It is likely that posttraumatic neurochemical alterations may involve changes in the synthesis and/or release of both endogenous “neuroprotective” and “autodestruc-tive” compounds. The identification of these compounds from the timing of the pathological cascade after brain injury provides a window of opportunity for treatment with pharmacological agents designed to modify gene expression, synthesis and release of transmitters, and receptor binding, or the physiological activity of these factors with subsequent prevention or attenuation of neu-ronal damage. Some of the more important changes are as follows.
Acetylcholine
An increase in the concentration of acetylcholine in the brain has been reported after experimental TBI. Other
studies have shown a decrease in the binding of cholin-ergic receptors, and fluid percussion brain injury in the rat significantly decreases the affinity of muscarinic and cholinergic receptor binding in both the hippocampus and brainstem, changes that may last as long as 15 days postinjury (Jiang et al. 1994; Lyeth et al. 1994). These and other data have led to the suggestion that activation of muscarinic cholinergic systems in the rostral pons medi-ates behavioral suppression associated with TBI, whereas lasting behavioral deficits result from pathological excita-tion of forebrain structures induced by the release of ace-tylcholine. More recently, it has been shown that con-trolled cortical impact in the rat causes an impairment of cholinergic neurons that produces enhanced vulnerability to disruption of cholinergically mediated cognitive func-tion, and previous studies have shown that the adminis-tration of the anticholinergic compound scopolamine reduces neurobehavioral dysfunction after experimental brain injury in rats. In a recent study of pre- and postsyn-aptic markers of cholinergic transmission in human post-mortem brains from patients who died after brain injury and matched controls, the mean value of choline acetyl-transferase activity was reduced by approximately 50% in the brain-injured group. In contrast, there was no differ-ence between the brain-injured and control groups in the levels of M1 or M2 receptor binding (Dewar and Graham 1996). Given the involvement of acetylcholine in cogni-tive function, it is possible to speculate that reduced cho-linergic acetyltransferase activity may be associated with cognitive impairment in patients who survive a brain injury (Murdoch et al. 2002).
Arachidonic Acid Cascade
Damage to the cell membrane by calcium-activated pro-teases and lipases induces the production of a variety of potentially pathogenic agents from a breakdown of endog-enous intracellular fatty acids. The formation of com-pounds such as arachidonic acid–activated phospholipase A2 lipooxygenase, cyclooxygenase, and leukotrienes;
thromboxanes; free-fatty acids; and other breakdown products with arachadonic acid cascade have been associ-ated with neuronal death and poor outcome in models of experimental brain injury (DeWitt et al. 1988; Ellis et al.
1989; Hall 1985; Nakashima et al. 1993; Shohami et al.
1987; Wei et al. 1982; Yergey and Heyes 1990).
Catecholamine and
Monoamine Neurotransmitters
Laboratory studies have shown that circulating levels of epinephrine and norepinephrine increase with increasing
severity of injury and that there are regional changes in the tissue concentration of them and of dopamine after experimental fluid percussion and controlled cortical impact brain injury in rats (McIntosh et al. 1994b; Prasad et al. 1992; Prasad et al. 1994). Changes in α1-adrenergic receptor binding in damaged cortex and hippocampus after experimental lateral fluid percussion in the rat have also been described (Prasad et al. 1994).
Activation of the serotonergic (5-HT) system has also been suggested to play a role in TBI, and an increase in 5-HT has been shown to be closely associated with the de-pression of local cerebral glucose utilization in regions showing extensive histological damage (Pappius 1981;
Prasad et al. 1992; Tsuiki et al. 1995).
Cytokines
There is an increased number of immunocompetent cells in the plasma of brain-injured patients, and it is possible that such cells, because the blood-brain barrier is opened, often for long periods, may enter the injured brain and exert a neurotoxic effect. Polymorphonuclear leucocytes accumulate within 24 hours in injured brain (Biagas et al.
1992; Zhuang et al. 1993), and this correlates with the onset of posttraumatic brain swelling in rats (Schoettle et al. 1990). However, experimentally induced neutropenia does not appear to influence the development of posttrau-matic edema or reduce cortical lesion volume, although a decrease in volume after occlusion of the middle cerebral artery in immunosuppressed (neutropenic) rats has been described (Chen et al. 1993). Macrophages undoubtedly play an important role in wound healing, and many of them secrete soluble factors, including cytokines that may influ-ence posttraumatic neuronal survivability and outcome.
Moreover, injured neuronal and nonneuronal cells within the central nervous system (CNS) can synthesize and secrete inflammatory cytokines that may mediate further brain damage. Among the cytokines implicated in this additional damage are tumor necrosis factor (TNF) and the interleukin family of peptides. For example, after mechanical trauma to the brain, there is a large increase in the regional brain concentration of interleukin-1, -6, and TNF, suggesting that the CNS-derived cytokines may play a role in the pathophysiological cascade of brain damage after trauma (Fan et al. 1995; Mocchetti and Wrathall 1995; Shohami et al. 1994). Studies have documented the beneficial effects of pharmacological blockade of interleu-kin-1β and TNF, suggesting that the release and/or upreg-ulation of these pathways may be either pathogenic (Wood-roofe et al. 1991) or protective (Dietrich et al. 1996).
Although many compounds have been measured after TBI, the identification of neuron-specific enolase and the
S-100 protein in the CSF or serum indicate nerve cell or glial damage (Herrmann et al. 2000; McKeating et al.
1998; Ogata and Tsuganezawa 1999; Singhal et al. 2002).
Endogenous Opioid Peptides
There is an increase in the regional immunoreactivity of the endogenous opioid dynorphin after a fluid percussion brain injury that has been shown to correlate with struc-tural brain damage and reductions in regional CBF (McIn-tosh et al. 1987a, 1987b). Furthermore, both the intracere-broventricular and intraparenchymal microinjection of dynorphin and other kappa-agonists worsens neurological injury, suggesting that, indeed, dynorphin has a pathogenic effect after brain injury (McIntosh et al. 1994a). However, pharmacological studies would suggest that the effect is indirect and that it may be mediated by other neurotrans-mitter or neurochemical systems, including the excitatory amino acids (EAAs) glutamate and aspartate, an effect that can be reversed by both competitive and noncompetitive N-methyl-D-aspartate (NMDA) antagonists (Isaac et al.
1990). Although the mechanisms by which dynorphin induces NMDA receptor–mediated activity remain specu-lative, some studies suggest that opioids may modulate the presynaptic release of EAA neurotransmitters, thereby contributing to regional neuronal damage during the acute posttraumatic period (Faden 1992).
Excitatory Amino Acids
There is a marked increase in the extracellular EAAs glutamate and aspartate after TBI (Jenkins et al. 1988;
Katayama et al. 1990; Nilsson et al. 1990; Palmer et al.
1993). Although the amount varies in different models of TBI, there is a close association between the increased intracellular concentration and total tissue concentrations of sodium and calcium (Olney et al. 1987; Rothman and Olney 1995). The exact mechanisms underlying EAA-mediated cell death are not well understood, but it has been postulated that the sustained release of glutamate with prolonged postsynaptic excitation causes the early accumulation of intracellular sodium, which in turn leads to acute neuronal swelling and delayed calcium influx that causes a cascade of metabolic disturbances within neurons that may lead eventually to cell death. These findings have suggested that posttraumatic cognitive deficits may result in part from excitotoxic events specifically targeting the hippocampus, inducing overt neuronal cell loss, cellu-lar stress, and/or dysfunction, thereby disrupting normal synaptic transmission (Smith and McIntosh 1996).
Laboratory evidence for the glutamate hypothesis is good, particularly in models of focal cerebral ischemia in
which treatment is started either immediately before or after the procedure. Cerebral ischemia is common after TBI, and because there is good evidence both in animal models of neurotrauma (Chen et al. 1991; Gordon and Bullock 1999; Landolt et al. 1998; Smith and McIntosh 1996) and in human TBI (Zauner and Bullock 1995) that glutamate is released in large amounts, it is logical to hy-pothesize that antagonists directed toward the NMDA receptor might be effective. However, the initial clinical trials have been disappointing (Narayan et al. 2002).
Growth Factors
The potential of neurons and glial cells to recover after TBI depends both on the posttraumatic ionic/rotransmitter environment and on the presence of neu-rotrophic substances (growth factors). They support nerve cell survival, induce the sprouting of neurites (plas-ticity), and facilitate the guidance of neurites to their proper target sites. The most well-characterized neu-rotrophic factors include nerve growth factor (NGF), basic fibroblast growth factor (FGF), brain-derived neu-rotrophic factor, glial-derived neuneu-rotrophic factor, and NT-3. Some studies have suggested that these factors are synthesized or released after traumatic CNS injury and that their concentration increases during the first few days after a number of experimental procedures (Conner et al. 1994; Varon et al. 1991). Relatively little is known about the neurotrophic factor response in experimental TBI (Leonard et al. 1994), but NGF- and FGF-like neu-rotrophic activity has been observed to increase in the CSF of brain-injured patients (Patterson et al. 1993). The intraparenchymal infusion of NGF over 14 days postin-jury has also been reported to reduce septohippocampal cellular damage and improve neurobehavioral motor and cognitive function after fluid percussion brain injury in the rat (Sinson et al. 1995). A neuroprotective effect of FGF has also been found in a rodent model of cortical contusion (Dietrich et al. 1996).
Ion Changes
The principal ion changes in TBI are in calcium, magne-sium, and potassium. Changes in calcium ion homeostasis are believed to be pivotal in the development of neuronal cell death. For example, total brain tissue calcium concen-trations have been found to be significantly elevated in injured areas after both experimental fluid percussion brain injury and cortical contusion in rats (Shapira et al.
1989a, 1989b). Furthermore, there is a significant increase in regional calcium accumulation that has been shown to persist for at least 48 hours after fluid percussion
brain injury in the rat (Hovda et al. 1991). In support of this hypothesis is the finding of increased expression of some of the immediate early genes after fluid percussion injury, because they are known to be activated by an increase in intracellular calcium (Raghupathi et al. 1995;
Yang et al. 1994).
Magnesium is involved in a number of critical cellular processes, and alterations in its tissue amounts impair maintenance of normal intracellular sodium and potas-sium gradients. After traumatic injury to the CNS, there is a reduction in brain magnesium that is hypothesized to impair glucose utilization, energy metabolism, and pro-tein synthesis, thereby reducing both oxidative and sub-strate phosphorylation (Vink and McIntosh 1990; Vink et al. 1990). Because magnesium has an important regula-tory role with respect to calcium transport and accumula-tion and cerebrovascular contractility, changes in intra-cellular magnesium could potentially contribute to posttraumatic calcium-mediated neurotoxicity and/or the regulation of regional posttraumatic blood flow.
After experimental brain injury, there is a rapid and massive increase in the release of potassium into the extra-cellular space that can be associated with burst discharges, depolarization, and spreading depression (Siesjo and Wie-loch 1985). The increase in extracellular potassium has been thought to contribute to disruption of energy homeo-stasis, cerebral vasoconstriction, changes in cerebral glycol-ysis, and loss of consciousness (Siesjo and Wieloch 1985).
The excess extracellular potassium is rapidly taken up by astrocytes: this may result in astrocytic edema, which in turn may impair neuronal oxygen transport.
Oxygen-Free Radicals and Lipid Peroxidation
Hypoperfusion of brain tissue may stimulate the genera-tion of oxygen-free radicals, principal amongst which is superoxide. Superoxide may arise from a number of sources that include the arachidonic acid cascade, the autooxidation of amine neurotransmitters, mitochondria leakage, xanthine oxidase activity, and the oxidation of extravasated hemoglobin (Hall 1996; Kontos and Pov-lishock 1986). Additional sources, at least in the first few hours and days after trauma, may be activated microglia, infiltrating neutrophils, and macrophages. Within the injured brain where pH is lowered, conditions are also favorable for the potential release of iron, which may then participate in the formation of hydroxy radical. Iron also promotes the process of lipid peroxidation. Multiple stud-ies have shown that in cats subjected to fluid percussion injury there is early generation of superoxide radicals in injured brain, and the generation of these radicals occurs in parallel with secondary injury to the brain and its
microvasculature, including the formation of vasogenic edema (Hall 1996; Kontos and Povlishock 1986; Siesjo and Wieloch 1985).