Altered respiratory motor drive after spinal cord injury: supraspinal and bilateral effects of a unilateral lesion

Abstract

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 C(2) hemisection. Phrenic neurograms were recorded under baseline conditions and during hypercapnic and hypoxic challenge in C(2) 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 C(2)-hemisected animals. To test whether C(2) 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 C(2) 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 C(2) hemisection that coincided with the spontaneous recovery of some ipsilateral phrenic function. Some of these effects may be modulated by serotonin-containing neurons.

Fig. 1.

Photomicrograph of a 10 μm transverse section through the cranial portion of the second cervical spinal segment. Notice complete hemisection with no spinal tissue remaining from the side of injury and the contralateral side remains intact.LF, Lateral funiculus; VF, ventral funiculus; DH, dorsal horn; VH, ventral horn; CC, central canal.

Fig. 2.

The contralateral phrenic neurogram response to hypercapnic challenge. The response to hypercapnia is presented at 1 month (A, B) and 2 months (C, D) after injury in control (▪), DHT-only (▿), C₂ hemisected (○), and DHT + C₂ hemisected (▴) rats. Aand C represent the change in respiratory rate (RR) during 5 min of hypercapnia. B and Drepresent the percentage of increase in phrenic amplitude (PA) from baseline in the contralateral phrenic nerve. DHT, 5,7-Dihydroxytryptamine. Means ± SE; *p < 0.05 relative to control; †p < 0.05 relative to DHT-only; ‡p < 0.05 relative to hemisected + DHT.

Fig. 3.

The contralateral phrenic neurogram response to hypoxic challenge. The response to hypoxia is presented at 1 month (A,B) and 2 months (C,D) after injury in control (▪), DHT-only (▿), C₂ hemisected (○), and DHT + C₂ hemisected (▴) rats. A and C represent the change in respiratory rate (RR) during 2 min of hypoxia. B andD represent the percentage of increase in phrenic amplitude (PA) from baseline in the contralateral phrenic nerve. Means ± SE; *p < 0.05 C₂hemisected group relative to control.

Fig. 4.

The hypoglossal neurogram amplitude response to hypercapnia. The amplitude response is presented at 1 month (A) and 2 months (B) after injury from both hypoglossal nerves in control rats (▪), and the ipsilateral (▿) and contralateral (▵) hypoglossal nerves in C₂ hemisected rats. HA, Hypoglossal neurogram inspiratory amplitude as a percentage of baseline. *p < 0.05 relative to control.

Fig. 5.

Integrated (top) and raw (bottom) neurograms from the contralateral (C) and ipsilateral (I) phrenic nerves in a C₂hemisected rat at 2 months after injury during normocapnia and asphyxia. Vertical scales (in millivolts) are the same for all traces. Note the presence of synchronous phasic inspiratory activity in the ipsilateral phrenic nerve (dotted arrows) and increased tonic activity during the expiratory phase in the ipsilateral but not the contralateral phrenic neurogram (solid arrows).

Fig. 6.

Photomicrographs of sections showing relative serotonin immunoreactivity from the caudal brainstem in normal (A) and DHT-only (B) rats, and the C₄ spinal segment in normal (C), DHT-only (D), C₂ hemisected (E), and DHT and C₂ hemisected (F) rats. Notice the paucity of immunoreactive cells (midline of each brainstem section) and varicosities (top aspect of each brainstem section) in rats that had received DHT. Also notice the bilateral distribution of immunoreactive varicosities in the C₄ spinal sections from the normal rat (C) and the loss of immunoreactivity on the side of injury in the C₂ hemisected rat (D). Both uninjured and injured rats that had received DHT had only sparse immunoreactive varicosities (D, F).