Using chemogenetics and novel tools to uncover neural circuit and behavioral changes after spinal cord injury

Abstract

Spinal cord injury (SCI) results in persistent neurological deficits and significant long-term disability. Stimulation of peripheral afferents by epidural electrical stimulation (EES) has been reported to reduce spasticity by reorganizing spared and disrupted descending pathways and local circuits. However, a current barrier to the field is that the plasticity mechanisms that underly improved recovery is unknown. Using the power of hM3Dq Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), we aim to accelerate the dissection of the mechanisms underlying enhanced recovery. In these studies, we identified the effect of clozapine-N-oxide (CNO) on the H-reflex of naïve animals; investigated the baseline influence of hM3Dq DREADDs in peripheral afferents in the intact animal using a novel behavioral tool, an addition of angled rungs to the horizontal ladder walking task; and began to uncover the neural and behavioral changes that accompany hM3Dq DREADDs activation in peripheral afferents after SCI. We observed no significant differences in the H-reflex with 4 mg/kg dosage of CNO administration (pre-CNO vs. CNO-active: p=0.82; CNO-active vs. CNO wash-out: p=0.98; n=6). On our novel ladder, we found significant differences in correct hind paw placement (p=0.0002, n=7) and incorrect placement (p=0.01) when DREADDs were activated with CNO (4 mg/kg). In our SCI study, we report that acute and chronic DREADDs activation may activate extensor muscles about the hip (32 cm/s: p=0.047; controls: n=6; DREADDs: n=8 and hereafter unless otherwise stated) as well as induce sprouting and synaptogenesis within motor pools and Clarke’s column in the lumbar spinal cord (motor pool: p=0.00053; Clarke’s column: p=0.021; controls: n=4; DREADDs: n=6). This muscle recruitment may have long-term effects such as increased hindquarter heights (e.g., 16 cm/s: p=0.017) and more frequent hindlimb coordination (p=0.002). Results from this study suggest hM3Dq DREADDs may have the potential to recapitulate EES-activation of afferents as well as provide a platform with which to functionally map changes that occur both within targeted afferents and second order neurons they effect. Future work, such as using C-Fos to examine and map changes in interneuronal networks, could seek to more directly tie changes in kinematics to observed changes in plasticity.