Soma size and oxidative enzyme activity in normal and chronically stimulated motoneurones of the cat's spinal cord

The vulnerability of spinal motoneurons and soma size plasticity in a mouse model of amyotrophic lateral sclerosis

KEY POINTS

Motoneuron soma size is a largely plastic property that is altered during amyotrophic lateral sclerosis (ALS) progression. We report evidence of systematic spinal motoneuron soma size plasticity in mutant SOD1-G93A mice at various disease stages and across sexes, spinal regions and motoneuron types. We show that disease-vulnerable motoneurons exhibit early increased soma sizes. We show via computer simulations that the measured changes in soma size have a profound impact on the excitability of disease-vulnerable motoneurons. This study reveals a novel form of plasticity in ALS and suggests a potential target for altering motoneuron function and survival.

ABSTRACT

α-Motoneuron soma size is correlated with the cell's excitability and function, and has been posited as a plastic property that changes during cellular maturation, injury and disease. This study examined whether α-motoneuron somas change in size over disease progression in the G93A mouse model of amyotrophic lateral sclerosis (ALS), a disease characterized by progressive motoneuron death. We used 2D- and 3D-morphometric analysis of motoneuron size and measures of cell density at four key disease stages: neonatal (P10 - with earliest known disease changes); young adult (P30 - presymptomatic with early motoneuron death); symptom onset (P90 - with death of 70-80% of motoneurons); and end-stage (P120+ - with full paralysis of hindlimbs). We additionally examined differences in lumbar vs. sacral vs. cervical motoneurons; in motoneurons from male vs. female mice; and in fast vs. slow motoneurons. We present the first evidence of plastic changes in the soma size of spinal α-motoneurons occurring throughout different stages of ALS with profound effects on motoneuron excitability. Somatic changes are time dependent and are characterized by early-stage enlargement (P10 and P30); no change around symptom onset; and shrinkage at end-stage. A key finding in the study indicates that disease-vulnerable motoneurons exhibit increased soma sizes (P10 and P30). This pattern was confirmed across spinal cord regions, genders and motoneuron types. This extends the theory of motoneuron size-based vulnerability in ALS: not only are larger motoneurons more vulnerable to death in ALS, but are also enlarged further in the disease. Such information is valuable for identifying ALS pathogenesis mechanisms.

Responses of diseased muscle to electrical and mechanical intervention

It is well established that the properties of muscle fibres are influenced by their neurons and that this is at least in part mediated by the pattern of activity. Application of this knowledge has led to the experimental trial of electrical stimulation in diseased muscle, both in the dystrophic mouse and in children with Duchenne muscular dystrophy. This has shown a beneficial effect of slow frequency stimulation. Another route through which muscle properties can be influenced is by changing the load by procedures such as tenotomy. This has been studied by complete tenotomy in normal animals and recently by selective partial procedures in human disease. Y. Rideau has shown that release of early shortening (contractures) of several muscles, a consistent feature in Duchenne muscular dystrophy, has a beneficial effect on muscle function. From personal observations on a number of Rideau's patients who have undergone this procedure the improvement in function seems disproportionate to what could be explained on simple biomechanical grounds alone and suggests some more fundamental change in the contractile properties of the muscle.

Rat alpha- and gamma-motoneuron soma size and succinate dehydrogenase activity are independent of neuromuscular activity level

The chronic level of neuromuscular activity, that is, activation and loading, strongly influences the morphological, metabolic, phenotypic, and physiological properties of skeletal muscles. The effects on the innervating motoneurons, however, are less established. We determined and compared the effects of 30 days of decreased activity (induced by a complete mid-thoracic spinal cord transection, ST) or near inactivity (induced by spinal cord isolation, SI) on the soma size and succinate dehydrogenase (SDH) activity of motoneurons innervating a predominantly slow ankle extensor (soleus) and a predominantly fast ankle flexor (tibialis anterior) muscle of adult rats. Soleus and tibialis anterior motoneuron pools were labeled retrogradely using nuclear yellow. The alpha- and gamma-motoneurons were classified based on soma size. Mean number of labeled motoneurons, and mean soma size and SDH activity for both alpha- and gamma-motoneurons were similar in control, ST, and SI rats. Compared to previous reports showing significant decreases in muscle fiber size and adaptations toward a "faster" metabolic profile following ST and SI, the results indicate that, unlike the muscles they innervate, the motoneurons are relatively unresponsive to chronic reductions in neuromuscular activity. The implication of these results is that mean size and SDH activity are independent of the number of action potentials generated by both alpha- and gamma-motoneurons and that even the absence of afferent input to the spinal cord has no influence on size and oxidative metabolic potential of the motoneuron soma.

Effects of hyperbaric exposure with high oxygen concentration on the physical activity of developing rats

The effects of hyperbaric exposure with high oxygen concentration on the physical activity of developing male rats were investigated. Five-week-old male rats were exposed to an atmospheric pressure of 1.25 with an oxygen concentration of 36.0% for 12 h (7.00-19.00 h) and exercised voluntarily for 12 h (19.00-7.00 h) daily for 8 weeks. The voluntary running activities were compared with those in age-matched rats without hyperbaric exposure. In addition, the properties of the soleus and plantaris muscle fibers and their spinal motoneurons were examined. The voluntary running activities of rats with or without hyperbaric exposure increased during development. However, the mean voluntary running activities were higher in rats with hyperbaric exposure (7,104 m/day) than in those without hyperbaric exposure (4,932 m/day). The oxidative capacities of the soleus and plantaris muscle fibers and their spinal motoneurons increased following hyperbaric exposure. It is suggested that adaptations of neuromuscular units to hyperbaric exposure with high oxygen concentration enhance the metabolism, and thus, the function of neuromuscular units is promoted.

Effects of exercise training on α-motoneurons

Evidence is presented that one locus of adaptation in the “neural adaptations to training” is at the level of the α-motoneurons. With increased voluntary activity, these neurons show evidence of dendrite restructuring, increased protein synthesis, increased axon transport of proteins, enhanced neuromuscular transmission dynamics, and changes in electrophysiological properties. The latter include hyperpolarization of the resting membrane potential and voltage threshold, increased rate of action potential development, and increased amplitude of the afterhyperpolarization following the action potential. Many of these changes demonstrate intensity-related adaptations and are in the opposite direction under conditions in which chronic activity is reduced. A five-compartment model of rat motoneurons that innervate fast and slow muscle fibers (termed “fast” and “slow” motoneurons in this paper), including 10 active ion conductances, was used to attempt to reproduce exercise training-induced adaptations in electrophysiological properties. The results suggest that adaptations in α-motoneurons with exercise training may involve alterations in ion conductances, which may, in turn, include changes in the gene expression of the ion channel subunits, which underlie these conductances. Interestingly, the acute neuromodulatory effects of monoamines on motoneuron properties, which would be a factor during acute exercise as these monoaminergic systems are activated, appear to be in the opposite direction to changes measured in endurance-trained motoneurons that are at rest. It may be that regular increases in motoneuronal excitability during exercise via these monoaminergic systems in fact render the motoneurons less excitable when at rest. More research is required to establish the relationships between exercise training, resting and exercise motoneuron excitability, ion channel modulation, and the effects of neuromodulators.

Hyperbaric exposure with high oxygen concentration enhances oxidative capacity of neuromuscular units

The effects of hyperbaric exposure with high oxygen concentration on spinal motoneurons and the skeletal muscle fibers that they innervate were investigated. Five-week-old male rats were exposed to a hyperbaric (1.25 atmospheric pressure) environment with a high oxygen concentration (35.0%) for 6h daily. The number, cell body size, and oxidative enzyme activity of motoneurons innervating the soleus and plantaris muscles were examined after 8 weeks of hyperbaric exposure. In addition, the fiber type distribution, cell size, and oxidative enzyme activity of the slow soleus and fast plantaris muscles were examined. The oxidative enzyme activity of alpha motoneurons innervating the soleus and plantaris muscles increased after hyperbaric exposure, irrespective of their cell body sizes. The percentage of high-oxidative fibers in the soleus and plantaris muscles increased after hyperbaric exposure. The oxidative enzyme activity of all types of fibers in the soleus and plantaris muscles increased after hyperbaric exposure. It is concluded that hyperbaric exposure with high oxygen concentration enhances the oxidative capacity of neuromuscular units.

Effects of running exercise during recovery from hindlimb unloading on soleus muscle fibers and their spinal motoneurons in rats

The effects of hindlimb unloading and recovery with or without running exercise on morphological and metabolic properties of soleus muscle fibers and their spinal motoneurons in rats were investigated. Ten-week-old rats were hindlimb suspended for 2 weeks and thereafter were rehabilitated with or without voluntary running exercise for 2 weeks. A decreased percentage of type I fibers and atrophy of all types of fibers were observed after hindlimb unloading. In addition, decreased oxidative enzyme activity of all types of fibers was observed after hindlimb unloading. In contrast, an improvement in the decreased percentage of type I fibers, decreased fiber cross-sectional area, and decreased fiber oxidative enzyme activity was observed after recovery with running exercise, but not without running exercise. There were no changes in the number, cell body size, or oxidative enzyme activity of motoneurons innervating the soleus muscle after hindlimb unloading or recovery with or without running exercise. These results indicate that running exercise is beneficial for the recovery of the decreased percentage of type I fibers and the atrophy and decreased oxidative enzyme activity of all types of fibers in the soleus muscle induced by hindlimb unloading and that there are no changes in morphological or metabolic properties of spinal motoneurons innervating the soleus muscle following decreased or increased neuromuscular activity.

Motoneuron and sensory neuron plasticity to varying neuromuscular activity levels

The size and phenotypic properties of the neural and muscular elements of the neuromuscular unit are matched under normal conditions. When subjected to chronic decreases or increases in neuromuscular activity, however, the adaptations in these properties are much more limited in the neural compared with the muscular elements.

Motoneuron and sensory neuron plasticity to varying neuromuscular activity levels

The size and phenotypic properties of the neural and muscular elements of the neuromuscular unit are matched under normal conditions. When subjected to chronic decreases or increases in neuromuscular activity, however, the adaptations in these properties are much more limited in the neural compared with the muscular elements.

Cell body size and succinate dehydrogenase activity of spinal motoneurons innervating the soleus muscle in mice, rats, and cats

The cell body sizes and succinate dehydrogenase (SDH) activities of motoneurons in the retrodorsolateral region of the ventral horn in the spinal cord innervating the soleus muscle in mice, rats, and cats were compared using quantitative enzyme histochemistry. There was an inverse relationship between cell body size and SDH activity of motoneurons in the three species. The mean cell body sizes of both gamma and alpha motoneuron pools were in the rank order of mice < rats < cats, while the mean SDH activities of both gamma and alpha motoneuron pools were in the rank order of mice > rats > cats. It is concluded that smaller motoneurons innervating the soleus muscle have higher SDH activities than larger motoneurons, irrespective of the species, and that motoneuron pools innervating the soleus muscle in smaller animals have smaller mean cell body sizes and higher mean SDH activities than those in larger animals.

Comparison of the response of motoneurons innervating perineal and hind limb muscles to spaceflight and recovery

The succinate dehydrogenase (SDH) activities and cell body sizes of motoneurons in the dorsomedial (DM) region of the ventral horn at the lower portion of the L5 and the L6 segmental levels of the rat spinal cord were determined following 14 days of spaceflight and after 9 days of recovery on Earth and compared with those in the retrodorsolateral (RDL) region of the ventral horn at the same segmental levels. No changes in the mean SDH activity of motoneurons in the DM region were observed following spaceflight or after recovery. However, a decrease in the mean SDH activity of motoneurons with cell body sizes between 500 and 900 microm(2) in the RDL region was observed following spaceflight and after recovery. These data indicate that moderate-sized motoneurons in the RDL region, which are most likely associated with the hind limb musculature, were responsive to the microgravity environment. In contrast, the motoneurons in the DM region associated with the perineal muscles (associated with predominantly fast, low-oxidative muscles which are recruited for relatively brief periods at high activation levels and have no load-bearing function at 1G) were not affected by microgravity.

Metabolic and morphological stability of motoneurons in response to chronically elevated neuromuscular activity

The purpose of this study was to determine the plasticity of spinal motoneuron size and succinate dehydrogenase activity in response to increased levels of neuromuscular activation and/or increased target size. The plantaris muscles of adult rats were functionally overloaded for one or 10 weeks via the removal of the soleus and gastrocnemius muscles bilaterally. In addition, one group of functionally overloaded rats at each time period was trained daily (1 h/day) on a treadmill. The plantaris muscle on one side in each rat was injected with the fluorescent tracer Nuclear Yellow two days prior to the end of the study to retrogradely label the associated motor pool. At one week, the plantaris weight was increased compared to control, whereas there was no change in motoneuron size. Succinate dehydrogenase activity was unaffected in either the muscle or motoneurons. At 10 weeks, the plantaris muscle weight was larger and the succinate dehydrogenase activity lower in the functionally overloaded rats compared to age-matched controls. Training further increased the hypertrophic response, whereas the succinate dehydrogenase activity returned to control levels. In contrast, mean motoneuron size and succinate dehydrogenase activity were similar among the three groups. These data indicate that overload of a specific motor pool, involving both an increase in activation and an increase in target size, had a minimal effect on the size or the oxidative potential of the associated motoneurons. Thus, it appears that the spinal motoneurons, unlike the muscle fibers, are highly stable over a wide range of levels of chronic neuromuscular activity.

Discharge properties of motoneurones: how are they matched to the properties and use of their muscle units?

A general survey is given of old as well as more recent findings concerning matches between electrophysiological properties of motoneurones and contractile properties of their muscle fibres. Mechanisms for creating and maintaining such matches are discussed. It is pointed out that it is not sufficient to describe the variation of functional motoneurone characteristics simply in terms of 'fast' or 'slow': all properties seem continuously graded and there is cytochemical evidence for several, seemingly independent parameters of functional specialisation.

Effects of 14 days of spaceflight and nine days of recovery on cell body size and succinate dehydrogenase activity of rat dorsal root ganglion neurons

The cross-sectional areas and succinate dehydrogenase activities of L5 dorsal root ganglion neurons in rats were determined after 14 days of spaceflight and after nine days of recovery. The mean and distribution of the cross-sectional areas were similar to age-matched, ground-based controls for both the spaceflight and for the spaceflight plus recovery groups. The mean succinate dehydrogenase activity was significantly lower in spaceflight compared to aged-matched control rats, whereas the mean succinate dehydrogenase activity was similar in age-matched control and spaceflight plus recovery rats. The mean succinate dehydrogenase activity of neurons with cross-sectional areas between 1000 and 2000 microns2 was lower (between 7 and 10%) in both the spaceflight and the spaceflight plus recovery groups compared to the appropriate control groups. The reduction in the oxidative capacity of a subpopulation of sensory neurons having relatively large cross-sectional areas immediately following spaceflight and the sustained depression for nine days after returning to 1 g suggest that the 0 g environment induced significant alterations in proprioceptive function.

Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. II. Motoneuron properties

Chronic stimulation (for 2-3 mo) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow (type SO). We tested the hypothesis that motoneurons of MG muscle thus made type SO by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously type SO (slowly contracting, nonfatiguing, nonsagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of type S, fatigue-resistant motor units. Two separate populations were identified (on the basis of afterhyperpolarization, rheobase, and input resistance) that likely correspond to motoneurons that had been fast (type F) or type S before stimulation. Type F motoneurons, although modified by chronic stimulation, were not converted to the type S phenotype, despite apparent complete conversion of their muscle units to the slow oxidative type (type SO). Muscle units of the former type F motor units were faster and/or more powerful than those of the former type S motor units, indicating some intrinsic regulation of motor unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia excitatory postsynaptic potential (EPSP) amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported, although the degree of control exerted by the periphery may vary: natural type SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin-(NT) 4/5 in muscle plays an important role in controlling muscle and motoneuron properties.

Calreticulin expression in spinal motoneurons of the rat

We have examined the expression of calreticulin in rat spinal motoneurons in order to reveal the occurrence and distribution of Ca2(+)-storage organelles in these neurons. Calreticulin, the non-muscle equivalent of calsequestrin, is the low-affinity, high-capacity calcium-binding protein responsible for intracompartmental Ca2(+)-storage in a number of different cell types. The results of the present immunohistochemical study show that all spinal motoneurons express calreticulin at approximately the same level; no significant differences in cytoplasmic immunostaining intensity were observed between different motoneuron pools or between small and large spinal motoneurons. Immunoelectron microscopy revealed that the intracellular localization of calreticulin within spinal motoneurons was confined to the endoplasmic reticulum and to spherical or pleiomorphic, frequently 'coated' vesicles with a diameter ranging between 120 and 150 nm. Some of these vesicles may represent the so-called calciosomes, the intracellular Ca2(+)-storage vesicles described in liver cells and in cerebellar Purkinje cells. The molecular components responsible for the uptake and release of Ca2+ from the Ca2(+)-storage organelles in spinal motoneurons still remain to be identified.

Comparison of succinate dehydrogenase activity and soma size relationships among neurons in dorsal root ganglia of rats and cats

Compared to dorsal root ganglion (DRG) neurons at L5 in rats, DRG neurons at L7 in cat have a larger mean soma size, a bimodal rather than unimodal distribution of sizes and lower succinate dehydrogenase (SDH) activities for neurons of all sizes. In contrast to spinal motoneurons in both cats and rats, the larger DRG neurons have the higher SDH activities. The 10-20% higher SDH activity of DRG cells in rats than in cats may reflect, in part, a species difference of about 40% in metabolic rates.

Succinate dehydrogenase activity and soma size of motoneurons innervating different portions of the rat tibialis anterior

The spatial distribution, soma size and oxidative enzyme activity of gamma and alpha motoneurons innervating muscle fibres in the deep (away from the surface of the muscle) and superficial (close to the surface of the muscle) portions of the tibialis anterior in normal rats were determined. The deep portion had a higher percentage of high oxidative fibres than the superficial portion of the muscle. Motoneurons were labelled by retrograde neuronal transport of fluorescent tracers: Fast Blue and Nuclear Yellow were injected into the deep portion and Nuclear Yellow into the superficial portion of the muscle. Therefore, motoneurons innervating the deep portion were identified by both a blue fluorescent cytoplasm and a golden-yellow fluorescent nucleus, while motoneurons innervating the superficial portion were identified by only a golden-yellow fluorescent nucleus. After staining for succinate dehydrogenase activity on the same section used for the identification of the motoneurons, soma size and succinate dehydrogenase activity of the motoneurons were measured. The gamma and alpha motoneurons innervating both the deep and superficial portions were located primarily at L4 and were intermingled within the same region of the dorsolateral portion of the ventral horn in the spinal cord. Mean soma size was similar for either gamma or alpha motoneurons in the two portions of the muscle. The alpha motoneurons innervating the superficial portion had a lower mean succinate dehydrogenase activity than those innervating the deep portion of the muscle. An inverse relationship between soma size and succinate dehydrogenase activity of alpha, but not gamma, motoneurons innervating both the deep and superficial portions was observed. Based on three-dimensional reconstructions within the spinal cord, there were no apparent differences in the spatial distribution of the motoneurons, either gamma or alpha, associated with the deep and superficial compartments of the muscle. The data provide evidence for an interdependence in the oxidative capacity between a motoneuron and its target muscle fibres in two subpopulations of motoneurons from the same motor pool, i.e. the same muscle.

Succinate dehydrogenase activity and soma size relationships among cat dorsal root ganglion neurons

A large range in succinate dehydrogenase (SDH) activity and soma size among neurons in the dorsal root ganglion (DRG) and the dorsolateral region of the ventral horn (DLVH) at spinal cord level L7 was observed. Mean soma sizes were similar for the two populations. DLVH, but not DRG, neurons showed an inverse relationship between SDH activity and soma size. DRG neurons had a higher mean SDH activity than DLVH neurons, reflecting the observation that there was a population of DRG neurons with a higher oxidative capacity than DLVH neurons.

Oxidative enzyme activity in the rat soleus muscle and its motoneurons during postnatal maturation

The effect of postnatal maturation on changes in the oxidative enzyme (succinate dehydrogenase) activity in the rat soleus muscle and its motoneurons was examined at 3, 6, and 12 weeks of age. The motoneurons innervating the soleus muscle were identified using the fluorescent retrograde neuronal tracer Nuclear Yellow. An inverse relationship between soma size and oxidative enzyme activity of soleus motoneurons was observed at 3 and 6 weeks of age, whereas there was no correlation between them at 12 weeks. Although the oxidative enzyme activity in the soleus muscle increased during postnatal maturation, it showed a decrease in the soleus motoneurons. These data demonstrate that the inverse relationship between soma size and oxidative enzyme activity of rat soleus motoneurons can only be detected in the early postnatal period and that the oxidative enzyme activity in the rat soleus muscle and its motoneurons can change independently during postnatal maturation.

Radiolabeling motoneuron proteins in the isolated frog spinal cord preparation

An in vitro method for radiolabeling protein in adult frog spinal motoneurons is described, with per cell incorporations which are 2-3 orders of magnitude higher than previously reported for mammalian brain neurons. In the procedure, isolated lumbar spinal cord preparations from Rana pipiens are labeled with 3H-L-leucine, motoneuron cell bodies are recovered and TCA-precipitated protein is analyzed by scintillation counting. The higher levels of labeling (> 90 cpm/cell body) allow one to quantify newly synthesized protein within individual or small groups of identified nerve cell bodies. Motoneuronal labeling correlates directly with cell body size, and other sources of variation in labeling and their control are identified and discussed.

Metabolic profiles of white and red-intermediate spinal motoneurons in the zebrafish

To study the interactions between the citrate cycle and amino acid metabolism in zebrafish spinal motoneurons, we composed enzyme histochemical profiles from the activities of NAD-linked isocitrate dehydrogenase (NAD-ICDH), glutamate dehydrogenase (GDH), succinate dehydrogenase (SDH) and glucose 6-phosphate dehydrogenase (G6PDH). The enzyme assays were performed on serially-sectioned motoneuron somata. The motoneurons were identified by retrograde tracing from the trunk muscle and classified, on the basis of their location in the motor column, as those innervating the white, fast glycolytic fibers (WMNs) or those innervating the red and intermediate slow oxidative fibers (RIMNs). We found the following relationships between enzyme activities in WMNs: GDH correlates with G6PDH activity (r = 0.31; p = 0.02) and NAD-ICDH correlates with GDH activity (r = 0.37; p < 0.01); correlations between NAD-ICDH and SDH and between SDH and GDH are not significant. In RIMNs we found correlations between NAD-ICDH and SDH (r = 0.34; p = 0.03), between NAD-ICDH and GDH (r = 0.41; p < 0.01) and between GDH and SDH (r = 0.50; p < 0.01); the correlation between GDH and G6PDH is not significant. The differences in metabolic profiles between WMNs and RIMNs can be explained in the following way: in WMNs, alpha-ketoglutarate is drawn off from the citrate cycle and is used in amino acid metabolism whereas in RIMNs the removal of alpha-ketoglutarate from the cycle is balanced by formation of alpha-ketoglutarate. The data suggest that the functional role of the citrate cycle differs in the two motoneuron populations: in RIMNs energy generation predominates but in WMNs a role in biosyntheses seems most important.

Relationship between soma diameter and oxidative enzyme activity of alpha-motoneurons

In order to determine whether there is a relationship between soma diameter and oxidative enzyme activity of alpha-motoneurons, we investigated the alpha-motoneurons innervating the different portions within a rat gluteus medius muscle. Two fluorescent neuronal tracers, Nuclear yellow and Fast blue, were used for labeling motoneurons innervating the deep (predominance of oxidative fibers) and superficial (predominance of non-oxidative fibers) portions of the muscle. An inverse relationship between soma diameter and oxidative enzyme activity was not seen in the motoneuron pool innervating either the deep or superficial portions. When the two portions were taken together, however, the inverse relationship was seen. The inverse relationship seemed to be demonstrated in a motoneuron pool which intermingled motoneurons innervating oxidative fibers and motoneurons innervating non-oxidative fibers. These results suggest that the oxidative enzyme activity of alpha-motoneurons is not correlated strictly with their soma size. We consider that the oxidative enzyme activity of alpha-motoneurons is correlated with oxidative enzyme activity of the muscle unit.

Adaptability of the oxidative capacity of motoneurons

Previous studies have demonstrated that a chronic change in neuronal activation can produce a change in soma oxidative capacity, suggesting that: (i) these 2 variables are directly related in neurons and (ii) ion pumping is an important energy requiring activity of a neuron. Most of these studies, however, have focused on reduced activation levels of sensory systems. In the present study the effect of a chronic increase or decrease in motoneuronal activity on motoneuron oxidative capacity and soma size was studied. In addition, the effect of chronic axotomy was studied as an indicator of whether cytoplasmic volume may also be related to the oxidative capacity of motoneurons. A quantitative histochemical assay for succinate dehydrogenase activity was used as a measure of motoneuron oxidative capacity in experimental models in which chronic electromyography has been used to verify neuronal activity levels. Spinal transection reduced, and spinal isolation virtually eliminated lumbar motoneuron electrical activity. Functional overload of the plantaris by removal of its major synergists was used to chronically increase neural activity of the plantaris motor pool. No change in oxidative capacity or soma size resulted from either a chronic increase or decrease in neuronal activity level. These data indicate that the chronic modulation of ionic transport and neurotransmitter turnover associated with action potentials do not induce compensatory metabolic responses in the metabolic capacity of the soma of lumbar motoneurons. Soma oxidative capacity was reduced in the axotomized motoneurons, suggesting that a combination of axoplasmic transport, intracellular biosynthesis and perhaps neurotransmitter turnover represent the major energy demands on a motoneuron. While soma oxidative capacity may be closely related to neural activity in some neural systems, e.g. visual and auditory, lumbar motoneurons appear to be much less sensitive to modulations in chronic activity levels.

Soma diameter and oxidative enzyme activity of identified alpha-motoneurons: application of a retrograde fluorescent neuronal tracer

Using a fluorescent compound as a retrograde neuronal tracer for the identification of a specific motoneuron pool, we examined the soma diameter and oxidative enzyme activity of alpha-motoneurons innervating the extensor digitorum longus (EDL) and the soleus (Sol) muscles in the rat. An inverse relationship between soma diameter and oxidative enzyme activity was demonstrated in the EDL, while there was no significant relationship between these variables in the Sol. These results suggest that an inverse relationship between soma diameter and oxidative enzyme activity cannot always be demonstrated in all of the motoneuron pools.

Enzyme histochemical profiles of fish spinal motoneurons after cordotomy and axotomy of motor nerves

Histochemical profiles were made of identified spinal motoneurons from normal adult zebrafish and from animals subjected to cordotomy or unilateral axotomy of the motor nerves. The lesions caused an increase of the myotomal area with oxidative muscle fibers. We studied the question: do changes in the myotomal muscle configuration concur with changes in the enzyme histochemical profiles of innervating motoneurons? Based on the location and size of cell somata, two categories of motoneurons are distinguished: large white (W) motoneurons that innervate the deep fast, glycolytic muscle fibers, and smaller red and intermediate (RI) motoneurons that innervate the superficial slow oxidative and intermediate muscle fibers. In normal animals, glucose-6-phosphate dehydrogenase activity is high in the large W motoneurons and relatively low in the small RI motoneurons. The reverse holds for succinate dehydrogenase activity is high in the large W motoneurons and relatively low in the small RI motoneurons. The reverse holds for succinate dehydrogenase activity. W and RI motoneurons show similar nicotinamide adenine dinucleotide diaphorase activity. Short- (2 weeks) and long- (8 weeks) term effects of lesions were studied. The results show that: (1) the 3 types of lesions lead to prolonged changes in the enzyme histochemical profiles of spinal motoneurons. The type of change depends on the type of lesion and on the type of motoneuron; (2) unilateral axotomy of the motor nerves affects the histochemical characteristics of spinal motoneurons and the myotomal muscle fiber type configuration on the ipsi- and contralateral side. The contralateral effects are conceived as adaptations to maintain a left-right symmetry in the motor output.

Menadione-linked alpha-glycerophosphate dehydrogenase activity of motoneurons in rat soleus and extensor digitorum longus neuron pools

After injection of horseradish peroxidase into the soleus (slow twitch) and extensor digitorum longus (fast twitch) muscles, glycolytic enzyme activity as reflected by alpha-glycerophosphate dehydrogenase activity of labeled motoneurons in the neuron pool was examined. No differences were found in glycolytic enzyme activity of motoneurons between slow twitch and fast twitch neuron pools.

Quantitative cytology of ganglion neurons and satellite glial cells in the superior cervical ganglion of the sheep. Relationship with ganglion neuron size

Neurons and glial cells of the superior cervical ganglion of sheep were investigated with morphometric methods in the light and electron microscope. The nerve cell sectional area (measured on nucleated cell profiles) ranged from 165 to 2500 microns2, which corresponds to range in cell diameter from 14 to 56 microns and a range in cell volume from 1600 to 93,000 microns3, i.e. a 60-fold volume difference between smallest and largest neurons. The distribution of cell sizes appeared unimodal, with a predominance of small neurons; there were no variations in different parts of the ganglion. This wide range in nerve cell sizes is discussed in the light of the suggestion that large neurons innervate a greater amount of target tissue (e.g. smooth muscle) and are less readily excitable than smaller neurons: it is thus possible that there is differential recruitment of ganglion neurons in autonomic reflexes. The ultrastructural features of ganglion neurons in the sheep were similar to those observed in small laboratory animals. The relative volumes of perikaryal cytoplasm occupied by mitochondria and Golgi apparatus were 8.5% and 4.8%, respectively, but the average values were the same in small and large neurons. Subsurface cisternae of endoplasmic reticulum were common in the perikaryon, while in the dendrites clusters of synaptic vesicles were found beneath the plasma membrane; the absence of a glial wrapping at the latter sites suggests that they are points of (non-synaptic) release of transmitters. The extent of the capsule that satellite cells form around each neurons was compared in size-based classes of neurons. There was no difference in the size of glial nuclei, and this suggests that glial cells are probably of uniform size. However, glial cells were more densely packed over the surface of large neurons than over the surface of small neurons--in fact the packing density was proportional to the ganglion neuron volume, rather than to its surface. The average thickness of the glial capsule was significantly greater around large than around small neurons. It is suggested that the matching of glial cell number and nerve cell volume is achieved during development by glial cell mitosis taking place long after the nerve cells have ceased dividing.

Postnatal growth of medial gastrocnemius motoneurons in the kitten

Cat muscle nerves and ventral roots for the hindlimbs show a unimodal distribution of axon diameter at birth, followed, at about 20 days postnatal (dPN), by a marked change to a bimodal distribution resembling that of the adult. However, volumes calculated for motoneuron somata retrogradely labeled with HRP have been reported to be divided into two size populations at birth in the kitten. In the literature it is suggested that a dissociation between axonal and somal growth appears at a very early age. This apparent dissociation, not present in adults, prompted us to examine the somal growth patterns of kitten lumbar motoneurons. In the present report we have examined somal size development in medial gastrocnemius (MG) motor nuclei in 18 cats aged 2 dPN to adulthood using retrogradely transported horseradish peroxidase to label the motoneurons. Measurements of minimum and maximum diameter somal size, volume calculations and a double circle technique relating the diameters of an estimated spherical volume contained within the soma to that of a second spherical volume enclosing the soma clearly distinguish two subpopulations in the adult, a small and a large population. In contrast, in the kitten we show there is a unimodal distribution of small motoneuron somata at birth which at 19-23 dPN differentiates into a bimodal population. This sudden differentiation of somal size coincides with that reported for MG motoneuron axonal calibre, ruling against a neonatal dissociation of somal and axonal size distributions, and appears to correspond to the time of onset of functional characteristics and the histochemical differentiation of fiber types in the MG muscle.

Soma size and oxidative enzyme activity of motoneurones supplying the fast twitch and slow twitch muscles in the rat

The relationship between soma size and oxidative enzyme activity of motoneurones supplying the fast twitch muscle and the slow twitch muscle was examined. Horseradish peroxidase was injected into the extensor digitorum longus (fast twitch) muscle and the soleus (slow twitch) muscle to retrogradely label corresponding motoneurones of the rat spinal cord. There was a negative relation between soma size and oxidative enzyme activity of motoneurones in a particular neurone pool. The alpha motoneurones supplying the slow twitch muscle had higher oxidative enzyme activities than identical size motoneurones supplying the fast twitch muscle. The present results suggest that there is a difference between oxidative capacities of fast twitch and slow twitch neurone pools.