2.1.6. Lipid peroxidation

The resulting ionic unbalance and edema contribute to the massive entry of Ca2+, which is intrinsically related to neurotoxicity by the exaggerated release of glutamate and the activation of proteases and phospholipases. This activation triggers the destruction of neurofilaments and the destabilization of key proteins for cellular support, favoring axonal collapse, and fragmentation in the first hours or days post-trauma [15]. In addition to phospholipase activation, the increase in Ca2+ contributes to the production of pro-inflammatory molecules, such as arachidonic acid, leukotriene, and thromboxane due to the release of fatty acids from membrane phospholipids [4]. Likewise, intracellular mobilization of cytosolic Ca2+ generates reactive oxygen species (ROS), energetic failure, cytoskeletal damage, and errors in protein

The sudden entry of intracellular Ca2+ likewise leads to the aforementioned glutamate excitotoxicity. These mechanisms conjointly contribute to immediate cell death or the activa-

SCI affects the regular equilibrium of glutamate and aspartate in the CNS, leading to significant alterations. Fifteen minutes after SCI, glutamate concentration increases to concentrations six times higher than physiological levels [18]. This increase is due to the overstimulation of ionotropic glutamate receptors (GluRs), provoked by the massive entry of Ca2+ and Na+

ion flow can induce a secondary increase of intracellular Ca2+, leading to an overstimulation of viable neurons and neuronal death. This toxic effect, known as excitotoxicity [19], leads to

This phenomenon is mainly evidenced in glial cells, with axonal-myelinating oligodendrocytes showing greater susceptibility. Excitotoxicity signals are regulated by 2-amino-3-hydroxy-5 methyl-4-isoxazolepropionic acid (AMPA) and kainate type GluRs; their overactivation facili-

Microvascular disruption, ionic deregulation, glutamate increase, mitochondrial dysfunction, and the activation of inflammatory mechanisms stimulate the formation of FR [4, 6, 22]. The cascade of FR production begins with intracellular Ca2+ elevation and the production of uncoupled electrons, which bind to O2 molecules, transforming them into superoxide radicals

) capable of increasing oxidative damage by promoting further FR formation [23].

Damage induced by FR, denominated oxidative stress or nitrosative stress, occurs when excessive amounts of ROS and reactive nitrogen species (RNS) are produced, along with low levels of antioxidant defenses. FR production following SCI can damage cellular lipids, proteins, deoxyribonucleic acid (DNA), and ribonucleic acid (RNA), causing mutations or irreversible damage, which leads to cellular death [24]. Moreover, peroxidase (Prx) 1/6 and manganese superoxide dismutase (MnSOD) are modified by phosphorylation, oxidation, or nitration during

tates oligodendrocyte death and consequent demyelination after SCI [21].

oxidative stress after injury, inhibiting their antioxidant functions [25].

. This

tion of calcium-dependent signaling pathways, which result in cellular death [15, 17].

folding [16].

2.1.4. Glutamate excitotoxicity

26 Essentials of Spinal Cord Injury Medicine

2.1.5. FR production

(O2

neuronal and oligodendrocytic death [18, 20].

One of the most important pathophysiological mechanisms derived from FR production is LP. ROS such as hydroxyl radicals (OH) and O2 , combine with nitric oxide (NO) to form the superoxidant agent peroxynitrite (ONOO). These reactants, in turn, can protonate at a physiological pH level, forming peroxynitrous acid (ONOOH) [23, 26].

At a physiological pH, ONOO reacts with polyunsaturated fatty acids (PUFAs), taking one electron to form a lipid radical (L•) that interacts with molecular oxygen to form peroxyl lipid radicals (LOO•). Without regulatory mechanisms, LP will result in membrane depolarization and ensuing demyelination in the SC [26, 27]. Substantial damage induced by FR involves an oxidative attack to the cellular membrane, which is made of PUFAs (arachidonic acid, linoleic acid, eicosapentaenoic acid, or docosahexaenoic acid) [28, 29]. Two aldehydic products arise from LP: 4-hidroxynonenal (4-HNE) y 2-propenal (acrolein). These molecules have been characterized in SCI models, forming covalent bonds with basic amino acids found in cellular proteins, and thus altering their structure and functional properties [28].

Likewise, the inflammatory response is partially responsible for FR production after SCI due to its stimulation of NO production. This molecule is produced by different cellular types after SCI and is capable of damaging medullar parenchyma when produced by inducible nitric oxide synthase (iNOS) [29]. High concentrations of NO, which are mainly produced by iNOS, require an immunological/inflammatory stimulus, such as inflammatory cytokines (IL-6, IL-1, and IFN-γ), resulting in nanomolar quantities produced for prolonged time periods [30, 31].

After SCI, high concentrations of NO (produced by iNOS) and peroxynitrite increase up to three or five times, reaching their peak 12 hours after injury [32]. Some studies have detected iNOS activity 3, 4, 24, and 72 hours following SCI, vinculating its presence to LP, and neural destruction [31–33]. High concentrations of NO simultaneously participate in cellular damage and increase vascular permeability. Consequently, NO contributes to the formation of edema, as well as excitotoxicity through the release of high concentrations of Ca2+ and glutamate. Furthermore, NO alters the electron transport chain in the mitochondria, generating further FR by affecting enzymes with a sulfuric catalytic center, such as ubiquinone succinate [34]. In addition, iNOS expression and production of NO have a retroactive effect on the development of the inflammatory response, due to their role in the production of cyclooxygenase 2 (COX)-2, which increases the levels of inflammatory products such as prostaglandins and thromboxane [34, 35].
