Neuroprotective effects of adenosine

Comparative neuroprotective effects of pentobarbital, vinpocetine, flunarizine and ifenprodil on ischemic neuronal damage in the gerbil hippocampus

We studied the protective effects of pentobarbital, vinpocetine, flunarizine, and ifenprodil on delayed neuronal death using Mongolian gerbils. The animals were allowed to survive for 7 days after 5 min of cerebral ischemia induced by bilateral occlusion of the common carotid arteries. Hippocampal cell loss was quantified histologically 7 days following ischemia. Intraperitoneal application of pentobarbital (40 mg/kg) 30 min and vinpocetine (50 and 100 mg/kg) 10 min before ischemia significantly reduced neuronal cell loss in the CA1 sector. However, the intraperitoneal administration of flunarizine (10 and 30 mg/kg) and ifenprodil (10 and 30 mg/kg) 15 min before ischemia was not protective. The results suggest that pentobarbital and vinpocetine prevent ischemic neuronal damage, but not flunarizine and ifenprodil. These findings are of interest in relation to the mechanism of delayed neuronal death.

Activation of adenosine receptors induces c-fos, but not c-jun, expression in neuron-glia hybrids and fibroblasts

Extracellular adenosine acts through specific cell surface receptors to modulate numerous physiological processes in both the CNS and peripheral tissues (e.g. neurotransmitter release and blood flow). Activation of A1 or A2 adenosine receptors leads to decreased or increased intracellular cAMP levels, respectively. Fos and Jun are nuclear proto-oncogene products, which, like cAMP, appear to act as intermediates in a number of signal transduction pathways. Since increases in both adenosine release and Fos and Jun expression occur in the brain following seizures, we wanted to determine whether Fos and Jun induction might occur as a result of adenosine receptor activation. 3T3 fibroblasts and NG108-15 neuroblastoma-glioma hybrid cells were chosen for study, since they were known to respond to adenosine agonists with changes in cAMP levels. The membranes of NG108-15 cells were shown to have A2-like binding activity in a competitive binding assay. Cultures of each cell line were treated with the adenosine agonists, CHA (A1-selective) and NECA (non-selective adenosine agonist). Both lines responded with a concentration-dependent transient increase in c-fos, but not c-jun, mRNA content after treatment with either agonist. The kinetics of the response were much more rapid for 3T3 cells (peak between 15 and 30 min) than for NG cells (peak between 60 and 90 min). The slower, more prolonged response in the NG108-15 cells is more similar to the time interval between adenosine release and the peak of c-fos mRNA induction in brains of animals following the administration of seizure-promoting drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine transport systems on dissociated brain cells from mouse, guinea-pig, and rat

The kinetics and sodium dependence of adenosine transport were determined using an inhibitor-stop method on dissociated cell body preparations obtained from mouse, guinea-pig and rat brain. Transport affinity (KT) values for the high affinity adenosine transport systems (KT(H] were significantly different between these three species; mean +/- SEM values were 0.34 +/- 0.1 in mouse, 0.9 +/- 0.2 in rat, and 1.5 +/- 0.5 microM in guinea-pig. The KT values for the low affinity transport system (KT(L) were not different between the three species. Brain cells from rat displayed a significantly greater maximal capacity to accumulate [3H]adenosine (Vmax) than did mouse or guinea-pig for the high affinity system, or than did mouse for the low affinity system. When sodium chloride was replaced in the transport medium with choline chloride, the KT(H) values for guinea-pig and rat were both increased by approximately 100%; only in rat did the change reach statistical significance. The sodium-dependence of adenosine transport in mouse brain was clearly absent. The differences between KT(H) values in mouse and those in guinea-pig or rat were accentuated in the absence of sodium. The differences in kinetic values, ionic requirements, and pharmacological characteristics between adenosine transporters in CNS tissues of mouse, guinea-pig and rat may help account for some of the variability noted among species in terms of their physiological responses to adenosine.

Effect of propentofylline (HWA 285) on extracellular purines and excitatory amino acids in CA1 of rat hippocampus during transient ischaemia

1. The adenosine uptake blocker propentofylline (HWA 285) has previously been shown to protect hippocampal CA1 pyramidal cells from ischaemia-induced delayed neuronal death. The influence of propentofylline, on the extracellular concentrations of purines, aspartate and glutamate in the CA1 of the rat hippocampus during transient forebrain ischaemia was investigated. 2. Twenty min of ischaemia was induced by four-vessel occlusion in Wistar rats, extracellular compounds were sampled by use of microdialysis and EEG was recorded by a tungsten electrode attached to the dialysis probe. 3. Propentofylline (10 mg kg-1 i.p.) did not influence the basal levels of any of the compounds in the hippocampal dialysates. 4. The EEG became isoelectric within 20 s after induction of ischaemia. 5. Extracellular adenosine, inosine, hypoxanthine, aspartate and glutamate increased several fold during ischaemia and remained elevated during early reflow. Within 2 h of reperfusion the concentration of all compounds was normalized. Xanthine increased upon reperfusion and remained elevated after 2 h. 6. Propentofylline (10 mg kg-1 i.p.) administered 15 min before ischaemia significantly enhanced the ischaemia-evoked increase of adenosine but attenuated the increases of the other purine catabolites and of glutamate. 7. In separate in vitro experiments, propentofylline did not inhibit adenosine deaminase activity. 8. The present data show that propentofylline enhances extracellular adenosine and lowers extracellular glutamate in vivo during ischaemia. These findings may be important in relation to the neuroprotective properties of propentofylline.

The pharmacotherapy of focal cortical ischaemia in the mouse

The measurement of cortical omega 3 (peripheral-type benzodiazepine binding) site densities provides an accurate index for the detection and quantification of ischaemic brain lesions following middle cerebral artery occlusion (MCAO) in mice. Here, we have used this marker to assess the neuroprotective activity of potential anti-ischaemic drugs belonging to several chemical classes. In untreated mice, the mean infarcted volume measured 96 h after unilateral coagulation of the middle cerebral artery was 27.9 +/- 4.3 mm3 (17.5% of the hemisphere volume) and omega 3 site densities (measured by incubation with 3H-PK 11195) were increased by 107.3 +/- 4.8% (cortical homogenates) or by 81% (coronal brain sections). The administration of the anti-ischaemic agent SL 82.0715 (10 mg/kg i.p.), 5 min, 6 h and 18 h after the occlusion and then twice daily until sacrifice evoked a decrease of similar magnitude (ca. 60-70%) in the volume of the infarction and in the proliferation of omega 3 sites. The constant tissue sparing effect of SL 82.0715 allowed the examination of the window of therapeutic opportunity. A significant diminution of cortical omega 3 sites was still noted when the first administration was delayed until 3 h post-occlusion. Moreover, the protective effect of SL 82.0715 was enhanced by repeated treatment for the first 36 h but not thereafter. Based on the histological, autoradiographic and homogenate binding results obtained with SL 82.0715, we studied the protective effects of several competitive and non-competitive NMDA receptor antagonists. When administered according to the above-described standard protocol, these drugs reduced omega 3 site levels in cortical homogenates from MCAO mice in a dose-dependent manner. The dose preventing by 50% the increase in omega 3 site levels (in mg/kg i.p.) and the maximal inhibition were respectively: MK-801 (0.2, 93%); TCP (1.6, 66%); kynurenate (260, 58%); ifenprodil (7.0, 58%); SL 82.0715 (1.1, 72%); CGS 19755 (46% at 10 mg/kg); dextromethorphan (46% at 30 mg/kg). In contrast, agents acting preferentially upon sigma (sigma) opiate receptors ((+)-3PPP, 1-10 mg/kg i.p. and haloperidol, 0.3-3 mg/kg i.p.) did not provide a significant protection. In general, calcium channel blockers (nimodipine, flunarizine, verapamil, perhexiline, diltiazem) were devoid of a clear neuroprotective potential when administered at non-toxic doses after the coagulation of the middle cerebral artery. Diltiazem (3 and 10 mg/kg i.p.) provided a significant protection when the first administration was performed 10 min prior to the occlusion. Limited protection was observed with adenosine A1 receptor agonists (N6-cyclohexyladenosine and 2-chloro-adenosine).(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate-evoked release of endogenous adenosine from rat cortical synaptosomes is mediated by glutamate uptake and not by receptors

L-Glutamate (10 microM-1 mM) released endogenous adenosine from rat cortical synaptosomes. Studies with excitatory amino acid antagonists, (+)-5-methyl-16,11,dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), 6,7-dinitroquinoxaline-2,3-dione (DNQX), Mg2+, and agonists N-methyl-D-aspartate (NMDA), kainate, and quisqualate, indicated that this release was not receptor mediated. D,L-2-Amino-4-phosphonobutanoic acid (APB) also did not affect glutamate-evoked adenosine release. Inhibition of glutamate uptake by dihydrokainate or replacement of extracellular Na+ blocked glutamate-evoked adenosine release. D-aspartate, which is a substrate for the glutamate transporter but is not metabolized, also released adenosine, suggesting that release was due to amino acid transport and not to its subsequent metabolism. D-Glutamate, a relatively poor substrate for the transporter, was correspondingly less potent than L-glutamate at releasing adenosine. Glutamate-evoked adenosine release was not Ca2+ dependent or tetrodotoxin sensitive and did not appear to occur on the bidirectional nucleoside transporter. Inhibition of ecto-5'-nucleotidase virtually abolished glutamate-evoked adenosine release, indicating that adenosine was derived from extracellular metabolism of released nucleotide(s). However, L-glutamate did not release ATP and did not appear to release cyclic AMP. Therefore, transport of glutamate into presynaptic terminals releases some other nucleotide which is converted extracellularly to adenosine. This adenosine could act at P1-purinoceptors to modulate glutamatergic neurotransmission.

Cellular and synaptic alterations in the aging brain

The morphological and functional impairments observed in the aging brain are discussed in the framework of theoretical concepts, such as the existence of different modalities of intercellular communication and of specific trophic features in the central nervous system. The relevance of changes at the cellular level (disappearance of neuronal cell bodies and proliferation of astroglial cells) and at the synaptic level (alterations in neurotransmitter and receptor levels) is discussed. Two, non-mutually exclusive hypotheses are advanced to explain the frequent absence of correlation between neuropathological findings and functional deficits in aged patients. According to the first, the physiological reshaping of brain circuits during aging may lead to "wrong" readjustments of neural networks (e.g. due to less effective endogenous and exogenous orienting signals) causing minor morphological alterations but marked functional deficits. The second hypothesis maintains that the absence of correlation between neuropathological and functional deficits is due to the impairment of restricted neuronal populations ("pacemaker and command neurons") which play a special role in the hierarchical organization of neuronal networks. These neurons (inter alia, peptidergic neurons) may also be involved in volume transmission (diffusion of electrical and chemical signals in the extracellular fluid to reach distant targets). Moreover, the relevance of glial cells, not only as regulators of the extracellular medium but also on the basis of their trophic links with neurons, is considered. Finally, the interplay between trophic factors and therapeutical experience for the maintenance and/or recovery of an impaired function in elderly patients is discussed.

Role of excitatory amino acid receptors in K+- and glutamate-evoked release of endogenous adenosine from rat cortical slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.

Alzheimer's disease: changes in hippocampal N-methyl-D-aspartate, quisqualate, neurotensin, adenosine, benzodiazepine, serotonin and opioid receptors--an autoradiographic study

The following receptors were assessed post-mortem in the hippocampi (anterior region) of eight patients with Alzheimer's disease and nine age-matched controls, using autoradiography: N-methyl-D-aspartate (including glutamate, phencyclidine and glycine binding sites), quisqualate, kainic acid, adenosine A1, benzodiazepine, serotonin (1 and 2), muscarinic cholinergic, beta-adrenergic, neurotensin and opioid receptors. In CA1 there were significant parallel losses of binding to the three N-methyl-D-aspartate-linked sites (average reduction 46%) and also losses of quisqualate (38%) and serotonin2 (58%) receptor binding, with a 47% loss of binding to A1 sites. Binding to all of these receptors was also reduced in CA3 (except binding to A1 sites which was normal) but only the serotonin2 receptor binding loss reached significance (52%). A significant reduction in binding was also observed in the entorhinal area to the N-methyl-D-aspartate receptor-linked sites (average reduction = 39%), benzodiazepine (40%) and serotonin2 receptors (45%), and there was a loss of binding to neurotensin (57%) and opioid receptors (42%). Significant reductions in the dentate gyrus molecular layer were seen for serotonin2 receptors (44%), and binding to opioid (44%) and A1 receptors (46%). Levels of ligand binding to muscarinic cholinergic, serotonin1, beta-adrenergic and kainic acid receptors were not significantly different from control values in any of the four areas examined. These results provide support for observations of selective receptor changes in Alzheimer's disease involving a broad range of receptor types which encompass both excitatory amino acid and other receptors (notably serotonin2, A1, benzodiazepine, neurotensin and opioid receptors). The implications of the pattern of receptor changes for the suggestion that excitotoxicity plays a role in the disease are discussed, as is the possible contribution of the receptor changes to the symptomatology of Alzheimer's disease.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Ischemia-induced slowly progressive neuronal damage in the rat brain

Ischemic neuronal damage has been believed to make rapid progress in the course of a few days even in delayed selective neuronal death, to say nothing of acute brain necrosis. In the present study, however, we demonstrate for the first time a new type of ischemia-induced neuronal damage which progresses in the course of several weeks or a few months and we tentatively call this process "slowly progressive neuronal damage". We have focused on the chronological changes of neuronal damage in the dorsolateral striatum and neocortex following various durations of transient middle cerebral artery occlusion, which does not cause cerebral infarction and is clinically designated "transient ischemic attack". In the rats subjected to 15 min middle cerebral artery occlusion, the neocortex and lateral striatum were rarely damaged, whereas the small to medium-sized neurons only in the narrow area restricted to the dorsal striatum showed slowly progressive neuronal damage. Prolongation of ischemic duration to 30 min accelerated the evolution of neuronal damage in the dorsolateral striatum and also extended the distribution of neuronal damage to the neocortex, especially to layer III and more superficial layers. Further prolongation of ischemic duration to 45 min resulted in more rapid progress of selective neuronal death in those areas described above, whereas no animal escaped 60 min ischemia, without acute total tissue necrosis in the middle cerebral artery territory. Ischemia-induced slowly progressive neuronal damage may be implicated in the pathogenesis of such slowly progressive neurologic deterioration as dementia or Parkinsonism in patients with cerebral arteriosclerosis.

Adenosine formation and release by embryonic chick neurons and glia in cell culture

Adenosine formation and release were studied in 48-h-old cultured ciliary ganglia and confluent peripheral and CNS glial cultures from embryonic chicks. Metabolic poisoning induced by 30 mM 2-deoxyglucose and 2 micrograms/ml oligomycin reduced ATP concentration by 90%. An increase in adenosine accounted for 15-40% of the fall in ATP. Dilazep (3 X 10(-6) M), a nucleoside transport inhibitor, decreased both incorporation of adenosine (an index of nucleoside transport) and release of adenosine by 80-90%. Dilazep trapped the newly formed adenosine intracellularly. A concentration of alpha, beta-methylene ADP that inhibited ecto-5'-nucleotidase by 80-90% did not alter the concentration of adenosine or AMP in neurons, glia, or medium. The results demonstrate that adenosine is formed intracellularly and exported out of the cell via the nucleoside transporter. The participation of ecto-5'-nucleotidase was excluded.

Prevention of ischemic neuronal damage by alpha 1-adrenoceptor agonist (methoxamine)

The effect of methoxamine, an alpha 1-adrenoceptor agonist, on ischemic neuronal damage was studied in the gerbil. The animals were subjected to 5 min of ischemia by bilateral common carotid arteries occlusion. Morphological changes and calcium accumulation were evaluated in the CA1 sector of the hippocampus after 7 days of survival. The degree of ischemic neuronal damage and calcium accumulation in the methoxamine-treated groups were significantly attenuated compared with the saline-treated ischemic group. The results suggest that alpha 1-adrenoceptor stimulation prevents ischemic neuronal damage.

Ischaemic damage in gerbil hippocampus is reduced following upregulation of adenosine (A1) receptors by caffeine treatment

We have used the fact that long-term caffeine treatment upregulates the adenosine receptors, to examine the role of adenosine in ischaemia. Four-week oral treatment with caffeine (0.2% in drinking water) caused a significant (10-17%) increase in the binding of the adenosine A1-receptor ligand [3H]cyclohexyl-adenosine (CHA) to several brain regions, including the hippocampal CA1 area, in Mongolian gerbils. Animals subjected to such treatment exhibited significantly less neuronal damage in the CA1 region following 5 min bilateral carotid occlusion than did control animals (50% of the caffeine-treated animals showed no damage at all compared to 11% in the control group). Our findings provide further evidence for a protective role of endogenous adenosine during ischaemia.

A serotonin S2 antagonist, naftidrofuryl, exhibited a protective effect on ischemic neuronal damage in the gerbil

The effect of a serotonin S2 antagonist, naftidrofuryl, on ischemic neuronal damage was examined in the gerbil. Naftidrofuryl was injected i.p. 5 min prior to a single 5-min forebrain ischemia or immediately after each of three 2-min forebrain ischemic insults at 60-min intervals. In both groups the number of intact hippocampal CA1 neurons were significantly higher than in the saline-treated group. These results indicate that serotonin S2 antagonists have a protective effect against ischemic neuronal damage.

Dorsal-ventral gradient in vulnerability of CA1 hippocampus to ischemia: a combined histological and electrophysiological study

Transverse hippocampal slices were prepared after 7 days survival from rats subjected to 8 min of global incomplete ischemia by temporary occlusion of both carotid arteries and hypotension. The slices demonstrated a dorsal-ventral gradient in the amount of ischemic neuronal necrosis in the CA1 region. Histologically ischemic cell change decreased from 90% dorsoseptally to 10% ventrotemporally. Electrophysiological analysis of the number of slices with viable synaptic transmission in CA1 also revealed a septotemporal gradient in susceptibility to ischemia.