Genetic targeting of adult Renshaw cells using a Calbindin 1 destabilized Cre allele for intersection with Parvalbumin or Engrailed1

Renshaw cells (RCs) are one of the most studied spinal interneurons; however, their roles in motor control remain enigmatic in part due to the lack of experimental models to interfere with RC function, specifically in adults. To overcome this limitation, we leveraged the distinct temporal regulation of Calbindin (Calb1) expression in RCs to create genetic models for timed RC manipulation. We used a Calb1 allele expressing a destabilized Cre (dgCre) theoretically active only upon trimethoprim (TMP) administration. TMP timing and dose influenced RC targeting efficiency, which was highest within the first three postnatal weeks, but specificity was low with many other spinal neurons also targeted. In addition, dgCre showed TMP-independent activity resulting in spontaneous recombination events that accumulated with age. Combining Calb1-dgCre with Parvalbumin (Pvalb) or Engrailed1 (En1) Flpo alleles in dual conditional systems increased cellular and timing specificity. Under optimal conditions, Calb1-dgCre/Pvalb-Flpo mice targeted 90% of RCs and few dorsal horn neurons; Calb1-dgCre/En1-Flpo mice showed higher specificity, but only a maximum of 70% of RCs targeted. Both models targeted neurons throughout the brain. Restricted spinal expression was obtained by injecting intraspinally AAVs carrying dual conditional genes. These results describe the first models to genetically target RCs bypassing development.

Unveiling the neuromechanical mechanisms underlying the synergistic interactions in human sensorimotor system

Motor synergies are neural organizations of a set of redundant motor effectors that interact with one another to compensate for each other’s error and ensure the stabilization of a performance variable. Recent studies have demonstrated that central nervous system synergistically coordinates its numerous motor effectors through Bayesian multi-sensory integration. Deficiency in sensory synergy weakens the synergistic interaction between the motor effectors. Here, we scrutinize the neuromechanical mechanism underlying this phenomenon through spectral analysis and modeling. We validate our model-generated results using experimental data reported in the literature collected from participants performing a finger force production task with and without tactile feedback (manipulated through injection of anesthetic in fingers). Spectral analysis reveals that the error compensation feature of synergies occurs only at low frequencies. Modeling suggests that the neurophysiological structures involving short-latency back-coupling loops similar to the well-known Renshaw cells explain the deterioration of synergy due to sensory deprivation.

Inhibitory interneurons show early dysfunction in a SOD1 mouse model of ALS

Few studies in amyotrophic lateral sclerosis (ALS) focus on the premotor interneurons synapsing onto motoneurons (MNs). We hypothesized inhibitory interneurons contribute to dysfunction, particularly if altered before MN neuropathology. We directly assessed excitability and morphology of ventral lumbar glycinergic interneurons from SOD1G93AGlyT2eGFP (SOD1) and wildtype GlyT2eGFP (WT) mice. SOD1 interneurons were smaller but density was unchanged. Patch clamp revealed dampened excitability in SOD1 interneurons, including depolarized PICs and voltage threshold. Renshaw cells (RCs; confirmed with immunohistochemistry) showed similar dampened excitability. Morphology and electrophysiology were used to create a “random forest” statistical model to predict RCs when histological verification was not possible. Predicted SOD1 RCs were less excitable (consistent with experimental results); predicted SOD1 non-RCs were more excitable. In summary, inhibitory interneurons show very early perturbations poised to impact MNs, modify motor output, and provide early biomarkers of ALS. Therapeutics like riluzole that universally reduce CNS excitability could exacerbate this dysfunction.

Two voltage-dependent currents can explain the functional diversity of embryonic Renshaw cells

Spontaneous neuronal activity occurs at the onset of the synaptogenesis in the central Nervous System and plays a major role in shaping developing neural networks. How intrinsic properties of neurons evolve during this critical developmental period remains largely unknown. We studied the Renshaw cells because they participate to the early-synchronized neuronal activity in the embryonic spinal cord. We found that these interneurons are subdivided into several functional clusters at the onset of the synaptogenesis and then display a transitory involution process during which they lose their ability to sustain tonic firing. This complex developmental trajectory results from the synergy between a persistent sodium inward current and a delayed rectifier potassium outward current, which are present in most neurons during development and in the adult. Taken together, our results reveal a core mechanism producing functional hetereogeneity in embryonic neurons and likely shaping the ongoing of early spontaneous neuronal activity.

Comparative Study between Magnesium Sulfate and Pethidine for Controlling Shivering after Spinal Anesthesia

Background Shivering is an involuntary muscular activity. Increased muscle tone during shivering is due to temperature-induced changes in neuronal activity in the reticular formation. Synchronization of motor neurons during shivering may be mediated by recurrent inhibition through renshaw cells. Aim of the Work To verify the efficacy of magnesium sulfate for controlling post spinal shivering, to compare the efficacy of magnesium sulfate and pethidine for controlling post spinal shivering and to detect the side effects of both magnesium sulfate and pethidine after their use for controlling post spinal shivering. Patients and Methods This prospective study was conducted at El-Matarya Teaching Hospital from 2018 till 2019. After obtaining approval from the Research Ethical Committee of Ain Shams University, informed patient consent was obtained before the procedure. After giving the spinal anesthesia, only patients who developed post-spinal shivering were followed for the study. 60 patients with post-spinal shivering were included with the following criteria: Results Regarding age, weight, height and duration of surgery; there were no statistically significant differences between the two studied groups. Comparison of the two studied groups revealed no statistically significant changes at all times of measurement. Conclusion Magnesium sulfate in a dose of 30 mg/kg IV infusion in 100 ml normal saline over 10 min is effective for control of post spinal shivering. Pethidine in a dose of 0.5 mg/kg IV bolus is effective for control of post spinal shivering.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Recurrent excitation between motoneurones propagates across segments and is purely glutamatergic

Spinal motoneurones constitute the final output for the execution of motor tasks. In addition to innervating muscles, motoneurones project excitatory collateral connections to Renshaw cells and other motoneurones, but the latter have received little attention. We show that motoneurones receive strong synaptic input from other motoneurones throughout development and into maturity with fast type motoneurones systematically receiving greater recurrent excitation than slow type motoneurones. Optical recordings show that activation of motoneurones in one spinal segment can propagate to adjacent segments even in the presence of intact recurrent inhibition. Quite remarkably, while it is known that transmission at the neuromuscular junction is purely cholinergic and Renshaw cells are excited through both acetylcholine and glutamate receptors, here we show that neurotransmission between motoneurones is purely glutamatergic indicating that synaptic transmission systems are differentiated at different post-synaptic targets of motoneurones.