Plasticity in adult and ageing sympathetic neurons

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.

Paracrine control of vascular innervation in health and disease

Proper vascular regulation is of paramount importance for the control of blood flow to tissues. In particular, the regulation of peripheral resistance arteries is essential for several physiological processes, including control of blood pressure, thermoregulation and increase of blood flow to central nervous system and heart under stress conditions such as hypoxia. Arterial tone is regulated by the periarterial autonomic nervous plexus, as well as by endothelium-dependent, myogenic and humoral mechanisms. Underscoring the importance of proper vascular regulation, defects in these processes can lead to diseases such as hypertension, orthostatic hypotension, Raynaud's phenomenon, defective thermoregulation, hand-foot syndrome, migraine and congestive heart failure. Here, we review the molecular mechanisms controlling the development of the periarterial nerve plexus, retrograde and localized signalling at neuro-effector junctions, the molecular and cellular mechanisms of vascular regulation and adult plasticity and maintenance of periarterial innervation. We particularly highlight a newly discovered role for vascular endothelial growth factor in the structural and functional maintenance of arterial neuro-effector junctions. Finally, we discuss how defects in neuronal vascular regulation can lead to disease.

Age-related changes in the sympathetic innervation of cerebral vessels and in carotid vascular responses to norepinephrine in the rat: in vitro and in vivo studies

We hypothesized that the density of sympathetic noradrenergic innervation of cerebral arteries and vasoconstrictor responses evoked in carotid circulation by norepinephrine (NE) increase with maturation and age. In rats of 4-5, 10-12, and 42-44 wk of age (juvenile, mature, middle aged), glyoxylic acid applied to stretch preparations showed the density of noradrenergic nerves in basilar and middle cerebral arteries was greater in mature than juvenile or middle-aged rats. In anesthetized rats, infusion of NE (2.5 mug/kg iv) increased mean arterial pressure (ABP) to approximately 180 mmHg in mature and middle-aged but to only approximately 150 mmHg in juveniles rats. Concomitantly, carotid blood flow (CBF) decreased in mature and middle-aged rats but remained constant in juveniles because carotid vascular conductance (CVC) decreased more in mature and middle-aged than juvenile rats. We also hypothesized that nitric oxide (NO) blunts cerebral vasoconstrictor responses to NE. Inhibition of NO synthase with l-NAME (10 mg/kg iv) induced similar increases in baseline ABP in each group, but larger decreases in CVC and CBF in mature and middle-aged than juvenile rats. Thereafter, the NE-evoked increase in ABP was similar in juvenile and mature but accentuated in middle-aged rats. Concomitantly, NE decreased CVC in juvenile and mature, but not middle-aged rats; in them, CBF increased. Thus, in juvenile rats, sparse noradrenergic innervation of cerebral arteries is associated with weak NE-evoked pressor responses and weak carotid vasoconstriction that allows autoregulation of CBF. Cerebral artery innervation density increases with maturation but lessens by middle age. Meanwhile, NE-evoked pressor responses and carotid vasoconstriction are stronger in mature and middle-aged rats, such that CBF falls despite the evoked increase in ABP. We propose that in juvenile and mature rats, NO does not modulate NE-evoked pressor responses, cerebral vasoconstriction, or CBF autoregulation, but by middle age, NO limits pressor responses and prevents breakthrough of CBF in the upper part of the autoregulatory range.

Aging impairs collateral sprouting of nociceptive axons in the rat

Sprouting of uninjured nociceptive axons was examined in young adult, middle aged and aged rats. Axon sprouting from the spared sural nerve, both into adjacent denervated skin and into end-to-side coapted nerve graft, was significantly higher in young rats than in aged rats. Cross-transplantations of the end-to-side coapted nerve grafts between young and aged rats demonstrated that axon sprouting from young recipient nerves into aged donor nerve grafts was significantly deteriorated, whereas the axon sprouting from aged recipient nerves into young donor nerve grafts was not statistically significantly affected. The levels of laminin polypeptides in peripheral nerves were 50-100% higher in young adult than in aged rats. However, the levels of peripherin, NGF isoforms and TrkA in skin, peripheral nerves and DRG, respectively, were not significantly reduced in aged rats. Therefore, impaired sprouting of nociceptive axons in aged rats is due rather to the alterations in peripheral neural pathways, than to the limited sprouting capacity of aged sensory neurons. Decreased levels of extracellular matrix components might be important in this respect.

Age-related changes in vagal afferents innervating the gastrointestinal tract

Recent progress in understanding visceral afferents, some of it reviewed in the present issue, serves to underscore how little is known about the aging of the visceral afferents in the gastrointestinal (GI) tract. In spite of the clinical importance of the issue-with age, GI function often becomes severely compromised-only a few initial observations on age-related structural changes of visceral afferents are available. Primary afferent cell bodies in both the nodose ganglia and dorsal root ganglia lose Nissl material and accumulate lipofucsin, inclusions, aggregates, and tangles. Additionally, in changes that we focus on in the present review, vagal visceral afferent terminals in both the muscle wall and the mucosa of the GI tract exhibit age-related structural changes. In aged animals, both of the vagal terminal types examined, namely intraganglionic laminar endings and villus afferents, exhibit dystrophic or regressive morphological changes. These neuropathies are associated with age-related changes in the structural integrity of the target organs of the affected afferents, suggesting that local changes in trophic environment may give rise to the aging of GI innervation. Given the clinical relevance of GI tract aging, a more complete understanding both of how aging alters the innervation of the gut and of how such changes might be mitigated should be made research priorities.

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.

Neurotoxic and neurotrophic roles of proNGF and the receptor sortilin in the adult and ageing nervous system

The precursor form of the nerve growth factor (proNGF), forms a heterotrimeric complex with the receptors p75 and sortilin; this complex has been implicated in neuron cell death. However, it is not known whether proNGF and the receptors p75 and sortilin contribute to age- and disease-related neurodegeneration. Here we show that proNGF induces cell death in subpopulations of basal forebrain and peripheral sympathetic neurons of old, but not of young, adult rodents. In contrast, proNGF appears to induce neurite outgrowth rather than cell death of young adult sympathetic neurons. We have examined the neurotoxic role of proNGF in old age, and find that proNGF protein is elevated during ageing in the projection areas of some populations of vulnerable central and peripheral neurons; caloric restriction, which has known neuroprotective effects, partially prevents these increases. Sortilin was found to play a significant part in the observed patterns of age-related proNGF-mediated neurotoxicity. In particular, survival of aged neurons was rescued by neurotensin, an alternative sortilin ligand that blocks the sortilin-mediated effects of proNGF. Furthermore, sortilin immunoreactivity increases markedly in ageing rodent basal forebrain and sympathetic neurons; in contrast, p75 levels are either unchanged or reduced. From these data we propose that selective age-related neuronal atrophy and neurodegeneration may be mediated by increased sortilin expression in neurons, together with elevated levels of proNGF expression in some targets.

Enteric neurodegeneration in ageing

The objective of this article is to review the clinical presentation and neurobiology of degeneration of the enteric nervous system with emphasis on human data where available. Constipation, incontinence and evacuation disorders are frequently encountered in the ageing population. Healthy lower gastrointestinal function is essential for successful ageing as it is critical to maintaining independence and autonomy to pursue further activity. One clinical expression of enteric neurodegeneration is constipation. However, the aetiology may be multifactorial as disturbances of epithelial, muscle or neural function may all result from neurodegeneration. There is evidence of loss of excitatory (e.g. cholinergic) enteric neurons and interstitial cells of Cajal, whereas inhibitory (including nitrergic) neurons appear unaffected. Understanding neurodegeneration in the enteric nervous system is key to developing treatments to reverse it. Neurotrophins have been shown to accelerate colonic transit and relieve constipation in the medium term; they are also implicated in maintenance programmes in adult enteric neurons through a role in antioxidant defence. However, their effects in ageing colon require further study. There is evidence that 5-HT(2) and 5-HT(4) mechanisms are involved in development, maintenance and survival of enteric neurons. Further research is needed to understand and potentially reverse enteric neurodegeneration.

Enteric neurodegeneration in ageing

The objective of this article is to review the clinical presentation and neurobiology of degeneration of the enteric nervous system with emphasis on human data where available. Constipation, incontinence and evacuation disorders are frequently encountered in the ageing population. Healthy lower gastrointestinal function is essential for successful ageing as it is critical to maintaining independence and autonomy to pursue further activity. One clinical expression of enteric neurodegeneration is constipation. However, the aetiology may be multifactorial as disturbances of epithelial, muscle or neural function may all result from neurodegeneration. There is evidence of loss of excitatory (e.g. cholinergic) enteric neurons and interstitial cells of Cajal, whereas inhibitory (including nitrergic) neurons appear unaffected. Understanding neurodegeneration in the enteric nervous system is key to developing treatments to reverse it. Neurotrophins have been shown to accelerate colonic transit and relieve constipation in the medium term; they are also implicated in maintenance programmes in adult enteric neurons through a role in antioxidant defence. However, their effects in ageing colon require further study. There is evidence that 5-HT(2) and 5-HT(4) mechanisms are involved in development, maintenance and survival of enteric neurons. Further research is needed to understand and potentially reverse enteric neurodegeneration.

Role of PACAP in the physiology and pathology of the sympathoadrenal system

Sympathetic neurons and chromaffin cells derive from common sympathoadrenal precursors which arise from the neural crest. Cells from this lineage migrate to their final destination and differentiate by acquiring a catecholaminergic phenotype in response to different environmental factors. It has been shown that the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its PAC1 receptor are expressed at early stages of sympathetic development, and participate to the control of neuroblast proliferation and differentiation. PACAP also acts as a neurotransmitter to stimulate catecholamine and neuropeptide biosynthesis and release from sympathetic neurons and chromaffin cells, during development and in adulthood. In addition, PACAP and its receptors have been described in neuroblastoma and pheochromocytoma, and the neuropeptide regulates the differentiation and activity of sympathoadrenal-derived tumoral cell lines, suggestive of an important role in the pathophysiology of the sympathoadrenal lineage. Transcriptome studies uncovered genes and pathways of known and unknown roles that underlie the effects of PACAP in the sympathoadrenal system.

Degeneration of vagal efferent axons and terminals in cardiac ganglia of aged rats

Baroreflex control of the heart rate is significantly reduced during aging. However, neural mechanisms that underlie such a functional reduction are not fully understood. We injected the tracer DiI into the left nucleus ambiguus (NA), then used confocal microscopy and a Neurolucida Digitization System to examine qualitatively and quantitatively vagal efferent projections to cardiac ganglia of young adult (5-6 months) and aged (24-25 months) rats (Sprague Dawley). Fluoro-Gold was injected intraperitoneally to counterstain cardiac ganglionic principal neurons (PNs). In aged, as in young rats, NA axons projected to all cardiac ganglia and formed numerous basket endings around PNs in the hearts. However, significant structural changes were found in aged rats compared with young rats. Vagal efferent axons contained abnormally swollen axonal segments and exhibited reduced or even absent synaptic-like terminals around PNs, such that the numbers of vagal fibers and basket endings around PNs were substantially reduced (P < 0.01). Furthermore, synaptic-like varicose contacts of vagal cardiac axons with PNs were significantly reduced by approximately 50% (P < 0.01). These findings suggest that vagal efferents continue to maintain homeostatic control over the heart during aging. However, the marked morphological reorganization of vagal efferent axons and terminals in cardiac ganglia may represent the structural substrate for reduced vagal control of the heart rate and attenuated baroreflex function during aging.

Age-dependent changes in Ca2+ homeostasis in peripheral neurones: implications for changes in function

Calcium ions represent universal second messengers within neuronal cells integrating multiple cellular functions, such as release of neurotransmitters, gene expression, proliferation, excitability, and regulation of cell death or apoptotic pathways. The magnitude, duration and shape of stimulation-evoked intracellular calcium ([Ca2+]i) transients are determined by a complex interplay of mechanisms that modulate stimulation-evoked rises in [Ca2+]i that occur with normal neuronal function. Disruption of any of these mechanisms may have implications for the function and health of peripheral neurones during the aging process. This review focuses on the impact of advancing age on the overall function of peripheral adrenergic neurones and how these changes in function may be linked to age-related changes in modulation of [Ca2+]i regulation. The data in this review suggest that normal aging in peripheral autonomic neurones is a subtle process and does not always result in dramatic deterioration in their function. We present studies that support the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age-related decline in the function of at least one of the neuronal calcium-buffering systems, smooth endoplasmic reticulum calcium ATPases, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a 'weak point' in cellular compensation as this over time may contribute to cell death. In addition, we present more recent studies on [Ca2+]i regulation in the form of the modulation of release of calcium from smooth endoplasmic reticulum calcium stores. These studies suggest that the contribution of the release of calcium from smooth endoplasmic reticulum calcium stores is altered with age through a combination of altered ryanodine receptor levels and modulation of these receptors by neuronal nitric oxide containing neurones.

Reactive oxygen species, dietary restriction and neurotrophic factors in age-related loss of myenteric neurons

We have studied the mechanisms underlying nonpathological age-related neuronal cell death. Fifty per cent of neurons in the rat enteric nervous system are lost between 12 and 18 months of age in ad libitum (AL) fed rats. Caloric restriction (CR) protects almost entirely against this neuron loss. Using the ROS-sensitive dyes, dihydrorhodamine (DHR) and 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (HPF) in vitro, we show that the onset of cell death is linked with elevated intraneuronal levels of reactive oxygen species (ROS). Treatment with the neurotrophic factors NT3 and GDNF enhances neuronal antioxidant defence in CR rats at 12-15 months and 24 months but not in adult or aged AL-fed animals. To examine the link between elevated ROS and neuronal cell death, we assessed apoptotic cell death following in vitro treatment with the redox-cycling drug, menadione. Menadione fails to increase apoptosis in 6-month neurons. However, in 12-15mAL fed rats, when age-related cell death begins, menadione induces a 7- to 15-fold increase in the proportion of apoptotic neurons. CR protects age-matched neurons against ROS-induced apoptosis. Treatment with neurotrophic factors, in particular GDNF, rescues neurons from menadione-induced cell death, but only in 12-15mCR animals. We hypothesize that CR enhances antioxidant defence through neurotrophic factor signalling, thereby reducing age-related increases in neuronal ROS levels and in ROS-induced cell death.

Increased NGF proforms in aged sympathetic neurons and their targets

Target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3) regulate sympathetic neuron survival. Here, NGF and NT-3 protein and transcript were examined in sympathetic neurons and targets in order to determine their role in age-related neuronal atrophy. One obvious alteration was a dramatic increase (up to 50-fold) in NGF protein forms, corresponding to proNGF-B, in the superior cervical ganglion (SCG) and targets where sympathetic innervation shows atrophy. In the iris, where sympathetic innervation is protected into old age, proNGF-B was decreased. Alterations in NGF transcript paralleled changes in NGF protein, albeit to a lesser degree. Though significantly increased in aged SCG, NT-3 protein, found primarily as the 'mature' form, showed only minor changes in most tissues, though NT-3 mRNA generally was decreased. In contrast, both NT-3 transcript and NT-3 precursors were increased in iris. The dramatic increases in proNGF, together with minimal changes in NT-3, suggest that alterations in NGF regulation may contribute to the loss of sympathetic innervation observed in many aged peripheral targets.

Ovarian expression of alpha (1)- and beta (2)-adrenoceptors and p75 neurotrophin receptors in rats with steroid-induced polycystic ovaries

Polycystic ovary syndrome (PCOS) is the main cause of infertility in women. Despite extensive research aimed at identifying the pathogenetic mechanism underlying this condition, the aetiology of the disease is still unknown. Evidence from studies on women with PCOS and on an experimental rat polycystic ovary (PCO) model suggests that the sympathetic regulatory drive to the ovary may be unbalanced. The present study was designed to investigate this hypothesis. Accordingly, we used the well-defined rat PCO model, where PCO is induced by a single intramuscular (i.m.) injection of estradiol valerate (EV), and compared the model with oil-injected controls. We studied the ovarian expression of the alpha1- and beta2-adrenoceptors (ARs), the neurotrophin receptor p75 (p75NTR), and the sympathetic marker tyrosine hydroxylase (TH) at two time points: 30 and 60 days after EV injection. Our data demonstrate for the first time that all of the alpha1-AR subtypes are expressed in normal rat ovaries at both the mRNA and the protein levels. Furthermore, the expression of the alpha1-AR subtypes was differentially modulated in a time- and subtype-dependent manner in rats with EV-induced PCO. The ovaries in rats with steroid-induced PCO are characterised by an early overexpression of these molecules and p75NTR, while the beta2-AR was downregulated. An increase in the expression of ovarian TH after EV injection was also detected, suggesting a structural and functional remodelling of ovarian sympathetic innervation in PCO rats. Our evidence strongly indicates that the role of the sympathetic nervous system is crucial in the pathogenesis of EV-induced PCO. Overall, our findings suggest that therapeutical approaches aimed at down-regulating the sympathetic tone to the ovary could be useful in the prevention and clinical treatment of PCOS.

Effect of electro-acupuncture on ovarian expression of alpha (1)- and beta (2)-adrenoceptors, and p75 neurotrophin receptors in rats with steroid-induced polycystic ovaries

BACKGROUND

Estradiol valerate (EV)-induced polycystic ovaries (PCO) in rats is associated with an increase in ovarian sympathetic outflow. Low-frequency (2 Hz) electro-acupuncture (EA) has been shown to modulate sympathetic markers as well as ovarian blood flow as a reflex response via the ovarian sympathetic nerves, in rats with EV-induced PCO.

METHODS

In the present study, we further tested the hypothesis that repeated 2 Hz EA treatments modulate ovarian sympathetic outflow in rats with PCO, induced by a single i.m. injection of EV, by investigating the mRNA expression, the amount and distribution of proteins of alpha1a-, alpha1b-, alpha1d-, and beta2-adrenoceptors (ARs), as well as the low-affinity neurotrophin receptor (p75NTR).

RESULTS

It was found that EV injection results in significantly higher mRNA expression of ovarian alpha1b- and alpha1d-AR in PCO rats compared to control rats. The p75NTR and beta2-ARs mRNA expression were unchanged in the PCO ovary. Low-frequency EA resulted in a significantly lower expression of beta2-ARs mRNA expression in PCO rats. The p75NTR mRNA was unaffected in both PCO and control rats. PCO ovaries displayed significantly higher amount of protein of alpha1a-, alpha1b- and alpha1d-ARs, and of p75NTR, compared to control rats, that were all counteracted by repeated low-frequency EA treatments, except for alpha1b-AR.

CONCLUSION

The present study shows that EA normalizes most of the EV-induced changes in ovarian ARs. Furthermore, EA was able to prevent the EV-induced up regulation of p75NTR, probably by normalizing the sympathetic ovarian response to NGF action. Our data indicate a possible role of EA in the regulation of ovarian responsiveness to sympathetic inputs and depict a possible complementary therapeutic approach to overcoming sympathetic-related anovulation in women with PCOS.

Interactions of sympathetic nerves with capsaicin-sensitive sensory nerves: neurogenic mechanisms for phenol-induced hypertension in the rat

BACKGROUND

Previous investigations have shown that norepinephrine is capable of inhibiting neurotransmission in capsaicin-sensitive sensory nerves via a prejunctional mechanism. The alteration in the activity of sympathetic or capsaicin-sensitive sensory nerves in the development of phenol-induced hypertension was observed separately in rats.

METHODS

In the present study, we examined interactions of adrenergic nerves with capsaicin-sensitive sensory nerves in phenol-induced hypertensive rats. Blood pressure, the synthesis and release of calcitonin gene-related peptide (CGRP) and the content of nerve growth factor in (NGF) arteries were determined.

RESULTS

Intrarenal injection of phenol caused a permanent elevation of blood pressure concomitantly with a decrease in the concentration of CGRP in plasma, the content of CGRP in dorsal root ganglia and the density of CGRP-containing nerves in the mesenteric artery, and vascular NGF content. Chronic treatment with prazosin (an alpha1-adrenoreceptor antagonist, 3 mg/kg per day) failed to alter the synthesis and release of CGRP and vascular NGF content, even though it completely normalized blood pressure. However, treatment with yohimbine (an alpha2-adrenoreceptor antagonist, 5 mg/kg per day) significantly increased CGRP level and vascular NGF content. Combined administration of prazosin and yohimbine not only significantly elevated the synthesis and release of CGRP and arterial NGF content, but also completely normalized blood pressure.

CONCLUSION

These results indicate that the decreased production and release of CGRP and reduced vascular NGF content are attributed to the activation of alpha2-adrenoreceptors in phenol-induced hypertensive rats.

The role of NGF uptake in selective vulnerability to cell death in ageing sympathetic neurons

We have examined the hypothesis that differences in nerve growth factor (NGF) uptake and transport determine vulnerability to age-related neurodegeneration. Neurons projecting to cerebral blood vessels (CV) in aged rats are more vulnerable to age-related degeneration than those projecting to the iris. Uptake of NGF was therefore examined in sympathetic neurons projecting from the superior cervical ganglion (SCG) to CV and iris in young and old rats by treating the peripheral processes of these neurons with different doses of I125-NGF. Total uptake of I125-NGF was reduced in old CV-projecting, but not iris-projecting, neurons. Numbers of radiolabelled neurons projecting to each target were counted in sectioned ganglia. The data showed age-related reductions in numbers of labelled neurons projecting to CV, but no change in numbers of neurons projecting to the iris. Calculation of uptake of I125-NGF per neuron unexpectedly showed no major age-related differences in either of the two neuron populations. However, uptake per neuron was considerably lower for young and old CV-projecting, compared to iris-projecting, SCG neurons. We hypothesized that variations in NGF uptake might affect neuronal survival in old age. Counts of SCG neurons using a physical disector following retrograde tracing with Fluorogold confirmed the selective vulnerability of CV-projecting neurons by showing a significant 37% loss of these neurons in the period between 15 and 24 months. In contrast, there was no significant loss of iris-projecting neurons. We conclude that vulnerability to, or protection from, age-related neurodegeneration and neuronal cell death are associated with life-long low, or high, levels of NGF uptake, respectively.

Nicotinic acetylcholine receptor subtypes in the rat sympathetic ganglion: pharmacological characterization, subcellular distribution and effect of pre- and postganglionic nerve crush

Nicotinic acetylcholine receptors (nAChRs) mediate fast synaptic transmission in autonomic ganglia, which innervate and control the activity of most visceral organs. By combining ultrastructural, immunocytochemical, and pharmacological analyses, we characterized the nAChR subtypes in the rat superior cervical ganglion (SCG) and the effect of pre- and postganglionic nerve crush on their number in the ganglion and their distribution at the intraganglionic synapses. Binding with radioactive nicotinic ligands, immunoprecipitation, and immunolocalization experiments revealed the presence of different nAChR subtypes: those containing the alpha3 subunit associated with beta4 and/or beta2 subunits that bind 3H-Epibatidine with high affinity, and those containing the alpha7 subunit that bind 125I-alphaBungarotoxin. After postganglionic nerve crush, the number of nicotinic receptors and immunopositive intraganglionic synapses for each nAChR subunit strongly decreased. Both the number of nAChRs and immunoreactivity recovered 26 days after injury, when regenerating postganglionic fibers had reinnervated the peripheral target organs, as shown by the restoration of tyrosine hydroxylase immunoreactivity in the iris. This observation and the lack of any effect of preganglionic nerve crush on the number of nicotinic receptors suggest that the peripheral targets affect the organization of intraganglionic synapses in adult SCG.

Neurotrophin presence in human coronary atherosclerosis and metabolic syndrome: a role for NGF and BDNF in cardiovascular disease?

The development of atherosclerotic cardiovascular disease is a common comorbidity in patients with the metabolic syndrome, a concurrence of cardiovascular risk factors in one individual. While multiple growth factors and adipokines are identified in atherosclerotic lesions, as well as neurotrophins implicated in both cardiac ischemia and lipid and glucose metabolism, the potential role of neurotrophins in human coronary atherosclerosis and in the metabolic syndrome still remains to be elucidated. Here we describe and discuss our results that represent a novel attempt to study the cardiovascular and metabolic biology of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and mast cells (MC). The local amount of NGF, the immunolocalization of p75 neurotrophin receptor (p75NTR) and the number of MC were correlatively examined in coronary vascular wall and in the surrounding subepicardial adipose tissue, obtained from autopsy cases in humans with advanced coronary atherosclerosis. We also analyzed the plasma levels of NGF, BDNF and leptin and the number of MC in biopsies from abdominal subcutaneous adipose tissue in patients with a severe form of the metabolic syndrome. The results demonstrate that NGF levels are decreased in atherosclerotic coronary vascular tissue but increased in the subepicardial adipose tissue, whereas both tissues express a greater number of MC and a stronger p75NTR immunoreactivity, compared to controls. Metabolic syndrome patients display a significant hyponeurotrophinemia and an increased number of adipose MC; the later correlates with elevated plasma leptin levels. In effect, we provide the first evidence for (i) an altered presence of NGF, p75NTR and MC in both coronary vascular and subepicardial adipose tissue in human coronary atherosclerosis, and (ii) a significant decrease in plasma NGF and BDNF levels and an elevated amount of plasma leptin and adipose MC in metabolic syndrome patients. Together our findings suggest that neuroimmune mediators such as NGF, BDNF, leptin and MC may be involved in the development of cardiovascular disease and related disorders.

Target-derived trophic effect on skeletal muscle innervation in senescent mice

In the present work, we tested the hypothesis that target-derived insulin-like growth factor-1 (IGF-1) prevents alterations in neuromuscular innervation in aging mammals. To explore this hypothesis, we studied senescent wild-type mice as a model of deficient IGF-1 secretion and signaling and S1S2 transgenic mice as a tool to investigate the role of sustained overexpression of IGF-1 in striated muscle in neuromuscular innervation. The analysis of the nerve terminal in extensor digitorum longus muscles from senescent mice showed that the decrease in the percentage of cholinesterase-stained zones (CSZ) exhibiting nerve terminal branching, number of nerve branches at the CSZ, and nerve branch points was partially or completely reversed by sustained overexpression of IGF-1 in skeletal muscle. Target-derived IGF-1 also prevented age-related decreases in the postterminal alpha-bungarotoxin immunostained area, as well as the reduction in the number and length of postsynaptic folds, and area and density of postsynaptic folds studied with electron microscopy. Overexpression of IGF-1 in skeletal muscle may account for the lack of age-dependent switch in muscle fiber type composition recorded in senescent mice. In summary, the use of the S1S2 IGF-1 transgenic mouse model allowed us to provide morphological evidence for the role of target-derived IGF-1 in spinal cord motor neurons in senescent mice.

Neural control of aging skeletal muscle

Functional and structural decline in the neuromuscular system with aging has been recognized as a cause of impairment in physical performance and loss of independence in the elderly. Alterations in spinal cord motor neurones and at the neuromuscular junction have been identified as evidence of denervation in skeletal muscles from aging mammals, including humans. However, the reciprocal influences of neurones on gene expression in muscle and of muscle on age-related neurodegeneration are poorly understood, and, as a result, interventions aimed at delaying or preventing degeneration of the neural component in aging muscle have been largely unsuccessful. The present article discusses the evidence for neural influence on age-related impairments of skeletal muscle, including a role in excitation-contraction uncoupling. The role of nerves in regulating the trophic actions of insulin-like growth factor-1 (IGF-1) and other neurotrophic factors is considered as a novel influence on the effects of aging on the neuromuscular junction. A better understanding of nerve-muscle interactions will allow for more rational interventions in the aging neuromuscular system.

Reduced age-related plasticity of neurotrophin receptor expression in selected sympathetic neurons of the rat

Selective vulnerability of particular groups of neurons is a characteristic of the aging nervous system. We have studied the role of neurotrophin (NT) signalling in this phenomenon using rat sympathetic (SCG) neurons projecting to cerebral blood vessels (CV) and iris which are, respectively, vulnerable to and protected from atrophic changes during old age. RT-PCR was used to examine NT expression in iris and CV in 3- and 24-month-old rats. NGF and NT3 expression in iris was substantially higher compared to CV; neither target showed any alterations with age. RT-PCR for the principal NT receptors, trkA and p75, in SCG showed increased message during early postnatal life. However, during mature adulthood and old age, trkA expression remained stable while p75 declined significantly over the same period. In situ hybridization was used to examine receptor expression in subpopulations of SCG neurons identified using retrograde tracing. Eighteen to 20 h following local treatment of iris and CV with NGF, NT3 or vehicle, expression of NT receptor protein and mRNA was higher in iris- compared with CV-projecting neurons from both young and old rats. NGF and NT3 treatment had no effect on NT receptor expression in CV-projecting neurons at either age. However, similar treatment up-regulated p75 and trkA expression in iris-projecting neurons from 3-month-old, but not 24-month-old, rats. We conclude that lifelong exposure to low levels of NTs combined with impaired plasticity of NT receptor expression are predictors of neuronal vulnerability to age-related atrophy.

Ultrastructural transynaptic effects of unilateral cochlear ablation in the gerbil medial superior olive

This study investigated the long-term effects of unilateral hearing loss on the structure of synapses within the gerbil medial superior olivary (MSO) nuclei. Five animals had complete (surgical) left cochlear ablation at postnatal day 18. Previous studies have shown this to produce, within 3 days, significant transneuronal atrophy in the left dendritic field of both MSOs. Electron micrographs from sagittal ultrathin sections through the MSOs of the cochlear-ablated animals were compared to those from unoperated normals. Qualitatively, the ultrastructural features were similar. Most of the axodendritic terminals were R-type (round-type vesicles, putative excitatory) whereas, in the central part of the nucleus, predominated by neuron soma profiles, terminals of P- and F-type (pleomorphic- and flattened-type vesicles, putative inhibitory) were present in equal numbers with R-type terminals. F-type terminals were infrequent and occurred most around lateral parts of the MSO somata. These three types of terminals seen around the somata and proximal dendrites all had extended profiles with multiple, discontinuous appositions. Quantitative analysis revealed that R-type axodendritic terminals became smaller and less densely populated with vesicles where they synapsed onto the remaining dendrites arrayed towards the ablated side of both MSOs, and axosomatic P-type afferent terminals were smaller in the contralateral nuclei. A significant reduction in the number of terminals and synapses occurred in the central, somatic, region of the ipsilateral MSO. However, the terminal vesicle concentration in the remaining terminals increased. The results indicate that cochlear ablation can induce transynaptic reduction in the size of afferent axon terminals within the MSO, and alter their vesicle concentration. These changes are likely to affect the probability of transmitter release and thus influence their signaling power within the nucleus.

The oestrogenized rat myometrium inhibits organotypic sympathetic reinnervation

Chronic administration of oestrogen to rats during the infantile/prepubertal period provokes, at 28 days of age, complete loss of noradrenaline-labelled intrauterine sympathetic nerves. It is not known whether oestrogen inhibits the growth or causes the degeneration of developing uterine sympathetic nerves, or whether the uterus recovers its innervation following cessation of infantile/prepubertal oestrogen treatment. In the present study, we analysed the time-course of the effects of oestrogen on the development of uterine sympathetic nerves in the rat, using histochemical methods. In addition, the pattern of sympathetic reinnervation of the uterus of intact and ovariectomised females was assessed 3 and 6 months after cessation of chronic oestrogen treatment. The ability of sympathetic nerves to reinnervate the oestrogenized uterine tissue was assessed in intraocular transplants of uterine myometrium into ovariectomised host rats. Early exposure to oestrogen did not inhibit the approach of sympathetic nerves to the uterus, but prevented the normal growth and maturation of intrauterine sympathetic fibres and abolished the innervation that reached the organ before initiation of treatment. Three or six months following cessation of oestrogen treatment, most of the sympathetic nerves were restricted to the mesometrium and mesometrial entrance, whereas intrauterine innervation remained persistently depressed as a consequence of a sustained oestrous-like state provoked by ovarian dysfunction (polycystic ovary). An organotypic regrowth of uterine sympathetic nerves was observed in ovariectomised infantile/prepubertal oestrogen-treated animals. After 5 weeks in oculo, the innervation of oestrogenized myometrial transplants was reduced by 50%, and substantial changes in the pattern of reinnervation were observed. In control transplants, 86% of the nerves were terminal varicose myometrial and perivascular nerve fibres, whereas 14% were preterminal nerve bundles. In oestrogenized myometrial transplants, 83% of the noradrenaline-labelled intercepting nerves were enlarged preterminal bundles and only 17% were terminal fibres. These results indicate that the oestrogenized myometrium is unattractive for sympathetic nerves and inhibits organotypic sympathetic reinnervation.

Evidence for reduced accumulation of exogenous neurotrophin by aged sympathetic neurons

The present study investigated the potential for neurotrophin uptake by cerebrovascular axons and subsequent accumulation in the aged superior cervical ganglion (SCG) following a two week intracerebroventricular infusion of nerve growth factor (NGF). In the SCG from aged rats, NGF protein levels declined significantly compared with the SCG from young adult rats. Following NGF infusion, perivascular axons from both young adult and aged rats showed intense NGF immunostaining. In addition, significant increases in NGF protein were shown using enzyme-linked immunosorbent assay (ELISA) and in counts of NGF immunopositive cell bodies in the SCG when compared with age-matched controls. NGF accumulation in ganglia from aged rats, however, was significantly less when compared with ganglia from young adult rats. The results of the present study suggest that NGF protein is significantly reduced in aged ganglia with the neurons retaining some capacity to take up and transport exogenous neurotrophin. Even so, the potential for NGF accumulation is dramatically reduced in aged rats when compared with that of young adult rats. While previous results have shown robust NGF-induced neurotransmitter responses by sympathetic neurons from the aged animal, the present finding of reduced accumulation of NGF in aged sympathetic neurons suggests an age-related difference in the utilization or transport of NGF.

Differential age-dependent trophic responses of nodose, sensory, and sympathetic neurons to neurotrophins and GDNF: potencies for neurite extension in explant culture

The age-dependent trophic responses of sympathetic, sensory, and nodose neurons to the neurotrophins NGF, BDNF, and NT-3 and to glial cell line-derived neurotrophic factor (GDNF) were examined by an explant culture system. Superior cervical ganglia (SCG), dorsal root ganglia (DRG), and nodose ganglia (NG) were removed from rat embryos (E18), neonatals (< or = 1 day old), young adults (3-6 months old), and aged adults (> 24 months old). The ganglia were cultured with and without each neurotrophic factor; the neurite extension and neurite density were then assessed. The SCG from rats of all ages were significantly influenced by NGF, NT-3, and GDNF; the effects of NT-3 and GDNF were reduced after maturation. The DRG from embryos and neonates were influenced by all neurotrophic factors; however, the effects of BDNF and NT-3 disappeared after maturation. The GDNF showed little effect on adult DRG and no effect on aged DRG. The effect of NGF was preserved over all ages of DRG. The NG from embryonic rats were significantly responsive to BDNF and GDNF; their effects decreased in the neonatal NG, but a minimum effect remained in the aged NG. These results indicate that age-dependent profiles of trophic effects differ extensively among the lineages of the peripheral nervous system and also among the individual neurotrophic factors.

Effects of infantile/prepubertal chronic estrogen treatment and chemical sympathectomy with guanethidine on developing cholinergic nerves of the rat uterus

The innervation of the uterus is remarkable in that it exhibits physiological changes in response to altered levels in the circulating levels of sex hormones. Previous studies by our group showed that chronic administration of estrogen to rats during the infantile/prepubertal period provoked, at 28 days of age, an almost complete loss of norepinephrine-labeled sympathetic nerves, similar to that observed in late pregnancy. It is not known, however, whether early exposure to estrogen affects uterine cholinergic nerves. Similarly, it is not known to what extent development and estrogen-induced responses in the uterine cholinergic innervation are affected by the absence of sympathetic nerves. To address this question, in this study we analyzed the effects of infantile/prepubertal chronic estrogen treatment, chronic chemical sympathectomy with guanethidine, and combined sympathectomy and chronic estrogen treatment on developing cholinergic nerves of the rat uterus. Cholinergic nerves were visualized using a combination of acetylcholinesterase histochemistry and the immunohistochemical demonstration of the vesicular acetylcholine transporter (VAChT). After chronic estrogen treatment, a well-developed plexus of cholinergic nerves was observed in the uterus. Quantitative studies showed that chronic exposure to estrogen induced contrasting responses in uterine cholinergic nerves, increasing the density of large and medium-sized nerve bundles and reducing the intercept density of fine fibers providing myometrial and perivascular innervation. Estrogen-induced changes in the uterine cholinergic innervation did not appear to result from the absence/impairment of sympathetic nerves, because sympathectomy did not mimic the effects produced by estrogen. Estrogen-induced responses in parasympathetic nerves are discussed, considering the direct effects of estrogen on neurons and on changes in neuron-target interactions.

Evidence for loss of myelinated input to the spinal cord in senescent rats

Impaired sensory perception is a well-established stigma of aging and whereas loss of dorsal root ganglion (DRG) neurons is marginal there is a specific pattern of reduced peripheral sensory innervation. To resolve if similar regressive processes occur in the central innervation, peripheral nerves were injected with markers for unmyelinated (isolectin B4) or myelinated (cholera toxin B subunit; CTB) DRG neurons. The results were a dramatic decrease of primary sensory endings in the spinal cord of aged rats following transganglionic labeling with CTB, and also to a lesser degree with B4. Profile counting and frequency estimates showed that the reduction of CTB labeled profiles not was caused by impaired axonal uptake, slowed axonal transport of CTB, or by a loss of myelinated fibers in the peripheral nerve. At the ultrastructural level, peripheral nerves showed the classical hallmarks of aging, with more pronounced alterations in myelinated than unmyelinated axons. Taken together, sensory deprivation in senescence appears to be a distal process in DRG neurons involving both peripheral and central target disconnection. Finally, preliminary data indicates that the substantial reduction in mechanoreceptive input to the central nervous system co-varies with the degree of sensorimotor impairment of the aged individuals.

Selective vulnerability in adult and ageing mammalian neurons

Data are presented in support of the idea of antagonistic pleiotropy that features which are adaptive during early life may become maladaptive during the ageing process, when selective pressure is reduced. A model of selective vulnerability to age-related neurodegeneration involving neighbouring subpopulations of vulnerable and protected sympathetic neurons is presented. The two groups of neurons are morphologically and physiologically distinct, indicating advantageous adaptation to particular functions. Neurotrophin signalling is investigated in these different groups of neurons, revealing significant differences between them: neurotrophic factor expression in their target tissues is markedly different, same as their neurotrophin uptake characteristics. Preliminary evidence is presented that the mechanism linking neurotrophin signalling and age-related neurodegeneration may involve the capacity of neurons to buffer free radical generation, hence reducing the effects of attrition by free radical damage.

Aging and neuronal plasticity: lessons from a model

In spite of many well-documented examples of age-related reductions in neuronal plasticity, the causes of such changes remain largely unknown. One example of age-reduced plasticity involves an aberrant sprouting response of mature rat sympathetic neurons into the CNS (hippocampal formation). This phenomenon has proven to be useful for exploring the relative contribution of target aging (extrinsic influences) versus neuronal aging (intrinsic influences) to reduced sprouting. Aged sympathetic neurons mount a robust growth response when confronted with young target tissue or when exposed to exogenous trophic factor in vivo. In contrast, the aged target tissue (the hippocampal formation in this example) exhibits reduced receptivity for sympathetic sprouting. This change in the target does not appear to be due to alterations in baseline levels of trophic or substrate support for axonal growth. Rather, aging appears to dampen the consequences of target denervation so that the aged target elicits less sprouting. Age-related reductions in neuronal sprouting are speculated to reflect increasing commitment to information storage at the expense of neuronal plasticity.

Impairment of peripheral sensory innervation in senescence

Sensorimotor disturbances are common among elderly and one of the main factors depreciating life quality in senescence. Mechanistically sensory deficits during aging include not only degenerative and regressive events but also phenotypic switches among sensory neurons as well as remodeling of sensory innervation. The pattern of changes suggests that an underlying mechanism is a sustained dependence of sensory neurons on target tissues, and that this dependence, at least in part, appears to be mediated through signaling by target-derived trophic factors. This review presents and discusses evidence supporting this notion.

Age-related sympathetic ganglionic neuropathology: human pathology and animal models

Systematic studies of the autonomic nervous system of human subjects and development of well-defined animal models have begun to substantially improve our understanding of the pathogenesis of autonomic dysfunction in aging and may eventually provide strategies for intervention. Neuropathological studies of the sympathetic ganglia of aged human subjects and rodent models have demonstrated that neuroaxonal dystrophy involving intraganglionic terminal axons and synapses is a robust, unequivocal and consistent neuropathological finding in the aged sympathetic nervous system of man and animals. Quantitative studies have demonstrated that markedly swollen argyrophilic dystrophic axon terminals develop in the prevertebral superior mesenteric (SMG) and coeliac, but to a much lesser degree in the superior cervical ganglia (SCG) as a function of age, sex (males more than females) and diabetes. Dystrophic axons were immunoreactive for neuropeptide Y, tyrosine hydroxylase, dopamine-beta-hydroxylase, trkA and p75NTR, an immunophenotype consistent with their origin from postganglionic sympathetic neurons, and contained large numbers of highly phosphorylated neurofilaments or tubulovesicular elements. The sympathetic ganglia of aged rodents also showed the hallmark changes of neuroaxonal dystrophy as a function of age and location (many more in the SMG than in the SCG). Plasticity-related synaptic remodeling could represent a highly vulnerable target of the aging process. The fidelity of animal models to the neuropathology of aged humans suggests that similar pathogenetic mechanisms may be involved in both and that therapeutic advances in animal studies may have human application.

Resistance of the dentate gyrus to induced apoptosis during ageing is associated with increases in transforming growth factor-beta1 messenger RNA

Up-regulation of endogenous neurotrophic factors may protect against apoptosis during ageing. Recent studies showed that the expression of several neurotrophic factors increased with age in specific regions of the rat brain. Previously, we showed that removal of trophic adrenal steroids by adrenalectomy induced apoptosis in the dentate gyrus of adult rats, which was accompanied by increased expression of transforming growth factor-beta1 (TGF-beta1) messenger RNA. In this study, we compared the relative densities of apoptotic cells in the dentate gyrus with TGF-beta1 messenger RNA expression in virgin male Fischer 344 rats ranging from 2 to 26 months of age across three treatment groups: adrenalectomy, adrenalectomy with corticosterone replacement, or sham operation. Seven days after adrenalectomy an increase in the density of apoptotic cells was observed in rats of all age groups compared with sham-operated and corticosterone-treated groups. By in situ hybridisation, the glial messenger RNAs, TGF-beta1 and glial fibrillary acidic protein as a marker of ageing, increased following adrenalectomy in the dentate gyrus in rats of all ages compared with control groups. Interestingly, within adrenalectomy groups, both the number and density of apoptotic cells decreased significantly by 6-8 months with a further decrease at 24-26 months of age. Furthermore, the amount of apoptosis corresponded to changes in TGF-beta1 expression, notably in: (i) adrenalectomised rats showing a significant inverse correlation and (ii) 24-26-month-old rats with the lowest induced apoptosis showing increased expression at the time of adrenalectomy. These studies show that resistance to adrenalectomy-induced apoptosis in the dentate gyrus is associated with increases in TGF-beta1 messenger RNA expression. Furthermore, the endogenous up-regulation of neurotrophic factors, such as the increase in TGF-beta1 expression in the oldest rats, suggests that the aged brain may have compensatory mechanisms, which protect against apoptosis.

Plasticity of aged perivascular axons following exogenous NGF: analysis of catecholamines

The present study investigated the atrophy of aged perivascular sympathetic axons and the response of these cerebrovascular neurons to the neurotrophin nerve growth factor (NGF). Using high performance liquid chromatography coupled with electrochemical detection (HPLC-ECD) to quantify catecholamines and immunohistochemical methods to quantify the density of TH immunoreactive fibers, we found a significant decrease in norepinephrine (NE) and TH in aged sympathetic axons. However, following in vivo administration of exogenous neurotrophin, aged neurons exhibited a robust response to NGF that was similar to the young adult, suggesting little decline in the capability of aged neurons to utilize exogenous neurotrophin. These results suggest that the age-related atrophy of aged sympathetic axons may result primarily from reduced availability of target-derived neurotrophin rather than from intrinsic alterations of neuronal function.

p75 and TrkA receptors are both required for uptake of NGF in adult sympathetic neurons: use of a novel fluorescent NGF conjugate

We have developed and tested the biological activity and specificity of a novel fluorescent dextran-Texas Red-nerve growth factor (DTR-NGF) conjugate. DTR-NGF was found to promote survival and neurite outgrowth in cultured dissociated sympathetic neurons similarly to native NGF. The conjugate was taken up and transported retrogradely by terminal sympathetic nerves innervating the iris to neurons in the ipsilateral superior cervical ganglion (SCG) of young adult rats. Uptake and transport was assessed by counting numbers of labelled neurons and by measuring intensity of neuronal labelling using confocal microscopy and image analysis. DTR-NGF labelling in SCG neurons was shown to be dose-dependent with an EC(50) of 75 ng. Similar concentrations of unconjugated DTR resulted in no neuronal labelling. DTR-NGF uptake was competed off using a 50-fold excess of native NGF, resulting in a 73% reduction in numbers of labelled neurons. Pretreatment of nerve terminals with function-blocking antibodies against the low (p75) and high (TrkA) affinity NGF receptors resulted in a large (85-93%) reduction in numbers of DTR-NGF labelled neurons. Anti-p75 and anti-TrkA antibodies had comparable effects which were concentration-dependent. These findings indicate that both receptors are required for uptake of NGF in adult rat sympathetic neurons. In particular, the results provide strong evidence that the p75 receptor plays a more active role in transducing the NGF signal than has been proposed.

Nerve growth factor levels and mast cell distribution in human coronary atherosclerosis

Nerve growth factor (NGF), in addition to its neurotrophic function, acts on a variety of non-neuronal cells including immune cells and vascular smooth muscle cells. The aim of the present study was to determine the NGF levels and the distribution of NGF and low-affinity NGF receptor (p75NGFR) and mast cells (MC) in human atherosclerotic coronary arteries. Specimens of human coronary arteries obtained from autopsy cases (n=12, subjects with atherosclerotic lesions; n=9, subjects without atherosclerotic lesions/controls) were used. The present study showed that in the atherosclerosis-lesioned arteries, the amount of NGF decreased, whereas the expression of p75NGFR immunoreactivity and the number, both of MC and vasa vasorum, particularly in the adventitia, significantly increased, compared with the control arteries. Cumulatively, our findings help to set the neurotrophic theory and its currently extended neuroimmune framework into the context of pathobiology of atherosclerosis, suggesting that altered presence of NGF, p75NGFR, and MC may play a role in neuroimmune mechanisms of human coronary atherosclerosis.

Neurotrophins, but not depolarization, regulate substance P expression in the developing optic tectum

Neurotransmitter expression can be regulated by both activity and neurotrophins in a number of in vitro systems. We examined whether either of these factors was likely to play a role in the in vivo optic nerve-dependent regulation of a substance P-like immunoreactive (SP-ir) population of cells in the developing optic tectum of the frog. In contrast to our previous results with the adult system, blocking tectal cell responses to glutamate release by retinal ganglion cells with 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX) did not affect the percent of SP-ir cells in the developing tectum. Treatment with d-(-)-2-amino-5-phosphonovaleric acid (d-AP-5) was also ineffective in this regard, although both it and CNQX treatment disrupted visual map topography. Chronic treatment with brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) produced increases in SP-ir cells in the treated lobes of normal animals, which were significant in the case of NT-4/5. Both substances also prevented the decrease of SP cells that would otherwise occur in the deafferented lobe of unilaterally optic nerve-transected tadpoles. These changes in the percent of SP-ir cells occurred without any detectable changes in the overall number of tectal cells. NGF had no effect on SP expression. Nor did it affect topographic map formation, which was disrupted by treatment with either BDNF or NT-4/5. Our results demonstrate that different mechanisms regulate SP expression in the developing and adult tectum. They indicate that neurotrophin levels in the developing optic tectum may selectively regulate a specific neuropeptide-expressing population of cells.

Cholecystokinin-8 promotes recovery of sympathectomy induced by 6-hydroxydopamine in adult mice

We used an experimental model of sympathetic neuropathy to investigate the effects of intraperitoneal cholecystokinin-8 (CCK-8) administration on the recovery of injured peripheral neurones. After treatment of adult mice with 6-hydroxydopamine (6-OHDA), which known to induce peripheral sympathectomy, nerve growth factor (NGF) in peripheral tissue first increased and then rapidly decreased to baseline levels. Following this observation, sympathectomised mice were treated with CCK-8 starting when the NGF levels lowered toward the control value. Our results show that injections with 8 nmol/kg of CCK-8 promote not only recovery of noradrenergic innervation but also NGF and neuropeptide Y (NPY) synthesis in peripheral tissue. This latter observation suggests that the effect of CCK-8 might be mediated through the stimulation of NGF synthesis.

Estrogen and neuroinflammation

Women have a higher incidence of inflammatory disorders than men and also appear to perceive painful stimuli differently. It has been suggested that neuroinflammation plays a role in painful bladder disorders of uncertain etiology, such as interstitial cystitis. Nerve growth factor (NGF) is a neurotrophin produced in peripheral tissues that can also mediate pain and inflammation. We found that treatment of mice with the estrogen antagonist ICI 182,780 had no effect on bladder NGF content but decreased bladder NGF messenger RNA. Using immunohistochemistry, we demonstrated that the mucosa is the primary source of NGF in the mouse bladder, and the bladder mucosa also expresses estrogen receptor (ER)-alpha, ER-beta, and the high-affinity NGF receptor tyrosine kinase A. Estrogen may also modulate neurogenic inflammation by interaction with other substances and cells that participate in the pathogenesis of neurogenic inflammation, including substance P, bradykinin, and mast cells. Collectively, these observations indicate that estrogen has the capacity to influence the onset and course of neurogenic inflammation of the bladder.

Zinc inhibits p75NTR-mediated apoptosis in chick neural retina

It has previously been documented that Zn2+ inhibits TrkA-mediated effects of NGF. To evaluate the ability of Zn2+ to attenuate the biological activities of NGF mediated by p75NTR, we characterized the effects of this transition metal cation on both binding and the pro-apoptotic properties of the NGF-p75NTR interaction. Binding of NGF to p75NTR displayed higher affinity in embryonic chick retinal cells than in PC12 cells. NGF induced apoptosis in dissociated cultures of chick neural retina. The addition of 100 microM Zn2+ inhibited binding and chemical cross-linking of 125I-NGF to p75NTR, and also attenuated apoptosis mediated by this ligand-receptor interaction. These studies lead to the conclusion that Zn2+ antagonizes NGF/p75NTR-mediated signaling, suggesting that the effect of this transition metal cation can be either pro- or anti-apoptotic depending on the cellular context.

Serum-free culture of dissociated, purified adult and aged sympathetic neurons and quantitative assays of growth and survival

In vitro studies of dissociated neurons have provided crucial data regarding the regulation of plasticity in embryonic and perinatal neurons from both central and peripheral nervous systems. There have been few attempts to apply these methods to adult or aged neurons and the methods that have been reported have not been able to dissect the possible confounding contributions of non-neuronal cells and serum. Furthermore, quantitative assays of cultured neurons, particularly of their growth, have rarely been described. We report here the development of a novel method for the dissociation, purification and culture of sympathetic neurons from the adult and aged rat SCG under serum free conditions and in defined media. The technique results in a relatively high yield of viable, growing neurons. We describe methods for assaying the total yield of neurons, the proportion of surviving neurons and the proportion of neurons initiating neurite outgrowth after plating. A novel semiautomated assay of neurite outgrowth is outlined using image analysis of composite images of immunofluorescence-stained single neurons.

Innervation of cerebral blood vessels: morphology, plasticity, age-related, and Alzheimer's disease-related neurodegeneration

The light microscopical and ultrastructural morphology of the innervation of the major cerebral arteries and pial vessels is described, including the origins of the different groups of nerve fibres and their characteristic neurotransmitter phenotype. Species and region specific variations are described and novel data regarding the parasympathetic innervation of cerebral vessels are presented. The dynamic nature, or plasticity, of cerebrovascular innervation is emphasized in describing changes affecting particular subpopulations of neurons during normal ageing and in Alzheimer's disease. The molecular controls on plasticity are discussed with particular reference to target-associated factors such as the neurotrophins and their neuronal receptors, as well as extracellular matrix related factors such as laminin. Hypotheses are presented regarding the principal extrinsic and intrinsic influences on plasticity of the cerebrovascular innervation.

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

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

Effects of electro-acupuncture on nerve growth factor and ovarian morphology in rats with experimentally induced polycystic ovaries

Despite extensive research on the pathogenesis of polycystic ovary syndrome (PCOS), there is still disagreement on the underlying mechanisms. The rat model for experimentally induced polycystic ovaries (PCO)-produced by a single injection of estradiol valerate-has similarities with human PCOS, and both are associated with hyperactivity in the sympathetic nervous system. Nerve growth factor (NGF) is known to serve as a neurotrophin for both the sympathetic and the sensory nervous systems and to enhance the activity of catecholaminergic and possibly other neuron types. Electro-acupuncture (EA) is known to reduce hyperactivity in the sympathetic nervous system. For these reasons, the model was used in the present study to investigate the effects of EA (12 treatments, approximately 25 min each, over 30 days) by analyzing NGF in the central nervous system and the endocrine organs, including the ovaries. The main findings in the present study were first, that significantly higher concentrations of NGF were found in the ovaries and the adrenal glands in the rats in the PCO model than in the control rats that were only injected with the vehicle (oil or NaCl). Second, that repeated EA treatments in PCO rats resulted in concentrations of NGF in the ovaries that were significantly lower than those in non-EA-treated PCO rats but were within a normal range that did not differ from those in the untreated oil and NaCl control groups. The results in the present study provide support for the theory that EA inhibits hyperactivity in the sympathetic nervous system.