Targeting the Full Length of the Motor End Plate Regions in the Mouse Forelimb Increases the Uptake of Fluoro-Gold into Corresponding Spinal Cord Motor Neurons

Abstract

Lower motor neuron dysfunction is one of the most debilitating motor conditions. In this regard, transgenic mouse models of various lower motor neuron dysfunctions provide insight into the mechanisms underlying these pathologies and can also aid the development of new therapies. Viral-mediated gene therapy can take advantage of the muscle-motor neuron topographical relationship to shuttle therapeutic genes into specific populations of motor neurons in these mouse models. In this context, motor end plates (MEPs) are highly specialized regions on the skeletal musculature that offer direct access to the pre-synaptic nerve terminals, henceforth to the spinal cord motor neurons. The aim of this study was two-folded. First, it was to characterize the exact position of the MEP regions for several muscles of the mouse forelimb using acetylcholinesterase histochemistry. This MEP-muscle map was then used to guide a series of intramuscular injections of Fluoro-Gold (FG) in order to characterize the distribution of the innervating motor neurons. This analysis revealed that the MEPs are typically organized in an orthogonal fashion across the muscle fibers and extends throughout the full width of each muscle. Furthermore, targeting the full length of the MEP regions gave rise labeled motor neurons that are organized into columns spanning through more spinal cord segments than previously reported. The present analysis suggests that targeting the full width of the muscles’ MEP regions with FG increases the somatic availability of the tracer. This process ensures a greater uptake of the tracer by the pre-synaptic nerve terminals, hence maximizing the labeling in spinal cord motor neurons. This investigation should have positive implications for future studies involving the somatic delivery of therapeutic genes into motor neurons for the treatment of various motor dysfunctions.

Differential Joint-Specific Corticospinal Tract Projections within the Cervical Enlargement

“…We examined the distribution of CST labeling in the C5/C6 segments and the C7/C8 segments. Retrogradely labeled deltoid, biceps, and wrist extensor motoneurons are present throughout these segments ( [22]; see below). Joint-specific sites projected consistently to a dense core region on individual spinal sections (typically a single site; occasionally 1 or 2 additional sites of much lesser density) within a more variable and broad sparsely labeled region (Fig.…”

“…For the shoulder, we injected the Deltoid muscle group (spino- and acromiodeltoidus). We have optimized the CTb injections to maximally label the motoneurons of injected muscles, taking advantage of recent findings that many forelimb motor pools have an extended rostrocaudal distribution [22].We inserted the Hamilton syringe into the muscle, advanced it along the long axis of the muscle, and injected CTb as the needle was withdrawn. As needed to maximally label the muscle, we often re-advanced the needle and injected more tracer.…”

Dendritic spine dysgenesis contributes to hyperreflexia after spinal cord injury

“…We began with a broad sample population of neurons by identifying Golgi-stained ␣-motor neurons located in the ventral spinal cord in Rexed lamina IX and with soma diameters Ͼ25 m (Hashizume et al 1988;Jacob 1998). Above the injury site (C4 -C5), we narrowed candidate neurons for analysis by selecting neurons from motor pools located in similar dorsolateral coordinates of ␣-motor neurons shown to innervate the extensor carpi radialis muscle group (ϳ1.5-2 mm deep, 1.5-2 mm lateral from midline), as we and others have demonstrated through intramuscular retrograde tracing studies (Sunshine et al 2013;Tan et al 2012a;Tosolini et al 2013). Below the injury site (L4 -L5), we narrowed our sampled ␣-motor neurons to those located in ventral motor pools with similar dorsolateral coordinates of motor pools known to innervate the plantar muscle (ϳ1.5-2.5 mm deep, 1.5-2.2 mm lateral from midline) as shown by retrograde tracing (Crockett et al 1987;Jacob 1998).…”

Viral-mediated optogenetic stimulation of peripheral motor nerves in non-human primates

“…Our second approach was to attempt to localize zones of high neuromuscular junction density near the motor end plate using electrical stimulation. Previous rodent studies have demonstrated that targeting muscle injections along motor endplates greatly enhances motor neuron transduction [42,48]. Targeting of the motor end plate as in these studies requires prior histological mapping of the motor end plate in a given muscle in situ followed by visual alignment of anatomical landmarks in the subject to be injected.…”