Post-Injury Administration of an NK1 Receptor Antagonist Ameliorates the Neuroinflammatory Response in a Rodent Model of Traumatic Brain Injury

Alan Nimmo1, Konrad Reardon1, Robert Vink2. 1James Cook University, Cairns, Queensland, Australia, 2University of Adelaide, Adelaide, South Australia, Australia

Introduction: It is now realised that inflammation plays a key role in many CNS pathologies. In the CNS neuroinflammation can be mediated via glial cells, but can also be associated with compromised blood-brain barrier (BBB) function that can follow acute insults, such as a traumatic injury (TBI). Using an animal model of TBI, we have demonstrated that selective NK1 antagonists are able to restore BBB function following injury (Donkin et al., 2009).The aim of the present study was to use a rodent model of TBI to assess whether post-injury administration of an NK1 receptor antagonist can ameliorate the neuroinflammatory response associated with injury. Raised serum IL-6 levels are recognised as a biomarker of neuroinflammation associated with TBI, particularly where intracranial pressure is raised (Hergenroeder et al., 2010). In this study we assessed the effect of post-injury administration of an NK1 antagonist on serum IL-6 levels, as well as on the levels of IL-1β, IL-6 and TNFα mRNA expression in the cerebral cortex.

Methods: Adult male Sprague-Dawley rats (350-450gms) were used in this study (n=20). Animals were anesthetized with isoflurane, and subject to either a sham injury (n=10) or a severe TBI (n=10). Severe TBI was induced using the impact-acceleration model of diffuse axonal injury as previously described (Foda,& Marmarou,1994). Animals in the injury and sham-injury groups were randomly assigned to receive either post-injury treatment with an NK1 antagonist (N-acetyl tryptophan; 2.4mg/kg) or drug vehicle (n=5/group). Animals were killed by decapitation 6 hours after injury, and serum samples collected. Brains were excised, and cortical tissues were snap-frozen in liquid N2.

Serum levels of IL-6 were determined by ELISA using a rat-specific monoclonal antibody. Total mRNA was extracted from cortical tissue samples and purified. TaqMan gene expression assays for IL-6, IL-1β and TNF-α were used, and quantitation relative to rpl32 expression was achieved by the comparative CT method. Statistical significance was determined using a one-way analysis of variance (ANOVA).

Results: In sham-injured animals, serum IL-6 levels were 103.7 ± 14.7pg/ml. Animals subject to TBI and treatment with drug vehicle showed a significant rise in IL-6 levels (289.3 ± 75.9pg/ml; p < 0.01). However, in animals treated with an NK1 antagonist 30mins after injury, there was no significant rise in serum IL-6 levels (127.6 ± 11.7pg/ml). RT-PCR revealed a significant increase in the mRNA levels of the inflammatory mediators IL-6, IL-1β and TNF-α, with a 134-fold, 32-fold and 45-fold increase respectively (p < 0.01). In animals treated with the NK1 antagonist following injury, there was no significant increase in the mRNA levels of any of these inflammatory mediators.

Conclusions: The results of this study indicate that post-injury administration of an NK1 antagonist may ameliorate the neuroinflammatory response following TBI, presumably through its restoration of BBB integrity.

Donkin et al., J Cereb Blood Flow Metab. 29:1388-98, 2009.

Hergenroeder et al. Journal of Neuroinflammation 7:19, 2010.

Foda,& Marmarou, Neurosurg.80: 301-13, 1994.