A NIVEL ESTATAL (CONSEJO SUPERIOR DE DEPORTES)

Traumatic brain injury is known to impair cerebral hemodynamic function.

Understanding and correcting hemodynamic dysfunction has become an impor-tant therapeutic target in arresting or reversing secondary damage resulting from brain trauma, ischemia, or neurodegeneration from worsening or becoming irreversible.

Restoring cerebral blood flow to preinjury levels is a key therapeutic target because when trauma, ischemia, or neurodegenerative damage occurs, nutrient delivery to the brain, including oxygen and glucose, which are the primordial mol-ecules necessary for maintaining normal brain cell function, can fail to supply brain cells’ demand to carry out all metabolic activities.

Since most of the energy of the brain is obtained exclusively from aerobic meta-bolic processes, a low supply and high demand for energy nutrients in the presence of trauma not only creates a neuronal energy crisis from the uncoupling of cerebral blood flow and metabolism but also ensures that motor, sensory, and cognitive func-tions remain impaired or irreparably lost. Additionally, the brain cannot manufac-ture or store energy nutrients such as glucose and is totally dependent on optimal cerebral blood flow to deliver these nutrients as needed.⁸⁹

We will see in the “Cerebral Hemodynamic Function in Brain Injury” section how brain trauma, stroke, and neurodegenerative conditions such as vascular demen-tia (VaD) and AD can uncouple cerebral blood flow and metabolism by involving a multifactorial list of factors that can act independently or as part of a pathologic cascade to alter blood flow to the brain. This analysis is relevant in attempting to

Prevents glutamate excitoxicity Free radical scavenger

Suppresses tissue factor

Blocks NMDA receptors

Blocks Na⁺,Ca⁺ cell influx

Inhibits toxic cytokines

Dimethyl sulfoxide in CNS injury

Anti-inflammatory Increases CBF (via prostaglandins)

Reduces brain edema Platelet deaggregator Vascular smooth muscle

cell relaxant

FIGURE 8.1 Reported biological activity for DMSO in central nervous system (CNS) injury. Most of the molecular and cellular factors listed in this illustration result from physical and physiological damages to the brain, but the chronology and the degree of damage caused by many of these factors on brain tissue have not been entirely clarified.

understand the role DMSO plays when used therapeutically to improve the outcome following primary injuries to the brain.

The most common factors influencing cerebral blood flow are cardiac output, ICP, blood gas content, vasoactive substances such as nitric oxide, PGs, endothelins, thromboxane A₂, endothelial-derived hyperpolarizing factor, adenosine, and K⁺ and H⁺ ions.³⁴

CPP is the driving force that moves blood flow through the cerebral circulation.

Cerebral blood flow varies directly with CPP, which is defined as the difference between mean arterial and ICPs, and inversely with cerebrovascular resistance. Normally, CPP can be considered equal to mean arterial pressure. Immediately after brain trauma, two compensatory mechanisms to counter low CPP from vessel stenosis or vessel obstruc-tion (e.g., platelet aggregaobstruc-tion) can be activated: autoregulaobstruc-tion and oxygen extracobstruc-tion fraction. As CPP falls, cerebral blood flow is maintained by the vasodilation of resis-tance arterioles, which results as reflex autoregulation is activated.⁹⁰

If cerebral blood flow falls beyond the autoregulatory range, oxygen extraction factor kicks in order to maintain oxygen levels, but up to a point.⁹¹ When CPP falls beyond the point where neither autoregulation nor increases in oxygen extraction fraction can compensate, ischemia resulting in an insufficient delivery of oxygen to meet metabolic demands will occur with almost certain damage to the brain.

Cerebrovascular resistance results mainly from the friction caused by the blood on the vessel wall and is determined by vessel length, which does not vary signifi-cantly in normal physiology, and from blood viscosity, which normally stays within a small physiological range. Blood viscosity, however, increases with blood loss, with ischemia, or with the inability of RBCs to deform in tight capillary lumens.⁹² Blood viscosity depends on hematocrit, erythrocyte aggregability, and plasma viscosity.⁹³

When blood viscosity increases, cerebral blood flow decreases, and inversely, when viscosity is decreased, cerebral blood flow is increased. This occurs because resistance arterioles regulate the amount of blood flow supplied to the brain by changing their diameters according to the viscosity and the friction of the blood passing through them, also called shear stress. Consequently, the most important aspect of resistance in regulating brain blood flow is the diameter of the vessel whose main activator is blood viscosity.

There is a vasomotor duality that produces either vasodilation or vasoconstriction of brain arterioles, and it is thought to occur from endogenous arteriolar generation of either 20-hydroxyeicosatetraenoic acid (20-HETE), a powerful vasoconstrictor derived from arachidonic acid (AA), or from conversion of AA to the vasodilator epoxyeicosatrienoic acid.⁹⁴

Normal cerebral blood flow (CBF) in adult humans is about 60 mL/100 g/min and, regionally, about 70 mL/100 g/min in gray matter, and 20 mL/100 g/min in white matter. These values normally decrease with age by an estimated rate of about 0.5% per year.⁹⁵ Between the ages of 20 and 65, normal CBF generally declines about 15%–20%.^96,97 It is generally accepted that when CBF reaches 30 mL/100 g/min following brain injury, neurologic symptoms can appear and when CBF falls to 15–20 mL/100 g/min, electrical failure or irreversible neuronal damage can occur even within minutes. Neuroimaging studies show that vascular disturbances can reduce CBF to generate ischemic neurons (<12 mL/100 g/min), penumbral neurons

(12–22 mL/100 g/min), or hypoperfused neurons (>22 mL/100 g/min. Penumbral neurons are functionally impaired but, unlike ischemic neurons, lack structural damage, while hypoperfused neurons may or may not exhibit anatomic or functional changes for extended time periods.²⁷

Penumbral and hypoperfused neurons therefore may be salvageable following brain trauma in the event that a safe and effective treatment can be found. Such a treatment should address most if not all of the following factors:

1. Inhibit platelet aggregability

2. Block vasoconstrictors including thromboxane A₂, endothelins, and 20-HETE 3. Suppress tissue factor to prevent thrombosis

4. Block Na⁺ and Ca⁺ intracellular influx

5. Block glutamatergic activation of NMDA receptors 6. Suppress inflammation

7. Inhibit free radical formation 8. Reduce brain edema

9. Stabilize cell membrane from damage 10. Inhibit toxic cytokines

11. Increase cerebral blood flow by (a) activating smooth muscle cells in brain to increase vasodilation and (b) stimulating PG vasodilators E₁ and prosta-cyclin (I₂)

These factors that are linked with injuries to the brain and spinal cord and the role DMSO plays in their activation are reviewed in the following text and summarized in Figure 8.1.

PROSTAGLANDINS

Concussive brain trauma is often accompanied by platelet aggregation and platelet dysfunction in pial arteries and arterioles due to coagulation disturbances caused by the sudden trauma.^98,99

In brain trauma patients, early coagulopathy is common on hospital admission with the coagulopathic process beginning at the time of trauma.¹⁰⁰ As discussed in the section “Cerebral Hemodynamic Function in Brain Injury” in this chapter, blocked or obstructed brain vessels can lead to energy delivery disturbances whose outcome is brain cell damage or death. For this reason, traumatic brain injury has been the focus of treatments using antiplatelet aggregators such as prostacyclin, a potent vasodilator and platelet deaggregant.¹⁰¹

Although microcirculation is improved following prostacyclin treatment after brain trauma,¹⁰¹ brain edema and cerebral blood flow do not necessarily improve.

This limitation has discouraged the use of prostacyclin and other antiplatelet aggre-gators both in basic and clinical strategies in treating brain trauma.

DMSO has been consistently reported to be an effective platelet deaggregator^101–103 and, unlike prostacyclin, has been shown, following severe insults to the brain, to improve brain edema, cerebral blood flow, CPP, and electroencephalographic activ-ity, while lowering ICP and cerebrovascular reactivity.^41,47–49

The first step in the PG synthesis is the release of the substrate fatty acid, such as AA, from the cellular phospholipids, by the action of the enzyme phospholipase A₂ (Figure 8.2).

There are three ways that PGs can be elaborated. The monoenoic pathway forms PGE₁ and PGE₁-alpha from dihomo-y-linolenate. The bisenoic pathway leads to the elaboration of the endoperoxides (PGG₂ and PGH₂), which are the controlling agents in subpathways producing PGI₂, TXA₂, and PGE₂ (Figure 8.2).

It can be seen from Figure 8.2 that PGI₂ (prostacyclin) and, to a lesser extent, PGE₂ act as potent vasodilators and platelet deaggregators. The antiplatelet aggregation activity by PGI₂ and PGE₂ results from their increase of cAMP levels in platelets.^104,105

The other products, especially TXA₂, are potent vasoconstrictors and platelet aggregators. There is a third trienoic pathway derived from eicosapentaenoic acid, which originates with the membrane-bound fatty acid, but little is known about the effects of its products, TXA₃ and PGI₃, on the cerebral vascular bed. However, it is of interest that the vascular-platelet action of PGI₃ is similar to that of PGI₂, although TXA₃ is not considered analogous to TXA₂ since the latter can increase the levels of cAMP in platelets, thus neutralizing their potential aggregation.^105,106

Formation of PGE₂ can stimulate the levels of adenosine diphosphate (ADP), a mild platelet deaggregator and antagonist of noradrenaline production.¹⁰⁷

Cellular phospholipids

FIGURE 8.2 Simplified scheme of prostaglandin biosynthesis, which starts with the release of a fatty acid nearly always derived from the 2-position of a membrane phospho-lipid. Arachidonic acid is created from diacylglycerol via phospholipase-A2 and then brought to either the cyclooxygenase pathway where either prostaglandins (PG) or thromboxane is formed from synthases that yield prostacyclin (PGI₂) and PGE₂ or thromboxane A₂. PGE₁, PGI₂, and PGE₂ are vasodilators, anti-inflammatory, and platelet deaggregators, while throm-boxane A₂ acts as a vasoconstrictor, inflammatory, and platelet aggregator. DMSO is reported to block thromboxane A2 and promote the release of PGE1, PGI2, and PGE2 either directly or via cAMP stimulation.

Vascular constriction and platelet clumping can be antagonized if PGI₂ is released from endothelial cells in the microvasculature.¹⁰⁴ PGI₂ appears to do this by stimu-lating adenylate cyclase in many tissues including peripheral neurons leading to an increase in cAMP levels in platelets.¹⁰⁸ An increase in PGI₂ synthesis or availability at the vessel wall would have a positive and direct effect on cerebral ischemia by antagonizing platelet aggregation and vasoconstriction (Figure 8.2).

DMSO also has a direct effect on platelet cAMP levels. It increases cAMP pre-sumably by inhibiting phosphodiesterase (PDE)^109,110 although an indirect action on PG1₂-induced elevation of platelet cAMP by DMSO can also be considered (Figure 8.2).

Research has revealed that of the 60 different platelet isoforms described in mam-malian species, platelets are known to possess three main PDEs: PDE₂, PDE₃, and PDE₅.^111,112 Despite many studies carried out in the past four decades examining the role of DMSO in PG synthesis, it remains unclear which of these PDEs DMSO tar-gets for the inhibition of platelet aggregation.

Platelet function is, to a large extent, dependent on PDEs, because their inhi-bition can dampen platelet aggregation by increasing cAMP and cyclic guano-sine 3′-5′-monophosphate (cGMP), an action that limits the levels of intracellular nucleotides.¹¹³

Cyclic AMP is a powerful platelet deaggregator, and evidence exists that DMSO can increase the levels of cAMP in various biological preparations108,110,114,115

(Figure 8.2). Increasing cAMP by DMSO suggests that DMSO can manipulate signal transduction mediated by second messenger molecules such as cAMP. This means that the inhibition of platelet signal transduction by DMSO may suppress platelet acti-vation regardless of the initial stimulus.¹¹¹ The mechanism that would allow DMSO to directly inhibit platelet-activating second messengers like cAMP and cGMP is by its interaction with intracellular signalling pathways. In addition, DMSO may act indirectly on the endothelium-derived PGI₂, which is known to increase intracellular platelet cAMP through the activation of membrane adenylate cyclase, a process that begins when PGI₂ binds to its receptors on the platelet surface.¹¹¹

The role of DMSO in PG biosynthesis remains partly theoretical, but a number of intriguing findings lend support to the specific actions by DMSO on PG activity and their catalytic enzymes (Figure 8.2).

For example, studies have reported that DMSO can increase the synthesis of PGE₁, a moderate vasodilator.^71,72 PGE₁ can reduce platelet aggregation by increasing cAMP levels, and this PG also inhibits the calcium-induced release of noradrenaline in nerve terminals, an effect that may antagonize vasoconstriction following tissue trauma with the resulting effect of preventing cerebral blood flow reduction¹¹⁶ (Figure 8.2).

An increase in PGI₂ synthesis or availability at the vessel wall would have a posi-tive and direct effect on ischemia by antagonizing platelet aggregation and vasocon-striction, thus allowing better blood perfusion of the hypoperfused tissue. DMSO may also antagonize the synthesis or release of thromboxane A₂^117,118 and thus indi-rectly counteract platelet aggregation and vasoconstriction. The presence of TXA₂ in blood will cause strong vasoconstriction and platelet aggregation, while the forma-tion of PGF₂α will also negatively affect the vascular and platelet systems, but the reaction will not be as severe as with TXA₂.¹¹¹

Theoretical considerations to explain platelet deaggregation by DMSO appears to involve PG synthesis in the brain.¹¹⁸

One study has shown evidence that DMSO has the ability to block the prothrom-botic release of thromboxane A₂.¹¹⁹ Although the exact mechanism of DMSO’s blocking action of thromboxane A₂ to reduce platelet aggregation has not been clari-fied, it may possibly mimic the action of hydralazine or dipyridamole since DMSO shares a number of similar biochemical features with these agents, specifically their increase of cAMP levels.^109,118

The actions of DMSO on the biosynthesis of specific PGs may have clinical appli-cation not only in cerebral ischemia secondary to traumatic brain injury but also in conditions where inflammatory cytokines, platelet aggregation, and vasospasm result from disease and physical injury.

DMSO is reported to exert protective activity on tissue factor expression in human endothelial cells in response to TNF-α or thrombin exposure.¹²⁰