The potential of melatonin in reducing morbidity–mortality after craniocerebral trauma

Authors


Address reprint requests to M.D. Maldonado M.D., Ph.D., Department Medical Biochemistry and Molecular Biology, University of Seville Medical School, Avda. Sánchez Pizjuán 4, 41009, Spain.
E-mail: aibar@us.es

Abstract

Abstract:  Craniocerebral trauma (CCT) is the most frequent cause of morbidity–mortality as a result of an accident. The probable origins and etiologies are multifactorial and include free radical formation and oxidative stress, the suppression of nonspecific resistance, lymphocytopenia (disorder in the adhesion and activation of cells), opportunistic infections, regional macro and microcirculatory alterations, disruptive sleep–wake cycles and toxicity caused by therapeutic agents. These pathogenic factors contribute to the unfavorable development of clinical symptoms as the disease progresses. Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine endogenously produced in the pineal gland and in other organs and it is protective agent against damage following CCT. Some of the actions of melatonin that support its pharmacological use after CCT include its role as a scavenger of both oxygen and nitrogen-based reactants, stimulation of the activities of a variety of antioxidative enzymes (e.g. superoxide dismutase, glutathione peroxidase, glutathione reductase and catalase), inhibition of pro-inflammatory cytokines and activation–adhesion molecules which consequently reduces lymphocytopenia and infections by opportunistic organisms. The chronobiotic capacity of melatonin may also reset the natural circadian rhythm of sleep and wakefulness. Melatonin reduces the toxicity of the drugs used in the treatment of CCT and increases their efficacy. Finally, melatonin crosses the blood–brain barrier and reduces contusion volume and stabilizes cellular membranes preventing vasospasm and apoptosis of endothelial cells that occurs as a result of CCT.

Introduction

Severe trauma to the head leads to primary and secondary brain injuries, often called craniocerebral trauma (CCT). Primary brain injury results from mechanical forces applied to the head at the time of impact (e.g. cranial fracture and hemorrhage). Secondary brain injury occurs following the primary impact. Numerous pathophysiological mechanisms have been postulated to explain the progressive tissue damage produced by secondary injuries (free radical generation, neuroinflammatory response, adhesion molecule generation, cytokine and chemokine production and infections, among others) [1, 2]. Thus, several pharmacological therapies have been developed in an attempt to limit these injuries [2, 3].

Melatonin is an indoleamine with a wide range of antioxidative properties including direct free radical scavenging and indirect stimulatory actions on a variety of antioxidative enzymes (e.g. superoxide dismutase, glutathione peroxidase, glutathione reductase, etc.) which further promote its ability to reduce the toxicity of radicals and their associated reactants [4–6]. Melatonin is endogenously produced in all vertebrates (in the pineal gland and a variety of other organs) and exogenously acquired (in the diet). Melatonin has been widely tested to determine its efficacy in protecting against free radical damage in experimental models of brain edema, hemorrhagic shock, cortical infarction, post-traumatic epilepsy (PTE), ischemia and reperfusion injury and other conditions [7–10]. In these experimental settings, melatonin proved highly effective in reducing molecular damage, cellular death and tissue loss and, when tested, organ function was also preserved. Melatonin is a lipophilic molecule and it crosses all morphophysiological barriers [11] including the blood–brain barrier [12, 13].

Normally, the brain is extremely prone to oxidative stress due, in part, to its high rate of oxygen consumption. Energy is constantly consumed in the brain by both sensory and motor nerves as well as by glia [14]. The brain also has a high mitochondrial population to support the demanding signal transduction activities. Mitochondria are the repository of enzymes which are involved in several aspects of cell metabolism, including the oxidative and phosphorylation enzymes of the Krebs cycle. Mitochondria are absolutely essential for the energy demands of the brain. CCT results in conspicuous mitochondrial damage by free radicals, which limits ATP production and promotes cellular death [15, 16]. By reducing free radical damage, melatonin improves mitochondrial ATP production [17, 18], reduces electron leakage from respiratory complexes and scavenges radicals that are generated at the mitochondrial level [19].

Craniocerebral trauma also results in a depression of the immune defenses. This opens the possibility for opportunistic infections and sepsis, which can lead to the evolution of the development of illness and even to death [20–22]. The lethal effects of opportunistic infections and sepsis are associated with the production and release of numerous pro-inflammatory biochemical mediators including cytokines, chemokines, adhesion and activation molecules, and the development of a profound T-cell lymphocytopenia by trauma [23–25]. As melatonin has potent actions as an antioxidant, a cytokine modulator, in reducing adhesion and activation of molecules, promoting sleep, and diminishing the toxicity of other drugs, this review summarizes published data which suggest the use of melatonin to improve the outcome of patients with CCT.

Craniocerebral trauma and free radical damage

Chemically, free radicals are unstable in nature and have a tendency to extract electrons from neighboring molecules leading to the formation of a new radical species. Once this activity has been initiated, it propagates and amplifies via chain reactions. A large number of free radicals and their metabolites contain oxygen or nitrogen atoms; thus, they are commonly referred to as reactive oxygen species (ROS) and reactive nitrogen species (RNS), respectively. Fig. 1 summarizes the generation of common ROS and RNS as they occur in the brain and elsewhere. Under normal conditions, the brain is particularly sensitive to lipid peroxidation because of its high concentration of polyunsaturated fatty acids, its low antioxidant capacity, and its high rate of oxygen consumption. Furthermore, brain tissue contains high levels of iron and copper, which promote the formation of oxygen free radicals. For these reasons, when the brain is exposed to the same concentrations of oxidants, the resulting molecular damage is several times greater than that which occurs in cells from other organs under the same circumstances [14]. Fig. 2 summarizes the conditions that occur during CCT which lead to tissue damage. CCT causes two types of injuries in neural tissue. One is the primary injury which occurs at the time of impact. The other is secondary injury and it is a result of the pathophysiological alterations secondary to trauma, some of which are hypotension, ischemia and reactants produced by the primary injury [26]. Monoamines, ROS/RNS, neuropeptides, arachidonic acid metabolites and changes in intracellular calcium are some of the alterations caused by the primary injury that take place and induce secondary injury.

Figure 1.

 Generation of reactive oxygen and nitrogen species in the brain. NADPH, nicotinamide adenine dinucleotide phosphate; NADH, reduced nicotinamide adenine dinucleotide; NOS, nitric oxide synthase; SOD, superoxide dismutase.

Figure 2.

 Causes of tissue damage that develop as a consequence of craniocerebral trauma. XO, xanthine oxidase.

A variety of experimental studies have documented that brain injury results in an increase of ROS and RNS. These agents contribute to a variety of diseases in the central nervous system (Fig. 2) [14, 27]. Both ROS and RNS incessantly mutilate beleaguered, essential molecules (e.g. lipids, proteins, DNA, RNA, etc.). The accumulated molecular debris, that is a consequence of these reactions, contributes to brain damage. The persistent mangling meted out by these agents leads to molecular, cellular and eventually organismal dysfunction, thereby compromising the quality of life and even survival. The damage resulting from these biological reactants is referred to as oxidative stress or the total oxidative load. Oxidative damage can be repaired but, when it is not, it leads to disorders [28].

Craniocerebral trauma is associated with a strong inflammatory response which may have a significant impact on brain edema formation and neuronal cell death. One critical event in the development of brain edema is breakdown of the blood–brain barrier which may be initiated and regulated by several proinflammatory mediators including ROS and RNS [1, 2, 29]. Various studies have clearly demonstrated that the nuclear factor-κB (NF-κB) plays a central role in the regulation of genes responsible for the generation of mediators or proteins in inflammation [e.g. tumor necrosis factor-α (TNF-α), interleukin (IL-1), VCAM-1, ICAM-1 and inducible NO synthase (iNOS)]. Under usual conditions, NF-κB is present within the cytoplasm in an inactive state, bound to its inhibitory protein (IκB-α). However, inflammatory situations initiate an intracellular signaling cascade which results in the phosphorylation of IκB-α at serine residues 32 and 36. Once released from its inhibitory protein, NF-κB translocates to the nucleus where it orchestrates the transcription of a number of pro-inflammatory genes [30–32]. NF-κB mediates some of the effects of ROS because the interaction between the inhibitory protein IκB and NF-κB is regulated by protein kinases that contain several redox-sensitive cysteine residues in critical kinase domains [33].

Craniocerebral trauma with hemorrhagic cortical infarction results in extravasation of blood and breakdown of red blood cells and hemoglobin. Iron released from hemoglobin, and hemoglobin itself, are associated with the generation of ROS and RNS. ROS and RNS are involved in the mechanism of seizures induced by iron in the rat brain, an experimental animal model for PTE [8]. ROS are responsible for the induction for peroxidation of neural lipids, i.e. injury of neural membranes, and also could induce disorders in excitatory and inhibitory neurotransmitters [7].

The transitory discontinuation of the blood supply to the brain is encountered in conditions such as accidental or iatrogenic trauma and sepsis; this creates periods of neural hypoxia/anoxia, and is followed by the reperfusion with oxygenated blood when the obstructed vessel is re-opened. The reoxygenation of the tissue leads to massive ROS/RNS generation [28]. When progressive post-traumatic ischemia occurs, it is imperative that the tissue is reoxygenated as quickly as possible because neural tissue deprived of oxygen dies within minutes. However, reperfusion of ischemic tissue with oxygenated blood causes further damage of the reperfused area. Thus, both ischemia and reperfusion (I/R) have negative consequences. As already noted, much of the molecular destruction that occurs in these situations is a consequence of the generation of ROS/RNS [34].

Antioxidants and craniocerebral damage

Melatonin, a product of tryptophan metabolism, is a multifaceted free radical scavenger and antioxidant [35]. A variety of experimental studies have demonstrated that melatonin has marked neuroprotective effects both in vitro and in vivo. Much of this protection is a consequence of its antioxidant activities; thus, melatonin (a) protects nuclear and mitochondrial DNA, membrane lipids, and cytosolic proteins from oxidative damage [36, 37]; (b) it blocks oxidative mediators that initiate the neuroinflammatory response after traumatic brain injury, e.g. by reducing NF-κB activation; (c) it inhibits the pro-oxidative enzyme nitric oxide synthase (NOS); (d) it detoxifies ROS and RNS; (e) it stimulates antioxidative enzymes; (f) it improves oxidative phosphorylation. Additionally, melatonin has a wide intracellular distribution and stabilizes neuronal membranes [12] (Fig. 3). The rapid transfer of melatonin into the central nervous system as well as its quick uptake into cells are essential features in reference to its ability of to protect against tissue loss. This contributes to melatonin's efficacy in reducing acute neural I/R injury, decreasing infarct volume, reducing the number of apoptotic neurons and glia, inhibiting gliosis and limiting cerebral edema and levels of toxic products that cause oxidative damage [38]. Furthermore, several melatonin metabolites that are generated when melatonin interacts with toxic reactants are themselves direct free radical scavengers, e.g. N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK). Furthermore, AMK is a potent inhibitor of the pro-oxidative enzyme, NOS [12, 39–43]. These combined actions greatly increase the efficacy of melatonin in protecting against free radical damage.

Figure 3.

 The multiple actions by which melatonin provides protection against craniocerebral trauma.

Compelling data documenting the protective effects of melatonin against brain damage from traumatic injury has been published by Sarrafzadeh et al. [14] and Kerman et al. [27]. These groups used models of brain injury in rats and rabbits, respectively. The animals received melatonin (10 mg/kg i.p.) 20 min before trauma, immediately after the injury, and 1 and 2 hr after trauma during daytime and nighttime. They demonstrated that melatonin significantly reduces contusion volume. However, when melatonin was injected in the late post-traumatic period of CCT it was ineffective in reducing neural damage [44]. Recently, Ozdemir et al. [10] used a model of hippocampal damage and spatial memory deficits in immature rats. The animals received melatonin at doses of 5–20 mg/kg of body weight i.p. immediately after induction of traumatic injury. They demonstrated that melatonin significantly attenuated trauma-induced neuronal death in hippocampal CA1 region, CA3 region and dentate gyrus, and improved spatial memory deficits when it was administered immediately after induction of traumatic injury. Beni et al. [32] investigated, in a model of closed head injury (CHI) in mice, the effects of melatonin on neurobehavioral recovery, low molecular weight antioxidants and activation of redox-sensitive transcription factor nuclear NF-κB and AP-1. Some of the mice received melatonin (1, 5 and 10 mg/kg i.p.) 1 hr after CHI and they were evaluated, by neurological severity score, at 24 hr and re-evaluated at 4 and 7 days after injury. Melatonin induced neuroprotection, only at a dose of 5 mg/kg. The neuroprotective dose facilitated clinical recovery during the first post-traumatic week. Melatonin potentiated post-CHI brain antioxidants and finally melatonin did not affect early phase CHI-induced activation of NF-κB and AP-1; however, it totally blocked the late-phase robust activation of NF-κB and reduced that of AP-1 to half the basal level. In this study, neuroprotection by melatonin was mediated via potentiation of other brain antioxidants (e.g. ascorbic acid) thus altering the redox state of the cell and consequently attenuating NF-κB and AP-1 activation. On the other hand, the decrease of NF-κB and A-P1 activation also reduced pro-inflammatory cytokine production and this would be associated with better immune state, fewer infections and a more favorable outcome.

Iron is associated with oxidative damage to the brain. The free iron concentration of brain is known to be elevated after head injury or hemorrhagic cortical infarction [7, 8] and these patients often manifest PTE [45, 46]. In these situations, iron ions generate oxygen free radical species via the Fenton reaction leading to neuronal macromolecular peroxidation and seizures. Melatonin inhibits iron-induced seizures by scavenging free radicals, and in a variety of models melatonin has proven to be an effective prophylactic agent against seizures.

Of interest is the work of Makarov et al. [47] who studied 66 patients with various diseases of the nervous system. Among them were patients with traumatic subarachnoid hemorrhage. In this study they demonstrated, using a fluorimetric method, that the melatonin concentration in cerebrospinal fluid was elevated as a result of the neural injures. They surmised that melatonin may exhibit a compensatory response due to damage and the indole may be protective in these situations.

Natural antioxidants, such as condensed tannins, β-carotene, vitamin E, vitamin C, glutathione, uric acid and antioxidative enzymes scavenge ROS and/or RNS and have prophylactic benefits in protecting against oxidative stress [7, 35]. Invariably, a great deal of molecular damage and cellular death occurs under conditions of CCT and all physiological antioxidants combined are incapable of resisting the tissue destruction that accompanies these damaging episodes. Thus, any antioxidant used to effectively reduce free radical damage under such severe conditions must be administered in pharmacological doses. In particular reference to the brain, melatonin has distinct advantages over some other commonly used free radical scavengers. Unlike vitamin E, which must be administered in high doses over long periods of time to substantially improve its levels in the brain, melatonin enters the brain within minutes after its administration even when peripherally given in doses much lower than the amounts of vitamin normally taken. Additionally, vitamin E, as a result of its high lipophilicity, is confined to the lipid-rich environment of the cell, e.g. membranes; this limits its efficacy in curtailing damage to cytosolic proteins, nuclear DNA and so on. By comparison, melatonin seemingly has a much wider intracellular distribution and is known to protect against lipid peroxidation in membranes, protein carbonyl formation in the cytosol, as well as nuclear and mitochondrial DNA fragmentation [28].

Another advantage melatonin has in reference to its protective antioxidative actions is related to its concentration. Studies indicate that melatonin levels in the cerebrospinal fluid of the third ventricle are orders of magnitude higher than those in the peripheral circulation [34, 48, 49]. From this fluid, melatonin could readily be absorbed into the surrounding brain tissue to protect it from oxidative damage and/or for other toxic actions.

Craniocerebral trauma and the immune system

Trauma is the principal cause of death of people from age 18–40 yr. Despite improved diagnosis and management, infection remains the most common complication in patients surviving the initial injury [20, 50–52]. Prolonged periods of intensive care and respiratory support predispose them to infective nosocomial complications. Some 5% are reported to die as a result of septic complications [20, 53, 54]. These patients, in the absence of significant systemic injury and as a result of severe head injury, are unable to mount an effective immune response and consequently are more susceptible to infection [22].

The immune system includes a complex series of cells distributed throughout the body, functioning in a coordinated manner in defense against infective agents. Because of a generalized decrease in immune function, CCT patients have an susceptibility to infectious diseases and a decreased ability to develop protective immunity. There are two general types of immune responses: humoral immunity, wherein pathogen-specific antibodies are secreted into the serum and body fluids, and cellular immunity, wherein activated lymphocytes foster inflammatory reactions and kill infected cells. Both types of immunity are markedly diminished in CCT patients. The most dramatic change is the hyporesponsiveness of the T helper cell (TH). Because TH plays an important role in cytotoxic T lymphocytes and B cell stimulation, all aspects of immunity are impaired [21].

Numerous data support the idea that trauma injury produces depressed immunological functions [21]. These parameters include low skin test reactivity, a profound T-cell lymphocytopenia, reversal of the T-helper/suppressor cell ratio, decreased neutrophil and B-cell functions, and increased acute phase reactants and pro-inflammatory cytokine (IL-1, IL-6, IL-8 and TNF-α) production. In severe trauma patients, concurrent with a substantial drop in absolute numbers of T-lymphocytes, there is an increase in T-cells bearing membrane molecules involved in activation (CD25, CD69 and CD71) and adhesion (CD11c, CD49a and CD54); this rise in lymphocyte adhesiveness may be responsible for a rapid recruitment of T-cells from the blood compartment, leading to a depletion of circulating T-lymphocytes [50, 54].

We postulate that the immunodeficiency suffered by CCT patients is not due solely to the loss of blood, but also to the lymphocytes that are adherent to the walls of the vessels and, therefore, they cannot perform their normal actions. These changes are transient with the lymphocyte counts returning to normal values within 5–7 days post-CCT [54] simultaneous with the disappearance of the activation–adhesion phenotype found in peripheral blood lymphocytes. In contrast, the patients who do not survive, do not recover the number of lymphocytes and the activation and adhesion molecules remain elevated [21, 55].

The relationship between melatonin and the immune system has been document in numerous investigations [56, 57]. The secretory product of the pineal gland, i.e. melatonin, plays an important role in modulating immune responses. Mice in which melatonin secretion by the pineal gland is inhibited by light exposure at night or after administration of the beta-adrenergic receptor blocker, propanolol, have associated suppressed humoral and cellular immunological responses. One important function of melatonin is to modulate of the production/secretion of cytokines [58, 59]. Chronic administration of melatonin increases T-helper cell activity and IL-2 production in human lymphocytes [60, 61]. Furthermore, as melatonin reduces the production of adhesion molecules (e.g. CD11c, CD49a and CD 54, which promote the adhesion of leucocytes to the endothelial lining of blood vessels) [34], the increases of these surface adhesion molecules in the traumatized brain may result in a depletion of circulating T-cells and cell redistribution from blood to the tissues. This may be responsible for the reduction in cellular immunity observed following severe head injury. In clinical trials carried out in neonates with sepsis, respiratory distress and bronchopulmonary dysplasia, melatonin reduced proinflammatory cytokines including IL-6, IL-8 and TNF-α and modified serum inflammatory parameters, improving the clinical course of surgically treated neonates [62–65]. The lethal effects of CCT and secondarily the septic shock are associated with the production and release of numerous pro-inflammatory biochemical mediators including cytokines IL-1, IL-6, IL-8 and TNF-α. As melatonin has a marked ability to reduce pro-inflammatory cytokines and adhesion molecules, melatonin favorably alters the course of immunology disorders [25, 59, 66].

Melatonin receptors are detected in various human immune cells (membrane MT1 and nuclear receptors RZR/RORα) [57, 67, 68] and the immune systems of mice have both MT1 and MT2 membrane melatonin receptor [57]. Blockade of membrane receptors with luzindole or nuclear receptors with CGP 55644 in human lymphocytes results in a drop in IL-2 and IL-2-receptor production [66]. Furthermore, cultured human lymphocytes synthesize and secrete melatonin suggesting that melatonin may also act through autocrine/paracrine mechanisms on the immune system [68, 69].

Considering the multiple actions of melatonin in the immune system, it appears that melatonin, which has little or no adverse effects, may be considered a safe and effective therapeutic agent for restoring depressed immunological function in CCT patients.

Craniocerebral trauma and sleep

Sleep is disrupted in patients with severe traumatic brain injury [70, 71]. This disruption has been documented by: (a) observation of their sleep–wake cycles, (b) self-reported sleep–wake patterns by CCT patients, and (c) objective measures (e.g. sleep architecture by electroencephalograph) [71]. Noise, light levels, data acquisition about vital signs and the trauma itself may relate to a profound impact on the overall health of the CCT patient, both from a physiological and a psychological standpoint. Also, sleep disruption changes cortisol levels, causes impaired immune responses and impairs cognitive function. Also, symptomatic and circadian disruption is linked to increased cancer risk [70, 72]. Disruption of sleep is also known to impair healing through many complex connections with other homeostatic processes.

There is a dearth of research on improving sleep and reversing the negative effects of sleep disruption on homeostasis in critically-ill patients. Melatonin is an agent with the capacity of entraining of circadian rhythms (chronobiotic effect) as well as having anti-insomnia effects and neuroprotective actions [3, 73]. These actions have been demonstrated in humans by clinical trials and in experimental animal models [71, 74]. In view of these observations and consistent with our findings, melatonin may provide a viable strategy to reduce circadian disruption and enhance quality of life in patients with severe traumatic brain injury.

Craniocerebral trauma and the toxicity caused by therapeutic agents

Craniocerebral trauma patients are treated with numerous drugs to improve their condition and to keep them alive. Some of the agents used include nonsteroidal anti-inflammatory drugs (NSAIDs), steroidal anti-inflammatory drugs (SAIDs), anti-psychotic, anti-agitation and anti-convulsive drugs (e.g. phenobarbital, diazepam, haloperidol), morphine and derivatives, metoclorpramide or transfusion of blood and iron, etc. Although these treatments have obvious beneficial effects, the drugs can also inflict collateral damage when administered. Thus, while they may be helpful for a specific condition, at the same time they can subvert molecular physiology and cellular function to the extent that they eventually compromise the overall well being of the organism. This damage if often mediated by free radicals and related reactants. Currently, there is great interest in the possibility of quelling this biological destruction with the use of agents that quench radical species and their toxic metabolites. Because of its actions, and possibly others that remain to be defined, melatonin reduces the toxicity and increases the efficacy of a large number of drugs whose adverse effects are well documented [12]. Herein, we summarize the beneficial effects of melatonin when it is used alone or combined with drugs currently administered for clinical treatment of patients with traumatic brain injury.

Melatonin

The full range of physiological actions of melatonin is not completely known. Therapeutic treatment of CCT using melatonin alone or in combination with other drugs deserves serious investigation. Prophylactic melatonin treatment in primary brain injury would block the progressive tissue damage produced by oxidative stress and immunology disturbances following secondary brain injury [9, 25, 75–77]. Furthermore, melatonin also has anxyolitic, sedative, and anticonvulsant properties, both in humans and animals [78]. These latter properties of melatonin are fundamental for the treatment of patients with CCT since they are drugs given to reduce agitation and convulsions after head trauma.

Nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs can induce ulcerative lesions of the gastrointestinal tract. The mechanism by which NSAIDs cause ulcers seems to involve inhibition of prostaglandin synthesis, a direct irritanting action on the mucosa and by the generation of ROS, as well as other yet undefined processes [79, 80]. Some NSAIDs also can provoke a reduction in Cu/Zn-SOD and mitochondrial Mn-SOD activity and GSH levels [81].

Recent studies [82, 83] demonstrate that melatonin protects against NSAID-induced gastric mucosal damage. The ability of melatonin to preserve the integrity of the mucosal epithelium may, in part, involve membrane melatonin receptors. Antioxidative processes mediated by melatonin may also have a role in melatonin's protective actions as the ability of melatonin to stimulate GSH synthesis and recycling, as well as the activity of antioxidative enzymes, are presumed to involve melatonin's interaction with receptors. Furthermore, the free radical scavenging actions of melatonin, processes that are receptor independent, may also aid this molecule in reducing NSAID-mediated gastric damage.

Steroidal anti-inflammatory drugs or corticosteroids

Craniocerebral trauma causes a number of biochemical and cellular alterations leading to tissue necrosis and cell death. SAIDs such as methylprednisolone, dexamethasone, cortisol and other glucocorticoids, by reducing edema and inflammation, are pharmacological agents with proven clinically benefits on CCT and in other post-traumatic neurological disorders [84]. Following CCT, there are significant increases in lipid peroxidation and in the neuroinflammatory response [1, 2]. This supports the use of SAIDs in situations in both animals and patients with damage to central or peripheral nervous system [85–87]. However, in vivo dexamethasone reportedly reduces the total number of leukocytes and lymphocytes (B and T-cell) in peripheral blood and bone marrow. Also, reductions in thymic and spleen activity and lymphoid tissue mass have been noted [86]. Immunosuppression generated by SAIDs would certainly be undesirable collateral damage which increases the risk and vulnerability of CCT patients to infection.

A functional connection between the neuroendocrine and the immune systems has been established. Of particular interest is the finding that melatonin and corticosterone exert modulatory effects on immune function [88]. Treatment of spinal cord injured animals with a combination of melatonin and methylprednisolone proved highly effective in protecting neurons and their adnexa [85]. Melatonin largely prevents the immunosuppression (leucocytopenia, reduction of B and T cell, bone marrow, lymphoid tissues, etc.) generated by SAIDs [86, 89]. Theses findings suggest that melatonin and SAIDs may have a synergistic actions in the modulation of acquired immune deficiency indicating that co-treatment with an SAID plus melatonin would be more effective than melatonin or SAID treatments alone.

In addition to the immunosuppression, SAIDs induce ulcerative lesions of the gastrointestinal tract [90]. Indeed, a randomized, placebo-controlled trial, showed that treatment with a high dose of corticosteroids was associated with the highest incidence of bleeding on the gastroduodenal mucosal. The mechanism by which SAIDs induce ulcers is not totally clarified but seems to involve an increase of release of hydrochloric acid and a direct irritant action on the mucosa [91]. In another study, it was shown that patients who are severely injured may develop stress ulcers [92]. The hypersecretion of catecholamines resulting from severe stress, such as occurs in hypovolemia, sepsis, shock, multiple organ failure, trauma, burns, and major surgery, contributes to ischemia of the gastric mucosa by producing vasoconstriction. At the same time, the ischemia induces diffusion of hydrogen ions and the breakdown of the gastric mucosal barrier [93]. Thus, the patients with CCT have a higher incidence and risk for the development of hemorrhage and perforating gastrointestinal lesions. Melatonin dose-dependently reduces gastric mucosal damage in ulcerative models of rats by scavenging hydroxyl radicals [94, 95] and it does so much more effectively than commonly-used anti-ulcer drugs, e.g. omeprazole and famotidine [96]. The ability of melatonin to preserve the integrity of the gastric mucosal epithelium that is normally aggravated by SAID treatment is not trivial considering the wide-spread use of SAID and the frequency with which they induce serious gastrointestinal complications.

Ranitidine and omeprazole

These two drugs are widely used as anti-ulcer medications [12, 94–96]. Ranitidine blocks histamine-stimulated acid secretion in the stomach while omeprazole, a substituted benzimidazole, completely interrupts acid secretion. Both drugs, however, have side effects including, among others, headache, dizziness, diarrhea and skin rashes. When melatonin is combined with either ranitidine or omeprazole, the prescription drugs are much more efficient in reducing stress-mediated mucosal breakdown in the stomach.

Haloperidol

Haloperidol, a dopamine-receptor antagonist, is used in a variety of clinical situations including CCT and for the treatment of psychoses and agitation. This drug is dosage-dependently cytotoxic especially for hippocampal cells and related areas [97, 98]; the basis for the toxicity is believed to be a result of induced oxidative stress. One side effect of haloperidol is clinically characterized by tardive dyskinesia which is a major limitation of neuroleptic therapy. Melatonin's protective effect against haloperidol was documented in patients in a thorough report by Shamir et al. [99]. In a double-blind, placebo-controlled, cross-over study, this group found that melatonin (10 mg daily), given for only 6 wk to patients exhibiting tardive dyskinesia due to antipsychotic drug treatment, markedly reduced the movement disorders as assessed by the Abnormal Involuntary Movement Scale. The difference, relative to the placebo-treated controls, was highly statistically significant and no adverse side effects of melatonin therapy were noted. Besides tardive dyskinesia associated with haloperidol (13 patients), this study also showed that the abnormal involuntary movements induced by other antipsychotics [i.e. chlorpromazine (four patients), perphenazine (three patients) and zuclopenthixol (two patients)], were also reduced when the patients were given melatonin.

While the sample size was small and the treatments varied, the universal response of the patients lends support to the authors’ suggestion that melatonin may be an effective treatment to attenuate tardive dyskinesia in patients being treated with antipsychotic mediations.

Phenobarbital

Phenobarbital is an antiepileptic agent widely used for treatment of partial and generalized tonic-clonic seizures; it also is used to treat CCT patients and for the treatment of convulsions and agitation [45]. A large amount of experimental work suggests that melatonin suppresses neuronal excitability [100, 101]; this prompted Molina-Carballo et al [102] to use melatonin as an adjunctive treatment in a child with myoclonic epilepsy. Within 1 month of beginning pharmacological doses of melatonin (100 mg daily) in combination with an ineffective dose of phenobarbital, the seizures abated. As the melatonin dose was reduced over time, the seizures reappeared and were again suppressed when melatonin was added. The mechanisms of melatonin's anticonvulsive activity may include inhibitory actions on glutamate receptors and a potentiations of the GABA-benzodiazepine receptor [12].

Morphine and derivatives.

Pain is a frequent symptom in patients with CCT. The causes are multiple: bone fracture, injury, loss of skin, hemorrhage, etc. This often necessitates the use of analgesic drugs such as morphine and derivatives. Melatonin by itself abates pain in some clinical situations [103, 104] but when combined with morphine or derivatives, the indole was shown to increase the antinociceptive effects of these drugs [105].

Blood transfusion and iron

In CCT patients suffering from anemia due to blood loss as a result of an accident, it may be necessary to administer blood transfusions or iron. Both of these agents have side effects which likely involve free radical damage. The administration of melatonin has been shown to prevent oxidative stress associated with both blood transfusions and iron treatment with no apparent side effects [12, 106].

Benefits of melatonin in the therapeutic treatment of patients with craniocerebral trauma

The evidence supporting the potential use of melatonin in reducing morbidity–mortality in patients with CCT is substantial and includes the following: (a) melatonin functions as a powerful free radical scavenger and antioxidant [5, 27, 44, 48], particularly in the central nervous system, and presumably has some basic actions within subcellular organelles which improve the ability of the cells (and thereby the tissues and organs) to function more efficiently and resist injury due to CCT. Indeed, melatonin seems to prepare cells to cope with disastrous situations. (b) As CCT is associated with lymphocytopenia, melatonin may favorably alter the course of neuroimmonological disorders and ameliorate the immunedeficiency that often results as a consequence of head injury. Leukocytes and other blood cells of the immune system possess functionally active membrane and nuclear melatonin receptors [57, 68, 69]. Consequently, melatonin has a variety of functions in immunomodulation and in inhibiting proinflammatory cytokines [21, 25, 57]. (c) CCT reportedly alters endogenous pineal melatonin secretion during the acute post-traumatic period. These disturbances are characterized by the absence of the normal melatonin circadian rhythm [3, 71]. In addition to its protective action as an antioxidant in the brain, the general opinion is that melatonin is also a synchronizer of circadian rhythms [107]. In humans melatonin reportedly improves sleep mechanisms which are important in patients with CCT who are often confined to intensive care units and have sleep deficiencies [73, 108]. (d) Melatonin is inexpensive and is available in pure form. Furthermore, when melatonin has been given in combination with drugs regularly used to treat CCT, the indole reduced their side effects and increased their efficacy. This has been shown for many chemotherapeutic agents habitually used to manage CCT patients [12]. Numerous studies in adult humans also indicate that melatonin has very low toxicity [109], and readily passes through all morphophysiological barriers [13]. It is much less toxic than conventional drugs used to treat CCT [12] and has even been used in newborn infants with no adverse side effects [62–65].

Concluding remarks

The data summarized herein are only a small sample of the massive literature which documents the ability of melatonin to reduce morbidity–mortality associated with CCT. What is obvious from the collective results of many investigations is the role of melatonin as a direct free radical scavenger and also its capacity to stimulate a number of antioxidative enzymes, which promotes its ability to reduce the toxic reactions of radicals and associated reactants in the central nervous system [5, 6, 110]. Melatonin is also an effective immunomodulatory agent in the treatment of immune dysfunction encountered after head trauma and hemorrhagic shock [59] and, thus, in may reduce the likelihood of opportunistic infections and septic shock in CCT patients [24, 25, 65, 111]. In view of the strikingly beneficial actions of this indole, as well as its low toxicity [12, 28, 112], it would seem imperative to make more widespread use of this molecule in medicine and specifically in intensive care units.

Acknowledgments

This work was supported in part by the research grant from Mapfre Foundation 2005–2006 and the Seville University (Spain), program number 2006/534, with code: 1808030101.

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