However, the μ-opioid receptor, which mediates the addictive potential of opioid agonists and their risk for respiratory depression, is only one component of the endogenous opioid system; there is growing pre-clinical and clinical evidence to suggest a rationale for targeting non-μ-opioid receptors, such as nociceptin and δ- and κ-opioid receptors [215]. Here, PNOC is linked to Respiratory insufficiency due to muscle weakness.