Decrease in cerebral free magnesium concentration following closed head injury and effects of VA-045 in rats

A Systematic Review of Closed Head Injury Models of Mild Traumatic Brain Injury in Mice and Rats

Mild TBI (mTBI) is a significant health concern. Animal models of mTBI are essential for understanding mechanisms, and pathological outcomes, as well as to test therapeutic interventions. A variety of closed head models of mTBI that incorporate different aspects (i.e., biomechanics) of the mTBI have been reported. The aim of the current review was to compile a comprehensive list of the closed head mTBI rodent models, along with the common data elements, and outcomes, with the goal to summarize the current state of the field. Publications were identified from a search of PubMed and Web of Science and screened for eligibility following PRISMA guidelines. Articles were included that were closed head injuries in which the authors classified the injury as mild in rats or mice. Injury model and animal-specific common data elements, as well as behavioral and histological outcomes, were collected and compiled from a total of 402 articles. Our results outline the wide variety of methods used to model mTBI. We also discovered that female rodents and both young and aged animals are under-represented in experimental mTBI studies. Our findings will aid in providing context comparing the injury models and provide a starting point for the selection of the most appropriate model of mTBI to address a specific hypothesis. We believe this review will be a useful starting place for determining what has been done and what knowledge is missing in the field to reduce the burden of mTBI.

Assessing Metabolism and Injury in Acute Human Traumatic Brain Injury with Magnetic Resonance Spectroscopy: Current and Future Applications

Traumatic brain injury (TBI) triggers a series of complex pathophysiological processes. These include abnormalities in brain energy metabolism; consequent to reduced tissue pO₂ arising from ischemia or abnormal tissue oxygen diffusion, or due to a failure of mitochondrial function. In vivo magnetic resonance spectroscopy (MRS) allows non-invasive interrogation of brain tissue metabolism in patients with acute brain injury. Nuclei with “spin,” e.g., ¹H, ³¹P, and ¹³C, are detectable using MRS and are found in metabolites at various stages of energy metabolism, possessing unique signatures due to their chemical shift or spin–spin interactions (J-coupling). The most commonly used clinical MRS technique, ¹H MRS, uses the great abundance of hydrogen atoms within molecules in brain tissue. Spectra acquired with longer echo-times include N-acetylaspartate (NAA), creatine, and choline. NAA, a marker of neuronal mitochondrial activity related to adenosine triphosphate (ATP), is reported to be lower in patients with TBI than healthy controls, and the ratio of NAA/creatine at early time points may correlate with clinical outcome. ¹H MRS acquired with shorter echo times produces a more complex spectrum, allowing detection of a wider range of metabolites.³¹ P MRS detects high-energy phosphate species, which are the end products of cellular respiration: ATP and phosphocreatine (PCr). ATP is the principal form of chemical energy in living organisms, and PCr is regarded as a readily mobilized reserve for its replenishment during periods of high utilization. The ratios of high-energy phosphates are thought to represent a balance between energy generation, reserve and use in the brain. In addition, the chemical shift difference between inorganic phosphate and PCr enables calculation of intracellular pH.¹³ C MRS detects the ¹³C isotope of carbon in brain metabolites. As the natural abundance of ¹³C is low (1.1%), ¹³C MRS is typically performed following administration of ¹³C-enriched substrates, which permits tracking of the metabolic fate of the infused ¹³C in the brain over time, and calculation of metabolic rates in a range of biochemical pathways, including glycolysis, the tricarboxylic acid cycle, and glutamate–glutamine cycling. The advent of new hyperpolarization techniques to transiently boost signal in ¹³C-enriched MRS in vivo studies shows promise in this field, and further developments are expected.

Molecular determinants of magnesium-dependent synaptic plasticity at electrical synapses formed by connexin36

Neuronal gap junction (GJ) channels composed of connexin36 (Cx36) play an important role in neuronal synchronization and network dynamics. Here we show that Cx36-containing electrical synapses between inhibitory neurons of the thalamic reticular nucleus are bidirectionally modulated by changes in intracellular free magnesium concentration ([Mg(2+)]i). Chimeragenesis demonstrates that the first extracellular loop of Cx36 contains a Mg(2+)-sensitive domain, and site-directed mutagenesis shows that the pore-lining residue D47 is critical in determining high Mg(2+)-sensitivity. Single-channel analysis of Mg(2+)-sensitive chimeras and mutants reveals that [Mg(2+)]i controls the strength of electrical coupling mostly via gating mechanisms. In addition, asymmetric transjunctional [Mg(2+)]i induces strong instantaneous rectification, providing a novel mechanism for electrical rectification in homotypic Cx36 GJs. We suggest that Mg(2+)-dependent synaptic plasticity of Cx36-containing electrical synapses could underlie neuronal circuit reconfiguration via changes in brain energy metabolism that affects neuronal levels of intracellular ATP and [Mg(2+)]i.

Intracellular magnesium-dependent modulation of gap junction channels formed by neuronal connexin36

Gap junction (GJ) channels composed of Connexin36 (Cx36) are widely expressed in the mammalian CNS and form electrical synapses between neurons. Here we describe a novel modulatory mechanism of Cx36 GJ channels dependent on intracellular free magnesium ([Mg(2+)]i). We examined junctional conductance (gj) and its dependence on transjunctional voltage (Vj) at different [Mg(2+)]i in cultures of HeLa or N2A cells expressing Cx36. We found that Cx36 GJs are partially inhibited at resting [Mg(2+)]i. Thus, gj can be augmented or reduced by lowering or increasing [Mg(2+)]i, respectively. Similar changes in gj and Vj-gating were observed using MgATP or K2ATP in pipette solutions, which increases or decreases [Mg(2+)]i, respectively. Changes in phosphorylation of Cx36 or in intracellular free calcium concentration were not involved in the observed Mg(2+)-dependent modulation of gj. Magnesium ions permeate the channel and transjunctional asymmetry in [Mg(2+)]i resulted in asymmetric Vj-gating. The gj of GJs formed of Cx26, Cx32, Cx43, Cx45, and Cx47 was also reduced by increasing [Mg(2+)]i, but was not increased by lowering [Mg(2+)]i; single-channel conductance did not change. We showed that [Mg(2+)]i affects both open probability and the number of functional channels, likely through binding in the channel lumen. Finally, we showed that Cx36-containing electrical synapses between neurons of the trigeminal mesencephalic nucleus in rat brain slices are similarly affected by changes in [Mg(2+)]i. Thus, this novel modulatory mechanism could underlie changes in neuronal synchronization under conditions in which ATP levels, and consequently [Mg(2+)]i, are modified.

Prolonged elevation of magnesium in the cerebrospinal fluid of patients with severe head injury

OBJECTIVES

Several works have investigated the role of serum magnesium (Mg) concentrations in traumatic brain injury. However, there is restricted information about cerebrospinal fluid (CSF) levels of Mg in patients with severe head injury (SHI). We assessed the changes of Mg concentrations in CSF and serum in patients with SHI during the first 10 days after the trauma.

METHODS

Eleven patients with SHI were studied prospectively on days 1-3, 5 and 10 with analysis of CSF and serum levels of Mg and Ca. The control group consisted of nine patients with hydrocephalus.

RESULTS

CSF levels of Mg were significantly higher in patients than controls in the corresponding time points except on days 5 and 10 of trauma. The CSF Mg levels tended to decrease and the highest level was found on day 1 after trauma (2.81 +/- 0.65 mg/dl). In the control group, the CSF level of Mg was 1.95 +/- 0.66 mg/dl. No significant difference can be detected between controls and patients regarding serum Mg and Ca levels. In addition, significantly higher values of Ca in the CSF were observed in all time points after trauma in patients with SHI than in the controls. There was no correlation between the CSF and serum levels of Mg and Ca levels.

DISCUSSION

Our study demonstrates that in patients with SHI, CSF levels of Mg and Ca are elevated during the whole observation period. Further works should be designed in order to show the role and importance of CSF levels of ionized Mg in outcome of patients with SHI.

Magnesium restores altered aquaporin-4 immunoreactivity following traumatic brain injury to a pre-injury state

Magnesium reduces edema following traumatic brain injury (TBI), although the associated mechanisms are unknown. Recent studies suggest that edema formation after TBI may be related to alterations in aquaporin-4 (AQP4) channels. In this study, we characterize the effects of magnesium administration on AQP4 immunoreactivity following TBI. Male Sprague-Dawley rats were injured by impact-acceleration diffuse TBI and a subgroup was administered 30 mg/kg magnesium sulphate 30 minutes after injury. Animals were fixed by perfusion 5 hours later, which corresponded to the time of maximum edema formation according to previous studies. One half of the brain was cut using a Vibratome and the other half blocked in paraffin wax. Wax and Vibratome sections were immunostained for detection of AQP4 by light and electron microscopy, respectively. In untreated animals, AQP4 immunoreactivity was increased in the subependymal inner glia limitans and the subpial outer glia limitans, and decreased in perivascular astrocytic processes in the cerebrum and brain stem. In contrast, animals treated with magnesium sulphate had AQP4 profiles similar to normal and sham control animals. We conclude that magnesium decreases brain edema formation after TBI, possibly by restoring the polarized state of astrocytes and by down-regulation of AQP4 channels in astrocytes.

Small Physiologic Changes in Calcium and Magnesium Alter Excitability and Burst Firing of CA1 Pyramidal Cells in Rat Hippocampal Slices

The effects of small physiologic changes in Ca and Mg concentrations on neuronal burst firing were examined. Intracellular electrophysiologic recordings were made from CA1 pyramidal neurons in rat hippocampal slices. There was no difference in the resting potential or the input resistance of neurons bathed in the lower Ca-Mg artificial Cerebrospinal fluid (aCSF) (1.4 mM Ca; 1.3 mM Mg) compared with the higher Ca-Mg aCSF (2 mM Ca; 2 mM Mg). However, neurons in the lower Ca-Mg aCSF, but not the higher Ca-Mg aCSF, demonstrated depolarizing waves and bursts of action potentials; no single component of the aCSF accounted for this difference. Reducing the Ca from 2 to 1 mM in the higher Ca-Mg aCSF increased the mean frequency of action potentials from 28 to 171 Hz; the addition of 6-cyano-7-nitroquinoxaline-2,3-dione did not reduce the frequency. The threshold of potassium for inducing bursts was 5.25 with 1 mM Ca, 7.25 with 2 mM Ca, and 11.25 with 3 mM Ca. When Mg was reduced from 2 to 1 mM, the number of potassium-induced bursts increased to 190%; increasing Mg from 2 to 3 mM reduced the bursts to 58% (frequency with 2 mM Mg set to 100%). Small decreases in extracellular Ca and/or Mg led to increased excitability and burst firing, which may alter physiologic and pathophysiologic processes such as enhancing long-term potentiation, pain transmission, epileptogenesis, and neuronal damage and decreasing anesthetic potency. Increases in extracellular Ca and/or Mg would have the opposite effect on these processes. These effects of extracellular divalent ions on burst firing may explain some of the pathophysiologic effects of hypocalcemia and hypomagnesemia.

Novel therapies in development for the treatment of traumatic brain injury

In industrialised countries, the mean per capita incidence of traumatic brain injury (TBI) that results in a hospital presentation is 250 per 100,000. In Europe and North America alone, this translates to > 2 million TBI presentations annually. Approximately 25% of these presentations are admitted for hospitalisation. Despite the significance of these figures, there is no single interventional pharmacotherapy that has shown efficacy in the treatment of clinical TBI. This lack of efficacy in clinical trials may be due, in part, to the inherent heterogeneity of the traumatic brain injury population. However, it is the multifactorial nature of secondary injury that also poses a major hurdle, particularly for those therapies that have been designed to specifically target an individual injury factor. It is now becoming increasingly recognised that any successful TBI therapy may have to simultaneously affect multiple injury factors, somewhat analogous to other broad spectrum interventions. Recent efforts in experimental TBI have therefore focussed on developing novel pharmacotherapies that may affect multiple injury factors and thus improve the likelihood of a successful outcome. While a number of interventions are noteworthy in this regard, this review will focus on three novel compounds that show particular promise: magnesium, substance P antagonists and cyclosporin A.

An overview of new and novel pharmacotherapies for use in traumatic brain injury

1. Although a number of interventional pharmacotherapies have undergone clinical trial in traumatic brain injury (TBI), none has shown considerable promise. The present short review will examine some of the more novel compounds that have been proposed recently as potential therapeutic agents for use in TBI. 2. Previous experimental studies have demonstrated that brain intracellular free magnesium significantly declines following TBI and that the administration of magnesium salts attenuates the post-traumatic neurological deficits. More recent studies have established that magnesium salts administered after trauma enter the brain intracellular space and reduce the size of the lesion volume. Such protection could be afforded through attenuation of both necrotic and apoptotic cell death. Magnesium salts are currently on clinical trial in TBI. 3. Cyclosporine A is known to inhibit opening of the mitochondrial permeability transition pore. Administration of cyclosporine A after TBI has been shown to attenuate axonal injury and decrease the resultant lesion volume. Therefore, inhibitors of mitochondrial transition pore opening and resultant attenuation of apoptosis show some promise as neuroprotective agents. 4. Recent evidence has shown that substance P antagonists may decrease lesion volume and improve neurological outcome after ischaemia. Similar findings have recently been reported in TBI. The fact that substance P antagonists are known to reduce neurogenic inflammation, oedema formation and are clinically being trialed as both antidepressants and antinociceptive agents suggests that these agents warrant further investigation as therapeutic agents following TBI. 5. There are numerous contradictions in the literature regarding the potential neuroprotective effects of the hormones oestrogen and progesterone. Recent studies suggest that both hormones are protective in TBI and further studies are required to ascertain the mechanisms associated with this protection and any potential for clinical application.

Subdural hematoma following traumatic brain injury causes a secondary decline in brain free magnesium concentration

A number of studies have demonstrated that neurologic motor and cognitive deficits induced by traumatic brain injury (TBI) can be attenuated with administration of magnesium salts. However, many severe traumatic brain injuries have a significant hematoma that develops subsequent to the primary events, and it is unclear whether magnesium salts are effective in this situation. In the present study, an impact-acceleration rodent model of TBI was used to produce an injury that causes an extensive subdural hematoma in over 50% of injured animals. At 30 min after TBI, rats were randomly administered 250 micromoles/kg intravenous MgSO4 or equal volume saline before being monitored by magnetic resonance spectroscopy for 8 h to determine brain intracellular free magnesium concentration. Animals were then assessed for neurologic motor deficits over 1 week using a rotarod device, followed by postmortem examination for presence of subdural hematoma. Animals with subdural hematoma treated with MgSO4 showed no improvement in motor outcome when compared to nontreated controls. Animals with no visible subdural hematoma demonstrated a significant improvement (p < 0.05 by ANOVA) in rotarod scores with MgSO4 treatment. Brain free magnesium concentration in the magnesium treated/hematoma group demonstrated a biphasic decline made up of an immediate initial decline, recovery of brain magnesium levels with MgSO4 treatment, and then a significant second magnesium decline (p < 0.05). Such a secondary decline did not occur in the Mg treated/no hematoma animals. Our results suggest that development of a subdural hematoma following TBI results in a decline in brain magnesium, even after bolus administration of magnesium salts. Such effects of hematoma development will need to be considered in trials examining efficacy of magnesium salts as an intervention following TBI.

Magnetic resonance spectroscopy in traumatic brain injury

Magnetic resonance spectroscopy (MRS) offers a unique non-invasive approach for assessing the metabolic status of the brain in vivo and is particularly suited to studying traumatic brain injury (TBI). In particular, MRS provides a noninvasive means for quantifying such neurochemicals as N-acetylaspartate (NAA), creatine, phosphocreatine, choline, lactate, myo-inositol, glutamine, glutamate, adenosine triphosphate (ATP), and inorganic phosphate in humans following TBI and in animal models. Many of these chemicals have been shown to be perturbed following TBI. NAA, a marker of neuronal integrity, has been shown to be reduced following TBI, reflecting diffuse axonal injury or metabolic depression, and concentrations of NAA predict cognitive outcome. Elevation of choline-containing compounds indicates membrane breakdown or inflammation or both. MRS can also detect alterations in high energy phosphates reflecting the energetic abnormalities seen after TBI. Accordingly, MRS may be useful to monitor cellular response to therapeutic interventions in TBI.

Altered cellular metabolism following traumatic brain injury: a magnetic resonance spectroscopy study

Experimental studies have reported early reductions in pH, phosphocreatine, and free intracellular magnesium following traumatic brain injury using phosphorus magnetic resonance spectroscopy. Paradoxically, in clinical studies there is some evidence for an increase in the pH in the subacute stage following traumatic brain injury. We therefore performed phosphorus magnetic resonance spectroscopy on seven patients in the subacute stage (mean 9 days postinjury) following traumatic brain injury to assess cellular metabolism. In areas of normal-appearing white matter, the pH was significantly alkaline (patients 7.09 +/- 0.04 [mean +/- SD], controls 7.01 +/- 0.04, p = 0.008), the phosphocreatine to inorganic phosphate ratio (PCr/Pi) was significantly increased (patients 4.03 +/- 1.18, controls 2.64 +/- 0.71, p = 0.03), the inorganic phosphate to adenosine triphosphate ratio (Pi/ATP) was significantly reduced (patients 0.37 +/- 0.10, controls 0.56 +/- 0.19, p = 0.04), and the PCr/ATP ratio was nonsignificantly increased (patients 1.53 +/- 0.29, controls 1.34 +/- 0.19, p = 0.14) in patients compared to controls. Furthermore, the calculated free intracellular magnesium was significantly increased in the patients compared to the controls (patients 0.33 +/- 0.09 mM, controls 0.22 +/- 0.09 mM, p = 0.03)). Proton spectra, acquired from similar regions showed a significant reduction in N-acetylaspartate (patients 9.64 +/- 2.49 units, controls 12.84 +/- 2.35 units, p = 0.03) and a significant increase in choline compounds (patients 7.96 +/- 1.02, controls 6.67 +/- 1.01 units, p = 0.03). No lactate was visible in any patient or control spectrum. The alterations in metabolism observed in these patients could not be explained by ongoing ischemia but might be secondary to a loss of normal cellular homeostasis or a relative alteration in the cellular population, in particular an increase in the glial cell density, in these regions.

Acute cytoskeletal alterations and cell death induced by experimental brain injury are attenuated by magnesium treatment and exacerbated by magnesium deficiency

Traumatic brain injury results in a profound decline in intracellular magnesium ion levels that may jeopardize critical cellular functions. We examined the consequences of preinjury magnesium deficiency and post-traumatic magnesium treatment on injury-induced cytoskeletal damage and cell death at 24 h after injury. Adult male rats were fed either a normal (n = 24) or magnesium-deficient diet (n = 16) for 2 wk prior to anesthesia and lateral fluid percussion brain injury (n = 31) or sham injury (n = 9). Normally fed animals were then randomized to receive magnesium chloride (125 micromol, i.v., n = 10) or vehicle solution (n = 11) at 10 min postinjury. Magnesium treatment reduced cortical cell loss (p < 0.05), cortical alterations in microtubule-associated protein-2 (MAP-2) (p < 0.05), and both cortical and hippocampal calpain-mediated spectrin breakdown (p < 0.05 for each region) when compared to vehicle treatment. Conversely, magnesium deficiency prior to brain injury led to a greater area of cortical cell loss (p < 0.05 compared to vehicle treatment). Moreover, brain injury to magnesium-deficient rats resulted in cytoskeletal alterations within the cortex and hippocampus that were not observed in vehicle- or magnesium-treated animals. These data suggest that cortical cell death and cytoskeletal disruptions in cortical and hippocampal neurons may be sensitive to magnesium status after experimental brain injury, and may be mediated in part through modulation of calpains.

Characterization of plasma magnesium concentration and oxidative stress following graded traumatic brain injury in humans

Plasma magnesium, calcium, and oxidative status were investigated in 31 male casualties with traumatic brain injury (TBI) during a 7-day posttraumatic period. The study group consisted of eight patients with mild closed head injury (Glasgow Coma Scale score [GCS] of 13-15), 10 patients with extensive penetrating head injury (GCS 4-6), and 13 patients with blast injuries but without direct head trauma. The latter group was included since previous experimental and clinical data have confirmed the development of indirect brain trauma in patients with blast injuries. Patients with multiple injuries were not included. Significant declines in plasma divalent cations were found in GCS 4-6 patients immediately after TBI and persisting for the entire 7-day study period. Similar changes in magnesium, but not calcium, were present in the GCS 13-15 and the blast injury groups, but only up until day 3 after injury. Alterations in lipid peroxidation products and superoxide anions were also observed following TBI. Increased lipid peroxidation was noted in all three groups over the entire posttraumatic period while increases in superoxide anion generation occurred transiently immediately following TBI. Thereafter, in the GCS 13-15 and blast injury groups, superoxide anions subsequently normalized, whereas in extensive head injury (GCS 4-6), superoxide anion generation significantly declined. A negative correlation between magnesium balance and oxidative stress was observed in all patients immediately after injury persisting in GCS 4-6 patients to the end of the observation period. Our findings suggest an interrelationship between magnesium changes and blood oxidants/antioxidants after TBI, which could be of both diagnostic and prognostic value in patients with neurotrauma.

Neuroprotective effects of MgSO4 and MgCl2 in closed head injury: a comparative phosphorus NMR study

Previous studies have shown that free magnesium levels decline after traumatic brain injury and that magnesium salt administration improves posttraumatic outcome. These earlier studies, however, have been limited to models of injury that do not produce a significant degree of diffuse axonal injury and have used either MgSO4 or MgCl2 as the magnesium salt. The present study compares the neuroprotective efficacy of MgSO4 and MgCl2 in a severe model of diffuse axonal injury in rats using phosphorus nuclear magnetic resonance spectroscopy and the rotarod test to monitor effects on metabolism and neurologic outcome, respectively. Both MgSO4 and MgCl2 given as a bolus of 100 micromoles/kg at 30 min after severe, closed head injury significantly improved brain intracellular free magnesium concentration and neurologic outcome. These findings suggest that both salts penetrate the blood-brain barrier after brain trauma, enter injured tissue, and subsequently improve neurologic outcome.