Experimental, mild blast-induced traumatic brain injury : focus on the monoamine and galanin systems

Abstract

Traumatic brain injury (TBI) is a common cause of mortality and morbidity among civilians and servicemen alike. The spectrum of TBIs encompasses mild to severe cases, with the predominate number of TBIs being mild (mTBI). Combat-related TBI inflicted by explosive blast (bTBI) is highly prevalent among military personnel. The injurious environment caused by explosive blast includes the high-energy shock wave that dissipates energy at the boundaries of anatomical structures with distinct acoustical impedance, and the blast wind that can propel the body resulting in acceleration/ deceleration type of injury. mTBI can also occur in the civilian population often as a result of contact sports and because of the mild acute symptoms it is often repeated. mTBI causes transient mood and cognitive changes, but it may also increase the risk for late onset chronic neurodegenerative conditions such as chronic traumatic encephalopathy. Little is known about the mechanism of mTBI and its post-sequelae. Monoamine projections from the brainstem play a key role in modulating the forebrain regions, and galanin, a neuropeptide that, in the rat, is co-localised with two of these key neurotransmitters. Dysfunctions in these systems have been associated with mood/anxiety disorders.

In this thesis we set out to examine changes in the monoamine and galanin systems following single and repeated blast exposure, in male and female rats. mTBI was induced using an experimental blast tube which uses real explosives. For one of the studies a shock tube that uses compressed air was also used to produce a blast-induced mTBI (mbTBI). The models appeared to cause mbTBI, given that no injury was observed when staining for degeneration, blood vessel damage, or disruption to the white matter tracts in either model, following single or repeated exposure.

The noradrenaline (NA) system was found to be particularly sensitive to mbTBI. The transcript levels of the biosynthetic enzyme tyrosine hydroxylase (TH) were found elevated immediately post-exposure bilaterally in the locus coeruleus (LC) in both males and females. This was concurrent with a transient increase in NA levels in a number of forebrain regions, and translated into decreased immobility in the forced swim test. This was only explored in the males and using the blast tube.

Sex-specific differences were found in the serotonin system (5-HT). Here the transcript levels of the biosynthetic enzyme tryptophan hydroxylase 2 (TPH2), were elevated across the mid/caudal-rostral dorsal raphe nucleus (DRN) in females. The elevation occurred acutely post-TBI and remained even after day (D)7, the last time point evaluated. In the males TPH2 was similarly elevated, but more modestly and only transiently. The increase was limited to only the mid/caudal part of the DRN, and by D3 TPH2 levels were similar to levels detected in sham animals. No changes in 5-HT levels were seen in the forebrain regions of male rats.

Exposure induced changes in the expression of galanin and its receptors were also examined by in situ hybridisation but in males only. Galanin mRNA levels increased bilaterally in the LC and gradually in the mid/caudal, but not in the rostral DRN and remained elevated, even at D7 in both nuclei.. However, quantitative polymerase chain reaction only confirmed the acute galanin increase in the LC, and in addition revealed galanin transcripts in the hippocampus. In terms of the galanin receptor 1-3 (GalR1-3), GalR1 was increased in the ventral periaqueductal grey and this increase persisted at D7 post-TBI, at this time-point GalR2 was decreased. In the forebrain regions GalR3 appeared to be the most dynamic receptor, decreasing in most regions immediately following TBI, and recovering on D7 post-exposure, except in the ventral hippocampus, where changes persisted. A cumulative effect of repeated exposure to blast was not apparent in the levels of the transcripts for TH, TPH2, and galanin using either the blast or shock tube in male rodents. Serum analyses revealed sex-specific differences acutely following a single blast exposure, including an increase in corticosterone and substance P in female and decreased BDNF levels in male exposed rats.

Taken together, these findings indicate a role for both the monoamine and the galanin systems following blast exposure, these changes appear robust across models and sexes (although some sex-specific differences are apparent). Hence these systems are possibly mediators of post-TBI sequelae and tartargeting the persistent dysfunctions in these systems may bring about therapeutic benefits.