A horseradish peroxidase labeling technique for correlation of motoneuron metabolic activity with muscle fiber types

Quantifying mitochondrial volume density in phrenic motor neurons

BACKGROUND

Previous assessments of mitochondrial volume density within motor neurons used electron microscopy (EM) to image mitochondria. However, adequate identification and sampling of motor neurons within a particular motor neuron pool is largely precluded using EM. Here, we present an alternative method for determining mitochondrial volume density in identified motor neurons within the phrenic motor neuron (PhMN) pool, with greatly increased sampling.

NEW METHOD

This novel method for assessing mitochondrial volume density in PhMNs uses a combination of intrapleural injection of Alexa 488-conjugated cholera toxin B (CTB) to retrogradely label PhMNs, followed by intrathecal application of MitoTracker Red to label mitochondria. This technique was validated by comparison to 3D EM determination of mitochondrial volume density as a "gold standard".

RESULTS

A mean mitochondrial volume density of ∼11 % was observed across PhMNs using the new MitoTracker Red method. This compared favourably with mitochondrial volume density (∼11 %) measurements using EM.

COMPARISON WITH EXISTING METHOD

The range, mean and variance of mitochondrial volume density estimates in PhMNs were not different between EM and fluorescent imaging techniques.

CONCLUSIONS

Fluorescent imaging may be used to estimate mitochondrial volume density in a large sample of motor neurons, with results similar to EM, although EM did distinguish finer mitochondrion morphology compared to MitoTracker fluorescence. Compared to EM methods, the assessment of a larger sample size and unambiguous identification of motor neurons belonging to a specific motor neuron pool represent major advantages over previous methods.

Neurotrophins and trk-receptors in adult rat spinal motoneurons: differences related to cell size but not to 'slow/fast' specialization

We have studied the mRNA expression of the neurotrophins brain-derived neurotrophic factor (BDNF), NT-3 and NT-4 and of their receptors trkB and trkC in individual retrogradely labeled lumbar spinal motoneurons of the adult rat, using quantitative non-radioactive in situ hybridization (ISH). We measured soma size and the optical density of the ISH reaction. We analyzed mRNA expression patterns in predominantly 'slow' or 'fast' motoneurons by comparing retrogradely labelled cells that innervated the slow m.soleus (Sol) with those innervating the fast m.extensor digitorum longus (EDL). Within individual motoneuron pools, there was a considerable variation in the intensity of mRNA expression encoding for BDNF, NT-3 and NT-4, as well as for the receptors trkB and trkC. The expression of trkC mRNA was significantly higher in small, presumed fusimotor neurons than in the larger alpha-motoneurons. Surprisingly, none of the variations in the level of motoneuronal mRNA expression for trkB, BDNF, NT-3 and NT-4 were related to soma size and no differences were found between EDL and Sol motoneurons. Apparently, the considerable variations in neurotrophin and neurotrophin receptor mRNA expression represent other parameters of motoneuronal specialization than those related to the well-established 'fast' vs. 'slow' categories of motoneurons.

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.

Morphological changes in the masseter muscle and its motoneurons during postnatal development

BACKGROUND

It has been suggested that the morphological properties of the masseter muscle are changed by the masticatory activity pattern. In the rat, the activity pattern of the muscle alters from sucking to biting around 3 weeks after birth. The working hypothesis in this study is that the unique alteration in masticatory activity has an important influence on the development of the masseter muscle and its motoneurons.

METHODS

We examined the morphological changes in the muscle fibers of the superficial masseter muscle and its motoneurons from 2 days to 280 days after birth in the rat. The change in masseter muscle activity sucking and biting was confirmed by electromyography. To label motoneurons innervating the muscle, horseradish peroxidase was injected into the muscle. The muscle and lower brain stem were sliced and processed histochemically to measure the diameters of muscle fibers and its motoneurons in the trigeminal motor nucleus. In addition, composition of myosin heavy chain (MHC) isoforms of the muscles were analyzed using gradient sodium dodecyl sulphate-polyacrylamide gel electrophoresis.

RESULTS

There was a rapid growth in both types of muscle fibers (fast-twitch oxidative glycolytic muscle fibers and fast-twitch glycolytic fibers) for 42 days after birth, and then a gradual growth lasting until 280 days after birth. Particularly, rapid growth of the muscle fibers was seen between 21 days and 42 days after birth. A large amount of neonatal type MHC disappeared between 21 days and 42 days after birth. In the motoneuron, there was a rapid growth of motoneurons by 42 days after birth but no significant growth was seen thereafter.

CONCLUSIONS

These results suggest that the alteration of mastication activity from sucking to biting has a significant influence on morphological development of both types of muscle fibers, but not on that of motoneurons innervating the masseter muscle.

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.

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.

Retrograde neuronal labeling of motoneurons in the rat by fluorescent tracers, and quantitative analysis of oxidative enzyme activity in labeled neurons

Extensor digitorum longus motoneurons in the rat spinal cord were identified by retrograde labeling with two fluorescent tracers, Fast blue (FB) and Nuclear yellow (NY). Labeled motoneurons had a blue fluorescent cytoplasm at 360 nm excitation wavelength with FB, and a golden-yellow fluorescent nucleus with NY on the cryostat section. Labeled motoneurons were further examined for succinate dehydrogenase activity on the same section used for identification of the motoneurons. This study demonstrates that fluorescent dyes are useful for neuroanatomical studies by the retrograde axonal transport method, and that quantitative analysis of metabolic activity in labeled motoneurons is also possible.

Effects of hypobaric hypoxia on the oxidative capacity of the extensor digitorum longus motor units in the rat

The fiber number, fiber type distribution, and succinate dehydrogenase activity were investigated from the fast-twitch extensor digitorum longus muscle of male rats exposed to 7 weeks of hypobaric hypoxia. The oxidative metabolic capacity of the motoneurons in the extensor digitorum longus neuron pool was also determined from quantitative histochemical analyses. The fiber number and oxidative enzyme activity of the muscle were not changed by hypoxia. An increase in the percentage of fast-twitch oxidative (FO) fibers and a concomitant decrease in the percentage of fast-twitch (F) fibers were observed in the hypoxic muscle. On the other hand, the oxidative capacity of small- to medium-sized alpha motoneurons (24-45 microns average soma diameter) was increased. The increase in the oxidative capacity of small- to medium-sized motoneurons and the type shift of muscle fibers from F (low-oxidative) to FO (high-oxidative) indicate that hypoxia enhances the oxidative capacity of particular motor units in the neuron pool.

Oxidative metabolism of the rat soleus neuron pool following hypobaric hypoxia

After 7 weeks of hypoxic acclimation, oxidative enzyme (succinate dehydrogenase) activity of motoneurons in the rat soleus neuron pool was examined. Although the total oxidative metabolism of the neuron pool was not changed, the oxidative enzyme activity of small to medium sized (25-45 microns average soma diameter) alpha motoneurons was increased. These results indicate that hypoxia preferentially enhances the oxidative capacity of specific motoneurons in the neuron pool.

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.

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.

Effects of ageing on the total number of muscle fibers and motoneurons of the tibialis anterior and soleus muscles in the rat

The age-related changes in the total number of muscle fibers and motoneurons of the tibialis anterior and soleus muscles were studied using 10-, 65-, and 135-week-old rats. The number of fast twitch muscle fibers was decreased at age 65 weeks, while the numbers of slow twitch fibers and of alpha motoneurons were decreased only later, at age 135 weeks. Therefore, the degenerative process of muscle fibers differs with the fiber type.

Distribution of vestibular afferents that innervate the sacculus and posterior canal in the gerbil

The central distribution of afferents that innervate the macula of the saccule and the crista of the posterior canal was assessed in the gerbil following the direct injection of horseradish peroxidase (HRP) separately into the sensory neuroepithelia of each peripheral receptor organ. Ganglion cells innervating the posterior canal were located in the caudal part of the inferior ganglion, while those cells innervating the saccule were located in the rostral part of the inferior ganglion, scattered in the superior ganglion, and concentrated at the junction (isthmus) between the two. The paths of the central axons of these two groups of ganglion cells through the vestibular root and their division into ascending or descending pathways were similar. However, the distributions of their terminals were different. The posterior canal projected to medial parts of the vestibular nuclear complex. Terminals were found in the medial and superior vestibular nuclei. The posterior canal also projected to the uvula of the cerebellum. The saccule projected to more lateral-lying brainstem areas. Terminal fields were located in the lateral and descending vestibular nuclei and cell group y. Saccule projections outside the vestibular complex were observed to the lateral cuneate nucleus, the N. gigantocellularis, and the cerebellar cortex. Of the eight areas receiving primary afferent projections from these two organs, only within the medial and descending vestibular nuclei and the cerebellar cortex were overlapping projections observed.

Double-labeling of rat alpha-motoneurons for cytochrome oxidase and retrogradely transported [3H]WGA

In this study, we have demonstrated the co-localization of a retrograde tracer and the reaction product of an oxidative enzyme within the same neurons in the same spinal cord section, using [3H] wheat germ agglutinin and cytochrome oxidase histochemistry. This approach allows unequivocal identification of the alpha-motoneurons innervating specific muscles. We have determined that there is a positive correlation between the distribution of cytochrome oxidase reactivity in alpha-motoneurons and the muscles that they innervate. The degree of cytochrome oxidase reactivity within the labeled alpha-motoneurons appears to be independent of spinal cord level and cell size.

Absence of postnatal death among motoneurones supplying the inferior gluteal nerve of the rat

Motoneurones innervating the caudal part of the gluteus maximus muscle of 0-2 day, 10-12 day and 2-3-month-old rats were labelled by a half-hour application of a solution of 30% horseradish peroxidase (HRP) and 2% lysophatidylcholine delivered by suction electrode to the cut inferior gluteal nerves. The numbers of motoneurones labelled 24-48 h later were not significantly different in the 3 age groups (mean = 58.75, 54.0, 57.5, respectively). When a simple 30% HRP solution was used in adult rats, the number of motoneurones labelled was significantly less (mean = 48.75). In contrast, application of 0.5 microliter of HRP in a pledget of gelfoam to either the cut or uncut inferior gluteal nerve of neonates labelled large numbers of motoneurones, presumably by diffusion into nearby muscles. It is concluded that no death of motoneurones innervating the gluteus maximus muscle occurs postnatally, and that spread of HRP to neighbouring muscles can give rise to spuriously high motoneurone counts in neonates, and that incomplete uptake or transport of HRP in adults can lead to incorrectly low counts.

Quantitative differences in horseradish peroxidase-labeling of alpha-motoneurons

Qualitative and quantitative differences in uptake and transport of exogenous proteins by different types of neurons are well known, but such differences in the same neuron type have not been previously reported. The current study addresses this problem with quantitative histochemical determinations of retrogradely transported horseradish peroxidase (HRP) in alpha-motoneuron cell bodies following intramuscular injections. The alpha-motoneurons innervating the rat tensor fascia lata and soleus muscles were used as examples of fast-twitch glycolytic (FG) and slow-twitch oxidative (SO) neurons, respectively. In 16 cases, the quantity of HRP in SO motoneurons was significantly (P less than 0.001) greater than that in FG motoneurons. We conclude that nerve terminal HRP uptake, transport and/or metabolism varies among the same neuron type and, in this instance, is related to the type of motor unit. The factors responsible for HRP-labeling differences in SO and FG motoneurons and the potential importance of these observations are discussed.