Propentofylline administered by microdialysis attenuates ischemia-induced hippocampal damage but not excitatory amino acid release in gerbils

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Adenosine signaling and function in glial cells

Despite major advances in a variety of neuroscientific research fields, the majority of neurodegenerative and neurological diseases are poorly controlled by currently available drugs, which are largely based on a neurocentric drug design. Research from the past 5 years has established a central role of glia to determine how neurons function and, consequently, glial dysfunction is implicated in almost every neurodegenerative and neurological disease. Glial cells are key regulators of the brain's endogenous neuroprotectant and anticonvulsant adenosine. This review will summarize how glial cells contribute to adenosine homeostasis and how glial adenosine receptors affect glial function. We will then move on to discuss how glial cells interact with neurons and the vasculature, and outline new methods to study glial function. We will discuss how glial control of adenosine function affects neuronal cell death, and its implications for epilepsy, traumatic brain injury, ischemia, and Parkinson's disease. Eventually, glial adenosine-modulating drug targets might be an attractive alternative for the treatment of neurodegenerative diseases. There are, however, several major open questions that remain to be tackled.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Adenosine uptake inhibitors

Adenosine is a purine nucleoside and modulates a variety of physiological functions by interacting with cell-surface adenosine receptors. Under several adverse conditions, including ischemia, trauma, stress, seizures and inflammation, extracellular levels of adenosine are increased due to increased energy demands and ATP metabolism. Increased adenosine could protect against excessive cellular damage and organ dysfunction. Indeed, several protective effects of adenosine have been widely reported (e.g., amelioration of ischemic heart and brain injury, seizures and inflammation). However, the effects of adenosine itself are insufficient because extracellular adenosine is rapidly taken up into adjacent cells and subsequently metabolized. Adenosine uptake inhibitors (nucleoside transport inhibitors) could retard the disappearance of adenosine from the extracellular space by blocking adenosine uptake into cells. Therefore, it is expected that adenosine uptake inhibitors will have protective effects in various diseases, by elevating extracellular adenosine levels. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain, and inflammatory diseases have been reported. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors.

Purines and neuroprotection

The activation of adenosine A1, A2 andA3 receptors can protect neurones against damage generated by mechanical or hypoxic/ischaemic insults as well as excitotoxins. A1 receptors are probably effective by suppressing transmitter release and producing neuronal hyperpolarisation. They are less likely to be of therapeutic importance due to the plethora of side effects resulting from A1 agonism, although the existence of receptor subtypes and recent synthetic chemistry efforts to increase ligand selectivity, may yet yield clinically viable compounds. Activation of A2A receptors can protect neurons, although there is much uncertainty as to whether agonists are acting centrally or via a peripheral mechanism such as altering blood flow or immune cell function. Selective antagonists at the A2A receptor, such as 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-yl-amino]ethyl)phenol (ZM 241385) and 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261), can also protect against neuronal death produced by ischaemia or excitotoxicity. In addition, A2A receptor antagonists can reduce damage produced by combinations of subthreshold doses of the endogenous excitotoxin quinolinic acid and free radicals. Since the A2A receptors do not seem to be activated by normal endogenous levels of adenosine, their blockade should not generate significant side effects, so that A2A receptor antagonists appear to be promising candidates as new drugs for the prevention of neuronal damage. Adenosine A3 receptors have received less attention to date, but agonists are clearly able to afford protection against damage when administered chronically. Given the disappointing lack of success of NMDA receptor antagonists in human stroke patients, despite their early promise in animal models, it is possible that A2A receptor antagonists could have a far greater clinical utility.

Mechanisms of glutamate release in the rat spinal cord slices during metabolic inhibition

Glutamate toxicity is a viable hypothesis to explain the expanding tissue degeneration occurring after traumatic or ischemic spinal cord injury. One important component in this process is the acute, excessive release of glutamate. In the current communication, the glycolytic inhibitor iodoacetate was used to induce metabolic inhibition in spinal cord slices and thereby provide an in vitro model to study the mechanisms of pathological glutamate release in the spinal cord. The evoked glutamate release was not Ca2+-dependent. Exclusion of NaCl reduced the evoked release of endogenous glutamate by 56%, while excluding Na+ increased release. Glutamate release was also reduced by the PLA2 inhibitors indomethacin (40%), arachidonyltrifluoromethyl ketone (45%) and 4-bromophenacyl bromide (36%). Blocking reverse glutamate transport by preincubation with 1 mM dihydrokainic acid reduced evoked release by 41%. However, when the dihydrokainic acid and arachidonyltrifluoromethyl ketone treatments were combined, no additive effect of the two substances was seen. These findings suggest that glutamate is released by three mechanisms from the energy compromised spinal cord: (1) in response to cellular swelling, most likely by the regulatory volume decrease, (2) by PLA2-mediated breakdown of the cell membrane and diffusion of glutamate down its concentration gradient, and (3) through reversal of the glutamate transporter.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

Effects of xanthine derivatives on electroretinographic responsiveness

In view of the use of synthetic propentofylline (PPF) as a protective agent in brain ischemia, its possible side effects on vision capacities have been explored by electroretinography in comparative experiments with theophylline. We used eyecup preparations of small-spotted dogfish sharks and of European eels, particularly suitable for long-lasting experiments. The drug exerted profound but reversible modifications of ERG records: (1) a dose-dependent increase of the amplitude and duration of the chemically isolated late receptor potential (LRP), (2) a partial unmasking of LRP, (3) a strong potentiation of the LRP-unmasking effect of low temperature, (4) a potentiation of light adaptation effects, and (5) a strong potentiation of the post-illumination hyperexcitability. The effects were explicable as due to a strong phosphodiesterase (PDE) inhibiting, cyclic guanosine monophosphate (cGMP) promoting, action of the drug. The effects were considerably stronger, or even of opposite sign, in comparison to those of the chemically related theophylline. PPF did not seriously affect the ERG c-wave originating in the pigment epithelium. The results suggested that the effects of PPF on vision may not seriously hamper the therapeutic use of the drug. They indicated, on the other hand, that PPF was a retinoactive drug of potential usefulness in the exploration of the complex biochemical events underlying visual transduction.

Vulnerability of the gerbil cochlea to sound exposure during reversible ischemia

Cochlear ischemia induces a sensorineural hearing loss, in part through a fast functional impairment of outer hair cellls. Assuming that the cochlea is rendered fragile during ischemia and reperfusion and that stimulation itself can jeopardize its functional recovery, we used a model of reversible selective cochlear ischemia in Mongolian gerbils to establish what type of sound exposure can be deleterious during and immediately after reversible ischemia. Several groups of gerbils were used, with different ischemia durations and levels of sound exposure. Control groups were only exposed to tones at 80 and 90 dB SPL during 30 min, while other groups underwent complete and fully reversible blockage of the labyrinthine artery, during 5.5 or 8 min, and were exposed to 60 or 80 dB SPL tones during 30 min. The amount of ischemia and reperfusion was measured by means of laser Doppler velocimetry, whereas outer hair cells' function was continuously monitored through distortion-product otoacoustic emissions (DPOAEs). The losses of DPOAE levels after 8 min transient ischemia and 60 dB SPL exposure were as large as those induced by 80 dB SPL exposures combined with 5.5 min ischemia, or 90 dB SPL exposures without ischemia, with a maximum loss around 25-30 dB, half an octave above the stimulus frequency. These results give evidence for an extremely high cochlear vulnerability to low-level sound exposure when associated with reversible ischemia. This vulnerability may have important clinical consequences in patients with cochlear circulatory disturbances.

Propentofylline protects neurons in culture from death triggered by macrophage or microglial secretory products

We recently demonstrated that conditioned medium (CM) from peritoneal macrophages or activated microglia triggers a predominantly apoptotic death in hippocampal neurons in culture. We tested the effects of propentofylline (ppf), an agent that is neuroprotective in focal ischemia and is also associated with reduced microglial antigen expression after insult. Ppf had no impact on the secretion of neurotoxin from microglia. However, ppf significantly attenuated the effects of macrophage and microglial conditioned medium on neurons. Ppf did not attenuate neuronal hypoxic injury but did reverse the exaggeration of hypoxic injury exerted by subsequent addition of macrophage CM. A1 and A2 adenosine receptor inhibitors and an inhibitor of adenosine uptake each mimicked the effect of ppf. Neither ATP nor a deaminase inhibitor blocked the effect of microglial CM. These findings may be relevant to the neuroprotective effects of ppf in ischemia and dementia.

Restoring adenine nucleotides in a brain slice model of cerebral reperfusion

Tissue adenine nucleotides are depleted during cerebral ischemia, impeding recovery after reperfusion. Although prior studies have attempted to prevent the initial loss of adenylates, the present study tests the hypothesis that stimulating synthesis of adenine nucleotides, through either adenosine kinase or adenine phosphoribosyltransferase, would result in significant cerebroprotection. To study the effects on neurons and glia directly while avoiding the influence of the cerebral vasculature, hippocampal brain slices were used for the model of transient ischemia with reperfusion. The standard brain slice insult of brief exposure to anoxia with aglycemia was modified based on studies which showed that a 30-minute exposure to air with 1 mmol/L glucose produced a stable, moderate reduction in ATP during the insult and that, 2 hours after return to normal conditions, there was moderate depletion of tissue adenine nucleotides and histologic injury. Treatments with 1 mmol/L adenosine, AMP, or adenine were equivalent in partially restoring adenine nucleotides. Despite this, only adenosine afforded histologic protection, suggesting a protective role for adenosine receptors. There also was evidence for metabolic cycling among adenine nucleotides, nucleosides, and purines. Adenosine may exert direct cerebroprotective effects on neural tissue as well as indirect effects through the cerebral vasculature.

Focal cerebral ischemia enhances glial expression of ecto-5'-nucleotidase

The effect of ischemia on the reactive expression of ecto-5'-nucleotidase in rat brain was studied 6 h and 1, 2 and 7 days after permanent middle cerebral artery occlusion (MCAO). The distribution of 5'-nucleotidase in the infarcted brain was compared to markers for astrocytes (glial fibrillary acidic protein (GFAP)) and microglia (complement receptor type 3, antibody OX42) using histological staining or immunohistochemistry. 5'-Nucleotidase could be associated with reactive astrocytes by immunohistochemistry and with reactive microglia by enzyme histochemistry. In the untreated control 5'-nucleotidase was associated with astrocytes only in the hippocampus and the submeningeal space. After ischemia the enzyme was expressed on reactive astrocytes in the tissue surrounding the volume of infarction. Individual reactive astrocytes were observed 6 h after MCAO and the astrocytic expression became continuously enhanced during the following days. An enzyme histochemical analysis of 5'-nucleotidase activity revealed a postischemic increase in reaction product around the infarcted tissue. Seven days after MCAO a discrete band (0.2-0.4 mm) of reaction product characterized the rim of the infarcted area. This band of activity of 5'-nucleotidase colocalized with a band of immunoreactivity for OX42, indicative of an intense accumulation of 5'-nucleotidase expressing microglia. Our results suggest that ischemia following permanent MCAO results in an upregulation of the capacity for the hydrolysis of nucleotides within the tissue adjacent to the infarcted volume. Nucleotides released from the damaged cells can be hydrolyzed and the adenosine eventually formed may exert neuroprotective functions limiting the extent of damage.

Monitoring of functional changes after transient ischemia in gerbil cochlea

Ischemia and reperfusion are involved in numerous sensorineural pathologies. A model of reversible cochlear ischemia has been designed in Mongolian gerbil. Selective labyrinthine ischemia of variable duration (4-10 min) was achieved through a posterior transcranial approach. Ischemia and reperfusion were controlled with the help of laser Doppler velocimetry. Functional changes were monitored every 1-10 s throughout experiments, using cochlear potentials and otoacoustic emissions. After interruption of blood flow, all signals rapidly began to decay. In contrast to cochlear potentials, otoacoustic emissions always exhibited a plateau before reaching noise floor only after approximately 4-5 min. Upon ischemia release, cochlear blood flow recovered instantly and completely and cochlear potentials rapidly improved in most cases, in contrast to otoacoustic emissions that underwent a delayed decay after immediate partial recovery. The phase and group latency of otoacoustic emissions exhibited only small changes throughout ischemia and reperfusion, suggesting adaptive rather than damaging mechanisms. Cochlear function returned to normal after 5 min 30 s ischemia but longer complete ischemia sometimes led to irreversible damage despite the systematic presence of some recovery just after ischemia release. This behavior suggests that reperfusion in itself can be deleterious to a sensorineural organ and this model can be useful for identifying the noxious mechanisms of ischemia and reperfusion.

Effects of an inhibitor of adenosine deaminase, deoxycoformycin, and of nucleoside transport, propentofylline, on post-ischemic recovery of adenine nucleotides in rat brain

The effects of an adenosine deaminase inhibitor (deoxycoformycin, 500 mu g/kg) and of an inhibitor of nucleoside transport (propentofylline, 10 mg/kg) on adenosine and adenine nucleotide levels in the ischemic rat brain were investigated. The brains of the rats were microwaved before, at the end of a 20 min period of cerebral ischemia (4 vessel occlusion + hypotension), or after 5, 10, 45, and 90 min of reperfusion. Deoxycoformycin increased brain adenosine levels during both ischemia and the initial phases of reperfusion. AMP levels were elevated during ischemia and after 5 min of reperfusion. ATP levels were elevated above those in the non-treated animals after 10 and 45 min of reperfusion. ADP levels were elevated above the non-drug controls at 90 min. These increases in ATP, ADP and AMP resulted in significant increases in total adenylates during ischemia, and after 10 min and 90 min of reperfusion. Propentofylline administration resulted in enhanced AMP levels during ischemia but did not alter adenosine or adenine nucleotide levels during reperfusion in comparison with non-treated controls.

Polyamines in brain tumor therapy

In the search for ways to augment current brain tumor therapies many have sought to exploit the fact that adult brain tissue is virtually lacking in cell division. This endorses a special appeal to therapeutic approaches which target the dependence on cell division for brain tumor growth. Polyamines play an essential role in the proliferation of mammalian cells and depletion results in inhibition of growth. As a result, there are investigations into the feasibility of controlling tumor growth by targeting the enzymes in polyamine metabolism with specific enzyme inhibitors. DFMO, an inhibitor of putrescine synthesis, is a cytostatic agent which in combination with tritiated radioemitters or cytotoxic agents such as, MGBG or BCNU is an effective antitumor agent, but the effectiveness of DFMO in vivo is reduced by tumor cell uptake of polyamines released into the circulation by normal cells and from gut flora or dietary sources. However, DFMO therapy combined with elimination of exogenous polyamines inhibits tumor growth but also results in body weight loss, reduced protein synthesis and evidence of toxicity. Furthermore, tumor growth recurs upon termination of treatment. In contrast, competitive polyamine analogs function in the homeostatic regulation of polyamine synthesis but fail to fulfill the requirements for growth and they continue to inhibit tumor growth for several weeks after cessation of treatment. Analogs are now in clinical trials. However, their action may be highly specific and differ from one cell type to another. We suggest that the effectiveness of polyamine based therapy would be enhanced by two approaches: local delivery by intracerebral microdialysis and tumor cell killing by internal radioemitters such as tritiated putrescine or tritiated thymidine which are taken up in increased amounts by polyamine depleted tumor cells. The growth inhibition by polyamine depletion prevents the dilution of the radioactive putrescine and thymidine. The overload of radioactivity kills the growth inhibited cells so that growth cannot recur when treatment terminates.