Differential effects of NT-3 on reinnervation of the fast extensor digitorum longus (EDL) and the slow soleus muscle of rat

Muscle specific nucleus ambiguus neurons isolation and culturing

BACKGROUND

Peripheral nerve injury leads to a regenerative state. However, the reinnervation process is highly non-selective. Growing axons are often misrouted and establish aberrant synapsis to abductor or adductor muscles. Determining the complex properties of abductor and adductor motoneurons in a neuron culture, may lay the groundwork for future studies on axon guidance, leading to a clinical treatment for a selective reinnervation.

NEW METHOD

In the present study we develop a neuron culture protocol to isolate recurrent laryngeal nerve abductor and adductor motoneurons in order to study their unique properties. Comparison with existing methods the best period to perform the present protocol for postnatal rat cranial motoneurons isolation was determined. In addition, the method allows identification of specific motoneurons from other primary motoneurons and interneurons within brainstem.

CONCLUSION

The present protocol will allow investigators to perform targeted and novel studies of the mechanisms of peripheral nerve regeneration.

Neuregulin 1 role in Schwann cell regulation and potential applications to promote peripheral nerve regeneration

Neuregulin 1 (NRG1) is a multifunctional and versatile protein: its numerous isoforms can signal in a paracrine, autocrine, or juxtacrine manner, playing a fundamental role during the development of the peripheral nervous system and during the process of nerve repair, suggesting that the treatment with NRG1 could improve functional outcome following injury. Accordingly, the use of NRG1 in vivo has already yielded encouraging results. The aim of this review is to focus on the role played by the different NRG1 isoforms during peripheral nerve regeneration and remyelination and to identify good candidates to be used for the development of tissue engineered medical devices delivering NRG1, with the objective of promoting better nerve repair.

Optimal time-point for neural stem cell transplantation to delay denervated skeletal muscle atrophy

INTRODUCTION

Transplantation of neural stem cells (NSCs) is a promising treatment to delay denervated skeletal muscle atrophy; however, the optimal time-point between peripheral nerve injury and NSC transplantation needs to be determined.

METHODS

Improvement in rat gastrocnemius muscle function was evaluated after NSCs were transplanted into sectioned distal tibial nerves. We also assessed survival and differentiation. ANOVA was used to compare the mean value of the number of neuron-like cells, cross-sectional area amelioration, the amount of activated fibers, and latency and amplitude of the gastrocnemius compound muscle action potential.

RESULTS

The group in which the NSCs were transplanted 1 week after tibial nerve transection had the largest number of neuron-like cells, maximum cross-sectional area amelioration, and maximum amount of activated gastrocnemius fibers compared with all other groups (P < 0.01).

CONCLUSIONS

The optimal time-point for NSC transplantation for delaying denervated skeletal muscle atrophy is 1 week after severing the nerve.

The recent understanding of the neurotrophin's role in skeletal muscle adaptation

This paper summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of the maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors influence not only the survival and function of innervating motoneurons but also the development and differentiation of myoblasts and muscle fibers. Muscle contractions (e.g., exercise) produce BDNF mRNA and protein in skeletal muscle, and the BDNF seems to play a role in enhancing glucose metabolism and may act for myokine to improve various brain disorders (e.g., Alzheimer's disease and major depression). In adults with neuromuscular disorders, variations in neurotrophin expression are found, and the role of neurotrophins under such conditions is beginning to be elucidated. This paper provides a basis for a better understanding of the role of these factors under such pathological conditions and for treatment of human neuromuscular disease.

Aerobic exercise alters analgesia and neurotrophin-3 synthesis in an animal model of chronic widespread pain

BACKGROUND

Present literature and clinical practice provide strong support for the use of aerobic exercise in reducing pain and improving function for individuals with chronic musculoskeletal pain syndromes. However, the molecular basis for the positive actions of exercise remains poorly understood. Recent studies suggest that neurotrophin-3 (NT-3) may act in an analgesic fashion in various pain states.

OBJECTIVE

The purpose of the present study was to examine the effects of moderate-intensity aerobic exercise on pain-like behavior and NT-3 in an animal model of widespread pain.

DESIGN

This was a repeated-measures, observational cross-sectional study.

METHODS

Forty female mice were injected with either normal (pH 7.2; n=20) or acidic (pH 4.0; n=20) saline in the gastrocnemius muscle to induce widespread hyperalgesia and exercised for 3 weeks. Cutaneous (von Frey monofilament) and muscular (forceps compression) mechanical sensitivity were assessed. Neurotrophin-3 was quantified in 2 hind-limb skeletal muscles for both messenger RNA (mRNA) and protein levels after exercise training. Data were analyzed with 2-factor analysis of variance for repeated measures (group x time).

RESULTS

Moderate-intensity aerobic exercise reduced cutaneous and deep tissue hyperalgesia induced by acidic saline and stimulated NT-3 synthesis in skeletal muscle. The increase in NT-3 was more pronounced at the protein level compared with mRNA expression. In addition, the increase in NT-3 protein was significant in the gastrocnemius muscle but not in the soleus muscle, suggesting that exercise can preferentially target NT-3 synthesis in specific muscle types.

LIMITATIONS

Results are limited to animal models and cannot be generalized to chronic pain syndromes in humans.

CONCLUSIONS

This is the first study demonstrating the effect of exercise on deep tissue mechanical hyperalgesia in a rodent model of pain and providing a possible molecular basis for exercise training in reducing muscular pain.

A conditioning lesion provides selective protection in a rat model of Amyotrophic Lateral Sclerosis

Background

Amyotrophic Lateral Sclerosis (ALS) is neurodegenerative disease characterized by muscle weakness and atrophy due to progressive motoneuron loss. The death of motoneuron is preceded by the failure of neuromuscular junctions (NMJs) and axonal retraction. Thus, to develop an effective ALS therapy you must simultaneously preserve motoneuron somas, motor axons and NMJs. A conditioning lesion has the potential to accomplish this since it has been shown to enhance neuronal survival and recovery from trauma in a variety of contexts.

Methodology/Principal Findings

To test the effects of a conditioning lesion in a model of familial ALS we administered a tibial nerve crush injury to presymptomatic fALS^G93A rats. We examined its effects on motor function, motoneuron somas, motor axons, and NMJs. Our experiments revealed a novel paradigm for the conditioning lesion effect. Specifically we found that the motor functional decline in fALS^G93A rats that received a conditioning lesion was delayed and less severe. These improvements in motor function corresponded to greater motoneuron survival, reduced motor axonopathy, and enhanced NMJ maintenance at disease end-stage. Furthermore, the increased NMJ maintenance was selective for muscle compartments innervated by the most resilient (slow) motoneuron subtypes, but was absent in muscle compartments innervated by the most vulnerable (fast fatigable) motoneuron subtypes.

Conclusions/Significance

These findings support the development of strategies aimed at mimicking the conditioning lesion effect to treat ALS as well as underlined the importance of considering the heterogeneity of motoneuron sub-types when evaluating prospective ALS therapeutics.

Bioengineered nerve regeneration and muscle reinnervation

The peripheral nervous system has the intrinsic capacity to regenerate but the reinnervation of muscles is often suboptimal and results in limited recovery of function. Injuries to nerves that innervate complex organs such as the larynx are particularly difficult to treat. The many functions of the larynx have evolved through the intricate neural regulation of highly specialized laryngeal muscles. In this review, we examine the responses of nerves and muscles to injury, focusing on changes in the expression of neurotrophic factors, and highlight differences between the skeletal limb and laryngeal muscle systems. We also describe how artificial nerve conduits have become a useful tool for delivery of neurotrophic factors as therapeutic agents to promote peripheral nerve repair and might eventually be useful in the treatment of laryngeal nerve injury.

Effect of neurotrophin-3 on reinnervation of the larynx using the phrenic nerve transfer technique

Current techniques for reinnervation of the larynx following recurrent laryngeal nerve (RLN) injury are limited by synkinesis, which prevents functional recovery. Treatment with neurotrophins (NT) may enhance nerve regeneration and encourage more accurate reinnervation. This study presents the results of using the phrenic nerve transfer method, combined with NT-3 treatment, to selectively reinnervate the posterior cricoarytenoid (PCA) abductor muscle in a pig nerve injury model. RLN transection altered the phenotype and morphology of laryngeal muscles. In both the PCA and thyroarytenoid (TA) adductor muscle, fast type myosin heavy chain (MyHC) protein was decreased while slow type MyHC was increased. These changes were accompanied with a significant reduction in muscle fibre diameter. Following nerve repair there was a progressive normalization of MyHC phenotype and increased muscle fibre diameter in the PCA but not the TA muscle. This correlated with enhanced abductor function indicating the phrenic nerve accurately reinnervated the PCA muscle. Treatment with NT-3 significantly enhanced phrenic nerve regeneration but led to only a small increase in the number of reinnervated PCA muscle fibres and minimal effect on abductor muscle phenotype and morphology. Therefore, work exploring other growth factors, either alone or in combination with NT-3, is required.

Investigation of differentially expressed proteins in rat gastrocnemius muscle during denervation-reinnervation

To have a better insight into the molecular events involved in denervation-induced atrophy and reinnervation-induced regeneration of skeletal muscles, it is important to investigate the changes in expression levels of a great multitude of muscle proteins during the process of denervation-reinnervation. In this study, we employed an experimental model of rat sciatic nerve crush to examine the differentially expressed proteins in the rat gastrocnemius muscle at different time points (0, 1, 2, 3, 4 weeks) after sciatic nerve crush by using two-dimensional gel electrophoresis (2-DE) followed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS), collectively referred to as the modern proteomic analysis. The results showed that 16 proteins in the rat gastrocnemius muscle exhibited two distinct types of change pattern in their relative abundance: (1) The relative expression levels of 11 proteins (including alpha actin, myosin heavy chain, etc.) were decreased either within 1 or 2 weeks post-sciatic nerve injury, followed by restoration during the ensuing days until 4 weeks. (2) The other 5 proteins (including alpha enolase, beta enolase, signal peptide peptidase-like 3, etc.) displayed an up-regulation in their relative expression levels within 1 week following sciatic nerve injury, and a subsequent gradual decrease in their relative expression levels until 4 weeks. Moreover, the significance of the changes in expression levels of the 16 proteins during denervation-reinnervation has been selectively discussed.

The role of neurotrophins in muscle under physiological and pathological conditions

This review summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of development, maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors not only modulates survival and function of innervating motoneurons and proprioceptive neurons but also development and differentiation of myoblasts and muscle fibers. Neurotrophins and neurotrophin receptors play a role in the coordination of muscle innervation and functional differentiation of neuromuscular junctions. However, neurotrophin receptors are also expressed in differentiating muscle cells, in particular at early developmental stages in myoblasts before they fuse. In adults with pathological conditions such as human degenerative and inflammatory muscle disorders, variations of neurotrophin expression are found, but the role of neurotrophins under such conditions is still not clear. The goal of this review is to provide a basis for a better understanding and future studies on the role of these factors under such pathological conditions and for treatment of human muscle diseases.

Are histochemistry and cytochemistry 'Omics'?

A plethora of new 'omics such as transcriptomics, proteomics, metabolomics (or metabonomics), pharmacogenomics, physiomics and cytomics are upon us, but can histochemistry be an 'omic? To be an 'omic a technique must take a 'global' and 'holistic' view of biology that addresses biological complexity head-on by synthesising multiparameter data into predictive models. Thus to be an 'omic, a histochemical technique should be as inclusive as possible in identifying as many targets as possible with equal likelihood and sensitivity of detection. Any technique capable of detecting only one or two targets is not within the spirit of an 'omic, ruling out it seems most of histochemistry. Nevertheless, new developments in high-throughput histochemistry and cytochemistry are making powerful claims to the title 'Histocytomics'. Histocytomics and all the other 'omics are components of the only real 'omic 'Biomics', that is, the integrated application of science into a coherent strategy for understanding biological complexity. In this paper, this strategy is presented for the investigation of the regulation of phenotypic change in skeletal and cardiac muscle in health and disease.

An electrophysiological study into the effect of neurotrophin-3 on functional recovery after lingual nerve repair

OBJECTIVE

Neurotrophin-3 (NT-3) is known to ameliorate central changes that result from peripheral nerve injury and may promote regeneration of myelinated axons. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat.

DESIGN

Six months after entubulation repair, with or without the incorporation of NT-3 at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents.

RESULTS

When compared with data from uninjured control animals, both repair groups showed persistent reductions in conduction velocity, receptor sensitivity, spontaneous discharge, proportion of gustatory and thermosensitive units, and rate of salivary secretion. Comparisons between the two repair groups revealed that in the NT-3 group, salivary secretion rate was lower and the activity evoked in the chorda tympani by gustatory or thermal stimuli was lower, but the spontaneous discharge rate was higher. Mechanosensitive units in the lingual nerve had slower conduction velocities but the mechanoreceptive field size, adaptation time and discharge frequency had increased.

CONCLUSIONS

Despite its known trophic role in the lingual somatosensory system, NT-3 did not enhance functional recovery from injury and had a negative effect on the long-term outcome for sensory and autonomic fibres.

Laryngeal abductor muscle reinnervation in a pig model

OBJECTIVE

To develop a large animal model for studies of laryngeal abductor reinnervation.

MATERIAL AND METHODS

Six minipigs underwent unilateral anastomosis of the phrenic nerve-abductor branch of the recurrent laryngeal nerve (RLN). Polyhydroxybutyrate (PHB) conduits were used for repair. At each of 30, 60 and 120 days, 2 animals underwent video laryngeal endoscopy (VLE) and were then killed. VLE was also performed in the 120-day pair at 60 days. Nerve-conduit-nerve-muscle samples were fixed for light and immunofluorescence (pan-neurofilaments, S-100) microscopy. Laryngeal muscles were harvested (myosin heavy chain analysis).

RESULTS

VLE showed recovery of abductor function in 1 animal at 60 days and in 1 at 120 days. Haematoxylin-eosin staining demonstrated a complex inflammatory response. Eosinophil recruitment was observed. Stepwise regeneration and reorganization of the distal nerve between 30 and 120 days was observed with pan-NF staining. The mean minimum diameter in the reinnervated posterior crico-arytenoids tended to increase for up to 120 days.

CONCLUSIONS

Anastomosis of the phrenic nerve-abductor branch of the RLN with a PHB conduit in a pig can result in functional and histological recovery within 2-4 months and appears to at least sustain abductor muscle fibre morphology. Recovery occurs despite a complex inflammatory response, which may be an essential part of healing rather than inhibitory.

A composite poly-hydroxybutyrate-glial growth factor conduit for long nerve gap repairs

There is considerable evidence that peripheral nerves have the potential to regenerate in an appropriate microenvironment. We have developed a novel artificial nerve guide composed of poly 3-hydroxybutyrate (PHB) filled with glial growth factor (GGF) suspended in alginate hydrogel. Gaps of 2-4 cm in rabbit common peroneal nerve were bridged using a PHB conduit containing either GGF in alginate hydrogel (GGF) or alginate alone (Alginate), or with an empty PHB conduit (Empty). Tissues were harvested 21, 42 and 63 days post-operatively. Schwann cell and axonal regeneration were assessed using quantitative immunohistochemistry. At 21 days, addition of GGF increased significantly the distance of axonal and Schwann cells regeneration in comparison with that observed in Alginate and Empty conduits for both gap lengths. The axons bridged the 2-cm GGF conduits gap by 63 days, with a comparable rate of regeneration seen in 4-cm conduits. Schwann cells and axonal regeneration quantity was similar for both gap lengths in each group. However, at all time points the quantity of axonal and Schwann cells regeneration in GGF grafts was significantly greater than in both Alginate and Empty conduits, the latter showing better regeneration than Alginate conduits. The results indicate an inhibitory effect of alginate on regeneration, which is partially reversed by the addition of GGF to the conduits. In conclusion, GGF stimulates a progressive and sustainable regeneration increase in long nerve gap conduits.

Effect of NT-4 and BDNF delivery to damaged sciatic nerves on phenotypic recovery of fast and slow muscles fibres

We investigated whether neurotrophin-4 (NT-4) and brain-derived neurotrophic factor (BDNF) affected the reinnervation of slow and fast motor units. Neurotrophin-impregnated or plain fibronectin (FN) conduits were inserted into a sciatic nerve gap. Fast extensor digitorum longus (EDL) and slow soleus muscles were collected 4 months postsurgery. Muscles were weighed and fibre type proportion and mean fibre diameters were derived from muscle cross-sections. All fibre types in muscles from FN animals were severely atrophied and this correlated well with type 1 fibre loss and atrophy in soleus and type 2b loss and atrophy in EDL. Treatment with NT-4 reversed soleus but not EDL mass loss above the FN group by significantly restoring type 1 muscle fibre proportion and diameters towards those of normal unoperated animals. BDNF did not increase muscle mass but did have minor effects on fibre type and diameter. Thus, NT-4 significantly improved slow motor unit recovery, and provides a basis for therapies intended to aid the functional recovery of muscles after denervating injury.

Denervation-induced changes in myosin heavy chain expression in the rat diaphragm muscle

Unilateral denervation (Dnv) of the rat diaphragm muscle (Diam) markedly alters expression of myosin heavy chain (MHC) isoforms. After 2 wk of Diam Dnv, MHC content per half-sarcomere decreases in fibers expressing MHC(2X) and MHC(2B). We hypothesized that changes in MHC protein expression parallel changes in MHC mRNA expression. Relative MHC isoform mRNA levels were determined by Northern analysis after 1, 3, 7, and 14 days of Dnv of the rat Diam. MHC protein expression was determined by SDS-PAGE. Changes in MHC isoform protein and mRNA expression were not concurrent. Expression of MHC(Slow) and MHC(2X) mRNA isoforms decreased dramatically by 3 days of Dnv, whereas that of MHC(2A) and MHC(2B) did not change. Expression of all MHC protein isoforms decreased by 3 days of Dnv. We observed a differential effect of rat Diam Dnv on MHC isoform protein and mRNA expression. The time course of the changes in MHC isoform mRNA and protein expression suggests a predominant effect of altered protein turnover rates on MHC protein expression instead of altered transcription after Dnv.

Neurotrophic factors and their receptors in axonal regeneration and functional recovery after peripheral nerve injury

Over a half a century of research has confirmed that neurotrophic factors promote the survival and process outgrowth of isolated neurons in vitro. The mechanisms by which neurotrophic factors mediate these survival-promoting effects have also been well characterized. In vivo, peripheral neurons are critically dependent on limited amounts of neurotrophic factors during development. After peripheral nerve injury, the adult mammalian peripheral nervous system responds by making neurotrophic factors once again available, either by autocrine or paracrine sources. Three families of neurotrophic factors were compared, the neurotrophins, the GDNF family of neurotrophic factors, and the neuropoetic cytokines. Following a general overview of the mechanisms by which these neurotrophic factors mediate their effects, we reviewed the temporal pattern of expression of the neurotrophic factors and their receptors by axotomized motoneurons as well as in the distal nerve stump after peripheral nerve injury. We discussed recent experiments from our lab and others which have examined the role of neurotrophic factors in peripheral nerve injury. Although our understanding of the mechanisms by which neurotrophic factors mediate their effects in vivo are poorly understood, evidence is beginning to emerge that similar phenomena observed in vitro also apply to nerve regeneration in vivo.

Differential tyrosine kinase C mRNA distribution in extensor digitorum longus and soleus motoneurons in adult rats: effect of axotomy and neurotrophin-3 treatment

Delivery of neurotrophin-3 (NT-3) to severed sciatic nerves results in specific normalization of atrophied fast 2b gastrocnemius muscle fibres, and promotes preferential neuromuscular junction maturation of fast extensor digitorum longus (EDL). To investigate the selective influence on fast muscle reinnervation due to NT-3 delivery, we analyzed tyrosine kinase C (trkC) mRNA differential expression in EDL and slow soleus motor pools of unoperated rats and at 1-week post-axotomy. Motoneurons (Mns) were identified using retrograde tracers. TrkC mRNA quantification was estimated by silver grain counting. TrkC mRNA expression was higher in EDL than in soleus Mns in unoperated rats. Axotomy decreased trkC mRNA expression only in EDL Mns, this downregulation being prevented by NT-3. These results suggest that differential expression of trkC receptor is the morphological correlate of the preferential effect of NT-3 upon EDL Mns.

A dose-dependent facilitation and inhibition of peripheral nerve regeneration by brain-derived neurotrophic factor

The time-dependent decline in the ability of motoneurons to regenerate their axons after axotomy is one of the principle contributing factors to poor functional recovery after peripheral nerve injury. A decline in neurotrophic support may be partially responsible for this effect. The up-regulation of BDNF after injury, both in denervated Schwann cells and in axotomized motoneurons, suggests its importance in motor axonal regeneration. In adult female Sprague-Dawley rats, we counted the number of freshly injured or chronically axotomized tibial motoneurons that had regenerated their axons 1 month after surgical suture to a freshly denervated common peroneal distal nerve stump. Motor axonal regeneration was evaluated by applying fluorescent retrograde neurotracers to the common peroneal nerve 20 mm distal to the injury site and counting the number of fluorescently labelled motoneurons in the T11-L1 region of the spinal cord. We report that low doses of BDNF (0.5-2 microg/day for 28 days) had no detectable effect on axonal regeneration after immediate nerve repair, but promoted axonal regeneration of motoneurons whose regenerative capacity was reduced by chronic axotomy 2 months prior to nerve resuture, completely reversing the negative effects of delayed nerve repair. In contrast, high doses of BDNF (12-20 microg/day for 28 days) significantly inhibited motor axonal regeneration, after both immediate nerve repair and nerve repair after chronic axotomy. The inhibitory actions of high dose BDNF could be reversed by functional blockade of p75 receptors, thus implicating these receptors as mediators of the inhibitory effects of high dose exogenous BDNF.

Sodium influx during action potential in innervated and denervated rat skeletal muscles

Resting Na(+) influx (J(i)(Na)) was measured in innervated and denervated (1-6 days) rat extensor digitorum longus muscle in the absence and presence of 2 micromol/L tetrodotoxin (TTX). The mean value of Na(+) permeability (P(Na)) in innervated muscles was 49.6 +/- 2.6 pm.s(-1). At the second day postdenervation, it decreased by about 45%. This was followed, between the second and fourth days, by a sharp rise, which by the sixth day reached a steady value approximately 2.5 times greater than that of innervated muscles. This, most likely, generated the 30% increase in internal [Na(+)] concentration ([Na(+)](I)) observed at this time. Tetrodotoxin reduced P(Na) of both innervated and denervated muscles by about 25%. In 6-day denervated muscles, virtually all the TTX effect on P(Na) represents the blockage of TTX-resistant Na(+) channels. Denervation produced a depolarization of about 20 mV by the sixth day. The extra J(i)(Na) per action potential (AP) decreased monotonically with time after denervation from 20.0 +/- 3.8 in innervated to 11.1 +/- 1.0 nmol.g(-1).AP(-1) in 6-day denervated muscles. The overshoot of the AP decreased from 15 +/- 1 in innervated to 7 +/- 1 mV in 6-day denervated muscles. Likewise, the maximum rate of rise (+dV/dt), an expression of the inward Na(+) current, fell from 305 +/- 14 in innervated to 188 +/- 18 V.s(-1) in 6-day denervated muscles. The estimated 6-day denervated/innervated ratio of peak Na(+) conductance (g(Na)) was 0.67. The changes in AP parameters promoted by denervation were substantially reduced when both innervated and denervated fibers were hyperpolarized to -90 mV. These results suggest that the depolarization, mainly due to the increase in P(Na) /P(K) ratio, increases Na(+) inactivation and consequently reduces peak g(Na), in spite of the absolute increment in resting TTX-sensitive P(Na). This, in addition to the moderate reduction in the inward driving force on Na(+), decreases the inward Na(+) current and the extra J(i)(Na) per AP.

Regulation of neurotrophin signaling in aging sensory and motoneurons: dissipation of target support?

A hallmark of senescence is sensorimotor impairment, involving locomotion and postural control as well as fine-tuned movements. Sensory and motoneurons are not lost to any significant degree with advancing age, but do show characteristic changes in gene-expression pattern, morphology, and connectivity. This review covers recent experimental findings corroborating that alterations in trophic signaling may induce several of the phenotypic changes seen in primary sensory and motoneurons during aging. Furthermore, the data suggests that target failure, and/or breakdown of neuron-target interaction, is a critical event in the aging process of sensory and motoneurons.