Mechanisms of Functional Recovery After Partial Nerve Injury

Veterans Health Administration, Decatur PA

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Abstract

DESCRIPTION (provided by applicant): Nerve injury is a common cause of permanent neurological disability in the combat military and veterans. It is well-known that victims of nerve injury follow highly variable clinical trajectories. Some patients make a full recovery, yet a very large number fail to regain normal function. Unfortunately, there are no treatments that improve chances to restore neurological function. A great deal of work has focused on encouraging axonal regrowth after nerve injury. In contrast, very little is known about how the nervous system functionally compensates after nerve damage; functional compensation therefore represents an underappreciated pathway to reducing disability from nerve injuries. We propose studies in animal models to demonstrate the efficacy of hormone receptor modulation for enhancing functional recovery without axonal growth. In preliminary work, we demonstrate a model system to study functional neuronal recovery without axon regeneration. Normally, the bilateral superior cervical ganglia (SCG) both innervate the pineal gland to stimulate nightly melatonin synthesis. In Fischer 344 (F344) rats, when we remove one SCG (1xSCGx), melatonin falls to 50% of basal levels; the day after this drop, melatonin spontaneously returns to 100% of basal levels, indicating functional recovery of the neural circuit occurs in the absence of axon regrowth. In Brown Norway (BN) rats, which carry a gain of function mutation in the mineralocorticoid receptor (MR), melatonin levels fall immediately and fail to make improvements. When BN rats are treated with corticosteroids or with antagonists of MR, rats make a rapid and permanent recovery in melatonin synthesis. These studies indicate that a neural circuit can spontaneously recover normal function without axon growth, and that steroids that act on MR or the glucocorticoid receptor (GR) are capable of enhancing functional neurological recovery. In this proposal, we will test the core hypothesis that MR antagonists and GR agonists promote function recovery when administered during specific periods after neuronal injury; we further hypothesize that MR and GR modulation after neuronal injury modifies the inflammatory response that is required for optimal compensation to neurological injury. The following specific aims will be tested. In Aim 1, we will test the time course of administration of MR antagonists for enhancing neurological recovery after partial nerve injury. In Aim 2, we will test the time course of administration of GR agonists for enhancing neurological recovery after partial nerve injury. Finally, in Aim 3, we will test the role of inflammatory cytokines on functional recovery from partial nerve injury. We hypothesize that a cascade of specific cytokines results from denervation of target tissue and that protective cytokine responses are driven by steroid hormone regulated transcription and correlate with long term functional recovery. These experiments promise to define time windows for potential clinical treatment of nerve injury and will identify molecular markers and signaling pathways that promote functional recovery after peripheral nerve injury.