UCLA

Brainstem and spinal cord neurons generate patterned activity necessary for the execution of respiration. The spinal circuit integrates sensory, propriospinal, and supraspinal inputs with endogenous rhythmic neuronal activity to coordinate excitation of phrenic motor neurons and other muscles with respiratory-related activity. In Chapter 2, we tested the hypothesis that dorsal cervical epidural electrical stimulation (CEES) would increase respiratory activity in anesthetized rats. Respiratory frequency and minute volume were significantly increased when CEES was applied to the cervical spinal cord between C2 and C6. We injected pseudorabies virus into the diaphragm to label respiratory-related neurons in the spine and brainstem and elicited c-Fos activity during CEES. We identified neurons in the dorsal horn of the cervical spine in which c-Fos and pseudorabies were colocalized, and these neurons also expressed somatostatin (SST). Using dual viral transfection to express the inhibitory Designer Receptor Exclusively Activated by Designer Drugs (DREADD), hM4D(Gi), selectively in SST-positive cells, we were able to inhibit SST-expressing neurons by administration of Clozapine N-oxide (CNO). The respiratory excitation elicited by CEES was diminished in the presence of CNO. We conclude that dorsal cervical epidural stimulation activated SST-expressing neurons in the cervical spinal cord, likely interneurons that communicated with more rostral elements of the respiratory pattern generating network to effect the changes in tidal volume and frequency that were observed. Respiratory complications are a leading cause of morbidity and mortality following cervical spinal cord injury. Mechanical ventilation is the main intervention that itself carries negative side-effects. Novel forms of neuromodulation have been explored to enhance respiratory activity in the injured spinal cord; yet, are often difficult to clinically execute. In Chapter 3, we explored dorsal CEES to increase diaphragm activity in anesthetized rats that underwent a C2 hemisection causing respiratory deficits. CEES increased the probability of rhythmic bursting in the once paralyzed (ipsilateral) diaphragm. The activity observed was significantly increased compared to sham trials. We found increases in rhythmic diaphragm activity waned after stimulation ceased. These results demonstrate that dorsal CEES can enhance respiratory activity after high cervical spinal cord injury. Further exploration could lead to a novel therapy for respiratory deficits after spinal cord injury.