Introduction

Trauma to the brain causes tissue damage by primary and secondary injury to the neural tissue. Primary injury due to initial mechanical trauma results in physical disruption of vessels, neurons and their axons [22, 39]. It is not possible to reverse the harmful outcomes of this primary impact on the neural tissue by medical or surgical means. However, secondary injury due to a series of complex events at the subcellular level following the primary impact causes the death of additional tissue at the peripheral zone of the initial damage and this type of injury is considered to be the main target of medical treatment.

As far as the mechanisms of secondary brain injury are considered, oxygen free radicals and lipid peroxidation are believed to play crucial roles [13, 25]. Trauma to the gray matter causes tissue damage and an increase of Ca2+ in the intracellular space. Increased intracellular Ca2+ activates phospholipases and destroys membrane phospholipids, including arachidonic acid. This stimulates the generation of oxygen free radicals and lipid peroxidation. Increased lipid peroxidation end-products after head injury and the protective effects of free-radical scavengers and lipid antioxidants are supporting findings of this theory [24, 25]. Another proposed mechanism is an excessive increase of excitatory amino acids (EAAs) in the synaptic cleft following trauma [7, 9, 14, 32]. The elevation of EAAs in the extracellular space stimulates the N-methyl-D-aspartate (NMDA) receptor on the cell membrane. This results in influx of Ca2+ into neurons and a consequent chain reaction. Cumulative studies have demonstrated that EAAs are elevated during and/or after traumatic brain injury [1, 9, 14, 16, 19, 27, 40, 41]. Additionally, treatments with EAA antagonists were found beneficial in several experimental models of traumatic brain injury [7, 10, 11, 17, 26]. However, the clinical potency of most of these drugs is hampered by their side effects [7, 18].

Memantine (1-amino-3,5-dimethyladamantane hydrochloride) is an uncompetitive NMDA receptor antagonist. Unlike other NMDA antagonists, it has been used clinically for years for the treatment of Parkinson’s disease, spasticity, and dementia without serious side effects [6, 29, 34]. A number of experimental in vivo and in vitro studies have shown that memantine can protect neural cells from harmful effects mediated by EAAs [3, 4, 23, 31, 45]. However, there have been only a few experimental studies that tested the therapeutic efficacy of memantine on traumatic brain injury [33].

In the present study, the time-course of lipid peroxidation by means of malondialdehyde (MDA) formation in a closed head trauma model in rats was studied and then the effects of memantine on lipid peroxidation were examined.

Materials and methods

A total of 132 adult male Sprague–Dawley rats, weighing 230–270 g, were used in this study. They were randomly divided into four groups: sham-operated (n=36), control (closed head trauma, n=36), sham-vehicle (closed head trauma model + saline, n=30), treatment (closed head trauma model + memantine, n=30). The design of the experiment is summarized in Table 1.

Table 1 The design of the experiment
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In the sham-operated group, only scalp incision was performed to expose the underlying skull. In the control group, cranial impact was delivered to the skull, just in front of the coronal suture, over the left hemisphere, from the height of 7 cm. The rats were euthanased at 0.5, 1, 2, 6, 24, 48 h after sham operation (groups Sa, Sb, Sc, Sd, Se, Sf) and closed head trauma (groups Ca, Cb, Cc, Cd, Ce, Cf) by intracardiac infusion of KCl. The brain was immediately removed. Brain tissue samples were taken at 5 mm away from the left frontal pole (subgroups Sa-left, Sb-left, Sc-left, Sd-left, Se-left, Sf-left), as well as from the symmetrical location of the contralateral hemisphere (subgroups Sa-right, Sb-right, Sc-right, Sd-right, Se-right, Sf-right) in the sham-operated group. Likewise, tissue samples from 5 mm away at the exact point in front of the left frontal hemisphere, exposed to cranial impact (subgroups Ca-left, Cb-left, Cc-left, Cd-left, Ce-left, Cf-left) and tissue samples from the symmetrical location of the right frontal hemisphere (subgroups Ca-right, Cb-right, Cc-right, Cd-right, Ce-right, Cf-right) as control group. Samples were frozen in liquid nitrogen and kept under −70°C until analysis.

In the first step of the experimental study, MDA levels in the sham-operated group and control group were evaluated. The results for both groups were compared statistically. Tissue MDA levels in some subgroups of the control group (Ca-left, Cb-right, Cb-left, Cc-right, Cc-left, Cd-left and Ce-left) showed a significant increase compared with the MDA levels of similar subgroups of the sham-operated group (Table 2).

Table 2 Mean values and standard deviations of MDA levels in all groups
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The second step of the experimental study was aimed at investigating the effectiveness of memantine treatment. Memantine was administered intraperitoneally 15 min after closed head injury at the dose of 10 mg/kg. The neuroprotective effect of this dose has been shown in various previous studies which used different models of central nervous system injury [10, 22, 32]. The sham-vehicle group received equal volumes of bacteriostatic saline intraperitoneally 15 min after the trauma. Samples were taken from the previously described regions and were frozen in liquid nitrogen and kept under −70°C until analysis.

All animals were allowed free access to food and water before and after the surgery at room temperature. All the rats were anesthetized with halothane. Maintenance of adequate anesthesia for the experimental procedure was confirmed by the animals’ loss of corneal and pupillary reflexes. Spontaneous breathing was ensured throughout the entire process. Before the initial incision was made, the scalp was infiltrated with 1 ml of 0.25% buvicaine. Body temperature was maintained at 37°C by using a heating pad.

Surgical procedure

All animal-use procedures were in strict accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. The basic surgical procedure for closed head trauma model was performed according to methods previously described [37]. This type of injury produces severe degree of supratentorial lesions and contusions resembling to cortical impact injury and lateral fluid percussion injury [15]. Briefly, the skull of the rat was exposed by a longitudinal incision of the skin. A cranial impact was induced to the left hemisphere 2 mm lateral to the midline, just in front of the coronal suture, and then the scalp incision was closed thoroughly. A 7-cm height of free fall was preferred, which is assumed to produced an impact energy of 0.5 J (N m) over the skull. A silicone tip was fixed to the center of a free-falling plate to avoid penetrating skull fractures.

Measurement of MDA activity

Tissue samples frozen at −70°C were irrigated well with a solution of NaCl (0.9%), and by admixing with the KCl (1.5%), homogenization at a ratio of 1:10 was achieved. Lipid peroxide level in the centrifuged tissue homogenates was measured according to the method by Ohkawa et al. [28]. The reaction product was assayed spectophotometrically at 532 nm. Lipid peroxide level was expressed as nmol of MDA per mg protein of tissue [21].

Statistical analysis

All measurements were expressed as mean ± standard deviation. Mann–Whitney U-test was used for comparison of groups in pairs. The Kruskal–Wallis ANOVA test was used for comparing them in sets of three. When discrepancies are noted, the Mann–Whitney U-test was used for exact determination. A value of P<0.05 was assumed to be significant.

Results

No animals died during or after the experimental study. Breathing was spontaneous, by a regular ventilatory pattern. Normothermia was maintained in all animals during the procedure. Mean values and standard deviations of MDA levels in all groups are shown in Table 2.

In the first step of the study, MDA levels were found to be significantly higher in the control groups [on the ipsilateral side of injured hemisphere at 0.5, 6 and 24 h (Ca-left, Cd-left and Ce-left) and on ipsilateral and contralateral of the injured hemisphere at 1 and 2 h (Cb-right, Cb-left, Cc-right, Cc-left) following trauma] than those of the sham group (P<0.0001). The MDA levels continued to increase for the first 2 h after the injury, then started to decrease gradually. In the latter step of our study, we found that memantine treatment significantly reduced lipid peroxidation levels (Ta-left, Tb-left, Tb-right, Tc-left, Tc-right, Td-left, Te-left) compared with the sham-vehicle and control groups (P<0.01) (Table 2). The decrease in MDA appeared to be the most clear (53.6% of MDA level of injured hemisphere in control group) at the second hour after injury.

Discussion

Indeed, a number of experimental studies suggest that both the above mechanisms (EAA toxicity and lipid peroxidation) interact with each other and lead to cell death as a chain reaction [1, 7, 35]. In our study, we tried to investigate the therapeutic efficacy of memantine on Ca2+-mediated neurotoxicity and its resultant lipid peroxidation in vivo after closed head trauma. To the best of our knowledge, there had been no previous experimental study testing the effect of memantine on lipid peroxidation after brain injury. Tissue lipid peroxidation was assessed by the measurement of MDA. The thiobarbituric acid method was used for the determination of MDA levels. This method as an indirect indicator of lipid peroxidation and it has been reviewed because TBA also reacts with other cellular elements besides lipid compounds [36]. However, it is the most widely used method to measure lipid peroxidation [8, 12, 30, 43, 44]. Our results showed that MDA values in the injured hemisphere significantly increased, starting from 30 min after the trauma, and reaching a maximum level at the second hour, then it started to decline gradually. Various experimental and clinical studies also demonstrated early elevation of MDA levels after head injury [8, 12, 30, 44]. All of these findings clearly show the role of lipid peroxidation in the evolution of the secondary damage after traumatic brain injury.

It was found that in various head injury models (weight drop injury, fluid-percussion-induced injury, controlled cortical impact injury), concentration of EAAs (aspartate and glutamate), measured by the microdialysis technique, increased suddenly and significantly, proportional to the severity of the injury [1, 9, 16, 20, 27, 40, 41]. Furthermore, the increase of extracellular glutamate levels in post-traumatic brain-injured adults who underwent implantation of cerebral microdialysis probe were evaluated in various studies previously [14]. Koura et al. [19] suggested that EAAs were released in highest concentrations in patients with cerebral contusions and there was a significant correlation between the concentration of glutamate and the patient outcome.

According to recent studies, treatment with both competitive and noncompetitive NMDA antagonists have been shown to have positive effects on various parameters in experimental head injury models [7, 10, 11, 17, 26]. Dizocilpine, which is the gold standard of neuroprotective NMDA antagonists, significantly attenuated the development of brain edema following experimental fluid-percussion brain injury [26]. This NMDA antagonist also decreased brain edema and improved neurological status in closed head injury model as well as reducing cerebral oedema and restoring brain barrier permeability at the penumbral zone of the lesion in cold injury mode [11, 38]. It has been also shown that dizocilpine reduced the volume of ischaemic damage after acute subdural haematoma in rats [42]. In spite of their potent therapeutic effects of dizocilpine or similar NMDA antagonists, their psychomimetic side effects cause serious problems [7, 18]. After dizocilpine has been investigated as a potential anticonvulsant in humans, side effects on cognition and behavior were reported [35].

Memantine has been in clinical use for the treatment of various cerebral disorders for many years with relatively few side effects [6, 29, 34]. In addition, it has been reported to be neuroprotective in animals subjected to experimental studies in recent years. We have also shown in an experimental study that memantine attenuated brain edema formation and restored blood brain barrier permeability at the periphery of the ischemia after focal cerebral ischemia and reperfusion in rat [10]. In the present study, memantine treatment significantly reduced lipid peroxidation levels in the treatment group compared with the sham-vehicle and control groups in the closed head trauma model. Memantine is a non-competitive open channel blocker; i.e., it influxes into the channels and inactivates them after the NMDA channels are stimulated and opened by an agonist like glutamate [2, 6]. Moreover, aside from other non-competitive NMDA receptor antagonists, memantine has a considerably low affinity and a fast kinetic action on NMDA receptor channels. Other NMDA receptor antagonists, like dizocilpine, influx into the NMDA channels over a long period of time and clear away slowly; thus they establish a complete and lengthy blockade. However, memantine appears to have a considerably fast on–off rate, and due to the partial blockage by memantine, they would be set free from many of the blocked channels more easily; thus, basal physiological activity of NMDA channels could be maintained at a moderate concentration of glutamate [2, 5].

Conclusion

The results of this study demonstrate that memantine may have utility in the treatment of traumatic brain injury because of its clinically usefulness in the treatment of other cerebral diseases for many years. Additional research will help further clarify the value of Memantine in the treatment of traumatic brain injury.