A Depletable Pool of Adenosine in Area CA1 of the Rat Hippocampus

Abstract

Adenosine plays a major modulatory and neuroprotective role in the mammalian CNS. During cerebral metabolic stress, such as hypoxia or ischemia, the increase in extracellular adenosine inhibits excitatory synaptic transmission onto vulnerable neurons via presynaptic adenosine A 1 receptors, thereby reducing the activation of postsynaptic glutamate receptors. Using a combination of extracellular and whole-cell recordings in the CA1 region of hippocampal slices from 12-to 24-d-old rats, we have found that this protective depression of synaptic transmission weakens with repeated exposure to hypoxia, thereby allowing potentially damaging excitation to both persist for longer during oxygen deprivation and recover more rapidly on reoxygenation. This phenomenon is unlikely to involve A 1 receptor desensitization or impaired nucleoside transport. Instead, by using the selective A 1 antagonist 8-cyclopentyl-1,3-dipropylxanthine and a novel adenosine sensor, we demonstrate that adenosine production is reduced with repeated episodes of hypoxia. Furthermore, this adenosine depletion can be reversed at least partially either by the application of exogenous adenosine, but not by a stable A 1 agonist, N 6 -cyclopentyladenosine, or by endogenous means by prolonged (2 hr) recovery between hypoxic episodes. Given the vital neuroprotective role of adenosine, these findings suggest that depletion of adenosine may underlie the increased neuronal vulnerability to repetitive or secondary hypoxia/ischemia in cerebrovascular disease and head injury.Key words: adenosine; hypoxia; ischemia; sensor; depletion; replenishment; glutamate; hippocampus; head injury; TBI; stroke; TIA; neuroprotection; adenosine deaminase; nucleoside phosphorylase; xanthine oxidase Extracellular adenosine in the CNS increases during pathological stimuli such as head injury (Nilsson et al., 1990;Headrick et al., 1994), epileptic seizures (Winn et al., 1980;Dunwiddie, 1999), and hypoxia/ischemia (Berne et al., 1974;Rudolphi et al., 1992;Sweeney, 1997;Von Lubitz, 1999). The increase in extracellular adenosine inhibits glutamate release via presynaptic adenosine A 1 receptors (Fowler, 1989;Katchman and Hershkowitz, 1993;Zhu and Krnjevic, 1993;Pearson and Frenguelli, 2000). In addition, simultaneous activation of postsynaptic A 1 receptors activates a potassium conductance leading to membrane hyperpolarization, thereby intensifying the magnesium block of the NMDA subtype of glutamate receptor (Erdemli et al., 1998;Von Lubitz, 1999). Together, these actions exert a powerful neuroprotective "retaliatory" (Newby, 1984) influence during traumatic or metabolic stress.Manipulations that increase extracellular adenosine, such as adenosine uptake inhibition (Gidday et al., 1995), inhibition of adenosine metabolizing enzymes (Phillis and O'Regan, 1989;Miller et al., 1996;Jiang et al., 1997), or activation of A 1 receptors by exogenous A 1 agonists (Rudolphi et al., 1992;Sweeney, 1997;Von Lubitz, 1999;de Mendonca et al., 2000) are all neuroprotective. Conversely, antagonism of ...

Normalization of A2A and A3 adenosine receptor up‐regulation in rheumatoid arthritis patients by treatment with anti–tumor necrosis factor α but not methotrexate

“…Adenosine concentrations were evaluated by using a specific sensor based on the principle of converting adenosine to inosine, hypoxanthine, and uric acid with the evolution of hydrogen peroxide (41). The 3 enzymes required for this process, adenosine deaminase, nucleosidase phosphorylase, and xanthine oxidase, were loaded into single-and double-barreled sensor probes obtained from Sycopel International (Jarrow, UK).…”

NAD(P)H Fluorescence Transients after Synaptic Activity in Brain Slices: Predominant Role of Mitochondrial Function

“…We take this as evidence pointing to the effectiveness of glycolytic inhibition with the inhibitors used here, although whether glycolysis is completely blocked cannot be determined from these results. Synaptic transmission loss was prevented by block of A1 receptors using DPCPX (Coelho et al, 2000;Pearson et al, 2001;Zhao et al, 1997), allowing evaluation of the nature of NAD(P)H transients triggered by a constant synaptic stimulus. It is possible that A1 receptor block could complicate interpretation of results here if, for example, adenosine receptor activation was a crucial link between synaptic stimulation and activation of glycolysis.…”

Measurement of purine release with microelectrode biosensors

“…However, from the point of view of cellular bioenergetics, this means that the ability of the brain to resynthesise ATP after metabolic stress is compromised, which may form the basis of a depletable pool of adenosine that we have described previously [67,68]. This is due to the reliance of the brain on the purine salvage pathway, an energy-efficient means by which to form ATP from its metabolites.…”

“…In these experiments, sensors were invaluable as they allowed the kinetics of enhanced adenosine release to be established, which would not have been possible through the use of receptor antagonists. Furthermore, they permitted the effects of inhibition of ecto-ATPases and ENTs on adenosine release to be established directly since the commonly used surrogate for adenosine release, the depression of excitatory synaptic transmission, is inhibited nonspecifically by POM1 [71], ARL 67156 [72] and by ENT inhibition [67,73]. Given that both D-ribose and adenine have been used safely in humans, these studies offer the possibility that RibAde treatment may be of value in a range of neurological conditions characterised by reductions in tissue ATP, as D-ribose has been for the energetically compromised heart, in vivo and in humans [74].…”

“…There are many studies both in vitro and in vivo that demonstrate the release of adenosine during ischemia and its in vitro analogue (oxygen-glucose deprivation) [22,25,67,68,77,[95][96][97][98][99]. Interestingly, adenosine can be released very early in the onset of an ischemic insult at points where, at least in vitro, complete recovery of neuronal and synaptic function is possible.…”