We have identified a novel property of the cervical spinal cord that modulates respiratory activity, both the rate and depth of breathing, such that the respiratory drive in opioid-suppressed states in mice and humans increases during spinal cord epidural stimulation. Respiratory rates are increased upon epidural stimulation of specific cervical spinal cord locations when there is spontaneous breathing, and rhythmic breathing can be generated when the respiratory state is depressed and spontaneous breathing is absent. This is an important observation because the brainstem, which contains the rhythm-generating center for respiration, is a difficult area for surgical and therapeutic access. If there are other accessible neural regions by surgery (i.e. epidural stimulation) or non-invasive means (i.e. transcutaneous stimulation), for example in the cervical spine, that influence respiration or contain their own rhythmic respiratory elements, these may represent potential therapeutic targets to reverse opioid-induced respiratory depression. Thus, we have proposed a strategy to characterize this novel cervical respiratory circuit. First, we will conduct extensive electrical mapping of the respiratory responsive elements of the cervical spinal cord in mice aided by machine learning strategies, and we will characterize the mechanistic basis for this response, the relation to opioid receptors to the respiratory response, and identity neurons responsible for the respiratory response using optogenetic techniques. Guided by the animal studies, we will then confirm the cervical respiratory responsive loci in humans. Third, we will subject the identified respiratory competent regions to increasing doses of opioid to further characterize the dose-response profile of these regions. Fourth, we will assess the feasibility and practical translation of this strategy to reverse opioid-induced respiratory depression in three patients implanted with spinal cord stimulators. These studies will provide an anatomical and electrophysiological characterization of the respiratory circuit within the cervical spine and provide practical information for the treatment of opioid-induced respiratory depression. The results obtained, especially because they are obtained in humans, will have an immediate translational impact on our understanding of the respiratory circuit that may, in turn, prevent deaths due to opioid overdose.
Opioid overdose often leads to death due to suppression of respiration; therefore, strategies to prevent respiratory depression in addition to treatment of opioid addiction are necessary to prevent deaths in the opioid-use epidemic. Respiratory suppression occurs because neurons in the brain that drive breathing are inhibited (turned off) by narcotics; thus, understanding the neural pathways for breathing will allow us to devise effective neuromodulatory strategies to treat opioid-induced respiratory depression. We have proposed to characterize a novel breathing circuit that, when activated by stimulation of the cervical spine, has the potential to be an effective treatment of opioid-induced respiratory depression, and it is our hope that findings from these studies may lead to new and more effective treatments for respiratory depression secondary to opioid overdoses.