Fellowship for Felicia Michael: Chemogenetic Silencing of Interneurons to Modulate Autonomic Dysreflexia

Traumatic spinal cord injury (SCI) leads to both motor and sensory dysfunction, but may also disrupt central autonomic pathways. Severe SCI to the upper thoracic cord often results in autonomic dysreflexia (AD), a condition that manifests as acute, episodic hypertension with concurrent bradycardia. AD is instigated by massive discharge of sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) that are reflexively activated by noxious stimuli beneath the injury, such as bladder or bowel distention. As a result of this burst in sympathetic outflow, widespread vasoconstriction occurs and produces a sudden and sharp increase in systemic blood pressure. Because the descending modulatory pathways that normally regulate sympathetic tone are interrupted after injury, this hypertensive state continues until the precipitating stimulus is removed. Left un-treated, AD can lead to cerebral hemorrhage, myocardial infarction, and even death. Although the loss of supraspinal modulation is a key component in the pathophysiology of AD, reorganization of spinal circuitry after injury also correlates with the development and severity of this insidious condition. Our lab has previously demonstrated two components of this maladaptive remodeling that may form the anatomical substrates linking sensory input to activation of cardiovascular SPN: 1) increased sprouting of CGRP+ primary afferents into the dorsal horn, and 2) sprouting of ascending propriospinal neurons that are believed to relay noxious sensory information rostrally to SPN in the thoracic cord. While earlier investigations show that inhibition of primary afferent sprouting reduces the severity of AD, it remains unknown whether a similar effect can be achieved by silencing ascending propriospinal neurons. In order to address this issue, we propose to reversibly silence ascending propriospinal neurons using viral vectors that deliver designer receptors exclusively activated by designer drugs (DREADDs), while simultaneously assessing the severity of noxious colorectal distention (CRD) induced hypertension in rats with complete SCI. DREADD technology is a powerful investigative tool that allows for precise spatiotemporal manipulation of discrete neural populations. We have access to a vector developed at Temple University that delivers the inhibitory hM4D(Gi)-mCherry DREADD through highly efficient retrograde transport. Silencing is then achieved through systemic (i.p.) administration of clozapine-n-oxide (CNO). Notably, CNO is an otherwise inert ligand that specifically stimulates the hM4D(Gi)-mCherry DREADD receptor, which subsequently hyperpolarizes infected neurons to silence them within 30 minutes of CNO administration (i.p.), and neurons fully recover within 12 hours. With this approach, we will test the hypothesis that silencing of ascending propriospinal neurons will mitigate AD in response to noxious CRD. Specific Aim 1: Dual vector spatiotemporal targeting of APN using retrograde HiRet-TRE-eGFP combined with localized AAV TetOn transduction. The goal of this aim is to infect APNs with two vectors i.e. retrogradely with Hiret-tre-eGFP at the thoracic IML and locally at the lumbosacral DGC, so that only cells with both vectors can express eGFP. Specific Aim 2: Use reversible chemogenetic silencing to characterize the relative contribution of APN fiber sprouting to the severity of AD. If successful, this investigation will provide direct evidence supporting an essential role of propriospinal neurons in the pathophysiology of AD. It also holds clinical relevancy, as it may one day be possible to treat AD in the chronic SCI population by specifically targeting discrete anatomical pathways.