Lack of Tryptophan Hydroxylase-1 in Mice Results in Gait Abnormalities

The role of peripheral serotonin in nervous system development is poorly understood. Tryptophan hydroxylase-1 (TPH1) is expressed by non-neuronal cells including enterochromaffin cells of the gut, mast cells and the pineal gland and is the rate-limiting enzyme involved in the biosynthesis of peripheral serotonin. Serotonin released into circulation is taken up by platelets via the serotonin transporter and stored in dense granules. It has been previously reported that mouse embryos removed from Tph1-deficient mothers present abnormal nervous system morphology. The goal of this study was to assess whether Tph1-deficiency results in behavioral abnormalities. We did not find any differences between Tph1-deficient and wild-type mice in general motor behavior as tested by rotarod, grip-strength test, open field and beam walk. However, here we report that Tph1 (−/−) mice display altered gait dynamics and deficits in rearing behavior compared to wild-type (WT) suggesting that tryptophan hydroxylase-1 expression has an impact on the nervous system

Spatiotemporal activation of lumbosacral motoneurons in the locomotor step cycle

The aim of this study was to produce a dynamic model of the spatiotemporal activation of ensembles of alpha motoneurons (MNs) in the cat lumbosacral spinal cord during the locomotor step cycle. The coordinates of MNs of 27 hindlimb muscles of the cat were digitized from transverse sections of spinal cord spanning the entire lumbosacral enlargement from the caudal part of L-4 to the rostral part of S-1 segments. Outlines of the spinal cord gray matter were also digitized. Models of the spinal cord were generated from these digitized data and displayed on a computer screen as three-dimensional (3-D) images. We compiled a chart of electromyographic (EMG) profiles of the same 27 muscles during the cat step cycle from previous studies and used these to modulate the number of active MNs in the 3-D images. The step cycle was divided into 100 equal intervals corresponding to about 7 ms each for gait of moderate speed. For each of these 100 intervals, the level of EMG of each muscle was used to scale the number of dots displayed randomly within the volume of the corresponding MN pool in the digital model. One hundred images of the spinal cord were thereby generated, and these could be played in sequence as a continuous-loop movie representing rhythmical stepping. A rostrocaudal oscillation of activity in hindlimb MN pools emerged. This was confirmed by computing the locus of the center of activation of the MNs in the 100 consecutive frames of the movie. The caudal third of the lumbosacral enlargement showed intense MN activity during the stance phase of locomotion. During the swing phase, the focus of activation shifted abruptly to the rostral part of the enlargement. At the stance-swing transition, a transient focus of activity formed in the most caudal part of the lumbosacral enlargement. This was associated with activation of gracilis, posterior biceps, posterior semimembranosus, and semitendinosus muscles. These muscles move the foot back and up to clear the ground during locomotion, a role that could be described as retraction. The spatiotemporal distribution of neuronal activity in the spinal cord during normal locomotion with descending control and sensory inputs intact has not been visualized before. The model can be used in the future to characterize spatiotemporal activity of spinal MNs in the absence of descending and sensory inputs and to compare these to spatiotemporal patterns in spinal MNs in normal locomotion

A novel approach for assigning levels to monkey and human lumbosacral spinal cord based on ventral horn morphology

Proper identification of spinal cord levels is crucial for clinical-pathological and imaging studies in humans, but can be a challenge given technical limitations. We have previously demonstrated in non-primate models that the contours of the spinal ventral horn are determined by the position of motoneuron pools. These positions are preserved within and among individuals and can be used to identify lumbosacral spinal levels. Here we tested the hypothesis that this approach can be extended to identify monkey and human spinal levels. In 7 rhesus monkeys, we retrogradely labeled motoneuron pools that represent rostral, middle and caudal landmarks of the lumbosacral enlargement. We then aligned the lumbosacral enlargements among animals using absolute length, segmental level or a relative scale based upon rostral and caudal landmarks. Inter-animal matching of labeled motoneurons across the lumbosacral enlargement was most precise when using internal landmarks. We then reconstructed 3 human lumbosacral spinal cords, and aligned these based upon homologous internal landmarks. Changes in shape of the ventral horn were consistent among human subjects using this relative scale, despite marked differences in absolute length or age. These data suggest that the relative position of spinal motoneuron pools is conserved across species, including primates. Therefore, in clinical-pathological or imaging studies in humans, one can assign spinal cord levels to even single sections by matching ventral horn shape to standardized series

Schematic drawings of lumbosacral enlargement sections in cat, monkey and human aligned via internal landmarks.

<p>Each schematic drawing represents the right side of every third section in a 1:5 series of 80μm sections (M5) or in a 1:6 80μm series in all 3 human cases. Cat data, with motoneuronal cell groups serving functionally distinct muscles marked by distinct colors, was adapted with permission from Vanderhorst and Holstege [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177243#pone.0177243.ref013" target="_blank">13</a>]. In all 3 species, series were aligned according to rostral (level 0) and caudal (level 100) anatomical landmarks as defined in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177243#pone.0177243.g001" target="_blank">Fig 1</a>. Lengths signify distances calculated in centimeters from level 0 to 100, based upon the thickness of each section and the number of sections, and after corrections for tissue shrinkage. Note that consistent changes in ventral horn shape are congruent across all 3 species, and among the 3 human subjects, despite obvious differences in size.</p

Comparison of three approaches to align the distribution of non-human primate lumbosacral motoneurons.

<p>Rostrocaudal distribution of iliopsoas, semimembranosus, and external sphincter motoneurons in 5 monkeys. Spinal cords were aligned among cases based upon (A) absolute length (millimeter distance calculated from the thickness of each section and the number of sections); (B) segmental level, starting at L4, and (C) internal landmarks representing level 0 (defined by the ventrolateral edge of the ventral horn extending laterally and representing the presence the most rostral leg motoneurons) and level 100 (defined by the ventral gray border changing from a curved protrusion to a straight edge or Onuf’s nucleus touching the ventral white matter. Note the size differences in (A) and the segmental differences in (B), which contrast the similarity in distribution of motoneurons across cases in (C).</p

Identification of rostral and caudal landmarks in the lumbosacral enlargement in rhesus monkey and humans.

<p>(A-C) Retrogradely labeled motoneurons in the rhesus monkey lumbosacral cord following CTb injections into the (A) iliopsoas, (B) semimembranosus, and (C) external sphincter muscle. Note that the iliopsoas motoneuron pool demarcates the rostral end of the enlargement, where the ventral horn extends laterally. The caudal landmark is demarcated by pelvic floor motoneurons of the external sphincter (Onuf’s nucleus) touching the edge of the ventral gray matter. (D-F) ChAT-IR neurons in the human lumbosacral cord (87 year old woman with Dementia with Lewy Bodies). Panels (D-F) are homologous to panels (A-C) in the monkey. Note that the changes in shape of the ventral horn, demarcating the rostral and caudal landmarks, can be identified in the human spinal cord. Similar to monkey, these changes in shape are determined by (ChAT-IR) motoneurons, similar to the rhesus monkey. Bars in (A-C), and (D-F) = 500μm.</p

Spinal projections of the A5, A6 (locus coeruleus), and A7 noradrenergic cell groups in rats

The pontine noradrenergic cell groups, A5, A6 (locus coeruleus), and A7, provide the only noradrenergic innervation of the spinal cord, but the individual contribution of each of these populations to the regional innervation of the spinal cord remains controversial. We used an adeno-associated viral (AAV) vector encoding green fluorescent protein under an artificial dopamine beta-hydroxylase (PRSx8) promoter to trace the spinal projections from the A5, A6, and A7 groups. Projections from all three groups travel through the spinal cord in both the lateral and ventral funiculi and in the dorsal surface of the dorsal horn, but A6 axons take predominantly the dorsal and ventral routes, whereas A5 axons take mainly a lateral and A7 axons a ventral route. The A6 group provides the densest innervation at all levels, and includes all parts of the spinal gray matter, but it is particularly dense in the dorsal horn. The A7 group provides the next most dense innervation, again including all parts of the spinal cord, but is it denser in the ventral horn. The A5 group supplies only sparse innervation to the dorsal and ventral horns and to the cervical and lumbosacral levels, but provides the densest innervation to the thoracic intermediolateral cell column, and in particular to the sympathetic preganglionic neurons. Thus, the pontine noradrenergic cell groups project in a roughly topographic and complementary fashion onto the spinal cord. The pattern of spinal projections observed suggests that the locus coeruleus might have the greatest effect on somatosensory transmission, the A7 group on motor function, and the A5 group on sympathetic function. J. Comp. Neurol. 520:19852001, 2012. (c) 2011 Wiley Periodicals, In