The functional role of heteromers of G-protein-coupled receptors is a matter of debate. In the present study, we demonstrate that

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ATP is released in an activity-dependent manner from different cell types in the brain, fulfilling different roles as a neurotransmitter, neuromodulator, in astrocyte-to-neuron communication, propagating astrocytic responses and formatting microglia responses. This involves the activation of different ATP P2 receptors (P2R) as well as adenosine receptors upon extracellular ATP catabolism by ecto-nucleotidases. Notably, brain noxious stimuli trigger a sustained increase of extracellular ATP, which plays a key role as danger signal in the brain. This involves a combined action of extracellular ATP in different cell types, namely increasing the susceptibility of neurons to damage, promoting astrogliosis and recruiting and formatting microglia to mount neuroinflammatory responses. Such actions involve the activation of different receptors, as heralded by neuroprotective effects resulting from blockade mainly of P2X7R, P2Y1R and adenosine A2A receptors (A2AR), which hierarchy, cooperation and/or redundancy is still not resolved. These pleiotropic functions of ATP as a danger signal in brain damage prompt a therapeutic interest to multi-target different purinergic receptors to provide maximal opportunities for neuroprotection.

ATP is released in a vesicular manner from nerve terminals mainly at higher stimulation frequencies. There is a robust expression of ATP (P2) receptors in the brain, but their role is primarily unknown. We report that ATP analogs biphasically modulate the evoked release of glutamate from purified nerve terminals of the rat hippocampus, the facilitation being mediated by P2X 1 , P2X 2/3 , and

The anti-Parkinsonian effect of glutamate metabotropic group 5 (mGluR5) and adenosine A 2A receptor antagonists is believed to result from their ability to postsynaptically control the responsiveness of the indirect pathway that is hyperfunctioning in Parkinson's disease. mGluR5 and A 2A antagonists are also neuroprotective in brain injury models involving glutamate excitotoxicity. Thus, we hypothesized that the antiParkinsonian and neuroprotective effects of A 2A and mGluR5 receptors might be related to their control of striatal glutamate release that actually triggers the indirect pathway. The A 2A agonist, CGS21680 (1-30 nM) facilitated glutamate release from striatal nerve terminals up to 57%, an effect prevented by the A 2A antagonist, SCH58261 (50 nM). The mGluR5 agonist, CHPG (300-600 lM) also facilitated glutamate release up to 29%, an effect prevented by the mGluR5 antagonist, MPEP (10 lM). Both mGluR5 and A 2A receptors were located in the active zone and 57 ± 6% of striatal glutamatergic nerve terminals possessed both A 2A and mGluR5 receptors, suggesting a presynaptic functional interaction. Indeed, submaximal concentrations of CGS21680 (1 nM) and CHPG (100 lM) synergistically facilitated glutamate release and the facilitation of glutamate release by 10 nM CGS21680 was prevented by 10 lM MPEP, whereas facilitation by 300 lM CHPG was prevented by 10 nM SCH58261. These results provide the first direct evidence that A 2A and mGluR5 receptors are co-located in more than half of the striatal glutamatergic terminals where they facilitate glutamate release in a synergistic manner. This emphasizes the role of the modulation of glutamate release as a likely mechanism of action of these receptors both in striatal neuroprotection and in Parkinson's disease.

Adenosine neuromodulation depends on a balanced activation of inhibitory A 1 (A 1 R) and facilitatory A 2A receptors (A 2A R). Both A 1 R and A 2A R modulate hippocampal glutamate release and NMDA-dependent long-term potentiation (LTP) but ageing affects the density of both A 1 R and A 2A R. We tested the effects of selective A 1 R and A 2A R antagonists in the modulation of synaptic transmission and plasticity in rat hippocampal slices from three age groups (young adults, 2-3 month; middle-aged adults, 6-8 months; aged, 18-20 months). The selective A 2A R antagonist SCH58261 (50 nm) attenuated LTP in all age groups, with a larger effect in aged ()63 ± 7%) than in middle-aged adults ()36 ± 9%) or young adult rats ()36 ± 9%). In contrast, the selective A 1 R antagonist DPCPX (50 nm) increased LTP magnitude in young adult rats (+42 ± 6%), but failed to affect LTP magnitude in the other age groups. Finally, in the continuous presence of DPCPX, SCH58261 caused a significantly larger inhibition of LTP amplitude in aged ()71 ± 45%) than middle-aged ()28 ± 9%) or young rats ()11 ± 2%). Accordingly, aged rats displayed an increased expression of A 2A R mRNA in the hippocampus and a higher number of glutamatergic nerve terminals equipped with A 2A R in aged (67 ± 6%) compared with middle-aged (34 ± 7%) and young rats (25 ± 5%). The results show an enhanced A 2A R-mediated modulation of LTP in aged rats, in accordance with the age-associated increased expression and density of A 2A R in glutamatergic terminals. This age-associated gain of function of A 2A R modulating synaptic plasticity may underlie the ability of A 2A R antagonists to prevent memory dysfunction in aged animals.

Adenosine and dopamine are two important modulators of glutamatergic neurotransmission in the striatum. However, conflicting reports exist about the role of adenosine and adenosine receptors in the modulation of striatal dopamine release. It has been previously suggested that adenosine A 1 receptors localized in glutamatergic nerve terminals indirectly modulate dopamine release, by their ability to modulate glutamate release. In the present study, using in vivo microdialysis, we provide evidence for the existence of a significant glutamate-independent tonic modulation of dopamine release in most of the analyzed striatal compartments. In the dorsal, but not in the ventral, part of the shell of the nucleus accumbens (NAc), blockade of A 1 receptors by local perfusion with the selective A 1 receptor antagonist 8-cyclopentyl-1,3-dimethyl-xanthine or by systemic administration of the non-selective adenosine antagonist caffeine induced a glutamate-dependent release of dopamine. On the contrary, A 1 receptor blockade induced a glutamate-independent dopamine release in the core of the NAc and the nucleus caudate-putamen. Furthermore, using immunocytochemical and functional studies in rat striatal synaptosomes, we demonstrate that a fraction of striatal dopaminergic terminals contains adenosine A 1 receptors, which directly inhibit dopamine release independently of glutamatergic transmission.

Please cite this article in press as: Canas, P.M., et al., Modification upon aging of the density of presynaptic modulation systems in the hippocampus, Neurobiol Aging (2008) Abstract Different presynaptic neuromodulation systems have been explored as possible targets to manage neurodegenerative diseases. However, most studies used young adult animals whereas neurodegenerative diseases are prevalent in the elderly. Thus, we now explored by Western blot analysis how the density of different presynaptic markers and receptors changes with aging in rat hippocampal synaptosomes (purified nerve terminals). Compared to synaptosomal membranes from 2-month-old rats, the density of presynaptic proteins (synaptophysin or SNAP-25) decreased at 18-24 months. In parallel, markers of glutamatergic terminals (vGluT1 or vGluT2) and cholinergic terminal markers (vAChT) constantly decreased with aging from 12 to 18 months onwards, whereas the densities of GABAergic (vGAT) only decreased after 24 months. Inhibitory A 1 and CB 1 receptor density tended to decrease with aging, whereas facilitatory mGluR5 and P2Y1 receptor density was roughly constant and facilitatory A 2A receptor density increased at 18-24 months. Thus aging causes an imbalance of excitatory versus inhibitory nerve terminal markers and causes a predominant decrease of inhibitory rather than facilitatory presynaptic modulation systems.