Microglia, especially those located at the periphery of the medulla, also play an important role in the secondary injury after an SCI. Activating microglia negatively affects neuronal function and leads to neuroinflammation, toxicity, and inhibition of neuronal cell growth by producing proinflammatory molecules [15,25].

However, it also has anti-inflammatory and neuroprotective effects to some extent, mainly within damaged nerves after an SCI [15]. Additionally, microglia have the ability to maintain calcium homeostasis and prevent calcium-dependent excitotoxicity. In the treatment of inflammation after an SCI, microglia accelerate the scarring of astrocytes, which prevent immune cells from entering the damaged spinal cord [18]. Depending on the stages of SCI, the role of microglia may change. An important goal of pharmacotherapy is to normalize microglia function to enhance the regeneration of damaged neurons [15].

In the treatment of inflammation after an SCI, bioactive lipids involved in the moderation of inflammation have also been shown to play an important role. The group of bioactive lipids includes many versatile mediators and regulators of inflammation, which include prostaglandins and the related eicosanoids. These compounds are formed from fatty acids that are bound to the cell membrane, the so-called phospholipids. For eicosanoids to be converted to active mediators of inflammation, the presence of several important enzymes is required, including the important phospholipase A2 (PLA2). On the other hand, PLA2 is responsible for the formation of AA, DHA, EPA acids and lysophospholipids by the same mechanism, which are used for the subsequent synthesis of bioactive lipid mediators with anti-inflammatory effects [26]. Thus, phospholipase A2 is involved in the initiation of inflammation in neurological disorders as well as in the resolution of inflammation. There are a very large number of isoforms of this enzyme, which differ biochemically, structurally, and functionally; this causes some difficulties in treatments. Pharmacotherapy of inflammation after SCI depends on the type and occurrence of cells that are responsible for the secretion of PLA2 enzyme isoforms. In SCIs, there is an imbalance between prostaglandins, leukotrienes, and free fatty acids (PUFA) (arachidonic acid (AA), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA)), as well as between the amount of free fatty acids alone [18,27]. After SCI, there is an increased level of free AA and decreased DHA at the site of the lesion. Because these two PUFAs have antagonistic effects on inflammation, this imbalance in PUFA homeostasis likely contributes to the exacerbated and chronic inflammatory response that occurs after neuro-injury. Despite the difficulty in treating inflammation after SCI with lipid mediators, there is hope of finding a promising molecular target that will prove effective in pharmacotherapy. Several strategies to increase n-3 PUFA levels after CNS injury have shown beneficial results. One example is supplementation with ALA, EPA, and DHA, which improve neuronal cell regeneration, minimize oligodendrocyte and neuronal loss, and further attenuate inflammation in various mouse and rat models of SCIs [28,29,30].

An important aspect of SCI pharmacotherapy is the modulation of inflammation in the injured spinal cord. Chemokines are small molecules that affect the movement of immune cells, causing chemotaxis. In addition, chemokines are involved in T cell growth and differentiation, apoptosis, cell cycle, angiogenesis, and metastatic processes. They can regulate the production of free radicals, nitric oxide, cytokines, and matrix metalloproteases. About 50 chemokine genes are currently known in humans. Due to the wide occurrence of receptor ligands, they are divided into four subfamilies that take into account their chemical structure. Chemokine CCL3 belongs to the group of inflammatory and inducible chemokines, which are regulated by transcriptional mechanisms during inflammation [31,32]. The chemokine ligand receptor system is considered an important factor involved in this inflammatory response. The chemokine CCL3, also called macrophage inflammatory protein (MIP)-1α, is regulated in a transcriptional mechanism during inflammation [33]; it is found in most mature hematopoietic cells, monocytes, macrophages, neutrophils, and in microglia and astrocytes. CCL3 is involved in stimulating immune cells, and its levels significantly increase after an SCI. Antisense oligonucleotides (ASOs) and 2’-deoxy-2-fluoro-D- arabinonucleic acid (FANA), which is an oligonucleotide analog that shows affinity for RNA, have been shown to form a FANA:RNA hybrid, reflecting the structure of the native DNA:RNA hybrid as a chimeric ASO FANA-DN [33,34]. This complex clearly modulates inflammation by suppressing CCL3 expression in the mouse spinal cord, leading to reduced levels of pro-inflammatory cytokines and improved functional recovery after spinal cord injury. This treatment strategy offers hope for improving pathological conditions not only after SCIs but also in other CNS diseases [16].

Heme oxygenases (HO) are enzymes that catalyze the breakdown of heme [35]. Heme oxygenase-1 (HO-1) is mainly induced in stress situations compared with heme oxygenase-2 (HO-2), which is constitutively present in cells but can also be induced [36].

Several studies have shown that HO-1 can play a protective role in the early phase of SCI inflammation, that is, when the blood–brain barrier has not yet been breached. In contrast, microglia play an anti-inflammatory and neuroprotective role on damaged core cells in the secondary phase of inflammation after SCI. One study tested the effect of heme oxygenase-1 on the microglia response in a rat model. The results partially confirmed the effect of HO-1 on inhibiting the inflammatory response in which microglia are involved. In addition, HO-1 enzyme alleviated neuroinflammation after SCI. This study raises the possibility of using the transplantation of modified microglia with HO-1 overexpression when treating patients after SCI, which can only have an impact on functional core regeneration [15].

An endogenous neuropeptide otherwise known as apelin is involved in the modulation of inflammation after SCI. Several studies have shown that the mRNA of apelin and its receptor (APJ) are expressed in the different parts of the CNS, such as the thalamus, hypothalamus, amygdala, substantia nigra, pituitary gland, medulla oblongata, and spinal cord [37,38]. Apelin mRNA and its receptor (APJ) have been shown to be expressed at various sites in the CNS. Additionally, this peptide is found in the cell bodies of neurons and oligodendrocytes. Apelin and its receptor APJ have beneficial effects on neuronal function by increasing neuronal survival and improving nerve conduction. It has been documented that apelin levels are significantly altered in pathological conditions of the central nervous system such as Alzheimer’s, Parkinson’s, and Huntington’s disease [37,38]. Studies have also shown an indirect reduction in the inflammatory process through the use of apelin-13, which inhibits the release of pro-inflammatory cytokines (PICs) such as IL-1β, IL6, and TNF-α [39], which are involved in the inflammation of spinal cord injury; apelin-13 also increases the levels of anti-inflammatory cytokines such as IL-10. In addition, apelin-13 may have the effect of increasing the volume of the medulla and increasing the number of nerve cells in that location. Thus, apelin shows neuroprotective effects and has become another new pharmacotherapy target for the treatment of inflammation after SCI and spinal cord reconstruction [19].

Nanotherapeutics are drugs combined with polymers to enhance the ability to target the drug to the affected area. These polymers exhibit biocompatibility with the diseased tissue, making treatment more effective. Such polymers include poly(lactic-co-glycolic acid) (PLAG), poly(ethyleneimine) (PEI) and methoxypolyethylene glycol (mPEG) [40]. More and more studies suggest the efficacy of this nanoparticle-based drug delivery system; as has been found, they are promising strategies for regulating inflammation [41]. Following SCI injury, hemorrhage and ischemia occur at the site of blood–spinal cord barrier damage. Immune cells such as macrophages also accumulate there. M1 macrophages are mainly responsible for inflammation, releasing inflammatory factors and reactive oxygen species (ROS) that lead to permanent tissue damage. The opposite is true for M2 macrophages, as they enhance axonal activity and repair of motor function. During blood–spinal cord barrier destruction, increased expression of matrix metalloproteinases (MMPs) is also observed, which exacerbates the blood–spinal cord barrier destruction [17]. An example of MMP-responsive molecules is activated cell-penetrating peptides (ACPPs). Hence, ACPP-modified nanoparticles have emerged as ideal nanocarriers that target damaged tissue across the damaged barrier. A biocompatible nanocarrier delivery system targeting the injured spinal cord was developed. This system is composed of PLAG, PEI, mPEG polymers to form a triblock comolymer (PPP), and an MMP-targeting peptide (ACPP) synthesizing the PPP-ACPP structure to load the anti-inflammatory drug, which is the TNF-α blocker etanercept (ET), to form the nanotherapeutic ET@PPP-ACPP. The main mechanism of the drug is to inhibit the secretion of TNF-α factor. Moreover, it is endocytosed and degraded by macrophages in the damaged tissue. It affects the regulation of the NF-κB signaling pathway, which is responsible for the transformation of M1 to M2 macrophages and polarizes M2 macrophages, resulting in a decreased secondary production of pro-inflammatory cytokines and increased production of anti-inflammatory cytokines. ET@PPP-ACPP has been shown to accumulate in the altered area of damaged tissue and achieve effective treatment of SCI. Additionally, in a rat model, promotion of locomotor regeneration was demonstrated [17].

In secondary injury after SCI, in addition to the inflammatory response, apoptosis and cellular autophagy also play an important role, contributing to neuronal destruction [21]. Autophagy, also called autophagocytosis (literally meaning “self-eating”), is a natural process of gaining additional energy by breaking down cellular particles, fragments, or organelles. This allows cells to differentiate and maintain homeostasis. Thus, enhancing autophagy can alleviate SCI in rats, resulting in a return to neurological function. Additionally, it has been demonstrated that autophagy can arrest enhanced apoptosis and exhibit protective effects on neurons [42]. An important regulator of autophagic pathways is Beclin 1, whose participation was found in SCI pathology. Apoptosis of nerve cells is influenced by the factors Bax and Bcl-2. Modulating the above factors associated with inflammation (mainly cytokines), apoptosis (Bax and Bcl-2), and cellular autophagy (Beclin-1) may be beneficial in reducing secondary damage in SCI [21].

FA is a derivative of cinnamic acid that is naturally found in cereals: rye, wheat, oats, but also in nuts and coffee beans, among others. When FA was tested for SCI in a rat model, it showed several important actions. FA in a behavioral study protected rats from SCI-induced motor dysfunction. The compound clearly has a neuroprotective effect, as tissue regeneration of spinal cord neurons was noted after 28 days. With chronic use of FA, a decrease in the expression of the inflammatory factors IL-1β, IL-6, and TNF-α was observed through the upregulation of NF-κB in the spinal cord. Furthermore, a decrease in the expression of COX-2 and iNOS enzymes was noted. Another important aspect is the degree of apoptosis, which is determined by the Bcl-2/Bax ratio. A decreased expression of Bcl-2 and increased dominant factor (Bax) can stimulate cell apoptosis in the injured core by SCI. After FA treatment, this ratio was reversed, confirming the efficacy of neuronal neuroprotection and thereby reducing cell apoptosis [21].

Neuroinflammation is mainly observed in the secondary phase of SCI. Activating microglia are produced with an increase in proinflammatory cytokines. A cascade of signaling pathways leads to an increase in numerous proinflammatory factors, and free radicals contribute to exacerbate secondary damage [43].

Mitogen-activated protein kinase (MAPK): it is now known that the ROS and MAPK signaling pathway can affect the activation of nuclear factor κB (NF-κB), which is an essential element involved in neuroinflammation and is activated by microglia after SCI. Inhibition of the NF-κB signaling pathway results in a decreased expression of inflammatory factors important for inflammation after SCI. These include IL-6, TNF- α, and IL-1β, among others. This fact favors pharmacotherapy of secondary damage caused by microglia-mediated inflammation [44].

Kaempferol: this compound plays an important role in neuroinflammation. This compound is one of the natural polyphenols most commonly found in tea, broccoli, grapefruit, kale, and cabbage. Kaempferol is a flavonoid with anti-inflammatory and antioxidant properties. In studies, kaempferol has been shown to alleviate oxidative stress mediated by microglia activation in the secondary phase of SCI. In addition, this flavonoid promoted recovery of front limb motor function in a rat model of SCI. Referring to the MAPKs responsible for inflammation, kaempferol was shown to inhibit the activation of cascade proteins and decrease the activity of the ROS-dependent MAPK-NF-κB signaling pathway, thereby reducing the levels of pro-inflammatory factors such as IL-1β, TNF-α, and iNOS in microglia. In addition, this compound inhibits the activation of the NLRP3 inflammasome-related pyroptosis pathway. This study showed that kaempferol may contribute to the reduction of oxidative stress and neuroinflammation. To date, this is the sole and primary pathophysiological mechanism that has been identified with respect to kaempferol’s neuroprotection in SCI [43].