Encapsulated cell biodelivery of nerve growth factor to the Basal forebrain in patients with Alzheimer's disease.

BACKGROUND/AIMS: Degeneration of cholinergic neurons in the basal forebrain correlates with cognitive decline in patients with Alzheimer's disease (AD). Targeted delivery of exogenous nerve growth factor (NGF) has emerged as a potential AD therapy due to its regenerative effects on the basal forebrain cholinergic neurons in AD animal models. Here we report the results of a first-in-man study of encapsulated cell (EC) biodelivery of NGF to the basal forebrain of AD patients with the primary objective to explore safety and tolerability. METHODS: This was an open-label, 12-month study in 6 AD patients. Patients were implanted stereotactically with EC-NGF biodelivery devices targeting the basal forebrain. Patients were monitored with respect to safety, tolerability, disease progression and implant functionality. RESULTS: All patients were implanted successfully with bilateral single or double implants without complications or signs of toxicity. No adverse events were related to NGF or the device. All patients completed the study, including removal of implants at 12 months. Positive findings in cognition, EEG and nicotinic receptor binding in 2 of 6 patients were detected. CONCLUSIONS: This study demonstrates that surgical implantation and removal of EC-NGF biodelivery to the basal forebrain in AD patients is safe, well tolerated and feasible.

Safety, Tolerability, Pharmacokinetics, and Food Effects on TAC-302 in Healthy Participants: Randomized, Double-Blind, Placebo-Controlled, Single-Dose and Multiple-Dose Studies.

TAC-302 stimulates neurite outgrowth activity and is expected to restore urinary function in patients with lower urinary tract dysfunction. We conducted 2 phase 1, randomized, placebo-controlled studies to confirm the safety and pharmacokinetics (PK) of TAC-302 in healthy adult Japanese male volunteers. In the first-in-human single-dose study (n = 60), TAC-302 was administered at doses from 100 to 1200 mg after an overnight fast. The effects of a meal on the PK of TAC-302 400 mg were also examined. A multiple-dose study (n = 36) evaluated the effects of meal fat content on the PK of single doses of TAC-302 (100, 200, or 400 mg) and multiple doses of TAC-302 administered for 5 days (100, 200, and 400 mg twice daily). TAC-302 showed linear PK up to doses of 1200 mg in the fasting state, and across the dose range of 100-400 mg in the fed state. No accumulation of TAC-302 was observed. Food, particularly with high fat content, increased TAC-302 plasma concentrations. No differences were observed in the adverse event incidence between the TAC-302 and placebo groups in either study. TAC-302 showed a wide safety margin.

Blood-brain barrier penetration and in vivo activity of an NGF conjugate.

Nerve growth factor (NGF) is essential for the survival of both peripheral ganglion cells and central cholinergic neurons of the basal forebrain. The accelerated loss of central cholinergic neurons during Alzheimer's disease may be a determinant of dementia in these patients and may therefore suggest a therapeutic role for NGF. However, NGF does not significantly penetrate the blood-brain barrier, which makes its clinical utility dependent on invasive neurosurgical procedures. When conjugated to an antibody to the transferrin receptor, however, NGF crossed the blood-brain barrier after peripheral injection. This conjugated NGF increased the survival of both cholinergic and noncholinergic neurons of the medial septal nucleus that had been transplanted into the anterior chamber of the rat eye. This approach may prove useful for the treatment of Alzheimer's disease and other neurological disorders that are amenable to treatment by proteins that do not readily cross the blood-brain barrier.

Pharmacokinetics and tissue distribution of neurotrophin 3 after intracochlear delivery.

Neurotrophin therapy has potential to reverse some forms of hearing loss. However, cochlear pharmacokinetic studies are challenging due to small fluid volumes. Here a radioactive tracer was used to determine neurotrophin-3 retention, distribution and clearance after intracochlear administration. 125I-neurotrophin-3 was injected into guinea pig cochleae using a sealed injection technique comparing dosing volumes, rates and concentrations up to 750 µg/mL. Retention was measured by whole-cochlear gamma counts at five time points while distribution and clearance were assessed by autoradiography. Smaller injection volumes and higher concentrations correlated with higher retention of neurotrophin-3. Distribution of neurotrophin-3 was widespread throughout the cochlear tissue, decreasing in concentration from base to apex. Tissue distribution was non-uniform, with greatest density in cells lining the scala tympani and lower density in neural target tissue. The time constant for clearance of neurotrophin-3 from cochlear tissues was 38 h but neurotrophin-3 remained detectable for at least 2 weeks. Neurotrophin-3 was evident in the semi-circular canals with minor spread to the contralateral cochlea. This study is the first comprehensive evaluation of the disposition profile for a protein therapy in the cochlea. The findings and methods in this study will provide valuable guidance for the development of protein therapies for hearing loss.

NGF receptor-mediated reduction in axonal NGF uptake and retrograde transport following sciatic nerve injury and during regeneration.

Injury to the rat sciatic nerve leads to the induction of nerve growth factor (NGF) receptors on the denervated Schwann cells and their disappearance on the regenerating axons of the axotomized, normally NGF-sensitive sensory and sympathetic neurons. This disappearance in the axonal expression and retrograde transport of NGF receptors is associated with a similarly dramatic reduction in the axonal uptake and retrograde transport of NGF following axotomy and during regeneration. In view of the massive NGF synthesis occurring in the injured nerve, these results suggest that, while sensory and sympathetic neurons are the primary targets of NGF in the normal peripheral nervous system, the denervated Schwann cells may become its primary target in the aftermath of nerve injury.

Controlled nerve growth factor release from multi-ply alginate/chitosan-based nerve conduits.

The delivery kinetics of growth factors has been suggested to play an important role in the regeneration of peripheral nerves following axotomy. In this context, we designed a nerve conduit (NC) with adjustable release kinetics of nerve growth factor (NGF). A multi-ply system was designed where NC consisting of a polyelectrolyte alginate/chitosan complex was coated with layers of poly(lactide-co-glycolide) (PLGA) to control the release of embedded NGF. Prior to assessing the in vitro NGF release from NC, various release test media, with and without stabilizers for NGF, were evaluated to ensure adequate quantification of NGF by ELISA. Citrate (pH 5.0) and acetate (pH 5.5) buffered saline solutions containing 0.05% Tween 20 yielded the most reliable results for ELISA active NGF. The in vitro release experiments revealed that the best results in terms of reproducibility and release control were achieved when the NGF was embedded between two PLGA layers and the ends of the NC tightly sealed by the PLGA coatings. The release kinetics could be efficiently adjusted by accommodating NGF at different radial locations within the NC. A sustained release of bioactive NGF in the low nanogram per day range was obtained for at least 15days. In conclusion, the developed multi-ply NGF loaded NC is considered a suitable candidate for future implantation studies to gain insight into the relationship between local growth factor availability and nerve regeneration.

Neurotrophins: from enthusiastic expectations through sobering experiences to rational therapeutic approaches.

Despite high enthusiasm, early attempts to develop clinical treatments based on animal research with neurotrophins were not successful. Here we survey clinical trials with neurotrophins, compared with neurotrophic factors of other gene families, and delineate the most likely reasons for their failure. We then suggest improved methods for regulated local supply of NTs to specific populations of neurons and discuss future therapeutic procedures evolving from the more detailed knowledge of the signal transduction pathways activated by neurotrophins via their receptors.

Intracerebral infusion of neurotrophic factors.

Neurotrophic factors are among the most potent neuroprotective and neuroregenerative agents known. However, they cross the adult mammalian blood-brain barrier very poorly and can have serious peripheral side effects. These problems can be solved by using chronic infusions with small pumps to directly deliver known quantities of these proteins into selected regions of the brains of small experimental animals such as rats and mice. The method consists of commercially available Alzet osmotic pumps that are placed under the skin and are connected to commercially available metal infusion cannulas whose tip can be stereotactically placed in virtually any location of the brain. Different models of pumps that fit comfortably in rodents can be selected for infusion between 1 and 28 days and at infusion rates ranging between 8 and 0.25 microL/h, respectively. Methodological details are provided for the successful use of proteins and to minimize the time of the surgery.

Stimulation of neurite outgrowth by neurotrophins delivered from degradable hydrogels.

Degradable hydrogels are useful vehicles for the delivery of growth factors to promote the regeneration of diseased or damaged tissue. In the central nervous system, there are many instances where the delivery of neurotrophins has great potential in tissue repair, especially for treatment of spinal cord injury. In this work, hydrogels based on poly(ethylene glycol) that form via a photoinitiated polymerization were investigated for the delivery of neurotrophins. The release kinetics of these factors are controlled by changes in the network crosslinking density, which influences neurotrophin diffusion and subsequent release from the gels with total release times ranging from weeks to several months. The release and activity of one neurotrophic factor, ciliary-neurotrophic factor (CNTF), was assessed with a cell-based proliferation assay and an assay for neurite outgrowth from retinal explants. CNTF released from a degradable hydrogel above an explanted retina was able to stimulate outgrowth of a significantly higher number of neurites than controls without CNTF. Finally, unique microsphere/hydrogel composites were developed to simultaneously deliver multiple neurotrophins with individual release rates.

Pharmacokinetics and biodistribution of a small radioiodine labeled nerve growth factor fragment.

Nerve growth factor (NGF) exerts various actions on neuronal and non-neuronal tissues and has potential therapeutic utility, but difficulties in using the whole protein have stimulated interest in small NGF fragments. We radioiodinated a small cyclic peptide derived from NGF using the Bolton-Hunter method [125I-C(92-96)], and confirmed binding to high affinity NGF receptors by cross-linkage analysis. Pharmacokinetic characteristics in intravenously injected mice were T 1/2 alpha 5.2 min, T 1/2beta 121.3 min, clearance 11.8+/-0.5 ml/min, and volume of distribution 69.7+/-4.6 ml. Dose-proportionate increases in areas-under-curve and peak-concentrations indicated linear pharmacokinetics. Biodistribution data revealed that clinically relevant doses allowed C(92-96) accumulation sufficient to elicit biological responses in receptor expressing organs including the lungs, liver, spleen, and pancreas.

Ligand-induced internalization of the p75 neurotrophin receptor: a slow route to the signaling endosome.

The nerve growth factor (NGF) family of neurotrophins binds two classes of cell-surface receptors, trk receptor tyrosine kinases and the shared p75 receptor. Rapid internalization and retrograde trafficking of neurotrophin-trk complexes have been demonstrated in a number of systems and are thought to transmit trophic signals from terminals to neuronal cell bodies. In contrast, the internalization and trafficking of neurotrophin-p75 complexes are not well understood. In this study, we used biotinylated NGF and a fluorescent-labeled anti-p75 antibody to follow the kinetics and route of ligand-induced internalization of the p75 receptor in cycling and differentiated PC12 cells. Binding of neurotrophins to p75 induced internalization at a rate approximately three times slower than that of transferrin and NGF-TrkA complexes in the same cells. The ligand-p75 complex was internalized via clathrin-coated pits into early endosomes and eventually accumulated in recycling endosomes in the cell body and vesicles colabeled by the cholera toxin B-subunit in the growth cones. Both internalized ligand and p75 were protected from proteolytic degradation and accumulated in vesicles that did not undergo acidification. Finally, NGF induced endosomal association of p75 and its MAGE interactors, necdin and NRAGE. These data suggest that signaling endosomes containing activated p75 are involved in neurotrophin signaling, and that such endosomes may be temporally and spatially distinct from those containing trk receptors.

GDNF delivery using human neural progenitor cells in a rat model of ALS.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive loss of spinal cord, brainstem, and cortical motor neurons. In a minority of patients, the disease is caused by mutations in the copper (2+)/zinc (2+) superoxide dismutase 1 (SOD1) gene. Recent evidence suggests that astrocytes are dysfunctional in ALS and may be a critical link in the support of motor neuron health. Furthermore, growth factors, such as glial cell line-derived neurotrophic factor (GDNF), have a high affinity for motor neurons and can prevent their death following various insults, but due to the protein's large size are difficult to directly administer to brain. In this study, human neural progenitor cells (hNPC) isolated from the cortex were expanded in culture and modified using lentivirus to secrete GDNF (hNPC(GDNF)). These cells survived up to 11 weeks following transplantation into the lumbar spinal cord of rats overexpressing the G93A SOD1 mutation (SOD1 (G93A)). Cellular integration into both gray and white matter was observed without adverse behavioral effects. All transplants secreted GDNF within the region of cell survival, but not outside this area. Fibers were seen to upregulate cholinergic markers in response to GDNF, indicating it was physiologically active. We conclude that genetically modified hNPC can survive, integrate, and release GDNF in the spinal cord of SOD1 (G93A) rats. As such, they provide an interesting source of cells for both glial replacement and trophic factor delivery in future human clinical studies.

Transport of nerve growth factor encapsulated into liposomes across the blood-brain barrier: in vitro and in vivo studies.

A nerve growth factor (NGF) was encapsulated into liposomes in order to protect it from the enzyme degradation in vivo and promote it permeability across the blood-brain barrier (BBB). RMP-7, a ligand to the B2 receptor on brain microvascular endothelial cells (BMVEC), was combined with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-n-[poly(ethylenegly-col)]-hydroxy succinamide (DSPE-PEG-NHS) to obtain DSPE-PEG-RMP-7. Then DSPE-PEG-RMP-7 was incorporated into the liposomes' surface to target sterically stabilized liposomes (SSL-T) to the brain. The highest percent of NGF encapsulated into liposomes was about 34%, and the average size of liposomes was below 100 nm. A primary model of BBB was established and evaluated by morphological, permeability, and transendothelial electrical resistance (TEER). The BBB model was employed to study the permeability of NGF liposomes in vitro. The results indicated that the liposomes could enhance transport of NGF across the BBB. The best transport rate was received with NGF-SSL-T. The brain distribution of NGF liposomes was studied in vivo, the amount of NGF in the brain was increased in the order: NGF-SSL-T>NGF-SSL+RMP-7>NGF-SSL>NGF-L. The maximum concentration of NGF was recorded in 30 min following the intravenous injection. In particular, a majority of NGF was distributed in striatum, hippocampus and cortex, and the concentration of NGF was relatively lower in olfactory bulb, cerebellum and brain stem. There was a close relationship between P(e) (permeability coefficient on in vitro BBB model) and T(e) (brain targeted coefficient in vivo) for NGF encapsulated into the liposomes.

Targeting delivery of liposomes with conjugated p-aminophenyl-α-d-manno-pyranoside and apolipoprotein E for inhibiting neuronal degeneration insulted with ß-amyloid peptide.

Liposomes with conjugated p-aminophenyl-α-d-manno-pyranoside (APMP) and apolipoprotein E (ApoE) (APMP-ApoE-liposomes) were employed to carry neuron growth factor (NGF) across the blood-brain barrier (BBB) and enhance the survival of degenerated neurons. APMP-ApoE-liposomes were used to deliver NGF across a monolayer of human brain-microvascular endothelial cells (HBMECs) regulated by human astrocytes (HAs) for rescuing SK-N-MC cells from an insult of ß-amyloid peptide 1-42 (Aß1-42). An increase in the APMP concentration enhanced the particle size, HBMEC and HA viability, permeability for propidium iodide (PI), and permeability for NGF, however, reduced the absolute value of zeta potential, APMP conjugation efficiency and transendothelial electrical resistance (TEER). In addition, an increase in the ApoE concentration increased the particle size, absolute value of zeta potential, HBMEC and HA viability, permeability for PI, permeability for NGF and SK-N-MC cell viability, however, decreased the ApoE conjugation efficiency and TEER. APMP and ApoE on liposomes can be promising surface moieties to carry NGF across the BBB, target degenerated neurons and inhibit Aß1-42-induced neurotoxicity in Alzheimer's disease.

Intravenous TAT-GDNF is protective after focal cerebral ischemia in mice.

BACKGROUND AND PURPOSE: Delivery of therapeutic proteins into tissues and across the blood-brain barrier is severely limited by their size and biochemical properties. The 11-amino acid human immunodeficiency virus TAT protein transduction domain is able to cross cell membranes and the blood-brain barrier, even when coupled with larger peptides. The present studies were done to evaluate whether TAT-glial line-derived neurotrophic factor (GDNF) fusion protein is protective in focal cerebral ischemia. METHODS: Anesthetized male C57BL/6j mice were submitted to intraluminal thread occlusion of the middle cerebral artery. Reperfusion was initiated 30 minutes later by thread retraction. Laser Doppler flow was monitored during the experiments. TAT-GDNF, TAT-GFP (0.6 nmol each), or vehicle was intravenously applied over 10 minutes immediately after reperfusion. After 3 days (30 minutes of ischemia), animals were reanesthetized and decapitated. Brain injury was evaluated by histochemical stainings. RESULTS: Immunocytochemical experiments confirmed the presence of TAT-GDNF protein in the brains of fusion protein-treated nonischemic control animals 3 to 4 hours after TAT fusion protein delivery. TAT-GDNF significantly reduced the number of caspase-3-immunoreactive and DNA-fragmented cells and increased the number of viable neurons in the striatum, where disseminated tissue injury was observed, compared with TAT-GFP- or vehicle-treated animals. CONCLUSIONS: Our results demonstrate that TAT fusion proteins are powerful tools for the treatment of focal ischemia when delivered both before and after an ischemic insult. This approach may be of clinical interest because such fusion proteins can be intravenously applied and reach the ischemic brain regions. This approach may therefore offer new perspectives for future strategies in stroke therapy.

Axonal transport of neurotrophins by visceral afferent and efferent neurons of the vagus nerve of the rat.

The receptor-mediated axonal transport of [125I]-labeled neurotrophins by afferent and efferent neurons of the vagus nerve was determined to predict the responsiveness of these neurons to neurotrophins in vivo. [125I]-labeled neurotrophins were administered to the proximal stump of the transected cervical vagus nerve of adult rats. Vagal afferent neurons retrogradely transported [125I]neurotrophin-3 (NT-3), [125I]nerve growth factor (NGF), and [125I]neurotrophin-4 (NT-4) to perikarya in the ipsilateral nodose ganglion, and transganglionically transported [125I]NT-3, [125I]NGF, and [125I]NT-4 to the central terminal field, the nucleus tractus solitarius (NTS). Vagal afferent neurons showed minimal accumulation of [125I]brain-derived neurotrophic factor (BDNF). In contrast, efferent (parasympathetic and motor) neurons located in the dorsal motor nucleus of the vagus and nucleus ambiguus retrogradely transported [125I]BDNF, [125I]NT-3, and [125I]NT-4, but not [125I]NGF. The receptor specificity of neurotrophin transport was examined by applying [125I]-labeled neurotrophins with an excess of unlabeled neurotrophins. The retrograde transport of [125I]NT-3 to the nodose ganglion was reduced by NT-3 and by NGF, and the transport of [125I]NGF was reduced only by NGF, whereas the transport of [125I]NT-4 was significantly reduced by each of the neurotrophins. The competition profiles for the transport of NT-3 and NGF are consistent with the presence of TrkA and TrkC and the absence of TrkB in the nodose ganglion, whereas the profile for NT-4 suggests a p75 receptor-mediated transport mechanism. The transport profiles of neurotrophins by efferent vagal neurons in the dorsal motor nucleus of the vagus and nucleus ambiguus are consistent with the presence of TrkB and TrkC, but not TrkA, in these nuclei. These observations describe the unique receptor-mediated axonal transport of neurotrophins in adult vagal afferent and efferent neurons and thus serve as a template to discern the role of specific neurotrophins in the functions of these visceral sensory and motor neurons in vivo.

Intraputamenal infusion of GDNF in aged rhesus monkeys: distribution and dopaminergic effects.

Site-specific delivery of trophic factors in the brain may be important for achieving therapeutic efficacy without unwanted side effects. This study evaluated the site-specific infusion of glial cell line-derived neurotrophic factor (GDNF) into the right putamen of aged rhesus monkeys. After 4 weeks of continuous infusion at a rate of 22.5 microg/day, GDNF had diffused up to 11 mm from the catheter openings in the putamen into the rostral putamen, internal capsule, external capsule, caudate nucleus, and globus pallidus. Anisotropic flow along the external capsule tracts carried GDNF into the anterior amygdaloid area. Backflow of GDNF along the catheter track from the frontal cortex infiltrated juxtaposed corpus callosal and cortical tissue. GDNF was carried by retrograde transport to dopamine neurons in the ipsilateral substantia nigra, stimulating an 18% increase in the number of tyrosine hydroxylase (TH)-positive dopamine neurons and a 28% increase in dopamine neuron perikaryal size. Also, TH-positive fiber density was increased in the ipsilateral globus pallidus, caudate nucleus, and putamen. Anatomic effects from GDNF stimulation of the dopaminergic system were restricted to the ipsilateral hemisphere. Retrograde GDNF labeling was also present in a few TH-positive neurons in the locus coeruleus and a large cluster of TH-negative neurons in the ventral anterior thalamus. Anterograde transport of GDNF was evident in axons in the pyramidal tract from the cerebral peduncle to the caudal spinal cord. Tissue injury from the intraparenchymal catheter and continuous infusion was confined primarily to a narrow zone surrounding the track and was mild to moderate in severity.

A non-invasive system for delivering neural growth factors across the blood-brain barrier: a review.

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, as well as protect these neurons against a variety of perturbations. Since neurotrophins do not pass the blood-brain barrier (BBB) in significant amounts, a non-invasive delivery system for this group of therapeutic molecules needs to be developed. We have utilized a carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. The biological activity of this carrier system was tested using in vitro bioassays and intraocular transplants; we were able to demonstrate that cholinergic markers in both developing and aged intraocular septal grafts were enhanced by intravenous delivery of the OX-26-NGF conjugate. In subsequent experiments, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate for 6 weeks, resulting in a significant improvement in spatial learning in previously impaired rats, but disrupting 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 as well as an upregulation of both low and high affinity NGF receptors in the medial septal region of rats initially impaired in spatial learning. Finally, OX-26-NGF was able to protect striatal cholinergic neurons against excitotoxicity and basal forebrain cholinergic neurons from degeneration associated with chemically-induced loss of target neurons. These results indicate the potential utility of the transferrin receptor antibody delivery system for treatment of neurodegenerative disorders with neurotrophic substances.

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.

Reduced transport of [125I]nerve growth factor by cholinergic neurons and down-regulated TrkA expression in the medial septum of aged rats.

Basal forebrain cholinergic neurons atrophy and degenerate in aging and Alzheimer's disease for unknown reasons. In this study, aged male Sprague-Dawley rats (26-30 months old) showed a significant 31% reduction in the number of septal cholinergic neurons which take up and retrogradely transport 125I-labelled nerve growth factor injected into their target hippocampus, as compared with young adult rats (three to six months old). In aged rats, cholinergic neurons not transporting nerve growth factor were severely atrophied and had a significant 60% reduction in mean cross-sectional area as compared with [125I]nerve growth factor transporting neurons. These changes were accompanied by a significant 43% decline in relative levels of messenger RNA encoding the high affinity nerve growth factor receptor TrkA, in the septal region of aged rats. There was no difference between young and aged rats in messenger RNA levels encoding the low affinity nerve growth factor receptor, p75NGFR. These findings suggest that aged basal forebrain cholinergic neurons exhibit a reduced capacity to sustain receptor mediated uptake and retrograde transport of target-derived neurotrophin. This reduced capacity is associated with severe neuronal atrophy and may contribute to the pronounced vulnerability of these neurons to degeneration in aging and Alzheimer's disease.