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

Loss of larger hypoglossal motor neurons in aged Fischer 344 rats

The intrinsic (longitudinal, transversalis and verticalis) and extrinsic (genioglossus, styloglossus, hyoglossus and geniohyoid) tongue muscles are innervated by hypoglossal motor neurons (MNs). Tongue muscle activations occur during many behaviors: maintaining upper airway patency, chewing, swallowing, vocalization, vomiting, coughing, sneezing and grooming/sexual activities. In the tongues of the elderly, reduced oral motor function and strength contribute to increased risk of obstructive sleep apnoea. Tongue muscle atrophy and weakness is also described in rats, yet hypoglossal MN numbers are unknown. In young (6-months, n=10) and old (24-months, n=8) female and male Fischer 344 (F344) rats, stereological assessment of hypoglossal MN numbers and surface areas were performed on 16µm Nissl-stained brainstem cryosections. We observed a robust loss of ~15% of hypoglossal MNs and a modest ~8% reduction in their surface areas with age. In the larger size tertile of hypoglossal MNs, age-associated loss of hypoglossal MNs approached ~30% These findings uncover a potential neurogenic locus of pathology for age-associated tongue dysfunctions.

Size-dependent differences in mitochondrial volume density in phrenic motor neurons

Neuromotor control of diaphragm muscle (DIAm) motor units is dependent on an orderly size-dependent recruitment of phrenic motor neurons (PhMNs). Slow (type S) and fast, fatigue resistant (type FR) DIAm motor units, which are frequently recruited to sustain ventilation, comprise smaller PhMNs that innervate type I and IIa DIAm fibers. More fatigable fast (type FF) motor units, which are infrequently recruited for higher force, expulsive behaviors, comprise larger PhMNs that innervate more type IIx/IIb DIAm fibers. We hypothesize that due to the more frequent activation and thus higher energy demand of type S and FR motor units, the mitochondrial volume density (MVD) of smaller PhMNs is greater compared with larger PhMNs. In eight adult (6 mo old) Fischer 344 rats, PhMNs were identified via intrapleural injection of Alexa488-conjugated cholera toxin B (CTB). Following retrograde CTB labeling, mitochondria in PhMNs were labeled by transdural infusion of MitoTracker Red. PhMNs and mitochondria were imaged using multichannel confocal microscopy using a ×60 oil objective. Following optical sectioning and three-dimensional (3-D) rendering, PhMNs and mitochondria were analyzed volumetrically using Nikon Elements software. Analysis of MVD in somal and dendritic compartments was stratified by PhMN somal surface area. Smaller PhMNs (likely S and FR units) had greater somal MVDs compared with larger PhMNs (likely FF units). By contrast, proximal dendrites or larger PhMNs had higher MVD compared with dendrites of smaller PhMNs. We conclude that more active smaller PhMNs have a higher mitochondrial volume density to support their higher energy demand in sustaining ventilation.NEW & NOTEWORTHY Type S and FR motor units, comprising smaller phrenic motor neurons (PhMNs) are regularly activated to perform indefatigable ventilatory requirements. By contrast, type FF motor units, comprising larger PhMNs, are infrequently activated to perform expulsive straining and airway defense maneuvers. This difference in activation history is mirrored in the mitochondrial volume density (MVD), with smaller PhMNs having higher MVD than larger PhMNs. In proximal dendrites, this trend was reversed, with larger PhMNs having higher MVD than smaller PhMNs, likely due to the maintenance requirements for the larger dendritic arbor of FF PhMNs.

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6 months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1 cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4 μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

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.

Morphological changes of the soleus motoneuron pool in chronic midthoracic contused rats

This study investigated the morphological features of the soleus motoneuron pool in rats with chronic (4 months), midthoracic (T8) contusions of moderate severity. Motoneurons were retrogradely labeled using unconjugated cholera toxin B (CTB) subunit solution injected directly into the soleus muscle of 10 contused and 6 age- and sex-matched, normal controls. Morphometric studies compared somal area, perimeter, diameter, dendritic length, and size distribution of labeled cells in normal and postcontusion animals. In normal animals, motoneurons with a mean of 110.4 +/- 5.2 were labeled on the toxin-injected side of the cord (left). By comparison, labeled cells with a mean of 93.0 +/- 8.4 (a 16% decrease, P = 0.006) were observed in the chronic spinal-injured animals. A significantly smaller frequency of very small (area, approximately 100 microm2) and medium (area, 545-914 microm2) neurons, and a significantly higher frequency of larger (area, >914 microm2) neurons was observed in the labeled soleus motoneuron pools of injured animals compared with the normal controls. Dendritic bundles in the contused animals were composed of thicker dendrites, were arranged in more closely aggregated bundles, and were organized in a longitudinal axis (rostrocaudal axis). Changes in soleus motoneuron dendritic morphology also included significant decrease of total number of dendrites, increased staining, hypertrophy of primary dendrites, and significant decreased primary, secondary, and tertiary branching. The changes in size distribution and dendritic morphology in the postcontusion animals possibly resulted from cell loss and transformation of medium cells to larger cells and/or injury-associated failure of medium cells to transport the immunolabel.

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.

Oxidative enzyme activity and soma size in motoneurons innervating the rat slow-twitch and fast-twitch muscles after chronic activity

The effects of chronic activity induced by running training on the activity of the mitochondrial enzyme succinate dehydrogenase (SDH) and soma size in motoneurons innervating the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles were studied in rats using the retrograde neuronal tracer Nuclear Yellow. Rats were assigned to control and trained groups that were subjected to treadmill running for 10 weeks (2 h/day, 30 m/min, 5 days/week). After training, both SOL and EDL muscles showed clear adaptations (citrate synthase activity in the SOL muscle, and the fast-twitch oxidative-glycolytic fiber area of the EDL muscle increased significantly after training). The SDH activity of the motoneurons innervating both SOL and EDL muscles was unchanged by training. However, SOL motoneurons of trained rats had a significantly larger soma size and a significantly higher total SDH activity [SDH activity x soma size) than those of control. Total SDH activity was calculated to examine the absolute SDH protein content of the motoneurons. On the other hand, there was no difference in both soma size and total SDH activity of EDL motoneurons between the two groups. These data demonstrate that chronic activity has a considerably stronger impact on soma size and total oxidative enzyme activity of motoneurons innervating slow-twitch rather than fast-twitch muscles.

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.

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.

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.