Thoracic VGluT2⁺Spinal Interneurons Regulate Structural and Functional Plasticity of Sympathetic Networks after High-Level Spinal Cord Injury

Abstract

Traumatic spinal cord injury (SCI) above the major spinal sympathetic outflow (T6 level) disinhibits sympathetic neurons from supraspinal control, causing systems-wide "dysautonomia." We recently showed that remarkable structural remodeling and plasticity occurs within spinal sympathetic circuitry, creating abnormal sympathetic reflexes that exacerbate dysautonomia over time. As an example, thoracic VGluT2 1 spinal interneurons (SpINs) become structurally and functionally integrated with neurons that comprise the spinal-splenic sympathetic network and immunological dysfunction becomes progressively worse after SCI. To test whether the onset and progression of SCI-induced sympathetic plasticity is neuron activity dependent, we selectively inhibited (or excited) thoracic VGluT2 1 interneurons using chemogenetics. New data show that silencing VGluT2 1 interneurons in female and male mice with a T3 SCI, using hM4Di designer receptors exclusively activated by designer drugs (G i DREADDs), blocks structural plasticity and the development of dysautonomia. Specifically, silencing VGluT2 1 interneurons prevents the structural remodeling of spinal sympathetic networks that project to lymphoid and endocrine organs, reduces the frequency of spontaneous autonomic dysreflexia (AD), and reduces the severity of experimentally induced AD. Features of SCI-induced structural plasticity can be recapitulated in the intact spinal cord by activating excitatory hM3Dq-DREADDs in VGluT2 1 interneurons. Collectively, these data implicate VGluT2 1 excitatory SpINs in the onset and propagation of SCI-induced structural plasticity and dysautonomia, and reveal the potential for neuromodulation to block or reduce dysautonomia after severe high-level SCI. Significance StatementIn response to stress or dangerous stimuli, autonomic spinal neurons coordinate a "fight or flight" response marked by increased cardiac output and release of stress hormones. After a spinal cord injury (SCI), normally harmless stimuli like bladder filling can result in a "false" fight or flight response, causing pathological changes throughout the body. We show that progressive hypertension and immune suppression develop after SCI because thoracic excitatory VGluT2 1 spinal interneurons (SpINs) provoke structural remodeling in autonomic networks within below-lesion spinal levels. These pathological changes can be prevented in SCI mice or phenocopied in uninjured mice using chemogenetics to selectively manipulate activity in VGluT2 1 SpINs. Targeted neuromodulation of SpINs could prevent structural plasticity and subsequent autonomic dysfunction in people with SCI.

Chronic demyelination and myelin repair after spinal cord injury in mice: A potential link for glutamatergic axon activity

“…Two‐week‐old Vglut2‐ires‐Cre mice were prepared for surgery as indicated above and then received two single‐level laminectomies at vertebral levels T5 and T7. Using the Nanoject system (World Precision Instruments, PV830 Pneumatic PicoPump), 750 nL of Gi DREADDs (Addgene Plasmid# 50475‐AAV8) or Gq DREADDs (Addgene Plasmid# 50474‐AAV8) were loaded into a custom‐pulled glass micropipette (World Precision Instruments, 1B150F‐4) and slowly microinjected into the intermediate gray matter (0.25 mm lateral and 0.5 mm deep) at T5 and T7 (Noble et al, 2022; Ueno et al, 2016). Prior work confirmed this injection paradigm labels intraspinal neurons and not descending brainstem neurons (Ueno et al, 2016).…”

Neuroplasticity and regeneration after spinal cord injury

“…Spinal interneuron subtype-specific circuit changes have been implicated in autonomic dysfunction [34 , 35] , neuropathic pain [36] , respiratory compensation [37] , [38] , [39] , locomotor recovery (reviewed below), and sensory-evoked muscle spasms (reviewed below) following SCI. Investigations into the interneurons involved in locomotor control have particularly revealed key pathophysiologic principles underlying the adaptive and maladaptive spinal circuit changes postinjury.…”

The neuronal architecture of autonomic dysreflexia

“…The resulting isolation of neurons in the spinal cord triggers a progressive maladaptive reorganization of neuronal projections throughout the spinal cord below the injury that permits the insidious emergence of uncontrolled hypertensive episodes, known as autonomic dysreflexia [1][2][3][4][5][6][7] . The resulting isolation of neurons in the spinal cord triggers a progressive maladaptive reorganization of neuronal projections throughout the spinal cord below the injury that permits the insidious emergence of uncontrolled hypertensive episodes, known as autonomic dysreflexia [1][2][3][4][5][6][7] . The consequence of these hypertensive episodes is a daily risk of life-threatening cardiovascular events [8][9][10][11][12][13][14][15][16][17][18] .…”

“… Autonomic dysreflexia is a life-threatening medical condition characterized by episodes of uncontrolled hypertension that occur in response to sensory stimuli after spinal cord injury (SCI) 1–7 . The fragmented understanding of the mechanisms underlying autonomic dysreflexia hampers the development of therapeutic strategies to manage this condition, leaving people with SCI at daily risk of heart attack and stroke 8–18 .…”

“…Spinal cord injury (SCI) disrupts the communication between the brainstem vasomotor centers and the regions of the spinal cord that regulate hemodynamics 19 . The resulting isolation of neurons in the spinal cord triggers a progressive maladaptive reorganization of neuronal projections throughout the spinal cord below the injury that permits the insidious emergence of uncontrolled hypertensive episodes, known as autonomic dysreflexia [1][2][3][4][5][6][7] . The resulting isolation of neurons in the spinal cord triggers a progressive maladaptive reorganization of neuronal projections throughout the spinal cord below the injury that permits the insidious emergence of uncontrolled hypertensive episodes, known as autonomic dysreflexia [1][2][3][4][5][6][7] .…”