NGF-induced remodeling of mature uninjured axon collaterals

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

Transcranial direct current stimulation (tDCS) affects neuroinflammation parameters and behavioral seizure activity in pentylenetetrazole-induced kindling in rats

Despite the introduction of new antiepileptic drugs, about 30 % of patients with epilepsy are refractory to drug therapy. Thus, the search for non-pharmacological interventions such as transcranial direct current stimulation (tDCS) may be an alternative, either alone or in combination with low doses of anticonvulsants. This study evaluated the effect of anodal (a-tDCS) and cathodal tDCS (c-tDCS) on seizure behavior and neuroinflammation parameters. Rats were submitted to the kindling model induced by pentylenetetrazole (PTZ) using diazepam (DZP) as anticonvulsant standard. tDCS groups were submitted to 10 sessions of a-tDCS or c-tDCS or SHAM-tDCS. Every 3 days they received saline (SAL), low dose of DZP (alone or in combination with tDCS) or effective dose of DZP 30 min before administration of PTZ, totaling 16 days of protocol. Neither a-tDCS nor c-tDCS reduced the occurrence of clonic forelimb seizures (convulsive motor seizures - stage 3 by the adapted Racine scale we based on). Associated with DZP, c-tDCS (c-tDCS/DZP0.15) increased the latency to first clonic forelimb seizure on the 10th and 16th days. Hippocampal IL-1β levels were reduced by c-tDCS and c-tDCS/DZP0.15. In contrast, these treatments induced an increase in cortical IL-1β levels. Hippocampal TNF-α levels were not altered by c-tDCS or a-tDCS, but c-tDCS and c-tDCS/DZP0.15 increased those levels in cerebral cortex. Cortical NGF levels were increased by c-tDCS and c-tDCS/DZP0.15. a-tDCS/DZP0.15 reduced hippocampal BDNF levels and c-tDCS/DZP0.15 increased these levels in cerebral cortex. In conclusion, c-tDCS alone or in combination with a low dose of DZP showed to affect neuroinflammation, improving central neurotrophin levels and decreasing hippocampal IL-1β levels after PTZ-induced kindling without statistically significant effect on seizure behavior.

Early BDNF treatment ameliorates cell loss in the entorhinal cortex of APP transgenic mice

Brain-derived neurotrophic factor (BDNF) improves molecular, cellular, and behavioral measures of neural dysfunction in genetic models of Alzheimer's disease (Blurton-Jones et al., 2009; Nagahara et al., 2009). However, BDNF treatment after disease onset has not been reported to improve neuronal survival in these models. We now report prevention of neuronal loss with early life BDNF treatment in mutant mice expressing two amyloid precursor protein (APP) mutations associated with early-onset familial Alzheimer's disease. APP transgenic mice underwent lentiviral BDNF gene delivery into the entorhinal cortices at age 2 months and were examined 5 months later. BDNF-treated mice exhibited significant improvements in hippocampal-dependent contextual fear conditioning compared with control-treated APP mice (p < 0.05). Stereological analysis of entorhinal cortical cell number demonstrated ∼20% reductions in neuronal number in layers II-VI of the entorhinal cortex in untreated APP mutant mice compared with wild-type mice (p < 0.0001), and significant amelioration of cell loss by BDNF (p < 0.001). Moreover, BDNF gene delivery improved synaptophysin immunoreactivity in the entorhinal cortex and, through anterograde BDNF transport, in the hippocampus (p < 0.01). Notably, BDNF did not affect amyloid plaque numbers, indicating that direct amyloid reduction is not necessary to achieve significant neuroprotective benefits in mutant amyloid models of Alzheimer's disease.

Motor neuron target selectivity and survival after prolonged axotomy

PURPOSE

After a cut peripheral nerve is repaired, motor neurons usually regenerate across the lesion site, however they often enter an inappropriate Schwann cell tube and may be directed to an inappropriate target organ such as skin, resulting in continued loss of function. In fact, only about 10% of adults who receive a peripheral nerve repair display full functional recovery. The reasons for this are many and complex, however one aspect is whether the motor neuron has undergone a prolonged period of axotomy prior to nerve repair. Previous studies have suggested a deleterious effect of prolonged axotomy.

METHODS

We examined the influence of prolonged axotomy on target selectivity using a cross-reinnervation model of rat obturator motor neurons regrowing into the distal femoral nerve, with its normal bifurcating pathways to muscle and skin.

RESULTS

Surprisingly, we found that a prolonged period of axotomy resulted in an increase in motor neuron regeneration accuracy. In addition, we found that regeneration accuracy could be increased even further by a simple surgical manipulation of the distal terminal nerve pathway to skin.

CONCLUSIONS

These results suggest that under certain conditions prolonged axotomy may not be detrimental to the final accuracy of motor neuron regeneration and highlight that a simple manipulation of terminal nerve pathways may be one approach to increase such regeneration accuracy.

Polysialic acid expression is not necessary for motor neuron target selectivity

INTRODUCTION

Recovery after peripheral nerve lesions depends on guiding axons back to their targets. Polysialic acid upregulation by regrowing axons has been proposed recently as necessary for this target selectivity.

METHODS

We reexamined this proposition using a cross-reinnervation model whereby axons from obturator motor neurons that do not upregulate polysialic acid regenerated into the distal femoral nerve. Our aim was to assess their target selectivity between pathways to muscle and skin.

RESULTS

After simple cross-repair, obturator motor neurons showed no pathway preference, but the same repair with a shortened skin pathway resulted in selective targeting of these motor neurons to muscle by a polysialic acid-independent mechanism.

CONCLUSION

The intrinsic molecular differences between motor neuron pools can be overcome by manipulation of their access to different peripheral nerve pathways such that obturator motor neurons preferentially project to a terminal nerve branch to muscle despite not upregulating the expression of polysialic acid.

Functional recovery of completely denervated muscle: implications for innervation of tissue-engineered muscle

Tissue-engineered muscle has been proposed as a solution to repair volumetric muscle defects and to restore muscle function. To achieve functional recovery, engineered muscle tissue requires integration of the host nerve. In this study, we investigated whether denervated muscle, which is analogous to tissue-engineered muscle tissue, can be reinnervated and can recover muscle function using an in vivo model of denervation followed by neurotization. The outcomes of this investigation may provide insights on the ability of tissue-engineered muscle to integrate with the host nerve and acquire normal muscle function. Eighty Lewis rats were classified into three groups: a normal control group (n=16); a denervated group in which sciatic innervations to the gastrocnemius muscle were disrupted (n=32); and a transplantation group in which the denervated gastrocnemius was repaired with a common peroneal nerve graft into the muscle (n=32). Neurofunctional behavior, including extensor postural thrust (EPT), withdrawal reflex latency (WRL), and compound muscle action potential (CMAP), as well as histological evaluations using alpha-bungarotoxin and anti-NF-200 were performed at 2, 4, 8, and 12 weeks (n=8) after surgery. We found that EPT was improved by transplantation of the nerve grafts, but the EPT values in the transplanted animals at 12 weeks postsurgery were still significantly lower than those measured for the normal control group at 4 weeks (EPT, 155.0±38.9 vs. 26.3±13.8 g, p<0.001; WRL, 2.7±2.30 vs. 8.3±5.5 s, p=0.027). In addition, CMAP latency and amplitude significantly improved with time after surgery in the transplantation group (p<0.001, one-way analysis of variance), and at 12 weeks postsurgery, CMAP latency and amplitude were not statistically different from normal control values (latency, 0.9±0.0 vs. 1.3±0.7 ms, p=0.164; amplitude, 30.2±7.0 vs. 46.4±26.9 mV, p=0.184). Histologically, regeneration of neuromuscular junctions was seen in the transplantation group. This study indicates that transplanted nerve tissue is able to regenerate neuromuscular junctions within denervated muscle, and thus the muscle can recover partial function. However, the function of the denervated muscle remains in the subnormal range even at 12 weeks after direct nerve transplantation. These results suggest that tissue-engineered muscle, which is similarly denervated, could be innervated and become functional in vivo if it is properly integrated with the host nerve.

Human neural stem cells genetically modified to express human nerve growth factor (NGF) gene restore cognition in the mouse with ibotenic acid-induced cognitive dysfunction

Alzheimer's disease (AD) is characterized by degeneration and loss of neurons and synapses throughout the brain, causing the progressive decline in cognitive function leading to dementia. No effective treatment is currently available. Nerve growth factor (NGF) therapy has been proposed as a potential treatment of preventing degeneration of basal forebrain cholinergic neurons in AD. In a previous study, AD patient's own fibroblasts genetically modified to produce NGF were transplanted directly into the brain and protected cholinergic neurons from degeneration and improved cognitive function in AD patients. In the present study, human neural stem cells (NSCs) are used in place of fibroblasts to deliver NGF in ibotenic acid-induced learning-deficit rats. Intrahippocampal injection of ibotenic acid caused severe neuronal loss, resulting in learning and memory deficit. NGF protein released by F3.NGF human NSCs in culture medium is 10-fold over the control F3 naive NSCs at 1.2 µg/10(6) cells/day. Overexpression of NGF in F3.NGF cells induced improved survival of NSCs from cytotoxic agents H2O2, Aβ, or ibotenic acid in vitro. Intrahippocampal transplantation of F3.NGF cells was found to express NGF and fully improved the learning and memory function of ibotenic acid-challenged animals. Transplanted F3.NGF cells were found all over the brain and differentiated into neurons and astrocytes. The present study demonstrates that human NSCs overexpressing NGF improve cognitive function of learning-deficit model mice.

Schwann cell influence on motor neuron regeneration accuracy

Extensive peripheral nerve injuries can result in the effective paralysis of the entire limb or distal portions of the limb. The major determinant of functional recovery after lesions in the peripheral nervous system is the accurate regeneration of axons to their original target end-organs. We used the mouse femoral nerve as a model to study motor neuron regeneration accuracy in terms of regenerating motor neurons projecting to their original terminal pathway to quadriceps muscle vs. the inappropriate pathway to skin. Using a variety of surgical manipulations and the selective removal of Schwann cells in the distal nerve via molecular targeting, we have examined the respective roles of end-organ influence (muscle) vs. Schwann cells in this model system. We found evidence of a hierarchy of trophic support that regulates motor neuron regeneration accuracy with muscle contact being the most potent, followed by the number or density of Schwann cells in the distal nerve branches. Manipulating the relative levels of these sources of influence resulted in predictable projection patterns of motor neurons into the terminal pathway either to skin or to muscle.

The specificity of motor neurone regeneration (preferential reinnervation)

The major determinant of functional recovery after lesions in the peripheral nervous system is the accurate regeneration of axons to their original target end-organs. Unfortunately, regenerating motor axons are often misrouted to sensory target end-organs, and sensory axons formerly innervating skin are often misrouted to muscle. As such regeneration is robust, but often inaccurate, a better understanding of how regenerating axons reinnervate terminal pathways would be of fundamental interest to basic and clinical neuroscience. This review will consider the underlying cellular and molecular mechanisms that influence the accuracy of peripheral nerve regeneration, within the context of 'preferential motor reinnervation' (PMR). Much previous work with PMR has utilized the rodent femoral nerve and has shown that regenerating motor axons preferentially, albeit incompletely, reinnervate a distal terminal nerve branch to muscle (quadriceps) vs. skin (saphenous). One interpretation of this body of work has been that Schwann cell tubes have a specific identity that can be recognized by regenerating motor axons and that influences their subsequent behaviour. We disagree with that interpretation, and suggest motor and cutaneous pathways are not inherently different in terms of their ability to support regeneration of motor axons. In fact, recent experiments indicate under certain conditions motor axons will preferentially reinnervate the inappropriate terminal cutaneous pathway instead of the appropriate pathway to muscle. We suggest that it is the relative level of trophic support provided by each nerve branch that determines whether motor axons will remain in that particular branch. Within the context of the femoral nerve model, our results suggest a hierarchy of trophic support for regenerating motor axons with muscle contact being the highest, followed by the length of the terminal nerve branch and/or contact with skin.

Bone marrow stromal cells reduce axonal loss in experimental autoimmune encephalomyelitis mice

We investigated the ability of human bone marrow stromal cell (hBMSC) treatment to reduce axonal loss in experimental autoimmune encephalomyelitis (EAE) mice. EAE was induced in SJL/J mice by injection with proteolipid protein (PLP). Mice were injected intravenously with hBMSCs or PBS on the day of clinical onset, and neurological function was measured daily (score 0-5) until 45 weeks after onset. Mice were sacrificed at week 1, 10, 20, 34, and 45 after clinical onset. Bielshowsky silver was used to identify axons. Immunohistochemistry was performed to measure the expression of nerve growth factor (NGF) and MAB1281, a marker of hBMSCs. hBMSC treatment significantly reduced the mortality, the disease severity, and the number of relapses in EAE mice compared with PBS treatment. Axonal density and NGF(+) cells in the EAE brain were significantly increased in the hBMSC group compared with the PBS group at 1, 10, 20, 34, and 45 weeks. Disease severity was significantly correlated with decreased axonal density and decreased NGF, and increased axonal density was significantly correlated with reduced loss of NGF expression after hBMSC treatment. Most of the NGF(+) cells are brain parenchymal cells. Under 5% of MAB1281(+) cells colocalized with NG2(+), a marker of oligodendrocyte progenitor cells. Nearly 10% of MAB1281(+) cells colocalized with GFAP, a marker of astrocytes, and MAP-2, a marker of neurons. Our findings indicate that hBMSCs improve functional recovery and may provide a potential therapy aimed at axonal protection in EAE mice, in which NGF may play a vital role.

Developmentally regulated changes in femoral nerve regeneration in the mouse and rat

The attractive influence of muscle on regenerating motor neuron axons is well-known. Less is known, however, about the intrinsic abilities of different nerve pathways to support these axons prior to end-organ contact. The age at which a nerve injury is sustained is also known to affect the relationship between regenerating motor axons and muscle. The femoral nerve model, with its distinct muscle and cutaneous pathways, is ideal to study intrinsic pathway properties because the influence of end-organs can easily be removed surgically. However, recent results using this model in adult mice are at odds with the same model in neonatal rats. To reconcile these discrepancies, we used the femoral nerve model to examine possible age differences in intrinsic pathway support for regenerating motor neurons in the mouse and rat. Rat motor neurons showed a preference to regenerate into the muscle pathway after axotomy at 3 weeks of age, but this preference was lost after axotomy at 6 weeks of age. Interestingly, mouse motor neurons showed no pathway preference after axotomy at 3 weeks of age but developed one for the cutaneous pathway after axotomy at 6 weeks of age. These results suggest that in the absence of end-organ contact there is no general preference for motor neurons to project to the muscle pathway.

Lack of dystrophin leads to the selective loss of superior cervical ganglion neurons projecting to muscular targets in genetically dystrophic mdx mice

Autonomic imbalance is a pathological aspect of Duchenne muscular dystrophy. Here, we show that the sympathetic superior cervical ganglion (SCG) of mdx mice, which lack dystrophin (Dp427), has 36% fewer neurons than that of wild-type animals. Cell loss occurs around P10 and affects those neurons innervating muscular targets (heart and iris), which, differently from the submandibular gland (non-muscular target), are precociously damaged by the lack of Dp427. In addition, although we reveal altered axonal defasciculation in the submandibular gland and reduced terminal sprouting in all SCG target organs, poor adrenergic innervation is observed only in the heart and iris. These alterations, detected as early as P5, when neuronal loss has not yet occurred, suggest that in mdx mice the absence of Dp427 directly impairs the axonal growth and terminal sprouting of sympathetic neurons. However, when these intrinsic alterations combine with structural and/or functional damages of muscular targets, neuronal death occurs.

Prolonged local neurotrophin-3 infusion reduces ipsilateral collateral sprouting of spared corticospinal axons in adult rats

The corticospinal tract is widely used to study regeneration and is essential for voluntary movements in humans. In young rats, corticospinal axons on the uninjured side sprout and grow into the denervated side. Neurotrophin-3 (NT-3) induces such crossed collateral sprouting in adults. We investigated whether local intraspinal NT-3 infusions would promote collateral sprouting of spared corticospinal terminals from within a partially denervated side, as this would be more appropriate for enhancing function of unilateral and specific movements. Adult rats received a partial bilateral transection of the pyramids, leaving approximately 40% of each tract intact. Vehicle or vehicle plus NT-3 (3 or 10 microg/day) was infused for 14 days into the left side of the cervical (C5/6) or lumbar (L2) cord. The corticospinal processes on the left side were anterogradely traced with cholera toxin B (CTB; which labeled gray matter processes more robustly than biotinylated dextran amine) injected into the front or hind limb area of the right sensorimotor cortex, respectively, 3 days before analysis. Unexpectedly, approximately 40% fewer CTB-labeled corticospinal processes were detectable in the cervical or lumbar gray matter of NT-3-treated rats than in vehicle-infused ones. Vehicle-infused injured rats had more corticospinal processes in the center of the cord than normal rats, evidence for lesion-induced collateral sprouting. NT-3 caused sprouting of local calcitonin gene-related peptide-positive fibers. These results suggest that NT-3 reduces collateral sprouting of spared corticospinal axons from within the denervated regions, possibly because of the injury environment or by increasing sprouting of local afferents. They identify an unexpected context-dependent outgrowth inhibitory effect of NT-3.

Chronic intracerebroventricular NGF administration improves working memory in young adult memory deficient rats

Although the beneficial effects of nerve growth factor (NGF) in age-related memory deficits are well documented, the therapeutic role of this neurotrophin in memory deficits occurring in young subjects remains unclear. In the present study, the effect of chronic NGF administration on spatial working memory was investigated in young adult memory deficient Wistar rats. Memory deficient rats were selected on the basis of their preoperative performance in delayed matching-to-position task (DMTP) carried out in the eight-arm radial maze. The delay between sample and test choices was prolonged stepwise from 10s, to 1, 5, and eventually 15 min. Rats that performance at the longest 15-min delay was at least 3 S.E.M. above the group mean were classified as "poor learners". They were randomly assigned to either Control or NGF group, and treated with either vehicle solution (artificial cerebrospinal fluid) or NGF at the total dose of 40 microg/rat. Intracerebroventricular (icv) drug infusion was made continuously over 28 days at the rate of 0.25 microl/h using Alzet 2004 osmotic mini-pump. The postoperative training included the same stages as the preoperative one. No significant between-group difference in the postoperative performance was noted at the shortest delay of 10s that could be bridged by the immediate memory. Conversely, at all three longer delays, postoperative performance in the NGF group was significantly better compared to control rats. The present study thus shows that NGF may have beneficial effects in memory-deficient young adults.

Pathway sampling by regenerating peripheral axons

A century ago, Ramon y Cajal described the generalized response of regenerating peripheral axons to their environment. By using mice that express fluorescent proteins in their axons, we are now able to quantify the response of individual axons to nerve transection and repair. Sciatic nerves from nonexpressing mice were grafted into those expressing a yellow variant of green fluorescent protein, then examined at 5, 7, or 10 days after repair. Regeneration was found to be a staggered process, with only 25% of axons crossing the repair in the first week. In the setting of Wallerian degeneration, this stagger will expose growth cones to an evolving menu of molecular cues upon which to base pathway decisions. Many axons arborize, allowing them to interact simultaneously with several pathways. Arborization could serve as the anatomical substrate for specificity generation through collateral pruning. Axons often travel laterally across the face of the distal stump before choosing a pathway. As a result, the average unbranched axon has access to over 100 distal Schwann cell tubes. This extensive access, however, does not ensure correct matching of axon and end organ, suggesting that pathway choice is made on the basis of factors other than end organ identity. These observations explain the failure of refined surgical techniques to restore normal function after nerve injury. The apparent wandering of axons across the repair defies surgical control and mandates a biological approach to reuniting severed axons with appropriate distal pathways.

Motor neurons can preferentially reinnervate cutaneous pathways

Previous work in the rat femoral nerve has shown that regenerating motor neurons preferentially reinnervate a terminal nerve branch to muscle as opposed to skin. This process has been termed preferential motor reinnervation (PMR) and has been interpreted as evidence that regenerating motor axons can differentiate between Schwann cell tubes that reside in muscle versus cutaneous terminal pathways. However, much of this previous work has been confounded by motor axons having access to target muscle during the regeneration period. The present experiments prevented muscle contact by regenerating motor axons. By 8 weeks under these conditions, significantly more motor neurons reinnervated the cutaneous pathway rather than the original muscle pathway. We propose that cutaneous and muscle terminal pathways are not inherently different in terms of their ability to support regeneration of motor neurons. Rather, we suggest that it is the relative level of trophic support provided by each nerve branch that determines whether motor axons will remain in that particular branch. Within the context of the femoral nerve model, our results suggest a hierarchy of trophic support for regenerating motor axons with muscle contact being the highest, followed by the length of the terminal nerve branch and/or contact with skin.

Structural and neurochemical features of postganglionic sympathetic neurons in the superior mesenteric ganglion of spontaneously hypertensive rats

Postganglionic sympathetic neurons, which are exquisitely sensitive to small changes in levels of target-derived nerve growth factor (NGF), express two transmembrane receptors: 1) the trkA receptor mediates neuron survival and neurite outgrowth; and 2) the p75 neurotrophin receptor (p75NTR) enhances neuronal responsiveness of trkA to NGF. Elevating levels of NGF induces several morphological and neurochemical alterations in sympathetic neurons, including axonal sprouting, increased levels of p75NTR mRNA relative to trkA mRNA, and increased accumulations of NGF in hypertrophied somata. Spontaneously hypertensive rats (SHR) display both elevated NGF levels and increased sympathetic axonal innervation of the mesenteric vasculature. In this investigation we assessed whether sympathetic neurons innervating the mesenteric vasculature of SHR display other features indicative of increased levels of target-derived NGF. In 5-week-old SHR, levels of both p75NTR and trkA mRNA in mesenteric sympathetic neurons were significantly elevated compared to levels in age-matched control rats. By 15 and 30 weeks of age, levels of p75NTR mRNA expression in mesenteric sympathetic neurons were similar between SHR and control rats. Accumulations of NGF were depleted in the sympathetic somata of 15- and 30-week-old SHR compared to age-matched control rats. Moreover, sympathetic neurons in SHR were not hypertrophied, as the sizes of somata were comparable between SHR and control rats. Our data illustrate that despite having augmented levels of NGF in the mesenteric vasculature, SHR do not display many of the morphological and neurochemical features that are associated with an enhanced responsiveness by sympathetic neurons to elevated levels of target-derived NGF.

Distribution of central sensory axons in transgenic mice overexpressing nerve growth factor and lacking functional p75 neurotrophin receptor expression

This study examined the roles of nerve growth factor (NGF) and the p75 neurotrophin receptor (p75NTR) in the growth of dorsal root ganglion (DRG) central processes in the dorsal horn. Two genetically modified mouse strains were used: transgenic mice that overexpress NGF in the CNS under the control of the glial fibrillary acidic protein promoter, and p75NTR exon III null mutant mice that express a hypomorphic form of this receptor. In both NGF transgenic and nontransgenic mice with hypomorphic expression of p75NTR, there is a significant loss of DRG neurons compared to mice with normal p75NTR expression. This reduction in neuron number has been shown to underlie a corresponding decrease in peripheral nociceptive sensory innervation. Within the CNS, however, nociceptive innervation of the dorsal horn appears to be unaffected by hypomorphic expression of p75NTR. Comparisons of calcitonin gene-related peptide immunoreactivity in the dorsal horn revealed that the area occupied by DRG central processes was not significantly different between p75NTR hypomorphic mice and wild-type siblings, or between NGF transgenic mice with either hypomorphic or normal expression of p75NTR. We propose that DRG central processes arborize extensively in both NGF-transgenic and nontransgenic p75NTR hypomorphic mice in order to compensate for the loss of DRG neurons and restore dorsal horn innervation to normal levels. We also present evidence suggesting that NGF plays only a minor role in the growth of DRG central processes.

Maternal glucocorticoid hormone influences nerve growth factor expression in the developing rat brain

Rat pups nursed from birth by mothers with increased plasma corticosterone show long-lasting biochemical and behavioral modifications. Here we have investigated nerve growth factor (NGF) concentrations in the basal forebrain, prefrontal cortex and hippocampus of both male and female offspring at 11 days of age. Maternal hypercorticosteronemia was achieved by giving corticosterone-enriched water (200 microg/ml) from delivery. There was a significant increase of NGF in the basal forebrain of both sexes and no changes in the prefrontal cortex. In the hippocampus, an increase in NGF was found in males. These results indicate that a moderate increase of corticosterone in the lactating mother modulates NGF in the developing rat. We propose that these effects contribute directly to the long-lasting behavioral and biochemical modifications in pups nursed by hypercorticosteronemic mothers.

Responsiveness of sympathetic and sensory iridial nerves to NGF treatment in young and aged rats

Altered neuronal responses to trophic factors may play a role in neuronal maintenance in adulthood and may also be involved in neuronal atrophy in old age. We have investigated this issue in the rat iris, studying responsiveness of sympathetic and sensory iridial nerves to a range of NGF concentrations in mature and aged rats. We show here that growth responses of sensory nerves to NGF, as measured by quantitative immunohistochemistry and image analysis, were unchanged in old rats. In contrast, there was a small but significant reduction in responsiveness of aged sympathetic neurons. The shapes of the dose-response curves for sensory and sympathetic neurons were different, with a larger response over a narrower range of concentrations in sensory neurons. Lower levels of p75 immunoreactivity were observed in iridial nerves from old compared to young rats. NGF treatment had no effect on receptor staining in young rats but restored 'young' levels of p75 staining in old rats. Our results do not support the hypothesis of a primary role for NGF in maintenance or atrophy of nerves in ageing.

Intraparenchymal NGF infusions rescue degenerating cholinergic neurons

Nerve growth factor (NGF) exerts both trophic (cell survival) and tropic (axonal growth-promoting) effects on several neuronal populations. In particular, its robust ability to prevent lesion-induced and spontaneous age-related basal forebrain cholinergic neuronal degeneration, and to promote mnemonic recovery, has suggested its potential use as a therapeutic agent in Alzheimer's disease. When infused intracerebroventricularly, however, NGF is associated with several adverse effects that make this delivery route impractical. The present study examined whether intraparenchymal infusions of NGF adjacent to cholinergic neuronal soma are an effective and well-tolerated means of providing NGF to degenerating cholinergic neurons. Cholinergic neuronal rescue together with axonal sprouting responses and local tissue damage in the brain were assessed in adult rats that underwent complete unilateral fornix transections, followed by intraparenchymal infusions of recombinant human NGF for a 2-week period. Intraparenchymal NGF infusions prevented the degeneration of 94.7+/-6.6% of basal forebrain cholinergic neurons compared to 21.7+/-2.6% in vehicle-infused animals (p < 0.0001). Cholinergic axons sprouted toward the intraparenchymal NGF source in an apparent gradient-dependent manner. Glial responses to intraparenchymal infusions were minimal, and no apparent toxic effects of the infusions were observed. Thus, when infused intraparenchymally, NGF rescues basal forebrain cholinergic neurons, alters the topography of axonal sprouting responses, and does not induce adverse affects over a 2-week infusion period. Intraparenchymal NGF delivery merits further study at longer term time points as a means of treating the cholinergic component of neuronal loss in Alzheimer's disease.

Enhanced neurotrophin-induced axon growth in myelinated portions of the CNS in mice lacking the p75 neurotrophin receptor

Axonal growth in the adult mammalian CNS is limited because of inhibitory influences of the glial environment and/or a lack of growth-promoting molecules. Here, we investigate whether supplementation of nerve growth factor (NGF) to the CNS during postnatal development and into adulthood can support the growth of sympathetic axons within myelinated portions of the maturing brain. We have also asked whether p75(NTR) plays a role in this NGF-induced axon growth. To address these questions we used two lines of transgenic mice overexpressing NGF centrally, with or without functional expression of p75(NTR) (NGF/p75(+/+) and NGF/p75(-/-) mice, respectively). Sympathetic axons invade the myelinated portions of the cerebellum, beginning shortly before the second week of postnatal life, in both lines of NGF transgenic mice. Despite the presence of central myelin, these sympathetic axons continue to sprout and increase in density between postnatal days 14 and 100, resulting in a dense plexus of sympathetic fibers within this myelinated environment. Surprisingly, the growth response of sympathetic fibers into the cerebellar white matter of NGF/p75(-/-) mice is enhanced, such that both the density and extent of axon ingrowth are increased, compared with age-matched NGF/p75(+/+) mice. These dissimilar growth responses cannot be attributed to differences in cerebellar levels of NGF protein or sympathetic neuron numbers between NGF/p75(+/+) and NGF/p75(-/-) mice. Our data provide evidence demonstrating that growth factors are capable of overcoming the inhibitory influences of central myelin in the adult CNS and that neutralization of the p75(NTR) may further enhance this growth response.

Targeted intraparenchymal delivery of human NGF by gene transfer to the primate basal forebrain for 3 months does not accelerate beta-amyloid plaque deposition

Nerve growth factor therapy has been proposed as a potential means of preventing degeneration of basal forebrain cholinergic neurons in Alzheimer's disease and thereby improving cognition. However, NGF has been reported to upregulate expression of the beta-amyloid precursor protein, which in turn could accelerate deposition of "mature" beta-amyloid in the brain. To address this possibility, the brains of 16 adult and aged rhesus monkeys were examined for beta-amyloid plaque deposition in the presence or absence of NGF treatment. Six aged monkeys received intraparenchymal grafts into the cholinergic basal forebrain of autologous cells genetically modified to secrete NGF, six aged monkeys received intraparenchymal grafts of autologous control cells expressing the reporter gene beta-galactosidase, and four adult nonoperated monkeys served as additional controls. All brains were examined for expression of mature beta-amyloid using an antibody recognizing amino acids 1-40 of the beta-amyloid peptide. Amyloid plaques were systematically quantified in representative sections of the temporal, frontal, cingulate, insular, and parietal cortices and in the amygdala and hippocampus. Results disclosed that aging resulted in an increase in amyloid plaque formation: no plaques at all were detected in nonaged monkeys, whereas a mean of 20 +/- 13 plaques per section were present in control-aged monkeys. Aged subjects with intraparenchymal NGF-secreting grafts for 3 months contained a mean of 29 +/- 14 plaques per section, an amount that did not differ significantly from control-aged monkeys (P = 0.66). Thus, 3 months of intraparenchymal NGF delivery did not significantly increase beta-amyloid deposition.

Overexpression of nerve growth factor in epidermis disrupts the distribution and properties of sympathetic innervation in footpads

Sympathetic and sensory neurons form distinct axonal arborizations in several peripheral targets. The developmental mechanisms responsible for partitioning sympathetic and sensory axons between potential target tissues are poorly understood. We have used rodent footpads to study this process because three populations of peripheral axons innervate topographically segregated targets in the footpad; cholinergic sympathetic axons innervate sweat glands, noradrenergic sympathetic axons innervate blood vessels, and sensory axons form a plexus at the epidermal/dermal junction. To examine how nerve growth factor (NGF), a trophic and survival factor for sympathetic and some sensory neurons, may contribute to the generation of the patterned distribution of axons among targets, we studied transgenic mice (K14-NGF mice) in which NGF expression was significantly increased in the epidermis. Whereas the temporal sequence in which sensory and sympathetic fibers arrived in the footpad was not affected, the normal partitioning of axons between target tissues was disrupted. The two sympathetic targets in footpads, sweat glands, and blood vessels lacked substantial innervation and instead a dense plexus of catecholaminergic sympathetic fibers was found commingled with sensory fibers in the dermis. Those sympathetic fibers present in sweat glands expressed an abnormal dual catecholaminergic/cholinergic phenotype. Our findings indicate that overexpression of NGF in skin interferes with the segregation of sensory and sympathetic axonal arbors and suggests a role for target-derived NGF in the establishment of distinct axonal territories. Our data also suggest that by determining where axon arbors form, NGF can indirectly influence the phenotypic properties of sympathetic neurons.

Early maternal separation increases NGF expression in the developing rat hippocampus

Nerve Growth Factor (NGF) is a neurotrophin involved in growth and differentiation of central cholinergic neurons. In this study a maternal separation paradigm was used to test whether levels of NGF might be affected by brief manipulations of rat pups early during ontogeny. The expression of NGF mRNA was examined in 3-day-old rat pups following 45 min maternal separation using in situ hybridization. Early maternal separation in neonatal rats resulted in increased expression of NGF mRNA in the dentate gyrus and the hilus of the hippocampus. NGF protein levels measured (by means of a sensitive ELISA assay) in the whole hippocampus the day following the separation procedure did not differ in separated vs. nonseparated pups. These data indicate that brief manipulations performed early during development can affect hippocampal NGF expression.

Effects of growth factors and estrogen on the development of septal cholinergic neurons from the rat

Cholinergic neurons of the septum are preferentially subject to degeneration in Alzheimer's disease. There is evidence that nerve growth factor, basic fibroblast growth factor, insulin-like growth factors, and estrogen all have effects on survival of this specific population of neurons at risk. We used a bilaminar culturing method to grow embryonic septal neurons from the rat in the presence of a separate glial plane but in the absence of serum. These neurons were treated with a number of factors, and neurite development of cholinergic neurons was assessed. Basic fibroblast growth factor and estrogen altered the number of primary neurites, number of secondary neurites, and mean total neurite lengths, while none of the other factors affected these end points. This would suggest a mechanism for the effects of these factors on memory.

Systemic administration of a nerve growth factor conjugate reverses age-related cognitive dysfunction and prevents cholinergic neuron atrophy

Intraventricular administration of nerve growth factor (NGF) in rats has been shown to reduce age-related atrophy of central cholinergic neurons and the accompanying memory impairment. Intraventricular administration of NGF is necessary because NGF will not cross the blood-brain barrier (BBB). Here we have used a novel carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. In our experiment, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate or a control solution (a mixture of unconjugated OX-26 and NGF) twice weekly for 6 weeks. The OX-26-NGF injections resulted in a significant improvement in spatial learning in previously impaired rats but disrupted the learning ability of previously unimpaired rats. Neuroanatomical analyses showed that OX-26-NGF conjugate treatment resulted in a significant increase in cholinergic cell size in the medial septal region of rats initially impaired in spatial learning. These results indicate the potential use of the transferrin receptor antibody delivery system for treatment of CNS disorders with neurotrophic proteins.

Autonomic nervous system as a model of neuronal aging: the role of target tissues and neurotrophic factors

The aim of this study was to determine the role of target tissues and neurotrophic factors in the growth and atrophy of autonomic neurons during development and aging. Using quantitative neuroanatomical techniques, it is shown that, although axonal and dendritic growth is apparent throughout postnatal development, different patterns of growth are found in autonomic neurons innervating different target tissues. For example, sympathetic neurons innervating the submandibular gland continue to grow well into maturity, but those innervating the iris cease net growth early in postnatal development. Similarly, although neuronal atrophy was observed in aged autonomic ganglia, this was not a general phenomenon but was specific to neurons innervating particular target tissues. Sympathetic neurons innervating the middle cerebral artery showed significant axonal and dendritic atrophy in old age, whereas neurons innervating the iris were morphologically unchanged. The trophic influence of peripheral target tissues on their innervating neurons has been shown to decline in old age possibly as a result of decreased availability of target-derived neurotrophic factors such as nerve growth factor (NGF) [Gavazzi et al. (1992) Neuroreport, 3:717-720]. Therefore, in an attempt to reverse neuronal atrophy where it occurred, NGF was infused via miniosmotic pumps over the peripheral axons of aged neurons. NGF induced increases in soma size, dendritic length and axonal arborization. However, in contrast to young adult neurons, no increase in the number of dendritic branch points or primary dendrites was observed, suggesting that some aspects of neuronal plasticity are impaired in old age. In sum, these results show a range of age- and target-specific differences in the axonal and dendritic morphology of autonomic neurons that may result from differing trophic interactions with their target tissues.

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.

Target-specific differences in the dendritic morphology and neuropeptide content of neurons in the rat SCG during development and aging

Our purpose in this work was to investigate the role of target tissues in the regulation of dendritic morphology from sympathetic neurons during development and aging. Neurons were retrogradely labeled from three targets, the iris, the submandibular gland (SMG), and the middle cerebral artery (MCA). They were then fixed and intracellularly injected to demonstrate their dendritic arborizations. Dendritic geometry varied quantitatively in sympathetic neurons innervating different target tissues at all stages of development. Neurons innervating the iris had the largest cell bodies and most extensive dendritic arborizations, whereas the vasomotor neurons were the smallest. The number of primary dendrites, however, did not vary significantly between the different neuronal populations. The growth of dendritic arborizations during postnatal development and their atrophy in old age were not concordant in the different neuron populations we studied. Neurons innervating the MCA and the iris ceased dendritic growth early in postnatal development, whereas the dendritic complexity of neurons supplying the SMG increased well into adulthood. By contrast, dendritic atrophy was seen in aged MCA- and SMG-projecting neurons but not in those innervating the iris, suggesting, with other evidence, correlated and distinct patterns of growth and atrophy in axons and dendrites of mature sympathetic neurons projecting to different targets. Swollen dendrites and protuberances on cell soma were a prominent feature of aged neurons. In addition to the target-specific variation in neuronal morphology, we observed diversity in neurotransmitter phenotype. For example, neuropeptide Y was expressed in iridial but not SMG-projecting neurons. These results show a range of age- and target-specific differences in the dendritic morphology and neuropeptide content of sympathetic neurons that may be a result of differing trophic interactions with their target tissues.

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.

The duration of sprouted cerebrovascular axons following intracranial infusion of nerve growth factor

We reported previously that a 2-week infusion of the trophic protein nerve growth factor (NGF) into the lateral ventricle of the adult rat brain elicits a sprouting response by perivascular axons associated with the intradural segment of the internal carotid artery. In the present study, we used electron microscopy to determine whether the sprouted axons persist following cessation of NGF delivery and, if not, to determine the time course of their disappearance. Our results demonstrate that NGF-induced sprouted axons do not persist following cessation of NGF delivery. The total number of axons at 1 week following the end of the NGF infusion was elevated compared to control values, but significantly reduced compared with NGF cases sacrificed immediately following the infusion period. Three weeks following the end of the NGF infusion, the total number of axons was similar to controls although there were no signs of axonal degeneration. These results suggest that continued elevation of NGF levels is necessary to maintain the sprouted axons and that endogenous levels of NGF, or other factors produced by the vascular target tissue, are not sufficient to maintain the newly formed axons. The demonstration that mature perivascular axons proliferate and disappear as a function of exogenous NGF exposure supports the accumulating evidence for continued plasticity in the mature nervous system.

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.

Nerve growth factor enhances the dendritic arborization of sympathetic ganglion cells undergoing atrophy in aged rats

We have investigated whether dendritic growth can be induced from sympathetic neurons of aged rats by the application of exogenous nerve growth factor to their target tissues. A previous study showed that significant dendritic atrophy (19%) occurs during aging in the sympathetic neurons innervating the middle cerebral artery and that dendritic atrophy correlated with loss of axon collaterals in the same population of neurons. Using retrograde tracing in conjunction with intracellular injection of fixed tissue and confocal microscopy, we now demonstrate that infusion of nerve growth factor over the peripheral processes of the same neurons from aged rats induces significant dendritic (45%) and cell body (60%) growth. However, not all aspects of the dendritic arborization were affected. Primary dendrites and branch points were not altered by nerve growth factor. In contrast, nerve growth factor induced a significant increase in the number of primary dendrites and branch points (100%) of neurons from young adults. Our results demonstrate that sympathetic neurons undergoing dendritic atrophy during aging can exhibit significant dendritic growth in response to the in vivo infusion of nerve growth factor, although the lack of regenerative response displayed by some parts of the dendritic tree leads us to believe that these neurons also show signs of reduced plasticity.

Intracranial infusion of purified nerve growth factor to an Alzheimer patient: the first attempt of a possible future treatment strategy

We report on the clinical outcome of a first case of intracranial infusion of nerve growth factor (NGF) to an Alzheimer patient. The therapeutic attempt is based on animal research showing that NGF stimulates central cholinergic neurons of the type known to be lost during the development of Alzheimer's disease (AD). Furthermore, our own previous clinical experience of infusing NGF to support the survival of intracranially transplanted adrenal chromaffin cells to Parkinsonian patients indicate this approach to be technically possible and safe and clinically of significant potential. Our first case was a 69-year-old woman, with symptoms of dementia since 8 years. Intraventricular infusion of 6.6 mg NGF over three months resulted in a marked transient increase in uptake and binding of [11C]nicotine in frontal and temporal cortex and a persistent increase in cortical blood flow as measured by PET as well as progressive decreases of slow wave EEG activity. After one month of NGF infusion, tests of verbal episodic memory were improved whereas other cognitive tests were not. No adverse effects of the NGF infusion were found. The results of this single case indicate that NGF may counteract cholinergic deficits in AD, and suggest that further clinical trials of NGF infusion in AD are warranted.

NGF can induce a 'young' pattern of reinnervation in transplanted cerebral blood vessels from ageing rats

Peripheral target tissues can determine age-related changes in their density and pattern of innervation. We have shown previously that middle cerebral arteries from young and old rats transplanted in oculo in young hosts become reinnervated with a density and pattern of innervation that is typical of the age of the donor, i.e., the density of reinnervation on old transplants is 50% lower than on young transplants. The alterations in the target tissues responsible for their decreased innervation in old age are still unknown. We have investigated the possibility that increasing the availability of nerve growth factor (NGF) might restore the pattern and density of perivascular nerves on old blood vessels to levels of innervation typical of young tissues. Old middle cerebral transplants were therefore treated with NGF or vehicle by three weekly transscleral injections. NGF treatment markedly increased the reinnervation of old transplants, restoring the density and pattern of innervation to one characteristic of young animals. NGF produced an equivalent increase in nerve growth on young and old transplants, thus confirming that the receptivity of old blood vessels to reinnervation is not impaired. Control experiments were performed by treating transplants with saline, bovine serum albumin, or cytochrome c. Unexpectedly, bovine serum albumin was shown to promote axonal growth, although to a lesser extent and with a different pattern than NGF.

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.

Distribution of [125I]nerve growth factor in the rat brain following a single intraventricular injection: correlation with the topographical distribution of trkA messenger RNA-expressing cells

The present study determined the topographical distribution of [125I] nerve growth factor in rat brain at various time points following an intraventricular injection. In addition, we quantified the tissue content of nerve growth factor in various brain tissues following the injection. Autoradiographic analysis of the distribution of [125] nerve growth factor indicated that the neurotrophin is rapidly distributed within the entire ventricular system. However, penetration of nerve growth factor into the brain parenchyma was very limited. At early time points following an injection of nerve growth factor, there was an accumulation of label in the immediate vicinity of the lateral ventricle and third ventricle with predominant labeling around the septum, hypothalamus and cerebellum. By 24 h following nerve growth factor administration, there was discreet labeling of the lateral septum, medial septum, diagonal band, hypothalamus, olfactory tubercle and nucleus of the olfactory tract, and some label was present in the hippocampus and subiculum. Quantitative ELISA of nerve growth factor in brain tissues 1 h following the injection indicated a 446% and 133% increase over basal levels of nerve growth factor in the basal forebrain and hippocampus, respectively. At 24 h nerve growth factor levels measured in brain were not significantly different from endogenous basal levels as determined by ELISA, whereas there were high quantities of 125I present in the thyroid gland, suggesting that the administered [125I] nerve growth factor was rapidly degraded following the intraventricular injection. We observed a similar labeling pattern of the medial septum/diagonal band cholinergic cell body group 24 h following either an intraventricular or intrahippocampal injection of [125I] nerve growth factor. There was a good correlation between the [125I] nerve growth factor labeling pattern and the presence of trkA messenger RNA. This suggested that, at least in the septohippocampal pathway, nerve growth factor accumulated in a region which contained trkA nerve growth factor receptors. Thus, this study shows that after a single unilateral intraventricular injection of nerve growth factor into rat brain there is effective uptake by diagonal band/septal cells on both sides of the brain, and by cells whose positions correlate with the locations of cholinergic and trk A messenger RNA-expressing cells. Significant uptake was also observed in the hypothalamus and cerebellum. The very limited penetration and rapid degradation of intraventricularly administered nerve growth factor suggests that tissue penetration may be a limiting factor when attempting to influence brain neurons by exogenous neurotropic factors.

Regulation of brain-derived neurotrophic factor messenger RNA and protein at the cellular level in pentylenetetrazol-induced epileptic seizures

We have examined the effects of pentylenetetrazol-induced epileptic seizures on brain-derived neurotrophic factor messenger RNA and protein and on the messenger RNA of its receptor in the rat. Pentylenetrazol, which acts at the picrotoxin recognition site of the GABAA receptor, was injected intraperitoneally and induced seizures by decreasing the inhibitory GABAergic activity. The effects of a single acute convulsive dose (50 mg/kg) of pentylenetetrazol were analysed at different time points by in situ hybridization or immunohistochemistry. Kindling was induced by daily subconvulsive injections (30 mg/kg) of pentylenetetrazol. At different time points during the kindling process, the messenger RNAs of brain-derived neurotrophic factor and trkB and the protein levels of brain-derived neurotrophic factor were analysed. We showed that brain-derived neurotrophic factor messenger RNA dramatically increased in neurons of the granule cell layer, piriform cortex and amygdala 3 h but not 6 h after an acute high dose of pentylenetetrazol, while brain-derived neurotrophic factor-like immunoreactivity was decreased in the granule cell layer and neurons of the hilus. The trkB messenger RNA was similarly increased 3 h and 6 h after the injection and returned to control levels after 24 h. The first change during the kindling development was seen after the first severe seizure: brain-derived neurotrophic factor messenger RNA was markedly increased in the piriform cortex and amygdala but not in the hippocampus. In fully kindled rats, which had several severe seizures, brain-derived neurotrophic factor messenger RNA and trkB messenger RNA were unaffected 3 h and 24 h after the last pentylenetetrazol injection. However, brain-derived neurotrophic factor-like immunoreactivity was markedly increased in the hippocampal formation 3 h, 24 h and three days after the last pentylenetetrazol injection, and still increased after 10 days. These results suggest that brain-derived neurotrophic factor may be involved in protection mechanisms after damage during seizures and in sprouting responses. The piriform cortex/amygdala seems to be an area of origin for the kindling development.

Nerve growth factor-induced sprouting of mature, uninjured sympathetic axons

The infusion of nerve growth factor (NGF) into the lateral ventricle of the mature rat brain elicits a sprouting response from axons associated with the intradural segment of the internal carotid artery. Using electron microscopic techniques, we observed a three-fold increase in the total number of perivascular axons. This NGF-elicited response is characterized by a dramatic reduction in glial cell ensheathment similar to that observed during development and by the presence of profiles devoid of organelles that may represent newly formed sprouts. In spite of the increase in axon number, no significant changes in the percentage of small, medium, or large axons were observed. The three-fold increase in the total number of axons was accompanied by an increase in the number of axons/fascicle but no change in the number of fascicles. This, along with the observation that a majority of sprouted axons were associated with other axons, supports the idea that the sprouted axons tend to associate preferentially with other axons. Bilateral superior cervical ganglionectomies following cytochrome C infusion indicate that approximately 60% of the axons associated with the internal carotid artery arise from the superior cervical ganglion and that the majority of axons contacting the smooth muscle layer arise from this ganglion. Sympathectomy following NGF infusion resulted in a 79% reduction in the total number of perivascular axons, demonstrating overwhelmingly that the majority of sprouted axons are sympathetic fibers. These results demonstrate that infusion of NGF into the mature rat brain results in the preferential sprouting of sympathetic axons associated with the internal carotid artery. These findings are consistent with the hypothesis that NGF normally plays a role in the regulation of autonomic cerebrovascular innervation in the adult animal and that mature, uninjured sympathetic neurons remain responsive to NGF.

Nerve growth factor and septal grafts: a study of behavioral recovery following partial damage to the septum in rats

Previous studies have produced conflicting results about the effects of intracerebral injection of NGF after septal damage in rats: in one experiment, behavioral deficits in maze tasks were exacerbated by NGF administration whereas they were alleviated in another one. The present investigation aimed to clarify the effects of NGF and to identify factors liable to induce different behavioral outcomes. Behavioral effects were assessed following a postsurgical delay of five months using various parameters: food consumption in a novel environment, spontaneous activity, locomotion in an open-field, immobility in a tail suspension test, spontaneous alternation in a T-maze and performance in a radial eight-arm maze. Possible influence of intrahippocampal sympathetic fiber ingrowth occurring after septal lesions was ruled out, as the comparison of rats subjected to superior cervical ganglia removal with their lesion-control counterparts showed few behavioral differences, even after NGF administration. All lesioned rats showed reduced adaptability in most of these tests. Grafts partially reversed the lesion-induced deficit in spontaneous alternation. A single intracerebral NGF injection was found to ameliorate radial maze performance, whether rats were grafted or not. However, it appeared that the number of strategies available to NGF-rats in the radial maze task was as limited as for lesion-control rats. These findings suggest that NGF-rats do not recover spatial abilities lost after septal lesions, but are able to make more efficient use of remaining capacities to master the maze task.

Nerve growth factor affects 11C-nicotine binding, blood flow, EEG, and verbal episodic memory in an Alzheimer patient (case report)

Based on animal research suggesting that nerve growth factor (NGF) can stimulate central cholinergic neurons, the known losses of cholinergic innervation of the cortices in Alzheimer's disease (AD), and our experience of infusing NGF to support adrenal grafts in parkinsonian patients, we have initiated clinical trials of NGF infusions into the brain of patients with AD. Here we report a follow-up of our first case, a 69-year-old woman, with symptoms of dementia since 8 years. Intraventricular infusion of 6.6 mg NGF during three months resulted in a marked transient increase in uptake and binding of 11C-nicotine in frontal and temporal cortex and a persistent increase in cortical blood flow as measured by PET as well as progressive decreases of slow wave EEG activity. After one month of NGF, tests of verbal episodic memory were improved whereas other cognitive tests were not. No adverse effects could be ascribed to the NGF infusion. Taken together, the results of this case study indicate that NGF may counteract cholinergic deficits in AD, and suggest that further clinical trials of NGF infusion in AD are warranted.

Neurotrophic factors in regeneration

Nerve growth factor, fibroblast growth factor, and ciliary neurotrophic factor can protect selected populations of neurons from some of the degenerative changes that otherwise follow axonal injury or other insults. The function of diffusible neurotrophic factors in axonal regeneration is still unclear, however. Knowledge of the nerve growth factor congeners, brain-derived neurotrophic factor and neurotrophin-3, is advancing rapidly as is the identification of neurotrophin receptors, several of which are membrane-bound tyrosine kinases.