Karen J. B. Martins

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Effect of satellite cell ablation on low‐frequency‐stimulated fast‐to‐slow fibre‐type transitions in rat skeletal muscle

The purpose of this study was to determine whether satellite cell ablation within rat fast‐twitch muscles exposed to chronic low‐frequency stimulation (CLFS) would limit fast‐to‐slow fibre‐type transitions. Twenty‐nine male Wistar rats were randomly assigned to one of three groups. Satellite cells of the left tibialis anterior were ablated by weekly exposure to a 25 Gy dose of γ‐irradiation during 21 days of CLFS (IRR‐Stim), whilst a second group received only 21 days of CLFS (Stim). A third group received weekly doses of γ‐irradiation (IRR). Non‐irradiated right legs served as internal controls. Continuous infusion of 5‐bromo‐2′‐deoxyuridine (BrdU) revealed that CLFS induced an 8.0‐fold increase in satellite cell proliferation over control (mean ±s.e.m.: 23.9 ± 1.7 versus 3.0 ± 0.5 mm−2, P < 0.0001) that was abolished by γ‐irradiation. M‐cadherin and myogenin staining were also elevated 7.7‐ and 3.8‐fold (P < 0.0001), respectively, in Stim compared with control, indicating increases in quiescent and terminally differentiating satellite cells; these increases were abolished by γ‐irradiation. Myonuclear content was elevated 3.3‐fold (P < 0.0001) in Stim, but remained unchanged in IRR‐Stim. Immunohistochemical analyses revealed attenuation of fast‐to‐slow fibre‐type transitions in IRR‐Stim compared with Stim. Comparable changes were observed at the protein level by SDS‐PAGE. It is concluded that although considerable adaptive potential exists within myonuclei, satellite cells play a role in facilitating fast‐to‐slow fibre‐type transitions.

Nitric oxide synthase inhibition prevents activity-induced calcineurin–NFATc1 signalling and fast-to-slow skeletal muscle fibre type conversions

•  Exercise is known to trigger skeletal muscle structural and functional adaptations. •  Control of these adaptive alterations is a complex process involving multiple signalling pathways and levels of regulation. •  The well‐characterized calcineurin–nuclear factor of activated T‐cells (NFATc1) signalling pathway is involved in the regulation of activity‐dependent alterations in skeletal muscle myosin heavy chain expression. Myosin heavy chain is a contractile protein that largely dictates a muscles speed of contraction. •  We show that a signalling molecule called nitric oxide may be regulating alterations in myosin heavy chain expression via activity‐modulated calcineurin–NFATc1 signalling. •  These findings increase our understanding of how skeletal muscle adaptive alterations are regulated.

Adaptive responses to creatine loading and exercise in fast-twitch rat skeletal muscle

We investigated the effects of chronic creatine loading and voluntary running (Run) on muscle fiber types, proteins that regulate intracellular Ca2+, and the metabolic profile in rat plantaris muscle to ascertain the bases for our previous observations that creatine loading results in a higher proportion of myosin heavy chain (MHC) IIb, without corresponding changes in contractile properties. Forty Sprague-Dawley rats were assigned to one of four groups: creatine-fed sedentary, creatine-fed run-trained, control-fed sedentary, and control-fed run-trained animals. Proportion and cross-sectional area increased 10% and 15% in type IIb fibers and the proportion of type IIa fibers decreased 11% in the creatine-fed run-trained compared with the control-fed run-trained group (P < 0.03). No differences were observed in fast Ca2+-ATPase isoform SERCA1 content (P > 0.49). Creatine feeding alone induced a 41% increase (P < 0.03) in slow Ca2+-ATPase (SERCA2) content, which was further elevated by 33% with running (P < 0.02). Run training alone reduced parvalbumin content by 50% (P < 0.05). By comparison, parvalbumin content was dramatically decreased by 75% (P < 0.01) by creatine feeding alone but was not further reduced by run training. These adaptive changes indicate that elevating the capacity for high-energy phosphate shuttling, through creatine loading, alleviates the need for intracellular Ca2+ buffering by parvalbumin and increases the efficiency of Ca2+ uptake by SERCAs. Citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities were elevated by run training (P < 0.003) but not by run training + creatine feeding. This indicates that creatine loading during run training supports a faster muscle phenotype that is adequately supported by the existing glycolytic potential, without changes in the capacity for terminal substrate oxidation.

Nitric oxide synthase inhibition delays low-frequency stimulation-induced satellite cell activation in rat fast-twitch muscle

This study examined the effect of nitric oxide synthase (NOS) inhibition via N(ω)-nitro-l-arginine methyl ester (l-NAME) administration on low-frequency stimulation-induced satellite cell (SC) activation in rat skeletal muscle. l-NAME only delayed stimulation-induced increases in SC activity. Also, stimulation-induced increases in hepatocyte growth factor (HGF) mRNA and protein expression were only abrogated at the mRNA level in l-NAME-treated animals. Therefore, early stimulation-induced SC activation appears to be NOS-dependent, while continued activation may involve NOS-independent HGF translational control mechanisms.

Fish oil mitigates myosteatosis and improves chemotherapy efficacy in a preclinical model of colon cancer

Background This study aimed to assess whether feeding a diet containing fish oil was efficacious in reducing tumor- and subsequent chemotherapy-associated myosteatosis, and improving tumor response to treatment. Methods Female Fischer 344 rats were fed either a control diet for the entire study (control), or switched to a diet containing fish oil (2.0 g /100 g of diet) one week prior to tumor implantation (long term fish oil) or at the start of chemotherapy (adjuvant fish oil). Chemotherapy (irinotecan plus 5-fluorouracil) was initiated 2 weeks after tumor implantation (cycle-1) and 1 week thereafter (cycle-2). Reference animals received no tumor or treatment and only consumed the control diet. All skeletal muscle measures were conducted in the gastrocnemius. To assess myosteatosis, lipids were assessed histologically by Oil Red O staining and total triglyceride content was quantified by gas chromatography. Expression of adipogenic transcription factors were assessed at the mRNA level by real-time RT-PCR. Results Feeding a diet containing fish oil significantly reduced tumor- and subsequent chemotherapy-associated increases in skeletal muscle neutral lipid (p<0.001) and total triglyceride content (p<0.03), and expression of adipogenic transcription factors (p<0.01) compared with control diet fed animals. The adjuvant fish oil diet was as effective as the long term fish oil diet in mitigating chemotherapy-associated skeletal muscle fat content, and in reducing tumor volume during chemotherapy compared with control fed animals (p<0.01). Conclusion Long term and adjuvant fish oil diets are equally efficacious in reducing chemotherapy-associated myosteatosis that may be occurring by reducing expression of transcription factors involved in adipogenesis/lipogenesis, and improving tumor-response to chemotherapy in a neoplastic model.