Serotonergic Modulation of Spinal Circuitry Restores Motor Function after Chronic Spinal Cord Injury

Abstract Electrical stimulation of the nervous system has been employed to enhance the recovery of motor function produced by use-dependent rehabilitation, which is the current gold standard of treatment, following spinal cord injury. However, the therapeutic effects almost always rely on the sustained activation of muscles or neurons, making the benefits largely contingent on continued delivery of stimulation. In the present study, we describe a neuromodulatory intervention that combined intraspinal delivery of serotonergic agonists with use-dependent rehabilitation to restore motor function after a chronic moderate-to-severe cervical contusion in rats that produces impairments in upper-limb movements and dexterity. We show that targeted delivery of quipazine, a broad-spectrum serotonergic agonist, caudal to the lesion increased the effectiveness of physical rehabilitation, leading to substantially improved motor-recovery outcomes in severely-injured, but not moderately-injured, animals. Delivery of quipazine significantly augmented recovery of skilled reach and grasp movements after a severe injury, but moderately-injured animals received no additional benefit from quipazine over physical rehabilitation alone. This difference was perhaps due to a greater loss of serotonin after a severe injury and a resulting environment in which exogenously-applied serotonin can improve circuit function. Our experiments highlight an important role for serotonin in restoration of motor function that is dependent on the severity of the spinal cord injury. They also allude to a potential role for residual serotonin as a biomarker of injury severity. Remarkably, quipazine-mediated behavioral improvements persisted for weeks after termination of neuromodulator delivery, signaling repair of severely-damaged adult spinal circuitry that drives lasting motor recovery. Significance Statement We describe a neuromodulatory intervention that combined intraspinal delivery of serotonergic agonists with use-dependent physical rehabilitation, which is the current standard of treatment, to promote motor recovery after a chronic moderate-to-severe spinal-contusion injury. Our results show that targeted delivery of serotonergic agonists caudal to the lesion increased the effectiveness of use-dependent rehabilitation, leading to substantially improved motor-recovery outcomes in severely-injured, but not moderately-injured, animals. Notably, therapeutic gains persisted for weeks after termination of neuromodulator delivery—a finding that is both unique and clinically relevant—signaling plasticity induction and repair in chronically-damaged adult spinal circuitry. Our experiments provide important insights into serotonergic modulation of spinal circuitry and highlight a potential role for residual serotonin as a neurochemical biomarker of injury severity.