Regulation of NGF and NT-3 protein expression in peripheral targets by sympathetic input

Pitavastatin attenuates neuropathic pain induced by partial sciatic nerve in Wistar rats

OBJECTIVES

Pitavastatin is a competitive HMG-CoA reductase inhibitor for lowering of cholesterol level and low density lipoprotein cholesterol. This study was designed to evaluate the effect of pitavastatin in neuropathic pain induced by partial sciatic nerve ligation along with neuronal changes in Wister rats.

METHODS

Pitavastatin was started three days prior to the surgery and continued for 14 days The pain was determined by thermal hyperalgesia and cold allodynia. The biochemical changes were estimated at the end of the study. The levels of cytokines were measured using an ELISA test. Western blot analysis was used to detect levels of expression of JNK, p-JNK, ERK, p-ERK, p38MAPK, p-p38MAPK. The sciatic nerve was investigated histopathologically.

KEY FINDINGS

Pitavastatin significantly ameliorated nerve pain induced by PSNL and also attenuated the biochemical changes in a dose-dependent manner. The levels of inflammatory mediators were inhibited by pitavastatin. There was significant improvement in sciatic nerve fibres histology. The levels of p-38, p-ERK, and p-JNK and their associated phosphorylated proteins were reduced after treatment with pitavastatin.

CONCLUSION

The present study indicates that treatment with pitavastatin reversed the PSNL-induced neuropathy in Wister rats and may be an additional therapeutic strategy in the management of neuropathic pain.

Early and Late Transcriptional Changes in Blood, Neural, and Colon Tissues in Rat Models of Stress-Induced and Comorbid Pain Hypersensitivity Reveal Regulatory Roles in Neurological Disease

Background

Irritable bowel syndrome (IBS) and temporomandibular disorder (TMD) are two chronic pain conditions that frequently overlap in the same individual, more commonly in women. Stress is a significant risk factor, exacerbating or triggering one or both conditions. However, the mechanisms underlying IBS–TMD co-morbidity are mostly unknown.

Aim

To detect both specific and common stress-induced visceral hypersensitivity (SIH) and comorbid TMD–IBS pain hypersensitivity (CPH) genetic signatures over time.

Method

Twenty-four female rats were randomly assigned to one of three experimental groups: naïve, SIH, and CPH (orofacial pain plus stress). RNA was extracted from blood, colon, spinal cord, and dorsal root ganglion 1 or 7 weeks after the stress paradigm. We combined differential gene expression and co-expression network analyses to define both SIH and CPH expression profiles across tissues and time.

Results

The transcriptomic profile in blood and colon showed increased expression of genes enriched in inflammatory and neurological biological processes in CPH compared to SIH rats, both at 1 and 7 weeks after stress. In lumbosacral spinal tissue, both SIH and CPH rats compared to naïve revealed decreased expression of genes related to synaptic activity and increased expression of genes enriched in “angiogenesis,” “Neurotrophin,” and “PI3K-Akt” pathways. Compared to SIH, CPH rats showed increased expression of angiogenesis-related genes 1 week after exposure to stress, while 7 weeks post-stress the expression of these genes was higher in SIH rats. In dorsal root ganglia (DRG), CPH rats showed decreased expression of immune response genes at week 1 and inhibition of nerve myelination genes at 7 weeks compared to naïve. For all tissues, we observed higher expression of genes involved in ATP production in SIH compared to CPH at 1 week and this was reversed 7 weeks after the induction of stress.

Conclusion

Our study highlights an increased inflammatory response in CPH compared to SIH rats in the blood and colon. DRG and spinal transcriptomic profiles of both CPH and SIH rats showed inhibition of synaptic activity along with activation of angiogenesis. Targeting these biological processes may lead to a more profound understanding of the mechanisms underlying IBS–TMD comorbidities and new diagnostic and therapeutic strategies.

Engineered Schwann Cell-Based Therapies for Injury Peripheral Nerve Reconstruction

Compared with the central nervous system, the adult peripheral nervous system possesses a remarkable regenerative capacity, which is due to the strong plasticity of Schwann cells (SCs) in peripheral nerves. After peripheral nervous injury, SCs de-differentiate and transform into repair phenotypes, and play a critical role in axonal regeneration, myelin formation, and clearance of axonal and myelin debris. In view of the limited self-repair capability of SCs for long segment defects of peripheral nerve defects, it is of great clinical value to supplement SCs in necrotic areas through gene modification or stem cell transplantation or to construct tissue-engineered nerve combined with bioactive scaffolds to repair such tissue defects. Based on the developmental lineage of SCs and the gene regulation network after peripheral nerve injury (PNI), this review summarizes the possibility of using SCs constructed by the latest gene modification technology to repair PNI. The therapeutic effects of tissue-engineered nerve constructed by materials combined with Schwann cells resembles autologous transplantation, which is the gold standard for PNI repair. Therefore, this review generalizes the research progress of biomaterials combined with Schwann cells for PNI repair. Based on the difficulty of donor sources, this review also discusses the potential of “unlimited” provision of pluripotent stem cells capable of directing differentiation or transforming existing somatic cells into induced SCs. The summary of these concepts and therapeutic strategies makes it possible for SCs to be used more effectively in the repair of PNI.

Triphala Churna-A Traditional Formulation in Ayurveda Mitigates Diabetic Neuropathy in Rats

Neuropathy is a common complication of diabetes affecting a large number of people worldwide. Triphala churna is a formulation mentioned in Ayurveda-a traditional system of medicine. It is a simple powder formulation consisting of powders of three fruits, Emblica officinalis L., Terminalia bellirica (Gaertn.) Roxb. and Terminalia chebula Retz. Individual components of Triphala churna have anti-diabetic and antioxidant activities. Hence, this study was designed to evaluate the effect of Triphala churna on diabetic neuropathy. Diabetes was induced with streptozotocin (STZ, 55 mg/kg, i. p.) in rats. Animals were grouped and treated orally with Triphala churna at a dose of 250, 500, and 1,000 mg/kg after 6 weeks of diabetes induction for the next 4 weeks. At the end of study, parameters such as body weight, plasma glucose level, motor nerve conduction velocity were determined. The effect of Triphala churna on thermal hyperalgesia, mechanical hyperalgesia, and mechanical allodynia was also determined at the end of study. The plasma cytokine levels like TGF-β1, TNF-α, and IL-1β were determined by ELISA assay. Histopathology study of the sciatic nerve was studied. Western blotting was performed to study the expression of neuronal growth factor.Treatment with Triphala churna showed a significant reduction in plasma glucose and a significant rise in body weight. Triphala treatment significantly increased the motor nerve conduction velocity and decreased the thermal and mechanical hyperalgesia, as well as mechanical allodynia. The treatment significantly inhibited levels of circulatory cytokines like TGF-β1, TNF-α, and IL-1β. Histopathology study confirmed the neuroprotective effect of Triphala churna. The expression of NGF was significantly increased in sciatic nerves after treatment with Triphala churna. From the results, it can be concluded that Triphala churna delays the progression of neuropathy in diabetic rats.

Role of Nerve Growth Factor in Orofacial Pain

Some chronic pain conditions in the orofacial region are common and the mechanisms underlying orofacial pain are unresolved. Nerve growth factor (NGF) is a member of a family of neurotrophins and regulates the growth, maintenance and development of neurons. Increasing evidence suggests that NGF plays a crucial role in the generation of pain and hyperalgesia in different pain states. This review investigates the role of NGF in orofacial pain and their underlying cellular mechanisms, which may provide essential guidance to drug-discovery programmes. A systemic literature search was conducted in Pubmed focusing on NGF and orofacial pain. Articles were reviewed, and those discussing in vitro studies, animal evidence, clinical course, and possible mechanisms were summarized. We found a hyperalgesic effect of NGF in peripheral sensitization in orofacial pain models. We also summarize the current knowledge regarding NGF-dependent pain mechanism, which is initiated by retrograde transport of the ligand-receptor complex, ensuing transcriptional regulation of many important nociceptor genes involved in nociceptive processing. Phase III trials suggest that anti-NGF drug is endorsed with anti-inflammatory and pain-relieving effects with good tolerance in a variety of pain conditions, including pain associated with osteoarthritis and chronic lower back pain. Based on the data reviewed herein, NGF is believed to be an important hyperalgesic mediator in orofacial pain. The identification of underlying mechanisms and pathways of orofacial pain opens new frontiers for pain management.

ProNGF and Neurodegeneration in Alzheimer's Disease

Profound and early basal forebrain cholinergic neuron (BFCN) degeneration is a hallmark of Alzheimer's disease (AD). Loss of synapses between basal forebrain and hippocampal and cortical target tissue correlates highly with the degree of dementia and is thought to be a major contributor to memory loss. BFCNs depend for their survival, connectivity and function on the neurotrophin nerve growth factor (NGF) which is retrogradely transported from its sites of synthesis in the cortex and hippocampus. The form of NGF found in human brain is proNGF. ProNGF binds to the NGF receptors TrkA and p75^NTR, but it binds more strongly to p75^NTR and more weakly to TrkA than does mature NGF. This renders proNGF more sensitive to receptor balance than mature NGF. In the healthy brain, where BFCNs express both TrkA and p75^NTR, proNGF is neurotrophic, activating TrkA-dependent signaling pathways such as MAPK and Akt-mTOR and eliciting cell survival and neurite outgrowth. However, if TrkA is lost or if p75^NTR is increased, proNGF activates p75^NTR-dependent apoptotic pathways such as JNK. This receptor sensitivity serves as a neurotrophic/apoptotic switch that eliminates BFCNs that cannot maintain TrkA/p75^NTR balance and therefore synaptic connections with their targets. TrkA is increasingly lost in mild cognitive impairment (MCI) and AD. In addition, proNGF accumulates at BFCN terminals in cortex and hippocampus, reducing the amount of trophic factor that reaches BFCN cell bodies. The loss of TrkA and accumulation of proNGF occur early in MCI and correlate with cognitive impairment. Increased levels of proNGF and reduced levels of TrkA lead to BFCN neurodegeneration and eventual p75NTR-dependent apoptosis. In addition, in AD BFCNs suffer from reduced TrkA-dependent retrograde transport which reduces neurotrophic support. Thus, BFCNs are particularly vulnerable to AD due to their dependence upon retrograde trophic support from proNGF signaling and transport.

Peripheral sympathetic mechanisms in orofacial pain

Sympathetic nervous system (SNS) is a part of the autonomic nervous system which involuntarily regulates internal body functions. It appears to modulate the processing of nociceptive information. Many orofacial pain conditions involve inflammation of orofacial tissues and/or injury of nerve, some of which might be attributed to SNS. Thus, the aim of this review was to bring together the data available regarding the peripheral sympathetic mechanisms involved in orofacial pain. A clearer understanding of SNS–sensory interactions in orofacial pain may provide a basis for novel therapeutic strategies for conditions that respond poorly to conventional treatments.

WITHDRAWN: Neurotrophins and specific receptors in the oviduct tracts of Japanese quail (Coturnix coturnix japonica)

The Publisher regrets that this article is an accidental duplication of an article that has already been published in Ann. Anat., 210 (2017) 112 – 120>, http://dx.doi.org/10.1016/j.aanat.2016.04.034. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

Evaluation of the efficacy of cervical perivascular sympathectomy on drooling in children with athetoid cerebral palsy

OBJECTIVE

To evaluate the efficacy of cervical perivascular sympathectomy (CPVS) for drooling in children with athetoid cerebral palsy (ACP).

METHODS

The severity and frequency of drooling and the amount of salivation of 32 ACP children with drooling were evaluated before CPVS and at 4th, 12th and 24 weeks postoperatively by the teacher drooling scale (TDS) and salivary flow rate (SFR).

RESULTS

Fifteen children exhibited improvements on drooling according to the TDS score at 4th week after surgery (P < 0.05). Later, the number of children decreased to 10 at 12th week (P < 0.05) and to 8 at 24 week after surgery (P < 0.05). SFR was 0.67 mg/min at baseline, which decreased to 0.58 mg/min (P < 0.05) at 4th week after surgery. However, SFR showed a gradual increase at 12th week and 24 week with no significant difference.

CONCLUSIONS

Although CPVS was effective in improving drooling in some children with ACP, the results were not satisfactory. Thus, CPVS still needs to be cautiously used. Furthermore, more rigorous clinical studies should be performed to detect the effectiveness and safety of this procedure.

Upregulation of BDNF and NGF in cervical intervertebral discs exposed to painful whole-body vibration

STUDY DESIGN

In vivo study defining expression of the neurotrophins, brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), in cervical intervertebral discs after painful whole-body vibration (WBV).

OBJECTIVE

The goal of this study is to determine if BDNF and NGF are expressed in cervical discs after painful WBV in a rat model.

SUMMARY OF BACKGROUND DATA

WBV is a possible source of neck pain and has been implicated as increasing the risk for disc disorders. Typically, aneural regions of painful human lumbar discs exhibit hyperinnervation, suggesting nerve ingrowth as potentially contributing to disc degeneration and pain. BDNF and NGF are upregulated in painfully degenerate lumbar discs and hypothesized to contribute to this pathology.

METHODS

Male Holtzman rats underwent 7 days of repeated WBV (15 Hz, 30 min/d) or sham exposures, followed by 7 days of rest. Cervical discs were collected for analysis of BDNF and NGF expression through RT-qPCR and Western blot analysis. Immunohistochemistry also evaluated their regional expression in the disc.

RESULTS

Vibration significantly increases BDNF messenger ribonucleic acid (mRNA) levels (P=0.036), as well as total-NGF mRNA (P=0.035). Protein expression of both BDNF (P=0.006) and the 75-kDa NGF (P=0.045) increase by nearly 4- and 10-fold, respectively. Both BDNF mRNA (R=0.396; P=0.012) and protein (R=0.280; P=0.035) levels are significantly correlated with the degree of behavioral sensitivity (i.e., pain) at day 14. Total-NGF mRNA is also significantly correlated with the extent of behavioral sensitivity (R=0.276; P=0.044). Both neurotrophins are most increased in the inner annulus fibrosus and nucleus pulposus.

CONCLUSION

The increases in BDNF and NGF in the cervical discs after painful vibration are observed in typically aneural regions of the disc, consistent with reports of its hyperinnervation. Yet, the induction of nerve ingrowth into the disc was not explicitly investigated. Neurotrophin expression also correlates with behavioral sensitivity, suggesting a role for both neurotrophins in the development of disc pain.

LEVEL OF EVIDENCE

N/A.

Neurotrophins regulate cholinergic synaptic transmission in cultured rat sympathetic neurons through a p75-dependent mechanism

The sympathetic nervous system regulates many essential physiological systems, and its dysfunction is implicated in cardiovascular diseases. Mechanisms that control the strength of sympathetic output are therefore potential targets for the management of these disorders. Here we show that neurotrophins rapidly potentiate cholinergic transmission between cultured rat sympathetic neurons. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), acting at the p75 receptor, increased the amplitude of excitatory postsynaptic currents (EPSCs). We observed increased amplitude but not frequency of miniature synaptic currents after p75 activation, suggesting that p75 acts postsynaptically to modulate transmission at these synapses. This neurotrophic modulation enhances cholinergic EPSCs via sphingolipid signaling. Application of sphingolactone-24, an inhibitor of neutral sphingomyelinase, blocked the effect of BDNF, implicating a sphingolipid pathway. Furthermore, application of the p75-associated sphingolipid second messengers C(2)-ceramide and d-erythro-sphingosine restricted to the postsynaptic cell mimicked BDNF application. Postsynaptic blockade of ceramide production with fumonisin, a ceramide synthase inhibitor, blocked the effects of BDNF and d-erythro-sphingosine, implicating ceramide or ceramide phosphate as the active signal. Together these data suggest that neurotrophin signaling, which occurs in vivo via release from sympathetic neurons and target tissues such as the heart, acutely regulates the strength of the sympathetic postganglionic response to central cholinergic inputs. This pathway provides a potential mechanism for modulating the strength of sympathetic drive to target organs such as the heart and could play a role in the development of cardiovascular diseases.

Transsynaptic activity-dependent regulation of axon branching and neurotrophin expression in vivo

The two major classes of activity-dependent neuroplasticity predict different consequences of activity alteration on circuit response. Hebbian plasticity (positive feedback) posits that alteration of neuronal activity causes a parallel response within a circuit. In contrast, homeostatic plasticity (negative feedback) predicts that altering neuronal activity results in compensatory responses within a circuit. The relative roles of these modes of plasticity in vivo are unclear, since neuronal circuits are difficult to manipulate in the intact organism. In this study, we tested the in vivo effects of activity deprivation in the superior cervical ganglion-pineal circuit of adult rats, which can be noninvasively silenced by exposing animals to constant light. We demonstrated that total deprivation of sympathetic activity markedly decreased the presence of axonal proteins in the pineal and reduced the density and thickness of sympathetic axonal arbors. In addition, we demonstrated that sympathetic inactivity eliminated pineal function and markedly decreased pineal expression of neurotrophins. Administration of β-adrenergic agonist restored the expression of presynaptic and postsynaptic proteins. Furthermore, compensatory axonal growth through collateral sprouting, normally seen following unilateral denervation of the pineal, was profoundly impaired in the absence of neural activity. Thus, these data suggest that sympathetic axonal terminals are maintained by neural activity that induces neurotrophins, which may act through a retrograde mechanism to preserve the integrity of axonal arbors via a positive feedback loop. Conversely, by using Hebbian-like neuroplasticity, silent yet intact circuits enter a hibernation mode marked by reduction of presynaptic axonal structures and dramatically reduced postsynaptic expression of neurotrophins.

Complement protein C1q-mediated neuroprotection is correlated with regulation of neuronal gene and microRNA expression

Activation of the complement cascade, a powerful effector mechanism of the innate immune system, is associated with neuroinflammation but also with elimination of inappropriate synapses during development. Synthesis of C1q, a recognition component of the complement system, occurs in brain during ischemia/reperfusion and Alzheimer's disease, suggesting that C1q may be a response to injury. In vitro, C1q, in the absence of other complement proteins, improves neuronal viability and neurite outgrowth and prevents β-amyloid-induced neuronal death, suggesting that C1q may have a direct neuroprotective role. Here, investigating the molecular basis for this neuroprotection in vitro, addition of C1q to rat primary cortical neurons significantly upregulated expression of genes associated with cholesterol metabolism, such as cholesterol-25-hydroxylase and insulin induced gene 2, and transiently decreased cholesterol levels in neurons, known to facilitate neurite outgrowth. In addition, the expression of syntaxin-3 and its functional association with synaptosomal-associated protein 25 was increased. C1q also increased the nuclear translocation of cAMP response element-binding protein and CCAAT/enhancer-binding protein-δ (C/EBP-δ), two transcription factors involved in nerve growth factor (NGF) expression and downregulated specific microRNAs, including let-7c that is predicted to target (and thus inhibit) NGF and neurotrophin-3 (NT-3) mRNA. Accordingly, C1q increased expression of NGF and NT-3, and small interfering RNA inhibition of C/EBP-δ, NGF, or NT-3 expression prevented the C1q-dependent neurite outgrowth. No such neuroprotective effect is seen in the presence of C3a or C5a. Finally, the induced neuronal gene expression required conformationally intact C1q. These results show that C1q can directly promote neuronal survival, thereby demonstrating new interactions between immune proteins and neuronal cells that may facilitate neuroprotection.

Laquinimod suppress antigen presentation in relapsing-remitting multiple sclerosis: in-vitro high-throughput gene expression study

Laquinimod (LAQ) is a new immunomodulatory drug shown to be effective in the treatment of relapsing-remitting multiple sclerosis (RRMS); however, its molecular target pathways are not well recognized. In this study we characterized in-vitro the molecular effects of LAQ in peripheral blood mononuclear cells (PBMC) of healthy subjects and RRMS patients by gene expression microarrays. We demonstrated that LAQ induced suppression of genes related to antigen presentation and corresponding inflammatory pathways. These findings were demonstrated mainly via the NFkB pathway. Analysis of PBMC subpopulations identified activation of Th2 response in CD14+ and CD4+ cells and suppression of proliferation in CD8+ cells.

Proneurotrophin-3 is a neuronal apoptotic ligand: evidence for retrograde-directed cell killing

Although mature neurotrophins are well described trophic factors that elicit retrograde survival signaling, the precursor forms of neurotrophins (i.e., proneurotrophins) can function as high-affinity apoptotic ligands for selected neural populations. An outstanding question is whether target-derived proneurotrophins might affect neuronal survival/death decisions through a retrograde transport mechanism. Since neurotrophin-3 (NT-3) is highly expressed in non-neural tissues that receive peripheral innervation, we investigated the localized actions of its precursor (proNT-3) on sympathetic neurons in the present study. Pharmacological inhibition of intracellular furin proteinase activity in 293T cells resulted in proNT-3 release instead of mature NT-3, whereas membrane depolarization in cerebellar granule neurons stimulated endogenous proNT-3 secretion, suggesting that proNT-3 is an inducible bona fide ligand in the nervous system. Our data also indicate that recombinant proNT-3 induced sympathetic neuron death that is p75(NTR)- and sortilin-dependent, with hallmark features of apoptosis including JNK (c-Jun N-terminal kinase) activation and nuclear fragmentation. Using compartmentalized culture systems that segregate neuronal cell bodies from axons, proNT-3, acting within the distal axon compartment, elicited sympathetic neuron death and overrode the survival-promoting actions of NGF. Together, these results raise the intriguing possibility that dysregulation of proneurotrophin processing/release by innervated targets can be deleterious to the neurons projecting to these sites.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

p75 and TrkA signaling regulates sympathetic neuronal firing patterns via differential modulation of voltage-gated currents

Neurotrophins such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) act through the tropomyosin-related receptor tyrosine kinases (Trk) and the pan-neurotrophin receptor (p75) to regulate complex developmental and functional properties of neurons. While NGF activates both receptor types in sympathetic neurons, differential signaling through TrkA and p75 can result in widely divergent functional outputs for neuronal survival, growth, and synaptic function. Here we show that TrkA and p75 signaling pathways have opposing effects on the firing properties of sympathetic neurons, and define a mechanism whereby the relative level of signaling through these two receptors sets firing patterns via coordinate regulation of a set of ionic currents. We show that signaling through the p75 pathway causes sympathetic neurons to fire in a phasic pattern showing marked accommodation. Signaling through the NGF-specific TrkA, on the other hand, causes cells to fire tonically. Neurons switch rapidly between firing patterns, on the order of minutes to hours. We show that changes in firing patterns are caused by neurotrophin-dependent regulation of at least four voltage-gated currents: the sodium current and the M-type, delayed rectifier, and calcium-dependent potassium currents. Neurotrophin release, and thus receptor activation, varies among somatic tissues and physiological state. Thus, these data suggest that target-derived neurotrophins may be an important determinant of the characteristic electrical properties of sympathetic neurons and therefore regulate the functional output of the sympathetic nervous system.

Chapter 26: Age-related differences in the reinnervation after peripheral nerve injury

Numerous and extensive functional, structural, and biochemical changes characterize intact aged peripheral nervous system. Functional recovery after peripheral nerve injury depends on survival of injured neurons and functional reinnervation of target tissue by regeneration of injured axons and collateral sprouting of uninjured (intact) adjacent axons. The rate of axonal regeneration becomes slower and its extent (density and number of regenerating axons) decreases in aged animals. Aging also impairs terminal sprouting of regenerated axons and collateral sprouting of intact adjacent axons, thus further limiting target reinnervation and its functional recovery. Decreased survival of aged noninjured and injured neurons, limited intrinsic growth potential of neuron, alteration in its responsiveness to stimulatory or inhibitory environmental factors, and changes in the peripheral neural pathways and target tissues are possible reasons for impaired reinnervation after peripheral nerve injury in old age. The review of present data suggests that this impairment is mostly due to the age-related changes in the peripheral neural pathways and target tissues, and not due to the limited intrinsic growth capacity of neurons or their reduced responsiveness to trophic factors. Age-related alterations in the soluble target derived neurotrophic factors, like nerve growth factor, and nonsoluble extracellular matrix components of neural pathways, like laminin, might be important in this respect.

Changes in NGF and NT-3 protein species in the superior cervical ganglion following axotomy of postganglionic axons

Mature sympathetic neurons in the superior cervical ganglion (SCG) are regulated by target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3). High molecular weight NGF species and mature NT-3 are the predominant NGF and NT-3 protein isoforms in the SCG, yet it is unknown whether the presence of these species is dependent on intact connection with the target tissues. In an attempt to determine the role of peripheral targets in regulating the neurotrophin species found in the SCG, we investigated the NGF and NT-3 protein species present in the SCG following axotomy (transection) or injury of the post-ganglionic axons. Following a 7 day axotomy, the 22-24 kDa NGF species and the mature 14 kDa NT-3 species in the SCG were significantly reduced by 99% and 66% respectively, suggesting that intact connection with the target is necessary for the expression of these protein species. As expected, tyrosine hydroxylase (TH) protein in the SCG was significantly reduced by 80% at 7 days following axotomy. In order to distinguish between the effects of injury and loss of target connectivity, the SCG was examined following compression injury to the post-ganglionic nerves. Following injury, no reduction in the 22-24 kDa NGF or 14 kDa mature NT-3 species was observed in the SCG. TH protein was slightly, yet significantly, decreased in the SCG following injury. The findings of this study suggest that the presence of the 22-24 kDa NGF and mature 14 kDa NT-3 species in the SCG is dependent on connection with peripheral targets and may influence, at least in part, TH protein expression in adult sympathetic neurons.