The Effects of Granulocyte-Colony Stimulating Factor on Regeneration in Nerve Crush Injuries in Rats

Granulocyte-colony stimulating factor (G-CSF) is widely known to have a neuroprotective effect, but its effects on function and morphology in mechanical nerve injury are not well understood. The aim of this study was to confirm the time course of the functional changes and morphological effects of G-CSF in a rat model of nerve crush injury. Twelve-eight rats were divided into three group: sham-operated control group, G-CSF-treated group, and saline treated group. 2 weeks after the nerve crush injury, G-CSF was injected for 5 days. After 4 weeks, functional tests such as motor nerve conduction velocity (MNCV), mechanical and cold allodynia tests, and morphological studies were performed. G-CSF-treated rats had significantly improved nerve function including MNCV and mechanical and cold allodynia. In addition, G-CSF-treated rats had significantly higher the density of myelinated fibers than saline-treated rats. In conclusion, we found that 100 μg/kg administration of G-CSF promoted long-term functional recovery in a rat model of nerve crush injury.     

Anti-Obesity Effects of Granulocyte-Colony Stimulating Factor in Otsuka-Long-Evans-Tokushima Fatty Rats

Granulocyte-colony stimulating factor (G-CSF) has molecular structures and intracellular signaling pathways that are similar to those of leptin and ciliary neurotropic factor (CNTF). It also has immune-modulatory properties. Given that leptin and CNTF play important roles in energy homeostasis and that obesity is an inflammatory condition in adipose tissue, we hypothesized that G-CSF could also play a role in energy homeostasis. We treated 12 38-week-old male Otsuka-Long-Evans-Tokushima fatty rats (OLETF, diabetic) and 12 age-matched male Long-Evans-Tokushima rats (LETO, healthy) with 200 µg/day G-CSF or saline for 5 consecutive days. Body weight reduction was greater in G-CSF-treated OLETF (G-CSF/OLETF) than saline-treated OLETF (saline/OLETF) following 8 weeks of treatment (−6.9±1.6% vs. −3.1±2.2%, p<0.05). G-CSF treatment had no effect on body weight in LETO or on food intake in either OLETF or LETO. Body fat in G-CSF/OLETF was more reduced than in saline/OLETF (−32.2±3.1% vs. −20.8±6.2%, p<0.05). Energy expenditure was higher in G-CSF/OLETF from 4 weeks after the treatments than in saline/OLETF. Serum levels of cholesterol, triglyceride, interleukin-6 and tumor necrosis factor-α were lower in G-CSF/OLETF than in saline/OLETF. Uncoupling protein-1 (UCP-1) expression in brown adipose tissue (BAT) was higher in G-CSF/OLETF than in saline/OLETF, but was unaffected in LETO. Immunofluorescence staining and PCR results revealed that G-CSF receptors were expressed in BAT. In vitro experiments using brown adipocyte primary culture revealed that G-CSF enhanced UCP-1 expression from mature brown adipocytes via p38 mitogen-activated protein kinase pathway. In conclusion, G-CSF treatment reduced body weight and increased energy expenditure in a diabetic model, and enhanced UCP-1 expression and decreased inflammatory cytokine levels may be associated with the effects of G-CSF treatment.     

Reestablishing Neuronal Networks in the Aged Brain by Stem Cell Factor and Granulocyte-Colony Stimulating Factor in a Mouse Model of Chronic Stroke

Stroke has a high incidence in the elderly. Stroke enters the chronic phase 3 months after initial stroke onset. Currently, there is no pharmaceutical treatment available for chronic stroke. We have demonstrated the therapeutic effects of the combination of stem cell factor (SCF) and granulocyte-colony stimulating factor (G-CSF) (SCF+G-CSF) on chronic stroke. However, it remains unclear how SCF+G-CSF repairs the brain in chronic stroke. In this study, we determined the effects of SCF+G-CSF on neuronal network remodeling in the aged brain of chronic stroke. Cortical brain ischemia was produced in 16–18 month-old transgenic mice expressing yellow fluorescent protein in layer V pyramidal neurons. SCF+G-CSF was subcutaneously injected for 7 days beginning at 3.5 months post-ischemia. Using both live brain imaging and immunohistochemistry, we observed that SCF+G-CSF increased the mushroom-type spines on the apical dendrites of layer V pyramidal neurons adjacent to the infarct cavities 2 and 6 weeks after treatment. SCF+G-CSF also augmented dendritic branches and post-synaptic density protein 95 puncta in the peri-infarct cortex 6 weeks after treatment. These data suggest that SCF+G-CSF treatment in chronic stroke remodels neural circuits in the aged brain. This study provides evidence to support the development of a new therapeutic strategy for chronic stroke.     

Granulocyte-Colony Stimulating Factor Related Pathways Tested on an Endometrial Ex-Vivo Model

Introduction

Recombinant human Granulocyte-Colony Stimulating Factor (rhG-CSF) supplementation seems to be a promising innovative therapy in reproductive medicine, used in case of recurrent miscarriage, embryo implantation failure or thin endometrium, although its mechanisms of action remain unknown. Our aim was to identify possible endometrial pathways influenced by rhG-CSF.

Materials and Methods

Hypothetical molecular interactions regulated by G-CSF were designed through a previous large scale endometrial microarray study. The variation of endometrial expression of selected target genes was confirmed in control and infertile patients. G-CSF supplementation influence on these targets was tested on an endometrial ex-vivo culture. Middle luteal phase endometrial biopsies were cultured on collagen sponge with or without rhG-CSF supplementation during 3 consecutive days. Variations of endometrial mRNA expression for the selected targets were studied by RT-PCR.

Results

At the highest dose of rhG-CSF stimulation, the mRNA expression of these selected target genes was significantly increased if compared with their expression without addition of rhG-CSF. The selected targets were G-CSF Receptor (G-CSFR), Integrin alpha-V/beta-3 (ITGB3) implicated in cell migration and embryo implantation, Plasminogen Activator Urokinase Receptor (PLAUR) described as interacting with integrins and implicated in cell migration, Thymidine Phosphorylase (TYMP) implicated in local angiogenesis, CD40 and its ligand CD40L involved in cell proliferation control.

Conclusion

RhG-CSF seems able to influence endometrial expressions crucial for implantation process involving endometrial vascular remodelling, local immune modulation and cellular adhesion pathways. These variations observed in an ex-vivo model should be tested in-vivo. The strict indications or counter indication of rhG-CSF supplementation in reproductive field are not yet established, while the safety of its administration in early pregnancy on early embryogenesis still needs to be demonstrated. Nevertheless, rhG-CSF appears as a promising therapy in some difficult and unsolved cases of reproductive failure. Indications of pre-conceptual rhG-CSF supplementation may derive from a diagnosed lack of endometrial expression of some target genes.     

G-CSF (Granulocyte-Colony Stimulating Factor) in the Central Nervous System

G-CSF (Granulocyte-colony stimulating factor) is a hematopoietic growth factor that has been known for 20 years, and has been named for its role in the proliferation and differentiation of cells of the myeloic lineage. We have uncovered a novel spectrum of activities of G-CSF in the central nervous system. G-CSF and its receptor are expressed by neurons in many brain regions, and are upregulated upon experimental stroke. In neurons, G-CSF acts anti-apoptotically by activating several protective pathways. In vivo, G-CSF decreases infarct volumes in acute stroke models in rodents. Moreover, G-CSF stimulates neuronal differentiation of adult neural stem cells in the brain, and improves long-term recovery in more chronic stroke models. Thus, G-CSF is a novel neurotrophic factor, and a highly attractive candidate for the treatment of neurodegenerative conditions. Here we discuss this new property of G-CSF in contrast to its known functions in the hematopoietic system, summarize data from other groups on G-CSF’s actions in cerebral ischemia, compare G-CSF to Erythropoietin (EPO) in the CNS and highlight clinical implications.     

Hedysari Extract Improves Regeneration after Peripheral Nerve Injury by Enhancing the Amplification Effect

Radix Hedysari is an herbal preparation frequently used in traditional Chinese medicine. It can promote regeneration after peripheral nerve injury, but its effect on the amplification ratio (the ratio of distal to proximal fibers) during peripheral nerve regeneration has not yet been examined. In this study, we explored the effect of Hedysari extract on the amplification ratio in the peripheral nerve. Male Sprague-Dawley rats were separated into three groups at random: normal group (without surgery), model group (given sleeve nerve bridging surgery, but without adjuvant treatment) and treatment group (given sleeve nerve bridging surgery and then given Hedysari extract as adjuvant treatment). Twelve weeks after surgery, general observations, electrophysiological examination, histological analysis, morphometric measurements, and amplification ratio calculations were made. The results showed that nerve conduction velocity, the fiber and axon diameter, the g-ratio, the number of regenerating nerve fibers and the amplification ratio were better in the treatment group than in the model group, suggesting that Hedysari extract can effectively promote the growth of lateral buds in the proximal nerve stump and substantially improve the amplification effect during peripheral nerve regeneration.     

乳腺癌患者加用重组人粒细胞集落刺激因子骨髓支持治疗的效果探讨

目的:探讨乳腺癌患者加用重组人粒细胞集落刺激因子(rh G-CSF)骨髓支持治疗的临床效果。方法:将我院2011年1月-2012年12月收治的106例乳腺癌患者随机分为对照组及观察组,每组53例。两组均给予常规化疗治疗,观察组在此基础上加用rh G-CSF骨髓支持治疗,比较两组治疗前后血象变化及骨髓毒性反应情况。结果:对照组治疗后WBC、ANC、Plt及Hb与治疗前比较均有显著降低,且差异具有统计学意义(P0.05);而观察组治疗后以上血象指标较治疗前变化不明显(P0.05),但与对照组同期比较具有显著性差异(P0.05);观察组骨髓毒性反应较对照组轻,Ⅱ度以上发生率为26.42%,显著低于对照组的47.17%(P0.05)。结论:乳腺癌患者加用rh G-CSF骨髓支持治疗可明显缓解患者化疗后的骨髓抑制反应,减少其骨髓毒性反应发生,对化疗导致白细胞数减少具有较好的预防和临床应用价值,值得进一步推广和应用。     

周围神经损伤的影像学研究进展

当代愈发频繁的交通事故和现代局部战争使外周神经损伤发生率持续升高.而在外周神经损伤治疗中,早期确定神经损伤类型及范围.尽早进行合理临床救治是改善预后的关键.因此,通过现代先进的影像学技术对早期周围神经损伤做出及时准确的诊断,成为摆在影像学者面前一个亟待解决的问题.本文将综述近年来CT、MR、超声等先进影像学技术在外周神经损伤诊断方面的研究进展,及其各自的优势与不足,以及本领域所面临的机遇和挑战.     

鼠神经生长因子对2型糖尿病周围神经病变的影响

目的:探讨鼠神经生长因子治疗2型糖尿病周围神经病变(DPN)临床疗效及安全性。方法:选择89例DPN患者分为观察组45例和对照组44例,两组都给予糖尿病的基础治疗,观察组在基础治疗基础上,加用注射用鼠神经生长因子,对照组加用甲钴胺注射液和丹红注射用,治疗前后测定正中神经和腓总神经的感觉传导速度(SNCV)、运动传导速度(MNCV),行多伦多临床评分系统(TCSS)评分。结果:治疗前,两组患者TCSS评分、正中神经、腓总神经的MNCV及SNCV相似,差异无统计学意义(P0.05);疗程结束时,两组TCSS评分较治疗前均下降,正中神经、腓总神经的MNCV及SNCV较治疗前均增加,差异有统计学意义(P0.05),但是观察组改善幅度较对照组更显著(P0.05)。观察组显效20例、有效23例和无效2例;对照组显效15例、有效21例和无效8例,差异有统计学意义(P0.05)。两组患者治疗期间未见严重不良反应发生。结论:加用鼠神经生长因子可以有效缓解2型糖尿病DPN的临床症状,提高神经传导速度,安全性高。     

Arylsulfatase B Improves Locomotor Function after Mouse Spinal Cord Injury

Bacterial chondroitinase ABC (ChaseABC) has been used to remove the inhibitory chondroitin sulfate chains from chondroitin sulfate proteoglycans to improve regeneration after rodent spinal cord injury. We hypothesized that the mammalian enzyme arylsulfatase B (ARSB) would also enhance recovery after mouse spinal cord injury. Application of the mammalian enzyme would be an attractive alternative to ChaseABC because of its more robust chemical stability and reduced immunogenicity. A one-time injection of human ARSB into injured mouse spinal cord eliminated immunoreactivity for chondroitin sulfates within five days, and up to 9 weeks after injury. After a moderate spinal cord injury, we observed improvements of locomotor recovery assessed by the Basso Mouse Scale (BMS) in ARSB treated mice, compared to the buffer-treated control group, at 6 weeks after injection. After a severe spinal cord injury, mice injected with equivalent units of ARSB or ChaseABC improved similarly and both groups achieved significantly more locomotor recovery than the buffer-treated control mice. Serotonin and tyrosine hydroxylase immunoreactive axons were more extensively present in mouse spinal cords treated with ARSB and ChaseABC, and the immunoreactive axons penetrated further beyond the injury site in ARSB or ChaseABC treated mice than in control mice. These results indicate that mammalian ARSB improves functional recovery after CNS injury. The structural/molecular mechanisms underlying the observed functional improvement remain to be elucidated.     

Alterations of Lipid Metabolism in Response to Nerve Growth Factor

Abstract: In response to nerve growth factor (NGF), clonal pheochromocytoma cells flatten and extend neurites capable of making functional synapses. Although no significant changes in overall phospholipid composition occur in the presence of NGF, there is increased incorporation of ³²PO₄ into phosphatidylinositol and phosphatidic acid within 10 min after the addition of NGF. NGF stimulates the incorporation of ³²PO₄ into other lipids, such as phosphatidylcholine, to a lesser extent. The kinetics of the NGF-induced phosphatidylinositol responses are different when the cells are in suspension from when they are attached to culture dishes. These changes in phospholipid metabolism are discussed with respect to their role in NGF-induced nerve differentiation.     

干细胞治疗周围神经损伤研究进展

周围神经损伤是临床常见的疾病。损伤后神经的修复和再生是复杂又漫长的过程。严重的神经损伤其预后效果并不令人满意,相应支配区域的功能难以恢复,这给患者及家人带来了极大的痛苦。因此如何更好的对周围神经损伤进行治疗一直是医学界的难题。在神经修复机制的研究中,科学家发现施万细胞对周围神经的修复和再生起到了非常重要的作用,但获取和扩增的困难限制了其临床的应用。随着生物医学的发展,人们把目光投向了干细胞,经实验发现干细胞不仅具有旺盛的增殖能力,而且可以分化为神经系细胞,还能分泌相关的神经营养因子促进神经的修复和再生,这为周围神经损伤后的治疗带来了新的希望。本文就近些年来应用于修复周围神经的干细胞及促进修复机制的研究做以综述。     

ErbB2 Receptor Over-Expression Improves Post-Traumatic Peripheral Nerve Regeneration in Adult Mice

In a transgenic mice (BALB-neuT) over-expressing ErbB2 receptor, we investigated the adult mouse median nerve in physiological and pathological conditions. Results showed that, in physiological conditions, the grip function controlled by the median nerve in BALB-neuT mice was similar to wild-type (BALB/c). Stereological assessment of ErbB2-overexpressing intact nerves revealed no difference in number and size of myelinated fibers compared to wild-type mice. By contrast, after a nerve crush injury, the motor recovery was significantly faster in BALB-neuT compared to BALB/c mice. Moreover, stereological assessment revealed a significant higher number of regenerated myelinated fibers with a thinner axon and fiber diameter and myelin thickness in BALB-neuT mice. At day-2 post-injury, the level of the mRNAs coding for all the ErbB receptors and for the transmembrane (type III) Neuregulin 1 (NRG1) isoforms significantly decreased in both BALB/c and BALB-neuT mice, as shown by quantitative real time PCR. On the other hand, the level of the mRNAs coding for soluble NRG1 isoforms (type I/II, alpha and beta) increased at the same post-traumatic time point though, intriguingly, this response was significantly higher in BALB-neuT mice with respect to BALB/c mice. Altogether, these results suggest that constitutive ErbB2 receptor over-expression does not influence the physiological development of peripheral nerves, while it improves nerve regeneration following traumatic injury, possibly through the up-regulation of soluble NRG1 isoforms.     

Peripheral Nerve Injury Modulates Neurotrophin Signaling in the Peripheral and Central Nervous System

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75^NTR and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.     

Peripheral Nerve Injury Is Associated with Chronic,Reversible Changes in Global DNA Methylation in the Mouse Prefrontal Cortex

Changes in brain structure and cortical function are associated with many chronic pain conditions including low back pain and fibromyalgia. The magnitude of these changes correlates with the duration and/or the intensity of chronic pain. Most studies report changes in common areas involved in pain modulation, including the prefrontal cortex (PFC), and pain-related pathological changes in the PFC can be reversed with effective treatment. While the mechanisms underlying these changes are unknown, they must be dynamically regulated. Epigenetic modulation of gene expression in response to experience and environment is reversible and dynamic. Epigenetic modulation by DNA methylation is associated with abnormal behavior and pathological gene expression in the central nervous system. DNA methylation might also be involved in mediating the pathologies associated with chronic pain in the brain. We therefore tested a) whether alterations in DNA methylation are found in the brain long after chronic neuropathic pain is induced in the periphery using the spared nerve injury modal and b) whether these injury-associated changes are reversible by interventions that reverse the pathologies associated with chronic pain. Six months following peripheral nerve injury, abnormal sensory thresholds and increased anxiety were accompanied by decreased global methylation in the PFC and the amygdala but not in the visual cortex or the thalamus. Environmental enrichment attenuated nerve injury-induced hypersensitivity and reversed the changes in global PFC methylation. Furthermore, global PFC methylation correlated with mechanical and thermal sensitivity in neuropathic mice. In summary, induction of chronic pain by peripheral nerve injury is associated with epigenetic changes in the brain. These changes are detected long after the original injury, at a long distance from the site of injury and are reversible with environmental manipulation. Changes in brain structure and cortical function that are associated with chronic pain conditions may therefore be mediated by epigenetic mechanisms.     

Simplified Large-Scale Refolding,Purification, and Characterization of Recombinant Human Granulocyte-Colony Stimulating Factor in Escherichia coli

Granulocyte-colony stimulating factor (G-CSF) is a pleiotropic cytokine that stimulates the development of committed hematopoietic progenitor cells and enhances the functional activity of mature cells. Here, we report a simplified method for fed-batch culture as well as the purification of recombinant human (rh) G-CSF. The new system for rhG-CSF purification was performed using not only temperature shift strategy without isopropyl-l-thio-β-d-galactoside (IPTG) induction but also the purification method by a single step of prep-HPLC after the pH precipitation of the refolded samples. Through these processes, the final cell density and overall yield of homogenous rhG-CSF were obtained 42.8 g as dry cell weights, 1.75 g as purified active proteins, from 1 L culture broth, respectively. The purity of rhG-CSF was finally 99% since the isoforms of rhG-CSF could be separated through the prep-HPLC step. The result of biological activity indicated that purified rhG-CSF has a similar profile to the World Health Organization (WHO) 2^nd International Standard for G-CSF. Taken together, our results demonstrate that the simple purification through a single step of prep-HPLC may be valuable for the industrial-scale production of biologically active proteins.     

Acute Regulation of the Epidermal Growth Factor Receptor in Response to Nerve Growth Factor

PC12 cells possess specific receptors for both nerve growth factor and epidermal growth factor, and by an unknown mechanism, nerve growth factor is able to attenuate the propagation of a mitogenic response to epidermal growth factor. The differentiation response of PC12 cells to nerve growth factor, therefore, predominates over the proliferative response to epidermal growth factor. We have observed that the addition of nerve growth factor to PC12 cells rapidly produces a decrease in surface 125I-epidermal growth factor binding capacity. Unlike previously described nerve growth factor effects on 125I-epidermal growth factor binding capacity, which required several days of nerve growth factor exposure, the decreases we report occur within minutes of nerve growth factor addition: A 50% decrease in 125I-epidermal growth factor binding capacity is evident at 10 min. This rapid nerve growth factor response is concentration dependent; inhibition of 125I-epidermal growth factor binding is detectable at nerve growth factor levels as low as 0.2 ng/ml and is maximal at approximately 50 ng/ml, consistent with known ranges of biological activity. No demonstrable differences in the rate of epidermal growth factor receptor synthesis or degradation were observed in cells acutely exposed to nerve growth factor. Scatchard analysis revealed that acute nerve growth factor treatment decreased the number of both high- and low-affinity 125I-epidermal growth factor binding sites, while the receptor affinity remained unchanged. We have also investigated the involvement of various potential intracellular mediators of nerve growth factor action and of known intracellular modulatory systems of the epidermal growth factor receptor for their capacity to participate in this nerve growth factor activity.     

Transplantation of Olfactory Ensheathing Cells to Evaluate Functional Recovery after Peripheral Nerve Injury

Olfactory ensheathing cells (OECs) are neural crest cells which allow growth and regrowth of the primary olfactory neurons. Indeed, the primary olfactory system is characterized by its ability to give rise to new neurons even in adult animals. This particular ability is partly due to the presence of OECs which create a favorable microenvironment for neurogenesis. This property of OECs has been used for cellular transplantation such as in spinal cord injury models. Although the peripheral nervous system has a greater capacity to regenerate after nerve injury than the central nervous system, complete sections induce misrouting during axonal regrowth in particular after facial of laryngeal nerve transection. Specifically, full sectioning of the recurrent laryngeal nerve (RLN) induces aberrant axonal regrowth resulting in synkinesis of the vocal cords. In this specific model, we showed that OECs transplantation efficiently increases axonal regrowth.OECs are constituted of several subpopulations present in both the olfactory mucosa (OM-OECs) and the olfactory bulbs (OB-OECs). We present here a model of cellular transplantation based on the use of these different subpopulations of OECs in a RLN injury model. Using this paradigm, primary cultures of OB-OECs and OM-OECs were transplanted in Matrigel after section and anastomosis of the RLN. Two months after surgery, we evaluated transplanted animals by complementary analyses based on videolaryngoscopy, electromyography (EMG), and histological studies. First, videolaryngoscopy allowed us to evaluate laryngeal functions, in particular muscular cocontractions phenomena. Then, EMG analyses demonstrated richness and synchronization of muscular activities. Finally, histological studies based on toluidine blue staining allowed the quantification of the number and profile of myelinated fibers.All together, we describe here how to isolate, culture, identify and transplant OECs from OM and OB after RLN section-anastomosis and how to evaluate and analyze the efficiency of these transplanted cells on axonal regrowth and laryngeal functions.     

Rapid Activation of Tyrosine Hydroxylase in Response to Nerve Growth Factor

Abstract: Nerve growth factor protein (NGF) was found to rapidly promote the activation of tyrosine hydroxylase in cultured rat PC 12 pheochromocytoma cells. PC 12 cultures were exposed to NGF for periods of less than 1 h and the soluble contents of homogenates prepared from the cells were assayed for tyrosine hydroxylase activity. Under these conditions, the specific enzymatic activity was increased by 60 ± 10% (n = 13) in comparison with that in untreated sister cultures. The increase was half maximal by 2–5 min of exposure and at NGF concentrations of about 10 ng/ml (0.36 n M ). Antiserum against NGF blocked the effect. Tyrosine hydroxylase activity could also be rapidly increased by NGF in cultures of PC12 cells that had been treated with the factor for several weeks in order to produce a neuron-like phenotype. This was achieved by withdrawing NGF for about 4 h and then readding it for 30 min. The NGF-induced increase of tyrosine hydroxylase activity in PC12 cultures was not affected by inhibition of protein synthesis and therefore appeared to be due to activation of the enzyme. Kinetic experiments revealed that NGF brought about no change in the apparent K_m of the enzyme for tyrosine or for co-factor (6-methyltetrahydropteridine), but that it did significantly increase the apparent maximum specific activity of the enzyme. These observations suggest that NGF (perhaps released by target organs) could promote a rapid and local enhancement of noradrenergic transmission in the sympathetic nervous system.