Effects of hindlimb unloading and reloading on c-fos expression of spinal cord evoked by vibration of rat Achille tendon

Disuse plasticity limits spinal cord injury recovery

Use-dependent plasticity after spinal cord injury (SCI) enhances neuromotor function, however, the optimal timing to initiate rehabilitation remains controversial. To test impacts of early disuse, we established a rodent model of transient hindlimb suspension in acute phase SCI. Early disuse in the first 2-week after SCI undermined recovery on open-field locomotion, kinematics, and swim tests even after 6-week of normal gravity reloading. Early disuse produced chronic spinal circuit hyper-excitability in H-reflex and interlimb reflex tests. Quantitative synaptoneurosome analysis of lumboventral spinal cords revealed shifts in AMPA receptor (AMPAR) subunit GluA1 localization and serine 881 phosphorylation, reflecting enduring synaptic memories of early disuse stored in the spinal cord. Automated confocal analysis of motoneurons revealed persistent shifts toward GluA2-lacking, calcium-permeable AMPARs in disuse subjects. Unsupervised machine learning associated multidimensional synaptic changes with persistent recovery deficits in SCI. The results argue for early aggressive rehabilitation to prevent disuse plasticity that limits SCI recovery.

Total rupture of Achilles tendon induces inflammatory response and glial activation on the spinal cord of mice

Rupture of Achilles tendon is a common accident affecting professional and recreational athletes. Acute and chronic pain are symptoms commonly observed in patients with rupture. However, few studies have investigated whether Achilles tendon rupture is able to promote disorders in the central nervous system (CNS). Therefore, the current study aimed to evaluate nociceptive alterations and inflammatory response in the L5 lumbar segment of Balb/c mice spinal cord after Achilles tendon rupture. We found increased algesia in the paw of the ruptured group on the 7th and 14th days post-tenotomy compared with the control group. This phenomenon was accompanied by overexpression of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase-2 (NOS-2) as well as hyperactivation of astrocytes and microglia in nociceptive areas of L5 spinal cord as evidenced by intense GFAP and IBA-1 immunostaining, respectively. Biochemical studies also demonstrated increased levels of nitrite in the L5 spinal cord of tenotomized animals compared with the control group. Thus, we have demonstrated for the first time that total rupture of the Achilles tendon induced inflammatory response and nitrergic and glial activation in the CNS in the L5 spinal cord region.

Effect of long-term weightlessness on retina and optic nerve in tail-suspension rats

AIM

To evaluate the effect of long-term weightlessness on retina and optic nerve in tail-suspension (TS) rats.

METHODS

A stimulated weightlessness model was established by suspending rats' tail. After 12wk, the ultrastructure and the number of optic nerve axons were observed by transmission electron microscope. The number of survival retinal ganglion cells (RGCs) was calculated by fluorescent gold retrograde labeling. Retina cells apoptosis was detected by TUNEL staining. The function of optic nerve and retina was evaluated by the visual evoked potential (VEP) and oscillatory potentials (Ops).

RESULTS

The optic nerve axons were swollen and sparsely aligned, and the lamellar separation and myelin disintegration occurred after 12wk in TS rats. The density of optic nerve axons was 32.23±3.92 (vs 37.43±4.13, P=0.0145), the RGCs density was 1645±46 cells/mm(2) (vs 1867±54 cells/mm(2) P=0.0000), the incidence rate of retinal cells apoptosis was 5.38%±0.53% (vs 4.75%±0.54%, P=0.0238), the amplitude of VEP-P100 was 15.43±2.14 µV (vs 17.67±2.17 µV, P=0.0424), the latency of VEP-P100 was 69.05±5.34ms (vs 62.43±4.87ms P=0.0143) and the sum amplitude of Ops was 81.05±8.34 µV (vs 91.67±10.21 µV, P=0.0280) in TS group and the control group, respectively.

CONCLUSION

Long-term weightlessness can induce the ultrastructural changes and functional depress of the optic nerve, as well as retinal cell damages in TS rats.

Effects of hindlimb unloading on neurotrophins in the rat spinal cord and soleus muscle

The aim of the present study was to investigate the effects of hindlimb unloading (HU) on the expression of neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), together with the expression of their high-affinity receptors tropomyosin receptor kinase C (TrkC) and tropomyosin receptor kinase B (TrkB), in lumbar (L4-6) segment of the spinal cord and in the soleus muscle. The mRNA and protein levels of the genes of interest were compared using quantitative PCR and western blot assays. Immunohistochemistry for NT-3 and BDNF was used to detect the levels of protein in the motoneurons in the lateral motor column. In this study, NT-3 and BDNF mRNA and protein expression were significantly increased in the spinal cord and soleus muscle after HU. NT-3 immunoreactivity, but not BDNF immunoreactivity, was significantly increased in the large motoneurons located in lateral motor column after 14 days of HU. The level of TrkC protein in the spinal cord and soleus muscle were significantly elevated after both 7 days and 14 days of HU. However, TrkC mRNA, TrkB mRNA and TrkB protein levels did not change significantly. Elevated BDNF, NT-3 and TrkC levels in the neuromuscular system indicate that neurotrophins are involved in HU-induced neuromuscular plasticity. NT-3 is a candidate to mediate the synaptic efficacy between alpha motoneurons and group Ia afferents.

Effects of simulated weightlessness on intramuscular hypertonic saline induced muscle nociception and spinal Fos expression in rats

We assessed the effects of simulated weightlessness, hindlimb unloading (HU) by 7 days of tail suspension, on noxious mechanically and heat evoked spinal withdrawal reflexes and spinal Fos expression during muscle nociception elicited by intramuscular (i.m.) injection of hypertonic (HT; 5.8%) saline into gastrocnemius muscle in rats. In HU rats, i.m. HT saline-induced secondary mechanical hyperalgesia was enhanced, and secondary heat hypoalgesia was significantly delayed. After 7 days of HU, basal Fos expression in spinal L4-6 segments was bilaterally enhanced only in superficial (I-II) but not middle and deep laminae (III-VI) of the spinal dorsal horn, which finding was not influenced by tail denervation. Unilateral i.m. HT saline injection increased spinal Fos expression bilaterally in both the control rats and 7 days of HU rats. The HT saline-induced bilateral increase of spinal Fos occurred within 0.5h and reached its peak within 1h, after which it gradually returned to the control levels within 8h. Spatial patterns of spinal Fos expression differed between the control group and 7 days of HU group. In superficial laminae, the HT saline-induced increases in Fos expression were higher and in the middle and deep laminae V-VI lower in the 7 days of HU than control rats. It is suggested that supraspinal mechanisms presumably underlie the effects of HU on spinally-organized nociception. Simulated weightlessness may enhance descending facilitation and weaken descending inhibition of nociception.

7-Nitroindazole potentiates c-fos expression induced by muscle tendon vibration in the spinal cord

INTRODUCTION

Expression of c-fos initiated by muscle proprioceptive signaling was studied in rats after inhibition of neuronal nitric oxide synthase (nNOS) with administration of 7-nitroindazole (7-NI).

METHODS

Fos-immunoreactive (Fos-ir) neurons were visualized immunohistochemically in the lumbar cord after vibration of the Achilles tendon and/or 7-NI systemic injections.

RESULTS

The total number of Fos-ir interneurons and motoneurons (per slice) was significantly greater in the 7-NI-pretreated and tendon-vibrated (7-NI + Tv) group than in the isolated tendon vibration group (Tv group). The greatest increases in the number of Fos-ir neurons were found in the L4 (+100%) and L5 (+105%) segments (P < 0.05).

CONCLUSIONS

Suppression of NO release after introduction of 7-NI was associated with potentiation of Fos immunoreactivity induced by muscle proprioceptive signaling within distinctive regions of the spinal cord.

Role of muscle spindle in weightlessness-induced amyotrophia and muscle pain

To date, the medium and long-term space flight is urgent in need and has become a major task of our manned space flight program. There is no doubt that medium and long-term space flight has serious damaging impact upon human physiological systems. For instance, atrophy of the lower limb anti-gravity muscle can be induced during the space flight. Muscle atrophy significantly affects the flight of astronauts in space. Most importantly, it influences the precise manipulation of the astronauts and their response capacity to emergencies on returning to the atmosphere from space. Muscle atrophy caused by weightlessness may also seriously disrupt the normal life and work of the astronauts during the re-adaptation period. Here we summarize the corresponding research concentrating on weightlessness-induced changes of muscular structure and function. By combining research on muscle pain, which is a common clinical pain disease, we further provide a hypothesis concerning a dynamic feedback model of "weightlessness condition right triple arrow muscular atrophy <--> muscle pain". This may be useful to explore the neural mechanisms underlying the occurrence and development of muscular atrophy and muscle pain, through the key study of muscle spindle, and furthermore provide more effective therapy for clinical treatment.