Selective spinal interneuron activation enhances the hypercapnic ventilatory response in chronic spinal cord injury

The ability to sense and regulate breathing in response to changes in carbon dioxide (CO₂) levels is vital for maintaining respiratory and systemic homeostasis. Impaired CO₂ chemosensitivity is a common feature of several debilitating conditions, including chronic obstructive pulmonary disease (COPD), central hypoventilation syndromes, and cervical spinal cord injury (cSCI) and contributes to significant respiratory complications such as reduced ventilatory drive, poor gas exchange, and diminished adaptability to metabolic demands. Chronic cSCI disrupts descending respiratory pathways, leading to impaired ventilatory function and, consequently disrupted CO₂ regulation leading to reliance on mechanical ventilation. Currently there are few effective treatments for spinal cord injury induced respiratory dysfunction, particularly in the chronic phase of injury where additional recovery is difficult to elicit. In this study, we investigated whether stimulation of excitatory spinal interneurons (eINs) could enhance respiratory responses to hypercapnia in health and during chronic cSCI. Mid-cervical eINs located in the intermediate lamina were chemogenetically activated via Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), and breathing was assessed during CO₂ challenges in awake, unrestrained animals using whole-body plethysmography. Acute stimulation of cervical eINs increased breathing in response to acute respiratory challenges in animals at 8-and 12-weeks post-traumatic cSCI and in uninjured animals. Importantly, this respiratory enhancement occurred without observable affects in non-respiratory motor functions, such as forelimb grip strength, suggesting that the effects of eIN activation were specific to respiratory circuits. Together these findings identify cervical eINs as a promising neuromodulatory target and a promising therapeutic avenue for improving ventilatory function in chronic SCI. More broadly, this approach may offer therapeutic potential for other respiratory conditions characterized by diminished chemosensitivity and ventilatory control, providing a foundation for future translational strategies in respiratory neurorehabilitation.