Gene expresion in rodent spinal neuronal populations and their response to injury

Motor neurons are the centre of convergence for all neural activity relating to movements. The activity integrated in the motor neurons is transmitted to appropriate muscles generating coordinated muscle contractions. Motor neurons, long considered passive integrators of the motor signal, have been shown to actively participate in shaping the output to the muscles during different behaviors, where active synaptic components resulting in plateau potentials and persistent inward currents can be activated during motor neuron recruitment. In the present thesis the functional significance of motor neurons during normal and injury states have been examined using a combination of electrophysiology and gene expression profiling. First the transmitter phenotype of motor neurons was examined. Motor neurons have long been thought to release only acetylcholine at their terminals thus following the central dogma proposed by Dale, stating that a neuron releases the same neurotransmitter from all its terminals. We find that motor neurons release not only acetylcholine but also glutamate at central synapses, whereas we did not discover any sign of glutamate release at the neuromuscular junction. This finding jeopardizes the central dogma, indicating a new level of possible modulation by motor neurons in shaping the motor output through a differentiated release of two fast neurotransmitters at distinct axon terminals. To further elucidate the functional role of motor neurons in relation to other spinal neuronal populations, the expression profiles of motor neurons and descending commissural interneurons (dCIN) were compared. This task required development of a method, which can be used for reliable gene expression profiling with RNA extracted from as few as 50 fluorescently identified and laser dissected cells. Based on this methodology, we find 49 significantly differentially expressed genes that may relate to the functional differences between motor neurons and dCINs in transmitting and shaping the motor output. Our method was subsequently used to measure the transcriptional response of motor neurons following spinal cord injury. Injury causes long-term changes in spinal networks located caudal to the injury resulting in maladaptive pathophysiological states including spasticity. In normal animals the expression of plateau potentials caused by persistent inward calcium and sodium currents (PICs) is conditional and depends on the presence of monoamines released from descending pathways. Motor neurons therefore lose the ability to express plateaus immediately after a spinal cord injury as the descending fibers are severed. The ability of motor neurons to express PICs reappears after a few weeks and has been implicated in injury-induced spasticity. We use the expression profiles of motor neurons to examine the molecular underpinnings of this return of plateaus in the late phase of the injury response, 21 and 60 days post injury. We find that the ancillary subunits of the channel complexes conducting the PICs, rather than the pore forming subunits, are subject to extensive regulation. Genes coding for receptors and intracellular pathways relating to the expression of plateau potentials also undergo regulation. Lastly, we examined the general transcriptional response of motor neurons throughout the injury response; 0, 2, 7, 21 and 60 days post injury and the underlying regulatory control of gene expression. We find that motor neurons are involved in the general injury response with a transient up-regulation of inflammatory and immunologically related processes in the early phase, while developmental pathways are up-regulated in late phases of the injury response. Promoter analysis conducted on expression clusters revealed general targets of regulation for identified transcription factors that participate in the injury response of the motor neurons. We conclude that the motor neurons engage an extensive molecular machinery to regulate and modulate their electrophysiological properties as a response to injury. This suggests that electrophysiological properties are subject to dynamic regulation that also could be at play in normal states of the spinal cord, thus modulating the functional response of the motor neurons and shaping the motor output. Together, the results presented in this thesis have provided new knowledge about the normal function of motor neurons and a novel insight into the development of spasticity that can help define new therapies for spinal cord injury.