Chemogenetic afferent modulation to understand spinal cord circuit function and plasticity post injury

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PROJECT SUMMARY Spinal cord injury (SCI) causes life-long neurological impairment, and there is currently no effective treatment. The premise of this proposal is recent work demonstrating that afferent stimulation paired with treadmill training can enhance standing, stepping, and volitional control in humans and animal models. Therefore, it is critically important to understand the mechanisms by which afferent stimulation drives motor improvement. Tools that can identify which afferents are necessary and sufficient to enhance recovery, and that can facilitate characterization of the helpful neural plasticity, are urgently needed. Our long-term goal is to develop approaches for selective afferent modulation, and apply them to the dissection of the mechanisms underlying recovery from SCI. The objective of this grant is to identify which sets of afferents are important for recovery and how spinal circuits change to facilitate it. To achieve selective modulation of afferents and enable genetic tracing we will use Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) that can modulate excitability in specific populations of neurons. To accurately quantify improvement, we will use Deep Learning to analyze large kinematic data sets. Our preliminary data shows strong expression of DREADDs in large diameter DRG neurons, that their activation by CNO can excite or inhibit the H-reflex, and that activation of excitatory DREADDs during treadmill training post-SCI improves stepping. Our main hypothesis is that activation of large afferents by the excitatory DREADD (hM3Dq) during treadmill training will enhance recovery, whereas inhibitory DREADDs (hM4Di) will suppress recovery. Four sub- hypotheses will test whether recovery is mediated by increased afferent projection onto 1) motor neurons, or 2) inhibitory interneurons; or by sprouting of 3) reticulospinal and 4) propriospinal circuits. Our Specific Aims are to determine whether selective expression of DREADDs in (Aim 1) all large diameter (proprioceptive and tactile) neurons and (Aim 2) large proprioceptive afferents only can enhance recovery. The rationale for these aims is that afferent stimulation is hypothesized to work through selective excitation of large diameter sensory afferents (LDSA) that both drive motor pools locally and facilitate proprio- and surpraspinal input. To date, it has not been possible to definitively determine which afferents were recruited after electrical stimulation, or to select between afferents of similar diameter. The significance of this work lies in determining whether recovery is mediated exclusively by proprioceptive axons or a combination of proprioceptive and tactile afferents, and uncovering the mechanisms of functional plasticity in the spinal cord.