Analysis of long-term gene expression in neurons of the hippocampal subfields following traumatic brain injury in rats

After experimental traumatic brain injury (TBI), widespread neuronal loss is progressive and continues in selectively vulnerable brain regions, such as the hippocampus, for months to years after the initial insult. To clarify the molecular mechanisms underlying secondary or delayed cell death in hippocampal neurons after TBI, we compared long-term changes in gene expression in the CA1, CA3 and dentate gyrus (DG) subfields of the rat hippocampus at 24 h and 3, 6, and 12 months after TBI with changes in gene expression in sham-operated rats. We used laser capture microdissection to collect several hundred hippocampal neurons from the CA1, CA3, and DG subfields and linearly amplified the nanogram samples of neuronal RNA with T7 RNA polymerase. Subsequent quantitative analysis of gene expression using ribonuclease protection assay revealed that mRNA expression of the anti-apoptotic gene, Bcl-2, and the chaperone heat shock protein 70 was significantly downregulated at 3, 6 (Bcl-2 only), and 12 months after TBI. Interestingly, the expression of the pro-apoptotic genes caspase-3 and caspase-9 was also significantly decreased at 3, 6 (caspase-9 only), and 12 months after TBI, suggesting that long-term neuronal loss after TBI is not mediated by increased expression of pro-apoptotic genes. The expression of two aging-related genes, p21 and integrin beta3 (ITbeta3), transiently increased 24 h after TBI, returned to baseline levels at 3 months and significantly decreased below sham levels at 12 months (ITbeta3 only). Expression of the gene for the antioxidant glutathione peroxidase-1 also significantly increased 6 months after TBI. These results suggest that decreased levels of neuroprotective genes may contribute to long-term neurodegeneration in animals and human patients after TBI. Conversely, long-term increases in antioxidant gene expression after TBI may be an endogenous neuroprotective response that compensates for the decrease in expression of other neuroprotective genes.

The 21-aminosteroid U-74389G reduces cerebral superoxide anion concentration following fluid percussion injury of the brain

We examined the effects of the 21-aminosteroid antioxidant U-74389G (16-desmethyl-tirilazad) on the concentration of extracellular superoxide anion following fluid percussion traumatic brain injury (TBI) measured by a cytochrome c-coated electrode and on local cerebral perfusion (CBFld) measured by laser Doppler flowmetry (LDF). U-74389G in a dose of 3 mg/kg reduced superoxide anion concentrations 60 min after TBI significantly but had no significant effect on CBFld. These results indicate that reduction of CBF after TBI can be dissociated from superoxide anion production. Persistent ischemia may limit neuroprotection efficacy and may contribute to divergent outcome results in clinical and animal trials using agents to modify reactive oxygen species.

Effects of nalmefene, CG3703, tirilazad, or dopamine on cerebral blood flow, oxygen delivery, and electroencephalographic activity after traumatic brain injury and hemorrhage

Hemorrhage after traumatic brain injury (TBI) in cats produces significant decreases in cerebral oxygen delivery (DcereO2) and electroencephalographic (EEG) activity. To determine whether effective treatments for the separate insults of TBI and hemorrhagic shock would also prove effective after the clinically relevant combination of the two, we measured the effects of a kappa-opiate antagonist (nalmefene), an inhibitor of lipid peroxidation (tirilazad), a thyrotropin-releasing hormone analog (CG3703), a clinically useful pressor agent (dopamine) or a saline placebo on cerebral blood flow (CBF), and EEG activity after TBI and mild hemorrhagic hypotension. Cats (n = 40, 8 per group) were anesthetized with 1.6% isoflurane in N2O:O2 (70:30) and prepared for fluid-percussion TBI and microsphere measurements of CBF. Cats were randomized to receive nalmefene (1 mg/kg), tirilazad (5 mg/kg), CG3703 (2 mg/kg), dopamine (20 microg x kg(-1) x min[-1]) or a saline placebo (2 ml, 0.9% NaCl). Animals were injured (2.2 atm), hemorrhaged to 70% of preinjury blood volume, treated as just described and resuscitated with a volume of 10% hydroxyethyl starch equal to shed blood. CBF was determined and EEG activity recorded before injury, after hemorrhage, and 0, 60, and 120 min after resuscitation (R0, R60, and R120). CBF increased significantly after resuscitation (R0) in the nalmefene- and CG3703-treated groups. CBF did not differ significantly from baseline in any group at R60 or R120. DcereO2 was significantly less than baseline in the saline-, dopamine-, and tirilazad-treated groups at R60 and in the dopamine-, tirilazad-, and CG3703-treated groups at R120. EEG activity remained unchanged in the nalmefene-treated group but deteriorated significantly at R60 or R120 compared to baseline in the other groups. Nalmefene and CG3703 preserved the hyperemic response to hemodilution (otherwise antagonized by TBI), and nalmefene prevented the deterioration in DcereO2 and EEG activity that occurs after TBI and hemorrhage.

NG-nitro-L-arginine methyl ester modifies the input function measured by dynamic susceptibility contrast magnetic resonance imaging

In rat brain dynamic susceptibility contrast magnetic resonance (MR) images, vessels visible on the same scan plane as the brain tissue were used to measure the characteristics of the input function of the MR contrast agent gadopentetate dimeglumine. MR images were acquired 30 and 60 minutes after intravenous injections of 3 mg/kg and 15 mg/kg NG-Nitro-L-arginine methyl ester (L-NAME) (n = 9). The time of arrival (TOA) and the mean transit time corrected for TOA of the input function were increased by 3 mg/kg or 15 mg/kg L-NAME. The area of the input function was increased by 15 mg/kg L-NAME. In two animals, similar modifications of the input function induced by 20 mg/kg L-NAME were reversed by infusion of sodium nitroprusside. In two other animals, MABP was increased by phenylephrine to a similar extent as in L-NAME experiments, but did not induce the same modifications of the input function, showing that the action of L-NAME on the input function was not simply caused by an effect on MABP. These results show that the input function can be significantly altered by manipulations widely used in cerebrovascular studies. These input function changes have important implications for calculation of cerebral blood flow.

Triiodothyronine (T3) antagonizes adverse effects of high circulating reverse-T3 (rT3) during hemorrhagic shock

To examine whether triiodothyronine (T3) could counteract the lethal effect of exogenous reverse T3 (rT3) in hemorrhagic shock, 21 anesthetized, heparinized mongrel dogs were given 15 micrograms/kg of rT3 IV. Thirty minutes later, the dogs were bled rapidly into a reservoir to achieve and maintain a mean arterial pressure of 40 mm Hg. After 60 minutes at 40 mm Hg (compensated shock), the reservoir line was clamped for 30 minutes (uncompensated shock). The shed blood was then reinfused over 30 minutes, and the dogs were monitored for an additional 60 minutes. At the start of uncompensated shock, 11 dogs were given at least 15 micrograms/kg of T3 IV, and 10 animals received saline. Before T3 treatment, there were no significant intergroup differences in the measured hemodynamic and blood gas variables. In the untreated group, 8 of 10 dogs (80%) died during uncompensated shock, in comparison to 3 of 11 dogs (27%) that received T3 (P less than 0.01). Long-term survival in the T3 group was 5/11 (45%), significantly higher than that (1/10, 10%) in the untreated group (P less than 0.05). These results, interpreted in relationship to previous studies, suggest that the therapeutic efficacy of T3 in canine hemorrhagic shock may be related to antagonism of adverse effects of endogenous rT3.

The effects of traumatic brain injury on regional cerebral blood flow in rats

Alterations in cerebral blood flow (CBF) are among the most important secondary pathophysiologic consequences of traumatic brain injury. The present study compared CBF in control rats (n = 20) and in rats that received a calibrated experimental traumatic brain injury (n = 17). The traumatized rats were anesthetized with ketamine (25 mg/kg) and xylazine (10 mg/kg), and prepared for fluid percussion injury (FPI). Twenty-four hours later, the rats were anesthetized with 1% halothane in nitrous oxide-oxygen (70:30) and the left atrium was catheterized via a thoroacotomy. The atrial cannula was used to inject 15 microns radiolabeled microspheres to measure CBF. Following surgery, the concentration of halothane was reduced to 0.5% and the rats were paralyzed with pancuronium bromide (0.1 mg/kg). Thirty minutes later, baseline microsphere determinations were made, and the rats were injured (2.47 +/- 0.08 atm). Each rat received additional injections of microspheres at two of the following four times (T): 5, 15, 30, and 60 min after the brain injury. The procedures for the control group rats were the same as described above except that the rats were not subjected to the craniotomy and the FPI. The traumatized group exhibited heterogeneous decreases in CBF following trauma. Global CBF in this group was 78% (p less than 0.01), 64% (p less than 0.05), 52% (p less than 0.001) of those in the control group at T5, 15, 30, and 60, respectively. In rats, the most prominent cerebral circulatory changes following fluid percussion injury were early reductions of CBF and an increasingly heterogeneous CBF pattern. Hemorrhage, edema, and elevated prostagandin levels are mechanisms that may contribute to these changes.

Experimental traumatic brain injury elevates brain prostaglandin E2 and thromboxane B2 levels in rats

Prostaglandin E2 (PGE2) and thromboxane B2 (TxB2) levels were measured in rats following experimental traumatic brain injury. Rats (n = 36) were prepared for fluid percussion brain injury under pentobarbital anesthesia. Twenty-four hours later, rats were lightly anesthetized using methoxyflurane, injured (2.3 atm), and killed 5 or 15 min later. Twelve of the rats died before and are not included in the analyses. The following groups were used for data analysis: group I (n = 6) were sham-injured rats prepared for injury but not injured: group II (n = 6) were injured and killed 5 min later; group III (n = 12) were injured and killed 15 min posttrauma. Thirty seconds prior to sacrifice by decapitation into liquid nitrogen, all rats were injected with indomethacin (3 mg/kg, intravenously [IV]) to prevent postmortem PG synthesis. After sacrifice, brains were removed, weighed, and homogenized in a small quantity of phosphate buffer with indomethacin (50 micrograms/ml). PGE2 and TxB2 levels were determined using double-label radioimmunoassays. Posttraumatic convulsions were observed in 5 of 12 rats in group III and these rats were analyzed separately. PGE2 and TxB2 levels increased significantly (p less than 0.05) in both hemisphere and brainstem 5 min posttrauma. Fifteen minutes after injury, both PGE2 and TxB2 levels remained elevated but the levels were lower than at 5 min in the rats that did not exhibit posttraumatic seizures. This decrease in PG levels at 15 min was not observed in the rats that had seizures after injury and both PGE2 and TxB2 levels remained high in hemispheres and brainstem. Thus, fluid percussion brain injury results in substantial elevations in PGE2 and TxB2 levels and posttraumatic seizures exacerbate the observed increases.

Combined pretrauma scopolamine and phencyclidine attenuate posttraumatic increased sensitivity to delayed secondary ischemia

Fasted Wistar rats were given a mild level of traumatic brain injury (TBI) and then subjected to 6 min of transient forebrain ischemia 24 h posttrauma. One group was given simultaneous 1 mg/kg scopolamine and 4 mg/kg phencyclidine intraperitoneally (IP) 15 min before trauma and another group an equal volume of plasmalyte A solution. After 7 days of postinjury survival, placebo-treated rats demonstrated increased posttraumatic vulnerability to secondary ischemic CA1 neuronal death even 24 h after trauma. This finding confirmed that increased posttraumatic ischemic vulnerability persists for at least 24 h even following mild trauma. Combined muscarinic receptor and N-methyl-D-aspartate (NMDA) receptor coupled ion channel blockade given and present during the mild TBI statistically attenuated this enhanced secondary ischemic CA1 neuronal death and thus posttraumatic increased ischemic vulnerability. Placebo-treated rats had 335.3 +/- 93.6 CA1 neurons/10(6) microns 2 and drug-treated rats had 844.8 +/- 184.9 CA1 neurons/10(6) microns 2. This result suggests that muscarinic and/or NMDA receptor-mediated events confined to TBI and the early posttraumatic period are in part responsible for the phenomenon of increased posttraumatic ischemic vulnerability.