Quantitative demonstration of the retrograde axonal transport of endogenous nerve growth factor

Rapid retrograde tyrosine phosphorylation of trkA and other proteins in rat sympathetic neurons in compartmented cultures

According to the current theory of retrograde signaling, NGF binds to receptors on the axon terminals and is internalized by receptor-mediated endocytosis. Vesicles with NGF in their lumina, activating receptors in their membranes, travel to the cell bodies and initiate signaling cascades that reach the nucleus. This theory predicts that the retrograde appearance of activated signaling molecules in the cell bodies should coincide with the retrograde appearance of the NGF that initiated the signals. However, we observed that NGF applied locally to distal axons of rat sympathetic neurons in compartmented cultures produced increased tyrosine phosphorylation of trkA in cell bodies/ proximal axons within 1 min. Other proximal proteins, including several apparently localized in cell bodies, displayed increased tyrosine phosphorylation within 5-15 min. However, no detectable 125I-NGF appeared in the cell bodies/proximal axons within 30-60 min of its addition to distal axons. Even if a small, undetectable fraction of transported 125I-NGF was internalized and loaded onto the retrograde transport system immediately after NGF application, at least 3-6 min would be required for the NGF that binds to receptors on distal axons just outside the barrier to be transported to the proximal axons just inside the barrier. Moreover, it is unlikely that the tiny fraction of distal axon trk receptors located near the barrier alone could produce a measurable retrograde trk phosphorylation even if enough time was allowed for internalization and transport of these receptors. Thus, our results provide strong evidence that NGF-induced retrograde signals precede the arrival of endocytotic vesicles containing the NGF that induced them. We further suggest that at least some components of the retrograde signal are carried by a propagation mechanism.

Islet amyloid polypeptide and calcitonin gene-related peptide expression are down-regulated in dorsal root ganglia upon sciatic nerve transection

Islet amyloid polypeptide (IAPP) is structurally related to calcitonin gene-related peptide (CGRP) and has been implicated in glucose homeostasis and diabetes pathogenesis because it is expressed in insulin cells and forms amyloid in pancreatic islets from type II diabetic patients. IAPP is also constitutively co-expressed with CGRP in rat sensory neurons. Whether expression of IAPP is altered by nerve injury with or without regeneration was investigated in adult rats subjected to unilateral sciatic axotomy; IAPP and CGRP expression were determined by quantitative in situ hybridization and immunocytochemistry at days 3, 10 and 30 after axotomy. In ipsilateral L4-L5 dorsal root ganglia (DRG), the percentages of nerve cell profiles labelled for IAPP and CGRP mRNA were reduced at all time points studied. IAPP and CGRP mRNA expression were lower in nerve cell profiles in ipsilateral DRGs compared to the contralateral side after axotomy alone whereas epineurial nerve suture maintained or restored IAPP and CGRP expression. The numbers of IAPP- and CGRP-immunoreactive DRG nerve cell profiles and dorsal horn fibers were reduced on the ipsilateral side at all time points. Thus, IAPP and CGRP expression are down-regulated upon axotomy. Nerve repair maintains or restores IAPP and CGRP expression in individual neurons but does not prevent the loss of CGRP/IAPP phenotype of some of these neurons in response to axotomy.

Retrograde transport and steady-state distribution of 125I-nerve growth factor in rat sympathetic neurons in compartmented cultures

We have used compartmented cultures of rat sympathetic neurons to quantitatively examine the retrograde transport of 125I-nerve growth factor (NGF) supplied to distal axons and to characterize the cellular events that maintain steady-state levels of NGF in cell bodies. In cultures allowed to reach steady-state 125I-NGF transport, cell bodies contained only 5-30% of the total neuron-associated 125I-NGF, whereas 70-95% remained associated with the distal axons. This was true over an 8 pM to 1.5 nM 125I-NGF concentration range, indicating that saturation of high affinity receptors could not account for the large fraction of 125I-NGF remaining in axons. Dissociation assays indicated that 85% of 125I-NGF associated with distal axons was surface-bound. At steady-state, only 2-25% of the distal axon-associated 125I-NGF was retrogradely transported each hour, with higher transport rates associated with younger cultures and lower 125I-NGF concentrations. The velocity of 125I-NGF retrograde transport was estimated at 10-20 mm/hr. However, as in a previous report, almost no 125I-NGF transport was observed during the first hour after 125I-NGF administration, indicating a significant lag between receptor binding and loading onto the retrograde transport system. During 125I-NGF transport through axons spanning an intermediate compartment in five-compartment cultures, little or no 125I-NGF was degraded or released from the axons. After transport, 125I-NGF was degraded with a half-life of 3 hr. In summary, although some cellular events promoted NGF accumulation in cell bodies, distal axons represented by far the principal site of NGF-receptor interaction at steady-state as a result of a low retrograde transport rate.

Tissue-specific and ontogenetic regulation of LIF protein levels determined by quantitative enzyme immunoassay

To define the physiological role of leukemia inhibitory factor (LIF), it is essential to localize sites of LIF synthesis in vivo. We generated polyclonal antibodies specific for native rat LIF, and developed a two-site immunoassay to detect 10 pg LIF/ml. Using this immunoassay, we determined LIF content of 18 organs, CNS regions, and ganglia throughout postnatal development of rats. High levels of LIF protein (1.0-11.0 ng/g tissue) are present in relatively few tissues: the uterus at late proestrus to estrus and on day 5 of pregnancy, ovary at estrus to early metestrus-1, footpads during early postnatal development and thymus throughout. Intermediate levels (0.5-1.0 ng) are detected in the gut, skin, skeletal muscle, pancreas and lung at one or more postnatal ages. Low levels (0.1-0.5 ng) are observed in most other non-nervous and nervous tissues. LIF protein levels do not completely correspond to reported LIF mRNA levels.

Detection of increased tissue concentrations of nerve growth factor with an improved extraction procedure

Nerve growth factor (NGF) is a protein essential for the survival and normal function of sympathetic neurons. Two-site immunoassays have been developed over the past decade in several laboratories and used to estimate its endogenous concentrations in a variety of effector tissues. However, levels appear restricted to a narrow range, display only a poor correlation with innervation density, and show obvious inter- and intralaboratory variations, the origins of which are unclear. This led us to examine alternative extraction procedures for NGF before quantification. In particular, we have found treatment of tissue extracts with high and low pH in the presence of detergent results in the detection of higher NGF concentrations in immunoassays using either polyclonal or commercially available monoclonal antibodies. These increases were tissue-specific (sciatic nerve, mesenteric arteries, and thoracic aorta > heart and brain > sympathetic ganglia > abdominal aorta) and as much as 10 times greater than the amounts detected by traditional procedures. The method should also prove useful for the assay of other members of the neurotrophin family when appropriate antibodies become available.

Control of ovarian innervation

The presence of intra-ovarian nerves was reported more than a century ago and successive investigations have demonstrated that the mammalian ovary is innervated by both sympathetic and sensory fibres distributed to the different compartments of the gland, such as blood vessels, ovarian stroma and follicle wall. Despite the extensive ovarian innervation and the experimental evidence indicating that neuromediators can influence sex steroid production, the role of the nervous system in the control of ovarian activity is still largely unknown.

Effects of nerve growth factor on splenic norepinephrine and pineal N-acetyl-transferase in neonate rats exposed to alcohol in utero: neuroimmune correlates

Prenatal alcohol exposure (FAE) has been associated with multiple anomalies, including a selective developmental delay of sympathetic innervation in lymphoid organs. Sympathetic neurons require nerve growth factor (NGF) for their development and maintenance, and recent evidence has suggested that alcohol impairs the synthesis and/or biological activity of NGF in selected central and peripheral neurons. Thus, the present study examined the hypothesis that NGF administration to FAE rats during early postnatal development would reverse some of the peripheral sympathetic deficits. Neonate rats, FAE and the corresponding control cohorts, received daily treatments of NGF or cytochrome C (0.3 mg/kg; s.c.) for various time intervals, and were killed 24 hr or 10 days after the last treatment. The measured parameters included norepinephrine (NE) concentrations in the spleen and heart, which receive nor-adrenergic innervation from the coeliac ganglion and the superior cervical ganglion (SCG), respectively. In addition, we measured the activity of pineal N-acetyltransferase (NAT), the rate-limiting enzyme of melatonin biosynthesis, which depends on sympathetic innervation from the SCG. The data show that chronic, but not acute, NGF treatments reversed the FAE-related deficits in splenic NE concentrations as well as in pineal NAT activity in a time- and age-dependent manner. Sympathetic neurons play an important role in immune modulation. Thus, the altered splenic NE levels and pineal NAT activity may play a role in immune deficits associated with exposure to alcohol in utero.

Paracrine and autocrine actions of neurotrophic factors

Neurotrophic factors are proteins that promote the survival and growth of neurons in the vertebrate nervous system. Although it is well known that many neurons obtain these factors from the regions to which their axons project, studies of the sites of neurotrophic factor synthesis have raised the possibility that at least some neurons may obtain these factors from other sources. Alternative sources of neurotrophic factors include cells along a neuron's axon shaft and cells or other axons terminals within the vicinity of a neuron's cell body and dendritic arbour. In addition, recent experimental studies have shown that at certain stages of development neurotrophic factor autocrine loops operate in some neurons. The evidence for and the potential physiological significance of these different modes of action of neurotrophic factors will be discussed.

The concept of uptake and retrograde transport of neurotrophic molecules during development: history and present status

In the present review honoring Hans Thoenen's contributions to the concept of uptake and retrograde transport of trophic molecules, I have attempted to identify the major historical pathways that had to converge before this concept could be accepted as a fundamental principle in neurobiology. Some of the critical events in this history which are discussed here include: neuron-target interactions, bidirectional trophic signals, axoplasmic transport, receptor-mediated endocytosis, transneuronal trophic signals, the discovery of NGF, the retrograde transport of NGF, and the production of NGF by target tissues. Only when all of these diverse pieces of the puzzle were in place was the concept finally confirmed as being the mechanism that mediates the many phenomena attributed to the regulation and maintenance of neurons by their targets.

Retrograde transport of neurotrophins from the eye to the brain in chick embryos: roles of the p75NTR and trkB receptors

The receptors involved in retrograde transport of neurotrophins from the retina to the isthmo-optic nucleus (ION) of chick embryos were characterized using antibodies to the p75 neurotrophin receptor and trkB receptors. Survival of neurons in the ION has been shown previously to be regulated by target-derived trophic factors with survival promoted or inhibited by ocular injection of brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF), respectively. In the present paper, we show that during the period of target dependence, these neurons express trkB and p75 neurotrophin receptor but not trkA or trkC mRNAs. We also show that BDNF and NT-3 were transported efficiently at low doses, whereas NGF was transported significantly only at higher doses. The transport of BDNF and NT-3 was reduced by high concentrations of NGF or by antibodies to either trkB or the p75 neurotrophin receptor. Thus both receptors help mediate retrograde transport of these neurotrophins. Ocular injection of the comparatively specific trk inhibitor K252a did not reduce transport of exogenous BDNF, but did induce significant neuronal death in the ION, which could not be prevented by co-injection of BDNF. Thus, transport of BDNF alone does not generate a trophic signal at the cell body when axonal trkB is inactivated. In summary, our results indicate that both p75 neurotrophin and trkB receptors can mediate internalization and retrograde transport of BDNF, but activation of trkB seems to be essential for the survival-promoting actions of this neurotrophin.

Attenuation and recovery of nerve growth factor receptor mRNA in dorsal root ganglion neurons following axotomy

The actions of nerve growth factor (NGF) are mediated by two receptor proteins, trk and p75. Recent evidence indicates that NGF upregulates the expression of both trk and p75 in responsive neurons including rat dorsal root ganglion (DRG) neurons. Axotomy by disconnecting the neuron from its source of target-derived NGF is predicted to lead to the downregulation of trk and p75 expression. However, previous studies of the effects of axotomy on trk and p75 mRNA expression in rat DRG have yielded discrepant results. We report that following sciatic nerve crush, trk and p75 mRNA levels in L4-L6 DRG decrease to approximately 50% of control levels at 4-14 days, return to control levels by 30 days, and are increased by approximately 30% at 60 days. Similar changes are observed following nerve transection although mRNA levels are slower in returning to normal and do not exceed control levels at later timepoints. Thus, trk and p75 expression decline early following target disconnection and later recover irrespective of target reinnervation. These observations indicate that target derived NGF is required for the maintenance of NGF receptor expression in adult rat DRG neurons and that non-target derived factors can appropriate this function following peripheral nerve injury.

Nerve growth factor alleviates a painful peripheral neuropathy in rats

Nerve growth factor (NGF) reverses some effects of axotomy and prevents toxic neuropathy in adult rodents. We tested the effect of NGF on behavioral hyperalgesia resulting from a chronic constriction injury (CCI) of the sciatic nerve in the rat [5]. CCI rats exhibit thermal hyperalgesia as demonstrated by a reduction of paw withdrawal latency to a noxious thermal stimulus applied to the paw on the side of injury. The mechanical sensitivity of the ipsilateral hindpaw, assessed with von Frey filaments, was also significantly increased. There were no significant changes in nociceptive thresholds on the contralateral side. When NGF was infused directly on the ligated nerve via an osmotic pump (0.5 microgram/microliter/h for 7 days) immediately after the ligation, thermal hyperalgesia was abolished from postoperative days 5 up to at least two weeks. The CCI-induced decrease in mechanical threshold was also abolished by NGF. However, NGF had only a minor effect on the abnormally long response duration, a second measure of mechanical sensitivity, to the mechanical stimulus. Delayed infusion of NGF four days after the ligation failed to block hyperalgesia. Infusion of NGF on the sciatic nerve of rats that had no CCI had no significant effect on paw withdrawal latency. Infusion of anti-NGF antiserum did not enhance hyperalgesia in CCI rats. These results suggest that alterations in neurotrophic factor(s) contribute to the development of behavioral hyperalgesia in an animal model of neuropathy and that NGF may have therapeutic value in the treatment of neuropathic pain in humans.

Retrograde axonal transport of signal transduction proteins in rat sciatic nerve

Neurons require a mechanism to transmit stable signals over the large distance from the nerve growth cone or terminal to the cell body, in order that information from the target tissue can be relayed to the cell body where it is required. Nerve growth factor (NGF), a target-derived neurotrophic factor, is thought to signal over this distance by receptor mediated internalization of NGF, followed by retrograde axonal transport of the NGF-receptor complex. In this paper we show, by immunohistochemistry of rat sciatic nerve, accumulation of phosphotyrosine immunoreactivity only on the distal side of a nerve crush, suggesting axonal transport of tyrosine kinases and/or tyrosine phosphorylated proteins primarily in a retrograde direction. Furthermore, we also show retrograde axonal transport of phosphoinositide 3-kinase, ERK, MEK and MEK kinase, of which all but MEK kinase are known to be activated downstream of tyrosine receptor kinase activation. The retrograde transport of these proteins suggests that they may be involved in transmission of signals along the axon, relaying neurotrophic factor receptor activation at the nerve terminal to the nerve cell body.

Predegeneration enhances regeneration into acellular nerve grafts

In the present study, we determined the regeneration rate and the initial delay in rat sciatic nerve grafts first made hypercellular by predegeneration then acellular by freeze-thawing. 7-day predegenerated nerve pieces from the distal nerve stump on the right side were made acellular by repeated freeze-thawing and inserted as grafts into a 10-mm long freshly created defect on the left contralateral side. Freshly made (no predegeneration period) acellular nerve grafts were used as controls. Both types of grafts supported outgrowth of regenerating axons as demonstrated by the sensory pinch test. However, the predegenerated acellular nerve grafts had a significantly shorter initial delay period (2.7 days) as compared with freshly made acellular nerve grafts (9.5 days). The initial delay period for predegenerated acellular nerve grafts was similar to that for fresh cellular nerve grafts but significantly longer than that for predegenerated cellular nerve grafts [24]. The rate of regeneration appeared independent of the type of grafts used. We suggest that modifications of the basal lamina and/or factors produced during the predegeneration period by non-neuronal cells survive the freeze-thawing cycle and account for the decrease in the initial delay period.

Human recombinant nerve growth factor replaces deficient neurotrophic support in the diabetic rat

Manipulation of neurotrophic support is a developing strategy for new therapy aimed at neurodegenerative diseases. This study demonstrates reduced content and retrograde transport of endogenous nerve growth factor (NGF) in sciatic nerve of diabetic rats. There were also reductions in the diabetic rats in NGF protein and mRNA in skin and muscle of the hindlimb. These deficits correlated with reductions in substance P and calcitonin gene-related peptide--both products of NGF-influenced genes in primary afferents. These manifestations of deficient neurotrophic support were corrected by intensive insulin treatment and surmounted by administration of exogenous human recombinant NGF in a dose-related manner. Impaired neurotrophic support may, therefore, participate in the pathogenesis of diabetic and other peripheral neuropathies.

Experimental model for local administration of nerve growth factor in microsurgical nerve reconnections

An experimental model for local administration of neuronotrophic substances at the site of peripheral nerve lesion is presented. The model consists of a subcutaneously located silicone reservoir and a connecting tube with its distal end fixed in the proximity of the severed and repaired nerve. The results of the preliminary tests of the model are presented. Sixty Sprague-Dawley rats were divided into two groups: control (saline-treated) (n = 30) and NGF-treated (n = 30). After axotomy of the sciatic nerve, an epineurial repair is performed. NGF or saline is injected daily into the subcutaneous reservoir during the first 3 weeks after axotomy and a single dose in the fourth week. The regenerated nerve observed in the NGF-treated group after four weeks of treatment presents a greater percentage of myelinated axons, thicker myeline sheaths, and more mature endoneurial layers. This experimental model provides a reliable and quantitative way to deliver neuronotrophic substances in site and at different administration rates.

Pre-degenerated nerve grafts enhance regeneration by shortening the initial delay period

In the present study we tested how nerve grafts with different pre-degeneration periods (1-28 days) influenced the early regenerative response in the rat sciatic nerve. The sciatic nerve on the right side was crushed and after 1-28 days of pre-degeneration, a 10 mm segment was used as an autologous nerve graft and transposed to a freshly made 10 mm long nerve defect on the left side. The regeneration distance was measured by the sensory pinch test 2-10 days after nerve repair. A newly developed mathematical model was used to calculate regeneration rates and initial delay periods from the measured regeneration distances. Pre-degenerated nerve grafts improved nerve regeneration by decreasing the initial delay period as compared to fresh nerve grafts without affecting the regeneration rate. Only one day of pre-degeneration was sufficient to reduce the initial delay period from 3.6 days to 1.7 days. The maximal effect on the initial delay period was achieved after 3 days of pre-degeneration. The initial delay period at later pre-degeneration intervals (7-14 days) was about 1 day. The effect persisted for at least 28 days of pre-degeneration. The regeneration rate was 1.5 mm/day for fresh nerve grafts and between 1.8-2.1 mm/day for pre-degenerated grafts. The results suggest that the effects of pre-degeneration are not only due to the increased cell proliferation in the graft, but that also trophic and/or inflammatory mechanisms may be of importance. Grafts pre-degenerated by crush may have clinical implications since they are easy to perform if an elective nerve grafting procedure is planned.

Nerve growth factor (NGF) uptake and transport following injection in the developing rat visual cortex

Recent investigations have shown that cortical nerve growth factor (NGF) infusions during the critical period inhibit ocular-dominance plasticity in the binocular portion of the rat visual cortex. The mechanisms underlying the effects of NGF on visual cortical plasticity are still unclear. To investigate whether during normal development intracortical and/or extracortical cells possess uptake/transport mechanisms for the neurotrophin, we injected 125I-NGF into the occipital cortex of rats at different postnatal ages. Within the cortex, only a few labelled cells were observed. These cells were confined to the vicinity of the injection site and their number depended on the animal's age at the time of injection. Labelled cells were absent at postnatal day (PD) 10 but could be detected between PD 14 and PD 18. They then decreased in number over the following period and were not detected in adult animals. Outside the cortex, neurons of the lateral geniculate nucleus (LGN) were not observed to take up and retrogradely transport NGF at any age after birth. In contrast, retrogradely labelled neurons were found in the basal forebrain. Labelled cells were first observed here at PD 14 and then increased in number until reaching the adult pattern. Our results show that intrinsic and extrinsic neurons are labelled following intracortical injections of iodinated NGF. In both neuronal populations, the uptake and transport of NGF is present over a period corresponding to the critical period for visual cortical plasticity. These findings suggest that NGF may play a role, both intra and extracortically, in plasticity phenomena.

The role of neurotrophins in the developing nervous system

Neurotrophins were originally identified by their ability to promote the survival of developing neurons. However, recent work on these proteins indicates that they may also influence the proliferation and differentiation of neuron progenitor cells and regulate several differentiated traits of neurons throughout life. Moreover, the effects of neurotrophins on survival have turned out to be more complex than originally thought. Some neurons switch their survival requirements from one set of neurotrophins to another during development, and several neurotrophins may be involved in regulating the survival of a population of neurons at any one time. Much of our understanding of the developmental physiology of neurotrophins has come from studying neurons of the peripheral nervous system. Because these neurons and their progenitors are segregated into anatomically discrete sites, it has been possible to obtain these cells for in vitro experimental studies from the earliest stage of their development. The recent generation of mice having null mutations in the neurotrophin and neurotrophin receptor genes has opened up an unparalleled opportunity to assess the physiological relevance of the wealth of data obtained from these in vitro studies. Here I provide a chronological account of the effects of members of the NGF family of neurotrophins on cells of the neural lineage with special reference to the peripheral nervous system.

Deficits in sciatic nerve neuropeptide content coincide with a reduction in target tissue nerve growth factor messenger RNA in streptozotocin-diabetic rats: effects of insulin treatment

The role of sub-optimal neurotrophic support in the aetiology of the sensory neuron dysfunction associated with diabetic neuropathy was investigated. The status of sciatic nerve neuropeptide content was related to target tissue nerve growth factor messenger RNA levels in streptozotocin-diabetic rats. The levels of substance P and calcitonin gene-related peptide in diabetic sciatic nerve were significantly lowered by approximately 50% and 28%, respectively, compared with aged matched controls and insulin-treated diabetic rats (P < 0.01) for both peptides and both comparisons). Measurements of nerve growth factor messenger RNA levels in sensory neuron target tissues, namely foot-skin and soleus muscle, revealed deficits of approximately 50% in diabetic rats, with insulin treatment reversing the decrease in foot-skin but not in soleus muscle. The results show a possible correlation between deficient neuropeptide gene expression in sensory neurons and reduced nerve growth factor messenger RNA levels in target tissue.

An improved procedure for the immunohistochemical localization of nerve growth factor-like immunoreactivity

Nerve growth factor (NGF) is a survival factor required by a number of neuronal populations including most post-ganglionic sympathetic neurones. NGF has been detected and quantified in many tissues but there is little information regarding its cellular localization. Although it has been argued that histological detection has proven difficult due to the low levels of NGF present, other factors may contribute to prevent its identification. In the present study, we report a method for the histological detection of NGF-like immunoreactivity in the rat superior cervical ganglia (SCG). Adult Wistar-Kyoto rats were perfused briefly with either a high or low pH buffer prior to fixation and routine immunohistochemistry. Polyclonal antibodies to native mouse NGF used in the present study recognized mouse NGF but not recombinant human neurotrophin 3 (rhNT3) or brain-derived neurotrophic factor (rhBDNF) by immunoblot analysis. NGF-like immunoreactivity was localized to most sympathetic neurones. Immunoreactivity was detected in the cytoplasm with dense labelling around nuclei. No stain was seen in sections incubated with normal sheep IgG or from animals perfused with phosphate buffer (pH 7.4) prior to fixation. In addition, axotomy resulted in the disappearance of NGF immunoreactivity which was confirmed by biochemical quantification. Finally, no NGF immunoreactivity was found in neurones of rats treated systemically with NGF antiserum 3 days earlier. Possible mechanisms underlying the improvement of NGF immunohistochemistry by pH manipulation before fixation are discussed.

Retrograde axonal transport of locally synthesized proteins, e.g., actin and heat shock protein 70, in regenerating adult frog sciatic sensory axons

The local synthesis and subsequent retrograde axonal transport of [35S]methionine-labelled proteins was studied in the in vitro regenerating adult frog sciatic sensory axons. By the use of a three compartment culture system, proteins in the outgrowth region were selectively labelled. After 2 days in culture a rise in TCA-insoluble radioactivity was detected in the dorsal root ganglia, which could be prevented by the addition of vinblastine or 2,4-dinitrophenol to the nerve proximal to the crush site. Two-dimensional polyacrylamide gel electrophoresis of ganglionic proteins revealed a pattern of 35 labelled polypeptides with apparent molecular masses (Mm) ranging from < 15 to 95 kDa and with isoelectric points (pI) ranging from 4.5 to 6.5. The major ones, representing about 75% of the activity in a gel, were of Mm/pI 47/5.4, 48/6.1,. 57/6.0, 62/5.2, 65/4.9-5.0, 65/5.2, and 81/5.4 respectively. One of these polypeptides (47/5.4) was identified as actin and another (81/5.4) as a member of the heat shock protein 70 family. The spots at 65/4.9-5.0 were tubulin isoforms. There was a striking similarity between transported proteins on one hand, and proteins synthesized in the injured nerve on the other, with respect to the Mm/pI of at least 14 protein species. The results suggest that a selected set of proteins, synthesized by non-neuronal cells, e.g., Schwann cells, is transferred to the ganglionic cell bodies by retrograde axonal transport.

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.

Pathophysiology of glial growth factor receptors

Activation and proliferation of glial cells are common events in the pathology of the nervous system. Although we are only beginning to understand the molecular signals leading to glial activation in vivo, there is increasing evidence that growth factors and their receptors may play an important part. In this paper we summarize the data on the pathophysiology of glial growth factor receptors and their ligands in the central and peripheral nervous systems.

NGF and the local control of nerve terminal growth

It is generally believed that the mechanism of action of neurotrophic factors involves uptake of neurotrophic factor by nerve terminals and retrograde transport through the axon and back to the cell body where the factor exerts its neurotrophic effect. This view originated with the observation almost 20 years ago that nerve growth factor (NGF) is retrogradely transported by sympathetic axons, arriving intact at the neuronal cell bodies in sympathetic ganglia. However, experiments using compartmented cultures of rat sympathetic neurons have shown that neurite growth is a local response of neurites to NGF locally applied to them which does not directly involve mechanisms in the cell body. Recently, several NGF-related neurotrophins have been identified, and several unrelated molecules have been shown to act as neurotrophic or differentiation factors for a variety of types of neurons in the peripheral and central nervous systems. It has become clear that knowledge of the mechanisms of action of these factors will be crucial to understanding neurodegenerative diseases and the development of treatments as well as the means to repair or minimize neuronal damage after spinal injury. The concepts derived from work with NGF suggest that the site of exposure of a neuron to a neurotrophic factor is important in determining its response.

The role of neurotrophins during successive stages of sensory neuron development

Neurotrophins comprise a family of basic homodimeric proteins. The isolation of the first two neurotrophins, nerve growth factor and brain-derived neurotrophic factor, was based on the ability of these proteins to promote the survival of embryonic neurons. However, the identification of additional neurotrophins by homology screening together with recent work on these proteins has shown that neurotrophins do more than just regulate neuronal survival. Neurotrophins influence the proliferation and differentiation of neuron progenitor cells and regulate the expression of several differentiated traits of neurons throughout life. Moreover, the influence of neurotrophins on survival is more complex than originally thought; some neurons switch their survival requirements from one set of neurotrophins to another during development and several neurotrophins may be involved in regulating the survival of a population of neurons at any one time. Most of what is known of the developmental physiology of neurotrophins has come from studying neurons of the peripheral nervous system. Quite apart from the accessibility of these neurons and their progenitor cell populations, investigation of the actions of neurotrophins on several well-characterised populations of sensory neurons has permitted the age-related changes in the effects of neurotrophins to be interpreted in the appropriate developmental context. In this review I provide a chronological account of the action of neurotrophins in neuronal development with special reference to sensory neurons.

Expression of mRNA for neurotrophic factors and their receptors in the rat dorsal root ganglion and sciatic nerve following nerve injury

The adult rat dorsal root ganglion (DRG) produces mRNA for the neurotrophic factors nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) and contains large populations of neurons responsive to these factors. We report that following a focal crush injury of the sciatic nerve, NGF mRNA expression increases threefold and BDNF mRNA two-fold, in the ipsilateral L4 and L5 DRGs. The mRNAs encoding the cognate neurotrophin receptors, p75NGFR, trkA, and trkB were expressed in the DRG throughout the post-injury time course, suggesting that DRG neurons remain responsive to both NGF and BDNF. p75NGFR mRNA levels became transiently depressed in the DRG during the first several days after the lesion but returned to normal within 1 week. trkB mRNA was expressed in the normal sciatic nerve and levels were not altered by nerve crush. RNase protection assays detected both full-length and truncated trkB transcripts in the DRG, but only truncated trkB mRNA, lacking the tyrosine kinase domain, was detected in the sciatic nerve. Likewise, trkA transcripts were not detected by RNase protection in normal sciatic nerve or in a segment of nerve distal to the crush site. These results are consistent with a model in which regenerating sensory neurons are supported by neurotrophic factors synthesized within the DRG.

Development of trophic interactions in the vertebrate peripheral nervous system

During embryogenesis, the neurons of vertebrate sympathetic and sensory ganglia become dependent on neurotrophic factors, derived from their targets, for survival and maintenance of differentiated functions. Many of these interactions are mediated by the neurotrophins NGF, BDNF, and NT3 and the receptor tyrosine kinases encoded by genes of the trk family. Both sympathetic and sensory neurons undergo developmental changes in their responsiveness to NGF, the first neurotrophin to be identified and characterized. Subpopulations of sensory neurons do not require NGF for survival, but respond instead to BDNF or NT3 with enhanced survival. In addition to their classic effects on neuron survival, neurotrophins influence the differentiation and proliferation of neural crest-derived neuronal precursors. In both sympathetic and sensory systems, production of neurotrophins by target cells and expression of neurotrophin receptors by neurons are correlated temporally and spatially with innervation patterns. In vitro, embryonic sympathetic neurons require exposure to environmental cues, such as basic FGF and retinoic acid to acquire neurotrophin-responsiveness; in contrast, embryonic sensory neurons acquire neurotrophin-responsiveness on schedule in the absence of these molecules.

Restoration of substance P and calcitonin gene-related peptide in dorsal root ganglia and dorsal horn after neonatal sciatic nerve lesion

Dorsal root ganglion (DRG) neurons decrease their substance P (SP) synthesis after peripheral nerve lesions. Levels in the dorsal horn also decline but return to normal if regeneration is successful. In adults, when regeneration is prevented, recovery of SP in the dorsal horn is slow and incomplete, whereas in newborns, recovery is rapid and complete even though retrograde cell death of DRG neurons is greater than in adults. We have examined the mechanisms that might account for the rapid and complete recovery of SP and calcitonin-gene related peptide (CGRP) in the dorsal horn after peripheral nerve injury in newborns. Peptides were compared in the L4 and L5 DRG and spinal cord segments of normal rats and in rats surviving 6 days to 4 months after sciatic nerve section/ligation within 24 hours of birth. Sciatic nerve section/ligation produced 50% neuron death in L4 and L5 DRGs, but immunocytochemical methods showed that both SP-immunoreactivity (-IR) and CGRP-IR recovered completely in dorsal horn. Radioimmunoassay confirmed that recovery of SP was not an artefact due to shrinkage. beta-Preprotachykinin (PPT)-mRNA hybridization and SP-IR were observed mostly in small neurons; alpha-CGRP-mRNA-hybridized and CGRP-IR neurons were more heterogeneous. The percentage of DRG neurons that contained SP (approximately 25%) or CGRP (approximately 50%) was the same in normal newborn and adult rats. Neither selective cell survival nor change in neuron phenotype was likely to contribute to the recovery seen in the dorsal horn, and DRG neurons ipsilateral to the lesion exhibited the same level of hybridized beta-PPT-mRNA and alpha-CGRP-mRNA as intact DRG neurons. Because neither the constitutive level of expression of the genes nor peptide levels increased above those observed in intact DRG neurons, these mechanisms were also not responsible. Axotomized DRG neurons, however, contributed to recovery. Recovery was also due to sprouting by neurons in intact DRGs rostral and caudal to L4 and L5.

Expression of NGF in sympathetic neurons leads to excessive axon outgrowth from ganglia but decreased terminal innervation within tissues

The effects of nerve growth factor (NGF) on sympathetic axon growth were investigated by generating transgenic mice in which the beta subunit of NGF was expressed in sympathetic neurons using the human dopamine beta-hydroxylase (DBH) promoter. In DBH-NGF mice, the sympathetic trunk and nerves growing to peripheral tissues were enlarged and contained an increased number of sympathetic fibers. Although sympathetic axons reached peripheral tissues, terminal sympathetic innervation within tissues was decreased in DBH-NGF mice. This effect could be reversed in the pancreas by overexpression of NGF in pancreatic islets. The observations are consistent with a model in which NGF gradients are not required to guide sympathetic axons to their targets, but are required for the establishment of the normal density and pattern of sympathetic innervation within target tissues.

Peripheral nerve regeneration: role of growth factors and their receptors

Growth factors play a central role in the regulation of normal and injury-induced regenerative cell growth. The purpose of this article is to summarize the available data on the expression of different growth factors and their receptors in the injured peripheral nervous system and to discuss their possible role in promoting peripheral nerve regeneration.

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.

Nerve growth factor and regeneration of peripheral nervous system

Nerve growth factor, a well-known neurotrophic factor, supports the survival, differentiation and maintenance of sensory and sympathetic neurons during embryonic development and in the adult. This paper summarises the data on its involvement in peripheral nerve regeneration.

Synthesis and localization of ciliary neurotrophic factor in the sciatic nerve of the adult rat after lesion and during regeneration

Ciliary neurotrophic factor (CNTF) is expressed in high quantities in Schwann cells of peripheral nerves during postnatal development of the rat. The absence of a hydrophobic leader sequence and the immunohistochemical localization of CNTF within the cytoplasm of these cells indicate that the factor might not be available to responsive neurons under physiological conditions. However, CNTF supports the survival of a variety of embryonic neurons, including spinal motoneurons in culture. Moreover we have recently demonstrated that the exogenous application of CNTF protein to the lesioned facial nerve of the newborn rat rescued these motoneurons from cell death. These results indicate that CNTF might indeed play a major role in assisting the survival of lesioned neurons in the adult peripheral nervous system. Here we demonstrate that the CNTF mRNA and protein levels and the manner in which they are regulated are compatible with such a function in lesioned peripheral neurons. In particular, immunohistochemical analysis showed significant quantities of CNTF at extracellular sites after sciatic nerve lesion. Western blots and determination of CNTF biological activity of the same nerve segments indicate that extracellular CNTF seems to be biologically active. After nerve lesion CNTF mRNA levels were reduced to less than 5% in distal regions of the sciatic nerve whereas CNTF bioactivity decreased to only one third of the original before-lesion levels. A gradual reincrease in Schwann cells occurred concomitant with regeneration.

The neurotrophins BDNF, NT-3, and NGF display distinct patterns of retrograde axonal transport in peripheral and central neurons

The pattern of retrograde axonal transport of the target-derived neurotrophic molecule, nerve growth factor (NGF), correlates with its trophic actions in adult neurons. We have determined that the NGF-related neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are also retrogradely transported by distinct populations of peripheral and central nervous system neurons in the adult. All three 125I-labeled neurotrophins are retrogradely transported to sites previously shown to contain neurotrophin-responsive neurons as assessed in vitro, such as dorsal root ganglion and basal forebrain neurons. The patterns of transport also indicate the existence of neuronal populations that selectively transport NT-3 and/or BDNF, but not NGF, such as spinal cord motor neurons, neurons in the entorhinal cortex, thalamus, and neurons within the hippocampus itself. Our observations suggest that neurotrophins are transported by overlapping as well as distinct populations of neurons when injected into a given target field. Retrograde transport may thus be predictive of neuronal types selectively responsive to either BDNF or NT-3 in the adult, as first demonstrated for NGF.

Effects of sciatic nerve transplants after fimbria-fornix lesion: examination of the role of nerve growth factor

At two weeks post-transplantation, sciatic nerves inserted into the lesioned septo-hippocampal pathway contain NGF levels more than twice that of normal nerves. These transplanted nerves also contain regenerating cholinergic axons. Moreover, transplanted animals exhibit septal NGF levels that are significantly greater than in animals with lesions only. These results suggest a role for NGF in the ingrowth of axons into the transplants and in the increase in ChAT(+) septal neurons previously observed at this post-transplant time.

Analytical purification of the slow, high affinity NGF receptor: identification of a novel 135 kd polypeptide

Although nerve growth factor (NGF) action is mediated by the slow, high affinity NGF receptor, little is known regarding its molecular composition or mode of action. We have used reversible chemical cross-linkers and affinity chromatography strategies to purify the slow NGF receptor covalently cross-linked to its NGF ligand. Subsequent uncoupling of the cross-links reveals that the receptor-ligand complex is composed of only a novel 135 kd polypeptide interacting with NGF. The previously characterized 85 kd fast, low affinity NGF receptor is not a component of the cross-linked slow receptor-ligand complex. This newly identified 135 kd polypeptide is either the entire slow NGF receptor, or it might be one component of a larger, multisubunit slow NGF holo-receptor.

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.

A ciliary neuronotrophic factor from peripheral nerve and smooth muscle which is not retrogradely transported

We have found that a CNTF-like molecule which supports ciliary and sympathetic neurons is not retrogradely transported in either sympathetic or parasympathetic nerves. The factor has an apparent Mr of 21 kDa, a pI of 4.9, and is present in peripheral nerves and smooth muscle of the chick. Our experiments indicate that CNTF-like activity does not accumulate on the distal side of ligated chick expansor nerves. In contrast, there is a clear accumulation of NGF. The activity further differs from NGF in that it is not removed from a smooth muscle of the chick wing by innervating sympathetic fibers. Transection of these fibers does not lead to an accumulation of ciliary activity in the expansor secundariorum muscle, suggesting that neurons do not actively deplete the muscle of factor by retrograde transport. Finally, recombinant CNTF or semi-purified preparations of CNTF-like activity labelled with 125I were not transported to the ciliary ganglion of chicks following injection of biologically active material into the eye. Our results suggest either that endogenous CNTF does not act as a survival factor in vivo, or that retrograde transport is not a property inherent to all neuronotrophic molecules.

Diabetes mellitus-associated decrease in nerve growth factor levels is reversed by allogeneic pancreatic islet transplantation

After an untreated 5-month duration of streptozotocin (STZ)-induced diabetes mellitus (DM), nerve growth factor (NGF) levels in BDE rats were decreased to 45-65% of control in the sympathetically innervated target organs iris and submandibular gland, in the superior cervical ganglion (containing NGF-dependent sympathetic perikarya projecting to the cranial targets), and in the NGF-transporting sciatic nerve. Successful allogeneic pancreatic islet transplantation (providing a physiological glucose homeostasis without immunosuppression) after 3-4 weeks of DM reversed the DM-related decrease in NGF levels 4 months after transplantation as compared with untreated diabetic rats. By contrast, NGF levels in the treated vas deferens (innervated by short postganglionic sympathetic neurons) remained increased as in the untreated diabetic rats (175% of control). Thus, DM-associated changes in endogenous NGF levels seem to be reversible by institution of metabolic control, at least at an early stage of DM when NGF-responsive neurons have not been deprived of NGF for a long time.

Histochemistry of sympathetic neurons allotransplanted from young and aged mice to the submandibular gland

Sympathetic ganglion tissue of young (3 months) and aged (24 months) NMRI mice was allotransplanted into the submandibular gland to study the influence of aging on the survival of grafted neurons. The submandibular gland (SMG) was chosen as a host tissue because of its high concentration of NGF and good blood supply. Four weeks postgrafting the viability of transplants was evaluated using the formaldehyde-induced fluorescence technique, tyrosine hydroxylase (TH) immunohistochemistry, and morphometry. The density of neurons, catecholamine fluorescence and TH immunoreactivity (TH-IR) appeared to be almost unchanged when the transplant was completely surrounded by the SMG tissue, whereas transplants located within the interlobular septum and capsule, or even outside the capsule, showed significantly reduced neuronal survival. The shape of most of the transplanted neurons was not different from those in the intact ganglia. The average diameter of the transplanted young neurons was significantly decreased; this was not the case with the aged neurons. The histograms of grouped diameter values showed a shift to smaller cells in ganglion transplants in both age groups. The transplants in mice treated with 6-OH-dopamine showed considerable regrowth of adrenergic nerve fibers. There seemed to be no marked difference in the survival of transplanted neurons between aged and young animals. The results indicate that the sympathetic neurons from both young adult and aged animals survive the allotransplantation procedure. The neurotrophic factors together with dense vascularization present in the mouse submandibular gland may be beneficial for the restoration of the integrity of mature and aged adrenergic neurons.

Immunohistochemical localization of nerve growth factor receptor in the cochlea and in the brainstem of the perinatal rat

Nerve growth factor receptor (NGF-R) localization was studied immunohistochemically in the cochlea and in the brainstem of the perinatal rat, using a specific monoclonal antibody directed against the rat NGF-R. In the cochlea, NGF-R immunoreactivity is positive during the whole perinatal period studied, and is located at the hair cell level, in fibers that reach the organ of Corti, in the intraganglionic spiral bundle and in some small bundles of fibers in the auditory nerve. In the brainstem, NGF-R is detected in auditory structures such as the ventral cochlear nucleus, the superior olivary complex, the nuclei of the trapezoid body and the trapezoid body. Many auditory structures labelled by the NGF-R antibody are implicated in the efferent cochlear innervation. These results suggest that NGF could be implicated in interactions between auditory receptors and efferent innervation of the developing cochlea. This coincides with findings on the immunohistochemical localization of NGF-like protein in the organ of Corti of the developing rat. Moreover, these observations could be related to an early prenatal development of auditory efferent innervation.

End structure and neuropeptide immunoreactivity of axons in sciatic neuromas following nerve section in neonatal rats

The formation of neuromas after neonatal nerve injury was studied in rats. In neonatal pups, the sciatic nerve was cut and tightly ligated, and a portion of the distal stump was removed. After 6-10 weeks, a nerve-end neuroma had formed in about 70% of the animals. In the remaining animals the nerve had grown on the side of the ligature. The end structure of the neuroma axons was studied using anterogradely transported WGA-HRP injected into the L5 dorsal root ganglion. HRP labeling occurred in the entire proximal sciatic nerve. In the neuroma, labeled fibers branched profusely and either terminated with minor end swellings or turned in the retrograde direction. Immunohistochemistry showed that the fibers which projected into the neuroma presented a moderate immunoreactivity to substance P and neuropeptide Y and a strong reactivity to calcitonin gene-related peptide. The results show that many sensory and sympathetic sciatic nerve fibers survive chronic axotomy in the newborn and contribute to the formation of nerve-end neuromas. There are, however, important structural differences between adult and neonatally induced neuromas.

Trophic influence of the distal nerve segment on GABAA receptor expression in axotomized adult sensory neurons

The depolarizing action of gamma-aminobutyric acid (GABA), or the GABAA receptor agonist muscimol, on rat dorsal root (L4 and L5) fibers is attenuated following transection, but not crush, of the sciatic nerve. Following discrete nerve crush, axons actively regenerate and contact both the distal nerve segment and the peripheral target tissues. The aim of the present study was to distinguish between these two regions as possible sources of trophic support for retrograde maintenance of dorsal root GABA receptor sensitivity. A surgical procedure was employed to permit a delimited segment of axonal regeneration while prohibiting reestablishment of end organ innervation; the sciatic nerve was crushed and a ligature was placed 3 cm distal to the crush site. Under these conditions, the injury-induced decrement in the dorsal root GABA response, observed between 12 and 21 postoperative days, was significantly attenuated relative to that of ligated nerves, in which regeneration into the distal stump does not occur. The data suggest that nerve transection by ligation restricts trophic support for maintenance of GABA receptor expression in dorsal root ganglion (DRG) neurons. Furthermore, during regeneration the denervated distal nerve segment assumes a neurotrophic role in the maintenance of dorsal root GABA sensitivity, consistent with the hypothesis that growth factors derived from reactive Schwann cells may positively regulate the expression of receptors on axotomized sensory neurons.

Endogenous levels of nerve growth factor (NGF) are altered in experimental diabetes mellitus: a possible role for NGF in the pathogenesis of diabetic neuropathy

Sympathetic and neural-crest derived sensory neurons consisting of unmyelinated and small myelinated fibers are known to be affected at an early stage in diabetes mellitus (DM). Since these peripheral neurons need nerve growth factor (NGF) for their development and maintenance of function in adulthood, changes in endogenous NGF levels could be of relevance for the pathogenesis of diabetic neuropathy (DNP). Using an improved two-site enzyme immunoassay for NGF, we have investigated whether endogenous NGF levels are altered in Sprague-Dawley rats with DM induced by a single injection of streptozotocin (STZ). STZ-treated rats are known to develop in many respects equivalents to neuropathic complications observed in human DM. We found in some sympathetically innervated target organs decreased NGF contents by maximally 56%: transiently in the iris 2 weeks and in the ventricle 12-24 weeks after DM induction and permanently in the submandibular gland already 3 days after DM induction. Several weeks after onset of DM, NGF content was increased by maximally 145-300% in most peripheral targets investigated, such as in iris, cardiac atrium and ventricle, spleen, prostate gland, and vas deferens. This is suggestive for an impaired NGF removal by NGF-sensitive neurons in diabetic rats. Moreover, NGF levels were decreased to minimally 42.6 +/- 4% of control in the NGF-transporting sciatic nerve. NGF levels began to decrease not before 3 weeks after DM induction and remained decreased with 54.0 +/- 5% of control even after 6 months duration of DM. About the same time (i.e., 2 weeks after induction of DM) NGF levels began to decrease in the superior cervical ganglion (where the sympathetic perikarya are located) to minimally 53.2 +/- 4% of control 12 weeks after DM induction. No altered NGF levels were observed during a 3-month duration of DM in the terminal ileum and sensory trigeminal ganglion. Since NGF exerts its neurotrophic action in the perikarya after its retrograde transport from the NGF-producing periphery, our results are consistent with the hypothesis that an alteration in NGF levels may play a role in the pathogenesis of DNP as far as sympathetic neurons are concerned. Thus, our results suggest that DM influences the production and/or transport of endogenous NGF and consequently, that a deprivation of this neurotrophic factor may account for some of the functional deficits known to occur in DNP, such as impaired catecholaminergic transmitter synthesis. This hypothesis possibly opens the way for new concepts in the therapy of DNP.

The ontogeny of specific retrograde transport of nerve growth factor by motoneurons of the brainstem and spinal cord

Radiolabeled Nerve Growth Factor (NGF) was injected into either the mandibular process of the first visceral arch or the limb bud of chick embryos at Days 3.5-14 or Days 4-13 of incubation, respectively. Control embryos received injections of labeled cytochrome-C or labeled NGF plus an excess of unlabeled NGF. The tissues were then processed for autoradiography. The 125I-NGF was retrogradely transported by motoneurons of the trigeminal (V) motor nucleus on Days 3.5-8 of incubation, but not at later stages. Similar transport was seen in motoneurons of the spinal cord lateral motor column from Days 4-10 of incubation, but not at later stages. Sensory neurons of the V ganglion and of the dorsal root ganglia transported NGF at all injection ages. In no instance was the 125I-cytochrome-C transported by sensory or motor neurons. The injection of an excess of cold NGF along with labeled NGF resulted in no evidence of retrograde transport of the labeled NGF indicating that the transport was saturable. The time of transport by these brainstem and spinal cord motoneurons corresponds closely to the points during development at which they have been found to exhibit specific NGF binding. The present results, then, provide further evidence for a possible biological role for NGF during early developmental stages of these motoneuron populations.