, both spatially and temporally, suggesting that NGF may direct the innervation of cholinergic afferents to this region. A spatial correlation was also observed between NGF-ir and cholinergic innervation within the retrosplenial cortex and tenia tecta. With our current techniques, however, we were unable to determine at what point during development the adult-like pattern of cholinergic terminal innervation in these regions occurred and, thus, were not able establish a temporal correlation in these regions. Within the cingulate cortex, there was no evidence suggesting that the developmental appearance of NGF-ir in this region was associated with a specific enhancement of cholinergic innervation. Thus, the results of the current investigation clearly identify the presence of transiently occurring zones of NGF-ir during postnatal CNS development, although defining their exact functional role will require additional investigation.

Axotomized septal cholinergic neurons rescued by nerve growth factor or neurotrophin-4/5 fail to express the inducible transcription factor c-Jun

The inducible transcription factor c-Jun increases in neurons in response to axotomy by unknown mechanisms, and it has been postulated that c-Jun may regulate genes involved in promoting either degeneration or regeneration of axotomized neurons. In this report, we investigated the effect of daily or twice daily intraventricular administration of the neurotrophins nerve growth factor or neurotrophin-4/5 on the decrease in choline acetyltransferase expression and the increase in c-Jun expression in rat medial septum/diagonal band neurons three, seven and 14 days following unilateral, complete, fornix fimbria lesion. We also examined whether medial septum/diagonal band neurons might die by apoptosis within two weeks of fornix fimbria lesion using terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labelling. Our results show that both nerve growth factor and neurotrophin-4/5 maintain the phenotype of basal forebrain cholinergic neurons following axotomy. Furthermore, using double-labelling immunofluorescence, we found that while c-Jun was expressed in cholinergic neurons in control-treated rats seven days following fornix fimbria lesion, cholinergic neurons rescued by either nerve growth factor or neurotrophin-4/5 in neurotrophin-treated rats failed to express c-Jun. At no time-point (three, seven or 14 days post-axotomy) did any neurons in the medial septum/diagonal band stain positive for terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling, suggesting that medial septum/diagonal band neurons do not undergo apoptosis within the first two weeks following axotomy at the time-points observed by us. Therefore, these results show that both nerve growth factor and neurotrophin-4/5 rescue the phenotype of axotomized cholinergic neurons and that these rescued neurons fail to express c-Jun in response to axotomy. In addition, since neither nerve growth factor nor neurotrophin-4/5 induced c-Jun in medial septum/diagonal band cholinergic neurons, it seems unlikely that the neurotrophic effects of nerve growth factor and neurotrophin-4/5 on cholinergic neurons are mediated via c-Jun expression. Furthermore, since axotomy failed to increase terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labelling in septal neurons, it appears unlikely that c-Jun expression in these axotomized neurons is related to neuronal degeneration via apoptosis.

Maintenance of sympathetic innervation into the hippocampal formation requires a continuous local availability of nerve growth factor

The sprouting of peripheral sympathetic fibers into the septally denervated hippocampal formation is a well-characterized model of lesion-induced plasticity. While various studies have demonstrated the importance of nerve growth factor for evoking sympathetic sprouting, little is known concerning whether nerve growth factor continues to be required for maintaining innervation once it has occurred. In the present study we have addressed this point by (i) investigating the consequences of withdrawing exogenous nerve growth factor support from rats in which sympathetic innervation was enhanced by a nerve growth factor infusion and (ii) using blocking antibodies to interfere with the actions of endogenous nerve growth factor. The results of this investigation clearly indicate that a continuous supply of nerve growth factor (either exogenous or endogenous) is required to maintain sympathetic innervation within the hippocampal formation. Evidence is also provided demonstrating that the nerve growth factor must be made available locally within a given region to evoke and maintain the sympathetic innervation within this location. Axonal rearrangement within the developing and adult brain is believed to be an important mechanism underlying learning and memory is crucial for lesion-related plasticity. In various experimental paradigms, nerve growth factor has been shown to be an important cue for initiating axonal remodeling. In the current study, we have demonstrated that once such rearrangements have taken place, nerve growth factor may also be required to maintain them.

Short increase of BDNF messenger RNA triggers kainic acid-induced neuronal hypertrophy in adult mice

Neurotrophin gene expression in adult brain varies according to physiological activity and following brain injury, suggesting a role in neuronal maintenance and plasticity. However, the exact roles and mechanisms of action of neurotrophins in the adult brain are still poorly understood. We have recently demonstrated that neurons of the adult mouse dentate gyrus can develop a conspicuous morphogenetic response to intrahippocampal injection of kainic acid. This response is correlated with long-lasting overexpression of the brain-derived neurotrophic factor gene, suggesting a causal relationship between molecular and structural changes. To test this hypothesis, brain-derived neurotrophic factor messenger RNA were sequestered in vivo by administration of antisense oligodeoxynucleotides. When administered before 20 h post-kainate, antisense oligodeoxynucleotides totally prevented the kainate-induced neuronal hypertrophy, while sense or missense sequences had no effect. On the other hand, the hypertrophic response was observed when antisense administration was begun 24 h post-kainate, indicating an involvement of brain-derived neurotrophic factor messenger RNA in the initiation of structural changes, but not in their evolution. The hypertrophy was blocked by inhibition of tyrosine kinase activities by K252a, suggesting an involvement of Trk high affinity receptors. Administration of human recombinant brain-derived neurotrophic factor without previous treatment by kainate failed to induce any morphogenetic response. These results show that a short activation of the brain-derived neurotrophic factor gene can, in association with neuronal activation by kainate, trigger dramatic and long-lasting morphological changes in adult neurons. A physiological role of brain-derived neurotrophic factor in adult brain could therefore be to link, by autocrine/paracrine action, activation of glutamate receptors and neuronal morphological adaptive responses.

NGF, BDNF and NT-3 play unique roles in the in vitro development and patterning of innervation of the mammalian inner ear

Developing cochleovestibular ganglion (CVG) neurons depend upon interaction with the otocyst, their peripheral target tissue, for both trophic support and tropic guidance. RT-PCR of E11 through E14 otocyst-CVG RNA extracts have shown that NGF as well as BDNF and NT-3 are expressed in the developing inner ear (in situ RT-PCR on tissue sections of E12 otocysts localized all three neurotrophins to the otocyst). To evaluate the functional significance of NGF, BDNF and NT-3 expression, E10.5 otocyst-CVG explants were treated with antisense oligonucleotides and compared to sense treated and control cultures. Confocal microscopic analysis revealed that treatment with BDNF antisense resulted in extensive neuronal cell death, downregulation of NGF caused an inhibition of neuritogenesis and a decrease in the neuronal population of the CVG, whereas treatment with NT-3 antisense resulted in a loss of target directed CVG neuritic ingrowth in this in vitro model. The effect of NGF or BDNF antisense treatment could be prevented by the simultaneous addition of the respective growth factor. These findings demonstrate that each of the three neurotrophins have important roles during the onset of neuritic ingrowth of the CVG neurons to the otocyst.

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 immunoreactivity and sympathetic sprouting in the rat hippocampal formation

Several lines of evidence support a role for nerve growth factor (NGF) in the sympathetic sprouting response that occurs following septal cholinergic denervation of the rat hippocampal formation. The present study was undertaken to compare the distribution of NGF-like immunoreactivity and the topography of sympathetic sprouting in rats receiving medial septal lesions. Comparisons were made using adjacent sections of the hippocampal formation stained either for NGF-like immunoreactivity or for NGF receptor-immunoreactivity (p75, to visualize sympathetic fibers). p75-immunoreactive sympathetic axons were localized within the same regions exhibiting NGF-like staining, i.e., the hilus of the dentate gyrus and stratum lucidum in the CA3 area. Furthermore, the sympathetic fibers that invaded the hippocampal formation exhibited NGF-like immunostaining. These results provide additional evidence in support of NGF's role in this collateral sprouting response in the mature rat CNS.

Nerve growth factor in CNS repair

The hypothesis that neurotrophic factors play important roles in the adult central nervous system (CNS) has been successfully investigated in the past decade with regard to experimental and pathologic situations. Trophic roles in adult CNS axonal regeneration, on the other hand, have received much less attention. We review three groups of recent studies that demonstrate the relevance of nerve growth factor (NGF) for the regeneration of selected axons into adult central nervous tissue. The first group concerns a septohippocampal model where transected septal cholinergic axons are allowed to regrow into the hippocampal formation through a peripheral nerve bridge implanted into the transection lesion gap. NGF is required in the bridge, enhances penetration of the hippocampal tissue when infused there, and both attracts and promotes sprouting within the septum when infused in the lateral ventricle or the septal tissue itself. The second group of studies concerns the development of a spinal cord sensory regeneration model, where dorsal root ganglionic axons regrow into a nerve bridge placed within the dorsal spinal cord. Preliminary data indicate that NGF infusion rostral to the bridge once again promotes substantial penetration of the adult cord tissue by the regenerating NGF-sensitive fibers. In the third group of studies, attention has been shifted to the location of endogenous NGF in the adult rat hippocampal formation and the normal or lesion-induced occurrence of extrasomal NGF immunoreactivity. These regions of anchored NGF have the ability to attract NGF-sensitive growing axons and may provide opportunities to investigate local cues for final definition of terminal fields.

Changes in nerve growth factor immunoreactivity following entorhinal cortex lesions: possible molecular mechanism regulating cholinergic sprouting

To assess the possible role of trophic factors in lesion-induced plasticity, we have used a sensitive immunohistochemical technique to evaluate changes in nerve growth factor (NGF) staining in the hippocampal formation 3, 8, 16, and 30 days following entorhinal cortex lesions. Our results indicate that a band of NGF immunoreactivity appears in the outer molecular layer of the ipsilateral dentate gyrus following entorhinal ablation. The distribution of the NGF-immunoreactive band exactly coincides with the distribution of sprouting cholinergic terminals revealed by acetylcholinesterase histochemistry or NGF-receptor immunostaining. Increased NGF-immunoreactivity is detectable at 3 days postlesion, is most intense at 8 days, and decreases to near control levels by 30 days. Lesion-induced increases in NGF immunostaining also occur in animals in which septohippocampal fibers had been removed by prior destruction of the fimbria-fornix. Increases in NGF-immunoreactivity, however, are substantially reduced in animals receiving intraventricular injections of colchicine, which presumably blocks NGF release. These results indicate that 1) increases in NGF immunostaining, which occur following entorhinal lesions, precede any changes in cholinergic sprouting parameters and are greatest during the period of maximal cholinergic sprouting; 2) increased NGF-immunoreactivity is not due to NGF binding by septohippocampal fibers; and 3) increased NGF-immunoreactivity appears to depend on the release of NGF by neurons that produce it. We hypothesize that, following entorhinal lesions, NGF immunostaining within the hippocampal formation may represent NGF "anchored" within the tissue and that NGF accumulation by such a mechanism may direct the sprouting response of NGF-sensitive cholinergic 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.

Embryonic Purkinje cells grafted on the surface of the adult uninjured rat cerebellum migrate in the host parenchyma and induce sprouting of intact climbing fibres

By grafting solid pieces of cerebellar anlage onto the surface of the adult rat cerebellum, we have investigated the problem of the interactions between embryonic and adult neurons in an intact brain. A few days after grafting, embryonic astrocytic processes crossed the graft--host interface and radiated into the recipient molecular layer. Several grafted Purkinje cells also migrated into the host brain along such processes as well as adult Bergmann glia. Adult climbing fibres, labelled by means of Phaseolus vulgaris leucoagglutinin (PHA-L), sprouted new collateral branches which terminated on embryonic Purkinje cells at both extra- and intraparenchymal levels. No sign of activation of host astroglia or microglia was evident in the host cerebellum in relation to these processes. Embryonic Purkinje cells which migrated into the host cerebellum developed an adult-like morphology. Intraparenchymal grafts of neocortical embryonic tissue induced conspicuous growth of host olivary axons, characterized by a pattern which was different from that observed following cerebellar grafts. By contrast, when neocortical tissue was placed onto the surface of the recipient cerebellum, graft--host interactions were limited and climbing fibre sprouting was rarely seen. These results show that (i) supernumerary Purkinje cells can penetrate and settle in the adult intact cerebellar cortex, (ii) adult climbing fibres are able to innervate these new targets in the absence of any injury or activation of non-neuronal cells of the adult brain, and (iii) in the absence of damage to the adult brain, the plasticity of adult olivary axons is specifically elicited and controlled by embryonic Purkinje cells.