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The Journal of Neuroscience, November 1, 2001, 21(21):8680-8689

Altered Respiratory Motor Drive after Spinal Cord Injury: Supraspinal and Bilateral Effects of a Unilateral Lesion

Francis J. Golder¹, Paul J. Reier², and Donald C. Bolser¹

¹ Department of Physiological Sciences, College of Veterinary Medicine, and ² Department of Neuroscience, University of Florida, Gainesville, Florida 32610

Because some bulbospinal respiratory premotor neurons have bilateral projections to the phrenic nuclei, we investigated whether changes in contralateral phrenic motoneuron function would occur after unilateral axotomy via C₂ hemisection. Phrenic neurograms were recorded under baseline conditions and during hypercapnic and hypoxic challenge in C₂ hemisected, normal, and sham-operated rats at 1 and 2 months after injury. The rats were anesthetized, vagotomized, and mechanically ventilated. No group differences were seen in contralateral neurograms at 1 month after injury. At 2 months, however, there was a statistically significant decrease in respiratory rate (RR) at normocapnia, an elevated RR during hypoxia, and an attenuated increase in phrenic neurogram amplitude during hypercapnia in the C₂-hemisected animals. To test whether C₂ hemisection had induced a supraspinal change in respiratory motor drive, we recorded ipsilateral and contralateral hypoglossal neurograms during hypercapnia. As with the phrenic motor function data, no change in hypoglossal output was evident until 2 months had elapsed when hypoglossal amplitudes were significantly decreased bilaterally. Last, the influence of serotonin-containing neurons on the injury-induced change in phrenic motoneuron function was examined in rats treated with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Pretreatment with 5,7-dihydroxytryptamine prevented the effects of C₂ hemisection on contralateral phrenic neurogram amplitude and normalized the change in RR during hypoxia. The results of this study show novel neuroplastic changes in segmental and brainstem respiratory motor output after C₂ hemisection that coincided with the spontaneous recovery of some ipsilateral phrenic function. Some of these effects may be modulated by serotonin-containing neurons.

Key words: hypercapnia; hypoglossal; hypoxia; phrenic; rats; serotonin; spinal cord injury; ventilation

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Copyright © 2001 Society for Neuroscience  0270-6474/01/21218680-10$05.00/0

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