The pathway mediating the monosynaptic stretch reflex has served as an important model system for studies of plasticity in the spinal cord. Its usefulness is extended by evidence that neurotrophins, particularly neurotrophin-3 (NT-3), which has been shown to promote spinal axon elongation, can modulate the efficacy of the muscle spindle-motoneurone connection both after peripheral nerve injury and during development. The findings summarized here emphasize the potential for neurotrophins to modify function of both damaged and undamaged neurones. It is important to recognize that these effects may be functionally detrimental as well as beneficial.

Neuropathic pain arises as a debilitating consequence of nerve injury. The etiology of such pain is poorly understood, and existing treatment is largely ineffective. We demonstrate here that glial cell line-derived neurotrophic factor (GDNF) both prevented and reversed sensory abnormalities that developed in neuropathic pain models, without affecting pain-related behavior in normal animals. GDNF reduces ectopic discharges within sensory neurons after nerve injury. This may arise as a consequence of the reversal by GDNF of the injury-induced plasticity of several sodium channel subunits. Together these findings provide a rational basis for the use of GDNF as a therapeutic treatment for neuropathic pain states.

The fidelity of impulse propagation through the complex axonal tree en route to the various target cells of that fiber is an important question in neurobiology. Anatomists can trace pathways, but if impulses fail to propagate down to the terminals to release transmitter onto the target cell, there is a significant `disconnect' between anatomy and physiology. These issues have been studied at length in the spinal cord of the cat where it has proven possible to examine the connections made by afferent fibers on motoneurons under different stimulus conditions. EPSP amplitude varies systematically during high frequency stimulation of the afferents according to the identity of the target motoneuron. This variation is a function of the state of the motoneuron's relation to its peripheral target. It changes after motoneuron axotomy and recovers with reinnervation of the periphery. Neurotrophins delivered to the axotomized motor axons fail to induce recovery. Chronic stimulation of the motor nerve alters muscle properties with coordinated changes in properties of the synapses on motoneurons innervating the stimulated muscle. We cannot yet definitively establish the mechanisms determining synaptic behavior during high frequency stimulation. However, the retrograde regulation of these properties suggests that it is an important variable and thus is worthy of intensive further study.

The effects of neurotrophin-3 (NT-3) and NT-4/5 on the function of axotomized group Ia afferents and motoneurons comprising the monosynaptic reflex pathway were investigated. The axotomized medial gastrocnemius (MG) nerve was provided with NT-3 or NT-4/5 for 8–35 d via an osmotic minipump attached to its central end at the time of axotomy. After this treatment, monosynaptic EPSPs were recorded intracellularly from MG or lateral gastrocnemius soleus (LGS) motoneurons in response to stimulation of the heteronymous nerve under pentobarbital anesthesia. Controls were preparations with axotomized nerves treated directly with vehicle; other axotomized controls were administered subcutaneous NT-3. Direct NT-3 administration (60 μg/d) not only prevented the decline in EPSP amplitude from axotomized afferents (stimulate MG, record LGS) observed in axotomy controls but, after 5 weeks, led to EPSPs larger than those from intact afferents. These central changes were paralleled by recovery of group I afferent conduction velocity. Removal of NT-3 4–5 weeks after beginning treatment resulted in a decline of conduction velocity and EPSP amplitude within 1 week to values characteristic of axotomy. The increased synaptic efficacy after NT-3 treatment was associated with enhanced connectivity of single afferents to motoneurons. NT-4/5 induced modest recovery in group I afferent conduction velocity but not of the EPSPs they elicited. NT-3 or NT-4/5 had no effect on the properties of treated motoneurons or their monosynaptic EPSPs. We conclude that NT-3, and to a limited extent NT-4/5, promotes recovery of axotomized group Ia afferents but not axotomized motoneurons or the synapses on them.

Several lines of evidence suggest that neurotrophin administration may be of some therapeutic benefit in the treatment of peripheral neuropathy. However, a third of sensory neurons do not express receptors for the neurotrophins. These neurons are of small diameter and can be identified by the binding of the lectin IB4 and the expression of the enzyme thiamine monophosphatase (TMP). Here we show that these neurons express the receptor components for glial-derived neurotrophic factor (GDNF) signaling (RET, GFRα-1, and GFRα-2). In lumbar dorsal root ganglia, virtually all IB4-labeled cells express RET mRNA, and the majority of these cells (79%) also express GFRα-1, GFRα-2, or GFRα-1 plus GFRα-2. GDNF, but not nerve growth factor (NGF), can prevent several axotomy-induced changes in these neurons, including the downregulation of IB4 binding, TMP activity, and somatostatin expression. GDNF also prevents the slowing of conduction velocity that normally occurs after axotomy in a population of small diameter DRG cells and the A-fiber sprouting into lamina II of the dorsal horn. GDNF therefore may be useful in the treatment of peripheral neuropathies and may protect peripheral neurons that are refractory to neurotrophin treatment.

This study of cat medial gastrocnemius (MG) muscle and motor unit (MU) properties tests the hypothesis that the normal ranges of MU contractile force, endurance, and speed are directly associated with the amount of neuromuscular activity normally experienced by each MU. We synchronously activated all MUs in the MG muscle with the same activity (20 Hz in a 50% duty cycle) and asked whether conversion of whole muscle contractile properties is associated with loss of the normal heterogeneity in MU properties. Chronically implanted cuff electrodes on the nerve to MG muscle were used for 24-h/day stimulation and for monitoring progressive changes in contractile force, endurance, and speed by periodic recording of maximal isometric twitch and tetanic contractions under halothane anesthesia. Chronic low-frequency stimulation slowed muscle contractions and made them weaker, and increased muscle endurance. The most rapid and least variable response to stimulation was a decline in force output of the muscle and constituent MUs. Fatigue resistance increased more slowly, whereas the increase in time to peak force varied most widely between animals and occurred with a longer time course than either force or endurance. Changes in contractile force, endurance, and speed of the whole MG muscle accurately reflected changes in the properties of the constituent MUs both in extent and time course. Normally there is a 100-fold range in tetanic force and a 10-fold range in fatigue indexes and twitch time to peak force. After chronic stimulation, the range in these properties was significantly reduced and, even in MU samples from single animals, the range was shown to correspond with the slow (type S) MUs of the normal MG. In no case was the range reduced to less than the type S range. The same results were obtained when the same chronic stimulation pattern of 20 Hz/50% duty cycle was imposed on paralyzed muscles after hemisection and unilateral deafferentation. The findings that the properties of MUs still varied within the normal range of type S MUs and were still heterogeneous despite a decline in the variance in any one property indicate that the neuromuscular activity can account only in part for the wide range of muscle properties. It is concluded that the normal range of properties within MU types reflects an intrinsic regulation of properties in the multinucleated muscle fibers.

We have tested the ability of neurotrophins to reverse axotomy-induced changes in adult motor and sensory neurons, using the physiological measure of conduction velocity. Five weeks after axotomy, sensory and motor conduction velocities were greatly reduced. NT-3 at 60 μg/d, pumped directly onto the cut nerve stump, largely prevented the change in sensory fibers. Lower doses were less effective, and NT-4/5 was without effect. In contrast, both NT-3 and NT-4/5 were effective at rescuing motoneurons, with similar dose dependencies. This amelioration of physiological deficits in adult mammalian neurons suggests possible therapeutic application of neurotrophins. We have also studied the physiological effects of neurotrophin deprivation on intact peripheral neurons. After 2 weeks of sequestration of trkB ligands (BDNF and NT-4/5), motor, but not sensory, neuron conduction was significantly slowed. Sequestration of NT-3 was found to affect both motor and sensory fiber velocities but more modestly and only with higher doses of sequestering agent. These data therefore suggest that peripherally produced neurotrophins are necessary for the maintenance of normal functional properties of peripheral neurons.

We tested the hypothesis that neurotrophin-3 (NT-3) in adult cats can rescue the central synapses made by muscle afferents from the effects of peripheral axotomy. The medial gastrocnemius (MG) muscle nerve in cats was axotomized and capped or axotomized and the distal end provided with either saline or NT-3 by mini-osmotic pump. Four to five weeks later monosynaptic excitatory postsynaptic potentials (EPSPs) elicited by electrical stimulation of the axotomized MG nerve were recorded in intact lateral gastrocnemius/soleus (LGS) motoneurons. The axotomized MG afferents without NT-3 treatment generated EPSPs averaging one-half of the amplitude of those generated by normal intact MG afferents. Axotomized MG afferents treated with NT-3 elicited EPSPs averaging 2.5 times normal amplitude and 5 times the amplitude of those from afferents axotomized but not treated. The very large EPSPs generated by NT-3-treated afferents remained as susceptible to depression during high-frequency stimulation (32 shocks at 167 Hz) as those elicited by untreated axotomized afferents. The arrival of the afferent volley of the cord dorsum potential and the onset of EPSPs were both delayed by axotomy of the group Ia afferents and were both restored by exposure to NT-3. This result suggests that the conduction velocity and thus the caliber of group Ia afferents are also controlled by NT-3. We conclude that the neurotrophin NT-3 has a continuing role in the maintenance of physiological function of muscle afferents in adult mammals.

Resistive force and electromyograms from triceps surae muscles were measured during dorsiflexion of both ankles of awake cats before and after interruption of one dorsolateral funiculus (DLF), DLF lesions produced ipsilateral increases in dynamic and static reflex force that persisted over 66 weeks. The increase in dynamic reflex force was velocity sensitive, as demonstrated by a greater effect for 60 degrees/sec than for 10 degrees/sec dorsiflexion. Also, the lesions increased dynamic force to a greater extent than static force (increased dynamic index). Background force (recorded immediately before each reflex response) was elevated ipsilaterally. However, increases in reflex force were observed when preoperative and postoperative background forces were matched within 10% and were associated with equivalent resting levels of electromyographic (EMG) activity. Resistive reflex force was significantly correlated with EMG responses to dorsiflexion and was not determined by nonreflexive mechanical stiffness of the muscles. Contralateral background and reflex force and associated EMG activity were decreased slightly, comparing preoperative and postoperative records.Clinical testing revealed ipsilateral postoperative increases in extensor tone, increased resistance to hindlimb flexion, hypermetria during positive support responses, and appearance of the Babinski reflex. However, the most reliable tests of DLF lesion effects were the quantitative measures of dynamic and static reflex amplitude, The enhancement of stretch reflexes is suggestive of spasticity. However, hyperactive stretch reflexes, hypertonicity, and the Babinski reflex were observed soon after interruption of the ipsilateral DLF, in contrast to a gradual development of positive signs that is characteristic of a more broadly defined spastic syndrome from large spinal lesions. Also, other signs that often are included in the spastic syndrome, including clonus, increased flexor reflex activity, and flexor spasms, did not result from DLF lesions. Thus, unilateral DLF lesions provide a model of spasticity but: produce only several components of a more inclusive spastic syndrome.

Glial cell‐line‐derived neurotrophic factor (GDNF) is a potent neurotrophic factor shown to rescue developing and adult motoneurons in vitro and in vivo from programmed and injury‐induced cell death. To test whether GDNF would rescue adult mammalian sensory and motor neurons from physiological consequences of injury, the tibial nerve of rats was axotomized and, after a 10 day delay to permit injury processes to proceed, vehicle or GDNF was supplied directly to the nerve for 2 or 4 weeks or GDNF intrathecally for 2 weeks. Conduction velocity (CV) of both sensory and motor axons declined during the initial 10 days, and even more so if then treated with vehicle. Treatment with GDNF resulted in marginal improvement of sensory axon CV. CV of motor axons recovered significantly in a dose‐ and time‐dependent manner. The results suggest that GDNF may have therapeutic potential in the treatment of peripheral neuropathies.

1. Trains of action potentials in motoneurones frequently commence with an initial doublet; i.e. a uniquely short interspike interval. Previous authors have speculated on the functional importance of initial doublets. Here we test the hypotheses that these doublets are associated with particular classes of motoneurones or particular physiological conditions. 2. Discharges of inspiratory motoneurones were recorded extracellularly in the thoracic ventral horn of anaesthetized, paralysed cats. Seventy units (35 each with axons in the internal and external intercostal nerves) were classified on the basis of their maximum firing rates, start times in the respiratory cycle and axonal destination. 3. Initial doublets were defined by an interspike interval < 14 ms. Of seventeen units firing initial doublets, fifteen had axons in the external intercostal nerve and two had axons in the internal intercostal nerve. Neither maximum firing rate nor start time during the respiratory cycle predicted the occurrence of doublets. 4. The chemical drive to breathe was manipulated by altering the CO2 content of the inspired gas or by briefly stopping the respiratory pump. Varying the chemical drive to breathe had no consistent effect on the occurrence of initial doublets. 5. These results support the view that initial doublets are part of the normal pattern of discharge of motoneurones. However, because the incidence of doublets does not consistently support previous functional hypotheses, we argue that the occurrence of doublets may not necessarily be dictated by the CNS, but in some circumstances it is an epiphenomenon dependent on the state of the motoneurone, in particular on the statistical properties of its synaptic inputs.

1. In this study we describe application of high-frequency stimulation to the group Ia afferent-to-motoneuron synapse of cats to determine the extent to which regeneration of axotomized muscle afferents and motoneurons into skin or into muscle rescues their ability to generate excitatory postsynaptic potentials (EPSPs). 2. The medial gastrocnemius (MG) muscle nerve was transected and 1) left chronically axotomized, 2) cross-united to the caudal cutaneous sural (CCS) nerve, or 3) self-united. The ability of the operated MG muscle afferents to generate EPSPs in normal lateral gastrocnemius-soleus (LGS) motoneurons and of normal LGS muscle afferents to generate EPSPs in the operated MG motoneurons was tested 5 wk-30 mo later. 3. EPSPs were generated by bursts of 32 shocks at 167 Hz and averaged in register. In normal cats, EPSP amplitude decreased (negative modulation) during these bursts in type S motoneurons and could increase or decrease in type F motoneurons (positive or negative modulation). 4. After axotomy, EPSPs generated both in axotomized motoneurons and by axotomized afferents showed only negative modulation during the burst, and the negative modulation was much greater than in normal animals. Regeneration of the muscle nerve into skin significantly decreased the negative modulation relative to axotomy. Regeneration of the muscle nerve into muscle restored the EPSP modulation behaviors even more, to essentially normal values. 5. We conclude that the ability of muscle afferents to generate EPSPs in motoneurons in response to high-frequency stimulation, and the ability of motoneurons to express those EPSPs, are both influenced by the target innervated by those neurons. Synaptic efficacy is severely reduced by target deprivation (axotomy), partially rescued by cross-regeneration into skin, and rescued virtually completely by regeneration into the native muscle. We speculate on the role of target-derived neurotrophins in these effects.

1. In this study we investigate the peripheral receptive field properties and spinal cord connections of low-threshold muscle afferent fibers cross-regenerated into the skin to determine whether a cutaneous target can rescue physiological functions lost after chronic axotomy. 2. In adult cats the medial gastrocnemius (MG) muscle nerve was coated with the distal cut end of either the caudal or lateral cutaneous sural nerves and allowed to regenerate into the hairy skin (postoperative period 6-30 mo). During terminal acute experiments we made recordings of single MG afferent fibers in dorsal root filaments and peripheral nerve. Conduction velocity and receptive field characteristics were determined for each fiber. In addition, the MG nerve was stimulated to elicit cord dorsum potentials and monosynaptic excitatory postsynaptic potentials (EPSPs) in heteronymous motoneurons. As controls, studies were carried out after MG nerve axotomy (postoperative period 2.5-12 mo). 3. After innervation of the skin, MG muscle afferent fibers exhibited firing characteristics and proximal segment conduction velocities like those of normal MG afferents. Responses to skin and hair stimulation consisted primarily of slowly adapting, stretch-sensitive, and steady discharge patterns, all common in normal muscle afferents but not in cutaneous afferents. These properties were observed despite the innervation of touch domes and single hairs, suggesting that the peripheral physiology of muscle afferents is a function of the axonal membrane and is not respecified by a cutaneous target and/or receptors. 4. Cord dorsum potentials were characteristic of those elicited by intact muscle afferents rather than skin afferents and showed recovery of configurations lost after chronic axotomy. 5. The monosynaptic EPSPs elicited in lateral gastrocnemius-soleus motoneurons also recovered from the reduction in amplitude observed after chronic axotomy. The configurations of these EPSPs were characteristic of muscle afferents rather than skin afferents. 6. These experiments demonstrate that the peripheral and central physiological properties of muscle afferents are rescued from the axotomy state if the afferents are allowed to reinnervate skin. We found no evidence that respecification had occurred to bring the function of muscle afferents into accord with the new cutaneous target.

Both motoneurons and the muscle fibers that they innervate can vary widely in their properties. The finding that these properties are highly correlated indicates that these cells are not specified independently, but rather interact in some manner to achieve the observed coherence. The direction of the interaction, i.e., whether orthograde or retrograde, requires analysis beyond the simple observation of these correlated properties. Evidence, largely from experiments involving reinnervation of muscle by the original or a foreign motor nerve, suggests the action of retrogradely transported factors from the muscle as well as orthograde ones. Various possible factors, specifically neurotrophins, are advanced as possible candidates for retrograde specification. In addition, synaptic input to motoneurons also varies in a coordinated fashion in a manner that suggests a retrograde determination from the muscle. However, motoneuron properties and their synaptic input change after peripheral nerve manipulations such that they are no longer in register. This indicates some independence in retrograde specification of motoneurons and their synaptic input.

For over a decade the commonly adopted definition of human spasticity has been “a motor disorder characterized by a velocity dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks” (Lance 1980). However, the manifestations of spasticity are heterogeneous, which complicates attempts to develop a test that encompasses all the clinical signs observed in human spinal cord injury (SCI; Wiesendanger 1985). To avoid this problem, Noth (1991) has suggested that the term spasticity be used “only when the typical increase in muscle tone is present.” Thus, progress in understanding human spasticity might arise from the study of “one typical subtype of ‘spasticity’“(Young 1980). Accordingly, we have operationally defined animal spinal spasticity as augmented muscle tone and stretch reflexes resulting from spinal injury (for a review on the mechanisms and quantitation of spastic hypertonia see Katz and Rymer 1989).

1. Retrograde neuronal transport following small iontophoretic injections of horseradish peroxidase was used to investigate the location of neurones projecting to the thoracic ventral horn of the cat. 2. A concentration of labelled neurones was seen in the contralateral medial ventral horn at the immediately opposite rostrocaudal position. 3. The participation of respiratory interneurones in the projection was investigated by means of spike‐triggered averaging, as follows. 4. Spike trains of single interneurones whose firing patterns were related to the central respiratory cycle were recorded extracellularly in the thoracic ventral horn of anaesthetized, paralysed cats. Firing patterns were defined by the construction of cycle‐triggered histograms. 5. Spike‐triggered averaging of the signal from an extracellular tungsten microelectrode in the opposite ventral horn was performed to test for the presence of axonal, terminal or focal synaptic potentials. 6. At least one of these types of potential was found for 34/55 units. Terminal potentials were found for thirty‐one units, accompanied by focal synaptic potentials for twenty‐seven units. Potentials were found for units with all types of firing patterns. Units whose activity elicited these potentials were generally located in the medial half of the ventral horn. 7. We conclude that at least 60% of the respiratory interneurones project to the immediately opposite ventral horn.

1. This study investigates the relation between the peripheral innervation of low-threshold cutaneous afferents and the postsynaptic potentials elicited by electrical stimulation of those afferents. 2. In cats deeply anesthetized with pentobarbital sodium, cord dorsum potentials (CDPs) and postsynaptic potentials (PSPs) in spinal motoneurons were elicited by stimulation of the caudal cutaneous sural nerve (CCS), the lateral cutaneous sural nerve (LCS), and the medial gastrocnemius (MG) muscle nerve. We tested 1) unoperated cats, and cats in which CCS has been 2) chronically axotomized and ligated, 3) cut and self-reunited, 4) cut and cross-united with LCS, or 5) cut and cross-united with the MG. Terminal experiments were performed 3-36 mo after initial surgery. 3. In cats in which the CCS had been self-reunited or cross-united distally with LCS, tactile stimulation of the hairy skin normally innervated by the distal nerve activated afferents in the CCS central to the coaptation, indicating that former CCS afferents had regenerated into native or foreign skin, respectively. 4. In cats in which the CCS had been cross-united distally with the MG, both stretch and contraction of the MG muscle activated the former CCS afferents. 5. In unoperated cats, CDPs elicited by stimulation of CCS and of LCS exhibited a low-threshold N1 wave and a higher-threshold N2 wave. These waves were greatly delayed and appeared to merge after chronic axotomy of CCS. Regeneration of CCS into itself, into LCS, or into MG restored the normal latencies and configurations of these potentials. 6. In unoperated cats, stimulation of CCS, of LCS, and of MG each produced PSPs of characteristic configurations in the various subpopulations of motoneurons of the triceps surae. CDPs and PSPs elicited by the CCS cross-regenerated into LCS or MG were typical of those generated by the normal CCS, i.e., there was no evidence of respecification of central synaptic connections to bring accord between center and periphery after cross-regeneration. 7. Chronic axotomy of CCS increased the latencies of PSPs from CCS; regeneration of CCS into the CCS, LCS, or MG restored normal latencies. 8. In summary, CDPs and PSPs in motoneurons from the cutaneous sensory nerve CCS are altered by chronic axotomy of CCS, thus indicating their target dependency. These alterations are restored by regeneration of CCS into not only a native or foreign cutaneous target, but also into skeletal muscle. We conclude that muscle as well as skin is capable of providing trophic support for cutaneous afferents.

1. In cats, we studied the physiological properties of regenerating sprouts of muscle afferent fibers and compared them with sprouts from cutaneous afferent fibers. 2. Muscle nerves to the triceps surae and cutaneous sural nerves were axotomized in the popliteal fossa, and the proximal ends were inserted into nerve cuffs. Six days later, we recorded action potentials from single Groups I and II muscle and mostly Group II cutaneous afferents driven by mechanostimulation of the cuff. 3. Most muscle afferent sprouts (91%) had a regular slowly adapting discharge in response to sustained mechanical displacement of the cuff, particularly to sustained stretch stimuli, whereas most cutaneous afferents (92%) did not. Muscle afferents were more likely to have a spontaneous discharge and afterdischarge. 4. Group II muscle afferent sprouts had lower stretch thresholds and a higher incidence of spontaneous discharge compared with Group I fiber sprouts, whereas Group I fibers had a higher incidence of high-frequency afterdischarge to mechanical stimuli. 5. We conclude that, 6 days after axotomy, regenerating sprouts of muscle afferents, particularly Group II afferents, have become mechanosensitive in the absence of a receptor target and exhibit physiological properties similar to those found when innervating their native muscle but significantly different from sprouts of cutaneous afferents. Expression of these native muscle afferent firing patterns after the inappropriate reinnervation of hairy skin may be due to inherent properties of the muscle afferent fiber.