Objective: To determine the mechanism responsible for ghrelin's neuroprotective effects after traumatic brain injury (TBI) and hemorrhagic shock.
Background: Ghrelin, a gastrointestinal hormone, has been demonstrated to possess multiple functions, including upregulation of uncoupling protein 2 (UCP2) and stimulation of the vagus nerve. Recent evidence has indicated that ghrelin is neuroprotective. We, therefore, hypothesized that ghrelin protects rats against TBI and hemorrhagic shock through upregulation of UCP2, involving stimulation of the vagus nerve.
Methods: Brain injury was induced by dropping a 450 g of weight from 1.5 m onto a steel helmet attached to the skull of male adult rats. Immediately after TBI, a midline laparotomy was performed, and both lumbar veins were isolated and severed at the junction with the vena cava. The abdomen was kept open for 20 minutes. At 45 minutes after TBI and uncontrolled hemorrhage (UH), ghrelin (4, 8, or 16 nmol/rat) or 1 mL of normal saline (vehicle) was intravenously administered. The Neurological Severity Scale (NSS), morphological alterations and β-amyloid precursor protein expression in the brain, systemic organ injury markers (ie, alanine aminotransferase, aspartate aminotransferase, and lactate), and UCP2 expression in the cortex were measured. To determine whether the protective effect of ghrelin is mediated through upregulation of UCP2, genipin, a specific UCP2 antagonist, was administered intravenously before the injection of ghrelin in animals with TBI and UH. The role of the vagus nerve was assessed by performing vagotomy immediately before ghrelin administration.
Results: Ghrelin attenuated brain injury and facilitated functional recovery after TBI and UH. Ghrelin increased UCP2 expression in the cortex, and administration of genipin abolished ghrelin's protection after TBI and UH. Furthermore, vagotomy prevented the beneficial effects of ghrelin and eliminated ghrelin-induced UCP2 upregulation after TBI and UH.
Conclusions: The protective effects of ghrelin after TBI and UH seem to be related to upregulation of UCP2 expression in the brain and requiring the intact vagus nerve.