Developmental changes of crossed phrenic activity following cervical spinal cord hemiseciton and correlated mechanisms

Yonglu Huang, Wayne State University

Abstract

Spinal cord hemisection rostral to the phrenic nucleus leads to paralysis of the ipsilateral hemidiaphragm and respiratory insufficiency. Recovery of the paralyzed hemidiaphragm may be induced by activating a latent respiratory motor pathway in adult rats. Although the pathway is latent in adults, it may not be latent in neonatal rats as shown by the spontaneous expression of activity over this pathway in an earlier in vitro study. Activity mediated over the latent pathway is known as "crossed phrenic activity". Whether crossed phrenic activity following C2 spinal cord hemisection occurs spontaneously in the neonatal rat in vivo is still unknown. We hypothesized that crossed phrenic activity may be spontaneously expressed in neonates in vivo and may be converted from a spontaneously active state to a latent and nonfunctional state during postnatal development. The mechanisms that contribute to the conversion of spontaneous crossed phrenic activity in postnatal rats are also unknown. Developmental change of the central respiratory control circuitry in neonates may underlie the expression of crossed phrenic activity in the neonatal rats. However, the anatomy of the pathway in neonatal rats is still unknown. In respiratory system, glutamate receptor is an excitatory neurotransmitter receptor and the primary mediator in descending the respiratory drive to phrenic motoneurons. Moreover, glutamate receptors also play an important role in the regulation of developmental and adult neural plasticity. In adult rats, crossed phrenic pathway can be activated by drug-induced upregulation of NR2A and GluR1 subunits in the phrenic nucleus following hemisection. Therefore, we hypothesize that the expressions of NR2A and GluR1 subunits on phrenic motoneurons are transiently high in neonates and the downregulation of these two subunits during development may play an important role in the inactivation of crossed phrenic pathway. We tested the functional status of the ipsilateral and contralateral hemidiaphragm following hemisection in postnatal rats at different age through EMG analysis. Crossed phrenic activity was expressed in ventral, lateral, and dorsal parts of the ipsilateral hemidiaphragm in P2. During postnatal development, the activity was observed only in the ventral area of the ipsilateral hemidiaphragm in P7, P14, P21 and P28 animals. Significant decreases in the extent of ventral crossed phrenic activity were observed from P2 to P28. The pathway generating this activity becomes latent by postnatal day 35. The present results suggest that spontaneous crossed phrenic activity occur in vivo following C2 hemisection and the activity gradually decreases during the first four postnatal weeks. To delineate this neural pathway in neonates, we injected WGA-HRP and BHRP individually to the ipsilateral phrenic nerve and hemidiaphragm following hemisection. The WGA-HRP labeling was observed only in the ipsilateral phrenic nucleus and ipsilateral rVRG in the P2, P7, and P28 hemisected rats. Bilateral labeling of rVRG neurons was shown in P35 rats. The BHRP study showed that many phrenic dendrites cross the midline in P2 neonatal rats at both rostral and caudal parts of the phrenic nucleus. There was a marked reduction of crossing dendrites observed in P7 and P28 animals and no crossing dendrites observed in P35 rats. The present results suggest that the crossed phrenic pathway in neonatal rats involves spinal decussating axon collaterals from the ipsilateral rVRG premotor neurons and midline-crossing dendrites of the ipsilateral phrenic motoneurons. In this dissertation, we also investigate the expression of NR2A and GluR1 on phrenic motoneurons during the change of crossed phrenic activity in postnatal rats via western blot and immunofluorescence study. The protein levels of NR2A and GluR1 were transiently high in postnatal day 2 (P2) rats and then was significantly reduced in P7 and P35 animals. Downregulation of these receptor subunits is coincident with the changes of crossed phrenic activity during postnatal development. Therefore, we indicate that downregulation of NR2A and GluR1 correlates to the conversion of crossed phrenic pathway in postnatal animals from an active state to a latent state.

Recommended Citation

Yonglu Huang, "Developmental changes of crossed phrenic activity following cervical spinal cord hemiseciton and correlated mechanisms" (January 1, 2009). ETD Collection for Wayne State University. Paper AAI3350060.
http://digitalcommons.wayne.edu/dissertations/AAI3350060