A Quantitative Study on the Uptake and Retrograde Transport of Horseradish Peroxidase by Adult Rat Phrenic Motorneurons after Axonal Crush Lesions
Furicchia, James Vincent
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The effect of both a double and single lesion crush on the uptake and retrograde transport of horseradish peroxidase (HRP) in phrenic neurons was studied quantitatively. The extent of HRP uptake and transport gives an indication of the metabolic state of the cell. In the control group, a single lesion crush was performed intrathoracicly followed immediately by application of HRP. In all animals, HRP was applied to the phrenic nerve after a cervical phrenicotomy. In the single crush experimental group, the phrenic nerve was crushed intrathoracicly followed some time later (5 to 50 days) by application of the HRP. In the second experimental group, two intrathoracic phrenic nerve crushes were performed five days apart. The second crush was performed proximal to the first crush. HRP was then applied five to fifty days after the second crush. The present study is significant since previous research in the CNS has determined that an initial injury to the spinal cord results in both cell death and metabolic changes in the axotomized neurons. Since there are both extrinsic (outside the cell) and intrinsic (inside the cell) factors that can contribute to these neuronal changes, it was unclear which of the factors could have contributed to the metabolic changes in the surviving neurons. Extrinsic factors include connective tissue and glial scarring around the cut axons, the connection of inappropriate synapses as a result of the absence of Schwann cells in the CNS, and the compactness of the spinal cord. Intrinsic factors include chromatolysis and changes in cell organelle production. The present study was designed such that the extrinsic factors are not present. The phrenic nerve, with axons projecting into the PNS,was used in this study. The PNS lacks the extrinsic factors that are present in the CNS. Another significant aspect of this project is that recent research has demonstrated that a double lesion to the hypoglossal nerve resulted in cell death in the hypoglossal nucleus. The present study employed a double crush lesion five days apart to see if cell death will result in the phrenic nucleus. In the present study, it was concluded that no significant cell death occurred from either the single crush or the double crush lesion. Also there was no significant difference in the uptake of HRP in both the experimental and control groups. This leads to the conclusion that the extrinsic factors were the more important factors contributing to both cell death and/or metabolic changes in neurons of the CNS.