Although neurotrophins are conventionally associated with the proper growth and survival of developing neurons, there is increasing evidence that they play an equally significant role in the functions of adult neurons. Specifically, brain derived neurotrophic factor activation of its preferred receptor TrkB is essential in the regulation of motoneuronal activity. Neurotrophin‐dependent and independent activation of TrkB regulates the motoneuronal dendritic integrity, and maintains unique classes of synapses. In addition, it regulates the expression and function of ion channels as well as the strength of inhibitory and excitatory synapses via different intracellular pathways. The recent physiological findings in the regulatory roles of TrkB are implicative of its presence on motoneuronal dendrites. Although, the expression of TrkB in the soma has long been confirmed, its distribution on the dendritic tree of motoneurons remains unknown. We aimed to examine the distribution of TrkB in the cytoplasm and membrane‐associated regions of the dendritic tree of adult neck motoneurons. We have determined, via confocal microscopy, that TrkB is present and abundant throughout the cytoplasm and the membrane‐associated regions of motoneuronal dendrites as well as the soma. TrkB is organized in clusters and its distribution is best described as punctated. We then developed a technique to separately extract and quantify the TrkB immunoreactivity associated with the membrane and the cytoplasm as function of distance from the soma and dendritic tree. We have demonstrated that there is no bias in TrkB immunoreactivity to a specific region of the dendritic tree in five trapezius motoneurons. These observations were confirmed for both cytoplasmic and membrane‐associated TrkB. There is compelling evidence that both mature full‐length and immature hypoglycosylated TrkB isoforms are active in strengthening the response to excitatory synapses in motoneurons. We identified the full length TrkB as well as 3 hypoglycosylated isoforms in cervical spinal segments that contain trapezius motoneurons and phrenic motoneurons. Taken together, these data indicate that TrkB is likely involved in regulating and maintaining different classes of ion channels and synapses on the dendrites as well as the soma. Various isoforms of TrkB may also be involved in regulating motoneuronal activity. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2014-01-14 12:48:21.357

The currents generated by excitatory and inhibitory synapses on motoneurons can be amplified by noradrenalin and serotonin. Both of these neurotransmitters act, and interact, via the same Gq-protein second-messenger system to modulate L-type Ca++, persistent-Na+, and leak K+ channels on motoneuron dendrites. However, noradrenergic and serotonergic synapses only modulate nearby excitatory and inhibitory synapses, so their relative distributions play a major role in the regulation of the overall output of the motoneuron. Moreover, the relative proximity between noradrenergic and serotonergic synapses may allow their individual effects to combine nonlinearly when co-activated, thereby regulating the magnitude of the amplification. The goal of the present study is to determine whether the distributions of noradrenergic and serotonergic synapses are biased along motoneuron dendritic trees. The dendritic trees of five intracellularly stained feline splenius motoneurons were reconstructed. On them were plotted the locations of noradrenergic and serotonergic contacts, as determined by immunohistochemistry. The distribution of noradrenergic contacts was moderately biased both dorsally and distally in all five cells. Serotonergic contacts on the same neurons showed a moderate ventral bias. These findings suggest that excitatory and inhibitory inputs located dorsally and/or distally are preferentially amplified by noradrenergic synapses. Also, those synapses which are located ventrally are favorably amplified by serotonergic synapses. Both serotonergic and noradrenergic contacts are strongly biased towards innervation along small diameter (<2μm) dendrites. The relative distributions between serotonergic and noradrenergic contacts have also been analyzed for all five cells. There was a bias towards minimizing the distance between like contacts (NE to NE and 5-HT to 5-HT). This increases the likelihood of interaction within populations when contacts are co-activated. Conversely, the distances between neighbouring noradrenergic and serotonergic contacts (NE to 5-HT and 5-HT to NE) were biased towards greater separation. This decreases the likelihood of interaction between populations when contacts are co-activated. In summary, these findings suggest that noradrenalin and serotonin, having different location biases along the dendritic tree, will amplify some synapses in a biased manner. Additionally, like synapses may work in a coordinated manner with respect to their relative proximity. Coordination between noradrenergic and serotonergic synapses is less likely. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2008-09-29 09:59:38.799

Ulm, Univ., Diss., 2008.

Würzburg, Univ., Diss., 2010. / Zsfassung in dt. Sprache.

Techn. Hochsch., Diss., 2000--Aachen.

Cellular proteome profiling revealed that most biomolecules do not exist in isolation, but rather are incorporated into modular complexes. These assembled complexes are usually very large, consisting of 10 subunits on an average and include either proteins alone, or proteins and nucleic acids. Consequently, such macromolecular assemblies rather than individual biopolymers perform the vast majority of cellular activities. The faithful assembly of such molecular assemblies is often aided by trans-acting factors in vivo, to preclude aggregation of complex components and/or non-cognate interactions. A paradigm for an assisted assembly of a macromolecular machine is the formation of the common Sm/LSm core of spliceosomal and histone-mRNA processing U snRNPs. The key assembly factors united in the Protein Arginine Methyltransferase 5 (PRMT5) and the Survival Motor Neuron (SMN) complexes orchestrate the assembly of the Sm/LSm core on the U snRNAs. Assembly is initiated by the PRMT5-complex subunit pICln, which pre-arranges the Sm/LSm proteins into spatial positions occupied in the mature U snRNPs. The SMN complex subsequently binds these Sm/LSm units, displaces pICln and catalyses the Sm ring closure on the Sm-site of the U snRNA. The SMN complex consists of the eponoymous SMN protein linked in a modular network of interactions with eight other proteins, termed Gemins 2-8 and Unrip. Despite functional and structural characterisation of individual protein components and/or sub-complexes of this assembly machinery, coherent understanding of the structural framework of the core SMN complex remained elusive. The current work, employing a combined approach of biochemical and structural studies, aimed to contribute to the understanding of how distinct modules within the SMN complex coalecse to form the macromolecular SMN complex. A novel atomic resolution (1.5 Å) structure of the human Gemin8:7:6 sub-complex, illustrates how the peripheral Gemin7:6 module is tethered to the SMN complex via Gemin8’s C-terminus. In this model, Gemin7 engages with both Gemin6 and Gemin8 via the N- and C-termini of its Sm-fold like domain. This highly conserved interaction mode is reflected in the pronounced sequence conservation and identical biochemical behaviour of similar sub-complexes from divergent species, namely S. pombe and C. elegans. Despite lacking significant sequence similarity to the Sm proteins, the dimeric Gemin7:6 complex share structural resemblance to the Sm heteromers. The hypothesis that the dimeric Gemin7:6 functions as a Sm-surrogate during Sm core assembly could not be confirmed in this work. The functional relevance of the structural mimicry of the dimeric Gemin7:6 sub-complex with the Sm heterodimers therefore still remains unclear. Reduced levels of functional SMN protein is the cause of the devastating neurodegenerative disease, Spinal Muscular Atrophy (SMA). The C-terminal YG-zipper motif of SMN is a major hot-spot for most SMA patient mutations. In this work, adding to the existing inventory of the human and fission yeast YG-box models, a novel 2.2 Å crystal structure of the nematode SMN’s YG-box domain adopting the glycine zipper motif has been reported. Furthermore, it could be assessed that SMA patient mutations mapping to this YG-box domain greatly influences SMN’s self-association competency, a property reflected in both the human and nematode YG-box biochemical handles. The shared molecular architecture and biochemical behaviour of the nematode SMN YG-box domain with its human and fission yeast counterparts, reiterates the pronounced conservation of this oligomerisation motif across divergent organisms. Apart from serving as a multimerization domain, SMN’s YG-box also acts as interaction platform for Gemin8. A systematic investigation of SMA causing missense mutations uncovered that Gemin8’s incorporation into the SMN complex is influenced by the presence of certain SMA patient mutations, albeit independent of SMN’s oligomerisation status. Consequently, loss of Gemin8 association in the presence of SMA patient mutations would also affect the incorporation of Gemin7:6 sub-complex. Gemin8, therefore sculpts the heteromeric SMN complex by bridging the Gemin7:6 and SMN:Gemin2 sub-units, a modular feature shared in both the human and nematode SMN complexes. These findings provide an important foundation and a prospective structural framework for elucidating the core architecture of the SMN complex in the ongoing Cryo-EM studies. / Systematische Untersuchungen von zellulären Bestandteilen haben gezeigt, dass viele Proteine nicht isoliert, sondern vielmehr in modularen Komplexen organisiert vorliegen. Mit durchschnittlich zehn Untereinheiten sind diese Komplexe sehr groß, wobei sie entweder ausschließlich aus Proteinen oder aber aus Proteinen und Nukleinsäuren bestehen können. Daher wird der Großteil zellulärer Aktivitäten nicht von einzelnen Biopolymeren, sondern von makromolekularen Komplexen verrichtet. Die Zusammenlagerung dieser Komplexe wird in vivo häufig von Hilfsfaktoren unterstützt, um die Aggregation der Einzelkomponenten und/oder unspezifische Wechselwirkungen zu verhindern. Ein Beispiel für eine derartige Zusammenlagerungshilfe ist die Bildung des Sm/LSm-Cores der mRNA-prozessierenden U snRNPs. Dabei wird die Anlagerung von Sm/LSm Proteinen an die U snRNAs durch eine Anzahl von Hilfsfaktoren orchestriert, die in Protein-Arginin-Methyltransferase 5 (PRMT5)- und dem Survival Motor Neuron (SMN)-Komplexen organisiert sind. Die Zusammenlagerung wird durch die PRMT5-Untereinheit pICln initiiert, die die räumliche Anordnung von Sm/LSm-Proteinen in höher-geordneten Komplexen stabilisiert. Diese werden anschließend auf den SMN-Komplex übertragen, wobei pICln verdrängt und die Verbindung mit der Sm-Seite der U snRNA sichergestellt wird. Der SMN-Komplex besteht aus dem SMN-Protein, das in einem modularen Netzwerk mit acht weiteren Proteinen (Gemins 2-8 und Unrip) interagiert. Auch wenn funktionale und strukturelle Charakterisierungen einzelner Proteinkomponenten und Module dieser Zusammenlagerungs-Maschinerie vorliegen, steht ein tiefergehendes Verständnis des strukturellen Organisation des Gesamt-Komplexes noch aus. In der vorliegenden Arbeit sollte unter Anwendung biochemischer und struktureller Techniken ein Beitrag dazu geleistet werden, die Interaktionen der verschiedenen Komponenten innerhalb des SMN-Komplexes zu verstehen, die so die dreidimensionale Organisation des SMN-Komplexes zu verstehen. Eine neuartige Kristallstruktur des humanen Gemin8:7:6-Subkomplexes bei einer Auflösung von 1.5 Å zeigt, wie der periphere Gemin7:6-Abschnitt durch den C-Terminus von Gemin8 zum SMN-Komplex dirigiert wird. In diesem Modell interagiert Gemin7 sowohl mit Gemin6 als auch Gemin8 über den N- und C-Terminus der Sm-ähnlichen Domäne. Dieser hochkonservierte Interaktionsmodus wird in der erwähnten konservierten Sequenz und dem gleichen biochemischen Verhalten ähnlicher Subkomplexe in divergenten Spezies einschließlich S. pombe und C. elegans widergespiegelt. Obwohl es keine signifikante Übereinstimmung mit der Sequenz von Sm-Proteinen gibt, weist der dimere Gemin7:6-Komplex markante strukturelle Ähnlichkeit mit dem einem Sm-Heterodimer auf. Die Annahme, der dimere Gemin7:6-Subkomplex würde als Hilfsfaktor über die direkte Interaktion mit Sm-Proteinen fungieren konnte in der vorliegenden Arbeit nicht bestätigt werden. Folglich bleibt die Funktion des dimeren Gemin7:6-Subkomplexes im Kontext der SMN-Zusammenlagerungsmaschinerie unklar. Verringerte Mengen des funktionellen SMN-Proteins sind die Ursache für die neurodegenerative Erkrankung Spinale Muskelatrophie (SMA). Das C-terminale YG-Zipper-Motiv von SMN stellt einen Hotspot für die meisten SMA-Mutationen dar. In dieser Arbeit wurde der bereits bekannten YG-Box aus H. sapiens und S. pombe eine neuartige Kristallstruktur der SMN YG-Box aus C. elegans mit einer Auflösung von 2.2 Å hinzugefügt. Zusätzlich wurde gezeigt, dass SMA-verursachende Missense-Mutationen in der YG-Box einen beträchtlichen Einfluss auf die Selbst-Interaktion von SMN haben, was aus biochemischen Versuchen mit der YG-Box aus H. sapiens und C. elegans ersichtlich wurde. Der molekulare Aufbau und das biochemische Verhalten der SMN YG-Box aus C. elegans, S. pombe und H. sapiens betont die Konservierung dieses Oligomerisierungsmotives über mehrere Organismen hinweg. Neben der Funktion als Multimerisationsdomäne dient die YG-Box von SMN auch als Interaktionsplattform für Gemin8. Eine systematische Untersuchung von SMA-verursachenden Missense-Mutationen ergab, dass die Einbindung von Gemin8 in den SMN-Komplex durch definierte Substitutionen massiv beeinflusst wird. Interessanterweise ist dieser Bindungsdefekt unabhängig vom SMN-Oligomerisierungsstatus. Demzufolge würde diese Klasse von SMA-Mutationen spezifisch die Inkorporation des Gemin7:6-Subkomplexes beeinflussen. Die Resultate dieser Arbeit bilden eine wichtige Grundlage für weitere strukturelle Untersuchungen des SMN-Komplexes über Kryo-Elektronenmikroskopie.

No description available.

Previous work has demonstrated that rhythmic motor outputs, such as locomotion and scratch induce a hyperpolarization of the voltage threshold (Vth) for action potential initiation in spinal motoneurons, enhancing their excitability. Descending monoamines were implicated in mediating this effect; however, the recent observation that changes in Vth persist during fictive scratch in cats following acute cervical transection revealed that intraspinal systems, of unknown neuromodulatory identity, also have the ability to regulate motoneuron excitability during motor behaviour. This thesis addresses: 1) whether acetylcholine (ACh) is able to modulate spinal motoneuron Vth, and 2) whether endogenous ACh modulates motoneuron excitability during motor activity without intact descending modulation. Our first study investigates whether ACh from exogenous and/or endogenous sources alters motoneuron Vth. We made intracellular recordings of lumbar motoneurons from neonatal rats to pharmacologically manipulate muscarinic and nicotinic receptor activity. Results show that ACh induces either Vth hyperpolarization, Vth depolarization or no change in Vth depending on the activity state of the network, the ACh concentration, and influences from other systems. Our second study investigates whether intraspinal cholinergic inputs induce Vth hyperpolarization during rhythmic motor output when descending projections are disrupted. For this we developed an in vitro neonatal rat spinal cord preparation to elicit rhythmic activity independently of brainstem or lumbar cord stimulation. Intracellular recordings from motoneurons allowed comparison of the Vth prior to and during rhythmic output, both in the absence and presence of cholinergic antagonists in the lumbar cord. Results show that intraspinal cholinergic mechanisms are active and importantly contribute to modulation of motoneuron Vth during motor output. We suggest that in addition to descending modulation, the spinal cholinergic system regulates motoneuron Vth to either facilitate or inhibit recruitment according to the motor network state. Motoneuron excitability regulation by modification of distinct membrane properties resulting from separate modulatory systems activation during diverse motor behaviours is discussed. This work is the first to demonstrate the role of cholinergic mechanisms in regulating motoneuron excitability through modulation of Vth in an activity based context, and therefore outlines a spinal modulatory system that would contribute to motor control in both normal and pathological states. / May 2016

In dieser Arbeit sollte die Funktion von RSK in Motoneuronen von Drosophila untersucht werden. Mutationen im RSK2-Gen verursachen das Coffin-Lowry-Syndrom (CLS), das durch mentale Retardierung charakterisiert ist. RSK2 ist hauptsächlich in Regionen des Gehirns exprimiert, in denen Lernen und Gedächtnisbildung stattfinden. In Mäusen und Drosophila, die als Modellorganismen für CLS dienen, konnten auf makroskopischer Ebene keine Veränderungen in den Hirnstrukturen gefunden werden, dennoch wurden in verschiedenen Verhaltensstudien Defekte im Lernen und der Gedächtnisbildung beobachtet. Die synaptische Plastizität und die einhergehenden Veränderungen in den Eigenschaften der Synapse sind fundamental für adaptives Verhalten. Zur Analyse der synaptischen Plastizität eignet sich das neuromuskuläre System von Drosophila als Modell wegen des stereotypen Innervierungsmusters und der Verwendung ionotroper Glutamatrezeptoren, deren Untereinheiten homolog sind zu den Untereinheiten der Glutamatrezeptoren des AMPA-Typs aus Säugern, die wesentlich für die Bildung von LTP im Hippocampus sind. Zunächst konnte gezeigt werden, dass RSK in den Motoneuronen von Drosophila an der präsynaptischen Seite lokalisiert ist, wodurch RSK eine Synapsen-spezifische Funktion ausüben könnte. Morphologische Untersuchungen der Struktur der neuromuskulären Synapsen konnten aufzeigen, dass durch den Verlust von RSK die Größe der neuromuskulären Synapse, der Boutons sowie der Aktiven Zonen und Glutamatrezeptorfelder reduziert ist. Obwohl mehr Boutons gebildet werden, sind weniger Aktive Zonen und Glutamatrezeptorfelder in der neuromuskulären Synapse enthalten. RSK reguliert die synaptische Transmission, indem es die postsynaptische Sensitivität, nicht aber die Freisetzung der Neurotransmitter an der präsynaptischen Seite beeinflusst, obwohl in immunhistochemischen Analysen eine postsynaptische Lokalisierung von RSK nicht nachgewiesen werden konnte. RSK ist demnach an der Regulation der synaptischen Plastizität glutamaterger Synapsen beteiligt. Durch immunhistochemische Untersuchungen konnte erstmals gezeigt werden, dass aktiviertes ERK an der präsynaptischen Seite lokalisiert ist und diese synaptische Lokalisierung von RSK reguliert wird. Darüber hinaus konnte in dieser Arbeit nachgewiesen werden, dass durch den Verlust von RSK hyperaktiviertes ERK in den Zellkörpern der Motoneurone vorliegt. RSK wird durch den ERK/MAPK-Signalweg aktiviert und übernimmt eine Funktion sowohl als Effektorkinase als auch in der Negativregulation des Signalwegs. Demnach dient RSK in den Zellkörpern der Motoneurone als Negativregulator des ERK/MAPK-Signalwegs. Darüber hinaus könnte RSK die Verteilung von aktivem ERK in den Subkompartimenten der Motoneurone regulieren. Da in vorangegangenen Studien gezeigt werden konnte, dass ERK an der Regulation der synaptischen Plastizität beteiligt ist, indem es die Insertion der AMPA-Rezeptoren zur Bildung der LTP reguliert, sollte in dieser Arbeit aufgeklärt werden, ob der Einfluss von RSK auf die synaptische Plastizität durch seine Funktion als Negativregulator von ERK zustande kommt. Untersuchungen der genetischen Interaktion von rsk und rolled, dem Homolog von ERK in Drosophila, zeigten, dass die durch den Verlust von RSK beobachtete reduzierte Gesamtzahl der Aktiven Zonen und Glutamatrezeptorfelder der neuromuskulären Synapse auf die Funktion von RSK als Negativregulator von ERK zurückzuführen ist. Die Größe der neuromuskulären Synapse sowie die Größe der Aktiven Zonen und Glutamatrezeptorfelder beeinflusst RSK allerdings durch seine Funktion als Effektorkinase des ERK/MAPK-Signalwegs. Studien des axonalen Transports von Mitochondrien zeigten, dass dieser in vielen neuropathologischen Erkrankungen beeinträchtigt ist. Die durchgeführten Untersuchungen des axonalen Transports in Motoneuronen konnten eine neue Funktion von RSK in der Regulation des axonalen Transports aufdecken. In den Axonen der Motoneurone von RSK-Nullmutanten wurden BRP- und CSP-Agglomerate nachgewiesen. RSK könnte an der Regulation des axonalen Transports von präsynaptischem Material beteiligt sein. Durch den Verlust von RSK wurden weniger Mitochondrien in anterograder Richtung entlang dem Axon transportiert, dafür verweilten mehr Mitochondrien in stationären Phasen. Diese Ergebnisse zeigen, dass auch der anterograde Transport von Mitochondrien durch den Verlust von RSK beeinträchtigt ist. / In this thesis the function RSK in motoneurons of Drosophila has been analyzed. Mutations in the RSK2-gene cause the Coffin-Lowry-Syndrome (CLS) which is characterized by mental retardation. RSK2 is predominantly expressed in regions of the brain where learning and formation of the memory take place. Even no obvious changes in brain structures could be observed at macroscopic level in mouse and Drosophila which serve as an animal model for CLS. However deficits in various learning tasks could be observed due to the loss of the RSK function. Synaptic plasticity and the following changes in synaptic properties are fundamental for adaptive behaviors. The neuromuscular system of Drosophila suits as a model for studies of the synaptic plasticity because of the stereotypic innervation pattern and the use of ionotropic glutamate receptors which subunits are homologous to the subunits of the mammalian AMPA-type of glutamate receptors which are essential for the formation of LTP in the hippocampus. This study shows that RSK is located at the presynaptic site of the motoneurons of Drosophila which indicates a synapse-specific function of RSK. The structural analysis of the neuromuscular junction (NMJ) show that the loss of RSK causes a reduction in size of the NMJ, boutons, active zones and glutamate receptor fields. More boutons were found at the NMJ, but less active zones and glutamate receptor fields were established. The localization of RSK at the postsynaptic side could not be detected in this study although RSK regulates the synaptic transmission by affecting the postsynaptic sensitivity but not the presynaptic neurotransmitter release. Hence RSK could take part in the regulation of synaptic plasticity. Immunohistochemical analysis could depict a novel function of RSK in the synapse-specific localization of ERK. Further this study show that due to the loss of RSK more activated ERK is located in den cell bodies of the motoneurons. RSK functions as a negative regulator of the ERK/MAPK signaling in the somata of motoneurons. Additionally, RSK could regulate the distribution of ERK in the different subcompartments of the motoneurons. Previous studies show ERK as a regulator of synaptic plasticity by influencing the insertion of AMPA receptors into the postsynaptic membrane during LTP. RSK is activated by the ERK/MAPK signaling and functions not only as an effector kinase but also as a negative regulator of this pathway. If the effect of RSK on synaptic plasticity is due to its function as a negative regulator of ERK should be clarified in this work. Analysis of the genetic interactions of rsk and rolled, the Drosophila homologue of mammalian ERK, show that the reduced number of active zones and glutamate receptor fields found at the NMJ of RSK null mutants is caused by the function of RSK as a negative regulator of ERK. In turn RSK affects the size of the NMJ, also the size of the active zones and glutamate receptor fields by its function as an effector kinase of the ERK/MAPK signaling. Several studies have shown that the axonal transport of mitochondria is affected in many neuropathological diseases. This work could uncover a novel function of RSK in the regulation of the axonal transport in motoneurons. The loss of RSK causes the formation of agglomerates of the presynaptic proteins BRP and CSP. Therefore RSK takes part in the regulation of the transport of presynaptic material. In absence of RSK less mitochondria are transported in anterograde direction and more mitochondria are pausing. This results implicate a function of RSK in regulating the anterograde transport of mitochondria.

Serotonin (5-HT) is a well-known modulator of spinal cord excitability and motor output. In the spinal cord, the actions of 5-HT are primarily mediated by the 5-HT1AR, 5-HT2Rs and the 5-HT7R. Following a spinal cord transection, which results in a loss of supraspinal input, 5-HT agonists such as quipazine are used to provide excitation to the spinal cord to facilitate locomotor recovery. This is characterized by rhythmic alteration of left and right hindlimbs and ipsilateral flexor and extensor muscles. However, whether 5-HT has a global effect on spinal cord excitability or is confined to a specific motor group (i.e. flexors or extensors) is currently unknown. Furthermore, quipazine is used in conjunction with activity based interventions to enhance recovery following a spinal cord injury. However, the influence of limb activity on the responsiveness of the injured spinal cord to quipazine has not been examined. Lastly, the recovery of locomotion is at least in part thought to occur through an up-regulation of 5-HT receptors, although this has not been investigated in lumbar spinal cord. Chapter 2 examines whether quipazine had a differential effect on flexor and extensor motor output assessed by recording flexor and extensor reflexes, motoneurons and Ia extracellular field potentials pre- and post-quipazine. It was determined that following an acute spinal transection, quipazine induced a larger flexor monosynaptic reflex (MSR) compared to the extensor MSR due to pre-synaptic but not motoneuron modulation. Chapter 3 examines the influence of a chronic spinal transection with and without passive cycling on the hindlimb flexor and extensor MSR, both pre- and post-quipazine. It was found that three months post STx, the extensor but not flexor MSR demonstrated a hyperexcitable response, which was attenuated with passive cycling. Further, three months of passive cycling extensor MSR response to quipazine was similar to that seen in the control intact group. Chapter 4 examined 5-HT receptor expression in flexor and extensor motoneurons three months post spinalization with or without passive cycling. Following a chronic STx, the 5-HT1AR and 5-HT2CR are down regulated, whereas the 5-HT2AR is up-regulated. Passive cycling further enhanced the 5-HT2AR expression as well as up-regulated the 5-HT7R in extensor but not flexor motoneurons. Chapter 5 discusses the results and significance of these findings in detail.