Traumatic brain injury (TBI) is among the most devastating types of injury and can result in a different profile of neurological and cognitive deficits, and even death in the most severe cases. TBI represents a large portion of the global injury burden and is caused mainly by falls, especially in old patients, and road injuries, often affecting young patients [1]. The effects of TBI are not limited to the patient suffering from this injury; it also affects families and societies, with a relevant financial burden. There is a solid agreement that the management of TBI must be focused to avoid brain injury. Upon clinical examination [2], TBI is classified into mild, moderate, and severe, based on the scores of the Glasgow Coma Scale (GCS). Such categories have been found to be predictive of a patient’s long-term outcome, although other measures and models have also been tested, as biomarkers [2,3].

As explained, the management of TBI has to be focused on the reduction of the severity of the sequelae and to improve the recovery of these. The improvement in monitoring and in the knowledge of the pathophysiology of TBI could change current management, allowing for more adequate interventions that could improve the final outcomes, reducing the associated disabilities [4]. This improvement has been based on a better understanding of the complex pathophysiology of TBI. Neuroprotection is considered one of the treatments among the improvements in the treatment of TBI [5,6,7,8,9].

Among the biochemical mechanisms implicated in the pathophysiology of TBI, the inflammatory processes [10,11,12,13,14,15,16], and the impairment of the phospholipid metabolism and its consequences [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35] play an important role. Because of these pathophysiological conditions, there is agreement on the need for drugs that may have a protective and restorative or reparative effect on the nervous system [36,37,38,39,40], the so-called “traumatic penumbra” being the target of their effect [22]. Citicoline is the generic name for cytidine-5′-diphosphocholine or CDP-choline, which is a normal component of the human metabolism as an intermediate in the synthesis of phosphatidylcholine, the main phospholipid in cellular membranes (Figure 1), and, when it is administered exogenously as a drug, has a wide range of biochemical and pharmacological actions becoming a putative treatment for some neurological diseases [41], including TBI [22].

Table 1 summarizes the most relevant preclinical studies evaluating the therapeutic efficacy and the mechanism of action of citicoline in different experimental models.

It has been demonstrated that citicoline is able to prevent degradation of choline and ethanolamine phospholipids during brain ischemia [42], and to restore the integrity of the blood−brain barrier [43]. Other old experimental studies have shown the protective effects of citicoline in neuronal cultures in hypocapnic conditions [44], and positive effects on the threshold for the arousal reaction, reducing the duration of coma induced by different mechanisms [45,46,47,48]. For example, Ogashiwa et al. [48] analyzed the dose-response of citicoline on post-traumatic disturbances of consciousness in mice. They used groups of 10 mice for every dose of citicoline administered by IV. The dose-escalation of citicoline was 0, 1, 2, 4, 8, 15, 30, 60, 125, and 250 mg/Kg and the assignment was randomized. Control group received comparable volumes of saline solution. The injury (mechanical impact) was induced 10 min after the administration of the drug. The degree of concussion was assessed measuring the time interval from the onset of coma and the return of the righting reflex (RR time) and the time between the onset of coma and subsequent appearance of spontaneous movement (SM time). A significant dose-dependent relationship was found on RR time in animals that received the doses of 60 mg/Kg or higher. Furthermore, for the same level of doses, a significant reduction of SM times was found, thus the authors concluded that with doses not less than 60 mg/kg IV, citicoline showed a statistically significant improvement in the disturbances of consciousness during the acute phase of head injury in mice.

Citicoline can increase the incorporation and the metabolism of glucose and reduce the levels of lactate in the brain during ischemia [49]. Citicoline can modulate the activity of some enzymes, such as cholinephosphotransferase or phospholipases, especially phospholipase A2 [50,51,52,53,54,55]. Arrigoni et al. [51] demonstrated that following a cryogenic injury of the brain, within the tissue surrounding the necrotic lesion, there was a clear activation of the phospholipase A2 at 2 and 4 h after the lesion that participated in the phospholipid breakdown. They also detected an activation of the cholinephosphotransferase 2 h post-injury, that could correspond to an early phospholipid resynthesis. For their experiment, the authors used cryogenic brain injury in rabbits that were randomly assigned to the control group or to the lesion group. Animals were killed by decapitation 2, 4 and 8 h after the lesion. Eighteen injured animals received 200 mg/Kg PO of citicoline 5 h before the injury, except for the 8 h-injured group which received 100 mg/Kg PO 5 h before and 1 h after the injury. Eighteen injured animals were used as controls. The enzymatic activities were measured in the brain homogenate obtained after the sacrifice. As described, a significant increase of the activity of the phospholipase A2 was detected 2 and 4 h after the injury. In injured animals, citicoline administration was associated with a significant prevention of the phospholipase A2 stimulation as compared to the untreated injured animals, whereas the drug did not affect the phospholipase A2 activity in control treated animals. In this model, citicoline did not affect the cholinephosphotransferase activity in any experimental conditions. The authors concluded that the beneficial effects of citicoline might be explained by a prevention of destruction rather than an enhancement of reconstruction of phospholipids. Kitazaki et al. [54] studied the effects of citicoline on membrane-associated phospholipase A2 using a synaptosome fraction from rat cerebral cortex and a crude membrane fraction from rat platelets. Citicoline, at a dose 1–10 mM, inhibited the activity of both enzymes I a dose-dependent manner, being this inhibition competitive.

In some experimental models, citicoline provided significant protection against the lethality, improving the survival quality [56,57,58]. Algate et al. [56] compared the effects of repeated oral administration of citicoline and vehicle on the EEG changes induced by epidural brain compression in anesthetized cats. Animals were treated with citicoline at doses 0.5 or 2.5 g/Kg per day for 5 days. After this 5 days of treatment, animals were anesthetized and the epidural compression was induced by a balloon catheter and platinum electrodes were placed stereotactically to record EEG. Citicoline-treated cats exhibited a statistically significant increase in resistance to the effects of mechanical compression of the brain when compared with the vehicle-treated group. The onset of abnormal EEG waveforms occurred at greater levels of brain compression in citicoline-treated animals, and these animals were less susceptible to cardiac and respiratory disorders. A statistically significant protection against the lethal effects of epidural compression was also noted in treated animals. The authors concluded that repeated oral treatment with citicoline can provide some protection against the effects of acute mechanical compression of the brain. Kondo [58] studied the effects of citicoline (15–20 mg/Kg) slowly injected into the common carotid artery or IV in a model of progressive brain compression in cats. Citicoline markedly improved the disturbances of consciousness and the EEG records. It has been possible to detect the labelled drug in the brain, notably in the affected areas in a model of cold brain injury in rats [59]. Boismare et al. [60] studied the effects of citicoline in a model of craniocervical trauma without direct blow (“whiplash”) in rats. The lesion was induced by means of a fierce acceleration and deceleration applied to the animals. Two days after the lesion animals presented an asymmetry in the activity of the latero-cervical muscles, a postural dysregulation of the brain circulation, and disturbances in conditioned avoidance responses. These alterations were related with an increase of the cerebral norepinephrine splitting. The intraperitoneal injection of citicoline (20 mg/Kg) one hour after the trauma prevented all the modifications on the central catecholamine metabolism and suppressed the behavioral disorders noticed in the control animals. In many different experimental models of brain edema a significant effect of citicoline on the reduction of the edema has been demonstrated, associating this effect with the restoration of the activity of the membranous ATPases that facilitate the reabsorption of the edema [17,18,61,62,63,64,65,66]. Clendenon et al. [61] studied the effects of citicoline (100–200 mg/Kg IV) on the ATPase activity in a model of spinal cord injury by impact in dogs. Two minutes after the injury a significant decrease of the Na+/K+-ATPase activity was found, and at 30 min, also decreased activity of the Mg++-ATPase. Administration of citicoline 5 min after injury prevented the decrease of the Mg++-ATPase activity, whereas the decrease of the Na+/K+-ATPase activity was not prevented, probably because it was already inactivated to a low level before the administration of the drug. Cohadon et al. [62] demonstrated that in the development of vasogenic cerebral edema there was a progressive quantitative and qualitative impairment of mitochondrial-ATPase and of Na+/K+-ATPase using a model of cryogenic brain edema in rats. Citicoline was administered 24 h after the injury at doses of 20 mg/Kg and continued daily until the sacrifice of the animals. Citicoline was able to correct this disturbed enzymatic activity and at the same time reduce the extent of the cerebral edema. Cervós-Navarro and Lafuente [63,64] studied the effects of citicoline in a model of ultraviolet-induced brain edema in cats. After the induction of the edema by exposure to ultraviolet light, eight cats were treated with a single dose of citicoline of 100 mg/Kg 1, 8 and 16 h after the lesion. Eight cats were used as controls and they received the same volume of solvent alone (1 mL/Kg). Tissue water content was measured by a microgravimetric method. It was shown that animals treated with citicoline had a less volumetric increase than those animals treated with placebo, demonstrating an effect of the drug accelerating the reabsorption of brain edema. Majem et al. [65] analyzed the effects of multiple oral doses of citicoline on the bioelectric changes induced by brain edema in a model of cryogenic brain edema in rats. Treated animals (n = 8) receive oral citicoline at a dose 1 g/Kg/d for 8 days, starting immediately after the lesion. Treated animals showed a significant increase of the theta band during their awake state at the expense of a decrease of delta band as compared to control animals. A minor interindividual dispersion in each of the frequency bands was observed in treated animals, that somehow reflected the protective effects of the drug. Roda [66] in his study assessing the effects of citicoline on two different animal models of cryogenic brain edema, demonstrated the efficacy of the drug by means of a significant reduction of the Evans blue extravasation and that this effect was more evident when the drug was administered before the induction of the injury. Citicoline has been able to show a significant effect on microvascular permeability during experimental endotoxemia [67] and in models of early burn edema [68], postulating for a significant anti-inflammatory effect of the drug.

The effects of exogenous administration of citicoline (100 mg/Kg/d/18 d IP beginning 1 day postinjury) on the motor consequences, spatial memory capacity, and acetylcholine levels in some areas of the brain, such as hippocampus and neocortex were analyzed in an experimental model of traumatic brain injury by a controlled lateral impact in a total of 50 rats. In the motor study, animals treated with citicoline had a significantly longer balance period than animals in the control group. In addition, the treatment with citicoline was significantly associated with less cognitive deficits. This effect could be explained by the rapid increase in acetylcholine production seen after a single administration of citicoline, in microdialysis studies [69].

Plataras et al. [70] investigated the effect of different citicoline concentrations (0.1–1 mM) on acetylcholinesterase, Na+/K+-ATPase and Mg++-ATPase activities in homogenates of adult rat whole brain and in pure (nonmembrane bound) enzymes. After a 1–3 h citicoline preincubation, this drug induced a maximal stimulation of 20–25% (p < 0.001) for acetylcholinesterase and 50–55% (p < 0.001) for Na+/K+-ATPase, but it did not influence Mg++-ATPase activity. Citicoline can stimulate brain acetylcholinesterase and Na+/K+-ATPase independently of acetylcholine and noradrenaline and these effects could account for the clinical effects of the drug [70].

Başkaya et al. [71] examined the effect of citicoline on secondary injury factors, brain edema and blood−brain barrier breakdown, after TBI using a model of controlled cortical impact in rats. Brain edema was evaluated using the wet-dry method 24 h postinjury, and blood−brain barrier breakdown was evaluated by measuring Evans blue dye extravasation with fluorescein 6 h after TBI. After the injury, treated animals received intraperitoneal injections of citicoline (50, 100, or 400 mg/kg two times after TBI (n = 8–10 animals in each group)) and controls animals received saline solution (8 animals). TBI produced an increase in the water content and in measuring Evans blue dye extravasation in the injured cortex and the ipsilateral hippocampus. Citicoline at a dose of 50 mg/kg had no significant effect. At a dose of 100 mg/kg, citicoline attenuated Evans blue dye extravasation in both regions, although it reduced brain edema only in the injured cortex. In both regions, 400 mg/kg of citicoline significantly decreased brain edema and blood−brain barrier breakdown. Thus, the authors demonstrated a dose-dependent neuroprotective effect of citicoline in a model of experimental TBI, suggesting that this drug could be an effective neuroprotective agent on secondary injuries that appear following TBI.

Dempsey et al. [72] investigated whether citicoline protects the hippocampal neurons after controlled cortical impact-induced TBI in adult rats. Citicoline (100, 200, and 400 mg/Kg) or saline were injected intraperitoneally into the animals twice (immediately postinjury and 6 h postinjury). Seven days after the injury, the rats were neurologically evaluated and killed, and the number of hippocampal neurons was estimated by examining thionine-stained brain sections. By 7 days postinjury, there was a significant amount of neuronal death in the ipsilateral hippocampus in the CA2 (53 ± 7%, p < 0.05) and CA3 (59 ± 9%, p < 0.05) regions and a contusion (volume 34 ± 8 mm³) in the ipsilateral cortex compared with sham-operated control animals. Rats subjected to TBI also displayed severe neurological deficit at 7 days postinjury. Treating rats with citicoline at doses of 200 and 400 mg/Kg significantly prevented TBI-induced neuronal loss in the hippocampus, decreased cortical contusion volume, and improved neurological recovery.

Menku et al. [73] demonstrated a synergistic effect in the association of propofol with citicoline in an experimental model of TBI in rats, resulting in a higher reduction of the lipidic peroxidation when the drugs were administered together, concluding that this combination therapy may become a feasible option for the treatment of head injury.

Jacotte-Simancas et al. [74], using a model of controlled cortical impact injury in rats, studied the effects of citicoline and/or voluntary physical exercise on the related memory deficits and on neurogenesis and neuroprotection. Forty-eight male Sprague Dawley albino rats, six-weeks old, were randomly assigned to one of five experimental conditions, according to whether they were sham operated or had received TBI, and according to the treatment administered: citicoline vs. vehicle and exercise (E) vs. sedentary conditions. Citicoline (200 mg/kg) was administered by intraperitoneal route 4 h after surgery, and thereafter every 24 h until completing five injections. Citicoline improved memory deficits at short and long-term, while physical activity only in the long-term test. Physical activity increased cell proliferation and neurogenesis, and citicoline reduced the interhemispheric differences in the volume of the hippocampal formation. The combined effects of citicoline and physical exercise did not show any synergy, even the present data could suggest that the combined treatment with citicoline and physical exercise should be avoided in patients after TBI.

Qian et al. [75] designed an experimental study to investigate the neuroprotective effects of citicoline on a model of closed head injury in rats. Citicoline (250 mg/Kg IV 30 min and 4 h after injury) lessened body weight loss, and improved neurological functions significantly at 7 days. Treatment with citicoline was associated with a decrease of brain edema and of blood−brain barrier permeability, an enhancement of the activities of superoxide dismutase and the levels of glutathione, and with a reduction of the levels of malondialdehyde and lactic acid. Moreover, citicoline suppressed the activities of calpain, and enhanced the levels of calpastatin, myelin basic protein and αII-spectrin in traumatic tissue 24 h after injury. Citicoline was also able to attenuate the axonal and myelin sheath damage in corpus callosum and the neuronal cell death in hippocampal CA1 and CA3 subfields 7 days after injury. Authors consider that these findings provide additional support for the use of citicoline in the management of TBI.

Gan et al. [76], in an in vivo TBI zebrafish model, demonstrated that microglia, considered the resident macrophages of the central nervous system, accumulated rapidly after the injury. To perform its function, activated microglia secreted two types of cytokines, including proinflammatory interleukins and anti-inflammatory factors, helping to remove injured neurons and restore the homeostasis of the central nervous system. Citicoline was able to induce a further activation of microglia, and this was related with the reduction of neuronal apoptosis and the promotion of neuronal proliferation around the lesioned site associated with the use of citicoline.

Furthermore, some positive neuroprotective effects of citicoline have been published on different models of traumatic spinal cord lesion [77,78,79,80].

Citicoline holds some biochemical, pharmacological, and pharmacokinetic characteristics to be a potentially useful drug for the management of TBI [81] and citicoline has an appropriate profile for the treatment of the different brain ischemia related disorders, having different neuroprotective and neurorestorative properties (Table 2) [82,83,84,85].

As it has been shown in the previous section about the experimental data showing that citicoline can induce significant positive effects in different experimental models of TBI, citicoline seems to be a suitable drug for the management of patients suffering TBI. Thus, it is possible to say that citicoline has a pleiotropic effect on several steps of the ischemic cascade involved in the development of the TBI [84].

Many years ago, the effect of citicoline stimulating the ascending reticular activating system at the brain stem level was postulated to explain the effects of the drug on the consciousness level [84]. As citicoline could be considered as a valid pharmacological treatment for TBI, many clinical studies have been performed over time to assess if the drug would have beneficial effects in the treatment of patients with TBI.

There are early published clinical data showing that citicoline can lead to recovery from neurological clinical symptoms and a return to a conscious state with an excellent safety profile [86].