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Dianne M. O'Dell and Robert J. Hamm

✓ The present experiment examined the efficacy of postinjury administration of MDL 26,479 (Suritozole), a negative modulator at the γ-aminobutyric acidA (GABAA) receptor that enhances cholinergic function, in attenuating spatial memory deficits after traumatic brain injury in the rat. Two experiments were performed. In the delayed-dosing experiment, rats received a moderate level (2.1 atm) of fluid-percussion brain injury and were tested in the Morris water maze 11 to 15 days following injury. These rats were injected with either 5 mg/kg (eight rats) or 10 mg/kg (eight rats) of MDL 26,479 60 minutes before each water maze test. Additional rats were injured and treated with saline (eight rats) or were surgically prepared but not injured (eight rats). In the second experiment, an early postinjury dosing procedure was followed. Rats were injured in the same manner but drug treatment began 24 hours after injury and continued daily through Day 15. Results indicated that the rats in the delayed chronic dosing regimen did not differ from the injured, saline-treated rats in their latency to reach the goal platform (p > 0.05). However, those treated chronically beginning 24 hours after injury had significantly shorter latencies than the injured, saline-treated rats (p < 0.05). These results suggest that administration of agents that enhance cholinergic function may be an appropriate strategy for promoting cognitive recovery when given after traumatic brain injury. Furthermore, prolonged treatment may be necessary to elicit beneficial effects.

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Robert J. Hamm, Brian R. Pike, Dianne M. O'Dell and Bruce G. Lyeth

✓ The authors have examined the effect of experimental traumatic brain injury on the amnesia produced by the N-methyl-D-aspartate (NMDA) antagonist MK-801. Rats were either subjected to a moderate level of fluid-percussion injury or prepared for injury but not injured (“sham injury”). Nine days following injury or sham injury, the rats were injected either with saline (sham/saline group, nine rats; injured/saline group, nine rats) or with 0.1 mg/kg of MK-801 (sham/MK-801 group, nine rats; injured/MK-801 group, eight rats) 30 minutes before being trained on a passive-avoidance task. Twenty-four hours later, the rats were tested for retention of the passive-avoidance task. Results revealed that the low dose of MK-801 did not significantly affect retention of the passive-avoidance task in the sham-injured group. In injured animals, administration of MK-801 produced a profound amnesia in contrast to the sham-injured animals treated with MK-801 and the injured animals treated with saline. To further investigate this enhanced sensitivity to the amnesic effects of MK-801 exhibited by the injured animals, nine injured and eight sham-injured rats were injected with 0.3 mg/kg of MK-801 15 minutes before injury. Results indicated that the animals treated with MK-801 before injury did not significantly differ from the sham-injured animals in retention of the passive-avoidance task. In addition, test results in the animals treated with MK-801 before injury and reinjected with MK-801 before passive-avoidance testing did not differ from those in untreated injured animals reinjected with saline before passive-avoidance testing. These findings indicate that MK-801 treatment before injury prevented the enhanced sensitivity to MK-801-induced amnesia that follows traumatic brain injury.

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Robert J. Hamm, Larry W. Jenkins, Bruce G. Lyeth, Daphne M. White-Gbadebo and Ronald L. Hayes

✓ Age of the patient is one of the most important predictors of outcome following human traumatic brain injury. This study employs the fluid-percussion model to investigate the effects of aging on outcome following traumatic brain injury in rats. The results revealed that there was an age-associated increase in mortality rate following both low (1.7 to 1.8 atm) and moderate (2.00 to 2.25 atm) levels of traumatic brain injury. Age-related changes in systemic physiological, neurological, and histopathological indexes of brain injury were also examined following a low level of traumatic brain injury. Traumatic brain injury produced equivalent acute hypertension and increased plasma glucose levels in both young adult and aging rats. Injury produced an acute increase in heart rate in the young adult rat group, while the heart rate decreased in the aged rats. At low levels of brain injury, no significant gross histopathological alterations were produced in either age group. Neurological outcome was assessed by measuring the duration of suppression of a number of nonpostural and postural reflexes and more complex somatomotor functions (righting, escape, head support). Except for head support, there was a significant age-related increase in the duration of the suppression of these reflexes following brain injury. These data demonstrate that aging is associated with an increased mortality rate and greater acute neurological deficits following traumatic brain injury. These data also demonstrate the usefulness of the fluid-percussion model for studying the mechanisms responsible for the age-related increase in vulnerability to brain injury.

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Ji Y. Jiang, Bruce G. Lyeth, Guy L. Clifton, Larry W. Jenkins, Robert J. Hamm and Ronald L. Hayes

✓ Recent work has shown that mild to moderate levels of hypothermia may profoundly reduce the histological and biochemical sequelae of cerebral ischemic injury. In the present study, the authors examined the effect of fluid-percussion injury on brain temperature in anesthetized rats and the effect of anesthesia on brain temperature in uninjured rats. The relationship between the brain, rectal, and temporalis muscle temperatures during normothermia, hypothermia, and hyperthermia was studied following a moderate magnitude of fluid-percussion brain injury (2.10 to 2.25 atmospheres) in rats. The results showed that mean brain temperature in 10 anesthetized injured rats, in 21 anesthetized uninjured rats, and in 10 unanesthetized uninjured rats was a mean (± standard error of the mean) of 36.04° ± 0.20°C, 36.30° ± 0.08°C, and 37.95° ± 0.09°C, respectively. There was no significant difference in temperature under general anesthesia between injured and uninjured rats (p > 0.05). In the absence of brain injury, mean brain temperature was significantly lower in anesthetized rats than in unanesthetized rats (p < 0.001). In anesthetized brain-injured rats, temporalis muscle temperature correlated well with brain temperature over a 30° to 40°C range, even when brain temperature was rapidly changed during induction of hypothermia or hyperthermia (r = 0.9986, p < 0.0001). In contrast, rectal temperature varied inconsistently from brain temperature. These observations indicated that: 1) brain injury itself does not influence brain temperature in this model; 2) anesthesia alone decreases brain temperature to levels producing cerebral protection in this model; and 3) external monitoring of temporalis muscle temperature can provide a reliable indirect measure of brain temperature in the course of experimental brain injury. The authors believe that it is essential to monitor or control brain temperature in studies of experimental brain injury.

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C. Edward Dixon, Bruce G. Lyeth, John T. Povlishock, Robert L. Findling, Robert J. Hamm, Anthony Marmarou, Harold F. Young and Ronald L. Hayes

✓ Fluid percussion models produce brain injury by rapidly injecting fluid volumes into the cranial cavity. The authors have systematically examined the effects of varying magnitudes of fluid percussion injury in the rat on neurological, systemic physiological, and histopathological changes. Acute neurological experiments showed that fluid percussion injury in 53 rats produced either irreversible apnea and death or transient apnea (lasting 54 seconds or less) and reversible suppression of postural and nonpostural function (lasting 60 minutes or less). As the magnitude if injury increased, the mortality rate and the duration of suppression of somatomotor reflexes increased. Unlike other rat models in which concussive brain injury is produced by impact, convulsions were observed in only 13% of survivors. Transient apnea was probably not associated with a significant hypoxic insult to animals that survived. Ten rats that sustained a moderate magnitude of injury (2.9 atm) exhibited chronic locomotor deficits that persisted for 4 to 8 days. Systemic physiological experiments in 20 rats demonstrated that all levels of injury studied produced acute systemic hypertension, bradycardia, and increased plasma glucose levels. Hypertension with subsequent hypotension resulted from higher magnitudes of injury. The durations of hypertension and suppression of amplitude on electroencephalography were related to the magnitudes of injury. While low levels of injury produced no significant histopathological alterations, higher magnitudes produced subarachnoid and intraparenchymal hemorrhage and, with increasing survival, necrotic change and cavitation. These data demonstrate that fluid percussion injury in the rat reproduces many of the features of head injury observed in other models and species. Thus, this animal model could represent a useful experimental approach to studies of pathological changes similar to those seen in human head injury.

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Zhengwen Zhou, Wilson P. Daugherty, Dong Sun, Joseph E. Levasseur, Nabil Altememi, Robert J. Hamm, Gaylan L. Rockswold and M. Ross Bullock

Object

Hyperbaric oxygen (HBO2) has been shown to improve outcome after severe traumatic brain injury, but its underlying mechanisms are unknown. Following lateral fluid-percussion injury (FPI), the authors tested the effects of HBO2 treatment as well as enhanced normobaric oxygenation on mitochondrial function, as measured by both cognitive recovery and cellular adenosine triphosphate (ATP) levels.

Methods

Adult male Sprague–Dawley rats were subjected to moderate lateral FPI or sham injury and were allocated to one of four treatment groups: 1) FPI treated with 4 hours of normobaric 30% O2; 2) FPI treated with 4 hours of normobaric 100% O2; 3) FPI treated with 1 hour of HBO2 plus 3 hours of normobaric 100% O2; and 4) sham-injured treated with normobaric 30% O2. Cognitive outcome was assessed using the Morris water maze (MWM) on Days 11 to 15 after injury. Animals were then killed 21 days postinjury to assess hippocampal neuronal loss. Adenosine triphosphate was extracted from the neocortex and measured using high-performance liquid chromatography. The results showed that injured animals treated with HBO2 or normobaric 100% O2 alone had significantly higher levels of cerebral ATP as compared with animals treated using normobaric 30% O2 (p ≤ 0.05). The injured animals treated with HBO2 had significant improvements in cognitive recovery, as characterized by a shorter latency in MWM performance (p ≤ 0.05), and decreased neuronal loss in the CA2/3 and hilar regions as compared with those treated with 30% or 100% O2 (p ≤ 0.05).

Conclusions

Both hyperbaric and normobaric hyperoxia increased cerebral ATP levels after lateral FPI. In addition, HBO2 treatment improved cognitive recovery and reduced hippocampal neuronal cell loss after brain injury in the rat.